Fuel 94 (2012) 1–33
Contents lists available at SciVerse ScienceDirect
Fuel
journal homepage: www.elsevier.com/locate/fuel
Review article
An overview of the organic and inorganic phase composition of biomass
Stanislav V. Vassilev a,b,⇑, David Baxter a, Lars K. Andersen a, Christina G. Vassileva b, Trevor J. Morgan a
a
b
Institute for Energy and Transport, Joint Research Centre, European Commission, P.O. Box 2, NL-1755 ZG Petten, The Netherlands
Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad. G. Bonchev Street, Block 107, Sofia 1113, Bulgaria
a r t i c l e
i n f o
Article history:
Received 19 May 2011
Received in revised form 7 September 2011
Accepted 14 September 2011
Available online 8 October 2011
Keywords:
Biomass
Phase and chemical composition
Structural components
Inorganic matter
Classifications
a b s t r a c t
An extended overview of the organic and inorganic phase composition of biomass was conducted. Some
general considerations and problems related to phase composition of biomass as a solid fuel were discussed initially. Then, reference peer-reviewed data including contents of cellulose, hemicellulose, lignin
and bulk extractives of 93 varieties of biomass were used and grouped for their comparison and classification. Additionally, reference peer-reviewed data and own investigations for various minor organic
components and minerals, and modes of element occurrence identified in biomass were also applied
and organised to describe the biomass systematically. It was found that the phase distinctions among
the specified natural and anthropogenic (technogenic) biomass groups, sub-groups and varieties are significant and relate to different biomass sources and origin. The phase composition of biomass is highly
variable due to the extremely high variations of structural components and different genetic types (authigenic, detrital and technogenic) of inorganic matter. The technogenic biomass group is quite complicated
as a result of incorporation of various non-biomass materials during biomass processing. It was identified
that the biomass phase composition is significantly different from that of coal. Correlations and associations among phase and chemical characteristics were studied to find some major trends and important
relationships occurring in the natural biomass system. Certain leading associations related to the occurrence, content and origin of elements and phases in biomass were identified and discussed, namely: (1)
CAH (mainly as authigenic cellulose, hemicellulose, lignin and organic extractives); (2) SiAAlAFeANaATi
(mostly as detrital silicates and oxyhydroxides, excluding authigenic opal); (3) CaAMgAMn (commonly
as authigenic oxalates and carbonates); and (4) NAKASAPACl (normally as authigenic phosphates, sulphates, chlorides and nitrates). Finally, it was emphasised that these important associations have potential applications and can be used for initial classifications or prediction and indicator purposes connected
with future advanced and sustainable processing of biomass to biofuels and chemical feedstock.
Ó 2011 Elsevier Ltd. All rights reserved.
Contents
1.
2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.
General considerations about biomass composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.
Common issues concerning phase-mineral composition of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Materials, methods and data used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.
General observations about phase-mineral composition of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.1.
Distribution of structural components and bulk extractives in biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.2.
XRD of biomass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1.3.
XRD of dry water-soluble residue (DWR) of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1.4.
XRD of biomass high-temperature ash (HTA). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2.
Organic matter of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2.1.
Cellulose (Cel) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2.2.
Hemicellulose (Hem) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.2.3.
Lignin (Lig) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
⇑ Corresponding author at: Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad. G. Bonchev Street, Block 107, Sofia 1113, Bulgaria. Tel.: +359
2 9797055; fax: +359 2 9797056.
E-mail address: [email protected] (S.V. Vassilev).
0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.fuel.2011.09.030
2
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
4.
5.
6.
3.2.4.
Bulk extractives (Ext) and minor organic components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.5.
Organic minerals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.
Fluid matter of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.
Inorganic matter of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.1.
Silicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.2.
Oxides and hydroxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.3.
Sulphates, sulphites and sulphides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.4.
Phosphates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.5.
Carbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.6.
Chlorides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.7.
Nitrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.8.
Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Potential applications of phase-mineral and chemical composition of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Potential biomass resources for biofuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
1.1. General considerations about biomass composition
Since it is considered that the biomass system and respective
biofuels as sub-systems do not contribute to the greenhouse effect
due to the CO2 neutral conversion, extensive investigations have
been carried out worldwide to enhance biomass use by substituting fossil fuels for energy conversion. The focus on bioenergy as
an alternative has increased tremendously during the last few
years because of global warming problems originated mostly from
fossil fuels combustion ([1] and references therein). Two fundamental aspects related to biomass use as fuel are: (1) to extend
and improve the basic knowledge on composition and properties;
and (2) to apply this knowledge for the most advanced and sustainable utilisation of biomass. The systematic identification, quantification and characterisation of chemical and phase composition
of a given solid fuel are the initial and most important steps during
the investigation and application of such fuel. This composition is a
fundamental code that depends on various factors and definite
properties, quality and application perspectives, as well as technological and environmental problems related to any fuel [1].
As a first step, an extended overview of the chemical composition of biomass was recently conducted [1]. This overview emphasised some general considerations and aspects related to biomass
and particularly problems associated with the composition of this
fuel. Reference peer-reviewed data for chemical composition of 86
varieties of biomass, including traditional and complete proximate,
ultimate and ash analyses (totally 21 characteristics), were used to
describe the biomass system. It was demonstrated that:
(1) The chemical composition of biomass and especially ash
components are highly variable due to the extremely high
variations of moisture, ash yield (shortly ash) and different
genetic types of inorganic matter in biomass.
(2) The chemical distinctions among the specified natural and
anthropogenic biomass groups and sub-groups are significant and they are related to different biomass source and
origin, namely from plant and animal products or from
mixtures of plant, animal and manufacture materials
(Table 1).
(3) The major and minor elements in biomass, in decreasing
order of abundance, are commonly C, O, H, N, Ca, K, Si, Mg,
Al, S, Fe, P, Cl, Na, Mn and Ti. The biomass chemical composition is significantly different from that of coal and the variations among biomass composition were also found to be
21
22
22
23
24
25
25
25
25
25
26
26
26
28
29
30
30
greater than for coal. Natural biomass is normally: (1) highly
enriched in Mn > K > P > Cl > Ca > (Mg, Na) > O > moisture >
volatile matter; (2) slightly enriched in H; and (3) depleted
in ash, Al, C, Fe, N, S, Si and Ti in comparison with coal.
(4) Some fundamental trends and important relationships were
also found in the natural biomass system based on the correlations and associations among chemical characteristics.
Five strong and important associations, namely: (1) CAH;
(2) NASACl; (3) SiAAlAFeANaATi; (4) CaAMgAMn; and
(5) KAPASACl; were identified. The potential applications
of these associations for initial and preliminary classification, prediction and indicator purposes related to biomass
were also introduced.
The chemical overview concluded that future detailed and systematised data on the phase-mineral composition of biomass are
required as a second step to explain chemical trends and associations. Therefore, the major purpose of this subsequent work is to
perform an overview of the organic and inorganic phase composition of biomass.
1.2. Common issues concerning phase-mineral composition of biomass
Some preliminary observations and major factors concerning
the different phase-mineral composition of biomass have also been
given in the former extended overview [1]. Briefly, biomass is
contemporaneous (non-fossil) and a complex biogenic organic–
inorganic solid product formed by natural and anthropogenic
(technogenic) processes, and comprises: (1) natural constituents
originated from growing land- and water-based vegetation via
photosynthesis or generated via animal and human food digestion;
and (2) technogenic products derived via processing of the above
natural constituents. Biomass, similar to solid fossil fuels, is a
complex heterogeneous mixture of organic matter and, to a lesser
extent, inorganic matter, containing various solid and fluid intimately associated phases (Table 2). The phases in biomass originate from natural (authigenic and detrital) and anthropogenic
processes during pre-syngenesis, syngenesis, epigenesis and postepigenesis of biomass according to the main formation process
and place, time and mechanism of phase formation (Table 3). The
contaminated biomass (semi-biomass) is more complex and quite
complicated in comparison with the natural biomass as a result of
incorporation of various non-biomass materials through processing steps (Table 1).
A detailed review of the scientific literature, including more
than 490 mostly peer-reviewed references, and data compilations
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
3
Nomenclature
adb
BC
CC
Cel
daf
db
DWR
Ext
FC
Hem
HTA
Lig
LTA
MM
PP
R2
RH
SG
SS
VM
WS
%
air-dried basis
beech wood chips
corn cobs
cellulose
dry, ash-free basis
dry basis
dry water-soluble residue
bulk extractives
fixed carbon
hemicellulose
high-temperature ash (>500 °C)
have been conducted to systematise the results obtained for
biomass composition. It was found that numerous studies on phase
composition of biomass have been performed worldwide and data
for different biomass varieties have been generated and published
[2–171]. As a result, significant information for organic matter (cellulose, hemicellulose, lignin, extractives and other minor phases)
and, to a lesser extent, inorganic matter (various minerals or
phases) and modes of element occurrence identified in biomass
lignin
low-temperature ash (100–250 °C)
marine macroalgae
plum pits
correlation coefficient
rice husks
switchgrass
sunflower shells
volatile matter
walnut shells
weight%
has been collected and arranged alphabetically and chronologically
in time (Table 4). These results provide a sound foundation for an
initial database that can be used for phase characterisation and
subsequent classification and sustainable exploitation of biomass
as fuel. It was also found that serious problems related to phase
investigations of biomass occur and some of them are similar to
those determined for chemical studies of biomass [1]. Therefore,
an attempt to summarise the problems related to phase investiga-
Table 1
General classification of biomass varieties as solid fuel resources according to their biological diversity, source and origin.
Biomass groups
Biomass sub-groups, species and varieties
1. Wood and woody biomass
Coniferous or deciduous, angiospermous or gymnospermous and soft or hard such as stems, barks, branches (twigs),
leaves (foliage), bushes (shrubs), chips, lumps, pellets, briquettes, sawdust, sawmill and others from various wood
species
2. Herbaceous and agricultural biomass
Annual or perennial, arable or non-arable and field-based or processed-based biomass from various species such as:
2.1. Grasses and flowers (alfalfa, arundo, bamboo, bana, cane, miscanthus, reed canary, ryegrass, switchgrass,
timothy, others)
2.2. Straws (barley, bean, corn, flax, mint, oat, paddy, rape, rice, rye, sesame, sunflower, triticale, wheat, others)
2.3. Stalks (alfalfa, arhar, arundo, bean, corn, cotton, kenaf, mustard, oreganum, sesame, sunflower, thistle,
tobacco, others)
2.4. Fibers (coconut coir, flax, jute bast, kenaf bast, palm, others)
2.5. Shells and husks (almond, cashewnut, coconut, coffee, cotton, hazelnut, millet, olive, peanut, rice, sunflower,
walnut, others)
2.6. Pits (apricot, cherry, olive, peach, plum, others)
2.7. Other residues (fruits, pips, grains, seeds, coir, cobs, bagasse, food, fodder, marc, pulps, cakes, others) from
various species
3. Aquatic biomass
Marine or freshwater, macroalgae or microalgae and multicellular or unicellular species (blue, blue-green, brown,
golden, green and red algae; diatoms, duckweed, giant brown kelp, kelp, salvinia, seaweed, sweet-water weeds,
water hyacinth, others)
4. Animal and human biomass wastes
Bones, chicken litter, meat-bone meal, sponges, various manures, others
5. Contaminated biomass and industrial biomass
wastes (semi-biomass)
Municipal solid waste, demolition wood, refuse-derived fuel, sewage sludge, hospital waste, paper-pulp sludge,
waste papers, paperboard waste, chipboard, fibreboard, plywood, wood pallets and boxes, railway sleepers, tannery
waste, others
6. Biomass mixtures
Blends from the above varieties
Table 2
Phase composition of biomass.
Matter
State and type of constituents
Phases and components
1. Organic matter
1.1. Solid, non-crystalline
1.2. Solid, crystalline
Structural ingredients (cellulose, hemicellulose, lignin), extractives, others
Organic minerals such as CaAMgAKANa oxalates, others
2. Inorganic matter
2.1. Solid, crystalline
2.2. Solid, semi-crystalline
2.3. Solid, amorphous
Mineral species from different mineral classes (silicates, oxyhydroxides, sulphates, phosphates, carbonates,
chlorides, nitrates, others)
Poorly crystallized mineraloids of some silicates, phosphates, hydroxides, chlorides, others
Amorphous phases such as various glasses, silicates, others
Fluid, liquid, gas
Moisture, gas and gas–liquid inclusions associated with both organic and inorganic matter
3. Fluid matter
(mostly inorganic)
4
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Table 3
Origin of phases in biomass.
Formation
process
Place of formation
Time of formation
Formation mechanism
1. Natural
1.1. Authigenic (formed in biomass)
1.1.1. Syngenetic (during
plant growing)
Generated by biogenic processes of growing plants (photosynthesis,
diffusion, adsorption, osmosis, pinocytose, endocytose, exocytose,
hydrolysis, precipitation, others)
Originated by natural processes after plants died (evaporation,
precipitation)
Pre-existing and finely dispersed mineral grains (commonly <1 lm)
introduced into the plant by water suspensions during syngenesis
(endocytose)
Pre-existing and fine-grained particles (normally <10–100 lm) introduced
by water and wind on plant surfaces and fixed in pores, voids and cracks
1.2. Detrital (formed outside biomass,
but fixed in/on biomass)
1.1.2. Epigenetic (after plant
died)
1.2.1. Pre-syngenetic
(before plant growing)
1.2.2. Pre-syngenetic,
syngenetic or epigenetic
2. Anthropogenic
Technogenic (formed outside or
inside biomass and fixed in/on
biomass)
Post-epigenetic (during and
after plant collecting)
tions of biomass as fuel was initially undertaken and is described
below:
(1) There is a huge amount of data on the phase composition of
biomass in Internet and numerous scientific reports from
projects, conference proceedings and workshop presentations, as well as in different databases and many publications; however, the use of such information is insecure
because the data are not peer-reviewed.
(2) The long term experience and knowledge achieved for the
phase composition of the most studied solid fuels such as coal,
peat, petroleum coke and municipal solid waste or refusederived fuel ([39,40,172–175] and references therein) have
not been implemented very successfully in the field of biomass (see below). Therefore, the use of unsuitable scientific
approaches and procedures, incomplete data or unusual and
sometimes inappropriate terms is common in many investigations of biomass.
(3) The lack of generally accepted terminology, classification systems and standards worldwide about the phase composition
of biomass lead to very serious misunderstanding during the
investigations. The biggest problem seems to be the absence
of appropriate phase classifications for biomass (Table 2) in
contrast to some other fuels. For example, there are systematic and relatively strict phase specifications for coal (probably also applicable to biomass) including organic
ingredients, namely lithotypes, microlithotype groups and
macerals, and inorganic matter such as mineral classes,
groups and species, which are well recognised worldwide
and relatively well characterised ([172,173,176] and references therein).
(4) A serious omission is the limited description of the specific
type, place and manner of collection, as well as storage
and processing conditions of biomass as samples or feedstock. The exact fuel status of the samples studied, namely
as-collected, as-received, air-dried or oven-dried basis, is
also very often not reported. These are serious problems
for the reliable identification and characterisation of phases
in biomass. Representative sampling problems associated
with biomass also occur and some of them have been discussed [71,134,177].
(5) The solid biomass fuels are a result of natural and anthropogenic formation processes (Table 3) and a significant part
(occasionally dominant) of contaminated biomass contains
other non-biomass phases [39,40]. Therefore, this semi-biomass should always be considered separately from natural
biomass due to the different origin, composition and potential use.
Natural and/or industrial components (dust, materials, additives,
contaminants, others) introduced in biomass during collecting, handling,
transport and subsequent processing steps
(6) The detailed and complete data from simultaneous chemical
and phase-mineral analyses for the biomass varieties are
very scarce. Thus, relatively little systematic and advanced
information is available from combined investigations
[1,11,178,179].
(7) The methods for phase and mineral investigations of biomass have not been developed and implemented very well.
For example, various extraction solvents and different procedures are used for the determination of structural components and bulk extractives in biomass. Unfortunately, the
direct methods for determination of the structural components are very rare. Additionally, the direct and indirect
identification and quantification of minerals in biomass are
almost absent due to their low contents and detection problems. Some analytical concerns associated with biomass
have been discussed ([71,134,163,177] and references
therein).
(8) Despite the traditionally dominant occurrence of organic
matter, the inorganic matter can also have a major role in
some biomass varieties. For example, the moisture content
(mineralised solution) and ash yield in biomass can reach
to 80% and 46%, respectively [1]. The inorganic matter in biomass has generally been divided mostly into two classes,
namely inherent (or intrinsic, included) and extraneous (or
extrinsic, adventitious, entrained, imbedded, introduced,
added, foreign, dirt), and occasionally even into unusual
structural and extractable classes. However, the actual origin
of inorganic matter in natural biomass (Table 3) could be
divided simply into detrital (terrigenous) and authigenic
classes which are more informative, well-known and
accepted for solid fossil fuels ([176,180] and references
therein). Furthermore, most studies used the data from ash
yield or the bulk chemical composition of ash to explain
mineral matter, mineral composition, inorganic matter or
inorganics in biomass, which is not fully correct and can lead
to confusion in many cases.
(9) The common scientific approach used is to study the concentration and behaviour of individual elements for explaining
and evaluating different technological and environmental
problems related to biomass processing. However, the actual
reasons for such problems are most likely connected with
modes of element occurrences in biomass or biomass products, namely specific phases and minerals that contain such
elements (similar to coal [173,180,181] and references
therein).
(10) Sequential chemical fractionation is mostly used to distinguish the speciation of elements in biomass fuels and their
products. Unfortunately, this indirect procedure cannot be
5
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Table 4
Phases, minerals, compounds and modes of element occurrence identified in biomass.
