Article pubs.acs.org/EF
Effect of Cellulose and Polyvinyl Chloride Interactions on the Catalytic Cracking of Tar Contained in Syngas Qunxing Huang, Yijing Tang, Shurong Wang,* Yong Chi, and Jianhua Yan State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: Tar contained in gasification syngas is one of the most problematic species that restricts the gasification efficiency and system availability. The catalytic decomposition of tar is a novel technology to clean the produced syngas. In this article, the interactions of cellulose and polyvinyl chloride (PVC) on the catalytic cracking of tar contained in municipal solid waste gasification syngas were experimentally studied. Results show that at the pyrolysis temperatures of 500 and 600 °C the catalytic cracking efficiency was 91−98% for tar derived from a single feedstock. When cellulose was mixed with PVC, the tar cracking efficiency decreased to 68−90% because of the interactions during the copyrolysis and decomposition reactions. Gas chromatography (GC) and gas chromatography−mass spectrometry (GC−MS) were employed to identify the main compositions of gaseous product and hydrocarbon species after cracking. The GC−MS results show that 99% of tar species have a carbon number less than 10 when the calcined dolomite catalyst was used. During the copyrolysis, PVC plays a dominant role in the formation of tar. The GC analysis shows that copyrolysis yields more CH4, C2H4, C2H6, and C2H2 and less CO than those from a single feedstock. Brunauer−Emmett−Teller, scanning electron microscopy, energy dispersive spectroscopy, and X-ray fluorescence analyses were used to characterize the catalyst before and after reaction. Results indicate that after mixing, the poisoning of catalyst by chlorine increased by 3−15 times, accounting for the low conversion ratio of tar derived from a mixture of cellulose and PVC.
1. INTRODUCTION As the most promising and environmental friendly thermal treatment method, pyrolysis/gasification has gained extensive interest and research for energy and resource recovery from municipal solid waste (MSW). However, due to the diversity of the feedstock, the syngas from gasification contains unacceptable high levels of various impurities, especially the liquid tar, which can cause serious operational problems in downstream facilities by blocking gas coolers, filter elements, or engine suction channels.1,2 In the past few years, considerable efforts have been devoted to tar removal from syngas using methods, such as thermal cracking, catalytic cracking,3 nonthermal plasmas,4 and mechanistic methods.5 As catalytic decomposition can increase gasification efficiency,6,7 it has been demonstrated to be one of the most effective methods and extensively reported in the literature.8 The catalysts used in tar reforming consist of metallic catalysts, mainly nickelbased catalysts,9,10 alkali catalysts, dolomites, olivine,11 or a combination of metals on mineral substrates.12 Luo et al.13 have investigated steam gasification with and without catalyst (NiO/γ-Al2O3 and calcined dolomite). They found there was a remarkable increase (45% and 26.5%) in gas content, and tar was not detected with the presence of catalyst. The catalysts have significantly improved the cracking of tar and the reforming of hydrocarbons to generate valuable gases. Li et al.14 developed a novel supported trimetallic catalyst (nano-NiLaFe/γ-Al2O3), which was promising for MSW steam gasification, showing more than 99% tar removal and an increased hydrogen yield. However, catalysts introduce operational challenges because of the reductions of catalyst activity, which are caused by poisoning, fragmentation, or carbon deposition typically.15 The main cause differs according to the feeding materials. Carbon deposition has been identified as the main cause of catalyst deactivation during biomass gasification,16 and poisoning is known as a serious problem if the feedstock contains chlorine or sulfur.17 © XXXX American Chemical Society
