Energy & Fuels 1987,1, 529-534
the organic matter in Green River shale can be isolated as carboxylic acid salts when treatment times are limited to about 18 min. Conclusions are as follows: (1) 22 w t % of the organic matter in Green River shale exists in the form of carboxylic acid salts, and (2) the s a l h are possibly converted at longer treatment times to free carboxylic acids through a proton exchange in the aqueous media. The authors also suggest that the chemical nature of the car-
529
boxylic acid salts influences the recovery of these materials from Green River shale. Acknowledgment. The authors express thanks and appreciation to the US. Department of Energy for fundihg of this work under Cooperative Agreement No. De-FC2183FE60177. Registry No. CH,OH, 67-56-1.
Explosive Comminution of Bituminous Coal Using Steam? Thong Hang, Mahendra P. Mathur,* Nand K. Narain, Dennis N. Smith, and John A. Ruether Pittsburgh Energy Technology Center, United States Department of Energy, Pittsburgh, Pennsylvania 15236 Received June 11, 1987. Revised Manuscript Received August 17, 1987
The explosive comminution of Illinois No. 6 coal (Burning Star mine) was performed with steam in a batch reactor. Parametric studies were conducted to investigate the effects of key variables, such as temperature (347-506 "C),pressure (2400-5000 psig), coal slurry concentration (14%-75%), and heating rate, on both the particle size reduction and the desulfurization. Data on particle size reduction and SEM (scanning electron microscopy) micrographs provided evidence that the thermoplastic properties (softening, swelling, etc.) probably played a major role in explosive comminution. Coal was comminuted most effectively at temperatures in the plastic range. Volume mean particle size was typically reduced from 90 pm to the range of 15-40 pm by treatment; using lump coal feed resulted in an even finer product. Sulfur removal as H2S fell in the range 6%-15%, but it could not be determined if any organic sulfur was removed.
Introduction Explosive comminution provides an alternative method to the traditional crushing and grinding techniques for achieving size reduction. As far back as 1870, the method was used on processes that cooked fibrous materials under pressure and disintegrated the resulting pulp by allowing it to discharge at inte~alsinto a low-pressure region.' The method was later developed to be used as a unit operation in the process industries to pulverize a number of materials, such as wood pulp, cereals: glass, and fused quartz? The applicability of explosive shattering to various ores was studied extensively by U S . Bureau of Mines invest i g a t o r ~ . Pulverization ~ as a means of preparing coal for use in gasification or combustion could be brought about by rapid release of high-pressure (1500 psi) steam used to saturate the coal.5 Yellott and Singh demonstrated the feasibility of such a technique in a continuous process.6 It is estimated that two-thirds of the sulfur dioxide generated in the United States comes from power plants burning fossil fuels.' Reduction of sulfur content in coal prior to combustion is potentially important in controlling noxious emissions. This can be done by coal-washing techniques, which in general include a size-reduction step. The finer the coal is comminuted, the greater the extent of mineral matter separation theoretically possible. Thus, t Reference in this report to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply its endorsement or favoring by the US.Department of Energy.
0887-0624/87/2501-0529$01.50/0
there is currently an interest in investigating methods for producing very finely divided coal. Within the last decade, increased interest has been shown in explosive comminution as a coal preparation technique. Both mineral matter and sulfur content of coal can be reduced by use of explosive comminution. For example, workers at Consolidated Natural Gas are developing a process to produce superfine clean coal by pressuring and heating a coal/water slurry to supercritical conditions and rapidly expanding it through an orifice to atmospheric ~ o n d i t i o n s . ~Ash ~~ then could be separated from the hydrocarbonaceous particles by using separators such as hydrocyclones. The principle of explosive comminution rests on the fact that porous materials, when exposed to compressed fluids under high pressure, undergo enormous interhal stresses upon a sudden release of the restraining pressure. Stresses exceeding the tensile strength of the materials result in explosive shattering.1° Since the mineral matter in coal (1) Blackman, H. U.S. Patent 369836,1887. (2)Meigs, D. Chem. Metall. Eng. 1941,48,122-125. (3)Poulter, T.J. Phys. Rev. 1932,40,877-880. (4)Dean, R.S.;Gross, J. U.S. Bur. Min. Rep. Znuest. 1932,-US. Bur. Mines No. 3118;1933, No. 3201. (5) Godwin, F. W. "Coal Pulverization by Internal Explosion"; Report from the Armour Research Foundation to the Peabody Coal Co., August, 1939. ~. .. (6) Yellott, J. I.; Singh, A. D. Power Plant Eng. 1945,49,82-86.
