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The effect of combined pretreatments on the pyrolysis of corn stalk
Bioresource Technology 281 (2019) 309–317
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
The effect of combined pretreatments on the pyrolysis of corn stalk a,b,c
Kuo Zeng a b c
c
a
a,⁎
T
a,c
, Xiao He , Haiping Yang , Xianhua Wang , Hanping Chen
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, PR China Shenzhen Huazhong University of Science and Technology Research Institute, Shenzhen 523000, PR China Department of New Energy Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Corn stalk Demineralization Torrefaction Pretreatment Pyrolysis
Untreated corn stalk (CS), deionized water washed CS (WCS), aqueous phase bio-oil washed CS (LCS), and 5% acetic acid washed CS (CCS) were torrefied at 230, 260, and 290 °C. The influences of washing, torrefaction, and combined washing-torrefaction pretreatments on corn stalk pyrolysis were investigated. The combined pretreatments, especially aqueous phase bio-oil washing-torrefaction improved fuel properties of pretreated samples largely by increasing their volatile and hydrogen contents. Absorption peaks of O–H and C]O groups in combined pretreatment samples increased when torrefaction temperature increased. In addition, CO, H2, and CH4 contents of pyrolysis gas increased, while CO2 decreased after combined pretreatments. The bio-oil yields from WCS290, LCS290, and CCS290 increased by 134.04%, 127.66%, and 129.79% respectively, compared with that from CS290. Similarly, their relative sugar contents (rich in levoglucosan) increased to 36.63%, 45.89%, and 52.34%, respectively. Aqueous phase oil washing-torrefaction is a promising pretreatment and acetic acid plays the most important role.
1. Introduction Corn stalk is the most abundant agricultural waste in China. In 2017, approximately 239 million tons of corn stalk were produced and were therefore potentially available for conversion into biofuels (biochar, bio-oil, and gas) by pyrolysis (Cen et al., 2016). Pyrolysis of corn
⁎
stalk not only disposes of agricultural waste, but also produces “green fuel”. The utilization of the products of pyrolysis, especially bio-oil, is greatly limited by poor fuel properties (Wigley and Alex, 2016; Wigley et al., 2015). The high oxygen content and alkali and alkaline earth metals (AAEMs) in biomass feedstock lead to high oxygen and moisture
Corresponding author. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.biortech.2019.02.107 Received 2 January 2019; Received in revised form 22 February 2019; Accepted 23 February 2019 Available online 25 February 2019 0960-8524/ © 2019 Published by Elsevier Ltd.
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investigate the effects of a combined pretreatment of washing with aqueous phase bio-oil from pyrolysis and torrefaction on subsequent pyrolytic behavior (Chen et al., 2017a; Zhang et al., 2016a). Among them, Chen et al. found that bio-oil yield increased, with increased phenol and decreased acid production in the bio-oil (Chen et al., 2017a). Aqueous phase bio-oil contains high water and acid contents and low levels of ketones and phenols. Karnowo et al. demonstrated that phenols promote leaching of AAEMs as it permeated into the organic matrix of the biomass, forming hydrophobic interactions and making them more accessible to acid and water (Karnowo et al., 2014). By comparing washing with different compounds (acid and water) we can determine the role of other compounds in aqueous phase bio-oil in leaching, torrefaction, and subsequent pyrolysis. However, examinations of the influence of combined pretreatments, including washing with different types of solutions and torrefaction at different temperatures, have not been reported. Similarly, pretreatment effects on biomass characteristics and its subsequent pyrolysis have not yet been investigated. The principal aim of this study was to demonstrate the feasibility of producing high quality pyrolysis products by combining demineralization and torrefaction pretreatments. Corn stalk was washed with different solutions (deionized water, aqueous phase oil, and 5% acetic acid) and then torrefied at different temperatures (230, 260, and 290 °C). Different pretreatment processes were evaluated and compared by analyzing the fuel characteristics of original and pretreated samples. Following this, the untreated and pretreated corn stalk was pyrolyzed and the yields and properties of the char, liquid, and gas products were analyzed. The use of a single washing compound (acid or water) compared to aqueous phase bio-oil has not been reported to-date. Therefore, the secondary objective of this study was to determine the roles of other compounds (acid or water) in aqueous phase bio-oil in leaching, torrefaction, and subsequent pyrolysis. The results of this study will be valuable for the high value utilization of corn stalk pyrolysis bio-oil.
content, and hence slagging/fouling shortcomings, of the bio-oil product. Therefore, biomass pretreatments prior to pyrolysis are necessary to improve feedstock properties and upgrade its final products (Carpenter et al., 2014). Torrefaction pretreatment releases oxygen from biomass as it is pyrolyzed at temperatures in the range of 200–300 °C (Chen et al., 2015). A lower oxygen and moisture content with higher energy density pretreated biomass feedstock is thereby obtained. Bio-oil products from subsequent pyrolysis have lower oxygen, water, and acid contents (Meng et al., 2012; Zeng et al., 2018a; Zheng et al., 2013). Variation in hemicellulose, cellulose, and lignin contents during torrefaction have a direct influence on subsequent pyrolysis products (Chen et al., 2014). The content of oxy-compounds (such as acetic acid and furan) in bio-oil decreased with increasing torrefaction temperature, while the content of aromatic hydrocarbons increased (Zeng et al., 2018a). Washing pretreatments remove inorganic content (including soluble salts or cations bound to reactive sites) from biomass as it is demineralized by leaching in water, light bio-oil, or dilute acid (Wigley et al., 2015). Soluble salts (about 30–80% of AAEMs) can be leached by water (Cen et al., 2016). The removal of soluble salts by water leaching has a negligible effect on the yield and composition of bio-oil, while the removal of acid-soluble cations using mineral acids (HCl and H2SO4) has been utilized (Dong et al., 2015). However, the use of mineral acids and organic acids is not practicable because of the problems of corrosion and cost (Karnowo et al., 2014). Bio-oil derived from biomass pyrolysis, especially aqueous phase bio-oil (light bio-oil) obtained from natural phase separation is considered an effective and cheap demineralization agent for removing AAEMs (Chen et al., 2017; Karnowo et al., 2014; Zhang et al., 2016a). Almost no soluble salts or cations are bound in aqueous phase bio-oil washed biomass feedstock (Chen et al., 2017a). The removal of acid-soluble cations via acid or bio-oil leaching increased the yield of bio-oil from subsequent pyrolysis and improved its quality by lowering the water content and enriching the content of levoglucosan (Mourant et al., 2011). There are some drawbacks reducing the benefits of torrefaction or washing pretreatments and restricting their individual use. First, torrefaction increases the inorganics in pretreated biomass, which lowers the bio-oil quality due to AAEMs catalyzing dehydration and cracking reactions of intermediate bio-oil (Saddawi et al., 2012). Second, the washed sample used as feedstock in subsequent pyrolysis must be dried to eliminate the washing solutions. One way to minimize the impacts of these limitations is to combine these two pretreatments. In such a combination, a washing pretreatment is used to remove inorganics, torrefaction is then performed to both reduce oxygen content and dry the washed sample. An aqueous phase bio-oil (light bio-oil) as a washing agent is optimal because it is cheap and has environmentally friendly properties (Karnowo et al., 2014). The quality of such pretreated biomass and its subsequent pyrolysis product are thereby enhanced. Few studies have been undertaken to examine the efficacy of a combined washing and torrefaction pretreatment on biomass pyrolysis. Some research groups have used combined water washing and torrefaction pretreatments to remove inorganic minerals and oxygen from biomass, which enhanced the quality and yield of bio-oil following subsequent pyrolysis (Cen et al., 2016; Zhang et al., 2016b, 2015). Zhang et al. (2018) and Ukaew et al. (2018) investigated the effects of combined torrefaction and leaching with inorganic acid (HCl) or organic acid (acetic acid) on biomass pyrolysis. Their results demonstrated that bio-oil quality was improved by enriching phenols and sugars, especially levoglucosan. A combined pretreatment with torrefaction and leaching using an acidic liquid (rich in acetic acid and formic acid) produced from torrefaction has been proposed (Chen et al., 2017a; Wigley and Alex, 2016; Wigley et al., 2015). Wigley et al. found that levoglucosan increased significantly with a combined pretreatment compared to washing or torrefaction alone (Wigley and Alex, 2016; Wigley et al., 2015). There are also some studies conducted to
2. Materials and methods 2.1. Experimental sample 2.1.1. Corn stalk Corn stalk was collected from Luoyang city, Henan province, China. Prior to the experiments, it was screened through a narrow particle size distribution within the range 125–250 μm to avoid the confounding effects of particle size, and then dried at 105 °C for 48 h. The dried corn stalk was denoted as CS. 2.1.2. Washing agents Pyrolysis oil was collected from a biomass-based pyrolytic polygeneration factory with a million-ton annual production, located in Ezhou city, Hubei province, China. It was prepared from corn stalk pyrolysis at 550 °C. Aqueous phase oil naturally separates from pyrolysis oil during storage. The water content of aqueous phase oil was about 91% by weight. Gas chromatography/mass spectrometry (GC/ MS) was used to analyze the chemical composition of the aqueous phase oil; acetic acid was abundant (peak area of 53.52%) in aqueous phase oil. Deionized water and 5% acetic acid were chosen as blank washing agents. The roles of the main compounds contained in aqueous phase bio-oil playing in leaching, torrefaction, and subsequent pyrolysis were determined. 2.2. Pretreatments 2.2.1. Washing The corn stalk was washed with either deionized water, aqueous phase oil, or 5% acetic acid. For each washing, 30 g of corn stalk was 310
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2018b). For washed samples (WCS, LCS, and CCS), torrefaction product yields followed the same trends, with rising temperature causing decreased solid product yield and increased yields of liquid and gas products. However, the torrefied solid yields from washed samples decreased more slowly compared to that from CS torrefaction. Differences in the yields of solid products from differently washed samples were very small at 290 °C, with increases of 68.87%, 68.50%, and 66.50% for WCS, LCS, and CCS, respectively (Fig. 1a). The increase of solid yields from washed samples was attributed to the removal of AAEMs, because they reduce pyrolysis temperature and facilitate the release of volatile compounds (Mourant et al., 2011).
added to 600 mL solutions at 30 °C and stirred for 4 h with a magnetic stirrer. After washing, the corn stalk was separated by vacuum filtration and washed with deionized water until the pH of deionized water after flushing was close to neutral. The washed corn stalk was then dried at 105 °C for 48 h in an oven, and subsequently denoted as WCS, LCS, and CCS, respectively. 2.2.2. Torrefaction after washing The torrefaction pretreatment after washing was conducted in a horizontal tube furnace. Details of the torrefaction setup and process have been reported in our previous studies (Zeng et al., 2018a,b). The corn stalk (CS), deionized water washed corn stalk (WCS), aqueous phase oil washed corn stalk (LCS), and 5% acetic acid washed corn stalk (CCS) were torrefied at three different temperatures (230, 260, and 290 °C) for 30 min. In view of the deoxygenating and upgrading effects of torrefaction pretreatment, moderate (230 °C) and severe (260 and 290 °C), torrefaction conditions were investigated in this study. The torrefied samples were labelled as CS230, CS260, CS290, WCS230, WCS260, WCS290, LCS230, LCS260, LCS290, CCS230, CCS260, CCS290.
