Lean Premixed Combustion of Undiluted Syngas at Gas Turbine Relevant Conditions: NOx Emissions and Lean Operational Limits

Abstract

The experimental work presented in this paper focuses on the characterization of four syngas mixtures, primarily in terms of NOx emissions and Lean Blow Out (LBO) limits; these mixtures were selected to simulate various syngas types derived from coal, refinery residues, biomass and co-firing of syngas with natural gas. These fuel mixtures are all of interest for applications in gasturbine processes for power generation. The experiments were carried out in a High Pressure Test Rig. Preheating of the fuel/air mixture to a temperature of 673 K, inlet bulk velocities between 40 and 80 m/s and operating pressures between 5 and 15 bars have been applied. The results show the expected strong difference between the CH4 containing mixture and all the other “pure” syngas mixtures concerning the “operational window”. As the focus of this paper is on lean extinction limits and NOx emissions, flashback phenomena are not discussed in further detail. Lean Blow Out limits were found to have weak pressure dependence for the methane containing mixture whereas for the other mixtures this dependence is slightly stronger: “Lean Blow Out” limits move to less lean condition with increasing pressure. No significant dependence on the Inlet velocity was found. Lean blow out occurs at much leaner conditions, ΦLBO≈0.25, for the pure syngas mixtures than for the methane containing fuel mixture (simulating co-firing of syngas with natural gas) which shows flame extinction already at ΦLBO≈0.38. NOx emissions show also a significant difference between the CH4 containing mixture and all other “pure” syngas mixtures with higher NOx emissions for the latter fuel gas mixtures. The difference can be attributed to the different O2 concentration in the hot exhaust gas after the flame front (for a given flame temperature) and to the difference in the flame length which leads to longer residence times in the post flame zone (for the much more compact syngas flames).