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Ind. Eng. Chem. Process Des. Dev. 1985,2 4 , 66-72
Coal Liquefaction Catalysis by Industrial Metallic Wastes Dlwakar Garg" and Edwin N. Glvens Corporate Resarch and Development Department, Air Prodocts and Chemicals, Inc., Allentown, Pennsylvania 18 105
Catalytic activity of industrial metallic wastes in coal liquefaction was examined in a lOO-lb/day continuous coal processing development unit. Red mud, a waste material from the aluminum industry, and an electric furnace flue dust containing nickel, molybdenum, cobalt, and iron showed a pronounced effect on the conversion of Eastern Kentucky Elkhorn No. 3 coal. Coal conversion and oil production increased significantly with the addition of red mud and flue dust. At 850 O F , oil yield increased from 20 to 26% with flue dust and from 20 to 34% with red mud; hydrocarbon gas yield and hydrogen consumption also increased. Significantly higher oil yield and lower gas production and hydrogen consumption were noted at a lower reaction temperature with red mud and flue dust, indicating the benefits of using milder reaction conditions. In the absence of coal, the solvent hydrogen content increased slightly with red mud and flue dust: however, it increased considerably in the presence of coal, indicating an interaction between coal and catalyst in coal liquefaction. Comparison of the catalytic activity of pyrite, red mud, and flue dust, based on selectivii analysis, showed that red mud was the most desirable disposable catalyst. However, if the primary goal of coal liquefaction is high oil production, irrespective of hydrogen consumption, pyrite is the most active catalyst among those discussed in the paper.
Introduction Several coal liquefaction processes are being examined by various companies to improve the overall efficiency of coal conversion. Many of these processes are similar in that coal is liquefied in the presence of hydrogen to produce oil and remove sulfur in the form of hydrogen sulfide. A two-stage coal liquefaction process is currently being designed by a joint venture of subsidiaries of Air Products and Chemicals, Inc., and Wheelabrator Frye Inc., sponsored by the U.S. Department of Energy. An improved overall utilization of hydrogen and a more flexible product slate are the main advantages of this two-stage liquefaction process. In this process, a coal/solvent mixture is liquefied in a dissolver in the presence of hydrogen gas in the first stage. The liquefaction product is distilled to recover the process solvent, which is recycled back to the front end of the process. Vacuum distillation bottoms containing liquefied coal, unreacted coal macer&, and ash are treated in a critical solvent deashing unit to separate the dissolved solid solvent-refined coal (SRC) from the residue. Solid SRC is further hydroprocessed in a catalytic step to yield high-value liquid product, which can be sold as a chemical feedstock or further processed to yield transportation fuel. Liquefaction in the first stage of the two-stage process occurs in the absence of any catalyst. The amount of solid SRC produced in the first stage or the amount to be further processed in the second stage can be altered by using a catalyst; this improves the overall hydrogen economy and it provides a flexible product slate. Use of a commercially available catalyst in a bed-type reactor in the first stage is costly because of severe catalyst deactivation associated with deposition of coke, ash, and metals on the catalyst surface. Inexpensive slurry catalysts are a means to circumvent this problem. Extensive research has been performed in the area of mineral catalysis in coal liquefaction (Wright and Severson, 1972; Mukerjee and Chowdhury, 1976; Hamrin, 1976; Granoff and Thomas, 1977;Tarrer et al., 1977; Guin et al., 1978,1979; Lee et al., 1978; Seitzer, 1978; Anderson, 1979; Gangwar and Prasad, 1979; Given, 1979; Granoff and Bacca, 1979; SRC Process, 1976; Curtis et al., 1981). It has been speculated that mineral matter catalyzes coal liquefaction by enhancing the transfer of hydrogen from the gas to the liquid phase; this maintains the hydrogen donor 0196-4305/85/1124-0066$01.50/0
capability of the process solvent. German researchers have reported that the addition of iron and iron compounds to the feed slurry improves the liquefaction of coal (Wu and Storch, 1968). The addition of pyrite has been shown to improve solvent hydrogenation and coal liquefaction reactions (Garg and Givens, 1982). It has also been concluded by several researchers that pyrite was by far the major, if not the only, mineral constitutent in coal to effectively catalyze coal conversion (Moroni and Fischer, 1980). The Bureau of Mines extensively tested the catalytic activity of iron and several other transition metals and their compounds in the liquefaction of U.S. coals (Weller et al., 1950; Pelipetz et al., 1948,1953; Schlesinger et al., 1962; Ginsberg, et al., 1960; Wright and Severson, 1972). Iron and iron compounds were shown to catalyze coal liquefaction significantly. In addition, transition metals, even at very low concentration, were shown to be active in coal conversion. Based on the available literature, pyrite seems to be a favorable disposable catalyst for use in U.S.coal liquefaction plants. It is found in most eastern US.coals, and it is readily available at coal beneficiation facilities. The major drawback of the pyrite catalyst system is the additional H2S generation. Transition metals can also be used in U.S. coal liquefaction plants, but their use will be limited because of their high cost and scarce supply. Like iron compounds,many other inexpensive industrial metallic wastes, such as red mud and electric furnace flue dust, are available in large quantities that can be used as disposable catalysts. Red mud was extensively used during World War I1 by the Germans to liquefy low-rank brown coal at extremely high pressures. However, little is known about the activity of red mud in the liquefaction of highrank bituminous coals at more reasonable pressures up to 3000 psig. In addition, the various industrial metallic wastes available in the U. S. offer an opportunity for commercial utilization and comparison of their activity with that of pyrite. Data on the catalytic activity of red mud and flue dust in the hydrogenation of process solvent and in coal liquefaction are discussed in this paper. The catalytic activity of industrial wastes in coal conversion is assessed by the product distribution, including hydrocarbon gas, oil, 0 1984 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985 67 FLARE
