The Formation of VOC, PAH and Dioxins During Incineration

0957±5820/00/$10.00+0.00 q Institution of Chemical Engineers Trans IChemE, Vol 78, Part B, January 2000

THE FORMATION OF VOC, PAH AND DIOXINS DURING INCINERATION H. K. CHAGGER, J. M. JONES, M. POURKASHANIAN and A. WILLIAMS Department of Fuel and Energy, University of Leeds, Leeds, UK

T

he formation of polynuclear aromatic hydrocarbons (PAH) dioxins and furans which arise from incinerators and coal-®red combustion systems has been the focus of attention for many years. Some of these compounds are considered to be formed in the combustion region and other in the cooler post-combustion environment. The accurate measurements of trace emissions such as those of dioxins are dif®cult and expensive; consequently it is useful to examine these from a modelling and thermodynamic point of view in order to make design predictions and derive a full analytical speci®cation from incomplete experimental data. The thermodynamic properties combined with the kinetic pathways have been used to examine the likely routes of formation of dioxins. These processes were modelled using the Sandia National Laboratories code PSR (Perfectly Stirred Reactor). The properties demonstrated that formation of these compounds occurred during the quenching process for the species which were thermodynamically favoured. Proper insight into the chemical features may help to improve incinerator installation so as to reduce, or eliminate, these emissions. Keywords: incineration; emissions; modelling.

INTRODUCTION

during the start-up of a plant, whilst Table 3 shows the emissions during start-up, burning and burn-out in a step grate incinerator. The objective of this paper is to simulate the formation of VOC and PAH during the start-up, burning and burn-out conditions for two types of incinerators: ¯uidized bed (FB) and kiln type combustion. The thermodynamic properties combined with the kinetic pathways have been used to examine the likely routes of formation of the TOC. It is proposed that the majority of the toxic organic species are derived from hydrocarbon radicals in the ¯ame, but their concentrations change during the cooling process. Although the presence of dioxins in cooling stack gases and on ¯y ash is well established, there are still uncertainties about the mechanism or kinetics of formation.

The growing concern over land use and the closing of existing land®lls has led many communities to consider energy-recovery municipal solid waste (MSW) incinerators. The advantage of this process is the signi®cant reduction of volume of collected material into an inert residue as well as the energy produced. However, incineration and combustion processes in general can produce hazardous or toxic compoundsÐfor example, polyaromatic hydrocarbons (PAH), volatile organic compounds (VOC) and the chlorinated dioxins and furans. The emissions of these toxic organic compounds (TOC) depend upon the waste composition and the operating parameters such as furnace temperature and excess air. Incinerators in general consist of primary and secondary combustion chambers. Solid waste combustion is a very complex phenomenon involving devolatilization, char formation and their combustion. The waste material is heated by hot combustion gases and radiation from the furnace walls, resulting in devolatilization and evolution of gases. These devolatilization products are not fully oxidized in either the primary or subsequent secondary combustion chamber and can pass through as turbulent-rich pockets of gas. These products are formed in the bed region which is characterized by a relatively low heating rate, inadequate mixing and insuf®cient oxygen. Concentrations of PAH were shown to be highest during start-up, and this was attributed to incomplete combustion at low temperatures. Typical emissions arising from large-scale incinerators are shown in Tables 1±3. Table 1 shows that emissions tend to be higher from grate combustion and lower for CFB units. Table 2 illustrates the wide range of products obtained

COMBUSTION MODELS EMPLOYED Equilibrium Model The equilibrium concentrations of major VOC, PAH and related compounds were calculated using a computer program based on the minimization of Gibbs Free Energy4. Thermochemical data was used from the Barin thermochemical database within the program4 or estimated using the NIST data5. Perfectly Stirred Reactor (PSR) In order to predict the formation of TOC in turbulent reacting ¯ows, a detailed kinetic mechanism needs to be used. The kinetic mechanism used here consists of 942 reactions and 170 chemical species. It is a compilation of previously published combustion kinetic schemes by the 53

CHAGGER et al.

