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Cracking of model tar species from the gasification of municipal solid waste using commercial and waste derived catalysts Qunxing Huang, Peng Lu, Binhang Hu, Yong Chi, and Jianhua Yan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00711 • Publication Date (Web): 01 Jun 2016 Downloaded from http://pubs.acs.org on June 7, 2016

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Cracking of model tar species from the gasification of municipal solid waste using commercial and waste derived catalysts Qunxing Huang*, Peng Lu, Binhang Hu, Yong Chi, Jianhua Yan State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, 310027, People′s Republic of China KEY WORDS: Municipal solid waste; tar; toluene; waste derived catalysts ABSTRACT: The cracking of model tar species from municipal solid waste (MSW) gasification using dried sewage sludge char (DSS char) and bottom ash catalyst (BAC) was investigated and catalytic performance was compared with that of well-studied calcined dolomite, NiO/γ-Al2O3 and non-catalytic thermal cracking. The effects of temperature, internal structure, chemical composition and functional groups on the performance of tar cracking were characterized. The results showed that when toluene was selected as model tar species, conversion ratios for all catalysts were over 94% at 950 °C. The cracking efficiency was ordered as, NiO/γ-Al2O3 > calcined dolomite > DSS char > BAC > thermal cracking. When temperature increased from 750 to 850 °C, the conversion ratios for DSS char and BAC raised from 68.8% and 40.1% to 81.5% and 63.2% respectively. H2 and coke were the major products of toluene cracking and catalysts promoted the yield of hydrogen. The LHV of the product gas followed the same rules of conversion ratio. Coke deposition from toluene cracking will decrease the BET surface area of the catalysts, inevitably leading to the deactivation. 1. Introduction The treatment of municipal solid waste (MSW) is a big challenge for all over the

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world especially for the developing countries where most of MSW has been landfilled. Although incineration is an efficient solution for MSW disposal with the advantages of dramatic volume reduction and energy recovery, it may cause secondary pollution due to imperfect control of pollutants, especially dioxins and heavy metals. Unlike incineration, gasification converts the combustible MSW components into light syngas and inorganics in bottom or fly ash. Generally, the combustion of syngas is much cleaner and more efficient than direct mass burning of mixed solid MSW. However, as an undesirable and harmful byproduct, tar will be produced during MSW gasification especially when the feedstock is not well prepared and gasified in fixed bed reactor. In addition to the traditional wet washing method, the removal of tar at high temperature has attracted extensively interests as the sensible heat of hot syngas can be used.1-5 Among the many developed solutions, catalytic cracking is considered to be more effective and economical as tar can be converted into combustible gases.6 Previous tar cracking studies7,8 found that in order to achieve high cracking efficiency, catalysts should be stable and active with essential tolerance for impurities contained in syngas during tar conversion. Moreover, catalysts should be inexpensive and abundant for massive usage in industrial system and also, the catalysts are expected to have selectivity to get desired products. Ni-based catalysts, Fe-based catalysts, zeolite, calcined dolomite, olivine and chars are most studied.9 Although, Ni-based catalysts have very high catalytic activity, they are expensive and become deactivated when acid gases are presented. A notorious characteristic of MSW is its high content of chlorine and during MSW gasification hydrogen chloride will be


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produced. For Ni-based catalysts, the existence of HCl results in a long term irreversible deactivation that is ascribed to a depletion of the Ni metal d-orbital population and a modification of the electronic structure of the supported metal phase to Ni-based catalyst.10 Other catalysts, like dolomite and Fe-based catalysts, will also lose their cracking activity because of the formation of chlorides with relatively low melting points and the molten chlorides will cover the fresh catalysts and inhibit its contacting with tar species. The calcined dolomite is very cheap and abundant, but it is significantly active only above 800 °C.11 Recently, waste derived catalysts become a hot research topic. Henrietta Essie Whyte et al.12 used refuse derived fuel (RDF) char and oyster shell as catalysts for the pyrolysis of RDF. They found the catalysts were effective in reducing tar yield and achieving high deoxygenation. Kanokwan Ngaosuwan et al.13 synthesized a green sulfonated carbon-based catalyst (SCAC catalyst) from coffee residue and a high catalytic activity and catalyst stability for esterification of HCp were achieved. Osman Nur Syazwani et al.14 used Cyrtopleura costata (Angel Wing Shell) to get low-cost solid catalyst for producing biodiesel using microalgae Nannochloropsis oculata oil for the first time, the calcined Angel Wing Shell at 900 °C can be reused more than three times with fatty acid methyl ester (FAME) yield greater than 65% with optimization condition. ShuMin Fan et al.15 studied the effects of composite catalyst (15% K2CO3 and 5% eggshell derived CaO) on the gasification of Indonesian sub-bituminous KPU coal in a fixed bed reactor at atmospheric pressure. A CO yield of 0.32 mol/mol-C was obtained by utilizing composite catalyst, and the yields of H2 increased by 6% and 125%


