Catalytic Cracking of Toluene as a Tar Model Compound Using


Sep 1, 2016 - Equipment Management Department of Zhejiang Energy Jiaxing Power Generation Company, Limited, Pinghu, Zhejiang 314201, People's ...
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Catalytic cracking of toluene as a tar model compound using sewage sludge derived char Peng Lu, Xiaofeng Qian, Qunxing Huang, Yong Chi, and Jianhua Yan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01832 • Publication Date (Web): 01 Sep 2016 Downloaded from http://pubs.acs.org on September 6, 2016

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Catalytic cracking of toluene as a tar model compound using sewage sludge derived char Peng Lua, Xiaofeng Qianb, Qunxing Huanga,*, Yong Chia, Jianhua Yana a

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, People’s Republic of China

b

Equipment management department of Zhejiang Energy jiaxing power generation co., LTD, Pinghu, 314201, People’s Republic of China

KEY WORDS: Tar; toluene; catalytic cracking; sewage sludge char; syngas component ABSTRACT: The catalytic cracking of model tar species (toluene) using sewage sludge char (SSC) was investigated. The effects of typical syngas components from municipal solid waste (MSW) gasification on toluene conversion ratio, cracking products distribution and characteristics of SSC catalyst were studied. Deposited coke significantly decreased the Brunauer-Emmett-Teller (BET) surface area from 74.213 to 51.782 m2/g in N2 atmosphere. CO2 and steam slowed down the decrease of BET surface area. HCl showed negative effect on the pore structure by formation of low-melting-point chlorides which melted above 750 °C and blocked the pores. The cracking efficiency in different reaction atmospheres was ordered as, CO2/H2O/N2 > H2/CO/H2O/N2 > H2/CO/H2O/HCl/N2 > N2. The conversion ratios were all above 65% at 750 °C, which reached more than 93% at 950 °C. The highest conversion ratio of 97.1% was achieved at 950 °C when CO2 and steam were presented. CO2 and steam significantly increased the tar conversion ratio and enhanced the resistance of

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deactivation. CO and H2 increased the tar conversion ratio and changed cracking products distribution mainly by gas phase reactions. Besides, when 300 ppm hydrogen chloride (HCl) was presented, toluene conversion ratio decreased from 86% to 81.2% at 850 °C. 1. Introduction As an undesirable and harmful byproduct from municipal solid waste (MSW) gasification, tar can be dealt with by physical separation, plasma methods, thermal cracking and catalytic cracking. Physical separation will cause secondary pollution and lose the chemical energy of tar.1-4 Thermal cracking needs high temperature, usually over 1000 °C and plasma methods require very high energy input.5 Compared with other methods, catalytic cracking has higher conversing ratio and overall energy efficiency.6 Although Ni-based catalysts, Fe-based catalysts, zeolite, calcined dolomite and olivine are widely studied for tar cracking, most of these catalysts are suffering from relatively high cost and ease to be poisoned or deactivated by hydrogen chloride, hydrogen sulfide and deposited coke.7,8 In recent years, char gains more attention for tar elimination due to the low cost, abundant sources and high resistance to poisoning/deactivation.9 Char can be produced continuously from hydrocarbon materials through pyrolysis or gasification process. The porous internal structure and rich functional groups on the surface of char along with the minor mineral impurities promote the adsorption and catalytic cracking capability for tar.10,11 Coal char and biomass char have been widely studied and proved to be effective in

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catalytic tar cracking.11-15 Wang et al.15 used coal char to catalyze the tar from pyrolysis and gasification of Shengli brown coal. They compared the catalytic performance of inactivated char and char activated with steam and found that the activated char showed a better tar cracking performance, especially for the tars from gasification. The tar yields were well reduced to 3.24% and 1.82% for pyrolysis and gasification at 950 °C due to the use of activated char as catalyst. Mani et al.14 used pine bark biochar as catalyst to decompose toluene (model tar compound) over a temperature range of 600-900 °C. The toluene conversion reached 94% at 900 °C and the activation energy was reduced by four fold compared to thermal cracking. Sun et al.11 investigated the thermal and catalytic effects of biomass char in a temperature range of 500-700 °C on the cracking of tar from the pyrolysis of pine wood. The yield of liquid product containing tar dropped from 35% without char to 11% with char at 700 °C. The biomass char was catalytically active for the tar cracking at 500-600 °C, while the thermal effect became a dominant mode of the tar cracking at 600-700 °C. Recently, sewage sludge char (SSC) has been reported to be effective for the elimination of tar from MSW gasification. SSC is derived from sewage sludge that is the product from the treatment of abundant municipal sewage. SSC has high ash content (normally higher than 80 wt.%) and low fixed carbon content (normally lower than 10 wt.%), which is different from other kind of chars. The relatively high contents of Fe, Ca, Mg, Na and K elements contained in the ash may have positive effects on tar cracking. At the same time, the sintering problem due to the low melting point of ash may be a major drawback. Diego et al.16 compared the catalytic

