A Mechanism Study on Hydrothermal Carbonization of Waste Textile

Aug 5, 2016 - In this work, waste textile (WT) was employed as one representative pseudo-component of municipal solid waste (MSW) to investigate the ...
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A Mechanism Study on Hydrothermal Carbonization of Waste Textile Yousheng Lin,* Xiaoqian Ma, Xiaowei Peng, and Zhaosheng Yu Guangdong Key Laboratory of Efficient and Clean Energy Utilization Institutes, School of Electric Power, South China University of Technology, Guangzhou 510640, People’s Republic of China S Supporting Information *

ABSTRACT: In this work, waste textile (WT) was employed as one representative pseudo-component of municipal solid waste (MSW) to investigate the mechanisms during hydrothermal carbonization (HTC) process. The experiments were examined at 230 and 280 °C with the residence time from 30 min to 90 min. The chemical component analysis showed that a significant fraction of fixed carbon was retained within the hydrochar, while ∼98% volatile matter was detected in the soluble fraction. Our results clearly demonstrate that decarboxylation was the most important defunctionalization process, whereas dehydration turned out to be less important. The combustibility index (S) and the combustion stability index (Rw) of the hydrochar were both greater than those of WT, suggesting hydrochar was superior in combustion performance. Fourier transform infrared (FTIR) analysis and solid-state 13C nuclear magnetic resonance (NMR) characterization displayed that large amount of aliphatic compounds decomposed, while enhancements of aromatic and carbonyl types carbons were observed in hydrochar and soluble. Higher temperature enhanced the breakage of unreacted feedstock to fragment and enforced the aromatization and repolymerization reactions to form the solid char and liquid phase. Between char and liquid phase, recombined, polycondensation and repolymerization reactions progressively occupied the dominant effects. These reactions promoted char and oil-range molecules to form hydrochar. On the basis of these outcomes, a conversion pathway of hydrothermal carbonization of WT was proposed.

1. INTRODUCTION In the past few years, considerable attention has been paid to produce hydrochar from hydrothermal carbonization (HTC) of wet waste biomass, such as waste biomasses,1−5 municipal solid waste,6−8 sludge,8−10 etc. Because of its simplicity to perform, the elimination of an energy-extensive drying process, the wide availability of raw materials, and fewer greenhouse gas emissions,9,11 HTC has been suggested as a relatively novel and promising technique to treat high-moisture solid materials.11,12 HTC occurred at relatively low temperatures (180−300 °C)4 under autogenous pressures in hot compressed liquid water for times ranging from 30 min to several hours,13 simultaneously generating gas, a liquid phase, and a carbon-rich solid residue termed “hydrochar”.1 During the HTC process, liquid water acts both as a reaction medium and a catalyst, because it shows a maximum ionic strength at ∼180−280 °C, enhancing dissolution of the feedstock and recondensation into the solid.14 The output of this process is a high-carbon-energydensity hydrochar (coal-like fuel),15 and it possesses a low moisture content8 and easier handling, transport, and storage, providing a more suitable solid fuel for further thermochemical conversion.16 On the other hand, 171 × 106 tons of municipal solid waste (MSW) were collected in China in 2013, and this number will increase at an annual rate of 8%−10%.17 Thus, both challenges and opportunities must be performed eagerly to implement MSW minimization and obtain energy-efficient utilization. The development of HTC technologies, such as a simulated natural coalification process,5 offers a new way to dispose of MSW.6,8 Previous works have distinctly verified that HTC was an environmentally advantageous process for treating MSW to value-added products, approaching the characteristics of low-rank natural coals.7,8,18,19 The energy could be recovered © XXXX American Chemical Society

by combusting or co-combusting MSW hydrochar with coal.8,10,13,20 It is well-known that complex reaction mechanisms occur during HTC, including hydrolysis of the feedstock, followed by dehydration, and then decarboxylation, condensation, polymerization, and aromatization.8,13,18,21 Although more-detailed reaction pathways are largely unknown, with the exception of the HTC of relatively single substances.10,21−23 He et al.10 proposed specific formation pathways of sludge hydrochar during HTC. Funke et al.21 summarized the HTC mechanism for solid biofuel of pure carbohydrates. Reza et al.24 determined that the degradation of selective hydrolyzed compounds followed first-order reaction kinetics during the HTC process. Wu et al.22 demonstrated kinetics and mechanism of hydrothermal decomposition of lignin model compounds. He et al.23 speculated general reaction pathways for proteinaceous wastes and plastic wastes. Unfortunately, the organic components of MSW are very complicated and varied with the seasons and regions.25 It is difficult to investigate the HTC reaction mechanisms of all real MSW components or units in detail. However, there are few studies on exploring the hydrothermal reaction of single representative or specific fractions of MSW.7,23,26 These results are only meaningful and reproducible for specific fractions of MSW. The research about specific formation pathways of MSW hydrochars is still very scarce. Therefore, the mechanisms of hydrothermal treatments of MSW are also unclear. To optimize the HTC conditions of MSW and develop this method, an in-depth understanding of Received: June 6, 2016 Revised: August 5, 2016

A

DOI: 10.1021/acs.energyfuels.6b01365 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Chemical Properties of Waste Textile (WT), Hydrochar, and Soluble Fraction on a Dry Basis (%) Ultimate Analysis (%)