54

Table 1. VOC emissions from different plants1. TVOC in mg nm±3 and other compounds in g nm±3 10% CO2. Emissions Benzene Toluene Phenol m-xylene 1,3,5-trimethylbenzene 2-ethylhexanol Naphthalene TVOC

A (Grate)

B (Grate)

C (CFB)

D (Grate)

E (Travelling grate)

2.46 1.40 28.98 0.47 2.61 63.02 0.09 0.56

14.37 4.75 25.40 0.48 5.70 50.17 0.37 0.43

0.79 1.65 7.58 0.58 2.69 22.02 0.18 0.12

1.32 0.40 20.18 0.08 0.45 8.11 ± 0.17

0.37 1.23 52.62 0.52 ± 36.65 0.07 0.90

following researchers, namely: · Konnov , for hydrocarbon chemistry, which incorporates a number of important reactions of hydrocarbon combustion at both high and low temperatures; and · Marinov et al.7, for hydrocarbon and PAH chemistry. 6

This combustion mechanism enables the satisfactory description of low temperature oxidation reactions during cooling as well as those at ¯ame temperatures. The thermochemical information for species was obtained largely from the CHEMKIN thermodynamic database but more complex species were calculated from other sources6±9.

Table 2. Concentration of compounds detected in the stack during start-up of the plant2. Compound (number of isomers)

Concentration, mg m ±3

Trichlorobenzenes (3) Naphthalene Methylnaphthalene (2) Tetrachlorophenols (3) Trichlorophenols (3) Biphenyl Dimethylnaphthalenes (4) Acenaphthylene Pentachlorobenzene Tetrachlorophenol Fluorene 1,3-Diphenyl propane 1,2-Diphenyl ethene Fluorene-9-one Dibenzothiophene Pentachlorophenol Phenanthrene Anthracene 1,2-bis-(4-methylphenyl)-ethane 3-Methylphenanthrene 2-Methylphenanthrene 4-Methylphenanthrene 9-Methylphenanthrene 1-Methylphenanthrene Dimethylphenanthrenes (2) Fluoranthane Pyrene Benzonaphthothiophene Benzo(ghi)¯uoranthene Benzo(a)anthracene Chrysene Unidenti®ed PAH Benzo(de)anthracenone Benzo(j+ k)¯uoranthene Benzo(e)pyrene Benzo(cd)pyren-6-one Quaterphenyl

9.4 114.0 40.0 37.0 9.2 2.3 15.0 15.0 6.5 15.0 12.0 2.5 2.8 14.0 1.5 8.0 43.0 3.7 8.8 3.3 4.3 1.4 2.6 3.8 1.0 11.0 6.8 1.9 2.1 1.1 3.0 47.0 0.1 1.9 0.7 0.4 2.6

