Energy & Fuels 1994,8, 388-394
388
Upgrading of Pitch Produced by Mild Gasification of Subbituminous Coal Robert L. McCormick' and Mahesh C. Jha Amax Research & Development Center, Golden, Colorado Received August 12, 1993. Revised Manuscript Received November 19, 199P
Alternative sources of anode binder pitch may be required in the future. One potential process for the production of pitch is low-temperature coal pyrolysis or mild gasification. Characterization data for a pitch produced by mild gasification of Powder River Basin subbituminous coal are reported. This pitch has acceptable sulfur, sodium, ash, and beta-resin levels but is shown to be deficient in aromatic carbon content and quinoline insolubles (QI). It also contains high levels of oxygen which leads to a high softening point and low coking value. Several strategies for upgrading this pitch to meet anode binder standards were investigated. These included heat treating, air blowing, solvent extraction, vacuum distillation, and addition of artificial &I. The addition of artificial &I (carbon lampblack) was the most successful of the methods investigated. Introduction Pitch is used as a binder in the manufacture of carbon anodes for aluminum production. The traditional source of binder pitch is from coal tar produced as a byproduct in coke ovens. Alternative sources of binder pitch may become necessary in the future because of declining coke oven capacity.l The decline in coke oven capacity is caused by decreased demand for coke and difficulties in adapting coke oven technology to strict environmental regulations. One potential alternative source may be coal tar pitch produced by low-temperature pyrolysis or mild gasification of coal. This paper presents the results of an investigation into methods for upgrading a mild gasification pitch produced from subbituminous coal to meet anode binder specifications. The effect of heat treatment, solvent extraction, vacuum distillation, and admixing of artificial quinoline insolubles (QI) on several important pitch properties, which are discussed below, were investigated. Mild gasification is a process wherein coal is heated under relatively mild temperature and pressure conditions to produce a low-Btu gas, tar, and char.2 Because mild gasification is a continuous process, it has several advantages over batch coke oven technology. Continuous processing can result in consistent product quality from day to day. Perhaps more importantly, continuous mild gasification may be more easily adapted to meet environmental regulations. The commercial viability of mild gasification depends upon upgrading and marketing of several coproducts made from a relatively inexpensive feedstock, coaL3 Market evaluation studies4 suggested that production of pitch from mild gasification of low-
* To whom correspondence should be addressed. Present address: Chemical Engineering and Petroleum Refining Department, Colorado School of Mines, Golden, CO 80401. Abstract published in Advance ACS Abstracts, January 1, 1994. (1)Anonymous. Coal Age 1986,4, 19. (2) Khan, M. R.; Kurata, T. M. "The Feasibility of Mild Gasification and Research Needs". DOE/METC-85/4019. 1985. (3) Jha, M. C.; Cha, C. Y. Mild Gasificationof Western Subbituminous Coal: Product Recovery and Upgrading. Proc. Sixth Annu. Pittsburgh Coal Conf. 1989. 765-773. (4) Cha.C.Y.:'Merriam.N. W.: Jha.M.C.:Breault.R. W. ''Develonment of Advanced, Continuous Mild GasificationProcess for the Prod;zi'on of Co-Products". DOE/MC/24268-2700,1988. 0887-0624/94/2508-0388$04.50/0
Table 1. Typical Anode Binder Pitch Swcifications
(I
property
specification
softening point, "C viscosity a t 105 "C, CP viscosity a t 150 "C,CP quinoline insoluble, w t % toluene insoluble, w t % coking value, w t % sulfur, wt 7% ash, w t % calcium, ppm sodium, ppm iron, wt % atomic C/H ratio specific gravity at 25 "C volatility (370 "C-),wt %
108-114 100004 5oooo 12-17 26-28 54-56 0.6" 0.35"
1W loo0 0.025' 1.70b 1.32 5.0
*
Maximum. Minimum.