Phase, mineral, compound, occurrence
Formula, element or note
Reference used
1. Organic matter
1.1. Organic phases and compounds
1.1.1. Structural components
Cellulose (polysaccharide, also known as glucan and
glucosan)
(C6H10O5)n, CH1.67O0.83, (C5H10O5)x,
C6H10O5 or C6(H2O)5
[2,5,6,8,13,14,17,19,21,24,28,32,33,42,44–46,49,52,53,55,56,
66,71,73,75,78,79, 85,87–89,91,93,95,100,102,108–111,120,
122–124,127,132,134–136,139,140, 144–147,150–154,156,
158–160,163,165,170]
[14,17,19,21,28,32,33,42,44,45,49,52,55,56,59,66,71,73,75,78,
79,85,87–89,93, 95,100,102,108–111,120,122–124,127,132,
134–136,139,140,144,145,147,
150–154,156,158–161,163,165,168,170]
[2,6,9,14,17,19,21,28,32,33,42,44,45,49,52,55–57,60,62,66,71,
73,75,77–79,85, 87–89,93,95,100,102,108–111,120–124,126,
127,132,134–136,139,140,144–147, 150–154,156,158–160,
163,165,170]
Hemicellulose (polysaccharide, also known as xylan,
polyose and pentosan)
(C5H8O4)n, CH1.64O0.78 or C5(H2O)4
Lignin (polyphenol, oxygenated aromatic hydrocarbon,
also known as lignen)
[C9H10O3(OCH3)0.91.7]n, C9H10O2,
C10H11O3.5, C10H12O3, C11H14O4,
C40H44O6 or C40H44O14 (ill-defined
composition)
1.1.2. Extractives (from successive leaching mainly with
water, ethanol, toluene and benzene, and occassionaly
acetone, alcohol, dichloromethane, ethers, heptane,
hexane, methanol, methylene chloride, petroleum spirit
and their mixtures)
Various components
[9,17,21,28,33,35,42,44,45,56,66,75,79,85,88,89,93,100,102,
109,123,128,137,139, 140,147,152–154,158,160,165,168–170]
C2H4O2 or CH3COOH
RACH(NH2)ACOOH
C6H12O6
C18H30O16X2
C6H12O6
C6H10O7
[87,100,102,132,159]
[76,141]
[120]
[100,120]
[9,44,93,165]
[66,100,102,120,127,135,160,170]
[15,32,35,44,62,69,77,87,89,100,102,110,120,125,128,132,
139–141,144,145,155, 156,160,162,163,165,170]
[35,95,128,132,142,160]
[160]
[93]
[30,155]
[9,44,93,165]
[93,100,102,122,135,160]
[66,102,127]
C6H12O6 or C6H10O6
C6H10O7
[9,44,93,120,165,170]
[30,45,62,66,71,91,93,100,102,120,122,127,134,135,155,160]
[9,66,102,127]
1.1.3. Oxygenated aliphatic hydrocarbons
Acetic acid (carboxylic acids)
Amino acids (amines and carboxylic acids)
Amylopectin (polysaccharide)
Amylose (polysaccharide)
Arabinan (polysaccharide)
Arabinose (monosaccharide)
Carbohydrates (saccharides, polyhydroxy aldehydes and
ketones)
Fatty acids (carboxylic acids)
Fatty esters
Fructans (polysaccharides)
Fructose (levulose, monosaccharide)
Galactan (polysaccharide)
Galactose (monosaccharide)
Galacturonic acid (polysaccharide, carboxylic acids,
aldehydes)
Glucan (polysaccharide)
Glucose (dextrose, monosaccharide)
Glucuronic and methylglucuronic acids
(monosaccharides, carboxylic acids)
Glycans (polysaccharides or oligosaccharides)
Glycerides (acylglycerols, fatty esters)
Glycerol (glycerine, alcohols, polyols)
Glycosides (saccharides)
Holocellulose (cellulose plus hemicellulose)
Inositol (alcohols, polyols)
Mannans (polysaccharides)
Mannose (monosaccharide)
Methanol (alcohols)
Methylglucuronic acid (monosaccharide, carboxylic
acids)
Mucilages (polysaccharides)
Nucleic acids (polysaccharides)
Pectates (saccharides)
Pectines (polysaccharides)
Pentosans (polysaccharides)
Proteins (amino acids, amines and carboxylic acids)
Resin acids (carboxylic acids)
Saccharides (sugars, carbohydrates divided into
monosaccharides, disaccharides, oligosaccharides
and polysaccharides)
Starches (polysaccharides)
Sterols (alcohols, steroids)
Sucrose (saccharose, disaccharide derived from glucose
and fructose)
Triglycerides (fatty esters)
Uronic acids (carboxylic acids)
Xylan (polysaccharide)
Xylose (monosaccharide)
C5H10O5
(CH2O)n
CH3(CH2)nCOOH
CH3(CH2)nACO2AR
C3H5(OH)3
C6H12O6, (CH2O)6 or (ACHOHA)6
C6H12O6
CH3OH
(C5H8O4)n
C19H29COOH
Cn(H2O)n or (CH2O)n, as n P 3
(C6H10O5)n
C12H22O11
CH2AOOCAR or C55H98O6
C5H10O5
1.1.4. Aliphatic hydrocarbons (includes alkanes, alkenes and alkynes)
Alkenes
CnH2n
[165]
[128]
[95]
[102,145,160]
[45,56,75,139]
[119,161]
[9,44,45,93,102,165]
[66,93,100,102,122,127,135,160]
[122]
[66,127]
[93,102]
[7,141,155]
[141]
[82,93,100,102,120,158,168]
[44]
[11,32,35,45,69,74,76,85,89,93,100,102,110,120,128,132,
139–141,145,151, 155,157,158,162,163,165]
[79]
[9,19,32,35,45,49,56,62,66,71,82,87,89,93,95,100,102,110,
120,122,127,132,134,135, 140,144,145,150,151,157,160,163,
165,170]
[32,35,87,89,93,95,100,102,120,140,145]
[100,128,151,160]
[30,93,100,155]
[95,100,128,167]
[93,100,146,150,165]
[9,33,44,45,87,93,100,102,120,122,165,170]
[66,100,102,120,122,127,135,160]
[122]
(continued on next page)
6
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Table 4 (continued)
Phase, mineral, compound, occurrence
Formula, element or note
Reference used
Fats (alkanes that are solid or liquid at ambient
temperatures)
Gums (usually polyalkenes such as butadiene polymers)
Hydrocarbons
Latex (polyalkenes, alcohols, sterols)
Lipids (also oxygenated aliphatic hydrocarbons)
Oils (polyalkanes that are liguid at ambient
temperatures)
Resins (also aromatic hydrocarbons)
Rubbers (elastomers, polyalkenes)
Terpenes
Terpenoids or isoprenoids
Waxes (polyalkanes that are solid at ambient
temperatures)
CnH2n+2
[35,69,88,90,102,128,140,142,151,154,160,167]
1.1.5. Oxygenated hydrocarbons
Acetyls (carboxylic acids, functional groups)
Alcohols (acyclic, ethanol, methanol, others)
Aldehydes (functional groups)
Carboxylic acids (organic acids)
Esters
Ethers
Ketones (acetone)
Phytates (alcohols, polyols)
1.1.6. Oxygenated aromatic hydrocarbons
Flavonoids (ketone-containing compounds)
Phenols (monophenols)
Saponins (phenols)
Tannins (polyphenols and ketones)
1.1.7. Nitrogenated (basic) hydrocarbons
Alkaloids
Amines (functional groups, with alone pair of electron)
CxHy
C30 triterpenoid
(C5H8)n, as n P 2
CH3(CH2)nCH3 and also O-containing
aliphatics
CH3COOA or C2H3O+
CnH2n+1OH, C2H5OH, CH3OH, others
RACHO
RACOOH
RCO2R0
CAOAC bonding
(CH3ACH2AOACH2ACH3)
R1ACOAR2 or RC(@O)R0
CaAMg(AK)-salt of C6H6[OPO(OH)2]6
C6H5OH or C6H6O
C, H and N plus some S, Cl, Br, P
(ANH13)
1.1.8. Aromatic hydrocarbons
Aromatics (arenes)
1.1.9. Organo-metallic complexes
Chelates
Organo-metallic compounds (mostly carboxylic acids
plus metal)
Phytoferritin (Fe-protein complex)
1.1.10. Dyes (pigments)
Chlorophyll (green pigment, also organo-metallic
complex)
Pigments
1.1.11. Others
Functional groups such as amine, carbonyl, carboxyl,
ester, ether, hydroxyl (phenolic or alcoholic), ketone,
methoxyl, methyl and other groups
Organic acids (carboxylic acids and others such as
sulphonic, lactic, acetic, formic, citric, oxalic, uric and
other acids)
Organic materials in semi-biomass
Organic sulphates (hydrocarbon that contains organic
sulphate)
Phytic acid (inositol hexaphosphate, organophosphate)
Sulpholipids (organic sulphate)
1.2. Organic minerals
CaAMn oxalate
Ca oxalate
Ca oxalate dehydrate
KANaACaAMg oxalates
K oxalate
Oxalates
Weddelite (Ca oxalate dihydrate)
Whewellite (Ca oxalate monohydrate)
1.3. Organically associated elements
Amino acids
Carbohydrates
Chelates
Chlorophyll
Citrates
Covalently bound
[93,102]
[32,35,48,67,75,87,89,100,104,128,145,151,159,160,168]
[128,151]
[7,32,35,76,89,93,95,110,132,145,155,160,162]
[35,71,95,100,102,120,128,134,135,139,140,142,151,154,159,167]
[35,79,88,100,102,145]
[100,128]
[35,79,88,100,102,128]
[35,79,95,100,128,160]
[46,95,100,102,128,150,156,160,165]
[9,93,146,165,170]
[35,45,70,98,100,122,159]
[160]
[77]
[70,76,122,142,146,160]
[2,70,98,102,122,159,160]
[122,159,160]
[71,119,134,141,161]
[79]
[35,88,93,98,102,122,132,151,159,160]
[102,128]
[35,74,168]
[100,102]
[45]
[6,10,35,45,70,100,111,121,122,127,144,146,151,159]
[10,18,141,143,160,164]
[22,72,106]
(FeO.OH)8(FeOOPO3H2)
[71,134,141]
C55H72MgN4O5
[7,11,71,74,100,134,141,160,165]
[35,100]
O-, S- and N-containing groups
[2,6,9,10,15,22,45,64,65,70,71,98,100,110,121,122,134,
136,142,144,146,148,160,161,168]
[35,159,163,165]
Various compounds
[39a]
[141]
C6H18O24P6
S-containing functional group
[141,161]
[141]
(Ca,Mn)C2O42H2O
CaC2O4nH2O
CaC2O4
Ca(CO2)2(H2O)2 or CaC2O42H2O
CaC2O4.H2O
[96,168]
[11,64,70,71,74,83,94,96,101,131,134,141,161,168,present study]
[131,161,168]
[15]
[161,168]
[15,74,79]
[70b,present study]
[50b,70b,present study]
Hg, N, S
Ca, K, Mg, Mn, Na
Fe
Cu, Mg, Mn, Mo, N, Ti
Ni
Cl, P, S
[74,141,166]
[15,141]
[18,141]
[7,11,71,74,134,141]
[74]
[83,161,168]
K2C2O4
7
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Table 4 (continued)
Phase, mineral, compound, occurrence
Formula, element or note
Reference used
Covalently or ionically bound
Esters
Ion-exchangeable bonds
Ion-exchanged sites in carboxylic groups
Ionically bound
Ionically bound in carboxylic groups
Nucleic acids
Organically bound (directly in organic groups,
complexes, matrix, structures)
Organic materials in semi-biomass (additives in
adhesives, briquettes, insecticides, lacquers,
lignosulphonates, paints, pellets, preservatives,
others)
Organic S (amino acids, sulphate esters, sulphates,
sulphites, sulpholipids, sulphonates, proteins)
Organo-metallic compounds (forms)
Oxalates
Oxygen-containing functional groups
Pectates
Phospholipids
Phytates and phytic acid
Phytoferritin
Proteins
Pyrophosphates
Salts in carboxylic acids
Ca, Fe, K, Mg, Mn, Na, P, S
P, Si
Al, Ca, Fe, K, Mg, Mn, Na, S
Ca, K
Al, Ca, Cl, Fe, K, Mg, Mn, P, S
Al, Ca, Fe, K, Mg, Na
Mg, P
Al, Ca, Cl, Fe, K, Mg, Mn, Na, P, S, Si
[22,47,65,71,131,134]
[15,18]
[18,47,116,141,168]
[65]
[18,168]
[15,161,168]
[7,71,134,141]
[61,64,71,74,94,134,141,168]
Cl, N, S plus various elements
[39a,40a, 62,80]
Fe, N, S
[18,71,76,80,134,141]
Hg, K
Ca, K, Mg, Mn, Na
K
Ca, Mg
P
P
Fe, P
I, Mg, Mn, N, P, S
P
Ca
[22,72,166]
[15,96,161,168]
[41]
[71,134,141]
[7]
[71,119,134,141,161]
[71,134,141]
[11,74,141]
[18]
[131]
NH4+
[74,168]
[74]
2. Fluid matter
Ammonium cation
Borate anions
Carbonate anion
Chlorides
Elements as anions (Br, Cl, Cr4, F, Mo6, S2, V5,
W6 plus As, Au, B, Ge, I, N, P, Sb, Se, Si, Ta, Te, others)
Elements as cations (Ag+, Al3+, Ca2+, Cr3+, 4+, Cu+, 2+, Fe2+,
3+
, K+, Mg2+, Mn2+, Na+, V6+ plus Ba, Be, Bi, Cd, Ce, Co,
Cs, Ga, Hg, In, La, Li, Ni, Pb, Sc, Sn, Sr, Ti, Tl, Y, Zn, Zr,
others)
Hydrocarbonate and hydrobicarbonate anions
Hydrogen phosphate anions
Hydroxide anion
Molybdate anion
Nitrate anion
Nitrates
Organic anions
Oxalate anion
Oxonium cation
Phosphate anion
Phosphates
Phosphoric acid (orthophosphoric acid)
Silicic acid
Sulphate anion
Water and gas phases
3. Inorganic matter
3.1. Silicates
Albite
Augite
Biotite
Chlorite
Clay minerals
Cristobalite
Cs silicate
Diopside
Feldspars
Forsterite
Illite
Kaolinite
K feldspar
Mg aluminosilicates
Montmorillonite
Muscovite
Opal (amorphous or crystalline, silicic acid polymerised,
biogenic silica, silica phytolith)
2
BO3
3 , B4 O7
[7,41]
CO2
3
CaCl2, KCl, MgCl2, NaCl
Various elements
[71,131,134,141]
[3,7,18,20,37,41,71,74,96,107,113,134,141,168]
Various elements
[3,18,20,37,41,65,74,96,107,113,168]
2
HCO
3 , HðCO3 Þ2
[20,41,113]
2
H2 PO
4 , HPO4
OH
[18,74,96,168]
MoO2
4
NO
3
Ca(NO3)2, KNO3, Mg(NO3)2, NaNO3
C2 O2
4
H3O+
PO3
4
Ca3(PO4)2, Mg3(PO4)2
H3PO4
[SiOX(OH)42X]n, Si(OH)4 or H4SiO4
SO2
4
H2O, N2, O2, Ar, CO2, H2
NaAlSi3O8
(Ca,Na)(Mg,Fe,Al,Ti) (Si,Al)2O6
K(MgFe)3AlSi3O10(OHF)2
(MgFe)5Al2Si3O10(OH)8
KANaACaAMgAFe aluminosilicates
SiO2
CaMg(SiO3)2
NaAlSi3O8ACaAl2Si2O8, KAlSi3O8
Mg2SiO4
(K,H2O)Al2(Al,Si)Si3O10(OH)2
Al2Si2O5(OH)4
KAlSi3O8
(NaCa)0.3(AlMgFe)2Si4O10(OH)2xH2O
KAl2AlSi3O10(OH)2
SiO2nH2O
[7,168]
[74]
[7,20,74,113]
[71,134,141]
[168]
[168]
[168]
[41,71,113,134,141,168]
[71,134,141]
[168]
[18,61,71,84,96,131,134,148,168]
[7,18,20,41,71,74,76,96,107,113,134,141,168]
[74]
[36c,50b,c]
[90c]
[39a]
[39a]
[12c,18c,28c,31d,32c,39a,51d,71c,84,116c,134c,145c,148,168]
[39a,90c]
[74]
[84]
[29c,32c,36c,39a,131c,145c,168]
[84]
[10c,39a]
[10c,39a,141]
[16c,36c,39a]
[171]
[39a]
[39a]
[4,15,18,32,41,54,59,61,64,71,72,74,94,96,103,113, 131,134,138,
141,161,168,present study]
(continued on next page)
8
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Table 4 (continued)
Phase, mineral, compound, occurrence
Formula, element or note
Reference used
Orthoclase
Plagioclases
Quartz
Sepiolite
Talc
Vermiculite
Wollastonite
Zeolites
KAlSi3O8
NaAlSi3O8ACaAl2Si2O8
SiO2
Mg8H6Si12O30(OH)40(H2O)6
Mg3Si4O10(OH)2
(MgFeAl)3(SiAl)4O10(OH)24H2O
Ca3Si3O9
(Ca,Sr,Ba,Na2,K2) Al2Si210O8242–
8H2O
[84]
[36c,39a,50b,c]
[10c,12c,16c,29c,39a,50b,c,70b,71,84,90c,114,134,141,168,171]
[39a]
[39a]
[39a]
[39a]
[39a,131e]
Al(OH)3
[168]
[84]
[39a]
[39a]
[74]
[74]
[74,141]
[39a]
[39a]
[39a]
[74]
[39a]
[29d,39a]
[74]
[39a]
[74]
[74]
3.2. Oxides and hydroxides
Al hydroxide
Al oxides
Brucite
Corundum
CuAAl oxides
CuAFe oxide
Fe oxide
Goethite
Hematite
Magnetite
Ni oxide
Portlandite
Rutile
Sn oxide
Spinels
Zn oxide
ZnAMn oxide
3.3. Sulphates, sulphites and sulphides
Anhydrite
Arcanite
Barite
Ettringite
Fe sulphate
FeAZn sulphide
Gypsum
Jarosite
Millosevichite
Pyrite
Thenardite
Sulphides
Sulphites
Mg(OH)2
Al2O3
a-FeOOH
Fe2O3
FeFe2O4
Ca(OH)2
TiO2
MgAl2O4AMg(AlFe)2O4
CaSO4
K2SO4
BaSO4
Ca6Al2(SO4)3(OH)1226H2O
Fe2(SO4)3
CaSO42H2O
KFe3(SO4)2(OH)6
Al2(SO4)3
FeS2
Na2SO4
Compounds with sulphide anion (S2)
Compounds with sulphite anion
(SO32)
Zn sulphide
3.4. Phosphates
Apatite
Fe phosphate
Hydroxyl-apatite
K hydrogen and K dihydrogen phosphates
MgACa phosphate
Mn phosphate
3.5. Carbonates
Ankerite
Calcite
Dolomite
KANaACaAMg carbonates
Magnesite (Fe-rich)
3.6. Chlorides
Bi chloride
Halite
KACa chloride
K perchlorate
Sylvite
3.7. Nitrates
Ca nitrate dihydrate
Nitrates
Nitre (niter)
Nitrocalcite
3.8. Other inorganic matter
AuANi phase
Bi-bearing phase
Glass
[39a,70b]
[28f,49f,50b,70b,76,84,161,168,present study]
[39a,74]
[39a]
[97e,129e,131e]
[74]
[39a]
[39a]
[129e,131e]
[10c]
[39a]
[10c,31,68,74,76,84,116,118,131]
[76]
[74]
Ca5(PO4)3(F,Cl,OH) or
Ca(PO4)3(F,Cl,OH)
FePO4
Ca(PO4)3(OH)
K2HPO4, KH2PO4
MgCaPO4
Ca(MgFe)(CO3)2
CaCO3
CaMg(CO3)2
(Mg,Fe)CO3
NaCl
KClO4
KCl
Ca(NO3)22H2O
[39a,74,84,99g]
[97e]
[7g,86f,115,149f]
[161]
[70b]
[74b]
[39a]
[10c,39a,70b,141,present study]
[39a,70b]
[15]
Present study
[74]
[18h,39a,70b,113,present study]
[171]
[18b]
[28f,39a,41,49f,70b,74,131,161,168,171,present study]
KNO3
Ca(NO3)24H2O
Present study
[70b]
[18b,50b]
Present study
Various elements
[74]
[74]
[39a]
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
9
Table 4 (continued)
Phase, mineral, compound, occurrence
Formula, element or note
Reference used
Metallic alloys and pure metals
Native Au, Fe, Mn and Zn
NiAFeAAs phases
Various elements
[39a,40a]
[74]
[74]
Ba, Be, Ca, F, Mg, Mn, P, Ra, Sr, Y, rareearth elements
Al, As, B, Ca, Cd, Cl, Cr, Cu, Fe, Hg, K, Mg,
Mn, N, Na, Ni, P, Pb, S, Si, Ti, Zn, others
[7]
Various elements
[10,22,32,64,71,134]
C, Ca, Cd, Cl, K, Mg, N, Na, P, S
Various elements
[15,18,27,28,34,46,49,58,63,71,86,92,105,130,134]
[50b,present study]
[71,134]
3.9. Inorganically associated elements
Bones
Contaminants as inclusions and impurities (additives,
atmospheric deposition, dirt, manufacture, paints,
preservatives, sands, sea water, soils, others)
4. Others
Atomically dispersed inorganic material within the
organic matter
Fertilizers
Inorganic amorphous material
Salts bound in the C structure
a
b
c
d
e
f
g
h
[12,16,18,20,22,23,25,26,29–32,34,38–
40,43,45,51,59,62,66,71,80,81,83,
86,90,92,96,105,106,110,112,117, 118,131,133,134,138,145,161]
Contaminant in municipal solid waste.
Low-temperature ash produced in oxygen plasma (100–250 °C).
Contaminant from soil.
