Numerical study of CO and CO_2 formation in CH_4/H_2 blended flame under MILD condition

摘要

Reduction of air pollutants formation from hydrocarbon combustion process requires improvements in combustion systems. The moderate and intense low oxygen dilution (MILD) combustion technique is an opportunity to achieve such a goal. MILD combustion is a combustion regime which can be attained by high temperature preheating and high level dilution. In this paper, the mechanism of CO and CO_2 formation for a CH_4/H_2 fuel mixture is studied under MILD combustion condition of a jet in hot coflow (JHC) burner. This investigation is done using the computational fluid dynamics (CFD) and also zero dimensional well-stirred reactor (WSR) analysis. The RANS equations with modified k-ε equations are solved in an axisymmetric 2D computational domain. The DRM-22 reduced mechanism is considered to represent the chemical reactions. The effects of oxidizer oxygen concentration and fuel hydrogen content are studied on methane oxidation pathways. Results show that the higher hydrocarbon oxidation pathways are effective on CO and CO_2 formation under MILD condition. In the methane oxidation mechanism, the ratio between the main route and ethane route is the main reason of CO increment at higher O_2 level under MILD condition in JHC laboratory burner. The WSR analysis illustrates that a decrease of O_2 concentration in oxidizer does not necessarily lead to lower production of CO and CO_2.