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performance of two model tars (toluene and naphthalene) over coal, coconut and sewage sludge chars. They found the cracking efficiency was ordered as coconut char>coal char>sewage sludge char, but the differences were quite small. However, tar elimination by sewage sludge char has not been well studied and the specific process or mechanism is still unclear. Syngas from MSW gasification mainly consist of H2, CO, CO2 and steam, as well as other harmful components like HCl. These syngas components have significant effects on the tar cracking because they will decide the specific reaction pathways, for example the inert atmosphere leads to the cracking pathway and the oxidizing atmosphere leads to the reforming pathway.17 The catalytic cracking of tar is a complicated process involving gas phase reactions, solid phase reactions and gas-solid phase reactions. Nitsch et al.18 found that the wood char was highly catalytic with phenol (the model tar) in the presence of steam and H2 seemed to have no significant influence on catalytic activity. Other researchers found the H2 in the atmosphere inhibited the polymerization of tar caused by thermal cracking and char gasification was decelerated by the presence of H2 and tars.19,20 Abu et al.12 compared the catalytic cracking of naphthalene and phenol with calcined dolomite, olivine, used fluid catalytic cracking catalyst, biomass ash and nickel catalyst in the atmosphere of CO2 and steam at 700 ~ 900 °C. They found the continuous supply of biomass char from the gasifier and activation by the steam and CO2 content in the producer gas make the biomass char more stable than the other catalysts. The existence of HCl in the atmosphere will cause the serious deactivation of Fe-based catalysts and dolomite

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catalysts by the formation of low-melting-point chlorides, and poison the Ni-based catalysts by some electronic structure reason.21 But the effects of HCl on char catalysts has not been well researched. Review of literature above reveals that the catalytic cracking of tar will be affected by syngas components to a large extent, but many details have not been well discussed. Thus, it is necessary to clarify the specific influence of syngas components on the catalytic cracking of tar and the characteristics of catalysts. In present work, performance of SSC catalyst produced from municipal sewage sludge for cracking major tar species from MSW gasification was studied. To quantitatively compare with previous results, toluene has been selected as model tar compound because toluene is the lightest tar component and the most stable alkyl-aromatic. The effects of H2, CO, CO2, steam and HCl on the products of tar cracking, tar conversion ratio and the pore structure, chemical composition, functional groups and stability of char have been examined. 2. Material and methods 2.1 Material The municipal sewage sludge was collected from Wenzhou Hongze Corp., China which was responsible for the municipal sewage treatment. The char was produced in a laboratory muffle furnace at 900 °C for one hour with a continuous supply of 0.1 L/min N2 flow. Raw char was sieved to particles between 1-1.7 mm, dried before tests. Similar to previous study,22 toluene was selected as model tar and its concentration was set at 12.9 g/Nm3 for comparing purpose.

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2.2 Experimental setup The experimental setup is shown in Figure 1 and the detailed description can be found elsewhere.22 The retention time was 0.3 s for all the tests. The volume fraction of H2, CO, CO2, steam and HCl and specific experimental conditions are presented in Table 5. Water was injected into the carrier gas through another syringe pump and heated up to 250 °C before entering the reactor. HCl was mixed in deionized water and fed along with it. After the carrier gas and toluene were fed into the reactor, liquid products were sampled every 15 min. 5 min after liquid products started sampled, gas products were sampled every 10 min. Each test lasted for 1 hour. When test was finished, SSC was cooled down by N2, weighed and stored for later analysis. GC-MS was used for the quantification of toluene. The contents of H2, CH4, CO and CO2 in the product gas were determined by gas chromatograph (GC), as other minor hydrocarbons were negligible and not discussed in this paper. Toluene conversion ratio ϕ is defined as:

ϕ=

Cin − Cout × 100% Cin

(1)

Here Cin and Cout are the inlet and outlet toluene concentration respectively. R is the ratio of the lower heating value (LHV) of dry product gas to the LHV of dry feed gas, which is calculated as:

R=

(1 − Fsteam ) (107.98PH + 126.36 PCO + 358.18PCH 3905 ( Cin M toluene ) + 107.98FH + 126.36 FCO 2

4

)

(2)

2

Here M toluene is the relative molecular mass of toluene, Fi is the volume fraction in feed gas, Pi is the volume fraction in dry product gas and the combustible gas

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species include H2, CH4 and CO. 2.3 Catalyst characterization The SSC properties were characterized by nitrogen adsorption measurements for pore structure, X-Ray Fluorescence (XRF) for composition of ash, Fourier transformation infra-red spectrometer (FTIR) for the determination of functional groups and scanning electron microscope (SEM) coupled with energy dispersive spectroscopy (EDS) for surface structure and chemical elements identification. 3. Results and discussion 3.1 The effects of syngas components on the characteristics of SSC catalyst Table 1 shows the properties of fresh SSC catalyst. The ash content was as high as 89.3 wt.%. In addition to the major component SiO2, 25.1 wt.% of CaO and 10.4 wt.% of Fe2O3 were found in the ash. The high content of Ca and Fe may be the partial reason of catalytic activity.23,24 The weight balance of char bed (W) is presented in Figure 2. All the tests showed the same trend that the W increased firstly and decreased above 850 °C. In N2 atmosphere (tests #1-3), deposited coke increased the mass of char but deeper char cracking also occurred which decreased the mass. Both of the process became stronger at higher temperature. At 750 °C and 850 °C, the amount of deposited coke was more than the weight loss of char cracking. So W was positive. When temperature reached 950 °C, char cracking was dominating and the final mass of char was even smaller than that of original char which resulted in a negative W. The adding of steam and CO2 (tests #4-6) significantly increased the W and reached 1.877 g/g-input toluene at 850 °C which was the largest in all tests.

