Sulfur from coal

Environmental considerations also were reflected in coal production and consumption statistics, including regional production patterns and economic sector utilization characteristics. Average coal sulfur content, as produced, declined from 2.3% in 1973 to 1.6% in 1980 and 1.3% in 1990. Coal ash content declined similarly, from 13.1% in 1973 to 11.1% in 1980 and 9.9% in 1990. These numbers clearly reflect a trend toward utilization of coal that produces less SO2 and less flyash to capture. Emissions from coal in the 1990s were 14 x 10 t /yr of SO2 and 450 x 10 t /yr of particulates generated by coal combustion at electric utiUties. The total coal combustion emissions from all sources were only slightly higher than the emissions from electric utiUty coal utilization (6). 

Nitrogen, unlike pyritic sulfur, is mosdy chemically bound in organic molecules in the coal and therefore not removable by physical cleaning methods. The nitrogen content in most U.S. coals ranges from 0.5—2.0 wt %. 

More recendy, the molten caustic leaching (MCL) process developed by TRW, Inc. has received attention (28,31,32). This process is illustrated in Eigure 6. A coal is fed to a rotary kiln to convert both the mineral matter and the sulfur into water- or acid-soluble compounds. The coal cake discharged from the kiln is washed first with water and then with dilute sulfuric acid solution countercurrendy. The efduent is treated with lime to precipitate out calcium sulfate, iron hydroxide, and sodium—iron hydroxy sulfate. The MCL process can typically produce ultraclean coal having 0.4 to 0.7% sulfur, 0.1 to 0.65% ash, and 25.5 to 14.8 MJ/kg (6100—3500 kcal/kg) from a high sulfur, ie, 4 wt % sulfur and ca 11 wt % ash, coal. The moisture content of the product coal varies from 10 to 50%. 

Where low-grade coal is burned, electrostatic precipitators or fabric filters may be required for flue-gas particulate collection and a wet desulfurization system (gas scrubbers) to remove sulfur from the flue gas. 

The vast majority of sulfur at any given time is in the lithosphere. The atmosphere, hydrosphere, and biosphere, on the other hand, are where most transfer of sulfur takes place. The role of the biosphere often involves reactions that result in the movement of sulfur from one reservoir to another. The burning of coal by humans (which oxidizes fossilized sulfur to SO2 gas) and the reduction of seawater sulfate by phytoplankton which can lead to the creation of another gas, dimethyl sulfide (CH3SCH3), are examples of such processes. 

Atlantic Richfield Company has reported strains of Pseudomonas sp. CB1 (ATCC 39381) [108] and Acinetobacter species CB2 [109] (ATCC 53515) to be effective for the removal of sulfur from organic molecules found in petroleum, coal, etc. In fact, the aerobic and heterotrophic soil microorganisms Pseudomonas CB1 and Acinetobacter CB2 were reported to convert thiophene sulfur into sulfate, using a bench-scale continuous bioreactor. The direct contact with Illinois 6 coal reduced the organic sulfur content in about 40% to 50%. As already mentioned, most of this work was carried out on coal. Further work was not pursued probably due to decrease in coal usage or due to the economics of the processes. 

The developed process aimed for the removal of iron and sulfur from coal. While several patents for microbial removal of pyrite from coal exist, this one is unique because of the microbial mixture developed by the group for this purpose, which is partially... 

Ledgemont A process for removing sulfur from coal by an oxidative leach with lime and ammonia. Developed by Hydrocarbon Research. 

Fluidized bed combustion is a newer technology that burns coal in an efficient manner and can produce both electricity and heat. A mixture of finely crushed coal and limestone rides on a stream of air, which allows the coal to be burned at temperatures lower than conventional coal burners. This reduces the nitrogen oxide produced. The limestone absorbs sulfur from the coal, which reduces the sulfur dioxide. 

Over a three-year period at Rheinau, 1922-1925, Bergius and his assistants tested more than 200 different kinds of coal. Starting from a relatively small scale, they eventually hydrogenated coal in quantities as large as 1,000 kg (1 ton). A typical reaction run contained 100 kg of coal mixed with 40 kg of heavy oil, 5 kg of hydrogen gas, and 5 kg of ferric oxide to remove any sulfur present in the coal. The reaction yielded 20 kg of gas and about 128 kg of oil and solids. Distillation of the oil produced 20 kg of gasoline. (14)... 

A renewed interest in the behavior of volatile electrolyte solutions appeared around 1975. It was raised by the need of better design of industrial processes, especially pollution control processes, elimination of acid gases from natural gas, removal of sulfur from liquid and solid fuels and more recently coal conversion processes. 

The goal of beneficiation is to remove as much sulfur from a fuel as possible before it is ever burned. When burned, fuel with lower sulfur content will produce less sulfur dioxide. Beneficiation is usually accomplished by a physical process that separates one form of sulfur, pyritic sulfur, from coal. Pyritic sulfur consists of sulfur minerals (primarily sulfides) that are not chemically bonded to coal in any way. The name is taken from the most common form of mineral sulfur usually found in coal, pyrite, or iron sulfide (FeS2). 

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