Effect of coal minerals on chlorine and alkali metals released during biomass/coal cofiring

Abstract

The combustion behavior, gaseous emissions, and alkali metals released during the combustion of several biomass/coal blends were investigated using a direct sampling, molecular beam mass spectrometer (MBMS) system in conjunction with a high-temperature alumina-tube flow reactor. Pittsburgh No. 8 and Eastern Kentucky coals were blended with various biomass samples such as red oak wood chips, Imperial wheat straw, and Danish wheat straw. The coal/biomass blends were 15% biomass on an energy input basis. All pure fuels and blends were subjected to combustion in 20% oxygen in helium at a furnace temperature of 1100 °C, and the products were monitored with the MBMS. The amounts of NO(g) and SO2(g) detected during the combustion of the coal/biomass blends suggested that any change was the result of diluting the nitrogen and sulfur present in the fuel blend. The amount of HCl(g) detected during the combustion of the coal/wheat straw blends was higher than expected based on the combustion results for the pure fuels. Conversely, the amounts of KCl(g) and NaCl(g) detected during the combustion of the coal/wheat straw blends was lower than expected. Chemical equilibrium calculations indicated that the amounts of condensed alkali species were significantly enhanced, usually in the form of Sanidine (K2O*Al2O3*6SiO2) and Albite (Na2O*Al2O3*6SiO2). Therefore, the lower-than-expected amounts of gas-phase potassium chloride detected during combustion of the wheat straw/coal blends was likely the result of potassium being sequestered in the condensed phase by the clay minerals in the coal.