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compared with that of pure K2CO3 and no catalyst at 800 °C. Waste derived char and CaO are the most studied catalysts because the former can be produced continually from waste pyrolysis, and the latter is natural abundant and inexpensive. In present work, two different waste derived catalysts, char from dry sewage sludge (DSS char) and bottom ash from municipal solid waste incineration (BAC), were used as catalysts for cracking major tar species from MSW gasification. The BAC mainly consists of CaO and iron oxides, which makes it very suitable for tar removal. Gasification of MSW results in volatile gases, tar and solid residue. In this paper, toluene was selected as a proxy of the tar species from MSW gasification because toluene is the most stable alkyl-aromatic and the lightest tar component. The conversion performance of waste derived catalysts was compared with calcined dolomite (C.D), NiO/γ-Al2O3 and non-catalytic thermal cracking. Attention has been paid to the influence of temperature, internal structure, chemical composition and functional groups on the performance of tar cracking. 2. Experimental method 2.1 Materials The sewage sludge was provided by Wenzhou Hongze Heat and Power Corp., China. DSS char was produced from dried sewage sludge in a laboratory muffle furnace at 900 °C for 1 hour under inert environment. Raw char was sieved to particles with size between 1-1.7 mm and dried at 105 °C for 10 hours and kept in dry atmosphere before the tests. The bottom ash was taken from Xiao Shan Jin Jiang Green Power plant, Hangzhou, China. This plant was equipped with three fluidized


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bed incinerators and MSW was shredded before feeding. The sample was collected from the bottom ash discharger and big particles including glass were screened out and then the sample was grinded to less than 2 mm and calcined at 900 °C in muffle furnace for 4 hours. Commercial dolomite was calcined at 900 °C for 4 hours with a particle size range of 3-5 mm. Ni (NO3)∙6H2O was added to ethanol, then γ-Al2O3 was added to the solution and stirred, followed by heating to 80 °C until ethanol completely evaporated. After that, sample was dried at 105 °C overnight and calcined for 3 hours at 800 °C to get γ-Al2O3 supported 15 wt. % NiO catalyst. The tar content from MSW gasification will be affected by the waste composition, reaction temperature, gasifying agent, residence time, equivalence ratio and catalyst. During test, the concentration of toluene was selected as 12.9 g/Nm3 which was an average concentration of tar presented in MSW gasification syngas from previous studies.3,8,16,17 2.2 Experimental setup The experimental apparatus is showed in Figure 1. The tests were carried out in a

laboratory fixed bed reactor which was made of a quartz tube with a length of 50 cm and an internal diameter of 3.5 cm. The reactor was supported and heated by a tubular electrical furnace and the temperature was controlled by a K-type thermocouple placed near the center of the bed. The variability of the temperature controlling system was ±1 °C. A silica wool filter was placed in the reactor to support the catalysts and catch the coke produced during the cracking tests. N2 was chosen as carrier gas whose flow rate was adjusted by a mass flow controller and heated up to


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250 °C before entering the reactor. Then toluene was injected into the carrier gas through a syringe pump. During test, the residence time for DSS char, calcined dolomite and NiO/γ-Al2O3 was 0.3 s so tar can have enough reacting time. For bottom ash catalyst (BAC), 0.3 s was not enough to get an acceptable conversion ratio considering its smaller specific surface area and weaker catalytic activity, so the residence time was increased to 1.5 s. Before each test, the pipes both connecting the input and output side of the reactor were preheated to 250 °C and tests were carried out between 750 ~ 950 °C under atmospheric pressure. Catalysts were stabilized for 5 min at desired temperature. Then, N2 and toluene were fed into the reactor by a mass flow controller and a syringe pump respectively. 2 min later, liquid products and gas products were sampled separately. Each test lasted for 30 min. When test was finished, catalyst was cooled down by N2, weighed and stored for later analysis. Liquid products were collected by two bottles filled with n-Hexane which is a fine solvent for toluene and other organic matters. Two bottles were placed in the ice-water mixture. After each test, the solvent was recovered and bottles were washed with acetone. The samples were analyzed by GC-MS (Thermo Scientific ISQ, detector type is TR-5MS, detector size is 30 mm × 0.25 mm × 0.25 mm, carrier gas is Helium.) for the quantification of toluene and qualitative analysis for other liquid products. Gas products were sampled every 5 min during the test and analyzed by gas chromatograph (Agilent Micro GC 490, the type of chromatographic columns are MS5A 10m BF and PPU 10m BF，the temperature of the columns is 80 °C.) and the


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contents of H2, CH4, C2H2, C2H4 and C2H6 were determined. Average values were taken from 6 gas samples of each test. Toluene conversion ratio ϕ is defined as:

ϕ=

Cin − Cout × 100% Cin

(1)

Here C in and Cout are the inlet and outlet toluene concentration respectively. Hydrogen production ratio ω is defined as18:

ω=

moles of H 2 produced × 100% 4 × moles of toluene input

(2)

After cracking, the lower heating value (LHV) of combustible gas species is calculated as: LHV = 108.0YH 2 + 126.4YCO + 358.2YCH 4 + 59.0YC2 H 4 +63.8YC2 H 6 +56.5YC2 H 2 (kJ/Nm 3 )

