Torrefaction and Degradative Solvent Extraction As Means of

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Torrefaction and degradative solvent extraction as means of processing rice husk waste Ryan Fitrian Sofwan Fauzan, Dong Hyuk Chun, Jiho Yoo, Sangdo Kim, Jeonghwan Lim, Youngjoon Rhim, Sihyun Lee, and Ho Kyung Choi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03378 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 31, 2018

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Torrefaction and degradative solvent extraction as means of processing rice husk waste Ryan Fitrian Sofwan Fauzan, Donghyuk Chun, Jiho Yoo, Sangdo Kim, Jeonghwan Lim, Youngjoon Rhim, Sihyun Lee, Hokyung Choi* Clean Fuel Laboratory, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon, Republic of Korea

ABSTRACT The high moisture level and ash content of biomass often hinder its further processing. In this study, torrefaction and degradative solvent extraction were employed to upgrade rice husk waste. Biomass torrefied at five different temperatures, in addition to dry raw biomass for comparison, was extracted in 1methylnaphthalene at 300o C for 1 hour. Two solid fractions were obtained: extracted biomass (EB) and residue biomass (RB). The extraction yields of the EB and RB were 12% to 19.3% and 31.7% to 52% (db), respectively. The torrefaction temperature affected the extraction yields and slightly influenced product characteristics. The EB had almost no ash content as it was concentrated in the RB. However, both the EB and RB had higher heat values and carbon content as well as lower oxygen content than the raw biomass. Therefore, our findings suggest that EB could be utilized not only as fuel, but also as functional materials.

Keywords: Torrefaction, degradative solvent extraction, upgrading, biomass waste, rice husks

* Corresponding author: [email protected]

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Introduction

In light of the rapidly depleting sources of fossil fuel and the environmental problems caused by its overuse, biomass has been deemed to be the most feasible alternative resource for providing energy and serving as building blocks for chemicals in the future. Biomass waste is especially abundant in some countries, and developing efficient processes to utilize it is of great importance. However, the nature of biomass, such as its high moisture content, which results in its low calorific value, oftentimes hinders its valorization. Therefore, dewatering the biomass is an important step before it is processed any further. Unfortunately, dewatered carbonaceous material has a high tendency to spontaneously combust due to its high oxygen content 1. In addition, other problems, such as its low energy density, grinding difficulty, and high ash content (especially grass-based biomass feedstock), need to be addressed. For these reasons, pretreatment to dewater and upgrade the properties of the biomass is necessary before it can be converted into fuel or other uses. One of the primary examples of biomass pretreatment is torrefaction. Torrefaction is a thermal treatment that is conducted in relatively low temperatures (around 200o C to 300o C) in the absence of oxygen 2. Torrefaction removes moisture and highly volatile compounds from the biomass and converts it into densified carbon material. In addition, for many forms of biomass utilization, a small particle size is necessary for technical reasons or to increase the yield of the product. Torrefaction improves the grindability of raw biomass and therefore can reduce the energy required to reduce the particle size by 80% to 90% 3. However, after the torrefaction process, the biomass generally still has a high oxygen content—around 25% to 40% (wt, daf)—and increased ash content compared to its parent biomass 4. Furthermore, torrefied biomass’ properties vary greatly depending on the feedstock used. As a solution to these problems pertaining to conventional pretreatment methods such as torrefaction, especially for feedstock with high ash content, several groups of researchers are developing a new method called degradative solvent extraction. This process has been proven to be effective for producing ash-free coal from 2 ACS Paragon Plus Environment

