Fluidized Bed Combustion
Beginning in 1970, with the passage of the first amendment to the Clean Air Act, air pollution regulations have placed strict limitations on the amount of sulfur in coal that can be burned in power plants. To comply with these regulations electric utilities have largely switched to burning very low ash, low sulfur coal. However, the demand for low ash, low sulfur coal focuses attention on what is actually a small percentage of Kentucky's coal resource. In reality, most of the coal in Kentucky contains moderate to high amounts of ash (especially in eastern Kentucky), and/or sulfur (especially in western Kentucky). As such, a significant portion of Kentucky's coal resource is usually bypassed. When this coal is mined, it typically undergoes intense beneficiation to the point where a majority of the mined product is separated out and discarded as a waste product. For example, one active mine in Johnson County in eastern Kentucky is extracting 9 to 10 feet of Coalburg coal, 70 percent of which will be separated out and discarded at the preparation facility, because of high ash content. Clearly, this is a very inefficient use of an important energy resource.
New clean coal technologies may help to better use resources of moderate to poor quality. Fluidized bed combustion plants (FBC), four of which have been proposed for construction in Kentucky, are capable of handling coal and preparation plant refuse with high levels of ash and sulfur, while effectively controlling S02, NOx and particulate emissions. Circulating fluidized bed combustion (CFBC) uses a sorbent such as limestone or dolomite to capture sulfur released by the combustion of coal. Jets of air suspend the mixture of sorbent and burning coal during combustion, converting the mixture into a suspension of red-hot particles that flow like a fluid. These boilers operate at atmospheric pressure. Pressurized fluidized bed combustion (PFBC) also uses a sorbent and jets of air to suspend the mixture of sorbent and burning coal during combustion. However, these systems operate at elevated pressures and produce a high-pressure gas stream at temperatures that can drive a gas turbine. Steam generated from the heat in the fluidized bed is sent to a steam turbine, creating a highly efficient combined-cycle system. In the case of both systems, combustion temperatures are kept well below the threshold where thermal NOx is produced, thus keeping NOx emissions to a minimum.
Integrated Gasification Combined Cycle
Integrated gasification combined cycle technology (IGCC) works by extracting synthetic gas (syngas) through a process that turns coal into a hydrogen-rich gas, rather than burning it directly. The hydrogen can then be used in a conventional combustion turbine, or used in a hydrogen fuel cell to produce clean electricity. Common air pollutants such as sulfur dioxide and nitrogen oxides are cleaned from the coal gases, and converted to useable by-products such as fertilizers and soil enhancers. Mercury pollutants are also removed. This technology could help western Kentucky coals, in particular, as their high sulfur contents have resulted in a 41 percent decrease in production between 1991 and 2001. Ultimately, this process could also become an important source of hydrogen for the new hydrogen economy that is being advocated for production of electric power, as well as a new fleet of hydrogen-powered cars and trucks.
A cooperative program with the USGS is currently underway to better evaluate the compatibility of Kentucky coals with these emerging technologies.
The Kentucky Geological Survey continues to maintain and update a comprehensive, computerized coal-quality database that includes trace-element measurements for Kentucky coal. Each year the Survey's coal analytical laboratories, which have been in operation since 1989, analyze several hundred coal samples. The laboratories routinely perform proximate (moisture, volatile matter, fixed carbon, and ash yield), ultimate (elemental carbon, nitrogen, hydrogen, and oxygen), total sulfur content, calorific value, ash fusion, and X-ray fluorescence analyses, and plans are being developed to expand the analytical capabilities to include testing for trace elements.