Coal Processing Methods

Coal Processing Methods

More coal processing is done in foreign coal-producing countries because of their requirements for a smokeless fuel, their need to make a satisfactory metallurgical coke from inferior quality coals, and their general lack of indigenous oil or gas from which to produce chemicals or other carbon-based products.

High-Temperature Carbonization

Conventional Coke Manufacture

The demand for coke in the United States has declined from a recent high in 1957 of 76.0 million tons to only 55.9 million tons in 1959 and 57.2 million tons in 1960. In addition to a decrease in coke demand in 1959 and 1960 because of a steel strike and general business recession, some technologic developments, such as supplemental fuel injection and better prepared burdens, tended to lower further the blast furnace coke rate, which reduced coke requirements for pig iron production. Nevertheless, because the cost of coke is important in pig iron manufacture, efforts to improve the methods and economics of carbonizing coal have not been reduced.

Blending of coals of different ranks and from different seams is used increasingly in the United States and in foreign countries to obtain an oven charge that will give coke of optimum quality and strength without damage to the carbonization equipment. All the 72 plants in operation in the United States use blends rather than single coals.

No major changes have been made in conventional oven equipment or processes used to produce coke in the United States for blast furnace use, but recent trends in automation of the plants to reduce labor costs have continued. Although the very large coke ovens used at several plants in Germany, Italy, and Russia have not been erected in this country, a construction firm has an agreement to build German-type ovens in the United States. Labor costs are claimed to be reduced significantly with these large ovens.

Because most European countries must use relatively poor coking coals to produce satisfactory coke, greater efforts to improve coke quality have been made there than in the United States. The charge of four large coke plants in France and about 90 percent of the coal carbonized in Poland are reported to be “prestamped.” This improves coke quality by increasing the bulk density of the charge while using less of the more expensive strongly coking coals in the blend. Another development, used in one plant in France, is “dry charging,” which gives a greater bulk density, greater coke production, and an improved coke strength for a given blend. In the United States a study showed that for each 1 percent of moisture reduction in the coal, oven throughputs increased 1.5 to 3.5 percent with no deterioration in coke properties.

Preheating the coal blend before charging into the ovens continues at the experimental level in several countries. The same improvement in coke strength for poorly coking coals results from preheating the charge as from drying the coals and results in further increased oven capacity. In the United States, the Bureau of Mines showed that preheating an Illinois No. 6 bed coal increased the coke strength and reduced the coking time. In other Bureau tests, increased strength of coke was obtained for all coals and blends tested; the greatest improvement occurred for those coals and blends making the poorest coke. In a study the Koppers Company obtained substantial improvement in coke quality by preheating the charge when high-oxygen coals were used.

After years of development in many different laboratories, small-scale test ovens, varying in size and used to predict the effect of changing variables on coke strength, have demonstrated their usefulness both in the United States and abroad. Four years of study in Marineau, France, showed that, except for material balances on new oven blends, complete comparability in tests is possible between full-scale ovens and a 400-kg. test oven. Similarly, the British Coke Research Association was able to relate test results of both a 10-ton and a 500-pound oven with commercial ovens.

With small-scale test ovens, the many variables that affect coke strength can be studied safely, and the economic importance of proposed changes in carbonization practice can be estimated. The effect on various blends of coals of such variables as particle size of the charge, surface-moisture, and oil additions on coke strength and expansion properties of the charge have been studied extensively in the United States, England, Germany, and France. Other variables, such as the effect of inerts, the use of blends of various coal tar pitches, the rates of heating the charge, and the effect of coking temperature, have been investigated and their interrelationship clarified. Based on this information, the addition of coke breeze or pulverized low-temperature char to increase coke strength has reached commercial application in France and Germany. In German experiments the effects of changes in operation of the oven and of different kinds of blends have been studied both with and without “stamping” of the charge. Other studies have shown that for coals that make a relatively weak coke, the Gieseler plastometer gives a good indication of how best to blend coal with higher ranking coals to make the lowest cost coke of required strength.

Improved small-scale tests have been developed to predict wall pressures that would be obtained in commercial practice. As a result, better information can now be obtained easily on the effect of bulk density and other variables on wall pressures.

In the last several years, success has been achieved in the United States and abroad in showing the comparability between the different laboratory tests used for measuring coke strength. The Micum and ASTM tumbler tests and a number of other standard tests can now be compared so that data collected by different test methods can now be used by all research workers.

Theoretical studies have continued, in all nations concerned with coal, on the mechanism and kinetics of the coking process. Much of this effort is centered about coal plasticity and the rate of weight loss as a function of temperature for different ranks of coal. This approach may lead to a better understanding of how coals and coal blends are transformed into coke. Other studies, such as the effect on hardness and other properties of coke caused by adding various amounts of pitch to coal and the effect of various petrographic constituents on coke strength, should lead to the development of improved coking methods and practices.

These theoretical studies and the experiments on small-scale and full-scale ovens have resulted in much progress in using lower-rank and poorer coking coals to make a coke of satisfactory strength.


Ferrocoke is not used extensively in the United States, but interest continues in its manufacture, particularly when a small pig iron plant cannot afford sintering equipment for blast-furnace flue dust. It has been demonstrated that making ferrocoke requires a higher bulk density of the charge and a longer coking time. Larger pieces of coke are formed and the shatter index is higher, but the tumbler index is lower for both the 1-inch and ¼-inch screen.

Research on ferrocoke continues overseas, and a study in India showed that 15 percent iron ore and 85 percent lignite gave a ferrocoke briquet of satisfactory strength. The addition of 10 percent low- or high-volatile bituminous coal to the mixture allowed the iron ore concentration to be increased to 30 percent without loss of briquet strength.