(7)Ember, L. R. Chem. Eng. News 1981,59(37),20-31. (8)Massey, L.G.; Brabets, R. I.; Abel, W. A. U S . Patent 4313737,
1982.
(9)Massey, L.G.; George, D. A.;Brabets, R. I.; Abel, W. A. U.S. Patent 4 421 722,1983.
0 1987 American Chemical Society
Hang et al.
530 Energy & Fuels, Vol. 1, No. 6, 1987 Internal Pressure Transducer
. ,Tk"ocoup'c
1
5000
.- 4000
::
, , i 3000
a
3
2000
E
4050 mL Receiver
IO00
0
i Vacuum
F i g u r e 1. Explosive comminution batch reactor.
is relatively nonporous, the compressed fluids affect it less than the organic matter, i.e., the hydrocarbon fraction. This feature of discriminatory size reduction should aid in the development of an operation to separate the organic and inorganic matter, which is needed after comminution. Though still not fully understood, the sulfur removal by explosive comminution of coal using steam can be partially explained as follows. At temperatures above 380 and below 680 "C,pyrite reportedly reacts with steam and decomposes to ferrous sulfide and elemental sulfur, as in the following reactions:'l 3FeS2 + 2Hz0 3FeS + 2H2S + SOz 2HzS + SO2 2HzO + 3 s
--
Thus, the net reaction is 3FeSz
HZO
3FeS
+ 3s
Elemental sulfur may react with the hydrocarbon matrix of coal to form organic carbon-sulfur bonds, which may be further converted to hydrogen sulfideall Hydrogen sulfide released from coal can then be carried away with steam. The concept of pores described above would probably apply well to materials with a fixed, definable pore structure. Its applicability to coal, however, is restricted because coal is an amorphous substance. Indeed, the coal structure undergoes a drastic change at high temperatures above the so-called softening temperature at which the plastic range begins. In that region, swelling of coal, plasticity, and thermal decomposition take place, and together they alter the chemical and physical nature of coal significantly. Our preliminary tests revealed that explosive comminution seemed to give best results at temperatures in the plastic range, indicating the importance of thermoplastic properties of coal on the process. This work attempts to investigate those aspects by studying the key process parameters. Experimental Section Apparatus a n d Test Procedure. The test apparatus consisted of a heating and pressuring section (reactor) and a depressuring section (receiver) separated by a rupture disk. A schematic diagram of the apparatus is shown in Figure 1. The 70-mL reactor was constructed of type 316 stainless steel and heated by external electric resistance heaters. The heating elements were connected to a transformer whose voltage could be set to obtain desired heating rates. The rupture disks were of 316 stainless steel. The receiver was a 4050-mL vessel of type (10)Perry, R. H.; Chilton, C. H. Chemical Engineers' Handbook, 5th ed.; McGraw-Hill: New York, 1973; p 8-56. ( 1 1 ) Tsai, S. C. Fundamentak of Coal Beneficiation and Utilization; Elsevier: Amsterdam, 1982, 222-273.
0
2
4
6
8
10
12
14
1618
TIME, min
F i g u r e 2. Typical pressure-time profile for batch reactor. 304 stainless steel. Temperatures and pressure were measured by means of thermocouples and a pressure transducer, as shown in Figure 1. During heating to and including the rupture of the disk, data were automatically monitored and recorded every 5 9.
