Article pubs.acs.org/EF
Direct Liquefaction of Brown Coal Using a 0.1 Ton/Day Process Development Unit: Effect of Hydrothermal Treatment on Scale Deposition and Liquefaction Yield Toshinori Inoue,† Osamu Okuma,‡ Kaoru Masuda,† Motoharu Yasumuro,§ and Kouichi Miura*,∥ †
Applied Chemistry Division, Kobelco Research Institute, Incorporated, 1-5-5, Takatsukadai, Nishi-ku, Kobe 651-2271, Japan Research Department, The New Industry Research Organization, 1-5-2, Minatojima-minamimachi, Chuo-Ku, Kobe 650-0047, Japan § Coal and Energy Technology Department, Kobe Steel, Limited, 2-3-1, Shinhama, Arai-cho, Takasago 676-8670, Japan ∥ Department of Chemical Engineering, Graduate School of Engineering, Kyoto University, Katsura, Nishigyo-ku, Kyoto 615-8510, Japan ‡
ABSTRACT: Scale deposition is a very troublesome problem for a long-term stable operation of a direct coal liquefaction plant. The scale reduction effect of hydrothermal treatment (HTT) for a brown coal liquefaction was investigated using a 0.1 ton/day process development unit (PDU). It was found that the amount of scale formed was reduced by half compared to non-treated coal when HTT coal treated at 325 °C was liquefied. This was because most carboxyl groups were decomposed and exchangeable cations, such as Ca and Na, precursors of the scale, such as CaCO3 and NaCl, were reduced during HTT. Furthermore, the formation of scale comprising Fe1−xS and SiO2 was also suppressed by HTT probably because of a decrease in the amounts of NaCl and CaCO3. Liquefying the HTT coal slightly decreased the oil yield compared to the non-treated coal. However, this disadvantage is compensated by the increase in the space time yield of the reactors liquefying HTT coal, because the coal concentration of the HTT coal−solvent slurry fed to the reactors can be increased from 28 to 42 wt % as a result of the reduction of viscosity, as reported in an our previous paper. The concentrations of major scale precursors in the HTT coal−solvent slurry of 42 wt % coal concentration are lower than those in the non-treated coal−solvent slurry of 28 wt % coal concentration. These results indicate that HTT is an effective pretreatment method not only to realize a long-term stable operation but also to improve the oil productivity of the liquefaction plant.
1. INTRODUCTION A brown coal liquefaction (BCL) process has been developed for the liquefaction of a Victorian brown coal.1 The BCL process is a two-stage hydrogenation process, which consists of four unit sections: dewatering, primary hydrogenation (liquefaction), de-ashing, and secondary hydrogenation, as shown in Figure 1. In the BCL process, pulverized raw coal is dewatered in a liquefaction solvent and then heated and hydroliquefied at 450 °C in the presence of hydrogen and a catalyst. 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 achieving the final target of 8000 h of continuous operation. During the operation of the plant, the scales consisting of NaCl, CaCO3, Fe1−xS, etc., which originated from the brown coal and the catalyst used, were formed on the inner walls of the preheater, reactors, and the pipes connecting them. It is known that most Na and Cl in the brown coal exist as water-soluble salts and most Ca and Mg exist as carboxylates.2 During the pilot-plant operation, the scale consisting of NaCl and Fe1−xS was formed mainly in the preheater and the scale consisting of carbonates of Ca, Mg, and Na was formed on the walls of the reactors and pipes connecting the reactors. Figure 2a shows the scale formed in the preheter of the pilot plant after 980 h of operation.3 A similar scale was also formed during the operation of a 0.1 ton/ © 2012 American Chemical Society
