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Extraction of coal using supercritical water
Extraction
of coal using supercritical
Girish V. Deshpande, and Irving Wender
Gerald
D. Holder,
Department of Chemical Engineering, University USA (Received 28 April 1983; revised 19 September
Alfred
A. Bishop,
of Pittsburgh,
water Jairam
Pittsburgh,
Gopal
PA 75267,
1983)
An experimental apparatus was developed to inject coal into an autoclave containing preheated supercritical water. The supercritical water appears to act as both solvent and reactant in the conversion of coal to gases and liquids. Experiments were carried out with German brown coal, lignite and bituminous coal and with glucose at both subcritical and supercritical water densities. A significantly larger quantity of char was obtained when operating at subcritical densities and when the coal was mixed with water before heating to supercritical conditions. Smaller amounts of char were obtained as density increased and as reaction time increased. (Keywords: coal; supercritical water; extraction)
Supercritical extraction is a process in which highly compressed gas is contacted with relatively non-volatile solid or liquid at temperatures at, or slightly above, the critical temperature of the gas’. Under such conditions, the condensed phase will begin to volatilize, which is interpreted as the dissolution of the condensed phase in the supercritical gas phase. At supercritical densities which are much larger than typical gas densities, the gas acts as a strong solvent for the condensed phase. When treated with fluids having a critical temperature near the temperature of devolatilization, coals with a high volatile content will yield substances whose volatility will be increased dramatically due to the presence of a supercritical fluid phase. Hence subsequent condensation or char formation may be greatly reduced’. In previous studies, glucose, cellulose and maple sawdust, when treated with supercritical water, were converted almost entirely to organic liquids and synthesis gas (plus CO, and CH,). Virtually no char is formed under these conditions. Water acts both as a solvent and as a reactant for the pyrolysis product; evidently a reforming reaction takes place during liquefaction to produce the gase?. Analogies between biomass, including lignin, and lowrank coals suggest that supercritical extraction of such coals with water could lead to an entirely new, simpler and less expensive method of obtaining synthetic fuels from coal. In this work, supercritical extraction was carried out with coal and with glucose, both at subcritical and supercritical densities. BACKGROUND The enhanced solvent power of compressed gases was known to Hannay and Hogarth in 1879, but was not applied in the energy industries until the late 1950s when Zhuze’ reported that supercritical propane/propylene mixtures could be used in deasphalting petroleum residue. The non-tarry materials volatilize in the supercritical gas phase which is then separated in a second low pressure separator6. The gas extraction technique has been em0016-2361/84/070956-05%3.00 @ 1984Butterworth & Co. (Publishers) Ltd
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ployed in a number of other processes including gas chromatography and the extraction of paraffin from mineral ores?. Its potential use in the extraction of food products which are often heat labile at low temperature, with either CO2 or ethylene, is well documented. To date, however, the areas of most interest in the energy industry are those where more desirable coal conversion products have been obtained through supercritical extraction’. One application with potential is the supercritical extraction of raw coal; this process has primarily been investigated by the National Coal Board, UK6. In the presence of a supercritical gas, the high-boiling compounds in coal tend to vaporize at temperatures considerably below their normal boiling points. It is thought that the supercritical fluid also serves to solvate reactive intermediates, thus preventing their recombination with the coal solids. It may be possible to use supercritical extraction to remove part of the vacuum bottoms remaining after distillation of coal liquefaction products. The supercritical fluid is chosen such that its critical temperature is close to the extraction temperature’- lo. The most effective solvents appear to be those with critical temperatures of 588-723 K. Pure solvents which have been tested include toluene, chlorobenzene, a-xylene, dodecane and p-cresol. Toluene is widely used since it is stable under extraction conditions, does not cause the coal to cake, and is relatively cheap and available. Much work has been carried out on the extraction of coal under supercritical conditions with various solvents. Toluene has been studied as a solvent for different bituminous coals in great detail by a number of workers. Ceylan and Olcay’ ’ treated Zonguldak coal with toluene at 633 K and found that the yield of extraction increased from 19 to 32% as extraction pressure increased from 11.6 to 29 MPa. Kershaw et al.’ studied the effect of solvent properties on the supercritical gas extraction of coal. To obtain a wider cross-section of solvents in the supercritical state, the extractions were carried out at 723K’. Correlations between conversion and critical temperature, density of supercritical fluid under experimental
Coal extraction using supercritical water: G. V. Deshpande et al.
conditions and the solubility parameter under experimental conditions were developed. Low conversions were obtained with water as the extraction was carried out with a reduced density of 0.28 (p = 87 kg mm3), and a reduced temperature which was also high during the extraction. Both factors result in lower solubility in the supercritical solvent phase of the coal’s less volatile components as compared with solubility when the extraction is carried out at the critical point”. Blessing and Ross’* studied the degree of coal dissolution with a number of organic solvents. The conversions were = 10% which is low and may be due to the presence of coal in the solution during the heat up period which, as the present results show, is unfavourable. Barton’ 3 studied supercritical liquefaction of Illinois No. 6 coal using supercritical water as the solvent and stannous chloride or molybdenum trisulphide as an impregnated catalyst. Hydrogen pressures were 2.65.4MPa, and the system pressure 22.3-34.6 MPa. Hydrogen consumption was 0.3-2 kg/kg coal and yields of gases and liquids up to 54 wt% daf were attained. Supercritical water distillation quantitatively separated the oil and asphaltenes from the coal char. Model1 and co-workers3,4 conducted a series of experiments in which glucose, cellulose, maple sawdust containing z 19% lignin, and coal were extracted with water at supercritical conditions (647 K, 22.3 MPa). Nearly 20% of the carbon in glucose and up to 40% in sawdust was converted to gaseous products (CO, H,, CO,, CH,). In the one experiment reported with coal (lignite) 8% gases and 20% liquids were obtained. Although char is the chief product when these substances are pyrolysed, no char was found at supercritical conditions in runs with glucose, cellulose and maple sawdust. The presence of supercritical water (SCW) appears to prevent the formation of condensation products by keeping the reactive intermediates in a highly solvated and dispersed state. At supercritical conditions, water is an excellent solvent for hydrocarbons (benzene and naphthalene are completely miscible with SCW), and for polar organics. Although the number of SCW-organic systems that have been studied is limited, SCW seems an excellent solvent for many organic substances that have little solubility at temperatures and pressures below supercritical conditions. Model1 and coworkers3q4 also noted that water at supercritical conditions can serve as a reforming reactant. In some cases, addition of a reforming catalyst increased the yield of synthesis gas. The reforming reaction of organic substrate with water seems to take place early in the reaction. In addition, Appell et a1.14 noted that the addition of H, and/or CO significantly reduced the amount of char produced when cellulose water slurries were heated to 523623 K in a batch autoclave. Lignin in the maple sawdust reacted completely with SCW14. The above work led to the present studies with coal. The structure of lignite, which contains >20% oxygen, includes many of the structures found in lignin (single benzene or phenolic rings with three carbon chains in various stages of oxidation”. Materials that have clear association with common degradation products of the constituents of higher plants, particularly of lignin, exist in both low- and high-rank coals’6v1 7. The extraction of coal using supercritical water is a promising area and, if successful, the conversion of coal to liquids and useful gases could be carried out under milder conditions without added hydrogen.