3.1.2. Metallic species Table 1 presents the contents of metallic species in the samples. The content of metallic species in corn stalk was reduced to various extents after washing with different agents, in agreement with previous studies (Cen et al., 2016; Chen et al., 2017a; Zhang et al., 2018). The removal rates of K, Na, Ca, Mg, and Fe were 79.88%, 77.93%, 16.57%, 32.50%, and 48.89%, respectively, when corn stalk was washed with deionized water. Removal rates increased to 97.53%, 81.38%, 27.16%, 84.86%, and 49.96% respectively, with aqueous phase oil as the washing agent. These reductions were very similar to those obtained when 5% acetic acid washing pretreatment was applied (97.46%, 80.00%, 21.42%, 89.51%, and 53.33%, respectively). Thus the removal rates of metallic species by aqueous phase oil and 5% acetic acid washing were higher than those obtained by deionized water washing. These results can be explained by the presence of not only acetic acid but also phenolic compounds in aqueous phase oil. These phenols permeated into the organic matrix of corn stalk by hydrophobic interactions, making the matrix more accessible to acetic acid and water, thereby promoting the removal of metallic species (Karnowo et al., 2014).
2.3. Characterization of raw and pretreatment corn stalk The ultimate and proximate analyses of corn stalk were performed with an elemental analyzer (2400 II, PerkinElmer, USA) and industrial analyzer (SDTGA5000a), respectively. The absolute contents of metallic species in the original and washed corn stalk samples were measured with inductively coupled plasma mass spectrometry (ICP-MS, ELAN DRC-e, PerkinElmer, USA). The functional groups of samples were analyzed with a Fourier transform infrared spectroscopy (FTIR) analyzer (VERTEX 70, BrukerBruker Bruker, Germany).
3.1.3. Fuel properties The main properties of samples (CS, WCS, LCS, and CCS) before and after torrefaction are shown in Table 2. The ash contents of samples subject only to washing (WCS, LCS and CCS) decreased by 50.20%, 66.51%, and 65.94%, respectively, compared to its content in unwashed CS. The volatile content was slightly increased. This can be explained by the removal of AAEMs and a small amount of organic matter by the washing pretreatment (Zhang et al., 2016a). There was almost no difference in elemental contents between CS and washed CS (WCS, LCS, and CCS), indicating that the washing pretreatments caused no change in corn stalk composition (Chen et al., 2017). The torrefaction pretreatment clearly affected the properties of CS and its effect was more obvious than that of washing pretreatments. The volatile contents decreased with increasing torrefaction temperature; however, ash contents and fixed carbon contents increased significantly (Table 2). These results can be explained by the enhanced decomposition of hemicellulose and cellulose, resulting in increased release of volatiles from corn stalk (Cen et al., 2016). Consequently, the remaining ash exhibited a relative increase of its mass fraction. The ash contents of corn stalk obtained from the combination of washing and torrefaction pretreatments were less than those of just the torrefaction pretreatment, and even less than that of the original corn stalk. Chen et al. found a similar change in ash content with a combined pretreatment, which implied that washing could offset the negative effect of torrefaction on ash content (Chen et al., 2017). In addition, the combined pretreatments caused an increase in the volatile content and a decline in the fixed carbon content in the sample compared to that of the torrefaction pretreated sample. This result might be due to the removal of most metallic species such as Na, Mg, and Ca by the washing pretreatment, which inhibited the release of volatile contents during subsequent torrefaction (Zeng et al., 2018a,b; Zhang et al., 2018). Torrefaction caused a significant increase in carbon content and a decline in oxygen content in corn stalk due to the release of CO2 and H2O. The carbon contents of WCS290, LCS290, and CCS290 were 49.56%, 49.54%, and 50.16%
2.4. Pyrolysis Pyrolysis was conducted in a fixed-bed reactor, as described in our previous study (Zeng et al., 2018b). A total of 3 g ( ± 5%) of each sample was placed in a quartz boat and quickly fed into the reactor when it reached 550 °C and held at that temperature for 30 min. Subsequently, 400 mL/min nitrogen was flushed through the reactor to remove the gaseous products during pyrolysis. The liquid product was collected in the condensation unit. The gas product was kept in a sampling bag. The solid product was left to cool and kept in the quartz boat. The liquid and solid yields were determined by the differences in mass of the quartz boat and condensation before and after pyrolysis, respectively. Gas yield was calculated by difference. Each run was repeated at least three times to ensure the Relative Standard Deviation (RSD) was less than 5%. Gas composition was determined by micro-gas chromatography (Micro-GC, Agilent 3000). The liquid product organic components were determined by gas chromatography-mass spectroscopy (GC–MS; 7890A GC- HP5975 MS) and individual compounds were identified based on a NIST mass spectral data library and our previous internal standard method (Zeng et al., 2018a). 3. Results and discussion 3.1. Effect of pretreatments on corn stalk characteristics 3.1.1. Pretreatment product distribution The yields of solid, liquid, and gas products obtained from torrefaction of original and washed corn stalk are presented in Fig. 1. For CS torrefaction (corn stalk), the solid yield decreased from 96.02% to 49.58% as torrefaction temperature increased from 200 to 290 °C (Fig. 1a). Simultaneously, the gas yield increased from 2.42% to 24.90% (Fig. 1b), and the liquid yield increased from 1.56% to 25.52% (Fig. 1c). These results are consistent with enhanced degradation of hemicellulose and cellulose in this temperature range (Zeng et al., 311
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Fig. 1. Torrefaction product distribution of original and washed corn stalk: (a) solid yield; (b) liquid yield; and (c) gas yield. Table The AAEM content and removal efficiency of original and washed corn stalk. Samples
AAEMs content (mg/kg)
CS WCS LCS CCS
AAEMs removal efficiency (%)
K
Na
Ca
Mg
Fe
K
Na
Ca
Mg
Fe
10,020 2016 250 255
290 64 54 58
2890 2411 2105 2271
1440 972 218 151
360 176 170 168
0 79.88 97.53 97.46
0 77.93 81.38 80
0 16.57 27.16 21.42
0 32.5 84.86 89.51
0 48.89 52.78 53.33
combination of washing and torrefaction pretreatment increased the hydrogen content and decreased the carbon content in pretreated samples.
Table 2 The fuel properties of original and pretreated corn stalk. Samples
CS CS 230 CS 260 CS 290 WCS WCS 230 WCS 260 WCS 290 LCS LCS 230 LCS 260 LCS 290 CCS CCS 230 CCS 260 CCS 290
Proximate analysis (wt.%, db)
Ultimate analysis (wt.%, db)
Volatile matter
Ash
Fixed carbon
Cd
Hd
Od
Nd
O/C
H/C
84.13 79.74 75.17 58.43 86.87 85.49 84.06 79.88 87.83 85.50 83.08 81.56 85.36 83.18 80.90 76.09
3.09 3.78 4.49 6.25 1.54 1.61 1.69 2.10 1.04 1.27 1.51 1.81 1.05 1.31 1.58 2.03
12.79 16.48 20.33 35.32 11.59 12.89 14.25 18.02 11.13 13.23 15.42 16.63 13.58 15.51 17.51 21.88
44.17 46.33 48.95 56.57 45.43 46.45 47.94 49.56 46.16 46.90 48.12 49.54 45.35 46.67 48.47 50.16
5.81 5.68 5.60 4.97 5.97 5.90 5.89 5.69 6.00 5.86 5.77 5.62 5.91 5.81 5.77 5.60
49.61 47.09 45.05 37.94 48.41 46.96 45.97 44.52 47.58 46.47 45.83 44.47 48.48 46.73 45.46 43.88
0.41 0.40 0.39 0.53 0.18 0.19 0.20 0.23 0.26 0.27 0.29 0.38 0.27 0.28 0.30 0.36
0.84 0.76 0.69 0.50 0.80 0.76 0.72 0.67 0.77 0.74 0.71 0.67 0.80 0.75 0.70 0.66
1.58 1.47 1.37 1.05 1.58 1.52 1.47 1.38 1.56 1.49 1.43 1.36 1.56 1.49 1.43 1.34
3.1.4. Functional groups The evolution of typical functional groups in corn stalk during pretreatment is presented. Absorption peaks centered at 3440 cm−1 (O–H) and 1730 cm−1 (C]O) initially increased with torrefaction temperature rising to 260 °C and then decreased at the higher temperature of 290 °C. Increasing amounts of hemicellulose decomposed with the increase of torrefaction temperature, which resulted in a loss of more functional groups (Correia et al., 2017; Zhang et al., 2016a). However, this also resulted in an increase in the relative content of cellulose when torrefaction temperature increased to 260 °C. Because cellulose contains a high content of O–H and C]O groups, the absorption peaks of O–H and C]O groups in CS230 and CS260 increased even with enhanced decomposition of hemicellulose. At a higher torrefaction temperature of 290 °C, the absorption peaks of O–H and C]O groups in CS290 slightly decreased due to the enhanced decomposition of cellulose. In addition, FTIR spectra of just washed samples (WCS, LCS, and CCS) were similar to that of original corn stalk. Thus, the washing pretreatment caused no obvious loss of organic content in biomass (Zhang et al., 2016a). The absorption peaks of O–H and C]O groups in the combined pretreatment samples increased with increased torrefaction temperature. This may be attributed to the removal of
lower than that of CS290 (56.57%), respectively. The hydrogen contents of WCS290, LCS290, and CCS290 were 5.69%, 5.62%, and 5.60% higher than that of CS290 (4.97%), respectively. In general, the 312
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Fig. 2. Effect of torrefaction temperature on the product distribution from subsequent pyrolysis of: (a) CS; (b) WCS, (c) LCS; and (d) CCS.