(3)

Here ܻ௜ is the volume fraction and the combustible gas species include H2, CH4, CO, C2H2, C2H4 and C2H6. The relative concentration factor (R) of liquid product is defined as: （1 − ϕ）× Area percent of liquid product R= ϕ × Area percent of balanced toluene

(4)

Here ϕ is the toluene conversion ratio, balanced toluene means that the rest of area percent by GC-MS belongs to toluene. 2.3 Catalysts characteristics Proximate analysis of DSS char was undertaken by a 5E-MAG6700 and ARL ADVANT’X IntelliPowerTM 4200 X-Ray Fluorescence (XRF) was used to identify the chemical composition of DSS char and BAC. Internal structures of catalysts were analyzed by ASAP 2020M Micropore adsorption instrument. The temperature for gas


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removal was 250 °C. Gas adsorption measurements were performed at -196 °C. Total surface area was analyzed by BET method. Micropore surface area and volume were determined by t-plot method. Nicolet 5700 Fourier transformation infra-red spectrometer (FTIR) was used to determine the functional groups on the surface of fresh and used DSS char. Char sample was mixed with KBr at a mass ratio of 1:100 and milled into powder. Then, the mixture was compressed under 15 Mpa for about 1 min. After that, the prepared sample was analyzed by FTIR analyzer. The resolution of FTIR analyzer was 0.09 cm-1, and the scanning range was from 400 to 4000 cm-1. S-4800 scanning electron microscope (SEM) was used to visualize the surface characteristics of waste derived catalysts both before and after each test. Besides, energy dispersive spectroscopy (EDS) was coupled with SEM to identify the deposited coke on the surface of waste derived catalysts and EDS was performed by an EDAX-TEAM instrument.

3. Results and discussion 3.1 Catalysts characteristics Previous studies have found that pore structure of catalyst can strongly affect its activity. For example, Sou Hosokai et al.19 observed when decomposition of aromatics occurred in the absence of steam, coking mainly affected micropores instead of mesopores or macropores. Thus, total surface area, total pore volume and micropores of the sample used in this paper were characterized. Table 1 presented the internal structure of the waste derived catalysts used. The total surface areas of fresh BAC, DSS char, calcined dolomite and NiO/γ-Al2O3 were 0.984 m2/g, 38.02 m2/g, 15.328


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m2/g and 100.846 m2/g. BAC was the least porous catalyst while NiO/γ-Al2O3 had highest porous textural structure. After the conversion tests, the total surface area of BAC gradually decreased with increasing temperature due to the coke deposition. For DSS char, when temperature was below 900 °C, the changes of internal structure followed the same trend of BAC. However, the total surface area at 950 °C was bigger than that at 850 °C. The possible reason is that when DSS char reacted at a temperature above 900 °C, deeper devolatilization occurred. The contents of moisture, ash, volatile and fixed carbon in DSS char were 0.51 wt. %, 89.31 wt. %, 2.38 wt. % and 7.8 wt. % respectively. Very high ash content is a main feature of DSS char. The chemical composition of catalysts were listed in Table 2. In addition to the major component SiO2, BAC contained 25.16 wt.% of Fe2O3 and 20.03 wt.% of CaO, the total amount of Fe2O3, CaO and MgO was over 50%. Henrietta Essie Whyte et al.12 found the surface area of RDF char was 26.4 m2/g while that of oyster shell was 7.1 m2/g, but the efficiency of tar elimination by oyster char was better than that of RDF char because Ca content was higher in oyster shell than in RDF char. Therefore the activity of catalysts can be promoted by AAEM (Alkali and alkaline earth metals) contents. The high contents of Ca, Mg, Fe and the existence of Na2O (1.59 wt.%) and K2O (1.26 wt.%) offer the potential for tar cracking. DSS char contained 25.09 wt.% of CaO, 4.11 wt.% of MgO and 10.43 wt.% of Fe2O3. A small portion of Na2O (1.00 wt.%) and K2O (0.853 wt.%) could also be found. As a carbon-based catalyst, Ca, Mg, Fe, K and Na enhanced its catalytic activity.


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In Figure 2, four typical peaks can be identified from FTIR spectrogram at 925 cm-1, 1049 cm-1, 1619 cm-1 and 3446 cm-1, which are respectively associated with aromatic C-H (at 925 cm−1), 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 DSS char. The absorbance of C-O groups was quiet strong and may overlap that of aromatic C-H. The absorbance of C-O groups changed dramatically under different conditions compared to C=C and –OH groups, suggesting that C-O groups of the DSS char played important role for catalytic conversion of toluene. Fresh char had the fewest functional groups compared with used char because new functional groups formed. DSS char is different from other three catalysts as it is carbonaceous material, the functional groups on the char surface will interact with toluene molecular which affects the adsorption ability and the toluene conversion ratio. Besides, the thermal cracking of DSS char may also produce carbon-containing gases, like CO and CO2, while these two gases were barely found in the product gas when the other three catalysts were used. Figure 3 presents the CO and CO2 volume fractions in the product gas using DSS char. The volume fraction of CO increased while CO2 decreased with temperature. This phenomenon suggested that the decomposition of carbon-oxygen compounds preferred to produce CO. The produced CO2 could also react with char and/or coke to form CO under high temperature. On the one hand, the coke produced by toluene cracking will react with the oxygen on the surface of char to form carbon-oxygen compounds. On the other hand, the carbon-oxygen compounds will disintegrate to form CO. The two possible reaction pathes are presented in Figure


