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Energy & Fuels 1998, 12, 284-288
Solvent-Free Liquefaction of Brown and Subbituminous Coals Using NiMo Sulfide Catalyst Supported on Carbon Nanoparticles Kinya Sakanishi,* Haru-umi Hasuo, Masahiro Kishino, and Isao Mochida Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816, Japan Received July 8, 1997
Single- and two-stage liquefaction processes of Yallourn (YLC), South Banko (SBC), and Tanitoharum (THC) coals were performed in an autoclave of 50-mL capacity at variable solvent (tetralin)/coal ratios from 0 to 1.5 under the reaction hydrogen pressure of 15 MPa, using NiMo sulfide supported on carbon nanoparticles or commercial NiMo/alumina and synthetic pyrite catalysts. Although the oil yield decreased very much with all the catalysts by reducing the amount of solvent to zero, the NiMo/carbon catalyst gave much higher oil yields of 52 and 64%, respectively, in the single-stage (450 °C, 60 min) and two-stage (360 °C, 60 min; 450 °C, 60 min) liquefaction under the solvent-free conditions compared with the yield of around 40% by the other two catalysts in both single- and two-stage liquefaction under the same conditions. The oil yield depended on the coal species under the solvent-free conditions, being in the order of SBC > YLC > THC regardless of the reaction conditions. SBC provided the highest oil yields of 60 and 68% in the single- and two-stage liquefaction, respectively, reflecting its higher reactivity and lower gas yield. THC gave the lowest oil yields among the coals examined, although the oil yield reached 60% by the two-stage liquefaction even under the solvent-free conditions. The oil produced with NiMo/carbon catalyst carried lighter fractions in the boiling range 100-300 °C than those with the other catalysts regardless of the reaction conditions and coal species. Such excellent performance of the NiMo/carbon catalyst reflects its higher hydrogenation activity as well as the high dispersion on the coal surface at the initial stage of coal liquefaction, suppressing the retrogressive reactions. It is confirmed that the major portion of solid coal was solubilized during the heating and the initial stage to work as the self-producing solvent under the solventfree reaction conditions. The design of coal liquefaction with the least use of solvent is discussed for the higher productivity.
The cost of the coal liquefaction process is still attempted to be reduced by 30% for its commercialization to meet the increasing demand of the transportation fuels in developing countries with large populations in Asia.1 There are four approaches to reduce the cost:2 (1) increasing oil yield with the least production of hydrocarbon gases; (2) increasing productivity of oil at a fixed reactor volume; (3) catalyst recycling after the separation from the residue; (4) simplifying the preheater and reactor configuration. The authors proposed a novel type of liquefaction catalyst with much higher activity and functions for recovery and repeated use.3,4 NiMo, FeMo, or FeNi bimetallic sulfides supported on a particular carbon black of nanoparticles is one of such catalysts to provide high distillate yields with the least residual product and to be recoverable from the solid residues by gravimetric
separation for repeated use.5,6 The catalyst increased significantly the oil yield by the two-stage process, which consists of a first stage at 360 °C and a second stage at 450 °C.7 A solution to a remaining problem of increasing the productivity for better feasibility is to increase the coal concentration in the slurry feed for the reactor. Jackson et al.8 reported a coal-liquefaction procedure without solvent by impregnating the catalyst species to be dispersed directly onto the solid coal matrix, although such impregnation of the catalyst onto the coal prior to the feeding and reaction is rather costly. In the present study, liquefaction of Australian brown and Indonesian brown and subbituminous coals, which are target coal species of the two Japanese liquefaction processes (brown coal and bituminous coal-liquefaction processes) developed by New Energy Industrial Technology Development Organization (NEDO) using NiMo sulfide supported on carbon nanoparticles, commercial
(1) Mochida, I.; Sakanishi, K.; Korai, Y.; Fujitsu, H. Fuel Process. Technol. 1986, 14, 113. (2) Mochida, I.; Sakanishi, K. Advances in Catalysis; Academic Press: New York, 1994; p 39. (3) Mochida, I.; Sakanishi, K.; Sakata, R.; Honda, K.; Umezawa, T. Energy Fuels 1994, 8, 25. (4) Sakanishi, K.; Hasuo, H.; Mochida, I.; Okuma, O. Energy Fuels 1995, 9, 995.
