Two-Stage Conversion of Low-Rank Coal or Biomass into Liquid Fuel

Apr 8, 2015 - various types of low-rank coals or biomass wastes into several ... to produce high-quality liquid fuel from low-rank coals and/or biomas...
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Two-Stage Conversion of Low-Rank Coal or Biomass into Liquid Fuel under Mild Conditions Xian Li,† Dedy Eka Priyanto, Ryuichi Ashida, and Kouichi Miura*,‡ Department of Chemical Engineering, Kyoto University, Katsura, Nishigyo-ku, Kyoto 615-8510, Japan ABSTRACT: The authors have recently proposed a novel degradative solvent extraction method that upgrades and fractionates various types of low-rank coals or biomass wastes into several different molecular weight fractions at around 350 °C. The lowest molecular weight fraction that was recovered as solid (termed soluble) by the yield of 19.4−71.7 wt % on a carbon basis had unique and almost raw-material-independent properties: almost free from ash and moisture, average molecular weight of around 300, carbon content of as high as 81.5−84.8 wt %, and oxygen content of as low as 6.5−12.1 wt %. In this work, the combination of the proposed extraction method and the liquefaction of the soluble under mild conditions was investigated as a two-stage liquefaction method to produce liquid fuel. A rice straw (RS) and a brown coal (LY) were extracted by the proposed extraction method to produce solubles by the yields of 31.5 and 21.7 wt %, respectively. The solubles produced were then liquefied at 400 °C using FeOOH/sulfur as the catalyst under the H2 pressure of as low as 2.0 MPa (at room temperature). Then, 62.1 wt % of RS soluble and 56.8 wt % of LY soluble were converted to the liquid fraction (oil), excluding H2O. The oxygen contents of the oils produced from the two-stage liquefactions of RS and LY were as low as 2.2 and 3.9 wt %, respectively, much lower than those from the direct liquefaction of the raw materials (single-stage liquefaction). Furthermore, both the amount of CO2 emitted and the amount of H2 consumed during the two-stage liquefaction were much less than those of the single-stage liquefaction. Thus, it was shown that combining the degradative solvent extraction method and the liquefaction of the soluble is a promising method to produce high-quality liquid fuel from low-rank coals and/or biomass wastes.

1. INTRODUCTION With a rapidly increasing demand in liquid fuels, direct coal liquefaction technologies have received the world’s attention again since the beginning of this century. The direct liquefaction of low-grade carbonaceous resources, such as low-rank coals and biomass wastes, to produce liquid fuel has been widely studied in recent years,1−6 owing to the projected rapid depletion of high-grade coals, such as bituminous coal. However, it has been reported that the direct liquefaction of low-rank coals is more difficult than the direct liquefaction of bituminous coal.7−9 This is because low-rank coals are rich in oxygen functional groups and the cross-linking reactions among the oxygen functional groups forming large-molecular-weight compounds proceed significantly at the temperature lower than the liquefaction temperature.10 The oxygen-functional-groupderived cross-links may change to stronger carbon−carbon covalent linkages, which suppress the formation of light hydrocarbons (liquid products) during the liquefaction process.1,7 Direct liquefaction of biomass compared to the direct liquefaction of coal is far away from economical and technical feasibilities. The core problem is also the high content of oxygen functional groups in biomass, which causes a series of problems during the liquefaction process and produces oils with too high of an oxygen content.3 Furthermore, liquefying low-rank coal and biomass with a high oxygen content consumes more H2 and produces more CO2, which reduces process efficiency significantly.11,12 If low-rank coal and biomass can be deoxygenated largely and separated into several fractions having different molecular weights, it is expected that the lowmolecular-weight fraction can be hydroliquefied readily under relatively mild conditions to produce low-oxygen-content liquid products by minimizing the cross-linking reactions. © XXXX American Chemical Society

