Polynuclear Aromatic Hydrocarbons in Fly Ash from Pressurized

Aug 26, 1999 - Polynuclear Aromatic Hydrocarbons in Fly Ash from Pressurized ... in the fly ash from a pressurized fluidized bed (PFB) air gasificatio...
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Energy & Fuels 1999, 13, 1067-1073

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Polynuclear Aromatic Hydrocarbons in Fly Ash from Pressurized Fluidized Bed Gasification of Fuel Blends. A Discussion of the Contribution of Textile to PAHs Nader Padban* and Ingemar Odenbrand Department of Chemical Engineering II, Chemical Center, Lund University, P.O.B. 124, SE-221 00 Lund, Sweden Received February 23, 1999. Revised Manuscript Received June 14, 1999

The identification and quantification of 20 different polynuclear aromatic hydrocarbons (PAHs) in the fly ash from a pressurized fluidized bed (PFB) air gasification system based on hot gas filtering were determined by GC-MS analysis. A comparison was made on the basis of the results from two different sets of experiments with varied gasifier feedstock. The first set included four experiments with pure wood biomass as the gasifier fuel. In the second set a mixture of biomass and 10 wt % textile waste was used. The comparison showed that the distribution of the PAHs in the fly ash was strongly dependent on the gasifier feedstock but that the operational parameters, such as pressure and air/fuel ratio, showed minor effects. The relative content of the heavier compounds was larger for the pure biomass experiments, while the lighter compounds became predominant in the case of the mixtures. The study showed that the decisive parameter influencing the formation was the structure of each PAH rather than its molecular weight. Thermal decomposition of the added textile resulted in the formation of phenylic radicals. The excessive occurrence of such intermediates/compounds favored the formation of the simple structured PAHs with relatively low molecular weights. The largest contribution of the textile was to PAHs consisting of two benzene rings with at least one C-C bond between, such as dibenzofurane, fluorene, and biphenyl. Formation of the PAHs consisting of two joined benzene rings, such as naphthalene and 1- and 2-methylnaphthalene, was also strongly affected by the textile addition. The smallest effect of the mixing was observed in the contents of the heavier compounds consisting of more than three benzene rings. The differences in the relative concentrations of some compounds, such as phenanthrene and triphenylene, can be explained in the terms of their reactivity.

Introduction Solving the problem of the increasing amount of industrial waste is one of the critical issues in the industrialized countries at the present time. The old methods of handling these materials, that in the majority of cases was to dump them in certain areas, is far from being an acceptable option nowadays. Increased environmental awareness, together with the newer and more restricted measures set by governments, create the necessity of finding new methods for handling this global problem. To have the materials inside the production and reusage cycle is the optimal solution and avoids the problem associated with industrial waste. For organic materials, however, the characteristics and quality are normally changed after recycling a few times so that reusage becomes impossible. Although all organic materials can be assumed to be a kind of energy source, it is more beneficial to extract their energy content as effectively as possible. The integrated gasification combined cycle (IGCC) processes have been the matter of study for several * Author to whom correspondence should be addressed.

development projects in both Europe and the United States within the last two decades.1-3 These systems, in combination with hot gas cleaning, are deemed to be a more efficient and environmentally friendly way of utilizing solid fuels. Woody biomass, or in other words the residues from the forestry industry, is one of the optional fuels for IGCC processes. Biomass utilization can decrease the dependence of the energy sector on fossil fuels and in this way contribute to a zero balance in CO2 emission, which is the largest contributor to the greenhouse phenomenon. Beside the positive effect of the biomass utilization concerning the greenhouse effect, biomass has the advantages of being clean from other elements that affect the environment, such as sulfur and lead, which are found in the widely used fossil fuels such as coal and oil. Effective usage of other types of organic solid wastes such as plastics, textiles, and recycled (1) Palonen, J.; Lundqvist, R. G.; Ståhl, K. IGCC Technology and Demonstration; Proceedings Power Production from Biomass II, VTT, Espoo, Finland, 27-28 March, 1995. (2) Babu, S.; Bain, R. L.; Craig, K. R. Thermal Gasification of Biomass Technology Development in USA; Proceedings Power Production from Biomass II, VTT, Espoo, Finland, 27-28 March, 1995. (3) Elliott, P. Biomass-Energy Overview in the Context of Brazilian Biomass-Power Demonstration; Bioresour. Technol. 1993, 46, (1 & 2).