The PSR computer program, CHEMKIN-II Version 2.310, was utilized to simulate a perfectly stirred ¯ow reactor in which combustion is permitted to be undertaken at a series of temperatures and residence times appropriate to the ¯ame (reaction) zone, and then the ¯ame species are subjected to a cooling process from ¯ame temperature to ¯ue gas exit temperature. The species input for both the models, both kinetic and thermodynamic, were calculated on the basis of the composition of gases and light tars produced during devolatilization of a coal measured by pyrolysis-GC, and are shown in Figure 1; this is typical of the products from a range of carbon/hydrogen materials, although that from biomass is slightly different. The measured concentrations were used to represent the composition of a rich pocket of gas in an incinerator, and these are, in mol fractions, 0.216 CH4, 0.001 C2H4, 0.027 C2H6, 0.013 C3H8, 0.005 C6H6, 0.005 C7H8, 0.433 H2, 0.06 O2 and 0.24 N2. Different cases were also studied in order to evaluate the effect of volatile composition on the types of PAH formed. In Case A the above composition was used and heavier hydrocarbons were considered to form soot outside the pocket and were not taken into account in the computation steps. For Case B the volatile species also included a representative aromatic molecule, naphthalene, and the composition used was 0.35 C10H8, 0.108 CH4, 0.005 C2H4, 0.0135 C2H6, 0.0065 C3H8, 0.0025 C6H6, 0.0025 C7H8, 0.212 H2, 0.01 HCl and 0.29 air; this case would be representative of rubber or plastic materials. For Case C, the volatile composition given above was modi®ed by the addition of naphthalene to represent the heavier species and a volatile composition, which includes oxygen-containing combustion species (CO, CO2, H2O) and hydrogen; this case is similar to the volatiles produced from biomass materials. The mixture composition used in this case was 0.0574 C10H8, 0.1372 CH4, 0.0049 C2H4, 0.021 C2H6, 0.0021 C3H6, 0.0039 C3H8, 0.0039 C4H6, 0.0042 C6H6, 0.0049 C7H8, 0.0276 CO, 0.0056 CO2, 0.2656 H2, 0.0084 HCl, 0.1533 H2O and 0.3 air. The volatile composition, including heavier species, was estimated using the FG-DVC pyrolysis computer model (Applied Fuel Research, USA) previously used by us11. For simplicity the heavier tar was considered to decompose to naphthalene and a carbonaceous residue. Any residual carbonaceous product was assumed to react outside the gaseous pocket. Mechanism for Dioxin Formation The proposed gas-phase reaction mechanism is shown in Table 4. It is based on the prediction by the main mechanism Trans IChemE, Vol 78, Part B, January 2000

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Table 3. PAH concentrations in the stack gas from step-grate incinerator3. PAH, m g m ±3 B(k)F + B(b)F

B(a)P

(B(GHI)P

SPAH

CO, ppm

4.6 0.9 0.8 0.6 0.4

1.1 0.4 0.1 0.2 0.2

1.2 0.1 <0.1 0.1 <0.1

0.8 <0.2 <0.2 <0.2 <0.2

7.7 1.4 0.9 0.9 0.6

20±1380 400±700 200±300 50±150 10±100

10.0 10.7 9.8

<0.1 <0.1 <0.1

0.2 0.1 0.2

<0.1 <0.1 <0.1

<0.2 <0.2 <0.2

0.2 0.1 0.2

0±20 0±5 0±5

16.0 19.0

<0.1 1.3

0.1 0.2

0.1 0.2

0.4 0.5

0.6 2.2

400±600 200±300

Combustion Temperature, 8 C

O2, %

Start-up

270±330 370±430 430±560 660±730 770±860

20.0 18.5 16.3 15.7 16.0

Burning

800±860 780±850 750±800

Burn-out

430±530 310±370

Combustion period

B(a)A

B(a)A: Benzo(a)anthracene, B(k)F: Benzo(k)¯uoranthene, B(b)F: Benzo(b)¯uoranthene, B(a)P: Benzo(a)pyrene, B(ghi)P: Benzo(ghi)perylene.

of the concentrations of the VOC including benzene, phenol and dibenzyl. These species act as precursors to the formation of furan and dioxin as shown in Table 4. This mechanism then assumes the chlorination of the furan and dioxin by HCl via a ¯y ash catalysed reaction to form the chlorinated species. The results obtained using non-steady state calculations are shown in Table 5 for the input composition given in the table. These compositions approximate to initial biomass pyrolysis products. It is clear that the oxygen concentration has a signi®cant effect on the formation of furan and dioxin; i.e., the concentration increases by a factor of ten in an oxygen-rich case. MODELLING PRODUCTS FROM INCINERATORS Modelling Results for Kiln Combustion Equilibrium model It is assumed that under kiln combustion conditions the temperatures can rise as high as 1900 K. The equilibrium concentration of VOC species was computed for a temperature range of 1900±773 K and is shown in Figure 2(a). These predictions show that the PAH concentrations are strongly dependent upon the temperature of the reactor. Hence, it can be concluded that: · The major hydrocarbon species at 1900 K is ethyne, but its concentration decreases at lower temperatures. · Species such as methane, benzene, toluene and PAH are predicted to be formed below 1900 K and peak in concentration between 1600 and 1350 K, but below this temperature range their concentrations decrease signi®cantly.