The authors have proposed a degradative solvent extraction method, which treats low-rank coals or biomass wastes in a non-hydrogen donor at around 350 °C, to dewater without phase change, remove oxygen functional groups, and separate them into several fractions having different molecular weights.13−15 The core concept of this method involves exposing the entire feedstock to thermal reactions at around 350 °C in a nonpolar solvent. The products formed during the extraction are then filtrated at the treatment temperature to obtain the extract and residue (termed residue). The extract is then separated to two fractions at room temperature: one is the fraction that precipitates as solid at room temperature (termed deposit), and the other is the fraction that is soluble in the solvent at room temperature (termed soluble). The solid soluble is finally obtained after removing the solvent by distillation. The carbon basis soluble yields were 19.4−31.2% for low-rank coals and were as high as 36.7−71.7% for biomass wastes. Solubles were almost free from water and ash. The carbon contents of solubles were as high as 81.5−84.8 wt %, and the oxygen contents were as low as 6.5−12.1 wt %. The solubles have the molecular weight around 300 and can soften and melt at less than 100 °C.14,16 Furthermore, the chemical structure and chemical and physical characteristics of the solubles obtained from both low-rank coals and various types of biomasses were almost independent of the raw materials and rather similar to each other.13,14 It was found that main reactions during this process are dehydrations and decarboxReceived: December 27, 2014 Revised: April 8, 2015

A

DOI: 10.1021/ef502574b Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Properties of Raw Materials, Solubles, HIs, and Oils ultimate analysis (wt %, daf) sample RS

LY

a

raw residue deposit soluble HI (two stage) HI (single rapid) HI (single TProg) oil (two stage) oil (single rapid) oil (single TProg) raw residue deposit soluble HI (two stage) HI (single rapid) HI (single Tprog) oil (two stage) oil (single rapid) oil (single TProg)

proximate analysis (%, db)

C

H

N

Oa

O/C

HHVb (MJ/kg)

volatile matter

fixed carbon

ash

46.2 61.7 87.5 80.1 79.4 67.4 67.8 85.9 78.0 67.8 66.7 77.4 77.5 81.8 79.6 79.9 81.0 86.4 80.5 83.6

6.7 5.0 6.2 6.4 6.4 5.2 4.7 11.2 8.5 9.4 4.7 4.0 5.0 7.5 5.2 5.1 4.9 9.5 8.5 3.7

1.6 1.7 2.9 1.4 1.7 1.6 1.2 0.7 0.6 3.3 0.9 1.0 1.0 0.5 1.0 0.9 0.8 0.2 1.5 2.3

45.5 31.6 3.4 12.1 12.5 25.9 26.3 2.2 12.8 19.5 27.7 17.6 16.9 10.2 14.2 14.0 13.2 3.9 9.5 10.4

0.74 0.38 0.03 0.11 0.12 0.29 0.29 0.02 0.12 0.20 0.31 0.17 0.16 0.09 0.13 0.13 0.12 0.03 0.09 0.09

17.1 22.4 37.9 36.0 33.8 25.6 25.0 41.9 36.3 33.0 24.3 28.8 30.4 36.6 31.9 31.9 32.1 42.2 37.8 31.7

