Experimental study on effects of combustion atmosphere and coal char on NO2 reduction under oxy-fuel condition

Nitrogen oxides (NO_x) as the principal air pollutants are mainly from the combustion of fossil fuels. Oxy-fuel combustion is a promising clean coal technology, by which carbon dioxide (CO₂) can be captured in large-scale and NO_x emission can be reduced significantly. The formation of nitrogen dioxide (NO₂) in oxy-fuel combustion exceeds that under traditional air condition. However, the specific studies on NO₂ chemistry under oxy-fuel condition are still insufficient and the functional mechanisms of minerals and combustion atmosphere on NO₂ reduction have yet to be fully understood. The objective of present study is to experimentally clarify the effects of combustion atmosphere and coal char on NO₂ reduction in oxy-fuel combustion using a fixed-bed reactor. Experimental results showed that the decomposition of NO₂ had a strong temperature dependence and the NO₂ reduction rate showed a positive variation with temperature. The strength of catalytic activity in NO₂ reduction to nitric oxide (NO) was Fe₂O₃ > MgO > CaO > Al₂O₃ > Na₂CO₃ > K₂CO₃ > SiO₂. In addition, the increased concentrations of carbon monoxide (CO) and CO₂ could promote the reduction of NO₂, while the low content of CO₂ only established a slight impact on NO₂ reduction. However, the increase of oxygen (O₂) concentration displayed an inhibition effect on NO₂ reduction to a certain extent. The variation of atmosphere in oxy-fuel combustion generated a substantial influence on the creation and reduction of NO₂. The char prepared in lower temperature exhibited a higher promotion effect on the consumption of NO₂. Higher contents of fixed carbon and basic oxides had more obvious stimulation effects on NO₂ reduction. Fixed carbon had a superior activity in NO₂ reduction than ash. The kinetic analysis indicated that high content of CO and the presence of char could reduce the apparent activation energy of NO₂ reduction. The present study can be helpful to improve the understanding of NO₂ chemistry in oxy-fuel combustion.