Single Particle Experiments for the Investigation of Unburned Carbon in Flyash

Recent industrial attempts to meet increasingly tight emissions restrictions have led to difficulties in maintaining acceptably low levels of unburned carbon in boiler flyash. The significance of this problem is not limited to the obvious loss of efficiency and may become more important in the future because of flyash recycling vs. disposal issues. This paper is a summary of the single particle work done at Sandia's Combustion Research Facility to address this issue. Two experimental techniques (an entrained flow reactor equipped with optical diagnostics, and a high resolution captive-particle imaging technique) are used to evaluate several issues related to char reactivity under pulverized-coal combustion conditions. This paper demonstrates that single particle laboratory studies can yield useful insights into the origin and causes of unburned carbon in flyash. Experimental results indicate that variation of char reactivity during combustion and, under certain conditions, the difference in reactivity from particle-to-particle can have a significant impact upon the time required to reach high levels of conversion. We have observed a loss of reactivity during the intermediate stages of char combustion for several bituminous coals, but not for low rank fuels (subbituminous coals, lignites, biomass).