Experimental and numerical study of the effects of N2 dilution and CH4 addition on the laminar burning characteristics of syngas

Abstract

The study of combustion characteristics of complex biomass syngas under changing conditions is helpful for its practical utilization. In this paper, the effects of N2 dilution (dilution rates from 0% to 60%) and CH4 addition (mixing ratios from 0% to 20%) on the laminar flame properties of H2/CO/N2 syngas were researched. The laminar burning velocities (LBVs) were obtained by constant volume combustion chamber system. Five common mechanisms were used for numerical simulation using the CHEMKIN software package and the results were compared with the measurement data. The Li mechanism that was well matched with the experimental data was selected for kinetic analysis. The results indicate that the LBVs of H2/CO/N2/air mixtures decrease due to the dilution of N2. The addition of CH4 decreases the LBVs of H2/CO/N2/CH4/air mixtures and the LBVs of fuel-rich mixtures decrease faster. Sensitivity analysis illustrates that R29 is the most important branching reaction affecting LBV in H2/CO/N2/air premixed flame, the net reaction rate of all branched reactions and the mole fraction of H and OH in the mixture decrease due to the increase of N2 content. After adding CH4, the dominant branching reaction becomes R1. Due to the increase of CH4 addition ratio, the concentrations of H and O decrease, but the concentrations of OH and CH3 increase slightly. The chemical effect of adding CH4 increases LBV, but the physical effect decreases LBV. The reduction in the proportion of chemical effect is the reason for the faster reduction of LBV in the fuel-rich region. Finally, the formation of NO was analyzed in detail using the GRI 3.0 mechanism. The results show that N2 dilution reduces the mole fraction of NO, but CH4 addition increases the mole fraction of NO. Pathway analysis shows that N2 dilution and CH4 addition decrease and increase the effect of thermal mechanism on NO generation, respectively.