Effects of Water Washing and Torrefaction Pretreatments on Corn

Nov 30, 2016 - ABSTRACT: The washing pretreatment and torrefaction pretreatment of corn stalk were performed in this study. The effects of both separa...
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Effects of Water Washing and Torrefaction Pretreatments on Corn Stalk Pyrolysis: Combined Study Using TG-FTIR and a Fixed Bed Reactor Kehui Cen,† Dengyu Chen,*,‡ Jiayang Wang,‡ Yitong Cai,‡ and Lei Wang‡ †

Nanfang College, Nanjing Forestry University, Nanjing 210037, China College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China

‡

ABSTRACT: The washing pretreatment and torrefaction pretreatment of corn stalk were performed in this study. The effects of both separate and combined pretreatments on the pyrolysis were studied using TG-FTIR and a fixed bed reactor. Washing pretreatment had little impact on the physicochemical properties of corn stalk, contributing mainly to the removal of some ash and metallic species. Torrefaction pretreatment, on the contrary, decreased the oxygen content but increased the ash content. TG-FTIR analysis showed that TG/DTG curves of corn stalk (CS), washed corn stalk (W-CS), torrefied corn stalk (T-CS), and torrefied-washed corn stalk (TW-CS), in turn, showed a right shift trend, and the initial decomposition temperatures increased obviously. Among the samples, W-CS had the lowest values of 27.58 and 29.76 wt % for the final residue mass in the TG curve and the biochar yield, respectively, and the highest bio-oil yield of 48.32 wt %, indicating that the removal of metallic species facilitated the pyrolysis of corn stalk and reduced the secondary cracking of pyrolysis volatiles. Moreover, torrefaction pretreatment greatly promoted the generation of combustible gases and phenols of bio-oil, whereas it remarkably reduced the acids of bio-oil. The combination of water washing and torrefaction preserved the advantages of each method on its own, illustrating this combination was a promising pretreatment for improving pyrolysis products.

1. INTRODUCTION As a clean renewable resource, biomass has great potential for producing biofuels. Pyrolysis is considered to be one of the most promising technologies for biomass utilization. Its efficient use, however, is generally limited by several factors, such as high moisture and oxygen content, low energy density, and difficulty to be ground.1 Because of this, biomass pretreatment technology aims to improve biomass quality and the resulting products, receiving much attention in recent years.2 Water washing is a very common and simple pretreatment that can remove some ash and alkali and alkaline earth metallic species (AAEMs) from biomass.3 The ash and AAEMs have an important effect on biomass pyrolysis, which increases the secondary cracking of pyrolysis volatiles directly, reduces bio-oil yield, and raises the water and acid contents of bio-oil, resulting in poor stability and low heating value of bio-oil.4,5 Previous studies have shown that water washing of biomass can effectively remove soluble alkali metals and part of the alkaline earth metals from biomass;6 and it can also reduce the catalytic effects of AAEMs on pyrolysis, improving the bio-oil yield and decreasing its acids and oxygen contents,3 as well as enhancing the formation of some valuable substances, such as laevoglucose, in bio-oil.4 Thus, water washing is advantageous for biomass pretreatment because of its application prospect, easy use, and low cost. Although corn stalk is one of the most common materials of pyrolysis, only a handful of studies have been performed on water washing pretreatment of corn stalk. Torrefaction pretreatment is another promising technology to improve the quality of biomass, a process by which biomass is mildly pyrolyzed at temperatures ranging from 200 to 300 °C under an inert atmosphere.7 Previous studies have shown that © XXXX American Chemical Society