New Processing Methods

The rising demand for coke breeze has increased the average reported price from $3.80 in 1949 to $8.27 in 1960. Because of this price increase, alternate methods of producing small-size coke that can be used when strength is not critical have been developed, particularly in those areas where the breeze must be transported long distances; thereby, the delivered price is raised appreciably.

A rotary coke oven developed by the Wise Coal and Coke Company reportedly produces a small-size coke or char suitable for a chemical reducing agent in some processes, such as electric- furnace production of steel and in phosphorus manufacture. Coal, fed in a bed 6 to 12 inches deep onto a rotating circular grate, is heated by combustion of volatile matter evolved from the coal as it is carbonized. In effect, the rotary oven operates as if it were a continuous beehive oven. No byproducts are recovered, but it is believed that low capital and operating costs can be achieved when low-cost coals are used.

Two other continuous methods, similar to the process used commercially in Canada by Shawinigan Chemicals, Ltd., for producing small-sized coke have been reported in the United States. Both of these methods use moving grates. In one series of industrial tests a water-cooled Vibra grate stoker installed in a Kewanee boiler was used to produce a 1- x 1/8-inch coke with a 53 to 54 percent yield. The feed material was a 2- x 1-¼-inch high-volatile A bituminous coal. In these tests about 38 percent of the heat value in the coal was recovered as steam, and about 50 percent remained in the coke. On the basis of these tests a commercial plant has been constructed at a cost of $600,000. The plant is reported to use a traveling grate but does not recover heat in the form of steam. No details have been released concerning the other continuous coker, which has been developed and which also reportedly uses a traveling grate.

Other developments using a traveling grate of different types to produce a chemical grade coke either commercially- or in pilot studies have been reported. The New Jersey Zinc Company is using a traveling grate to carbonize a zinc ore-coal briquet. A traveling grate stoker for producing coke was investigated by the Central Fuel Research Institute in India, a company in South Africa, and Bituminous Coal Research, Inc., in the United States.

In Western United States, reserves of good grades of coking coals that are comparable to those in Eastern United States are in short supply, and two new processes for making coke have been announced. The Food Machinery and Chemical Corporation and the United States Steel Corporation have developed a process, which is said to produce either a coke strong enough for use in blast furnaces or a product that can be used as a substitute for coke breeze in phosphorus reduction. The process is reported to be carried out in two steps a low-temperature treatment of the coal, followed by briquetting and further processing of the briquet.

In the other process developed by United States Fuel Company, fine coal is pelletized with an inert binder and water. The pellets are dried and charged to the top of an externally heated retort and flow countercurrent to a stream of natural gas fed at the bottom. Natural gas requirements of the operation are about 18,000 cubic feet per ton of coal, 10,000 cubic feet for drying and preheating, and 8,000 cubic feet per ton of coal for the thermal decomposition reaction. Tar in 40- to 50-gallon amounts per ton of coal and the combination of decomposition products from natural gas and coal yield 18,000 to 20,000 cubic feet of “reactor gas” containing 85 percent hydrogen. Volatile matter in the coke produced is about 1 percent.

Low-Temperature Carbonization

The apparently attractive potentials of low-temperature carbonization of coals as a coal-processing method are so great that hundreds of patents have been issued in the last 50 years on different methods of carrying out this process. Millions of dollars have been spent on research to develop plants that would be commercially successful, but in the United States today only two low-temperature carbonization plants exist, each reported to be operating only part time. The Disco plant of Consolidation Coal Company processes a bituminous coal and produces a smokeless fuel, which meets air pollution regulations in the Pittsburgh area. The other plant at Dickinson, North Dakota, using lignite as a raw material produced a lignite char, which was subsequently briquetted and sold as a domestic solid fuel. In the last several years the Dickinson plant was reported to be making barbecue briquets.

Low-temperature carbonization processes, using any rank of coal, can be economically successful, if either the char or the tar can be sold at a premium price. Because the United States has ample supplies of some of the world’s best coking coals that are obtainable at low costs, no premium could be expected for the chars in metallurgical outlets in the United States, except in special places or under unusual conditions. Moreover, because the competing fuels, oil and gas, are available locally at reasonable prices in cities where air pollution is a problem, no large-scale premium outlet exists in the United States in the foreseeable future for the char as a domestic smokeless fuel. Also, the development of commercial outlets for the tar at prices that would make low-temperature carbonization attractive have not been successful. For these reasons, commercial exploitation of low-temperature carbonization has been limited.

Either coking or noncoking coals can be used to prepare chara, but usually a noncoking or weakly coking coal is employed because of its availability and price. In the United States, however, low-temperature carbonization has mostly been attempted with coking coals, which constitute over 90 percent of our production but which introduce serious operating problems. The largest plant constructed in recent years for testing the economics of low-temperature carbonization is the semi-commercial unit of the Aluminum Co. of America in Rockdale, Texas, which uses a Texas lignite that is carbonized in an externally heated fluid-bed reactor. Research on the Parry process, which is employed at the Aluminum Co. plant, is continuing at the Bureau’s Denver Coal Research Laboratory with experiments aimed at determining the optimum ratio of internal to external heating and developing methods of using strongly coking coals.

The previously mentioned Food Machinery and Chemical Corporation development is reported to use a fluid-bed process for low-temperature carbonization as a first step in the process, although no description has been published.

During the 1950’s experiments were conducted on low-temperature carbonization of coking coal by United Engineers and Constructors, Southern Research Institute, Consolidation Coal Company, and Montana State College. None of these processes, as originally, tested, have reached commercial application as yet, but these research activities plus a steady stream of patents on the subject indicate a continuing interest in the subject.