73.0 24.1 37.0 74.8 53.8 36.3 c

11.9 22.7 61.0 24.4 45.0 27.5 c

15.1 53.2 2.1 0.8 1.2 36.2 c

51.5 31.8 39.4 83.4 58.5 42.4 c

47.0 66.0 59.9 16.3 40.8 54.7 c

1.5 2.3 0.7 0.3 0.7 2.9 c

Calculated by difference. bHigher heating value. cNot determined. 140 °C under reduced pressure. The three solid products (residue, deposit, and soluble) were then dried under vacuum at 150 °C for 5 h to remove the residual 1-MN. The gaseous products were collected in a gas bag through a pressure relief valve after the reactor was cooled to room temperature and before the reactor was opened. They were then analyzed by a gas chromatograph to be quantified. The degradative solvent extraction was also performed using a small-batch reactor (12 mL) for the water yield determination. The reactor, charged with the raw material and 1-MN at the same ratio as the extraction using the stainless-steel autoclave described above and purged sufficiently by N2, was heated by immersing it in a sand bath at a heating rate of 5 K/min up to 350 °C, where it was kept for 60 min. The reactor was shaken by a shaker to ensure good mixing of all of the substances during the extraction. After the extraction, the reactor was taken out from the sand bath and cooled to room temperature. The reactor was then opened and washed carefully by excess tetrahydrofuran (THF) to collect all of the substance into a sealed bottle. After 1 h of still standing, a small amount of the upper liquid was taken from the bottle to determinate the water content in the THF solution by the Karl Fischer titration. A light hydrocarbon (oil) yield of the extraction was calculated by difference. 2.3. Liquefaction. The second stage of the two-stage liquefaction is the liquefaction of soluble produced by the degradative solvent extraction. The liquefaction was performed at 400 °C by a small-batch reactor (12 mL) using tetralin as the solvent and iron hydroxide (FeOOH) and sulfur as the catalyst (the molar ratio of Fe/sulfur was 1:2) in the presence of gaseous hydrogen (H2). The reactor was charged with 0.5 g of soluble (dry basis), 2 g of tetralin, and catalyst. The amount of catalyst added was 1.0 wt % as Fe to soluble. The reactor was then purged several times by 0.5 MPa of H2 and finally charged with 2.0 MPa of H2. Then, the reactor was immersed in a sand bath to be heated to 400 °C in 10 min and kept for 30 min at 400 °C. This liquefaction method is the so-called rapid liquefaction17,18 that is said to be effective to increase the oil yield. The reactor was shaken by a shaker to ensure good mixing of the substances during the liquefaction. The pressure change of the reactor was carefully monitored using a high-precision pressure transducer. After the liquefaction, the reactor was taken out from the sand bath and cooled to room temperature. The gaseous products with remaining H2 were collected in a gas bag and analyzed by a gas chromatograph to be quantified. The amount of H2 consumed during the liquefaction was

ylations without primary decomposition reactions accompanying the cracking of C−C bonds for low-rank coals13 and are the reactions associated with a kind of “coalification process” for biomass wastes.14 In this work, the possibility of a two-stage liquefaction method combining the proposed degradative solvent extraction method and the liquefaction of soluble to produce high-quality liquid fuel was examined with the reference of single-stage liquefaction for a low-rank coal and a biomass waste.

2. EXPERIMENTAL SECTION 2.1. Samples and Solvents Used. Loy Yang coal (LY, brown coal from Australia) and a rice straw (RS, from Thailand) were used as raw materials. The properties of the raw materials are shown in Table 1. A non-hydrogen donor, 1-methylnaphthalene (1-MN), was used as the solvent for the degradative solvent extraction. Tetralin was used as a hydrogen donor for the liquefaction of raw materials and the solubles. 2.2. Degradative Solvent Extraction. The first stage of the twostage liquefaction is the degradative solvent extraction. A detailed procedure of the extraction was described in our previous works.13,14 It is briefly introduced here. A stainless-steel autoclave was charged with as-received raw material of around 13 g (on a dry basis) and 300 mL of 1-MN. A stainless filter was equipped at the bottom of the autoclave. After the autoclave was sufficiently purged by N2, the autoclave, sealed with 0.5 MPa of N2, was heated to 150 °C at a heating rate of 5 K/min, where the inherent water was liberated as liquid water from the sample into the solvent. The reactor was then cooled to room temperature. The solvent together with the liberated water was recovered by opening the valve between the autoclave and the reservoir. Then, after the autoclave was charged again with 300 mL of 1-MN and 0.5 MPa of N2, it was heated to 350 °C at a heating rate of 5 K/min and kept for 60 min. The extract together with the solvent were separated from the unextacted fraction (residue) at 350 °C by opening the valve under the filter. The extract and solvent were collected in the reservoir, which was at room temperature. A part of the extract in the reservoir precipitated as solid, which was called deposit in this work. The extract that was still soluble in the solvent at room temperature (soluble) with the solvent was filtrated by a membrane filter to be separated from the deposit. The solid soluble was recovered by evaporating the solvent at B