Additive for paper production.
Contaminant in sewage sludge as detergent, flocculant or precipitation agent.
Contaminant from fertilizers.
From animal bones.
Contaminant from sea transport.
applied to identify the actual modes of element occurrence
in a multicomponent system. Leaching alone has many limitations and can be used only as preliminary information for
some possible associations of elements in phases. Other
direct methods commonly applied for coal and coal products
([174,175] and references therein) could provide better
information and they should be always used additionally
for such a purpose.
(11) It is commonly accepted that biomass contains less phases of
toxic elements in comparison with coal. However, such
phases in biomass fuels and their products tend to occur in
much more mobile and hazardous forms than in coals and
their products [40,71,134,182–184]. Systematic studies
about these phases in biomass are only at an initial stage
of investigation.
(12) Washing to remove water-soluble phases prior to use of biomass fuels may reduce some technological and environmental problems. However, such future large-scale leaching may
create new environmental concerns related to the fate of
mobile phases containing Cl, S, P and some hazardous trace
elements associated with them.
(13) Regulations exist in some countries which specify the limiting and guiding values for the contents of some elements
(Ca, Cd, Cl, Co, Cr, Cu, K, N, Ni, Pb, S, V, Zn) in biomass fuels
and their products in respect of their unrestricted use. However, the bulk concentrations of these elements are less
informative than the abundance and behaviour of their
forms in which they are present.
The above listed problems show that additional, systematic
and detailed phase studies based on proved, improved or new
approaches are required to reduce uncertainties. Therefore, from
a critical review of publications and some own investigations an
attempt will be undertaken: (1) to compile and systematise a
reliable phase database and to define the basic phase findings;
(2) to supply additional phase results and to clarify some of
the problems related to phase composition, properties and perspectives of biomass; and finally (3) to understand how the fun-
damental knowledge on the phase composition may be
implemented for the most advanced and sustainable utilisation
of biomass as fuel. Peer-reviewed data and own key studies on
biomass, other solid fuels and their products will be used to support or not the reference achievements. For better differentiation, the results in this overview are specified to: (1) peerreviewed investigations; and (2) peer-reviewed data including
our own new data and studies with novel interpretation; and referred to as ‘‘reference investigations’’ and ‘‘present study’’
throughout the text, respectively.
2. Materials, methods and data used
The present study includes a combination of reference and own
data as specified above. The phases, minerals, compounds and
modes of element occurrence identified or assumed (with uncertain identification) in biomass are given in Table 4 using numerous,
mostly peer-reviewed investigations. It was found that the traditional, complete and peer-reviewed structural and extractive data
for many varieties of biomass are quite limited. Therefore, complete data for the structural components (normalised to 100%) plus
incomplete results for the bulk extractives of 93 varieties of biomass (197 samples) were collected for the present study (Table
5). The data used are from 25 references, namely 23 peer-reviewed
articles [17,28,33,42,44,52,56,66,73,75,100,109,110,120,123,139,
140,147,151,153,160,165,170], one peer-reviewed monograph
[85] and one conference proceeding [21]. Some of these data are
mean values from numerous determinations for a given biomass
variety. It was also identified that the traditional and complete
proximate, ultimate, ash [1], structural and extractive data (Table
5) occur only for 28 of the 93 biomass varieties studied (samples
3, 4, 6, 9, 10, 16, 19, 34–36, 38, 42, 45, 46, 48, 51–53, 56, 65, 67,
68, 70–74 and 87 in Table 5). These data were subjected to Pearson’s correlation test to calculate correlation coefficient values
among 24 characteristics for 28 varieties of natural biomass,
excluding moisture due to different and uncertain biomass basis
used for its measurement.
10
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Eight biomass samples were collected and studied for additional clarification of the phase composition in the present work
(Table 6). They include beech wood chips (BC), corn cobs (CC),
marine macroalgae (MM), plum pits (PP), rice husks (RH),
switchgrass (SG), sunflower shells (SS) and walnut shells (WS).
The majority of investigations up to now have been concentrated
mostly on woody biomass and wheat straw as solid biofuels.
Most of these eight samples are herbaceous and agricultural residues and algae and much less is known about them. The selection of these samples is also based on their highly variable: (1)
structural composition (Fig. 1); and especially (2) inorganic
chemical composition (Fig. 2) according to the reference data
[1,17,18,21,34,42,66,75,100,107,110,115,120,123,139,151,170,
185–191]. Another big advantage of these samples for comparative
investigations is that they belong to all four inorganic chemical
types of biomass (S, C, K, and CK types) and different sub-types
that have been specified recently [1]. Additionally, five samples
from them belong to the low acid K and CK sub-types, which
are among the most problematic biomass resources from technological and environmental points of view (Fig. 2 and see also
Section 4).
The samples collected were between 2 and 5 kg in weight
and they were air dried under ambient conditions during a period of over 12 months. After that, visible contaminants such as
sand, soil or shell particles were eliminated manually from RH
and MM. Then the samples were cut/crushed (CC, PP, WS) and
ground (BC, CC, MM, PP, RH, SS, SG, WS) to a particle size
of <500 lm (BC, MM, RH, SG, SS) and <1000 lm (CC, PP, WS).
They were investigated using light microscopy, powder X-ray
diffraction (XRD) and some leaching, precipitation and ashing
procedures. An ordinary stereomicroscope under transmitted
and reflected light was used for optical observations. XRD patterns of finely ground samples were recorded by D2 Phaser Bruker AXS and DRON 3 M diffractometers, and collected at 10–70°
2h using Cu and Co Ka radiations, respectively. Dry, water-soluble residues (DWRs) were isolated from the biomass samples
by evaporation and precipitation of the water-soluble solutions
leached. The leachates from this extraction procedure were generated from 10 g of biomass samples placed in glass containers
with 100 ml of distilled water and soaked for 24 h at ambient
temperature. The suspensions were periodically stirred and finally decanted and filtered. The pH and electrical conductivity values were measured in the generated water solutions by a bench
pH-mV-conductivity meter PC 5000L (VWR International Ltd.).
Then the leachates were placed in a drying oven at 80 °C for
evaporation and crystallization giving DWRs. This leaching and
precipitation procedure is similar to those applied for coals
[183,192] and refuse-derived fuels [39]. The high-temperature
ashes (HTAs) of biomass samples were produced in an electric
furnace with static air at 500 ± 10 °C for 2 h with a heating
and cooling rate of about 5 °C min1. The reference data and
own results obtained for these eight biomass varieties were also
subjected to Pearson’s correlation test to calculate correlation
coefficient values among the characteristics studied.
3. Results and discussion
3.1. General observations about phase-mineral composition of biomass
Biomass can be regarded as a complex heterogeneous mixture
of main structural organic components, namely cellulose (Cel),
hemicellulose (Hem) and lignin (Lig), and associated with these
matrices major, minor and accessory organic and inorganic compounds represented by various solid and fluid phases with different contents and origin (Tables 2 and 3).
3.1.1. Distribution of structural components and bulk extractives in
biomass
The proportions of structural components and bulk extractives
(Ext) in biomass varieties, as well as among the different biomass
groups and sub-groups specified are highly variable (Table 5).
Therefore, the contents of Cel, Hem and Lig (normalised to 100%)
for 93 varieties of biomass (Fig. 3) and the mean values for biomass
groups and sub-groups (Fig. 4) were plotted for comparison. The
distribution of structural components shows:
(1) The similarity between sub-groups such as wood twigs,
barks and leaves, and some herbaceous and agricultural
sub-groups, namely the couples grasses and straws, stalks
and fibres, and shells–husks and pits;
(2) The differentiation between the biomass groups and the
characteristic distinction for some sub-groups, namely
between the wood stems and other tree parts (twigs, barks
and leaves).
The biomass varieties with similar structural proportions in the
above listed sub-groups may be combined in some cases and their
future separate characterisation could be avoided depending on
the application purposes. It is interesting to note that a similar
chemical composition was also identified for the sub-groups of
grasses and straws based on the ultimate and ash analyses [1].
Figs. 3 and 4 were divided into six sectors labelled (CHL, CLH,
HCL, LCH, HLC and LHC) in such a way that each point in the given
sector is characterised by the consecutive decreasing quantities of
the three structural components. For example, the CHL sector includes only biomass varieties with concentrations that fit in the
structural order: Cel > Hem > Lig. It can be seen that most of the
groups and sub-groups (Fig. 4) belong to the sectors with proportions amounting to less than 50% of the structural components,
excluding only the dominant (>50%) occurrence of: Cel in contaminated biomass (mostly paper-based semi-biomass), herbaceous
and agricultural fibres and stalks, and wood stems; Hem in wood
twigs; and Lig in fruit pits (Figs. 3 and 4 and Table 5). The six structural orders specified include the following biomass groups, subgroups and varieties:
(1) Cel–Hem–Lig (CHL) order for 44 varieties that belong mostly
to groups and sub-groups of wood stems, herbaceous and
agricultural biomass (grasses, straws, stalks, fibres, shells–
husks, residues) and contaminated biomass (semi-biomass);
(2) Cel–Lig–Hem (CLH) order for 21 varieties that include commonly the sub-groups of wood stems and agricultural
shells–husks;
(3) Hem–Lig–Cel (HLC) order for 14 varieties that belong normally to sub-groups of wood barks, twigs and leaves;
(4) Hem–Cel–Lig (HCL) order for six varieties that include
mostly the sub-groups of tree leaves and grasses;
(5) Lig–Cel–Hem (LCH) order also for six varieties that belong
commonly to groups and sub-groups of some herbaceous
and agricultural biomass (shells–husks, pits, residues) and
animal biomass (only a cattle manure);
(6) Lig–Hem–Cel (LHC) only for two varieties, namely wood
bark and hazelnut shells.
11
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Table 5
Contents of structural components (cellulose, hemicellulose and lignin) in 93 varieties of biomass on dry ash-free basis and normalised to 100.0%. The extractive contents are
additionally given on dry ash-free or occasionally dry basis, wt.%.
Biomass
Cellulose
Hemicellulose
Lignin
Sum
Type
Extractives
Samplesa
1. Wood and woody biomass (WWB)
Mean
39.5
Minimum
12.4
Maximum
65.5
1.1. Stems (WWS)
1. Albizzia wood
59.5
2. Aspen
64.2
3. Beech wood
45.2
4. Birch
50.2
5. Birch wood
49.1
6. Eucalyptus
52.7
7. Hardwood
46.8
8. Hardwood stems
46.3
9. Oak
58.4
10. Pine
48.1
11. Premna wood
65.5
12. Pterospermum wood
60.2
13. Softwood
43.3
14. Softwood stems
44.2
15. Spruce
43.6
16. Spruce wood
47.0
17. Subabul wood
44.9
18. Syzygium wood
61.1
19. Wood
42.0
Mean
51.2
34.5
6.7
65.6
26.0
10.2
44.5
100.0
CHL
3.1
1.0
9.9
35
35
35
6.7
20.2
32.7
32.8
31.6
15.4
30.4
32.1
31.4
23.5
12.6
10.7
27.4
27.9
27.4
25.3
27.1
7.5
22.0
23.4
33.8
15.6
22.1
17.0
19.3
31.9
22.8
21.6
10.2
28.4
21.9
29.1
29.3
27.9
29.0
27.7
28.0
31.4
36.0
25.4
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
CLH
CHL
CHL
CHL
CHL
CLH
CHL
CHL
CHL
CLH
CLH
CLH
CLH
CLH
CLH
CLH
CLH
CLH
CLH
CLH
1.9
4
1
2
2
1
1
3
2
1
4
4
4
3
1
1
2
1
4
1
19
1.2. Barks (WWBA)
20. Albizzia bark
21. Premna bark
22. Pterospermum bark
23. Syzygium bark
24. Wood bark
Mean
22.5
19.0
20.7
22.6
25.2
22.0
44.9
60.4
53.2
46.4
30.3
47.0
32.6
20.6
26.1
31.0
44.5
31.0
100.0
100.0
100.0
100.0
100.0
100.0
HLC
HLC
HLC
HLC
LHC
HLC
1.3. Twigs (WWT)
25. Albizzia twigs
26. Premna twigs
27. Pterospermum twigs
28. Syzygium twigs
Mean
15.7
19.8
13.7
12.4
15.4
61.7
60.0
61.9
65.6
62.3
22.6
20.2
24.4
22.0
22.3
100.0
100.0
100.0
100.0
100.0
1.4. Leaves (WWL)
29. Albizzia leaves
30. Premna leaves
31. Pterospermum leaves
32. Syzygium leaves
Mean
25.3
30.3
22.1
28.1
26.5
44.6
50.8
50.6
42.9
47.2
30.1
18.9
27.3
29.0
26.3
1.5 Others (WWO)
33. Pine cones
34. Pine sawdust
35. Woody biomass
Mean
34.3
45.9
50.6
43.6
39.5
26.4
24.7
30.2
2. Herbaceous and agricultural biomass (HAB)
Mean
46.1
Minimum
23.7
Maximum
87.5
2.1. Grasses (HAG)
36. Bamboo
43.9
37. Bermuda grass
37.3
38. Elephant grass
31.5
39. Esparto grass
42.8
40. Grasses
34.2
41. Orchard grass
41.7
42. Reed canary grass
49.2
43. Ryegrass
49.1
44. Sugar cane
45.8
45. Sweet sorghum grass
50.6
46. Switchgrass
48.7
47. Timothy grass
38.0
Mean
42.7
2.2. Straws (HAS)
48. Barley straw
49. Flax straw
48.6
36.7
1.1
3.0
2.2
3.9
3.5
1.9
1.8
2.5
9.9
1.4
3.0
4.5
2.6
3.1
3.0
Reference used
[56]
[147]
[75,139]
[21,85,147]
[139]
[100]
[21,66,140]
[120,151]
[147]
[21,85,100,147]
[56]
[56]
[21,66,140]
[120]
[85]
[75,139]
[17]
[56]
[52]
[56]
[56]
[56]
[56]
[21]
3.3
4
4
4
4
1
5
HLC
HLC
HLC
HLC
HLC
1.9
2.0
1.0
1.6
1.6
4
4
4
4
4
[56]
[56]
[56]
[56]
100.0
100.0
100.0
100.0
100.0
HLC
HCL
HLC
HLC
HCL
3.0
4.6
3.7
3.3
3.7
4
4
4
4
4
[56]
[56]
[56]
[56]
26.2
27.7
24.7
26.2
100.0
100.0
100.0
100.0
HCL
CLH
CHL
CHL
4.8
4.6
[153]
[139]
[73]
4.7
1
1
1
3
30.2
12.3
54.5
23.7
0.0
54.3
100.0
CHL
13.7
1.2
86.8
53
53
53
26.5
53.2
34.3
35.5
44.7
52.2
39.1
41.4
31.3
24.7
38.4
33.1
37.9
29.6
9.5
34.2
21.7
21.1
6.1
11.7
9.5
22.9
24.7
12.9
28.9
19.4
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
CLH
HCL
HLC
CHL
HCL
HCL
CHL
CHL
CHL
CHL
CHL
CHL
CHL
2.8
47.2
25.0
8.0
16.6
20.5
2
1
1
1
1
2
2
2
1
1
5
3
12
29.7
34.4
21.7
28.9
100.0
100.0
CHL
CHL
14.8
20.0
5
3
23.3
[44,151]
[120]
[151]
[151]
[120]
[139,151]
[110]
[151]
[100]
[100]
[66,100,110,120,151]
[160]
[28,151,160,165]
[160]
(continued on next page)
12
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Table 5 (continued)
Cellulose
Hemicellulose
Lignin
Sum
Type
Extractives
50. Legume straw
51. Oat straw
52. Rape straw
53. Rice straw
54. Rye straw
55. Triticale straw
56. Wheat straw
Mean
29.2
44.8
54.8
52.3
49.9
47.9
44.5
45.4
35.5
33.4
23.2
32.8
29.6
31.5
33.2
31.5
35.3
21.8
22.0
14.9
20.5
20.6
22.3
23.1
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
HLC
CHL
CHL
CHL
CHL
CHL
CHL
CHL
3.8
11.2
18.0
9.3
11.0
21.7
12.4
13.6
1
2
2
5
2
2
8
9
[139]
[151,165]
[28]
[17,21,120,139,151]
[28,151]
[165]
[17,21,28,66,75,120,151,160,165]
2.3. Stalks (HAST)
57. Corn stalks
58. Cotton stalks
59. Oreganum stalks
60. Sunflower stalks
61. Tobacco stalks
Mean
49.0
66.2
62.6
71.6
44.6
58.8
37.9
18.4
20.2
12.3
30.2
23.8
13.1
15.4
17.2
16.1
25.2
17.4
100.0
100.0
100.0
100.0
100.0
100.0
CHL
CHL
CHL
CLH
CHL
CHL
10.5
2
1
1
1
2
5
[17,21,123]
[123]
[123]
[123]
[123,139]
2.4. Fibers (HAF)
62. Flax fibers
63. Jute bast fibers
64. Kenaf bast fibers
Mean
75.9
53.3
47.0
58.8
20.7
21.2
30.2
24.0
3.4
25.5
22.8
17.2
100.0
100.0
100.0
100.0
CHL
CLH
CHL
CHL
25.6
[109]
[151]
[151]
25.6
1
1
1
3
2.5. Shells and husks (HASH)
65. Almond shells
66. Cashewnut shells
67. Coconut shells
68. Hazelnut shells
69. Millet husks
70. Olive husks
71. Peanut shells
72. Rice husks
73. Sunflower shells
74. Walnut shells
Mean
50.7
41.3
40.3
26.6
44.9
25.0
42.2
43.8
56.5
28.1
40.0
28.9
18.6
27.8
30.0
36.2
24.6
22.1
31.6
28.0
26.6
27.4
20.4
40.1
31.9
43.4
18.9
50.4
35.7
24.6
15.5
45.3
32.6
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
CHL
CLH
CLH
LHC
CHL
LCH
CLH
CHL
CHL
LCH
CLH
2.4
8.7
8.4
3.9
12.7
8.9
10.9
6.6
2.6
2.7
6.8
1
1
1
2
1
2
1
2
2
2
10
2.6. Pits (HAP)
75. Apricot pits
23.7
22.0
54.3
100.0
LCH
5.4
1
2.7. Other residues (HAR)
76. Bagasse
77. Banana residue
78. Coconut coir pith
79. Coconut coirs
80. Corn cobs
81. Corn grains
82. Corn stovers
83. Cotton gin residue
84. Cotton seed hairs
85. Flax bast fiber seeds
86. Flax shives
87. Sugar cane bagasse
88. Tea residue
Mean
47.4
31.4
38.1
52.2
48.1
27.3
47.4
82.8
87.5
49.5
39.9
42.7
33.3
48.3
29.1
35.3
20.3
28.4
37.2
54.5
30.3
17.1
12.5
26.3
26.8
33.1
23.2
28.8
23.5
33.3
41.6
19.4
14.7
18.2
22.3
0.1
0.0
24.2
33.3
24.2
43.5
22.9
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
CHL
HLC
LCH
CHL
CHL
HCL
CHL
CHL
CHL
CHL
CLH
CHL
LCH
CHL
9.4
2
1
1
1
5
1
1
1
2
1
1
1
2
13
3. Animal biomass (AB)
89. Cattle manure
32.7
24.5
42.8
100.0
LCH
1
[120]
92.5
45.6
74.3
60.0
68.1
45.6
92.5
0.0
31.3
17.1
20.0
17.1
0.0
31.3
7.5
23.1
8.6
20.0
14.8
7.5
23.1
100.0
100.0
100.0
100.0
100.0
CLH
CHL
CHL
CHL
CHL
1
1
1
1
4
4
4
[120]
[120]
[120]
[120]
44.4
12.4
92.5
31.2
0.0
65.6
24.4
0.0
54.3
100.0
CHL
9.5
1.0
86.8
93
93
93
43.3
12.4
87.5
31.8
6.7
65.6
24.9
0.0
54.3
100.0
CHL
9.5
1.0
86.8
89
89
89
4. Contaminated biomass (CB)
90. Paper
91. Paper (newspaper)
92. Paper (waste pulps)
93. Refuse-derived fuel
Mean
Minimum
Maximum
All varieties of biomass (AVB)
Mean
Minimum
Maximum
Natural biomass
Mean
Minimum
Maximum
a
Samplesa
Biomass
15.6
13.1
17.4
6.9
7.0
86.8
7.3
1.2
7.7
7.8
16.8
Reference used
[75]
[33]
[17]
[75,139]
[17]
[21,75]
[17]
[17,21,42]
[75,120]
[75,120]
[139]
[17,44]
[151]
[17]
[17]
[17,75,120,123,170]
[100]
[100]
[17]
[120,151]
[151]
[165]
[151]
[75,139]
Some of these data are mean values from numerous determinations for a given biomass variety.
