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

• A concentrating photothermal reactor at heating rate of 4800 °C/min was conducted. • Combustion characteristics in O 2 /N 2 , O 2 /CO 2 and O 2 /H 2 O at fast heating rate were studied. • Faster consumption rates of functional groups and higher condensed char in O 2 /H 2 O were evidenced. • Second-order function relationship expressions between R and I (Gr+Vl+Vr) /I D were observed. To comprehensively investigate the combustion characteristics and structural evolutions of Zhundong coal pellet at fast heating rate in O 2 /N 2 , O 2 /CO 2 and O 2 /H 2 O atmospheres, the real-time mass loss, temperature and combustion images were directly obtained on a novel concentrating photothermal reactor with heating rate of 4800 °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 H 2 O concentration but decreased with the increasing of CO 2 concentration. Under the experimental condition with the fast heating rate, CO 2 prolonged while H 2 O 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 /H 2 O while slower consumption rates were evidenced in O 2 /CO 2 compared with those in O 2 /N 2. Raman band area ratio I (Gr+Vl+Vr) /I D of chars also exhibited that the less condensed char was observed in O 2 /CO 2 but more condensed char was obtained in O 2 /H 2 O. Such results indicated that the coal conversion was slowed down with the addition of CO 2 but accelerated with the addition of H 2 O 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 /CO 2 and O 2 /H 2 O atmospheres, and they were accorded with the second-order function relationship expressions. [ABSTRACT FROM AUTHOR]