the torrefaction pretreatment increased the carbon content and energy density of biomass, decreased the volatiles, water, and oxygen contents, and improved the grinding and hydrophobic properties of biomass.8−10 In terms of biomass pyrolysis, although torrefaction pretreatment decreased the bio-oil yield, it reduced the water and acids contents, improved the heating value, and increased the high valuable chemicals of bio-oil.11−15 Therefore, torrefaction pretreatment of biomass is also promising in promoting the use of bio-oil. However, torrefaction pretreatment cannot remove ash from biomass, resulting in increased ash content of biomass after torrefaction. On the other hand, AAEMs still present in biomass after torrefaction aggravated the secondary cracking of bio-oil and thus decreased its quality.16 Therefore, a combination of both processes, water washing and torrefaction pretreatment, has great potential to enhance the quality of both biomass and its pyrolysis products. Corn stalk, large amounts of which are generated annually in China, could be used as an energy source, reducing pollution while also generating economic outcomes. However, to our knowledge, the effects of combination of water washing and torrefaction pretreatment on the quality of corn stalk and its pyrolysis behavior have not yet been investigated. Moreover, with the advantages of less sample requirement, high accuracy, and real-time analysis, thermogravimetry− Fourier transform infrared spectroscopy (TG-FTIR) is widely used in pyrolysis of biomass, which can not only record the weight loss but also evaluate the functional groups of pyrolysis volatiles.17,18 Nevertheless, TG-FTIR could not collect and Received: October 27, 2016 Revised: November 26, 2016 Published: November 30, 2016 A

DOI: 10.1021/acs.energyfuels.6b02813 Energy Fuels XXXX, XXX, XXX−XXX

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2.5. Products characterization. 2.5.1. Analysis of noncondensable gas. The noncondensable gas was detected with a gas chromatograph analyzer (GC-TCD 7890, Shanghai Tianmei, China). It is worth noting that only four gas products (CO2, CO, H2, and CH4) were determined exactly by GC, and there were other gases (C2, C3, etc.). Hence, it was difficult to calculate the exact HHV of the gas products. In this study, the HHV of the noncondensable gas was calculated approximatively by summing each gas concentration with its HHV, following: HHVgas (MJ/N m3) = VolCO (%) × HHVCO (MJ/N m3) + VolH2 (%) × HHVH2 (MJ/N m3) + VolCH4 (%) × HHVCH4 (MJ/N m3) + VolC2+ (%) × HHVC2+ (MJ/N m3). The overall density of the noncondensable gas was calculated from the density of each gas. 2.5.2. Analysis of bio-oil. The bio-oil was analyzed as follows: ultimate analysis was performed using an elemental analyzer (Vario macro cube, Elementar, Germany); water content was analyzed by Karl Fischer titration; HHV was estimated in an adiabatic oxygen bomb calorimeter (XRY-1A, Changji Geological Instruments, China); pH was measured with a digital pH meter (PHS-3C, Shanghai leici, China); and 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 μm × 0.25 μm). Helium (99.999%) was used as a carrier gas at a flow rate of 1.0 mL/min, and 1 μL of sample was injected into the column. The compounds were identified by comparing the spectral data with the NIST library and literature data. 2.5.3. Analysis of corn stalk and biochar. Proximate and ultimate analysis of samples was performed, respectively, according to the Chinese National Standards GB/T28731-2012 and an elemental analyzer (Vario macro cube, Elementar, Germany). Oxygen was estimated by difference. The HHV of biochar was measured using an adiabatic oxygen bomb calorimeter (XRY-1A, Changji Geological Instruments, China). The contents of metallic species contained in corn stalk samples before and after pretreatments were determined with an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 7300 DV, PerkinElmer, USA). In addition, biochar was added to the deionized water in a mass ratio of 1:20 for measuring its pH using a pH meter. 2.5.4. 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%. The standard deviation of the data was also listed in this study.

determine the solid, liquid, and gaseous products of pyrolysis. It is therefore essential to perform pyrolysis using a fixed or fluidized bed reactor in order to obtain enough information on product properties. From the literature point of view, few research studies have been conducted on corn stalk pyrolysis though a combined study using TG-FTIR and a fixed bed reactor. In this study, different pretreatments (water washing, torrefaction, and combined water washing and torrefaction) were evaluated by analyzing the fuel properties of the original and pretreated corn stalk. The pyrolysis characteristics were then investigated in detail using TG-FTIR and a fixed bed reactor. The objective of this work was to demonstrate the effect of different pretreatments on the pyrolysis behavior of corn stalk. It was expected to provide basic data for high-quality products preparation.

