By William H. Allen

Coal research aimed at No. 1 enemy: sulfur

Scientists are waging a multiple-front research war to solve a problem that has crippled exploitation of the state's massive reserves of coal — too much sulfur.

AT FIRST glance, it looks as if David Rapp is making a large, licorice milk shake. He switches on an electric motor that spins a steel shaft reaching down into a big glass container filled with a black, foamy brew. Under the surface a propeller-like mechanism churns the solution, which rises in a froth and spills out over the lowered front lip of the container into a collecting tray. The concoction — a mixture of Illinois No. 6 coal, water, kerosene and a foaming agent — finally stops bubbling. He switches off the motor and carries the tray away for chemical analysis.

Rapp, a scientist at the Illinois State Geological Survey, is experimenting in his Champaign lab with a process called "aggregate flotation coal cleaning." He and his colleagues are trying to develop this new way of taking sulfur, a major pollutant, out of high-sulfur coal before it is burned in power plants and other industries. The project is one of dozens sponsored by the Department of Energy and Natural Resources' Coal Development Board through the fledgling Center for Research on Sulfur in Coal (CRSC). Established in 1982, the center is coordinating a scientific attack on a problem of great economic importance: the high-sulfur content of coals in the Illinois Basin, a massive reserve that covers parts of Indiana and Kentucky and most of Illinois. Since it began three years ago, the CRSC has marshaled about $4.7 million in state funds and nearly $1 million in federal and industry money to support the research of 150 scientists at various institutions in Illinois.

William L. Wells, director of the Center for Research on Sulfur in Coal, with analytical equipment used by scientists affiliated with the center.

The story behind the CRSC begins with the simple chemistry of coal combustion. Coal, which is ages-old, decayed organic matter, consists primarily of carbon and hydrogen atoms combined chemically. Ideally, when coal is burned, the chemical bonds between these atoms are broken and energy is released in the form of heat. Atoms of carbon and hydrogen combine with oxygen in the air during combustion to form carbon dioxide, carbon monoxide and a small amount of gaseous water (steam). The solid residue that remains is drained of its energy. This residue, or ash, comes from mineral matter in the coal. That's the ideal case.

For Illinois Basin coal the process is not so simple. The coal does have a high heating value, an advantage in the marketplace. But besides carbon and hydrogen it also contains relatively large amounts of other elements, especially sulfur, and this is its big disadvantage. The sulfur, when burned with the rest of the coal, combines with oxygen to become primarily sulfur dioxide. To make a long story short, clean air regulations restrict the amount of sulfur dioxide a power plant can release through its smokestack, and the regulations may become even stricter. This has reduced the economic potential of Illinois' high-sulfur coals. If the sulfur problem can be minimized or eliminated, the market for the coal could increase. The state has turned to its scientists for help.

Current regulations for new plants require that at least 90 percent of the sulfur in Illinois coal be removed before it gets to the atmosphere, says William L. Wells, director of the CRSC, headquartered in Champaign. Wells came to the CRSC in June 1983 from the Tennessee Valley Authority, where he led research on coal pollution-control technologies. "We're encouraging basic research to try and spot those techniques and discoveries that look promising," he said. When a new idea works in the lab, its economics will be analyzed. If it still looks good, more research will be done to develop the process so that it can be applied commercially. Because of the complex nature of Illinois coal, a successful treatment, if discovered, probably will comprise two or more techniques.

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Sulfur must be removed from coal before, during and/or after combustion. Researchers associated with the CRSC are working in six major areas: (1) "coal characterization," (2) coal cleaning by physical, chemical and biological means, (3) fuels and chemicals from coal, including "multiple-phase" products, (4) combustion technology, such as fluidized-bed combustion, (5) gas cleanup, including the "scrubber" process, and (6) related desulfurization studies, such as economic feasibility and waste management. Emphasis has been placed on pre-combustion methods — the main focus of the first three areas listed — because that is where the most questions remain unanswered. To find answers, CRSC scientists are investigating the basic chemical structure of various kinds of Illinois coals, the way these coals burn, and what physical and chemical methods best remove the sulfur before it is burned in a power plant.