shown here are relative to the output of computation of PSR after reaction time. Figure 2(b) illustrates that three regions of the reactor can be identi®ed: · The high temperature `reaction zone’, during which the initial volatiles are consumed and the major oxidation products, such as CO, are produced, and the major remaining hydrocarbon is ethyne. · An initial region of post-¯ame cooling (1500±1200 K) in which methane and benzene are predominantly formed. Other high molecular weight species, such as naphthalene, are also formed in this region. · A third zone (below 1200 K) which can be identi®ed as a `quenching zone’ in which the concentrations of methane, benzene and other species level off at approximately the equilibrium values at this temperature. A similar situation is observed when Case B species are used; this is illustrated in Figure 3. It can be noted that the unburned hydrocarbons (VOC, PAH etc.) are in above equilibrium concentrations because of the existence of rich pockets which aise from turbulent mixing. The carbon monoxide is also found experimentally in supra-equilibrium levels which is impossible to model by conventional CFD modelling methods. Experimental measurements of carbon monoxide provide an excellent predictive method for TOC via calculations of the type shown in Figure 3.

These conclusions are effectively the same as we have previously found using similar calculations for coal combustion11. Perfectly stirred reactor Equilibrium predictions are valid for the high temperature experienced in a kiln-like combustor where all reaction rates are fast, but will tend to fail at lower temperatures, and the products will be quenched at concentrations appropriate to equilibrium values at higher temperatures. In order to investigate at which temperature this occurs, a non-steady state calculation of the gas composition was undertaken. The results obtained by this model for the same case, Case A, are shown in Figure 2(b). The data points Trans IChemE, Vol 78, Part B, January 2000

Figure 1. Composition of gases and light tars produced during coal devolatilisation at 1273 K.

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Table 4. Proposed dioxin formation mechanism. Furan formation

Dioxin formation

C12 H10 + OH=C12 H10 O + H C12 H10 O + H=C12 H9 O + H2 O C12 H9 O=C12 H8 O + H C6 H5 O + C6 H5 O=C12 H8 O + H2 O C6 H5 OH + C6 H6 = C12 H8 O + H2 + H2 C6 H5 O + C6 H5 OH=C12 H10 O2 + H C6 H4 OH + C6 H5 O=C12 H8 O + H2 O

MODELLING PIC PRODUCTS FROM FB COMBUSTION The situation in a ¯uidized bed combustor is signi®cantly different to that in kiln combustion because the temperature of the devolatilization zone and the combustion bed and free board is in the range 1173±1273 K. Consequently there is no possibility that the devolatilization products are converted to ethyne, as is the case for kiln combustion. However, the steadily operating FB units are, in effect, stirred reactors in which decomposition of refuse-derived fuel and product reactions are occurring. Published data regarding the oxidative pyrolysis of plastics in FB in the temperature range of 1073±1223 K have shown that the hydrocarbons ranging from C10±C35 were produced. At 1223 K similar hydrocarbons were identi®ed but PAH like benzo(b)¯uoranthene and benzo(a)pyrene were predominant in condensed phase. The high temperature promotes PAH formation, lower temperatures promote alkane formation. It was also concluded that complete combustion did not occur, despite the fact that samples appeared to have ignited. Besides this, chemical reactions and transport phenomena occur in the condensed phase of gas as well as interface between the two. Melting and evaporation also combine with thermal degradation or pyrolysis, gas diffusion, mass transfer as well as homogeneous oxidation. The thermoplastics partially devolatilize and form char residues, and thus in addition to the above phenomena they undergo heterogeneous oxidation. However, devolatilization and movement of the bed result in pockets of unreacted gas moving upwards12. This process is important for the conversion of fuel products and formation of VOC and PAH and is indeed the only way in which these products can result in oxygen-containing ¯ue gases. Hence, it is possible to conclude that during the devolatilization of the gases under oxidizing conditions in MSW,