More commercial success in low-temperature carbonization has been realized in other nations, where high-quality coking coals are expensive and in short supply and where an extensive market for smokeless fuels exists. A premium price is offered for the chars produced in both of these markets. Two commercial processes (Phurnacite and Rexco) have been used for a number of years in England to make a smokeless fuel. Both processes carbonize a weakly coking coal in an externally heated vessel with no briquetting of the charge. Low-volatile fines, which have been briquetted with a pitch binder and then subjected to a low-temperature carbonization are also used in another process to make a domestic fuel. Similar processes are in commercial operation in other European countries.

Vigorous efforts are being made in England to produce chars that can be used to make a suitable smokeless fuel, and a systematic study is being made of a number of English coals in an 8-inch diameter fluid bed carbonizer to determine yields and properties of the products Larger diameter fluid carbonizers of 18 and 24 inches also have been tested, and a commercial plant with a 5-ton per hour capacity is being constructed on the basis of pilot plant results.

In England recent carbonization experiments using a disperse phase, in which coal particles are suspended in a hot gas stream, indicated that volatile matter can be reduced from 37 to 23 percent, in 1 or 2 seconds. If a process based on this principle could be devised, important savings in plant cost should be possible.


Manufactured Gas

In the past, a considerable quantity of coal and coke derived from coal was used in producing “manufactured gas.” When long distance pipeline transmission of natural gas from Texas was introduced to the heavily populated sections of the country, the production of manufactured gas declined rapidly. In 1960, only 722,000 tons of solid fuels were used in producing manufactured gas by all utilities in the United States.

Some gas from coal is still produced for internal use at manufacturing plants, but no reliable figures are available on the quantity of coal used for this purpose. The introduction of readily available natural gas, however, has almost completely displaced coal in markets such as this.

Because the average delivered price of natural gas has risen from about 22 cents per 1,000 cubic feet in 1945 to over 50 cents in 1960 and the price of coal has remained relatively constant during that same time, an opportunity exists for coal to compete with natural gas by producing a cheap hot producer gas for use in such commercial applications as lime burning and glass making. At one commercial plant, a study was made of the economics and optimum operating conditions of gas producers using bituminous coal, but otherwise little or no systematic research has been conducted on production of manufactured gas declined rapidly. In 1960, only 722,000 tons of solid fuels were used in the production of manufactured gas by . all utilities in the United States.

Some gas from coal is still produced at manufacturing plants for internal use in the plant, but no reliable figures are available on the quantity of coal used for this purpose. However, it is known that the introduction of readily available natural gas has almost completely displaced coal in this market, too.

As the average delivered price of natural gas has gone up from about 22 cents per 1,000 cubic feet in 1945 to over 50 cents in 1960 and the price of coal has remained relatively constant during the period 1948 to 1960, an opportunity exists for coal to compete with natural gas by producing a cheap hot producer gas for use in such commercial applications as lime burning, glass making, etc. However, except for one commercial plant where a study has been made of the economics and optimum operating conditions of gas producers using bituminous coal, little or no systematic research has been conducted on this problem. Some gas producers using coke and anthracite, however, are being used in this country and information on operating experience and economics should be available when using these noncoking fuels.

In foreign countries, because of the absence of adequate supplies of indigenous natural gas, coke oven gas has been distributed for commercial and residential use. To release more coke oven gas for this purpose, research has been conducted on methods of

When gasification to make “water gas” was common in the United States, cyclic processes operating at atmospheric pressure and using coke or noncoking coals were used. These were high-cost operations, but test showed that water gas generators could be made continuous by using oxygen instead of air as the oxidizing medium; this procedure should reduce costs. These modifications in fixed- bed cyclic operations can be made to remove the ash in dry form or as slag. The kinds of coals that can be used, however, are still limited.

Pressure operation has significant economic advantages, and the Lurgi fixed-bed gasifier was developed to exploit these advantages. Commercial installations have been made in many countries using non-coking coals or weakly coking sized coals. The gasifying medium is a steam-oxygen mixture, and the process can be operated up to 30 atmospheres. The chief limitations of this process are the special properties required of the coals, the excess steam needed to prevent ash clinkering so that it can be removed in a dry state, and the relatively small throughput per unit of reactor volume. To overcome these difficulties, the Bureau of Mines, Grand Forks Lignite Research Laboratory, North Dakota, is conducting research to develop a method of removing the ash as slag. This method would permit greater throughput per unit volume of reactor and lower steam consumption. Two separate investigations are underway in England with the same objective, the first study by the Gas Research Council and the second study by the British Coal Utilization Research Association under the auspices of the Ministry of Fuel and Power.

The major limitation of fixed-bed gasifiers is the requirement that the coals be either noncoking or weakly coking. In the heavily populated areas of the United States, where much of the demand for synthesis gas or hydrogen would be concentrated, nearly all of the coals are strongly coking. Consequently, fixed-bed gasifiers cannot be operated unless the coal is pretreated. Pretreatment to destroy coking properties will probably be relatively expensive because methods of treatment at a reasonable cost still need to be developed.

As a result, research efforts in the United States have been aimed at developing gasification processes that can use any rank of coal and can be operated continuously. A Koppers-Totzek, atmospheric entrained gasifier using 1 ton of oxygen per hour was installed at the former Bureau of Mines Demonstration Plant, Louisiana, Missouri, and was successfully operated for several years. At the Morgantown Coal Research Center of the Bureau, several entrained atmospheric pressure gasifiers using both upward and downward flow of the reactants were tested. On the basis of these investigations, Babcock and Wilcox Company in cooperation with the Dupont Company constructed in West Virginia an atmospheric gasifier, which produced approximately 1 million cubic feet of carbon monoxide and hydrogen per hour. Because of economic considerations work was discontinued after successful operation of this unit.

