Very little research on gasification of coal for low-Btu fuel has been done in the United States. For the production of high-Btu gas from coal four welldefined processes exist. By substituting air for oxygen in the initial step of any one of these four processes, a low-Btu gas can be made. However, the best process for pipeline gas may not be the most suitable for power gas (a large methane yield is desirable for high-Btu gas but not for low-Btu gas). There are so many options available for each step in low-Btu coal gasification that a very great number of different complete processes are imaginable. Coals melt and become sticky; at least seven ways to deal with this problem have been proposed. At least three approaches are available for the problem of gasification. Then there are six or seven ways to remove ash after gasification (it should preferably be removed with no carbon). Clearly a very large tableau would be necessary to describe all the options of different approaches to the three problems of coal handling, gasification, and ash removal.

Few of the possible coal gasification options have been studied. The only system that is now commercially available was developed in Germany in the 1930's. It is a "gravitating bed" gasifier, manufactured by Lurgi Gesellshaft für Mineralöltechnik GmbH in West Germany. The Lurgi system has several important limitations. The products of combustion of the Lurgi power gas contain large amounts of water vapor, the coal particles must not be smaller than 3 millimeters in diameter, and the size of the gasification unit appears to be quite small for the scale of U.S. power generation. More than 20 Lurgi gasifiers of the current design would be needed for a 1000-megawatt power station.

However, Lurgi gasifiers are available now, and the company is putting into operation at Lünen, Germany, an installation to supply power gas to a combined-cycle system, which has a gas turbine that will generate 74 megawatts of electricity and a steam turbine that will generate 98 megawatts. In the United States, the Commonwealth Edison Company of Chicago has plans to install three Lurgi units to gain experience with coal gasification, but the power gas will not be used to fuel a combined-cycle system.

The possibility of coal gasification to make a low-Btu power gas has been heavily publicized in the United States in the last year, and the Office of Coal Research of the Department of the

Interior is now considering proposals for the development of such a process. But the first new experiment on a gasifier to supersede the Lurgi design will almost certainly be performed in Europe. In Paris, France Albert Godel is planning to test a new design for a gasifier in early 1973. A coal gasifier called the Ignifluid boiler was developed by Godel and Babcock-Atlantique 17 years ago. The unit has been widely used (except in the United States), and successfully makes low-Btu gas with a fluidized-bed, a method for burning coal that has several advantages over a fixed bed. Air and steam are injected rapidly enough to buoy up the granular material (coal that is not carefully sized can be used). Because the fluidization allows easy movement of the solids from one part of the boiler to another, the temperature of the fluidized bed is uniform. Because steam is utilized much more efficiently in a fluidized bed than in a fixed bed, almost no steam appears in the power gas. However, because of the design of the grating on which coal rests, the Ignifluid boiler cannot be operated at high pressures. The new design, soon to be tested by Godel, has a different grating that is suitable for high pressure operation.

Other proposals for fluidized-bed processes have been studied on paper (or with small bench-scale experiments), but have never been tested. Arthur Squires and his colleagues at City College have proposed an elaboration of Godel's design that would treat fine carbon particles and ash differently. The City College gasifier would be shaped so that the fine particles would form a turbulent fluidized bed (called a "fast" fluidized bed) in the high velocity gases rising from a fluidized bed composed of larger coal sizes. The City College gasifier would incorporate Godel's ingenious discovery, used in the Ignifluid boiler, to collect the coal ash. Squires and his colleagues are also studying ways to clean hydrogen sulfide from power gas at high temperatures. The methods discussed earlier for cleaning power gas from residual fuel oil could be used for cleaning power gas from coal, but require that the gas be cooled.