DOI: 10.1021/ef502574b Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels calculated from the H2 concentration and volume of the gaseous phase in the reactor before and after the liquefaction. The reactor was then washed carefully by excess cyclohexane to collect all of the substance. The cyclohexane-soluble fraction (abbreviated to HS) together with cyclohexane and residue was separated by filtration. The residue was then dried at 100 °C under vacuum for 3 h to recover cyclohexaneinsoluble fraction (HI) by removing the residual cyclohexane completely. The yield of HI was determined by measuring its weight. Because H2 consumed during the liquefaction is distributed to all of the products, the yields of hydrogen containing gaseous products and HI contain the part of H2 consumed. The yield of HS was intended to be estimated by difference, because HS could not be completely separated from tetralin. The yield of HS was therefore equated to 1 + the weight of H2 consumed per unit weight of soluble − the yield of gaseous products − the HI yield. Then, the total yield exceeds 1 slightly by the weight of H2 consumed per unit weight of soluble. Hydrogen transferred from tetralin to the products must be taken into account to calculate the product yields, but the quantification of tetralin was not successful. Then, we were obliged to assume that tetralin did not change during the liquefaction when calculating the yields. All of the experiments were repeated under the same conditions to determine the water yield by the same method as the water yield determination of the degradative solvent extraction stage, as described above. The oil yield of the liquefaction was calculated by subtracting the water yield from the HS yield. Direct liquefaction of the raw materials (single-stage liquefaction) was also performed by two methods for comparison purposes. One follows exactly the procedure employed for the liquefaction of soluble, and the method is referred to as single-rapid liquefaction in this work. The other employed the following heating profile: the reactor with raw material, catalyst, solvent, and H2 (2.0 MPa) was heated to 350 °C at the heating rate of 5 K/min, kept for 60 min at 350 °C, then heated rapidly to 400 °C, and kept for 30 min at that temperature. The second method, which is referred to as single-TProg (temperatureprogramed) liquefaction in this work, was employed to simulate the heating profile during the degradative solvent extraction and the rapid liquefaction of soluble. 2.4. Product Analyses. The water content in the THF solution was determined by a Karl Fischer moisture titrator (KEM, MKS510N). The anode reagent of the titrator consists of amine, 2methoxyethanol, SO2, and I2. The chemical component of the oil was identified by gas chromatography−mass spectrometry (GC−MS, Shimadzu GCMS-QP5050A). GC analysis conditions were set as follows: column, DB-5 (0.25 mm inner diameter × 30 m, 0.25 μm film thickness); carrier gas, 50 mL/min of helium; detector temperature, 280 °C; injector temperature, 280 °C; and oven temperature, programmed from an initial temperature (40 °C) to 260 °C at a heating rate of 5 K/min. MS conditions were as follows: interface temperature, 280 °C; and detector voltage, 1.5 kV. Prior to GC−MS measurement, the oil was diluted in hexane and 2 μL of solution was injected to GC−MS for the analysis. Most of the components detected were identified by referring to a library and software [NIST/EPA/NIH Mass Spectral Library (NIST 05) and NIST Mass Spectral Search Program, version 2.0d] attached to GC−MS. The proximate analyses of the raw materials, residues, deposits, solubles, and HIs were performed using a thermogravimetric analyzer (Shimadzu, TGA50). For each run, about 10 mg of sample placed in a platinum pan was heated to 900 °C in a N2 atmosphere at the heating rate of 10 °C/min. The elemental analyses were performed on a CHN corder (Yanaco, CHN MT-6M).