corn stalk occurs in response to the torrefaction pretreatment (Chen et al., 2017), while the ash content in combined pretreatment samples was less than that for samples exposed only to the torrefaction pretreatment due to the removal of most metallic species by washing (Chen et al., 2017). Consequently, the decrease of yield of the pyrolysis liquid was more obvious for just the torrefaction pretreatment sample, with more catalytic secondary reactions of volatiles than that occurring in combined pretreatment samples (Zhang et al., 2018). Compared with CS290 pyrolysis (Fig. 2a), the pyrolysis liquid yields of WCS290, LCS290, and CCS290 increased by 134.04% (Fig. 2b), 127.66% (Fig. 2c), and 129.79% (Fig. 2d), respectively. In contrast, the pyrolysis char yields decreased by 48.51%, 46.27%, and 46.27%, respectively. In addition, for the combined pretreated samples, the liquid yield decreased while char yield increased with increasing torrefaction temperature (Fig. 2b, c, and d).
metallic species by washing, which inhibited the decomposition of cellulose during subsequent torrefaction (Lv and Wu, 2012). The absorption peaks of O–H in combined pretreated samples were higher than those in CS and its torrefied samples. This was due to the reduction of hydration reactions during the process of torrefaction, caused by the removal of AAEMs during the washing pretreatments (Wigley and Alex, 2016). 3.2. Effect of pretreatment on subsequent pyrolysis 3.2.1. Pyrolysis product distribution Fig. 2 presents the effects of pretreatments on product yields from subsequent pyrolysis. For CS and torrefied CS pyrolysis, char yield increased from 26.50% by weight to 54.45% by weight when torrefaction temperature rose to 290 °C (Fig. 2a) and liquid yield decreased from 41.45% by weight to 18.70% by weight, while gas yield slightly decreased from 32.00% by weight to 26.85% by weight. Lignin (the major source of char) relative content increased with rising torrefaction temperature, resulting in an increased char yield (Zeng et al., 2018a). The decreased yields of liquid and gas were due to enhanced devolitization, cross-linking and carbonization in torrefied corn stalk when torrefaction temperature increased (Zheng et al., 2012). Compared with CS pyrolysis, the pyrolysis liquid yields of WCS, LCS, and CCS increased by 26.21% (Fig. 2b), 24.27% (Fig. 2c), and 37.86% (Fig. 2d), respectively. This was because the washing pretreatments caused some C-O bonds to break, leading to the chains of each component (hemicellulose, cellulose, and lignin) becoming shorter, and thus it was easier to convert them into liquid products (Kumagai et al., 2015). Furthermore, washing pretreatments removed most metals, especially AAEMs, which inhibited the catalytic decomposition of volatiles (Chen et al., 2017). Consequently, the pyrolysis gas yields of WCS, LCS, and CCS decreased by 8.64%, 12.35%, and 24.69% respectively, compared to that of CS. Lower char yields were also observed for WCS, LCS, and CCS samples. The change rate of pyrolysis products (liquid and char) with torrefaction temperature for combined pretreatment samples became more slowly than that of corn stalk. The enrichment of metallic species in
3.2.2. Gas product composition The effects of pretreatment on the composition of gas products from subsequent pyrolysis are shown in Fig. 3. CO2, CO, H2, and CH4 were the main gas components. For CS and torrefied CS pyrolysis, CO volume fractions decreased from 59.26 vol% to 25.00 vol% when torrefaction temperature was increased to 290 °C, while CO2 volume fractions increased from 19.36 vol% to 48.58 vol%. The CO decrease and CO2 increase in pyrolysis gas products were related to the thermal degradation characteristics of hemicellulose and cellulose during biomass torrefaction (Yang et al., 2006). A small amount of CH4 and H2 were produced by demethylation of lignin and this was slightly increased with rising torrefaction temperature (Wigley et al., 2015). For the combination pretreatments (Fig. 3b, c, and d), the CO2 volume fractions decreased with increasing torrefaction temperature, while volume fractions of CO, CH4, and H2 slightly increased. These results are in accordance with previous studies (Liu et al., 2013; Zhang et al., 2016a). The reduced production of CO2 might be due to the removal of alkali metals by washing, which inhibited the formation of carboxyl groups from cellulose during torrefaction and its catalytic cracking into CO2 (Hu et al., 2015). The removal of AAEMs by washing pretreatments would slightly inhibit the decomposition of 313
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Fig. 3. Effect of torrefaction temperature on the composition of gas products from subsequent pyrolysis of: (a) CS; (b) WCS; (c) LCS; and (d) CCS.
selectivity formation of sugars (Wang et al., 2011). Phenol relative contents were 25.92%, 20.08%, 18.42%, and 18.11% from the pyrolysis of CS, WCS, LCS, and CCS, respectively. For only torrefaction pretreatment feedstock pyrolysis, phenol contents sharply increased when torrefaction temperature rose (47.32% for CS290). Torrefaction increased the lignin content and phenol mainly came from lignin decomposition (Zheng et al., 2013) and the relative content of phenol from combined pretreatment feedstock pyrolysis decreased with increasing torrefaction temperature. Thus, the relative content of phenol declined from 20.08% (WCS), 18.42% (LCS), and 18.11% (CCS) to 15.15% (WCS290), 9.76% (LCS290), and 14.49% (CCS290) when torrefaction temperature rose to 290 °C. Although torrefaction pretreatment changed the structure of feedstock by increasing the lignin relative content, washing pretreatments removed the AAEMs, thereby inhibiting the decomposition of lignin, which could ultimately cause the decrease of phenol content (Zhang et al., 2016a). Compared to acid relative content in liquid products from pyrolysis of CS, the acid relative content of WCS, LCS, and CCS decreased from 20.79% to 14.66%, 11.21%, and 12.01%, respectively. This result might be attributed to the washing pretreatments removing a lot of potassium and thus inhibiting catalytic reactions such as glucose fragmentation into acetic acid (Zhang et al., 2016a,b). For the combination of washing and torrefaction pretreatments, the acid relative content decreased with increased torrefaction temperature. More hemicellulose would decompose at higher torrefaction temperatures, which results in less hemicellulose remaining after the combined pretreatments, and less acetic acid was derived from hemicellulose pyrolysis (Branca et al., 2014). In addition, cross-linking reactions among hemicellulose, cellulose, and lignin was enhanced with the increase of torrefaction temperature, which caused the carbonization of them and their intermediates (Ru et al., 2015). Consequently, the formation of ketones in subsequent pyrolysis was also gradually decreased with increased torrefaction temperature. The relative contents of typical chemical compounds in the liquid product of pyrolysis are compared in Fig. 5 to determine the influence of combined pretreatments. Washing with aqueous phase oil caused a decline in acetic acid relative content in LCS compared with that in CS.
hemicellulose, cellulose, and lignin following the torrefaction pretreatment (Zhang et al., 2016b). CO was mainly formed by cracking of carboxyl groups in cellulose; CH4 and H2 were derived from the reforming of rings in lignin (Yang et al., 2006). Consequently, the higher contents of cellulose and lignin in combined pretreatment samples would produce more CO, CH4, and H2 in the subsequent pyrolysis. Compared with CS290 pyrolysis, the CO volume fractions of WCS290, LCS290, and CCS290 increased by 16.06% (Fig. 3b), 1.16% (Fig. 3c), and 1.82% (Fig. 3d), respectively. The CH4 volume fractions increased by 16.06% (Fig. 3b), 1.16% (Fig. 3c), and 1.82% (Fig. 3d), respectively. The H2 volume fractions increased by 46.38% (Fig. 3b), 14.50% (Fig. 3c), and 17.69% (Fig. 3d), respectively, while, the CO2 volume fractions decreased by 21.26%, 6.78%, and 7.66%, respectively.
3.2.3. Liquid product characterization Fig. 4 shows the relative contents of different groups of liquid products from the pyrolysis of original and pretreated corn stalk. Six main functional groups (phenols, acids, sugars, ketones, furans, and aldehydes) were compared according to the relative content (percentage area). The washing pretreatments that greatly increased sugar relative contents relative to that from original corn stalk pyrolysis are shown in Fig. 4a. The sugar relative contents from WCS, LCS, and CCS increased to 11.65%, 20.80%, and 25.75%, respectively. This could be due to the removal of AAEMs, which inhibited the decomposition and secondary reactions of sugar intermediates (such as glucose units) (Oudenhoven et al., 2015). Furthermore, with torrefaction temperature rising to 290 °C, the sugar relative contents from combination pretreatment feedstocks continued to increase to 36.63%, 45.89%, and 52.34% for WCS290 (Fig. 4b), LCS290 (Fig. 4c), and CCS290 (Fig. 4d), respectively. Acid-soluble cations were tightly bound to biomass polymers and facilitated dehydration and cracking reactions. Washing pretreatments removed these cations and their sites were replaced by H+, which led to the cross-links in the biomass decomposing into a comparatively looser structure (Yang et al., 2006; Zhou et al., 2013). The interactions between hemicellulose and cellulose mitigated the looser structure that formed when torrefaction temperature increased. These structural changes and weakened interactions increased the 314
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Fig. 4. Effect of torrefaction temperature on the relative content of groups in liquid products from subsequent pyrolysis of: (a) CS; (b) WCS; (c) LCS; and (d) CCS.
decomposition was enhanced as torrefaction temperature rose, and acetic acid was mainly formed by hemicellulose thermal degradation (Wang et al., 2016). In addition to acetic acid, washing pretreatments caused an increased content of furfural, but this decreased with increased torrefaction severity. Furfural is mainly derived from the depolymerization and dehydration reactions of xylose units in cellulose
Washing pretreatments removed a lot of AAEMs. Consequently, ring fission or fragmentation of glucose into smaller molecular compounds (such as acetic acid), reactions catalyzed by K and Na, would be inhibited (Karnowo et al., 2014; Oudenhoven et al., 2015). In addition, acetic acid relative content decreased with increasing torrefaction temperature. This might be due to the fact that hemicellulose
Fig. 5. Effect of torrefaction temperature on the relative content of typical chemical compounds in liquid products from subsequent pyrolysis of CS and LCS. 315
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and hemicellulose (Yang et al., 2006). Washing pretreatments increased the amorphous and loose structure in cellulose and hemicellulose, which facilitates the formation of small molecule compounds (such as furfural) (Carpenter et al., 2014). Torrefaction pretreatment increased the lignin relative content. As a result, more phenols (such as phenol and o-Cresol) were produced with increasing torrefaction severity since phenols were mainly derived from lignin pyrolysis (Chen et al., 2017). The guaiacol in the LCS pyrolysis liquid product decreased to 2.35% compared with that from the CS pyrolysis liquid product. AAEMs, particularly Na, K, and Ca, can inhibit guaiacol fragmentation (Oudenhoven et al., 2015). However, they were removed a lot by the aqueous phase oil washing pretreatment and the guaiacol fragmentation reaction was promoted. The relative content of hydroxyacetone was reduced by washing treatments. This is because ketones were mainly formed through secondary decomposition reactions in the glucose units of pyrolysis intermediate products (Zeng et al., 2018b). The removal of AAEMs inhibited these secondary reactions (Zeng et al., 2018a; Zhang et al., 2016a). For LCS pyrolysis, 2,3-Dihydrobenzofuran decreased with increasing torrefaction temperature. Torrefaction reduced the quantity of hemicellulose and mitigated its interaction with cellulose (Zhang et al., 2018). Their interaction supports the formation of furans. There were small quantities of D-Allose and levoglucosan detected in the liquid product from pyrolysis of CS and torrefied CS. However, they increased to 5.33% and 13.01% respectively, in the LCS pyrolysis liquid product. This finding indicated that the removal of AAEMs can inhibit the secondary cracking of glucose units in the pyrolysis intermediate products (Zhang et al., 2016a). The highest levoglucosan yield of 37.46% was obtained at the torrefaction temperature of 290 °C with the aqueous phase oil washing pretreatment sample (LCS).