10.1021/ef990031j CCC: $18.00 © 1999 American Chemical Society Published on Web 08/26/1999

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papers can also, in the long term, decrease the dependence of the energy sector on fossil fuels. Low ash content and high reactivity of the organic solid wastes resemble the specifications of the woody biomass. Technically, this means it is possible to cogasify these kind of fuels without any major modifications in the biomass handling processes. On the other hand, mixing these kind of fuels with biomass will certainly change the quality and composition of the end products because, despite the similarity in reactivity, these fuels have a different organic and inorganic composition than biomass. The main products of a gasification process are combustible gases, which are burned in a gas turbine chamber after filtration and cleaning. In the process a number of high-molecular-weight polynuclear aromatic hydrocarbons (PAHs) are formed. Most of these compounds will follow the gas stream and will finally be combusted in the turbine, but a fraction of the PAHs (depending on the filter temperature) will condense on the particle surface of the fly ash and will leave the system at fly ash discharge. The identification and quantification of PAHs in the fly ash becomes a very important and crucial matter because of their highly hazardous and, in some cases, highly carcinogenic characteristics. The aim of this work, besides the characterization and quantification of the PAHs in the fly ash from a pressurized gasification system, was to study how the composition of the feedstock can affect the distribution of different PAHs in the fly ash. Although the used textile had a known chemical and structural composition, the background parameters that influence the formation of different PAHs could be explained. The work presents the results from identification and quantification of 20 PAHs in 10 different fly ash samples from a 90 kW(th) pressurized, fluidized bed gasifier. The PAHs ranged from naphthalene (MW 128) up to perylene (MW 252). Two kinds of feedstock, pure biomass and biomass mixed with 10 wt % textile waste, were used during the experiments. The effect of the mixing on the composition of the fly ash PAH is discussed. Experimental Section Gasification Experiments. The reactor consists of a 3.7 m high, temperature-resistant, stainless steel tube with a 102 mm inner diameter. The fuel is continuously fed from the lower part of the reactor from a position 35 cm above the fluidizing nozzle at the bottom. The bed material is fed batchwise from the top of the reactor. The product gas then continues to the filtering unit consisting of one SiC-based ceramic candle filter, which is cleaned from time to time by back pulsing. Fine particulate bed material and fly ash that are accumulated on the filter surface drop down during the back pulsing. The gasifier is preheated by means of electrical furnaces before gasification experiments. This allows reaching the steady-state operation within 15 to 20 min after starting the fuel and air feeding. The steady state is defined as the combination of the uniform temperature profile along the reactor, stable product gas flow, and constant gas composition. The duration of each experiment depending on the purposes vary between 2 and 6 h. After each experiment when the temperature and pressure inside the reactor are low, the filter residues consisting of bed material fines and fly ash are discharged. A schematic illustration of the gasifier is shown in Figure 1. The more

