Experimental and chemical kinetic study of CO and NO formation in oxy-methane premixed laminar flames doped with NH3

The present work focuses on the oxy-fuel combustion of methane doped with ammonia in a premixed laminar burner operating at atmospheric pressure, and includes both experiments and a chemical kinetic study. CO and NO formation/emission were examined as a function of the stoichiometry and oxidizer composition. The experimental results showed that, for all oxidizer compositions studied, an increase in the excess oxygen coefficient generally decreases both the CO and NO emissions. Moreover, for the O2/CO2 environments, decreasing the oxygen concentration in the oxidizer, for a given excess oxygen coefficient, leads to higher CO emissions, but lower NO emissions. In air firing, the CO emissions were found to be significantly lower than those measured under oxy-fuel conditions, while the NO emissions were higher than those from the oxy-fuel cases. The chemical kinetic study allowed to identify the main reactions that directly (with the aid of a rate-of-production analysis) and indirectly (through a sensitivity analysis) influence both the CO and NO emissions. Under oxy-fuel conditions, CO2 + H ↔ CO + OH and 1CH2 + CO2 ↔ CH2O + CO significantly contribute to CO formation additionally to those reactions found in air-fired combustion, while CO oxidation takes place through CO + OH ↔ CO2 + H for all studied conditions. Formation of NO occurs for all conditions mainly with HNO as intermediate, particularly through HNO + H ↔ H2 + NO. Once NO is formed, interconversion to NO2 occurs.