There have been some statements that the structural approach
for biomass classification has been shown to be unreliable due to
variations depending on the biomass source [193]. However, the
present study reveals that the different structural orders include
Table 6
Data for eight biomass samples (air-dried basis) additionally studied, wt.% (indicated otherwise).
Beech wood chips
Corn cobs
Marine macroalgae
Plum pits
Rice husks
Switchgrass
Sunflower shells
Walnut shells
Code
Source
BC
JRS Rosenberg,
Germany
01.2009
Produced size is
1–4 mm
<0.5
454
CC
Debnevo, Bulgaria
MM
Tsarevo, Bulgaria
RH
Kovachevo, Bulgaria
SG
Mead, Nebraska, USA
08.2008
08.2007
Cleaned from sand and
soil grains
<0.5
451
2003
Two mixed harvests
(50:50 vol.%)
<0.5
263
SS
Billa Sofia,
Bulgaria
08.2008
WS
Debnevo,
Bulgaria
08.2008
<1
412
08.2009
Cleaned from sand and
shell grains
<0.5
954
PP
Debnevo,
Bulgaria
08.2008
Weathered for
1 year
<1
889
<0.5
250
<1
833
479
435
1250
1000
476
286
275
883
5.15
0.70
5.32
1.60
6.75
19.81
5.57
0.67
6.66
2.12
5.20
1.63
5.12
3.47
5.28
0.88
1.4
0.8
15.1
0.3
1.5
7.8
2.9
0.9
1.3
25.0
14.5
17.0
1.72
1.47
42
2.1
16.0
12.0
11.0
1.33
1.45
25
28.2
8.5
6.0
4.0
1.42
2.13
29
0.7
13.0
12.5
17.0
1.04
0.76
4
18.5
21.0
10.5
10.5
2.00
2.00
50
5.1
26.0
12.0
13.0
2.17
2.00
54
3.0
6.0
4.0
6.0
1.50
1.00
33
0.9
13.5
8.5
12.0
1.59
1.13
37
82.0
17.2
0.8
100.0
47.2
46.6
6.1
0.1
0.01
100.01
0.01
45.2
32.7
22.1
100.0
1.1
81.2
16.8
2.0
100.0
47.9
45.7
5.9
0.5
0.01
100.01
0.23
48.1
37.2
14.7
100.0
7.0
50.5
25.9
23.6
100.0
43.2
45.8
6.2
2.2
2.60
100.00
3.34
No data
No data
No data
No data
No data
80.8
17.8
1.4
100.0
49.9
42.4
6.7
0.9
0.08
99.98
0.01
23.7
22.0
54.3
100.0
5.4
62.8
19.2
18.0
100.0
49.3
43.7
6.1
0.8
0.08
99.98
0.12
43.8
31.6
24.6
100.0
6.6
80.4
14.5
5.1
100.0
49.7
43.4
6.1
0.7
0.11
100.01
0.08
48.7
38.4
12.9
100.0
8.0
76.0
20.9
3.1
100.0
50.4
43.0
5.5
1.1
0.03
100.03
0.10
56.5
28.0
15.5
100.0
2.6
59.3
37.9
2.8
100.0
49.9
42.4
6.2
1.4
0.09
99.99
0.15
28.1
26.6
45.3
100.0
2.7
27.65
13.19
35.49
6.96
2.49
2.05
1.55
7.14
1.26
0.13
1.44
10.79
13.37
8.50
0.74
10.89
1.63
22.42
17.32
0.01
3.59
14.65
44.88
20.12
0.11
11.62
0.68
2.47
0.46
0.02
94.38
0.97
2.29
0.54
0.21
0.19
0.22
0.92
0.16
0.02
66.09
10.19
9.62
3.91
2.21
4.70
1.36
0.83
0.58
0.28
23.46
15.18
28.29
7.07
8.67
7.27
4.23
4.03
0.79
0.15
23.29
16.70
32.99
6.20
2.40
13.49
1.50
2.20
1.00
0.10
Collection time
Note
Size reduction (mm)
Loose bulk density
(kg m3)
Shaken bulk density
(kg m3)
pH of leachate
Electrical conductivity
(mS cm1)
Dry water-soluble
residue
A (500 °C/2 h)
M22 (mm)
M18 (mm)
M15 (mm)
M22/18
M22/15
CrI
Reference data
VM (db)
FC (db)
A (db) (550–600 °C)
Sum
C (daf)
O (daf)
H (daf)
N (daf)
S (daf)
Sum
Cl (db)
Cel (daf)
Hem (daf)
Lig (daf)
Sum
Ext (daf)
High-temperature ash (550–600°C)
SiO2
12.33
CaO
67.80
K2O
2.59
P2 O5
2.29
Al2O3
0.12
MgO
11.43
Fe2O3
1.09
SO3
0.80
Na2O
0.89
TiO2
0.10
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Characteristic
(continued on next page)
13
Abbreviations: A, ash yield; Cel, cellulose; CrI, Segal crystallinity index of cellulose based on the formula CrI = ((I22 I18)/I22)) 100, where I22 and I18 are respectively the maximums (mm) at 22° and 18° 2h for Cu Ka from XRD
patterns [53]; daf, dry ash-free basis; db, dry basis; Ext, extractives; FC, fixed carbon; Hem, hemicellulose; Lig, lignin; M15, M18, M22 and their ratios (M22/18 and M22/15) are the maximums at 15, 18 and 22° 2h for Cu Ka from
XRD patterns; VM, volatile matter.
[1,75,120,188]
[1,75,120,188,190]
[1,34,66,100,110,120,151]
([1,75,139] and
own data)
Reference used
[17,18,75,115,120,123,170,186]
[1,107]
[1,139,185,189]
[1,17,21,42,187,191]
Sunflower shells
Switchgrass
0.16
0.07
100.00
Rice husks
0.08
0.02
100.00
Plum pits
1.35
0.05
100.00
12.86
0.03
100.00
Marine macroalgae
Corn cobs
Cl2O
MnO
Sum
2.01
0.08
100.00
Beech wood chips
0.16
0.40
100.00
Characteristic
Table 6 (continued)
0.85
0.01
100.00
Walnut shells
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
0.12
0.01
100.00
14
samples that belong preferentially to some specific biomass groups
and sub-groups. Hence, the biomass varieties can be grouped simply into six structural types, which are identical to the above orders
in agreement with the distribution of structural components in
Figs. 3 and 4.
The present study is also in accordance with many of the reference findings. For example, the identified orders of structural components in the literature were: (1) Cel > Hem > Lig for biomass
[66,71,100,120,127,134,135,151,158,165], wood [88], deciduous
trees [88] and hardwoods [140]; (2) Cel > Hem = Lig for woody biomass [73]; (3) Cel > Lig > Hem for wood [52], coniferous trees [88]
and softwoods [140]. However, it should be noted that in contrast
to the reference investigations, the present compilation of structural data (Table 5) is based on many more biomass varieties and
sets of samples.
The decreasing mean values for Cel, Hem, Lig and Ext (normalised to 100%) are also variable among the biomass groups and
sub-groups specified (Table 7). Additionally, the typical decreasing
orders among the mean contents of these characteristics for the
biomass groups and sub-groups are shown (Table 8). Seven major
orders were identified for these four characteristics as the sequence Cel > Hem > Lig > Ext is the most common for biomass.
They are similar to the above structural orders as only Ext changes
to higher order positions in some sub-groups such as herbaceous
and agricultural grasses and fibres (Tables 7 and 8).
3.1.2. XRD of biomass
The XRD patterns of eight biomass samples (Fig. 5) show a
typical non-crystalline character similar to those for wood char,
coal, coal char and petroleum coke (see below). There are only
some traces of crystalline and semi-crystalline (poorly crystallized or cryptocrystalline) phases, which are close to the XRD
detection limit and probably belong to some minerals commonly
identified in biomass (Table 4). These minerals are opal, Ca oxalates (weddelite, whewellite), calcite and halite (Fig. 5). Hence,
the actual identification of phases in bulk biomass samples only
by XRD is quite limited. However, the XRD patterns reveal some
very informative positions and shapes (maximums) of the amorphous halo. This hump in the range of 10–50° (2h for Cu Ka radiation and everywhere below) on the base line of the
diffractograms is due to diffuse scattering of the X-ray caused
by the unordered to partially ordered structures or regions
(mostly organic), which is also typical for other solid fuels (see
below). The biomass amorphous halo is intensive between 10°
and 27° for all samples and for some of them (PP, SG, RH,
MM) it disappears gradually up to 50° (Fig. 5). Two major maximum positions of the halo as large peaks (broad diffusion reflections) at about 15° and 22° were observed for all samples.
Additionally, two minor maximum positions of the halo at about
31° and 44° only for one sample (PP) were also identified. These
observations (especially for 15° and 22°) require an additional
clarification because the position of the amorphous halo depends
on: (1) the intermolecular interferences; (2) the degree of packing of molecules in the amorphous phase; and (3) the most frequently occurring intermolecular distance as the smaller angle of
the halo shows the higher intermolecular distance and the smaller density of amorphous phases [194].
It is well known that XRD patterns of biomass are characterised
by intensive amorphous halo with a major maximum at 20–23°
and minor maximum at 13–17°, and they indicate mostly the
occurrence of Cel. It has been shown in previous studies of algae,
cotton–ramie, pine wood and different straws, grasses and stalks
treated or not treated by solvents [13,53,144,150,160]. It has been
suggested that such degree of crystallinity at 20–23° is related to
the contents of: (1) Cel [13,53,144,150,160]; (2) wax [160]; (3)
two Cel polymorphs (triclinic and monoclinic) [13]; and (4) com-
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
BC - Beech wood chips
CC - Corn cobs
PP - Plum pits
RH - Rice husks
SG - Switchgrass
SS - Sunflower shells
WS - Walnut shells
Fig. 1. Structural composition of seven biomass samples, wt.%.
BC - Beech wood chips
CC - Corn cobs
MM - Marine macroalgae
PP - Plum pits
RH - Rice husks
SG - Switchgrass
SS - Sunflower shells
WS - Walnut shells
Fig. 2. Positions of eight biomass samples in the chemical classification system of
the inorganic matter in high-temperature biomass ashes based on [1], wt.%.
plex nature of bonding between the three structural components
[160]. Some investigations show that the halo maximum at 13–
17° is a peak of Cel [13,53,150] and probably more characteristic
of triclinic Cel polymorph [13]. However, the occurrence of a strong
peak at 15° for purified Lig was also identified [60]. It is interesting
to note that the halo for organic matter of rice husk char [42], wood
char [114], different coals [195–197], coal char [198] and petroleum coke [199] has a similar shape to that of biomass, but their
halo positions are normally at higher angles, namely between
15° and 35°, with a maximum at about 25°. On the other hand,
the characteristic graphite d-spacings (the distance between the
regularly spaced planes) are at 26–27° and, to a lesser extent, at
42°, 44–45°, 51°, 55°, and 60°. Additionally, the minor halo maximum at 44° for PP (Fig. 5) was also observed for wood char [114]
and coal char [198]. Hence, the halo position and major halo maximums of biomass tend to have higher d-spacings than those of
carbonised fuels and graphite due to an increasing aromatisation
of molecular structures in the latter fuels and the crystal structure
of graphite.
Surprisingly, the present data show that there are some biomass
samples (PP, SS, WS) where the maximum at 15° is higher or equal
to this at 22° (Fig. 5). These samples with unusual XRD patterns be-
15
long to the LCH and CHL structural types (Table 5) and the inorganic K type (Fig. 2) of biomass. It can also be seen that a more
intensive broad peak at 15° is characteristic for the varieties most
abundant in Lig such as PP, WS, BC and RH (Fig. 5 and Table 6).
There is an absence of any large peak at 15° for MM (Fig. 5), but
it is well known that marine macroalgae normally do not contain
Lig [132]. In contrast, the most intensive occurrence of the large
peak at 22° is typical for the varieties highly enriched in Cel such
as CC, BC, SG and RH (Fig. 5 and Table 6). However, the most abundant in Cel variety (SS) shows the less ordered character among the
XRD patterns.
The calculated Segal crystallinity index of Cel based on the ratio
of maximums respectively at 22° (the peak intensity on the 002
crystallographic lattice plane) and 18° (the peak intensity of
amorphous diffraction zone) for the amorphous halo [53] is:
SG > RH > BC > WS > SS > MM > CC > PP (Table 6). Additionally, the
peaks at 22° for SG, RH and BC (Fig. 5) are significantly sharper than
for other samples. This indicates more developed Cel ordering in
these biomass varieties, which is also in agreement with the crystallinity index. On the other hand, the calculated height ratios between the maximums at 22° and 15° (the peak intensity on the
1 0 1 and 1 0 1 crystallographic lattice planes of Cel) for the amorphous halo are: MM > RH = SG > BC > CC > WS > SS > PP (Table 6)
and this order is quite different in comparison with Cel crystallinity. Therefore, the XRD patterns were arranged in the opposite order for 22/15° ratio from up to down in Fig. 5 for better illustration
of the first maximum (15°).
The present observations indicate that the amorphous halo
maximum at 15° is more complex and probably also related to
some abundant occurrence of Lig (see below) instead of contents
and crystallinity of Cel. Moreover, the two halo maximums also
coexist with the major diffraction peaks of Ca oxalates (at about
15°) and silica minerals such as opal plus some low-temperature
cristobalite, tridymite and quartz (at about 22°) (Fig. 5). Certainly,
these minerals are limited in the bulk biomass (excluding probably
some Ca oxalates in BC and opal in RH and SG) to provoke these
broad and intensive reflections (Table 6). Such observations additionally complicate the biomass XRD patterns, but they have not
been discussed in the literature. In contrast to Cel, the position of
Hem in the XRD patterns is unclear, but it could be intermediate
between both maximums. Finally, Ext can be various non-crystalline, semi-crystalline and crystalline organic and inorganic components. The data from correlation analysis (Table 9) for eight
biomass samples reveal:
Significant positive correlations among the maximums (22°, 18°
and 15°) and between the 22/15°ratio and Cel crystallinity
index;
Significant positive correlations of Cel crystallinity index with
Hem and insignificant with Cel;
The maximums and their ratios show significant (22° and 22/
15°) and insignificant (18° and 22/18°) positive correlations
with Hem, and to a lesser extent with Cel (22°, 22/15°, 22/
18°) and Lig (18°);
The maximum at 15° shows insignificant positive correlation
with Lig and negative with Cel and Hem;
Therefore, despite the typical and well known maximums of
Cel at 22° and 15° some additional relationships in the biomass
system were observed. The trends reveal that the maximums
at 22° and 18° and ratios between the maximums at 22/18°
(identical to Cel crystallinity) and 22/15° correlate also
with Hem, while the maximum at 15° and 18°correlate also
with Lig.
In summary, the present data indicate that the organic matter in
biomass has:
16
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Wood and woody biomass (samples 1-35)
Stems (samples 1-19)
Barks (samples 20-24)
Twigs (samples 25-28)
Leaves (samples 29-32)
Others (samples 33-35)
Herbaceous and agricultural biomass (samples 36-88)
Grasses (samples 36-47)
Straws (samples 48-56)
Stalks (samples 57-61)
Fibers (samples 62-64)
Shells and husks (samples 65-74)
Pits (sample 75)
Other residues (samples 76-88)
Animal biomass (sample 89)
Contaminated biomass (samples 90-93)
Fig. 3. Structural composition of 93 varieties of biomass specified in different groups and sub-groups, wt.%.
WWB - Wood and woody biomass
WWS - Stems
WWBA - Barks
WWT - Twigs
WWL - Leaves
WWO - Others
HAB - Herbaceous and agricultural biomass
HAG - Grasses
HAS - Straws
HAST - Stalks
HAF - Fibers
HASH - Shells and husks
HAP - Pits
HAR - Other residues
AB - Animal biomass
CB - Contaminated biomass
AVB - All varieties of biomass
NB - Natural biomass
Fig. 4. Mean structural composition of different biomass groups and sub-groups based on 93 varieties of biomass, wt.%.
(1) Non-crystalline character and only limited crystallinity
occurs in the organic structures with d-spacings in the range
of 0.3 and 0.9 nm;
(2) Amorphous to semi-crystalline phases with larger intermolecular distances (between 0.330 and 0.884 nm and mostly
at 0.404 and 0.590 nm) in comparison with carbonised fuels
(between 0.256 and 0.590 nm and mostly at 0.356 nm) due
to low aromatisation of biomass;
(3) Characteristic d-spacing at about 0.590 nm (15°) related to
complex Lig–Hem–Cel–oxalate associations;
(4) Typical intermolecular distance at about 0.404 nm (22°) connected with complex Cel–Hem–opal occurrences.
17
Abbreviations: AB, animal biomass; AVB, all varieties of biomass; CB, contaminated biomass; HAB, herbaceous and agricultural biomass; HAF, herbaceous and agricultural fibers; HAG, herbaceous and agricultural grasses; HAP,
herbaceous and agricultural pits; HAR, herbaceous and agricultural residues; HAS, herbaceous and agricultural straws; HASH, herbaceous and agricultural shells and husks; HAST, herbaceous and agricultural stalks; WWB, wood
and woody biomass; WWBA, wood and woody barks; WWL, wood and woody leaves; WWO, other wood and woody biomass; WWS, wood and woody stems; WWT, wood and woody twigs.
Order for groups and sub-groups
CB68.1 > HAST51.1 > WWS49.6 > HAF43.8 > WWO41.5 > HAR40.2 > AVB40.1 > HAB39.8 > HAS39.3 > WWB38.3 > HASH37.3 > HAG33.9 > AB32.7 > WWL25.5 > HAP22.4 > WWBA21.3 > WWT15.2
WWT61.3 > WWL45.5 > WWBA45.4 > WWB33.4 > HAG30.1 > WWO28.8 > AVB28.3 > HAS27.2 > HAB26.1 > HASH25.6 > AB24.5 > HAR24.0 > WWS22.7 > HAP20.8 > HAST20.6 > HAF17.9 > CB17.1
HAP51.4 > AB42.8 > HASH30.3 > WWBA30.0 > WWL25.3 > WWB25.2 > WWO25.0 > WWS24.7 > AVB22.1 > WWT21.9 > HAB20.4 > HAS19.9 > HAR19.0 > HAG15.5 > HAST15.2 > CB14.8 > HAF12.7
HAF25.6 > HAG20.5 > HAR16.8 > HAB13.7 > HAS13.6 > HAST13.1 > AVB9.5 > HASH6.8 > HAP5.4 > WWO4.7 > WWL3.7 > WWBA3.3 > WWB3.1 > WWS3.0 > WWT1.6
Cellulose
Hemicellulose
Lignin
Extractives
Characteristic
Table 7
Decreasing mean values for cellulose, hemicellulose, lignin and extractives (normalised to 100.0%) among the biomass groups (bold font) and sub-groups specified.
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Table 8
Characteristic decreasing orders among the structural components and extractives
(mean values normalised to 100.0%) for the biomass groups (bold font) and subgroups specified.
