THE COMBUSTION OF PULVERIZED COAL IN SIMULATED TURBINE EXHAUST-GAS - EFFECTS ON NOX, COMBUSTION EFFICIENCY AND FLAME STRUCTURE

A large proportion of electrical power generation world-wide is derive d from the combustion of pulverised coal in large power-plant boilers coupled to steam turbines, with overall efficiencies typically in the range 35-39%. One method of improving the overall efficiency of such a power plant or to increase the power output is to re-power the existi ng boilers using gas turbines in a combined-cycle system. In this conf iguration the hot turbine exhaust gas is used as the oxidant for burni ng the pulverised coal in the existing boiler, to raise steam. Because of the low oxygen content and the high temperature of the turbine exh aust gas, many issues exist as to the flexibility of this technology i n practice, particularly with respect to flame stability, burnout and pollutant emissions such as NO(x), CO and unburnt hydrocarbons. This p aper presents the results of a preliminary study on the combustion of pulverised coal in simulated turbine exhaust gas at 1.3 MW thermal inp ut. A generic experimental burner was used. Simulated turbine exhaust gas was produced from two natural-gas-fired turbine exhaust-gas genera tors with independent control of oxygen and temperature levels. The si mulated turbine exhaust gas was used as main oxidant and as the coal-t ransport medium. In the simulated turbine exhaust gas, oxygen concentr ation ranged between 10 and 21%, and temperatures ranged between 400 a nd 600-degrees-C. Two coals were fired-Gottelborn (HVBa) and Obed Moun tain (HVBc). Stable combustion was obtained for turbine exhaust-gas ox ygen levels down to 11%; this observation was relatively insensitive t o turbine exhaust-gas temperature. Dependent on coal type, NO(x) emiss ions ranged from 120 ppm (0% O2) for turbine exhaust-gas oxygen levels of 11%, to 1100 ppm (0% O2) at 21% oxygen. Burnout expectedly fell as turbine exhaust-gas oxygen level was reduced. This latter phenomenon was more significant for the Gottelborn coal.