Thermochemical conversion of biomass
Sanjay P Mande, Fellow
Energy–Environment Technology Division, T E R I , New Delhi
Biomass conversion routes 705
Biomass densification
technologies 706
Biomass combustion 715
Biomass stoves 729
Biomass pyrolysis 738
Biomass gasification 746
Guidelines for designing
downdraft gasifiers 766
Tar formation and
reduction 770
Inverted downdraft gasifier
stove 781
Nomenclature 783
References 785
Annexe 793
Biomass conversion routes
Biomass can be used for different purposes such as
cooking, process heating, electricity generation,
steam generation, and mechanical or shaft power
applications. It also produces a variety of chemicals as by-products. Various biomass conversion
processes used for achieving these objectives can
broadly be classified as follows.
Physical
• Fuel processing: chopping, shredding, pulverizing, and densification into briquettes/
pellets
Thermochemical
• Combustion
• Pyrolysis
• Gasification
• Liquification
• Ammonia production
Chemical
• Acid hydrolysis
Biochemical
• Anaerobic digestion to methane
• Ethanol fermentation
The first two biomass conversion routes,
namely physical and thermochemical (combustion, pyrolysis for charcoal production, and
gasification), are covered in this chapter, while
biochemical conversion routes will be covered in
the next chapter.
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Biomass densification technologies
Many non-woody biomass residues suffer from major disadvantage of having
low bulk densities for their efficient utilization. For example, the bulk density
of the majority of agro-residues lies in the low range of 50–200 kg/m3 (Table
13.1) as compared to 800 kg/m3 for coal of the same size. This results in huge
storage space requirements, difficulty in handling, and higher transportation
costs, which makes them uneconomical as a marketable commodity (Dhingra,
Mande, and Kishore 1996). Also, low bulk densities and the loose nature of
available biomass are associated with faster burning of fuels resulting in higher
flue gas losses (lower operating thermal efficiencies) and emissions in the form
of fly ash or particulates in the atmosphere. This makes them poor quality
biomass fuels. In order to improve the marketability of the available loose
biomass as fuel, pre-processing becomes necessary (TERI 2004).
Densification of biomass can be done by the briquetting or pelletizing
technology that compresses loose biomass into densified forms. This reduces
the transportation and storage costs, and improves the effectiveness of
biomass for use as a combustible fuel (Mande and Lata 2005).
Densified briquettes/pellets produced from biomass are fairly good substitutes for coal, lignite, and firewood and have several advantages.
They are renewable and sustainable sources of energy.
They are cheaper than coal.
They are of consistent quality and size.
They have better thermal efficiency than loose biomass.
High density (800–1200 kg/m3) compared to loose biomass (50–200 kg/m3).
They provide value addition for rural biomass.
Table 13.1 Bulk density of selected loose biomass
Bulk density (kg/m3)
Biomass material
Saw dust
Saw dust
Straw
Straw
Coir pith
Jute dust
Groundnut shell
Bagasse pith
Sugar cane leaves
Loose
Briquetted
Loose
Bales
—
—
Pulverized
—
Pulverized
177
350–400
80
320
47
74
165
74
167
Source Iyer, Rao, and Grover (2002)
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Appropriate biomass residue for briquetting
Almost all types of biomass can be densified. However, many factors need to
be considered before biomass qualifies as an appropriate feedstock for
briquetting. The main characteristics of an appropriate biomass residue for
briquetting are discussed below.
Moisture content
Higher moisture content poses problems in grinding and requires higher energy
for the drying operation. Therefore, biomass with lower moisture content,
preferably below 10%–15%, is desirable.
Ash content and composition
The majority of biomass residues (except rice husk with 20% ash) have a low
ash content but they contain higher percentages of alkaline (especially potash)
minerals which contribute towards lowering the sintering temperature leading
to ash deposition. Higher ash content thus increases the slagging tendency,
which becomes more acute with biomass containing more than four per cent
ash.
Flow characteristics
Fine granular material with uniform size flows easily in the fuel hoppers and
storage bins/silos. Thus, some of the appropriate biomass materials for
briquetting include sawdust, coffee husk, groundnut shell, pulverized mustard
stalk, and cotton sticks.
Binding mechanism of densification
Briquetting is one of the several agglomeration techniques used for the
densification of biomass residues. On the basis of compaction, briquetting
technologies can be classified as follows.