Order
Group and sub-group
1. Cellulose > hemicellulose > lignin > extractives
AVB, HAB, HAR, HAS,
HAST, WWB, WWO
CB
HAG
HASH, WWS
HAF
WWL
WWBA, WWT
HAP
AB
1a. Cellulose > hemicellulose > lignin
2. Cellulose > hemicellulose > extractives > lignin
3. Cellulose > lignin > hemicellulose > extractives
4. Cellulose > extractives > hemicellulose > lignin
5. Hemicellulose > cellulose > lignin > extractives
6. Hemicellulose > lignin > cellulose > extractives
7. Lignin > cellulose > hemicellulose > extractives
7a. Lignin > cellulose > hemicellulose
Abbreviations: AB, animal biomass; AVB, all varieties of biomass; CB, contaminated
biomass; HAB, herbaceous and agricultural biomass; HAF, herbaceous and agricultural fibers; HAG, herbaceous and agricultural grasses; HAP, herbaceous and
agricultural pits; HAR, herbaceous and agricultural residues; HAS, herbaceous and
agricultural straws; HASH, herbaceous and agricultural shells and husks; HAST,
herbaceous and agricultural stalks; WWB, wood and woody biomass; WWBA, wood
and woody barks; WWL, wood and woody leaves; WWO, other wood and woody
biomass; WWS, wood and woody stems; WWT, wood and woody twigs.
Certainly, such preliminary observations require more detail
and precise future investigations including preferably direct methods with quantitative determinations especially of biomass varieties among different groups and sub-groups not treated by
solvents.
3.1.3. XRD of dry water-soluble residue (DWR) of biomass
The XRD patterns of DWR generated from eight biomass samples (Fig. 6) show the complex crystalline, semi-crystalline and
amorphous character of the samples. The presence of sylvite, halite, arcanite, weddelite, whewellite and amorphous material, as
well as traces of Ca nitrate hydrate, nitrocalcite and Fe-rich magnesite, was identified. The overlapping of some diffraction peaks and
their low intensity made it difficult to identify some species. It is
well known that the generated DWR: (1) normally resembles the
original water-soluble phases (plus moisture composition) in biomass; and (2) concentrates these phases in the residue for their
more secure subsequent identification [39,174,183,192]. The present work shows that the most mobile water-soluble phases in biomass are chlorides, sulphates, oxalates and nitrates plus some
carbonates and amorphous material. These phases could be related
to their authigenic formation, namely as syngenetic and especially
epigenetic precipitations (Table 3) of fluid matter in biomass (see
Section 3.3). The occurrence of amorphous material is clearly seen
on the XRD patterns as amorphous halo in the interval 10–50° at
different maximums (Fig. 6). It should be stated that some of this
amorphous material is also a result of sample gelification (particularly for SG > SS > CC > WS) during the sample storage and XRD
measurement due to strong hydrophilic properties of some phases
(probably hydrated chlorides) and oils in DWR. This amorphous
material has both inorganic and organic character. It seems that
the amorphous inorganic material is dominant in MM, SS, RH, CC
and WS, while the amorphous organic material is abundant in
PP, BC and SG according to the maximums of the amorphous halo
to smaller or higher intermolecular distances, respectively.
3.1.4. XRD of biomass high-temperature ash (HTA)
The XRD results for eight biomass ashes (Fig. 7) are shown just
for illustration and comparison of the dominant non-crystalline
biomass (Fig. 5), semi-crystalline DWR (Fig. 6) and crystalline
HTA (Fig. 7) characters of the samples. Most of the HTAs produced
at 500 °C/2 h are highly crystalline materials with limited occurrence of inorganic amorphous material and organic matter at this
temperature. Only a few individual char particles were recognised
18
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Fig. 5. XRD patterns of eight biomass samples. Abbreviations: BC, beech wood
chips; CC, corn cobs; Cc, calcite; Cps, counts per second; Ha, halite; MM, marine
macroalgae, Op, opal; PP, plum pits; RH, rice husks; SG, switchgrass; SS, sunflower
shells; Wd, weddelite; Wh, whewellite; WS, walnut shells.
Fig. 6. XRD patterns of dry water-soluble residues (DWRs) generated from eight
biomass samples. Abbreviations: Arc, arcanite; BC–DWR, beech wood chips; CaNH,
Ca nitrate hydrate; CC–DWR, corn cobs; Cps, counts per second; Ha, halite; MFe, Ferich magnesite; MM–DWR, marine macroalgae; NCc, nitrocalcite; PP–DWR, plum
pits; RH–DWR, rice husks; SG–DWR, switchgrass; SS–DWR, sunflower shells; Sy,
sylvite; Wd, weddelite; Wh, whewellite; WS–DWR, walnut shells.
Table 9
Significant positive (+) and negative () correlation coefficient values (R2) at 95% confidence levels (bold font)a, and insignificant (normal font) R2 values for some characteristics
of 8 biomass samples additionally studied.
Characteristic
Correlation coefficient value with:
M22
(+) M18(0.81); M22/18(0.69); Hem(0.64); M15(0.64); CrI(0.60); MnO(0.57); SiO2(0.54); M22/15(0.41); VM(0.40); TiO2(0.39); CaO(0.36); Ext(0.29);
O(0.25); H(0.19); C(0.14); Cel(0.10); () N(0.70); Fe2O3(0.61); K2O(0.59); SO3(0.55); Al2O3(0.52); P2O5(0.51); Cl2O(0.48); EC(0.47); FC(0.46); Cl(0.43);
Na2O(0.43); S(0.41); MgO(0.31); Lig(0.30); DWR(0.19); A(0.18); pH(0.16);
M18
(+) M15(0.88); M22(0.81); MnO(0.63); VM(0.58); H(0.50); CaO(0.45); Hem(0.31); O(0.26); Ext(0.24); SiO2(0.15); M22/18(0.14); C(0.12); TiO2(0.11);
Lig(0.10); CrI(0.03); M22/15(0.02); () N(0.76); Fe2O3(0.73); Al2O3(0.68); EC(0.55); SO3(0.50); FC(0.49); Cl(0.47); S(0.45); Na2O(0.45); Cl2O(0.44);
DWR(0.42); A(0.40); Cel(0.29); pH(0.25); K2O(0.14); MgO(0.11); P2O5(0.02);
M15
(+) M18(0.88); M22(0.64); VM(0.63); MnO(0.56); H(0.55); CaO(0.50); Lig(0.48); C(0.41); P2O5(0.19); MgO(0.18); TiO2(0.10); K2O(0.09); M22/18(0.01);
SiO2(0.00); () N(0.73); EC(0.73); SO3(0.70); Cl(0.67); A(0.64); Na2O(0.64); Cl2O(0.64); S(0.63); DWR(0.62); Cel(0.60); Fe2O3(0.56); Al2O3(0.50);
pH(0.47); M22/15(0.37); FC(0.29); Hem(0.13); CrI(0.11); O(0.08); Ext(0.06);
M22/18
(+) CrI(0.97); SiO2(0.79); M22(0.69); M22/15(0.65); Hem(0.62); TiO2(0.52); Cel(0.45); Ext(0.23); A(0.20); C(0.16); MnO(0.13); pH(0.05); O(0.02);
DWR(0.15); M18(0.14); M15(0.01); () P2O5(0.82); K2O(0.79); Lig(0.55); MgO(0.44); SO3(0.36); Cl2O(0.31); H(0.28); N(0.25); Na2O(0.21); Cl(0.19);
S(0.18); EC(0.17); FC(0.16); Fe2O3(0.16); VM(0.05); Al2O3(0.05); CaO(0.01);
M22/15
(+) Hem(0.80); A(0.74); DWR(0.67); M22/18(0.65); CrI(0.65); pH(0.60); Ext(0.52); S(0.51); EC(0.51); SiO2(0.50); Cl(0.51); Na2O(0.49); O(0.47);
Cel(0.45); Cl2O(0.43); M22(0.41); SO3(0.39); N(0.19); TiO2(0.08); MnO(0.02); M18(0.02); () K2O(0.78); P2O5(0.62); Lig(0.61); C(0.56); MgO(0.45);
VM(0.42); M15(0.37); Al2O3(0.36); Fe2O3(0.32); CaO(0.19); FC(0.17); H(0.09);
CrI
(+) M22/18(0.97); SiO2(0.71); M22/15(0.65); Hem(0.63); M22(0.60); Cel(0.53); TiO2(0.49); A(0.22); MnO(0.17); DWR(0.16); O(0.13); Ext(0.08);
C(0.07); CaO(0.07); Al2O3(0.04); pH(0.03); M18(0.03); () P2O5(0.92); K2O(0.81); Lig(0.61); H(0.43); MgO(0.40); SO3(0.27); Cl2O(0.25); N(0.20);
Na2O(0.14); VM(0.13); Cl(0.13); S(0.13); M15(0.11); EC(0.10); FC(0.04); Fe2O3(0.04);
DWR
(+) EC(0.90); S(0.89); Cl(0.88); Na2O(0.88); Cl2O(0.85); SO3(0.80); A(0.70); N(0.68); Hem(0.57); pH(0.51); Cel(0.48); Ext(0.41); O(0.31); Fe2O3(0.12);
MgO(0.11); FC(0.06); TiO2(0.01); () C(0.73); Lig(0.55); VM(0.54); K2O(0.36); CaO(0.22); MnO(0.18); SiO2(0.14); Al2O3(0.07); P2O5(0.06); H(0.01);
pH
(+) A(0.95); S(0.68); Cl(0.67); EC(0.67); Cl2O(0.63); Na2O(0.63); SO3(0.58); N(0.56); DWR(0.51); Ext(0.39); H(0.26); SiO2(0.23); O(0.17); Lig(0.14);
FC(0.11); () VM(0.74); TiO2(0.68); C(0.56); Al2O3(0.45); CaO(0.44); Fe2O3(0.40); K2O(0.36); MnO(0.33); MgO(0.21); Cel(0.15); Hem(0.09); P2O5(0.07);
EC
(+) S(0.99); Cl(0.99); Na2O(0.99); Cl2O(0.98); SO3(0.96); DWR(0.90); N(0.81); A(0.79); Cel(0.74); pH(0.67); O(0.38); FC(0.22); MgO(0.19); Hem(0.14);
Fe2O3(0.14); Ext(0.08); P2O5(0.06); H(0.00); () C(0.85); VM(0.70); Lig(0.59); TiO2(0.37); SiO2(0.33); MnO(0.23); CaO(0.21); K2O(0.21); Al2O3(0.08);
Abbreviations: A, ash yield; Cel, cellulose; CrI, Segal crystallinity index of cellulose; DWR, dry water-soluble residue; EC, electrical conductivity; Ext, extractives; FC, fixed
carbon; Hem, hemicellulose; Lig, lignin; VM, volatile matter. M15, M18, M22 and their ratios (M22/18 and M22/15) are the maximums at 15, 18 and 22° 2h for Cu Ka from
XRD patterns.
a
The significant R2 values at 95% confidence level are: P0.63 and 60.63,and P 0.67 and 60.67 for 8 and 7 variables, respectively.
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
19
by light microscopy in these ashes as black particles particularly
encapsulated by silica material. The XRD data reveal that there
are many newly formed minerals from carbonate, sulphate, silicate
and phosphate classes in biomass HTAs. Some of them are very
interesting species from technological and environmental points
of view ([181,200] and see also Section 4) because they are not typical of coal ashes [176,201]. The phase composition of HTAs and
phase transformations during biomass combustion are beyond
the subject of the present study and they will be characterised
and discussed further in separate publications.
tural integrity of plant cells and provide the mechanical support
and strength for plants [102,135,160]. On microscopic scale, plant
cell walls consist of so-called Cel microfibrils, which are bundles of
Cel molecules coated with Hem as Lig is deposited in between the
microfibrils [120,152]. The strongly intermeshed structural components are chemically bound by non-covalent forces or by covalent cross-linkages [151]. Various minor and accessory organic
components also occur in biomass (Table 4). The concentration of
each organic compound in biomass varies depending on species,
type of plant tissue, stage of growth and growing conditions [32].
3.2. Organic matter of biomass
3.2.1. Cellulose (Cel)
The reference investigations show that Cel is mostly described
as: (1) a saturated linear polysaccharide from long-chain natural
polymers [19,32,45,66,87,93,100,127,145,160,163]; (2) glucose
polymer [56,66,71,100,120,122,134,135,151,160]; or (3) highmolecular-weight linear polymer [102]. This carbohydrate has
fibrous structure with smooth surface [33] and it is occasionally
protected by a waxy outer surface as in the case for the Cel structural tubes in straws [46]. Cel forms long chains that are bonded to
each other by a long network of hydrogen bonds [102]. Biomass
contains various proportions of two distinct crystalline polymorph
phases of Cel, namely: (1) the form Ia which is triclinic, metastable
and dominant for lower plants; and (2) the form Ib which is monoclinic, stable and dominant for higher plants [5,8,13,53,87]. It was
also proposed that Cel can even exist in four recognised crystal
structures known as celluloses I–IV [120]. Cel is completely insoluble in water [87,102]. It was found that the amount of Cel normally increases with increasing particle size (from < 150–180 lm
to > 850 lm) of biomass fractions [165]. Holocellulose (Cel plus
Hem) is composed entirely of sugar units and has relatively low
calorific value because of their high level of oxidation. In contrast,
Lig and Ext have a lower degree of oxidation and a considerably
higher heat of combustion [56,75,89,145,202].
The present study shows that Cel content (normalised structural basis) in biomass (Table 5) is highly variable and the mean
Cel values normally decrease in the biomass group order: contaminated biomass > herbaceous and agricultural biomass > wood and
woody biomass > animal biomass (Table 7). The high Cel value is
characteristic of some sub-groups such as wood stems and herbaceous and agricultural stalks and fibres (Table 7). The extremely
high Cel content is typical of some varieties, namely paper, cotton
(seed hairs and gin residue), flax fibres, sunflower stalks and waste
paper pulps (Table 5). In contrast, the low Cel concentration is
characteristic of some sub-groups such as agricultural pits and
wood twigs, barks and leaves (Table 7). The extremely low Cel value is typical of some twigs for different trees (Table 5). These data
are also in accordance with some of the literature findings. The correlation analysis (Table 10) shows only significant negative associations of Cel with Lig (Fig. 8), Na2O and N. The reasonable negative
correlation between Cel and Lig also confirms to some extent the
distinctive XRD patterns for both structural components in biomass. The other negative correlations are probably related to some
depletion of N- and Na-bearing phases in Cel matrix. For example,
the tree stems are highly enriched in Cel (Table 5) and normally
show the lowest N concentration in comparison with the other biomass sub-groups [1]. A similar statement is probably also valid for
Na if the biomass materials are not influenced by halite solution
during their transport and processing [1]. It should be noted that
the loose and shaken bulk densities of biomass samples (Table 6)
have significant and very strong negative correlation (R2 = 0.98)
with Cel, probably due to porous cell texture of Cel.
The basic organic constituents of biomass have a leading importance for the phase composition and they include three main structural biopolymers, namely Cel, Hem and Lig plus some extractives
(Table 2). The structural components have authigenic (syngenetic)
formation in biomass (Table 3) and these matrices contain various
major, minor and accessory organic and inorganic phases. Hence,
the structural organic components in biomass play a role similar
to that of lithotypes in coal. For example, it is well known that
Cel and Lig are thought to be the main precursor materials of the
vitrinite maceral (the main component in humic coals) [6],
whereas Lig was occasionally described even as a low-sulphur
immature coal [135].
The reference investigations show that biomass is a flexible
composite of Cel, Hem and Lig, which serve to maintain the struc-
Fig. 7. XRD patterns of high-temperature ashes (HTAs) generated from eight
biomass samples at 500 °C/2 h. Abbreviations: A, anhydrite; Ap, apatite; Arc,
arcanite; BC–HTA, beech wood chips; Cc, calcite; CC–HTA, corn cobs; Cps, counts
per second; Cr, cristobalite; D, dolomite; F, fairchildite; G, gypsum; Ha, halite; K,
kalicinite; KF, K feldspar; MM–HTA, marine macroalgae; N, natrite; NF, natrofairchildite; Pm, picromerite; Pl, plagioclase; PP–HTA, plum pits; Q, quartz; RH–HTA,
rice husks; SG–HTA, switchgrass; SS–HTA, sunflower shells; Sy, sylvite; Tr,
tridymite; WS–HTA, walnut shells.
3.2.2. Hemicellulose (Hem)
The reference investigations show that Hem is commonly described as: (1) xylan, pentosan or polyose [33,44,102,122]; (2) a
20
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Table 10
Significant positive (+) and negative () correlation coefficient values (R2) at 95% confidence levels (bold font)a, and insignificant (normal font) R2 values for the structural
components and extractives with the chemical composition of 28 varieties of natural biomass.
Characteristic
Correlation coefficient value with:
Cellulose
(+) Ext(0.19) CaO(0.18) S(0.16) VM(0.15) SO3(0.12) SiO2(0.11) A(0.09) O(0.07) H(0.04) Al2O3(0.04) C(0.03); () Lig(0.83) Na2O(0.44) N(0.43) FC(0.29)
K2O(0.22) MgO(0.22) Cl(0.19) TiO2(0.07) P2O5(0.06) Fe2O3(0.06) Hem(0.04) Mn(0.01)
Hemicellulose
(+) S(0.54) SiO2(0.43) A(0.38) Cl(0.27) N(0.23) K2O(0.10); () Lig(0.52) Mn(0.38) CaO(0.36) SO3(0.32) MgO(0.31) Al2O3(0.28) FC(0.26) Fe2O3(0.20)
Na2O(0.20) VM(0.10) Ext(0.05) Cel(0.04) C(0.04) O(0.03) H(0.03) TiO2(0.03) P2O5(0.02)
Lignin
(+) Na2O(0.49) FC(0.39) Mn(0.36) MgO(0.36) N(0.24) Fe2O3(0.16) K2O(0.13) Al2O3(0.12) SO3(0.07) TiO2(0.07) P2O5(0.06) CaO(0.05) Cl(0.01); ()
Cel(0.83) Hem(0.52) S(0.44) SiO2(0.33) Ext(0.14) A(0.29) VM(0.07) O(0.04) H(0.02) C(0.01)
Extractives
(+) SiO2(0.52) O(0.34) S(0.34) Cl(0.32) H(0.29) Cel(0.19) A(0.18) SO3(0.18) Na2O(0.08) TiO2(0.08) VM(0.04); () Mn(0.58) MgO(0.50) C(0.43) CaO(0.35)
P2O5(0.26) FC(0.24) K2O(0.20) Lig(0.14) Fe2O3(0.11) Al2O3(0.10) N(0.08) Hem (0.05)
Abbreviations: A, ash yield; Cel, cellulose; Ext, extractives; FC, fixed carbon; Hem, hemicellulose; Lig, lignin; VM, volatile matter.
a
The significant R2 values at 95% confidence level are: P0.36 and 60.36, P0.39 and 60.39, P0.42 and 60.42, and P0.53 and 60.53 for 28, 24 (Cl), 21 (Ext) and 12
(Mn) variables, respectively.