Proximate Analysis (%)

sample

C

H

N

S

O

volatile matter

fixed carbon

ash

higher heating value, HHV (MJ/kg)

raw WT H-230−30 H-230−60 H-230−90 H-280−30 H-280−60 H-280−90 S-230−30 S-230−60 S-230−90 S-280−30 S-280−60 S-280−90

50.29 54.43 55.43 56.64 58.68 59.24 60.15 49.92 48.49 48.72 50.59 50.17 49.84

5.19 5.12 4.77 4.24 3.99 4.08 4.12 5.93 6.36 6.80 6.70 6.71 6.99

1.98 1.86 0.36 0.22 0.57 0.62 0.69 1.65 3.10 3.43 3.72 4.25 4.51

0.47 0.09 0.04 0.01 0.04 0.00 0.00 0.68 0.78 0.85 0.94 1.01 1.03

41.79 38.07 38.86 38.31 36.12 35.47 34.41 40.69 40.24 39.01 36.60 36.05 35.50

81.13 77.23 74.81 73.65 74.11 73.04 74.35 98.50 98.71 98.65 98.50 98.13 97.75

18.59 22.33 24.65 25.77 25.28 26.37 25.01 0.37 0.26 0.17 0.05 0.06 0.12

0.28 0.44 0.54 0.58 0.61 0.59 0.64 1.13 1.03 1.18 1.46 1.82 2.13

19.36 21.11 21.08 20.89 21.57 21.89 22.55 20.03 19.51 19.56 20.41 20.43 20.28

oven-dried at 105 °C for 24 h. The fraction still dissolved in the water was referred as soluble and recovered as a solid through evaporating the filtrate at 65 °C to remove the water by a evaporator.28 Each experiment was conducted in triplicate to ensure reproducibility and consistency, and the experimental errors were within 3%. The product was labeled using a nomenclature format of “x-xxx−xx”, where “x” represents hydrochar and soluble, “xxx” represents the HTC temperature, and “xx” represents the residence time. For example, “H-230−30” and “S-230−30” respectively represent the hydrochar and soluble obtained at 230 °C for 30 min. 2.3. Product Analyses. The gaseous phase composition was quantitatively analyzed using a gas chromatography system (Agilent, Model 7890A GC) that was equipped with a thermal conductivity detection (TCD) device. The elemental composition was determined by an Elementar (Vario EL cube). The higher heating values (HHVs) of the samples were measured using an oxygen bomb calorimeter. The proximate analysis and decomposition behavior of solid products were performed using a thermogravimetric analysis (TGA) system (Mettler−Toledo). The sample was heated from ambient temperature to 1000 °C in an air atmosphere at a flow rate of 100 mL/min, with a heating rate of 20 °C/min. Fourier transform infrared (FTIR) spectrometry (Nicole, Model iS10) was employed to analyze various functional groups contained in the samples. The FTIR spectra were collected using a mixture of sample and KBr in a sample/KBr ratio (w/ w) of 1:50. The chemical structure of solid products was analyzed through a 13C superconducting nuclear magnetic resonance (NMR) spectrometer (Bruker, Model Avance III HD 400). To evaluate the carbon skeleton structures, Gaussian multipeaks fitting of 13C NMR curve was employed to obtain the relative percentage of each type of carbon (see the Supporting Information).

this process mechanism is distinctly needed. To overcome the above-mentioned difficulties and separate interactions between components, we try to establish a representative pseudocomponents simulation system during the HTC of MSW. Each representative pseudo-component will be investigated separately. The proposed HTC conversion mechanisms of each representative pseudo-component will be discussed. Subsequently, reaction pathways and synergistic interactions between pseudo-components will also be researched further. Through the representative pseudo-components simulation system, envisaged integrated and systematic reaction mechanisms during the HTC of complicated MSW or mixtures will be speculated. MSW is a complex mixture and usually divided into six compnents in China, including food residue, wood waste, paper, plastics, textiles, and rubber.25 In our current research, the waste textile was first chosen to be investigated. In reviewing the recent literature, such an investigation has not been sufficiently performed. The nature cotton and chemical fiber are most commonly contained in waste textiles. The specific objectives of this work include (1) evaluating the hydrochar fuel properties of waste textile and (2) speculating the possible detailed reaction mechanisms during the HTC of waste textile.

2. MATERIALS AND METHODS 2.1. Materials. Our preinvestigation showed that the actual proportion (as-received basis) of waste textile in MSW ranged from 3.16% to 11.09% in China. The waste textile (abbreviated as WT) was collected on the campus (Guangzhou, China), which contained natural cotton and synthetic fiber. Before the experiment, the raw WT was carefully cut into shreds by a scissor. The proximate analysis and elemental compositions of WT is presented in Table 1. 2.2. Equipment and Procedures. HTC experiments were carried out using a 250 mL autoclave reactor.27 The reactor was filled with 5 ± 0.1 g of raw WT and 100 ± 1 mL of deionized water. Before each test, nitrogen was flushed within the reactor for 2 min to purge it from the presence of air. The reactor then was heated up to the desired temperature (230 and 280 °C) at a heating rate of ∼4 °C/min and maintained for a designated residence time (30, 60, and 90 min). The stirring rate was set to 500 rpm throughout the reaction process. After reaction, the reactor was rapidly cooled to room temperature with water and an electric fan. The pressure release valve was opened to collect the gaseous products, using an aluminum foil bag. The fraction precipitated as a solid was termed hydrochar and recovered by filtration. The liquid products and hydrochar were separated by filtration through a preweighted fiberglass filter paper (0.47 μm), then

3. RESULTS AND DISCUSSION 3.1. Ultimate and Proximate Analyses. Figure 1 presented the appearance of the raw WT, hydrochars, and anthracite coal (for a comparison) from different reaction temperatures and residence times. After being subjected to the HTC process, the volume of hydrochar obviously shrank, compared to raw WT. Besides, it appeared that hydrochar became powderlike, and the color varied from light (H-230− 30) to darker (H-280−90). Furthermore, the uniformity of hydrochar was clearly observed. These features of hydrochar enhanced the potential to be employed in thermal utilization as an alternative fuel. Meanwhile, Figure S1 in the Supporting Information displays the process filtrate (precursor of soluble) under different HTC conditions. The latter five process filtrates were much darker, compared to the first one (230 °C, 30 min). This implied that the HTC reaction was moderate in the first 30 min at 230 °C. In addition, the system seemed more reactive B