Environmental Damage from Mining Although coal is so plentiful the U.S. supply may last 500 years, it cannot be removed from the earth without paying a high price for the upheaval of topsoil, the pollution of streams, and the safety and health of miners. Deep mining is one of the

most dangerous jobs in the country, and the US. techniques for deep mining are very inefficient compared to European methods. Deep mining causes subsidence of the land, apparent throughout Appalachia, and severe pollution of streams because the "run off" water from mines is heavily laden with acid. Most of the lands that have been strip-mined in the United States have not been reclaimed, and many observers question how effective any reclamation program can be. Demand for coal from western states such as Montana is growing because of its low sulfur content (about 1 percent versus 4 percent for many eastern coals), and stripmining techniques are being used now to "open" the western coal fields. Some observers argue that reclaiming land in the plains will be relatively easy and effective compared to the difficulties of reclaiming hillsides in West Virginia. However, much of the West is underlaid with a hard caprock that would be broken by extensive mining, and environmentalists afraid that the water table might fall significantly as a result of strip-mining.

Limited Funds for Coal Research

are

Nevertheless, the rate of progress of research in coal gasification could be very important for supplying energy in the future. Coal may be the only fuel available after gas and oil are depleted, especially if the output of nuclear power stations is limited by technical or environmental problems. But the total amount of money spent on coal degasification (about $40 million in the last 11 years) is less than one-third the amount spent on fission every year.

Through various coincidences of history, economics, and politics, coal research has been badly neglected in the United States. The program for production of high-Btu gas at the Office of Coal Research has still not produced a working pilot plant, and research on . low-Btu gas production was simply not funded until $3 million was provided for initial studies last summer. The shortage of natural gas is here, and the shortage of low-sulfur fuel is imminent. Coal gasification techniques to replace these fuels are needed, but so far the options are not available.

-WILLIAM D. METZ

References

1. A. J. Giramonti, "Advanced power cycles for Connecticut electric utility stations," Report L971090-2 prepared for the Connecticut Development Commission by United Aircraft Research Laboratories, East Hartford, Connecticut (1972).

Natural gas is one of our most precious

ENERGY energy resources. Not

only is it the cheap

est and most versatile

fossil fuel available but, perhaps most important, it is also the least polluting. As industry and utilities have been forced to meet ever more stringent air pollution regulations, this fortuitous combination of attributes has elicited a strong surge in demand for natural gas. Domestic consumption rose to 22.7 trillion standard cubic feet (1 scf 28.3 liters) in 1971, an increase of more than 73 percent since 1961.

This increased demand has pushed natural gas producers and distributors to the limits of their capacity. Many distributors no longer accept new customers, and some are even restricting the amounts of gas available to their present customers. Others are meeting their requirements only by importing liquefied natural gas at prices (about $1 per 1000 scf) that are generally more than twice the U.S. rate. Proven U.S. reserves, furthermore, are sufficient for only about 13 more years at the present rates of consumption (although the American Petroleum Institute projects that 66 percent of U.S. reserves has not been discovered yet). Some studies indicate that by 1980, demand could outstrip supply by 20 billion sef per day.

Frequently, however, a crisis proves to be the catalyst for new technological developments. The potentially massive gap between natural gas supplies and demand has thus brought to the forefront a host of both old and new technologies for producing synthetic natural gas (SNG) from other fossil fuels. Industrial plants incorporating this technology should begin contributing to gas supplies within 2 years, and by 1985 may contribute as much as 15 percent of the total.

The basic chemistry of gasification is simple. Carbon from coal or naphtha-the petroleum fraction with a boiling point between 175° and 240°C -is combined with water at high temperature to form methane, the principal constituent of natural gas. The overall reaction requires several steps, however, and is much more complex.

Naphtha gasification is considerably

simpler than coal gasification and is in a much more advanced state of development. Three different naphtha processes have been commercialized: the catalytic rich gas (CRG) process developed by the British Gas Council, the methane rich gas (MRG) process developed by Japan Gas Company, and the Gasynthan process developed by West Germany's Lurgi Mineraloeltechnik GmbH and Badische Anilin-und Soda-Fabrik AG. These processes are very similar in concept. The main difference is in the catalysts used.

