ABSTRACT
Computer-controlled scanning electron microscopy (CCSEM), x-ray absorption fine structure (XAFS) spectroscopy, and Mössbauer spectroscopy have been used to carry out a systematic investigation of the transformations and reactions of the inorganic phases in a suite of coals during pulverized coal (PC) combustion. Combustion experiments were carried out at several different scales at PSI Technology (PSIT), Massachusetts Institute of Technology (MIT), the University of Arizona (UA), and the State Electricity Commission of Victoria (SECV) in Australia. Perhaps the most extensive work was carried out on Beulah lignite. In the current paper, ternary composition - volume percentage diagrams derived from the CCSEM data on ash and coal minerals is coupled with XAFS spectroscopic data on calcium and potassium, and Mössbauer spectroscopic data on iron to determine the mineral interactions that occur during the combustion of this coal. The Beulah ash products from the UA and SECV tests exhibited three principal phases: a Ca-aluminosilicate slag or glass; a Na-aluminosilicate phase with compositions centered around nepheline (NaAlSiO4); and a Na-Ca-sulfate. The composition ranges of these phases were observed to become tighter as the particle sizes decreased. Na interacted more strongly with aluminosilicates in the SECV run, while exhibiting a stronger tendency for sulfate formation in the UA runs. For the UA runs, Ca-aluminosilicates were dominant in the larger ash particles, while Na-Ca-sulfates were the dominant phase in the small particle ash fraction. Na-aluminosilicate was the most abundant phase in all SECV ash samples. Drop tube combustion experiments were carried out on a potassium cation-exchanged Beulah lignite in which much of the carboxyl bound sodium and calcium had been replaced by potassium. The potassium in the ash was found to be divided between potassium in aluminosilicate slag and potassium sulfate (K2SO4). The potassium slag phase had a rather tight composition, similar to the nepheline phase formed by sodium.