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low-rank coal 5,6. In Japan, this research has been carried out since 1999 to produce the so-called hyper-coal 7. This method uses a nonpolar solvent to treat the coal at a temperature below 350o C under inert conditions. The extraction fractionates biomass into two solid products, namely extracted biomass (EB) and residue biomass (RB); gas (mostly CO2); and liquid (mostly H2O) products 8–10. Zhu et al. (2016)8 has proposed a possible mechanism underlying this method by extracting woody biomass using 1-methylnaphthalene (1-MN) at 350o C. They found that the process involves deoxygenation and aromatization reactions, with the oxygen mainly removed as CO2 and H2O. Wannapeera et al.9 found that the chemical and physical properties of EB from eight different types of biomass waste were very similar and that almost all of the ash content of the EB was removed. The EB yield from their work has ranged from 25.1% (wt, daf) for Napier grass to 50.2% (wt, daf) for Eucalyptus. In addition, other previous work has shown a significant increase in carbon and a drop in oxygen content in both EB and RB 8–10. The results of work by Zhu et al.8 show that the carbon content increased 56.3% (wt, daf) from raw biomass to both EB and RB, while the oxygen content dropped as much as 67.8% and 63.7% (wt, daf) from the raw biomass to EB and RB, respectively. Finally, Fujitsuka et al.11 found the extracted fraction of low-rank coal to be much less prone to spontaneous combustion due to its low oxygen adsorption rate, demonstrating this method’s ability to suppress the low-temperature reactivity of low-rank carbonaceous material. This nearly ash-free, high-carbon, deoxygenated EB is promising for further utilizations, and the possibility of using it as fuel with high calorific value and as a precursor for the development of other functional materials has been investigated in previous work. Li et al.12 found that the oils produced from the liquefaction of EB, which mainly consist of aliphatic and aromatics compounds, have a lower oxygen content (2.2% wt, daf) compared to that produced from direct liquefaction of raw feedstock (19.5% wt, daf). Zhu et al.13 also studied the conversion of biomass into bio-oils by solvent extraction followed by pyrolysis. The result was positive, showing that the bio-oils from EB have a significantly higher carbon content and higher heat value (HHV), along with a higher 3 ACS Paragon Plus Environment

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content of value-added aromatics, compared to the bio-oils from its parent biomass. On the other hand, the oxygen, water, and corrosive acid levels were much lower compared to the bio-oils from raw biomass pyrolysis. Zhu et al.14 also found that blending EB with caking coal in making coke improved the coke quality and its thermoplastic properties, with as much as a 9o C increase in the plastic range. Another promising result was also obtained by Li et al.15, who used EB and extracted low-rank coal produced from degradative solvent extraction as precursors for carbon fiber, a high value-added functional material. As for RB, it has been found to have higher combustion and gasification activity than raw feedstock, which makes it possible to use it in boilers and gasifiers 10. However, despite the known feasibility of solvent extraction as biomass pretreatment on its own and the potential usefulness of its products, only a few researchers have explored the feasibility of combining this process with other pretreatment methods, such as torrefaction. Torrefaction is known to be advantageous in high-scale biomass processing as it enables the biomass to be stored and transported easily before any further conversions. Hidayat et al.16 tried to investigate the effect of torrefaction prior to solvent extraction on woody biomass, which has low ash content. Agro-industrial waste, however, are mostly grass-based biomass, which has a much higher ash content. Therefore, in this study, the feasibility of employing torrefaction prior to solvent extraction of grass-based biomass was investigated. Rice husk waste, which has a high ash content (11% wt, as received), was chosen as the biomass of interest. Torrefaction of the rice husk waste was conducted prior to degradative solvent extraction at five different temperatures (200o C, 250o C, 270o C, 300o C, and 330o C). Dry raw biomass, in addition to torrefied biomass, was then extracted at 300o C using 1-MN as the solvent. This temperature was used after investigating the effects of extraction temperature on product yield and properties. The yields, chemical composition, physicochemical characteristics, and thermal decomposition behavior of the solid


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products (EB and RB) were then obtained and analyzed. The magnitude of the effect of torrefaction on the overall process was also assessed.

2.

Materials and Methods 2.1.

Materials. Rice husk waste was used as the raw material in this study. Proximate and ultimate

analyses, as well as the calorific value of the raw biomass, are shown in Table 1. 1-MN was employed as the solvent for the extraction. For comparison purposes, three main constituents of biomass purchased from SigmaAldrich—cellulose (fibrous powder), hemicellulose (powder), and lignin (alkali, powder)—were also used in this work.

2.2.

Drying and torrefaction of biomass. The raw biomass was sieved to obtain 1 to 2-mm-sized

particles for drying and torrefaction. The drying process was conducted using an oven at 107o C for 24 hours under an N2 atmosphere to obtain the T107 sample. The rest of the samples (T200, T250, T270, T300, and T330) were made by torrefying the sieved rice husks at five different respective temperatures (200o C, 250o C, 270o C, 300o C, and 330o C) for 30 minutes, also under an N2 atmosphere. Torrefaction was carried out in a ceramic crucible that was put inside a lab-scale furnace. The samples were then ground prior to solvent extraction, resulting in final samples less than 850 mesh in size.

2.3.

