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The use of torrefaction and solvent extraction to produce ashless biomass as solid fuel feedstock for co-firing Sukma Hidayat, Ryan Fitrian Sofwan Fauzan, Seongha Jeong, Dong Hyuk Chun, Jiho Yoo, Sangdo Kim, Jeonghwan Lim, Youngjoon Rhim, Sihyun Lee, and Ho Kyung Choi Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 07 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017
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The use of torrefaction and solvent extraction to produce ash-less biomass as solid fuel feedstock for co-firing Sukma Hidayat, Ryan Fitrian Sofwan Fauzan, Seongha Jeong, 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, South Korea
ABSTRACT Due to the characteristics of biomass, biomass exploitation as a secondary fuel feedstock in co-firing is only technically and economically acceptable in the range of 5 to 10% w/w. In this study, torrefaction and solvent extraction as biomass pretreatments were conducted to address feedstock limitations, making a larger share of biomass for co-firing feasible. The physical and chemical characteristics of the solvent-extracted biomass were investigated using proximate analysis, ultimate analysis, thermogravimetric analysis and FTIR spectroscopy. Biomass torrefied at 200, 250, 270, 300, and 330°C were extracted by non-polar organic solvent 1methylnaphthalene (1-MN) at 350°C for 1 hour to produce ash-less biomass (extracted biomass). A dry, raw, woody biomass was also extracted under the same conditions to see the effect of torrefaction on the characteristics of extracted biomass and residue biomass. The result shows that solvent extraction was effective in producing ash-less biomass with less than 0.1% (wt., db) ash content remaining in the extracted fraction. It was also found that torrefaction temperature had an effect on the extracted biomass yield and slightly influenced the physical and chemical properties of the extracted biomass. The unique properties of the extracted biomass make it possible to utilize it for other purposes, namely as high value functional materials. Keywords: ash, ash-less biomass, co-firing, solvent extraction, torrefaction *
Corresponding author:
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1. Introduction Currently, there are more than 7000 coal-fired power plants supplying 41% of the world’s electrical supply.1, 2 The idea of substituting coal with biomass in power plants is considered as one of the most profound fast-tracks to overcome global warming as it is a near-term, low-risk, and sustainable option. It is estimated that 10% biomass co-firing in coal power plants could reduce CO2 emissions up to 450 million ton/year by 20353.
More than 150 coal-fired power plants have been reported as having some co-firing activities in many countries, mostly in the US and Europe.3, 4 Most of these installations use direct co-firing, because it is the simplest and cheapest option. Among examples are 635 MWe EPON Project of Gelderland Power Station in Holland which uses direct co-firing with waste wood and the 150 MWe Studstrup Power Plant, Unit 1, near Aarhus, Denmark that is co-firing coal with straw. 5
Despite its tremendous availability, biomass exploitation as a secondary fuel in co-firing is only technically and economically acceptable in the range of 5 to 10% w/w for a continuous operation.3, 6 High moisture content, low bulk density, and low energy density of biomass lead not only to storage and transportation problems, but also decreases in the energy output of electric power generation. Moreover, even though biomass generally contains less mineral content compared to coal, it typically has a low ash-fusion temperature, especially because of larger sodium (Na), potassium (K), and phosphorous (P) contents that responsible for slagging and corrosion problems.3, 7–10 In the US, it is reported that corrosion contributes 10% of the cost of electricity and is said to be the main factor of economic loss in power plant. Inoperable power plant due to failure by corrosion more than 1,250 days in 1992 caused an economic loss of $250,000 per day.11 While it is true that co-firing ability depends on the biomass and combustor type, the corrosion/agglomeration caused by the ash content of biomass remains a problem hindering its efficiency which leads to economic loss.
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Many efforts have been made to overcome these shortcomings. To increase the energy density of biomass, torrefaction has been successfully applied to various biomass feedstocks.12 Torrefaction is a thermolysis process that subjects the feedstock to thermal treatment at low temperatures of 200-300°C in the absence of oxygen.13 Besides improving the energy density of biomass, torrefaction is also able to decrease moisture content and improve hydrophobicity, grindability, and reactivity of biomass.12 These changes can have significant advantages such as preventing biomass from biological decomposition and reducing the energy required for size reduction. However, since torrefaction only induces volatilization, the ash content of biomass remains in torrefied biomass in higher fractions. Consequently, combustion of torrefied biomass in power plants could cause severe problems. Therefore, a demineralization process should be applied to address this issue.