'Unlocking' sulfur

Sulfur is locked into the coal in two basic ways. When it is bonded with the carbon atoms, it is called organic sulfur. When it is enmeshed with the mineral matter, it is called inorganic. Most of the inorganic form occurs as iron disulfide, also known as pyrite. Coal from different parts of the Illinois Basin can have different proportions of organic and pyritic sulfur, but the ratio is generally 50-50. That's important. The organic sulfur can be removed from coal only through chemical means, the pyrite primarily and most economically through physical means, although chemical removal, is an option. Scientists accordingly have concentrated on both physical and chemical methods of getting rid of the sulfur before the coal is burned.

Characterizing coal. Essential, basic information in the research attack is provided by scientists working on coal characterization, the study of the organic chemistry of high-sulfur coal, how the sulfur atoms are bonded in the coal, what happens if those atoms are removed, and how well the coal burns afterward. Coal characterization relies on such state-of-the-art techniques as nuclear magnetic resonance spectroscopy, scanning-electron microscopy and multiplex gas chromatography to analyze minute quantities. Multiplex gas chromatography is one of several methods used at Southern Illinois University at Carbondale by Gerald V. Smith and colleagues, who are attempting to detail the various types of organic sulfur in coal. They know from previous research that coal with what is known as aromatic sulfur is more difficult to desulfurize than coal with other kinds of organic sulfur. They are testing the properties of coal samples that have several kinds of organic sulfurs. Using X-ray detection techniques in an electron microscope, Charles Wert and colleagues at the University of Illinois at Urbana-Champaign have measured the loss of organic sulfur from Illinois No. 5 coal as it was heated to successively higher temperatures. By 600 degrees Celsius (1112 degrees Fahrenheit), the organic sulfur declined to about half its original concentration, but it did not drop much more until heated well above 800 degrees Celsius (1472 degrees Fahrenheit).

Chao-li Liu of the Illinois Stale Geological Survey with the pyrolysis and combustion system with which he and his colleagues conduct "stable isotope analysis" of high-sulfur coal.

Fines and flotations. While these scientists seek basic understanding through coal characterization, others are grinding coal, mixing it with chemicals and exposing it to microbes. By applying various physical, chemical and biological methods, they are trying to find the most effective ways to remove sulfur. In the foamy aggregate flotation method mentioned earlier, Rapp was working on a variation of a traditional physical coal-cleaning strategy called "froth flotation." In froth flotation, ground-up coal is vigorously mixed in water. Air is bubbled through the water; the coal particles selectively attach to bubbles in the agitated water and rise to the surface. The mineral matter remains behind. But because the coal particles still have pyritic sulfur attached to them, they must be ground finer. The trouble is that when the partides are ground to the required dimensions — less than 400 mesh, or about 1/625th of an inch in diameter — they are too small to be captured by bubbles in the froth-flotation mixture. So Rapp and his colleagues are experimenting with the process by adding kerosene and foaming agents, which cause the fine coal particles, called "fines," to agglomerate into clusters that are picked up by the bubbles. Success with batch-processing experiments has led to work on a small pilot-plant process.

"New mining techniques are creating more fines," said Henry P. Ehrlinger III, a Geological Survey minerals engineer who leads the aggregate flotation project. "With an increasing amount of fines coming to plants and with changing economic and environmental conditions, using agglomeration techniques makes more and more sense." In work with Illinois No. 6 coal the group has found that the aggregate flotation method can remove about 84 percent of the pyritic sulfur and retain 92 percent of the coal's heating value. Among other materials the researchers are investigating are fines from the waste stream of a southern Illinois mine.

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Aggregate flotation is not the only physical coal-cleaning method under scrunity. For instance, R.D. Doctor and colleagues at Argonne National Laboratory are investigating new approaches to magnetic separation of pyrites using a superconducting magnet. Dimitri Gidaspow and Darsh Wasan of the Illinois Institute of Technology are studying electrostatic removal methods.

Breaking the bond. But even if the physical techniques are perfected and all the pyritic sulfur is removed, half of the sulfur in the coal remains. "The organic sulfur is the real problem," said director Wells. "We want a clean, solid product, and today there's nothing economically sound or commercially available to get at the organic. That's where most of our money has been going.

Scientists concerned with chemical coal cleaning are mainly trying to find substances that will react with the organic sulfur. "Our strategy is to find some chemical reagent that can break the coal-sulfur bond and separate out the sulfur so we can literally wash it away," said Karl S. Vorresof Argonne. Among several chemical projects are experiments by Vorres and John E. Young, also at the national lab, with copper compounds reportedly capable of breaking the coal-sulfur bond. They also will be investigating several chemical "cousins" of the copper compounds.