Although the catalytic decomposition of tar has been intensively studied, few researchers have concerned the interaction effects of feeding materials on the catalytic cracking of tar or the deactivation of catalysts, although these interactions have been demonstrated during pyrolysis and gasification in many recent researches. Matsuzawa et al.18 have investigated the pyrolysis of cellulose and polymers, including polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyvinylidene chloride (PVdC) by thermal analysis. Cellulose was shown to interact with PVC and PVdC during pyrolysis. In addition, the interactions of three MSW components (rice, poplar wood, and PVC) during pyrolysis were investigated using the technology of thermogravimetric analyzer coupled with Fourier transform infrared spectrometer (TG-FTIR) by Zhou et al.,19 and the interactions between PVC and rice or poplar wood were strong, whereas the interaction between rice and poplar wood was negligible. Moreover, Liu et al.20 have also investigated the pyrolysis of a mixture of hemicellulose, cellulose, and lignin by TG-FTIR. The results demonstrated the interactions among these components, especially the influence of lignin to hemicellulose and hemicellulose to cellulose. Previous research has shown that major combustible components of MSW are fossil fuel hydrocarbons, such as plastics, and renewable hydrocarbons, such as cellulose.18,21 For plastics, PVC attracts the highest attention because it is the most widely used halogen-containing polymer, accounting for half the hydrogen chloride emitted from waste incineration.22 The annual production of PVC was estimated to be more than 35 million tons in 1997, increasing globally by 3.8% per annum to 500 million tons Received: February 24, 2016 Revised: May 24, 2016
A
DOI: 10.1021/acs.energyfuels.6b00432 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Ultimate and Proximate Analyses (wt%) of the Model Compounds ultimate analysis
proximate analysis
compounds
Cad
Had
Nad
Stad
Oad
Clad
Mad
Aad
Vad
FCad
Qgr,ad(kJ/kg)
cellulose PVC
40.0 39.1
5.9 4.1
0.1 0.1
0.2 0.8
47.5 0.0
0.0 38.6
6.2 0.8
0.1 16.4
88.4 66.9
5.3 15.9
16284 19211
in 2012.23,24 As the thermal treatment of PVC will lead to release of hazardous chlorinated compounds and dioxins at elevated temperatures, the disposal of PVC by thermal methods, such as energy reclamation (incineration) and thermal degradation, is known to cause catalyst poisoning and increase capital costs (due to corrosion of plant equipment).25 Hence, cellulose and PVC were chosen as the principal components of MSW for studying the effect of feeding material interactions on catalytic tar cracking. In this study, the catalytic cracking of tar derived from cellulose, PVC, and their mixtures in weight ratios of 3:1, 1:1, and 1:3 at the pyrolysis temperatures of 500 and 600 °C was investigated. Calcined dolomite was used as the catalyst and the catalyst bed temperature was set as 850 °C.26,27 The properties of dolomite before and after use were respectively examined by Brunauer−Emmett−Teller (BET), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray fluorescence (XRF) analyses to assess the effect of cellulose and PVC interactions on cracking behavior. The composition of final tar after decomposition was analyzed by gas chromatography− mass spectroscopy (GC−MS), and the gaseous product composition was analyzed by gas chromatography (GC). The results of this paper provide useful information for tar catalytic cracking and MSW sorting.
composition of PVC ash was analyzed via EDS and presented in Table 2. It can been discovered that the major elements in PVC ash are Ca, O, Cl, C, and Si, which take up more than 95% by weight. The dolomites were activated via calcination at 900 °C for 4 h with a N2 flow at 100 mL/min. Calcined dolomite was ground into 3−6 mm particles. SEM analysis of dolomite was carried out using a SIRON instrument from the Dutch FEI company. EDS was performed using an EDAX−TEAM instrument and XRF was performed using an ARL ADVANT’X 4200 instrument (Thermo Fisher Co., USA) equipped with a rhodium X-ray tube to examine the element composition. In addition, the specific surface area, pore volume, and pore size of the calcined dolomite were measured by N2 adsorption at −196 °C on an Autosorb-1-c instrument (Quantachrome Co., USA). The results are shown in Table 3. 