The only full-scale entrained pressure gasifier operated in the United States was that erected by the Olin Mathieson Company in Morgantown, using the Texaco process. The gasifying medium at this plant, however, was air rather than oxygen. The Bureau of Mines and the Institute of Gas Technology have operated pilot-plant sized entrained gasifiers under pressure using oxygen. The Institute of Gas Technology gasifier operated at 105 pounds per square inch , while the Bureau of Mines gasifier operated at 450 pounds, per square inch. Refractory difficulties and erosion were problems in both reactors.

Fluid-bed gasifiers have been investigated in Europe and the United States. The major limitation on the fluid-bed gasifier is that it requires a noncoking or weakly to moderately caking coal. Commercial processes using this method of solid-gas contacting have been developed in Germany, and the Winkler, Winkler-Flesch, and Basf-Flesch-Demag processes have been commercially installed in several countries to make synthesis gas.

In France a pilot plant using the fluidized technique has been developed recently, which it is claimed can use any rank of solid fuel if the particle size and moisture content fall within specified ranges.

In the United States, Consolidation Coal Company is reported , to have operated a fluidized gasifier under pressure using a strongly coking bituminous coal. No reports have been published of this research. Hydrocarbon Research in the United States also operated a fluid-bed gasifier under pressure, but anthracite fines were used so that the coking problem was avoided. Up to 650,000 cubic feet per day of combustible gas were produced in this plant, and high reaction rates between carbon and steam were obtained.

Two new methods of gasifying coal have been proposed by Rummel in Germany. Both processes use a slag bath as a heat transfer medium. In the first, a single shaft gasifier, a mixture of coal, oxygen (or air), and steam is tangentially injected into a bath of molten slag. The coal remains in the slag at elevated temperatures long enough to complete its gasification. The slag is claimed also to act as a catalyst for the reaction. In the second process, in which a double shaft gasifier is used, a rotating slag bath is separated by a dividing wall to form two chambers. Air and coal are introduced into the slag in one chamber, and the coal is burned to raise the temperature of the slag. The slag circulates to a second chamber into which steam and coal are added and where the steam-carbon reaction takes place to produce synthesis gas. The double shaft process has been reported to have been operated satisfactorily, and on the basis of these investigations,.the North Thames Gas Board in England is planning to install a large pilot plant. The chief disadvantage of the double shaft process in producing synthesis gas or hydrogen is that it does not operate under pressure. The advantages, however, are that it requires no oxygen and should be able to handle any type of coal. On the other hand, the single shaft process, which has not been extensively investigated, should be adaptable to pressure operation, using either oxygen to produce synthesis gas or air to make producer gas.

Production of Liquid Fuels

Coal Hydrogenation

One of two processes used by Germany during World War II to produce large quantities of liquid fuels from coal was the Bergius process in which a powdered coal is mixed with a recycle oil stream and reacted with hydrogen in the presence of a catalyst in a two-step process. In the first reactor, operated at 10,000 p.s.i. and 900° F., a liquid phase is maintained, and the product is a middle oil. The middle oil is reacted with hydrogen in a vapor phase in the second step of the process, using a catalyst and operating at pressure of 10,000 p.s.i. and temperature of 920° F.

The liquid products made by the Bergius process are expensive because the extreme temperatures and pressures used necessitate high capital plant costs and the complexity of the process makes for high operating and labor costs. Immediately after the war several countries tried to make the process more economic in competing with natural petroleum products but were not successful; coal hydrogenation plants have been converted to other uses. In some of these plants, coal tars in place of coal were hydrogenated to liquid products because milder operating conditions and less hydrogen are needed for tars, but no plants in the free world are operating now, even in this more favorable manner.

Nevertheless, research on coal hydrogenation in the United States and abroad has resulted in many process improvements and improved economics. Improved catalysts and methods of using them have been discovered, and in the United States, the Bureau of Mines has demonstrated that when molybdenum concentrations are as low as 0.01 percent, Wyoming coals can be converted at 8,000 p.s.i. and 480° C. with throughput four times as great as the Germans were able to obtain with iron catalysts. In another attempt to reduce plant costs and complexity and thus product costs, the Bureau studied a one step process. However, coke formation occurred at lower catalyst concentrations where temperature control was possible; at higher catalyst concentrations, temperature control was not possible.

Interest in the higher boiling products developed in the United States as the result of increased demand for jet and gas turbine fuels, and other Bureau experiments were made at 2,000 p.s.i. and 465° C. to produce these types of fuels rather than the normal range of petroleum products. The much lower pressure used would reduce capital costs appreciably. At these conditions, 95 percent conversion of coal was obtained with throughput comparable to the high-pressure Bergius process. Lower catalyst concentrations resulted in lower throughput, but product characteristics remained the same. Tests using iron as the catalyst at several concentrations showed that 2 percent iron was equal to 0.1 percent molybdenum in tests with 1 percent iron, the throughput was reduced 25 percent.

Autoclave experiments were conducted by the Bureau in an effort to elucidate the mechanism of coal hydrogenation and to develop new and improved catalysts that would permit high conversion and large throughput at lower temperatures and pressures. No major breakthrough was discovered that would make a major change in the economics of the process.

No other research at the engineering level is reported in the United States or abroad, although Consolidation Coal Company and Standard Oil Company of Ohio have announced a joint research program involving hydrogenation of coal. At the laboratory level, some research is still underway on catalyst development and on basic studies that may reveal ways of hydrogenating coal more economically. For example, it has been suggested that, if a method could be devised to use the hydrogen-rich portion of the coal to produce liquid products and leave a less reactive char for producing the hydrogen required, significant savings would result. Some of the laboratory studies now underway may demonstrate how this can be achieved.