EXPERIMENTAL The experimental apparatus is shown in Figure I and consists of a custom-made 1 dm3 autoclave reactor with a Magnedrive II agitator. The reactor has various ports for thermocouples, venting, pressure gauges, solids injection, draining the reactor contents and sampling the fluid. It is provided with a single zone electric furnace whose temperature is controlled by a proportional controller. A 16 channel data logger was installed for recording temperatures and pressures throughout the run. The solids are injected through an electrically actuated ball valve. Analyses of the coal (100-200 mesh, 74-149 pm) and type of glucose used are given in Table I. Procedure A
This method was used for studying the injection of solids into preheated supercritical water. It was the most successful in producing gases and liquids. Figure 2 shows the procedure followed in the analysis of the extraction/reaction products obtained. The autoclave was charged with the amount of distilled water required for the fluid to be at the desired supercritical density under reaction conditions (i.e., 0.0031 m3 gave a density of 310 kgme3). A known weight of solids was placed in the charging bomb where it remained at ambient temperature until injection. The reactor was then heated to ~3 K above the desired operating temperature. A portion (WT 1) of the solid was then forced into the
figure 1 Schematic diagram of the experimental apparatus. 1, solvent pump; 2, solvent heater; 3, solids reservoir; 4, Magne drive; 5. reactor; 6, agitator; 7, cooling coil; 8, electric furnace; 8. rupture disc; 10. pressure gauge; 11, back pressure regulator; 12. three way valve; 13, flashing valve; 14, ice bath; 15, liquid trap; 16, wet gas meter; 17, drain valve; 18, cooling water; 19. thermocouple; 20, pressure transducer; 21, solvent inlet valve; 22, sample line; 23, temperature controller
Table 1 Analysis of coal used (wt%) GBC Ash Volatile C H s N
matter
BBC
5.ioa 54.21 a
4.9oa -
66.846 5.35b 0.05 b l.o5b
82.696 5.666 1.46b 1.726
a dry basis b dry ash free basis Glucose used was the D type in anhydrous form
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Coal extraction using supercritical water: G. V. Deshpande et al.
WTl-WT2-G*MW WTl > where: G=moles of gas formed in the reaction; and MW = its molecular weight. The amount of gas formed due to reaction was calculated using: %liquids = 100
Figure 2 Analysis procedure of extraction/reaction G, gases; L, liquids. Other terms explained in text
products.
reactor with argon at 6.2MPa greater than the reactor pressure. The weight of the charging bomb after injection was subtracted from the original weight giving the mass of coal injected (WT 1). Because argon is entering the reactor, its pressure (which was previously z 22 MPa) rises by 0.7-1.4MPa. The reaction/extraction was then carried out for a preset time (15560 min). At the end of the run, the reactor was quenched by passing water through the internal cooling coils. The reactor pressure dropped very rapidly (to 7 MPa in 2 min) and the reactor temperature cooled to 473 K in 5 min. The total amount of gas in the reactor was determined from the ideal gas law using the known reactor volume (less the volume of water), ambient temperature and reactor pressure (usually ~0.6 MPa) at ambient temperature. A gas sample was taken and then the reactor contents were drained through an extraction thimble. The reactor was then washed with 0.001 m3 water and the washings drained through the same thimble which, complete with contents was dried at 343 K for 24 h and then weighed. This weight (WT 2) gave the amount of undissolved solids and by difference the amount of liquids and gases (see formulae below). The reactor was then washed with 0.001 m3 of THF and the washing passed through the same extraction thimble that contained the water-free solids. The solid residue was extracted with THF in a soxhlet extraction apparatus for 24 h, then dried for a further 24 h. The solid residue (WT 3) remaining is the THF insolubles (THFI). The gas sample was analysed on a Perkin-Elmer Sigma IB gas chromatographic system fitted with a $n molecular sieve and kin Porapak P concentric columns and a TCD detector. A known calibration gas mixture was used to determine the response factors. Procedure B
This method was used for studying the substrate (coal for example) being mixed with water prior to heating to reaction conditions. The procedure is similar to procedure A except that the reactor was charged with a known quantity of slurry (i.e., coal and water) prior to heating. The slurry was heated to the desired temperature which was maintained for a preset time and then quenched to room temperature. Data analysis
The overall conversion of coal to liquid, gaseous and THF-soluble solid products was obtained from the formula :
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FUEL, 1984, Vol 63, July
G=P*(y-
(I- Y4) TI,)*RT
where: R=gas constanf; P=pressure of reactor after cooling; V,= liquid volume charged to reactor; Y, = mole fraction of argon in gas phase; T = temperature of reactor after cooling; and V,= reactor volume. In several runs with coal, an elemental analysis was carried out by Micro Analysis Inc. of Delaware and the ash analyses in the coal laboratory at the University of Pittsburgh. RESULTS AND DISCUSSION Tables 2-4 summarize the results obtained for German brown coal (GBC), Bruceton bituminous coal (BBC) and glucose, respectively. The glucose runs were conducted to verify that the results of Model1 et al3 could be reproduced. The results were similar in the quantity of gases formed and in that almost no char was formed using procedure A (injection). As with most extractions involving coal, the physical processes involved are complex, but several observations can be made. The results obtained for runs 2,7 and 8 and for runs 9 and 10 show that the experiments give reproducible results. Run 9 was conducted with argon and run 10 without. The similarity of the results indicates that argon does not affect the extraction. An important observation can be made by comparing the results shown in Table 2, using procedure A, where coal is injected into