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detoxification of HTC. As expected, the mass of fixed carbon in the hydrochar increased, which ranged from 22.33% to 26.37%. The partial enhancement in yields of fixed carbon was attributed to the reduction in volatile matter. In addition, the enhancement of the polymerization reaction due to severe conditions led to the increased fixed carbon content in the hydrochars.9 In terms of soluble fraction, the yield of volatile matter reached ∼98%. However, an extremely small fixed carbon content was detected in the soluble fraction. Thus, the HTC process could effectively recover the carbon into the hydrochar. In addition, because of the heat and compression, the ash was concentrated in liquid phase during HTC. As a result, the ash contents of soluble were higher than that of WT and hydrochar. A Van Krevelen diagram (H/C vs O/C) allows for delineation of the reaction pathways.31 Figure 2 presents the

Figure 1. Appearance of raw WT and hydrochars at different HTC temperatures and residence times.

with increasing residence time or increasing temperature, promoting feedstock transform to an aqueous fraction. Meanwhile, as displayed in Figure S2 in the Supporting Information, mass balances also confirmed this trend. After the process filtrate was dried, a black semisolid state product was generated. Figure S3 in the Supporting Information exhibited the appearance of soluble S-280−90, which was a type of soft and sticky product. Table 1 shows the proximate analysis, ultimate analysis, and HHV of all samples. It could be observed that the carbon contents of all hydrochars were higher than that of raw feedstock (50.29%). The carbon contents of hydrochars ranged from 54.43% to 60.15%. In contrast, except for S-280−30 (50.59%), the carbon contents of solubles were slightly smaller than that for WT. This indicated that the HTC of WT resulted in a significant fraction of carbon being retained within the hydrochar. Besides, the oxygen contents of hydrochars decreased from 41.79% (WT) to 34.41% (H-280−90). This was also the case for the solubles. This clearly demonstrated that the raw WT was indeed deoxygenated. It also could be found that the oxygen content of solid product (both hydrochar and soluble) obtained from more severe conditions was lower. However, when it came to the hydrogen, nitrogen, and sulfur content, hydrochar and the soluble fraction presented the opposite variation tendency. As shown in Table 1, the hydrogen, nitrogen, and sulfur contents of hydrochar were all smaller than WT. However, for solubles, these contents all displayed a gradually increasing variation. Note that the nitrogen and sulfur contents of the hydrochar were much lower than those of WT and the solubles. In terms of nitrogen, most of the nitrogen was originated from proteins contained in natural cotton, as a composition of WT. The published investigations had reported the organic nitrogen in protein was decomposed into an ammonium-N presence in the soluble phase during the HTC process.8,9,29 This also applied to sulfur content. The organic sulfur content contained in natural cotton was degraded into a liquid phase, such as aliphatic sulfur.30 Prolonging the residence time or raising the reaction temperature could promote the removal of sulfur from hydrochar to solubles. Therefore, the reduction of nitrogen and sulfur content in hydrochars implied hygienization/

Figure 2. Van Krevelen diagram of raw WT and hydrochar.

Van Krevelen diagram of raw WT and hydrochar, in comparison with four types of coal. As it is illustrated, hydrochars obtained from different operating conditions were similar to that of lignite, which had lower atomic H/C and O/C ratios. For the soluble fraction (Figure S4 in the Supporting Information), the O/C ratios were slightly lower than that of WT, while the H/C ratios were much higher than that of WT. Thus, the solubles were far away from the coal area. The coal ranks of hydrochars were upgraded by removing low-grade energy substance (soluble) from raw WT. It could be speculated that the HTC can discard the dross and select the essence of raw feedstock. As shown in Figure 2, the formation of hydrochar was predominantly governed by the dehydration reaction (lower H/C) and decarbonylation/decarboxylation (lower O/C), while the demethylation reaction had a relatively small effect. This finding was similar to that observed for the HTC process of paper sludge, mixed MSW, and biomass.21,27,32 CO2 detection was an indirect way to prove the existence of the decarboxylation reaction. Figure 3 showed the noncondensable gaseous phase composition proportion (%) under different HTC conditions. The GC analysis results revealed that CO2 and CO were the dominant gaseous species, which occupied ∼95%−100% of the volume. This insinuated that a decarboxylation reaction was occurring during the HTC of WT. At 230 °C with 30 and 60 min, almost all the gases were composed of CO2 (∼90%) and CO. As the temperature increased to 280 °C, the CO2 production decreased C

DOI: 10.1021/acs.energyfuels.6b01365 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. Gaseous phase composition proportion (%) under different HTC conditions.

significantly while the CO production increased. A possible explanation to this phenomenon could be that the decarboxylation reaction was favored at low temperatures, while high temperatures had a tendency to favor decarbonylation.33 Because of the dehydration and decarboxylation reactions, most of the oxygen atoms were removed. The remaining 5% were mostly CH4 and C2H4, especially for 280 °C. This suggested that the demethylation reaction was sensitive to temperature. Because of the dehydration reaction, the enhancement in carbon content, the decreased oxygen content, and the carbonization effect, the HHV value of hydrochar was upgraded after the HTC process, as given in Table 1. As the residence time and temperature increase, the HHV of hydrochar increased from 21.11 MJ/kg (H-230−30) to 22.55 MJ/kg (H-280−90). 3.2. Combustion Behavior Analyses of WT and Hydrochar. Figure 4 displayed TG and DTG curves of raw WT and hydrochars at a heating rate of 20 °C/min. Table 2 presents the combustion characteristic parameters of all samples. The method how to define the combustion parameters, such as ignition (Ti), burnout temperature (Tf), combustibility index (S), and combustion stability index (Rw) could be found in the Supporting Information. According to Figure 4b, there were three peaks for WT during the entire combustion process. These three peaks respectively corresponded to three combustion stages: the combustion of light volatile components (325−379 °C), the combustion of heavy volatile components (379−483 °C), and the combustion of fixed carbon (483−688 °C). Because of the high volatile matter content in WT, the two front peaks, which were centered at 348 °C (−17.03%/min) and 440 °C (−18.44%/min) were more remarkable than the later one at 551 °C (−3.21%/min). After the HTC process, strikingly differences in the TG and DTG profiles were observed for hydrochar. As shown in Table 2, the Ti values of all three hydrochars obtained at 230 °C were lower than WT (325 °C). However, the Ti values of all three hydrochars obtained at 280 °C were higher than that of WT. Unlike Ti, all Tf values were lower than that of WT. This phenomenon indicated that the HTC process will make the combustion curve of hydrochar shift to the low-temperature combustion zone. As presented in Figure 4b, apart from H-230−30 and H-230−60, the DTG curves of other hydrochars could be divided into two regions.