Degradative solvent extraction. Before extracting the samples, a preliminary experiment to

determine the optimum treatment temperature for rice husk extraction was conducted. The solvent extraction process was carried out at 300o C using a small-batch autoclave reactor (Ilshin Autoclave) for 1 hour after purging the reactor with N2. The detailed reactor configuration was discussed in previous publication

16

. The

extraction process was done by mixing and heating the 30 grams of each biomass sample with 1-MN with a 5 ACS Paragon Plus Environment

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weight ratio of 1 to 9. The slurry was then mixed with an agitator at 60 rpm. The heating rate was 5oC per minute before reaching the setting temperature. As the working temperature was reached, the working pressure varied between 1.5 to 2.5 MPa, depending on the sample. A metal filter fitted to the bottom of the reactor allowed in-situ filtration, separating the EB and RB by opening the valve connecting the reactor with a reservoir. Filtration was done without cooling the sample first for two reasons. First, as the temperature decreased, the viscosity of the solution increased, which could have caused clogging in the filter. Second, as the solution cooled down, precipitation of the extracted fraction occurred, decreasing its yield accordingly. Both the EB solution and RB slurry were then dried using an oven at 260o C, and a vacuum condenser was used to recover the 1-MN. Each experiment was conducted at least two times to ensure reproducibility and then averaged. Cellulose, hemicelluloses, and lignin standards were also extracted under the same conditions.

2.4.

Analyses. The chemical compositions, physicochemical characteristics, and thermal behavior of

the raw biomass and the solid products (EB and RB) were characterized using various analyses. A proximate analysis was conducted using LECO TGA701, and an ultimate analysis was performed using the Thermo Scientific FLASH 2000 series analyzer. The calorific values of the samples were determined using the Parr 6400 Calorimeter. Thermogravimetric (TG) and derivative thermogravimetric (DTG) analyses were conducted using the TA SDT Q-600, in which approximately 10 mg of each sample was heated to 900o C with a heat rate of 10o C per minute under a flow of 100 mL/min of N2. The thermal degradation behavior of the cellulose, hemicelluloses, and lignin standards was also analyzed by this method. Finally, the chemical functional groups of all samples were analyzed using the Fourier transform infrared (FTIR) spectrometer NICOLET 6700 FT-IR from Thermo Scientific.


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2.5.

Equations. Mass yield (wt%, db), energy yield (%, db), and carbon and oxygen distribution

(wt%, daf) of the samples were calculated using the equations below. In this paper, all mass is given in grams, and calorific values are in MJ/kg. TB1 mass yield = (mass of TB obtained / mass of raw biomass fed) × 100

(1)

EB2 mass yield = (mass of EB obtained / mass of TB fed) × 100

(2)

RB3 mass yield = (mass of RB obtained / mass of TB fed) × 100

(3)

TB energy yield = (TB mass yield × CV4 of TB / CV of T107) × 100

(4)

EB energy yield = (EB mass yield × CV of EB / CV of TB) × 100

(5)

RB energy yield = (RB mass yield × CV of RB / CV of TB × 100

(6)

Overall EB mass yield = EB mass yield × TB mass yield

(7)

Overall RB mass yield = RB mass yield × TB mass yield

(8)

Overall EB energy yield = EB energy yield × TB energy yield

(9)

Overall RB energy yield = RB energy yield × TB energy yield

(10)

Total overall mass yield = Overall EB mass yield + Overall RB mass yield

(11)

Total overall energy yield = Overall EB energy yield + Overall RB energy yield (12) C distrib. = (C content of sample/C content of raw biomass) × mass yield (daf)

(13)

O distrib. = (O content of sample/O content of raw biomass) × mass yield (daf)

(14)

3.

Results and Discussion

1

TB: torrefied biomass EB: extracted biomass 3 RB: residue biomass 4 CV: calorific value 2


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Effect of torrefaction. Torrefaction was carried out in 200o C, 250o C, 270o C, 300o C, and 330o

C for 30 minutes. Figure S1 shows the morphology of the torrefied biomass samples. The mass and energy yield of the rice husk torrefaction are presented in Table 2, with the dry raw biomass sample (T107) as the basis. The results generally agree with other studies, as presented in Table 3. It can be seen that the yield decreased with increasing temperatures. However, a significant decrease only started to occur at 270o C, whereas the yield for T200 and T250 only slightly changed from T107. Additionally, T330 had the lowest mass and energy yield of 74.4% (wt, db) and 80.4% (db), respectively. Moreover, the composition of the torrefied biomass was also affected by temperature. The volatile matter decreased with increasing temperatures, while the ash and fixed carbon content increased, as seen in Table 2. Among the samples, T330 experienced the most severe alteration in its composition after torrefaction. Calorific value increased from 16.95 MJ/kg of T107 to 19.43 MJ/kg of the T330 sample. The increase in heat value may indicate the oxygen removal from the biomass into gas products during torrefaction. Figure 1 presents the results of the TG and DTG analysis of the torrefied biomass alongside those of its main constituents (cellulose, hemicelluloses, and lignin). Decomposition of cellulose occurred at the range of 275o C to 375o C, with the peak at around 335o C; for hemicellulose, the decomposition temperature ranged from 250o C to 350o C, peaking at 300o C. Lignin was the most difficult to decompose; this process started at 160o C and continued to occur until 900o C with a low weight-loss rate. The thermal degradation profiles for cellulose and lignin look similar to the findings of Yang et al.17, whereas the peak of hemicellulose was around 268o C in their work. This slight difference may be due to the different material used. Looking at the DTG curves of the torrefied biomass samples, the highest peak was that of T270, which was most likely due to a cellulose peak as most of the hemicellulose content should have been degraded at that temperature. On the other hand, the lowest peak was that of T330 because most of the cellulose and hemicellulose should have already been decomposed at 330o C. 8 ACS Paragon Plus Environment