40
%
0.2
0.1
60
0
20
25
30
35
Hemicellulose (daf)
3
%
0
0
10
20
Extractives (daf)
30
%
20
10
40
20
0
30
35
0.2
40
%
R2 = -0.44
0.1
0.0
20
40
Lignin (daf)
%
25
Hemicellulose (daf)
%
10
0
MgO (in ash)
1
35
R2 = 0.36
%
2
30
20
40
R2 = -0.58
25
S (daf)
50
R2 = 0.38
Hemicellulose (daf) %
%
R2 = -0.52
30
0
20
15
60
0
%
R2 = -0.50
10
5
0
20
40
Lignin (daf)
%
C (daf)
% 100
Lignin (daf)
R2 = 0.54
0.0
0
Cellulose (daf)
Mn (in ash)
0.3
Ash (db)
40
20
%
%
S (daf)
R2 = -0.83
MgO (in ash)
Lignin (daf)
% 80
60
60
%
R2 = -0.43
50
40
0
10
20
Extractives (daf)
30
%
0
10
20
Extractives (daf)
30
%
Fig. 8. Some significant relationships based on correlation coefficient (R2) among the structural components, extractives and chemical characteristics of 28 varieties of natural
biomass, wt.%.
complex mixture of heterogeneous and branched-chain polysaccharides containing five C5 and C6 monosaccharides plus glucuronic and galacturonic acids [19,32,45,56,66,71,87,93,100,102,
127,134,135,145,160,163,165]; (3) macromolecular substance of
different sugars [102,144,151]; (4) sugar heteropolymer [120]; or
(5) high-molecular-weight polymer [102]. Hem particles have
irregular shape and cracks on their surface [33]. This carbohydrate
has a random and amorphous structure and is highly branched
[100,122,135]. Hem binds tightly, but non-covalently, to the surface of each Cel microfibril [66]. The five different sugars in Hem
are covalently joined together in long chains [135]. This constituent is soluble in diluted alkali solutions [87]. The bark has maximum Hem contents in comparison to other tree parts [56]. On
the other hand, hardwoods contain more Hem than softwoods
[71,102,134]. The amount of Hem increases with increasing particle size (from <150–180 lm to >850 lm) of biomass fractions
[165], which is similar to Cel.
The present study shows that Hem content (normalised structural basis) in biomass (Table 5) is highly variable and the mean
Hem values normally decrease in the biomass group order: wood
and woody biomass > herbaceous and agricultural biomass > animal biomass > contaminated biomass (Table 7), which is quite different to that of Cel. The high Hem value is characteristic of wood
twigs, leaves and barks, as well as some grasses (bermuda and
orchard) and corn grains (Tables 5 and 7). In contrast, the low
Hem concentration is characteristic of some sub-groups such as
herbaceous and agricultural fibres, stalks and pits (Table 7). The extremely low Hem value is typical of some varieties, namely some
tree stems, paper, sunflower stalks and cotton seed hairs (Table
5). These data are also in accordance with some of the literature
findings. The correlation analysis (Table 10) shows: (1) significant
positive relationship of Hem with S (Fig. 8), SiO2 and ash yield
(Fig. 8); and (2) significant negative association of Hem with Lig
(Fig. 8) and CaO. It is interesting to note that the positive correla-
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
tions of Hem with inorganic matter and Si, plus highly mobile elements in biomass such as S, Cl, N and K (Table 10 and [1]), strongly
indicate the abundance of authigenic opal, sulphates, chlorides and
nitrates in the Hem matrix (see also Section 3.2.4). The loose and
shaken bulk densities of biomass samples (Table 6) have significant
and strong negative correlations (R2 = 0.74 and 0.76) with Hem,
which is similar to Cel.
3.2.3. Lignin (Lig)
The reference investigations show that Lig is commonly described as: (1) irregular polymer of four or more substituted phenylpropane units based upon different alcohols as basic building
blocks [32,45,100,120,127,135,145,151]; (2) large, highly branched
and substituted polyaromatic polymer [19,87,100,122,127,151];
(3) high-molecular-weight compound [66,71,110,127,134]; (4)
non-sugar polymer [71,89,134]; (5) cross-linked polymer with aromatic nature and without exact structure [102,111]; (6) phenolic
polymer [160,163,202]; (7) complex macromolecule with a great
variety of functional groups [121,144]; or (8) long-chain natural
polymer [66]. This aromatic biopolymer is the main binder for
the fibrous components in plants [44,102]. Lig particles normally
present a hemispherical shape [33] and they are most often adjacent to Cel as an embedding material [87,127,202]. For example,
it was found that Lig: (1) encrusts the cell walls; (2) cements the
cells together; (3) binds and agglomerates the cellulosic fibres;
and (4) holds the microfibrils with relatively high structural rigidity in a lignocellulosic complex [44,62,66,87,88,160,163]. In the
cell-walls Lig is enriched particularly in the middle lamellae between adjacent cell walls, but also forms part of the primary and
secondary cell-wall materials [6,202]. Woody plant species are typically composed of tightly bound fibres, while herbaceous plants
have loosely bound fibres, indicating a lower proportion of Lig
[66]. Lig is amorphous [66,102,127,160] and its structure leads to
a large number of possible inter-linkages between individual units
[102,121,122]. Ether bonds predominate between Lig units, but
CAC and covalent linkages also exist [19,102,160]. The covalent
linking between Lig and polysaccharides strongly enhances the
adhesive bond strength between Cel fibres and Lig ‘‘potting
matrix’’ [102]. The order of decreasing Lig contents is normally:
softwoods > hardwoods > herbaceous and agricultural biomass or
non-woody biomass [44,66,87,102]. The annual biomass species
have commonly lower Lig value than perennial biomass species
[19]. However, it was also found that the Lig concentrations are
normally higher in leaves and young shoots than in stems for different woods [56,202]. The Lig values among different grasses
commonly follow the order: stems > leaves [111]. As mentioned
above, the macroalgae normally does not contain Lig [132].
The reference investigations show that some properties of Lig
are quite interesting. For instance, the role of Lig in water transport, mechanical support and biodefense for the evolution of plants
is well-known [202]. Lig provides the hydrophobic surface that allows plants to transport water [202]. Lignification supplies the
necessary mechanical strength that supports plants and provides
their ability to withstand aggressive environmental conditions
[71,88,111,134,163,202]. Lig also serves as a physical and chemical
barrier (shield) against: (1) the invasion of pests and pathogens;
(2) the rapid microbial or fungal destruction of cellulosic fibres;
and (3) the hydrolysis of Cel and Hem [87,102,158,202]. Lig is
one of the most persistent biological molecules and is highly resistant to natural degradation and biological digestion processes in
contrast to other common bioorganic components [32,87,110].
Lig is almost insoluble in strong H2SO4 and this constituent is also
quite resistant to conversion by many other chemical agents [87].
High-molecular-weight Lig is acid-insoluble in sulphuric acid,
whereas low-molecular-weight Lig is considered to be acid-soluble
[163]. The amount of acid-insoluble Lig increases with increasing
21
particle size (from <150–180 lm to >850 lm) of biomass fractions
[165], which is similar to Cel and Hem. On the other hand, biomass
treated by alkaline solution removes Lig from the biomass [158].
The high Lig content contributes to the relatively high heating value of biomass [32,44,56,75,89,145,202]. For instance, Lig has higher energy content (about 30%) than Cel and Hem [202].
The present study shows that Lig content (normalised structural
basis) in biomass (Table 5) is highly variable and the mean Lig values normally decrease in the biomass group order: animal biomass > wood and woody biomass > herbaceous and agricultural
biomass > contaminated biomass, which is opposite to the Cel order (Table 7). The high Lig value is characteristic of some subgroups such as wood barks and herbaceous and agricultural pits,
shells and husks (Table 7). The extremely high Lig content is typical
of some varieties, namely apricot pits, olive husks, some shells
(walnut, hazelnut and cashewnut), tea residue and coconut coir
pith (Table 5). In contrast, the low Lig concentration is characteristic of some sub-groups such as herbaceous and agricultural fibres,
stalks and grasses (Table 7). The extremely low Lig value is typical
of some varieties, namely cotton (seed hairs and gin residue), flax
fibres, certain grasses (orchard, bermuda and rye), paper and waste
paper pulps (Table 5). These data are also in accordance with some
of the literature findings. The correlation analysis (Table 10)
shows: (1) significant positive relationship of Lig with Na2O, fixed
C (FC) and MgO (Fig. 8); and (2) significant negative association of
Lig with Cel (Fig. 8), Hem (Fig. 8) and S (Fig. 8). These correlations
are probably related to higher contents of aromatics, some enrichment of Na- and Mg-bearing phases and depletion of S-bearing
phases in Lig. The loose and shaken bulk densities of biomass samples (Table 6) have significant and very strong positive correlations
(R2 = 0.97 and 0.98, respectively) with Lig as a compact binder, in
contrast to Cel and Hem. This is also in agreement with the above
XRD results for structural components (see Section 3.1.2).
3.2.4. Bulk extractives (Ext) and minor organic components
The reference investigations show that bulk Ext consist of various organic and inorganic components extracted by different polar
or non-polar solvents from biomass, but commonly water, ethanol,
benzene and toluene and their mixtures (Table 4). They are defined
as those compounds that are not an integral part of the biomass
structure [100]. The type, composition and quantity of the determined Ext depend on biomass varieties and solvents, as well as
extraction time and methods used. The studied Ext components
(Table 4) include: (1) commonly various saccharides and other carbohydrates, proteins, hydrocarbons, oils, aromatics, lipids, fats,
starches, phenols, waxes and inorganic materials; (2) to a lesser extent, chlorophyll, resins, terpenes, terpenoids, acetyls, uronic acids,
organic acids, sterols and glycosides; and (3) occasionally other
minor organic components such as alkaloids, gums, mucilages,
dyes, saponins, tannins, flavonoids, others.
The reference investigations show that the water–ethanol
extraction removed approximately 90% of the Ext and their removal significantly reduced ash yield, acid-insoluble Lig and soluble sugars [165]. Herbaceous biomass is more enriched in Ext than
woody biomass [163]. The orders of decreasing amount of Ext in
wood and woody biomass are normally: hardwoods > softwoods
[140]; wood barks > wood stems [35,79,88]; wood leaves > wood
stems, barks and twigs [56]. Ext decrease with the rotation age
(from 6 to 1 year) of tree stems [35] as the young (1-year-old)
wood barks contains slightly more Ext than old (3–4-year-old)
wood barks [79]. The quantities of Ext decrease significantly
(1.5 times) with increasing particle size (from <150–180 lm
to >850 lm) of biomass fractions [165]. Certain plants such as
rapeseeds and soybeans can have large amounts of Ext [100]. The
higher heating value of leaves, branches and tops compared to
stems and barks for some tree species could associate with higher
22
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
content of Ext (particularly oils) and lower degree of oxidation in
the former tree parts [35,56,88,169].
The present study illustrates that bulk Ext can be both organic
and inorganic components (see Sections 3.1.3, 3.2.5 and 3.3–3.4)
with authigenic (syngenetic, epigenetic) formation (Tables 3 and
4). The Ext content in biomass (Table 5) is highly variable and
the mean Ext concentrations normally decrease in the biomass
group order: herbaceous and agricultural biomass > wood and
woody biomass (Table 7). The high Ext value is characteristic of
some sub-groups such as herbaceous and agricultural fibres,
grasses and residues (Table 7). The extremely high Ext content is
typical of some varieties, namely corn grains, sugar cane, flax fibres, some straws (triticale, flax) and sweet sorghum grass (Table
5). In contrast, the low Ext concentration is characteristic of some
sub-groups such as wood twigs, stems, barks and leaves (Table 7).
The extremely low Ext value is typical of some varieties, namely
beech wood, cotton gin residue, syzygium wood and twigs of different trees (Table 5). These data are in accordance with some of
the literature findings. The correlation analysis (Table 10) shows:
(1) significant positive relationship of Ext with SiO2; and (2) significant negative association of Ext with Mn (Fig. 8), MgO (Fig. 8) and
C (Fig. 8). The positive correlations of Ext with Si plus ash yield and
highly mobile S and Cl (Table 10) indicate that some opal, sulphates and chlorides are mobile during biomass treatment by solvents. On the other hand, the negative correlation of Ext with Mn,
Mg, C and Ca indicates that carbonates and oxalates are less mobile
when biomass is treated by such solvents.
The reference investigations also show that different plants can
have large amounts of minor organic components. For example,
some of these components can even have major occurrences in
some biomass varieties and their contents can reach:
6–71% of proteins, 2–40% of lipids and up to 6% of nucleic acid in
algae [155].
1–11% of proteins in some straws, pine, spruce, birch, switchgrass, flax shives and corn grains [85,100,165].
1–5% of resins in pine, spruce and birch [88].
2–4% of acetyls in some straws and flax shives [165].
1–4% of uronic acids in some straws, pine, eucalyptus, sweet
sorghum grass and flax shives [100,165].
It should be stated that the organic composition of aquatic biomass is different in comparison with terrestrial biomass. Algae
(macro and micro) normally have high lipid contents, but some
of them also contain large amounts of proteins and carbohydrates
[100,155,162]. For example, the organic composition of algae commonly includes: proteins > carbohydrates > lipids > nucleic acid
[155]. Algae consist of cells and the intercellular water is fixed by
this network structure, while the soft walls consist mainly of complex sugars and proteins [157].
It was also found that the ash yield and Ext and protein contents
decreased with increasing particle size (from <150–180 lm
to >850 lm) of biomass fractions [165]. Hence, Ext and proteins
show some association with the inorganic material of biomass.
The minor components play a different role in the plants. For instance, starches are commonly found in the vegetable kingdom
(corn, rice, wheat, beans, and potatoes) [32,100,203]. These noncrystalline glucose polysaccharides [100,120] are reserve sources
of carbohydrates in biomass [87]. Pectins are structural polysaccharides present in cell walls of all plant tissues [82,93] and they
function as intercellular and intracellular cementing materials
[82]. Uronic acids are sugars that have been oxidised to acids [100].
The most interesting observation from the present correlation
analysis of structural components and Ext is that Hem associates
with highly mobile elements in biomass. It seems that Hem plays
a role of conducting and concentrating tissue for mineralised solu-
tions abundant in sulphates, chlorides, nitrates and silicic acid in
plants. Such observation is also supported by the fact that Hem
is mostly abundant in annual and fast-growing twigs and leaves
of trees, grasses and straws among biomass sub-groups (Table 7).
In contrast, Cel, Lig and Ext show associations with different nonmobile and slightly to highly mobile elements in biomass. This
observation is somehow not in agreement with the opinion that
Lig is particularly enriched in the water conducting xylem tissues
[110].
Finally, it is worth noting that there are different indirect chemical methods (ASTM, Browning, Klason, Koch, NREL, Scott, Van Soest and Wine, and their modifications) for the determination of
structural components and Ext by various solvents and procedures
[9,19,35,44,49,56,79,93,110,111,120,124,136,158,160,163,204].
Therefore, the data can differ considerably depending on the methods used and this has been emphasised [163]. The correlation analysis shows that there are relatively limited significant associations
for the structural components and Ext in biomass (Table 10) and
one of the reasons for that may be the different methods used. A
proved chemical method for determination of structural components or a new validated and worldwide accepted chemical procedure for more accurate determinations is required. Other direct
methods such as light and electron microscopy could assist in such
a procedure.
3.2.5. Organic minerals
Oxalates are minerals, but they are specified to a special group
because they are organic species, which is one of the exceptions in
mineralogical sense [205]. The reference investigations show that
Ca oxalates are typical minerals of plants [64,70,74,79,83,94,96,
131,141,168]. These bio-minerals are an end product of plant
metabolism [70] and commonly present in cytoplasm [64]. Calcium accumulates in plants through the precipitation of Ca oxalates as Mn co-precipitates in the oxalates as solid solutions [96].
It was found that the amount of oxalates is higher in the bark
and foliage than in the wood [79,96]. The crystallization of Ca oxalates gives structural stability of plants due to formation of rigid
microstructure body in some species [141]. For example, mineral
grains (2–10 lm) and cubic crystals (5–20 lm) of Ca oxalates were
found as inclusions in the plant structure, as well as on detrital Sirich mineral grains in wood stems and especially in barks
[74,83,94]. Whewellite (CaC2O4H2O) was also identified frequently
in low-temperature ashes produced from different biomass varieties [50,70]. It should be noted that low-temperature ashes are prepared in oxygen plasma at low temperatures (100–250 °C) and this
technique is generally able to isolate the inorganic matter in biomass with relatively little alteration of its original form [70].
The present data show that minor and trace occurrences of
weddelite (CaC2O42H2O) and whewellite were observed in most
of the biomass samples and their DWR (Figs. 5 and 6). Calcium
dihydrate and monohydrate oxalates are characteristic of RH, PP,
BC and SG. This confirms the above statement for their typical
occurrence in biomass. These relatively mobile minerals have
authigenic (syngenetic, epigenetic) formation in biomass (Table
3). It was noted that Ca oxalates are only soluble in acidic water
solutions [94]. However, the present results reveal that they are
also partly soluble in slightly acidic to neutral conditions during
water treatment (Table 6 and Fig. 6), similar to coal [192].
3.3. Fluid matter of biomass
The fluid matter in biomass is a mineralised aqueous solution
containing various cations, anions or non-charged species (Table
4). This constituent has complex origin and can include detrital,
authigenic and technogenic origins of components (Table 3). The
moisture commonly consists of water with dissolved free ions in
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
cell fluid [37,131,134]. Its content in biomass (at different basis)
normally varies in the interval of 10–60% and it can reach even
80–90% for some raw biomass species [1,74]. The fluid matter plays
an important role for the composition of biomass because of: (1)
high water content in the living cells; (2) variable total mineralisation of water; and (3) different chemical composition of these solutions [1]. For example, it is well known that annual and fast
growing crops (foliage and small branches of trees, short-rotation
woods, straws, grasses, fruits) show the highest contents of water,
inorganic matter and elements such as Cl, K, Mg, N, P, S and occasionally Na and Ca ([1,22,31,32,56,58,65,80,96,168,179] and references therein). These are typical mobile elements not only in the
plant physiology [96], but also in the natural water system
[206,207]. The plants transport the mineral-rich water from the
roots via stems to their upper branches and leaves. Some water
is lost during this transport through evaporation (especially from
leaves and branches) and the minerals tend to precipitate in the
plant [88]. Additionally, there is also intensive epigenetic mineral
precipitation from a saturated solution due to moisture evaporation after biomass harvesting and during biomass drying [1]. This
process results mostly in consecutive formation of water-soluble:
(1) phosphates; (2) carbonates; (3) sulphates; (4) chlorides; and
(5) nitrates, which are normally a general sequence of precipitation
from less soluble to highly soluble minerals in the water system
[1,183,206,207]. Such epigenetic mineral formations are the reason
for enhanced leaching of Ca, Cl, K, Mg, Na, P and S from biomass
harvested and left in the field for a prolonged period of time ([1]
and references therein]).
The reference investigations show that the water-soluble
phases in biomass include mostly inorganic material, N-containing
material, sugar acids and non-structural sugars, which interfere
with structural carbohydrate analysis [163]. It was also found that
the water-soluble fraction in straws, grasses, halophytic plants and
woody biomass is 0.3–8.8% (mean 4.0%) [20,43,56,113], while the
pH of water leachates from rice straw, lichen and woody biomass
is 4–6 [168,208,209]. The electrical conductivity for water leachates from banagrass and straws varies between 0.4 and
23.8 mS cm1 (mean 6.0) at solid/liquid weight ratio of 1:8–70
[20,30,49]. The water-soluble elements in biomass have been
studied extensively [18,20,22,28,30,31,34,41,43,47,49,55,56,64,
68,74,79,83,86,94,111,113,117,118,131,133,141,161,163,164,168,
208–220]. It was identified that their proportions leached are
highly variable for different biomass sources. For instance, the calculated decreasing order (based on mean and range values) of
water-soluble elements leached from various biomass varieties,
namely agricultural residues (olive pits, sugarcane bagasse, vine
shoots), grasses (banagrass, switchgrass), straws (rice, wheat),
wood (olive, citrus, spruce, pine, birch, aspen) and meat–bone
meal are: Cl (87, 59–98%) > K (71, 25–93%) > Na (60,18–96%) > P
(51,4–97%) > S (46, 15–100%) > N (37%) > Mg (36, 7–68%) > Mn
(23%) > Al (9, 0–40%) > Ca (7, 3–15%) > Fe (4, 0–12%) > Si (2, 0–5%)
plus some As, Ba, Co, Cu, Mo, Pb, Sr, Ti, V and Zn [18,20,
22,30,34,86,118,133,168,208,215].