day (dry basis) process development unit (PDU). Figure 2b shows the scale formed in the pipes connecting the reactors of the PDU after 200 h of operation. These scale depositions may cause significant operational problems by increasing the pressure drops in the preheater and pipes. In a previous paper,4 the authors showed that hydrothermal treatment (HTT) at 300−350 °C can remove most Na and Cl, parts of Ca and Mg, and most carboxyl groups contained in the brown coal, and hence, the deposition of scale consisting of NaCl and the carbonate, such as CaCO3, is expected to be suppressed when HTT coal is liquefied. Another concern of HTT of the brown coal is its effect on the liquefaction reactivity of coal. Serio et al. reported that HTT at 300 °C does not affect the reactivity but HTT at 350 °C may decrease the liquefaction reactivity.5 Furthermore, Morimoto et al. reported that molecular weight distribution of the brown coal shifted to a higher molecular weight region after the HTT at 350 °C.6,7 It is, therefore, important to examine the effect of HTT on the liquefaction reactivity using a liquefaction facility that is large enough to simulate the 50 ton/day pilot plant. In this study, the effects of HTT at 325 °C were investigated to estimate the validity of the HTT as a pretreatment method of the liquefaction process from the viewpoints of both the Received: June 13, 2012 Revised: August 15, 2012 Published: August 16, 2012 5821
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Figure 1. Simplified flow diagram of the BCL process.1
Figure 2. Scale deposited in pipes: (a) preheater (60 mm inner diameter) of the pilot plant after 980 h of operation and (b) pipe (6 mm inner diameter, CP-1) connecting the first and second reactors of the PDU after 200 h of operation.
Table 1. Properties of Coals and Solvent Coals proximate analysis (wt %, dry basis) fixed carbon
volatile matter CD coal HTTcoal
CD coal HTTcoal
44.4 41.5
ultimate analysis (wt %, daf) ash
C
54.0 1.6 66.8 56.9 1.6 70.4 metal content (mg/kg, dry basis)
Ca
Mg
Na
Si
1880 1600
1830 1660
480 110
270 280
liquefaction condition boiling point range ultimate analysis (wt %, daf) structural parameters (1H NMR) representative hydrogen-donor compounds (by GC)
Al 1760 1990 Solvent
H
N
S
O (difference)
4.8 4.5
0.5 0.6
0.3 0.3
27.6 24.2
Fe
Cl
carboxyl groups (mol/kg, daf)
3790 3910
440 210
1.63 0.80
450 °C, 20 MPa, 1 h, catalyst Fe2O3 + S, 3 wt % as Fe on daf coal, S/Fe = l.2 180−420 °C C, 88.1; H, 9.3; N, 1.0; S, 0.1; O (difference), 1.5 fa, 0.52; σ, 0.38; Ln, 2.75; Har/Car, 0.95 2 rings (tetralins), 4.0 wt %; 3 rings (2H-anthracenes, etc.), 1.8 wt % 4 rings (2H-naphthacenes, 2H-pyrenes, etc.), 0.6 wt %
2. EXPERIMENTAL SECTION
suppression of scale deposition and the improvement of oil yield during the liquefaction. These investigations were performed using a 0.1 ton/day (dry basis) PDU, which consisted of three preheaters and three reactors connected in series.