Experiments of coal comminution were carried out in the following manner. Known quantities of coal and water were charged to the reactor, and the reactor was sealed. Because the system was closed, the reactor pressure increased with temperature. The testing conditions, i.e., temperature and pressure a t which depressurization took place, were achieved by use of an appropriate rupture disk. By trial, thicknesses of rupture disks were determined that burst at the desired temperatures and pressures. The burst of the rupture disk imposed a very rapid (on the order of a few hundred microseconds) and large pressure drop in the reactor because it was then open to the attached receiver, which had been evacuated and maintained at about 180 "C. Upon depressurization, the feed coal was shattered and discharged into the receiver. Figure 2 presents a typical profile of pressure vs time in a coal comminution batch reactor experiment. The temperatures and pressures of reaction reported are the final values before the rupture disk burst. Samples a n d Analyses. In this study, explosive comminution of HvBb Illinois No. 6 coal from the Burning Star mine was investigated. The coal was air-dried at room temperature overnight, ground under nitrogen, and screened to collect the -60-mesh fraction. The volume mean particle size of the sample was about 170 mesh (-90 pm). Ultimate and proximate analyses of the feed coal are given in Table I. For an explosive comminution test, weighed amounts of the feed coal and water were charged to the reactor. After reaction, the receiver was opened and washed with water to minimize the loss of comminuted fine coal particles. The wet product was filtered, air-dried overnight, and subjected to analyses for determination of particle sizes and the extent of sulfur removal. Typical recovery of moisture-free coal after reaction was greater than 90%. Particle size analysis was performed by using two techniques: hand wet sieving and analysis with a Leeds & Northrup Microtrac particle size analyzer. The wet-sieving technique provided a simple and reliable procedure to fractionate a sample. Isopropyl alcohol was used to disperse agglomerates of fine coal particles. The dispersion was treated for 30 min in an ultrasonic bath before sieving. In this work, the fraction of particle sizes less than 25 pm was determined as an easily measured indicator of the extent of particle size reduction. Particle size distribution was measured on the whole coal sample by the Microtrac instrument. The Microtrac analyzer utilizes the phenomenon of low-angle, forward-scattering light from a laser beam to determine the size distribution of particles. Structural features of treated coal particles were observed by use of scanning electron microscopy (SEM). Coal analyses (proximate, ultimate, sulfur, and sulfur forms) were carried out by Huffman Laboratories, Wheat Ridge, CO. Although the gas phase was sampled for analysis, the readsorption of hydrogen sulfide on the freshly created coal surface, as discussed below, hindered the reliability of the analytical procedure. The sulfur-removal data presented
Explosive Comminution of Coal Table I. Proximate, Ultimate, and Ash Analyses of Illinois No. 6 Coal (Burning Star Mine) Proximate Analysis (wt W )As Received 8.85 moisture volatile matter 35.39 fixed carbon 45.17 ash 10.59 heating value, Btu/lb 11 173 Ultimate Analysis (wt W )Moisture Free H 4.85 C 69.22 N 1.55 0 (ind) 9.14 S 3.62 ash 11.62 sulfur forms pyritic 1.36 sulfate 0.24 organic 2.02 Fusibility of Ash ("C) initial deformation 1110 softening temp 1140 hemi temp 1160 fluid temp 1266 Major Elements in Ash (wt %) SiOz 47.49 A1203 18.90 13.30 TiOz 0.67 CaOz 6.02 MgOz 0.83 NazO 0.55 K2O 1.09 below are based on analyses of the feed and product coal. The batch reactor reproduced the testing conditions within 3.0% for temperature ("C) and 7.0% for pressure (psi). Reproducibility was obtained within 3.0% for the sieving procedure. Microtrac measured the particle sizes within 10% based on calibration with standard samples. Huffman Laboratories reported an average absolute error of 10.3% for C, H, N, S, and ash analyses. S u l f u r Forms. One of the objectives of this study was to investigate the possibility of removal of organic sulfur from coal as hydrogen sulfide via explosive comminution. Data on sulfur forms in both the feed and the product are required for that purpose. Sulfur in coal is commonly classified as inorganic, which includes pyrites and sulfate groups, and organic." ASTM methods were applied for measuring sulfur forms in the feed and product coals. Sulfate is determined by extraction with hydrochloric acid, pyrites are determined by extraction with dilute nitric acid following sulfate removal, and organic sulfur is determined by difference between total and inorganic sulfur. However, applying this procedure to coals that had been subjected to explosive comminution gave unreliable results. In most cases, treatment of the coal resulted in an apparent decrease in both the total and the inorganic sulfur and an increase in the organic sulfur. The same trend was obtained in an investigation of the airlwater oxydesulfurization of coal by Warzinski et a1.12 These workers concluded that the measurements of total sulfur were reliable but that determinations of organic sulfur by difference for treated coals were not. The method was thought to break down when applied to treated coals because of an unknown and irreproducible reaction of pyrites during the oxydesulfurization reaction. We believe the same situation applied in the present work. Conse(12) Warzinski, R. P.; Friedman, S.; Ruether, J. A.; Lacount, R. B. "Air/ Water Oxydesulfurization of Coal-Laboratory Investigation"; U.S.
