Effects of steam and CO2 on the characteristics of chars during devolatilization in oxy-steam combustion process

Oxy-steam combustion is considered as one of promising technologies for the next generation oxy-fuel combustion, in which coal mainly burns in O2/H2O atmosphere. This work aims to investigate the devolatilization characteristics of coal in oxy-steam combustion and clarify the potential effects of steam and CO2 on the char characteristics. Chars of a Chinese lignite were prepared in a fixed-bed reactor at 1050°C under various steam/CO2/N2 atmospheres and characterized by a FT-IR/Raman spectrometer and thermogravimetric analyzer. The results indicate the char yield during devolatilization in oxy-steam combustion was lower than that in N2 atmosphere, and the steam gasification reaction played the key role. The high concentration of steam would significantly accelerate the condensation of aromatic rings in the char, and more condensed char would form during devolatilization in oxy-steam combustion compared to that in N2 atmosphere. CO2 in high concentration of steam had little effects on the char yields but it could participate in the cross-linking reactions on the char surface and partly reduce the condensation of the chars during devolatilization. Steam and CO2 gasification reactions can not only speed up the consumption of the original O-containing functional groups in the coal but also bring some additional O-containing functional groups in the char. The reactivity of the char formed during devolatilization in oxy-steam combustion was lower than that in N2. A good linear relationship between the Raman band area ratio I(Gr+Vl+Vr)/ID and char reactivity confirmed the decrease of the char reactivity was mainly attributed to the condensation of the char.