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Usibelli tries clean coal technology.

Mixing government seed money with innovative private sector technology, Usibelli Coal Mine Inc. and several partners will test prospects for burning coal more cleanly. Usibelli is supplying the coal and a site at Healy for construction of a new power plant, which will be built and operated by Golden Valley Electric Association to supply power to its growing market in Fairbanks.

The U.S. Department of Energy is putting up $93.2 million and the Alaska Legislature has appropriated $29 million for the project that will be owned by the Alaska Industrial Development and Export Authority.

At the heart of the project is a leading-edge coal combustion unit designed by TRW Combustion Business Unit. Besides burning coal with a lower level of waste emission, the technology provides other cost-saving and conversion advantages. According to Charles Green, a Usibelli marketing specialist, the system reduces maintenance costs and allows retro fitting existing coal- and oil-burning plants with lower capital costs.

Green says the'clean coal'project is a scaled back version of original plans that called for waste heat from the plant to be used to dry Usibelli coal to create a more exportable fuel for sale to the Orient. According to Usibelli consultants, early coal drying tests were not sufficiently conclusive to warrant construction of a plant for that purpose. The experts recommended that the coal-drying element of the project be dropped to strengthen the proposal for federal funding.

What's left, says Green, is 'essentially a demonstration use of new technology to burn coal cleaner than it's been done before.'

Experiments in coal beneficiation the process of upgrading raw coal to increase its quality - will continue, according to Green. In fact, techniques such as mild gasification and conversion of coal to liquids have shown somewhat more promise than drying and will be pursued by Usibelli on a path parallel to the Healy clean coal project.

According to the federal Department of Energy, a number of recent advances in coal-burning technology have focused on combining high combustion efficiency with pollutant removal. A number of these new developments are based on the 'cyclone' combustor concept. The coal is burned in a separate chamber outside the furnace cavity. The hot combustion gases then pass into the boiler where the actual heat exchange takes place.

The advantage of a cyclone combustor is that the ash is kept out of the furnace cavity where it could collect on boiler tubes and lower heat transfer efficiency. To keep ash from being blown into the furnace, the combustion temperature is kept so hot that mineral impurities melt, forming slag that is forced to the outer walls of the combustor where it can be removed.

The high temperature is also the greatest disadvantage of cyclone combustion: It creates unacceptable levels of nitrogen oxide.

Another recently developed coal burning process is accomplished with a fluidized bed combustor, and involves suspension of crushed coal and limestone on a bed of air. As the coal burns, the limestone absorbs the sulfur released by combustion.

According to literature provided by TRW, the entrained coal combustion planned for use in the Healy project differs significantly from the processes described above. In entrained coal combustion technology, pulverized coal is injected into a precombustion chamber where it is mixed with swirling air and partially burned. Ash is converted to slag and drained off. Removal of more than 80 percent of the ash before reaching the boiler minimizes conventional maintenance resulting from corrosion and allows the unit to be used with boilers originally intended for other fuels.

The next stage of the entrained coal combustion process provides additional air in a secondary burner where further combustion of the hot gases takes place. At this point, limestone is injected into the gases to react with and provide initial removal of sulfur dioxide. Temperatures and oxygen levels throughout combustion are controlled to minimize formation of nitrogen oxides and to maximize sulfur removal.

Tests of entrained coal combustion technology have been very encouraging, with carbon conversion levels of 99.5 percent routinely being achieved. Besides high energy efficiency, the system demonstrates a high level of reliability and emissions of sulfur and nitrogen oxide comparable to or better than other new technologies.

Another advantage is its relatively small size, making it readily adaptable to smaller-scale coal plants.

The Healy clean coal project also will feature an emission control system developed by Joy Technologies that further reduces sulfur in the flue gases.

The project will employ about 200 workers during a two-year construction period, and provide about 50 year-round jobs when completed. Usibelli is hoping the new plant can be completed by 1995, with a one year startup and test program to follow.

For Golden Valley Electric Association, the 50 megawatt plant means a new reliable source of energy to meet demands that have been growing steadily for a decade. But in the longer term, successful application of the entrained coal combustion technology in Alaska, as well as Usibelli's experiments in coal enhancement and by-products, may further strengthen the state's position in world coal markets.

Says Green: Looking at the future for Alaskan coal, particularly Alaskan sub-bituminous coal, this technology may directly affect marketability in Asian markets. The day may well come when there will be a significant trade in low-sulfur, sub-bituminous coal' .
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Title Annotation:Usibelli Caol Mine Inc.
Author:Richardson, Jeffrey
Publication:Alaska Business Monthly
Date:Sep 1, 1990
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