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10(c). Within the temperature range of tests, the formation rate of carbon-oxygen compounds (r1) was bigger than the cracking rate of carbon-oxygen compounds (r2), which could also explain the absorbance of used DSS chars was stronger than that of fresh char in Figure 2. One thing must be noticed that the absorbance at 950 °C was weaker than the absorbance at 850 °C and the production of CO increased dramatically when temperature raised from 850 to 950 °C. Because the rise of toluene conversion ratio was only 5.6%, which limited the increase of r1, but the rise of r2 was dramatic. The cracking of DSS char could be seen clearly from the weight balance of catalysts in Figure 4. BAC, calcined dolomite and NiO/γ-Al2O3 could not be cracked and the changes of weight were only caused by the deposition of coke. The weight balance of BAC, calcined dolomite and NiO/ γ -Al2O3 were near the weight of converted toluene which indicated that coke was the major product of toluene cracking and the amount of coke increased with increasing temperature. However, the weight balance of DSS char was quiet different, smaller than that of other three catalysts at the same temperature due to the cracking of itself. At 950 °C, the weight balance of DSS char was even negative, indicating that the effect of char cracking was even bigger than that of coke deposition at high temperature.

3.2 Tar conversion over different catalysts Gas components and toluene conversion ratios over different catalysts are shown in Figures 5. Tests were performed at empty reactor firstly to study the thermal cracking of toluene. At 750 °C and 850 °C, the conversion ratios by thermal cracking were only


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8.1% and 19.3%. The concentration of H2 in the product gas was quiet low. The conversion ratio reached 62.1% when temperature increased to 950 °C. The results showed that toluene is quiet stable and direct thermal cracking is unlike to happen strongly if the temperature is below 950 °C. When calcined dolomite was used as catalyst, toluene conversion ratio was 82.6% at 750 °C, and the toluene was completely cracked at 950 °C with the hydrogen production ratio reaching 95.2%. NiO/γ-Al2O3 showed the best performance for toluene cracking. At 750 °C, toluene conversion ratio was 85.9%, when temperature reached 850 °C, the conversion ratio increased to 97.5%. Toluene conversion ratio of DSS char was 68.8% at 750 °C, and the ratio increased to 94.5% at 950 °C. Many factors may have contributed to such high ratio. Firstly, char has highly porous textural structure, which can improve the tar adsorption capacity and facilitate the transport of reactant molecules.20 Besides, the surface of char is abundant of oxygen/nitrogen functional groups which can improve the adsorption capacity by physical and chemical mechanism.21 Moreover, alkali metals and alkaline earth metals in the char also play important role for tar cracking. Tsukasa Sueyasu et al.22 used char from the pyrolysis of K-loaded cedar as catalyst to reform tar at 600 °C and 700 °C, the concentration of heavy tar in the product gas was as low as 20 mg/Nm3. Yun-liang Zhang et al.23 also found out that the char added with K2CO3 show higher catalytic activity for tar conversion than the water washed char which will lose inherent alkali metal elements. The conversion performance of BAC was the worst compared to that of other three


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catalysts. The conversion ratio was 40.1% at 750 °C, higher than thermal cracking. Fortunately, it reached 94.2% when temperature increased to 950 °C which was acceptable for tar elimination. Moreover, BAC is useless and other unburned organic pollutant species contained in the BAC can also be removed during the cracking process, such as dioxins. Above all, the conversion ratio can be ordered as: NiO/γ-Al2O3 > calcined dolomite > DSS char > BAC > thermal cracking. The major products of the toluene cracking were coke and H2. There were also a small portion of CH4, C2H4, C2H6, C2H2 and condensable hydrocarbons. For all the tests, the conversion ratios and the amounts of coke and H2 increased with temperature. But for CH4, C2H4, C2H6 and C2H2, as they can be further cracked into coke and H2, the amounts of them were not linear related with temperature. Figure 6 presents the hydrogen production ratios under different experimental conditions. Temperature significantly affected the hydrogen production via stimulating the thermal and catalytic cracking which are endothermic reactions. The difference of hydrogen production ratios was small at 950 °C because thermal cracking dominated at such high temperature. Hydrogen production ratios were always close to toluene conversion ratios except for the thermal cracking tests, indicating that the catalysts promoted the yield of hydrogen. Figure 7 shows the lower heating value (LHV) of product gas. The LHV of inlet gas was 522.06 kJ/Nm3. Nevertheless, the LHV of the product gas was no more than 140 kJ/Nm3 because much coke was formed during the conversion process. Most of the heating value of toluene was transferred to coke. The surface of BAC and DSS char before and after cracking tests at 950 °C were


