Effects of CO2 and H2O on oxy-fuel combustion characteristics and structural evolutions of Zhundong coal pellet at fast heating rate

To comprehensively investigate the combustion characteristics and structural evolutions of Zhundong coal pellet at fast heating rate in O-2/N-2, O-2/CO2 and O-2/H2O atmospheres, the real-time mass loss, temperature and combustion images were directly obtained on a novel concentrating photothermal reactor with heating rate of 4800 degrees C/min. The structural features of the coal/char with different holding times were analyzed by FT-IR and Raman. The experimental results showed that the maximum temperature and the maximum mass loss rate of coal pellet increased with the increasing of H2O concentration but decreased with the increasing of CO2 concentration. Under the experimental condition with the fast heating rate, CO2 prolonged while H2O shortened the ignition delay of coal pellet. In the initial stage of coal combustion, faster consumption rates of all functional groups were evidenced in O-2/H2O while slower consumption rates were evidenced in O-2/CO2 compared with those in O-2/N-2. Raman band area ratio I( Gr+Vl+Vr)/ID of chars also exhibited that the less condensed char was observed in O-2/CO2 but more condensed char was obtained in O-2/H2O. Such results indicated that the coal conversion was slowed down with the addition of CO2 but accelerated with the addition of H2O in the initial stage at fast heating rate. The specific reaction rates were reasonably characterized by the Raman band area ratio of char in O-2/N-2, O-2/CO2 and O-2/H2O atmospheres, and they were accorded with the second-order function relationship expressions.