(5) Sakanishi, K.; Hasuo, H.; Kishino, M.; Mochida, I.; Okuma, O. Energy Fuels 1996, 10, 216. (6) Sakanishi, K.; Taniguchi, H.; Hasuo, H.; Mochida, I. Ind. Eng. Chem. Res. 1997, 36, 306. (7) Hasuo, H.; Sakanishi, K.; Taniguchi, H.; Kishino, M.; Mochida, I. Ind. Eng. Chem. Res. 1997, 36, 1453. (8) Hulston, C. K.; Redlich, P. J.; Jackson, W. R.; Larkins, F. P.; Marshall, M. Fuel 1996, 75, 1387.
Introduction
S0887-0624(97)00113-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/05/1998
Solvent-Free Liquefaction of Coals
Energy & Fuels, Vol. 12, No. 2, 1998 285
Table 1. Elemental Analysis of Coals Used in the Present Study coal
C
Yallourn Tanitoharum South Banko
66.9 71.6 63.8
wt % (daf) H N (O + S) 4.7 5.6 5.7
0.5 1.6 1.1
27.9 21.2 29.4
ash (wt %)
(H/C)
1.6 3.9 1.9
0.84 0.93 1.07
Table 2. Some Properties of Catalysts surface area (m2/g) particle size (µm) apparent density (g/L) Ni (wt %) Mo (wt %)
NiMo/Al2O3
NiMo/KB
FeS2
400 YLC > THC in this order regardless of reaction conditions. In the single-stage liquefaction, THC, which was the highest-rank coal among the three coals, particularly gave the lowest oil yield of 46%. SBC of an Indonesian brown coal with lower oxygen content provided the higher oil yield than YLC of an Australian brown coal, reaching 60 wt % with less gas formation.
Solvent-Free Liquefaction of Coals Table 3.
Energy & Fuels, Vol. 12, No. 2, 1998 287 13C
NMR Analysis of Oil Products from Yallourn Coal (%)a
catalysts NiMo/KB NiMo/Al2O3 synthetic FeS2
conditions
Co
Ci
Cp
Cal
CR
Cβ
Cs/Ca
single-stage two-stage single-stage two-stage single-stage two-stage
0.0 1.4 3.7 6.1 8.0 6.8
5.3 11.6 18.5 20.7 26.8 22.0
45.3 38.8 33.6 29.0 27.3 31.9
41.8 40.8 33.2 28.6 25.1 29.4
4.8 5.2 6.4 7.1 6.6 6.0
2.8 2.2 4.6 8.5 6.2 3.9
0.98 0.96 0.85 0.89 0.70 0.73
a C : oxygenated carbons. C : internal or alkylated aromatic carbons. C : protonated aromatic carbons. C : methyl and naphthenic o i p a1 carbons. CR: R-methylene carbons, Cβ: β-methylene carbons. Cs ) Ca1 + CR + Cβ; Ca ) Ci + Cp.
Table 4. Analysis of Gas Products in the Solvent-Free Liquefaction of Yallourn Coal wt % (daf base)
Figure 6. Effect of coal species on the boiling-point distribution of the oil fraction produced by the solvent-free liquefaction: (a, b) Yallourn coal; (c, d) South Banko coal; (e, f) Tanitoharum coal. Parts a, c, and e are for the single-stage process, and parts b, d, and f are for the two-stage process. The following are the reaction conditions: single stage, 450°C, 60 min; two-stage process, 360 °C, 60 min and 450 °C, 60 min; reaction H2 pressure ) 15 MPa; coal/solvent/catalyst ) 3/0/ 0.09 weight ratio; catalyst ) NiMo/KB; stirring speed ) 1300 rpm; heating rate ) 20 °C/min.
