Upgrading and multistage separation of rice straw by...

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY

Volume 43, Issue 4, April 2015 Online English edition of the Chinese language journal

Received: 05-Dec-2014. Revised: 25-Jan-2015. Foundation item: Supported by the Study on National Natural Science Foundation of China (21306059), Xinjiang Uygur Autonomous Region International Science and Technology Cooperation Projects (20136007) and the Independent Innovation Fund from Huazhong University of Science and Technology (2013TS077). *Corresponding author: Tel: 027-87542417, E-mail: [email protected]. Copyright 2015, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

RESEARCH PAPERCite this article as: J Fuel Chem Technol, 2015, 43(4), 422428

Upgrading and multistage separation of rice straw by degradative solvent extraction ZHU Xian-qing1, ZHANG Zong1, ZHOU Qi-xiong2, CAI Ting1, QIAO En2, LI Xian1,*, YAO Hong1 1State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China. 2School of Chemistry and Chemical Engineering, Xinjiang University, Urumchi 830000, China

Abstract: Upgrading and multi-stage separation (UMSS) of rice straw was conducted at different temperatures using

1-methylnaphthalene (1-MN) as solvent. Three main solid products were obtained: low molecular weight extract (soluble), high

molecular weight extract (deposit) and extraction residue (residue). The elemental composition, chemical structure and

physicochemical characteristic of each component were analyzed in detail. Alkali and alkaline earth metal (AAEM) contents of solid

products and rice straw were also measured by ICP-MS. The results showed the yield of soluble increased with temperature, and the

carbon-based yield of soluble reached 33.48% at 350°C. The carbon content and oxygen content of three solid products (soluble,

deposit, and residue) increased and decreased with temperature, respectively. The carbon content of soluble and deposit reached up to

82.36% and 80.59% respectively. Meanwhile the oxygen contents of them were as low as 9.50% and 12.03% respectively. More than

86.99% oxygen of rice straw was removed as H2O and CO2. Soluble was almost free from ash, and the ash content of deposit was also

less than 1.50%. The higher heating values (HHV) of three solid products were significantly higher than that of rice straw. The FT-IR

results indicated that not only dehydration reaction and decarboxylation reaction occurred, but also including obvious aromatization

reaction. The contents of Na, Mg and K of soluble and deposit were extremely low, and they decreased with temperature gradually. In

conclusion, the degradative solvent extraction method realized dehydration, deoxygenation and multistage separation of biomass under

mild condition, and obtained a variety of products of low ash and oxygen content as well as high carbon content and HHV.

Keyword: degradative solvent extraction; biomass; multistage separation; upgrading; alkali and alkaline earth metals

The fast consumption and overuse of fossil energy have resulted in serious pollution problem (such as acid rain, greenhouse gases emission and atmospheric haze)[1]. Researchers are actively seeking new energy substitute for fossil fuel. Biomass, which is characteristics of low sulfur, low nitrogen content and zero net emissions of carbon dioxide[2–5], has shown great potential for replacing fossil fuel. Furthermore, biomass is also one of the cheapest and most important renewable energy resources[6,7], accounting for 14%–15% of the world’s energy consumption[8]. Therefore, the exploitation and utilization of biomass resources can effectively mitigate the great pressure caused by fossil energy shortage and environmental pollution. However, the high moisture content, low oxygen content and low carbon content characteristic of biomass give rise to its low heating value, easy to spontaneous combustion, inconvenient for storage and transportation, and great difficulty in being directly applied

totraditional processing technologies. In consequence, it is very necessary to deoxygenate and upgrade biomass prior to use them as fuels. Thermochemical conversion is the major biomass conversion technologies, mainly including pyrolysis, gasification and direct liquefaction[9]. But in order to improve the yield and quality of target products, most of these technologies require rather severe conditions, such as high temperature and high pressure, requesting catalysts or hydrogen. But even so, the quality of products still can not fully meet the requirements[10]. Also, the wide diversity of physicochemical properties of different types of biomass also result in the great difference in derived products[11]. Therefore, it is of great significance for biomass utilization if biomass upgrading can be realized under mild conditions. The authors have proposed a degradative solvent extraction method to achieve the upgrading and multi-stage separation (UMSS) low rank carbonaceous resources[10,12–15] (such as low-rank coal