COAL
GASILIOUID SEPARATOR
PRODUCT LlOUlD
Figure 1. Coal liquefaction unit. Table I. Chemical Analysis of Elkhorn No. 3 Coal
Table 111. Analysis of Red Mud
wt%
wt%
Ultimate Analysis (as Received) 69.40 carbon hydrogen 4.88 oxygen 8.18 sulfur 1.94 nitrogen 1.00
A1203
Fe203 SiOz TiOz CaO NazO
Proximate Analysis (as Received) 37.56 volatile matter fixed carbon 46.03 ash 14.60 moisture 1.81
moisture
wt%
w t %, 550-850 OF
element
cut of heavy distillate
carbon hydrogen oxygen nitrogen sulfur
88.79 7.40 1.96 1.20 0.48
89.44 7.21 1.70 1.10 0.55
Solvent Separation oil asphaltene preasphaltene insoluble organic material (IOM)
90.8 8.9 0.4
93.8 5.0 0.4 0.8
0.0
Distribution of Hydrogen in the Oil Fractionb Haromatic WAR)
Htmnaylic (Ha) Hother (Ha)
3.24 2.26 2.22
9.3
Table IV. Analysis of Flue Dust
Table 11. Detailed Analysis of Process Solvents
fuel oil blend"
15.0 51.5 1.7 6.7 7.0 1.0
3.20 2.02 1.99
"Process solvent was used in the previous paper by Garg and Givens (1982). bDistribution of hydrogen was determined by proton NMR; oil-pentane solubles; asphaltene-pentane insoluble, benzene solubles; preasphaltene-benzene insolubles, pyridine solubles; IOM-pyridine insolubles; HAR= concentration of aromatic protons; Ha = concentration of a protons defined as protons on carbon atoms immediately adjacent to an aromatic ring; H, = concentration of @ and higher protons defined as those protons residing on two or more carbon atoms removed from an aromatic ring.
asphaltene, and preasphaltene yields, and the degree of coal conversion. The catalytic activity of industrial wastes for hydrogenation of solvent is measured for solvent alone as well as in the presence of coal. In addition, the catalytic activity of industrial wastes in coal liquefaction is compared to that of pyrite.
Experimental Section Materials. Kentucky Elkhorn No. 3 was a bituminous run-of-mine sample taken from a mine in Floyd County.
Ni Cr Mo co Fe C Mn
S SiOz cu Pb Sn A1 Ti
v
Mg moisture
10.98 5.13 0.61 7.66 20.32 3.00 0.52 0.50 5.36 1.01 0.57 0.01 2.46 0.30 0.05 3.00
0.01
The coal sample was ground to 95%-200 mesh particles, dried in air, and screened through a 150-mesh sieve before use. A detailed analysis of the screened coal is reported in Table I. The 550-850 O F cut of SRC-I1 heavy distillate supplied by the Pittsburgh and Midway Coal Mining Co. was used as a process solvent. As shown in Table 11, the solvent contained 93.8% pentane-soluble oil, 5.0% asphaltene, 0.4% preasphaltene, and 0.8% insoluble organic material. A red mud sample received from Kaiser Aluminum Co. was ground to -200 US.mesh size in the presence of liquid nitrogen. As shown in Table 111, the red mud contained approximately 52% iron in the form of iron oxide, which appeared to be the principal catalytic agent. Fine electric furnace flue dust containing nickel, molybdenum, cobalt, and iron was received from the Metal Recovery Division of Air Products. Table IV shows that the BET surface area of the waste materials was approximately 14 m2/g. The X-ray diffraction analysis of the waste materials is presented in Table V. Equipment. Process studies were done in a continuous 100 lb/day coal liquefaction unit equipped with a 1-L continuous stirred autoclave. A detailed description of the
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985
Table V. X-ray Diffraction Analysis of the Red Mud and Flue Dust before and after the Coal Liquefaction Reaction cat. phase orig material after reaction red mud major Fe203 FeaO4 minor quartz, CaCO,, A1203 quartz, CaCO,, A1203, Fez03, FeS flue dust major Fea04, NiFe204, FeCrz04 minor FeS, ZnS
Fea04, NiFe204, FeCr20, FeS, ZnS
coal liquefaction unit is given in Figure 1. The use of a stirred tank reactor ensured that solvent vaporization matched that of an actual SRC-I dissolver and that coal minerals did not accumulate. Since there was no slurry preheater, all of the sensible heat had to be provided by resistance heaters on the reactor. Because of this high heat flux,the reactor wall was about 27 OF hotter than the bulk slurry. Multiple thermocouples revealed that the slurry temperature inside the reactor varied by only 9 O F from top to bottom. The products were quenched to 320 O F before flowing to a gas/liquid separator that was operated at system pressure. The slurry was throttled into the product receiver while the product gases were cooled to recover the product water and organic condensate. The product gases were then analyzed by an on-line gas chromatograph. Procedure. Coal liquefaction runs were performed at 825 and 850 OF, 2000 psig hydrogen pressure, 1000 rpm stirrer speed, a hydrogen feed rate equivalent to 5.5 wt % of the coal, and a superficial slurry space velocity of 1.5 h-l. The coal concentration in the feed was 30 wt % . The concentration of industrial wastes used was 10 wt % of feed slurry. At least 10 reactor volumes of the product were discarded before samples were collected. A complete set of samples consisted of one 8-02 product slurry, one l-L product slurry (asback-up), a light condensate sample, and a product gas sample. The product slurry from the continuous reactor was solvent-separated into four fractions: (1) pentane-soluble material (oil), (2) pentane-insoluble and benzene-soluble material (asphaltene), (3) benzene-insoluble and pyridine-soluble material (preasphaltene), and (4) pyridineinsoluble material. The latter contained insoluble organic material (IOM) and mineral residue. The overall coal conversion was calculated as the fraction of organic material (moisture-ash-free coal) soluble in pyridine.