2. MATERIALS AND METHODS 2.1. Materials. Corn stalk used in this study was collected from Fuyang city of China. Prior to the experiments, the corn stalk was screened into particles with a size of 40−60 mesh and then dried at 105 °C for 6 h. The dried corn stalk was denoted as CS. 2.2. Water washing and torrefaction process. In the water washing pretreatment process, corn stalk (CS, 5g) was added to a beaker containing 500 mL of deionized water and stirred for 6 h in a thermostat at 60 °C. The sample was then dried in an oven at 105 °C for 12 h. The resulting samples were washed corn stalk and named as W-CS. In the torrefaction pretreatment process, corn stalk (5 g) was put in the tubular furnace and then torrefied in the N2 atmosphere at 250 °C for 30 min to induce a solid torrefied corn stalk which was called as TCS. More details of the torrefaction pretreatment process can be found in our previous study.14 The W-CS was subjected to the same torrefaction conditions, and the solid product was torrefied-washed corn stalk, coded as TW-CS. Mass yield (Ymass) and energy yield (Yenergy) calculations of the pretreatment process are shown as

Ymass = M pre /Mori × 100%

(1)

Yenergy = Ymass·HHVpre/HHVori × 100%

(2)

3. RESULTS AND DISCUSSION 3.1. Properties of corn stalk before and after pretreatments. The basic properties of the CS, W-CS, TCS, and TW-CS are shown in Table 1. Compared to the properties of CS, the ash content of W-CS decreased from 7.26 wt % to 5.06 wt %, while the volatile and fixed carbon contents slightly increased, resulting in a slightly higher HHV. The element contents of W-CS were similar to those of CS, indicating that water washing pretreatment had no obvious effect on the chemical composition of corn stalk. However, the torrefaction pretreatment had a significant effect on the properties of CS. Compared to the basic properties of CS, the HHV and fixed carbon content of T-CS rose; however, the volatile content and oxygen content obviously reduced. This was due to the decomposition of biomass components during torrefaction pretreatment.19 Because of the poor thermal stability, a large amount of hemicellulose was decomposed, whereas cellulose and lignin underwent a partial degradation to release CO2, CO, and water vapor. The volatiles were released, and the ash remained in the corn stalk, meaning an increase in its content in the corn stalk after torrefaction. The contents of ash, volatiles, and fixed carbon of TW-CS were found to be between the values of W-CS and T-CS. In addition, the HHV of TW-CS was very close to that of T-CS, but obviously higher

where M is the mass of sample and HHV is the higher heating value of the sample. The subscripts “pre” and “ori” represent the pretreated sample and original sample, respectively. 2.3. TG-FTIR experiments. Pyrolysis experiments of CS, W-CS, T-CS, and TW-CS were performed using a TG-FTIR instrument that consisted of a thermogravimetric analyzer (TGA Q500, TA Instrument, USA) and a Fourier-transform infrared spectrometer (Nicolet 6700, Thermo Scientific, USA). For each experiment, the samples (20 mg) were heated from 50 to 650 °C at a heating rate of 10 °C/min with a flowing rate of 70 mL/min nitrogen. The pyrolysis volatiles were detected online by FTIR, in which IR spectra were recorded at 4000−400 cm−1 with a resolution of 1 cm−1. The TG/DTG curves and 3D FTIR spectrograms were obtained using a computer which recorded the experiment data of TG-FTIR in real time. 2.4. Pyrolysis experiments. Pyrolysis experiments of CS, W-CS, T-CS, and TW-CS were also performed using a fixed bed pyrolysis reactor, which was demonstrated in our previous study.12 For each experiment, when the temperature of the reactor reached a steady state of 500 °C, the samples (5 g) were quickly fed into the reactor and then retained for 15 min. High-purity nitrogen was used as carrier gas with a flow rate of 300 mL/min. After the experiment, solid (biochar), liquid (bio-oil), and gas (noncondensable gas) products were collected in the reactor, quartz condenser, and gas bag, respectively. The biochar and bio-oil yields were obtained by weighing while that of noncondensable gas was obtained by difference. B

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Ymass (%)

100 98.01 ± 0.35 92.91 ± 0.35 95.40 ± 0.32 ± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.01

16.41 16.85 18.29 18.33

0.35 0.30 0.34 0.41

HHV (MJ/kg) [N]