2.2. Experimental Setup. Catalytic decomposition test of pyrolysis tar was performed in a quartz tubular furnace 1200 mm in length and 32 mm in inner diameter. The horizontal chamber of the furnace was separated into two parts acting as a two-stage fixed-bed reactor. Pyrolysis took place in the first part and tar species carried by N2 at 0.1 L/min were then decomposed in the second part. The reaction temperatures of these two zones were controlled by external electrical heat sources with individual temperature controllers. The pyrolysis temperatures were set as 500 and 600 °C, and the catalytic bed temperature was 850 °C. The gas velocity was 0.85 cm/s in the second zone. About 4 g of sample was used for each test, and during the cracking test, 40 g of calcined dolomite was filled in the cracking zone with a bed length of 3−5 cm for an enough contact time of 3.5−5.9 s. Quartz wool was filled downstream the catalyst to avoid fine particulates from flowing into the follow-up apparatus. In each experimental run, both parts of the reactor tube were heated to the target temperatures and maintained for 10 min. Then the sample was pushed into the first zone where pyrolysis took place. After 20 min, the final gas products were entrained to a flask coupled with a reflux condenser. Remaining tar (with molecular weights higher than benzene)1,28 species were extracted from the liquid products and weighed. The noncondensed (gaseous) products were collected with syringes every 2 min. Each test was performed at least 3 times to reduce experimental uncertainty. The composition of tar was measured by a GC−MS analyzer (Trace GC, ISQ MS, Thermo Scientific Co.) equipped with a TR-5MS capillary column (30 m length, 0.25 mm inner diameter, and 0.25 μm film thickness). The injector and the transfer line were set at 270 and 250 °C, respectively. The oven was maintained at 50 °C for 5 min, increased to 270 °C at a rate of 15 °C/min, and then maintained at 270 °C for 10 min. In the mass spectrometer, electron ionization (EI) energy was used for ionization. The ion source temperature was maintained at 200 °C. The volume of each injection was 0.2 μL and the split ratio was 10:1. The produced gas, which mainly comprised H2, CH4, CO, CO2, C2H4, C2H6, and C2H2, was quantitatively analyzed by Agilent 490 Micro GC.
2. EXPERIMENTAL SECTION 2.1. Materials. Powder cellulose particles below 125 μm was purchased from Henan province, China. Raw PVC materials obtained from Zhejiang province were sieved into particles below 300 μm. Before the test, the materials were oven-dried at 105 °C for 6 h to remove moisture. The ultimate and proximate analyses are listed in Table 1. Elemental
Table 2. EDS Analysis of Ash in PVC element
C
O
Mg
Al
Si
Cl
Ca
Ti
weight content (%)
11.2
23.4
0.9
1.6
8.9
18.0
34.1
1.9
Table 3. Properties of the Calcined Dolomite properties
calcined dolomite
elemental composition (wt%)
Ca Mg O Fe
35.9 23.5 40.3 0.3 22.3 0.22 40.2
BET surface area (m2/g) pore volume (cm3/g) average pore size (nm)
Table 4. Tar Cracking Ratio tar cracking ratio (%) cellulose: PVC 3:1
1:1
1:3
temperature (°C)
cellulose
exp.
cal.
exp.
cal.
exp.
cal.
PVC
500 600
98.0 ± 0.5 98.0 ± 0.5
70.0 ± 0.4 68.4 ± 0.3
97.4 ± 0.4 96.3 ± 0.5
89.2 ± 0.3 85.9 ± 0.2
96.8 ± 0.4 94.7 ± 0.5
80.5 ± 0.4 69.8 ± 0.3
96.2 ± 0.4 93.0 ± 0.4
95.7 ± 0.5 91.3 ± 0.4
B
DOI: 10.1021/acs.energyfuels.6b00432 Energy Fuels XXXX, XXX, XXX−XXX
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600 C
26.4 ± 0.2 20.5 27.4 ± 0.1 20.5 37.5 ± 0.3 27.5 10.0 ± 0.2 7.2 36.1 ± 0.1 52.0 62.6 ± 0.3 72.3 21.9 ± 0.3 14.3 20.2 ± 0.2 14.3 37.5 ± 0.2 22.1 17.5 ± 0.2 9.1 40.6 ± 0.4 63.7 62.3 ± 0.1 76.7
29.6 ± 0.3 22.7 29.7 ± 0.3 22.7 37.5 ± 0.3 29.7 10.0 ± 0.2 7.5 32.9 ± 0.3 47.7 60.3 ± 0.2 69.9
1:1 8.0 ± 0.2 / 8.0 ± 0.1 / 16.7 ± 0.2 / 11.0 ± 0.2 / 75.3 ± 0.2 / 81.0 ± 0.3 / 10.7 ± 0.1 / 10.7 ± 0.1 / 21.0 ± 0.2 / 13.3 ± 0.2 / 68.3 ± 0.2 / 76.0 ± 0.1 /
24.5 ± 0.2 16.7 25.0 ± 0.2 16.7 35.0 ± 0.2 25.3 17.5 ± 0.2 10.4 40.5 ± 0.4 58.0 57.5 ± 0.1 72.9
3:1 500
catalytic
non gas
catalytic
non liquid
solid
non
catalytic
exp. cal. exp. cal. exp. cal. exp. cal. exp. cal. exp. cal.