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Except for the coke deposition and char cracking discussed above, steam and CO2 could react with coke to decrease the mass and may react with metals contained in char and formed metal oxides to increase the mass. The oxidation was the strongest cause for the increase of W because the W in tests #4 -12 was much larger than that of tests #1-3. At the same time, the reduction of metal oxides may cause the decrease of char mass when H2 and CO were added. In addition, the free radicals, like H, OH and O radicals, provided by H2 and steam in the atmosphere will form new functional groups on the surface of char, like C-H, C-O, C=O and –OH groups which will also increase the mass of char.25-27 The pore structures of fresh char and chars after different tests at 850 °C are shown in Table 2. The fresh char had the largest BET surface area, micropore area, total pore volume and smallest average pore diameter compared with other four used samples, indicating that the deposited coke has blocked the pores and decreased the surface area and pore volume. In N2 atmosphere, the BET surface area and pore volume decreased from 74.213 m2/g and 0.165 cc/g (fresh char) to 51.782 m2/g and 0.131 cc/g (test #2). CO2 and steam consumed the deposited coke, which retarded the drop of surface area and pore volume. When CO2 and steam were added, the BET surface area and pore volume only decreased to 62.536 m2/g and 0.159 cc/g (test #5). CO and H2 showed no significant effect on the pore structure when compared tests #8 with #5. The existence of HCl decreased the BET surface area and pore volume from 62.222 m2/g and 0.162 cc/g (test #8) to 57.73 m2/g and 0.153 cc/g (test #11) by formation of low-melting-point chlorides which melted above 750 °C and blocked the pores,

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indicating a negative effect on pore structure of char. The fresh SSC and used SSC were examined for surface characteristics by SEM (Figure 3) and EDS (Table 3). The appearance and surface characteristics of fresh SSC are shown in Figure 3 (a) and (b) respectively. Cylindrical and round particles were the main structure of original coke and C, O, Fe, Al, Si, P and Ca were the major elements on the surface of fresh SSC. The contents of Fe and Ca reached 18.09 wt.% and 6.01 wt.% . Besides, small amounts of Na and K were found as well. Fe, Ca, Na and K contained in char benefit the catalytic cracking of tar. In N2 atmosphere (in Figure 3 (c)), the deposited coke from toluene cracking was mainly round particles, showing sparse distribution and agglomerating tendency.5 Steam and CO2 (in Figure 3 (d)) consumed both the original coke and deposited coke, making the surface much cleaner. The result of adding HCl is presented in Figure 3 (f) with blocked pores due to melted low-melting-point chlorides. After test #11, the contents of C and O elements increased from 26.86 wt.% and 15.9 wt.% in fresh SSC to 34.82 wt.% and 24.11 wt.% respectively, while the contents of other elements decreased. The increase of C element was due to coke deposition generated from toluene cracking on SSC and the increase of O element was due to oxidation of ash metals mainly by steam. As a carbonaceous material, char has rich functional groups that was prior to other catalysts because the functional groups on the char surface will interact with tar molecular which affects the adsorption ability and tar conversion ratio.28 Thus, many researchers studied the surface chemistry of char from pyrolysis or gasification. Sharma et al.29 used pectin to produce char under oxidative and inert atmospheres at

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temperature ranging from 150-550 °C. They found the char lost its aliphatic character and became more aromatic at high temperature. Besides, the hydroxyl and carbonyl functionalities were lost progressively as the temperature increased, which was in agreement with the results of Esin et al.30 As shown in Figure 4, there were three typical functional groups, which were C-O of alcohols and/or phenolic (at 1049 cm-1), C=C groups (at 1619 cm-1) and –OH groups (at 3446 cm-1) on the surface of SSC. Among these functional groups, the concentration of C-O of alcohols and/or phenolic (at 1049 cm-1) was the biggest. Fresh SSC had the fewest functional groups compared with other four tests because new functional groups formed by the reactions of toluene cracking coke, original coke, toluene molecular and free radicals. CO and H2 (tests #11 and #8) enhanced while CO2 (test #5) decreased the yields of functional groups when compared with test #2. Because CO2 consumed the coke which restricted the formation of new functional groups. H2 could provide H radical and steam could provide OH, O and H radicals, which took part in the formation of new functional groups. 3.2 The effects of syngas components on the conversion ratio and cracking products Table 4 shows the main reactions involved in toluene cracking and the average composition of product gas, LHV ratio of dry product gas to dry feed gas (R) and toluene conversion ratio under different experimental conditions are given in Table 5. The conversion ratio increased with temperature no matter in what reaction atmosphere. When temperature was 750 °C, the conversion ratios were all above 65%,