The volume of liquid fuels produced by the Fischer-Tropsch process was never as great in Germany as production by the Bergius process. When an early decision was needed to freeze production plant design, the Bergius process was selected because at that time it had been investigated longer rather than because of any consideration of the relative economics. On the contrary, cost estimates made after World War II in the United States seem to indicate that the Fischer-Tropsch process would be more economic.

In the Fischer-Tropsch process, coal is gasified completely to make synthesis gas, and the gas is reacted over a catalyst to make a variety of organic products. The major engineering problem is the removal of heat because the reactions are highly exothermic. The first Fischer-Tropsch plants used cobalt catalysts operating at pressures from atmospheric to 100 p.s.i. in fixed-bed reactors. The catalyst was surrounded by water-cooled heat exchangers to maintain temperature control. Later German plants used higher pressures and iron catalysts.

Unlike the coal hydrogenation plants, some Fischer-Tropsch plants recently were still operating in Europe but produced mostly a variety of organic compounds and small quantities of liquid fuels. It is not known if any Fischer-Tropsch plants are operating at present. In the United States a Fischer-Tropsch plant constructed at Brownsville Texas, used natural gas as the raw material but stopped production of liquid fuels in 1957. In South Africa, the SASOL plant with a 5,000 bbl. per day liquid product capacity began operating in 1957 and has been successfully producing since then. A recent announcement stated that the plant would be expanded to almost double its present capacity. Because there are no indigenous liquid or gaseous fuels in South Africa, this plant is important for strategic reasons. Moreover, foreign currency considerations, the low-cost coal available, the desire to use a native resource, and the high transportation cost of liquid fuels imported to the densely populated Johannesburg area where the SASOL plant is located all affect the overall economic and political decisions to expand the plant.

Several important engineering modifications have been made in the Fischer-Tropsch process to remove the exothermic heat, and major improvements in catalysts have reduced costs. All modern processes use pressures of 25 to 30 atmospheres. The hourly space velocity of a new fixed-bed process, installed at SASOL is 500 cubic feet per cubic foot catalyst hour, contrasted with only 100 cubic feet per cubic foot catalyst hour for the early German plants. The second process used at SASOL is the Kellogg entrained process operated at 300° to 340° C. and with hourly space velocities of 400. The fixed-fluidized process operated in Brownsville, Texas, used a mill-scale iron catalyst at 300° to 340° C. and hourly space velocities of 440 V/V-hr.

In a fixed bed pilot plant, experiments have shown that the heat of reaction can be removed by circulating oil over the catalyst bed. At 280° C. and hourly space velocities of 600, either a fused iron or an iron lathe turning catalyst can be used. In the “slurry process,” the catalyst is suspended in the oil that is used to remove the heat. The process operates at temperatures of 280° C. and at hourly space velocities of 300, using precipitated, fused iron, or mill-scale catalysts.

The most recent engineering modification of the Fischer-Tropsch tested in a pilot plant is the “hot gas recycle process,” in which the heat of reaction is removed by recirculating 15 to 20 volumes of cooled “make gas” for each volume of fresh gas. This process can be accomplished at moderate pressure drops with a catalyst of iron turnings and hourly space velocities of 1,000 and temperatures of 300° to 340° C.

Major advances have been made in recent years in developing iron catalysts that can withstand the mechanical and other burdens imposed by each of the engineering modifications of Fischer-Tropsch process. Moreover, all the catalysts have good selectivity, activity, life and low initial costs.

Research is continuing in the United States and abroad on improved catalysts and engineering modifications of the Fischer-Tropsch process, mostly directed toward more basic studies rather than pilot plant experiments. However, a pilot plant has been constructed in India capable of producing 100 gallons of oil per day.

Important technologic breakthroughs will be necessary to make liquid fuels .produced by Fischer-Tropsch process economic in most countries, and the first plants will have to depend on income from the chemicals produced.

Production of High-B.t.u. Gas

Development of a substitute for natural gas is of interest, but because gas of 800 B.t.u. content or more is not used commercially in many other countries, most of the research in producing gas of high heating value is being conducted in the United States. The Gas Research Council in England, however, has done considerable research on the direct hydrogenation of coal and investigations have been made in Australia using this process with brown coal.

Direct Hydrogenation

Production of methane from coal by direct hydrogenation would require only ½ the amount of synthesis gas that is needed for catalytic methanation. Because synthesis gas costs represent over 80 percent of the final gas cost, smaller gas requirements could have important economic implications. The Bureau of Mines and the Institute of Gas Technology are now experimenting to determine the optimum residence times and temperatures and pressures for this reaction. Pressures have been varied from 500 to 6,000 p.s.i. and temperatures from 1,000° to 1,800° F. Both entrained- and fluid-bed methods of gas-solid contacting have been tested. Long residence times and high carbon conversions are obtained using a fluid bed, but pretreatment of the coal is necessary to prevent coking. The product gas from fluid-bed reactors is high in methane and low in unreacted hydrogen, whereas the gas from an entrained reactor has a low methane content and would require a methane-hydrogen separation to obtain a high-B.t.u. gas. Entrained reactors, however, can use any kind of coal and require only short residence times.

Because the hydrogenation of coal to methane is exothermic, steam could be added to hydrogen to modulate the reaction and provide part of the hydrogen required. The endothermic steam-carbon reaction would use the heat released by the coal-hydrogen reaction. The potentials for using a steam-hydrogen mixture in a fluid-bed reactor are now being investigated by the Institute of Gas Technology.

In a balanced process, part of the carbon is used to produce the hydrogen required for the hydrogasification of the coal. Numerous methods have been proposed for using the more reactive portions of the coals high in hydrogen, to produce the methane. Low carbon conversions would be acceptable, because the unreacted char would be used to make hydrogen and in gasification an unreactive char would not be a serious drawback.