ZHU Xian-qing et al / Journal of Fuel Chemistry and Technology, 2015, 43(4): 422428

and biomass) on the basis of previous work. This method treats low rank carbonaceous materials in a specially designed batch autoclave using non-polar solvents below 350°C, and separates them into three solid products: solvent-insoluble residue (termed as residue), the extract insoluble in solvent at room temperature (deposit), the extract soluble in solvent at room temperature (soluble) at room temperature, little gaseous product (gas, mainly CO2), and liquid products (liquid, mainly H2O). The results showed as follows: the physicochemical properties of solubles and deposits produced from different biomass materials were very similar to each other[10,13]. The higher heating value and carbon content of solubles and deposits were rather higher than those of raw feedstocks, which could be utilized as fuel of high calorific value and precursor for preparation of functional carbon material[16]. The low temperature oxidation reactivities of soluble and deposit were remarkably lower than those of raw feedstocks, which could effectively inhibit the spontaneous combustion of low rank carbonaceous resources[14], residue also shows higher combustion and gasification activity[15], meaning that it could be used as fuels in traditional boilers and gasifiers, or preparation of activated carbon[17]. Despite of the high feasibility of this method proven in our previous works, the in-depth mechanism of this method still remains unknown. Moreover, biomass contains significant amounts of alkali metal and alkaline earth metals (AAEM), which is very likely to retain in solid products, and then seriously affecting the quality and the subsequent utilization of the solid products[2,18,19]. The migration and distribution of alkali and alkaline earth metals in solid products has also not been investigated in preliminary work.

So, based on our previous work[16], in this study the biomass UMSS process were conducted at different temperatures (250, 300, 350°C). The yields and properties of the products obtained at different temperatures were investigated in detail. The mechanism of thermal solvent upgrading of biomass was discussed. Also, the migration and distribution of alkali and alkaline earth metals in solid products during the thermal solvent upgrading process was investigated. 1 Experimental 1.1 Biomass sample

A typical Chinese agricultural waste (rice straw) was

chosen in the present study. The elemental analysis and proximate analysis of the rice straw were shown in Table 1. 1-methylnaphthalene (1-MN) which can be easily separated form products was used as the upgrading solvent.

1.2 Experimental procedure

Fig. 1 Schematic diagram of degradative solvent extraction system

Fig. 2 Schematic diagram of upgrading and multistage separation of

rice straw

1.2.1 Upgrading and multi-stage separation of biomass

The upgrading of rice straw was conducted in a batch

autoclave (500 mL) as shown in Figure 1[15]. The whole experimental apparatus consists of two parts. The above part is reactor with magnetic stirring devices, and the below part is reservoir. A stainless steel filter was equipped at the bottom of the reactor. On each trial, about 20 g rice straw and 300 mL 1-MN were placed in the autoclave. After the autoclave was fully purged with nitrogen, it was heated to required temperature (250, 300 and 350°C) at an average heating rate of 5 K/min and held for 90 min. The final pressure of each experiment was in a narrow range of 2.0–3.0 MPa. The solvent and liquid products in the reactor flowed into the reservoir quickly by opening the valve at the process temperature after 90 min, achieving the in-situ separation from extraction residue. The reservoir was cooled with circulating cooling water at room temperature. A part of the extract precipitated as solid from solvent, which was the deposit. Another part of extract still dissolved in the solvent. The deposit and the liquid substance (soluble and solvent) were separated by filtration under reduced pressure. Soluble was recovered as a solid by reduced pressure distillation. The gaseous product (gas) was collected in a gas bag and quantitatively analyzed by gas chromatography.


Fig. 3 Product yield distribution upgrading and multistage

separation of rice straw

The three solid products obtained were dried under vacuum

at 150°C for 5 h to remove residual solvent. The yield of the liquid products (mainly H2O) was calculated by difference. The detailed schematic diagram of the thermal solvent upgrading process is shown in Figure 2.

1.2.2 Digestion and elemental concentration determination

The raw rice straw and upgraded products were digested

through microwave digestion system (ETHOS E, Italian Milestone Corporation) and mixed reagents of HNO3 (65%) and H2O2 (30%). The digestion procedure was as follows: 50 mg of sample was digested in a mixture reagents of 10 mL HNO3 and 2 mL H2O2 in a Teflon tube. The tube was then heated up to 180°C and held at the temperature for 30 min. The concentration of alkali and alkaline earth metal was measured by inductively coupled plasma mass spectrometry (PerkinElmer, ELAN DRC-e).

