Advances in Utilizing Wood Residue and Bark as Fuel for a Gas Turbine

Supplies of mill residues for fuel are decreasing with increased demand. Harvesting residues may be in demand 5 to 10 years in the future, and after that coal may be the most available fuel source. Sawmills in the future may operate without steam, depending on electrical power for mechanical drives and direct firing of dryers. A modified gas turbine system is suitable where a variety of fuel are used to produce a high ratio of electrical power to steam power. A gas turbine compressor section supplies combustion air to a fluid bed, where inert sand-size particles are buoyed up and mixed with combustion air. Combustion maintains particles and combustion products at the turbine operating temperature of 1.500 to 1,600?F. The fluid bed allows rapid and complete combustion and suppresses pollutants, while particulate matter is removed by collectors and a granular filter. Some steam is generated by a boiler in the turbine exhaust system. With fluid bed and collector devices operating at the same pressure as the turbine (50 to 150 psig), volumetric flow through the system is reduced and smaller components are needed. Efficiency of the simple cycle is low due to excess air (200 to 300 percent) used to control the temperature of the gas going to the turbine. Efficiency can be improved by extracting heat in the form of steam from the fluidized bed during operation; tubes in the fluid bed produce steam and maintain bed temperature while allowing operation at low levels of excess air. The steam can be used in a back-pressure turbine to create electrical power before being used as process steam. To increase the ratio of electrical to steam power, another cycle uses the steam from the waste heat boiler to increase mass flow through the turbine. The treated water going through the boiler is not recovered; however, this problem can be solved by using direct water injection into or downstream from the burner, in which case water treatment is not necessary since boiling is accomplished in suspension. In the modified cycles, a separate expander is required to handle the increase in mass flow due to higher fuel flow rate and steam or water injection. Tabulating typical power generation costs (nuclear, coal steam, wood steam, oil steam, wood gas turbine, and oil gas turbine), point out the economic advantages of the modified turbine cycles. Relatively low capital costs and low fuel costs yield appreciable savings. Preliminary tests on a process development unit (PDU) are described, including data on system control and operational experience. Two components of the system–the dry scrubber and the fluid bed burner–are already operating in industry. Operation of these components is outlined.

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