Investigation on NO reduction and CO formation over coal char and mixed iron powder

Abstract

NO reduction by CO in a rotary reactor, separating NO adsorption and reduction processes in two different zones, was demonstrated by our group in previous studies. As an indispensable part of this system, it’s necessary to further reduce the residue NOx at the outlet of reduction zone and supply fresh reducing gas CO for this reactor. The aim of this paper is to look for some cheap materials to meet both aims and investigate the corresponding reaction mechanism. Coal char was chosen for this purpose since it could serve as both reducing agent and catalyst of NO reduction, and could also be the source of CO production. For some cases, simple iron powder was also physically mixed which coal char, expecting to enhance the reaction rates of NO reduction. Some key factors were investigated for their influence of target reactions, such as temperature, oxygen concentration, CO concentration, and presence of H2O and SO2. Experimental results showed that mixed iron powder could effectively serve as catalyst for NO reduction, lowering the required reaction temperature. Higher oxygen concentration could promote both NO reduction and CO production. Both coal char and produced CO act as the reducing agent of NO reduction. In order to explore the reaction mechanism, various tests were carried out for samples and reactions under different conditions, such as SEM, TG, XPS and in situ DRIFT. SEM images illustrated that coal char is sintered in the absence of oxygen at high temperatures, but Fe could inhibit this sintering process. TG tests showed that reaction rate of coal char is the fastest in the presence of oxygen, and iron powder would inhibit the weight loss rate. XPS results displayed that both elevated temperatures and the addition of iron species could increase surface oxygen content. In situ DRIFT spectra revealed that the introduction of NO formed various nitrates on the surface of the coal char. Based on the experimental results and characterizations, reaction mechanism over coal char with/without mixed iron powder was proposed.