supercritical water, and procedure B, where coal and water were mixed and then heated to supcrcritical conditions. In the first case (runs 2, 7, 8) higher conversion to THF-soluble products is obtained than when coal and water are mixed before heating (runs 9, 10). Similar relative results were obtained for the glucose runs (Table 4). Run 5 was carried out at a lower water density, and produced a greater portion (60.6%) of THF insolubles. Apparently, the water must be near its critical density (310 kgmm3) for this extraction to be effective. In Table 3, the results for supercritical extraction of BBC are reported. Initially a much smaller percentage of this coal was converted to gases and liquids, but by increasing the stirrer speed to 25OOrevmin-’ and lengthening the extraction time, higher conversions were obtained. No experiments were carried out at 500 rev min - 1 stirrer speed or with an extraction time of > 15 min. Figure 3 shows how the percentage of gases and liquids increased and the percentage of THF insolubles decreased with extraction time; an equilibrium conversion was obtained at 1 h. Another important variable is the density of the supercritical water phase. Figure 4 shows the percentage of THF insolubles obtained as a function of water density.
Coal extraction using supercritical water: G. V. Deshpande et al. Table 2 Summary
of batch extraction/reaction
of brown coal using supercritical Run No. 2 A 314.0 18.1 647.8 22.52 23.33 15 500 97.5 24.8 4.5 not measured -
Procedure Water density (kg rne3) Coal charged x 103 (kg) Temperature (K) Water pressure prior to injection (MPa) Pressure (water and argon) (MPa) Reaction time (minj Stirrer speed (rev min-1) THFI, feed coal (wt%) THFI, solid product (wt%) Gaseous product (wt%j Solid product (wt%) b Liquids (by diff) fwt%) Ash in THFI fwt%)
not measured
water
Run No. 5
Run No. 7
Run No. 8
Run No. 9
Run No. 10
A
A
163.3 16.1 655.7 21.30a 20.41 15 500 97.5 60.6 14.6 not measured 6.96
308.9 14.6 649.8 20.97 22.35 15 500 97.5 29.6 8.0 not measured not measured
A 353.8 18.1 648.7 22.81 24.15 15 500 97.5 25.4 11.1 not measured 8.04
0 291.8 14.1 648.8 NA 22.54 15 500 97.5 41.4 10.9 53.5
B 318.6 14.2 648.8 NAc 23.03 15 500 97.5 41.9 15.7 55.7
a Pressure is higher than (water and argon) pressure due to higher temperature b Note some of the solid products are soluble in THF. Hence the THFI % is always less than the solid product c NA, not applicable The pH of the water was not measured
Table 3 Summary of batch extraction/reaction using supercritical water
Procedure Water density (kg m-3) Coal charged x 103 (kg) Temperature (K) Water pressure prior to injection (MPa) Pressure (water and argon) (MPaj Reaction time fminj Stirrer speed (rev min-tj THFI, feed coal fwt%j THFI, solid product fwt%j Gaseous product (wt%) Solid products fwt%j a Liquids (by diffj (wt%j Ash in THFI fwt%) pH of water
of bituminous
coal
Run No. 14
Run No. 16
Run No. 17
Run No. 18
A 351.3 14.4 650.0 22.37
A 350.0 9.3 649.7 22.86
A 352.4 15.5 649.1 23.16
A 350.0 14.1 649.8 22.90
23.51
24.07
24.82
24.74
30 5:: 2500 91.3 91.3 84.9 47.9 0.6 0.7 98.0 52.9 1.4 46.4 5.83 7.44 not 4.6 measured
15 2500 91.3 64.1 1.1 67.1 31.8 5.48 4.3
60 2500 91.3 42.1 0.7 49.2 50.1 6.14 4.6
a Note some of the solid products are soluble in THF. Hence the THFI fwt%I is always less than the solid product fwt%)
This does not indicate that limiting values had been obtained and higher water densities may result in greater yields. Finally, two other factors should be noted. First the acidity of the product water (pH =4.3-4.6) is high for BBC which has a sulphur content of 1.46 wt%, daf. This is attributed to H,S dissolved in the water. The SCW thus seems to be leaching out some of the inorganic material. Table 5 indicates that a high percentage of the ash is being extracted. the amount of which was determined by making an ash balance on the coal injected and the residue obtained after THF extraction. Table 6 shows a typical gas analysis obtained with the two types of coal used in this investigation. Carbon dioxide was the major gaseous product obtained with both coals. The amount of gases obtained with the brown coal was considerably higher than with the bituminous coal. A small quantity of methane was also formed in both cases. The table shows the range of gas compositions (in ~01%) and the range of
35.6 not measured
28.8 6.41
fwt%)
wt% of coal converted to gases, carbon dioxide and methane in various experiments. In addition to the runs reported in Tables 2 and 3 one run was carried out using a high-sodium lignite. At similar conditions only 2% of the coal was converted to gases and liquids, but experimental error may be present. No detailed analysis was conducted. CONCLUSION The object of the present study was not to develop design data, but to investigate the phenomenon of the behaviour of supercritical water extraction of coal and to determine the variables important in coal conversion. The results indicate that the water density must be near or above the ciritical density and that extraction times of 2 1 h may be needed for higher rank coals. However the extraction time may decrease as the water density increases. These results explain why Kershaw2 considered Table 4 Summary of batch extraction/reaction supercritical water
Procedure Water density (kg m-3) Glucose charged x 103
of glucose using
Run No. 1
Run No. 11
Run No. 12
A 311.2 25.2
B 339.9 29.8
B 68.1 29.7
647.8 NAb
649.5 NAb
(kg) Temperature (Kl 649.2 Water pressure prior to 22.48 injection (MPa) Pressure (water and 24.57 argon) (MPaj Reaction time (min) 17 Stirrer speed (rev min-tj 500 THFI. glucose fwt%) 100.0 1.6 THFI. solid product fwt%j Gaseous product (wt%) 5.1 Solid products fwt%) a Not measured Liquid (by diffj fwt%)
23.35
12.69
15 500 100.0 9.5 12.6 11.3
15 500 100.0 40.8 16.8 40.9
76.1
42.3
a Note some of the solid products are soluble in THF. Hence the THFI (wt%) is always less than the solid product fwt%) b NA, not applicable The ash in THFI and the pH of the water were not measured
FUEL, 1984, Vol 63, July
959
Coal extraction using supercritical water: G. V. Deshpande et al. Table 5 Ash extraction
15
“0
30 Reoctron
by water
Run No.