The present data (Table 6 and Fig. 9) show that the pH of leachates from water extraction procedures of 8 biomass samples varies
between 5.1 and 6.8 (mean 5.7). The solutions are slightly acidic
(BC, CC, PP, SS, SG, WS) to neutral (MM, RH) and their decreasing
order of pH values is: MM > RH > PP > CC > WS > SG > BC > SS. Thus,
this characteristic correlates positively and significantly with inorganic matter and mobile S, Cl and Na components (Table 9 and
Fig. 9). For comparison, the pH values for coal are typically more
variable (2.2–7.5, mean 5.9) due to the highly changeable proportions of Fe sulphides, organic acids and carbonate concentrations
[181]. It is well known that some Cel is converted into water-soluble products under acidic conditions, whereas under alkaline conditions some Lig tends to decompose [2]. Hence, the detected
23
amorphous organic material in DWR (Fig. 6) could originate from
the decomposition of some metastable Cel (for example, for SS,
BC, SG, WS, CC, and PP) and Lig (for instance, for MM and RH) under
such conditions.
The present data (Table 6 and Fig. 9) show that the total mineralisation or DWR yield of the biomass leachates varies between 0.3
and 15.1% (mean 3.8%). They are salty (BC, CC, PP, RH, WS), sea
salty (SS) and brine (MM, SG) according to the classification of natural waters [206,207]. The decreasing order of these total dissolved
solids is: MM > SG > SS > RH > BC > WS > CC > PP. As expected, this
characteristic correlates positively and significantly with electrical
conductivity, inorganic matter and mobile S, Cl, Na and N components (Table 9 and Fig. 9). In contrast, the DWR values for coal are
normally lower (0.2–8.4%, mean 2.1%) than for biomass due to decreased occurrence of water-soluble salts after the biomass lithification [1,181]. Finally, the electrical conductivity for biomass
leachates varies significantly (0.7–19.8, mean 3.9 mS cm1) (Table
6 and Fig. 9). The decreasing order of electrical conductivity values
for these leachates is: MM > SS > RH > SG > CC > WS > BC > PP,
which is similar to that of the total mineralisation. The electrical
conductivity correlates positively and significantly with DWR,
inorganic matter, pH, cellulose and mobile S, Cl, Na and N components (Table 9 and Fig. 9). It is well known, that the conductivity of
water solutions is highly dependent mostly on the concentration of
dissolved salts. Hence, the leachates of PP, BC and WS are relatively
more salt-free solutions (with small charge imbalances) than others with higher ionic strength (particularly MM and SS) according
to the above sequence. It can be assumed that the measured pH,
DWR and electrical conductivity values are probably similar to
the original values of water solutions in the plants studied. Additionally, the present data for these three characteristics are in a
good agreement with reference observations, as well as with the
detected mineral composition in DWRs (see Section 3.1.3).
3.4. Inorganic matter of biomass
The inorganic matter has also an important role for the composition of biomass. It includes mostly mineral matter, namely mineral species and poorly crystallized mineraloids from different
mineral groups and classes, as well as amorphous inorganic phases
(Table 2) with authigenic, detrital and technogenic origin (Table 3).
This constituent in biomass varies in the interval of 0.1–46% (mean
6.8%) based on ash yield (dry basis) [1]. For comparison, this characteristic in coal is in the more narrow range of 6–52% (commonly
6–30% and mean about 21%) [1]. The inorganic matter in biomass is
normally much less than in solid fossil fuels, excluding animal biomass and some varieties from herbaceous, agricultural and contaminated biomass. Summarised data about the ash yield of
biomass have been reported recently [1].
The inorganic phases identified in biomass are listed in Table 4.
The reference investigations show that the high variability in
mineral content among plants can be considerable as it depends
on genetic and environmental factors, as well as physiological
and morphological differences among crops [179]. For example,
wood bark contains much more inorganic material than the wood
stem [15,16] as bark concentrates a considerable amount of inorganic impurities like sand or soil [23,25]. It was found that most
of the mineral constituents in biomass are located in the middle lamella and primary cell wall, but also in the secondary wall of wood
[15]. It was also found that the inorganic matter increases with
decreasing particle size (from <150–180 lm to >850 lm) of biomass fractions [165].
Little information is available on mineral matter in biomass
[1,11,188] and the combined qualitative and quantitative determinations are almost missing in contrast to coal ([174,176,180] and
references therein). It is well known that the methodology from
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
%
10
1
0.1
5
10
15
DWR
10
1
0
5
DWR
15
5
1
50
45
10
15
5.5
6
pH
6.5
7
10
15
DWR
20
%
R2 =0.95
10
1
0.4
5
20
%
R2 =-0.68
0.3
0.2
5.5
6
6.5
7
pH
%
60
50
R2 = 0.74
40
30
20
0
5
5
0.1
5
0.1
0.1
0
20
%
% 100
DWR
%
10
15
R2 = -0.73
0
%
R2 =0.67
10
40
20
TiO2
EC mS/cm
100
10
0.001
DWR
0.1
1
0.01
0
R2 = 0.88
0.1
0.001
% 55
R2 = 0.88
10
0.01
%
C (daf)
Na2O
% 100
20
1
0.1
A (db)
0
%
R2 = 0.89
10
Cel (daf)
R2 = 0.90
S (daf)
EC mS/cm
100
Cl (db)
24
5
5.5
6
pH
6.5
7
0
1
2
EC
3
4
mS/cm
Fig. 9. Some significant relationships based on correlation coefficient (R2) among the chemical characteristics of water leachates and dry water-soluble residues (DWRs)
generated from eight biomass samples, wt.%.
coal experiments can be applied to biomass [10]. It was also noted
that the mineralogy of biomass determines many important mechanisms and properties [10]. For example, the minerals in coal are
particularly illuminating in considering interpretations of coal formation, because frequently conditions for the development of certain minerals are better understood than conditions for the
formation of a given organic component (maceral) [221]. Additionally, there are predictable end products of a definite set of biological, chemical and physical conditions that provide an environment
in which the minerals could be deposited or in which they could
form [222]. Hence, the occurrence, abundance and distribution of
minerals in biomass and coal can be strong indicators for the conditions and processes occurring in these systems [180]. Moreover,
the mineral composition of coal and biomass has a strong impact
on processing, application and environmental and technological
concerns related to these fuels [181].
3.4.1. Silicates
Silicates identified in biomass are listed in Table 4. They have
one of the most complex origin among other mineral classes because they can be both natural (detrital and authigenic) and
anthropogenic species in biomass. It is well known that silicates
and especially quartz, feldspars (plagioclases and K feldspars), clay
minerals (kaolinite, montmorillonite, illite, chlorite) and mica minerals (muscovite, biotite) are typical soil minerals [205]. Hence,
these less mobile minerals can be detrital (pre-syngenetic, syngenetic and epigenetic), pre-existing and fine-grained particles (normally <10–100 lm) introduced by water and wind and fixed on
plant surfaces in natural biomass (Table 3). Additionally, some
detrital (pre-syngenetic), pre-existing and finely dispersed mineral
grains (commonly <1 lm) can also be introduced into the plant by
water suspensions through endocytose process during syngenesis
(Table 3). In contrast, opal has authigenic (syngenetic and epigenetic) origin in natural biomass (see below). The present optical
and XRD results (Fig. 5) indicate that opal can be an important
mineral in RH, SG, PP and WS, while the other silicates were not
identified due to their low contents and detection problems (Figs.
5 and 6). Additionally, the above minerals plus other silicates are
typical technogenic (post-epigenetic) species of semi-biomass
due to their presence as additives or contaminants during processing of natural biomass (Tables 3 and 4).
Silica minerals in biomass are a very interesting case due to
their complex origin in biomass. The authigenic silica occurs by
absorption of silicic acid from the soil solutions [84,131] and their
precipitation in plants [4,18,54,61,72,84,96,103,113,131,138,141].
These solutions flocculate in cell structures [103], deposit between
and within plant cells [138] and undergo polymerisation [4]. As a
result organically deposited silica hydrate (opal) is formed, which
is dominantly in an amorphous state and occasionally in crystalline
forms [4,18,54,61,72]. Such silica formations in plants are also described in the literature as phytoliths [54,74,113,138,141]. Opal
was found as spicules of fresh-water sponges and in the cell wall
or the lumen of the epidermis of some plants [113]. Silica in rice
straw was observed to concentrate in separate small inclusions
that were embedded in the organic matrix. Small (1–2 lm) nearly
pure silica particles were also observed in wheat straw [41]. Additionally, the entire outer shell of the rice husk, which is highly enriched in silica, was found to contain a distinct pure silica layer
[94]. The silicon distribution is highly dependent on the plant species [4]. For example, grasses and straws contain a high amount of
silica for the plant’s rigidness to maintain its posture [61,103]. Similarly, silica in wood occurs mainly in supportive tissues such as
bark [131]. Hence, silica also plays a role similar to structural component in some plant tissues because it forms a rigid microstructure that gives structural stability of plants [138,141]. It is noted
that a similar silicification process is also well known for some
coals. It represents initial formation of opal and its subsequent
dehydration and recrystallization to cryptocrystalline chalcedony
and later to more stable low-temperature quartz and cristobalite
in/on cell walls [176].
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
Silicates with dominant detrital and technogenic occurrences
in biomass were also identified. For example, silicon-rich mineral
grains, coming from dust, soil and dirt were found in wood chips
[83]. Quartz and albite were also identified as originating from
volcanic rock contamination in the region where the trees were
grown [50]. Additionally, the presence of quartz, cristobalite and
augite in meat–bone meal has been explained by cow digestion,
as particles of soil are swallowed by ruminants when they eat
grass [90]. Quartz with unclear origin was commonly identified
in low-temperature ashes of different biomass varieties [50,70].
The presence of quartz, diopside, orthoclase, forsterite, Cs silicate
and Mg aluminosilicate microcrystals was identified in bagasse,
peanut shells, hemlock needles and olive pits, but also without
specification of their origin [74,84,171]. Further, some semibiomass contains high contents of zeolites from detergents
(washing powders) [131]. Additives such as clay fillers are commonly used in paper production [31,32]. Various silicates were
also identified in municipal solid wastes and their probable origin
was also described [39].
3.4.2. Oxides and hydroxides
Oxyhydroxides identified in biomass are listed in Table 4 and
they can be both detrital and technogenic in origin. It is well
known that oxides and hydroxides of Fe (hematite, magnetite, goethite, lepidocrocite), Al (corundum, boehmite, diaspore, gibbsite)
and Ti (rutile, anatase, brookite) are typical soil minerals [205].
However, these less mobile mineral classes are not characteristic
of natural biomass due to its low content of Fe, Al and Ti and their
relatively limited occurrence as water-soluble forms in the system.
The present results confirmed such observation because oxyhydroxides were not identified in biomass due to their low contents
and detection problems (Figs. 5 and 6). The possible occurrence of
Al, Cu, Fe, Mn, Ni, Sn and Zn oxides in conifer bark, twigs and needles with unclear origin has been illustrated or mentioned [74].
Hence, the presence of oxyhydroxides in the natural biomass system could be mostly related to their detrital origin (similar to most
silicates). In contrast, the occurrence of Fe-, Al-, Mg-, Ca- and Tibearing oxyhydroxides in semi-biomass is mostly technogenic as
a result of additives and contaminants during biomass processing
(Table 4). For example, the presence of rutile in some semi-biomass
is caused during paper production where this oxide is often used as
a white pigment [29]. Various oxyhydroxides were also identified
in municipal solid wastes and their probable origin was also described [39].
3.4.3. Sulphates, sulphites and sulphides
Sulphates, sulphites and sulphides identified in biomass are
listed in Table 4 and they can be authigenic, technogenic and probably detrital in origin. These relatively mobile mineral classes are
not typical of biomass, excluding algae and semi-biomass. The
highly mobile sulphates can be authigenic and especially epigenetic in origin due to the evaporation and precipitation of water
in natural biomass. For example, arcanite was identified in DWR
from RH and this mineral probably also occurs in other DWRs,
but its X-ray diffraction peaks are overlapped (Fig. 6). The reference
investigations show that the presence of microcrystals of K and Ba
sulphates, and Fe and Zn sulphides were detected in bagasse,
peanut shells, hemlock bark and twigs, and sagebrush twigs, but
without specification of their origin [74,84]. Additionally, the
low-temperature ashes of different biomass samples reveal the
occurrence of arcanite and anhydrite [50,70]. However, such sulphate formation is commonly attributed to interaction, upon oxygen plasma treatment, between acidic SO2 and SO3 gases (formed
from biomass S compounds) and basic alkaline and alkaline-earth
ions bound to the organic matter of biomass as exchangeable elements [70].
25
The occurrence of sulphites and sulphides in biomass has been
mentioned in some investigations (Table 4). However, the presence
of these mineral classes (especially sulphides) is arguable because
their formation requires a combination of specific reducing and
acid to weak alkaline environment, occurrence of S-depositing bacteria, hydrogen sulphide and Fe sulphate solutions [223,224]. Such
conditions are typical for coal, but not for the natural biomass system. Further, the possible occurrence of detrital sulphides from soil
in biomass [10,131] is also questionable because sulphides are not
stable minerals during weathering. In contrast to sulphides and
sulphites, sulphates are characteristic of semi-biomass as a result
of their common use as additives during processing of natural biomass (Table 4). For example, sewage sludge contains large amounts
of Fe and Al sulphates used as flocculants and precipitation agents
for removal of P at the wastewater treatment plants [97,129]. Various sulphates were also identified in municipal solid wastes and
their probable origin was also described [39].
3.4.4. Phosphates
Phosphates identified in biomass are listed in Table 4 and they
can be detrital, authigenic and technogenic in origin. The genesis of
these minerals can be terrigenous for less mobile phosphates and
authigenic for mobile phosphates in natural biomass. It is well
known that apatite is a typical and stable soil mineral [205]. However, the biogenic (authigenic) origin of phosphorites was also suggested as a result of P-bearing spores, pollens and albumen of
vegetable matter, as well as microorganisms [221,225] and animal
bones [7,99]. Phosphates were not identified in the present work
due to their low contents and relatively lower solubility in water
(Figs. 5 and 6). However, the reference investigations show that
the presence of microcrystals of apatite and Mn phosphate was detected in bagasse, peanut shells, hemlock twigs and conifer wood
with unclear origin [74,84]. It was noted that animal bones consist
of apatite or Ca (hydroxyl, chloro, fluoro) phosphate [99]. It was
also suggested that the occurrence of hydroxyl-apatite in olive
biomass is most probably associated with the use of P-containing
fertilizers [86,149]. Additionally, the low-temperature ashes of
different biomass samples show the presence of MgCaPO4 [70].
Further, phosphates are characteristic of semi-biomass as a result
of their use as additives during processing of natural biomass
(Table 4). For instance, sewage sludge contains large amounts of
FePO4 used as precipitation agent at the wastewater treatment
plants [97]. Apatite was also identified in municipal solid wastes
and its probable origin was also described [39].
3.4.5. Carbonates
Carbonates identified in biomass are listed in Table 4 and these
relatively less mobile minerals can be detrital, authigenic and technogenic in origin. The present work shows that traces of calcite were
observed in MM, BC and SG (Fig. 5), whereas traces of magnesite
were detected in DWR of MM (Fig. 6). The detrital carbonate occurrence in MM is probably a result of some fine shell remains despite
the preliminary manual elimination of such particles from these
algae (Table 6). The detrital presence of calcite in biomass as soil
contaminant has also been mentioned elsewhere [10]. The lowtemperature ashes of different biomass samples show the
occurrence of calcite and dolomite [70]. Further, carbonates are
characteristic of semi-biomass due to their common use as
additives during processing of natural biomass [39]. For example,
various carbonates were also identified in municipal solid wastes
and their probable origin was also described [39].
3.4.6. Chlorides
Chlorides identified in biomass are listed in Table 4 and they
can be authigenic and technogenic in origin. This highly mobile
mineral class can be epigenetic in origin due to the evaporation
26
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
and precipitation of water in natural biomass (similar to sulphates). Traces of halite were observed in MM (Fig. 5), sylvite
was identified in all DWRs, whereas halite was found in DWRs
from MM and probably BC (Fig. 6). The reference investigations
show that interstitial halite crystals (0.1–0.2 lm) were observed
inside the vacuoles (0.3–4 lm) of the gland cells for halophyte
plant types [113]. Particles of sylvite deposited on straws [41]
and in olive pits [171], K and Bi chlorides in hemlock wood [74]
and KACa chloride in olive pits [171] with unclear origin were also
identified. The low-temperatures ashes of different biomass samples reveal the occurrence of sylvite, halite and K perchlorate
[18,70]. Further, chlorides are also characteristic of semi-biomass
as a result of their common use as additives during processing of
natural biomass. For example, various chlorides were identified
in municipal solid wastes and their probable origin was described
[39]. It is interesting to note that the biomass composition may be
slightly altered during sea transportation due to the intrusion of
halite originated from seawater [18].
3.4.7. Nitrates
Nitrates identified in biomass are listed in Table 4 and they can
be authigenic and technogenic in origin. This highly mobile mineral class can be authigenic and especially epigenetic in origin
due to the evaporation and precipitation of water in natural biomass (similar to sulphates and chlorides). Traces of Ca nitrate hydrates (Ca nitrate dihydrate and nitrocalcite) were observed in
DWRs from BC, PP and SG (Fig. 6). The reference investigations
show that small amounts of nitrate groups and nitre (KNO3) were
identified in low-temperature ashes of different biomass samples
[50,70]. Such nitrate formation is similar to that of sulphates and
it is commonly attributed to interaction, upon oxygen plasma
treatment, between acidic NO and NO2 gases (formed from biomass N compounds) and basic alkaline and alkaline-earth ions
bound to the organic matter of biomass as exchangeable elements
[70]. The anthropogenic occurrence of nitrates in biomass is commonly from the use of N-containing fertilizers and additives.
3.4.8. Others
Inorganic phases such as glass, metallic alloys and pure metals
with technogenic origin were commonly identified in semibiomass (Table 4). Such phases are characteristic of municipal solid
waste and sewage sludge, while they generally do not occur in natural non-contaminated biomass. However, the possible occurrence
of native Au, Fe, Mn and Zn, as well as some AuANi, Bi and
NiAFeAAs phases in hemlock bark and sagebrush twigs with unclear origin has been also mentioned [74]. A small amount of amorphous inorganic material with unclear composition was noticed in
low-temperature ashes of eucalypt barks and stems [50]. A nongranular portion of inorganic material (so called atomically
dispersed material) has also been reported in the literature as a
common mode of occurrence of inorganic matter in biomass
[10,22]. It was claimed in these studies that this complex material
occurs as dissolved salts in inherent moisture, cations attached to
carboxylic and other functional groups, complex ions and chemisorbed material.
The inorganic data show that there are some leading indicative
trends, namely the potential of natural biomass to have: higher
values of carbonates, chlorides, organically bound inorganic
elements, oxalates, phosphates and water-soluble components;
and lower values of inorganic matter, oxyhydroxides, silicates,
sulphates–sulphides and ash–fusion temperatures; in comparison
with coal [1,174,176,180,182,183,192,195–197].
4. Potential applications of phase-mineral and chemical
composition of biomass
The former [1] and present overviews on phase-mineral and
chemical composition of biomass may have potential implications
for future advanced and sustainable processing of biomass to biofuels and chemical feedstock, including collection, transportation,
storage, upgrading, anaerobic digestion, combustion, gasification,
pyrolysis and liquefaction. Some potential applications of biomass
composition or its use for prediction and indicator purposes are
listed below:
(1) The phase and chemical composition of biomass varieties
and their classification (such as Figs. 1–4 and 10, and [1])
have a key importance in both fundamental and applied
aspects related to biomass fuels and their products (similar
to coal [181]) and can assist directly or indirectly in:
Establishment of a uniform nomenclature and standards;
Characterisation of composition and prediction of
properties;
Elucidation of the behaviour of organic and inorganic
matter during processing;
Identification of appropriate or potential new modes of
utilisation; and
Prediction, estimation, reduction or elimination of technological, environmental and health problems or benefits
such as:
– Economically valuable or environmentally hazardous
components;
– Advanced (effective, muticomponent, wasteless) and
environmentally safe utilisation;
– Biological, physical, chemical and thermo-chemical
performance in industrial installations; and
– Global and local environmental contamination of the
air, water, soil and plants by toxic and potentially
toxic compounds.