0.43 0.37 0.49 0.44 ± ± ± ± 43.96 45.79 38.88 41.35 0.18 0.18 0.15 0.17 ± ± ± ±

[H]

CS), the washed samples (W-CS and TW-CS) showed an obvious decrease in K element content. More than 85% of the K element was extracted by water washing, indicating that water washing was an efficient pretreatment method in removing it from corn stalk. However, the removal of Ca, Mg, Fe, Al, and Na by water washing was weak. During the torrefaction process, the metallic species were not released, instead remaining in the corn stalk. Thus, the metallic species contents of T-CS were generally higher than those of CS and W-CS. Additionally, the mass yield (95.45%) and energy yield (98.01%) of W-CS indicated that the water washing eliminated some metallic species as well as a small quantity of organic matter of corn stalk. The weight losses of T-CS and TW-CS were obviously higher than those of CS and W-CS. However, the energy yields of T-CS and TW-CS had no obvious difference from those of CS and W-CS, because the HHV of corn stalk was improved after the torrefaction process. It is noteworthy that the mass and energy yields of TW-CS were higher than those of T-CS. This may be due to the effect of the K element. Previous studies showed that the K element could reduce degradation temperature and encourage volatiles release during torrefaction.20,21 Thus, it can be concluded that the combined water washing and torrefaction pretreatment was efficient in improving the fuel properties of corn stalk, because it not only removed some ash and troublesome inorganic elements but also improved the HHV of corn stalk and retained a relatively high energy yield. 3.2. TG-FTIR analysis. 3.2.1. TG analysis. The TG and DTG curves of CS, W-CS, T-CS, and TW-CS are shown in Figure 2. Clearly, there were three stages presented in the pyrolysis process of all the samples, namely Stage I (moisture evaporation, 50−150 °C), Stage II (main devolatilization, 150− 450 °C), and Stage III (carbonization process, 450−650 °C). The mass loss of the first period was slight, while it was remarkable in the second stage, which involved fast devolatilization of cellulose, hemicellulose, and lignin. And

6.64 6.65 5.52 5.73

[O]

0.59 0.55 0.62 0.71

Figure 1. Metallic species contents of CS, W-CS, T-CS, and TW-CS (μg/g).

± ± ± ± 41.71 42.13 46.85 46.03 0.19 0.15 0.14 0.19 ± ± ± ±

Ash

± ± ± ± ± ± ± ± 73.92 75.21 65.57 69.79

0.67 0.73 0.76 0.69

18.82 19.73 26.17 23.75

0.26 0.29 0.38 0.31

Fixed Carbon Volatile Sample

CS W-CS T-CS TW-CS

7.26 5.06 8.26 6.45

[C]

0.57 0.51 0.53 0.68

Ultimate analysis (wt %, db) Proximate analysis (wt %, db)

Table 1. Proximate and Ultimate Analysis, HHV, Mass Yield, and Energy Yield of Original and Pretreated Corn Stalk

than that of CS and W-CS. Thus, the effect of the combined water washing and torrefaction processes on corn stalk quality was better than that of water washing or torrefaction pretreatment separately. The metallic species contents of the four samples are shown in Figure 1. Compared to the unwashed samples (CS and T-

100 95.45 ± 0.53 83.36 ± 0.46 85.41 ± 0.65

Yenergy (%)

