Hazardous chemicals from coal conversion processes? Maybe. Because unexpected chemicals harmful to health and the environment may be produced, these methods and their products should be carefully studied now David W. Koppenaal and Stanley E. Manahan Department of Chemistry University of Missouri Columbia, Mo. 6520 1 The use of coal for energy is certain to increase in the U.S. during the next several decades. Utilization of domestically abundant coal for a majority of U.S. energy needs will require mining, transportation and processing operations on a scale not previously approached for any mineral resource. Environmental, occupational, sociological and economic changes resulting from the projected uses of coal will be substantial, and proper account must be taken of them in developing this resource. This is emeciallv true if conversion of coal to hvdrocarbons develoDS on‘a large scale. The largest synthetic fuel industrv to date was develoDed in Germany our’ng-worldWar 11. In 1944 peak production reached 100 000 bbllday-the equivalent of the crude oil consumption of one moderately large petroleum refinery. It is dwarfed by current U.S. consumption of about 20 million barrels a day.
Reactive C
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On an enormous scale The commonly accepted figure for the most efficient size of a synthetic natural gas (SNG) plant is 250 million ft31day. Approximately 176 sites have been identified in the US. that have sufficient coal and water to support a plant of this size for at least 20 yr. Typically one of these plants would require about 15 000 tpd of coal. Approximately the same quantity of steam would be required. Oxygen would be consumed at the rate of airnost 3000 tpd. Solid wastes from the gasifier and from coal pretreatment would amount to approximately 2500 tpd. If a typical high-sulfur Eastern coal is used for feedstock, approximately 600 tpd of sulfur would be produced. Assuming this coal has a heat value of 11 000 Btullb and a waste heat loss of approximately 35% during processing, 1.1 X 10” Btulday would be released to the environment. Gaseous waste effluents would be produced in appreciable quantities. Some processes would require as much as 3 million gallday of water to quench and scrub the raw gas product. This water would pick up sub1104 Environmental Science & Technology
stantially more than 1000 ppm each of phenols, ammonia, and COD and appreciable levels of suspended solids, thiocyanate, and cyanide. It must be treated before it is used as cooling tower makeup water or it is released to receiving waters. Production of the order of 5 X IO9 scflday of SNG is reasonably feasible by 1990. This would require 20 standard-size SNG plants. The amount of coal required would be 110 million tonslyear. A total of 11 pilot plants for various coal-conversion processes are operating or have been demonstrated in the U.S. The latest of these is a 70 tpd Synthane piant dedicated by the US. Bureau of Mines in November 1975. During that same month a $237 million demonstration plant to be constructed by Coalcon was authorized for Athens, 111. If constructed this plant, which will process 2600 tpd of coal and produce 22 million scflday of SNG and 3900 bbilday of liquid hydrocarbons, will be the first commercial operation for synfuels production in the U.S. The production of “exotic chemicals”-species which in trace amounts have profound environmental and health effects-has received only scant attention in studies of coalconversion processes. (Much of what is known has been inferred from studies of occupational health and environmental impact in the coking industry). The chemical nature of coal and the mineral elements contained in it are conducive to the formation of a variety of toxic substances under conditions that are attained during typical coal-conversionprocesses. If coal conversion is developed on the mammoth scale requiredto satisfy an appreciable fraction of domestic demand for hydrocarbon fuels, the environmental and occupational health problems will affect vast areas of the country and substantial numbers of people. Chemical nature of coal The organic portion of coal is largely composed of polycyclic aromatic structures and. in addition to carbon and hydrogen, contains significant quantities of organic oxygen, nitrogen and sulfur, residual biological compounds modified through the coalification process, and organically bound metals. Functional groupings include methoxy, hydroxyl, carbonyl, and carboxyl. Nitrogen, ,oxygen, and sulfur may exist within relatively nonreactive cyclic configurations. Aliphatic carbon and hydrogen are also present to some extent. Coal mineral matter contains appreciable quantities of practically all of the elements in the periodic table. Physically, coal has a widely varying porous structure. The physical behavior of heated coal is especially important to its reactivity and handling in a conversion process. Eastern US. coal resources swell and cake when,heated. These coals cannot be used with the Lurgi gasification process.