cost, low-rank coal might be economically attractive. A particularly attractive feedstock is Powder River Basin (PRB) subbituminous coal because the resulting pitch would be expected to have a low sulfur content. Anode Binder Property Requirements. The carbon anodes used in the production of aluminum are made from binder pitch and a filler consisting of petroleum coke and recycled anode butts. The first step in anode fabrication is the formation of a paste from the melted pitch and filler. This material is formed into the desired shape and then baked to produce the finished anode. During the baking process, pitch is carbonized to form binder coke which holds the anode together. Table 1 lists typical aluminum industry specifications for binder pitch. These specifications are based on handling requirements during anode fabrication, environmental requirements, and the high efficiencyrequired in aluminum oxide reduction cells. Softening point and viscosity are important properties because of the complex mixing and rheology encountered in paste formation and anode forming. The binder must wet and coat the filler particles under the conditions of paste formation. Quinoline insolubles are important because they determine the wetting and penetration of the petroleum coke by the pitch during bakinge5rs As &I is increased, wetting and penetration decrease, leading to a higher pitch 0 1994 American Chemical Society
Upgrading of Pitch
requirement. However, binder coke strength, density, and coking value increase with increasing &I. &Ialso causes the formation of a highly disordered graphite structure in the binder coke.7 Highly disordered binder cokes have increased strength and reduced porosity relative to ordered structures.8 The optimum level appears to be in the range of 10-15 wt % ~ 1 . 9 Quinoline insolubles (&I)consist of carbon, coke, coal, ash, and mesophase particles which are typically less than 3 km in diameter. Mesophase is a liquid-crystal coke precursor which can be formed upon heat treating of pitch. Mesophase is referred to as secondary &I. High levels of mesophase can interfere with mixing of the pitch and coke and inhibit wetting and penetration of the coke by the pitch. Mesophase is, therefore, undesirable in anode binder pitch. Toluene insolubles contain the &Iplus additional high molecular weight components. The difference between TI and &I is called beta-resin. Beta-resins contribute to coking value, as does &I, but unlike &I,beta-resins take part in the binding action between the filler coke and the p i t ~ h .Beta-resin ~ content in the range of 12-16 w t 5% is considered optimum. Beta-resin content can be increased by carefully controlled heat treatment (avoiding the formation of secondary &I)which also results in increased pitch viscosity. The coking value gives a laboratory estimate of the coke yield that will be obtained from the binder when used in an actual In principle, the highest possible coke yield is desired because this should maximize carbon product density and strength. Experience has shown that very high coking value pitches have excessively high viscosity and softening point. The optimum coking value appears to be around 55 wt % . High volatility and water content can lead to a low coking value and, therefore, are undesirable. Volatiles and water can also lead to lowdensity binder coke. The atomic C/H ratio of pitch is an indication of aromaticity which correlates with coking value. NMR studies indicate that typical pitches have carbon aromaticities around 95 mol % .l0J1 Sulfur in pitch is converted to SO2 during baking or aluminum production and is undesirable because of environmental concerns. Ash does not contribute to carbon yield and can cause processing problems. Ash constituents such as calcium,sodium, and iron can catalyze reactions of carbon with 02 and COz, leading to excess carbon consumption in the reduction cell. The allowable levels of the catalytic materials are, therefore, carefully controlled. Experimental Section Production of Raw Pitch Samples. The material studied under this program was produced in a 100 lb/h process research unit at the Western Research Institute in Laramie, Wyoming.12 (5) Pinoir, J.; Hyvernat, P. Light Metals 1981, 487-517. (6) Couderc, P.; Hyvernat, P.; Lemarchand, J. L. Fuel 1986,65,281287. (7) Jones, S. S.; Hildebrandt, R. D. Light Metals 1975, 291-302. (8)Jones, S. S.; Bart, E. F. Light Metals 1990, 611-627. (9) Jones, S. S.; Bart, E. F. Light Metals 1991,609-613. (10) Sfihi, H.; Tougne, P.; Legrand, A. P.; Couderc, P.; Saint Romain, J. L. Fuel Process. Technol. 1988,20, 43-50. (11) Snape, C. E.;Kenwright,A. M.;Bermejo, J.;Femandez, J.;Moinelo, S. R. Fuel 1989,68,1605-1608. (12) Merriam, N. W.; Jha, M. C. "Development of Advanced, Continuous Mild Gasification Process". DOE/METC-91/6123,Vol. 1, 1991.
