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Combined pretreatment with torrefaction and washing using torrefaction liquid products to yield upgraded biomass and pyrolysis products
Bioresource Technology 228 (2017) 62–68
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Combined pretreatment with torrefaction and washing using torrefaction liquid products to yield upgraded biomass and pyrolysis products Dengyu Chen a, Jiaming Mei a, Haiping Li a, Yiming Li a, Mengting Lu a, Tingting Ma a, Zhongqing Ma b,⇑ a
College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China School of Engineering, National Engineering & Technology Research Center of Wood-Based Resources Comprehensive Utilization, Zhejiang Agriculture & Forestry University, Lin’an, Zhejiang 311300, China b
h i g h l i g h t s Torrefaction liquid washing pretreatment greatly reduced the metals of cotton stalk. Torrefaction improved the fuel property while it increased metals contents of sample. Torrefaction liquid washing reduced the secondary cracking of pyrolysis volatiles. Torrefaction greatly reduced the acids while it increased the phenols in bio-oil. Combined washing and torrefaction pretreatment preserved advantages of each method.
a r t i c l e
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Article history: Received 6 November 2016 Received in revised form 15 December 2016 Accepted 22 December 2016 Available online 24 December 2016 Keywords: Torrefaction Pyrolysis Washing Cotton stalk Bio-oil
a b s t r a c t This study presented an approach to upgrade biomass and pyrolysis products using a process based on torrefaction liquid washing combined with torrefaction pretreatment. The torrefaction of cotton stalk was first conducted at 250 °C for 30 min and then the resulting torrefaction liquid products were collected and reused to wash cotton stalk. The pyrolysis of the original and pretreated cotton stalk was performed at 500 °C for 15 min in a fixed-bed reactor. The results indicated that the combined pretreatment obviously reduced the metallic species in cotton stalk, decreased the water and acids contents while promoted phenols in bio-oil, declined the ash content in biochar, as well as improved the heating value of non-condensable gas. Overall, the combined pretreatment did not only allow to reuse the liquid products issued from torrefaction pretreatment but also improved the quality of biomass and the pyrolysis products, making it a novel promising pretreatment method. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Biomass is a promising renewable energy resource, which is mostly utilized through processes based on polygeneration of biochar, bio-oil and non-condensable gas from stalks pyrolysis (Yang et al., 2016). However, the quality of utilized biomass materials is generally low due to its elevated hydrophilicity, high oxygen content, low energy density, the complexity of storage, and dispersed areas of biomass production (van der Stelt et al., 2011). The latter led to higher transportation costs, storage and utilization of biomass as an energy resource, which, in turn, limited the development of biomass (Carpenter et al., 2014). To improve the quality ⇑ Corresponding author. E-mail address: [email protected] (Z. Ma). http://dx.doi.org/10.1016/j.biortech.2016.12.088 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.
of biomass and the pyrolysis products, pretreatment processes are hence required before the utilization of biomass (Chew and Doshi, 2011; Deng et al., 2013; Dong et al., 2015; Pecha et al., 2015). The most common used pretreatment techniques include water washing, acid washing, and torrefaction. Each process has its advantages and drawbacks. For instance, water washing pretreatment removes some of the metals from biomass, such as K, Ca, and Mg, as well as it increases the high heating value (HHV) of biomass (Liaw and Wu, 2013; Mourant et al., 2011; Zhang et al., 2015). However, since the chemical structure of the biomass is not affected by water washing pretreatment, it has limited influence on the quality of biomass and the pyrolysis products. On the other hand, acid washing pretreatment employs acid solutions, such as HCl, H2SO4 and HNO3, to remove large amounts of metals from the biomass (Dong et al., 2015). Furthermore,
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because acids could modify the chemical structure of biomass, the quality of pyrolysis products like bio-oil could significantly be improved (Ly et al., 2015; Pecha et al., 2015). Nevertheless, the high cost of acid washing pretreatments limited their practical usage. Also, the use of acids could introduce some unnecessary elements in the biomass, such as Cl, S and N, which could be incorporated into the pyrolysis products (Zhang and Xiong, 2016). The torrefaction pretreatment process consists of heating biomass up to 200–300 °C in an anoxic environment in order to remove water and some of the volatiles (Chen et al., 2015a). This treatment improves several aspects of biomass, including destroying the fiber structure, efficiently reduces the oxygen content, increases energy density, improves grinding property, enhances C/O ratio, and boosts hydrophobicity of the biomass (Arias et al., 2008; Chen et al., 2014d, 2015c; Gil et al., 2015). Meanwhile, torrefaction pretreatment positively impacts the biomass pyrolysis products by reducing water content, acid substances, and oxygen content in bio-oil (Chen et al., 2015a; Zheng et al., 2013). Moreover, the torrefaction process effectively raises contents of high-value chemicals in bio-oil, as well as enhances heat value of the gas products issued from biomass pyrolysis (Chen et al., 2016, 2014e). In recent years, torrefaction pretreatment of biomass has attracted increasing attention due to the numerous advantages cited previously. However, torrefaction pretreatment induces partial removal of volatiles from biomass, which, in turn, raises ash and metals contents after the torrefaction pretreatment. The latter negatively affects the pyrolysis of biomass by blocking and eroding pipelines, as well as inducing catalytic secondary decomposition of pyrolysis volatile compounds and reducing the yield of bio-oil. It is worth noting that the torrefaction pretreatment process produces small amounts of liquid products called ‘‘torrefaction-li quid”, mostly composed of water and small amounts of organic compounds like acids, ketones and phenols, depending on the materials and torrefaction conditions (Chen et al., 2015b; Zheng et al., 2012). Previous studies have shown that organic acids, such as acetic acid, had higher efficiencies for removal of alkali and alkaline earth metals (Oudenhoven et al., 2013; Pecha et al., 2015). Therefore, torrefaction-liquid is expected to potentially remove ash and metal species from biomass. However, a little of attention was devoted to investigating torrefaction-liquid, and its usage for improving biomass and pyrolysis products has not been reported. This paper aimed to utilize torrefaction-liquid as an approach in combination with pretreatment methods to yield improved quality of biomass and pyrolysis products. 2. Materials and methods 2.1. Materials Cotton stalk, a typical biomass, was used as the experiment material to investigate and compare the influence of different pretreatments, torrefaction-liquid washing torrefaction, and combined pretreatment, on the quality of cotton stalk and the resulting pyrolysis products, including bio-oil, biochar, and noncondensable gas. Prior to performing the experimental, the cotton stalk was first crushed into granular particles with 40–60 mesh sizes, and then dried at 105 °C for 6 h. The resulting dried cotton stalk particles were represented as CS. 2.2. Torrefaction A tube furnace (OTL1200, Nanjing University Instrument Plant, China) was used in torrefaction experiments of cotton stalk. The reaction tube consisted of a high-temperature resistant corundum tube with an inner diameter of 100 mm and a length of 1000 mm.
