Applied Energy 60 (1998) 101±114
Formation of dioxins and other semi-volatile
organic compounds in biomass combustion
H.K. Chagger a, A. Kendall b*, A. McDonald b,
M. Pourkashanian a, A. Williams a
a
Department of Fuel and Energy, University of Leeds, Leeds LS2 9JT, UK
b
Department of Geography, University of Leeds, Leeds LS2 9JT, UK
Abstract
This paper identi®es advantages of using biofuels and biomass mixed with coal in combustion. The availability of biomass with regard to landuse is reviewed, followed by a brief
account of the combustion process and the concomitant formation of semi-volatile organic
compounds. Chemical compositions of selected biofuels and coal are presented. Routes of
formation for polychlorinated dibenzodioxins/furans (dioxins and furans) are illustrated with
subsequent reference to associated emissions. Graphs in the paper show coal and biofuel
propensities for forming dioxin and furan isomers followed by methods for predicting emission levels and isomer distributions within combustion systems. The ®nal sections of the paper
summarise recent equilibrium concentration studies and discuss the ongoing combustion
experiments being conducted in the University of Leeds' Department of Fuel and Energy.
Preliminary results are presented and discussed, ®nishing with three main experimentallydrawn conclusions. # 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction
The use of solid biofuels alone or the co-combustion of biomass and coal is a
technology which has numerous advantages [1]:
. It reduces net CO2 emissions. The carbon dioxide released during combustion
of biomass equals that which is taken in during growth. A full or partial
replacement of fossil fuels with biomass would therefore reduce net emissions
of CO2.
* Corresponding author.
0306-2619/98/$Ðsee front matter # 1998 Elsevier Science Ltd. All rights reserved.
PII: S030 6-2619(98)0002 0-8
102
H.K. Chagger et al./Applied Energy 60 (1998) 101±114
. The use of surplus biomass, like municipal waste, sewage and agricultural/forestry residues, alleviates mounting pressures to ®nd disposal routes.
. Biofuels encourage the preservation of diminishing resources of fossil fuels,
because most biomass resources are abundant in nature, as shown in Table 1.
Straw has proven to be a signi®cant crop with production values varying by
country. Despite the small land area of Denmark, it produces signi®cant quantities
of straw. Surplus straw poses a signi®cant problem to which the response has been
to attempt to use the surplus as a feedstock for heat-and-power generation. Local
farmland in Grenaa, Denmark produces 125,000 tonnes of surplus straw annually,
of which, 75,000 tonnes are used by the local cogeneration company and the farmer's
co-operative. As would be expected for Denmark, the percentage of land occupied
by forest and woodlands is moderate but not extensive, so indicating that wood and
wood residues would perhaps not be the preferred biomass feedstocks for energy
production. Ireland, like Denmark, is small in size but produces neither straw nor
forest/wood biomass in signi®cant quantities. This, however, has not prevented the
Republic of Ireland from investing time and money into the research and assessment of
energy potential from biomass. Ireland has a strong agricultural base and an interest
in reaping the bene®ts of socio-economic and employment stimulation via the
establishment of a bioenergy industry. CAP reforms, set-aside and the need for
Table 1
Availability of selected biomass fuels [2±4]
Country
Austria
Belgium
Denmark
Finland
France
Germany
Greece
Iceland
Ireland
Italy
Luxembourg
The Netherlands
Norway
Portugal
Spain
Sweden
UK
Total
a
Total land
area
(1000 km2)
Cropland
1991±1993
(1000 km2)
Straw: total
production/
total land
area
(t/km2)
Land area
occupied by
forests/woodland
1991±1993
(1000 km2)
Total
Roundwood
production
1991±1993
(1000 m3)
83.0
33.0
42.0
305.0
550.0
349.0
129.0
100.0
69.0
294.0
3.0
34.0
307.0
92.0 (4)
499.0
412.0
242.0
3543.0
15.1
10.1
(1)a 25.5
25.6
(5) 193.0
(6) 120.1
35.0
0.1
9.3
(2) 119.0
N/A
9.2
8.9
32.0
(3) 199.0
27.8
64.4
894.1
N/A
61.0
152.0
N/A
86.0
N/A
34.0
N/A
28.0
59.0
23.0
38.0
N/A
N/A
N/A
N/A
88.0
73.0
(3) 32.2
7.0
4.5
(1) 232.0
148.8
(6) 107.0
26.2
1.2
3.2
67.7
N/A
3.4
83.3
(4) 33.0
(5) 159.7
(2) 280.0
24.2
1213.4
13.76
4.41
2.25
37.66
43.62
36.25
2.73
N/A
1.80
9.24
N/A
1.40
10.52
11.41
15.22
59.91
6.20
256.38
Ranks shown for crop and woodland cover are based on percentage of total land area calculations.
