Kinetics of post-combustion nitric oxide reduction by waste biomass fly ash

This work demonstrates that biomass fly ash, a carbon-containing by-product of a commercial pine-bark fired grate boilers, is a more viable and effective NO reducing agent than lignite char in a gasstream containing NO, O2 and He in a tubular reactor at 300 to 600 °C that simulates the post-combustion zones. It requires no chemical or physical activation. It seems to follow the reburning mechanisms reported earlier. CO is a pivotal reaction intermediate. Alkali and alkaline earth metals catalyze both carbon oxidation leading to CO formation, and oxygen tranfer leading to CO scavenging of surfaceoxides formed after adsorption of NO. Empirical rate expressions for carbon oxidation and NO reduction are developed. The following rate model is used to recover the rate constants of carbon oxidation in the simulated flue gas.dXdt=k(1−X)(Poxygen)mA exp(−kchardeactivationt)where X, t, k, Poxygen, m, A and kchar deactivation denote carbon conversion, reaction time, rate constant based on per unit surface area, partial pressure of O2 at the inlet, reaction order, N2 BET surface area per unit weight of the sample, and rate of carbon deactivation, respectively. A first-order rate expression is adopted for recovering the rate constants of NO reduction.

Research highlights
► Biomass fly ash is an effective NO reduction agent in post-combustion zone.
► We report the kinetics of NO reduction on carbon-containing flyash.
► We report the kinetics of carbon in biomass flyash.
► CO serves as pivotal intermediate in the carbon oxidation and NO reduction.
► Minerals in flyash catalyze carbon oxidation and are oxygen shuttling agent.