Effect of minerals on surface morphologies and competitive reactions during char gasification in mixtures of O2 and CO2

This study aims to investigate the effect of minerals on surface morphologies and competitive reactions during char gasification in mixtures of O-2 and CO2. Char gasification experiments were performed in O-2/Ar, Ar/CO2, and O-2/CO2 environments to study char structure evolution and competitive reactions between char-O-2 and -CO2 gasification. Chars gasified under Ar/CO2 or O-2/CO2 were observed to have large pores on their surface, while chars gasified under O-2/Ar did not have them. The reaction between catalyst/carbon interfaces was key in explaining the difference in surface morphologies of chars gasified under O-2 and CO2. High specific surface area and large pores produced by gasification in CO2 increased char gasification rates in the internal diffusion-controlled regime. Then, two coals of different ash composition (including high alkali and alkaline earth metallic (AAEM) and low AAEM species) were used to study competitive reactions between O-2 and CO2 on the active sites. When chars prepared from coals including a low AAEM species were used, the char gasification rate in O-2/CO2 environments was almost 60% of the sum of those in O-2/Ar and Ar/CO2 environments, regardless of specific surface area. This indicated that char-O-2 and -CO2 reactions were competitive. However, when chars prepared from coals including a high AAEM species were used, higher Ca and Na concentrations were detected on the char surface, and the char gasification rate in O-2/CO2 environments was almost the same as the sum of gasification rates in O-2/Ar and Ar/CO2 environments, implying that Ca and Na played an important role as active sites. Unlike air environments, all surrounding gases were reactive in the O-2/CO2 environments. Therefore, a sufficient active site, mainly consisting of catalyst in this study, was important not only to enhance char gasification reactions but also to prevent competitive reactions between char-O-2 and CO2 gasification. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.