cellulose
Table 5. Experimental and Calculated Mass Fractions of Reaction Products (%)
600
500
cellulose: PVC
600
500
where Xcellulose and XPVC are the cracking ratio of single cellulose and PVC, respectively, wcellulose and wPVC are their weight percentages, and X(cal) is the linear weighted combination of Xcellulose and XPVC. As listed in Table 4, the conversion ratios of tar derived from cellulose or PVC were higher than 90% at 500 and 600 °C. However, the conversion ratios of tar derived from the 3:1, 1:1, and 1:3 mixtures of cellulose and PVC decreased from 70%, 89.2%, and 80.5% at 500 °C, respectively, to 68.4%, 85.9%, and 69.8% at 600 °C, respectively. It can be discovered that the tar derived from a single feeding material showed a significantly higher conversion ratio than those generated from the mixtures. The experimental results of cracking ratios were approximately 7−30% lower than the calculated values, indicating that copyrolysis reduced catalytic cracking efficiency. When the weight ratio of cellulose to PVC changed from 3:1 to 1:1 to 1:3, the tar conversion ratio increased first and then decreased. The highest conversion ratio was for the 1:1 weight ratio of cellulose to PVC, whereas the conversion ratio was the lowest for the cellulose to PVC ratio of 3:1. In addition, the deviations between experimental results and calculation values became larger at the pyrolysis temperature of 600 °C, indicating that the interactions are more remarkable at a higher temperature. Mass fraction (wt%) of reaction products of solid and liquid can be calculated by the ratio of solid residues and liquid products to feeding material. The gaseous product was obtained by mass balance, and the product distributions are listed in Table 5. Interactions were observed because the yields for solid and liquid products are higher than the linear combination of yields from single feedstocks, whereas the gaseous product shows the opposite result. As shown in our previous research, the copyrolysis of cellulose and PVC has the potential to reduce the tar yield.30 So thermal cracking has led to an opposite result, indicating that tar derived from cellulose is easily decomposed at a high temperature, whereas tars derived from PVC and mixture are not. In addition, the yields of gaseous product at pyrolysis temperatures of 500 and 600 °C were enhanced by an average of 23% and 22%, respectively, for catalytic cases, which were in agreement with Gusta et al.’s report.31 They noted that dolomites improved tar conversion to gaseous products by an average of 21% over noncatalytic results at 750 °C. 3.2. Composition of the Catalytic Decomposition Tar. The main components and C/H ratio of tar were identified by GC−MS. The number of benzene rings was deduced. The results are listed in Tables 6 and 7. About 5−8 components can be identified in the tar after a catalytic reaction. As shown in our previous research as well as other studies, over 70 species can be
32.1 ± 0.4 28.7 33.1 ± 0.1 28.7 37.5 ± 0.3 34.0 10.0 ± 0.1 4.6 30.4 ± 0.1 37.3 56.9 ± 0.1 66.8
1:3
(2)
feedstock
X(cal) = wcelluloseXcellulose + wPVCXPVC
500
where Ptar,C and Ptar are tar yields with and without catalyst. To evaluate the interactions of cellulose and PVC on tar cracking, the measured cracking ratios were compared with those obtained by linear calculations. The expected proportionate results calculated from the pyrolysis of individual single components (eq 2) were used to compare with the experimental results.29