Results and Discussion In a previous paper, the liquefaction of Eastern Kentucky Elkhorn No. 3 coal in a process solvent [550-850 OF cut of SRC-I1 fuel oil blend (FOB)] yielded a lower conversion than typical Western Kentucky coals (Garg and Givens, 1982);this is in agreement with the work reported earlier in the literature (Granoff and Thomas, 1977). Another process solvent used in this study, the 550-850 O F cut of SRC-I1 heavy distillate (HD), yielded a slightly higher coal conversion than FOB, as shown in Table VI. The elemental analysis of the two solvents, summarized in Table 11, showed that HD contained slightly lower hydrogen, oxygen, and nitrogen than FOB. Solvent separation of the two solvents showed that HD contained more oils and less asphaltenes compared with FOB. Finally, the distribution of hydrogen in the oil fraction showed similar concentrations of H A R in both solvents, but considerably lower Ha and H, contents in HD than FOB. The above data indicate that the solvent quality determined by the s u m of Ha and H, values is significantly lower for HD than FOB.
Table Vi. Elkhorn No. 3 Coal Liquefaction (Feed Composition, 70% Solvent 30% Coal) solvent fuel oil heavy blend" distillate temperature, OF a50 a50 2000 2000 pressure, psig residence time, min 38 38 19.9 18.6 hydrogen treat rate, MSCF/T production distribution, wt % MAF coal HC 4.2 6.8 co, coz 1.0 1.0 0.2 1.3 H2S oil 27.3 20.4 14.8 29.2 asphaltene 25.4 preasphaltene 30.1 18.1 15.8 IOM 1.2 3.2 water conversion, wt % MAF 81.9 84.2 1.4 hydrogen consumption, wt % MAF 0.9
+
oil hydrogen content, wt % start finish SRC sulfur, % selectivity, oil/Hz consumption
7.7 7.5 0.6 19.5
7.2 7.3 0.5 22.7
aData are taken from the paper by Garg and Givens (1982).
In contrast to higher coal conversion, lower oil production and higher hydrocarbon gas yield were noted with HD than with FOB (Table VI). The hydrogen consumption was lower with HD, resulting in slightly higher selectivity for oils. The improvements in IOM, SRC sulfur content, and selectivity would undoubtedly favor the use of HD as process solvent; however, oil yield was considerably lower with HD. The use of FOB, on the other hand, would be favorable if the primary goal of coal liquefaction is high oil yield, irrespective of hydrogen consumption. The difference in the production of H2S could have been due to the use of the sample bomb with HD rather than on-line GC as with FOB. The H2S gas could have reacted either with the walls of the bomb or with other compounds in the gas sample with time. These reactions would eventually lower the concentration of H2S determined by GC. The hydrogen content of oils generated with HD increased, whereas it decreased with FOB. This observation appears to indicate that the solvent generated by coal liquefaction are approaching a common hydrogen concentration value. Starting process solvents of different origin and composition have been known to influence coal liquefaction. Solvents having high hydrogen donor capability (solvent quality) have been reported in the literature to improve coal liquefaction. Furthermore, the addition of high boiling reaction products like asphaltenes to the recycle solvent has been shown to give higher liquid yield and improved product selectivity and operating stability (Ansell et al., 1980; Curtis et al., 1981). Because of the importance of starting process solvents in coal liquefaction, the properties of FOB and HD were compared to ascertain the differences between their liquefaction performance. Lower concentration of asphaltenes coupled with lower solvent quality of HD appear to be the main reasons for its inferior performance compared with FOB. However, more work is required to further quantify the differences in the performance of the two solvents. Since the objective of this paper is to evaluate the activity of various industrial metallic wastes, no attempts are made to further elaborate upon the differences in the performance of the solvents. The coal liquefaction data obtained with HD, as shown in Table VI, were therefore used as a base case in this paper. The effect of adding red mud and electric furnace flue dust