Padban and Odenbrand

Figure 1. PFB gasification plant. detailed description of the test rig can be find elsewhere.4-6 To be able to prepare a homogeneous mixture of biomass (dp: 1-2 mm) and textile waste, the later one was torn to particle sizes between 2 and 3 mm. The experiments with pure biomass were carried out at 16 bar pressure and at an air:fuel ratio of about 0.32. The first set of experiments with biomass-textile mixtures (TP1, TP2, TP3) were carried out at pressures of 9, 13, and 17 bar, respectively. The air:fuel ratio was around 0.33 for all experiments. During the second set, the pressure was kept constant while the air:fuel ratio was changed. TR1 has the highest air: fuel ratio (0.44) while the value for both TR2 and TR3 was 0.34. The temperatures inside the gasifier (830-850 °C) and filtering unit (650 °C) were the same during all experiments. Table 1 shows the summary of the experimental conditions. Sample Preparation for GC-MS Analysis. Toluene (80 mL) was added to 20 g of filter residues in a beaker. The mixture was stirred thoroughly and allowed to stand for 1 h. A constant time was chosen to be sure that all soluble compounds were dissolved in the solvent and to reduce the time effect during the dissolving process. The mixture was then filtered after a final stirring and any possible residues in the filter cake were washed with 25-30 mL of pure toluene until the filtrate volume was exactly 100 mL. Analyses Instrumentation. A Varian GC/MS equipped with an on-column injector, together with a 30 m × 0.252 mm capillary column coated with DB-Wax (DB-5), was used for the analyses. The GS-MS is equipped with a Star 3000 autosampler with a capacity of loading 48, 2 mL vials. The instrument settings during PAH analysis were as follow. Gas Chromatograph: column, DB-5 30 m × 0.32 mm ID × 0.25 mm; oven program, 110 °C for 3 min, program to 300 °C at 5 °C/min, and held for 28 min; injector program, 130 °C for 0.5 (4) Hellgren, R. Thermochemical Conversion of Biomass, A Technical Feasibility Study Concerning Wood Pyrolysis at High Temperatures and Biomass Gasification at Elevated Pressures. Licentiate Thesis, Department of Chemical Engineering II, Lund University, Lund, Sweden, 1995. (5) Gasification Group. Progress report Jou2-CT93-0347 - Comparison of Entrained Phase and Fluidised Bed Gasification of Biomass with Respect to Problems Related to Feeding and Hot Gas Cleaning. Department of Chemical Engineering II, Lund University, Lund, Sweden, 1994. (6) Gasification Group. Final report Jou2-CT93-0431- Hot gas cleaning in Pressurised Gasification of Biomass. Departement of Chemical Engineering II, Lund University, Lund, Sweden, 1996.

Polynuclear Aromatic Hydrocarbons in Fly Ash

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Table 1. Experimental Parameters set II: biomass + 10 wt % textile

set I: pure biomass task

B1

B2

B3

B4

TP1

TP2

TP3

TR1

TR2

TR3

pressure (bar) air (Nm3/h) fuel (kg/h) ERa gas rtb (s) gas vc (m/s)

16 27 18.7 0.32 13.6 0.22

16 27 19 0.32 13.6 0.22

16 27 18.8 0.32 13.6 0.22

16 27 18.7 0.32 13.6 0.22

9 15.0 10.6 0.30 13.2 0.23

13 21 13.7 0.31 13.8 0.22

17 25.8 17.3 0.32 14.4 0.21

13 21 10.2 0.44 13.6 0.22

13 21 13.2 0.34 13.6 0.22

13 21 16.8 0.34 13.6 0.22

a

Air: fuel ratio. bGas residence time in the reactor; cFluidization velocity. Table 2. Calibration Ranges for PAH compound

concentration range (ng/mL)

compound

concentration range (ng/mL)

1,2-benzanthracene 1-methylnaphthalene 2,3-benzoflueren 2-methylnaphthalene acenaphthene acenaphthylene anthracene benz[e]acephenanthrylene benzo[a]pyrene benzo[k]fluoranthene

0.57-40 0.53-137 0.54-38 0.06-63 0.05-53 0.09-92 0.06-67 0.47-33 0.41-29 0.54-38

biphenyl chrysene dibenzofuran fluoranthene fluorene naphthalene perylene phenanthrene pyrene triphenylene

0.06-62 0.57-40 0.07-69 0.14-145 0.06-65 0.13-130 0.59-41 0.17-173 0.20-200 0.70-49