High-pressure compaction
Medium-pressure compaction
Low-pressure compaction
Normally, binders are not required in high- and medium-pressure
compaction, but sometime pre-heating of biomass is used to enhance the
compaction process. In all compaction processes, individual particles are
pressed together in a confined volume. In case of biomass, the binding
mechanism under pressure can be divided into cohesive and adhesive forces,
Van der Waal’s forces of attraction between solid particles, and mechanical
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interlocking bonds under pressure, which create strength in bonding during
compaction. Binders, highly viscous bonding media such as tar and other
molecular organic liquid or cow dung, are used in low-pressure compaction to
enhance adhesion among biomass particles by creating solid–liquid bridges.
The lignin present in the biomass helps in creating such bonds due to its softening at higher temperatures and its adsorption on solid particle layers. The
strength of the resulting agglomeration depends on the type of interaction
and the material characteristics. Some important parameters for agglomeration are discussed below.
Particle size
Granular biomass material of 6–8 mm size with about 10%–20% powdery
(<4 mesh) material normally gives the best results. Though high-pressure
(1000–1500 bar) compaction machines such as piston press or screw extruder
can briquette larger particle-sized biomass, it can lead to choking of the
entrance to ram or die portion of the briquetting machine. The condensation
of vapour released from the larger particles onto finer particles can create
lumps, which affect free flow. However, presence of only the finer material is
not always good due to its low density and flowability. Presence of differentsized particles improves the packing dynamics contributing to higher
strength.
Moisture
The level of moisture content is a very critical factor as presence of the right
amount of moisture (7%–10%) leads to the development of self-bonding properties in lignocellulosic substances at elevated temperatures and pressures
prevailing in piston press and screw extruder briquetting machines. A higher
moisture content can result in poor and weak briquettes having cracks due to
the escape of steam. It also results in erratic operation as the feed flow chokes
due to steam formation. The briquettes produced should have a moisture content higher than the equilibrium value, otherwise they would regain moisture
from the atmosphere, resulting in swelling during storage, transportation, and
disintegration when exposed to humid conditions.
Biomass feed temperature
At higher temperatures, moisture of biomass gets converted into steam under
higher prevailing pressures, which hydrolyses the hemicellulose and lignin portions of the biomass into lower molecular carbohydrates, lignin products,
sugar polymers, and other derivatives. These act like in situ binding material.
Better compaction gives higher briquette density and strength with higher
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biomass feed temperature and pressure (Figure 13.1). This also softens the
fibres and their resistance to briquetting, which results in lower power
requirements and reduction in the wear of the contact parts. However, the
temperature should be kept lower than 300 ºC, beyond which biomass starts
pyrolysing.
Briquetting can be done with or without binder material. No binders are
generally required in high-pressure briquetting. Prior to the briquetting process, the biomass has to be broken up into small pieces and then dried to a
Figure 13.1 Variation in briquette density with pressure at different biomass feed
temperatures. (a) Mustard stalk (b) groundnut shell
Reproduced with permission from FAO
Source FAO (1996a, b)
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moisture content of about 12%–15%. The briquetting plants in India use sawdust, bamboo dust, groundnut shell, mustard stalk, cotton stalk, coffee husk,
baggasse, sugar mill waste mud (commonly called press mud), jute waste, coir
pith, etc. as raw material. All these biomass briquettes, except for press mud,
have good calorific values of the order of 3800–4000 kcal/kg.
Commercially, briquetting of biomass without binders is done by
briquetting machines based on the following technololgies.
Screw press
Ram-piston press
In both the piston and screw-press technologies, the application of high
pressure increases the temperature of the biomass, and thus the lignin present
in the biomass partially liquefies and acts as a binder.
Ram and piston press technology
Biomass briquetting using ram and piston press technology involves drying,
grinding, sieving, and compacting. Moisture is removed from the loose
biomass with the help of a dryer and then the biomass is ground in a hammer
mill grinder. The ground material is transported using pressurized air, separated from air using a cyclone separator, and then sieved and stored in a bin
above the hopper for ensuring a continuous flow of biomass material into the
press. Biomass is then punched into a die by a reciprocating ram to produce
briquettes (Figure 13.2). The ram moves about 250–300 times per minute in
the process. In the briquetting machine, due to wear of contact parts (here
ram and piston), frequent maintenance and/or replacement is required. The
average frequencies of replacement for some of the machine components are:
Figure 13.2 Ram and piston type briquetting press
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Table 13.2 Contribution of different heads (in %) for unit cost of briquettes
Briquetting machine production capacity (kg/h)
Input cost component
250
500
750
1000
1500
2250
Capital
Raw material
Operation
Electricity
Repair and maintenance
19.2
41.3
10.1
25.5
3.9
14.6
54.5
6.6
20.3
4.0
12.1
59.6
4.8
19.9
3.6
9.7
57.1
4.4
24.7
4.1
8.8
64.3
3.3
19.9
3.7
7.7
67.2
3.2
18.0
3.9
Source Tripathi, Iyer, and Kandpal (1998)
Table 13.3 Power requirement for briquetting units
Power requirement (kW)
Briquetting production
capacity (kg/h)
Fine granular
Dry
Wet
Coarse granular
Dry
Wet
Stalky material
Dry
Wet
250
500
750
1000
1500
2250
17.5
25.0
32.5
50.5
65.5
98.0
36.0
43.5
58.5
101.0
108.5
141.0
43.5
51.0
73.5
116.0
123.5
156.0
26.5
34.0
41.5
65.5
80.5
113.0
45.0
52.5
67.5
116.0
123.5
156.0
2.5
60.0
82.5
131.0
138.5
171.0
Source Tripathi, Iyer, and Kandpal (1998)
300 hours for ram, scrapper and wear rings, and 500 hours for taper, split die,
and hammers.