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985
69
Table VII. Liquefaction of Elkhorn No. 3 Coal in the Presence and Absence of Red Mud and Flue Dust feed compn 60% solv + 30% coal 60% solv + 30% coal 70% solv 10% red mud + 10% flue dust 30% coal temperature, O F 850 825 850 825 850 pressure, psig 2000 2000 2000 2000 2000 39 residence time, min 39 37 38 38 24.1 24.8 22.8 23.5 hydrogen treat rate, MSCF/T 18.6 product distribution, wt % MAF coal 5.3 8.7 4.5 9.2 HC 6.8
+
co, cop
+
1.0 0.2 20.4 29.2 25.4 15.8 1.2
1.3 0.0 35.9 10.6 28.4 16.6 1.9
1.7 0.0 33.6 18.5 22.1 12.8 2.6
1.3 0.0 30.9 26.3 19.6 14.2 3.2
1.7 0.0 25.5 33.7 18.8 7.1 4.0
84.2 0.9
83.4 1.1
87.2 1.9
85.8 1.4
92.9 2.2
oil hydrogen content, wt % start finish
7.2 7.3
7.2 7.7
7.2 7.5
7.2 7.1
7.2 7.7
SRC sulfur, %
0.5
0.6
0.5
0.6
0.5
22.7
32.6
17.7
22.1
11.6
H2S oil asphaltene preasphaltene IOM water
conversion, wt % MAF hydrogen consumption, wt % MAF
selectivity, oil/H, consumption
on coal liquefaction was evaluated using the same coal and HD as process solvent. Comparisons were made to the noncatalyzed coal case, as shown in Table VII. Liquefaction Catalysis by Red Mud. The catalytic activity of red mud, which was measured a t 850 O F , increased coal conversion from 84 to 87% (Table VII) and altered product yield distribution. An increased hydrocarbon gas yield resulted from an increase in cracking of the process solvent. The solubility fractions changed such that oil yield increased significantly from 20 to 34% and the asphaltene and preasphaltene yields decreased from 29 to 19% and from 25 to 22%, respectively. Clearly, red mud is active in converting both the asphaltenes and preasphaltenes to lower molecular weight products. In the absence of red mud, the hydrogen content of the oil fraction increased only slightly during liquefaction, whereas a much greater increase in hydrogen content was noted with red mud (Table VII). The increase in yields of hydrocarbon gases, oils, and water sharply increased hydrogen consumption from 0.9 to 1.9%. Addition of red mud did not change the sulfur contents of the SRC (Table VII) or the other fractions (Table VIII). However, the Ha that was generated was scrubbed out by the iron in the catalyst (Table VII). Carbon and nitrogen contents of various fractions did not change significantly with the addition of red mud. Reducing temperature from 850 to 825 OF significantly influenced catalyst selectivity. The production of hydrocarbon gases decreased from 8.7 to 5.3%, which reduced overall hydrogen consumption from 1.9 to 1.1%. Surprisingly, oil production increased slightly with decreasing temperature (Table VII). Coal conversion, however, dropped from 87 to 83%. The selectivity of oils yield over hydrogen consumption showed a significnat improvements; it increased from 18 to 33. The hydrogen content of the generated solvent also increased from 7.5 to 7.7% as temperature dropped, which is normally observed at lower reaction temperatures. The X-ray diffraction analysis of liquefaction residue showed partial conversion of Fe203to Fe304and FeS (Table V). No elemental iron was detected, probably because all the elemental iron that would have been produced during the reaction was sulfided by the H2S generated upon desulfurization of the coal.
Table VIII. Distribution of Elements in Various Liquefaction Reaction Fractions in the Presence and Absence of Red Mud and Flue Dust catalyst none redmud fluedust temperature, OF 850 850 850 oil fraction, wt % C 89.7 89.3 89.1 H 7.3 7.5 7.7 0" 1.7 1.7 1.7 N 0.7 0.9 1.0 S 0.6 0.6 0.6 asphaltene fraction, wt % C 86.1 85.5 85.9 H 6.1 6.2 6.2
0" N S
preasphaltene fraction, wt % C H 0" N S
4.9 2.4 0.5
5.6 2.3 0.4
5.2 2.3 0.4
86.2 5.1 5.9 2.5 0.5
85.4 5.0 6.3 2.8 0.5
85.4 5.3 6.2 2.6 0.6
Oxygen is determined by difference.