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The FTIR spectra for the torrefied biomass, standard cellulose, hemicelluloses, and lignin are shown in Figure S2. The peaks can be generally grouped into three ranges: C-H aliphatic stretch (3,000 to 2,650 cm-1), carbonyl/C=O stretch (1,850 to 1,600 cm-1), and C-O and C-C stretch (1,500 to 900 cm-1). Those peaks indicate the existence carbonyl, aromatic hydrocarbons, and phenolic compounds. Torrefaction consists of three stages: depolymerization, devolatilization, and carbonization; the formation of those compounds is a characteristic of the devolatilization step 18. The spectra show that all of these peaks were stronger with increasing torrefaction temperatures, indicating the formation of those compounds as well as the exposure of lignin as the hemicellulose and cellulose decomposed. The torrefaction process was conducted for 30 minutes, which means that it was most likely still at the end of the devolatilization stage when the torrefaction ended.

3.2.

Effect of extraction temperature on product characteristics. To determine the optimal

extraction temperature for the samples, the dry raw biomass sample (T107) was extracted at three different temperatures (250o C, 300o C, and 350o C). The mass yields for both the EB and RB are provided in Table 4. It can be seen that extraction at 250o C yielded the least amount of EB (7.3 wt%, db), whereas at both 300o C and 350o C, the extraction yielded around 19% (wt, db) EB. However, extraction at 350o C yielded a slightly less amount of RB than extraction at 300o C, which apparently also has more heating value than the dry raw biomass (T107). Table 4 shows the results of the proximate analysis and ultimate analysis and the calorific values of both the EB and RB produced at different extraction temperatures. While the ash content of all EB samples was less than 1% (wt, db), the volatile matter increased with higher extraction temperatures. On the other hand, less volatile matter was left on the RB fraction as the extraction temperatures increased. Furthermore, the ash content of the RB increased with higher temperatures, as was expected. As for the fixed carbon content, the trend was the opposite of the volatile matter: It decreased for the EB and increased for the RB as the extraction temperature


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increased. Overall, the results show that more volatile matter content was extracted as the extraction temperature increased. Moreover, the carbon content and oxygen content of both the EB and RB were higher and lower than the raw biomass, respectively, except for the RB obtained at 350o C (R107-350). In addition, the carbon and oxygen contents were increased and decreased with temperature, respectively. Figure 2 shows the carbon and oxygen distributions of the products. Increased extraction temperatures improved the transfer of carbon to the EB fraction, from 15.9% (wt, daf) at 250o C to 46.1% at 350o C, whereas the RB showed the opposite trend, decreasing from 48.2% at 250o C to 17.1% at 350o C. While the EB obtained at 350o C had the highest carbon content among all the EB samples, the solid products obtained at 300o C had the highest total carbon content of all, measuring 68.6% (wt, daf) from both the EB and RB, compared to 63.2% (wt, daf) at 350o C. Therefore, since the aim of the extraction was to utilize both solid products, 300o C was chosen to extract the rest of the samples. Figure 3 shows the TG and DTG curves for the EB and RB produced at different extraction temperatures. The thermal decomposition of EB started at around 200o C due to the high volatile matter content. The RB, on the other hand, mainly decomposed at higher temperatures due to the lower volatile matter content and higher content of ash and fixed carbon 10. These results are consistent with the proximate analysis. The TG and DTG curves also show that as the extraction temperature increased, the produced EB decomposed faster. On the other hand, the RB produced at higher extraction temperatures decomposed slower. These results indicate that there was a change in composition for both the EB and RB; in particular, more volatile matter content was extracted as the extraction temperature increased. In addition, RT107-250 showed another peak at around 300o C, which suggests the existence of low-molecular-weight compounds that may be derived from the unextracted hemicellulose/cellulose from the parent biomass. This also agrees with the proximate analysis result that shows a high amount of volatile matter in the RT107-250 sample, implying that 250o C was not a high 10 ACS Paragon Plus Environment

