Kinetic study on in-situ and cooling char combustion in a two-step reaction analyzer

Cooling char, which is prepared in an inert atmosphere and then reacted with oxygen after cooling, has been widely used in the kinetic study of char combustion. However, there is no cooling process in commercial combustion systems and thus the impacts of the cooling process on char reactivity remain unclear. To illustrate this question, a two-step reaction analyzer was developed to study the combustion kinetics of in-situ char, which was produced at a constant high temperature in an inert atmosphere and then directly reacted with oxygen at different temperatures. Two types of Chinese coal, Shenhua sub-bituminous coal and Yangquan anthracite, were selected as samples. The results showed that the reaction rate of in-situ char was about 1.1-1.25 times of that of rapid cooling char, suggesting that the cooling process has a significant effect on char reactivity. Then the combustion rate and structure of chars cooled at different cooling rates were investigated to determine the reason for char deactivation during the cooling process. The cooling char reactivity decreased with the decreasing cooling rate, but the specific surface area did not decreased considerably. This result revealed that char deactivation during the cooling process was independent of specific surface area. As the cooling rate decreased, the oxygen chemisorption capacity (active surface area) of cooling char decreased for Shenhua coal, indicating that the observed char deactivation during the cooling process might be caused by the decrease of active surface area. In addition, Yangquan anthracite, as a high rank coal, was less sensitive to the cooling process due to its higher carbon crystallites and provision of fewer active sites for oxidation.