Density of sew (kg m-s)
Ash extracted
5 10 8
160 320 350
17.3 47.2 59.9
(wt%)
60
45 trme (mm)
Table 6 Typical
gas analysis (~01%) -___ Bruceton bituminous
Coal Ar co2 ‘3’4
I
I
15
I
I
30 Reoctron time
Coal to gas (wt%) Coal to CH, (wt%) Coal to CO, (wt%)
98.1 -99.0 0.80-l .3 0.15-0.56 0.60--l .1 0.03-0.14 0.58-0.94
coal
German brown coal 68.2-85.6 14.1-31.2 0.29-0.82 4.5-15.7 0.03-0.14 4.47-l 5.6
60
45 (mm)
solvent and, as these studies demonstrate, it can be quite effective.
Figure 3 Effect of reaction time on conversion of coal to gases and liquids and on percentage of THF insolubles. Bruceton coal (bituminous) was used. Starting density, 0.35 g/cc; stirrer speed, 2500 rev mint; temperature, 649.5 +0.8 K; pressure, 24.55 kO.43 MPa
ACKNOWLEDGEMENT
I
IOOC
This work was carried out under DOE Grant No. DEFG22-81PC40800 and this assistance is gratefully acknowledged. REFERENCES Gangoli, N. and Thodos, G. Ind. Eng. Chem. Prod. Res. Dev. 1977, 16,209 Jezko, J., Gray, D. and Kershaw, J. R. Fuel Process. Technol. 1982, 5,229
04to.10 ’
I
I
I
I
I
0.15
0.20
0.25
0.30
0.35
Oensrty of supercritical
I
phase (g/cc) 4
Figure 4 Effect of density of the supercritical phase on THF insolubles. German brown coal was used. Reaction time 15 min; temperature, 651.4 & 1.5 K; pressure, stirrer speed, 500 rev min-‘; 22.30 +0.52 MPa
Model], M. ‘Gasification and Liquefaction of Forest Products in Supercritical Water’, AIChE 89th National Meeting, Oregon, 1980 Model& M., Reid, R. C. and Amin, S. I. US Pate& Application 41 I3446 1978
5 6
Paul, P. F. and Wise, W. S. ‘The Principles of Gas Extraction’, Mills and Boon Ltd, London, 1971 Zhuse,T. P. Vestnik Akad. Nauk SSSR 1959,29(11),47; Petroleum 1960,23,298
water a weak solvent as he carried out experiments at 25% of the critical density. Another important conclusion is that the temperature time history of the coal drastically affects its conversion. By injecting ambient coal into SCW, the coal is heated in a stepwise fashion to 647K. By doing this, retrogressive reactions at intermediate temperatures may be prevented. At supercritical conditions reactive intermediates may be extracted before condensation reactions can occur. Finally, more work needs to be done, investigating the effects of particle size, coal to water ratios, temperature, density and reaction time. In addition, more comprehensive chemical analyses of the extract are needed. An attractive aspect of this work is that no hydrogen needs to be added in the coal conversion. Water is an economical
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FUEL, 1984, Vol 63, July
I 8 9 10
11 12
13
14 15 16 17
Whitehead, J. C. and Williams, D. F. J. Inst. Fuel 1975,48, 182 Bartle, K. D., Jones, D. W., Calimli, A.,Olcay, A. and Snape, C. E. Nature 1979,217,284 Bartle, K. D., Calimli, A., Jones, D. W., Matthews, S. R. and Olcay, A. Fuel 1979,58, 423 Tugrul, T. and Olcay, A. Fuel 1978,57,415 Ceylan, R. and Olcay, A. Fuel 1981,60, 197 Blessing, J. E. and Ross, D. S. ‘Supercritical Solvents and the Dissolution of Coal and Lignite’, Am. Chem. Sec. Symp. Ser. 1978, 71,171 Barton, P. ‘Supercritical Separation in Aqueous Coai Liquefaction with Impregnated Catalyst’, 184th National ACS meeting, Kansas City, 1982 Appell, H. R., Fu, Y. C., Friedman, S., Yovorsby, P. M. and Wender, I. Report of Investigations 7560, Bureau of Mines, 1971 Wender, I. Catal. Rev. Sci. Eng. 1976, 14,97 Raj, S. PhD Dissertation Pennsylvania State University, 1976 Given, P. H. and Dickinson, E. J. ‘Soil Chemistry’ Vol. 3, Marcel Dekker, New York, 1975
of coal using supercritical
Girish V. Deshpande, and Irving Wender
Gerald
D. Holder,
Department of Chemical Engineering, University USA (Received 28 April 1983; revised 19 September
Alfred
A. Bishop,
of Pittsburgh,
water Jairam
Pittsburgh,
Gopal
PA 75267,
1983)
An experimental apparatus was developed to inject coal into an autoclave containing preheated supercritical water. The supercritical water appears to act as both solvent and reactant in the conversion of coal to gases and liquids. Experiments were carried out with German brown coal, lignite and bituminous coal and with glucose at both subcritical and supercritical water densities. A significantly larger quantity of char was obtained when operating at subcritical densities and when the coal was mixed with water before heating to supercritical conditions. Smaller amounts of char were obtained as density increased and as reaction time increased. (Keywords: coal; supercritical water; extraction)
Supercritical extraction is a process in which highly compressed gas is contacted with relatively non-volatile solid or liquid at temperatures at, or slightly above, the critical temperature of the gas’. Under such conditions, the condensed phase will begin to volatilize, which is interpreted as the dissolution of the condensed phase in the supercritical gas phase. At supercritical densities which are much larger than typical gas densities, the gas acts as a strong solvent for the condensed phase. When treated with fluids having a critical temperature near the temperature of devolatilization, coals with a high volatile content will yield substances whose volatility will be increased dramatically due to the presence of a supercritical fluid phase. Hence subsequent condensation or char formation may be greatly reduced’. In previous studies, glucose, cellulose and maple sawdust, when treated with supercritical water, were converted almost entirely to organic liquids and synthesis gas (plus CO, and CH,). Virtually no char is formed under these conditions. Water acts both as a solvent and as a reactant for the pyrolysis product; evidently a reforming reaction takes place during liquefaction to produce the gase?. Analogies between biomass, including lignin, and lowrank coals suggest that supercritical extraction of such coals with water could lead to an entirely new, simpler and less expensive method of obtaining synthetic fuels from coal. In this work, supercritical extraction was carried out with coal and with glucose, both at subcritical and supercritical densities. BACKGROUND The enhanced solvent power of compressed gases was known to Hannay and Hogarth in 1879, but was not applied in the energy industries until the late 1950s when Zhuze’ reported that supercritical propane/propylene mixtures could be used in deasphalting petroleum residue. The non-tarry materials volatilize in the supercritical gas phase which is then separated in a second low pressure separator6. The gas extraction technique has been em0016-2361/84/070956-05%3.00 @ 1984Butterworth & Co. (Publishers) Ltd