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enough temperature for the extraction. All in all, the TG and DTG curves were totally different from those of the raw biomass, suggesting that both the EB and RB consist of different chemical make-ups than the raw biomass. Regarding chemical structure, there were no striking differences in the chemical properties of the samples extracted at different temperatures, as shown in Figure S3. However, it can be seen that the spectra of the EB samples are totally different than the raw biomass (T107), whereas there is a resemblance in the spectra of RB samples.

3.3.

Yield distribution of extraction. After choosing 300o C as the ideal temperature for extraction,

the rest of the torrefied biomass samples (T200, T250, T270, T300, and T330) were also extracted with 1-MN. The yield distributions of the extraction products are shown in Figure 4. Extraction was also conducted for the standard samples of main biomass contents (cellulose, hemicelluloses, and lignin) under the same conditions. These results are also shown in Figure 4. It can be seen that the yield of the EB was rather low (12 wt% to 19.3 wt%, db) compared to the yields in similar studies 1,8–10. This was most likely caused by the torrefaction process, during which the extractable fractions were thermally decomposed prior to extraction. In addition, rapid filtration at a temperature past the solvent’s boiling point could cause flash evaporation of the extracted fractions, leading to higher loss. However, this procedure was necessary to prevent bigger technical problems caused by filtration at lower temperature, as the extract solution could become more viscous and precipitate before the filtration. The overall trend was that the yield for EB decreased as the torrefaction temperature increased because more extractable components were being removed during torrefaction at higher temperatures. However, the T200 and T250 samples did not follow this trend. For these samples, increasing the torrefaction temperature from 107o C to 200o C apparently decreased the extraction yield of the EB, whereas increasing the torrefaction temperature 11 ACS Paragon Plus Environment

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from 200o C to 250o C increased the yield. This could have been caused by polycondensation reactions experienced by the hemicellulose and lignin that may occur under 200o C and become more severe as the temperature increases. The reaction could induce the formation of a stable cross-linked carbohydrate polymer that leads to the formation of char, making the sample hard to extract 16. Up to 200o C, the polycondensation reaction during torrefaction accounted for a decrease in the EB yield of the T200 extraction. Then, as shown in Figure 1, hemicellulose started to decompose; the lignin continued to do so at 200o C, slowly suppressing the reaction accordingly. Because of this, coupled with the higher cellulose content in the sample, the EB yield increased to 19.3 wt% (db) for T250. Chen and Kuo19 drew a similar conclusion that the heating value from a torrefied biomass in mild conditions (260o C) is mainly contributed by cellulose. Beyond 250o C, as the cellulose started to decompose rapidly during torrefaction, the extraction yield decreased accordingly. Hemicellulose acts as the link between cellulose fibers, and its decomposition at lower temperatures leads to a more fragile structure of the biomass 20. Thus, the decreasing yield was most likely attributed to cellulose degradation during torrefaction prior to extraction. Moreover, among the three individual biomass constituents, cellulose had the highest extracted fraction yield at 19% (wt, db). This is a similar figure to the highest EB yield among the samples, which was the extraction of T250 at 19.3% (wt, db). This similarity suggests that the sample torrefied at 250o C had already lost some of its hemicellulose content and that most of the yield was contributed by cellulose. Extraction of T330 had a significantly lower yield of EB (12 wt%, db) and much higher yield of RB (51.9 wt%, db) compared to the other samples. This may have been caused by the severe cellulose degradation and initiated pyrolysis during torrefaction above 300oC 21, 22.Torrefaction at this temperature generates more unextractable fraction (char) and explains the significantly lower and higher yield of EB and RB, respectively 16. 3.4.