C6 H4 OH + C6 H5 O=C12 H9 O2 + H C6 H4 OH + C6 H5 O=C12 H9 O2 + H2 C6 H5 O + C6 H5 O=C 12 H9 O2 + H C6 H5 O + C6 H5 O=C 12 H8 O2 + H2 C12 H9 O2 =C12 H8 O2 + H C12 H9 O2 + OH=C12 H8 O2 + H2 O C12 H9 O2 + H=C 12 H8 O2 + H2 C12 H9 O2 + O=C 12 H8 O2 + OH

the char and tars break down to low molecular weight PAH compounds. In addition to the routes described in the previous section, there are two further possible routes leading to the formation of PAH and VOC in lower temperature combustion9 in a ¯uidized bed: (i) incomplete combustion, in which fragments of the aromatic structure of the fuel and waste particles are emitted; and (ii) reactions such as cyclization of alkyl chains and radical condensations of devolatilization products in fuel-rich regimes of the ¯ame, and these can particularly take place at ¯uid bed temperatures. In the next section, the possible reaction routes are examined. Modelling Results for FB Model Equilibrium model The equilibrium concentrations of the species were computed for a temperature range of 1573±673 K for all three cases. Figures 4(a) to (c) show equilibrium predictions of PAH for Cases A to C as discussed earlier. The equilibrium concentrations at 1227 K are different to those observed in the kiln. In particular the model does not predict naphthalene and phenanthrene to be present in the highest concentrations. Perfectly stirred reactor The kinetic model used for this study was the same as used for the kiln combustion; the only difference in this case was that the species experience cooling from 1100 to 800 K for Case A to C. The results obtained by this model are shown in Figures 5(a) to (c) and can be summarized as: · In the case of ¯uidised bed combustion, the temperature is not high enough for the devolatilization products to be converted to ethyne. · The temperatures in the FB are close to where quenching reactions take place, and there is an approximately

Table 5. Results obtained for oxygen rich and lean cases from non-steady state calculations when quenched from 800 to 600 K. C6H5OH 0.35, H2O 0.35, O2 0.06 and N2 0.24 Species C6H6 C6H5O C6H5OH C12H8O C12H9O C12H10O2 C12H9O2 C12H8O2

800 K 1.72 ´ 10ê 3.77 ´ 10ê 3.49 ´ 10ê 4.0 ´ 10ê 1.82 ´ 10ê 2.17 ´ 10ê 9.07 ´ 10ê 3.87 ´ 10ê

C6H5OH 0.35, H2O 0.35, O2 0.24 and N2 0.06

600 K 04 06 01 05 07 05 11 04

1.38 ´ 10ê 3.80 ´ 10ê 3.49 ´ 10ê 7.85 ´ 10ê 2.66 ´ 10ê 3.39 ´ 10ê 3.89 ´ 10ê 7.40 ´ 10ê

800 K 04 08 01 05 10 04 12 05

4.44 ´ 10ê 5.96 ´ 10ê 3.47 ´ 10ê 9.62 ´ 10ê 1.52 ´ 10ê 8.95 ´ 10ê 1.32 ´ 10ê 1.47 ´ 10ê

600 K 04 06 01 05 07 05 09 03

1.24 ´ 10ê 7.32 ´ 10ê 3.46 ´ 10ê 4.17 ´ 10ê 8.93 ´ 10ê 1.01 ´ 10ê 1.84 ´ 10ê 5.56 ´ 10ê

04 09 01 04 11 03 12 04

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Figure 2. (a) Thermodynamic predictions of the equilibrium concentration of gases for Case A high temperature model (mole fraction: pyrolysis gas 0.35, C 10H8 0.35, 0.01 HCl and 0.29 air) ( e C2H2 ethyne, Œ C6H6 ê ê ê ê benzene, ‚ C7H8 toluene, e C8H8 styrene, C10H8 *ê ê ê ê ê ê ê ê H naphthalene; C12H8 acenaphthalene, C f F 13 10 ¯uorene, ê ê ê ê ê ´ C14Hê 10ê anthracene, C14H10Xê phenanthracene, s ‚ ê ê ê ê ê ê ê ê ê ê ê ê C18H12 chrysene, C20H12 benzopyrene). (b) Non-steady state e ê ê ê ê calculation of the change in the composition of the main species from Case A high temperature model using PSR ( * CO carbon-monoxide, ± CO2 ê ê ê ê carbon-dioxide, F CH4 methane, e C2H2 ethyne, ‚ C 6H6 benzene, ê ê ê ê ê ê + C6H5CH3 toluene, C9H8 Œ C6H5C2H phenyl acetylene, e êindene, ê ê ê ê ê ê ê C10H8 naphthalene, C12H10 biphenyl). e ê ê *ê ê ê ê ê ê