H.K. Chagger et al./Applied Energy 60 (1998) 101±114
103
alternative economic activities on agricultural land are encouraging land owners to
consider energy crops such as willow, poplar, and Miscanthus as a new farming
enterprise [5].
The percentage of cropland coverage with respect to total land area can be calculated using ®gures from Table 1. These calculations show that 60% of Denmark's
land is used to produce crops. Short and long term phases of crop surpluses and
diseases of various sorts have, in the past, resulted in signi®cant biomass wastages.
Denmark is presently experiencing straw surpluses, which are partly dealt with using
bioenergy routes of disposal and partly through simple open-®eld incineration.
Over 75% of Finland's total land area is covered by forest and woodland. Sweden
follows closely with over 65% and then Austria with over a third of woodland coverage. Further analysis would determine the portion of these lands managed for
wood and timber production along with quantities of resultant wastes (woodchippings, branches, foliage etc.).
With regard to crop and woodland coverage, the three countries showing signi®cant
resources are Spain, Portugal and Germany. Although these countries have the
resources, it does not follow that they will invest the required time and money into
stimulating a market for bioenergy technology. There must be strong motivations and
sucient resources, aside from the natural ones, to make such an investment tempting.
Motivations may include: (1) the need to secure additional energy sources, (2) pressure
to reduce net harmful emissions and (3) the need to safely dispose of surplus biomass.
Resources that enable this are: (1) money to invest, (2) research and technical experience to employ the technology and (3) political support for this alternative energy.
Atmospheric pollution and government are terms often used interdependently.
International treaties pertaining to emission guidelines a€ect all governments
involved by setting speci®c standards for various pollutants. However, these standards are carefully set according to related economic factors and, to a lesser extent,
public opinion. Hence, it is extremely important to use any time and money invested
into bioenergy research to gain a thorough understanding of the potential and
harmful emissions and to develop economically feasible methods for reducing output levels and/or impacts. Research to date has identi®ed a group of potentially
harmful emissions from the combustion of biomass including crops and wood. The
compounds associated with these emissions are semi-volatile organic compounds
(SVOCs). The rest of this paper reviews the basic process of combustion, routes and
mechanisms of SVOC formation and the methods used to predict these emissions
using fuel input information. The ®nal section uses results from some laboratory
tests and predictive model runs to arrive at some general conclusions about
SVOCs associated with biomass combustion.
2. Combustion process and routes for the formation of semivolatile organic
compounds (SVOCs)
Combustion takes a typical particle of coal or biomass and decomposes it into
fractions of char and volatiles, the former burning slowly. In general the initial
104
H.K. Chagger et al./Applied Energy 60 (1998) 101±114
combustion or gasi®cation process can be separated into two main reactions, one of
them being the devolatilisation process which involves the breakdown of initial coal/
biomass into light gases and tars, which subsequently form soot. Numerous studies
have indicated that the thermochemical conversion of biomass is similar to that of
coal, although the amount of char is signi®cantly smaller [6±10].
In general the ratio of volatile products and chars in the case of coal gasi®cation is
almost unity, but, in the case of biomass, the volatile content is around 80% and rest
is char/ash. A comparative elemental analysis of biofuels and various coals in terms
of their elemental contents is given in Tables 2 and 3.
