Effect of reaction conditions on the evolution of surface functional groups in O2/H2O combustion process of demineralized coal char

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O2/H2O combustion is now regarded as a novel and promising technology of coal utilization for next-generation oxy-fuel combustion. The combustion reaction mainly occurs in the mixture of O2 and H2O instead of recycled flue gas (mainly CO2). The purpose of this research is to clarify the evolution characteristics of O/N-containing complexes (C(O)/C(N)) during O2/H2O combustion. Fourier transform infrared spectroscopy (FT-IR), temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were employed in this study. The addition of H2O in the atmosphere promoted the generation of NO and HCN in char combustion. There was a good linear relationship between the C(O) amount and char reactivity, as well as the NO conversion ratio, during the O2/H2O combustion process. Moreover, trace amounts of the precursors of CO2 with low thermal stability remained on the particle surface in the O2/H2O combustion. The enhancement of the reaction temperature revealed positive effects on inhibiting NO emissions in the O2/H2O combustion. The relative amount of pyrrole (N-5), which was the precursor of HCN, increased apparently at the expense of pyridine (N-6) and quaternary nitrogen (N-Q) in the early reaction stage. When the char conversion degree exceeded 50%, N-Q and N-6 became the dominant forms of C(N).