Formation and Reduction of NO₂ in Fixed Bed Combustion of Coal Char under Oxy–Fuel Conditions: Experimental and Density Functional Theory Analysis

Abstract

Nitrogen dioxide (NO 2 ) has been attracting a lot of attention because of its toxicity and potential in contaminating land, air, and water. However, research on NO 2 release behavior under oxy−fuel conditions is still insufficient. This paper aims to explore the formation and reduction mechanisms of NO 2 during oxy−fuel combustion of char by using both experimental and density functional theory (DFT) methods. Results showed that with the increase of temperature, the energy barrier of the NO oxidation reaction increases, while that of the NO 2 reduction reaction decreases. These facts lead to a higher peak concentration of NO and lower NO 2 emissions. Because of a larger effect of temperature on the rate constant of the NO 2 reduction reaction than that of the NO oxidation reaction, the NO 2 concentration decreases dramatically and rapidly even if the temperature increases only slightly. As the temperature increases to above 1273 K, the equilibrium constant of the NO oxidation reaction falls to below 10 5 . That is, the oxidation of NO to NO 2 is incomplete and reversible over the temperature range of 1273−1673 K. By contrast, the reduction of NO 2 to NO is complete and irreversible over this temperature range. As a result, almost no NO 2 is observed at high temperatures. In the presence of H 2 O, DFT calculations show that the water-gas-shift reaction is less important in contrast to the H 2 O−carbon reaction, which is consistent with previous research. In this case, NOx emissions under an O 2 /CO 2 /H 2 O atmosphere are lower than those under an O 2 /CO 2 atmosphere. Overall, combined experiments with DFT calculations offer a new approach to study the microcosmic mechanisms of NO 2 formation and reduction under oxy−fuel conditions during isothermal combustion of char.

The Influence of NH3 on Coal Combustion Kinetics during Coal/NH3 Cocombustion: Experiments and ReaxFF MD Simulations

“…Each coal sample utilized weighed approximately 0.08 g with a precision of ±0.0001 g. The total gas flow rate was 2.67 L/min. Our prior research demonstrated that the gas flow rate of 2.67 L/min can mitigate the impact of external diffusion on experimental outcomes. − At least three experiments per group were conducted to reduce experimental errors. Table shows the detailed experimental conditions.…”