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Prior to experiments, cotton stalk samples (about 200 g each) were put in several quartz boats and placed inside the tube furnace away from the non-heating zone. After heating the furnace to a specific temperature, the quartz boats were then rapidly pushed into the heating zone of the furnace. The torrefaction process was performed at 250 °C for a period of 30 min. High-grade purity nitrogen was used as a carrier gas and was circulated at a flow rate of 500 mL/min. The torrefaction gaseous products first passed through the quartz tube coated with a heating jacket at 150 °C, and then entered the condensing tube placed in the liquid nitrogen, where the torrefaction liquid product (torrefaction-liquid) was collected. The experiments were repeated several times under the same conditions to collect enough torrefaction-liquid. After completion of the experiments, the tube furnace was opened and the torrefied samples were obtained and marked as T-CS. 2.3. Washing with torrefaction liquid products The water content in the torrefaction-liquid was determined to be 88.73 wt.%. The peak areas% of acetic acid, 2,3-dihydrobenzofuran, hydroxyacetone, 1-hydroxy-2-butanon and furfural, were recorded as 31.26%, 27.35%, 14.61%, 3.44% and 3.59%, respectively. It can be seen that the acids were present at high contents than the others. A cotton stalk sample (10 g) was first mixed with 200 mL torrefaction-liquid then the mixture was constantly stirred at 60 °C for 2 h. The cotton stalk sample was collected by filtration followed by washing with deionized water until reaching neutral pH. The cotton stalk was then dried at 105 °C for 12 h, and the resulting torrefaction-liquid washed samples were labeled as W-CS. 2.4. Torrefaction of washed samples The previously mentioned tube furnace was also used for torrefaction pretreatment of W-CS (about 10 g per experiment) at 250 °C for 30 min. The previously mentioned experimental procedure of cotton stalk was applied for W-CS samples, and the obtained solid samples were labeled as TW-CS. 2.5. Mass and energy yields of torrefaction and washing process Mass yield (Ymass) and energy yield (Yenergy) calculations of the torrefaction and washing process are shown as:
Y mass ¼ Mpre =M ori 100%
ð1Þ
Y energy ¼ Y mass HHV pre =HHV ori 100%
ð2Þ
where M is the mass of sample and HHV is the higher heating value of the sample. Subscripts ‘‘pre” and ‘‘ori” represent the pretreated sample and original sample, respectively. 2.6. Pyrolysis The pyrolysis of 3 g of each sample, CS, W-CS, T-CS and TW-CS, was carried out at 500 °C in a small-sized downdraft fixed bed pyrolysis reactor. The details of the pyrolysis device and the procedures could be found in the literature (Chen et al., 2015a). Highgrade purity nitrogen was used as a carrier gas with a flow rate of 200 mL/min and the pyrolysis process was performed for 15 min. After completion of each experiment, the resulting products, solid pyrolysis (biochar), liquid pyrolysis (bio-oil) and gas pyrolysis (non-condensable gas) were collected from the reactor, condensing tube and gas collecting bag, respectively. The yields of biochar and bio-oil were estimated by measuring their masses, and the yield of non-condensable gas was obtained by difference.
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2.7. Products characterization 2.7.1. Analysis of non-condensable gas The non-condensable gas was detected by a gas chromatograph analyzer (GC-TCD 7890, Shanghai Tianmei, China). Its HHV was obtained by summing each gas concentration with its HHV, following: HHVgas (MJ/Nm3) = Vol.CO (%) HHVCO (MJ/Nm3) + Vol.H2 (%) HHVH2 (MJ/Nm3) + Vol.CH4 (%) HHVCH4 (MJ/Nm3) + Vol.C2+ (%) HHVC2+ (MJ/Nm3). The overall density of the noncondensable gas can be calculated from the density of each gas. 2.7.2. Analysis of bio-oil Water content of bio-oil was analyzed by Karl-Fischer titration. HHV was estimated using an adiabatic oxygen bomb calorimeter (XRY-1A, Changji Geological Instruments, China). The pH value was measured using a digital pH meter (PHS-3C, Shanghai leici, China). Organic components were determined by gas chromatography coupled to a mass spectrometer (GC/MS 7890A/5975C, Agilent Company, USA) equipped with an HP-5MS column (30 m 250 lm 0.25 lm). Helium (99.999%) was used as a carrier gas at a flow rate of 1.0 ml/min, and 1-ll sample was injected into the column. The compounds were identified by comparing the spectral data with the NIST library and literature data. 2.7.3. Analysis of cotton stalk and biochar Proximate analysis of samples was performed according to the Chinese National Standards GB/T28731-2012. Ultimate analysis was performed using an elemental analyzer (Vario macro cube, Elementar, Germany), and oxygen was estimated by the difference. The HHV of biochar was measured using an adiabatic oxygen bomb calorimeter (XRY-1A, Changji Geological Instruments, China). The contents of metallic species of the cotton stalk samples were determined using an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 7300 DV, PerkinElmer, USA). Moreover, biochar was added to the de-ionized water in a mass ratio of 1:20 for measuring its pH using a pH meter. 2.8. Experiment repeatability The analysis experiments were replicated three times, and the averaged data were used. Most results were uniform between each run, and the relative error was generally less than 5%. Standard deviation of the data was also listed in this study. 3. Results and discussion 3.1. Physicochemical properties of samples 3.1.1. Metallic species Fig. 1 shows the resulting contents of the metallic species present in the samples. It can be observed that metallic species contents in cotton stalk were reduced to various extents after the torrefaction-liquid washing process. The removal of K was the most apparent with a removal rate reaching up to 82.30%. The removal of other elements, including Na, Mg, Ca and Fe, were recorded in the range of 15%–79%, indicating that the torrefactionliquid was effective in removing metallic species from cotton stalk. A number of previously published studies reported that both water and acid (mineral or organic acids) washing processes effectively reduce the content of metallic species (Deng et al., 2013; Ly et al., 2015; Oudenhoven et al., 2013; Pecha et al., 2015; Zhang et al., 2015). The present study also confirmed that the torrefaction-liquid washing pretreatment was effective for removal of metallic species. Furthermore, it was observed that the contents of the metallic species in cotton stalk increased after
Fig. 1. The contents of metallic species in CS, W-CS, T-CS, and TW-CS.
the torrefaction pretreatment. However, the contents of the metallic species of TW-CS were found to be lower than those of T-CS. This was caused by the large amounts of metals, which had already been removed during the washing pretreatment.
3.1.2. Fuel characteristics The main properties of the prepared samples, CS, W-CS, T-CS and TW-CS, are listed in Table 1. The ash content of W-CS decreased to 3.92 wt.% compared to its content in CS (5.54 wt.%). Meanwhile, the contents of volatiles and fixed carbon increased in W-CS with respect to CS, along with carbon content. The increase in amounts of combustible components in cotton stalk also resulted in a slight increase in the HHV of W-CS. The impact of the torrefaction pretreatment appeared more obvious when comparing with the torrefaction-liquid washing pretreatment. If compared to CS and W-CS, the fixed carbon content of T-CS obviously increased, whereas the content of volatile compounds declined. This could be attributed to the decomposition of hemicellulose and partial degradation of both the cellulose and lignin. In turn, this released larger amounts of small gas molecules like CO2 and CO, as well as some condensed gas phase products, such as H2O and acetic acid (Chen et al., 2014a,c; Ma et al., 2015). One of the important roles of torrefaction pretreatment was to raise the carbon content and reduce the oxygen, which should result in decreasing the molar ratios of H/C and O/C and increasing HHV of the cotton stalk. However, as ash and metallic species remained in the cotton stalk, this should increase their contents after torrefaction, which was unfavorable for fuel quality of biomass and subsequent utilizations. If compared to direct torrefaction pretreatment, the process based on torrefaction-liquid washing combined with torrefaction pretreatment obviously influenced the basic properties of cotton stalk. As shown in Table 1, the values of proximate analysis, ultimate analysis and HHV of TW-CS were found to locate between the values of W-CS and T-CS. Besides, HHV and carbon content values of TW-CS were close to those of T-CS, whereas the volatile content values approached those of W-CS. After the different pretreatment processes, some changes in mass and energy yields of cotton stalk appeared and the results are gathered in Table 1. The torrefaction-liquid washing process clearly showed that in addition to removal of metals from the cotton stalk, low amounts organic components were eluted and led to a decline in the energy yield of W-CS to less than 100%. The torrefaction pretreatment resulted in a greater mass loss mainly caused by the decomposition of cellulose, hemicellulose,
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D. Chen et al. / Bioresource Technology 228 (2017) 62–68 Table 1 The fuel properties, mass and energy yields of original and pretreated cotton stalk. Sample
CS W-CS T-CS TW-CS
Proximate analysis (wt.%, db)
Ultimate analysis (wt.%, db)
Volatile
Fixed Carbon
Ash
[C]
[H]
[O]
[N]
76.93 ± 1.13 79.13 ± 1.08 64.83 ± 0.87 68.53 ± 1.02
17.53 ± 0.35 16.95 ± 0.34 27.81 ± 0.49 26.41 ± 0.52
5.54 ± 0.11 3.92 ± 0.08 7.36 ± 0.13 5.06 ± 0.09
45.16 ± 0.91 45.89 ± 0.86 51.85 ± 0.87 52.27 ± 0.89
6.28 ± 0.13 6.45 ± 0.12 5.79 ± 0.12 5.78 ± 0.11
42.35 ± 0.76 43.06 ± 0.82 34.39 ± 0.58 36.25 ± 0.72
0.67 ± 0.01 0.68 ± 0.01 0.61 ± 0.01 0.64 ± 0.01
HHV (MJ/kg)
Ymass (%)
Yenergy (%)
16.83 ± 0.36 17.18 ± 0.32 18.96 ± 0.39 20.13 ± 0.44
100 96.92 ± 0.85 80.25 ± 1.13 77.63 ± 0.98
100 98.94 ± 0.33 90.41 ± 0.38 92.85 ± 0.39
and lignin. However, the energy yields remained higher than mass yields under the same conditions, indicating that energy per unit mass was improved after the pretreatment of cotton stalk.