1.2.3 Characterization of products

The elemental analysis and proximate analysis of rice straw and solid products were performed by an industrial analyzer (Las Navas, TGA2000) and an elemental analyzer (Vario, CHN EL-2). The pyrolysis properties were analyzed by a thermogravimetric analyzer (NETZSCH, STA 449 F3). On each run the sample was heated up to 900°C at the heating rate of 10 K/min under nitrogen. The FT-IR spectra were recorded by a Fourier transform infrared spectrometer (Bruker, VERTEX 70) with a resolution of 4 cm–1 and scan range of 4000–400 cm–1. 2 Results and discussion 2.1 Yield distribution of upgraded products

Figure 3 shows the yield distribution of the products derived

from different temperatures. The yield of residue decreased significantly from 49.09% to 20.24%. While the yield of soluble increased from 11.82% to 19.92%. Biomass is mainly composed of cellulose, hemicellulose and lignin. Their major pyrolysis temperature range are 315–400, 220–315 and 160–900°C, respectively[20]. Soluble and deposit obtained at 250°C might mainly come from the decomposition of hemicellulose owing to its low thermal stability. Cellulose decomposed intensively and generated a large number of small molecular weight compounds, thus increasing the yield of soluble. The yield of gaseous product rarely changed, which mainly consisted of CO2 formed by the deoxygenation of rice straw. Comparing the results obtained at 300 and 350°C, the yield of soluble increased 5.33% and the yield of deposit decreased 2.46% with temperature increasing. Meanwhile, the yield of residue decreased relatively slightly. The sum yields of gaseous and liquid products almost keep unchanged. This indicates that the deposit decomposed secondarily to form relatively small molecular weight compounds (namely soluble).

Table 1 Ultimate and proximate analyses of rice straw and products

Temperature t/°C Sample Ultimate analysis wdaf /% Proximate analysis wd/%

C H N O* V FC A

RS 49.01 5.54 0.81 44.64 75.39 14.63 9.98

250 soluble 75.19 7.10 1.39 16.32 80.83 18.87 0.30

deposit 61.14 5.67 1.49 31.70 81.05 18.80 0.14

residue 53.60 5.37 1.00 40.03 68.26 13.21 18.53

300 soluble 77.58 6.75 1.51 14.16 81.10 18.84 0.06

deposit 71.64 5.85 1.60 20.91 61.57 37.66 0.76

residue 65.15 4.49 1.21 29.15 45.47 21.62 32.91

350 soluble 82.36 6.86 1.28 9.50 78.31 21.24 0.45

deposit 80.59 5.56 1.82 12.03 53.65 44.84 1.50

residue 77.53 4.63 1.29 16.55 40.91 24.24 34.84

*: by difference


Fig. 4 Carbon distribution of products from upgrading and

multistage separation of rice straw

Fig. 5 Oxygen distribution of products from upgrading and

multistage separation of rice straw

The authors’ previous research showed that the soluble has

higher utility value[10,16]. So 350°C should be more appropriate for rice straw UMSS treatment. 2.2 Ultimate and proximate analyses

The ultimate and proximate analyses of rice straw and solid

products are shown in Table 1. The carbon content and oxygen content of the three solid products were apparently higher and lower than rice straw, respectively. Additionally, they increased and decreased with temperature, respectively. The carbon contents of soluble and deposit reached up to 82.36% and 80.59%, respectively. In addition, the oxygen contents of them were as low as 9.50% and 12.03% at 350°C, respectively. The elemental compositions of soluble and deposit are very similar to those of bituminous coal. Furthermore, the soluble was almost free from ash, and the ash content of deposit was also less than 1.5%. The properties of the soluble and deposit demonstrate that they could be utilized as high calorific value fuel or precursor for preparation of high added value functional carbon material. Also, the residue can be used as better fuel than the raw rice

straw because of its higher carbon content and lower oxygen content than the raw rice straw, especially for the residue obtained at 350°C. 2.3 Carbon and oxygen distribution in products