In summary, red mud addition at 850 O F increased the yield of oils and hydrocarbon gases at the expense of a sizable increase in hydrogen consumption. To utilize the catalytic activity of red mud, hydrogen consumption must be kept at its lowest possible level. Solvent Hydrogenation. Solvent hydrogenation in the absence of coal was conducted at conditions equivalent to the coal liquefaction experiments. In the absence of coal and any catalyst, only minor changes were observed. The hydrogen content of the solvent oil fraction increased slightly as hydrogen consumption increased to 0.2 wt 9'0 (Tables IX and X). The distribution of hydrogen in the product showed that aromatic hydrogen content (HAR) decreased slightly with a slight increase in He. Overall, the combined concentration of He and H,increased marginally (Table X). Only minor changes in the distribution of the different solubility classes were observed (Table IX). In particular, the concentration of asphaltenes decreased only slightly. The production of hydrocarbon gases resulting
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Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985
Table IX. Solvent Hydrogenation in the Presence and Absence of Red Mud and Flue Dust feed compn orig 100% 90% solv + 90% solv + solv solv 10% red mud 10% flue dust temperature, OF 850 850 850 pressure, psig - 2000 2000 2000 1.27 hydrogen treat 1.08 1.31 rate, wt % solvent reaction time, min 38 39 38 product distribution, wt 70 HC 0.9 1.0 0.7 oil 93.8 93.4 95.1 96.8 5.0 3.4 1.2 1.6 asphaltene 0.8 0.6 0.4 1.0 preasphaltene IOM 0.8 1.0 0.4 0.9 0.5 water 0.4 0.3 0.2 hydrogen 0.3 0.2 consumption, wt '70 solvent Table X. Elemental Analysis of Oil Fraction in Solvent Hydrogenation Reaction feed c o m m orig 100% 90% solv + 90% solv + solv solv 10% red mud 10% flue dust temperature, O F 850 850 850 carbon 89.7 89.5 89.6 89.5 hydrogen 7.2 7.3 7.4 7.4 1.6 1.5 1.5 oxygen (direct) 1.4 nitrogen 1.1 0.9 0.9 1.0 sulfur 0.7 0.6 0.6 0.6 HAR
Ha Ho
Distribution of Protons, % 44.4 42.9 44.9 28.0 29.2 27.4 27.6 27.9 27.7
44.4 29.0 26.6
from thermal cracking amounted to only 0.9 wt ?& of the solvent. In the presence of red mud, the process solvent was hydrogenated (Table IX), but still to only a very small extent. Hydrocarbon gas production by catalytic hydrogenation was similar to that noted with thermal hydrogenation of the solvent. The concentration of lower molecular weight oils increased marginally from 93.8 to 95.170 with addition of red mud, and asphaltene concentration decreased from 5 to 1.6% (Table 1x1. Overall, catalysis did not appreciably change the hydrogen consumption or the nitrogen, oxygen, and sulfur contents in the solvent. Hydrogen consumption, which was due to hydrocarbon gas formation, solvent hydrogenation, and water production, was unaffected by catalyst. The distribution of protons was also very similar (Table X). Therefore, under typical coal liquefaction conditions, process solvent did not hydrogenate to any appreciable degree regardless of the presence or absence of catalyst. The production of hydrocarbon gases was also similar and represented only a small fraction of the process solvent. Liquefaction Catalysis by Electric Furance Flue Dust. With the addition of a flue dust containing nickel, molybdenum, cobalt, and iron coal at 850 OF, conversion increased significantlyfrom 84 to 93% (Table VI). As with red mud, the oil yield increased from 20 to 26% and the preasphaltenes decreased from 25 to 19%. Similarly, the addition of flue dust increased the yields of hydrocarbon gases and water. Like red mud, flue dust increased hydrogen consumption from 0.9 to 2.2 % , showing that the increase in oil yield was achieved at the expense of valuable hydrogen. The hydrogen content of the oil fraction increased with flue dust. Once again, the increase in hy-
Table XI. X-ray Diffraction Analysis of Red Mud and Flue Dust before and after the Solvent Hydrogenation Reaction cat. phase orig material after reaction red mud major Fez03 minor quartz, CaC03, A1203
Fe304 CaC03, quartz, Fe, FeS, Fe203,Alz03
flue dust major Fe304,NiFez04,FeCrZO4 Fe30,, NiFeZO4, FeCrzO, minor FeS, ZnS FeS, ZnS
drogen content of the solvent was much greater in the presence of coal and flue dust than with either of them alone. Flue dust addition resulted in no change in SRC sulfur contents and various other fractions (Tables VI and VIII). Again, all the H2Sgenerated by desulfurization of coal was scrubbed out by the flue dust. No major differences in the carbon and nitrogen contents of the various fractions were noted. X-ray diffraction analysis, given in Table V, showed no change in the chemical form of flue dust before and after the reaction. The selectivity for oils make over hydrogen consumption with flue dust was much lower than that observed in the absence of the additive (Table VI). To utilize the catalytic activity of flue dust, the reaction temperature was reduced from 850 to 825 OF. Reducing the temperature decreased coal conversion from 93 to 86% and increased oil production (Table VI) while decreasing hydrogen consumption from 2.2 to 1.4%. The increase in oil production and the decrease in hydrogen consumption resulted in an increase in selectivity from 12 to 22, which was comparable to the selectivityobserved at 850 O F in the no-additive run (Table VI). The generated solvent oil hydrogen content remained constant at 7.7%. Finally, the selective behavior of higher oil production, lower hydrogen consumption, and lower hydrocarbon gas production at the lower temperature makes flue dust a highly promising catalyst candidate. Solvent Hydrogenation. In the presence of flue dust, solvent hydrogenation was quite similar to that with red mud (Table IX). The hydrocarbon gas make was the same as in the solvent case without catalyst, demonstrating that these particular catalysts have no increased cracking function over