It is generally conceded that coal is made up largely of condensed aromatic hydrocarbon lamellae (Figure I ) held together physically or linked through chemical bridging groups. The molecular weights of coal species are thought to fall mostly within the range of 2000-12 000. These molecular weights are relatively low compared to those of tars and residual fuel oils (whose molecular weights may range into the millions) so that coal is a more reactive feedstock for conversion processes'than petroleum residues. Functional groups containing oxygen, nitrogen and sulfur may have a pronounced influence on coal reactivity and products. They lead to the formation of contaminants including phenols, aromatic n.itrogen compounds and catalyst-poisoning sulfur compounds in coal liquids. The functional groups of oxygen are understoodto a relatively greater extentthan those of nitrogen or sulfur. The oxygen in coal is primarily incorporated into hydroxyl (phenoxy) and carbonyl groups. A small amount is incorporated as heterocyclic oxygen and, particularly in low rank coals, carboxyl groups. As noted above, oxygen in coal may lead to formation of various phenolic compounds in coal liquids. Oxygen functional groups are also involved in the binding of metal ions. Nitrogenous functional groups in coal are not so well characterized. Nitrogen may exist in concentrations up to 5 % , although it is generally present from l-2% by weight. Nitrogen in coal is probably the major source of NO, compounds. which are produced when coal is burned. The same is true of nitrogen in coal liquids. Sulfur presents several technological and environmental problems. Therefore, all major conversion processes provide for the elimination of sulfur from Coal. The sulfur in coal may be either organic or inorganic. The inorganic portion exists primarily as pyritic iron sulfide, FeS2, which is fairly easy to remove. A small portion also exists as sulfate ion. Most of the organic sulfur is incorporated into heterocyclic configurations. AS such, it is somewhat harder to remove, and ultimately contributes to air
pollution. Sulfur concentrations in coal may range between 0-1 0%. There has been surprisingly little work on the analysis of the various functional groups of sulfur in coal. In view of the problems that sulfur presents, this should be an area of research priority. Coal-conversion processes The primary coal-conversion processesthat are planned for commercial development in the US. are: coal gasification coal liquefaction solvent-refinedcoal (SRC). Figure 2 illustrates basic flow diagrams for each of these conversion processes. Of these, the coal gasification and SRC processes are receiving the most attention and several commercial plants are being planned. in the gasification process, the coal is ground, dried,.and sent to a preheater where it is mixed with O2 and steam. It is then gasified at temperatures of about 1000 OC and pressures of 500-1000 psi. The gas product is "scrubbed" to remove tar, and then sent to the "shift" reactor where the HzICO ratio is optimized. Acidic gases are removed, and me product is methanated over a catalyst. The gas is then dehydrated and compressed to form pipeline gas. In the SRC process (€S&T, June 1974, p 510). the pulverized coal is mixed with a coalderived aromatic solvent. This coal slurry is then reacted with hydrogen in a dissolver at 300-400 "C and 1000-2000 psig. It is believed that coal minerals catalyze the hydrogenation. While in the dissolver, most of the coal (95%) is dissolved, almost half of the organic sulfur is converted to HZS, and hydrogen, amounting to I-2% of the weight of the coal, is consumed. Liquid, gaseous and solid phases are then separated. Effective removal of W2S. C02 and other hydrocarbon gases is necessary. These gases are consequently treated andlor recycled. The mineral ash is removed by centrifugation or filtration. Removal of up to 99.9% of the ash is possible. The solvent is
F G JI1E 2
Schematic of coal conversion I I
relined coal
:ycled
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removed by flash evaporation and recycled. The remaining liquid product is then solidified to form a brittle, hard, clean-burning solid. Or, if desired, the SRC liquid may be further hydrogenated over a catalyst to produce a liquid hydrocarbon fuel of high8tu value. The coal-liquefaction process closely resembles that of the SRC process, except that the former has a catalytic hydrogenation unit. The coal is hydrogenated over a catalyst at high temperatures and pressures. Hydrogen is supplied to the unit continuously. Solids and gases are then separated, and the resulting liquid undergoes a distillation and flash-evaporation process like that of the SRC process. The primary product of the liquefaction process is a synthetic crude oil that can be refined to produce light oils and gasoline. These conversion processes take a dirty, bulky, inconvenient solid fuel and transform it into a clean, easily transportable solid, liquid or gaseous fuel. The heat content per unit weight of the product normally is higher than that of the original fuel. Overall, the products are more acceptable environmentally, especially with respect to air pollutants. However, there is some potential for the formation of hazardous organometallic and/or carcinogenic chemicals. Formation of organometallics The chemical nature of coal combined with the conditions under which it is liquefied or gasified are conducive to the formation of a variety of organometallic compounds consisting of 1106