led to an increase in yield of bio-oil by 16.11% and biochar by 9.58%, while the yield of non-condensable gas decreased by 23.17%.
3.2. Effect of pretreatments on distribution of pyrolysis products
3.3. Effect of pretreatments on pyrolysis products properties
The product yields of pyrolysis of CS, W-CS, T-CS and TW-CS, are shown in Fig. 2. Compared with pyrolysis of CS, the bio-oil yield of W-CS slightly increased while the biochar yield somewhat declined. This was mainly caused by the torrefaction-liquid washing pretreatment which greatly removed the metals, especially the alkali and alkaline earth metals. This process changed the pyrolysis pathway to some extent, decreased the secondary decomposition of volatiles, increased the production of bio-oil, and reduced the yield of biochar. The torrefaction pretreatment had an obvious influence on the distribution of the pyrolysis products. Compared with CS and W-CS, the bio-oil yield of T-CS reduced by almost 26–29% while the biochar yield increased by 15–19%. Large amounts of hemicelluloses were decomposed during the torrefaction process due to the poor thermal stability, while small amounts of cellulose and lignin decomposed because of their relatively better thermal stabilities. This resulted in the increased content of lignin in biomass after torrefaction, while hemicellulose content dramatically reduced. A previous study showed that biochar yield of lignin was the highest with 50–75%, followed by xylan with 25–35%, and then cellulose with 5–23%; meanwhile, xylan and cellulose were found to have higher liquid yields of 40–50% and gas yields of about 30% (Xin et al., 2013). Therefore, torrefaction pretreatment increased the yield of biochar and reduced that of bio-oil. The combined pretreatment also greatly influenced the pyrolysis products. Although the bio-oil yield of TW-CS was lower to those of CS and W-CS, it was clearly superior to that of T-CS. With respect to torrefaction pretreatment, the combined pretreatment
3.3.1. Non-condensable gas The composition of the pyrolysis gas products included CO, CO2, CH4, H2 and small amounts of C2+ gases, where volume fractions of these gases are shown in Fig. 3. It can be observed that the volume fraction of CO2 in CS was the highest with 43.11 vol.%. On the other hand, the volume fractions of CO, CH4 and H2 were recorded as 28.72 vol.%, 16.35 vol.% and 9.21 vol.%, respectively. If compared to CS, the torrefaction-liquid washing pretreatment slightly increased and decreased contents of CO and CO2, respectively. It also somewhat influenced the contents of CH4 and H2. The torrefaction pretreatment process greatly affected the volume fractions of the non-condensable gases. When compared with CS, the CO2 volume fraction declined by 12.30% for T-CS, whereas the volume fractions of CH4 and H2 increased by 9.72% and 26.09%, respectively. Interestingly, the combined pretreatment further raised the volume fractions of CO and H2 while declined that of CO2. The HHVs of the non-condensable gases obtained with CS, W-CS, T-CS and TW-CS were estimated as 13.43 MJ/m3, 13.13 MJ/m3, 14.34 MJ/m3 and 14.70 MJ/m3, respectively. Overall, both processes, torrefaction and combined pretreatments, obviously enhanced the total volume fractions of the combustible gases, such as H2, CO, and CH4. This, in turn, led to increased HHV of the non-condensable gases when compared to the original cotton stalk.
Fig. 2. The yields of products generated from pyrolysis of CS, W-CS, T-CS, and TWCS.
Fig. 3. The volume fraction of gas products issued from pyrolysis of CS, W-CS, T-CS, and TW-CS.
3.3.2. Bio-oil The water content, HHV and pH value of bio-oil are listed in Table 2. The water content of bio-oil obtained from CS was estimated to 58.19 wt.%. However, all the three pretreatment processes,
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Table 2 The water content, HHV and pH values of bio-oil issued from pyrolysis of CS, W-CS, TCS, and TW-CS. Bio-oil
Water content (wt.%)
HHV (MJ/kg)
pH
CS W-CS T-CS TW-CS
58.19 ± 0.92 55.22 ± 0.95 41.39 ± 0.83 37.08 ± 0.46
11.34 ± 0.28 12.57 ± 0.31 14.83 ± 0.25 15.56 ± 0.23
2.95 ± 0.06 3.04 ± 0.05 3.23 ± 0.04 3.34 ± 0.05
torrefaction-liquid washing, torrefaction and combined pretreatments, produced bio-oils with reduced water contents. Table 2 clearly stated that water content in bio-oil obtained from TW-CS was the lowest with a value of 37.08 wt.%. Meanwhile, HHV of bio-oil increased from 11.34 MJ/kg for CS to 12.57 MJ/kg for W-CS. It then gradually increased to 14.83 MJ/kg for T-CS bio-oil and to finally reach a maximum value of 15.56 MJ/kg for bio-oil issued from TW-CS. The pH value of bio-oil increased after the pretreatments, suggesting that acidity of the bio-oil was reduced. These findings indicated that although all pretreatments positively impacted the quality of bio-oil, the combined pretreatment was more effective than the others. The composition of bio-oil is complex as it contains hundreds of chemicals worth identifying. These chemicals could be categorized into six groups based on their functional groups: acids, ketones, aldehydes, phenols, furans, and anhydrosugars. Fig. 4 shows the peak areas% of each group among the six groups found in bio-oil. It can be seen that the relative contents of acids, furans, ketones and phenols were relatively high in the bio-oil issued from CS pyrolysis. All pretreatments obviously influenced distribution of the components in bio-oil. When compared with CS, the relative contents of acids in W-CS, T-CS and TW-CS decreased by 12.13%, 30.99% and 38.65%, respectively. Meanwhile, the relative contents of phenols increased by 8.06%, 75.27% and 91.40% for W-CS, T-CS and TW-CS, respectively. Besides, the ketone and furan contents declined while phenols and anhydrosugars contents rose after subjecting to both the torrefaction and the combined pretreatments. This could mainly be related to two reasons. On the one hand, large amounts of hemicellulose were decomposed during the torrefaction pretreatment to yield acetic acid as the main pyrolysis product of hemicellulose. This indicated that a small amount of acetic acid was produced during the pyrolysis of T-CS. On the other hand, the torrefaction pretreatment increased the lignin content of biomass,
and phenols were the main liquid products issued from pyrolysis of lignin. This, in turn, raised the relative content of phenols. The presence of metallic species like K and Na prevented the decomposition of cellulose to sugars. The catalytic role played by metals in the decomposition of glucose units led to large numbers of low molecular weight compounds, such as aldehydes and ketones (Lv and Wu, 2012). The torrefaction-liquid washing pretreatment greatly removed the metallic species from cotton stalk, which subsequently led to the formation of sugars, including laevoglucose. In addition, as previously reported, the interaction between cellulose, xylan and lignin suppressed the formation of water-soluble tar like levoglucosan (Fushimi et al., 2009). However, the torrefaction pretreatment destroyed the fiber structure of cotton stalk and reduced the interaction between the three components, thus yielding more sugars during the pyrolysis process. It should be noted that elevated acid contents (mainly acetic acid) and water produced during pyrolysis were responsible for the high acidity and poor stability of bio-oil (Chen et al., 2014b). Bio-oil often contains some valuable chemical components, such as aromatic hydrocarbons, guaiacol, and laevoglucose. The combined pretreatment declined the water and acids contents while increased contents of some valuable products and the HHV of the bio-oil. This suggested that torrefaction-liquid utilized as pretreatment to wash biomass was promising in terms of reuse of torrefaction-liquid and improved quality of the pyrolysis products. 