Reduction of the flue gas recirculation rate in oxycoal processes by means of nonstoichiometric burner operation

One of the current research foci at the oxyfuel test rig at Technische Universität München is to investigate non-stoichiometric burner operation as a means to control the temperature distribution and heat exchange with low recirculation rates. Thus, the additional energetic and tangible expenditure of oxyfuel operation compared to the retrofit case with its higher flue gas recirculation rates up to 70% can be lowered. Non-stoichiometic burner operation leads to gradual release of combustion enthalpy and thus to lower (adiabatic) flame temperatures. In order to achieve an overall stoichiometry between λ=1.0-1.15 and low oxygen concentrations at the combustion chamber outlet a multiburner arrangement of up to three vertically aligned burners are currently under investigation. Next to the burners three radially arranged measurement ports are used for measurements with a suction pyrometer, flue gas analysis, video surveillance and heat flux/radiation measurements. Based on experiences with staged natural gas oxyfuel combustion and thermodynamic simulations a first coal burner design was developed and investigated under non-stoichiometric conditions. The data obtained was then used to modify models for CFD-based burner optimization with respect to aerodynamic behaviour. Recently a second-generation aerodynamically optimized burner was characterized in under-and over-stoichiometric operation. First results show that the flame temperature profile obtained by increasing the stoichiometry fits well to the calculated adiabatic reference case. A clear trend of increased NOx formation at higher stoichiometries could be observed. Radial gas profile analyses for flame characterization have shown a clear distinction between flame core (low oxygen and NOx concentration) and outer combustion zone (high oxygen and NOx concentration). Radial gas profiles taken below the flame nevertheless show rather constant values indicating that the main combustion reactions are finished. The characterization of non-stoichiometric multiburner operation with respect to flame temperatures, gas and heat flux profiles is subject to further investigations.