Naphtha, Steam Form Synthetic Gas In the generalized process, vaporized naphtha is superheated under pressure and catalytically desulfurized. The sulfur-free vapor is then reacted with superheated steam at high temperature (500° to 540°C) and pressure (34 atm) to form "synthesis gas"-a mixture of methane, hydrogen, and carbon monoide-and carbon dioxide. This gas is then subjected to a catalytic methanation in which three molecules of hydrogen are combined with one of carbon monoxide to form more methane. After carbon dioxide and water are removed, the product gas is about 95 to 98 percent methane with an energy content between 980 and 1035 British thermal units (1 Btu = 1054 joules) per scf-the same as that of natural gas.

Each of the three processes has been proved possible in small-scale plants abroad, but there are as yet no commercial plants in this country. American utilities and gas producers have, however, ordered at least 25 such plants with a total capacity of more than 3 billion scf per day, and the first of these should begin production in less than 2 years. Some 17 of these plants will be based on the CRG process.

But these plants may represent no more than a stopgap to the natural gas crisis, for they themselves face a future feedstock shortage. Almost all of the naphtha to be used in these plants must be imported and is thus subject to both rigorous U.S. import quotas and the caprice of the producing countries. Increasing quantities of naphtha are also being used as raw material by the world chemical industry, and this competition will further

limit the amount of naphtha available

for gasification. Gas producers will thus be forced to seek more abundant supplies of raw material, and the sole suitable alternative is coal.

The United States has massive reserves of coal-enough to last more than 500 years at current rates of consumption. Yet much of this coal contains such high quantities of sulfur that environmental regulations prevent its use, or the energy content is so low that transportation of the coal to power plants is economically prohib itive. Conversion of this coal to sulfurfree SNG may thus prove to be the only feasible way to use it.

Coal gas has, of course, been suc cessfully used in the past in the United States and, particularly, in Europe. In its manufacture, however, no attempt is made to maximize the production of methane. The product thus usually contains more than 50 percent hydrogen and carbon monoxide, and the energy content is never higher than 450 Btu per scf. Such gas is useful in some applications and may, in fact, eventually find widespread use for onsite generation of electrical power. But it is not interchangeable with natural gas, and it cannot be transported economically over long distances.

The basic thrust of modern coal gasification technology has been to optimize methane production by conducting the initial hydrogasification reaction at much higher temperatures and pressures than are used in the production of coal gas. There are five major processes for coal gasifica tion, but only one-developed by Lurgi-has been commercialized, and then only in Europe. A commercial facility based on the Lurgi process may be operating in this country by 1976, however; El Paso Natural Gas Company, Houston, Texas, has announced that it will build a $250 million, 250 millionscf-per-day facility in northwest New Mexico and, subject to Federal Power Commission approval, construction should begin within a year.

The four U.S. processes are the Hygas process developed by the American Gas Association and the Institute of Gas Technology, Chicago, Illinois; the CO, Acceptor process developed by Consolidation Coal Company, Pittsburgh, Pennsylvania; the Bi-Gas process

developed by Bituminous Coal Research Inc., Monroeville, Pennsylvania; and the Synthane process developed by the U.S. Bureau of Mines. Major funding for the first three processes has come from the Interior Department's Office of Coal Research (OCR), which has spent about $40 million on coal gasification during the past 11 years and is expected to spend another $125 million by 1975. The Hygas and CO2 Acceptor processes are being tested in pilot plants; construction was begun this summer on a pilot plant for the Bi-Gas process, and the design work for a Synthane pilot plant is nearly complete.

The Lurgi process is based on technology that was originally developed in the 1930's for the production of coal gas. The technology has been updated, but the reactor is still a low pressure (28 atm), fixed-bed unit with a very small capacity. Consequently, a 250 million-scf-per-day plant based on the Lurgi process requires 31 gasifiers, whereas a comparable plant based on U.S. technology requires only two or three. Although the initial cost of the two plants is comparable, the Lurgi plant would thus be expected to require much more maintenance and may also be less reliable.

