Formation of dioxins and other semi-volatile organic compounds in biomass combustion

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

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. 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.

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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

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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

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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

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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.

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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

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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

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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

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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

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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

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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

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