Catalytic Methanation

High-B.t.u. gas can also be produced by reacting synthesis gas over a catalyst. In the first experiments conducted in England, a nickel catalyst was used in a fixed-bed reactor. Nickel catalysts are sensitive to poisoning, and highly purified gas must be used. In later experiments at the Bureau of Mines, a Raney nickel catalyst was used in a fluid-bed reactor and produced satisfactory high-B.t.u. gas. Hydrogen was added at a number of inlets to insure good temperature control of the highly exothermic reaction. If it did not become sulfur poisoned, Raney nickel could be regenerated several times by a caustic soda treatment, thus giving good catalyst life.

The fluid-bed reactor provides a better means of modulating the heat of the reaction than a fixed-bed reactor, but internal water cooling will be required for each. In recent experiments by the Bureau of Mines, the hot gas recycle reactor developed for the production of liquid fuels has been shown to be an excellent method for producing methane. Moreover, iron lathe turnings which are less sensitive to sulfur poisoning and cheaper than nickel catalysts, can be used to produce a gas with heating value of 800 B.t.u. per cubic foot or more at pressures of 400 p.s.i., 340° F., and hourly space velocities of 800 to 1,000. If a gas of higher heating value were needed, the product from the iron catalyzed reaction could be passed over a Raney nickel catalyst to bring the heating value up to approximately 1,000 B.t.u. per cubic foot. This method of producing gas requires much less expensive nickel catalyst. The heat release problem in the nickel catalyst reactor would be much reduced because high concentrations of methane and low concentrations of synthesis gas are treated.

Production of Chemicals

In the United States, tar and light oil recovered at high- temperature carbonization plants are the only chemicals produced in significant amounts from coal. The production of many of these same chemicals from petroleum is increasing each year, and the petrochemical industry has had an important impact on coal chemical prices. Future production of chemicals from coal by carbonization or some other method will depend largely on whether the competition from the petrochemical industry can be met.

The production of chemicals from coal by extraction, oxidation, and other methods, and by low-temperature carbonization is discussed elsewhere in this paper. Two other methods can be used to make chemicals from coal. Synthesis gas, made by the gasification of coal, can be reacted over catalysts at different process conditions to make a variety of organic chemicals. Presently in the United States, synthesis gas is used commercially to make ammonia and alcohols, but all new installations in the last 15 years have used natural gas as the raw material. Whether coal will be used again to supply this market will depend upon the future cost of natural gas, at any location, compared with the cost of producing synthesis gas from coal by gasification. Improved and more economic coal gasification processes are, therefore, extremely interesting not only for producing liquid fuels and high-B.t.u. gas but also for manufacturing ammonia and methanol.

The other method of producing chemicals will be as a byproduct from plants making liquid fuels by the Fischer-Tropsch or direct hydrogenation process, when such commercial plants become economic. In a typical Fischer-Tropsch plant, 15 percent of the product could be water soluble alcohols (methanol to pentanols and higher), aldehydes (acetal to butyral), ketones (acetal to methyl butyl), and acids (acetic to valeric and higher), but if there were demand for these chemicals, the operation could be adjusted or the catalyst.could be changed to produce an even greater percentage of the plant products. For example, using a nitrided catalyst results in producing 40 percent of the product as alcohols. Chemical credits, however, will prove to be difficult to obtain because one 10,000-bbl, per day Fischer-Tropsch plant, supplying about 0.1 percent of domestic petroleum demand, would produce 216 million pounds of chemicals per year. Some of the chemicals produced by this one small plant would exceed their current market demand.

A similar economic situation exists for chemicals produced from coal hydrogenation plants, although aromatic the chemicals are made rather than the aliphatics produced in/Fischer-Tropsch process. A 10,000-bbl. per day plant, using an Illinois No. 6 bed coal, would produce 572 million pounds of phenols (carbolic acid to xylenol) and aromatics (benzene to naphthalenes). This one plant would supply 5 percent of the phenol requirements of the United States and more than the present market requirement for some of the other chemicals.

In 1952 Union Carbide Co. announced the development of a one-step process for hydrogenating coals directly to aromatic chemicals, using short residence times. No commercial plants have been constructed, but it is reported that Union Carbide and other chemical companies are still doing basic research on developing cheaper methods of producing chemicals from coal.

Briquetting and Recarbonizing to Metallurgical Fuel

Techniques to prepare a “formed coke” are not used as yet in the United States, but research and development in foreign countries have resulted in commercial use of this method of producing a metallurgical fuel. In Australia a brown coal is being dried to 15 percent moisture and briquetted under pressure without the use of binder. The resulting briquet is carbonized slowly in three stages of heating over a period of more than 20 hours, using a hot inert gas for carbonization. Despite a 52 percent shrinkage of the briquet, a strong metallurgical fuel is produced.

In Germany, preparing a strong hard briquet to produce a satisfactory metallurgical fuel has been emphasized rather than relying on special treatment during the carbonization step. Low-rank coals or chars are crushed and briquetted at optimum conditions to produce the hardest briquet possible and then carbonized at 900° to 1,000° C. Every effort is made to increase the density and strength of the briquet before carbonization is begun.

Also in Germany, coke breeze is converted into a metallurgical fuel by briquetting it with a mixture of 7 to 8 percent tar and 10 percent bituminous coal. The resulting briquet is then carbonized at 900° to 1,000° C.

In the United States the Bureau of Mines has conducted tests directed toward making a satisfactory metallurgical fuel from anthracite fines briquetted with bituminous coals and coal tar pitch and then calcined for 3 hours at 1,750° F. Good briquets can be made with many different proportions of bituminous coal and pitch. The effects of such variables, of coal-size consist, binder, briquetting pressures, and calcining temperatures have also been studied.

Foundry tests with this material indicated lower metal temperatures and the need for closer operating control than when using coke.