min, program to 280 °C at 0.88 min, and held for 15 min. Mass Spectrometer: scan range, 50-650; scan time (3 µscans), 1 min; background mass, 91 m/z; manifold temperature, 260 °C; transfer line temperature, 300 °C. The vials were filled with 2 mL of the extracted solutions from the fly ash. For each analysis 0.5 µL of the solution was injected into the GC-MS. Data Evaluation. Eight standards ranging from approximately 0.50 to 150 ng/mL of 20 PAHs were prepared in toluene. The retention time for each component was determined by injecting a toluene solution of the pure compound into the CS-MS. Table 2 shows the retention times and calibration concentration ranges for each compound. The calibration files were created on the basis of the analysis of these standard solutions. For some isomeric couples such as 1,2-benzanthracene-chrysene and benz[e]acephenanthrylenebenz[k]phenanthrene, the coelution makes that quantification of these compounds less ideal. To reduce the uncertainty in calculations, the peak heights for these compounds were used for quantification procedures instead of the peak areas. The calibration curves for some compounds are far from linear, especially at higher concentrations. The problem is more noticeable in the case of higher-molecular-weight compounds such as benz[a]pyrene and perylene. For these compounds the quadratic curve fit method gives a more satisfactory result. Quantification of Analyzed Data. Quantification of the analyzed data is made by the GC-MS instrument software, Saturn (Varian). This program can automatically integrate the analysis files and quantify each compound on the basis of an internal standard or external calibration files. One important problem during data evaluation arises when the concentration of a compound in the sample is very low and near to zero. In these cases the calculated values by Saturn become unreliable. The reason is that the extrapolation of the calibration curves, even the linear curves, does not normally go through the origin. To overcome this problem, the concentrations of some compounds were calculated manually on the basis of new curves. These new curves were created on the basis of concentrations and peak areas for these compounds at the low concentration range and were forced to go through the origin.

Results The total amount of the analyzed PAHs in the fly ash varies between 30 and 100 mg/kg ash. The lowest value corresponds to the experiment TR1, e.g., textile/biomass

mixture with the air:fuel ratio 0.44. The highest value (100 mg/kg) belongs to the experiment (TP3) with textile/biomass mixture at 17 bar and with an ER value 0.32. It is not possible to draw any conclusion about the effect of the mixing on the total amount of tar on the fly ash. This is so for two important reasons. First, the fly ash is diluted by the fine particulate bed material and the dilution degree is not the same for each of these experiments. Second, although the same amount of bed material was used during the experiments, the amount of fuel was not the same. However, the composition of the tar and the relationship between the different tar components should not be affected by the abovementioned factors. It was expected that the different experimental conditions, such as different pressures and ER values, would change the composition of the tar in the fly ash. The results show that the tar composition is mostly affected by the feedstock chemical properties and that the variations in the experimental conditions, within the previously mentioned ranges, does not drastically change the tar composition. Table 3 shows the detailed GC-MS analysis results. Comparison of the concentrations within the same series shows a large spreading of the measured data points even for experiments with similar conditions (B1-B4). One explanation can be that only a small part (1-3%) of the total fly ash has been used to determine the PAHs content in the ash. For the compounds such as 1,2benzanthracene-chrysene and benz[e]acephenanthrylene-benz[k]phenanthrene, the coelution can also affect the measurements. For the experiments with biomass/ textile mixtures, there is a clear difference in the concentrations of some of the compounds such as 1,2benzanthracene, chrysene, and dibenzofuran. However, a study of the experiment as a whole shows a clearly distinguishable difference in the concentrations of different components if the biomass/textile mixture experiment is compared with the pure biomass experiments. For the mixture, naphthalene is the largest component having an average concentration of about 23%. This is followed by acenaphthene, phenanthrene, and pyrene with average concentrations of 15%, 7%, and 7%,

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Table 3. GC-MS Analyses of the Samples compound (wt % of total PAHs)

B1

1,2-benzanthracene 1-methylnaphthalene 2,3-benzofluorene 2-methylnaphthalene acenaphthene acenaphthylene anthracene benz[e]acenaphthrylene benzo[a] pyrene benzo[k]fluoranthene biphenyl chrysene dibenzofuran fluorene fluoranthene naphthalene perylene phenanthrene pyrene triphenylene total

7.59 0.12 0.29 0.11 0.29 1.27 0.32 16.2 15.8 4.77 0.16 3.72 0.01 0.06 5.88 3.23 5.03 12.9 21.8 0.36 100

pure biomass B2 B3 8.44 0.40 0.52 0.47 0.56 5.06 0.67 10.2 12.9 9.01 0.67 3.42 0.07 0.15 5.55 8.67 2.46 12.6 18.0 0.21 100