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which reached more than 93% at 950 °C. At 850 °C, the conversion ratio was 80.9% in inert atmosphere, which was the lowest because thermal and catalytic cracking were the main conversion paths and the problem of deposited coke was the most serious. The conversion ratio in CO2/H2O/N2 atmosphere was 86.5%, which was the highest compared with other atmospheres by the participation of CO2/H2O in dry and steam reforming and coke gasification reactions. When CO/H2/H2O were presented in atmosphere, the conversion ratio was 86.0% because hydrocracking of toluene enhanced the conversion but the extent was smaller than the effects of dry reforming and coke gasification by CO2. The adding of HCl showed negative effect and the conversion ratio was only 81.2%. Thus, it can be concluded that toluene conversion ratio was promoted by thermal cracking, catalytic cracking, hydrocracking, reforming reactions and inhibited by coke deposition. CO2 and steam (tests #4-6) significantly enhanced the yields of H2 and CO compared with control tests (tests #1-3) by steam reforming (R4), dry reforming (R6), Boudouard (R7) and water gas reactions (R8). When temperature increased from 750 °C to 950 °C, the volume fraction of CO2 decreased from 14.02 to 10.40%, while the content of CO increased from 0.96 to 6.83% because Boudouard and water gas reactions were both endothermic. The adding of H2 and CO promoted the yields of CH4 and CO2 and the existence of CO restricted the formation of itself by hydrocracking (R3), water gas shift (R9) and methanation reactions (R10). In test #7 and 8, the volume fraction of CO decreased from 5.24 to 5.01% and the content of CO2 increased from 1.46 to 2.28% mainly by water gas shift reaction (R9), which was

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in agreement with the research of Gilbert et al.13 However, the volume fraction of CO increased to 7.48% and the content of CO2 dropped to 1.64% when temperature raised to 950 °C because dry reforming and Boudouard reactions occurred to a greater extent. Compared with tests #7-9, the adding of HCl (tests #10-12) slightly enhanced the production of CH4 and CO2, reduced that of H2 and CO. Figure 2 and Figure 5 indicate that the adding of H2, CO, CO2 and steam significantly increased the total mass of newly produced gases (M) and the LHV ratio of dry product gas to dry feed gas (R). In N2 atmosphere (tests #1-3), the M and R were quiet small and the main contributions were from H2 and CO. The M and R were only 0.239 g/g-input toluene and 0.28 at 950 °C because the major products of tar cracking were coke and H2 and most of the energy contained in toluene was transferred to coke.22 Some gases, like steam, CO2 and H2, contained in the syngas from MSW gasification were thought to be able to improve the energy utilization efficiency of tar cracking.9,12,31 When CO2 and H2O were added (tests #4-6), the main components of newly produced gases were H2 and CO. The mass proportions of CO were all over 50 wt.%. Therefore, the R was larger than other tests and reached 2.67 at 950 °C. H2 and CO2 were the dominant components when CO and H2 were added (tests #7-12). The contributions of CO2 to the M were all over 50 wt.%. So the LHV of product gas increased not as much as that of tests #4-6 but the M was larger and reached 6.0 g/g-input toluene in test #12. The M and R increased with increasing temperature because all the reactions involved in toluene cracking were endothermic except for R9 and R10. High temperature enhanced the yields of H2 and CO. The

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yields of H2 showed no significant differences in tests #4-12, which indicated that water was the dominant factor of H2 yield. Wu et al.32 also found steam showed the highest efficiency in tar elimination, while the presence of CO2 and O2 induced the formation of OH, H and O radicals which promoted hydrocarbon conversion. Above analysis shows that water plays an important role during tar cracking process by steam reforming, water gas and water gas shift reactions. In Figure 6, the water consumption ratio increased with increasing temperature because steam reforming and water gas reactions were both endothermic. And the differences of water consumption ratio in different atmospheres increased from 5.5% at 750 °C to 15.27% at 950 °C because a higher temperature promoted steam reforming for CO2 (R5). For tests #4-12, the water consumption ratios were smaller than 35% at 750 °C, increased to more than 55% at 950 °C. Besides, the adding of CO and H2 consumed more water than that of CO2 by enhancing water gas shift reaction (R9) and steam reforming for CO2 (R5), and the existence of CO restricted steam reforming for CO (R4). The highest water consumption ratio of 72.92% was achieved in test #12. As mentioned in section 2.2, each test had 6 gas samples and 4 liquid samples with time intervals of 10 and 15 min. The evolution of product gas volume fraction and toluene conversion ratio over time in one hour test are presented in Figure 7 and 8. Toluene cracking involved gas phase and gas-solid phase reactions. Since gas phase was stable with time, the changes of gas product and conversion ratio were due to the gas-solid phase reactions. Thus, Boudouard and water gas reactions were the major factors affecting gas contents and conversion ratio. In N2 atmosphere, the contents of

