Combined solar energy and combustion of hydrogen-based fuels under MILD conditions

This work presents the stability and performance characteristics of a Hybrid Solar Receiver Combustor operating in the Moderate or Intense Low oxygen Dilution (MILD) combustion regime. The device was operated at 12-kW_th in two different modes of operation, i.e. combustion-only (MILD) and mixed (combustion and solar introduced into the device simultaneously), using natural gas (NG), liquefied petroleum gas (LPG), hydrogen (H₂), NG/H₂ and LPG/H₂ blends. A 5-kW_el xenon-arc lamp was used to simulate the concentrated solar radiation introduced into the device. The influence of the mode of operation and fuel composition on the combustion stability, thermal efficiency, energy balance, pollutant emissions, heat losses and distribution of heat flux within the receiver are presented for a range of values of the heat extraction. It was found that MILD combustion can be successfully stabilised within the HSRC over a broad range of operating conditions and fuel type, and in mixed operations, with low CO (for carbon-based fuels) and NO_x emissions. The addition of H₂ and/or concentrated solar radiation to the MILD process was found to increase its stability limits. Mixed and combustion-only operations showed similar performance, regardless of the fuel type, providing further evidence that the fuel flow rate can be used dynamically to compensate for variability in the solar resource. Also, the heat extracted from the heat exchanger and the specific fuel consumption were found to increase and decrease, respectively, by adding H₂ to the system for both modes of operation, showing that hydrogen addition is beneficial. The numerical analysis revealed that the higher performance with H₂ is attributable to a higher radiative heat transfer rate than for NG and LPG under MILD conditions.