Figure 4. Thermogravimetry (TG) and differential thermogravimetry (DTG) combustion curves of (a) waste textile (WT) and hydrochar, and (b) WT and solubles.

In addition, each combustion peak center and peak value was quite distinctive, when compared to WT. The DTG curve shape of H-230−30 was similar to that of WT. However, all three peak temperatures were lower, compared to WT. As the residence time increased, the distinct peak attenuated at 370− 480 °C for H-230−60 and almost disappeared for H-230−90 and H-280-xx (where “xx” refers to 30, 60, and 90 min, the same as that described below). This result suggested that the heavy volatile components contained in WT would decompose into simpler chemical substances at long residence times or high reaction temperatures. As is known, the main components of waste textile are cotton and polyester fiber or synthetic fiber,34 which consists of cellulose, hemicelluloses, pectin, wax, and protein. Previously published studies showed these substances break down into different components in the presence of hot compressed water.5,21,27,29 Besides, there was a shoulder centered on 323 °C in front of the first peak for H230−60. The position of this shoulder was precisely at the first peak center of H-230−30. This observation confirmed the decomposition and the formation of different components. This was further evidence for the transformation from hydrochar to soluble and gas. As a result, H-230−60, H230−90, and H-280-xx intensively combusted in the range from 300 °C to 380 °C, compared to WT and H-230−30. Meanwhile, the mass loss rate in this region increased significantly. As displayed in Table 2, the DTGmax values of D

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Energy & Fuels Table 2. Combustion Characteristics Parameters of Waste Textile and Hydrochars Mass Loss Ratea (%/min)

a

sample

ignition temperature, Ti (°C)

burnout temperature, Tf (°C)

combustion residue mass, Mf (%)

DTGmax

DTGmean

index S (× 10−7 min−2 °C−3)

index Rw (× 10−5 min−1 °C−2)

raw WT H-230−30 H-230−60 H-230−90 H-280−30 H-280−60 H-280−90

325 296 299 321 334 327 331

688 649 562 540 560 566 568

0.28 0.44 0.57 0.72 0.61 0.57 0.64

−18.44 −19.10 −34.83 −42.87 −40.60 −43.39 −44.27

−2.235 −2.232 −2.234 −2.258 −2.233 −2.229 −2.227

5.67 7.50 15.49 17.40 14.51 15.98 15.84

8.25 9.94 20.73 24.73 21.71 23.44 23.55

DTGmax is the mass loss rate according to max peak; DTGmean is the average mass loss rate.

H-230−60, H-230−90, and H-280-xx were much larger than those of WT and H-230−30. The H-280−90 had the biggest DTGmax value (−44.27%/min), which was ∼2.5 times greater than that of WT. Expectedly, the index S and Rw values of hydrochar were both greater than those of WT, which suggested that the combustion performance of hydrochar was superior to that of WT. The H-230−90 achieved the best combustion performance, with an index S value of 17.40 × 10−7 min−2 °C−3 and an index Rw value of 24.73 × 10−5 min−1 °C−2, which were much bigger than that of lignite and bituminous coal (see the Supporting Information). Moreover, it was worth noting that the combustion characteristic parameters of H-280xx were similar to each other, which indicated that prolonging the residence time at 280 °C would have a slight influence. 3.3. FTIR Spectra Analyses. FTIR analysis was performed to characterize what type of functional groups were enhanced or weakened. As displayed in Figure 5, remarkable differences were found over the reaction temperature and residence time, confirming the occurrence of chemical transformation during the HTC process. Apparently, the functional groups contained in raw WT and hydrochars were much more abundant than those of solubles. As shown in Figure 5a, the intensity of the wide bands at ∼3200−3600 cm−1 (zone 1), which was attributed to OH-stretching vibration bands of hydroxyl and carboxyl groups. This peak in hydrochars gradually became less intense as the HTC temperature and time increased, compared with that found in raw WT. The published literatures had proved that the reduction of this peak was due to the dehydration reaction during HTC.28,35,36 This also indicated the decrease of hydroxyl and carboxyl contents contained in waste textile, yielding primarily H2O and CO2, which improved the hydrophobicity of hydrochar.8 The intensity of the band due to the aliphatic −CH3 asymmetric stretching vibration (2900−2970 cm−1, zone 3) group in hydrochars was lower than the intensity of the WT, implying the loss of a −CH3 group. It could be observed from Figure 5a that the aromatic groups prevailed in the spectra of hydrochars. The absorbance of the bands at 3000−3100 cm−1 (zone 2), 1460−1600 cm−1 (zone 5), and 750−875 cm−1 (zone 9), which were completely or partially due to vibrations of the aromatic benzene ringC−H stretching vibration, aromatic C−C stretching vibration, and aromatic out-of-plane C−H bending, respectively. These peaks all increased after HTC and were enhanced further as the temperature or time increased, especially for aromatic out-ofplane C−H bending at 750−875 cm−1. These enhancements clearly revealed that the aromatization reactions obviously occurred during the HTC process, in accord with an increase in fixed carbon, as shown in Table 1. The molecular structures of

Figure 5. Fourier transform infrared (FTIR) spectra curves of (a) WT and hydrochar and (b) solubles.