All of the U.S. processes have been proved feasible on a laboratory scale. The various pilot plant programs will attempt to solve the mechanical and chemical engineering problems inherent in scaling a laboratory reaction up to commercial size. The programs will also permit assessment of the economic feasibility of the processes.

The basic unit in each process is the gasifier, operating at pressures ranging from 20 to more than 70 atm and temperatures as high as 1500°C. In the generalized reaction scheme (Fig. 1), coal is admitted to the reactor under pressure and brought into contact with synthesis gas at temperatures of 600 to 800°C to drive off volatile components. The system may be designed to convert these components to methane or to capture them for use by the chemical industry.

The devolatilized coal is then transferred to the second stage of the reactor, where it is brought into contact with steam at temperatures greater than 900°C to form synthesis gas containing 40 to 65 percent methane. If necessary, the synthesis gas is subjected to a catalytic shift conversion similar to that employed in naphtha gasification. Carbon dioxide, hydrogen

Copyright

sulfide, organic sulfides, and water vapor are then stripped from the gas, and it is subjected to catalytic methanation. After further removal of water, the product is identical to the SNG produced by naphtha gasification.

The principal differences between the processes include the manner in which the coal is admitted to the gasifier, the type of reactor bed used (fixed or fluidized), and the source of heat for the gasification reaction. In the Hygas process, for example, the coal is admitted as a slurry in organic solvents, while in the other processes it is admitted as powder or lumps through a lock-hopper. In the Hygas and Synthane processes, moreover, some types of coal must be treated beforehand to prevent agglomeration in the reactor. Heat for the gasification process is generally provided by burning char-partially gasified coal-in either oxygen or air. In the CO, Acceptor process, however, heat is supplied by the exothermic reaction of calcined dolomite (CaO) with carbon dioxide to produce dolomite (CaCO).

Each pilot plant, furthermore, will incorporate a different system for shift conversion, cleanup, and methanation. By the time a proposed 60 to 70 million-scf-per-day demonstration plant is operating in 1976, therefore, most potentially useful processes will have been tested, and the best can be incorporated in the plant. It is quite possible, though, that no one process will be best in all cases. The CO2 Acceptor process, for example, currently seems best suited for the more reactive lignite and subbituminous coals found in the western United States.

whereas the Bi-Gas process appears best for eastern bituminous coals.

Numerous engineering problems have been encountered in operation of the pilot plants. Foremost among them has been the problem of melding all the subprocesses into a continuous one operating at high temperatures and pressures. Other problems have included structural failures and corrosion at high pressures, packing of the reactor beds caused by agglomeration of the coal, and the mechanical problems of transporting the coal through the system.

Despite these problems, OCR is optimistic about progress at the pilot plants. If work continues on the present schedule and at present funding levels, says Edward Larson, chief of OCR's division of contracts and administration, design of a $250 million, 250 million-scf-per-day plant (to be built with private funds) could begin by 1977, with the first such plant going into operation by 1981. From 12 to 37 such plants could be in operation by 1985, he suggests, with a total production of more than 9 billion scf per day.

Many subsidiary benefits could arise from the construction of such plants. Since all the materials used in the plant are domestic, they would help ease the growing balance of payments deficit. Total capital investment for each plant, including mines to produce 15,000 tons of coal per day per plant, will approach $350 million, bringing a modest boom to the chemical construction industry and pouring hitherto unprecedented amounts of capital into the economically depressed regions where most coal resources are located. -THOMAS H. MAUGH II

Reprinted from

27 October, 1972, volume 178, pages 386-387

ENERGY

Coal is the largest source of energy which is available now in the United States, but research into methods of improving the use of coal as a fuel has languished for lack of support. In recent years, however, more attention has been given to cleaning up coal-burning power plants, which despite their disadvantages are likely to continue as the mainstay of the utility industry for the rest of the century. Magnetohydrodynamic (MHD) generators that convert heat from combustion gases directly into electricity constitute one possible alternative, in that the high efficiency attainable with this technology would lead to reduced consumption of fossil fuels and markedly reduced thermal pollution. Perhaps surprisingly, it now appears that MHD generators also offer one of the best methods of eliminating sulfur oxide and reducing nitrogen oxide emissions from coal-fired power plants.