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both examined for coking and the SEM/EDS results are shown in Figure 8 and Figure 9. The carbon content increased from 6.31 wt.% (Fresh BAC) and 15.97 wt.% (Fresh DSS char) to 16.48 wt.% and 27.56 wt.% at 950 °C, resulting from the deposition of coke. The surface of fresh BAC was clean and less porous than DSS char, but it became rough after coke deposition. The fresh char was quiet rough and porous. As temperature increased, more coke deposited on the surface. The main structure of the coke was round particle. Table 3 and Table 4 show the GC-MS analysis of liquid species (Toluene was not included) from non-catalytic (thermal cracking) and catalytic cracking test using DSS char. The relative concentration of different liquid species at 950 °C and 850 °C were compared and evaluated through relative concentration factor (R). A larger R means more liquid product was produced. From the results, Styrene was the only aromatic species detected both in catalytic and non-catalytic cracking liquid products. Moreover, Styrene and (Z)-9-Octadecenamide were the dominant component respectively in the liquid products of thermal and catalytic cracking. Styrene may be formed from benzyl radical and methyl radical. When DSS char was used, the Nitrogen and Oxygen contained in the char took part in the reactions and formed (Z)-9-Octadecenamide. The R of thermal cracking was much bigger than that of catalytic cracking, indicating that catalytic cracking was more thorough than thermal cracking and less liquid products were formed.

3.3 Mechanism of toluene cracking Upper results show that cracking of model tar or toluene is very complicated


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involving both thermal and catalytic contributions. High temperature is needed for thermal cracking because the aromatic structure is stable and quiet difficult to break. According to the previous studies24,25, possible reaction paths of thermal cracking are presented in Figure 10(a). Taichang Zhang et al.26 concluded that Path 1 and 2 were the primary pyrolysis pathways of toluene and the decomposition reactions of toluene could be classified as three types, namely scission of C-H or C-C bond, isomerization, and subsequent dissociation of the isomers. Hui Wang et al.27 found when temperature was below 690 °C, path 1 dominated, otherwise path 2 was the main reaction. The C-H bond of methyl was the first bond to break for forming benzyl radical and hydroxyl radical. As temperature increased, the C-C bond between benzene ring and methyl broke and formed methyl radical and phenyl radical. After that, the further decomposition of benzyl radical and phenyl radical, the interactions of radicals and the polymerization reactions occurred quickly and produced H2, coke and other hydrocarbons.28-32 Because styrene was the major liquid product detected, path 3 was another possible reaction path. Benzyl radical reacted with methyl radical, producing styrene and hydrogen. Figure 10(b) and (c) show the main reaction paths for catalytic cracking. Toluene molecular was adsorbed to the surface of catalysts by chemical adsorption. Active sites (∙Cf) in the catalysts combined with benzyl, phenyl, methyl and hydroxyl radicals in various forms and made it easier to break the aromatic structure.20 Path 4 and path 5 were the most possible ways in catalytic cracking considering the mechanism of thermal cracking. From the results presented in Figure 5, the ratio of CH4 to H2 for


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catalytic cracking was much smaller than that of thermal cracking, which showed the catalytic cracking tends to form H2 instead of CH4 and the catalysts promoted the breaking of C-H bond of methyl (path 4) or the consumption of methyl radical. The ratios of C2H4, C2H6, C2H2 to H2 for catalytic cracking were smaller, fewer hydrocarbons in the product gas were found, which is in agreement with the results of liquid products detected by GC-MS. DSS char is a carbon-based catalyst and has more complicated chemical composition than the other three catalysts. DSS char contains K, Na, Ca, Mg and Fe as well as functional groups or active carbon atom, and all of them may be the active sites for tar cracking. The existence of K and Na can increase the amounts of activated carbon on the surface of DSS char.33 Ca, Mg and Fe themselves have catalytic activity for toluene conversion.34,35 Coke produced by toluene cracking can form new functional groups, which at the same time, may crack into CO, H2, CO2 at high temperature. As for BAC, although the specific surface area is small, high contents of Fe, Ca, Mg, Na and K still ensure an acceptable cracking efficiency.

4. Conclusions (1)

Waste derived catalysts (DSS char and BAC) were used for tar cracking, and

their conversion performance were compared with calcined dolomite and NiO/γ-Al2O3. The ordered conversion ratio is, NiO/γ-Al2O3 > calcined dolomite > DSS char > BAC > thermal cracking. (2)

DSS char had functional groups and porous internal structure, which were

good for adsorption of toluene molecular. When DSS char was used, the toluene


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conversion ratio was 68.8% at 750 °C and 94.5% at 950 °C. (3)

BAC is produced from MSW incinerators directly with simple preparation,

so the cost is quiet low compared to other catalysts. BAC contains much CaO, MgO and Fe2O3 which have catalytic activity for tar cracking. The present work is the first time to use BAC as catalyst for tar removal. The toluene conversion performance of BAC was 40.1% at 750 °C which was far higher than that of thermal cracking. The conversion ratio could reach 94.2% at 950 °C. (4)

The major products for tar cracking were coke and H2. The LHV of the

product gas followed the same trends of conversion ratio. The difference of hydrogen production ratios was small at 950 °C because thermal cracking dominated at such high temperature. Hydrogen production ratios were always close to toluene conversion ratios except for the thermal cracking tests, indicating that the catalysts promoted the yield of hydrogen. (5)

Coke deposition that occurred during the cracking tests would cause the

serious deactivation. Some liquid products (like styrene and chain hydrocarbons) were formed through thermal cracking but no significant liquid products were detected when catalysts were used.