In the case of two-stage liquefaction, the oil yield increased to 60-68 wt % regardless of the coal species by reducing the heavier fractions yields (asphaltene, preasphaltene, and residue) without increasing the gas yield. In particular, the two-stage liquefaction of SBC provided the highest oil yield of 68 wt % even under solvent-free conditions. Analysis of Product Oil. Figure 6 illustrates the boiling-point distributions of the hexane-soluble fraction (oil) produced under the solvent-free liquefaction conditions. The solvent-free liquefaction allowed a smallscale liquefaction to evaluate accurately the boilingpoint distributions of the oil because no contamination derived from solvent existed in the gas chromatogram. The major products under the present conditions were always the second-lightest fraction of the 200-300 °C boiling range regardless of the catalysts, coal species, and reaction conditions. The NiMo/KB catalyst provided greater amounts of the lighter oil fractions of 100200 °C and 200-300 °C than the amounts from conventional catalysts of NiMo/Al2O3 and synthetic pyrite as reported in a previous paper.7 The boiling-point distributions of the hexane-soluble fraction (oil) produced from SBC and THC exhibited a trend similar to that from YLC. The oil fraction from SBC included a higher yield of 38% of 200-300 °C fraction than the yield of other coals in the two-stage reaction. The heavier oil fraction from SBC, in contrast to THC and YLC, decreased in the two-stage reaction because of the higher conversion to the lighter fractions. It is noted that SBC has the highest reactivity to give the highest yield of the light distillate with the highest total oil yield among the coals investigated in the present study. Table 3 compares the 13C NMR carbon distributions of oil fractions produced with NiMo/KB, NiMo/Al2O3,
gas product
single-stage
two-stage (total)
first stage
second stage
CH4 C2H6 C2H4 C3H8 C3H6 i-C4H10 n-C4H10 CO CO2
2.8 1.7 0.0 1.8 0.1 0.1 0.6 2.5 13.8
2.1 1.4 0.0 1.7 0.1 0.1 0.6 2.2 13.4
0.6 0.0 0.0 0.0 0.0 0.0 0.0 1.6 11.6
1.5 1.4 0.0 1.7 0.1 0.1 0.6 0.6 1.8
total
23.3
21.6
13.8
7.8
and FeS2 in the single- and two-stage liquefaction without solvent. First of all, a much higher Cal (aliphaticcarbon content)/Car (aromatic-carbon content) ratio was noted with NiMo/KB regardless of liquefaction conditions compared with those with NiMo/Al2O3 and FeS2. A much higher activity of NiMo/KB for the hydrogenation of the aromatic ring is definitely required to produce the naphthenic ring. NiMo/Al2O3 gave a larger Cal/Car ratio than FeS2, suggesting the poor aromatic hydrogenation by the latter catalyst. Second, extensive deoxygenation of the oil product was noted with NiMo/KB, hydrodeoxygenation taking place very effectively on the catalyst. The two-stage liquefaction was more effective for the hydrogenation with NiMo/Al2O3 and FeS2 catalysts than the single-stage liquefaction, while NiMo/KB gave a slightly smaller Cal/Car ratio and more Ci (substituted aromatic carbon) content in the two-stage liquefaction. Extensively hydrogenated products in the first stage may perform further hydrocracking reactions in the second stage to give lighter products. A similar tendency was observed with the oil fractions produced from SBC and THC coals. Analysis of Gas Products. Table 4 summarizes the analyses of gaseous products provided from the solventfree liquefaction of Yallourn coal. The CO2 and CH4 were major product gases at yields of 13-14 and 2-3 wt %, respectively, with much lower yields of olefins (C2H4, C3H6) regardless of reaction conditions. In the two-stage liquefaction, the major portion of CO and CO2 was produced with a minor portion of methane in the first stage, the hydrocarbon gases being mainly produced in the second stage, suggesting the dominant contributions of dehydroxylation and decarboxylation in the first stage and hydrocraking including dealkylation in the second stage. It should be noted that the twostage conditions provided lower yields of C1-C3 of hydrocarbon gases as well as CO and CO2 compared with yields from the single-stage reaction.
288 Energy & Fuels, Vol. 12, No. 2, 1998
Sakanishi et al.