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. Cen, K., Chen, D., Wang, J., Cai, Y., Wang, L., 2016. Effects of water washing and torrefaction pretreatments on corn stalk pyrolysis: combined study using tg-ftir and a fixed bed reactor. Energy Fuel 30 (12), 10627–10634. Chen, D., Cen, K., Jing, X., Gao, J., Li, C., Ma, Z., 2017a. An approach for upgrading biomass and pyrolysis product quality using a combination of aqueous phase bio-oil washing and torrefaction pretreatment. Bioresour. Technol. 233, 150–158. Chen, D., Mei, J., Li, H., Li, Y., Lu, M., Ma, T., Ma, Z., 2017b. Combined pretreatment with torrefaction and washing using torrefaction liquid products to yield upgraded biomass and pyrolysis products. Bioresour. Technol. 228, 62–68. Chen, W., Peng, J., Bi, X.T., 2015. A state-of-the-art review of biomass torrefaction, densification and applications. Renew. Sustain. Energy Rev. 44, 847–866. Chen, Y., Yang, H., Yang, Q., Hao, H., Zhu, B., Chen, H., 2014. Torrefaction of agriculture straws and its application on biomass pyrolysis poly-generation. Bioresour. Technol. 156, 70–77. Correia, R., Goncalves, M., Nobre, C., Mendes, B., 2017. Impact of torrefaction and lowtemperature carbonization on the properties of biomass wastes from Arundo donax L. and Phoenix canariensis. Bioresour. Technol. 223, 210–218. Dong, Q., Zhang, S., Zhang, L., Ding, K., Xiong, Y., 2015. Effects of four types of dilute acid washing on moso bamboo pyrolysis using Py-GC/MS. Bioresour. Technol. 185, 62–69. Hu, S., Jiang, L., Wang, Y., Su, S., Sun, L., Xu, B., He, L., Xiang, J., 2015. Effects of inherent alkali and alkaline earth metallic species on biomass pyrolysis at different temperatures. Bioresour. Technol. 192, 23–30. Karnowo, Zahara, Z.F., Kudo, S., Norinaga, K., Hayashi, J., 2014. Leaching of alkali and alkaline earth metallic species from rice husk with bio-oil from its pyrolysis. Energy Fuel 28 (10), 6459–6466. Kumagai, S., Matsuno, R., Grause, G., Kameda, T., Yoshioka, T., 2015. Enhancement of bio-oil production via pyrolysis of wood biomass by pretreatment with H2SO4. Bioresour. Technol. 178, 76–82. Liu, H., Zhang, L., Han, Z., Xie, B., Wu, S., 2013. The effects of leaching methods on the combustion characteristics of rice straw. Biomass Bioenergy 49, 22–27. Lv, G., Wu, S., 2012. Analytical pyrolysis studies of corn stalk and its three main components by TG-MS and Py-GC/MS. J. Anal. Appl. Pyrol. 97, 11–18. Meng, J., Park, J., Tilotta, D., Park, S., 2012. The effect of torrefaction on the chemistry of fast-pyrolysis bio-oil. Bioresour. Technol. 111, 439–446. Mourant, D., Wang, Z., He, M., Wang, X.S., Garcia-Perez, M., Ling, K., Li, C., 2011. Mallee wood fast pyrolysis: effects of alkali and alkaline earth metallic species on the yield and composition of bio-oil. Fuel 90 (9), 2915–2922. Oudenhoven, S.R.G., Westerhof, R.J.M., Kersten, S.R.A., 2015. Fast pyrolysis of organic acid leached wood, straw, hay and bagasse: improved oil and sugar yields. J. Anal. Appl. Pyrol. 116, 253–262. Ru, B., Wang, S., Dai, G., Zhang, L., 2015. Effect of torrefaction on biomass physicochemical characteristics and the resulting pyrolysis behavior. Energy Fuel 29 (9), 5865–5874. Saddawi, A., Jones, J.M., Williams, A., Le Coeur, C., 2012. Commodity fuels from biomass through pretreatment and torrefaction: effects of mineral content on torrefied fuel characteristics and quality. Energy Fuel 26 (11), 6466–6474. Ukaew, S., Schoenborn, J., Klemetsrud, B., Shonnard, D.R., 2018. Effects of torrefaction temperature and acid pretreatment on the yield and quality of fast pyrolysis bio-oil from rice straw. J. Anal. Appl. Pyrol. 129, 112–122. Wang, S., Dai, G., Ru, B., Zhao, Y., Wang, X., Zhou, J., Luo, Z., Cen, K., 2016. Effects of torrefaction on hemicellulose structural characteristics and pyrolysis behaviors. Bioresour. Technol. 218, 1106–1114. Wang, S., Guo, X., Wang, K., Luo, Z., 2011. Influence of the interaction of components on the pyrolysis behavior of biomass. J. Anal. Appl. Pyrol. 91 (1), 183–189. Wigley, T., Alex, C.K.Y.S., 2016. Pretreating biomass via demineralisation and torrefaction to improve the quality of crude pyrolysis oil.pdf. Energy 109, 481–494. Wigley, T., Yip, A.C.K., Pang, S., 2015. The use of demineralisation and torrefaction to improve the properties of biomass intended as a feedstock for fast pyrolysis. J. Anal. Appl. Pyrol. 113, 296–306. Yang, H., Yan, R., Chen, H., Zheng, C., Lee, D.H., Liang, D.T., 2006a. Influence of mineral matter on pyrolysis of palm oil wastes. Combust. Flame 146 (4), 605–611. Yang, H.P., Yan, R., Chen, H.P., Zheng, C.G., Lee, D.H., Liang, D.T., 2006b. In-depth investigation of biomass pyrolysis based on three major components: Hemicellulose, cellulose and lignin. Energy Fuel 20 (1), 388–393. Zeng, K., Yang, Q., Zhang, Y., Mei, Y., Wang, X., Yang, H., Shao, J., Li, J., Chen, H., 2018a. Influence of torrefaction with Mg-based additives on the pyrolysis of cotton stalk. Bioresour. Technol. 261, 62–69. Zeng, K., Yang, Q., Che, Q., Zhang, Y., Wang, X., Yang, H., He, X., Wright, S., Chen, H., 2018b. Effects of temperature and Mg-based additives on properties of cotton stalk torrefaction products. Energy Fuel 32 (9), 9640–9649. Zhang, S., Dong, Q., Chen, T., Xiong, Y., 2016a. Combination of light bio-oil washing and torrefaction pretreatment of rice husk: its effects on physicochemical characteristics and fast pyrolysis behavior. Energy Fuel 30 (4), 3030–3037. Zhang, S., Dong, Q., Zhang, L., Xiong, Y., 2016b. Effects of water washing and torrefaction on the pyrolysis behavior and kinetics of rice husk through TGA and Py-GC/MS. Bioresour. Technol. 199, 352–361. Zhang, S., Dong, Q., Zhang, L., Xiong, Y., Liu, X., Zhu, S., 2015. Effects of water washing and torrefaction pretreatments on rice husk pyrolysis by microwave heating. Bioresour. Technol. 193, 442–448. Zhang, S., Su, Y., Xu, D., Zhu, S., Zhang, H., Liu, X., 2018. Effects of torrefaction and organic-acid leaching pretreatment on the pyrolysis behavior of rice husk. Energy
4. Conclusions Combined pretreatments, especially aqueous phase bio-oil washingtorrefaction can improve the fuel properties of pretreated corn stalks largely by removing most AAEMs and enhancing its volatile/hydrogen content. Compared with CS290 pyrolysis, the bio-oil yields from WCS290, LCS290, and CCS290 increased by 134.04%, 127.66%, and 129.79%, respectively. Their relative sugar contents (rich in levoglucosan) also increased to 36.63%, 45.89%, and 52.34%, respectively. CO, H2, and CH4 in pyrolysis gas increased while CO2 decreased after combined pretreatments. Aqueous phase bio-oil washing-torrefaction was the most promising pretreatment for improving biomass properties and its pyrolysis products, in which acetic acid plays the most important role. Acknowledgement This work was supported by the National Natural Science Foundation of China (51706083), Natural Science Foundation of Shenzhen (JCYJ20170818164006890), Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development (Y807s11001) and the Foundation of Institute for Clean and Renewable Energy at Huazhong University of Science & Technology (3011120011). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.02.107. References Branca, C., Di Blasi, C., Galgano, A., Brostrom, M., 2014. Effects of the torrefaction conditions on the fixed-bed pyrolysis of norway spruce. Energy Fuel 28 (9), 5882–5891.
316
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K. Zeng, et al.
128, 370–377. Zhou, S., Osman, N.B., Li, H., McDonald, A.G., Mourant, D., Li, C., Garcia-Perez, M., 2013. Effect of sulfuric acid addition on the yield and composition of lignin derived oligomers obtained by the auger and fast pyrolysis of Douglas-fir wood. Fuel 103, 512–523.
149, 804–813. Zheng, A., Zhao, Z., Chang, S., Huang, Z., He, F., Li, H., 2012. Effect of torrefaction temperature on product distribution from two-staged pyrolysis of biomass. Energy Fuel 26 (5), 2968–2974. Zheng, A., Zhao, Z., Chang, S., Huang, Z., Wang, X., He, F., Li, H., 2013. Effect of torrefaction on structure and fast pyrolysis behavior of corncobs. Bioresour. Technol.