2.1. Coal and Solvent. A Yallourn raw coal was used for the experiments. It was pulverized to particles of less than 2 mm in diameter and was dried in a N2 atm at 110 °C using a tubular dryer to prepare a conventionally dried coal (CD coal). The analyses of CD 5822
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Figure 3. Simplified flow diagram of the PDU. coal are given in Table 1. Properties of a solvent used for the liquefaction experiments are also shown in Table 1. It was a recycled solvent in the BCL pilot plant operated in Australia,1 and it was sampled after the primary hydrogenation section. Its boiling point range was 180−420 °C, and its main components were aromatic compounds of 2−4 rings and polar compounds, such as phenols. 2.2. HTT. HTT of the raw Yallourn coal was carried out using a continuous 1 ton/h tubular reactor. A mixture of the raw coal and water at the ratio of 3:5 by weight was treated at 325 °C and 14.5 MPa at 17 min of residence time. After the treatment, the HTT coal was recovered from the coal−water slurry by filtration and dried in a N2 atmosphere at 110 °C using a tubular dryer under the same conditions for preparing CD coal. 2.3. Liquefaction. A 0.1 ton/day (dry basis) PDU, which consisted of three preheaters and three reactors, was used for the liquefaction experiments. Figure 3 shows the simplified flow diagram of the PDU. The former two preheaters were made of a stainless-steel tube of 6 mm in inner diameter and 30 m in length and heated by a low-frequency induction heater. The third preheater was made of a stainless-steel tube of 6 mm in inner diameter and 10 m in length and heated by a brass-casting electric furnace. Each of the three liquefaction reactors had the same configuration as shown in Figure 4. It was 5.2 L (4 L of liquid phase and 1.2 L of gas phase) of continuous stirred tank reactor (CSTR) equipped with a magnetdriven stirrer. For normal PDU experiments, the pipes connecting the three reactors and gas−liquid separator were 6.0 mm inner diameter and 1 m long. Figure 2b shows the cross-section of the pipe connecting the first and second reactors after 200 h of normal operation. In this work, on the other hand, the pipes were replaced by those of 3.0 mm inner diameter to detect the scale formation more precisely from the pressure drops along the pipes. The pressure drops were monitored by sensitive differential gauges of 30 kPa full scale of ΔP-1, ΔP-2, and ΔP-3 (Fuji Electric, custom-made) attached to the connecting pipes CP-1, CP-2, and CP-3. The liquefaction conditions of PDU experiments were as follows: Coal was fed at the rate of 3 kg/h [dry and ash-free (daf) basis] as a coal−solvent slurry [the solvent/coal ratio (S/C) was 2.5 by weight]. A pyrite (FeS2) of 250 mesh was added to the slurry as a catalyst by 3 wt % as Fe to coal (daf). The slurry and the pressurized hydrogen gas were supplied at the flow rate of 12 L/h and 3.3 m3/h [normal temperature and pressure (NTP), 10 wt % feed coal (daf)], respectively. They were mixed before entering the first preheater and heated to 430 °C in three preheaters before entering the first reactor. The liquefaction temperature, pressure, and nominal residence time (reactor volume/volumetric flow rate of the feed slurry) were 450 °C, 14.8 MPa, and 60 min, respectively. The content of each reactor was stirred at 1000 rpm. Every PDU operation was continued for more than 200 h.
Figure 4. Schematic drawing of the liquefaction reactor. 2.4. Analysis. CD coal and HTT coal were characterized by ultimate analysis (using a Yanaco CHN analyzer, MT-500), proximate analysis (JIS M 8812 and 8813), amounts of carboxyl groups and carboxylates, and metal content. The amounts of carboxyl groups and carboxylates were determined by Schafer’s method.8 To estimate the metal content, about 100 mg of ash prepared from coal was dissolved in 100 mL of 2 M aqueous HCl solution, and then the metal concentration of the solution was measured by atomic absorption spectrometry (Valian, AA240FS) and inductively coupled plasma (ICP) atomic emission spectrometry (Shimadzu, ICPV-1017). The Cl content was determined by ion chromatography (Dionex, DX500). The yield of HTT coal through HTT was determined by weight, and the yields of gaseous products were quantified by the flow rate and composition. The flow rate was measured by a dry gas meter (Shinagawa, DC-2), and the composition was determined by gas chromatography (Shimadzu, GC-8A-TCD and GC-8A-FID) for CO, CO2, and C1−C4. The yield of water, which was assumed to be a main product, except for HTT coal and gas,4 was estimated by difference. 5823