Wood as a fuel is characterised by low ash contents, high calori®c values and large
amounts of ®xed carbon. Straw is low in moisture content with high heating
values and a high percentage of hydrogen and oxygen and has proven to have
low concentrations, if any, of iron, lead, zinc and copper. However, its high
chlorine content can be a signi®cant drawback. Miscanthus is a highly volatile fuel
with a high heating value due to its large proportions of carbon and oxygen: it
contains almost no iron, lead, zinc or copper, thus making it a fairly clean and
e€ective biomass fuel. Hence, a typical biofuel has a high volatile content and low
®xed carbon content, the nitrogen content varies and, generally low chlorine levels
altogether except straw which shows the highest chlorine concentration.
The combustion or incineration of various wastes or natural materials containing
chlorine can lead to the formation and emission of, polynuclear aromatic hydrocarbons (PAH), dioxins, furans, chlorohydrocarbons and other species. Dioxin is a
general term for a group of chemical compounds consisting of 75 polychlorinated
dibenzo para dioxins (PCDDs) and 135 polycyclic dibenzofurans (PCDFs). They are
structurally very similar, only di€ering in the number and spatial arrangement of
chlorine atoms in the molecule. Fig. 1 shows the basic structure of these two subgroups. Each of these structures represents a whole series of discrete compounds
which are present as trace amounts in the atmosphere and some of these isomers
have been shown to be extremely toxic, mutagenic and linked to the suppression of
the immune system in humans [17±19].
As a result of dioxin-contamination in Seveso, Italy in 1976, the European Community introduced the Seveso Directive in 1982 obliging dangerous chemicals manufacturers to identify risks present in their factories and informing the local residents of
the potential dangers. This directive also lists the amounts of dangerous chemicals that
can be stored safely within 500 m of each other. The United Kingdom complied with
Table 2
Carbon, hydrogen and oxygen compositions for selected biomass fuels [11±14]
Biomass
Wood
Straw
MSW
Sewage sludge
Miscanthus
Carbon
(% daf)
Hydrogen
(% daf)
Oxygen
(% daf)
Chlorine
(mg/g)
Nitrogen
(% daf)
45±50
40±45
30±35
40±45
50±55
5±6
5±6
1±2
5±6
4±5
40±45
40±45
20±25
20±25
40±45
0.04±0.06
0.34±0.36
0.15±0.20
0.10±0.15
0.16±0.18
0.3±0.5
0.5±0.7
1.0±1.5
3.5±4.0
0.4±0.6
H.K. Chagger et al./Applied Energy 60 (1998) 101±114
105
this Directive by introducing the Control of Industrial Major Accident Hazards
(CIMAH) regulations in 1984.
Apart from these dioxins, the combustion process also leads to the formation of
NOx, which could react with the PAH and other species present in the ¯ue gases to
give nitrogen containing compounds like polychlorinated pyridines, polychlorinated
anilines and polychlorinated benzophenols: polychlorinated benzonitriles have also
Table 3
Chemical analyses of fuels (dry) [15,16]
Typical
UK hard
coal
Beech
Pine
Straw
Miscanthus
Volatiles
Ash
Fixed C
C
H
O
N
S
Cl
Hu (MJ/kg)
Ashhemisphere
37.1
5.0
53.2
85.1
5.9
5.7
2.12
0.84
0.41
±
1080
83.2
0.34
16.5
48.7
5.7
±
0.13
<0.05
<0.1
18.5
1420
82.1
0.45
17.5
53
4.8
±
0.11
<0.05
<0.1
19.3
1110
78.8
3.66
17.6
47.4
4.5
±
0.5
0.1
0.4
17.09
1140
78.2
4.9
17
50.7
4.4
±
0.3
0.2
0.2
18.0
1170
67.2
1.6
22.2
47.1
±
±
0.43
0.26
0.02
18.7
±
47.94
0.75
9.82
47.6
6.11
44.34
0.65
±
<0.1
9.55
±
Ash analysis (% wt. ash)
SiO2
Al2O3
TiO2
Fe2O3
CaO
MgO
SO3
Na2O3
K2O
P2O5
31.47
17.6
0.6
23.2
12.5
0.6
2.6
4.2
1.5
6.6
15.2
2.65
0.26
3.8
37.3
8.5
3.0
3.0
8.6
13.7
28.6
2.5
0.1
6.5
35.8
5.2
3.0
1.9
9.2
3.3
56.2
1.2
0.06
1.2
6.5
3.0
1.1
1.3
28.7
4.4
70.6
1.1
0.06
1.0
7.5
2.5
1.7
0.17
12.8
2.0
±
±
±
±
±
±
±
±
±
±
6.45
0.66
0.02
2.05
73.05
0.54
1.93
0.17
4.5
1.90
±
±
±
±
±
±
±
±
±
±
±
±
±
<20
<30
1080
185
1.2
495
95
1530
605
<5
<30
84
550
<20
<30
1610
170
1.0
260
115
385
<30
<5
<30
25
320
<20
32
90
45
3
58
<20
85
45
<5
<30
28
125
<20
<30
70
<30
0.5
<30
<20
<30
<30
<5
<30
38
226
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
mg/kg ash
Sb
As
Ba
Pb
Cd
Cr
Co
Cu
Ni
Hg
Se
V
Zn
Bark Wood
106
H.K. Chagger et al./Applied Energy 60 (1998) 101±114
been observed (see Fig. 2). This indicates that there could be di€erent ways of
incorporation or addition of nitrogen in the aromatics, so suggesting that nitrogen
containing radicals (CN or NR) may join the formation reactions [20,21].