WT underwent some cleavage under hot compressed water, and the side chains were shortened due to the cracking of some weak chemical bonds, resulting in an increase in the nonsubstituted aromatic carbon content. The HTC process resulted in an increase in the relative intensity of the peak at 1600−1800 cm−1 (zone 4), as shown in Figure 5a, and the IR absorption especially increased dramatically in the spectra of H280−90. This absorption was probably attributed to the −C O group. The transfer hydrogenation reaction during the cracking of molecular structures could promote the formation of the −CO group. However, this speculative group required E

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three key regions: aliphatic carbon (0−90 ppm), aromatic carbon (90−165 ppm), and carbonyl carbon bands (>165 ppm). The aliphatic carbon was the most abundant in raw WT, and there was no obvious peak ascribed to carbonyl carbon. However, most of the standard signals from 0 to 90 ppm of the hydrochar and solubles exhibited considerably lesser intensity. For instance, the aliphatic carbon of H-280-xx almost disappeared. This indicated that a large amount of aliphatic compounds decomposed during the HTC process. As a consequence, aromatic carbon occupied the dominant compositions in the hydrochar and solubles. In comparison to WT and solubles, the chemical structures of the hydrochar was rather similar to sub-bituminous coal or lignite.28 Tables S1 and S2 in the Supporting Information give the Gaussian curve-fitting results, and the chemical shifts for different carbon types were assigned according to previous literature.28,38−40 As listed in Tables S1 and S2, the carbon types in raw WT mainly included −O−CH3, aliphatic carbons attached to oxygen, −O−CH2−, protonated aromatic and aromatic ArC-O, etc. During the HTC process, the aliphatic carbon structures and side chain functional groups experienced decreases. As shown in Tables S1 and S2, aliphatic carbon types, such as −CH3, −O−CH3, aliphatic carbons attached to oxygen, −O−CH2−, −NCH, and RCH2OH or >CHOH, were rarely detected or disappeared. Meanwhile, the small molecules rearranged through condensation, aromatization reaction, and repolymerization, resulting in increases in the protonated aromatic, bridgehead junction aromatics, and carbonyl carbon of hydrochars and solubles (Table S1). It is remarkable that the nonprotonated aromatic of hydrochar weakened and disappeared with increasing reaction temperature, implying that the antioxidant activity of hydrochar was improved after HTC. Table S2 shows that the −CH2 was observed in S-280-xx. One possible explanation was that this aliphatic carbon atom acted as a bridge, connecting different aromatic structural units (bridgehead junction aromatic) in the soluble structure. The increase in the content of −COOH and −COOR in the solubles was caused by the presence of carboxylic acids that were derived from the decomposition of organic compounds. From Table 3, we could clearly observe the change regularity about aliphaticity, aromaticity, and carbonylation ratio. The aliphaticity value of WT (59.9%) was much higher than that of hydrochars and solubles. After HTC, the aliphaticity yield

further investigation and confirmation. The absorbance located at 1200−1360 cm−1 (zone 6) may be the esters C−O−C stretching vibration, which became more significant with temperature and time extension. The peak at 1100−1160 cm−1 (zone 7) was assigned to the aliphatic ether C−O or alcohol C−O stretching vibration. The relative intensity of this peak weakened for hydrochar, compared with that for raw WT. The decomposition of these groups probably converted to CO2 or CO (gas), demonstrating the occurrence of a decarboxylation reaction. Interestingly, as shown in Figure 5a, there was a newly added peak only for hydrochar at 900−980 cm−1 (zone 8), which was sharp and had increased intensity. This difference was probably due to the aldehydes −CHO stretching vibration, partially indicating that the aldehyde structures were favorably formed during the HTC process. With regard to Figure 5b, fewer types of functional groups were observed in the solubles, compared to that of hydrochar. Furthermore, the spectrum of solubles exhibited the opposite variety in some of the same functional groups, providing direct evidence of a mutual transformation between hydrochars and solubles. Unlike hydrochar, the intensity of the peak located at 3200−3600 cm−1 (zone I) increased significantly after the HTC process. For instance, a broad and strong absorption peak in this zone was found in S-280−90. As mentioned above, this peak was attributed to the OH-stretching vibration bands of hydroxyl and carboxyl groups. First, the solubles almost consisted of water-soluble fractions. These fractions have different degrees of hydrophilic ability, resulting in the −OH groups concentrated in the solubles. In addition, published literature work has reported that acetic acid was detected in the HTC process water, which was due to the thermal degradation of hydrolysis products.32 Therefore, the presence of carboxylic acids in the water-soluble phase also made a contribution to this absorption peak. In terms of the intensity of the −CO group peak at 1600−1800 cm−1 (zone II), on the contrary, HTC resulted in a decrease in solubles. Similar to the hydrochar, the alcohols C−O stretching vibration of solubles at 1020−1150 cm−1 (zone V) was weakened. The peak of aromatic C−C stretching vibration centered at 1420 cm−1 (zone III) exhibited greater intensity with time extension at 230 °C. However, a slight enhancement of this peak was observed when extending the residence time from 30 min to 90 min at 280 °C. The possible reason was that this aromatic compound reached a quasi-equilibrium state during the temperature ramping, which might result in a balance between the decomposition and repolymerization reactions.37 The peaks located at 780−825 cm−1 (zone VI) and 1220−1360 cm−1 (zone IV) were assigned to aromatic ring out-of-plane C−H bending and esters C−O−C stretching vibration, respectively. These two peaks gradually became considerably less significant with time or temperature extension. As displayed in Figure 5b, these two peaks in S-230−30 were much more sensitive than other solubles. As previously discussed, the system seemed more reactive with increased residence time or increased temperature during the HTC process of WT. In the late stage of the reaction process, polycondensation and polymerization reactions prevailed. As a result, both of these two functional groups of solubles were largely recombined or adsorbed on the surface of the hydrochars, accumulating parts of the functional groups into the hydrochar. 3.4. Solid-State 13C NMR Analyses. The 13C NMR analyses of all samples were performed in Figure S3. As illustrated in Figure S3, the 13C NMR spectra mainly consist of