In India, piston-press briquetting machines are commercially available.
They are available in different capacity ranges, from 250 to 2250 kg/h. Table
13.2 gives the share of various input costs in the production of biomass briquettes. It can be seen that at higher production capacities, the raw material
contributes more as input cost and the share of other costs diminishes due to
the scale of production. Table 13.3 gives the power requirement for different
capacity machines for briquetting various types of biomass materials. Power
requirement increases with capacity of the machine. For given production capacity, it increases from fine to coarse to stalky biomass due to an increasing
pre-processing requirement of biomass. For a given type of biomass, wet
biomass requires higher power for an additional drying operation.
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Screw-press technology
In this process, biomass is dried to get an optimum moisture content value
by passing hot air produced by burning part of the briquettes in the furnace.
Using the heated oil obtained through a heat exchanger, biomass is further
pre-heated to about 100–120 ºC so as to minimize the wear of the dies and improve the true density of the briquettes formed. This pre-heated material is
then fed to the screw extruder where a revolving screw (at about 600–700
RPM) continuously compacts the material through a tapered die, which is
heated externally to reduce friction between the biomass and the die surface
(Figure 13.3). The briquette obtained through this technology has a hole in the
centre and its outer surface is partially carbonized.
The briquettes extruded using screw press are more homogeneous (as
output is continuous and not in strokes) and have better crushing strength and
combustion properties (due to larger combustion area per unit weight). Since
the outer surface of the briquettes is carbonized, it facilitates easy and clean
ignition, and also this impervious layer provides protection from moisture,
thereby increasing its storage life. However, power consumption and wear of
screw are higher than those in the ram and piston type of reciprocating machines (Table 13.4).
Figure 13.3 Schematic of screw type briquetting machine with biomass fired die heater
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Table 13.4 Comparison between piston press and screw extruder
Parameter
Piston press
Screw extruder
Optimum moisture content of raw material
Wear of contact parts
Output from machine
Power consumption
Density of briquette
Maintenance
Combustion performance of briquettes
Homogeneity of briquettes
10%–15%
Lower
In strokes
50 kWh/tonne
1–1.2 g/cm 3
Higher
Good
Less homogenous
8%–9%
Higher
Continuous
60 kWh/tonne
1–1.4 g/cm 3
Lower
Better
More homogenous
Source FAO (1996a)
Pelletizing machine
Pelletization produces somewhat lighter, and smaller pellets of biomass compared to briquetting. The pelletization machines are based on fodder making
technology. Pelletizing generally requires conditioning of biomass material
either by mixing with a binder or by raising its temperature through direct
addition of steam or both. The material is dropped in the pressing chamber of
the pellet mill where it forms a carpet on the die surface. The rollers roll over
this layer and press it through tapered die holes. The pressing force keeps on
increasing during rolling in the direction of the die holes. With each roll, a
small disc is formed in the die hole that gets attached to the pressed piece
already in the hole. The plugs are pushed forward uniformly and the hot
pellets are ejected out at about 50–90 ºC, which are cooled on conveyor belts
before their storage (Figure 13.4). Compared to briquetting machines, pellet
mills are simpler, and since they operate at lower pressures, the power
consumption is lower. The processing requirements are also relatively less
rigid for pelletizing.
Low-density briquettes using binder
In remote areas, for decentralized operation, a simple low-pressure briquettng
technique using binder material like tar or cow dung can be adopted. T E R I
has developed a simple screw extruder (Figure 13.5) coupled to a small motor,
which produces medium density briquettes (400–600 kg/m3). The system
was operated in village Dhanwas in Haryana for converting locally available
agro-residue (mustard stalk) into briquettes (after pre-processing like
chopping and pulverizing) using cow dung, biogas plant effluent slurry or
clay as binder material. These briquettes were used as a gasifier fuel. The
gasifier system provided electricity to the village community besides supplying
parasitic power for the briquetting machine (Raman, Mande, and Kishore
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Figure 13.4 Schematic diagram of pelletizing machine
1993). Briquettes made from pyrolysed biomass were also sold as fuel substitutes for Hara (a local stove using dung cakes for simmering milk) in the village
households and the road-side restaurants for cooking.