The major source of dioxin appears to occur during the incineration of municipal,
chemical and hospital wastes, during the combustion of oils, wood, gasoline, smelting of copper and scrap metals, recovery of plastic-coated (PVC) wire and natural
combustion such as forest ®res [22,23]. Greenpeace claims that PVC manufacturing
is the largest source of dioxins [24]. PCDFs and PCDDs have also been identi®ed in
various samples from pulp millsÐincluding e‚uents, sludge, pulp and paper products. It is considered that these products are formed during the bleaching processes:
the subsequent combustion of chlorine containing bleach plant waste causes the
emission of HCl, which leads to the formation of dioxins.
Although the mechanism of formation of dioxins remains speculative, two principal routes have been proposed. The ®rst being the de novo synthesis which postulates heterogeneous catalytic assembly of chlorinated dioxin structures from a
carbon, oxygen and chlorine source at a temperature window of 300±325 C in the
post combustion zones [25±27]. The second suggested mechanism for the formation
of PCDD/F refers to a multi-step reaction in the post-combustion zones including
Fig. 1. Molecular structure of dioxin, furan and 2,3,7,8-tetra isomers and PAH.
H.K. Chagger et al./Applied Energy 60 (1998) 101±114
107
Fig. 2. Halogenated aromatic polychlorinates.
aromatisation of aliphatic compounds and subsequent chlorination by molecular
chlorine formed from an equilibrium of HCl and oxygen in cooler parts of the
reactor [28±30]. Catalytic reaction of chlorinated aromatic precursors on ¯y ash in
the post-combustion zone has also been observed [31,32], although major questions
about the formation of dioxins remains unanswered. Gas-phase coupling reactions
of chlorinated precursors such as chlophenols, chlorobenzenes followed by the
adsorption on the organic particular phase occur. Copper and iron have a major
catalytic e€ect; copper being 20 times as e€ective as iron [27,33±35].
In contrast, the PAH species are involved in the ¯ame chemistry in the formation
of soot or smoke particles. They are produced from the acetylene (ethyne) which
forms benzene and multi-ringed PAH structures. The concentration of PAH compounds in a soot-forming ¯ame is controlled by kinetic factors, but the number of
rings and their isomers are determined by thermodynamics. The acetylene is formed
at high temperatures, but benzene and other aromatic structures are more readily
formed at lower temperatures. Fig. 3 illustrates the routes of formation of PAH
which can subsequently act as precursors for dioxin formation.
Tables 4 and 5 give typical UK emissions for dioxins and other possible species
which could lead to the formation of dioxins.
The collective concentrations of PCDDs and PCDFs are expressed by assigning
toxic equivalent factors (TEQ) to each congener. These allow the concentrations to
be weighted against the most toxic dioxin 2,3,7,8-TCDD. The TEQ values from a
number of sources are given in Tables 6 and 7.
The above table indicates that coal and municipal solid wastes (MSW) have the
highest propensity for dioxins/furan formation and emission. Sewage sludge (using
ESP) and wood waste appear to lower toxicological equivalents.