DOE/PETC/TR-80/6; Pittsburgh Energy Technology Center: Pittsburgh, PA, 1980. (13)Loison, R.;Peytavy, A,; Boyer, A. F.; Grillot, R. In Chemistry of Coal Utilization;Lowry, H. H., Ed.; Wiley, New York, 1963;Suppl. Vol., Chapter 4. (14)Habermehl, D.; Orywal, F.; Beyer, H.-D. In Chemistry of Coal Utilization;Elliott, M. A., Ed.; Wiley, N e w York, 1981;2nd Suppl. Vol., Chapter 6.
Energy & Fuels, Vol. 1, No. 6,1987 531 TYPICAL DILATATION OF COAL AT ATMOSPHERIC PRESSURE
I", I kT, Softening temperature Tc Contraction temperature Te Maximum swelling temperature TR Resolidification temperature VC I n i t i a l contraction volume V, Maaimum swelling volume VR Volume change on resolidification
Figure 3. Typical dilatation of coal a t atmospheric pressure. quently, except in Table I1 where sulfur forms are given to demonstrate the problem, only results for total sulfur removal are presented. Tsai discusses reactions of organic and inorganic sulfur in coal in detail." The extent of sulfur reduction is determined as follows: % sulfur reduction = % S in feed coal - % S in product coal x 100% % S in feed coal where the sulfur percents of the feed and the product coal are on a moisture-free basis.
Results and Discussion Although t h e pore structure undoubtedly plays a crucial p a r t i n explosive comminution of coal, t h e thermoplastic properties, the hardness, and t h e distribution of mineral m a t t e r are believed to be significant factors also. I n this study, t h e effects of four parameters t h o u g h t t o be imp o r t a n t t o explosive comminution were tested: temperature, pressure, coal slurry concentration, a n d heating rate. I n addition, i t was experimentally verified that mineral matter particle size was largely unaffected by t h e process. A small n u m b e r of experiments were performed t o investigate t h e effects of feed coal particle size a n d use of carbon dioxide as t h e working fluid in explosive comminution. Effect of Temperature. A set of experiments was carried o u t at a b o u t 4000 psig with a coallwater weight ratio of 2:l. T h e results showing the effect of temperature o n particle size of the product coal, o n t h e total sulfur reduction, and o n t h e sulfur forms are presented in Table
11. The finest coal was produced around t h e t e m p e r a t u r e of 400 O C . T h e plastic properties of bituminous coals at elevated temperatures are well-known.13J4 After a n initial volume contraction, coal expands due to heating. W i t h a further increase in temperature, t h e swelling reaches its maximum value. As t h e resolidification temperature is exceeded, t h e plastic mass changes into t h e solid state of coke. Figure 3, reproduced from t h e work of K h a n a n d J e n k i n ~ , ' shows ~ a typical dilatometric s t u d y of t h e Pittsburgh seam HvAb coal, in which t h e volume changes (15) Khan, M.R.; Jenkins, R. G. Fuel 1984,63, 109-115.
Hang et al.
532 Energy & Fuels, Vol. 1, No. 6, 1987 Table 11. Effect of Temperature on Explosive Comminution of Illinois No. 6 Coal" wt % C 25 pm
expt no. PT6-20 PT6-21 PT6-22 PT6-23 PT6-24 PT6-26 PT6-25
temp, OC 347 380 390 398 439 458 506
(sieving) 67 73 79 82 78 74 .61
vol mean diameter, pm (Microtrac) 20 15 19 18 19 19 34
tot. s redcn, % 7.7 8.6 10.5 6.1 8.3 12.0 15.0
% S (moisture free)
pyritic 0.85 0.98 0.97 0.87 0.76 0.72 0.49
sulfate 0.29 0.29 0.28 0.28 0.11 0.12 0.10
organic 2.20 2.04 1.99 2.25 2.45 2.35 2.49
"Note: Pressure and coal/water weight ratio were kept at about 4000 psig and 2:1, respectively.