Figure 1. Product yields of extractions and soluble liquefactions on a sample basis.

the first-stage extractions were only 4.6 and 0% for RS and LY, respectively. The soluble yields through the first-stage extraction were 31.5 and 21.7 wt % for RS and LY, respectively. As high as 62.1 and 56.8 wt % of solubles were then converted to oils through the second-stage liquefaction for RS and LY, respectively. The sums of CO2 and H2O yields through the first stage (extraction) were as high as 42.9 and 15.8 wt % for RS and LY, respectively, indicating that a large amount of oxygen was removed from raw materials as either CO2 or H2O during the extraction stage. The chemistry involved during the degradative solvent extraction was examined in the previous works, as mentioned in the Introduction. It was found that the degradative solvent extraction of RS may be regarded as a rapid “coalification”.14 The CO2 and H2O yields through the secondstage liquefaction were very small. Next, the product yields through the two-stage liquefaction were compared to the product yields through single-rapid and single-TProg liquefactions in Figure 2. The yields are all shown

Figure 2. Product yields of two-stage, single-rapid, and single-TProg liquefactions.

on a dry and ash-free raw material basis. The product yields through the two-stage liquefaction are the sum of the product yields through the first- and second-stage liquefactions. The oil yields through the two-stage liquefactions were 23.8 and 13.0 wt % for RS and LY, respectively. They were slightly smaller than the oil yields through either single-rapid or single-TProg liquefaction for the both of the raw materials. However, it is noteworthy that the CO2 yields through the two-stage liquefactions were much smaller than the CO2 yields of either single-rapid or single-TProg liquefaction for the both of the raw

3. RESULTS AND DISCUSSION 3.1. Product Yields through Two- and Single-Stage Liquefactions. Figure 1 shows the product yields through the degradative solvent extraction (first stage) and the liquefaction of solubles (second stage). The yields are all shown by weight percent on a dry and ash-free sample basis. The total liquefaction yields exceed 100% slightly, because they include the amount of H2 consumed, as stated above. The oil yields of C

DOI: 10.1021/ef502574b Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 3. GC−MS total ion chromatograms of oils produced from two-stage and single-rapid liquefactions of RS.

3.2. Elemental Analyses of Oils and Byproducts. The elemental compositions of solubles, deposits, residues, and the liquefaction products are listed with those of the raw materials in Table 1, where the elemental compositions of the oils were estimated by difference. First, the elemental compositions of the oils produced through the two-stage liquefaction were compared to those of the oils produced through the singlerapid and single-TProg liquefactions. The carbon contents of the oils produced through the two-stage liquefaction were as high as 85.9 and 86.4% for RS and LY, respectively. They were much larger than those of the oils produced through either single-rapid or single-TProg liquefaction. The big differences come mainly from the differences in the oxygen contents. The oxygen contents of the oils produced through the two-stage liquefactions were as low as 2.2 and 3.9% for RS and LY, respectively. On the other hand, the oxygen contents of the oils produced by the single-rapid and single-TProg liquefactions were as large as 12.8 and 19.5% for RS and as large as 9.5 and

materials. On the contrary, the H2O yields through the twostage liquefaction were larger than the H2O yields of either single-rapid or single-TProg liquefaction for the both of the raw materials. It is also stressed that most CO2 and H2O were formed during the first-stage extraction for the two-stage liquefaction, as shown in Figure 1. The two-stage liquefaction produced solid byproducts, deposit and residue, during the first-stage extraction. The sum yields of deposit and residue were 16.9 and 62.2 wt % for RS and LY, respectively. The oil yields through the two-stage liquefaction were slightly smaller than that through either single-rapid or singleTProg liquefactions on a weight basis, as mentioned above. However, the quality of oil is another concern when we discuss the relative merits between the two- and single-stage liquefactions. The qualities of deposit and residue are also a concern when we use them as raw materials for the gasification or solid fuel. Then, the quality of oil and the qualities of the byproducts are examined next. D