Figure 4. (a) Thermodynamic predictions of the equilibrium concentration from Case A FB model ( e C2H2 ethyne, e C4H6 butadiene, Œ ê ê ê Hê C6H6 benzene, ‚ C7H8 toluene, C8ê H8ê styrene, C10 e 8 ê ê ê ê ê ê *ê ê ¯uorene, naphthalene, ê e C 12H8 acenaphthalene, Cê 13H F 10 ê ê ê ê ê ê ê ê ´ C 14H10 anthracene, C14H10X phenanthracene, s ‚ ê ê ê ê êC ê H ê ê ê ê ê ê C20H12 benzopyrene). (b) Thermodynamic f 18 12 chrysene, ê ê ê ê predictions of the equilibrium concentration from Case B FB model ( e ê ê C2H2 ethyne, e C4H6 butadiene, Œ C6H6 benzene, ‚ C7H8 toluene, ê ê styrene, ê C ê H naphthalene,ê ê C8H C e e 8 10 8 12H8 ê ê ê ê ê ê *ê ê ê ê ê ê acenaphthalene, C13H10 ¯uorene, ´ C14H10 anthracene, F ê ê ê ê H ê C14H10Xê phenanthracene, C18 s ‚ f 12 chrysene, ê ê ê ê ê ê ê ê predictions ê ê ê ê C20H12 benzopyrene). (c) Thermodynamic of the equilibrium concentration from Case C FB model ( e C2H2 ethyne, e C4H6 ê ê butadiene, Œ C6H6 benzene, ‚ C7H8 toluene, C8êH8ê styrene, e ê naphthalene, ê ê ê ê ê ê Cê10H C 12H8 acenaphthalene, e F 8 ê ê *ê ê ê ê ê ê ê ê ê ê C13H10 ¯uorene, ´ C14H10 anthracene, C14H10X phenans ê ê ê ê ê ê ê ê thracene, C18H12 chrysene, C20H12 benzopyrene). ‚ f ê ê ê ê ê ê ê ê

Figure 3. Non-steady state calculation of the change in the composition of the main species from Case B high temperature model using PSR ( * CO ê ê carbon-monoxide, ± CO2 carbon-dioxide, F CH4 methane, e ê H ê ê ê toluene, ê ê C2H2 ethyne, ‚ C C6H5CH Œ C6H5C2H 6 6 benzene, e 3 ê ê ê ê naphthalene, ê + phenyl acetylene, C9H8ê indene, C H 10 8 ê ê ê ê ê ê *ê ê C12H10 biphenyl). e ê ê ê ê

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CHAGGER et al. DISCUSSION Kiln Type Combustion Chamber Model