Recently, T E R I has successfully carried out a feasibility study of making
briquettes from oil refinery waste sludge by mixing it with locally available
loose biomass material. This will not only solve the waste disposal problem of
hazardous waste from the oil refinery but can also yield easy-to-use briquettes
for substituting coal in the surrounding region (T E R I 2001).
In IIT Delhi, a simple hand operated briquetting plant was developed
for making beehive charcoal briquettes which can be used as a clean burning
fuel. Locally available leafy biomass, lantana or pine needles are carbonized
using a simple drum charring system. This is then mixed with a suitable binding material like clay or cow dung and pressed into a hand mould to form large
Figure 13.5 Low–density screw extruding briquetting machine
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cylindrical briquettes of about 90 mm height and 125 mm diameter, with 19
parallel vertical holes in it (Figure 13.6). One person can make about 30 briquettes (weighing about 1/2 kg after drying) per hour if the charcoal–clay
mixture is kept ready. These briquettes after drying can be used in an angeethi
(a charcoal burning stove), which burns slowly for long durations without
smoke. With about a 20% clay content in hardwood charcoal powder, the calorific value is around 18 MJ per kg or 9 MJ per briquette, and one briquette
keeps burning for about an hour. A single briqutte is used in the Indian chulha
but multiple, vertically stacked briquettes for long-duration operations are
commonly used in the high altitude areas such as Tibet (Neinhuysm 2003).
Biomass combustion
Combustion is the most direct process of biomass conversion into energy that
can be used for a variety of applications. The difficulty in combustion is in
starting the process, as high temperatures – at least 550 ºC (Quaak, Knowf,
and Stassen 1999; TNO 1992) – are required for the ignition of biomass. However, once ignition starts, the combustion process will continue if sufficient
air supply is available and if the moisture of biomass is not too high, till the
biomass is completely converted into residual ash. Figure 13.7 shows the fire
triangle delineating the components, namely, fuel, air, and heat, essential for
combustion. Fire can be extinguished by breaking the triangle, that is, either
by removing the fuel, by smothering (removing air), or by cooling (spraying
with water).
Biomass combustion is employed for a variety of applications such as
cooking, process heating, power generation and cogeneration. With the rising
prices of fossil fuels, biomass is gaining importance and there is increasing sophistication of biomass combustion devices to increase efficiency and to
reduce emissions. In order to harness biomass energy to the maximum extent,
it is important to understand biomass combustion. This includes understanding properties of biomass fuels (see Chapter 12), and the fundamentals
of numerous complex reactions associated with biomass combustion.
Figure 13.6 Low-density beehive briquetting mould
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Figure 13.7 Fire triangle
Biomass combustion process
Combustion is a process whereby the carbon and hydrogen in the fuel react
with oxygen ultimately to form carbon dioxide and water through a series of
free radical reactions resulting in the liberation of heat. General combustion
mechanisms have been postulated defining the various stages of chain reactions, namely, initiation, propagation, and termination. Generally, these stages
include the following.
Heating and drying
Pyrolysis and reduction
Gas phase pre-combustion and combustion reactions
Char oxidation reactions
Post-combustion reactions
Figure 13.8 depicts the processes associated with combustion.
Figure 13.8 Representative model of biomass combustion mechanism
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Biomass can be represented as CxHyOz and the overall combustion
reaction can be written as
C x H y Oz + ( x + 0.25y − 0.5z)O2 → x CO2 + (0.5y ).H2 O
Before the onset of combustion, physico-chemical processes such as
drying, pyrolysis, etc., take place, as described below.
Heating and drying
In these processes, physical reactions dominate chemical reactions and hence
there is a strong influence by the fuel particle size and moisture content. The
presence of moisture enhances the ability to conduct heat to the centre of the
fuel particle, but it also increases the energy requirement for heating and drying. About 2.8 MJ of energy is required to drive out one kilogram of moisture
in the fuel.
Pyrolysis and reduction
Pyrolysis is of central importance in the flaming combustion of fuel though
sufficient information is not yet available about this complex process for
quantitative prediction of the pyrolysis kinetics. The carbon is left behind, as
charcoal is the only solid produced in biomass pyrolysis. As the temperature of
dried fuel is elevated to about 225–325 ºC, pyrolysis of hemicelluloses begins.