3.3.3. Biochar The results of the proximate analysis, ultimate analysis, HHVs, pH value and specific surface areas of biochars are listed in Table 3. After the torrefaction-liquid washing pretreatment, the ash content in biochar decreased. The combined pretreatment resulted in minimum ash content in the biochar. When compared to the biochar generated from pyrolysis of CS, the fixed carbon contents of biochar formed during pyrolysis of pretreated cotton stalk increased to different extents. With respect to elemental content, both processes based on torrefaction and combined pretreatment slightly increased the carbon content, which, in turn, raised the HHV of biochar. Table 3 revealed that pH of the samples was overall alkaline, mainly caused by the presence of carbonates and some metals in the biochars. If compared to biochar produced from CS pyrolysis, the pH values of biochars produced from W-CS and TW-CS pyrolysis declined. The latter resulted from the removal of large amounts of metals during the torrefaction-liquid washing pretreatment. On the other hand, the high pH value recorded for biochar produced from T-CS pyrolysis was attributed to the torrefaction pretreatment in which the metallic species contents remained high in the cotton stalk. With respect to fuel properties of biochars, overall no significant differences were observed between the three tested pretreatment methods. However, the impact on the structure of biochar can clearly be distinguished. Table 3 depicted that all the three pretreatment methods increased the specific surface area of biochar, where biochar obtained from the pyrolysis of W-CS showed the largest value. This phenomenon may be ascribed to removal of large amounts of impurities and metallic species present in cotton stalk using the torrefaction-liquid washing pretreatment, which benefited the formation of porous structures in biochar (Klasson et al., 2014). The increase in the specific surface area was beneficial for subsequent utilizations of biochar, such as preparation of biochar-based fertilizers and activated carbons. 3.4. Schematic flowsheet of pretreatments and pyrolysis
Fig. 4. The relative content of different groups of bio-oil generated from pyrolysis of CS, W-CS, T-CS, and TW-CS.
Fig. 5 shows the schematic flowsheets of pretreatments and subsequent pyrolysis procedure. In order to clear the effect of the pretreatment methods on cotton stalk and its pyrolysis products,
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D. Chen et al. / Bioresource Technology 228 (2017) 62–68 Table 3 The properties of biochar issued from pyrolysis of CS, W-CS, T-CS, and TW-CS. Biochar
CS W-CS T-CS TW-CS
Proximate analysis (wt.%, db)
Ultimate analysis (wt.%, db)
Volatile
Fixed carbon
Ash
[C]
[H]
[O]
[N]
26.79 ± 0.41 25.27 ± 0.46 24.65 ± 0.43 25.28 ± 0.39
54.69 ± 0.58 60.35 ± 0.61 56.17 ± 0.56 60.38 ± 0.59
18.52 ± 0.27 14.38 ± 0.31 19.18 ± 0.35 14.34 ± 0.28
70.32 ± 0.63 72.78 ± 0.58 69.58 ± 0.62 72.75 ± 0.69
2.26 ± 0.03 2.32 ± 0.04 2.15 ± 0.03 2.33 ± 0.03
7.58 ± 0.15 9.17 ± 0.16 7.75 ± 0.13 9.13 ± 0.18
1.32 ± 0.02 1.35 ± 0.03 1.34 ± 0.02 1.45 ± 0.02
HHV (MJ/kg)
pH
SBET (m2/g)
26.59 ± 0.38 27.07 ± 0.43 26.23 ± 0.46 27.15 ± 0.39
9.02 ± 0.05 8.37 ± 0.04 9.85 ± 0.05 9.26 ± 0.05
134.63 ± 6.73 189.27 ± 9.25 178.12 ± 9.49 170.53 ± 9.36
Fig. 5. Schematic flowsheet of cotton stalk pretreatments and subsequent pyrolysis procedure.
the mass yields obtained using the different processes were also depicted in Fig. 5. The initial mass of cotton stalk was assumed to be 100, and the mass yield after each process was estimated based on the initial mass of cotton stalk (dry base). For example, for an initial mass of cotton stalk of 100 g, the produced W-CS after torrefaction-liquid washing pretreatment was 96.92 g. In turn, the 96.92 g W-CS produced 77.63 g TW-CS after the torrefaction pretreatment, and the 77.63 g TW-CS yielded respectively 27.09 g bio-oil, 30.20 g biochar, and 20.34 g gas products. The pretreatments of cotton stalk greatly changed the product distribution. Although the biomass washing pretreatment using torrefactionliquid has yet not been reported, a number of studies have already used washing pretreatment based on water or diluted acid solutions to pretreat rice husk, wheat straw, corn stalk, moso bamboo, and douglas fir wood (Deng et al., 2013; Dong et al., 2015; Pecha et al., 2015; Zhang et al., 2015). In terms of removal of metallic species from biomass, washing pretreatments with diluted acid solutions were found to be better than those based on water. In addition, the structure of biomass changed to some extent after the acid washing, resulting in a great change in distribution and quality of the pyrolysis products. In this study, the effects of washing pretreatment with torrefaction-liquid on biomass were similar to those based on dilute acid solutions reported in previous studies, especially in terms of removal of metallic species, improving the yield of bio-oil, decreasing acids content in bio-oil, and promoting the formation of phenols. The combined pretreatment process did not only allow to reuse the liquid products issued from the torrefaction pretreatment but also improved the quality of biomass and the pyrolysis products, making it a novel promising pretreatment method.
4. Conclusions The novel process based on torrefaction-liquid washing combined with torrefaction pretreatment not only allowed to reuse the torrefaction-liquid but also greatly impacted the quality of biomass and pyrolysis products. The process reduced the metals contents and improved the fuel quality of cotton stalk. It also declined the water and acids in bio-oil while increased several
aspects, such as HHV of bio-oil and non-condensable gas, some high-value compounds, and specific surface area of biochar. Therefore, the combined pretreatment method seemed to be promising as a new potential approach to improve biomass and pyrolysis products. Acknowledgements This work was supported by the Natural Science Foundation of Jiangsu Province (BK20151521), Natural Science Foundation of Zhejiang Province (LQ17E060002), University Science Research Project of Jiangsu Province (16KJB480001), Practice Innovation Training Program for College Students in Jiangsu Province (201610298025Z), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The authors also acknowledge the Advanced Analysis & Testing Center of Nanjing Forestry University for testing service. References Arias, B., Pevida, C., Fermoso, J., Plaza, M.G., Rubiera, F., Pis, J.J., 2008. Influence of torrefaction on the grindability and reactivity of woody biomass. Fuel Process. Technol. 89 (2), 169–175. Carpenter, D., Westover, T.L., Czernik, S., Jablonski, W., 2014. Biomass feedstocks for renewable fuel production: a review of the impacts of feedstock and pretreatment on the yield and product distribution of fast pyrolysis bio-oils and vapors. Green Chem. 16 (2), 384–406. Chen, D.Y., Zhou, J.B., Zhang, Q.S., 2014a. Effects of torrefaction on the pyrolysis behavior and bio-oil properties of rice husk by using TG-FTIR and Py-GC/MS. Energy Fuels 28 (9), 5857–5863. Chen, D.Y., Zhou, J.B., Zhang, Q.S., Zhu, X.F., 2014b. Evaluation methods and research progresses in bio-oil storage stability. Renewable Sustainable Energy Rev. 40, 69–79. Chen, D.Y., Zhou, J.B., Zhang, Q.S., Zhu, X.F., Lu, Q., 2014c. Torrefaction of rice husk using TG-FTIR and its effect on the fuel characteristics, carbon, and energy yields. BioResources 9 (4), 6241–6253. Chen, D.Y., Zhou, J.B., Zhang, Q.S., Zhu, X.F., Lu, Q., 2014d. Upgrading of rice husk by torrefaction and its influence on the fuel properties. BioResources 9 (4), 5893– 5905. Chen, Y.Q., Yang, H.P., Yang, Q., Hao, H.M., Zhu, B., Chen, H.P., 2014e. Torrefaction of agriculture straws and its application on biomass pyrolysis poly-generation. Bioresour. Technol. 156, 70–77. Chen, D.Y., Zheng, Z.C., Fu, K.X., Zeng, Z., Wang, J.J., Lu, M.T., 2015a. Torrefaction of biomass stalk and its effect on the yield and quality of pyrolysis products. Fuel 159, 27–32.