Energy & Fuels

C

DOI: 10.1021/acs.energyfuels.6b02813 Energy Fuels XXXX, XXX, XXX−XXX

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washing, and some volatiles were released by torrefaction. Thus, the water washing preferment of corn stalk reduced the biochar formation while torrefaction preferment promoted the biochar formation. 3.2.2. 3D FTIR analysis. The TG-FTIR analysis results are presented in Figure 3, which shows the 3D FTIR spectrograms of pyrolysis volatiles of CS, W-CS, T-CS, and TW-CS at the heating rate of 10 °C/min from 50 to 650 °C. The absorbance of one component showed a linear relationship with its content ratio. Thus, the variation of the components with increasing temperature could be determined. Figure 3 clearly showed that the variation FTIR spectra with increasing temperature contained three pyrolysis stages which were consistent with the TG/DTG curves shown in Figure 2. Generally, there were almost no compounds found in the 3D FTIR at first, because the weight loss of samples was very little during the first stage. Afterward, the pyrolysis products, such as furans, phenols, ketones, and aldehydes as well as CO2, H2O, CH4, and CO, were gradually generated. Their characteristic bands clearly appeared in the 3D FTIR, and the strongest spectral intensity corresponded to the mass loss peak of the DTG curve. Finally, the absorbance of pyrolysis products gradually decreased. The typical pyrolysis volatiles can be identified by their characteristic absorbances.17 The characteristic absorbances of H2O, CH4, CO2, and CO were found at 4000−3400 cm−1, 3050−2650 cm−1, 2400−2240 cm−1, and 2230−2000 cm−1, respectively. The obvious bands between 1880−1620 cm−1 representing the CO stretching absorbance were related to some organic components, such as aldehydes, ketones, and acids.23 The characteristic absorbances of the organics, such as alcohols, aldehydes, acids, and phenols, were also found at 1600−400 cm−1, but they were different to identify because these products were very complex. Moreover, it can be seen from Figure 3 that water washing and torrefaction pretreatments had a remarkable effect on the products evolved during pyrolysis. The 3D FTIR spectrograms of CS and W-CS had obvious shoulder peaks, whereas they disappeared in the 3D FTIR spectrograms of T-CS and TWCS. Meanwhile, compared to CS, the absorption intensity of the pyrolysis products of W-CS was enhanced, whereas that of T-CS was reduced. These results suggested that the removal of metallic species and ash facilitated the pyrolysis of corn stalk and would increase the bio-oil yield. 3.3. Distribution of pyrolysis products. The samples were pyrolyzed using a fixed bed pyrolysis reactor, and the product distributions are shown in Figure 4. If compared to the bio-oil yield of CS, the bio-oil yield of W-CS increased by 8.29% and the biochar yield reduced by 8.59%, while the noncondensable gas yield remained unchanged. This phenomenon was due to water washing having removed soluble alkali metals such as K from the corn stalk, lessening catalytic decomposition of the bio-oil and consequently raising the biooil yield and reducing biochar yield. Compared to yields with pyrolysis products in CS and W-CS, the yields of biochar and noncondensable gas of T-CS obviously increased while that of bio-oil obviously reduced to 28.73 wt %. This was due to the different decomposition characteristics of the three components of biomass. The bio-oil was mainly produced by hemicellulose and cellulose pyrolysis while the biochar was mainly obtained from lignin pyrolysis. A large amount of hemicellulose was decomposed during the torrefaction pretreatment. However, lignin had a strong thermal stability and wide decomposition range of temperatures,

Figure 2. TG (a) and DTG (b) curves of CS, W-CS, T-CS, and TWCS.

then, the mass loss decreased slowly to the final temperature and biochar was formed. The different pretreatments had an obvious effect on the pyrolysis, such as initial decomposition temperature, maximum loss rate, shoulder peaks, and final residue mass. As can be seen from Figure 2, the TG/DTG curves of CS, W-CS, T-CS, and TW-CS, in turn, showed a right shift trend. Meanwhile, the initial decomposition temperatures of W-CS, T-CS, and TWCS increased obviously, and the W-CS had the highest temperature of maximum loss rate, indicating that both water washing and torrefaction pretreatments enhanced the thermal stability of samples. Moreover, as seen from DTG curves, the shoulder peaks were obvious for CS and W-CS at about 260−280 °C. However, they disappeared for T-CS and TW-CS. This was mainly related to the different thermal stabilities of the three components of biomass. A previous study indicated that the shoulder peak corresponded to the decomposition of hemicellulose while the main peak corresponded to the decomposition of cellulose.22 During the torrefaction process, most of the hemicellulose of T-CS and TW-CS was decomposed because of its poor thermal stability. In addition, it can be seen from TG curves that the order of the final residue mass of the four samples was as follows: T-CS > TW-CS > CS > W-CS. This phenomenon was due to the fact that some ash and metallic species were removed by water D

DOI: 10.1021/acs.energyfuels.6b02813 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 3. 3D FTIR spectrograms of pyrolysis volatiles of the samples: CS (a), W-CS (b), T-CS (c), and TW-CS (d).