Table 3. Important Carbon Structural Parameters of WT, Hydrochar, and Solubles from 13C NMR Analysis aliphaticity definition WT H-230−30 H-230−60 H-230−90 H-280−30 H-280−60 H-280−90 S-230−30 S-230−60 S-230−90 S-280−30 S-280−60 S-280−90 F

aromaticity

fal = f1 + f 2 + ··· + f 7 + f 8 fa = f 9 + ··· + f13 59.9 40.08 43.56 44.78 31.26 47.98 15.30 54.37 3.77 61.40 2.46 62.04 2.84 61.85 30.06 58.84 16.42 60.92 11.48 63.07 13.77 51.72 13.94 49.44 11.86 50.85

carbonylation ratio fc = f14 + ··· + f16 0 11.66 20.76 29.24 34.58 35.49 35.27 11.09 22.65 25.43 34.50 36.61 37.28

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Figure 6. Proposed conversion pathway of the hydrothermal carbonization of waste textile.

decreased from 43.56% to 15.30% at 230 °C with increasing the residence time from 30 min to 90 min. However, an aliphaticity of only ∼2.46%−3.77% was detected in H-280-xx, which was an approximately stable value. It was also the case for the aromaticity and carbonylation ratio in hydrochars, whose values were ∼61.40%−62.04% and ∼34.58%−35.49%, respectively. This implied that prolonging the residence time at high reaction temperature (e.g., 280 °C) would have a slight influence on the chemical structure of hydrochars, which was similar to the combustion behavior analysis. In term of aromaticity and carbonylation ratio, the values of hydrochars experienced noticeable enhancement, compared to raw WT. For example, the aromaticity and carbonylation ratio of hydrochars increased from 44.78% to 54.37% and 11.66% to 29.24%, respectively. With regard to solubles, the change regularities of aliphaticity and carbonylation ratio were similar to those observed for hydrochars. The former chemical structure reduced after the HTC process, and the latter one increased. Interestingly, the aromaticity of solubles exhibited a different variation tendency from any other structural parameters. As given in Table 3, the aromaticity values of solubles initially increased from 58.84% to 63.07% and then decreased later. This clearly demonstrated that parts of aromatic compounds underwent recombined and repolymerization reactions, adsorbing at the char surface and being retained in hydrochars. This outcome was very consistent with the above FTIR analysis. 3.5. HTC Conversion Mechanism of Waste Textile. According to the outcomes obtained in this work, the possible HTC conversion process of waste textile was proposed. In the HTC process, water acted as both a solvent and a reactant, which reacted with feedstock in three parallel primary reactions.41 Because of the hot compressed water, the reaction mechanism is initiated by the hydrolysis and depolymerization of raw WT (most cotton), resulting in three primary products (liquid, gas, and char). In the initial stage, hydrolysis, dehydration, decarboxylation, deoxygenation, and aromatization reactions of raw WT occurred significantly. Large amounts of aliphatic compounds were broken, while enhancements in the aromatic and carbonyl carbons were observed. Meanwhile, part of the chemical structures degraded to gaseous products, mainly CO2, in this stage. With further increases in the reaction temperature or reaction time, higher temperatures enhanced the breakage of unreact feedstock to fragment (most synthetic fiber) and promoted the aromatization and repolymerization

reaction to form the solid char and liquid phase. Besides, some carbon-containing functional groups contained in the char were further degraded to secondary gaseous compounds, such as CO, CO2, and CH4. In this stage, the decarbonylation reaction prevailed, whereas demethylation occurred slightly. Between char and the liquid phase, recombined, the polycondensation and repolymerization reactions progressively occupied the dominant effects. These reactions promoted the formation of solid hydrochar from char and oil-range molecules. In addition, some small molecular components were also adsorbed by the char surface and were retained in hydrochar. Certainly, the char was still subjected to hydrolysis, dehydration, and decarboxylation reactions, being converted to acids or H2O, and becoming part of the soluble fraction. Finally, the HTC system reached a quasi-equilibrium state with three products (hydrochar, soluble, and gas). The proposed conversion pathway regarding the HTC process of waste textile is presented in Figure 6.

4. CONCLUSIONS In this paper, we try to establish a representative pseudocomponents simulation system during the hydrothermal carbonization (HTC) process of municipal solid waste (MSW). Waste textile (WT) was first employed to investigate the reaction mechanism. After the HTC process, both the carbon content and fixed carbon content were enhanced in hydrochars, and ∼98% volatile matter was detected in the soluble fraction. In addition, a decrease in the oxygen content was observed in both the hydrochar and the solubles. A Van Krevelen diagram illustrated that the coal ranks of hydrochars were upgraded by the removal of low-grade energy substances (solubles) from raw WT. It also showed that the formations of hydrochar were predominantly governed by dehydration and decarbonylation/decarboxylation reactions. Gas chromatography (GC) analysis results revealed CO2 and CO to be the dominant gaseous species. HTC had a remarkable effect on the combustion behavior of hydrochars, leading to better combustion performance. The FTIR and 13C NMR analysis clearly showed that significant decreases of aliphatic carbons and enhancements of both aromatic and carbonyl-type carbons in the hydrochar and solubles were observed. Increasing the temperature can enhance the aromatization, repolymerization, polycondensation, and adsorption reactions between the hydrochar and the solubles. Finally, the HTC system reached a quasi-equilibrium state and a value-added hydrochar was G