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Combined pretreatment with torrefaction and washing using torrefaction liquid products to yield upgraded biomass and pyrolysis products Dengyu Chen a, Jiaming Mei a, Haiping Li a, Yiming Li a, Mengting Lu a, Tingting Ma a, Zhongqing Ma b,⇑ a
College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China School of Engineering, National Engineering & Technology Research Center of Wood-Based Resources Comprehensive Utilization, Zhejiang Agriculture & Forestry University, Lin’an, Zhejiang 311300, China b
h i g h l i g h t s Torrefaction liquid washing pretreatment greatly reduced the metals of cotton stalk. Torrefaction improved the fuel property while it increased metals contents of sample. Torrefaction liquid washing reduced the secondary cracking of pyrolysis volatiles. Torrefaction greatly reduced the acids while it increased the phenols in bio-oil. Combined washing and torrefaction pretreatment preserved advantages of each method.
a r t i c l e
i n f o
Article history: Received 6 November 2016 Received in revised form 15 December 2016 Accepted 22 December 2016 Available online 24 December 2016 Keywords: Torrefaction Pyrolysis Washing Cotton stalk Bio-oil
a b s t r a c t This study presented an approach to upgrade biomass and pyrolysis products using a process based on torrefaction liquid washing combined with torrefaction pretreatment. The torrefaction of cotton stalk was first conducted at 250 °C for 30 min and then the resulting torrefaction liquid products were collected and reused to wash cotton stalk. The pyrolysis of the original and pretreated cotton stalk was performed at 500 °C for 15 min in a fixed-bed reactor. The results indicated that the combined pretreatment obviously reduced the metallic species in cotton stalk, decreased the water and acids contents while promoted phenols in bio-oil, declined the ash content in biochar, as well as improved the heating value of non-condensable gas. Overall, the combined pretreatment did not only allow to reuse the liquid products issued from torrefaction pretreatment but also improved the quality of biomass and the pyrolysis products, making it a novel promising pretreatment method. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Biomass is a promising renewable energy resource, which is mostly utilized through processes based on polygeneration of biochar, bio-oil and non-condensable gas from stalks pyrolysis (Yang et al., 2016). However, the quality of utilized biomass materials is generally low due to its elevated hydrophilicity, high oxygen content, low energy density, the complexity of storage, and dispersed areas of biomass production (van der Stelt et al., 2011). The latter led to higher transportation costs, storage and utilization of biomass as an energy resource, which, in turn, limited the development of biomass (Carpenter et al., 2014). To improve the quality ⇑ Corresponding author. E-mail address: [email protected] (Z. Ma). http://dx.doi.org/10.1016/j.biortech.2016.12.088 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.
of biomass and the pyrolysis products, pretreatment processes are hence required before the utilization of biomass (Chew and Doshi, 2011; Deng et al., 2013; Dong et al., 2015; Pecha et al., 2015). The most common used pretreatment techniques include water washing, acid washing, and torrefaction. Each process has its advantages and drawbacks. For instance, water washing pretreatment removes some of the metals from biomass, such as K, Ca, and Mg, as well as it increases the high heating value (HHV) of biomass (Liaw and Wu, 2013; Mourant et al., 2011; Zhang et al., 2015). However, since the chemical structure of the biomass is not affected by water washing pretreatment, it has limited influence on the quality of biomass and the pyrolysis products. On the other hand, acid washing pretreatment employs acid solutions, such as HCl, H2SO4 and HNO3, to remove large amounts of metals from the biomass (Dong et al., 2015). Furthermore,
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because acids could modify the chemical structure of biomass, the quality of pyrolysis products like bio-oil could significantly be improved (Ly et al., 2015; Pecha et al., 2015). Nevertheless, the high cost of acid washing pretreatments limited their practical usage. Also, the use of acids could introduce some unnecessary elements in the biomass, such as Cl, S and N, which could be incorporated into the pyrolysis products (Zhang and Xiong, 2016). The torrefaction pretreatment process consists of heating biomass up to 200–300 °C in an anoxic environment in order to remove water and some of the volatiles (Chen et al., 2015a). This treatment improves several aspects of biomass, including destroying the fiber structure, efficiently reduces the oxygen content, increases energy density, improves grinding property, enhances C/O ratio, and boosts hydrophobicity of the biomass (Arias et al., 2008; Chen et al., 2014d, 2015c; Gil et al., 2015). Meanwhile, torrefaction pretreatment positively impacts the biomass pyrolysis products by reducing water content, acid substances, and oxygen content in bio-oil (Chen et al., 2015a; Zheng et al., 2013). Moreover, the torrefaction process effectively raises contents of high-value chemicals in bio-oil, as well as enhances heat value of the gas products issued from biomass pyrolysis (Chen et al., 2016, 2014e). In recent years, torrefaction pretreatment of biomass has attracted increasing attention due to the numerous advantages cited previously. However, torrefaction pretreatment induces partial removal of volatiles from biomass, which, in turn, raises ash and metals contents after the torrefaction pretreatment. The latter negatively affects the pyrolysis of biomass by blocking and eroding pipelines, as well as inducing catalytic secondary decomposition of pyrolysis volatile compounds and reducing the yield of bio-oil. It is worth noting that the torrefaction pretreatment process produces small amounts of liquid products called ‘‘torrefaction-li quid”, mostly composed of water and small amounts of organic compounds like acids, ketones and phenols, depending on the materials and torrefaction conditions (Chen et al., 2015b; Zheng et al., 2012). Previous studies have shown that organic acids, such as acetic acid, had higher efficiencies for removal of alkali and alkaline earth metals (Oudenhoven et al., 2013; Pecha et al., 2015). Therefore, torrefaction-liquid is expected to potentially remove ash and metal species from biomass. However, a little of attention was devoted to investigating torrefaction-liquid, and its usage for improving biomass and pyrolysis products has not been reported. This paper aimed to utilize torrefaction-liquid as an approach in combination with pretreatment methods to yield improved quality of biomass and pyrolysis products. 2. Materials and methods 2.1. Materials Cotton stalk, a typical biomass, was used as the experiment material to investigate and compare the influence of different pretreatments, torrefaction-liquid washing torrefaction, and combined pretreatment, on the quality of cotton stalk and the resulting pyrolysis products, including bio-oil, biochar, and noncondensable gas. Prior to performing the experimental, the cotton stalk was first crushed into granular particles with 40–60 mesh sizes, and then dried at 105 °C for 6 h. The resulting dried cotton stalk particles were represented as CS. 2.2. Torrefaction A tube furnace (OTL1200, Nanjing University Instrument Plant, China) was used in torrefaction experiments of cotton stalk. The reaction tube consisted of a high-temperature resistant corundum tube with an inner diameter of 100 mm and a length of 1000 mm.