Figure 4. Yields of the products obtained from pyrolysis of CS, W-CS, T-CS, and TW-CS.

Figure 5. Volume fraction of gas products from pyrolysis of CS, W-CS, T-CS, and TW-CS.

resulting in the content of lignin in corn stalk being substantially increased after the torrefaction pretreatment. Thus, the bio-oil and biochar yields of TW-CS were recorded to be between those of W-CS and T-CS. 3.4. Properties of pyrolysis products. 3.4.1. Noncondensable gas. Figure 5 clearly depicted a high CO2 volume fraction of noncondensable gas produced by CS that reached 40.10 vol %, while those corresponding to CO, CH4, and H2 were 31.18 vol %, 17.12 vol %, and 8.89 vol %, respectively. Compared with CS, the CO2 volume fraction of W-CS pyrolysis slightly reduced, whereas those of CO, CH4, and H2 slightly increased

or basically remained unchanged. These changes were related to the catalytic role played by the metallic species in the pyrolysis, but the overall impact was very low. Water washing pretreatment removed some soluble metals but had no effect on the chemical structure of corn stalk. However, changes in the three groups of hemicellulose, cellulose, and lignin induced by torrefaction pretreatment markedly influenced the pyrolysis products of corn stalk. The effect of torrefaction pretreatment on the noncondensable gas was also clearly shown in Figure 5. Compared to CS, the volume fraction of CO2 decreased by 11.46% while that of CO E

DOI: 10.1021/acs.energyfuels.6b02813 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels and H2 increased by 4.49% and 32.58% for T-CS, respectively. The combined water washing and torrefaction pretreatment also influenced the gas products of corn stalk. With respect to CS and W-CS, the volume fraction of CO2 from TW-CS declined, whereas that corresponding to CO and H2 rose slightly. However, TW-CS had no clear advantage compared to T-CS, as the CO and CH4 volume fractions of TW-CS were slightly higher than those of T-CS, whereas the H2 volume fraction was lower than that of T-CS. Some C2+ gases, including C2H2, C2H4, C2H6, C3H6, and C3H8, were also detected in the gas products but at incredibly low volume fractions. The variation of gas volume fraction could be explained by the change of biomass structure, especially the variation of some characteristic functional groups during torrefaction pretreatment. CO and CO2 are likely to be formed by the cracking and reforming of carbonyl (CO), ether (C−O−C), and carboxylic acid (COOH) of hemicellulose and cellulose; H2 and CH4 are mainly derived from the aromatic ring and fracture of O−CH3 groups of lignin. Previous study has shown the following: lignin owned the highest H2 and CH4 yield; cellulose had the highest CO yield because of the higher carbonyl content; and hemicellulose showed the highest CO2 yield due to the higher carboxyl content.24 Differing decomposition degrees among cellulose, hemicellulose, and lignin during torrefaction changed the gas volume fractions in the subsequent pyrolysis. The HHV of the noncondensable gas of CS, W-CS, T-CS, and TW-CS was calculated as 14.05 MJ/N m3, 14.03 MJ/N m3, 14.63 MJ/N m3, and 14.52 MJ/N m3, respectively. It was important to note that the density of the noncondensable gas could be calculated to obtain the HHV with the MJ/kg unit. The water washing induced only a slight increase in the HHV of the noncondensable gas while the torrefaction pretreatment influenced it markedly. Compared to CS and W-CS, T-CS and TW-CS incremented the total volume concentration of the combustible gases (H2, CO, and CH4). This, in turn, improved the HHV of the noncondensable gas. 3.4.2. Bio-oil. The water content, HHV, and pH value of biooil are listed in Table 2. Bio-oil’s water content decreased from

Figure 6. Relative content of different groups of bio-oil from pyrolysis of CS, W-CS, T-CS, and TW-CS.