The high oxygen content and low carbon content of

biomass limit its utilization as fuel. Therefore, the primary goal of biomass upgrading is to increase the carbon content and reduce the oxygen content of target product. Figure 4 shows the carbon distribution in products, namely the carbon-based yield of products. The amount of carbon recovered to soluble increased clearly from 18.1% to 33.5% with temperature increasing from 250 to 350°C. Elevated temperature obviously enhanced the transfer of carbon to soluble, which was due to the increasement of the yield and the carbon content of soluble as shown in Figure 3 and Table 1. The amount of carbon recovered to deposit almost unchanged. It is because the carbon content of deposit increased with temperature, but the yield of deposit decreased with temperature. The amount of carbon recovered to residue reduced remarkably by 50% from 250 to 300°C, mainly because of the yield reduction of residue. The carbon in gaseous products mainly existed in the form of CO2. Figure 5 shows the oxygen distribution to products. More oxygen was transferred into liquid and gaseous products with increasing temperature. More than 86% of oxygen was removed into gaseous and liquid products at 350°C. On the contrary, the oxygen distributions in deposit and soluble were as low as 1.83% and 4.24% at 350°C, respectively. Elevated temperature clearly enhanced the removal of oxygen during the rice straw UMSS treatment. The molar ratio of H and O in liquid product which was calculated by elemental balance was approximately 2:1, indicating that its main component was water. Therefore, the UMSS treatment is rather effective for the carbon recovery and oxygen removal for the biomass. 2.4 Heating value and heat balance estimation

Fig. 6 High heating value of three solid products and raw rice straw


Fig. 7 High heating value balance during upgrading and multistage

separation of rice straw

The high heating value (HHV) of rice straw and the three

solid products prepared at different temperatures were measured through Dulong equation[21]:

QHHV [MJ/kg, daf] = (338.1wC + 1441.8wH – 180.2wO)/1000 (1)

Where wH, wC, wO refers to the mass fraction of hydrogen, carbon and oxygen, respectively. The high heating value of three solid products and rice straw is shown in Figure 6. The HHV of rice straw was only 16.5 MJ/kg, significantly lower than the three solid products. The HHVs of soluble and deposit were greater than 32.7 and 23.1 MJ/kg. The HHVs of deposit and soluble reached as high as 33.1 and 36.0 MJ/kg respectively at 350°C, which was close to those of bituminous coal. To sum up, this method could upgrade and convert the low rank biomass feedstock with low calorific value into high quality fuel products which are free from water and ash, and similar to bituminous coal. Figure 7 shows the heat balance calculated by the yield distribution of solid products and their corresponding HHVs. No obvious heat loss was observed during the process. As much as 91.0% of heat was transferred into the three solid products at 350°C. This further illustrates the superiority of the biomass UMSS treatment method. 2.5 Pyrolysis characteristic of rice straw and solid products

The thermal decomposition characteristics of the solid

products are of prime importance for in-depth understanding of the influence of temperature on the biomass UMSS treatment and the subsequent utilization of the products.

The TG curves of raw rice straw and the three solid products are shown in Figure 8. The soluble obtained at 250°C was abbreviated as 250-soluble, and by this analogy. The TG curves of solubles prepared at different temperatures were exceedingly similar to each other. The thermal decomposition characteristic of rice straw differed apparently from those of solubles. The pyrolysis of soluble started at low temperature

point, because soluble mainly consists of low molecular weight compounds which are likely to decompose. But the weight loss rates of solubles in 300–400°C were significantly lower than that of rice straw, which was mainly due to the intense decomposition of cellulose in rice straw. The main pyrolysis temperature of deposit was in the range of 200–400°C, also lower than that of rice straw. And the weight loss rate of deposit decreased with temperature, indicating that secondary decomposition or condensation reaction occurred to deposit to form soluble at 250–350°C. Unlike soluble and deposit, the main pyrolysis temperature of residue shifted to higher temperature region, mainly because low molecular weight substances were extracted into soluble and deposit. So the residue was principally composed of ash and large molecular weight substances which are difficult to decompose. The temperature corresponding to the maximum weight loss rate of 250-residue was similar to that of rice straw. But the weight decreasing profiles of 300-residue and 350-residue were notably different from those of rice straw. No apparent weight loss peak below 400°C was observed for 300-residue and 350-residue. 350-residue exhibited the minimum volatile content being consistent with the results of proximate analysis.