956
FUEL, 1984,
Vol
63, July
ployed in a number of other processes including gas chromatography and the extraction of paraffin from mineral ores?. Its potential use in the extraction of food products which are often heat labile at low temperature, with either CO2 or ethylene, is well documented. To date, however, the areas of most interest in the energy industry are those where more desirable coal conversion products have been obtained through supercritical extraction’. One application with potential is the supercritical extraction of raw coal; this process has primarily been investigated by the National Coal Board, UK6. In the presence of a supercritical gas, the high-boiling compounds in coal tend to vaporize at temperatures considerably below their normal boiling points. It is thought that the supercritical fluid also serves to solvate reactive intermediates, thus preventing their recombination with the coal solids. It may be possible to use supercritical extraction to remove part of the vacuum bottoms remaining after distillation of coal liquefaction products. The supercritical fluid is chosen such that its critical temperature is close to the extraction temperature’- lo. The most effective solvents appear to be those with critical temperatures of 588-723 K. Pure solvents which have been tested include toluene, chlorobenzene, a-xylene, dodecane and p-cresol. Toluene is widely used since it is stable under extraction conditions, does not cause the coal to cake, and is relatively cheap and available. Much work has been carried out on the extraction of coal under supercritical conditions with various solvents. Toluene has been studied as a solvent for different bituminous coals in great detail by a number of workers. Ceylan and Olcay’ ’ treated Zonguldak coal with toluene at 633 K and found that the yield of extraction increased from 19 to 32% as extraction pressure increased from 11.6 to 29 MPa. Kershaw et al.’ studied the effect of solvent properties on the supercritical gas extraction of coal. To obtain a wider cross-section of solvents in the supercritical state, the extractions were carried out at 723K’. Correlations between conversion and critical temperature, density of supercritical fluid under experimental
Coal extraction using supercritical water: G. V. Deshpande et al.
conditions and the solubility parameter under experimental conditions were developed. Low conversions were obtained with water as the extraction was carried out with a reduced density of 0.28 (p = 87 kg mm3), and a reduced temperature which was also high during the extraction. Both factors result in lower solubility in the supercritical solvent phase of the coal’s less volatile components as compared with solubility when the extraction is carried out at the critical point”. Blessing and Ross’* studied the degree of coal dissolution with a number of organic solvents. The conversions were = 10% which is low and may be due to the presence of coal in the solution during the heat up period which, as the present results show, is unfavourable. Barton’ 3 studied supercritical liquefaction of Illinois No. 6 coal using supercritical water as the solvent and stannous chloride or molybdenum trisulphide as an impregnated catalyst. Hydrogen pressures were 2.65.4MPa, and the system pressure 22.3-34.6 MPa. Hydrogen consumption was 0.3-2 kg/kg coal and yields of gases and liquids up to 54 wt% daf were attained. Supercritical water distillation quantitatively separated the oil and asphaltenes from the coal char. Model1 and co-workers3,4 conducted a series of experiments in which glucose, cellulose, maple sawdust containing z 19% lignin, and coal were extracted with water at supercritical conditions (647 K, 22.3 MPa). Nearly 20% of the carbon in glucose and up to 40% in sawdust was converted to gaseous products (CO, H,, CO,, CH,). In the one experiment reported with coal (lignite) 8% gases and 20% liquids were obtained. Although char is the chief product when these substances are pyrolysed, no char was found at supercritical conditions in runs with glucose, cellulose and maple sawdust. The presence of supercritical water (SCW) appears to prevent the formation of condensation products by keeping the reactive intermediates in a highly solvated and dispersed state. At supercritical conditions, water is an excellent solvent for hydrocarbons (benzene and naphthalene are completely miscible with SCW), and for polar organics. Although the number of SCW-organic systems that have been studied is limited, SCW seems an excellent solvent for many organic substances that have little solubility at temperatures and pressures below supercritical conditions. Model1 and coworkers3q4 also noted that water at supercritical conditions can serve as a reforming reactant. In some cases, addition of a reforming catalyst increased the yield of synthesis gas. The reforming reaction of organic substrate with water seems to take place early in the reaction. In addition, Appell et a1.14 noted that the addition of H, and/or CO significantly reduced the amount of char produced when cellulose water slurries were heated to 523623 K in a batch autoclave. Lignin in the maple sawdust reacted completely with SCW14. The above work led to the present studies with coal. The structure of lignite, which contains >20% oxygen, includes many of the structures found in lignin (single benzene or phenolic rings with three carbon chains in various stages of oxidation”. Materials that have clear association with common degradation products of the constituents of higher plants, particularly of lignin, exist in both low- and high-rank coals’6v1 7. The extraction of coal using supercritical water is a promising area and, if successful, the conversion of coal to liquids and useful gases could be carried out under milder conditions without added hydrogen.