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H2, CH4 and CO2 slightly decreased with time. In addition, the toluene conversion ratios decreased from 70.12% and 95.63% to 62.99% and 91.79% at 750 °C and 950 °C respectively, indicating that the deposited coke was bad for catalytic activity. The drop of conversion ratio decreased, while the change of product gas volume fraction increased with increasing temperature. When CO2 and steam were added, the contents of H2 and CO decreased from 10.368 to 4.13% and from 11.778 to 3.05% respectively, while the content of CO2 increased from 3.773% to 13.879% with time at 950 °C. This was because the Boudouard and water gas reactions between CO2/H2O and original coke of char were strong at the beginning of each test. With time went by, the reactions became weaker due to the consumption of original coke. At the same time, the drops of conversion ratio (tests #4-6) were the smallest compared with other tests because the strong effects between CO2/H2O and deposited coke. The conversion ratios only dropped from 73.54% and 99.32% to 65.99% and 95.65% at 750 °C and 950 °C. When CO and H2 were added, the contents of H2, CO2 and CO followed the same trend as tests #4-6, but not as significant as the later tests because the existence of H2 and CO restricted the formation of themselves. The adding of HCl (tests #10 and 11) decreased the conversion ratios significantly from 76.34% and 86.28% to 61.11% and 78.14% at 750 °C and 850 °C, dropping faster even compared with tests #1 and 2 because the melting of low-melting-point chlorides blocked the pores and weakened the adsorption ability for toluene molecular. However, the drop of conversion ratio slowed down at 950 °C (test #12). This was because the gas phase reactions, including thermal cracking and reforming reactions, dominated and the

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melting effect was not competitive at such high temperature. 4. Conclusions When toluene and sewage sludge char were selected as model tar and catalyst respectively, typical syngas components from MSW gasification showed significant effects on conversion ratio, cracking products distribution and catalyst characteristics and stability. The major conclusions were summarized as follows: (1) BET surface area of fresh SSC was 74.213 m2/g and the ash content was as high as 89.3 wt.%. 25.1 wt.% of CaO and 10.4 wt.% of Fe2O3 were found in the ash. C-O of alcohols and/or phenolic was the richest functional group on the surface of SSC. These properties guarantied an acceptable tar cracking efficiency. Fresh SSC had the fewest functional groups because new functional groups formed by the reactions of toluene cracking coke, original coke, toluene molecular and free radicals. Besides, fresh SSC had the biggest BET surface area, micropore area, total pore volume and smallest average pore diameter, indicating that the coke deposition blocked the pores. CO2 and steam showed positive effect and HCl showed negative effect, while CO and H2 showed no significant effect on the pore structure of char. (2) The cracking efficiency influenced by typical syngas components was ordered as, CO2/H2O/N2 > H2/CO/H2O/N2 > H2/CO/H2O/HCl/N2 > N2. When temperature was 750 °C, the conversion ratios were all above 65%, which reached more than 93% at 950 °C. CO2 and H2O significantly enhanced the yields of H2 and CO by reforming, Boudouard and water gas reactions. H2 and CO promoted the yields of CH4 and CO2 and the existence of CO restricted the formation of itself by hydrocracking, water gas

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shift and methanation reactions. CO2 and steam can react with deposited coke and enhance the energy utilization efficiency of tar cracking. Steam played an important role during tar cracking process by steam reforming, water gas and water gas shift reactions. (3) The drop of toluene conversion ratio with reaction time decreased at higher temperature. CO2 and H2O in the atmosphere enhanced the resistance of deactivation by char gasification, while HCl decreased it by the formation of low-melting-point chlorides. Gas phase reactions, including thermal cracking and reforming reactions dominated at high temperature. CO2 and steam affected toluene cracking by gas phase and gas solid phase reactions, while CO and H2 mainly took part in gas phase reactions and HCl affected the cracking performance by changing the characteristics of char.

Author information Corresponding Author

*Telephone: +86-571-87952834; Fax: +86-571-87952438. E-mail: [email protected]

Acknowledgment Acknowledgment is gratefully extended to Environment Protection Special Funds for Public Welfare (201509013) and the Project “Experimental study of efficient upgrading technology for syngas derived from municipal solid waste” with Covanta Energy, LLC for their financial support.

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References (1) Nair, S. A.; Yan, K.; Pemen, A. J. M.; van Heesch, E. J. M.; Ptasinski, K. J.; Drinkenburg, A. A. H. Tar Removal from Biomass Derived Fuel Gas by Pulsed Corona Discharges. Chemical Kinetic Study II. Ind. Eng. Chem. Res. 2005, 44(6), 1734-1741. (2) Asadullah, M. Biomass gasification gas cleaning for downstream applications: A comparative critical review. Renewable Sustainable Energy Rev. 2014, 40, 118-132. (3) Nakamura, S.; Kitano, S.; Yoshikawa, K. Biomass gasification process with the tar removal technologies utilizing bio-oil scrubber and char bed. Appl. Energy 2016, 170, 186-192. (4) Guan, G.; Kaewpanha, M.; Hao, X.; Abudula, A. Catalytic steam reforming of biomass tar: Prospects and challenges. Renewable Sustainable Energy Rev. 2016, 58, 450-461. (5) Zhu, F.; Li, X.; Zhang, H.; Wu, A.; Yan, J.; Ni, M. et al. Destruction of toluene by rotating gliding arc discharge. Fuel 2016, 176, 78-85. (6) Dabai, F.; Paterson, N.; Millan, M.; Fennell, P.; Kandiyoti, R. Tar Formation and Destruction in a Fixed Bed Reactor Simulating Downdraft Gasification: Effect of Reaction Conditions on Tar Cracking Products. Energy Fuels 2014, 28(3), 1970-1982. (7) Shen, Y.; Yoshikawa, K. Recent progresses in catalytic tar elimination during