(2) There is a potential advantage when the biomass classification systems are based on the identification and implementation of significant relationships between elemental
concentrations, combined chemical and phase-mineral associations and especially on an application of genetic
approaches for specification of the phases (similar to
Fig. 10). In this case the reasons for different problems or
benefits during biomass processing can be systematically
identified. For example, certain major trends and important
relationships were identified in the natural biomass system
based on correlations and associations among phase and
chemical characteristics. These associations are related to
the occurrence, content and origin of elements and phases
in biomass and they include: (1) CAH (mainly as authigenic
cellulose, hemicellulose, lignin and organic extractives); (2)
SiAAlAFeANaATi (mostly as detrital silicates and oxyhydroxides, excluding authigenic opal); (3) CaAMgAMn (commonly as authigenic oxalates and carbonates); and (4)
NAKASAPACl (normally as authigenic phosphates, sulphates, chlorides and nitrates). These important and systematic associations have potential applications (see also below)
and can be used for initial classifications or prediction and
indicator purposes connected with future advanced and sustainable processing of biomass to biofuels and chemical
feedstock.
(3) The data show some indicative trends of natural biomass in
comparison with coal, namely the potential of the former
fuel to have normally:
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
27
WWB - Wood and woody biomass
HAB - Herbaceous and agricultural biomass
HAG - Herbaceous and agricultural grass
HAS - Herbaceous and agricultural straw
HAR - Herbaceous and agricultural residue
AB - Animal biomass
MB - Mixture of biomass
CB - Contaminated biomass
AVB - All varieties of biomass
P - Peat
L - Lignite
S - Sub-bituminous coal
B - Bituminous coal
A - Algae
Fig. 10. Position areas of 86 biomass samples and 38 solid fossil fuels in the chemical classification system of the inorganic matter in biomass based on [1,180], wt.%.
Higher values of Ca, Cl, H, K, Mg, Mn, Na, O, P, moisture,
volatile matter, structural plant constituents, carbohydrates, light hydrocarbons, extractives, chelates, oxalates,
oxygen-containing functional groups that are with highly
reactive functionalities, carbonates, chlorides, phosphates, organically bound inorganic elements and
water-soluble components; and
Lower values of Al, C, Fe, N, S, Si, Ti, calorific value,
carbonised structural plant constituents, aromaticity,
functionalities, hydrocarbons, inorganic matter, oxyhydroxides, silicates, sulphates–sulphides, bulk density, friability and ash–fusion temperatures.
Exploiting of these differences is important and even critical for
proper use of biomass and/or co-processing with coal and other
fuels (see below).
(4) The data for chemical composition ([1] and Fig. 10]) and
structural components and extractives (Table 5 and Figs. 3
and 4) are informative for selecting specific biomass groups,
sub-groups and varieties as potential feedstocks abundant in
Cel, Hem, Lig or Ext for more effective biomass processing to
specific products. For example, the relative proportion of Cel
and Lig is one of the determining factors in identifying the
suitability of plant species for subsequent processing and
production of major organic compounds [66]. Further, a high
content of Ext is a strong indicator for possible recovery and
use of some minor components from biomass such as lipids,
terpenes, tannins, resins, sugars, starches, fats, oils, proteins,
organic acids and inorganic salts [35,79,100,160,162,165].
(5) It is well known that high quality and active adsorbents are
produced from some biomass resources such as agricultural
shells, husks and pits, and certain wood barks [25,187,226,
227]. It can be seen that the above resources include mostly
sub-groups and varieties enriched in Lig (LCH, LHC and HLC
structural types in Figs. 1, 3 and 4) and from inorganic lowacid K type (Figs. 2 and 10). The presence of different authigenic inorganic compounds in biomass (Fig. 10) can also
favour the production of chars with specially tailored sorption properties.
(6) It was found that the contents of organic elements in biomass are much less variable than those of inorganic elements [1]. Biomass contains significant to high levels of
inorganic matter (0.1–43% ash and 3–63% moisture on as
measured basis) and many of the technological and environmental problems encountered with the biomass processing
are associated with the occurrence, proportion, origin and
behaviour of inorganic constituents. It is supposed that the
genetic inorganic types in biomass (Table 3) may have a
leading importance for such problems (similar to coal
[181]). Therefore, the concept of dividing inorganic matter
of biomass into detrital, authigenic and technogenic types
has both fundamental and applied importance. For instance,
the detrital minerals (silicates and oxyhydroxides typical of
inorganic S type) are commonly stable during weathering,
less mobile (water-insoluble) and less reactive and with
high-melting temperatures during biomass processing. In
contrast, the authigenic minerals (opal, oxalates, carbonates,
phosphates, sulphates, chlorides and nitrates characteristic
of inorganic C, K and CK types) are normally unstable during
weathering, highly mobile (water-soluble) and reactive, and
with low decomposition or melting temperatures during
biomass processing (Fig. 10). Further, the technogenic inorganic matter includes various mineral species with more
variable properties and behaviour in comparison with the
natural inorganic constituents. Therefore, the inorganic matter types, mineral classes and groups and specific mineral
species are likely to be the major reasons for many problems
during biomass processing (similar to coal [181]). For
example:
The authigenic minerals can be highly responsible for
enhanced leaching behaviour, low-temperature transformations, partitioning behaviour and emission (or capture) of many volatile elements and hazardous
components, corrosion, agglomeration, deposits formation, slagging, fouling, bed defluidization and composition of residues during biomass processing;
The detrital minerals can be important for enhanced
abrasion–erosion (hard and angular quartz, feldspars,
rutile, corundum), formation of some low-temperature
28
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
(7)
(8)
(9)
(10)
(11)
eutectics, partitioning element behaviour and for
decreasing combustion efficiency and increasing operating costs for the handling of inert materials during biomass processing;
The technogenic minerals can also be responsible for
many of the above listed problems plus enhanced pollution by heavy metals, and this is because most semi-biomass fuels contain high levels of trace elements
[39,40,181]; and
The unusually high ash yield determined in some samples can be a very strong indicator for contamination of
biomass by detrital and technogenic materials [1].
Organic matter and authigenic minerals in biomass are intimately mixed with each other and their physical isolation
(in contrast to chemical leaching), namely separation by
screening, dense media treatment or flotation can be difficult during biomass upgrading. In contrast, detrital minerals
in biomass occur as physically more easily separable particles (similar to coal [181]).
It seems that many of the key constraints in the efficient
thermal conversion of biomass may arise mostly from authigenic minerals. However, such minerals can also have a catalytic effect on pyrolysis and combustion [89]. It should be
stated that many inorganic elements in biomass are also
bound in organic matter (Table 4), but these organically
associated elements and their phases react during the thermal treatment of biomass and appear dominantly as inorganic matter in residues (similar to municipal solid waste
[39] and coal [196,197,201]).
Biomass fuels with similar bulk chemical composition may
behave quite differently and may generate distinctly different solid products during biomass processing due to different modes of element occurrence in them. Therefore, the
specified structural and chemical types and mineral classes
(Table 5 and Figs. 3, 4 and 10) can be used for some preliminary elucidation of biomass behaviour during combustion,
pyrolysis and gasification. The organic phases and mineral
classes, groups and species (Table 4) have different behaviour during processing and their actual identification, quantification and associations in a specific biomass can predict
the behaviour of fuel during conversion. For example, such
information in coal has been used to predict the typical composition of combustion residues [181,196,197,200]. The
knowledge of phase, mineral and element occurrences in
biomass and their transformations during biomass thermal
treatment is also one of the leading factors for determining
formation mechanism and future application of biomass
residues.
Fluidized-bed combustion requires additional use of quartz,
some silicates or carbonates as a bed material. Hence, biomass highly enriched in Ca and Mg (animal and wood biomass) and silica (straws, grasses and rice husks) [1] may
reduce need to add the above additives in such combustion
chambers.
The release of volatile hazardous air pollutant elements and
compounds during biomass combustion mostly depends on
modes of element occurrence in biomass and combustion
and cleaning technologies used. Emission is related to the
association of these elements with both organic and inorganic matter and the common transformations and reactions
of these constituents at different temperatures during combustion. An interesting phenomenon in some coal and biomass co-combustion power plants could be the increased
retention of some volatile elements (in particular trace elements) in combustion residues. This phenomenon is known
as the concept of self-cleaning fuels, where specific minerals
and chars correlate with this capture behaviour [181]. Some
biomass is highly enriched in alkaline-earth and alkaline
constituents (C, CK and K inorganic types of biomass in
Fig. 10) and this favourable composition could contribute
significantly to the capture of volatile C, S, Cl and trace elements in combustion residues (see also below) by the subsequent inorganic matter transformations in the system [181].
For instance, the phase composition of biomass ashes produced at 500 °C demonstrates the presence of such favourable minerals (Fig. 7). The future large-scale co-firing and
co-gasification of biomass with coal and other fuels highly
contaminated by hazardous elements (municipal solid
waste, refuse derived char, petroleum coke, sewage sludge)
is a challenge because it will influence dramatically the
behaviour of hazardous compounds in such power plants
and the composition and properties of combustion and gasification residues. Additionally, the different phase and
chemical composition of biomass fuels give the possibility
of improving the performance and/or reducing of harmful
emissions by modification of the feed fuel composition
through tailored and self-cleaning fuel mixtures. The use of
such an advanced approach is of particular interest because
these cleaner fuel blends may contribute to reduce or avoid
additional installations of expensive cleaning systems in
power plants.
(12) It is well known that biofuels as renewable energy resources
do not contribute to the greenhouse effect due to the CO2
neutral conversion [1,105,134,151]. However, the present
work indicates that the biomass energy can be not only carbon–neutral, but also with some extra carbon-capture and
storage effect due to fixation of CO2 in the combustion residues. For example, the mineral composition of biomass
ashes (500 °C) clearly show the intensive formation of various newly formed carbonates (Fig. 7). These carbonates are
a result of solid–gas reaction between the volatile CO2
(released from biomass or occurring naturally in the atmosphere) and alkaline-earth and alkaline oxyhydroxides
formed during biomass combustion. Such carbonates are relatively stable during weathering and thermal treatment up
to 900 °C [197].
(13) There are biomass fuels such as reed canary grass, sorghastrum grass, rice straw and rice husks highly enriched
in silica [1]. The transformation of this opal type silica to
fine crystalline and high-temperature silica minerals
(especially cristobalite) in the combustion solid products
and their possible emission to atmosphere may cause a
health problem. This commonly fine-grained and fibrous
cristobalite needs careful control because it is a respiratory hazard and is the source of concern for lung diseases
[31,32,145].
5. Potential biomass resources for biofuels
This and the previous publication [1] have presented detailed
information on the phase and chemical composition of a relatively
large range of biomass varieties mainly with a view to the technological use of biomass resources. However, this type of information
might also be relevant to studies of the balance, regeneration, biodiversity, biocoenosis and life cycle assimilation in natural ecosystems (forests, tundra, grasslands, prairies, pastures, peatlands,
wetlands, rivers, lakes, seas, oceans). One of the biggest problems
for large-scale biofuel production is the availability of sustainable
biomass resources generated for that purpose. The conversion of
biomass sources from natural ecosystems into energy resources
may lead to serious environmental problems because the natural
biomass is a renewable energy source, while biomass fuel is still
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
an incomplete renewable energy resource at present [1], when
considering the complete life cycle assessment [134]. Like many
others we speculate that not enough knowledge is available to
truly comprehend the importance of the natural ecosystems. Given
this lack of knowledge humans should be very careful not to further disturb and destroy these native systems. In the current trend
towards large-scale production of fuel and energy from biomass resources it would thus be desirable to limit the exploitation of natural ecosystems and instead derive the biomass from a limited
number of feedstocks, namely:
Non-edible agricultural residues;
Semi-biomass (contaminated biomass and industrial biomass
wastes); and
Short-rotation energy crops such as cultivated grass, forest and
algae plantations, but grown only on existing low productive,
degraded or contaminated non-arable land and in wastewater
or contaminated ponds.
Thus, the potential biomass resources for biofuel production
could preferably always be divided initially into sustainable and
unsustainable management resources under specified criteria.
Subsequently, when there is confirmation of their sustainable origin and/or production only after that they can be assigned to the
currently used terms such as primary and secondary biofuels or
biofuel generations (I–IV) and products. It is noted that this might
very-well reduce the overall potentially available biomass resource. Still, biomass could play an important role as a fuel and energy resource when combined with other efforts, e.g. other types of
renewable energy and overall reducing the fuel and energy
consumption.
(5)
(6)
(7)
(8)
(9)
6. Conclusions
Some conclusions based on the present overview of the phase
and mineral composition of biomass can be made:
(1) Biomass is a complex heterogeneous mixture of organic
matter and, to a lesser extent, inorganic matter, containing
various solid and fluid phases with different contents. The
main structural organic components in biomass are cellulose, hemicellulose and lignin and these matrices contain
various major, minor and accessory organic and inorganic
phases with natural (authigenic and detrital) and anthropogenic origin.
(2) Biomass has highly variable composition and properties,
especially with respect to structural components, extractives
and inorganic constituents. These characteristics differ significantly in comparison with those of coal and they are critical for proper use of biomass and/or co-processing with coal
and other fuels.
(3) The organic matter (54–99%, mean 93%, at dry basis) in biomass has: non-crystalline character and only limited crystallinity with larger d-spacings (between 0.33 and 0.88 nm) in
comparison with carbonised fuels such as coal, char and coke
(between 0.26 and 0.59 nm) due to low aromatisation of biomass structure; and characteristic intermolecular distances
at about 0.59 nm and 0.40 nm related to complex Lig–Hem–
Cel–oxalate and Cel–Hem–opal associations, respectively.
(4) The structural components in biomass varieties belong to six
orders (or types) as the most common orders include: Cel > Hem > Lig; Cel > Lig > Hem; and Hem > Lig > Cel. Hemicellulose is abundant in annual and fast-growing plants or plant
parts and this constituent plays a role of conducting and
concentrating tissue for mineralised solutions abundant in
(10)
(11)
29
sulphates, chlorides, nitrates and silicic acid in plants. Cel,
Lig and Ext show associations with different non-mobile
and mobile elements in biomass.
Bulk extractives (1–87%, mean 10%, at dry, ash-free basis)
consist of various organic and inorganic components and
their contents are normally higher in herbaceous and agricultural biomass than in wood and woody biomass.
The fluid matter (3–63%, mean 14%, at different basis) is a
mineralised aqueous solution associated with both inorganic
and organic matter. The pH of leachates from water extraction procedures of biomass has slightly acidic to neutral
character and these solutions have salty, sea salty to brine
total mineralisation with variable electrical conductivity.
These three characteristics correlate positively with one
another and commonly with inorganic matter and mobile
S, Cl, Na and N components.
The inorganic matter (0.1–46%, mean 7%, at dry basis) in biomass includes various minor and accessory mineral species
and poorly crystallized mineraloids from various mineral
groups and classes, as well as some amorphous inorganic
phases with natural and technogenic origin. Dry water-soluble residues generated from biomass show that the most
mobile water-soluble phases in biomass are chlorides, sulphates, oxalates and nitrates plus some carbonates and amorphous material with both organic and inorganic character.
Biomass ashes (500 °C/2 h) have highly crystalline character
with limited occurrence of inorganic amorphous material
and organic matter. Various species of carbonate, sulphate,
silicate and phosphate classes were identified in the ashes
and most of them are newly formed minerals during biomass combustion.
Certain major associations related to the occurrence, content
and origin of elements and phases were identified in natural
biomass system and they include: (1) CAH (mainly as authigenic Cel, Hem, Lig and organic extractives); (2)
SiAAlAFeANaATi (mostly as detrital silicates and oxyhydroxides, excluding authigenic opal); (3) CaAMgAMn (commonly as authigenic oxalates and carbonates); and (4)
NAKASAPACl (normally as authigenic phosphates, sulphates, chlorides and nitrates). These systematic associations have a key importance in both fundamental and
applied aspects, namely their potential applications for classifications or prediction and indicator purposes connected
with advanced and sustainable processing of biomass.
The occurrence, content, origin and association of inorganic
matter types, mineral classes and groups and specific species
are likely to be the major reasons for many technological and
environmental concerns during biomass processing. The
authigenic minerals can be responsible for enhanced leaching,
separation, emission/capture, corrosion, agglomeration, slagging, fouling, bed defluidization, and waste and health problems during biomass processing. The detrital minerals can be
important for enhanced abrasion–erosion, low-temperature
eutectics, and partitioning and operating cost problems, as
well as for low combustion efficiency during biomass processing. The technogenic minerals can also be responsible for many
of the above listed problems plus enhanced pollution by heavy
metals. The unusually high ash yield determined in some samples can be a very strong indicator for contamination of biomass by detrital and technogenic materials.
Some biomass is highly enriched in alkaline-earth and alkaline constituents (C, CK and K inorganic types of biomass)
and this favourable composition could contribute significantly to the capture of volatile C, S, Cl and trace elements
in combustion residues. Hence, there are potential possibilities for reduction of harmful emissions by modification of
30
S.V. Vassilev et al. / Fuel 94 (2012) 1–33
the feed fuel composition through tailored and self-cleaning
fuel mixtures during co-firing or co-gasification of biomass
with other fuels.
(12) The present work indicates that the biomass energy can be
not only carbon–neutral, but also with some extra carboncapture and storage effect due to fixation of CO2 in the combustion residues. The mineral composition of biomass ashes
(500 °C) clearly show the intensive formation of various
newly formed carbonates as a result of solid–gas reaction
between the volatile CO2 (released from biomass or occurring naturally in the atmosphere) and alkaline-earth and
alkaline oxyhydroxides formed during biomass combustion.
The conversion of biomass sources from natural ecosystems
into energy resources may lead to serious environmental problems. The natural ecosystems should be avoided, to a maximum
extent, as resources for biofuel production. The potential favourable resources for that purpose should be focused preferably on:
(1) non-edible agricultural residues; (2) semi-biomass (contaminated biomass and industrial biomass wastes); and (3) short-rotation energy crops (grass, forest and algae plantations), but grown
only on existing low productive, degraded or contaminated nonarable land and in wastewater or contaminated ponds. The potential biomass resources for biofuel production should always be divided initially into sustainable and unsustainable management
resources under specified criteria.
Finally, the lack of enough knowledge and data appear to be the
biggest problem for the future advanced and sustainable management of biomass as a large-scale fuel resource. Much more complete
and detailed investigations are required to reach the level of knowledge achieved for coal. In order to fully understand the biomass
potential it is of vital importance to gain better fundamental knowledge of biomass composition and properties. Therefore, a lot of
future work related to systematic studies of biomass fuels is needed
and it includes: the development and standardisation of reliable
approaches and methods; combined chemical, phase and mineral
characterisation; elucidation of modes of element occurrence and
their behaviour; determination of trace elements concentration
and speciation; evaluation of phase transformations and chemical
reactions; and clarification of composition, formation and utilisation
of the waste products generated during biomass processing. For that
purpose, a better coordination and collaboration between the
leading scientific groups worldwide in this field is required.
The phase and mineral composition of biomass ashes and phase
transformations during biomass combustion, as well as variation in
composition of biomass and biomass ashes will be characterised in
future publications.
Acknowledgements
The present work was carried out in part within the European
Commission’s research programme and in part within the research
programme of the Bulgarian Academy of Sciences. Stanislav Vassilev would like to express his gratitude to the Joint Research Centre
(European Commission) for the possibility to perform studies at
the Institute for Energy and Transport (Petten, The Netherlands)
as a Detached National Expert.
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