Broad ranges must be given for PCDD/F because experimental results show that,
for any feedstock and biomass, the quantities and toxic equivalents will vary signi®cantly depending on the plant and operating conditions [38]. What is necessary is
108
H.K. Chagger et al./Applied Energy 60 (1998) 101±114
Fig. 3. Formation of PCDDs and PCDFs.
Table 4
UK dioxin emissions [36]
Market sector
UK coal consumption
(k tonnes)
Domestic
Bituminous
Anthracite
Manufacture smokeless
Indust./commercial
Power generation
Total
1988
1992
4350
1391
1443
9955
82465
99604
2853
1325
1090
8807
77028
91123
Total
emission (g)
1988
23
3
7
4
5
42
1992
120
11
21
546
693
1390
15
3
5
4
5
32
80
11
16
483
647
1240
Table 5
Releases to air [37]
Release achievable (mg/m3)
Substance
HCl
CO
Dioxins and furans
VOCs as carbon
HCN
Total particulate matter
RDF
Tyres
30
100
10
50
Ð
25
Ð
Poultry
litter
Clean
wood
30
Ð
200
100
0.1±0.5 ng/m3
20
Ð
Ð
25
Treated
wood
Straw
30
100
30
250
5
Ð
25
H.K. Chagger et al./Applied Energy 60 (1998) 101±114
109
Table 6
Dioxin and furan ¯ue-gas emissions from combustion of biomass and coal fuels [38]
Biomass/coal
Operating
conditions
Dioxins
(ng/kg
feed)
TEQa
(ng/kg
feed)
Furans
(ng/kg
feed)
TEQa
(ng/kg
feed)
Wood waste
Straw
MSW
Sewage sludge
Coal
Continuous
Continuous
Low ®re/High ®re
w/ESPsb
Low ®re/High ®re
231
328
4550/1925
203
1034/802
6.9
35.0
115/46.4
6.2
23.8/23.8
324
794
18570/5006
140
1703/1900
12.4
80.5
616/196
13.8
77.8/85.2
a
b
Toxicological equivalent amount.
Electrostatic precipitators.
Table 7
Dioxin and furan distribution for complete combustion process using coal and biomass fuel (percentages)
[38]
Biomass
Grate ash
Flue gases
Grit ash
Dioxins
Furans
Dioxins
Furans
Dioxins
Furans
Wood waste
Straw
2±10
0.5
4±20
0.6
70±90
99.4
60±90
99.4
MSW
Sewage sludge
Coal
0.7
10±15
<10
<1.0
<0.5
<5
75±85
45±75
60±90
13±20
75±100
75±100
6±21
No grit ash
produced
15±20
25±50
30±40
6±20
No grit ash
produced
5±10
0±25
20±30
to devise a method for optimising combustion systems taking into account PAH
formation and destruction.
3. Predicting dioxin and furan emissions
Emissions of many species can be calculated from the amount of carbon consumed during biomass combustion. Using the carbon consumption values along
with the respective emission ratios relative to the total carbon emissions, it is possible to calculate the total CH4, CO, N2O and NOx emissions arising from the biomass combustion. However, research into PCDD and PCDF formation and
destruction has not revealed con®dent patterns to support formulae for predicting
emissions such as the one presented for carbon associated emissions. It has been
generally accepted that fuel composition, temperature of combustion and other
operational parameters have signi®cant impacts on dioxin and furan formation/
destruction, but the relationships between such parameters are still not quantitative.
It has been observed that the maximum dioxin formation occurs around a temperature
110
H.K. Chagger et al./Applied Energy 60 (1998) 101±114
window of 300±325 C and above 400 C the residence time seems unimportant.
Rapid quenching of ¯ue gases to below 260 C leads to a reduction in the PCDD/F
concentrations. Low-temperature surface catalysed reactions can also occur in the
cooler parts of the reactor. The optimal residence-time of ¯ue gases in a typical
combustor is around 2±5 s. It has been reported that rapid formation of dioxins can
occur within 1.6 s in the cooler parts of an incinerator or combustor. Typical rates of
formation of PCDD/F were found to be of the order of 210ÿ2 g/g per m [39]. A
correlation between PAH and poor combustion conditions, particularly CO concentrations has been reported [40±42]. However, there are con¯icting data regarding
the correlation between PCDD/F and PAH [30]. Some suggest that the PAH suppresses the formation of PCDD/Fs and others have reported a simultaneous
increase [30]. Other workers have observed that the concentration of formation of
these compounds in incinerators is independent of the PAH concentration [42].