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biomass gasification or pyrolysis—A review. Renewable Sustainable Energy Rev. 2013, 21, 371-392. (8) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. A review of the primary measures for tar elimination in biomass gasi cation processes. Biomass Bioenergy 2003, 24, 125-140. (9) Hosokai, S.; Kumabe, K.; Ohshita, M.; Norinaga, K.; Li, C.; Hayashi, J. Mechanism of decomposition of aromatics over charcoal and necessary condition for maintaining its activity. Fuel 2008, 87 (13-14), 2914-2922. (10) Thaha, P.; Tomoaki, N.; Kunio, Y. Tar removal from biomass pyrolysis gas in two-step function of decomposition and adsorption. Appl. Energy 2010, 87, 2203-2211. (11) Sun, Q.; Yu, S.; Wang, F.; Wang, J. Decomposition and gasification of pyrolysis volatiles from pine wood through a bed of hot char. Fuel 2011, 90 (3), 1041-1048. (12) Abu El-Rub, Z.; Bramer, E. A.; Brem, G. Experimental comparison of biomass chars with other catalysts for tar reduction. Fuel 2008, 87 (10-11), 2243-2252. (13) Gilbert, P.; Ryu, C.; Sharifi, V.; Swithenbank, J. Tar reduction in pyrolysis vapours from biomass over a hot char bed. Bioresour. Technol. 2009, 100 (23), 6045-6051. (14) Mani, S.; Kastner, J. R.; Juneja, A. Catalytic decomposition of toluene using a biomass derived catalyst. Fuel Process. Technol. 2013, 114, 118-125. (15) Wang, F.; Zhang, S.; Chen, Z.; Liu, C.; Wang, Y. Tar reforming using char as catalyst during pyrolysis and gasification of Shengli brown coal. J. Anal. Appl.

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Pyrolysis 2014, 105, 269-275. (16) Fuentes-Cano, D.; Gómez-Barea, A.; Nilsson, S.; Ollero, P. Decomposition kinetics of model tar compounds over chars with different internal structure to model hot tar removal in biomass gasification. Chem. Eng. J. 2013, 228, 1223-1233. (17) Zhang, Y.; Wu, W.; Zhao, S.; Long, Y.; Luo, Y. Experimental study on pyrolysis tar removal over rice straw char and inner pore structure evolution of char. Fuel Process. Technol. 2015, 134, 333-344. (18) Nitsch, X.; Commandré, J.; Valette, J.; Volle, G.; Martin, E. Conversion of Phenol-Based Tars over Biomass Char under H2 and H2O Atmospheres. Energy Fuels 2014, 28 (11), 6936-6940. (19) Bayarsaikhan, B.; Sonoyama, N.; Hosokai, S.; Shimada, T.; Hayashi, J.; Li, C. et al. Inhibition of steam gasification of char by volatiles in a fluidized bed under continuous feeding of a brown coal. Fuel 2006, 85 (3), 340-349. (20) Jess, A. Mechanisms and kinetics of thermal reactions of aromatic hydrocarbons from pyrolysis of solid fuels. Fuel 1996, 75 (12), 1441-1448. (21) Shin, E.; Keane, M. A. Gas phase catalytic hydrodechlorination of chlorophenols using a supported nickel catalyst. Appl. Catal. B 1998, 18 (3-4), 241-250. (22) Huang, Q.; Lu, P.; Hu, B.; Chi, Y.; Yan, J. Cracking of Model Tar Species from the Gasification of Municipal Solid Waste Using Commercial and Waste-Derived Catalysts. Energy Fuels 2016, 30(7), 5740-5748. (23) Zhang, Y.; Ashizawa, M.; Kajitani, S.; Miura, K. Proposal of a semi-empirical