bio-oil. Previous study showed that although GC/MS analysis did not achieve bio-oil’s quantitative results, the chromatographic peak area% was linear with its content.25 Bio-oil’s identified components were divided into five main groups according to their functional groups, such as acids, ketones, furans, phenols, and anhydrosugars. It was worth noting that the sum of the total peak area% was less than 100%, which should be due to the unidentified compounds on the ion chromatograms. Acids detected in the bio-oil of CS reached the relative content of 40.96% and were the most dominant chemical group. The washing and torrefaction pretreatments greatly reduced acids in the bio-oil. Compared to the relative acid content in bio-oil for CS, that from W-CS, T-CS, and TWCS declined obviously, by 20.93%, 32.56%, and 41.34%, respectively. In contrast, the relative content of phenols greatly increased after water washing and torrefaction pretreatments, with phenols from W-CS, T-CS, and TW-CS ranging from 31.41% to 36.52%, clearly higher those of CS at 15.64%. The relative content of furans and anhydrosugars showed no obvious change. Potassium promoted catalytic reactions such as ring fission and fragmentation of a glucose unit, increasing the yield of low molecular weight compounds such as acetic acid and ketones.4 Water washing pretreatment removed a large amount of K and thus promoted conversion of pyrolysis to phenols. Torrefaction pretreatment changed the internal structure of corn stalk by reducing the hemicelluloses content and increasing the lignin content. As a result, it produced fewer acids and ketones, and more phenols during pyrolysis. The acids hinder the stability of bio-oil, while the phenols are raw chemical materials with high added value. The lowest relative content of acids and highest relative content of phenols came from pyrolysis of TW-CS. Thus, compared to the separate pretreatment of water washing or torrefaction, the combined pretreatment further improved the quality of bio-oil. 3.4.3. Biochar. The basic properties of biochar are shown in Table 3. The ash content of biochar for W-CS and TW-CS was slightly lower than that of CS and T-CS. Generally, the biochar from pyrolysis of corn stalk before and after pretreatment did not differ, indicating that water washing and torrefaction pretreatment had no obvious impact on biochar’s basic properties. Previous studies showed biochar containing

Table 2. Water Content, HHV, and pH Value of Bio-oil from Pyrolysis of CS, W-CS, T-CS, and TW-CS Bio-oil CS W-CS T-CS TW-CS

Water content (wt %) 56.18 54.34 40.32 39.41

± ± ± ±

1.05 1.06 0.94 0.65

HHV (MJ/kg) 10.61 11.92 14.09 14.91

± ± ± ±

0.32 0.37 0.35 0.31

pH 2.83 3.16 3.05 3.38

± ± ± ±

0.05 0.06 0.03 0.08

56.18 wt % from CS to 54.34 wt % from W-CS and further to 40.32 wt % from T-CS. The lowest water content of bio-oil was 39.41 wt % for TW-CS. Meanwhile, the HHV of bio-oil increased 10.63 MJ/kg from CS to 14.91 MJ/kg from TW-CS. The pH value of bio-oil tended to increase after pretreatment of corn stalk. These results can be attributed to removal of alkali metals by water washing pretreatment. This pretreatment decreased the occurrence of catalytic reactions forming water and dehydration reactions during torrefaction.6 In turn, water generation lessened in subsequent pyrolysis. The chemical composition of the bio-oil was analyzed using GC/MS to further evaluate the effect of combined water washing and torrefaction pretreatment on the pyrolysis of corn stalk. Figure 6 shows the relative contents of different groups of F

DOI: 10.1021/acs.energyfuels.6b02813 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 3. Properties of Biochar from Pyrolysis of CS, W-CS, T-CS, and TW-CS Proximate analysis (wt %, db) Biochar CS W-CS T-CS TW-CS

Volatile 17.29 17.98 17.63 18.14

± ± ± ±

0.26 0.41 0.34 0.45

Fixed carbon 63.06 63.29 63.16 62.65

± ± ± ±

0.87 0.68 0.65 0.78

Ultimate analysis (wt %, db) Ash

19.65 18.73 19.21 19.01

± ± ± ±

[C] 0.91 0.79 0.73 0.90

67.13 67.21 68.01 67.59

± ± ± ±

[H] 0.83 0.86 0.79 0.72

1.75 1.73 1.87 1.89

± ± ± ±

[O]

0.03 0.03 0.03 0.04

10.34 11.27 9.87 10.44

± ± ± ±

[N] 1.23 1.17 1.08 1.15

1.13 1.06 1.04 1.08

± ± ± ±

0.03 0.02 0.03 0.03

HHV (MJ/kg) 24.75 25.93 23.68 25.26

± ± ± ±

0.51 0.39 0.41 0.46

pH 9.82 8.95 10.01 9.16

± ± ± ±

0.05 0.03 0.06 0.08

Figure 7. Schematic flowsheet of pretreatments and subsequent pyrolysis procedure.