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Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
The effect of combined pretreatments on the pyrolysis of corn stalk a,b,c
Kuo Zeng a b c
c
a
a,⁎
T
a,c
, Xiao He , Haiping Yang , Xianhua Wang , Hanping Chen
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, PR China Shenzhen Huazhong University of Science and Technology Research Institute, Shenzhen 523000, PR China Department of New Energy Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Corn stalk Demineralization Torrefaction Pretreatment Pyrolysis
Untreated corn stalk (CS), deionized water washed CS (WCS), aqueous phase bio-oil washed CS (LCS), and 5% acetic acid washed CS (CCS) were torrefied at 230, 260, and 290 °C. The influences of washing, torrefaction, and combined washing-torrefaction pretreatments on corn stalk pyrolysis were investigated. The combined pretreatments, especially aqueous phase bio-oil washing-torrefaction improved fuel properties of pretreated samples largely by increasing their volatile and hydrogen contents. Absorption peaks of O–H and C]O groups in combined pretreatment samples increased when torrefaction temperature increased. In addition, CO, H2, and CH4 contents of pyrolysis gas increased, while CO2 decreased after combined pretreatments. The bio-oil yields from WCS290, LCS290, and CCS290 increased by 134.04%, 127.66%, and 129.79% respectively, compared with that from CS290. Similarly, their relative sugar contents (rich in levoglucosan) increased to 36.63%, 45.89%, and 52.34%, respectively. Aqueous phase oil washing-torrefaction is a promising pretreatment and acetic acid plays the most important role.
1. Introduction Corn stalk is the most abundant agricultural waste in China. In 2017, approximately 239 million tons of corn stalk were produced and were therefore potentially available for conversion into biofuels (biochar, bio-oil, and gas) by pyrolysis (Cen et al., 2016). Pyrolysis of corn
⁎
stalk not only disposes of agricultural waste, but also produces “green fuel”. The utilization of the products of pyrolysis, especially bio-oil, is greatly limited by poor fuel properties (Wigley and Alex, 2016; Wigley et al., 2015). The high oxygen content and alkali and alkaline earth metals (AAEMs) in biomass feedstock lead to high oxygen and moisture
Corresponding author. E-mail address: [email protected] (X. Wang).
https://doi.org/10.1016/j.biortech.2019.02.107 Received 2 January 2019; Received in revised form 22 February 2019; Accepted 23 February 2019 Available online 25 February 2019 0960-8524/ © 2019 Published by Elsevier Ltd.
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investigate the effects of a combined pretreatment of washing with aqueous phase bio-oil from pyrolysis and torrefaction on subsequent pyrolytic behavior (Chen et al., 2017a; Zhang et al., 2016a). Among them, Chen et al. found that bio-oil yield increased, with increased phenol and decreased acid production in the bio-oil (Chen et al., 2017a). Aqueous phase bio-oil contains high water and acid contents and low levels of ketones and phenols. Karnowo et al. demonstrated that phenols promote leaching of AAEMs as it permeated into the organic matrix of the biomass, forming hydrophobic interactions and making them more accessible to acid and water (Karnowo et al., 2014). By comparing washing with different compounds (acid and water) we can determine the role of other compounds in aqueous phase bio-oil in leaching, torrefaction, and subsequent pyrolysis. However, examinations of the influence of combined pretreatments, including washing with different types of solutions and torrefaction at different temperatures, have not been reported. Similarly, pretreatment effects on biomass characteristics and its subsequent pyrolysis have not yet been investigated. The principal aim of this study was to demonstrate the feasibility of producing high quality pyrolysis products by combining demineralization and torrefaction pretreatments. Corn stalk was washed with different solutions (deionized water, aqueous phase oil, and 5% acetic acid) and then torrefied at different temperatures (230, 260, and 290 °C). Different pretreatment processes were evaluated and compared by analyzing the fuel characteristics of original and pretreated samples. Following this, the untreated and pretreated corn stalk was pyrolyzed and the yields and properties of the char, liquid, and gas products were analyzed. The use of a single washing compound (acid or water) compared to aqueous phase bio-oil has not been reported to-date. Therefore, the secondary objective of this study was to determine the roles of other compounds (acid or water) in aqueous phase bio-oil in leaching, torrefaction, and subsequent pyrolysis. The results of this study will be valuable for the high value utilization of corn stalk pyrolysis bio-oil.
content, and hence slagging/fouling shortcomings, of the bio-oil product. Therefore, biomass pretreatments prior to pyrolysis are necessary to improve feedstock properties and upgrade its final products (Carpenter et al., 2014). Torrefaction pretreatment releases oxygen from biomass as it is pyrolyzed at temperatures in the range of 200–300 °C (Chen et al., 2015). A lower oxygen and moisture content with higher energy density pretreated biomass feedstock is thereby obtained. Bio-oil products from subsequent pyrolysis have lower oxygen, water, and acid contents (Meng et al., 2012; Zeng et al., 2018a; Zheng et al., 2013). Variation in hemicellulose, cellulose, and lignin contents during torrefaction have a direct influence on subsequent pyrolysis products (Chen et al., 2014). The content of oxy-compounds (such as acetic acid and furan) in bio-oil decreased with increasing torrefaction temperature, while the content of aromatic hydrocarbons increased (Zeng et al., 2018a). Washing pretreatments remove inorganic content (including soluble salts or cations bound to reactive sites) from biomass as it is demineralized by leaching in water, light bio-oil, or dilute acid (Wigley et al., 2015). Soluble salts (about 30–80% of AAEMs) can be leached by water (Cen et al., 2016). The removal of soluble salts by water leaching has a negligible effect on the yield and composition of bio-oil, while the removal of acid-soluble cations using mineral acids (HCl and H2SO4) has been utilized (Dong et al., 2015). However, the use of mineral acids and organic acids is not practicable because of the problems of corrosion and cost (Karnowo et al., 2014). Bio-oil derived from biomass pyrolysis, especially aqueous phase bio-oil (light bio-oil) obtained from natural phase separation is considered an effective and cheap demineralization agent for removing AAEMs (Chen et al., 2017; Karnowo et al., 2014; Zhang et al., 2016a). Almost no soluble salts or cations are bound in aqueous phase bio-oil washed biomass feedstock (Chen et al., 2017a). The removal of acid-soluble cations via acid or bio-oil leaching increased the yield of bio-oil from subsequent pyrolysis and improved its quality by lowering the water content and enriching the content of levoglucosan (Mourant et al., 2011). There are some drawbacks reducing the benefits of torrefaction or washing pretreatments and restricting their individual use. First, torrefaction increases the inorganics in pretreated biomass, which lowers the bio-oil quality due to AAEMs catalyzing dehydration and cracking reactions of intermediate bio-oil (Saddawi et al., 2012). Second, the washed sample used as feedstock in subsequent pyrolysis must be dried to eliminate the washing solutions. One way to minimize the impacts of these limitations is to combine these two pretreatments. In such a combination, a washing pretreatment is used to remove inorganics, torrefaction is then performed to both reduce oxygen content and dry the washed sample. An aqueous phase bio-oil (light bio-oil) as a washing agent is optimal because it is cheap and has environmentally friendly properties (Karnowo et al., 2014). The quality of such pretreated biomass and its subsequent pyrolysis product are thereby enhanced. Few studies have been undertaken to examine the efficacy of a combined washing and torrefaction pretreatment on biomass pyrolysis. Some research groups have used combined water washing and torrefaction pretreatments to remove inorganic minerals and oxygen from biomass, which enhanced the quality and yield of bio-oil following subsequent pyrolysis (Cen et al., 2016; Zhang et al., 2016b, 2015). Zhang et al. (2018) and Ukaew et al. (2018) investigated the effects of combined torrefaction and leaching with inorganic acid (HCl) or organic acid (acetic acid) on biomass pyrolysis. Their results demonstrated that bio-oil quality was improved by enriching phenols and sugars, especially levoglucosan. A combined pretreatment with torrefaction and leaching using an acidic liquid (rich in acetic acid and formic acid) produced from torrefaction has been proposed (Chen et al., 2017a; Wigley and Alex, 2016; Wigley et al., 2015). Wigley et al. found that levoglucosan increased significantly with a combined pretreatment compared to washing or torrefaction alone (Wigley and Alex, 2016; Wigley et al., 2015). There are also some studies conducted to
2. Materials and methods 2.1. Experimental sample 2.1.1. Corn stalk Corn stalk was collected from Luoyang city, Henan province, China. Prior to the experiments, it was screened through a narrow particle size distribution within the range 125–250 μm to avoid the confounding effects of particle size, and then dried at 105 °C for 48 h. The dried corn stalk was denoted as CS. 2.1.2. Washing agents Pyrolysis oil was collected from a biomass-based pyrolytic polygeneration factory with a million-ton annual production, located in Ezhou city, Hubei province, China. It was prepared from corn stalk pyrolysis at 550 °C. Aqueous phase oil naturally separates from pyrolysis oil during storage. The water content of aqueous phase oil was about 91% by weight. Gas chromatography/mass spectrometry (GC/ MS) was used to analyze the chemical composition of the aqueous phase oil; acetic acid was abundant (peak area of 53.52%) in aqueous phase oil. Deionized water and 5% acetic acid were chosen as blank washing agents. The roles of the main compounds contained in aqueous phase bio-oil playing in leaching, torrefaction, and subsequent pyrolysis were determined. 2.2. Pretreatments 2.2.1. Washing The corn stalk was washed with either deionized water, aqueous phase oil, or 5% acetic acid. For each washing, 30 g of corn stalk was 310
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2018b). For washed samples (WCS, LCS, and CCS), torrefaction product yields followed the same trends, with rising temperature causing decreased solid product yield and increased yields of liquid and gas products. However, the torrefied solid yields from washed samples decreased more slowly compared to that from CS torrefaction. Differences in the yields of solid products from differently washed samples were very small at 290 °C, with increases of 68.87%, 68.50%, and 66.50% for WCS, LCS, and CCS, respectively (Fig. 1a). The increase of solid yields from washed samples was attributed to the removal of AAEMs, because they reduce pyrolysis temperature and facilitate the release of volatile compounds (Mourant et al., 2011).
added to 600 mL solutions at 30 °C and stirred for 4 h with a magnetic stirrer. After washing, the corn stalk was separated by vacuum filtration and washed with deionized water until the pH of deionized water after flushing was close to neutral. The washed corn stalk was then dried at 105 °C for 48 h in an oven, and subsequently denoted as WCS, LCS, and CCS, respectively. 2.2.2. Torrefaction after washing The torrefaction pretreatment after washing was conducted in a horizontal tube furnace. Details of the torrefaction setup and process have been reported in our previous studies (Zeng et al., 2018a,b). The corn stalk (CS), deionized water washed corn stalk (WCS), aqueous phase oil washed corn stalk (LCS), and 5% acetic acid washed corn stalk (CCS) were torrefied at three different temperatures (230, 260, and 290 °C) for 30 min. In view of the deoxygenating and upgrading effects of torrefaction pretreatment, moderate (230 °C) and severe (260 and 290 °C), torrefaction conditions were investigated in this study. The torrefied samples were labelled as CS230, CS260, CS290, WCS230, WCS260, WCS290, LCS230, LCS260, LCS290, CCS230, CCS260, CCS290.