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kinetic model to reconcile with gasification reactivity profiles of biomass chars. Fuel 2008, 87(4-5), 475-481. (24) Huang, Y.; Yin, X.; Wu, C.; Wang, C.; Xie, J.; Zhou, Z.; Ma, L.; Li, H. Effects of metal catalysts on CO2 gasification reactivity of biomass char. Biotechnol. Adv. 2009, 27 (5), 568-572. (25) Song, Y.; Wang, Y.; Hu, X.; Hu, S.; Xiang, J.; Zhang, L. et al. Effects of volatile–char interactions on in situ destruction of nascent tar during the pyrolysis and gasification of biomass. Part I. Roles of nascent char. Fuel 2014, 22, 60-66. (26) Song, Y.; Wang, Y.; Hu, X.; Xiang, J.; Hu, S.; Mourant, D. et al. Effects of volatile–char interactions on in-situ destruction of nascent tar during the pyrolysis and gasification of biomass. Part II. Roles of steam. Fuel 2015, 143, 555-562. (27) Shen, Y. Chars as carbonaceous adsorbents/catalysts for tar elimination during biomass pyrolysis or gasification. Renewable Sustainable Energy Rev. 2015, 43, 281-295. (28) Gaspard, S.; Altenor, S.; Dawson, E. A.; Barnes, P. A.; Ouensanga, A. Activated carbon from vetiver roots: Gas and liquid adsorption studies. J. Hazard. Mater. 2007, 144 (1-2), 73-81. (29) Sharma, R. K.; Wooten, J. B.; Baliga, V. L.; Hajaligol, M. R. Characterization of chars from biomass-derived materials: pectin chars. Fuel 2001, 80, 1825-1836. (30) Apaydın-Varol, E.; Pütün, A. E. Preparation and characterization of pyrolytic chars from different biomass samples. J. Anal. Appl. Pyrolysis 2012, 98, 29-36 (31) Burhenne, L.; Aicher, T. Benzene removal over a fixed bed of wood char: The

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effect of pyrolysis temperature and activation with CO2 on the char reactivity. Fuel Process. Technol. 2014, 127, 140-148. (32) Wu, W.; Luo, Y.; Su, Y.; Zhang, Y.; Zhao, S.; Wang, Y. Nascent Biomass Tar Evolution

Properties

under

Homogeneous/Heterogeneous

Decomposition

Conditions in a Two-Stage Reactor. Energy Fuels 2011, 25 (11), 5394-5406.

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Table 1. Property of sewage sludge char Proximate and ultimate analysis of sewage sludge char (dry basis, wt.%) C 7.1 H 0.9 O 1.6 N 0.8 S 0.2 Volatile matter 2.4 Ash 89.3

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Composition of ash (wt.%) SiO2 Al2O3 MgO CaO Fe2O3 K2O Na2O

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31.5 15.8 4.1 25.1 10.4 0.9 1.0

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Table 2. Pore structures of fresh SSC and used SSC after different tests at 850 °C BET surface area (m2/g)

Micropore area (m2/g)

Total pore volume (cc/g)

Average pore diameter (nm)

Fresh SSC

74.213

11.424

0.165

8.898

CO2/H2O/N2(#5)

62.536

8.116

0.159

9.043

CO/H2/H2O/N2(#8)

62.222

7.081

0.162

10.432

CO/H2/H2O/HCl/N2(#11)

57.73

5.659

0.153

11.277

N2(#2)

51.782

6.911

0.131

12.082

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Table 3. Composition of fresh SSC and used SSC after test #11 by EDS (wt.%) C O Na Mg Al Si P K Ca Fe Fresh SSC 26.86 15.9 2.06 0.44 8.68 7.73 7.61 0.77 6.01 18.09 Test #11 34.82 24.11 1.35 0.42 5.11 5.74 6.81 0.67 4.94 14.69

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Table 4. Main reactions involved in toluene cracking Reaction Chemical formula

No.

Thermal cracking

C 7 H 8 → C oke + H 2 + CH 4

R1

Catalytic cracking

char C7 H 8 → Coke + H 2 + CH 4 + CO + CO2

R2

Hydrocracking

C7 H 8 + H 2 → Coke + H 2 + CH 4

R3

Steam reforming

C 7 H 8 + 7 H 2 O → 11H 2 + 7CO − 869 kJ / mol

R4

C7 H 8 + 14 H 2 O → 18 H 2 + 7CO2 − 581 kJ / mol

R5

Dry reforming

C 7 H 8 + 7CO2 → 4 H 2 + 14CO − 1157 kJ / mol

R6

Boudouard

C + CO2 → 2CO − 172 kJ / mol

R7

Water gas

C + H 2 O → H 2 + CO − 131 kJ / mol

R8

Water gas shift

CO + H 2 O → H 2 + CO2 + 41 kJ / mol

R9

Methanation

CO + 3 H 2 → CH 4 + H 2 O + 206 kJ / mol

R10

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Table 5. Catalytic cracking results, toluene concentration = 12.9 g/Nm3, retention time = 0.3 s #1 #2 #3 #4 #5 #6 #7 #8 #9 Operational conditions 750 850 950 750 850 950 750 850 950 Temperature (°C) 12.5% CO2, 15% H2O, 5% CO, 6% H2, Feed gas composition (vol.%) N2 72.5% N2 15% H2O, 74% N2

#10

#11

#12

750 850 950 5% CO, 6% H2, 15% H2O, (300 ppm) HCl, 74% N2

Average product gas composition (vol.%, dry) H2 0.68 0.95 CH4 0.01 0.02 CO 0.05 0.06 0.02 0.02 CO2

1.12 0.04 0.11 0.01

3.22 0.09 0.96 14.02

5.48 0.13 3.68 11.88

6.67 0.16 6.83 10.40

10.31 0.12 5.24 1.46

13.03 0.16 5.01 2.28

14.37 0.28 7.48 1.64

9.46 0.10 4.88 1.89

12.85 0.24 4.46 3.00

14.22 0.33 7.16 2.02

R Toluene conversion ratio (%)