Figure 4. SEM micrographs for Illinois No. 6 (River King mine) coal comminuted at 374 "C and 3200 psig.
and the characteristic temperatures were measured at atmospheric pressure. In Figure 3, T,is the softening temperature and TRis the resolidification temperature. The plastic range is the region between T,and TR.For Illinois coals, the plastic phase is approximately in the range 350-450 "C, with the maximum swelling around 400 "C.16 At elevated pressures, a decrease in the dilatation was noted. It is known that the viscosity of coal initially decreases and then increases with rising temperature as coal is heated through the plastic region from T,to Tp Let toughness be defined in the sense used in metallurgy: a measure of the specific energy a stressed solid absorbs before mechanical failure. We suggest that the toughness of coal goes through a minimum in the plastic region and that the maximum particle size reduction due to explosive comminution occurs by operation at a temperture in the vicinity of that for minimum toughness. Furthermore, when coal is heated to the plastic state,the pore structure tends to c01lapse.l~This reduction in permeability would also tend to increase particle disintegration upon rapid depressurization in explosive comminution. With pores closed, gases trapped within the coal in the high-pressure state could only escape by breaking up the particle. Preliminary experiments with another Illinois No. 6 coal (River King mine) had indicated that the plastic properties of coal were important in explosive comminution. SEM photomicrographs of the coal explosively comminuted with steam at 374 "C and 3200 psig are shown in Figure 4. Vacuoles of two different sizes are evident. We believe that these were formed by escaping steam or autogenous gases and were "frozen" when the coal cooled after depressurization.17 Relative to the results shown in Table 11, the extent of comminution achieved with the coal shown in (16) Walters, J. G.; Ode, W. H.; Spinetti, L. Bull.-U.S. Bur. Mines 1963, No. 610, 41. (17) Howard, J. B. In Chemistry of Coal Utilization; Elliott, M. A., Ed.; Wiley, New York, 1981; 2nd Suppl. Vol., p 675.
I
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(Comminution at 400 OC and 4,000psi)
10 IO0 PARTICLE SIZE, microns
IO00
Figure 5. Particle size distribution of feed and product coals (Illinois No. 6).
Figure 4 was small. This is thought to be due to the use of lower temperature and pressure. Vacuoles are detectable in coals comminuted more extensively, such as those reported in Table 11, but they are less pronounced than those in Figure 4. The size distribution of the product coal resulting from run PT6-23 is compared with that of the feed coal in Figure 5. Figure 5 shows considerable size reduction of the feed coal by explosive comminution. The particle size distribution for the product coal apparently is log-normal, but the one for the feed coal is not, because the feed coal was screened to -60 mesh. The extent of sulfur reduction is not simply correlated with temperature of operation or degree of comminution achieved (see Table 11). In considering the effects of operating conditions on sulfur liberation, a factor that may complicate data interpretation should be mentioned. The sulfur is liberated largely, if not completely, as hydrogen sulfide. Some of this gas may readsorb on the freshly created coal surface following explosive comminution. If
Energy & Fuels, Vol. 1, No. 6,1987 533
Explosive Comminution of Coal Table 111. Effect of Pressure on Explosive Comminution of Illinois No. 6 Coal" vol mean wt % < 25 pm diameter, pm tot. S exut no. P.,. usie., (sieving) (Microtrac) redcn, % 46 46 6.1 PT6-11 2417 71 38 10.0 PT6-10 3035 PT6-22 3929 79 19 10.5 I
Note: Temperature and coal/water weight ratio were kept at about 390 " C and 2:1, respectively. a
Table V. Effect of Heating Rate on Explosive Comminution of Illinois No. 6 Coal" vol mean heating rate, w t % < 25 pm diameter, pm tot. S expt no. OC/min (sieving) (Microtrac) redcn, % PT6-17 4.5 78 18 10.2 PT6-21 12.5 73 15 8.6 Note: Temperature, pressure, and coal/water weight ratio were kept at about 380 "C, 4000 psig, and 2:1, respectively.