30.7 ± 0.3 26.8 31.1 ± 0.2 26.8 35.0 ± 0.3 32.9 12.5 ± 0.1 5.2 34.3 ± 0.2 40.3 56.4 ± 0.3 68.0
(1)
34.7 ± 0.3 / 34.7 ± 0.3 / 38.3 ± 0.3 / 1.7 ± 0.1 / 27.0 ± 0.1 / 63.7 ± 0.2 /
PVC
× 100
600
Ptar
500
Ptar − Ptar,C
temperature (°C)
X=
600
3. RESULTS AND DISCUSSION 3.1. Catalytic Efficiency. In this article, tar cracking ratio X (%) was calculated using eq 1,
33.0 ± 0.3 / 33.0 ± 0.2 / 38.3 ± 0.3 / 3.3 ± 0.1 / 28.7 ± 0.1 / 63.7 ± 0.3 /
Energy & Fuels
DOI: 10.1021/acs.energyfuels.6b00432 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 6. Components of Tar Derived from Catalytic Reaction relative content (%) cellulose: PVC number
cellulose
3:1
1:1
1:3
PVC
T = 500 °C T = 600 °C T = 500 °C T = 600 °C T = 500 °C T = 600 °C T = 500 °C T = 600 °C T = 500 °C T = 600 °C
components
C
H
benzene ethane, 1,1-diethoxycyclohexene toluene cyclobutene, 2-propenylidenestyrene benzene, 1-ethynyl-4methyl1H-indene, 1-methylenenaphthalene azulene acenaphthylene phenanthrene cyclobutane, 1,2-diphenyl-
6 6 6 7 7
6 14 10 8 8
74.6 15.9
74.6 15.6
2.2
3.4
8 9
8 8
0.6 0.4
10 10 10 12 14 16
8 8 8 8 10 16
5.7
89.0
93.4
94.6
92.9
93.0
94.8
92.9
94.9
3.1 3.6
2.1 2.1
2.4
2.2 2.8
2.8
2.6
2.7
2.6
1.6
1.4
0.7
1.2
1.9 0.6 0.6
0.6 0.3
0.3
1.0 4.6
0.6
0.4
0.4
1.9
1.0
2.5
1.2
0.2
0.3 0.4
0.2
2.1 3.1 0.3
0.5
1.9
0.1
0.2
0.2
Table 7. Analysis of Tar Components cellulose: PVC feedstock
cellulose
temperature (°C) ratio of carbon to hydrogen relative content (%)
number of benzene rings
0 1 2 3
3:1
1:1
1:3
PVC
500
600
500
600
500
600
500
600
500
600
10.1
10.1
11.8
11.9
11.8
11.9
11.8
11.9
11.8
11.8
15.9 83.5 0.6 0
15.6 79.3 5.1 0
6.2 93.5 0.3 0
2.1 95.8 2.1 0
4.3 95.6 0.1 0
4.1 95.7 0.2 0
4.2 95.6 0.2 0
4.0 95.8 0 0.2
3.4 95.9 0.3 0.4
3.8 96.0 0 0.2
detected in the tar before cracking.32,33 Therefore, the thermal and catalytic decomposition of tar can significantly reduce the diversity of tar components. Moreover, the molecular weight of residue tar species after cracking is reduced dramatically. The main components for tar from cellulose were benzene and ethane, 1, 1-diethoxy-, which accounted for more than 90% in total. Between 75% and 85% of tar was one benzene ring component, and the C/H ratio was 10.1. For the tar derived from PVC, benzene and cyclohexene were more than 95% of the components. Moreover, components with one benzene ring took up at least 95% of the tar samples and the C/H ratio was 11.8. When cellulose and PVC were mixed together, the generated tar was affected more by PVC than cellulose because their components and C/H ratios were similar to those of PVC. These tar samples mainly comprised benzene and cyclohexene, and the C/H ratios were in the range of 11.8−11.9. Therefore, during the copyrolysis of cellulose and PVC, PVC plays a dominant role in tar formation. Among the three ratios of cellulose to PVC (3:1, 1:1, and 1:3), the relative contents of components without benzene ring or with only one benzene ring shared the same variation tendency with the tar conversion ratio. It has been reported that although calcined dolomite catalysts show good activity for the decomposition of phenols and oxygenated compounds, they are less effective for the removal of polycyclic aromatic hydrocarbons (PAHs).34−36 When cellulose and PVC were mixed at the ratio of 1:1, the tar after catalytic cracking owned most components with one benzene ring or without a benzene ring, indicating a nearly full conversion. In addition, the number of carbons for tar derived from cellulose was less than 12 while the number of hydrogen
Figure 1. Gaseous product components.