EXPERIMENTAL The experimental apparatus is shown in Figure I and consists of a custom-made 1 dm3 autoclave reactor with a Magnedrive II agitator. The reactor has various ports for thermocouples, venting, pressure gauges, solids injection, draining the reactor contents and sampling the fluid. It is provided with a single zone electric furnace whose temperature is controlled by a proportional controller. A 16 channel data logger was installed for recording temperatures and pressures throughout the run. The solids are injected through an electrically actuated ball valve. Analyses of the coal (100-200 mesh, 74-149 pm) and type of glucose used are given in Table I. Procedure A
This method was used for studying the injection of solids into preheated supercritical water. It was the most successful in producing gases and liquids. Figure 2 shows the procedure followed in the analysis of the extraction/reaction products obtained. The autoclave was charged with the amount of distilled water required for the fluid to be at the desired supercritical density under reaction conditions (i.e., 0.0031 m3 gave a density of 310 kgme3). A known weight of solids was placed in the charging bomb where it remained at ambient temperature until injection. The reactor was then heated to ~3 K above the desired operating temperature. A portion (WT 1) of the solid was then forced into the
figure 1 Schematic diagram of the experimental apparatus. 1, solvent pump; 2, solvent heater; 3, solids reservoir; 4, Magne drive; 5. reactor; 6, agitator; 7, cooling coil; 8, electric furnace; 8. rupture disc; 10. pressure gauge; 11, back pressure regulator; 12. three way valve; 13, flashing valve; 14, ice bath; 15, liquid trap; 16, wet gas meter; 17, drain valve; 18, cooling water; 19. thermocouple; 20, pressure transducer; 21, solvent inlet valve; 22, sample line; 23, temperature controller
Table 1 Analysis of coal used (wt%) GBC Ash Volatile C H s N
matter
BBC
5.ioa 54.21 a
4.9oa -
66.846 5.35b 0.05 b l.o5b
82.696 5.666 1.46b 1.726
a dry basis b dry ash free basis Glucose used was the D type in anhydrous form
FUEL,
1984,
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63, July
957
Coal extraction using supercritical water: G. V. Deshpande et al.
WTl-WT2-G*MW WTl > where: G=moles of gas formed in the reaction; and MW = its molecular weight. The amount of gas formed due to reaction was calculated using: %liquids = 100
Figure 2 Analysis procedure of extraction/reaction G, gases; L, liquids. Other terms explained in text
products.
reactor with argon at 6.2MPa greater than the reactor pressure. The weight of the charging bomb after injection was subtracted from the original weight giving the mass of coal injected (WT 1). Because argon is entering the reactor, its pressure (which was previously z 22 MPa) rises by 0.7-1.4MPa. The reaction/extraction was then carried out for a preset time (15560 min). At the end of the run, the reactor was quenched by passing water through the internal cooling coils. The reactor pressure dropped very rapidly (to 7 MPa in 2 min) and the reactor temperature cooled to 473 K in 5 min. The total amount of gas in the reactor was determined from the ideal gas law using the known reactor volume (less the volume of water), ambient temperature and reactor pressure (usually ~0.6 MPa) at ambient temperature. A gas sample was taken and then the reactor contents were drained through an extraction thimble. The reactor was then washed with 0.001 m3 water and the washings drained through the same thimble which, complete with contents was dried at 343 K for 24 h and then weighed. This weight (WT 2) gave the amount of undissolved solids and by difference the amount of liquids and gases (see formulae below). The reactor was then washed with 0.001 m3 of THF and the washing passed through the same extraction thimble that contained the water-free solids. The solid residue was extracted with THF in a soxhlet extraction apparatus for 24 h, then dried for a further 24 h. The solid residue (WT 3) remaining is the THF insolubles (THFI). The gas sample was analysed on a Perkin-Elmer Sigma IB gas chromatographic system fitted with a $n molecular sieve and kin Porapak P concentric columns and a TCD detector. A known calibration gas mixture was used to determine the response factors. Procedure B
This method was used for studying the substrate (coal for example) being mixed with water prior to heating to reaction conditions. The procedure is similar to procedure A except that the reactor was charged with a known quantity of slurry (i.e., coal and water) prior to heating. The slurry was heated to the desired temperature which was maintained for a preset time and then quenched to room temperature. Data analysis
The overall conversion of coal to liquid, gaseous and THF-soluble solid products was obtained from the formula :
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FUEL, 1984, Vol 63, July
G=P*(y-
(I- Y4) TI,)*RT
where: R=gas constanf; P=pressure of reactor after cooling; V,= liquid volume charged to reactor; Y, = mole fraction of argon in gas phase; T = temperature of reactor after cooling; and V,= reactor volume. In several runs with coal, an elemental analysis was carried out by Micro Analysis Inc. of Delaware and the ash analyses in the coal laboratory at the University of Pittsburgh. RESULTS AND DISCUSSION Tables 2-4 summarize the results obtained for German brown coal (GBC), Bruceton bituminous coal (BBC) and glucose, respectively. The glucose runs were conducted to verify that the results of Model1 et al3 could be reproduced. The results were similar in the quantity of gases formed and in that almost no char was formed using procedure A (injection). As with most extractions involving coal, the physical processes involved are complex, but several observations can be made. The results obtained for runs 2,7 and 8 and for runs 9 and 10 show that the experiments give reproducible results. Run 9 was conducted with argon and run 10 without. The similarity of the results indicates that argon does not affect the extraction. An important observation can be made by comparing the results shown in Table 2, using procedure A, where coal is injected