Cellulose gets pyrolized at a temperature range of 325–375 ºC while lignin
starts pyrolyzing at a temperature range of 350–500 ºC (Shafizadeh and Chin
1997). Pyrolysis gases escape and char layer is formed on the fuel particle. This
layer or reaction front hampers further pyrolysing of inner layers of fuel particles. Various pyrolysis pathways leading to gaseous products, tars, and char are
heavily influenced by the fuel particle size, heating rate of the particle, and the
ultimate pyrolysis temperature attained. Higher proportion of gaseous volatile
products is obtained due to a faster heating rate of smaller fuel particles to
higher temperatures, while the heating of larger particles, at a slower rate, to
lower ultimate temperatures favours char formation (Wenzl 1970).
Gas phase reactions
The gaseous compounds produced during biomass pyrolysis are further
fragmented before undergoing actual combustion sequences. These
fragmentation reactions can be represented by decarboxylation and
decarbonylation of acetic acid and acetaldehyde produced by pyrolysis of
holocellulose as shown below.
CH3COOH → CH4 + CO2
CH3CHO → CH4 + CO
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Chain initiation commences after such fragmentation reactions with further breaking down of volatiles, and one such probable sequence begins with
the ethane evolved through pyrolysis.
C2H6 + M → 2CH3 + M
2CH3 + 2C2H6 → 2CH4 + 2C2H5
C2H5 + M → C2H4 + H + M
H + C2H6 → H2 + C2H5
Chain propagation commences, once chain initiation occurs. Among almost infinite number of chain propagation reactions, the most commonly
cited ones are as follows.
CH3 + O2 + M → CH3O2 + M
CH3O2 → CH2O + OH
CH2O is a key combustion intermediate whose concentration reaches
the maximum in flames at 1050 ºC, that later forms HCO and in turn reacts
with the hydroxyl radical OH to form carbon monoxide (Palmer 1974). Among
the post-combustion or chain termination reactions which follow the dominant sequences are
HCO + OH → CO + H2O
CO + OH → CO2 + H
These fast reactions are complemented by slower oxidation reactions of
carbon monoxide.
CO + O2 → CO2 + O
Final concluding reactions forming carbon dioxide and water are as
follows.
H + OH → H2O
CO + O → CO2
Char oxidation reactions
Typical biomass chars, having empirical formulae C6.7H3.3O, are highly reactive
(Bradbury and Shafizadeh 1980a,b; Mulcahy and Young 1975) and proposed
char oxidation mechanisms are as follows.
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The hydroxyl radical (OH) required can come from the dissociation of
water (H2O) or from the hemolytic cleavage of hydroxyl functional groups during pyrolysis. Gasification reactions such as the Bouduard and the
steam-carbon reactions, as shown below, are the other two final char oxidation
mechanisms used in grate-fired, pile-burning systems, inclined grate furnaces,
and gasifiers.
C + CO2 → 2CO
C + H 2 O → CO + H 2
Combustion stoichiometry
The stoichiometric equation for combustion of any fuel is the one that represents a balanced equation of complete combustion of all reactants in the fuel
with no excess air. The principal reactions for the combustion of any fuel are
summarized below.
2OH + C → CO + H 2 O
OH + CO → CO2 + H
C + O2 → CO2
1
CO + O2 → CO2
2
1
H 2 + O2 → H 2 O
2
S + O2 → SO2

n
n
C m H n +  m +  O2 → mCO2 + H 2 O
4
2

where CmHn represents a volatile hydrocarbon present in the fuel.
During the normal combustion process, air is a common oxidizer, which
is a mixture of 21% oxygen and 79% nitrogen on volume basis (that is, 1 mole
of oxygen is accompanied by 3.76 (79/21) moles of inert nitrogen). Thus, the
combustion of carbon with air can be rewritten as
C + O2 + 3.76N 2 → CO2 + 3.76N 2
Stoichiometric air requirement for fuel combustion is the minimum
theoretical air required for complete combustion based on the chemical composition of fuel. In reality, some excess air is always required for complete
combustion to occur, value of which depends on the design of the combustion
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chamber and the type of fuel. Natural gas-fired boilers operate with about five
per cent excess air, coal-fired boilers operate with about 20% excess air, while
gas turbines operate with very lean mixtures with excess air levels of the order
of 400%. Rich mixtures (with less excess air) result in incomplete combustion,
leading to emissions in the form of PICs (products of incomplete combustion)
such as carbon monoxide, methane, and NMOC (non-methane organic
carbon), which is also associated with loss of energy.
If fuel composition is known, the theoretical or stoichiometric air can be
calculated using the above-mentioned equations by the mass balance method
or the mole method.