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The liquefaction products of the PDU experiments were quantified by analyzing the samples taken from the outlet stream of the third reactor at every 3 h intervals. More than three samples during the stable operation, where mass balance was established within 100 ± 2%, were selected and analyzed. The analyses of the three samples were averaged and employed as the yields. The yields of gaseous products were determined using the same method as for the experiment of the HTT. Liquid products were distillated under vacuum (ASTM D1160) to be fractionated into H2O, naphtha fraction (Nap., bp (corrected in atmospheric pressure) < 180 °C), solvent fraction (Solv., bp = 180− 420 °C), and residue called coal liquid bottom (CLB, bp > 420 °C). Oil was defined as the sum of Nap. and Solv. in this study. The yields of all products were expressed on the basis of raw coal (daf). CLB was fractionated by sequential solvent extraction using n-hexane, benzene, and pyridine to determine the yields of hexane-soluble (HS), hexaneinsoluble−benzene-soluble (HI−BS), benzene-insoluble−pyridinesoluble (BI−PS), and pyridine-insoluble (PI) fractions.9 After each PDU experiment, the scales deposited inside the pipes connecting the reactors and gas−liquid separator were recovered by the following procedure. First, the pipes were washed by acetone to remove organic components involved in the scale deposited on the inner wall. Then, the pipes were washed by a 50 wt % aqueous acetic acid solution. The acetic acid solution washed away the whole scale, and carbonates, such as CaCO3, and insoluble matter, such as SiO2, were recovered as solutions and solid particles, respectively. The particles and solution were separated by filtration. Then, the particles and solution were weighed and analyzed for their compositions. The particles were dissolved in 2 M aqueous HCl solution before analysis. Their metal contents were analyzed using the same method as for the analysis of ash in the coals. The minerals constituting the scale were identified by the X-ray diffraction (XRD) measurement. Total amounts of scales and their composition were determined on the basis of these analytical results.
Figure 5. HTT and liquefaction yields and extraction yields of CLB using a 0.1 ton/day PDU.
of the viscosity of the coal−solvent slurry prepared from the HTT coal. The authors have reported that the viscosity of the coal−solvent slurry prepared from the HTT coal was much smaller than the viscosity of the coal−solvent slurry prepared from the raw coal at the same coal concentrations. Because the HTT coal−solvent slurry of 42 wt % coal concentration showed the same viscosity of the raw coal−solvent slurry of 28 wt % coal concentration, the coal concentration of the coal− solvent slurry fed to the primary hydrogenator could be increased from 28 to 42 wt % using the HTT coal.4 The increase in the coal concentration in the coal−solvent slurry was estimated to increase the space time yield (the amount of the coal liquefied per unit reactor volume during unit time) of oil by 20%. Actually, it was reported that a high coal concentration in the coal−solvent slurry increased the oil yield when liquefaction was carried out under the same catalyst/coal ratio.10 On the basis of these results, we conclude that HTT is an effective pretreatment method to improve the oil productivity of the liquefaction plant. 3.3. Scale Deposition from HTT Coal. Panels a and b of Figure 6 show the pressure drops measured during the liquefaction of the CD coal and the HTT coal along the pipes connecting the liquefaction reactors, CP-1 and CP-2, and the pipe connecting the third reactor and the gas/liquid separator, CP-3, respectively. The pressure drops along all of the connecting pipes increased monotonously with increasing operation time during the liquefaction of the CD coal. During the liquefaction of the HTT coal, on the other hand, the pressure drop along CP-1 was null and the pressure drops along CP-2 and CP-3 increased at much smaller rates than those in the case of liquefaction of the CD coal and reached plateaus after about 400 h of operation. The pressure drop increase was judged to be caused by the scale formation in the connecting pipes. Then, the thickness of the scale was estimated using the Fanning equation, which correlates the pressure drop ΔP with the velocity V and the diameter of the flow channel D by ΔP = kV2/D, with a constant k. It was confirmed that the slurry flow was a turbulent flow for