Characteristics of the EB and RB. As seen in Table 5, at the same extraction temperatures, the

volatile matter content of both the EB and RB generally decreased as the torrefaction temperature increased. 12 ACS Paragon Plus Environment

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This agrees with their respective parent biomass, which had a lower volatile matter content as the torrefaction temperatures increased (Table 2). The carbon and oxygen content for all EB and RB samples were higher and lower than the raw biomass, respectively. Torrefaction temperature seemed to have a slight effect on the carbon and oxygen content, showing an overall increase and decrease, respectively, for both EB and RB. The carbon contents of the EB and RB increased to 79.85% (wt, daf) and 52.19%, respectively, compared to 40.87% of the raw biomass. Oxygen content, on the other hand, plunged to as low as 14.09% (wt, daf) for the EB and 43.97% for the RB compared to 53.09% of the raw biomass. Figure 4 also shows the carbon and oxygen distributions in the products. Carbon content was distributed quite uniformly in the EB, RB, and gas/liquid products for all samples, except the one that torrefied at 330o C, measuring around one-third for each fraction. In contrast, the oxygen content was mostly concentrated in the gas/liquid products, leaving less than 6% of total oxygen content in the EB. This was expected, since most oxygen should have been removed as CO2 and H2O during the extraction process. Figure 5 shows the TG and DTG curves for the EB and RB samples. It can be seen that the weight loss peak was shifted to a higher temperature as the torrefaction temperature increased for both the EB and RB. This clearly indicates a change in composition of each torrefied biomass sample, particularly pointing out the decreased volatile matter content of the EB and RB as the torrefaction temperature increased. These results are supported by the proximate analysis provided in Table 5. The TG and DTG curves of both the extracted and residue cellulose, hemicelluloses, and lignin are also presented in Figure 5. It can be seen that the peak positions of ET107, ET200, ET250, and ET270 are close to the extracted cellulose peak. This suggests that for samples treated at 107o C, 200o C, 250o C, and 270o C before the extraction, cellulose was the main contributor to the formation of EB, generating low-molecular-weight compounds during extraction. The ET300 and ET330 samples, on the other hand, had already lost most of their 13 ACS Paragon Plus Environment

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cellulose content, and therefore the peaks were shifted to higher temperatures. This may have resulted from carbonization during torrefaction, generating high-molecular-weight compounds. These claims are also supported by the proximate analysis provided in Table 5f. On the other hand, the TG and DTG curves of the RB samples were quite similar with each other, with no resemblance to that of the individual cellulose, hemicelluloses, and lignin curves. This may be because the RB samples most likely consisted of the polymerized product from the polycondensation reaction. Moreover, the calorific values for both the EB and RB were higher than those of the raw and torrefied biomass. Therefore, both EB and RB could be utilized as potential new forms of fuel. As for the chemical groups, the FTIR spectra (Figure S4) portrayed a high similarity among the EB and RB samples, which indicates that the torrefaction temperature does not affect the chemical structures of the EB and RB.

3.5.

Overall yield of the torrefaction extraction process. The overall mass and energy yield for the

torrefaction extraction process are presented in Figure 6. While the effect of torrefaction prior to extraction was not as good as expected, the extraction of T250 surprisingly gave the highest mass and energy yield of 18.92% (wt, db) and 36.83% (db), respectively. This was higher than the extraction of the non-torrefied T107 sample. However, if the RB is taken into account, the overall mass yield for the solid products was consistent at around 50% for every sample. As for the overall total energy yield, it varied from 67% to 74% for the total of the EB and RB. While torrefaction seems to have no apparent positive effect on yield, its role is quite important. First of all, torrefaction can improve the grindability of biomass, which makes it easier to obtain the smaller particles needed to achieve higher extraction yields. Furthermore, torrefaction reduces the working pressure significantly. The T200 extraction took place under 2.5 MPa, while the T330 extraction took place under 1.5 MPa. Finally, besides its improved grindability, the higher energy density and hydrophobicity of the torrefied biomass as the intermediate product of this two-step process could solve transportation and storage problems related to biomass 14 ACS Paragon Plus Environment

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Energy & Fuels

utilization. Therefore, with optimal operation conditions, pairing torrefaction with solvent extraction could be advantageous in high-scale biomass processing facilities.

4.

Conclusions

Torrefaction and solvent extraction were used to generate products with desirable properties. The two solid products, extracted biomass (EB) and residue biomass (RB) were obtained. The yield for EB was 12-19% (wt., db), with torrefaction seems to negatively affect the extraction yield. The highest yield was obtained by sample torrefied at 250o C at around 19% (wt., db) which is similar to the yield of cellulose extraction. EB had an ash content of less than 1% (wt, db), had twice the heat value, lower oxygen and higher carbon content compared to raw biomass. Therefore, EB has the potential to be used as fuel and functional materials. In addition, RB, aside from its ash content, has all the positive traits of EB. Thus, the possibility of its utilization remains open. Finally, while the effect of torrefaction on extraction yield has been less than favorable, it could be advantageous to employ it in high-scale operations in term of storage since torrefied biomass has higher density and hydrophobicity.