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


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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) Tao, J.; Lu, Q.; Dong, C.; Du, X.; Dahlquist, E. Effects of electric current upon catalytic steam reforming of biomass gasification tar model compounds to syngas. Energy Convers. Manage. 2015, 100, 56-63. (2) Sun, Y.; Li, R.; Yang, T.; Kai, X.; He, Y. Gasification of biomass to hydrogen-rich gas in fluidized beds using porous medium as bed material. Int. J. Hydrogen Energy

2013, 38 (33), 14208-14213. (3) Li, J.; Liao, S.; Dan, W.; Jia, K.; Zhou, X. Experimental study on catalytic steam gasification of municipal solid waste for bioenergy production in a combined fixed bed reactor. Biomass Bioenergy 2012, 46, 174-180. (4) Wang, J.; Cheng, G.; You, Y.; Xiao, B.; Liu, S.; He, P.; Guo, D.; Guo, X.; Zhang, G. Hydrogen-rich gas production by steam gasification of municipal solid waste (MSW) using NiO supported on modified dolomite. Int. J. Hydrogen Energy 2012, 37 (8), 6503-6510. (5) Świerczyński, D.; Libs, S.; Courson, C.; Kiennemann, A. Steam reforming of tar from a biomass gasification process over Ni/olivine catalyst using toluene as a model compound. Appl. Catal., B 2007, 74 (3-4), 211-222. (6) Liu, H.; Chen, T.; Chang, D.; Chen, D.; He, H.; Frost, R. L. Catalytic cracking of tar derived from rice hull gasification over palygorskite-supported Fe and Ni. J. Mol. Catal. A: Chem. 2012, 363-364, 304-310. (7) 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.


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(8) 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. (9) Shen, Y.; Yoshikawa, K. Recent progresses in catalytic tar elimination during biomass gasification or pyrolysis—A review. Renewable Sustainable Energy Rev.

2013, 21, 371-392. (10) 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. (11) He, M.; Hu, Z.; Xiao, B.; Li, J.; Guo, X.; Luo, S.; Yang, F.; Feng, Y.; Yang, G.; Liu, S. Hydrogen-rich gas from catalytic steam gasification of municipal solid waste (MSW): Influence of catalyst and temperature on yield and product composition. Int. J. Hydrogen Energy 2009, 34 (1), 195-203. (12) Whyte, H. E.; Loubar, K.; Awad, S.; Tazerout, M. Pyrolytic oil production by catalytic pyrolysis of refuse-derived fuels: Investigation of low cost catalysts. Fuel Process. Technol. 2015, 140, 32-38. (13) Ngaosuwan, K.; Goodwin, J. G.; Prasertdham, P. A green sulfonated carbon-based catalyst derived from coffee residue for esterification. Renewable Energy 2016, 86, 262-269. (14) Nur Syazwani, O.; Rashid, U.; Taufiq Yap, Y. H. Low-cost solid catalyst derived from waste Cyrtopleura costata (Angel Wing Shell) for biodiesel production using microalgae oil. Energy Convers. Manage. 2015, 101, 749-756. (15) Fan, S.; Yuan, X.; Zhao, L.; Xu, L.; Kang, T.; Kim, H. Experimental and kinetic study of catalytic steam gasification of low rank coal with an environmentally friendly,


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inexpensive composite K2CO3–eggshell derived CaO catalyst. Fuel 2016, 165, 397-404. (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) Gai, C.; Dong, Y. Experimental study on non-woody biomass gasification in a downdraft gasifier. Int. J. Hydrogen Energy 2012, 37 (6), 4935-4944. (18) Zhu, F.; Li, X.; Zhang, H.; Wu, A.; Yan, J.; Ni, M.; Zhang, H.; Buekens, A. Destruction of toluene by rotating gliding arc discharge. Fuel 2016, 176, 78-85. (19) 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. (20) Shen, Y. Chars as carbonaceous adsorbents/catalysts for tar elimination during biomass pyrolysis or gasification. Renewable Sustainable Energy Rev. 2015, 43, 281-295. (21) Considine, R.; Denoyel, R.; Pendleton, P.; Schumann, R.; Wong, S. The influence of surface chemistry on activated carbon adsorption of 2-methylisoborneol from aqueous solution. Colloids Surf., A 2001, 179 (2–3), 271-280. (22) Sueyasu, T.; Oike, T.; Mori, A.; Kudo, S.; Norinaga, K.; Hayashi, J. Simultaneous Steam Reforming of Tar and Steam Gasification of Char from the Pyrolysis of Potassium-Loaded Woody Biomass. Energy Fuels 2012, 26 (1), 199-208. (23) Zhang, Y.; Wu, W.; Zhao, S.; Long, Y.; Luo, Y. Experimental study on pyrolysis