4. CONCLUSION Water washing pretreatment removed some ash and metallic species from corn stalk, while torrefaction pretreatment reduced the oxygen content and increased both the ash content and HHV of corn stalk. The variation FTIR spectra with increasing temperature contained three pyrolysis stages which were consistent with the TG/DTG curves. The shoulder pyrolysis peaks were obvious for CS and W-CS, whereas they disappeared for T-CS and TW-CS, which was due to decomposition of hemicellulose during torrefaction. The final residue mass of the four samples indicated that the removal of metallic species and ash by water washing pretreatment facilitated the pyrolysis of corn stalk. Torrefaction pretreatment and combined pretreatment each enhanced the quality of the gas products, compared to water washing pretreatment. For bio-oil, the combined process greatly reduced the acid and water content, and raised both the phenol content and its HHV. These findings indicated that a combination of water washing and torrefaction was effective and a promising pretreatment for improving the qualities of pyrolysis products.

carbonates (such as CaCO3 and MgCO3) and alkali and alkaline earth metals (such as K, Na, Ca, and Mg).6,26 Thus, its pH values tended to be alkaline. The lowest pH value for the biochar from pyrolysis of W-CS was 8.95, possibly attributed to the elimination of some metallic species by water washing pretreatment. 3.5. Schematic flowsheet of pretreatments and pyrolysis. In order to intuitively show the influence of water washing and torrefaction pretreatments on corn stalk pyrolysis, the schematic flowsheets of the pretreatments and subsequent pyrolysis procedure are presented in Figure 7. The initial mass of corn stalk sample was assumed as 100%, and the pyrolysis products yields were all based on the initial mass of corn stalk (dry base). For example, 1 kg of corn stalk produced 0.95 kg of W-CS after water washing, and then the pyrolysis of 0.95 kg of W-CS produced 0.29 kg of biochar, 0.46 kg of bio-oil, and 0.20 kg of gas products with HHV of 25.93 MJ/kg, 11.92 MJ/kg, and 11.34 MJ/kg, respectively. The corn stalk clearly lost about 5−17% of its mass during water washing and torrefaction pretreatment. The reason was that a small amount of organic matter was leached out during water washing.27 A small amount of gas and liquid products were also released out during torrefaction. The mass loss also removed some energy, but the HHV of the pretreated corn stalk increased, keeping more than 93% of the energy in the pretreated corn stalk. Water washing and torrefaction pretreatment of corn stalk had no obvious effect on the biochar. For bio-oil, however, a large difference from pyrolysis of original and pretreated corn stalk can be found. Although the bio-oil obtained from TW-CS had low production, its qualities were greatly improved. For noncondensable gas, torrefaction pretreatment, and combined water washing and torrefaction pretreatment decreased the production of noncondensable gas but improved the HHV. Pyrolysis polygeneration has received increasing attention in recent years.28,29 Biochar, bio-oil, and noncondensable gas are all important products of corn stalk pyrolysis. Although water washing and torrefaction pretreatment changed the products distribution, their overall qualities generally improved, and thus it was advantageous to subsequent use of the products.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. ORCID

Dengyu Chen: 0000-0002-5275-3149 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Jiangsu Province (No. BK20151521), University Science Research Project of Jiangsu Province (No. 16KJB480001), National Natural Science Foundation of China (No. 51406089), Students Practice Innovation Training Program of Nanjing Forestry University (No. 2016NFUSPITP066), and Priority Academic Program Development of Jiangsu Higher G

DOI: 10.1021/acs.energyfuels.6b02813 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

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Education Institutions (PAPD). The authors also acknowledge the Advanced Analysis & Testing Center of Nanjing Forestry University for testing service.

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DOI: 10.1021/acs.energyfuels.6b02813 Energy Fuels XXXX, XXX, XXX−XXX