Figure 5. (a) Non-steady state calculation of the change in the composition of the main species from Case A FB model using PSR ( * CO carbonê ê monoxide, ± CO2 carbon-dioxide, F CH4 methane, e C2H2 ê ê ê ethyne, ‚ ê Cê 6H6 benzene, e C6Hê 5CH toluene, l C6H5C2H3 3 ê ê ê ê ê ê styrene, + C H5C2H5 ethylbenzene, C H8 naphthalene, ê Cê H 6 biphenyl) ê *ê ê (b) 10Non-steady (Reference ê (11). state e 12 10 êcalculation ê ê ê of the change in the composition of the main species from Case B FB model using PSR ( * CO carbon-monoxide, ± CO2 carbondioxide, F CH4 methane, ê eê C2H2 ethyne, ‚ C6ê H6ê benzene, e ê ê ê ê ê ê ê C6H5CH3ê toluene, l C6H5C 2H3 styrene, + C6H5C2H5 ethylbenzene, ê ê ê ê + C9H8 indene, C10H7CH3 methylnaphthalene, ‚ êC ê H ê naphthalene. ê ê ê ê ê 11). (c) Non-steady state calculation ê ê of*ê the ê (Reference 10 8 change in the composition of the main species from Case C FB model PSR ( * CO carbon-monoxide, ± CO2 carbon-dioxide, F CH4 methane, ê ê C H ethyne, ê ê ê ê toluene, C6H5CH ‚ C6H6 benzene, e e l 2 2 3 ê ê ê ê ê ê ê ê + C6H5C2H3 styrene, + C6H5C2H5 ethylbenzene, C9H8 indene, ê ê ê Cê ê H C H ethylC10H7CHê 3 methylnaphthalene, ‚ e 10 7 2 5 ênaphthalene, ê ê ê ê ê C 10H8 naphthalene, ê ê e C12H10 biphenyl). * ê ê ê ê ê ê ê ê (Reference 11).

equilibrium distribution between the PAH species and lower molecular weight compounds. · The major oxidation products are CO and CO2, and other predominant lower weight hydrocarbon species are methane, benzene and toluene. · Styrene and ethylbenzene have not reached equilibrium in Case B and Case C, but these compounds are in equilibrium in Case A. A similar trend is observed for methylnaphthalene and ethylnaphthalene for Case B and Case C. High concentrations of intermediates such as styrene, ethylbenzene and indene suggest that these species also lead to formation of PAH. · In Case B and Case C the major species produced are two ringed compounds like naphthalene and indene as shown in Table 1.

From the results of the equilibrium and kinetic models computations it is possible to compare the calculated concentrations for hydrocarbon gases and VOC such as benzene, toluene and naphthalene. Direct comparison of the two methods at temperatures below , 1373 K indicates that the thermodynamic predictions underestimate the amounts of hydrocarbon gases (with the exception of methane) and the VOC compared to the kinetic model, i.e. quenching of the reaction occurs at temperatures above , 1273 K, and the species concentrations are kinetically `frozen’ at below these temperatures. Thus, the thermodynamic prediction that the PAH concentration decreases at low temperatures (<1273 K) is not valid. However, equilibrium calculations predict the same trends in concentrations of gases and VOC as the kinetic calculations; namely, that these species concentrations increase as they experience cooling from 1900 K to between 1500 and 1373 K. Thus, the equilibrium model can be used to predict the PAH concentrations in the ¯ue gases, by assuming that the concentrations at , 1373 K are kinetically stable due to quenching. Hence, the thermodynamic predictions together with the kinetic modelling indicate that benzene, anthracene, phenanthracene, pyrene, benzo(a)pyrene, benzo(k)¯uoranthene and benzo(b)¯uoranthene are amongst the most abundant, and this agrees well with MSW data (as shown in Tables 2 and 3). There are some discrepancies in the predicted and experimental order of PAH concentrations in the emissions, which may be sensitive to the exact volatile composition in the volatile-rich pockets. Certainly, in Case C, the presence of oxygenated species in the volatiles results in initial consumption of certain species (such as toluene) followed by formation of all VOC and PAH in the `post-combustion’ temperature range, and the order of the concentrations of these compounds changes. It should be noted that the examples chosen are representative of a real system. In turbulent diffusion MSW systems, unburned hydrocarbons may be produced directly from the nature of non-premixed combustion. This may be due to the presence of very lean or very rich regions created within the combustion chamber which do not support rapid combustion, and lead to local ¯ame extinction. Fluidized Bed Model It is possible to compare concentrations of some lower weight hydrocarbon species and PAH from the results of the equilibrium and kinetic models predictions. This comparison indicates that the thermodynamic predictions underestimate the amount of PAH compounds produced at low temperatures. Predictions that PAH concentrations decrease at low temperatures are not valid under these conditions unless the reactions are catalysed by metals in the coal ash and waste products. In the case of the kinetic predictions the initial temperature is not high enough to form ethyne, and the major compounds formed are two and three ringed species like naphthalene and indene which are abundant in Case B and Case C. This agrees with the experimental data as shown in Tables 2 and 3. However, this is not observed for Trans IChemE, Vol 78, Part B, January 2000