DOI: 10.1021/ef502574b Energy Fuels XXXX, XXX, XXX−XXX

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3.3. GC−MS Analyses of Oils. The oils produced from both RS and LY were analyzed by GC−MS. Figure 3 typically compares the total ion chromatograms of the oils produced from RS through the two-stage and single-rapid liquefactions. The main components in both oils were single and double aromatic ring compounds and their derivatives as well as longchain aliphatic compounds. However, the oil produced through single-rapid liquefaction contains much more oxygen-containing compounds, such as phenolic compounds and long-chain aliphatic ester, as expected from the elemental composition data shown in Table 1. The long-chain aliphatic compounds in the oil produced through the two-stage liquefaction were mainly long-chain alkanes. Quantitative analyses of organic compositions of the oils were performed by normalization of the peak area of the GC− MS chromatogram. The relative abundances of acids (−COOH-containing compounds), esters (−COOC-containing compounds), phenols, aliphatic hydrocarbons, aromatic hydrocarbons, and others in the oils were estimated from the areas of MS spectra and shown in Figure 4. It was found that

10.4% for LY, respectively. The atomic O/C ratio of the oil produced from RS by the two-stage liquefaction is significantly lower than some published results of direct liquefaction of biomasses.3,19,20 The elemental compositions of the oils obtained from two-stage liquefactions of low-rank coal and biomass were rather similar to each other. It was attributed to the similar elemental compositions, chemical structure, and chemical and physical characteristics of the solubles obtained from the both raw materials, as mentioned above. The heating value may be a more direct index to evaluate the quality of fuel. Then, the higher heating values (HHVs) of the oils were estimated by the Dulong equation21 from the elemental compositions and listed in Table 1. The HHVs of the oils produced through the two-stage liquefactions were as large as 41.9 and 42.2 MJ/kg for RS and LY, respectively, similar to the HHV of commercial diesel oil, and were much higher than the HHVs of the oils produced through the single-stage liquefactions. These results clearly show that the two-stage liquefaction produces oil of much higher quality compared to the single-stage liquefactions, single-rapid and single-TProg liquefactions. This merit will compensate for the smaller oil yield of the two-stage liquefaction on a weight basis compared to the single-stage liquefactions. One of the significant problems of direct liquefaction of biomasses lies in too high of an oxygen content of the liquid product, as mentioned above. In this sense, the two-stage liquefaction proposed may solve the problem. The high quality of the oil produced through the two-stage liquefaction, low atomic O/C ratio, and high HHV is judged to be realized by the removal of most oxygen in the raw materials during the first stage of degradative solvent extraction, as clearly shown by small oxygen contents of solubles as low as 12.1 and 10.2% for RS and LY, respectively. Next, we focused on the elemental compositions of the byproducts obtained through the two-stage liquefaction, deposit and residue. Because the yields of deposit and residue were as low as 2.3 and 14.6 wt % for RS, respectively, it may be of little use to discuss the quality of these byproducts. Then, we focused on the deposit and residue produced from LY in the yields of as large as 11.1 and 51.1 wt %. First, both deposit and residue are almost completely free from water. The carbon contents of deposit and residue were 77.5 and 77.4%, respectively, and the oxygen contents of them were 16.9 and 17.6%, respectively. These elemental compositions give the HHVs as large as 30.4 and 28.8 MJ/kg for deposit and residue, respectively. The elemental composition and HHVs well correspond to those of sub-bituminous coal. Furthermore, it was found in our previous works that the residue has better combustion characteristics22 and higher gasification reactivity than the corresponding raw coal.23 These examinations show that deposit and residue can be used as high-quality solid fuel or high-quality raw materials for gasification. The yields of HI from single-rapid and single-TProg liquefactions of LY were also as high as 48.8 and 58.8%, respectively. Their effective uses are also essential, if the single-stage liquefactions are performed. Because this work focuses on the two-stage liquefaction and the use of the coal liquefaction residue (HI) has been studied widely,24−26 no further examinations were performed on the HI obtained from the single liquefactions in this work. The above discussion showed that the proposed two-stage liquefaction produces both high-quality oil and high-quality solid fuels that can be used as raw materials for gasification or combustion.

