Emissions of NO and CO from counterflow combustion of CH4 under MILD and oxyfuel conditions

Abstract

This paper reports on the NO and CO emission characteristics of counterflow combustion of methane simulated under MILD or/and oxyfuel conditions. Simulations using CHEMKIN are conducted for various injection conditions of fuel and oxidizer. Note that the terms “oxyfuel”, “MILD-N2” and “MILD-CO2” combustion adopted hereafter represent the conventional oxy-combustion and those MILD combustions diluted by N2 and CO2, respectively. It is observed that the NO emission of MILD-CO2 combustion is ultra-low for all cases of investigation, even when increasing the combustion temperature up to 2000 K or adding more N2 (up to 20%) to either the fuel stream (to simulate nitrogen-containing fuels like biomass) or the oxidizer stream (to simulate the air-ingress). A higher temperature allowed under MILD-CO2 combustion suggests the improvement of energy efficiency for the MILD combustion technology. Moreover, the presence of steam in the oxidant reduces both NO and CO emissions of combustion for all cases.The relative importance analysis reveals that the N2O-intermediate mechanism for producing NO prevails in MILD-CO2 combustion while the prompt and thermal mechanisms predominate MILD-N2 and oxyfuel combustion, respectively. In addition, the sensitivity analysis identifies those main reactions that play important roles for the NO emission under these combustion conditions.