Environmental Science & Technology
metals bonded to organic groups or ligands. Coal liquefaction in particular may produce organometallic compounds. Elemental analyses of a variety of coals (101) has revealed the presence of well over half of the elements in the periodic table (Table 1). Many metal or metalloid elements that might be expected to form some types of organometallic compounds or stable organic complexes occur at relatively high levels in these coals. By way of comparison, some values are given for elements analyzed in an ash-free, solvent-refined coal product prepared from Kentucky No. 9 coal. The solvent refining process yields impressive reductions in trace element content; however, a fraction of the metals remaining may be organically bound. Some organometallic compounds may result from the chemical interaction of trace metals in the ash with the organic portion of coal. Surprisingly, very little work has been done on elucidating the exact nature and effects of organometallic compounds that may be produced in coal-conversion processes. Some idea of the various types of organometallic compounds that may be formed can be inferred from the structure of coal, the biological origin of coal, and the bonding tendencies of known organometallic species. Some of the major types of organometallic compounds that may be formed include: 9 Metal-porphyrin compounds. The biological origin of coal makes it probable that porphyrin-type compounds are common components. Porphyrins are capable of binding metal atoms, and are known to be important carriers of vanadium and nickel in crude oil. It is possible that various metal porphyrins indigenous to coal may survive the processing intact or in an altered form. Metal-carbonyls. The relatively high-partial pressures of carbon monoxide during various stages of the conversion process may lead to the formation of metal carbonyls. Carbon monoxide may react with most of the transition series metals under certain conditions of temperature and pressure to form carbonyls. Nickel, iron and cobalt carbonyls are the most significant carbonyls in the petroleum industry and may be expected to arise from coal-conversion processes. Although the extreme temperatures will normally preclude the persistence of large amounts of these compounds, trace amounts may escape during particular phases of the process. All of the metal carbonyls are reactive and several are acutely toxic. Metallocenes. Also known as -9 complexes, these compounds consist of aromatic rings bounded to metal atoms. Usually, the metal atom is "sandwiched" between two aromatic systems--9 bonds linking the metal to the carbon atoms of the rings. Iron, nickel, chromium, vanadium, tantalum, molybdenum and tungsten metallocenes have been observed. Although the iron cyclopentadienyl compound (ferrocene) is quite stable, other metallocenes are relatively less stable. Introduction of various functional groups on the ring portion of the compound may have significant effects on the stability of these compounds, however. The possibility of the formation of quite stable metallocenes must not be overlooked. Arene carbonyls. Arene carbonyls are organometallic species in which the metal atom is bonded to both an aromatic ring system and to carbon monoxide molecules. Certain arene carbonyls may be more stable than their metallocene counterparts. Many transition metal arene carbonyls have been synthesized. Because of the high pressures of carbon monoxide in many conversion processes, the aromatic nature of coal, the presence of metals that form arene carbonyls, and the stability of these compounds, they are one of the most likely types of organometallics to be found in coal liquids. Metal alkyls. Many transition metal alkyls are known. In addition, lead, tin, aluminum and silicon may form metal-alkyl combinations. Because of the limited stability of these compounds, it is not anticipated that they will be common constituents of the final conversion product. However, as they may be
In addition, PAH's have been found in atmospheric particulate matter around coal liquefaction plants. Therefore, the release of hazardous compounds during process failure and shutdown may be of considerable importance. These compounds may also be found in scrub waters, which are eventually released to the environment. Contact with coal-conversion products may be hazardous. Worker contact with these products may be frequent, particularly when considering the problems that occur during the initial start-up of a new industry. The production and release of carcinogenic compounds, in addition to the hazard of personal contact with the conversion product, makes the consideration of these chemicals as a carcinogenic risb very important. A closer look at the occupational hazards of the industry is certainly warranted. involved in the transport and mobilization of heavy metals in coal liquids, their presence must not be completely discounted. Organo hydrides. Several metal and metalloid elements are known to form relatively stable organo hydrides of the general formula