Energy & Fuels, Vol. 8,No. 2, 1994 389 The main processingunits are a shallow inclined fluid-bed dryer and a shallow inclined fluid-bed pyrolysis reactor. Pyrolysis temperatures were in the range of 590-620 "C. The feed coal was PRB subbituminous coal from the Eagle Butte Mine near Gillette, Wyoming. In the inclined fluid-bed reactors, gas and solids are contacted in cross flow. This allows plug flow of solids to be achieved so that reactor volume is minimized for a given conversion. Also, the cross-flow configuration leads to rapid removal of volatile products during coal pyrolysis. Because of this rapid removal, secondary reactions of the volatiles with the char are minimized, theoretically leading to a highly aromatic, high molecular weight liquid product. The pitch studied was produced in mild gasification test MG122 in the terminology of Merriam and co-workers.'* Raw tar was collected in condensersand distilled to produce a pitch boiling range fraction (370 "C+). First, the tar was distilled to an end point of 215 "C in a 30-galglass batch still at atmospheric pressure to remove water and a light oil fraction. The residue from this distillation was then distilled in a small pilot scale continuous vacuum still to produce the pitch fraction as residue. Roughly 50 gal was processed. Characterization Methods. Raw pitch and upgraded samples were characterized by chemical analysis, solvent fractionation, and a variety of tests specific to anode binder materials. Analyses for C, H, N, S, and 0 were performed using a Carlo Erba Model EA1108 elemental analyzer. This instrument analyzes sample combustion products for C, H, N, and S and, in a separate experiment, analyzes pyrolysis products for oxygen. Metals analyses were performed using flame atomic absorption. The samples were ashed, fused with lithium borate, and digested in acid before analysis. Solubility fractionation is commonly employed to provide qualitative chemical information in studies of fossil fuel derived materials. In addition to providing useful information on hydrocarbon chemistry, solvent extraction can be utilized as an upgrading technique. Solvent extraction in pentane, hexane, cyclohexane,and toluene was conducted in a Soxhlet extraction apparatus utilizing a modified ASTM D4072 procedure. Quinoline insolubles were measured using the ASTM D2318-86 procedure. Softening point was measured using a modified ASTM D2319 procedure. This procedure is commonly known as the cube-inair method. Coking value was measured as the Conradsen carbon residue using the ASTM D2416-84method. Ash analyses were performed using the ASTM D2415 method. Upgrading Tests. Upgrading tests were conducted using various heat treating methods, solvent extraction, vacuum distillation, and addition of artificial &I. In each test series, the goal of the testing was to determine in a preliminary way if the upgrading method under study had some potential for success. No attempt was made to optimize conditions or to apply combinations of upgrading techniques to produce an optimized pitch sample. Heat treating of raw pitch was performed under a nitrogen atmosphere. The reaction vessel was a 500-mL glass resin kettle with glass stirrer,thermometer, and a dipper for product sampling. Vaporized material was condensed in a side arm. To begin a run, pitch was charged to the stirred reactor, the reactor was purged with nitrogen, and heating to reaction temperature was initiated. Reaction temperature was achieved in about 1 h for tests at 300350 OC. Tests were typically continued for 4-6 h. In tests conducted at 400 O C , 2.5 h were required to obtain the desired temperature. This is because 400 "C- material has to boil off before this temperature can be obtained. In tests at this temperature, pitch viscosity increased greatly and tests were terminated early because of difficulty in stirring. Air blowing is similar to heat treating except that lower temperatures are employed and air is bubbled up through the sample during the treatment. Air blowing is commonly used to increasethe softeningpoint of paving asphalt. Chemicalchanges in pitch during air blowing have been discussed by Barr and
McCormick and Jha
390 Energy & Fuels, Vol. 8, No. 2, 1994
Lewis13 who demonstrated that air oxidation induces dehydrogenative polymerization without introduction of oxygen into the products. Air-blowing testa were conducted in the same reactor used forheat-treating studies. Air was sparged into the bottom of this vessel through a tube. The test procedure was to charge the vessel with pitch and heat to a temperature above the melting point but somewhat below the desired treatment temperature. Airflow was then initiated. The heat of reaction and the electric furnace were then used to bring the reaction mixture to the desired temperature within 30 min. Air-blowing tests were generally of about 3-h duration. Volatilized material was condensed in a side arm. In petroleum refining, upgrading by solvent extraction is called deasphalting and is generally done with liquid propane or butane as the solvent.14 In refining, the goal is to remove aromatics and polar compounds from the aliphatics by precipitation. The resulting aliphatic fraction is then converted to gasoline by catalytic cracking. In pitch production, the desired product is aromatic, so the goal is to remove the aliphatics and recover the aromatics and polar compounds. The aromatic precipitate should have improvedproperties as pitch relative to the starting material. Impurities such as sulfur, nitrogen, and ash might also concentrate in the precipitate. Solvent extraction upgrading testa were conducted in a Soxhlet extraction apparatus unless otherwise noted. Lighter fractions can be removed from the pitch at temperatures lower than their normal boiling points by vacuum distillation. The goal of these testa was to reduce hydrogen content and increasecarbon content by removingthe more volatile fraction of the pitch. The advantage of vacuum distillation for pitch upgrading is that the temperature can be kept below the level at which mesophase (secondary QI) formation or cracking reactions begin. Coking value and T I would also be expected to increase. Distillation tends to concentrate impurities such as sulfur, nitrogen, and ash in the pitch. The maximum actual temperature obtained during vacuum distillation was 325 O C . Vacuum distillation testa were performed using an ASTM D1160 apparatus and procedure. The absolute pressure in these distillations was 0.5 mmHg. As noted below, the raw pitch contains very low levels of primary QI. One method for increasingQI without the production of mesophase is to add a fine carbon such as carbon black to the pitch. To test this idea, carbon lampblack was mixed at loadings of 5, 10, and 20 w t %. This mixing was accomplished by dry mulling of solid pitch samples and lampblack. The intimate solid mixture was then melted in order to obtain good contact between the pitch and lampblack.