Figure 4. Compositions of oils produced from two-stage and singlerapid liquefactions of RS and LY.

the oils produced through the two-stage liquefaction mainly consist of aliphatic and aromatic hydrocarbons, which account for 63.4 and 83.9% for RS and LY, respectively. The proportions of oxygen-containing compounds (such as phenols and acid) in the oils were less than 13.1 and 4.1% for RS and LY, respectively. In contrast, the proportions of oxygencontaining compounds in the oils produced by single-rapid liquefaction were as high as 62.7 and 21.2% for RS and LY, respectively. If we examine the difference in the yield of oxygencontaining compounds in the oils produced through the twostage and single-rapid liquefactions in more detail, the ester content of the oil produced from RS through the single-rapid liquefaction was as high as 46.7%, whereas the ester content of the oil produced through the two-stage liquefaction was almost 0%. The organic acid content of the oil produced from RS through the two-stage liquefaction was also much smaller than that of the oil produced from RS through single-rapid liquefaction. For the case of LY, the phenol content of the oil produced through the two-stage liquefaction was much smaller than that of the oil produced by single-rapid liquefaction. Thus, the removal of oxygen during the degradative solvent extraction stage of the raw materials E

DOI: 10.1021/ef502574b Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels significantly reduced the formation of oxygen-containing compounds during the liquefaction stage of solubles. 3.4. Amount of H2 Consumed for Producing Oils. One of the drawbacks of the direct coal liquefaction lies in its high H2 consumption, and it is highly requested to minimize the H2 consumption producing undesired products, such as H2O and residue.11 Then, the amount of H2 consumed to produce 1 kg of oil was compared between the two-stage and single-rapid liquefactions in Figure 5. It shows that the amount of H2

emitted per 1 kg of oil produced between the two-stage and single-rapid liquefactions. The CO2 emission on an oil basis is more meaningful than that on a raw material basis, as shown in Figure 2 for practical discussion. The amount of CO2 emitted through the two-stage liquefaction was 0.54 and 0.44 kg/kg of oil for RS and LY, respectively. On the other hand, the amount of CO2 emitted through the single-rapid liquefaction was as large as 1.26 and 1.07 kg/kg of oil for RS and LY, respectively. In other words, the amount of CO2 emitted through the twostage liquefaction was less than half of that emitted through single-rapid liquefaction. Besides, the CO2 emission of the twostage liquefaction mainly occurred during the degradative solvent extraction stage, as discussed above. The lower CO2 emission of the two-stage liquefaction did not produce highoxygen-content oil, because most oxygen was removed as H2O during the degradative solvent extraction stage, as discussed above. Thus, the two-stage liquefaction is more preferable than the single-stage liquefaction from the viewpoint of reducing CO2 emission during the coal or biomass conversion process. 3.6. Additional Advantages of the Two-Stage Liquefaction. The proposed two-stage liquefaction is judged to have several advantages over the single-stage liquefaction, as stated in detail above. The advantages include the significant reduction of the oxygen content of oil produced and the significant reduction of the amount of hydrogen consumption and CO2 emission. The two-stage liquefaction is expected to have other advantages over the single-stage liquefaction. The solubles that are served to the second stage of the two-stage liquefaction are small-molecular-weight compounds with small oxygen content, almost completely free from mineral matters, and completely soluble to solvent, as stated above. The unique property of the soluble is expected to solve several problems encountered in practical operation of a direct coal liquefaction plant. A brown coal liquefaction (BCL) process has been developed for the liquefaction of a Victorian brown coal.27 A 50 ton/day (dry basis) pilot plant of the BCL process was constructed and successfully operated in Australia in the 1990s. However, a continuous operation reaching 1770 h showed that the development of countermeasures for scale deposition is essential for commercialization of the process. During the operation of the plant, the deposits consisting of NaCl, CaCO3, Fe1−xS, etc., which originated from the brown coal and catalyst used, were formed on the inner walls of the preheater, reactors, and pipes connecting them. It has been shown that the scale deposition can be minimized by removing minerals and −COOH groups in the coal.28,29 Because soluble is almost free from mineral matters and contains little oxygen functional groups, as stated above, liquefaction of soluble is expected to minimize the scale deposition. The size of the liquefaction reactor may also be reduced because only soluble is fed to the liquefaction reactor. Thus, the proposed two-stage liquefaction is judged to have several merits over the single-stage liquefaction from practical viewpoints also.