12.7 0.17 0.44 0.20 0.41 1.50 0.49 17.5 15.4 6.53 0.22 6.10 0.03 0.08 3.78 4.28 3.12 11.3 15.2 0.61 100

B4

TP1

TP2

7.65 0.30 0.62 0.22 1.18 2.74 0.68 12.2 13.3 5.50 0.27 3.63 0.05 0.20 6.73 5.05 2.62 14.4 22.2 0.48 100

1.34 1.18 1.95 1.43 2.91 19.1 0.86 4.36 2.46 3.97 5.69 2.83 1.81 3.69 2.32 30.9 0.46 5.76 6.37 0.57 100

2.14 0.74 2.35 0.92 2.83 15.1 0.78 4.64 3.02 7.31 4.95 3.73 2.01 4.94 2.54 25.1 1.21 7.83 6.96 0.91 100

biomass + 10 wt % textile TP3 TR1 TR2 2.15 0.87 2.38 1.24 2.94 16.9 1.18 5.35 2.82 6.79 6.25 2.59 2.90 5.64 1.86 21.6 0.76 8.46 6.58 0.68 100

4.60 0.79 2.10 1.07 2.48 14.0 0.63 5.98 4.04 3.75 5.21 2.89 0.56 5.05 2.06 27.9 1.20 7.43 6.83 1.48 100

4.27 0.67 3.79 1.17 1.99 11.8 1.99 7.23 7.23 5.01 4.47 5.37 0.65 5.78 2.19 19.5 1.16 6.81 7.72 1.20 100

TR3 5.83 0.61 3.39 0.91 2.07 12.6 2.13 5.67 6.36 6.99 4.96 7.24 0.46 6.19 2.47 13.5 2.40 7.18 7.20 1.85 100

Figure 2. Accumulative concentrations of different PAHs in each experiment.

respectively. In the case of pure biomass, pyrene is the largest component (19%), followed by benzo[a]pyrene (14%), benz[e]acephenanthrylene (14%), and phenanthrene (13%). The content of some compounds such as dibenzofuran, biphenyl, 1-methyl-naphthalene, and 2-methyl-naphthalene is very small in the pure biomass fly ash. The corresponding values for these compounds become much larger for the textile/biomass mixtures. In general, the concentration of the heavier compounds is larger in the fly ash from pure biomass than from the biomass/textile mixture. Figure 2 shows a comparison between the accumulative concentrations of different compounds for the experiments with the pure biomass and the mixture. As observed in the figure, the concentration profiles show the same trend within the groups of similar feedstock. The compounds with a molecular weight larger than 168 (dibenzofuran) make up about 84% to 95% of the total PAHs for the pure biomass, while the corresponding value for the mixture is below 55% at its highest value. For biomass samples, in the molecular weight range 142-168, only acenaphthylene (MW 152) shows a considerable proportion. For

the mixture samples the contribution of the 1- and 2-methylnaphthalene to the total PAHs content is negligible. Despite their low content, the low-molecular-weight PAHs become interesting if their relative amounts in the two different sets of experiments are compared. The results obtained from such a comparison indicate differences of a 10-fold magnitude for some compounds. These differences are discussed later in the text. Discussion The reactions involved in formation of PAHs from gasification or pyrolysis of the biomass can be of a very complicated nature that still is not a well studied subject. It is believed that primary tars are formed in the first steps of pyrolysis or gasification. Increased temperature and residence time favors conversion of the primary tars to secondary and alkyl tertiary tars. At higher temperatures reactions involving dealkylation, chain break followed by repolymerization, and condensation give origin to tertiary tars. Tertiary tars are less oxygenated and exhibit a much higher degree of aromacity.

