Silverado Green Fuel, Inc. (Vancouver BC, Canada; edlinks.chemengonline.com/6900-533) has entered into a memorandum of understanding with the Mississippi Development Authority (Jackson) to build a demonstration plant in Choctaw County to produce a clean-burning “green” liquid fuel from the state’s plentiful lignite reserves. Scheduled for startup in 2009, the plant will convert 30,000 ton/yr of lignite into a coal-water fuel, equivalent to about 55,000 bbl of oil, that can be injected into oil-burning boilers. Plans call for half the $26-million cost to be provided by Silverado, other companies and state agencies, and half by the federal government.
Silverado’s process (diagram) is designed for inexpensive, low-rank coal, which constitutes about half of U.S. coal reserves. Pulverized coal is slurried with recycled process water and fed to a continuous reactor, where it is heated to approximately 300°C and 1,800 psi. A key feature is that “under these conditions coal is dehydrated without evaporating the water,” says Warrack Willson, a Silverado vice-president, who notes that low-rank coal can contain up to 60% moisture. Also important is the fact that waxes and resins are melted by the heat and seal off the coal’s micropores when cooled, thereby preventing reabsorption of moisture.
The resultant product is a coal-water fuel, resembling heavy oil, that can be injected into a boiler designed for oil with only a minor modification of the burners, says Willson. The energy content is 10–15% higher than that of the original coal, and the fuel’s water content moderates combustion temperatures and eliminates hot spots, thereby reducing the generation of thermal oxides of nitrogen (NOx). The process has been piloted at a scale of 6–7 ton/d, says Willson. He estimates that a commercial plant could produce boiler fuel for about $15/bbl (oil-equivalent).