into supercritical water, and procedure B, where coal and water were mixed and then heated to supcrcritical conditions. In the first case (runs 2, 7, 8) higher conversion to THF-soluble products is obtained than when coal and water are mixed before heating (runs 9, 10). Similar relative results were obtained for the glucose runs (Table 4). Run 5 was carried out at a lower water density, and produced a greater portion (60.6%) of THF insolubles. Apparently, the water must be near its critical density (310 kgmm3) for this extraction to be effective. In Table 3, the results for supercritical extraction of BBC are reported. Initially a much smaller percentage of this coal was converted to gases and liquids, but by increasing the stirrer speed to 25OOrevmin-’ and lengthening the extraction time, higher conversions were obtained. No experiments were carried out at 500 rev min - 1 stirrer speed or with an extraction time of > 15 min. Figure 3 shows how the percentage of gases and liquids increased and the percentage of THF insolubles decreased with extraction time; an equilibrium conversion was obtained at 1 h. Another important variable is the density of the supercritical water phase. Figure 4 shows the percentage of THF insolubles obtained as a function of water density.
Coal extraction using supercritical water: G. V. Deshpande et al. Table 2 Summary
of batch extraction/reaction
of brown coal using supercritical Run No. 2 A 314.0 18.1 647.8 22.52 23.33 15 500 97.5 24.8 4.5 not measured -
Procedure Water density (kg rne3) Coal charged x 103 (kg) Temperature (K) Water pressure prior to injection (MPa) Pressure (water and argon) (MPa) Reaction time (minj Stirrer speed (rev min-1) THFI, feed coal (wt%) THFI, solid product (wt%) Gaseous product (wt%j Solid product (wt%) b Liquids (by diff) fwt%) Ash in THFI fwt%)
not measured
water
Run No. 5
Run No. 7
Run No. 8
Run No. 9
Run No. 10
A
A
163.3 16.1 655.7 21.30a 20.41 15 500 97.5 60.6 14.6 not measured 6.96
308.9 14.6 649.8 20.97 22.35 15 500 97.5 29.6 8.0 not measured not measured
A 353.8 18.1 648.7 22.81 24.15 15 500 97.5 25.4 11.1 not measured 8.04
0 291.8 14.1 648.8 NA 22.54 15 500 97.5 41.4 10.9 53.5
B 318.6 14.2 648.8 NAc 23.03 15 500 97.5 41.9 15.7 55.7
a Pressure is higher than (water and argon) pressure due to higher temperature b Note some of the solid products are soluble in THF. Hence the THFI % is always less than the solid product c NA, not applicable The pH of the water was not measured
Table 3 Summary of batch extraction/reaction using supercritical water
Procedure Water density (kg m-3) Coal charged x 103 (kg) Temperature (K) Water pressure prior to injection (MPa) Pressure (water and argon) (MPaj Reaction time fminj Stirrer speed (rev min-tj THFI, feed coal fwt%j THFI, solid product fwt%j Gaseous product (wt%) Solid products fwt%j a Liquids (by diffj (wt%j Ash in THFI fwt%) pH of water
of bituminous
coal
Run No. 14
Run No. 16
Run No. 17
Run No. 18
A 351.3 14.4 650.0 22.37
A 350.0 9.3 649.7 22.86
A 352.4 15.5 649.1 23.16
A 350.0 14.1 649.8 22.90
23.51
24.07
24.82
24.74
30 5:: 2500 91.3 91.3 84.9 47.9 0.6 0.7 98.0 52.9 1.4 46.4 5.83 7.44 not 4.6 measured
15 2500 91.3 64.1 1.1 67.1 31.8 5.48 4.3
60 2500 91.3 42.1 0.7 49.2 50.1 6.14 4.6
a Note some of the solid products are soluble in THF. Hence the THFI fwt%I is always less than the solid product fwt%)
This does not indicate that limiting values had been obtained and higher water densities may result in greater yields. Finally, two other factors should be noted. First the acidity of the product water (pH =4.3-4.6) is high for BBC which has a sulphur content of 1.46 wt%, daf. This is attributed to H,S dissolved in the water. The SCW thus seems to be leaching out some of the inorganic material. Table 5 indicates that a high percentage of the ash is being extracted. the amount of which was determined by making an ash balance on the coal injected and the residue obtained after THF extraction. Table 6 shows a typical gas analysis obtained with the two types of coal used in this investigation. Carbon dioxide was the major gaseous product obtained with both coals. The amount of gases obtained with the brown coal was considerably higher than with the bituminous coal. A small quantity of methane was also formed in both cases. The table shows the range of gas compositions (in ~01%) and the range of
35.6 not measured
28.8 6.41
fwt%)
wt% of coal converted to gases, carbon dioxide and methane in various experiments. In addition to the runs reported in Tables 2 and 3 one run was carried out using a high-sodium lignite. At similar conditions only 2% of the coal was converted to gases and liquids, but experimental error may be present. No detailed analysis was conducted. CONCLUSION The object of the present study was not to develop design data, but to investigate the phenomenon of the behaviour of supercritical water extraction of coal and to determine the variables important in coal conversion. The results indicate that the water density must be near or above the ciritical density and that extraction times of 2 1 h may be needed for higher rank coals. However the extraction time may decrease as the water density increases. These results explain why Kershaw2 considered Table 4 Summary of batch extraction/reaction supercritical water
Procedure Water density (kg m-3) Glucose charged x 103
of glucose using
Run No. 1
Run No. 11
Run No. 12
A 311.2 25.2
B 339.9 29.8
B 68.1 29.7
647.8 NAb
649.5 NAb
(kg) Temperature (Kl 649.2 Water pressure prior to 22.48 injection (MPa) Pressure (water and 24.57 argon) (MPaj Reaction time (min) 17 Stirrer speed (rev min-tj 500 THFI. glucose fwt%) 100.0 1.6 THFI. solid product fwt%j Gaseous product (wt%) 5.1 Solid products fwt%) a Not measured Liquid (by diffj fwt%)
23.35
12.69
15 500 100.0 9.5 12.6 11.3
15 500 100.0 40.8 16.8 40.9
76.1
42.3
a Note some of the solid products are soluble in THF. Hence the THFI (wt%) is always less than the solid product fwt%) b NA, not applicable The ash in THFI and the pH of the water were not measured
FUEL, 1984, Vol 63, July
959
Coal extraction using supercritical water: G. V. Deshpande et al. Table 5 Ash extraction
15
“0
30 Reoctron
by water
Run No.
