Experimental and density functional theory study on role of calcium in NO reduction by NH3 on char surface during ammonia co-firing with pulverized coal

Abstract

As a carbon-free energy, NH3 has attracted increasing attention from scholars. Ammonia-coal co-firing, as a technology for ammonia energy utilization, has great application potential. However, the NOx generated during ammonia combustion cannot be ignored due to the N element in NH3. At present, the reaction mechanism of NH3 and NO on char, together with the influence mechanism of mineral Ca, in the combustion reduction zone during ammonia-coal co-firing is still unclear. In this study, the effects of Ca on the NO reduction with NH3 on the char surface have been investigated in the high-temperature reaction system. The NO reduction efficiency and the evolutions of char structural characteristics during the reaction processes were analyzed for both demineralized coal char and CaCl2-loaded coal char. The reaction mechanism was also investigated at the molecular level using density functional theory (DFT) to clarify NO reduction and CO release pathways with and without Ca. The DFT results showed that during NO reduction on char surface, NNH group formation and CO desorption are crucial steps in the presence of NH3. Ca plays a catalytic role in NO reduction reaction, which affects NO reduction by NH3 on char surface in two ways. On the one hand, Ca facilitates the reduction of NO to N2 by lowering the energy barrier for NNH group formation (20.98 kJ/mol) and N2 release (93.70 kJ/mol). On the other hand, the formation of Ca–O–C structure increases the energy barrier for CO release from char surface (377.25 kJ/mol), which is unfavorable for NO reduction. The experimental results showed that between 1300 °C and 1500 °C, Ca increases the NO reduction efficiency by 5 %, promoting NO reduction, because Ca lowers the energy barrier of the rate-determining step, thus accelerating the reaction. At this temperature, the inhibition effect of Ca on CO release is not obvious. However, between 1000 °C and 1300 °C, Ca exhibits an inhibition effect on NO reduction. This is because Ca significantly inhibits CO release. At this temperature, the formation rate of NNH species is relatively slow, and NO reduction is influenced by homogeneous reduction. This study aims to provide fundamental information for exploring the application potential of ammonia coal co-firing in industrial furnaces.