97/02094 Development of a stack plume opacity index for subbituminous coal-fired utility boilers

Abstract

Powder River Basin subbituminous coals were burned using conventional and low-NO{sub x} combustion conditions in a drop-tube furnace equipped with a multicyclone ash collection device. Fine ash fractions (< 2 {micro}m in diameter) collected during the tests were analyzed using computer-controlled scanning electron microscopy (CCSEM). Advances in particulate sample preparation methods enabled the CCSEM analysis of individual ash particles with submicron diameters as small as 0.1 {micro}m. The fine ash samples produced from the conventional combustion of coal consisted of discrete spherical particles, whereas particle agglomerates were characteristic of the low-NO{sub x} ash samples. Particle-size distributions of the low-NO{sub x} fine ash fractions were coarser because of the agglomeration. Theoretical light-scattering calculations indicate that for a given coal, the ash produced in low-NO{sub x} conditions causes less opacity as compared to conventional combustion conditions. The following phases were abundant in the ashes: Ca aluminosilicate, Ca aluminate, aluminosilicate, silica, (Ca, Mg)O, CaSO{sub 4}, Na{sub 2} SO{sub 4}, and (Na, K)Cl. Primary mechanisms that produced the fine ash include the thermal metamorphism of small (0.1 to 5 {micro}m) mineral grains and the vaporization and subsequent condensation of organically bound Na, Mg, and Ca, Empirical equations for estimating the concentration of fine ashmore » produced from burning subbituminous coals were formulated into an opacity index based on CCSEM coal mineral and fine ash analyses and on drop-tube furnace testing results. The effects of ash electrical resistivity on electrostatic precipitator collection efficiency are also considered in the index.« less