3. RESULTS AND DISCUSSION 3.1. Products Obtained through HTT. The yields of CO2 and H2O through HTT were 4.5 and 3.0 kg/100 kg of raw coal (daf), respectively. The yield of C1−C4 was as small as 0.1 kg/ 100 kg, and the yield of CO was negligibly small. The contents of oxygen and carboxyl groups in the HTT coal were smaller than those in the CD coal, as shown in Table 1. The contents of Na and Cl in the HTT coal were less than half of those in the CD coal, and the contents of Ca and Mg in the HTT coal were slightly smaller than those in the CD coal. The contents of Si and Fe did not change through HTT. 3.2. Liquefaction Yields. Figure 5 compares the product yields between the liquefaction of the CD coal and the liquefaction of HTT coal. The product yields through HTT are added to the liquefaction yields for the HTT coal. All yields are expressed on the basis of 100 kg of raw coal (daf). The yield of CO was negligibly small for each coal. The yields of CO2 and H2O through HTT and liquefaction were almost equal to those for the CD coal liquefaction. The oil (Nap. + Solv.) yields were 24 and 19 kg/100 kg, respectively, and the CLB yields were 43 and 48 kg/100 kg for the CD coal and HTT coal, respectively. These results indicate that HTT decreased the oil yield on liquefaction by increasing the CLB yield instead. Figure 5 also shows the yields of fractions quantified by the sequential solvent extraction of CLB. The yields of BI−PS and HI−BS fractions from the HTT coal were larger than those from the CD coal. This slightly low liquefaction reactivity of the HTT coal compared to the CD coal may come from the polymerization reaction of the brown coal molecules during HTT at 325 °C for 17 min.6,7 However, the decrease of the oil yield for the HTT coal liquefaction is well-compensated by the advantage of reduction 5824
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Figure 6. Pressure drops in the pipes connecting the reactors and gas/liquid separator (see Figure 3).
0.4 mm and those of the CP-1 and CP-2 were less than half of those for the CD coal liquefaction. The scale thickness for the HTT coal liquefaction was 0−0.8 mm, even at 500 h of operation time, at which the pressure drops along CP-2 and CP-3 reached plateaus and the scale formation stopped. The scale formation will stop when its rate balances with the rate of scale erosion caused by the slurry flow. The scale formation rate for the HTT coal liquefaction was found to be small enough to be suppressed by the slurry flow, clearly indicating that HTT is very effective to suppress the scale formation. Table 3a shows the elemental analyses of the scales formed in these pipes at 220 h of operation time for the CD coal liquefaction and 500 h of operation time for the HTT coal liquefaction, and Table 3b shows the deposition ratio of each element calculated from the elemental analyses and the amount of element supplied to the reactor. Many operation data of the pilot plant showed that scale compounds change with the progress of liquefaction as follows:3 Scale consisting of NaCl and Fe1−xS was formed at the early stage of liquefaction in the preheater and first reactor. The carbonate formed first was
the Fanning equation to be applied. Table 2 shows the diameters of the flow channel estimated and the scale Table 2. Scale Thickness Estimated from the Pressure Drops feed coal
operation time (h)
CD coal
220
220 HTT coal 500
pipe number
diameter of the flow channel (estimated) (mm)
scale thickness (estimated) (mm)
CP-1 CP-2 CP-3 CP-1 CP-2 CP-3 CP-1 CP-2 CP-3
1.0 1.0 2.4 3.0 2.2 2.4 3.0 1.8 1.4
1.0 1.0 0.3 0 0.4 0.3 0 0.6 0.8
thicknesses calculated from the diameters of the flow channel. During comparison at 220 h of operation time, the scale thicknesses for the HTT coal liquefaction were as small as 0−
Table 3. Results of (a) Elemental Analysis for the Scale and (b) Deposition Ratio of Each Element coal
operation time (h)
CD coal
220
HTT coal
500
CD coal
220
HTT coal
500
pipe number
Ca
Mg
Na
(a) Elemental Analysis of Scale (wt % on Scale) CP-1 7.6 4.6 22.6 CP-2 25.1 2.5 10.8 CP-3 24.9 5.6 6.9 CP-1 40.0