0.28 93.5

0.82 69.2

1.79 86.5

2.67 97.1

0.86 69.0

0.99 86.0

1.23 95.6

0.79 68.4

0.96 81.2

1.21 95.5

0.16 65.2

0.23 80.9

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Figure 1. Schematic drawing of experimental apparatus. 12 6 4

3

7 5 8

2 7

1

11

9

1

N2 CO2 CO H2

Gas outlet

10

GC/MS

GC

1. n-Hexane, 2. Ice-water mixture, 3. Silica wool filter, 4. Sewage sludge char, 5. Quartz tubular reactor, 6. Tubular furnace, 7. Syringe pump, 8. Toluene, 9. Gas bag, 10. Tar sample vials, ,11. Temperature controller, , 12. Deionized water (contain HCl or not)

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Figure 2. The mass of newly produced gases (M) and weight balance of char (W) in different reaction atmospheres. 8 7

Converted toluene Mass of newly produced gases (M) Weight balance of char (W) CO/H2/H2O/N2

CO/H2/HCl/H2O/N2

CO2/H2O/N2

6

g/g-input toluene

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5 4 3 2

N2

1 0

0.009

-1

750 850 950 750 850 950 750 850 950 750 850 950

Temperature (°°C)

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Figure 3. (a) Picture of fresh SSC. (b) ~ (f) Pictures of fresh SSC and used SSC after different tests at 850 °C by SEM. (b) Fresh SSC, (c) N2 (#2), (d) CO2/H2O/N2 (#5), (e) CO/H2/H2O/N2 (#8), (f) CO/H2/H2O/HCl/N2 (#11).

(a)

(b) EDS

(c)

(d)

(e)

(f) EDS

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Figure 4. Fourier transform infrared (FTIR) spectra of fresh SSC and used SSC after different tests at 850 °C. Fresh char CO2+H2O (#5)

Absorbance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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N2 (#2) CO+H2+H2O+HCl (#11) CO+H2+H2O (#8)

-OH 3446 cm-1

C=C -1 C-O 1049 cm-1 1619 cm 500

1000

1500

2000

2500

3000

3500

Wavenumber (cm-1)

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4000

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Figure 5. The LHV ratio of product gas to feed gas. 3.0

The ratio of LHVproduct to LHVfeed (R)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.5

750 °C 850 °C 950 °C

2.0

1.5

1.0

0.5

0.0

N2

CO2/H2O/N2

CO/H2/H2O/N2 CO/H2/H2O/HCl/N2

Different atmospheres

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Figure 6. Water consumption ratio. 100 90

Water consumption ratio (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

750 °C 850 °C 950 °C

70 60 50 40 30 20 10 0

CO2/H2O/N2

CO/H2/H2O/N2

CO/H2/H2O/HCl/N2

Different atmosphere

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Figure 7. The changes of product gas volume fraction over time. 1.6

28

1.4

CO2

CH4

H2

26 950 °C (#3)

(a)

1.2

850 °C (#2)

1.0

750 °C (#1) 0.8 0.6 0.4 0.2

Volume fraction of product gas (%)

Volume fraction of product gas (%)

CO

(b)

850 °C (#5)

24

950 °C (#6)

750 °C (#4)

22 20 18 16 14 12 10 8 6 4 2

0.0

0 1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18

1

N2

(c)

950 °C (#9) 850 °C (#8)

24 22

750 °C (#7)

20

26

Volume fraction of product gas (%)

26

2

3

4

5

6

7

18 16 14 12 10 8 6 4

8

9

10 11 12 13 14 15 16 17 18

CO2/H2O/N2

28

28

Volume fraction of product gas (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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950 °C (#12)

(d)

850 °C (#11)

24 22 20 750 °C (#10)

18 16 14 12 10 8 6 4 2

2

0

0 1

2

3

4

5

6

7

8

9

10 11 12 13 14 15 16 17 18

1

2

3

4

5

6

CO/H2/H2O/N2

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7

8

9

10 11 12 13 14 15 16 17 18

CO/H2/H2O/HCl/N2

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Figure 8. The change of toluene conversion ratio over time. 78

750 °C

Toluene conversion ratio (%)

76 74 72 70 68 66 64 N2 (#1) CO2/H2O/N2 (#4)

62

CO/H2/H2O/N2 (#7)

60

CO/H2/H2O/HCl/N2 (#10)

10

20

30

40

50

60

50

60

50

60

Time (min)

90

850 °C Toluene conversion ratio (%)

88 86 84 82 80 N2 (#2)

78

CO2/H2O/N2 (#5) CO/H2/H2O/N2 (#8)

76

CO/H2/H2O/HCl/N2 (#11)

10

20

30

40

Time (min) 100

950 °C Toluene conversion ratio (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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98

96

94 N2 (#3) CO2/H2O/N2 (#6)

92

CO/H2/H2O/N2 (#9) CO/H2/H2O/HCl/N2 (#12)

10

20

30

40

Time (min)

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