A combined experimental and theoretical study of micronized coal reburning

Abstract

Micronized coal reburning (MCR) can not only reduce carbon in fly ash but also reduce NO_x emissions as compared to the conventional coal reburning. However, it has two major kinetic barriers in minimizing NO_x emission. The first is the conversion of NO into hydrogen cyanide (HCN) by conjunction with various hydrocarbon fragments. The second is the oxidation of HCN by association with oxygen-containing groups. To elucidate the advantages of MCR, a combination of Diffuse Reflection Fourier Transform Infrared (FTIR) experimental studies with Density Functional Theory (DFT) theoretical calculations is conducted in terms of the second kinetic barrier.

FTIR studies based on Chinese Tiefa coal show that there are five hydroxide groups such as OH-π, OH-N, OH-OR₂, self-associated OH and free OH. The hydroxide groups increase as the mean particle size decreases expect for free OH. DFT calculations at the B3LYP/6-31 G(d) level indicate that HCN can be oxidized by hydroxide groups in three paths, HCN+OH→HOCN+H (path 1), HCN+OH→HNCO+H (path 2), and HCN+OH→CN+H₂O (path 3). The rate limiting steps for path 1, path 2 and path 3 are IM2→P1+H (170.66 kJ/mol activated energy), IM1→IM3 (231.04 kJ/mol activated energy), and R1+OH→P3+H₂O (97.14 kJ/mol activated energy), respectively. The present study of MCR will provide insight into its lower NO_x emission and guidance for further studies.

Key words： hydroxyl radicals; Fourier transform infrared spectroscopy (FTIR); density functional theory (DFT); homogeneous reaction mechanism; NO_x

Acknowledgements

This work was supported by Doctoral Fund of Ministry of Education of China (2010), the National Natural Science Foundation of China (Grant No. 50876060) and China Postdoctoral Science Foundation funded project (2012M511091).

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