Example 1
Calculate the stoichiometric air requirement to burn 1 kg fuel, if the biomass
fuel composition on mass basis is carbon 44%, hydrogen 15%, nitrogen 1%,
oxygen 28%, moisture 10%, sulphur 0.5%, and ash 1.5%.
Solution
On mass basis
For 1 kg fuel, the weight of oxygen required to burn its various combustible fuel constituents is as follows.
For carbon:
For hydrogen:
For sulphur:
0.44 × (32/12) = 1.173 kg
0.15 × (16/2)
= 1.200 kg
0.005 × (32/32) = 0.005 kg
Thus, the total oxygen required is 2.378 kg per kg of fuel, of which 0.28
kg is already present in the fuel. Therefore, the net oxygen required from the
air supply is 2.098 kg (2.318 – 0.28). As air contains 23% oxygen by weight, the
stoichiometric air required to be supplied for complete combustion is
9.123 kg (2.098 × 100/23) per kg of fuel.
Considering the molecular weight of air as 29, the air density at NTP
(normal temperature and pressure) works out to be 1.295 kg/m3 (29/22.4, where
22.4 is molecular volume of air at NTP). Thus, the minimum amount of air
required to be supplied for complete combustion is 7.045 m3 (9.123/1.295).
Example 2
Calculate the composition of dry flue gas if 20% excess air is supplied for the
combustion of fuel with the composition given in Example 1.
Solution
Let us first convert the fuel composition from the mass basis to the mole
basis for 100 kg fuel. Then from the basic combustion reactions given earlier,
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calculate the amount of oxygen required in kmol basis (1 kmol oxygen is
required for 1 kmol of carbon and sulphur, while 0.5 kmol oxygen is required
for 1 kmol of hydrogen).
Fuel constituent
Mass (kg)
kmol
Carbon (C)
Hydrogen (H 2)
Nitrogen (N 2)
Oxygen (O2)
Moisture (H 2O)
Sulphur (S)
44
15
1
28
10
0.5
44/12
15/2
1/28
28/32
10/18
0.5/32
kmol O2 required
=
=
=
=
=
=
3.667
7.5
0.036
0.875
0.556
0.016
3.667
3.750
0.016
Thus, the stoichiometric oxygen requirement is 7.432 kmol, the net
minimum oxygen required to be supplied is 6.557 kmol (7.432 – 0.875), and the
stoichiometric air requirement is 31.225 kmol (6.557 × 100/21) per 100 kg of fuel
or 699.4 m3 of air (as 22.4 m3 is the volume per kmol at NTP).
As 20% excess air is supplied, the actual amount of air supplied is
37.470 kmol (31.225 × 120/100) per 100 kg fuel. Of this, oxygen is 21%, that is
7.869 kmol and the balance is nitrogen. Since the stoichiometric oxygen
requirement is 6.557 kmol, the excess 1.331 kmol (7.869 – 6.557) oxygen will
appear as it is in flue gases. Thus, the dry flue gases after the combustion of
100 kg fuel with 20% excess air would be
Carbon dioxide = 3.667 kmol from combustion of carbon
Sulphur dioxide = 0.016 kmol from combustion of sulphur
Nitrogen
= 29.637 kmol (29.601 and 0.036 kmol from air and
fuel, respectively)
Oxygen
= 1.331 kmol as excess oxygen
Thus, the total volume of dry flue gases is 34.651 kmol. Therefore, the
volumetric composition of dry flue gases works out as follows.
Carbon dioxide = 10.58% (3.667/34.651)
Sulphur dioxide = 0.05% (0.016/34.651)
Nitrogen
= 85.53% (29.637/34.651)
Oxygen
= 3.84% (1.331/34.651)
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Biomass combustion for useful heat production
Direct combustion systems are used to produce heat, which can be either used
directly (for example, brick making) or transferred to a working fluid, such as
steam, for further use in process heat or in steam engines or turbines for
power production. The combustion efficiency can be defined as follows
The combustion efficiency is mainly determined by the completeness of
the combustion process. The flame temperature plays an important role in
deciding the overall efficiency of the combustion device. For boiler, the
following simplified regression equation can be used to calculate the approximate adiabatic flame temperature (Tad in ºC) (Tillman and Anderson 1983).
Tad =420 − 10.1( MC) w + 1734λ + 0.6( Tin − 25)
where (MC)w is the moisture content on wet basis, λ is the excess air factor
(ratio of actual fuel:air ratio to stoichiometric fuel:air ratio) and Tin is the temperature of the combustion air (ºC ) entering the combustor. It can be seen
that excess air has more influence than the moisture content on the adiabatic
flame temperature (Figure 13.9). Theoretically, the highest temperature can
be achieved with stoichiometric air supply (λ = 1) but in practice, excess air is
Figure 13.9 Effect of moisture (MCw), ash content (ACd), and excess air (λ) on adiabatic
flame temperature
Reproduced with permission from the World Bank
Source Quaak, Knowf, and Stassen (1999)
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ηcomb =
T
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always supplied to ensure complete combustion. The optimal values of λ depend on the furnace design, fuel type, and fuel feeding/firing system used. For
well-designed furnaces/combustion devices, the λ values are in the range
1.6–2.5, while in poorly designed furnaces, value of λ reaches as high as 4–5.
