Effect of steam concentration on demineralized coal char surface behaviors and structural characteristics during the oxy-steam combustion process

Oxy-steam combustion, fuel combusts in O2/H2O atmosphere, is treated to be a novel and promising technology for the next generation oxy-fuel combustion. The purpose of this study is to identify the effects of steam concentration on NO emission characteristics, functional group distribution and char NO reducibility during oxy-steam combustion process. A typical bituminous coal (SH) was employed for demineralization, devolatilization, combustion in O2/H2O environment, and investigated by Temperature Programmed Desorption/Reduction (TPD/TPR) methods and Raman spectrometer. The combustion results illustrated that char N/NO conversion during the oxy-steam combustion tests initially decreased with an increasing steam concentration in low range (1.2–8.5 vol.%), while increased apparently in high steam range (8.5–20 vol.%). Decomposition of H2O molecules promoted the formation of C(O) and small aromatic rings on particle surface at the low steam range; the high concentration of steam could promote the condensation of aromatic ring structures in char, leading to significant generation of large aromatic ring structures with a lower reactivity and the reduction of surface active sites (Cf). The TPD results illustrated that the chars oxidized at moderate steam concentration (8.5 vol.%), and the chars with moderate burnout degrees (such as S3, S6 and S11) had the largest amount of C(O). Due to the decomposition of the C(O) into new Cf and massive CO molecules, a rapid acceleration of NO reduction rate occurred in the temperature range of 1000–1400 K during the TPR process of each sample obtained from identical reaction conditions. Thus, variations in char surface behavior and microstructure might be the primary reasons affecting char-N emission.