DOI: 10.1021/acs.energyfuels.6b01365 Energy Fuels XXXX, XXX, XXX−XXX

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characteristics and combustion behavior. Appl. Energy 2013, 111, 257− 266. (11) Guo, S.; Dong, X.; Wu, T.; Shi, F.; Zhu, C. Characteristic evolution of hydrochar from hydrothermal carbonization of corn stalk. J. Anal. Appl. Pyrolysis 2015, 116, 1−9. (12) Román, S.; Nabais, J. M. V.; Laginhas, C.; Ledesma, B.; González, J. F. Hydrothermal carbonization as an effective way of densifying the energy content of biomass. Fuel Process. Technol. 2012, 103, 78−83. (13) Kambo, H. S.; Dutta, A. Comparative evaluation of torrefaction and hydrothermal carbonization of lignocellulosic biomass for the production of solid biofuel. Energy Convers. Manage. 2015, 105, 746− 755. (14) Wirth, B.; Reza, M. T. Continuous Anaerobic Degradation of Liquid Condensate from Steam-Derived Hydrothermal Carbonization of Sewage Sludge. ACS Sustainable Chem. Eng. 2016, 4 (3), 1673− 1678. (15) Escala, M.; Zumbühl, T.; Koller, C.; Junge, R.; Krebs, R. Hydrothermal Carbonization as an Energy-Efficient Alternative to Established Drying Technologies for Sewage Sludge: A Feasibility Study on a Laboratory Scale. Energy Fuels 2013, 27 (1), 454−460. (16) Pala, M.; Kantarli, I. C.; Buyukisik, H. B.; Yanik, J. Hydrothermal carbonization and torrefaction of grape pomace: A comparative evaluation. Bioresour. Technol. 2014, 161, 255−262. (17) Yuan, H.; Wang, Y.; Kobayashi, N.; Zhao, D.; Xing, S. Study of Fuel Properties of Torrefied Municipal Solid Waste. Energy Fuels 2015, 29 (8), 4976−4980. (18) Lu, L.; Namioka, T.; Yoshikawa, K. Effects of hydrothermal treatment on characteristics and combustion behaviors of municipal solid wastes. Appl. Energy 2011, 88 (11), 3659−3664. (19) Hwang, I. H.; Aoyama, H.; Matsuto, T.; Nakagishi, T.; Matsuo, T. Recovery of solid fuel from municipal solid waste by hydrothermal treatment using subcritical water. Waste Manage. 2012, 32 (3), 410− 416. (20) Muthuraman, M.; Namioka, T.; Yoshikawa, K. Characteristics of co-combustion and kinetic study on hydrothermally treated municipal solid waste with different rank coals: A thermogravimetric analysis. Appl. Energy 2010, 87 (1), 141−148. (21) Funke, A.; Ziegler, F. Hydrothermal carbonization of biomass: A summary and discussion of chemical mechanisms for process engineering. Biofuels, Bioprod. Biorefin. 2010, 4 (2), 160−177. (22) Wu, X.; Fu, J.; Lu, X. Kinetics and Mechanism of Hydrothermal Decomposition of Lignin Model Compounds. Ind. Eng. Chem. Res. 2013, 52 (14), 5016−5022. (23) He, W.; Li, G.; Kong, L.; Wang, H.; Huang, J.; Xu, J. Application of hydrothermal reaction in resource recovery of organic wastes. Resour., Conserv. Recycl. 2008, 52 (5), 691−699. (24) Reza, M. T.; Wirth, B.; Lueder, U.; Werner, M. Behavior of selected hydrolyzed and dehydrated products during hydrothermal carbonization of biomass. Bioresour. Technol. 2014, 169, 352−361. (25) Meng, A.; Chen, S.; Long, Y.; Zhou, H.; Zhang, Y.; Li, Q. Pyrolysis and gasification of typical components in wastes with macroTGA. Waste Manage. 2015, 46, 247−256. (26) Poerschmann, J.; Weiner, B.; Woszidlo, S.; Koehler, R.; Kopinke, F. D. Hydrothermal carbonization of poly(vinyl chloride). Chemosphere 2015, 119, 682−689. (27) Lin, Y.; Ma, X.; Peng, X.; Hu, S.; Yu, Z.; Fang, S. Effect of hydrothermal carbonization temperature on combustion behavior of hydrochar fuel from paper sludge. Appl. Therm. Eng. 2015, 91, 574− 582. (28) Zhu, X.; Xue, Y.; Li, X.; Zhang, Z.; Sun, W.; Ashida, R.; Miura, K.; Yao, H. Mechanism study of degradative solvent extraction of biomass. Fuel 2016, 165, 10−18. (29) Deb-Choudhury, S.; Haines, S.; Harland, D.; Clerens, S.; van Koten, C.; Dyer, J. Effect of cooking on meat proteins: mapping hydrothermal protein modification as a potential indicator of bioavailability. J. Agric. Food Chem. 2014, 62 (32), 8187−8196.

generated. Based on the above analysis, we proposed the possible conversion pathway during the HTC process of WT.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01365. Discussion of the volatile matter content, ignition and burnout temperature, combustibility index, combustion stability index, process filtrate, mass balances, solubles, 13 C solid-state NMR analyses, and Gaussian multipeaks fitting of 13C NMR curves (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 20 87110232. Fax: +86 020 87110613. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the support given by the National Natural Science Foundation of China (Nos. 51406058, 51476060); Guangdong Key Laboratory of Efficient and Clean Energy Utilization (No. 2013A061401005); the Fundamental Research Funds for the Central Universities (No. 2015ZZ015); Guangdong Natural Science Foundation (Nos. 2015A030311037, 2015A030313227); China Postdoctoral Science Foundation (No. 2015M582382).