Sulfur-eating 'bugs.' Organic sulfur also can fall prey to microorganisms, or "bugs," as microbiologists affectionately term the object of their study. Bacteria have long been known to consume organic sulfur found in the natural world, including that in high-sulfur coal. The main drawback is that bugs do not "eat" sulfur fast enough for a microbial coal-cleaning process to pay off economically. Scientists are trying to link a microbial process with some other, nonmicrobial sulfur-removal technique to get rid of the organic sulfur rapidly and inexpensively. "It's a viable technique, but we've got to overcome the rate problem," said J. Bruno Risatti, a microbial geochemist at the Geological Survey. A fine-texture coal product might be desulfurized more rapidly, since bacteria would have more opportunity to come into contact with exposed sulfur, Risatti noted. He and his colleagues are investigating how well a bacterium called Sulfolobus acidocaldarius removes sulfur from coal products that remain after the coal has been heated under various conditions.

Chars and feedstocks. Scientists in the research area of "fuels and chemicals from coal" are taking highsulfur coal and, through elaborate processes, changing it into useful gas (gasification), liquid (liquefaction) or solid (charring) products that can then be burned in a power plant. Although gasification and liquefaction research is under way, much of the interest — and funding — is focused on "multiple-phase" products. Multiple phase refers to the different physical states (gas, liquid and solid) of the products that result when the coal is chemically processed or heated. These researchers are trying to develop economical methods of removing sulfur in the temperature range of 250 to 600 degrees Celsius (482 to 1112 degrees Fahrenheit) — leaving a "clean" product that will then burn well in a plant.

A possible breakthrough in multiple-phase
research came this fall when a Geological Survey group
took advantage of an accident

Carl W. Kruse leads the work on the "low-temperature charring" technique at the Geological Survey. In this technique, coal is heated in the absence of air to expel volatile components such as hydrocarbons, hydrogen, hydrogen sulfide and organic sulfur compounds. The gases, which can be converted into useful fuels and chemical feedstocks, are collected. The solid product, called "char," is a finely divided fuel, with a reduced sulfur content that can be reduced even more by chemical processing.

Another multiple-phase products technique, called "supercritical extraction," is being investigated at SIU-C by Charles B. Muchmore, Juh W. Chen and colleagues. In this process a mixture of coal and alcohol is heated under high pressure to selectively remove the organic sulfur from the coal.

Scientists at SIU-C and the Geological Survey are working on a third multiple-phase process, "In-Situ Preparation of Iron Sulfide Catalysts." Smith of SIU-C, Richard H. Shiley of the survey and others are refining the three-step process. They heat coal in the presence of carbon monoxide to remove much of the pyrite, then treat it with ethanol to get rid of organic sulfur. The third step is to remove iron sulfide, which catalyzed some of the chemical reactions.

Low-temperature charring, supercritical extraction and the iron sulfide catalyst process have two major hurdles to overcome: economic infeasibility and the inability to remove enough sulfur to meet environmental requirements.

Serendipity

A possible breakthrough in multiplephase research came this fall when a Geological Survey group took advantage of an accident. Dennis D. Coleman, Chao-li Liu and colleagues conduct "stable isotope analysis" of high-sulfur coal to understand better what chemical reactions occur during low-temperature charring. Their analytical method measures different forms — isotopes — of sulfur to obtain a "fingerprint " of the organic and pyritic sulfur in a coal sample. They found that when their coal samples were heated to just over 500 degrees Celsius (932 degrees Fahrenheit), more than half of the organic sulfur came off, but little or no pyritic sulfur did. How, then, to get rid of the pyrite? A potential answer was revealed when Liu one day noticed that a heated coal sample contained a form of highly magnetic pyrite that could be separated easily from the rest of the sample with a magnet. He then discovered a small leak that let a trace of oxygen into the experimental system used to heat the samples. Apparently the small amount of oxygen caused the conversion to the magnetic form. "We don't completely understand the reactions yet, but this has opened up a new area of work," said Coleman, who coordinates the project. "It's very exciting." It also is a good example of the unpredictability, serendipity and value of basic scientific investigation.

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Commercial technologies

Scientists associated with the CRSC are conducting during-combustion research to help answer some of the questions that remain about "young" coal-burning technologies already adopted by industry. Existing or planned industry demonstration plants use such technologies as atmospheric-fluidized-bed combustion, pressurized-fluidized-bed combustion, limestone-injection multiphase burning, and circulating-bed combustion. In general, these methods capture sulfur at the point when the coal is burned in a power plant, that is, during combustion. The basic idea is to heat a mixture of limestone and coal so that when the sulfur in the coal forms sulfur dioxide, the limestone immediately captures the gas.