Polynuclear Aromatic Hydrocarbons in Fly Ash

PAHs investigated in this work belong to tertiary tar compounds. It must be emphasized that the objective of the discussion is not to explain the formation mechanisms of these PAHs, but to study to what extent the content of the different PAHs are affected by the textile addition. With this purpose, and to make the comparison more clear we introduced a relative frequency (RF) factor defined as

relative frequency (RF) ) % compound in the mixture fly ash % compound in the biomass fly ash By this definition, the RF factor will show to what extent the formation of each compound is affected by the mixing effect. The prerequisite for the above statement are the two following assumptions: (A) the amount of the PAHs condensed on the surface of the fly ash particles is dependent only on the total concentration of each compound inside the gasifier; (B) the condensation mechanism is a function of only the filter temperature, which was kept constant in all the experiments. A corresponding RF factor less than unity for a compound means that there is no contribution from the textile for this compound. On the other hand, a higher RF shows that the mechanism of the formation of the compound is strongly affected by the thermal decomposition of the textile. Table 4 shows the average concentration for the two different feedstocks and the calculated RF value. The compounds are arranged based on a decreasing RF factor. A close study of the table shows a close relationship between the RF factor and the structural formulas of the compounds. The compounds consisting of only two benzene rings with at least one C-C bond between, are placed at the top of the table and have a RF larger than 15. Structures with only two joined benzene rings have RF values between 3 and 6. One interesting point is that 2,3-benzofluorene, with a relatively high molecular weight, has a RF factor that places it in this group. In the third group with RF values between 2 and 3, anthracene (3 benzene rings) and triphenylene (4 benzene rings) can be found. Group 4, which contains components with RF values of less than unity, consists predominantly of high-molecular-weight PAHs. The differences in the RF factor for different compounds can actually be explained by the thermal degradation mechanism of the textile waste. The textile waste used in the experiment was a refractory product from carpeting factories whose structural fiber composition is described in Table 5. Fourier transform infrared (FTIR) analyses of the products yielded from pyrolysis of the textile and the biomass at the department, showed a clear difference between these two fuels. While CO was the predominate compound followed by methane and ethane for biomass, the products from textile could be arranged as following: toluene > buthane > methane > styrene > CO. This difference in pyrolitic property of the fuels changed the product distribution during the gasification experiments, too. For instance, the benzene yield was almost doubled when textile was mixed with biomass. As shown in Table 5, 23.1 wt % of the waste consists of polyesters. Polyethene terphthalate (PET) is a widely

Energy & Fuels, Vol. 13, No. 5, 1999 1071

used polymer in the manufacturing of synthetic fibers and has the following chemical formula:

The kinetics and mechanisms of PET thermal degradation at low temperatures (260 to 450 °C) are briefly described in the literature.7-13 Some of the products and intermediates of the PET thermal decomposition, according to these studies, are listed below:

It is more likely that at higher temperatures, which is the case during gasification, that the compounds with structures (I) to (V) will decompose further and contribute to the formation of structures with the formulas (VI) to (IX). All compounds with RF higher than 15 consist of two benzene rings with at least one C-C bond in between. The structures for these compounds, e.g., fluorene, dibenenzofuran, and biphenyl can easily be derived from the phenylic radicals and species described in formulas (VI) to (IX). The necessary conditions for the formation of structures belonging to group 2 (RF between 3 and 6)sif the excess parts originate from the polyestersis that a phenyl radical reacts consecutively with several alkene groups followed by one or two cyclization reactions. The complexity of the reaction pathway along with the limited availability to the alkene groups (compared to the cyclic radicals) can explain why the RF factors for group 2 become smaller than those for group one. (7) The Merck Index of Chemicals and Drugs, 7th ed; Merck & Co: Rahway, N J, 1960. (8) Sax, N. I. Dangerous Properties of Industrial Materials, 5th ed.; Reinhold Company: New York, 1979. (9) Chemical Carcinogens, ACS Monograph 173; Searle, C. E., Ed.; American Chemical Society: Washington, DC, 1976; pp 274-277. (10) Birladeanu, C.; Vasile, C.; Schneider, I. A. Makromol. Chem. 1976, 177, 121. (11) Spanninger, P. A. J. Polym. Sci., Polym. Chem. Ed. 1974, 12, 709. (12) Yoda, K.; Tsuboi, A.; Wada, M.; Yamadera, R. J. Appl. Polym. Sci. 1970, 14, 2357. (13) Goodings, E. P. Soc. Chem. Ind., London Chem. Eng. Group Proc., Monogr. 13, Society of Chemical Industry, London, 1961; p 211.

