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 O-2/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 O-2/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 (C-f). The TPD results illustrated that the chars oxidized at moderate steam concentration (8.5 vol.%), and the chars with moderate burnout degrees (such as S-3, S-6 and S-11) had the largest amount of C(O). Due to the decomposition of the C(O) into new C-f 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. (C) 2019 Elsevier Ltd. All rights reserved.