Supporting Information Effect of torrefaction on surface morphology of rice husk waste (Figure S1) and FTIR spectra of: torrefied biomass, cellulose, hemicellulose, lignin (Figure S2); extract and residue from dry biomass (ET107 and RT107) at different extraction temperatures (Figure S3); and extract and residue from torrefied biomass at 300oC (Figure S4).

Acknowledgments 15 ACS Paragon Plus Environment

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This work was conducted under the framework of Research and Development Program of the Korea Institute of Energy Research (KIER) (B7-2436).

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Figure Captions Figure 1. TG and DTG curves of cellulose, hemicellulose and lignin (left) and torrefied biomass (right). 17 ACS Paragon Plus Environment

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Figure 2. Carbon and oxygen distribution of the extraction products. Thirty grams of biomass sample was extracted at three different extraction temperatures (250, 300 and 350oC) for one hour. Figure 3. TG and DTG curves of extracted biomass (left) and residue biomass (right) at different extraction temperatures (250, 300 and 350oC). Figure 4. Extraction yield and distribution of biomass samples, cellulose, hemicellulose and lignin extraction (top) and carbon and oxygen distribution of extraction products (bottom). Thirty grams of dry biomass sample (T107) and samples of biomass torrefied at five different temperatures (T200, T250, T270, T300, T330) were extracted at 300oC for one hour. Cellulose, hemicellulose and lignin samples were also extracted at the same condition as reference. Figure 5. TG and DTG curves of extracted biomass (left) and residue biomass (right), cellulose, hemicellulose and lignin. Ten milligrams of each sample were loaded into TA SDT Q-600 TG analyzer and then heated to 900o C with a heat rate of 10o C per minute under a flow of 100 mL/min of N2. Figure 6. Overall mass yield and overall energy yield of the rice husk torrefaction-extraction. Thirty grams of dry biomass sample (T107) and samples of biomass torrefied at five different temperatures (T200, T250, T270, T300, T330) were extracted at 300oC for one hour.

List of Tables Table 1 Properties of raw rice husk. Proximate analysis (wt%, ar)a

Ultimate Analysis (wt%, daf)b


Chemical Composition (wt%, ar)

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Table 2 Mass and energy yields, proximate

Moisture Fixed Carbon Volatile Matter Ash

9.12 61.90 17.89 11.09

Calorific Value (MJ/kg, ar) a ar = as received b daf = dry, ash-free basis c by difference

Carbon (C) Hydrogen (H) Oxygen (O)c Nitrogen (N) Sulfur (S) 16.01

40.87 5.59 53.09 0.45 0.00

Cellulose Hemicellulose Lignin Extractives and Ash

analysis and heat value of torrefied biomass.

Samplea

Mass yield (wt%, db)

Energy yield (%, db)

100.0 99.1 T107 99.6 100.4 T200 97.8 100.4 T250 93.0 96.7 T270 90.4 94.2 T300 74.4 80.4 T330 a Tx = Biomass dried/torrefied at x°C b db = dry basis

Proximate Analysis (wt%) Volatile Fixed Ash Matter Carbon (db)b (daf) (daf) 76.79 12.84 23.21 76.84 12.96 23.16 76.31 12.96 23.69 74.58 13.68 25.42 73.94 14.20 26.06 62.84 17.90 37.16

Heat Value (MJ/kg) 16.95 17.22 17.55 17.93 18.08 19.44


34 28 18 20

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Table 3 Comparison of the results of rice husk torrefaction studies. Values shown are in percentage of variations between runs at different temperatures or in relation to the raw biomass. All experiments were conducted in 30 minutes.

Studies

Proximate Analysis (wt%) Volatile Moisture (ar) Matter Ash (db) (db) -89% -1% 7% -95% -2% 7% -94% -6% 17% -43% -5% 10% 0% -1% -16% 15% -38% 46% -38% 48% 0% -1% -16% 14% -39% 42% -39% 44% -68% -7% 41% -68% -31% 81% -27% -39% 99%

Variations

This study

Chen et al. (2014a) 2

Chen et al. (2014b) 18

Pinto et al. (2017) 23

Raw to 200oC Raw to 250oC Raw to 300oC 200 to 300oC Raw to 200oC Raw to 260oC Raw to 290oC 200 to 290oC Raw to 200oC Raw to 260oC Raw to 290oC 200 to 290oC Raw to 275oC Raw to 300oC 200 to 300oC

Heat Value (MJ/kg) 8% 10% 13% 5% 10% 16% 18% 8% 1% 9% 17% 16% 10% 11% 6%

Table 4 Proximate and ultimate analysis of EB and RB from dry biomass (T107) extraction at different temperatures. Mass yield (wt%, db)