Figure 7. Liquefaction scheme of Yallourn coal for higher efficiency with catalyst recovery and least use of solvent. Numbers indicate approximate amount based on the dry coal.
Discussion The present findings revealed that the two-stage liquefaction of brown and subbituminous coals catalyzed by NiMo supported on carbon nanoparticles (KB) achieved a remarkable oil yield above 60% even under the solvent-free conditions. One of the key factors in the liquefaction without solvent is that the stirring speed during the heating must be carefully controlled at as low as 500 rpm until the temperature reached 300 °C for the sufficient mixing of the coal particles with the catalyst nanoparticles by the stable stirring of their whole components without loss from splashing and sticking onto the reactor wall. Above 300 °C, the stirring speed can be increased 1300 rpm because a considerable amount of coal-derived solvent fraction was obtained to give a slurry of coal particles and the catalyst particles are well dispersed in the viscous matrix, enhancing the catalytic hydrogenation of the primary heavy products at 360 °C in the first-stage reaction with minimal retrogressive reactions. The product distribution after the first stage as illustrated in Figure 4 confirmed that the major portion of solid coal was solubilized and hydrogenated to be ready for the upgrading in the successive steps. The successive second-stage reaction at 450 °C effectively hydrocracked the hydrogenated products in the first stage, producing the relatively lighter fractions in the distillate at a very high yield above 60%. The NiMo/KB catalyst has two advantages in the solvent-free coal liquefaction. One is its nanoscale particle size with its high surface area, hollow structure, low density, and relatively lipophilic surface nature for the dispersion in the viscous matrix of the primary coal liquid. The authors reported that the higher stirring speed was very effective for achieving a remarkably high oil yield above 70% in the liquefaction of Tanitoharum coal using NiMo/KB in tetralin solvent.11 The nanosize particles of NiMo/KB can be well dispersed by rapid stirring with the aid of a small amount of liquid fraction produced in the initial stage of the reaction even if no solvent was externally added. The other advantage of the NiMo/KB catalyst is the possibility of its recovery from the residual products by the simple gravimetric separation method, since the hollow carbon nanopar-
ticles of the NiMo/KB catalyst are nonpolar and floating, being be dispersed well in the liquid phase and with a very small amount of catalyst precipitation taking place with the large particles of the residue products. We reported in a previous paper5 that the recovered NiMo/ KB catalyst together with the THF-insoluble residue exhibited an activity similar to the activity of a virgin catalyst for the liquefaction of Wyoming coal through the resulfiding treatment, suggesting that no irreversible deactivation may take place during the coal liquefaction. For practical application, recycled use of the NiMo/KB catalyst can be designed with the heavy distillable product and/or residue by the bottom-recycle mode or heavy-solvent recycle mode. In such an approach with less solvent, a certain amount of volatile initial solvent as a transportation mediator can be used in this type of coal feeding for the preparation of slurry of low viscosity to transfer it smoothly into the preheater. This type of solvent fraction can be easily vaporized by increasing the gas flow rate in the first reactor as reported by NBCL (Nippon Brown Coal Liquefaction) group.12 Such feeding of solid coal and catalyst with the least amount (about 30% based on dry coal) of initial solvent into the first reactor is schematically described in Figure 7. Furthermore, the two-stage process of the present scheme may not require the preheater because of the effective utilization of the exothermic heat of hydrogenation in the first stage, where the inlet and outlet of the first reactor may be heated to 360 °C and 450 °C, respectively, by controlling the additional hydrogen-gas charge. Thus, NiMo supported on carbon nanoparticles with high activity in the two-staged configuration is expected to overcome some hurdles encountered at the present commercialization of coal liquefaction by the beginning of the next century. EF970113C (11) Sakanishi, K.; Taniguchi, H.; Hasuo, H.; Mochida, I. Energy Fuels 1996, 10, 260. (12) Yasumuro, M.; Katsushima, S.; Kageyama, Y.; Matsumura, T. In Coal Science; Pajares, J. A., Tascon, J. M. D., Eds.; Elsevier Science, 1995; p 1371.
