I ) INVESTIGATION OF MINERAL TRANSFORMATIONS AND ASH DEPOSITION DURING STAGED COMBUSTION Quarterly Technical Progress Report

This report describes work completed during the eleventh quarter of a DOE-funded study of mineral transformations and ash deposition during staged combustion. Accomplishments included repairs and improvements to the laboratory combustor, collection of deposits and observation of deposition behavior as a function of operating conditions, and analysis of deposit samples collected under both staged and conventional conditions. The water cooling shell for the brass burner was rebuilt this quarter. It was necessary to rebuild the cooling shell because of frequently occurring leaks which could no longer be repaired. No new leaks have occurred since the repairs were made. Deposits were collected for several stoichiometries between SR=0.65 and SR=0.75. A changeover from carbon-rich deposits to deposits which contained little carbon was observed at about SR=0.73 for the coal and natural gas feed rates used the experiments. However, even deposits which contained relatively little carbon had a carbon-enriched layer at the tube surface. This layer was due to carbon which did not burn out completely after impaction. A similar type of carbon layer may affect deposit formation in utility boilers. One of the key contributions this quarter was the analysis of deposit samples collected under both staged and conventional conditions. These initiai results indicate that deposits formed during staged combustion of a Pittsburgh #8 are similar to those formed during conventional combustion. Some small differences were observed and it is not yet known how significant these differences may be. However, substantial differences between the deposits and ash samples were observed. These Qfferences were apparent in both the bulk compositions and “species” distributions. Continuing work will focus on clarifying and quantifying the effect of staged combustion on deposit formation and strength development. OBJECTIVES AND SCOPE A. Background A thorough understanding of the fundamental processes which govern the mineral behavior is essential to the development of tools to predict and manage ash deposition. The purpose of the current project is to perform a fundamental study of mineral transformations and ash deposition during staged combustion of pulverized coal. Staging of combustion air is a strategy used to reduce NOx emissions from coal-fired units. It is applicable to both advanced combustion systems currently under development (e.g. HITAF) and low NOx retrofits for existing units. These low NOx combustion strategies produce fuel rich or reducing conditions in the lower furnace. Therefore, the combustion history of the coal particles is significantly changed from that experienced under normal combustion conditions. A carefully designed experimental study is needed to examine the effects of altered combustion conditions on mineral matter release, fly ash formation, particle stickiness and deposit formation. This project uses state-of-the-art analflcal equipment and a well-characterized laboratory combustor to address this need.