Despite its promise, MHD has yet to be demonstrated as a practical technology, in part because support for construction of large-scale experimental facilities has not been available. Substantial technical problems associated with the endurance of the equipment remain to be resolved-MHD generators operate at elevated temperatures, typically 2400°C in the gas entering the generator, and the hot residues from coal combustion are extremely corrosive-although most scientists in the field are confident that these will not present serious obstacles. Because the technology has not been demonstrated, its economic prospects are still uncertain, but preliminary estimates are favorable.

Research on MHD is becoming worldwide, with active efforts in Japan and several European countries. Several

laboratories in this country are working on MHD with support from the U.S. Department of Interior and the utility industry. A more ambitious effort is being conducted in the U.S.S.R., which is already testing an experimental 75 megawatt power plant incorporating an MHD generator. Recent U.S. visitors report that the plant has so far produced up to 4 Mw for brief periods and seems to be operating successfully. The Russian program is primarily oriented toward the use of natural gas as the fuel-a choice that makes design of the generator not as difficult as for ash-laden fuels such as coal.

The MHD generator is basically an expansion engine in which hot, partially ionized gases flow down a duct lined with electrodes and surrounded by coils that produce a magnetic field across the duct. Unlike the gas in a turbine, the expanding gas propels only itself, and the movement of the electrically conducting gas through the magnetic field generates a current in the gas that is collected at the electrodes. Thus MHD generators are compact, have no moving parts, and can potentially accommodate temperatures and corrosive gases that would destroy conventional turbines. Very high temperatures would be necessary to ionize combustion gases; but with the addition of small amounts of potassium or other alkali metals, temperatures in the range 2000° to 2500°C provide sufficient ionization to allow the process to work.

Power plants incorporating MHD generators would include, in addition to the generator itself, pressurized combustion chambers for burning the fuel and heat exchangers or other equipment for preliminary heating of the air fed to the combustor. The preliminary heating appears to be necessary to reach the required temperatures, unless oxygen in large quantity is added to

the fuel mixture, a procedure that would be uneconomical at present. Exhaust gases from the MHD generators themselves would be used, in full scale power plants, to generate additional electricity with conventional steam turbines; in most designs, MHD would provide about half of the electricity from the combined plant. The overall efficiency of the combined generating facility is expected to reach about 50 percent in the first full-scale MHD plant, as com pared to 40 percent for the best conventional or nuclear power plants; and with more sophisticated MHD designs the efficiency could reach 60 percent.

MHD generators need stronger magnets than ordinary generators do because of the lower conductivity of gases as compared to copper, and superconducting magnets will probably be used in commercial plants. Large superconducting magnets have been built for applications in high energy physics, but relatively few have been built for MHD purposes and they are still very expensive. Research with the field strengths equivalent to those that will probably be used in MHD power plants (50,000 gauss) is only beginning The electricity produced from MHD generators is inherently direct current, which must be converted before transmission over existing networks.

Endurance of the generator remains the most substantial problem facing those working on MHD. Only limited experience with long-term operation has been gained a few-kilowatt generator at the Avco Corporation in Everett. Massachusetts, has been operated for several hundred hours and a 70-kw generator has been run for 500 hours in the U.S.S.R. The major question about long-term durability is whether leakage of current and arcing between electrodes due to condensation and penetration of the seed material into the

generator wall can be avoided. Other potential problems include plasma instabilitics in the ionized gas arising from interactions between the flow and the magnetic field; corrosion of the generator walls or of the air heater by coal ash may also present difficulties.