the thermal case alone. As with red mud, the change in solvent hydrogen in the absence of coal was quite small compared with the result observed when coal was present. Clearly, coal has a dramatic effect in the hydrogenation step. The spent catalytic agents were separated by filtration from the solvent hydrogenation reaction and analyzed by X-ray diffraction. No changes were noted in the chemical form of flue dust, whereas part of the Fe203present in the red mud was converted to Fe, Fe304,and FeS (Table XI). Although this transformation should increase water production and hydrogen consumption, the data in Table IX were not sufficiently sensitive to detect such an increase. Activity Comparison. A comparison of red mud, flue dust, and pyrite at 850 O F is presented in Table XII. Data on pyrite catalysis at 10% concentration are taken from the work of Garg and Givens (1982). Again, SRC-I1 fuel oil blend was used in generating the data on pyrite catalysis. The change in process solvent from FOB to HD may change the catalytic activity of pyrite, but it is considered to be small in the activity comparison. Flue dust addition results in the highest coal conversion and hydrocarbon gas production; it also results in the highest conversion of preasphaltenes to lower molecular weight compounds, as well as in the lowest oil production and asphaltene conversion. SRC sulfur content is the same with red mud and
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 1, 1985
Table XII. Comparison of Catalytic Activity of Various Catalysts cat. pyrite" r e d mud flue d u s t ~
catalyst concn, wt % slurry p r o d u c t i o n distribution, wt % MAF coal
HC oil asphaltene preasphaltene conversion H2consumption, wt % MAF coal SRC sulfur, % selectivity, oil/H, consumption
~
~
~~~
10.0
10.0
10.0
5.3 41.0 11.3 24.1 89.9 2.5* 0.7 16.4
8.7 33.6 18.5 22.1 87.2 1.9 0.5 17.7
9.2 25.5 33.7 18.8 92.9 2.2 0.5 11.6
" D a t a are t a k e n f r o m t h e paper by Garg a n d Givens (1982). reducing FeS, t o FeS.
* H y d r o g e n consumption includes t h e hydrogen required f o r
flue dust, but is higher with pyrite. Hydrogen consumption, however, is the highest with pyrite. The catalytic activity of pyrite, red mud, and flue dust is further compared in terms of the desirable and undesirable elements in the coal liquefaction reaction. The desirable elements are high coal conversion and high oils production, while the most undesirable behavior is high hydrogen consumption. These functions are best evaluated in terms of selectivity of oil production to hydrogen demand. Based on this selectivity analysis, red mud is the most desirable catalyst. However, if the primary goal of coal liquefaction is high oil yield, irrespective of hydrogen consumption, pyrite will be the most desirable among those discussed in this paper. Reaction Mechanisms. Unquestionably, liquefaction can be catalyzed by any number of materials (Garg and Givens, 1982). Within the normally accepted liquefaction scheme, in which hydrogen is first added to the solvent range material and then transferred to the reacting coal moieties, a catalyzed step would result in improved oil yield and incorporation of hydrogen in the solvent. Typically, the rate acceleration will occur in the rate-limiting step if the reaction sequence is sufficiently straightforward and simple. In a coal system in which the reactions are overwhelmingly complex, such an application of fundamentals may not be so straightforward, especially where two or more of the steps may have reasonably close values for the rate constants. In the generally accepted reaction scheme of hydrogen entering the reaction through a solvent hydrogenation step, this step is most likely the slowest since considerable data reported in the literature point to the rapid transfer of hydrogen from solvent to coal. It is well-known that solvent can be depleted of hydrogen quite readily, suggesting that the relative rate of the hydrogen transfer from solvent to coal moieties is considerably higher than the transfer of hydrogen from gas to solvent. These two steps now appear to be interconnected; that is, the presence of coal has an effect on the reaction rate of the first step, the solvent hydrogenation reaction. Furthermore, the catalysis of solvent hydrogenation and the involvement of coal in solvent hydrogenation definitely demonstrate a combined role of catalyst and coal in the direct liquefaction step.
Conclusions Addition of red mud and flue dust affects solvent hydrogenation. Under the reaction conditions employed here, improved solvent hydrogenation was noted with red mud and flue dust compared with thermal noncatalytic hydrogenation of solvent. Addition of red mud and flue dust increases the concentration of oil and decreases that of asphaltene over the noncatalyzed reaction. Red mud and
71
flue dust addition also catalyzes the coal liquefaction reaction. These catalysts improve coal conversion, increase oil and gas production, and increase hydrogen consumption. The increase in solvent hydrogen content with red mud and flue dust is higher in the presence of coal than in its absence, indicating an interaction between coal and catalyst. Both red mud and flue dust result in higher oil production and lower hydrogen consumption and hydrocarbon gas production at reduced reaction temperature. In addition, the selectivity of both the red mud and the flue dust increase with decreasing reaction temperature. Therefore, the proper selection of the reaction conditions is very important to selectively utilize the catalytic activity of red mud and flue dust in coal liquefaction. Finally, based on selectivity analysis, red mud is shown to be a more desirable catalyst for coal liquefaction. However, if the primary goal of coal liquefaction is high oil production, irrespective of hydrogen consumption, pyrite is the most active catalyst among those discussed in the paper.
