This project investigates the effects of chemisorption dynamics and structure on the reactivity of coal chars and carbons. Understanding of the reactivity of carbon is very important in the efficient utilization of carbon materials and coals. The project concerns some chemical and physical aspects during the gasification of coal char/carbon in air and carbon dioxide, which help the understanding of the reactivity.
The effect of chemisorption dynamics on thermogravimetric determination of carbon reactivity is studied, based on the validity of the pseudo-steady state assumption. It is shown that in the typical conditions of thermogravimetric studies, the effect of chemisorption dynamics is always negligible for gasification of carbon in carbon dioxide while it is generally important for gasification in oxygen unless very low oxygen pressure is used (< 0.1 atm). Subsequently a new approach is proposed for obtaining the activation energy distributions for chemisorption of oxygen on carbon, without the normal assumption of a known distribution function and a constant pre-exponential factor. The activation energy distribution function is found to consist of two discrete flat distributions, suggesting two groups of active sites. Following the chemisorption studies, the energetics of another important reaction, i.e. the reaction of CO desorption from graphite edge site is calculated using the ab initio molecular orbital theory. The energy of the reaction is found to be sensitive to the carbon structure and the coverage of active sites.
The new approach is also used to obtain the activation energy distribution of thermal annealing of a bituminous coal in an entrained flow reactor. The distribution function again appears to consist of two discrete functions. The characteristic time of thermal deactivation is found to be comparable to that of combustion, indicating the importance of thermal deactivation in coal combustion. The variation of structure of an anthracite as well as its reactivity with heat treatment time at various temperatures is then studied. It is demonstrated that the reactivity can be correlated with the fraction of organized carbon in char, suggesting the importance of the structural ordering in thermal deactivation. Iron is found to catalyze the process of structural ordering, evidenced by the observation of nearly perfect structure around the iron particles.
The pore structure as well as the crystallite structure during gasification of coal chars heat treated at different temperatures is then studied to improve the understanding of the gasification process. The effect of heat treatment temperature on the development of the pore structure is different for different coals. The results also show that air gasification is different from CO2 gasification in the development of the small micropores (< l0Å). The surface area and volume of the small micropores increases quickly initially and remains almost constant during air gasification while it keeps rising sharply during CO2 gasification. The difference is attributed to the different rates of opening of the closed pores. It is also shown that the variation of the crystallite structure can be different for two coal chars when that of the pore structure is similar.
Percolative fragmentation of char particles has been confirmed to occur during gasification of several coal chars. The electrical resistivity of the coal chars is measured and found to increase sharply after a certain conversion, indicative of percolative fragmentation. Two percolation models are applied successfully to fit the variation of the electrical resistivity and the percolation thresholds are obtained. The partially gasified char particles are observed under an optical microscope and number of fragments are found to increase with the increase of conversion, confirming the occurrence of fragmentation.