Effect of CO₂ and H₂O on Char Properties. Part 2: <i>In Situ</i> and <i>Ex Situ</i> Char in Oxy-Steam Combustion

Abstract

The physical and chemical properties of H2O (oxy-steam combustion) and N2 (air combustion) are different, which has influence on the char properties in oxy-steam combustion. Measuring in situ char (without any cooling) kinetic parameters is very important, especially at oxy-steam atmosphere. Both in situ and ex situ char properties were investigated in this paper. Ex situ char was obtained at different cooling rates. Char properties were measured by X-ray diffraction, H/C ratio, Raman spectrum, and specific surface area. The effect of steam on in situ and ex situ char combustion kinetics were determined. We found that the combustion reaction rates of different chars are in order of in situ > rapid cooling (103–104 K/s) > medium cooling (10–100 K/s) > slow cooling char (0.1 K/s). The rapid cooling char structure is disordered and has a high active surface area, which results in the high reactivity. In situ char has a shorter burn-out time, a faster combustion rate, and a higher reaction constant. For example, at 903 K, it appears the reaction rate of 0.048 s–1 for ex situ char (Vientiane coal char) in oxy-steam atmosphere (30% O2 + 15% H2O), while that of in situ char is 0.07 s–1. The reactivity of in situ char is higher than that of ex situ char. The activation energy of in situ char (e.g., 20.9 kJ/mol) is much lower than that of ex situ char (e.g., 82.24 kJ/mol) in oxy-steam atmosphere (e.g., 30% O2 + 10% H2O). The reaction reactivity of both ex situ and in situ char increase in oxy-steam atmosphere when compared with those from O2/CO2 combustion.