3.1.2. Metallic species Table 1 presents the contents of metallic species in the samples. The content of metallic species in corn stalk was reduced to various extents after washing with different agents, in agreement with previous studies (Cen et al., 2016; Chen et al., 2017a; Zhang et al., 2018). The removal rates of K, Na, Ca, Mg, and Fe were 79.88%, 77.93%, 16.57%, 32.50%, and 48.89%, respectively, when corn stalk was washed with deionized water. Removal rates increased to 97.53%, 81.38%, 27.16%, 84.86%, and 49.96% respectively, with aqueous phase oil as the washing agent. These reductions were very similar to those obtained when 5% acetic acid washing pretreatment was applied (97.46%, 80.00%, 21.42%, 89.51%, and 53.33%, respectively). Thus the removal rates of metallic species by aqueous phase oil and 5% acetic acid washing were higher than those obtained by deionized water washing. These results can be explained by the presence of not only acetic acid but also phenolic compounds in aqueous phase oil. These phenols permeated into the organic matrix of corn stalk by hydrophobic interactions, making the matrix more accessible to acetic acid and water, thereby promoting the removal of metallic species (Karnowo et al., 2014).
2.3. Characterization of raw and pretreatment corn stalk The ultimate and proximate analyses of corn stalk were performed with an elemental analyzer (2400 II, PerkinElmer, USA) and industrial analyzer (SDTGA5000a), respectively. The absolute contents of metallic species in the original and washed corn stalk samples were measured with inductively coupled plasma mass spectrometry (ICP-MS, ELAN DRC-e, PerkinElmer, USA). The functional groups of samples were analyzed with a Fourier transform infrared spectroscopy (FTIR) analyzer (VERTEX 70, BrukerBruker Bruker, Germany).
3.1.3. Fuel properties The main properties of samples (CS, WCS, LCS, and CCS) before and after torrefaction are shown in Table 2. The ash contents of samples subject only to washing (WCS, LCS and CCS) decreased by 50.20%, 66.51%, and 65.94%, respectively, compared to its content in unwashed CS. The volatile content was slightly increased. This can be explained by the removal of AAEMs and a small amount of organic matter by the washing pretreatment (Zhang et al., 2016a). There was almost no difference in elemental contents between CS and washed CS (WCS, LCS, and CCS), indicating that the washing pretreatments caused no change in corn stalk composition (Chen et al., 2017). The torrefaction pretreatment clearly affected the properties of CS and its effect was more obvious than that of washing pretreatments. The volatile contents decreased with increasing torrefaction temperature; however, ash contents and fixed carbon contents increased significantly (Table 2). These results can be explained by the enhanced decomposition of hemicellulose and cellulose, resulting in increased release of volatiles from corn stalk (Cen et al., 2016). Consequently, the remaining ash exhibited a relative increase of its mass fraction. The ash contents of corn stalk obtained from the combination of washing and torrefaction pretreatments were less than those of just the torrefaction pretreatment, and even less than that of the original corn stalk. Chen et al. found a similar change in ash content with a combined pretreatment, which implied that washing could offset the negative effect of torrefaction on ash content (Chen et al., 2017). In addition, the combined pretreatments caused an increase in the volatile content and a decline in the fixed carbon content in the sample compared to that of the torrefaction pretreated sample. This result might be due to the removal of most metallic species such as Na, Mg, and Ca by the washing pretreatment, which inhibited the release of volatile contents during subsequent torrefaction (Zeng et al., 2018a,b; Zhang et al., 2018). Torrefaction caused a significant increase in carbon content and a decline in oxygen content in corn stalk due to the release of CO2 and H2O. The carbon contents of WCS290, LCS290, and CCS290 were 49.56%, 49.54%, and 50.16%
2.4. Pyrolysis Pyrolysis was conducted in a fixed-bed reactor, as described in our previous study (Zeng et al., 2018b). A total of 3 g ( ± 5%) of each sample was placed in a quartz boat and quickly fed into the reactor when it reached 550 °C and held at that temperature for 30 min. Subsequently, 400 mL/min nitrogen was flushed through the reactor to remove the gaseous products during pyrolysis. The liquid product was collected in the condensation unit. The gas product was kept in a sampling bag. The solid product was left to cool and kept in the quartz boat. The liquid and solid yields were determined by the differences in mass of the quartz boat and condensation before and after pyrolysis, respectively. Gas yield was calculated by difference. Each run was repeated at least three times to ensure the Relative Standard Deviation (RSD) was less than 5%. Gas composition was determined by micro-gas chromatography (Micro-GC, Agilent 3000). The liquid product organic components were determined by gas chromatography-mass spectroscopy (GC–MS; 7890A GC- HP5975 MS) and individual compounds were identified based on a NIST mass spectral data library and our previous internal standard method (Zeng et al., 2018a). 3. Results and discussion 3.1. Effect of pretreatments on corn stalk characteristics 3.1.1. Pretreatment product distribution The yields of solid, liquid, and gas products obtained from torrefaction of original and washed corn stalk are presented in Fig. 1. For CS torrefaction (corn stalk), the solid yield decreased from 96.02% to 49.58% as torrefaction temperature increased from 200 to 290 °C (Fig. 1a). Simultaneously, the gas yield increased from 2.42% to 24.90% (Fig. 1b), and the liquid yield increased from 1.56% to 25.52% (Fig. 1c). These results are consistent with enhanced degradation of hemicellulose and cellulose in this temperature range (Zeng et al., 311
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Fig. 1. Torrefaction product distribution of original and washed corn stalk: (a) solid yield; (b) liquid yield; and (c) gas yield. Table The AAEM content and removal efficiency of original and washed corn stalk. Samples
AAEMs content (mg/kg)
CS WCS LCS CCS
AAEMs removal efficiency (%)
K
Na
Ca
Mg
Fe
K
Na
Ca
Mg
Fe
10,020 2016 250 255
290 64 54 58
2890 2411 2105 2271
1440 972 218 151
360 176 170 168
0 79.88 97.53 97.46
0 77.93 81.38 80
0 16.57 27.16 21.42
0 32.5 84.86 89.51
0 48.89 52.78 53.33
combination of washing and torrefaction pretreatment increased the hydrogen content and decreased the carbon content in pretreated samples.
Table 2 The fuel properties of original and pretreated corn stalk. Samples
CS CS 230 CS 260 CS 290 WCS WCS 230 WCS 260 WCS 290 LCS LCS 230 LCS 260 LCS 290 CCS CCS 230 CCS 260 CCS 290
Proximate analysis (wt.%, db)
Ultimate analysis (wt.%, db)
Volatile matter
Ash
Fixed carbon
Cd
Hd
Od
Nd
O/C
H/C
84.13 79.74 75.17 58.43 86.87 85.49 84.06 79.88 87.83 85.50 83.08 81.56 85.36 83.18 80.90 76.09
3.09 3.78 4.49 6.25 1.54 1.61 1.69 2.10 1.04 1.27 1.51 1.81 1.05 1.31 1.58 2.03
12.79 16.48 20.33 35.32 11.59 12.89 14.25 18.02 11.13 13.23 15.42 16.63 13.58 15.51 17.51 21.88
44.17 46.33 48.95 56.57 45.43 46.45 47.94 49.56 46.16 46.90 48.12 49.54 45.35 46.67 48.47 50.16
5.81 5.68 5.60 4.97 5.97 5.90 5.89 5.69 6.00 5.86 5.77 5.62 5.91 5.81 5.77 5.60
49.61 47.09 45.05 37.94 48.41 46.96 45.97 44.52 47.58 46.47 45.83 44.47 48.48 46.73 45.46 43.88
0.41 0.40 0.39 0.53 0.18 0.19 0.20 0.23 0.26 0.27 0.29 0.38 0.27 0.28 0.30 0.36
0.84 0.76 0.69 0.50 0.80 0.76 0.72 0.67 0.77 0.74 0.71 0.67 0.80 0.75 0.70 0.66
1.58 1.47 1.37 1.05 1.58 1.52 1.47 1.38 1.56 1.49 1.43 1.36 1.56 1.49 1.43 1.34
3.1.4. Functional groups The evolution of typical functional groups in corn stalk during pretreatment is presented. Absorption peaks centered at 3440 cm−1 (O–H) and 1730 cm−1 (C]O) initially increased with torrefaction temperature rising to 260 °C and then decreased at the higher temperature of 290 °C. Increasing amounts of hemicellulose decomposed with the increase of torrefaction temperature, which resulted in a loss of more functional groups (Correia et al., 2017; Zhang et al., 2016a). However, this also resulted in an increase in the relative content of cellulose when torrefaction temperature increased to 260 °C. Because cellulose contains a high content of O–H and C]O groups, the absorption peaks of O–H and C]O groups in CS230 and CS260 increased even with enhanced decomposition of hemicellulose. At a higher torrefaction temperature of 290 °C, the absorption peaks of O–H and C]O groups in CS290 slightly decreased due to the enhanced decomposition of cellulose. In addition, FTIR spectra of just washed samples (WCS, LCS, and CCS) were similar to that of original corn stalk. Thus, the washing pretreatment caused no obvious loss of organic content in biomass (Zhang et al., 2016a). The absorption peaks of O–H and C]O groups in the combined pretreatment samples increased with increased torrefaction temperature. This may be attributed to the removal of
lower than that of CS290 (56.57%), respectively. The hydrogen contents of WCS290, LCS290, and CCS290 were 5.69%, 5.62%, and 5.60% higher than that of CS290 (4.97%), respectively. In general, the 312
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Fig. 2. Effect of torrefaction temperature on the product distribution from subsequent pyrolysis of: (a) CS; (b) WCS, (c) LCS; and (d) CCS.