was below 14. For tar derived from PVC, components showed a carbon number less than or equal to 14. Most components of tar derived from the mixtures were distributed in the range of C6 to C10. The carbon number was below 16, indicating an increase in carbon number during the copyrolysis and decomposition. The reason may be that during the copyrolysis reaction, hydrogen chloride (HCl) evolution from PVC affected cellulose pyrolysis through catalysis as a Lewis acid37 and suppressed depolymerization. Therefore, the tar will contain heavier hydrocarbons and be more difficult to decompose, which may contribute to the lower conversion ratio of tar derived from a mixture of cellulose and PVC than those from single feeding materials. D
DOI: 10.1021/acs.energyfuels.6b00432 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 2. SEM and EDS analyses of calcined dolomite (a) before catalytic reaction; (b) after catalytic reaction with the feeding material of cellulose; after catalytic reaction with the feeding material of cellulose and PVC at the weight ratio of (c) 3:1, (d) 1:1, and (e) 1:3; and (f) after catalytic reaction with the feeding material of PVC. (The pyrolysis temperature was 500 °C and the catalytic temperature was 850 °C.)
As the pyrolysis temperature increased from 500 to 600 °C, the components without benzene ring decreased. Simultaneously, the C/H ratio increased, indicating the formation of heavy hydrocarbons, which agreed with the lower conversion ratios at
higher temperature. Comparing the experimental results with calculated C/H ratios and the relative contents of different tar components, the deviations were more distinct at the pyrolysis temperature of 600 °C. In conclusion, the interaction is more E
DOI: 10.1021/acs.energyfuels.6b00432 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 8. Carbon and Chlorine Contents in Dolomites at the Pyrolysis Temperature of 500 °C weight content in dolomite (%) feedstock cellulose: PVC 3:1
1:1
1:3
element
before use
cellulose
exp.
cal.
exp.
cal.
exp.
cal.
PVC
C Cl
2.6 0
3.2 0
2.9 1.6
3.4 0.1
3.7 0.8
3.5 0.2
4.2 1.4
3.7 0.3
3.8 0.4
intense at higher temperature. When cellulose and PVC are pyrolyzed together, heavier hydrocarbons are yielded and PVC plays a crucial role in the formation of tar. 3.3. Gas Composition. The produced gases (H2, CH4, CO, CO2, C2H4, C2H6, and C2H2) from pyrolysis at 500 and 600 °C were analyzed using GC. N2 and O2 were excluded from the syngas, and the gas composition is shown in Figure 1. The presence of dolomite increased the production of H2, whereas CO production decreased notably. The existence of dolomite also promoted the yield of CO2, especially when the feeding material was a mixture of cellulose and PVC. Cellulose produced a high amount of CH4, whereas high amounts of H2 and CO were generated from PVC when they were fed individually. Moreover, for both catalytic and noncatalytic tests, as the pyrolysis temperature increased from 500 to 600 °C, the volume fraction of H2 deceased slightly. However, when the feeding material was a mixture, the volume fraction of H2 increased. Unlike H2, the volume fraction of CO increased with the pyrolysis temperature for various feedstock runs. When cellulose was fed with PVC, the yields of H2 were slightly lower than the linear combination of the yields from the single cases, which showed that the copyrolysis restricted H2 formation. Obvious interaction can be noticed concerning the yields of CH4, CO, CO2, C2H4, C2H6, and C2H2. When the mixture ratios were 3:1, 1:1, and 1:3, the yields of CH4, C2H4, C2H6, and C2H2 were greater than their linear calculations, among which the increases of volume fraction of C2H4, C2H6, and C2H2 were more than 100% compared with those of the calculation results, showing a considerable acceleration of their formations. However, compared with linear calculations, the yield of CO decreased by 15−35%, indicating that the mixture of cellulose and PVC may inhibit CO production. 