The first law efficiency can typically be calculated using the heat loss
method and for typical biomass-fired boilers, efficiencies are in the range
60%–80%, depending on the ultimate analysis of fuel, moisture content, and
excess air factor (which normally ranges from 25% to 100%, depending on
combustor design and fuel quality). The boiler efficiency can also be approximated (to an accuracy of ± 2%) using the following regression equation.
η = 96.84 – 0.28MCg – 0.064Ts – 0.065EA
where MCg is the moisture content of flue gas on wet basis, Ts is the flue gas
temperature in ºC, and EA is the excess air (percentage) that has a strong influence on the efficiency (Tillman 1981).
Emissions during biomass combustions
Biomass combustion for producing useful heat is associated with airborne
emissions, which are undesirable but at the same time unavoidable too.
Particulate emission has long been considered as a major problem associated
with biomass combustion, while NOx has emerged recently with the development of higher temperature combustion devices using fuels such as rice hulls,
cotton processing wastes, etc.
Fly ash is largely governed by fuel type (fines in fuel) used, fuel feed rate,
excess air used, and its distribution. Particulate emissions are minimized in
staged combustion where the stoichiometric air is supplied under the grate
while excess air (of the order of 50%–60%) is supplied as over-fire air.
Particulate emissions range from a sub-micron size to 2 mm with typical concentrations of the order of 30–100 g/m3 (Jung 1979). The fly ash problem can
be controlled by using cyclone separators, dry scrubbers, electrostatic
precipitators, and bag-house filters.
Two sources of NOx exist during biomass combustion, namely fuel NOx
and thermal NOx. Fuel NOx is governed by the concentration of nitrogen
in fuel, which is mainly in amine form (Cowling and Kirk 1976). Fuel NOx is
formed by the oxidation of the reduced form of nitrogen contained in the fuel
and is generally not sensitive to temperature (Edwards 1974). Since nitrogen
contained in biomass fuels volatilizes preferentially, fuel NOx can be controlled by staged combustion.
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Thermal NOx is formed by the oxidation of nitrogen in the combustion
air, which is a high temperature reaction that is not favoured at flame
temperatures below 1500–1600 ºC. With the development of advanced
combustion devices with refractory lining and low excess air combustors,
thermal NO x has become a significant problem. Thermal NOx can be
regulated by proper control of excess air and by not over-emphasizing
the pre-heating of combustion air. Normally, NOx emissions are observed in
the range 0.4–1.2 kg/MJ, which are significantly lower than the emissions
from coal combustion (Kester 1980; Munro 1983).
Types of combustors
Biomass combustors are designed in such a way that the combustion mechanisms/processes described earlier are controlled to release heat through the
oxidation process of various chemical constituents of the fuel in the most
practical optimum manner. Various types of combustors are:
Fixed-bed or grate-fired combustors
Suspension burners
Fluidized bed systems
Fixed-grate systems
Fixed-grate systems were, for many years, the most common biomass combustion devices. Fixed-bed systems are mainly distinguished by the type of grate
used and the mechanism used to supply or transport fuel through the furnace.
Fixed-bed systems include manually fed systems, spreader-stoker systems, under-screw systems, through-screw systems, static grate, or inclined grates, and
travelling grate systems. In the simplest form of fixed-grate systems, primary
air for combustion of char is supplied under the grate while secondary air for
combustion of volatiles is supplied above the grate. Primary air continues the
combustion of char on the grate and the heat released during this process enables the pyrolysis of the fresh fuel added, releasing volatiles in it. The
secondary air completes the combustion process to exploit the heat content in
the released volatile gases. Typical combustion bed temperatures in fixed-grate
systems are of the order of 900–1400 ºC, and ash is removed below the grate.
As compared to coal, biomass has a higher volatile fraction, and hence larger
combustion space above the grate is provided. Therefore, biomass requires a
higher proportion of secondary to primary air supply as compared to coal.
In inclined grate systems (developed for coal during the 1920s–30s),
fuel is supplied at the top and gradually moves downwards during the
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combustion process (Figure 13.10). The first moving sloping grate system was
introduced in 1940s. In this system, the residence time of fuel is fixed by the
speed of the moving grate. The combustion chamber can be made further
compact (higher heat release rate per unit grate area) for uniform-size
biomass and with complete combustion occurring in different stages.