Several generator designs have been proposed, but not yet tested, to overcome these problems. In the Avco design, coal ash will be deliberately allowed to condense on the generator walls, building up a protective layer that R. T. Rosa of Avco believes will keep seed material from shorting the electrodes. Research groups at the University of Tennessee Space Institute in Tulahoma, Tennessee, at Stanford University in California, and at the Westinghouse research laboratories in Pittsburgh, Pennsylvania, have proposed still other designs. For example, the Westinghouse team, headed by Stuart Way, prefer a design with hotter wall temperatures that will prevent ash buildup; Way believes that cleaner fuels than coal-such as char, which is produced as a byproduct in coal gasification plants-may ultimately be preferable for MHD. But there appears to be general agreement that the problems can be solved, and that long-term testing and further experience with a pilot plant will be necessary to arrive at the best design.

Perhaps the most promising aspect of MHD power generation is the potential environmental advantages of these plants over traditional coal burning plants. Experimental work directed by Daniel Bienstock at the U.S. Bureau of Mines laboratory in Pittsburgh, Pennsylvania, has confirmed that the seed particles react with sulfur to form potassium sulfate or similar compounds which are relatively easy to precipitate (1). Because the alkali particles are

recovered from the exhaust gases by cloth traps or electrostatic precipitators -indeed, must be recovered and recycled for economic reasons-essentially all the sulfur can be removed from even high-sulfur coals.

Nitrogen oxide emissions, which at one time were expected to be a problem because of the high temperatures in MHD plants, can also be reduced, Bienstock finds, if the coal is burned in a fuel-rich mixture and excess air is added further downstream. Work at Avco and in Japan has confirmed the Bureau of Mines results. Detailed calculations at Avco of the kinetics of the nitrogen oxide production and consumption indicate that the temperature and composition of the combustion gases can be controlled to reduce NO, concentrations to acceptable levels.

If the predicted efficiencies can be achieved, and several independent analyses of the potential of MHD indicate that they can be (2), thermal pollution resulting from discharge of waste heat into cooling water would be reduced substantially, and would be less than that arising from any existing power plant. If gas turbines were used as the second half of an MHD power plant, the steam cycle and the need for cooling water could be eliminated.

Reliable estimates of the cost of MHD plants are not yet available, although there appears to be general agreement that construction costs should be approximately the same as those for traditional coal-fired plants and that, because of the more efficient use of fuel, operating costs per kilowatt would be significantly lower. The cost of the magnets is the largest single item, and the air preheaters are also expected to be expensive. Hence reductions in the cost of superconducting magnets or the cost of oxygen, which by enriching

the fuel mixture would reduce the preliminary heating and hence the cost of the equipment, could further improve the prospects for this source of power. Developing the technology will be neither cheap nor without some financial risk, so that in the absence of substantial federal funding-which has not been forthcoming; current R&D spending is at the rate of about $3 million per year-only limited progress can be made. Both demonstration and commercial power plants will probably have to be large, because the power output of an MHD generator increases in proportion to its volume, while most of the losses increase in proportion to its surface area; 100 Mw is estimated to be the smallest size feasible for a plant that does not use oxygen enrichment.

The use of coal or fuels derived from coal to generate electricity carries with it the high cost to the environment and to human health that is associated with the mining of coal. But this energy source will not be replaced overnight; indeed the production of coal is expected to increase in coming years, so that efforts to improve the use of this fuel would seem well worthwhile. MHD generating plants would, at the very least, diminish the impact of mining by producing equivalent amounts of power with less fuel. What appears to be needed, as one proponent put it, is to "give MHD a chance."

-ALLEN L. HAMMOND

References

1. D. Bienstock, R. J. Denski, J. J. Demeter, "Environmental Aspects of MHD." in Proceedings of the 1971 Inter-Society Energy Conversion Engineering Conference (Society of Automotive Engineers, New York, 1971). 2. "Open Cycle Coal Burning MHD Power Generation: An Assessment and a Plan for Action," Report of the MHD Power Generation Study Group, Massachusetts Institute of Technology (U.S. Government Printing Office. Washington, D.C., 1971).

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