Coal Pretreatment-Destruction of Coking Properties

To successfully carbonize the strongly coking coals of the Eastern United States at low temperature, their coking properties must be altered. Many methods have been suggested for this purpose although all the operations are costly. At the present time, there is renewed interest in destroying these coking properties because a noncoking-sized coal is required for either dry-ash or slagging fixed-bed pressure gasification, and this method now seems to be the most economic process for the complete conversion of coal to synthesis gas and for direct hydrogenation to high-B.t.u. gas in fluid-bed reactors.

Coking properties of coals can be destroyed by oxidation at elevated temperatures, but if large lumps are pretreated in this way, only surface coking properties are affected. If the oxidized lump coal is recrushed and new unreacted surfaces are exposed, the coal again becomes coking. Moreover, the oxidation technique is relatively costly because long residence times are required. For example, a 1/8- by 3/8-inch Pittsburgh seam coal requires 50 hours of oxidation at 250° C. to destroy its coking properties. Even at 390° C., which approaches the softening temperature of the coal, 3 hours are required and at these higher temperatures considerable volatile matter is lost.

The degree of pretreatment required to prepare a suitable feed for a fixed-bed gasifier will probably be even more severe because exposure to hydrogen at high pressures tends to reverse the oxidation process and to restore the coking properties of coal. The Bureau of Mines is now testing to determine the degree of pretreatment that coal will require when exposed to a mixture of CO and H2 at the pressures’ used in fixed-bed gasification. In another series of tests the effect of oxygen concentration in the oxidizing gases on the rate of destruction of coke properties will be studied.

The pretreatment of coal to destroy its coking properties is expected to become more important in the future, and additional research is needed to solve this difficult problem.

Manufacture of Smokeless Fuels

Demand in the United States for smokeless fuels that meet air pollution regulations is limited, but in other countries a smokeless solid fuel made from indigenous coals has a good market potential. A small market for the smokeless fuel produced from several commercial low-temperature carbonization processes (Phurnacite and Rexco) has existed for a number of years in England, A process using low-volatile coals and a pitch binder was described under the section, Low-Temperature Carbonization.

Two other processes to make a suitable smokeless fuel have been investigated extensively in the last several years in England. In both processes, a char is made first by low-temperature carbonization of a high-volatile coal. In one process hot char is briquetted without binder by the “shape” principle, a briquetting operation that uses a shear compression. In the other process, briquetting the hot char with a minimum amount of binder forms a briquet that is satisfactory yet smokeless; a pilot plant to produce 120 tons of fuel per day is now under construction in England.

Research is now being conducted in England to produce a briquet from anthracite fines suitable for use in small boilers employed for domestic hot water and central heating. In this process the anthracite is heated to 530° C., mixed with pitch and coking coal, and briquetted at 430° C. This treatment produces a fuel with satisfactory strength and combustion characteristics.

Other foreign countries also are interested in producing a smokeless fuel for use in open grates for heating or cooking. Experiments in several countries to prepare a smokeless product from low-rank or high-volatile coals have used approaches similar to those being tested in England.

Use of Coal in Iron Ore Processing

Anthracite and coke breeze have been used for many years to sinter flue dust recovered at blast furnace operations. The use of magnetic taconites, which requires that the ore be ground to very fine sizes before it can be beneficiated, made necessary the development of a method to reconstitute the recovered iron and to make a suitable charge for the blast furnace. Finely ground ores are first pelletized with the addition of binders and then hardened in an oxidizing atmosphere. This hardening can be done in shaft furnaces, continuous chain grates, or chain grate-rotary kiln combinations. In the shaft furnace an external source of heat must be used and between 800,000 and 1-½ million B.t.u.’s per ton of pellets must be supplied when hematite is charged. Less B.t.u.’s per ton are required when magnetite is the raw material. In this method of hardening pellets, coal can be used as the source of heat.

When pellets are hardened on traveling grates, about 4 percent of coal is added to support combustion on the grates and coke breeze, anthracite, and other low-volatile coals are used.

A better charge to the blast furnace would be a pellet which, instead of being hardened in an oxidizing atmosphere, was hardened in a reducing atmosphere during which some prereduction of the pellet would take place. In pilot plants, several processes have been investigated to obtain such a prereduced pellet. The Dwight-Lloyd-McWane, Orcarb, and Freeman processes all produce either iron or sponge iron from pellets into which coal in some form has been incorporated. Other new processes for reducing iron ore also use coal but generally as the source of heat for a kiln or as the source of the reducing gas for producing the sponge iron or iron product.

The Bureau of Mines is currently investigating a process, in which lignite or lignite char would be added externally to the pellet and would produce a partly upgraded iron pellet of only 80 percent iron (contrasted with the 90-100 percent sought for other processes) and hard enough to withstand transport. The use of carbon from lignite for this purpose would be particularly attractive because the lignite deposits are nearer than other coal deposits to the iron ore deposits.

Other Coal Processing Methods

Many methods to process coal other than the foregoing have been suggested, and some have been tested in laboratory and pilot plant scales. Because coal is so cheap, less than ¼ cent per pound at the mine, it is a potentially attractive raw material for many uses. At present, however, there is still only very limited commercialization of any of these processing methods not only in the United States but also abroad where competing raw materials are relatively more expensive. Very attractive economic possibilities exist, however, because the finished products sell at many times the price of the coal used.


Coal has been subjected to many oxidizing agents, and uses for the humic acids produced have been investigated extensively. The caustic soda-oxygen oxidation appears to be the most attractive, and the acids produced have been reacted with polyamines and alkanolamines, which on further heating form a strong heat- and water-resistant resin. The humic acids produced by the oxidation also appear satisfactory for the warp sizing of many synthetic fibers.

