Influence of char physicochemical features on the flue gas nitric oxide reduction with chars

Nitric oxide is formed during the combustion of fossil fuels and is subsequently oxidized in the atmosphere to NO2. The increasing knowledge of pollution problems arising from NOx had led to the introduction of stringent environmental regulations concerning NOx emissions that in most cases, cannot be met by the only application of combustion modifications, so postcombustion methods have been developed. The selective catalytic reduction (SCR) is the only flue gas denitrification technique so far which has been proven to be very effective. However, improvements of the SCR method can be made in two aspects: using low-temperature gas (avoiding reheating) and cheap catalysts. This paper is focused on the reduction of NO to N-2 from stack gases of stationary sources using nonactivated low-rank coal chars. Physical and chemical properties of the chars were characterized by different methods and techniques: proximate and ultimate analysis, N-2 and CO2 surface areas, acid-base titration, AA, XRD, and TPD. The flue gas has been simulated by varying the water vapor and oxygen content passing through a bed of char with varying reaction temperature and the space velocity. The effect of the different operation conditions on the NO removal capacity of the chars were examined. Correlations between the performance of these chars and their physicochemical features were investigated. An optimal amount of oxygen-functional groups on the surface char is needed to obtain high NO removal performance. This optimal value can be tailored by char oxidation. The effects of varying the gas-phase and surface oxygen concentrations were also studied.