RnMH4-,. (R represents an organic group and M a metal or metalloid.) Organo hydrides of Pb, Sn, Ge, and Si may form in the reducing atmosphere of the conversion process. Their relative stability makes their persistence likely. Metal chelates. Coal contains many chelating structures (phenolic OH, carboxylic acid groups, and amino groups) that can effectively bind metal atoms. These structures may have various effects on the mobilization and release of many metals. Figure 3 shows hypothetical chelates that may have an influence on metal distributions in coal-conversion products. In addition to the above mentioned possibilities, other unpredictable compounds involving metal interactions with the organic portion of coal should be considered. Although ash removal is very efficient in most conversion processes, small, but important quantities of many metals may be left organically bound in the conversion product. It is important that the organometallic portion of coal conversion products be well characterized. These compounds may be toxic-carcinogenic in some cases-and detrimental to the use of coal liquids as a fuel or feedstock for further refining processes. The release of such compounds to the atmosphere or water systems will pose significant pollution problems. In addition, such compounds may foul catalysts and interfere with the conversion process. On the other hand, such compounds may be found to have a catalytic effect, or prove to be a valuable source of some metals. Formation of PAH's Considering the aromatic nature of coal, it is not surprising that the polycyclic aromatic hydrocarbon (PAH) content of coal-conversion products is high. This is of concern because of the carcinogenic activity of many PAH's. It has been known since the early eighteenth century that a carcinogenic hazard is present when working with coal tar or its by-products. Skin cancer in particular was a common occupational hazard to chimney sweeps, dyestuff workers, and coke oven workers. The first chemical agent in coal tar separated and tested for its carcinogenic activity was 3,4-benzopyrene. It was found to be extremely carcinogenic to experimental animals and is now known to be carcinogenic to humans. It is often analyzed as an indicator of possible carcinogenic hazards. Many other PAH's have been found to be carcinogenic. The formation of PAH's in coal-conversion processes is well known. There have been studies on the carcinogenic hazard associated with coal liquefaction processes. The aromatic content of coal liquefaction products is generally in the range of 50-70%, depending on the boiling point range considered. The aromatic content generally increases with boiling point. Certain aromatic amines are also known to be carcinogenic. Polycyclic aromatic hydrocarbons and aromatic amines are thermally stable, and are present in coal-conversion products.
Associated hazards It is evident that some health hazards are to be expected with the advent of a large-scale coal-conversion industry. In particular, the hazards associated with carcinogenic polycyclic aromatic hydrocarbons, aromatic amines, toxic metals and organometallic compounds need to be investigated. It is important that these processes and their products be methodically examined for these hazards, especially in view of the possibility of a massive coal-conversion industry in the US. Although these processes look promising with respect to the removal of "traditional pollutants," it is likely that they will produce chemicals that have not even been identified or characterized with respect to their health and environmental effects. The development of any new technology requires investigation of toxicological and environmental impacts of that technology. This is especially true of coal-conversion processes because of the diverse nature of the raw material. In order to prevent any delay in the large-scale development of this industry, a major effort should be made now, before developments proceed beyond the pilot-plant stage, to eliminate potential hazards that may be imposed upon the environment and the health of personnel. Additional reading "Carcinogenic Potential of Coal and Coal Conversion Products," A Batelle Energy Program Report, R. I. Freudenthal, G. A. Lutz, and R. I. Mitchell, 1975. "Coal Structure and Reactivity," A Batelle Energy Program Report, G. L. Tingley, and J. R. Morrey, 1973. "industrial Carcinogens," R. E. Eckhardt, Grune & Stratton, New York, N.Y., 1959.
David W. Koppenaal is working on his Ph.D. degree in analytical chemistry at the University of Missouri. He is interested in new energy systems and their environmental effects, analytical instrumentation, and the occurrence and distribution of chelating agents in natural waters. Stanley E. Manahan is currently associate professor of chemistry at the University of Missouri where he directs a research program in environmental and analytical chemistry. Dr. Manahan's recent studies have focused on coal humic acids as they relate to pollution control and the fate of trace elements in solvent-refined coal. He is the author of some 30 technical publications and one textbook (Environmental Chemistry, 2nd ed., Willard Grant Press, 1975). Coordinated by LRE L'olume IO, Number 12, November 1976
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