As the utility industry proceeds with demonstration plants for these technologies, various CRSC-affiliated researchers are studying such phenomena as the movement of particles in a circulating bed of coal and limestone and the corrosion and erosion of tubes in the boilers.

In the gas cleanup area, post-combustion research is under way on processing the "hot gas" of various coal-burning technologies and on the scrubber. Scrubbers essentially remove sulfur from the gas that rises up through power plant smokestacks. After some initial stumbling, the scrubber is now a widely adopted technology, Wells said.

"One of the biggest problems was the chemistry of the scrubber, and I think it has been solved," he said. Such difficulties as nozzles clogged by solid precipitates were remedied by better understanding of the chemical reactions that occur when sulfur dioxide is removed from the hot gas. Progress has been made on materials issues, too. Many pipes dissolved and pumps failed, but scientists learned what alloys best resisted the highly corrosive chemicals in the scrubber systems.

They are still, however, confronted by the challenge of how to deal with sludge, the end-product of the scrubber process. CRSC-affiliated scientists in the "related desulfurization studies" area have launched several investigations of such disposal problems as contamination of groundwater and reducing the volume of sludge. Other researchers in this area go beyond the science of high-sulfur coal to its economics. They investigate the economic problems that face Illinois Basin coal and the feasibility of the methods being developed to clean it.

Coal 'bank,' communication

To facilitate the attack on sulfur in coal, the Coal Development Board maintains a coal "sample bank." Several tons of coal from each of four active Illinois mines are kept at the Geological Survey in plastic bags in dozens of 55-gallon steel drums. A nitrogen atmosphere is maintained inside the drums to keep the chemistry of the coal samples from changing. Coal from a fifth mine is expected to be added during the current fiscal year. A "premium" coal bank, which contains samples that are packaged more carefully, is being filled at Argonne for the U.S. Department of Energy. Such banks are important because they help eliminate confusion among scientists when comparing experimental results. "It's very important to have the same starting point," Wells said.

The CRSC is an important bridge for communication between industry and scientists. A new committee of industry representative helps review and select research projects, and for the first time in the center's short history, an ad hoc group consisting of almost all of the coal companies in Illinois contributed $392,000 to the center's fiscal year 1985 research budget.

"We think the scientists can benefit from hearing the practical problems and direct some of the research and thinking towards that end," Wells said. "And on the other hand, we think the industries can benefit because many times some of the research ideas can turn a commercial operation around." To enhance the interchange, the CRSC and the Coal Development Board sponsor a technical meeting each fall at which scientists present the latest research findings.

Perhaps more important than fostering a dialogue between scientists and industry is the center's role in increasing collaboration among scientists at the state's major research institutions. At first, the CRSC concentrated the facilities and expertise at the Geological Survey, University of Illinois at Urbana-Champaign and Southern Illinois University at Carbondale. In January 1984, Argonne, Northwestern University and the University of Illinois at Chicago became member institutions as well. In December 1985, further reorganization opened membership in the CRSC to all the state's scientific institutions.

"We've got people in Illinois who two years ago didn't even know each other existed and who are now collaborating actively on projects," Wells said. "They are sharing coal samples, lab equipment and ideas. That's a very positive step."

Continuity and consistency in research funding will be necessary to give the scientists associated with the CRSC a chance to win the battle against high-sulfur coal. Research programs are not built overnight; ideas must be developed, labs established, equipment acquired, technicians trained. "It has taken three years for us to get moving fairly well," Wells said. "It's going to take a while more, but we're starting to get somewhere."

Wells credits the legislature, Gov. James R. Thompson, the Coal Development Board and the Department of Energy and Natural Resources with establishing a long-term commitment to basic research on the high-sulfur issue. The institutions that conduct such research generate essential ideas and train students who go to industry. Historically, it takes 20 years or more to move from an idea to a commercial product, with no guarantees of success, Wells pointed out.

"But you're certainly not going to get an answer to these problems if you don't do research," he said. "We may not get it, but we know we won't if we don't try."

William H. Allen is senior science editor in the university News Bureau and a visiting lecturer in the Department of Journalism at the University of Illinois at Urbana-Champaign.

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