Density of sew (kg m-s)
Ash extracted
5 10 8
160 320 350
17.3 47.2 59.9
(wt%)
60
45 trme (mm)
Table 6 Typical
gas analysis (~01%) -___ Bruceton bituminous
Coal Ar co2 ‘3’4
I
I
15
I
I
30 Reoctron time
Coal to gas (wt%) Coal to CH, (wt%) Coal to CO, (wt%)
98.1 -99.0 0.80-l .3 0.15-0.56 0.60--l .1 0.03-0.14 0.58-0.94
coal
German brown coal 68.2-85.6 14.1-31.2 0.29-0.82 4.5-15.7 0.03-0.14 4.47-l 5.6
60
45 (mm)
solvent and, as these studies demonstrate, it can be quite effective.
Figure 3 Effect of reaction time on conversion of coal to gases and liquids and on percentage of THF insolubles. Bruceton coal (bituminous) was used. Starting density, 0.35 g/cc; stirrer speed, 2500 rev mint; temperature, 649.5 +0.8 K; pressure, 24.55 kO.43 MPa
ACKNOWLEDGEMENT
I
IOOC
This work was carried out under DOE Grant No. DEFG22-81PC40800 and this assistance is gratefully acknowledged. REFERENCES Gangoli, N. and Thodos, G. Ind. Eng. Chem. Prod. Res. Dev. 1977, 16,209 Jezko, J., Gray, D. and Kershaw, J. R. Fuel Process. Technol. 1982, 5,229
04to.10 ’
I
I
I
I
I
0.15
0.20
0.25
0.30
0.35
Oensrty of supercritical
I
phase (g/cc) 4
Figure 4 Effect of density of the supercritical phase on THF insolubles. German brown coal was used. Reaction time 15 min; temperature, 651.4 & 1.5 K; pressure, stirrer speed, 500 rev min-‘; 22.30 +0.52 MPa
Model], M. ‘Gasification and Liquefaction of Forest Products in Supercritical Water’, AIChE 89th National Meeting, Oregon, 1980 Model& M., Reid, R. C. and Amin, S. I. US Pate& Application 41 I3446 1978
5 6
Paul, P. F. and Wise, W. S. ‘The Principles of Gas Extraction’, Mills and Boon Ltd, London, 1971 Zhuse,T. P. Vestnik Akad. Nauk SSSR 1959,29(11),47; Petroleum 1960,23,298
water a weak solvent as he carried out experiments at 25% of the critical density. Another important conclusion is that the temperature time history of the coal drastically affects its conversion. By injecting ambient coal into SCW, the coal is heated in a stepwise fashion to 647K. By doing this, retrogressive reactions at intermediate temperatures may be prevented. At supercritical conditions reactive intermediates may be extracted before condensation reactions can occur. Finally, more work needs to be done, investigating the effects of particle size, coal to water ratios, temperature, density and reaction time. In addition, more comprehensive chemical analyses of the extract are needed. An attractive aspect of this work is that no hydrogen needs to be added in the coal conversion. Water is an economical
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FUEL, 1984, Vol 63, July
I 8 9 10
11 12
13
14 15 16 17
Whitehead, J. C. and Williams, D. F. J. Inst. Fuel 1975,48, 182 Bartle, K. D., Jones, D. W., Calimli, A.,Olcay, A. and Snape, C. E. Nature 1979,217,284 Bartle, K. D., Calimli, A., Jones, D. W., Matthews, S. R. and Olcay, A. Fuel 1979,58, 423 Tugrul, T. and Olcay, A. Fuel 1978,57,415 Ceylan, R. and Olcay, A. Fuel 1981,60, 197 Blessing, J. E. and Ross, D. S. ‘Supercritical Solvents and the Dissolution of Coal and Lignite’, Am. Chem. Sec. Symp. Ser. 1978, 71,171 Barton, P. ‘Supercritical Separation in Aqueous Coai Liquefaction with Impregnated Catalyst’, 184th National ACS meeting, Kansas City, 1982 Appell, H. R., Fu, Y. C., Friedman, S., Yovorsby, P. M. and Wender, I. Report of Investigations 7560, Bureau of Mines, 1971 Wender, I. Catal. Rev. Sci. Eng. 1976, 14,97 Raj, S. PhD Dissertation Pennsylvania State University, 1976 Given, P. H. and Dickinson, E. J. ‘Soil Chemistry’ Vol. 3, Marcel Dekker, New York, 1975