Even if future analyses indicate that compliance with optimal composition, combustion temperature and other parameters will achieve the desired control of such
emissions, it is not likely that such measures will be economically feasible [38].
The objective of this paper is to simulate a simpli®ed case of biomass combustion.
In this case a one-dimensional drop tube reactor has been considered, which represents a very simpli®ed practical set-up. Equilibrium concentrations of the dioxin
species and hydrocarbon species over a range of temperatures has been calculated
using a commercially-available equilibrium programme, Equitherm. This programme calculates the equilibrium composition of a particular system through the
minimisation of the Gibbs free energy. For a model gas, the importance of reaction
coal and biomass combustion and subsequent cooling of the ¯ue gas with varying
oxygen concentrations was investigated.
4. Mechanism of formation of SVOCs and dioxins
The formations of furan, dioxin and their isomers have previously been studied at
equilibrium concentration as a function of temperature and oxygen content. It has
been shown that [43]:
1. n-chlorodibenzofurans were more stable than n-chlorodibenzodioxins at higher
temperatures over the range 100±400 C (the cross-over point is 250 C);
2. the oxygen concentration is important in the equilibrium between furan and
dioxin, as might be expected thermodynamically (i.e. more oxygen equals less
dioxins); and
3. there are still considerable uncertainties in the scarce experimental thermodynamic data that are availableÐmost data are estimated on the basis of
group additives.
However, these studies maintain that the high-temperature combustion zone produces reactive species from the volatiles which subsequently produce PAHs at the
higher temperatures and dioxins during the cooling process (being more stable at
H.K. Chagger et al./Applied Energy 60 (1998) 101±114
111
lower temperatures). The temperatures in the cooling region are suciently high
enough to enable reactions to occur reasonably quickly. Hence, it can be deduced
that, predictions of equilibrium in the model gas are strongly dependent upon the
oxygen concentrations and temperatures chosen, with maximum dioxin concentrations occurring in cooled gases around 400 C.
The nature of the free energy of the species involved in the devolatilisation of
biomass or coal±biomass mixtures means that the species which dominate at the
initial high-temperature stage are acetylene (ethyne) and hydrogen. However, on
cooling to approximately 400 CÐa typical end-of-heat-exchanger temperatureÐthe
tendency is to form alkanes>aromatics>>alkenes>alkynes. Soot formation
which occurs at high temperature is controlled by a kinetic factor and does not
conform to the above statement. In practical combustion systems, the combustion
products are diluted by air (or oxygen-depleted air) and the pool of C, H and Cl
species change as the combustion proceeds giving CO2 etc. The overall CO concentration can be used as an indicator of the degree of combustion, i.e. the extent of
PAH and dioxin formation as represented in Table 8.