Figure 5. H2 consumptions of two-stage and single-rapid liquefactions.

consumed during the two-stage liquefaction was as low as 3.52 and 5.26 mol/kg of oil for RS and LY, respectively, and was much smaller than the amount of H2 consumed during the single-rapid liquefaction of the raw materials, especially for RS. This is because a large amount of H2 was consumed to produce H2O during the single-rapid liquefaction, as stated earlier. Most oxygen was removed as either CO2 or H2O during the firststage degradative extraction for the two-stage liquefaction, as stated above, which minimized the consumption of H2 forming H2O during the second-stage liquefaction of soluble. Thus, the proposed two-stage liquefaction was judged to be more advantageous than the single-rapid liquefaction from the viewpoint of H2 consumption. 3.5. Amount of CO2 Emitted for Producing Oils. A large amount of CO2 emission is an issue of concern during the practical conversion process of coal and/or biomass to fuels or value-added products. Figure 6 compares the amount of CO2

4. CONCLUSION Two-stage liquefaction of low-rank coal and biomass waste to produce high-quality liquid fuel was proposed in this work. The liquefaction consisted of producing soluble from low-rank coal or biomass waste by the degradative solvent extraction and liquefying the soluble under mild conditions to produce liquid fuel. The oils produced by the two-stage liquefaction have rather lower oxygen contents and mainly consist of aliphatic and aromatic hydrocarbons with a rather small amount of

Figure 6. CO2 emissions of two-stage and single-rapid liquefactions. F

DOI: 10.1021/ef502574b Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

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oxygen-containing compounds compared to the oils produced by the single-stage liquefactions of the raw materials, singlerapid and single-TProg liquefactions. More oxygen in the raw materials was removed as H2O rather than CO2 during the twostage liquefaction compared to the single-stage liquefactions. Most oxygen was removed during the degradative solvent extraction stage for the two-stage liquefaction. The amount of H2 consumed and the amount of CO2 emitted to produce the same amount of oil through the two-stage liquefaction were all much less than those through the single-rapid liquefaction. Additionally, the solid products (residue and deposit) of the two-stage liquefaction had high possibilities as high-quality solid fuel or high-quality raw materials for gasification. The scale deposition problem in practical operation is also expected to be minimized by the two-stage liquefaction. Thus, the possibility of combining the degradative solvent extraction method and the soluble liquefaction under mild conditions as a two-stage liquefaction method to produce high-quality liquid fuel from low-rank coals or biomass wastes was shown.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-774-38-3420. Fax: +81-774-38-3426. E-mail: [email protected]. Present Addresses

† Xian Li: State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China. ‡ Kouichi Miura: Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work was performed under the framework of the Japan−Thailand Science and Technology Research Partnership for Sustainable Development (SATREPS) Project: Development of Clean and Efficient Utilization of Low-Rank Coals and Biomass by Solvent Treatment.

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DOI: 10.1021/ef502574b Energy Fuels XXXX, XXX, XXX−XXX