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Table 4. Calculated RF, Average and Standard Deviation for Different PAHs

a This column shows the hazardous properties of each compound: u: Unknown, ir: Irritant, al: Allergen, T: Toxicity, m: Mutagen. The figures inside the parentheses show the carcinogenic activity for the compound; the higher the number, the higher the activity. These data are collected from references [7-9].

One interesting observation is the placing of 2,3benzofluorene in group 2, a compound with a relatively high molecular weight and consisting of 4 rings. The high RF for this compound might be explained by the higher concentration of fluorene in the case of mixture

gasification. For the case of pure biomass gasification, the average concentration of fluorene is around 0.2% of the total, while the corresponding value for the mixture case exceeds 5%. It is possible that a part of flourene in the later case undergoes combination reactions with

Polynuclear Aromatic Hydrocarbons in Fly Ash Table 5. Chemical Composition of the Textile Waste polyamide polyester wool cotton viscose polyacryl + polyurethane

7.8% 23.1% 21.3% 12.1% 0.4% 35.3%

alkene groups followed by cyclization, resulting in formation of 2,3-benzofluorene. Group 3 contains two compounds, namely triphenylene and anthracene. Anthracene has a higher molecular weight and also a more complex structure than the compounds in groups 1 and 2. Therefore it is expected that its RF factor will be smaller than group 1 and 2 compounds. A similar argument used in the case of 2,3benzofluorene can also be used to relate the higher concentration of anthracene to the high naphthalene concentration of mixture samples. While the higher concentrations of 2,3-benzofluorene and anthracene can be related to the higher concentrations of some compounds within the previous groups, to explain the higher concentration of the triphenylene the focus must be directed to the next group and the compound phenanthrene. The addition reactions across the 9,10 bond of phenanthrene occur fairly easily. This double bond in the central ring of phenanthrene exhibits properties which are similar to those of an isolated ethylenic double bond.9 The study of the triphenylene structure shows that this compound can be formed in the addition reactions of phenanthrene with some alkene groups. Although the availability to the alkene groups in the mixture gasification is higher than that for the pure biomass, it is possible that a part of phenanthrene reacts further and forms triphenylene. This reaction could also, on the other hand, explain the decreased concentration of phenanthrene in the mixture gasification. With the exception of phenanthrene, which was discussed previously, the compounds in group 4 have a higher molecular weight and contain at least four benzene rings. The fact that the concentration of these compounds is not affected by the textile (polyester)

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addition, might show that their formation mechanisms are quite different from the pathways that were discussed above. Conclusions The PAH content of the pure biomass fly ash is dominated by high-molecular-weight compounds consisting of more than 3 benzene rings. Naphthalene and acenaphthylene are the only low-molecular-weight compounds that show significant concentrations in these samples. The organic composition is dominated by pyrene followed by benzo[a]pyrene, benz[e]acephenanthrylene, phenanthrene, and 1,2-benzanthracene in decreasing order of amounts. In the textile/biomass mixture fly ashes, the lighter compounds clearly predominate. Naphthalene makes up almost 25% of the total PAH content and the corresponding value for the compounds containing less than three benzene rings is larger than 50%. The textile addition influences the distribution figures for different PAHs. This effect can be assigned to the structural composition of the textile fibers, especially polyethene terephthalate (PET). The degree of the influence is, in the first place, a function of the PAH structure. The compounds that can easily be formed by combination or addition reactions of the phenylic radicals or other PET thermal decomposition intermediates/products are affected most, and their contents become larger for the mixture samples. For anthracene, triphenylene, 2,3-benzofluoerene, and phenanthrene, the reactivity and the high concentration of the other compounds seems to be the major concentration-determining factor. Acknowledgment. The gasification tests were performed within the framework of EU-project JOF3-CT95 -0010. The Swedish energy company Sydkraft AB has financially supported this particular part of the research. EF990031J