INFLUENCE OF COAL QUALITY ON COMBUSTION PERFORMANCE

Three coals have been fired in a tangentially and an opposed fired ful l scale power plant (about 400 MWe) and in a pilot scale test rig (160 kW(t)) in order to investigate the influence of coal quality on nitro gen oxide (NO) formation and unburned carbon in relation to furnace de sign. In-flame and furnace gas temperatures have been measured, and su bstantial differences in temperature between the coals were observed i n the full scale furnaces. Identical trends in NO emission as a functi on of coal type were obtained for the three furnaces. The emissions co rrelated well with the high temperature volatile yield obtained from h eated wire mesh analysis. Under air staging conditions the pilot scale test rig was able to reproduce quantitatively the amount of NO from t he tangentially fired plant, which operates with over fire air. This i s probably due to the relatively small influence of the near burner mi xing pattern on NO formation under these conditions. Emissions from th e opposed fired plant with all combustion air introduced through the b urners could only be qualitatively reproduced by the pilot furnace. Un der single stage conditions the test rig provided higher NO levels. Ca rbon in ash levels did not show any correlation between the coals and the furnaces. An engineering, mathematical model has been developed de scribing radiation heat transfer and coal combustion in full scale fur naces. The model has been validated against measured temperatures and the amount of carbon in fly ash. The model was well able to predict av erage temperature and carbon in ash levels, but failed to predict the influence of coal quality on both temperature and carbon in ash. This may be due to a lack of knowledge on char reactivity and deactivation as a function of temperature and coal type. A brief parametric study h as been performed on important model parameters. (C) 1998 Elsevier Sci ence Ltd. All rights reserved.