Kinetics of coal char gasification in steam at 1000-1400 degrees C and 0.1-1.5 MPa

Kinetic data of char gasification in steam were measured in a unique high temperature and pressure thermogravimetric analysis system, to simulate the fast heating rate of a commercial gasifier. The coal either undergoes devolatilization and gasification in steam continuously, or is devolatilized in nitrogen and then gasified in steam. The coal chars thus produced are designated as in-situ char or nitrogen char respectively. These two types of char are prepared from bituminous coal, subbituminous coal and lignite. The coal chars are gasified at the same pyrolysis temperatures ranging from 1000-1400{dollar}spcirc{dollar}C, and a total pressure from 0.1-1.46 MPa in 76 mole% steam concentration, using an average particle size of 178 {dollar}mu{dollar}m. A unique feature of this study is to measure the reactivity data of in-situ chars.; Initial reactivities of in-situ chars were found to be two to six times higher than that of nitrogen chars. For nitrogen chars, the reactivity-conversion curves exhibited a maximum at approximately 0.05-0.1 conversion, whereas for in-situ chars, the reactivities decreased monotonically with conversion. At 0.1 MPa, the activation energies for the three coal chars ranged from 57 to 85 kJ/mole for the in-situ chars, and 63 to 99 kJ/mole for the nitrogen chars. As the total pressure increased from 0.1 to 1.46 MPa, the activation energies for the subbituminous in-situ chars varied from 61 to 46 kJ/mole. The gasification reaction rate at 1000{dollar}spcirc{dollar}C increased as the total pressure increased from 0.1 to 0.78 MPa and eventually reached a constant value.; When conversion was plotted versus dimensionless time {dollar}tau{dollar} ({dollar}tau{dollar} = t/t{dollar}sb{lcub}0.5{rcub}{dollar}, t is reaction time, and t{dollar}sb{lcub}0.5{rcub}{dollar} is time required to reach 50% conversion) for the same type of char, the rate data obtained at various temperatures, total pressures, or particle sizes were unified into a single characteristic curve. A rate expression was developed based on the reactivity data and the carbon dioxide surface area data, and used to describe the unified conversion-{dollar}tau{dollar} relations. Also, it was found that the product of half-life of conversion and reference surface area, t{dollar}sb{lcub}0.5{rcub}{dollar}S{dollar}sp{lcub}*{rcub}{dollar}, can be used to characterize the char gasification reaction and calculate the activation energy of char gasification reaction.