Table IV. Effect of Coal/Water Slurry Concentration on Explosive Comminution of Illinois No. 6 Coal" vol mean wt coal/ wt % < 25 pm diameter, pm tot. S (Microtrac) redcn, % (sieving) expt no. wt water 36 11.0 72 1:6 PT6-41 1:3 68 35 12.0 PT6-1 1:2 87 18 11.6 PT6-37 1:l 82 17 6.3 PT6-43 2:l 82 18 6.1 PT6-23 3:l 81 29 13.0 PT7-11 "Note: Temperature and pressure were kept at about 400 "C and 4000 psig, respectively.
this happens, the steps of sulfur liberation and readsorption could not be detected by analyses for total sulfur in the coal. Some workers believe a significant fraction of the liberated sulfur is readsorbed on the coal.18 Data given in Table I1 indicate an apparent decrease in the inorganic sulfur (pyrite sulfate) and an increase in the organic sulfur, as discussed above. Organic sulfur apparently increases with rising temperature. That is probably due to the effect of temperature on the reaction products of pyrites.12 Effect of Pressure. The effect of pressure on explosive comminution was tested in the range 2400-4000 psig. The temperature and the coallwater weight ratio were about 390 "C and 2:1, respectively. The results are listed in Table 111. The particle size of comminuted coal decreases with increasing reactor pressure (see Table 111). At constant temperature, one would expect the physical properties of coal, such as hardness and plasticity, to remain relatively unchanged, although an increase in pressure may decrease swelling, as noted earlier by Khan and Jenkins.16 Increasing pressure increases the concentration of supercritical fluid within the coal prior to depressurization and increases the specific internal energy of the fluid. Both effects would act to increase the amount of energy released per unit mass of coal upon depressurization. Therefore the increasing degree of comminution with increasing pressure is expected. The degree of sulfur removal also increased with pressure. Effect of Coal Slurry Concentration. In a series of six experiments conducted at 400 "C and 4000 psig, the coal slurry concentration was varied from 14% to 75%. The results are shown in Table IV. No clear trends are observable with respect to the extent of comminution achieved. While there is unexplained variation in extent of comminution with slurry concentration, the data indicate that particle size reduction is independent of slurry concentration at constant steam temperature and pressure. Similarly, the data for sulfur removal allow no conclusions to be drawn about a dependency on slurry concentration.
+
(18) Sresty, G. C.; Brabets, R. I. IIT Research Institute, Chicago, 1984, private communication.
-
Before Comminution Af?er Comminution
0
--
20 40 6 0 8 0 PARTICLE SIZE, microns
Figure 6. Particle size distribution of mineral matter in Illinois No. 6 coal.
