Effect of fuel type on the MILD combustion of syngas

Abstract

The MILD (Moderate or Intense Low-oxygen Dilution) combustion of four typical syngas fuels was examined in CRN (chemical reaction network) model and in parallel jet forward flow combustor. The results were presented on the thermodynamics, ignition kinetics and NO chemistry using CRN calculation. The flow field mixing was obtained from numerical simulation, the global flame signatures and exhaust emissions from experiments. The syngas fuel with higher content of inert gas (N2 and CO2) and lower concentration of active components (H2 and CO) exhibited higher mixture ignition temperature under MILD condition, as a result, greater gas recirculation ratio was required to establish the thermodynamic condition of MILD scheme. H2-enriched syngas and CO-enriched syngas showed different chemical kinetics in the ignition process of MILD mixture. In the NO chemistry under MILD condition, it was revealed that the H2 containing syngas fuels produce the NO emission mainly through the N2O-intermediate and NO-reburning mechanisms, whereas for pure H2 MILD combustion, the NNH route plays a more important role. The N2 component in the syngas fuel had physical effect in delaying the interaction between the air and fuel stream and in favoring the turbulent mixing as well as the gas recirculation. The CO2 component in the syngas fuel had chemical effect in lowering the reaction rate. The critical equivalence ratio above which MILD combustion occurs (with the reaction region covering the entire combustor) was identified for the four syngas fuels. It was observed that the syngas fuel with higher N2 and CO2 content can achieve MILD combustion at leaner condition and with lower NOx emissions. Reduced CO emission and single-digit NOx emissions were achieved for the four syngas fuels.