ET107-250

7.3

ET107-300

Proximate Analysis (wt%)

Ultimate Analysis (wt%, daf)

Volatile Matter (daf)

Ash (db)

Fixed Carbon (daf)

C

H

O

N

Heat Value (MJ/kg)

13.8

49.27

0.67

50.73

76.16

5.80

17.05

0.99

31.97

18.3

35.3

59.47

0.07

40.53

76.36

6.15

16.80

0.69

32.67

ET107-350

19.3

40.2

61.65

0.15

38.35

83.36

5.97

9.90

0.77

35.24

RT107-250

43.0

55.1

53.08

24.17

46.92

52.01

4.03

43.38

0.58

21.72

RT107-300

31.7

37.7

49.68

34.98

50.32

48.99

3.58

46.89

0.54

20.16

RT107-350

27.4

21.0

49.08

42.18

50.92

38.69

1.51

59.26

0.54

13.00

Sample

a d

a

ET107-x = Extracted biomass from T107 extraction at x°C RT107-x = Residue biomass from T107 extraction at x°C


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Energy & Fuels

Table 5 Proximate and ultimate analysis of extracted and residue biomass extracted at 300oC. Proximate Analysis (wt%) Volatile Fixed Ash Matter Carbon (db) (daf) (daf)

Mass yield (wt%, db)


ET107-300

18.7

36.3

59.47

0.07

ET200-300

16.1

31.3

59.36

ET250-300

19.3

36.4

ET270-300

17.8

ET300-300

Ultimate Analysis (wt%, daf) C

H

O

N

Heat Value (MJ/kg)

40.53

76.36

6.15

16.80

0.69

32.67

0.13

40.64

77.74

6.24

15.36

0.67

33.43

50.81

0.12

49.19

77.67

5.84

15.79

0.71

33.05

32.7

55.53

0.11

44.47

77.12

6.03

16.19

0.67

32.94

16.6

30.8

42.28

0.00

57.72

79.40

5.59

14.33

0.68

33.41

ET330-300

12.0

20.5

38.95

0.45

61.05

79.85

5.35

14.09

0.71

33.12

RT107-300

32.1

38.3

49.68

34.98

50.32

48.99

3.58

46.89

0.54

20.16

RT200-300

31.6

36.5

46.25

35.26

53.75

49.07

3.40

47.07

0.46

19.83

RT250-300

31.4

36.6

47.50

34.21

52.50

49.88

3.58

46.08

0.46

20.44

RT270-300

35.1

40.8

46.70

32.18

53.30

50.70

3.57

45.25

0.47

20.83

RT300-300

34.8

39.1

43.69

34.59

56.31

49.55

3.39

46.61

0.46

20.26

RT330-300

51.9

58.5

41.40

30.91

58.60

52.19

3.32

43.97

0.52

21.89

Sample

a d

a

ETx-300 = Extracted biomass from Tx extraction at 300°C RTx-300 = Residue biomass from Tx extraction at 300°C


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List of Figures

Figure 1. TG and DTG curves of cellulose, hemicellulose and lignin (left) and torrefied biomass (right).


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Figure 2. Carbon and oxygen distribution of the extraction products. Thirty grams of biomass sample was extracted at three different extraction temperatures (250, 300 and 350oC) for one hour.


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Figure 3. TG and DTG curves of extracted biomass (left) and residue biomass (right) at different extraction temperatures (250, 300 and 350oC).


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Figure 4. Extraction yield and distribution of

biomass samples, cellulose, hemicellulose and lignin

extraction (top) and carbon and oxygen distribution of extraction products (bottom). Thirty grams of dry biomass sample (T107) and samples of biomass torrefied at five different temperatures (T200, T250, T270, T300, T330) were extracted at 300oC for one hour. Cellulose, hemicellulose and lignin samples were also extracted at the same condition as reference.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 5. TG and DTG curves of extracted biomass (left) and residue biomass (right), cellulose, hemicellulose and lignin. Ten milligrams of each sample were loaded into TA SDT Q-600 TG analyzer and then heated to 900o C with a heat rate of 10o C per minute under a flow of 100 mL/min of N2.


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Figure 6. Overall mass yield and overall energy yield of the rice husk torrefaction-extraction. Thirty grams of dry biomass sample (T107) and samples of biomass torrefied at five different temperatures (T200, T250, T270, T300, T330) were extracted at 300oC for one hour.


Torrefaction and Degradative Solvent Extraction As Means of

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