corn stalk occurs in response to the torrefaction pretreatment (Chen et al., 2017), while the ash content in combined pretreatment samples was less than that for samples exposed only to the torrefaction pretreatment due to the removal of most metallic species by washing (Chen et al., 2017). Consequently, the decrease of yield of the pyrolysis liquid was more obvious for just the torrefaction pretreatment sample, with more catalytic secondary reactions of volatiles than that occurring in combined pretreatment samples (Zhang et al., 2018). Compared with CS290 pyrolysis (Fig. 2a), the pyrolysis liquid yields of WCS290, LCS290, and CCS290 increased by 134.04% (Fig. 2b), 127.66% (Fig. 2c), and 129.79% (Fig. 2d), respectively. In contrast, the pyrolysis char yields decreased by 48.51%, 46.27%, and 46.27%, respectively. In addition, for the combined pretreated samples, the liquid yield decreased while char yield increased with increasing torrefaction temperature (Fig. 2b, c, and d).
metallic species by washing, which inhibited the decomposition of cellulose during subsequent torrefaction (Lv and Wu, 2012). The absorption peaks of O–H in combined pretreated samples were higher than those in CS and its torrefied samples. This was due to the reduction of hydration reactions during the process of torrefaction, caused by the removal of AAEMs during the washing pretreatments (Wigley and Alex, 2016). 3.2. Effect of pretreatment on subsequent pyrolysis 3.2.1. Pyrolysis product distribution Fig. 2 presents the effects of pretreatments on product yields from subsequent pyrolysis. For CS and torrefied CS pyrolysis, char yield increased from 26.50% by weight to 54.45% by weight when torrefaction temperature rose to 290 °C (Fig. 2a) and liquid yield decreased from 41.45% by weight to 18.70% by weight, while gas yield slightly decreased from 32.00% by weight to 26.85% by weight. Lignin (the major source of char) relative content increased with rising torrefaction temperature, resulting in an increased char yield (Zeng et al., 2018a). The decreased yields of liquid and gas were due to enhanced devolitization, cross-linking and carbonization in torrefied corn stalk when torrefaction temperature increased (Zheng et al., 2012). Compared with CS pyrolysis, the pyrolysis liquid yields of WCS, LCS, and CCS increased by 26.21% (Fig. 2b), 24.27% (Fig. 2c), and 37.86% (Fig. 2d), respectively. This was because the washing pretreatments caused some C-O bonds to break, leading to the chains of each component (hemicellulose, cellulose, and lignin) becoming shorter, and thus it was easier to convert them into liquid products (Kumagai et al., 2015). Furthermore, washing pretreatments removed most metals, especially AAEMs, which inhibited the catalytic decomposition of volatiles (Chen et al., 2017). Consequently, the pyrolysis gas yields of WCS, LCS, and CCS decreased by 8.64%, 12.35%, and 24.69% respectively, compared to that of CS. Lower char yields were also observed for WCS, LCS, and CCS samples. The change rate of pyrolysis products (liquid and char) with torrefaction temperature for combined pretreatment samples became more slowly than that of corn stalk. The enrichment of metallic species in
3.2.2. Gas product composition The effects of pretreatment on the composition of gas products from subsequent pyrolysis are shown in Fig. 3. CO2, CO, H2, and CH4 were the main gas components. For CS and torrefied CS pyrolysis, CO volume fractions decreased from 59.26 vol% to 25.00 vol% when torrefaction temperature was increased to 290 °C, while CO2 volume fractions increased from 19.36 vol% to 48.58 vol%. The CO decrease and CO2 increase in pyrolysis gas products were related to the thermal degradation characteristics of hemicellulose and cellulose during biomass torrefaction (Yang et al., 2006). A small amount of CH4 and H2 were produced by demethylation of lignin and this was slightly increased with rising torrefaction temperature (Wigley et al., 2015). For the combination pretreatments (Fig. 3b, c, and d), the CO2 volume fractions decreased with increasing torrefaction temperature, while volume fractions of CO, CH4, and H2 slightly increased. These results are in accordance with previous studies (Liu et al., 2013; Zhang et al., 2016a). The reduced production of CO2 might be due to the removal of alkali metals by washing, which inhibited the formation of carboxyl groups from cellulose during torrefaction and its catalytic cracking into CO2 (Hu et al., 2015). The removal of AAEMs by washing pretreatments would slightly inhibit the decomposition of 313
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Fig. 3. Effect of torrefaction temperature on the composition of gas products from subsequent pyrolysis of: (a) CS; (b) WCS; (c) LCS; and (d) CCS.
selectivity formation of sugars (Wang et al., 2011). Phenol relative contents were 25.92%, 20.08%, 18.42%, and 18.11% from the pyrolysis of CS, WCS, LCS, and CCS, respectively. For only torrefaction pretreatment feedstock pyrolysis, phenol contents sharply increased when torrefaction temperature rose (47.32% for CS290). Torrefaction increased the lignin content and phenol mainly came from lignin decomposition (Zheng et al., 2013) and the relative content of phenol from combined pretreatment feedstock pyrolysis decreased with increasing torrefaction temperature. Thus, the relative content of phenol declined from 20.08% (WCS), 18.42% (LCS), and 18.11% (CCS) to 15.15% (WCS290), 9.76% (LCS290), and 14.49% (CCS290) when torrefaction temperature rose to 290 °C. Although torrefaction pretreatment changed the structure of feedstock by increasing the lignin relative content, washing pretreatments removed the AAEMs, thereby inhibiting the decomposition of lignin, which could ultimately cause the decrease of phenol content (Zhang et al., 2016a). Compared to acid relative content in liquid products from pyrolysis of CS, the acid relative content of WCS, LCS, and CCS decreased from 20.79% to 14.66%, 11.21%, and 12.01%, respectively. This result might be attributed to the washing pretreatments removing a lot of potassium and thus inhibiting catalytic reactions such as glucose fragmentation into acetic acid (Zhang et al., 2016a,b). For the combination of washing and torrefaction pretreatments, the acid relative content decreased with increased torrefaction temperature. More hemicellulose would decompose at higher torrefaction temperatures, which results in less hemicellulose remaining after the combined pretreatments, and less acetic acid was derived from hemicellulose pyrolysis (Branca et al., 2014). In addition, cross-linking reactions among hemicellulose, cellulose, and lignin was enhanced with the increase of torrefaction temperature, which caused the carbonization of them and their intermediates (Ru et al., 2015). Consequently, the formation of ketones in subsequent pyrolysis was also gradually decreased with increased torrefaction temperature. The relative contents of typical chemical compounds in the liquid product of pyrolysis are compared in Fig. 5 to determine the influence of combined pretreatments. Washing with aqueous phase oil caused a decline in acetic acid relative content in LCS compared with that in CS.
hemicellulose, cellulose, and lignin following the torrefaction pretreatment (Zhang et al., 2016b). CO was mainly formed by cracking of carboxyl groups in cellulose; CH4 and H2 were derived from the reforming of rings in lignin (Yang et al., 2006). Consequently, the higher contents of cellulose and lignin in combined pretreatment samples would produce more CO, CH4, and H2 in the subsequent pyrolysis. Compared with CS290 pyrolysis, the CO volume fractions of WCS290, LCS290, and CCS290 increased by 16.06% (Fig. 3b), 1.16% (Fig. 3c), and 1.82% (Fig. 3d), respectively. The CH4 volume fractions increased by 16.06% (Fig. 3b), 1.16% (Fig. 3c), and 1.82% (Fig. 3d), respectively. The H2 volume fractions increased by 46.38% (Fig. 3b), 14.50% (Fig. 3c), and 17.69% (Fig. 3d), respectively, while, the CO2 volume fractions decreased by 21.26%, 6.78%, and 7.66%, respectively.
3.2.3. Liquid product characterization Fig. 4 shows the relative contents of different groups of liquid products from the pyrolysis of original and pretreated corn stalk. Six main functional groups (phenols, acids, sugars, ketones, furans, and aldehydes) were compared according to the relative content (percentage area). The washing pretreatments that greatly increased sugar relative contents relative to that from original corn stalk pyrolysis are shown in Fig. 4a. The sugar relative contents from WCS, LCS, and CCS increased to 11.65%, 20.80%, and 25.75%, respectively. This could be due to the removal of AAEMs, which inhibited the decomposition and secondary reactions of sugar intermediates (such as glucose units) (Oudenhoven et al., 2015). Furthermore, with torrefaction temperature rising to 290 °C, the sugar relative contents from combination pretreatment feedstocks continued to increase to 36.63%, 45.89%, and 52.34% for WCS290 (Fig. 4b), LCS290 (Fig. 4c), and CCS290 (Fig. 4d), respectively. Acid-soluble cations were tightly bound to biomass polymers and facilitated dehydration and cracking reactions. Washing pretreatments removed these cations and their sites were replaced by H+, which led to the cross-links in the biomass decomposing into a comparatively looser structure (Yang et al., 2006; Zhou et al., 2013). The interactions between hemicellulose and cellulose mitigated the looser structure that formed when torrefaction temperature increased. These structural changes and weakened interactions increased the 314
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Fig. 4. Effect of torrefaction temperature on the relative content of groups in liquid products from subsequent pyrolysis of: (a) CS; (b) WCS; (c) LCS; and (d) CCS.
decomposition was enhanced as torrefaction temperature rose, and acetic acid was mainly formed by hemicellulose thermal degradation (Wang et al., 2016). In addition to acetic acid, washing pretreatments caused an increased content of furfural, but this decreased with increased torrefaction severity. Furfural is mainly derived from the depolymerization and dehydration reactions of xylose units in cellulose
Washing pretreatments removed a lot of AAEMs. Consequently, ring fission or fragmentation of glucose into smaller molecular compounds (such as acetic acid), reactions catalyzed by K and Na, would be inhibited (Karnowo et al., 2014; Oudenhoven et al., 2015). In addition, acetic acid relative content decreased with increasing torrefaction temperature. This might be due to the fact that hemicellulose
Fig. 5. Effect of torrefaction temperature on the relative content of typical chemical compounds in liquid products from subsequent pyrolysis of CS and LCS. 315
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and hemicellulose (Yang et al., 2006). Washing pretreatments increased the amorphous and loose structure in cellulose and hemicellulose, which facilitates the formation of small molecule compounds (such as furfural) (Carpenter et al., 2014). Torrefaction pretreatment increased the lignin relative content. As a result, more phenols (such as phenol and o-Cresol) were produced with increasing torrefaction severity since phenols were mainly derived from lignin pyrolysis (Chen et al., 2017). The guaiacol in the LCS pyrolysis liquid product decreased to 2.35% compared with that from the CS pyrolysis liquid product. AAEMs, particularly Na, K, and Ca, can inhibit guaiacol fragmentation (Oudenhoven et al., 2015). However, they were removed a lot by the aqueous phase oil washing pretreatment and the guaiacol fragmentation reaction was promoted. The relative content of hydroxyacetone was reduced by washing treatments. This is because ketones were mainly formed through secondary decomposition reactions in the glucose units of pyrolysis intermediate products (Zeng et al., 2018b). The removal of AAEMs inhibited these secondary reactions (Zeng et al., 2018a; Zhang et al., 2016a). For LCS pyrolysis, 2,3-Dihydrobenzofuran decreased with increasing torrefaction temperature. Torrefaction reduced the quantity of hemicellulose and mitigated its interaction with cellulose (Zhang et al., 2018). Their interaction supports the formation of furans. There were small quantities of D-Allose and levoglucosan detected in the liquid product from pyrolysis of CS and torrefied CS. However, they increased to 5.33% and 13.01% respectively, in the LCS pyrolysis liquid product. This finding indicated that the removal of AAEMs can inhibit the secondary cracking of glucose units in the pyrolysis intermediate products (Zhang et al., 2016a). The highest levoglucosan yield of 37.46% was obtained at the torrefaction temperature of 290 °C with the aqueous phase oil washing pretreatment sample (LCS).