FORMATION OF VOC, PAH AND DIOXINS DURING INCINERATION Case A. This could be attributed to the presence of tar molecules in the initial unburnt pocket in varying concentrations in both Case B and Case C. Hence, the major route to PAH formation in the FB is from the dissociation or rearrangement of aromatic tars, resulting in the two and three ring PAH being the most abundant and thermodynamically stable. Similar conclusions have been reached for coal combustion using similar data11. CONCLUSIONS VOC and PAH and their emissions are a factor of the combustor size, operating conditions, type of fuel used and burn-out rates of the rich pockets of gas formed during turbulent combustion. The results obtained indicated that: · At high temperatures above 1373 K coal/waste molecules decomposed to form light ole®ns, mainly ethyne, which subsequently reacts to VOC and PAH during cooling and quenching. · The PAH found in ¯ue gases were predominantly the more volatile PAH compounds. · PAH were high during start-up because of incomplete combustion at low temperature and the changes in the PAH emissions were mainly attributed to the post-combustion gas temperature and residence time. However, the species initially released from low temperature combustion in ¯uidized beds were predominantly large complex molecules with a small concentration of aliphatic material. The lower furnace temperature in case of FB was not suf®cient to sustain uniform ignition of the particles. · In case of FB incinerators the PAH were high because combustion was more unstable than in the kilns. · Organic carbon dust was normally highest during start-up and lowest during burn-out.

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· The concentration of dioxins is a function of the oxygen concentration and the unburned hydrocarbon in the reactor. REFERENCES 1. Zhang, X. J., 1998, J Environ Sci Health, A33(2): 279. 2. Anders, L. C., Yngve, U. Z. and Conny, E. O., 1986, Atmospheric Environment, 20: 2279. 3. Yasuda, K. and Takahashi M., 1998, J Air Waste Management Association, 48: 441. 4. Barin, I., 1992, Thermochemical Data of Pure Substances, Second edition (VCH Verlagsgesellschaft Weinheim). 5. Stein, S. E., 1994, NIST Database 25, Structures and properties (NIST, Gaithersburg). 6. Konnov, A. A., 1997, http://homepages.vub.ac.be/, akonnov 7. Marinov, N. M., Pitz, W. J., Westbrook, C. K., Castaldi, M. J. and Senkan, S. M., 1996, Combust Sci Technol, 116: 211. 8. Burcat A., ftp://ftp:technion.ac.il.pub/supported/aetdd/thermodynamics/burcat.thr 9. Muller, C., Michel, V., Scacchi, G. and Come, G. M., 1995, J Chim Phys, 92: 1154. 10. Glarborg, P., Kee, R. J., Grear, J. F. and Uiller, J. H., 1990, SAND8608209, UC-4 (Sandia National Laboratories). 11. Chagger, H. K., Jones, J. M., Pourkashanian, M., Williams, A., Owen, A. and Fynes, G., 1999, Fuel, 78: 1527±1538. 12. Leckner, Bo, 1998, Prog Energy Combust Sci, 24: 31.

ACKNOWLEDGEMENTS The research has been funded by European Commission, JOULETHERMIE- R & D Programme `Clean Technologies for Solid Fuels’, Contract No. JOR3-CT95-0057 and INCO-COPERNICUS.

ADDRESS Correspondence concerning this paper should be addressed to Professor A. Williams, Department of Fuel and Energy, University of Leeds, Leeds LS2 9JT, UK. This paper was presented at the 2nd International Symposium on Incineration and Flue Gas Treatment Technologies, organized by IChemE and held at the University of Shef®eld, UK, 4±6 July 1999.