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REFERENCES

(1) Hoekman, S. K.; Broch, A.; Robbins, C. Hydrothermal Carbonization (HTC) of Lignocellulosic Biomass. Energy Fuels 2011, 25 (4), 1802−1810. (2) Liu, Z.; Balasubramanian, R. Upgrading of waste biomass by hydrothermal carbonization (HTC) and low temperature pyrolysis (LTP): A comparative evaluation. Appl. Energy 2014, 114, 857−864. (3) Parshetti, G. K.; Kent Hoekman, S.; Balasubramanian, R. Chemical, structural and combustion characteristics of carbonaceous products obtained by hydrothermal carbonization of palm empty fruit bunches. Bioresour. Technol. 2013, 135, 683−689. (4) Zhu, X.; Liu, Y.; Qian, F.; Zhang, S.; Chen, J. Investigation on the Physical and Chemical Properties of Hydrochar and Its Derived Pyrolysis Char for Their Potential Application: Influence of Hydrothermal Carbonization Conditions. Energy Fuels 2015, 29 (8), 5222− 5230. (5) Zhang, L.; Wang, Q.; Wang, B.; Yang, G.; Lucia, L. A.; Chen, J. Hydrothermal Carbonization of Corncob Residues for Hydrochar Production. Energy Fuels 2015, 29 (2), 872−876. (6) Prawisudha, P.; Namioka, T.; Yoshikawa, K. Coal alternative fuel production from municipal solid wastes employing hydrothermal treatment. Appl. Energy 2012, 90 (1), 298−304. (7) Li, L.; Diederick, R.; Flora, J. R. V.; Berge, N. D. Hydrothermal carbonization of food waste and associated packaging materials for energy source generation. Waste Manage. 2013, 33 (11), 2478−2492. (8) Zhao, P.; Shen, Y.; Ge, S.; Chen, Z.; Yoshikawa, K. Clean solid biofuel production from high moisture content waste biomass employing hydrothermal treatment. Appl. Energy 2014, 131, 345−367. (9) Areeprasert, C.; Zhao, P.; Ma, D.; Shen, Y.; Yoshikawa, K. Alternative Solid Fuel Production from Paper Sludge Employing Hydrothermal Treatment. Energy Fuels 2014, 28 (2), 1198−1206. (10) He, C.; Giannis, A.; Wang, J.-Y. Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: Hydrochar fuel H

DOI: 10.1021/acs.energyfuels.6b01365 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels (30) Wu, J.; Liu, J.; Yuan, S.; Zhang, X.; Liu, Y.; Wang, Z.; Zhou, J. Sulfur Transformation during Hydrothermal Dewatering of Low Rank Coal. Energy Fuels 2015, 29 (10), 6586−6592. (31) Erdogan, E.; Atila, B.; Mumme, J.; Reza, M. T.; Toptas, A.; Elibol, M.; Yanik, J. Characterization of products from hydrothermal carbonization of orange pomace including anaerobic digestibility of process liquor. Bioresour. Technol. 2015, 196, 35−42. (32) Berge, N. D.; Ro, K. S.; Mao, J.; Flora, J. R.; Chappell, M. A.; Bae, S. Hydrothermal carbonization of municipal waste streams. Environ. Sci. Technol. 2011, 45 (13), 5696−5703. (33) Basso, D.; Patuzzi, F.; Castello, D.; Baratieri, M.; Rada, E. C.; Weiss-Hortala, E.; Fiori, L. Agro-industrial waste to solid biofuel through hydrothermal carbonization. Waste Manage. 2016, 47, 114− 121. (34) Gholamzad, E.; Karimi, K.; Masoomi, M. Effective conversion of waste polyester−cotton textile to ethanol and recovery of polyester by alkaline pretreatment. Chem. Eng. J. 2014, 253, 40−45. (35) Gao, P.; Zhou, Y.; Meng, F.; Zhang, Y.; Liu, Z.; Zhang, W.; Xue, G. Preparation and characterization of hydrochar from waste eucalyptus bark by hydrothermal carbonization. Energy 2016, 97, 238−245. (36) Reza, M. T.; Uddin, M. H.; Lynam, J. G.; Coronella, C. J. Engineered pellets from dry torrefied and HTC biochar blends. Biomass Bioenergy 2014, 63, 229−238. (37) Brand, S.; Hardi, F.; Kim, J.; Suh, D. J. Effect of heating rate on biomass liquefaction: Differences between subcritical water and supercritical ethanol. Energy 2014, 68, 420−427. (38) Liu, F.-J.; Wei, X.-Y.; Zong, Z.-M.; Fan, M. Characterization of the Oxygenated Chemicals Produced from Supercritical Methanolysis of Modified Lignites. Energy Fuels 2016, 30 (4), 2636−2646. (39) Lin, H.-l.; Li, K.-j.; Zhang, X.; Wang, H. Structure Characterization and Model Construction of Indonesian Brown Coal. Energy Fuels 2016, 30 (5), 3809−3814. (40) Baccile, N.; Laurent, G.; Babonneau, F.; Fayon, F.; Titirici, M.M.; Antonietti, M. Structural Characterization of Hydrothermal Carbon Spheres by Advanced Solid-State MAS 13C NMR Investigations. J. Phys. Chem. C 2009, 113 (22), 9644−9654. (41) Zhang, B.; Huang, H. J.; Ramaswamy, S. A Kinetics Study on Hydrothermal Liquefaction of High-diversity Grassland Perennials. Energy Sources, Part A 2012, 34 (18), 1676−1687.

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DOI: 10.1021/acs.energyfuels.6b01365 Energy Fuels XXXX, XXX, XXX−XXX