Nitric acid oxidation of bituminous coal in laboratory tests have shown that the potassium and ammonia salts of the humic acids produced may be a satisfactory fertilizer. A semi-plant-scale process is being tested in Japan.

Recently, use of humic acids produced by oxidation to prepare a very low-ash char, which could be used for electrode carbon, has been proposed. In this process the humic acids would be reacted with alkali and the soluble salts would be separated from the ash, reprecipitated, and then carbonized to produce a tar and low-ash char. As yet, this process has not been applied commercially.

Naturally oxidized lignite (leonardite) is used as an additive to stabilize drilling muds, and it has been shown that oxidation of lignite can produce a material with the same properties as leonardite. This process may prove to be a more economic source of drilling mud additive in oilfields that are remote from the leonardite deposits and near to sources of other lignitic coals that could be oxidized satisfactorily.

Other Chemical Reactions

Research is under way at the Bureau of Mines to react coal with different chemical reagents, such as alkali metals and metal amines, to see if useful products can be produced. At the same time, studies are being started that may lead to using coal as the source of carbon for producing such chemicals as hydrogen cyanide, carbon disulfide, carbon black, and acetylene.

The reactions of lignite or of humic acids produced from any kind of coal with ammonia to form a fertilizer is being actively investigated in several laboratories in the United States and abroad. Ammoniated lignite or coal might be particularly Beneficial because it could serve not only as the source of nitrogen but as a soil conditioner.

The reaction of lignite and bituminous coals with sulfuric acid apparently produces a resin suitable for treating hard water, comparing favorably with other ion exchange materials. One manufacturer is reported to be producing this material commercially. A similar ion exchange material made from any kind of coal has been reported from India. This material is made by nitration and reduction and may find use in water softening or other special purposes, such as extraction and recovery of metals in analytical work.

A Wyoming subbituminous coal has been hydrolyzed, using caustic soda, and many potentially valuable chemicals have been identified, indicating that such a process may be used for converting this rank of coal to chemicals.

Work in England on reacting coal with gaseous trifluorides has resulting in forming liquid products similar to those of commercial fluorocarbons. No commercial production has been reported, but the products can be considered for use as hydraulic fluids, high-temperature lubricants, and other applications, when high-thermal stability is important.


Considerable research effort has been devoted to testing different solvents to upgrade coal. Commercial applications of some of these solvents have been made in some foreign countries,, and a few industrial developments have resulted from research in the United States. In Germany coal is extracted at elevated temperatures with coal tar and then filtered to produce a material that can be carbonized to form a low-ash carbon suitable for electrode production. A series of patents have been issued in Japan for producing electrode carbons by treating coals with an organic solvent at elevated temperatures.

Montan wax is produced commercially in the United States by extraction of a California lignite deposit using petroleum solvents and producing 280 pounds of crude wax per ton of lignite. The resins and waxes from a Utah coal have been extracted and are being used to produce satisfactory dielectric enamels. Five to seven percent of the coal is recovered in the extract. In England a research program to extract montan wax from lignite and peat was not successful, but the higher molecular weight compounds that were extracted appear to be a possible raw material for plastic production. Research toward developing such a process is now underway in Ireland.

Extraction of a North Dakota leonardite with an alkali solution is used commercially to produce a satisfactory wood- staining material.

To find another method of producing chemicals from coal, research at Pennsylvania State University has been directed toward showing that solution of the coal with aromatic oils may lead to a direct production of aromatic chemicals from coal.

Activated Carbon and Other Special Carbons

At one plant in the United States activated carbon has been produced commercially from a Texas lignite for almost 40 years but no other commercial operations have been reported as using other lignites. Bituminous coal is reported to be used for manufacturing granular activated carbon in at least two plants.

In 1958 production of electric furnace graphite from anthracite, petroleum coke, or coal tar pitch coke was reported by five companies operating nine plants in the United States.


Research aimed at reducing the sulfur in coke by adding a chemical to the raw coal during carbonization was conducted at Pennsylvania State University, but no success was reported for the different compounds tested.

Several organizations have attempted to dissolve coal in either coal tar road oils or in asphalt to produce a cheaper or more satisfactory road-paving material. No commercial applications have been reported.

Lump anthracite has been calcined under a variety of conditions to produce a metallurgical fuel for blast furnace use, and laboratory tests of its physical properties indicate that this product may prove to be a satisfactory metallurgical fuel.

Briquetting of coal fines in the United States and other countries to make effective use of coal or coke fines produced either in mining or during transportation is widely practiced. Coal or coke breeze is briquetted with a petroleum asphalt binder and sold for domestic use.

Coal Waste Utilization

The conversion of coal refuse to useful products serves a twofold economic purpose it produces income and eliminates disposing of the waste product. Using the waste or upgrading the byproducts from coal preparation plants and from the carbonization and combustion of coal has been a goal of the coal industry and its consumers for many years. Fly ash is used widely as a pozzolanic material for concrete manufacture in the United States and abroad. Fly ash is also used in manufacturing lightweight aggregates, in producing cinder blocks, as a drilling mud additive, and in combination with lime as a soil stabilizer and road base material.

Sintering of the refuse from coal preparation plants to produce a lightweight aggregate is practiced commercially at several plants. The use of cinders from coal combustion furnaces in cinder block manufacturing and the use of slag from wet bottom furnaces for road construction and for other purposes is growing.

Coal refuse is reported to be used atone plant to prepare aluminum sulfate by reacting the refuse with sulfuric acid, filtering the iron and aluminum sulfate solution from the refuse, and separating the two sulfates by fractional crystallization.

Carbon dioxide produced during the combustion of coals is being used to produce dry ice. Recovering sulfur dioxide from flue gases of boiler plants has been proposed, although no commercial plants are in operation.