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Prior to experiments, cotton stalk samples (about 200 g each) were put in several quartz boats and placed inside the tube furnace away from the non-heating zone. After heating the furnace to a specific temperature, the quartz boats were then rapidly pushed into the heating zone of the furnace. The torrefaction process was performed at 250 °C for a period of 30 min. High-grade purity nitrogen was used as a carrier gas and was circulated at a flow rate of 500 mL/min. The torrefaction gaseous products first passed through the quartz tube coated with a heating jacket at 150 °C, and then entered the condensing tube placed in the liquid nitrogen, where the torrefaction liquid product (torrefaction-liquid) was collected. The experiments were repeated several times under the same conditions to collect enough torrefaction-liquid. After completion of the experiments, the tube furnace was opened and the torrefied samples were obtained and marked as T-CS. 2.3. Washing with torrefaction liquid products The water content in the torrefaction-liquid was determined to be 88.73 wt.%. The peak areas% of acetic acid, 2,3-dihydrobenzofuran, hydroxyacetone, 1-hydroxy-2-butanon and furfural, were recorded as 31.26%, 27.35%, 14.61%, 3.44% and 3.59%, respectively. It can be seen that the acids were present at high contents than the others. A cotton stalk sample (10 g) was first mixed with 200 mL torrefaction-liquid then the mixture was constantly stirred at 60 °C for 2 h. The cotton stalk sample was collected by filtration followed by washing with deionized water until reaching neutral pH. The cotton stalk was then dried at 105 °C for 12 h, and the resulting torrefaction-liquid washed samples were labeled as W-CS. 2.4. Torrefaction of washed samples The previously mentioned tube furnace was also used for torrefaction pretreatment of W-CS (about 10 g per experiment) at 250 °C for 30 min. The previously mentioned experimental procedure of cotton stalk was applied for W-CS samples, and the obtained solid samples were labeled as TW-CS. 2.5. Mass and energy yields of torrefaction and washing process Mass yield (Ymass) and energy yield (Yenergy) calculations of the torrefaction and washing process are shown as:
Y mass ¼ Mpre =M ori 100%
ð1Þ
Y energy ¼ Y mass HHV pre =HHV ori 100%
ð2Þ
where M is the mass of sample and HHV is the higher heating value of the sample. Subscripts ‘‘pre” and ‘‘ori” represent the pretreated sample and original sample, respectively. 2.6. Pyrolysis The pyrolysis of 3 g of each sample, CS, W-CS, T-CS and TW-CS, was carried out at 500 °C in a small-sized downdraft fixed bed pyrolysis reactor. The details of the pyrolysis device and the procedures could be found in the literature (Chen et al., 2015a). Highgrade purity nitrogen was used as a carrier gas with a flow rate of 200 mL/min and the pyrolysis process was performed for 15 min. After completion of each experiment, the resulting products, solid pyrolysis (biochar), liquid pyrolysis (bio-oil) and gas pyrolysis (non-condensable gas) were collected from the reactor, condensing tube and gas collecting bag, respectively. The yields of biochar and bio-oil were estimated by measuring their masses, and the yield of non-condensable gas was obtained by difference.
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2.7. Products characterization 2.7.1. Analysis of non-condensable gas The non-condensable gas was detected by a gas chromatograph analyzer (GC-TCD 7890, Shanghai Tianmei, China). Its HHV was obtained by summing each gas concentration with its HHV, following: HHVgas (MJ/Nm3) = Vol.CO (%) HHVCO (MJ/Nm3) + Vol.H2 (%) HHVH2 (MJ/Nm3) + Vol.CH4 (%) HHVCH4 (MJ/Nm3) + Vol.C2+ (%) HHVC2+ (MJ/Nm3). The overall density of the noncondensable gas can be calculated from the density of each gas. 2.7.2. Analysis of bio-oil Water content of bio-oil was analyzed by Karl-Fischer titration. HHV was estimated using an adiabatic oxygen bomb calorimeter (XRY-1A, Changji Geological Instruments, China). The pH value was measured using a digital pH meter (PHS-3C, Shanghai leici, China). Organic components were determined by gas chromatography coupled to a mass spectrometer (GC/MS 7890A/5975C, Agilent Company, USA) equipped with an HP-5MS column (30 m 250 lm 0.25 lm). Helium (99.999%) was used as a carrier gas at a flow rate of 1.0 ml/min, and 1-ll sample was injected into the column. The compounds were identified by comparing the spectral data with the NIST library and literature data. 2.7.3. Analysis of cotton stalk and biochar Proximate analysis of samples was performed according to the Chinese National Standards GB/T28731-2012. Ultimate analysis was performed using an elemental analyzer (Vario macro cube, Elementar, Germany), and oxygen was estimated by the difference. The HHV of biochar was measured using an adiabatic oxygen bomb calorimeter (XRY-1A, Changji Geological Instruments, China). The contents of metallic species of the cotton stalk samples were determined using an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 7300 DV, PerkinElmer, USA). Moreover, biochar was added to the de-ionized water in a mass ratio of 1:20 for measuring its pH using a pH meter. 2.8. Experiment repeatability The analysis experiments were replicated three times, and the averaged data were used. Most results were uniform between each run, and the relative error was generally less than 5%. Standard deviation of the data was also listed in this study. 3. Results and discussion 3.1. Physicochemical properties of samples 3.1.1. Metallic species Fig. 1 shows the resulting contents of the metallic species present in the samples. It can be observed that metallic species contents in cotton stalk were reduced to various extents after the torrefaction-liquid washing process. The removal of K was the most apparent with a removal rate reaching up to 82.30%. The removal of other elements, including Na, Mg, Ca and Fe, were recorded in the range of 15%–79%, indicating that the torrefactionliquid was effective in removing metallic species from cotton stalk. A number of previously published studies reported that both water and acid (mineral or organic acids) washing processes effectively reduce the content of metallic species (Deng et al., 2013; Ly et al., 2015; Oudenhoven et al., 2013; Pecha et al., 2015; Zhang et al., 2015). The present study also confirmed that the torrefaction-liquid washing pretreatment was effective for removal of metallic species. Furthermore, it was observed that the contents of the metallic species in cotton stalk increased after
Fig. 1. The contents of metallic species in CS, W-CS, T-CS, and TW-CS.
the torrefaction pretreatment. However, the contents of the metallic species of TW-CS were found to be lower than those of T-CS. This was caused by the large amounts of metals, which had already been removed during the washing pretreatment.