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tar removal over rice straw char and inner pore structure evolution of char. Fuel Process. Technol. 2015, 134, 333-344. (24) Zhang, L.; Cai, J.; Zhang, T.; Qi, F. Kinetic modeling study of toluene pyrolysis at low pressure. Combust. Flame 2010, 157 (9), 1686-1697. (25) Oehlschlaeger, M. A.; Davidson, D. F.; Hanson, R. K. Thermal decomposition of toluene: Overall rate and branching ratio. Proc. Combust. Inst. 2007, 31 (1), 211-219. (26) Zhang, T.; Zhang, L.; Hong, X.; Zhang, K.; Qi, F.; Law, C. K.; Ye, T.; Zhao, P.; Chen, Y. An experimental and theoretical study of toluene pyrolysis with tunable synchrotron VUV photoionization and molecular-beam mass spectrometry. Combust. Flame 2009, 156 (11), 2071-2083. (27) Wang, H.; Yang, H.; Ran, X.; Wen, Z.; Shi, Q. The pyrolysis mechanism of carbon matrix precursor toluene used as carbon material. J. Mol. Struct.: THEOCHEM 2002, 581 (1–3), 1-9. (28) Jones, J.; Bacskay, G. B.; Mackie, J. C. Decomposition of the Benzyl Radical: Quantum Chemical and Experimental (Shock Tube) Investigations of Reaction Pathways. J. Phys. Chem. A 1997, 101 (38), 7105-7113. (29) Brouwer, L. D.; Mueller-Markgarf, W.; Troe, J. Thermal Decomposition of Toluene: A Comparison of Thermal and Laser-Photochemical Activation Experiments. J. Phys. Chem. 1988, 92 (17), 4905-4914. (30) Oehlschlaeger, M. A.; Davidson, D. F.; Hanson, R. K. Experimental Investigation of Toluene + H → Benzyl + H2 at High Temperatures. J. Phys. Chem. A 2006, 110 (32), 9867-9873.


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2006, 110 (30), 9388-9399. (32) Oehlschlaeger, M. A.; Davidson, D. F.; Hanson, R. K. High-Temperature Thermal Decomposition of Benzyl Radicals†. J. Phys. Chem. A 2006, 110 (21), 6649-6653. (33) Chen, S. G.; Yang, R. T. Unified Mechanism of Alkali and Alkaline Earth Catalyzed Gasification Reactions of Carbon by CO2 and H2O. Energy Fuels 1997, 11 (2), 421-427. (34) Clemens, A. H.; Damiano, L. F.; Matheson, T. W. The effect of calcium on the rate and products of steam gasification of char from low rank coal. Fuel 1998, 77 (9-10), 1017-1020. (35) Asami, K.; Sears, P.; Furimsky, E.; Ohtsuka, Y. Gasification of brown coal and char with carbon dioxide in the presence of finely dispersed iron catalysts. Fuel Process. Technol. 1996, 47 (2), 139-151.


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Table 1. Internal structure of catalysts Catalysts

BAC

DSS char C.D NiO/γ-Al2O3

Status

Fresh 750 °C 850 °C 950 °C Fresh 750 °C 850 °C 950 °C Fresh Fresh

Total surface area (BET) m2/g 0.984 0.642 0.384 0.314 38.020 34.365 33.765 34.133 15.328 100.846

Micropore area m2/g 0.311 0.295 0.157 0.106 4.736 3.481 2.846 2.004 3.837 11.832


Micropore volume cm3/g 0.001 0.000 0.000 0.000 0.005 0.003 0.002 0.002 0.002 0.006

Total pore volume cm3/g 0.009 0.006 0.004 0.002 0.186 0.117 0.103 0.106 0.098 0.866

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Table 2. Chemical composition of waste derived catalysts (wt.%) BAC DSS char C.D NiO/γ-Al2O3

SiO2 35.26 31.51 0.42 -

Al2O3 9.93 15.77 85

MgO 6.76 4.11 54.88 -

CaO 20.03 25.09 44.04 -

Fe2O3 25.16 10.43 0.07 -


K2O 1.26 0.853 -

Na2O 1.59 1.00 -

NiO 15

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Table 3. Liquid species detected by GC-MS (thermal cracking, temperature = 950 °C, toluene conversion ratio = 62.1%, balanced toluene; catalytic cracking with DSS char, temperature = 850 °C, toluene conversion ratio = 81.5%, balanced toluene)

Thermal cracking at 950 °C

Catalytic cracking at 850 °C

No.