2.6 FT-IR analysis of rice straw and solid products

The FT-IR spectra of rice straw and the products were

recorded as shown in Figure 9. The spectrum of rice straw showed broad stretching vibration attributed to O−H stretching vibration (3100–3600 cm–1) and C−O stretching vibration (1000–1100 cm–1) arising from the major component of biomass (cellulose). The spectra of solubles were distinctly different from those of deposits. In comparison to rice straw, the major change in the spectra of soluble was as follows: weakening of the O−H and C−O stretching vibration. appearance of distinct and sharp aliphatic groups: C−H, C−H2, C−H3 stretching vibration (2850–2960 cm–1), aromatic ring stretching vibration (1460–1520 cm–1) and aromatic out-of plane stretching vibration (770 cm–1). The spectra of soluble also slightly varied with temperature: gradual weakening of O−H and C−O stretching vibration, and enhancement of the relative intensity of aromatic C=C stretching vibration (1600 cm–1) to C=O (1720 cm–1). On the other hand, The spectra of deposit obviously changed with rising temperature: distinct weakening of O−H and C−O stretching vibration, and appearance of distinct aliphatic C−H, C−H2, C−H3 stretching vibration, aromatic C=C stretching vibration, aromatic ring and aromatic out-of plane stretching vibration. The larger changes in the spectra of deposit with temperature compared to those of soluble indicate that solubles are rather stable product, while more extra secondary pyrolysis and condensation reaction occurred to deposits during the UMSS treatment.


Fig. 8 TG curves of raw rice straw and three solid products

Fig. 9 FT-IR spectra of raw rice straw and solid products

Table 2 AAEM content of raw rice straw and solid products

Sample Concentration w/%

Na Mg K Ca

RAW 1.797 0.093 0.296 0.171

250-soluble 1.081 0.019 0.064 0.809

250-deposit 0.754 0.020 0.115 0.230

300-soluble 1.213 0.010 0.064 0.429

300-deposit 1.287 0.018 0.094 0.388

350-soluble 0.108 0.000 0.023 0.000

350-deposit 0.000 0.000 0.097 0.000

This corresponds to the results of TG curves and proximate

analysis. Both soluble and deposit displayed remarkable aliphatic C−H, C−H2, C−H3 stretching vibration, aromatic ring, aromatic out-of plane stretching vibration and weak C=O stretching vibration. These results clearly demonstrate that significant aromatization reaction occurred as well as the decarboxylation and dehydration reaction during the process. 2.7 AAEM contents of rice straw and solid products

Biomass contains considerable content of alkali and

alkaline earth metal (AAEM), which can cause serious problems during the thermal conversion and utilization process of biomass, such as fouling, slagging and corrosion in combustion boiler or gasifier. Therefore, the AAEM contents of soluble and deposit were estimated and the results are shown in Table 2. It can be observed that the content of Na, Mg and K in soluble and deposit were apparently lower than raw rice straw except Ca. Furthermore, the contents of Na, Mg

and K in soluble and deposit decreased with increasing temperature, which might be owing to cleavage of the organic associated structure between metal and organic matrix (such as –COOM) and then these metal turned into solvent insoluble substance. The Ca content of soluble and deposit were higher than those of soluble and deposit obtained at 250 and 300°C. This might because the Ca in rice straw may be mainly comprised of organometallic Ca which cross-linked with biomass organic matrix, and the stability of the bond between divalent metal and organic matrix makes it hard to crack[18] at 300°C or lower temperature. All AAEM contents at 350°C were less than 0.108%, which were markedly lower than that of rice straw. Although the migration and transformation of AAEM during the biomass UMSS treatment still needs further research, the results above could enough declare that this method could efficiently remove the AAEM in biomass. 3 Conclusions


In this study rice straw was upgraded and separated into three main solid products by degradative solvent extraction method. The yield of high value-added product (soluble) increased with temperature. The carbon-based yield (carbon distribution) of soluble reached 33.48% at 350°C. The carbon contents and oxygen contents of three solid products (soluble, deposit, residue) increased and decreased with temperature, respectively. The carbon contents and oxygen contents of soluble and deposit at all treatment temperature were apparently higher and lower than rice straw, respectively. The carbon contents of soluble and deposit respectively reached up to 82.36% and 80.59%. The oxygen contents of them were respectively as low as 9.50% and 12.03%. More than 86.99% oxygen in rice straw was removed as H2O and CO2. The soluble was almost free from ash, and the ash content of deposit was also less than 1.5%. The higher heating values (HHV) of the three solid products were significantly higher than rice straw. No obvious heat loss was observed during the treatment process. Significant aromatization reaction occurred except the decarboxylation and dehydration reaction during the treatment process. The contents of Na, Mg and K of soluble and deposit were extremely lower than that of rice straw, indicating the UMSS method could efficiently remove the AAEM in biomass. Therefore, the UMSS method achieves effective deoxygenation and upgrading of biomass under mild conditions, obtaining two solid extracts with rather low ash and oxygen content, high carbon content and high HHV. The promising application prospect of the UMSS method is shown.

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