Results Characterizationof Raw Pitch. Table 2 lists characterization results for the raw pitch. Mild gasification pitch is characterized by low coking value, low softening point, low TI and &I,and low atomic C/H ratio. However, beta-resin content (TI less QI) is in the desired range. Ash, sulfur, and sodium contents are in the acceptable range. Calcium content is high. I n view of the high Ca content of Powder River Basin coal (1w t % ), it is suspected that ash carryover from the pyrolysis reactor was probably too high. This problem can be easily corrected by proper design and operation of the reactor. Iron content is also very high. It is believed that this is because the high oxygen content leads to an acidic pitch which dissolves iron from the steel cans in which it is stored. Use of appropriate stainless steel or pitch processing to remove oxygen immediately after production should eliminate this problem. (13) Barr, J. B.; Lewis, I. C. Carbon 1978,16,439-444. (14)Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1980;pp 405-409.
Table 2. Properties of Raw Mild Gasification Pitch carbon, wt % 82.0 hydrogen, w t % 7.8 atomic C/H ratio 0.88 sulfur, wt % 0.3 0.6 nitrogen, w t % oxygen, wt % 8.2 ash, w t % 0.2 Fe, w t % 0.101 Na, ppm 54 Ca, P P ~ 171 pentane insoluble,wt % 66 hexane insoluble,w t % 49 cyclohexane insoluble, wt % 40
TI,& %
16
&I,& % coking yield, wt % softening point, "C specific gravity at 25 O C volatility (370 "C-), wt %
28 102
1.0 1.158 9.1
The low atomic C/H ratio of the raw pitch is particularly important. This suggests that aromaticity is much less than for a typical coke oven pitch. This was confirmed by solid-state 13CNMR which indicated that the aromatic carbon content was approximately 60 mol % .I5 Results of extraction of mild gasification pitch with pentane, hexane, and cyclohexane also provide support for this conclusion. The fraction soluble in pentane is unlikely to contain highly aromatic structures. As much as 34 wt 7% is pentane soluble. The difference between cyclohexane and normal hexane solubilities suggests that roughly 9 wt % of the pitch molecules are predominantly naphthenic ring structures. Naphthenic rings might be dehydrogenated during upgrading to produce aromatic rings in some cases. The low aromaticity is responsible in part for the low coking value and softening point. The high oxygen content may also contribute to these problems. Quinoline insolubles are very low and contribute to low coking value. These results indicate that mild gasification pitch will require considerable upgrading in order to be utilized as an anode binder for aluminum production. Raw Pitch Upgrading Tests. The characterization results reported above suggest certain general features required of any upgrading process. The goal of upgrading must be to increase coking value while at the same time producing a pitch that can adequately wet and penetrate the anode filler. Specifically,the upgrading process must (1)increase carbon aromaticity (reduce hydrogen content); (2) reduce oxygen content; (3)increase &I; and (4)reduce impurities and not introduce new impurities. Several approaches to accomplishing these goals were investigated. Heat Treating. One approach is treatment of the pitch at elevated temperature. Heat treating runs were made at 300, 350,and 400 "C. Results for TI, C/H ratio, and oxygen content are shown in Figures 1,2,and 3. The data indicate that heat treating can have a significant effect on TI. As a comparison of Figures 1,2,and 3 indicates, higher temperatures allow reduced heat treatment times for a given level of TI. C/H atomic ratio was also increased by heat treating but not up to the desired level. Higher temperatures produced greater increasesin C/H. Increases in C/H ratio occurred early during heat treating runs at 300 and 350 O C and then stabilized. At 400 OC, C/H increased throughout the test. Oxygen content was reduced but not below about 4%. Higher temperatures (15)Miknis, F. P., private communication.
Upgrading of Pitch
Energy h Fuels, Vol. 8, No. 2, 1994 391
0 Oxygen
1.5
0 C/H Ratio
60
F-
50
40
p
1.2
1
-
3
c
-
50
2V
1.1
1.0 0
20 10
0
1
2
3
4
5
6
7
8
0.8
Time, hours
Figure 1. Results of pitch heat treating at 300 "C.
&
::
0
5
70
0 TI
60
0 Oxygen 0 C/H Ratio
2 2
1.2
50
F
2
1.5
40
-
30
- 1.0 ?v
0
1.1
U
ri
2
0" E
3m
20
- 0.9 10
0.8 0
1
2
3
4
5
6
7
8
lime, hours
Figure 2. Results of pitch heat treating at 360 OC. 1.4
1.5
1.2 ,o
P V
*g
1.1
4