Leaching is known as one of the demineralization methods to produce upgraded coal. It uses strong acids to eliminate the minerals.14–16 The similar approach is reported to be used for biomass demineralization. Asadieraghi et al.17 carried out lignocellulosic biomass demineralization by diverse acid solutions (H2SO4, HClO4, HF, HNO3, and HCl). Different types of dried palm oil biomass were subjected to those acids at room temperature for 48 hours and then filtered and washed with distilled water. The leached biomass samples were dried in oven at 105°C for over 24 hours. The result shows that demineralization using HF produced the best result in respect to mineral content reduction. It was reported that it could reduce mineral content of empty fruit bunches from 7.0% to 0.43%. Eom et al.18 also performed a similar experiment. He tried to demineralize biomass not only by acid, but also by water. It was found that the treatment by acid and water could reduce the ash content from 0.70% to 0.11% and 0.50%, respectively. Biomass leaching was also investigated by Stefanidis et al.19 and Wigley et al.20 with similar reported results. However, despite its ability to reduce the mineral content, acid is known to be corrosive and toxic, whereas water is not effective enough to use. For these reasons, an alternative treatment for biomass demineralization is required.
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Another approach to eliminate mineral content is solvent extraction. This method has been applied to coal for Ash-Free Coal (AFC) production by extracting the ash-less fraction of coal, leaving its residue with highly concentrated ash.21–30 The result shows that the process is effective to produce AFC with negligible ash content. In contrast, studies related to biomass demineralization by solvent extraction are limited. The study of biomass extraction mostly focuses on the organosolv pulping process to isolate fiber, hemicellulose and lignin from biomass for and production of oil and chemicals.31–36 Recent studies focusing on solid product through biomass extraction was carried out by Zhu et al.37–39 and by Wannapeera et al.39 They conducted a degradative solvent extraction to produce high-grade carbonaceous material from biomass wastes using non-polar organic solvent 1methylnaphthalene (1-MN). They found that the optimum condition for biomass extraction was at 350°C for 1 hour. The mass yield of extracted biomass (deposit and soluble) and residue biomass ranged from 26.6% to 50.2%-dafb and 8.8% to 20.4%-dafb, respectively, depending on biomass type. In addition, due to unique characteristics of extracted biomass, several other works was also conducted to evaluate the possibility of using it as functional materials such as carbon fiber and coke blending material.37, 40
In this study, biomass torrefied at 200, 250, 270, 300, and 330°C was extracted by 1-methylnaphthalene (1-MN) at 350°C for 1 hour to produce ash-less biomass. A dry, raw biomass was also extracted under the same conditions to see the effect of torrefaction on extracted biomass and residue biomass. The physical and chemical characteristics of the ash-less biomass (extracted biomass) and the residue biomass were investigated using proximate analysis, ultimate analysis, thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR).
2. Experimental Section 2.1. Materials. A woody biomass sample from trunk and branches of Zelkova tree (Zelkova serrata) was used in this study. The composition of the biomass was determined by proximate and ultimate analysis (Table 1). A non-polar solvent 1-methylnaphtalene (1-MN) was selected for biomass solvent extraction due to its 4 ACS Paragon Plus Environment
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favorable recoverability and selectivity against minerals.28–33 Three main biomass constituents which are cellulose (powder fibrous form), hemicellulose (powder form), and lignin (alkali, brown powder), all purchased from Sigma-Aldrich, were also used in this study.
2.2. Drying and Torrefaction of Biomass. Prior to drying and torrefaction, the raw biomass was cut to reduce the size into small chips around 4 mm in size. The chips were then dried for 24 hours in oven at 107°C to make the T107 sample. Other samples were made by torrefying the biomass chips for 30 minutes using a rotary type reactor flooded with nitrogen at atmospheric pressure. The samples were torrefied at 200°C, 250°C, 270°C, 300°C, and 330°C to produce T200, T250, T270, T300, and T330, respectively. After torrefaction, T107, T200, T250, T270, T300, and T330 were ground and sieved to obtain the final samples with