3.1.2. Fuel characteristics The main properties of the prepared samples, CS, W-CS, T-CS and TW-CS, are listed in Table 1. The ash content of W-CS decreased to 3.92 wt.% compared to its content in CS (5.54 wt.%). Meanwhile, the contents of volatiles and fixed carbon increased in W-CS with respect to CS, along with carbon content. The increase in amounts of combustible components in cotton stalk also resulted in a slight increase in the HHV of W-CS. The impact of the torrefaction pretreatment appeared more obvious when comparing with the torrefaction-liquid washing pretreatment. If compared to CS and W-CS, the fixed carbon content of T-CS obviously increased, whereas the content of volatile compounds declined. This could be attributed to the decomposition of hemicellulose and partial degradation of both the cellulose and lignin. In turn, this released larger amounts of small gas molecules like CO2 and CO, as well as some condensed gas phase products, such as H2O and acetic acid (Chen et al., 2014a,c; Ma et al., 2015). One of the important roles of torrefaction pretreatment was to raise the carbon content and reduce the oxygen, which should result in decreasing the molar ratios of H/C and O/C and increasing HHV of the cotton stalk. However, as ash and metallic species remained in the cotton stalk, this should increase their contents after torrefaction, which was unfavorable for fuel quality of biomass and subsequent utilizations. If compared to direct torrefaction pretreatment, the process based on torrefaction-liquid washing combined with torrefaction pretreatment obviously influenced the basic properties of cotton stalk. As shown in Table 1, the values of proximate analysis, ultimate analysis and HHV of TW-CS were found to locate between the values of W-CS and T-CS. Besides, HHV and carbon content values of TW-CS were close to those of T-CS, whereas the volatile content values approached those of W-CS. After the different pretreatment processes, some changes in mass and energy yields of cotton stalk appeared and the results are gathered in Table 1. The torrefaction-liquid washing process clearly showed that in addition to removal of metals from the cotton stalk, low amounts organic components were eluted and led to a decline in the energy yield of W-CS to less than 100%. The torrefaction pretreatment resulted in a greater mass loss mainly caused by the decomposition of cellulose, hemicellulose,
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D. Chen et al. / Bioresource Technology 228 (2017) 62–68 Table 1 The fuel properties, mass and energy yields of original and pretreated cotton stalk. Sample
CS W-CS T-CS TW-CS
Proximate analysis (wt.%, db)
Ultimate analysis (wt.%, db)
Volatile
Fixed Carbon
Ash
[C]
[H]
[O]
[N]
76.93 ± 1.13 79.13 ± 1.08 64.83 ± 0.87 68.53 ± 1.02
17.53 ± 0.35 16.95 ± 0.34 27.81 ± 0.49 26.41 ± 0.52
5.54 ± 0.11 3.92 ± 0.08 7.36 ± 0.13 5.06 ± 0.09
45.16 ± 0.91 45.89 ± 0.86 51.85 ± 0.87 52.27 ± 0.89
6.28 ± 0.13 6.45 ± 0.12 5.79 ± 0.12 5.78 ± 0.11
42.35 ± 0.76 43.06 ± 0.82 34.39 ± 0.58 36.25 ± 0.72
0.67 ± 0.01 0.68 ± 0.01 0.61 ± 0.01 0.64 ± 0.01
HHV (MJ/kg)
Ymass (%)
Yenergy (%)
16.83 ± 0.36 17.18 ± 0.32 18.96 ± 0.39 20.13 ± 0.44
100 96.92 ± 0.85 80.25 ± 1.13 77.63 ± 0.98
100 98.94 ± 0.33 90.41 ± 0.38 92.85 ± 0.39
and lignin. However, the energy yields remained higher than mass yields under the same conditions, indicating that energy per unit mass was improved after the pretreatment of cotton stalk.
led to an increase in yield of bio-oil by 16.11% and biochar by 9.58%, while the yield of non-condensable gas decreased by 23.17%.
3.2. Effect of pretreatments on distribution of pyrolysis products
3.3. Effect of pretreatments on pyrolysis products properties
The product yields of pyrolysis of CS, W-CS, T-CS and TW-CS, are shown in Fig. 2. Compared with pyrolysis of CS, the bio-oil yield of W-CS slightly increased while the biochar yield somewhat declined. This was mainly caused by the torrefaction-liquid washing pretreatment which greatly removed the metals, especially the alkali and alkaline earth metals. This process changed the pyrolysis pathway to some extent, decreased the secondary decomposition of volatiles, increased the production of bio-oil, and reduced the yield of biochar. The torrefaction pretreatment had an obvious influence on the distribution of the pyrolysis products. Compared with CS and W-CS, the bio-oil yield of T-CS reduced by almost 26–29% while the biochar yield increased by 15–19%. Large amounts of hemicelluloses were decomposed during the torrefaction process due to the poor thermal stability, while small amounts of cellulose and lignin decomposed because of their relatively better thermal stabilities. This resulted in the increased content of lignin in biomass after torrefaction, while hemicellulose content dramatically reduced. A previous study showed that biochar yield of lignin was the highest with 50–75%, followed by xylan with 25–35%, and then cellulose with 5–23%; meanwhile, xylan and cellulose were found to have higher liquid yields of 40–50% and gas yields of about 30% (Xin et al., 2013). Therefore, torrefaction pretreatment increased the yield of biochar and reduced that of bio-oil. The combined pretreatment also greatly influenced the pyrolysis products. Although the bio-oil yield of TW-CS was lower to those of CS and W-CS, it was clearly superior to that of T-CS. With respect to torrefaction pretreatment, the combined pretreatment
3.3.1. Non-condensable gas The composition of the pyrolysis gas products included CO, CO2, CH4, H2 and small amounts of C2+ gases, where volume fractions of these gases are shown in Fig. 3. It can be observed that the volume fraction of CO2 in CS was the highest with 43.11 vol.%. On the other hand, the volume fractions of CO, CH4 and H2 were recorded as 28.72 vol.%, 16.35 vol.% and 9.21 vol.%, respectively. If compared to CS, the torrefaction-liquid washing pretreatment slightly increased and decreased contents of CO and CO2, respectively. It also somewhat influenced the contents of CH4 and H2. The torrefaction pretreatment process greatly affected the volume fractions of the non-condensable gases. When compared with CS, the CO2 volume fraction declined by 12.30% for T-CS, whereas the volume fractions of CH4 and H2 increased by 9.72% and 26.09%, respectively. Interestingly, the combined pretreatment further raised the volume fractions of CO and H2 while declined that of CO2. The HHVs of the non-condensable gases obtained with CS, W-CS, T-CS and TW-CS were estimated as 13.43 MJ/m3, 13.13 MJ/m3, 14.34 MJ/m3 and 14.70 MJ/m3, respectively. Overall, both processes, torrefaction and combined pretreatments, obviously enhanced the total volume fractions of the combustible gases, such as H2, CO, and CH4. This, in turn, led to increased HHV of the non-condensable gases when compared to the original cotton stalk.
Fig. 2. The yields of products generated from pyrolysis of CS, W-CS, T-CS, and TWCS.
Fig. 3. The volume fraction of gas products issued from pyrolysis of CS, W-CS, T-CS, and TW-CS.