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. Cen, K., Chen, D., Wang, J., Cai, Y., Wang, L., 2016. Effects of water washing and torrefaction pretreatments on corn stalk pyrolysis: combined study using tg-ftir and a fixed bed reactor. Energy Fuel 30 (12), 10627–10634. Chen, D., Cen, K., Jing, X., Gao, J., Li, C., Ma, Z., 2017a. An approach for upgrading biomass and pyrolysis product quality using a combination of aqueous phase bio-oil washing and torrefaction pretreatment. Bioresour. Technol. 233, 150–158. Chen, D., Mei, J., Li, H., Li, Y., Lu, M., Ma, T., Ma, Z., 2017b. Combined pretreatment with torrefaction and washing using torrefaction liquid products to yield upgraded biomass and pyrolysis products. Bioresour. Technol. 228, 62–68. Chen, W., Peng, J., Bi, X.T., 2015. A state-of-the-art review of biomass torrefaction, densification and applications. Renew. Sustain. Energy Rev. 44, 847–866. Chen, Y., Yang, H., Yang, Q., Hao, H., Zhu, B., Chen, H., 2014. Torrefaction of agriculture straws and its application on biomass pyrolysis poly-generation. Bioresour. Technol. 156, 70–77. Correia, R., Goncalves, M., Nobre, C., Mendes, B., 2017. Impact of torrefaction and lowtemperature carbonization on the properties of biomass wastes from Arundo donax L. and Phoenix canariensis. Bioresour. Technol. 223, 210–218. Dong, Q., Zhang, S., Zhang, L., Ding, K., Xiong, Y., 2015. Effects of four types of dilute acid washing on moso bamboo pyrolysis using Py-GC/MS. Bioresour. Technol. 185, 62–69. Hu, S., Jiang, L., Wang, Y., Su, S., Sun, L., Xu, B., He, L., Xiang, J., 2015. Effects of inherent alkali and alkaline earth metallic species on biomass pyrolysis at different temperatures. Bioresour. Technol. 192, 23–30. Karnowo, Zahara, Z.F., Kudo, S., Norinaga, K., Hayashi, J., 2014. Leaching of alkali and alkaline earth metallic species from rice husk with bio-oil from its pyrolysis. Energy Fuel 28 (10), 6459–6466. Kumagai, S., Matsuno, R., Grause, G., Kameda, T., Yoshioka, T., 2015. Enhancement of bio-oil production via pyrolysis of wood biomass by pretreatment with H2SO4. Bioresour. Technol. 178, 76–82. Liu, H., Zhang, L., Han, Z., Xie, B., Wu, S., 2013. The effects of leaching methods on the combustion characteristics of rice straw. Biomass Bioenergy 49, 22–27. Lv, G., Wu, S., 2012. Analytical pyrolysis studies of corn stalk and its three main components by TG-MS and Py-GC/MS. J. Anal. Appl. Pyrol. 97, 11–18. Meng, J., Park, J., Tilotta, D., Park, S., 2012. The effect of torrefaction on the chemistry of fast-pyrolysis bio-oil. Bioresour. Technol. 111, 439–446. Mourant, D., Wang, Z., He, M., Wang, X.S., Garcia-Perez, M., Ling, K., Li, C., 2011. Mallee wood fast pyrolysis: effects of alkali and alkaline earth metallic species on the yield and composition of bio-oil. Fuel 90 (9), 2915–2922. Oudenhoven, S.R.G., Westerhof, R.J.M., Kersten, S.R.A., 2015. Fast pyrolysis of organic acid leached wood, straw, hay and bagasse: improved oil and sugar yields. J. Anal. Appl. Pyrol. 116, 253–262. Ru, B., Wang, S., Dai, G., Zhang, L., 2015. Effect of torrefaction on biomass physicochemical characteristics and the resulting pyrolysis behavior. Energy Fuel 29 (9), 5865–5874. Saddawi, A., Jones, J.M., Williams, A., Le Coeur, C., 2012. Commodity fuels from biomass through pretreatment and torrefaction: effects of mineral content on torrefied fuel characteristics and quality. Energy Fuel 26 (11), 6466–6474. Ukaew, S., Schoenborn, J., Klemetsrud, B., Shonnard, D.R., 2018. Effects of torrefaction temperature and acid pretreatment on the yield and quality of fast pyrolysis bio-oil from rice straw. J. Anal. Appl. Pyrol. 129, 112–122. Wang, S., Dai, G., Ru, B., Zhao, Y., Wang, X., Zhou, J., Luo, Z., Cen, K., 2016. Effects of torrefaction on hemicellulose structural characteristics and pyrolysis behaviors. Bioresour. Technol. 218, 1106–1114. Wang, S., Guo, X., Wang, K., Luo, Z., 2011. Influence of the interaction of components on the pyrolysis behavior of biomass. J. Anal. Appl. Pyrol. 91 (1), 183–189. Wigley, T., Alex, C.K.Y.S., 2016. Pretreating biomass via demineralisation and torrefaction to improve the quality of crude pyrolysis oil.pdf. Energy 109, 481–494. Wigley, T., Yip, A.C.K., Pang, S., 2015. The use of demineralisation and torrefaction to improve the properties of biomass intended as a feedstock for fast pyrolysis. J. Anal. Appl. Pyrol. 113, 296–306. Yang, H., Yan, R., Chen, H., Zheng, C., Lee, D.H., Liang, D.T., 2006a. Influence of mineral matter on pyrolysis of palm oil wastes. Combust. Flame 146 (4), 605–611. Yang, H.P., Yan, R., Chen, H.P., Zheng, C.G., Lee, D.H., Liang, D.T., 2006b. In-depth investigation of biomass pyrolysis based on three major components: Hemicellulose, cellulose and lignin. Energy Fuel 20 (1), 388–393. Zeng, K., Yang, Q., Zhang, Y., Mei, Y., Wang, X., Yang, H., Shao, J., Li, J., Chen, H., 2018a. Influence of torrefaction with Mg-based additives on the pyrolysis of cotton stalk. Bioresour. Technol. 261, 62–69. Zeng, K., Yang, Q., Che, Q., Zhang, Y., Wang, X., Yang, H., He, X., Wright, S., Chen, H., 2018b. Effects of temperature and Mg-based additives on properties of cotton stalk torrefaction products. Energy Fuel 32 (9), 9640–9649. Zhang, S., Dong, Q., Chen, T., Xiong, Y., 2016a. Combination of light bio-oil washing and torrefaction pretreatment of rice husk: its effects on physicochemical characteristics and fast pyrolysis behavior. Energy Fuel 30 (4), 3030–3037. Zhang, S., Dong, Q., Zhang, L., Xiong, Y., 2016b. Effects of water washing and torrefaction on the pyrolysis behavior and kinetics of rice husk through TGA and Py-GC/MS. Bioresour. Technol. 199, 352–361. Zhang, S., Dong, Q., Zhang, L., Xiong, Y., Liu, X., Zhu, S., 2015. Effects of water washing and torrefaction pretreatments on rice husk pyrolysis by microwave heating. Bioresour. Technol. 193, 442–448. Zhang, S., Su, Y., Xu, D., Zhu, S., Zhang, H., Liu, X., 2018. Effects of torrefaction and organic-acid leaching pretreatment on the pyrolysis behavior of rice husk. Energy
4. Conclusions Combined pretreatments, especially aqueous phase bio-oil washingtorrefaction can improve the fuel properties of pretreated corn stalks largely by removing most AAEMs and enhancing its volatile/hydrogen content. Compared with CS290 pyrolysis, the bio-oil yields from WCS290, LCS290, and CCS290 increased by 134.04%, 127.66%, and 129.79%, respectively. Their relative sugar contents (rich in levoglucosan) also increased to 36.63%, 45.89%, and 52.34%, respectively. CO, H2, and CH4 in pyrolysis gas increased while CO2 decreased after combined pretreatments. Aqueous phase bio-oil washing-torrefaction was the most promising pretreatment for improving biomass properties and its pyrolysis products, in which acetic acid plays the most important role. Acknowledgement This work was supported by the National Natural Science Foundation of China (51706083), Natural Science Foundation of Shenzhen (JCYJ20170818164006890), Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development (Y807s11001) and the Foundation of Institute for Clean and Renewable Energy at Huazhong University of Science & Technology (3011120011). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.biortech.2019.02.107. References Branca, C., Di Blasi, C., Galgano, A., Brostrom, M., 2014. Effects of the torrefaction conditions on the fixed-bed pyrolysis of norway spruce. Energy Fuel 28 (9), 5882–5891.
316
Bioresource Technology 281 (2019) 309–317
K. Zeng, et al.
128, 370–377. Zhou, S., Osman, N.B., Li, H., McDonald, A.G., Mourant, D., Li, C., Garcia-Perez, M., 2013. Effect of sulfuric acid addition on the yield and composition of lignin derived oligomers obtained by the auger and fast pyrolysis of Douglas-fir wood. Fuel 103, 512–523.
149, 804–813. Zheng, A., Zhao, Z., Chang, S., Huang, Z., He, F., Li, H., 2012. Effect of torrefaction temperature on product distribution from two-staged pyrolysis of biomass. Energy Fuel 26 (5), 2968–2974. Zheng, A., Zhao, Z., Chang, S., Huang, Z., Wang, X., He, F., Li, H., 2013. Effect of torrefaction on structure and fast pyrolysis behavior of corncobs. Bioresour. Technol.
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