For small- to medium-size particle fuels, screw feeder systems are developed to push the fuel to the centre of the combustion and to take out the ash
from other side. For large-size fuels and fuels with high ash content, a
through-screw system is used. Here, the fuel is burned while being screw-fed
through the combustion zone and the remaining ash is deposited into the ash
pit at the end of the screw. For relatively small operations, special feed systems
have been developed consisting of screw and spreader stokers. Fuel particles
are spread above the reaction zone with spreader stokers, which resembles
like suspension burning. Part combustion occurs when the fuel particle is in
suspension while moving through the gas above the grate (Figure 13.11). A
comparison of various fixed bed combustion systems is given in Table 13.5.
Grate-fired systems can handle fuels of larger particle sizes and with
higher moisture contents (up to 50% wet basis). They are also capable of fuel
utilization up to 600 GJ/h with complete combustion in various stages.
Figure 13.10 Sloping grate combustion chamber
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Figure 13.11 Spreader–stoker grate combustor
Table 13.5 Comparison of fixed-bed combustion systems
Fuel size
(mm)
System
<φ100 × 300
<40 × 30 × 15
>20 × 20 × 10
Through screw
<φ50 × 100
Moving or inclined grate <300 × 100 × 50
Spreader–stoker
<40 × 40 × 40
Static grate
Under screw
Maximum
moisture
content
(per cent
wet basis)
Fuel supply
Ash removal
50
40
40
40
50
50
Manual/automatic
Automatic
Automatic
Automatic
Automatic
Automatic
Manual/automatic
Manual/automatic
Manual/automatic
Automatic
Automatic
Manual/automatic
Inclined grate systems normally operate with lower heat release rates of about
3.5 GJ/m2 of grate surface and about 500 MJ/m3 of combustion chamber
volume. Spreader–stoker can achieve higher heat release rates of the order
of 10.2 GJ/m2 of grate surface and 500–750 MJ/m3 of combustion chamber
volume.
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Suspension burner
These are special purpose burners, similar to pulverized coal-fired burners,
developed for biomass. They have increased specific capacity (per volume
of reactor) and produce an oil-type combustion flame, but require more extensive fuel preparation and storage than grate-fired systems (Figure 13.12). The
heat release rate of suspension burners is in the range 500–600 MJ/m3 of combustion volume. They require fine biomass particulate size (< 2 mm) with less
than 15% moisture (wet basis). The main drawback of the suspension burner is
a low operating efficiency as high level of excess air (more than 100%) is
required to prevent the build-up of slag in the burner/combustor. In the slagging mode, with low excess air levels, higher temperatures of the order of
1600–1700 ºC can be achieved. Another drawback is that in the absence of
staged combustion, suspension burning results in high fly ash and also higher
fuel nitrogen conversion to oxides of nitrogen as compared to spreader–stoker
firing.
Fluidized bed systems
In a fluidized bed combustor, fuel is burned in a hot (800–1000 ºC ) turbulent
bed of non-combustible material (sand, limestone, etc.), which acts as a
medium of heat transfer. The bed is fluidized by using fans to blow air through
Figure 13.12 Suspension burner
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a perforated bottom plate. Fluidization facilitates high heat transfer rates by
creating a large heat transfer surface. This helps in complete combustion with
low excess air levels (25%–35%), resulting in a high overall efficiency. A high
thermal mass of inert material also enables good combustion of very wet fuels.
Fluidized bed combustors are gaining increased acceptance, especially for
loose biomass combustion, due to the several advantages associated with it.
These are:
Flexibility to accommodate changes in fuel properties, size, and shape
Capability to handle high moisture (up to 65% wet basis) content fuels
Capability to handle high ash content (up to 50%) fuels like rice husk
Depending on the air velocity, either a BFB (bubbling fluidized bed) or a
CFB (circulating fluidized bed) is created. In a BFB, the combustor is divided
into two zones, namely a zone containing free-moving sand-bed particles supported by air flowing upward giving the resemblance of bubbling fluid, and a
free board zone above the fluidized bed (Figure 13.13). In a CFB, the velocity is
so high that the lighter bed and fuel particles get carried away with the flow in
circulating motion and get separated in cyclone and later return to the reactor
(Figure 13.14). Thus, light fuel particles burn during circulation while larger/
heavier particles burn until they become light enough to join the circulating
stream. BFB has a heat release rate of about 5.6 GJ/m2 grate equivalent and
470 MJ/m3 of reactor volume (Envirosphere 1980). Higher rates can be
achieved with CFB systems.
Figure 13.13 Bubbling fluidized bed combustion system
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