Acknowledgment The authors wish to thank Kaiser Aluminum Company and Dave Taschler of Air Products for providing samples of red mud and flue dust. The authors also wish to express their appreciation to Richard Hamilton of Air Products for performing the X-ray diffraction analyses, and to Marianne Phillips of International Coal Refining Company for her help in editing the manuscript. The support of the U.S. Department of Energy and Air Products is also gratefully acknowledged. Registry No. A1,03, 1344-28-1; pyrite, 1309-36-0.
Literature Cited Ansell, L. L.; Trachte, K. L.; Taunton, J. W. EPRI Contractors Meeting, Palo Atto, CA. May 1980. Anderson, R. P. "Low Cost Additives in the SRC Processes"; Presented at the Department of Energy Project Review Meeting on Disposable Catalyst In Coal Liquefaction held at Albuquerque, NM, June 1979. Curtls, C. W.; Guin, J. A.; Jeng, J. F.; Tarrier, A. R. Fuel 1981, 60, 667. Gangwar, T. E.; Prasad, H. Fuel 1979, 58, 577. Garg, D.; Givens, E. N. Ind. Eng. Chem. Process Des. Dev. 1982. 27, 113. Ginsberg, H. H.; Friedman, S.; Lewis, P. S.; Schlesinger. M. D.; Stewart, A. J.; Hiteshue, R. W. "Hydrogenating Coal in a Pilot Plant with a Molybdenum Catalyst"; Rep. Invest. U S .Bur. Mlnes No. 5673 1980. Ginsberg, H. H.; Lewis, P. S.; Anderson, R. B.; Hiteshue. R. W. "Producing Heavy Fuel Oil by Hydrogenating Bituminous Coal"; Rep. Invest. U . S . Bur. Mines No. 5674 1980. Given, P. H. "Catalysis of Liquefaction by Iron Sulfuide from Coals, With Some Thoughts on Coal Mineral Analysis"; Presented at the Department of Energy Project Review Meeting on Disposable Catalysts in Coal Liquefaction held in Albuquerque, NM, June 1979. Granoff, B., Thomas, M. G. "Mineral Matter Effects in Coal Liquefaction: Autoclave Screening Study"; Presented at ACS Division of Fuel Chemistry Meeting, Chicago, Sept 1977. Granoff, B.; Baca, P. M. "Mineral Matter Effects and Catalyst Characterization in Coal Liquefaction"; Annual Report (Oct 1977-Sept 1978), Sandia Laboratories (SAND-79-0505), Albuquerque, NM, April 1979. Guin, J. A.; Tarrer, A. R.; Prather, J. W.; Johnson, D. R.; Lee, J. M. Ind. Eng. Chem. Process Des. Dev. 1978, 17, 118. Guin, J. A.; Tarrer, A. R.; Lee, J. M.; Lo, L.; Curtis, C. W. Ind. Eng. Chem. Process Des. Dev. 1979, 78, 371. Hamrin, C. E., Jr. "Catalytic Activity of Coal Mineral Matter"; Interim Report for the Period April-Sept, 1976; Prepared for ERDA by University of Kentucky (FE-2233-2). Lee, J. M.: VanBrackle, H. F.; Lo, Y. L.; Tarrer, A. R.;Guin, J. A. "Catalytic Actlvity of Coal Minerals in Coal Liquefaction"; Presented at the 84th National AIChE Meeting, Atlanta, Feb 1978. Moroni, E. C.; Fischer. R. H. "Disposable Catalysts for Coal Liquefaction"; Presented at the 179th National Meeting of the American Chemical Society, Houston, March 1980. Mukherjee, D. K.; Chowdhury, P. B. Fuel 1978, 55, 4. Pelipetz, M. G.; Kuhn. E. M.; Friedman, S.; Storch, H. H. Ind. Eng. Chem. 1948, 40(7), 1259. Schlesinger, M. D.; Frank, L. V.; Hiteshue. R. W. "Relative Activity of Impregnated and Mixed Molybdenum Catalysts for Coal Hydrogenation"; Rep. Invest. U S . Bur. Mlnes No. 6021 1962. Seitzer, W. H. "Miscellaneous Autoclave Liquefaction Studies"; Final Report Prepared for EPRI AF-612 (RP-779-7), Feb 1978. Solvent Refined Coal (SRC) Process, Quarterly Technical Progress Report FE1496-155 Prepared for US. Department of Energy by the Pittsburgh and Mdway Coal Mining Co., Shawnee Mission, KS. March 1979.
Ind. Eng. Chem. Process Des. Dev. 1985, 2 4 , 72-77
72
Tarrer, A. R.; Guin, J. A.; Pitts, W. s.; Henley, J. P.; Prather, J. W.; Styles, G. A. "Effect of Coal Minerals on Reaction Rates During Coal Liquefaction"; PreDrints. "Liauid Fuel from Coal": Academic Press: New York. 1977. Weller; S.;Pelipetr, M. D.; Friedman, 5.; Storch, H. H. Ind. and€ng.'Chem. 1950. 42(21. 330. Wright, 6. H:;'Severson, D. E. Am. Chem. SOC. Div. Fuel Chem. Prepr. 1972, 16(2),68.