3.4. Catalyst Analysis. The morphologies of calcined dolomites before and after the catalytic reaction were examined by SEM. Figure 2a shows that calcined dolomite before reaction had a dense surface with randomly distributed irregular particles. Moreover, the calcined dolomite showed many fine and sharpedged particles on the dolomite crystals. These particles were generated from the breakdown of larger grains to small fine grains during the calcination process. Figure 2b−f illustrates the scanning electron micrographs of the used dolomite catalysts at the pyrolysis temperature of 500 °C and the catalytic temperature of 850 °C. In these figures, spherical coke particles are distributed densely upon the dolomite surface. The continuous accumulation of coke particles will cause carbon deposition on surface during processing and hinder the catalytic decomposition efficiency. The tendency of carbon to deposit depends on the nature and properties of the surface species. The carbon species can either react with water to form gas products (hydrogen, carbon monoxide, and carbon dioxide) or undergo a series of reactions leading to solid coke deposition on catalyst surface. Table 8
shows the carbon and chlorine content in dolomites examined by EDS and XRF. Compared with dolomite before use, the changes in carbon contents of used dolomite samples were not obvious, indicating that carbon deposition was relatively slight in this case. When cellulose and PVC were mixed, the chlorine contents of the used catalyst increased by around 3−15 times than the linear combination of those from the single cases. This result is in accordance with the cracking ratio as the lower cracking ratio and the higher chlorine content present in the dolomite. Therefore, the copyrolysis of cellulose and PVC accelerates the poisoning of dolomite by chlorine and this finding may account for the high conversion ratio for tar derived from single feeding materials and the much lower efficiencies for that derived from their mixtures.
4. CONCLUSION The understanding of interactions during copyrolysis on the catalytic cracking of tar and catalyst will benefit the source sorting of different MSW components and help optimize gasification systems. In this study, cellulose and PVC were chosen as principal components of MSW, and their interactions on pyrolysis tar cracking were experimentally investigated in a two-stage fixed-bed reactor. The results showed that the copyrolysis of their mixture leads to an obvious reduction of catalytic tar cracking efficiency. The GC−MS analysis of catalytic decomposition tar showed that the carbon numbers of 99% of the tar species were below 10. During the copyrolysis of cellulose and PVC, PVC played a dominant role in the formation of tar. Their interactions on the cracking ratio and tar components became stronger at the higher temperature (600 °C) than those at 500 °C. EDS and XRF results indicated that the poisoning of catalyst was very serious during copyrolysis, whereas the carbon content was almost unchanged from single to mixed conditions. After mixing, catalyst poisoning caused by chlorine increased by 3−15 times compared with the linear combination of those from the single cases, indicating that poisoning was the key factor for the deactivation of dolomite rather than carbon deposition. To conclude, the interaction between cellulose and PVC promotes the production of heavier hydrocarbons on one hand and, on the other hand, it accelerates the poisoning of dolomite. Both contribute to the low cracking efficiency of tar.
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AUTHOR INFORMATION
Corresponding Author
* Telephone: +86-571-87952801; E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Acknowledgment is gratefully extended to the Environment Protection Special Funds for Public Welfare (201509013), and the Project “Experimental study of efficient upgrading technology for F
DOI: 10.1021/acs.energyfuels.6b00432 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
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syngas derived from municipal solid waste” with Covanta Energy, LLC for their financial support.
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DOI: 10.1021/acs.energyfuels.6b00432 Energy Fuels XXXX, XXX, XXX−XXX