Effect of Heating Rates. To investigate the effect of heating rates on the explosive comminution process, a number of tests were conducted in which heating rates were varied between 4.5 and 12.5 "Clmin. Results of two tests are shown in Table V. In both of these tests, the temperature, the pressure, and the coallwater weight ratio were kept approximately at 380 "C, 4000 psig, and 2:1, respectively. No effect of the heating rate was observed on the extent of comminution or sulfur removal. This observation is consistent with the findings of Khan and Jenkins,15who reported that at atmospheric pressure the swelling of coal is dependent on heating rates but for pressures above 400 psia no such dependence on heating rates was observed even if the rates increased from 25 to 65 "Clmin. Comminution of the Mineral Matter. The purpose of this experiment was to test the effect of explosive comminution on the particle size distribution of the mineral matter. Coal was subjected to comminution with steam at about 400 "C and 4000 psig. The coallwater weight ratio was 1:l. Mineral matter was obtained from the feed coal and the product coal by the ASTM high-temperature ashing procedure. The maximum temperature of the ashing procedure was 750 "C, which was substantially below the ash fusion temperature (see Table I). Thus, the ash particle size distribution could not be significantly affected by the high-temperature ashing. Particle size distributions of both the feed and the product mineral matter were determined by Microtrac and are given in Figure 6. The results show only a marginal change of the mineral particle sizes, confirming that the mineral matter is essentially unaffected by explosive comminution. Comminution of Lump Coal. The influence of particle size on the softening behavior of coal has been explored in detail elsewhere; the results for coal with different sizes varying from 0.075 to 0.6 mm, as well as those for lump coal, indicated a decrease in dilatation and an increase in viscosity of the plastic mass with decrease in particle size.14 That is, lump coal exhibited the most pronounced plastic properties. From the foregoing discussion relating coal properties to particle fracturing in explosive comminution,
534 Energy & Fuels, Vol. 1, No. 6, 1987 Table VI. Explosive Comminution of Illinois No. 6 Lump Coal"
wt % tot. s expt feed coal temp, P, < 25 pm redcn, no. size "C psig (sieving) % 394 4162 93 27.0 PT5-25 1 in. X 3 / 4 in. X 3 / 4 in. PT5-30 3 / 4 in. X 3 / 4 in. X 3 / 4 in. 397 4216 88 28.0 PT6-1 -60 mesh 386 4059 68 12.0 PT6-23 -60 mesh 398 4011 82 6.1 "Note: The coallwater weight ratio was 1:3 for PT5-25, PT5-30, and PT6-1 and 2:l for PT6-23. Table VII. Explosive Comminution of Illinois No. 6 Coal with C0.j'
wt % vol mean tot. S expt temp, P, < 25 pm diameter, pm no. psig (sieving) (Microtrac) redcn, '70 "C 33 2220 13 130 PT4-15 b PT4-21 69 1794 14 133 b PT4-22 76 2363 9 110 b PT4-23 133 2917 11 134 b 31 58 PT6-31 397 3359 14.0 PT6-33 391 5354 39 51 13.0 "Note: Volume mean particle size of the feed coal was about 160 pm in the PT4 series and 90 pm in the PT6 series. bNot determined.
one would expect better comminution with lump coal than with ground coal. Experimental results shown in Table VI confirm this prediction. Two tests were conducted with lump coal having particle diameters about 280 times larger than those used in the previous tests. As seen in Table VI, both particle size reduction and sulfur reduction were greater for the lump coal. Alternative Fluid. Currently a number of workers are investigating the processing of coal in the presence of supercritical water or organic liquids to increase liquid yields in p y r o l y s i ~ .Results ~ ~ ~ ~ are sometimes explained in terms of the properties of the carrier fluid in the vicinity of its critical point. The conditions of temperature and pressure used for the experiments reported in Tables I1 and VI are also in the vicinity of the critical point of water. I t was decided to test if this was fortuitous, or if the reduced temperature and reduced pressure of the working fluid were significant variables. The reduced temperature, T,, is defined as the ratio of the temperature to the critical temperature, TIT,,and the reduced pressure, P,, is similarly defined. Several comminution experiments were carried out with carbon dioxide as the working fluid. Carbon dioxide is capable of penetrating the coal structure extensively, which has led to its use for measuring the internal pore area of coals.21 It also has significantly lower critical temperature and pressure than water. By performing explosive comminution experiments with the two fluids at similar values of reduced temperature, it was possible to investigate if temperature influenced process results through its effect on the coal or on the working fluid. Volume mean particle size of the feed coal for experiments using carbon dioxide was 160 pm, somewhat larger than the feed for the experiments using water reported in Table 11. Experiments with carbon dioxide covered both (19) Bienkowski, P. R.; Narayan, R.; Greenhorn, R. A.; Chao, K. C. 2nd. Eng. Chem. Res., in press. (20) Towne, S. E.; Shah, Y. T.; Holder, G. D.; Deshpande, G. V.; Cronauer, D. C. Fuel 1986, 64, 883-889. (21) Fuller, E. L. In Coal Structure; Gorbaty, M. L., and Ouchi, K., Eds.; Advances in Chemistry 192; American Chemical Society: Washington, DC, 1979; pp 293-309.
Hang et al. Table VIII. Explosive Comminution of Illinois No. 6 Coal with Different Fluids wt%