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Received for review March 2, 1983 Revised manuscript received January 16, 1984 Accepted March 12, 1984
Improving the Action of Sulfur Sorbents in the Fluidized-Bed Combustion of Coal Dlck Schmal Division of Technology for Society TNO, Department of Chemisw, 2600 AE Delft, The Netherlands
As part of an investigation into the fluidized-bed combustion of coal and its environmental aspects, TNO is studying the sorption of sulfur dioxide as a combustion product. Special attention is being paid to reactivation of fluidiid-bed ash by treatment with water or water vapor, followed by heating to fluidized-bed temperatures. A laboratory research program was started on sulfation and reactivation of sorbents on their own and of sorbent-containing ashes from TNO's 4 MW atmospheric fluidized-bed boiler (AFBB). The experiments show that reactivation can be a versatile method for improving the efficiency of the sorbents.
Introduction In the combustion of coal, sulfur dioxide is formed as a major air pollutant. Its adverse effects on the environment and on human health, and the continuing replacement of oil as a fuel by coal, have prompted a great deal of research into economical methods of removing it from the combustion process. In fluidized-bed combustion, sulfur dioxide is removed by adding sorbents, such as limestone or dolomite, to the bed. Normally, however, only some tens of percent of the calcium oxide added are converted into calcium sulfate, and so methods of improving its sorption efficiency are being widely studied. One method involves pyrolytic regeneration of the sorbent, the sulfur dioxide that evolves being concentrated and converted into sulfur or sulfuric acid (see, e.g., Montagna et al., 1977; Yang and Shen, 1979). This method has the disadvantage of being chemically complex and being applicable only a few times on the same limestone particles. Another method is to add salts (e.g., sodium or calcium chloride) which improve the sorption power of calcium oxide (see, e.g., Shearer et al., 1977; Van Houte et al., 1978). Although the results of this method are encouraging, it has not been employed so far because suitable salts are strongly corrosive to the metals of the boiler. Promising results using iron oxide coated dolomite particles were recently published (Desal and Yang, 1983). It has been shown, through a model, that for 90% sulfur retention a 40% reduction of the sorbent requirement can be achieved over the noncatalyzed case. For limestone, however, the iron oxide coating had an inhibiting effect. The starting point of our investigations was a method proposed by the Argonne National Laboratory (Shearer et al., 1980),i.e., reactivation of partially sulfated limestone by hydration. I t has the advantage of being relatively simple and cheap: apart from water, no other chemicals are needed. The aim of our investigation has been to attain maximum sorption of sulfur dioxide with a minimum of sorbent. This article is a condensed version of a report on the same subject (Schmal, 1982). Chemical Processes This section deals with the chemical processes occurring in the calcination of limestone, the sorption of sulfur dioxide by calcium oxide, and the reactivation by water and
Table I. Equilibrium Pressures of Calcium Carbonatea C02-equilib C02-equilib temp, "C press., Pa temp, "C press., Pa 550 5 x 10' 800 2.4 x 104 605 3 x 102 852 5.0 x 104 701 3 x 103 898 1.0 x 105 736 7.1 x 103 937 1.8 x 105 1.9 x 106 777 1.4x 104 1158 2.0 x 104 1241 3.9 x 106 795 ""Handbook
of Chemistry and Physics", 42nd ed., 1960.
heat treatment of the sorption products. 1. Calcination. The dissociation by heat of calcium carbonate obeys the equation CaC03 + CaO + C02 (1) The equilibrium is temperature dependent. The equilibrium pressures of carbon dioxide are given in Table I. The equilibrium temperature in air (partial COzpressure 3 X lo1 Pa) is about 550 "C, and in flue gas (partial COz pressure 1.5 X lo4Pa) about 780 "C. The release of carbon dioxide from the limestone grains causes formation of pores through which sulfur dioxide diffuses into the grains. 2. Sulfation. The sorption of sulfur dioxide by calcium oxide obeys the equation CaO + SO2 + 1/202 -+CaS04
(2)
Although its mechanism has not yet been established with absolute certainty, there is good evidence (see, for example, Burdett et al., 1979) that sulfur trioxide is formed first SO2 + 1/202 SO3
SO,
+ CaO + CaS04
(24
(2b)
As calcium sulfate has a much larger molecular volume than calcium oxide (or calcium carbonate), it tends to plug the pores of the oxide and thereby to reduce its efficiency as a sulfur dioxide sorbent. The degree of conversion of calcium oxide into calcium sulfate depends on temperature, the highest conversions being reached between 800 and 900 "C (Christman and Edgar, 1980). The chemical background of this maximum is not yet fully clear. Of several explanations in the literature, one (Christman and Edgar, 1980) is that the rise of conversion with temperature up to about 850 "C par-
0196-4305/85/1124-0072$01.50/00 1984 American Chemical Society