3.3.2. Bio-oil The water content, HHV and pH value of bio-oil are listed in Table 2. The water content of bio-oil obtained from CS was estimated to 58.19 wt.%. However, all the three pretreatment processes,
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Table 2 The water content, HHV and pH values of bio-oil issued from pyrolysis of CS, W-CS, TCS, and TW-CS. Bio-oil
Water content (wt.%)
HHV (MJ/kg)
pH
CS W-CS T-CS TW-CS
58.19 ± 0.92 55.22 ± 0.95 41.39 ± 0.83 37.08 ± 0.46
11.34 ± 0.28 12.57 ± 0.31 14.83 ± 0.25 15.56 ± 0.23
2.95 ± 0.06 3.04 ± 0.05 3.23 ± 0.04 3.34 ± 0.05
torrefaction-liquid washing, torrefaction and combined pretreatments, produced bio-oils with reduced water contents. Table 2 clearly stated that water content in bio-oil obtained from TW-CS was the lowest with a value of 37.08 wt.%. Meanwhile, HHV of bio-oil increased from 11.34 MJ/kg for CS to 12.57 MJ/kg for W-CS. It then gradually increased to 14.83 MJ/kg for T-CS bio-oil and to finally reach a maximum value of 15.56 MJ/kg for bio-oil issued from TW-CS. The pH value of bio-oil increased after the pretreatments, suggesting that acidity of the bio-oil was reduced. These findings indicated that although all pretreatments positively impacted the quality of bio-oil, the combined pretreatment was more effective than the others. The composition of bio-oil is complex as it contains hundreds of chemicals worth identifying. These chemicals could be categorized into six groups based on their functional groups: acids, ketones, aldehydes, phenols, furans, and anhydrosugars. Fig. 4 shows the peak areas% of each group among the six groups found in bio-oil. It can be seen that the relative contents of acids, furans, ketones and phenols were relatively high in the bio-oil issued from CS pyrolysis. All pretreatments obviously influenced distribution of the components in bio-oil. When compared with CS, the relative contents of acids in W-CS, T-CS and TW-CS decreased by 12.13%, 30.99% and 38.65%, respectively. Meanwhile, the relative contents of phenols increased by 8.06%, 75.27% and 91.40% for W-CS, T-CS and TW-CS, respectively. Besides, the ketone and furan contents declined while phenols and anhydrosugars contents rose after subjecting to both the torrefaction and the combined pretreatments. This could mainly be related to two reasons. On the one hand, large amounts of hemicellulose were decomposed during the torrefaction pretreatment to yield acetic acid as the main pyrolysis product of hemicellulose. This indicated that a small amount of acetic acid was produced during the pyrolysis of T-CS. On the other hand, the torrefaction pretreatment increased the lignin content of biomass,
and phenols were the main liquid products issued from pyrolysis of lignin. This, in turn, raised the relative content of phenols. The presence of metallic species like K and Na prevented the decomposition of cellulose to sugars. The catalytic role played by metals in the decomposition of glucose units led to large numbers of low molecular weight compounds, such as aldehydes and ketones (Lv and Wu, 2012). The torrefaction-liquid washing pretreatment greatly removed the metallic species from cotton stalk, which subsequently led to the formation of sugars, including laevoglucose. In addition, as previously reported, the interaction between cellulose, xylan and lignin suppressed the formation of water-soluble tar like levoglucosan (Fushimi et al., 2009). However, the torrefaction pretreatment destroyed the fiber structure of cotton stalk and reduced the interaction between the three components, thus yielding more sugars during the pyrolysis process. It should be noted that elevated acid contents (mainly acetic acid) and water produced during pyrolysis were responsible for the high acidity and poor stability of bio-oil (Chen et al., 2014b). Bio-oil often contains some valuable chemical components, such as aromatic hydrocarbons, guaiacol, and laevoglucose. The combined pretreatment declined the water and acids contents while increased contents of some valuable products and the HHV of the bio-oil. This suggested that torrefaction-liquid utilized as pretreatment to wash biomass was promising in terms of reuse of torrefaction-liquid and improved quality of the pyrolysis products. 3.3.3. Biochar The results of the proximate analysis, ultimate analysis, HHVs, pH value and specific surface areas of biochars are listed in Table 3. After the torrefaction-liquid washing pretreatment, the ash content in biochar decreased. The combined pretreatment resulted in minimum ash content in the biochar. When compared to the biochar generated from pyrolysis of CS, the fixed carbon contents of biochar formed during pyrolysis of pretreated cotton stalk increased to different extents. With respect to elemental content, both processes based on torrefaction and combined pretreatment slightly increased the carbon content, which, in turn, raised the HHV of biochar. Table 3 revealed that pH of the samples was overall alkaline, mainly caused by the presence of carbonates and some metals in the biochars. If compared to biochar produced from CS pyrolysis, the pH values of biochars produced from W-CS and TW-CS pyrolysis declined. The latter resulted from the removal of large amounts of metals during the torrefaction-liquid washing pretreatment. On the other hand, the high pH value recorded for biochar produced from T-CS pyrolysis was attributed to the torrefaction pretreatment in which the metallic species contents remained high in the cotton stalk. With respect to fuel properties of biochars, overall no significant differences were observed between the three tested pretreatment methods. However, the impact on the structure of biochar can clearly be distinguished. Table 3 depicted that all the three pretreatment methods increased the specific surface area of biochar, where biochar obtained from the pyrolysis of W-CS showed the largest value. This phenomenon may be ascribed to removal of large amounts of impurities and metallic species present in cotton stalk using the torrefaction-liquid washing pretreatment, which benefited the formation of porous structures in biochar (Klasson et al., 2014). The increase in the specific surface area was beneficial for subsequent utilizations of biochar, such as preparation of biochar-based fertilizers and activated carbons. 3.4. Schematic flowsheet of pretreatments and pyrolysis
Fig. 4. The relative content of different groups of bio-oil generated from pyrolysis of CS, W-CS, T-CS, and TW-CS.
Fig. 5 shows the schematic flowsheets of pretreatments and subsequent pyrolysis procedure. In order to clear the effect of the pretreatment methods on cotton stalk and its pyrolysis products,
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D. Chen et al. / Bioresource Technology 228 (2017) 62–68 Table 3 The properties of biochar issued from pyrolysis of CS, W-CS, T-CS, and TW-CS. Biochar
CS W-CS T-CS TW-CS
Proximate analysis (wt.%, db)
Ultimate analysis (wt.%, db)
Volatile
Fixed carbon
Ash
[C]
[H]
[O]
[N]
26.79 ± 0.41 25.27 ± 0.46 24.65 ± 0.43 25.28 ± 0.39
54.69 ± 0.58 60.35 ± 0.61 56.17 ± 0.56 60.38 ± 0.59
18.52 ± 0.27 14.38 ± 0.31 19.18 ± 0.35 14.34 ± 0.28
70.32 ± 0.63 72.78 ± 0.58 69.58 ± 0.62 72.75 ± 0.69
2.26 ± 0.03 2.32 ± 0.04 2.15 ± 0.03 2.33 ± 0.03
7.58 ± 0.15 9.17 ± 0.16 7.75 ± 0.13 9.13 ± 0.18
1.32 ± 0.02 1.35 ± 0.03 1.34 ± 0.02 1.45 ± 0.02
HHV (MJ/kg)
pH
SBET (m2/g)
26.59 ± 0.38 27.07 ± 0.43 26.23 ± 0.46 27.15 ± 0.39
9.02 ± 0.05 8.37 ± 0.04 9.85 ± 0.05 9.26 ± 0.05
134.63 ± 6.73 189.27 ± 9.25 178.12 ± 9.49 170.53 ± 9.36
Fig. 5. Schematic flowsheet of cotton stalk pretreatments and subsequent pyrolysis procedure.
the mass yields obtained using the different processes were also depicted in Fig. 5. The initial mass of cotton stalk was assumed to be 100, and the mass yield after each process was estimated based on the initial mass of cotton stalk (dry base). For example, for an initial mass of cotton stalk of 100 g, the produced W-CS after torrefaction-liquid washing pretreatment was 96.92 g. In turn, the 96.92 g W-CS produced 77.63 g TW-CS after the torrefaction pretreatment, and the 77.63 g TW-CS yielded respectively 27.09 g bio-oil, 30.20 g biochar, and 20.34 g gas products. The pretreatments of cotton stalk greatly changed the product distribution. Although the biomass washing pretreatment using torrefactionliquid has yet not been reported, a number of studies have already used washing pretreatment based on water or diluted acid solutions to pretreat rice husk, wheat straw, corn stalk, moso bamboo, and douglas fir wood (Deng et al., 2013; Dong et al., 2015; Pecha et al., 2015; Zhang et al., 2015). In terms of removal of metallic species from biomass, washing pretreatments with diluted acid solutions were found to be better than those based on water. In addition, the structure of biomass changed to some extent after the acid washing, resulting in a great change in distribution and quality of the pyrolysis products. In this study, the effects of washing pretreatment with torrefaction-liquid on biomass were similar to those based on dilute acid solutions reported in previous studies, especially in terms of removal of metallic species, improving the yield of bio-oil, decreasing acids content in bio-oil, and promoting the formation of phenols. The combined pretreatment process did not only allow to reuse the liquid products issued from the torrefaction pretreatment but also improved the quality of biomass and the pyrolysis products, making it a novel promising pretreatment method.
4. Conclusions The novel process based on torrefaction-liquid washing combined with torrefaction pretreatment not only allowed to reuse the torrefaction-liquid but also greatly impacted the quality of biomass and pyrolysis products. The process reduced the metals contents and improved the fuel quality of cotton stalk. It also declined the water and acids in bio-oil while increased several
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