Table 8 gives the products of the pyrolysis and oxidation of the volatiles produced
by an idealised biomass fuel. The actual biomass composition used is that given in
Table 8
Concentration of di€erent species obtained from biomass pyrolysis combustion at di€erent temperatures
Species
Temperature
Pyrolysis
CH4
C2H2
C2H4
C2H6
C3H6/P
C3H8
C6H6
C7H8 (toluene)
C8H10/E
CH3Cl
C2H3Cl
C2H5Cl
CH2O
C6H6O
CO
CO2
COCl
COCl2
H2
HCl
H2O
O2
Oxygen lean
Oxygen rich
1500 C
400 C
1500 C
400 C
1500 C
400
3.4E-3
0.071
6.2E-4
5.7E-7
7.9E-11
7.5E-11
4.9E-5
7.1E-8
2.0E-11
1.5E-7
3.9E-8
5.9E-11
1.4E-7
6.5E-12
0.58
8.2E-7
1.0E-9
3.0E-15
0.36
4.4E-3
1.9E-6
4.9E-20
013
1.6E-10
4.9E-6
1.1E-3
0.30
2.6E-7
0.105
0.148
7.5E-4
1.7E-9
4.6E-15
1.94E-10
1.8E-18
2.7E-10
1.4E-4
7.4E-7
1.7E-23
2.4E-29
0.457
2.6E-6
2.0E-4
2.11E-34
4.6E-8
2.2E-11
1.6E-13
8.3E-17
2.4E-21
2.7E-25
1.5E-33
0.00
0.00
2.1E-12
1.1E-17
1.3E-20
1.2E-7
1.0E-35
0.587
0.043
9.9E-10
2.8E-15
0.296
3.8E-3
0.083
1.4E-11
0.37
6.9E-11
3.9E-6
1.5E-4
1.5E-7
2.5E-7
8.0E-3
5.7E-4
1.2E-6
2.1E-8
4.7E-13
1.3E-10
2.2e-9
4.9E-9
0.282
0.349
1.0E-16
4.4E-19
2.9E-3
6.4E-4
2.96E-4
2.5E-35
4.8E-25
1.4E-32
0.00
0.00
0.00
0.00
0.00
0.00
0.00
2.5E-26
0.00
0.00
1.2E-15
0.00
1.0E-4
0.402
1.5E-12
3.6E-17
1.6E-5
2.4E-4
0.241
0.369
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
2.7E-18
0.402
4.6E-27
4.6-27
9.0E-23
2.4E-4
0.241
0.369
112
H.K. Chagger et al./Applied Energy 60 (1998) 101±114
Table 9
Assumed biomass composition and devolatilised products
Bark elemental composition
C48.3H7.9O42.9Cl0.56
Devolatilisation products (moles)
C3H6 (0.33)
CO2 (0.66)
HCl (0.01)
Cf (0.33) char
Table 9 which is made up of an assumed devolatilisation gas mixture and a ®xed
carbon content (Cf).
Table 9 shows the elemental composition of the used biomass, which is similar to
that of bark, and it has been assumed that the products obtained during devolatilisation are propylene, carbon dioxide and HCl and their molar concentrations are
given below.
5. Conclusions
From the calculated thermodynamic concentrations calculated on the basis of
suppressed carbon formation, several conclusions can be made:
1. Low temperature (400 C) equilibrium concentrations of the relative organic
compounds follow the trend observed experimentally.
2. PAH, naphthalene and the higher PAH compounds are formed from benzene
and acetylene. The PAH compounds only exist at high temperatures (1500 C)
and for equilibrium reasons do not exist at lower temperatures. Typically
naphthalene and coronene (under pyrolysis at 1500 C) would have a concentration of approximately 0.1 times the value of benzene, and anthracene
would be slightly smaller (10ÿ3). At 400 C the PAH concentrations become
very small essentially because of kinetic factors. A similar situation applies to
dibenzofuran and dibenzodioxin, which have concentrations similar to naphthalene but are controlled by the availability of oxygen as well.
3. The formation of chlorinated hydrocarbons acts as a precursor to dioxin formation, which occurs on the way from hot ¯ames to cooler parts of the
combustor (i.e. post-combustion regions where the temperatures are low).
Excess oxygen plays a critical role in dechlorination reactions and will promote
production of chlorinated species. The possible reformation scheme is that the
fuel molecules are broken into smaller species C1 and C2, which recombine in
post-combustion regions to form larger species like chlorobenzene and chlorophenols. Other possibilities could be chlorination of unburnt fuel escaping
from post-combustion regions which arise from wall quenching. It has also
been suggested that, unless the concentration of aromatics and chlorine is
extremely high, ring addition cannot occur during the combustion process.
Besides this, in the presence of H atoms, the dechlorination reactions are
favoured over chlorination [44]. Hence, in the ¯ame region, where temperatures are high, dechlorination is favoured over chlorination. However, in the
post-combustion regions, where temperatures are low, chlorination is
H.K. Chagger et al./Applied Energy 60 (1998) 101±114
113
predominant (especially if catalytic) and competes with dechlorination. Essentially the small concentration of dibenzofuran and dibenzodioxin become
chlorinated to yield n-chlorodibenzofuran and n-chlorodibenzodioxin in
very small concentrations as observed.
Acknowledgements
This work is supported by the EU Commission Framework Joule-Thermie (contract J0F3CT950024) and University of Leeds (A.K.)
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