Retention time(min)

Species

Area percent (%)

R 10-3

1 2 3 4 5 6 7 8

7.37 9.81 12.30 14.23 15.90 17.39 18.74 19.96

1 2 3 4 5 6 7 8 9 10

7.40 9.80 12.29 13.59 14.22 15.89 17.38 18.73 19.96 20.53

Styrene 4,7-dimethyl-Undecane Pentadecane Heptadecane 2,6,11,15-tetramethyl-Hexadecane Eicosane 2,6,10,15-tetramethyl-Heptadecane 7-hexyl-Eicosane Total Styrene 4,7-dimethyl-Undecane 2,7,10-trimethyl-Dodecane 2,6,11-trimethyl-Dodecane Hexadecane 2,6,11,15-tetramethyl-Hexadecane 2-methyl-Nonadecane 9-octyl-Heptadecane 2,6,10,15-tetramethyl-Heptadecane (Z)-9-Octadecenamide Total

3.08 1.04 1.21 2.73 2.26 2.86 1.85 1.97 17.00 0.20 0.25 0.19 0.13 0.28 0.36 0.30 0.27 0.19 1.05 3.22

22.65 7.65 8.90 20.07 16.62 21.03 13.60 14.49 125.00 0.47 0.59 0.45 0.31 0.66 0.84 0.70 0.63 0.45 2.46 7.55


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Figure 1. Schematic drawing of the experimental apparatus 6 4

3

5 8

2

7

1

N2

1

9

10 GC-MS

GC

Gas outlet

11

1. n-Hexane, 2. Ice-water mixture, 3. Silica wool filter, 4. Catalyst, 5. Quartz tubular reactor, 6. Tubular furnace, 7. Syringe pump, 8. Toluene, 9. Gas bag, 10. Tar sample vials，， 11. Temperature controller


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Figure 2. Fourier transform infrared (FTIR) spectra of DSS char before and after the tests 1.0

C-O -1 1049 cm

Fresh

0.8

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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750 °C

C-H -1 925 cm

850 °C 950 °C

0.6

0.4

-OH -1 3446 cm

C=C -1 1619 cm

0.2

0.0 500

1000

1500

2000

2500

3000

-1 Wavenumber (cm )


3500

4000

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Figure 3. Volume fractions of CO and CO2 in the product gas with DSS char

Volume fractions in the 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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0.20

CO CO2 0.15

0.10

0.05

0.00 750

800

850

Temperature (°°C)


900

950

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Figure 4. Weight balance of catalysts and relationship with converted toluene Converted toluene Weight balance of catalysts

2.0

Calcined dolomite

NiO/γγ-Al2O3

BAC

Weight (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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1.6 1.2

DSS char

0.8 0.4 0.0 -0.4 750

850

950

750

850

950 750

850

Temperature (°°C)


950

750

850

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Figure 5. Gas products and conversion ratios at (a) 750 °C, (b) 850 °C, (c) 950 °C 1.2 1.0

H2

(a)

CH4 C2H4 C2H6

80

C2H2

0.8

100

Toluene conversion ratio

60

0.6 0.4

40

0.04 20 0.02 0.00 Thermal cracking BAC

0

DSS char

C.D

Catalysts


NiO/γγ-Al2O3

Toluene conversion ratio (%)

Volume fraction of gas components (%)

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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1.2 1.0 0.8

Page 32 of 40

H2

(b)

CH4

100

C2H4 C2H6

80

C2H2 Toluene conversion ratio

60

0.6 0.4

40

0.04

20 0.02 0.00 Thermal cracking

0 BAC

DSS char

C.D

Catalysts


NiO/γγ-Al2O3


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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1.2 1.0

H2

(c)

CH4

100

C2H4 C2H6 C2H2

0.8

Toluene conversion ratio

80

0.6 0.4 60 0.08 0.04 0.00

40

Thermal cracking

BAC

DSS char

Catalysts


C.D


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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Figure 6. Hydrogen production ratio

100

Hydrogen production 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

Page 34 of 40

750 °C 850 °C 950 °C

80

60

40

20 0.08 0 Empty reactor

BAC

DSS char

C.D

Catalysts


NiO/γγ-Al2O3

Page 35 of 40

Figure 7. Lower heating value of the product gas

Lower heating value of the syngas (kJ/Nm3)

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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140 120 100 80 60

750 °C 850 °C 950 °C

40 20 0 Empty reactor

BAC

DSS char

C.D

Catalysts


Ni-based catalyst

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Figure 8. Surface characteristics of BAC by SEM and EDS. (a) Before test, (b) After test at 950 °C

(a)

C: 6.31 wt.%

50µm (b)

C: 16.48 wt.%

50µm


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Figure 9. Surface characteristics of DSS char by SEM and EDS. (a) Before test, (b) After test at 950 °C

(a) C: 15.97 wt.%

50µm (b) C: 27.56 wt.%

50µm


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Figure 10. Main reaction paths of toluene cracking.16,24 (a) Thermal cracking, (b) and (c) Catalytic cracking


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(c)

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Chemical adsorption Active sites H2, coke, CxHy,...

Coke

Catalyst

Calcined dolomite; NiO/γ-Al2O3; BAC.

CO,CO2,... Cracking of FG

Functional groups(FG)

Formation of FG


DSS char

Cracking of Model Tar Species from the ... - ACS Publications

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