Development of porosity during coal char combustion

Abstract

A detailed study has been conducted on the nature of porosity developed during the combustion of coal chars. This information is used to provide information on the mode of char combustion in full-scale pulverized coal boilers. Both laboratory-prepared and actual commercial boiler chars have been examined. The characterization of porosity involved the use of nitrogen and carbon dioxide adsorption. Porosity development in the laboratory chars, in air under zone I reaction conditions, followed a trend dictated by rank. Porosity generally increases with decreasing rank. The development of porosity under zone II conditions was also explored and found to follow a different course than that under zone I burning: significantly less microporosity was developed under these conditions. It can be inferred from the development of porosity in commercial chars, when burned under laboratory zone I conditions, that the utility boilers were operating under zone II conditions. Comparison of the laboratory chars with the commercial chars also supports this conclusion. The conclusion that nitrogen is a better general probe of commercial (high burnoff) char than carbon dioxide receives further support. A hypothesis is advanced regarding the origin of the very steep increase in apparent nitrogen-accessible porosity at low burnoffs.