Experimental investigation of the influence of solar-to-fuel ratio on performance and stability characteristics of hybrid solar-MILD hydrogen processes

Abstract This study presents an investigation of the influence of solar-to-fuel energy input ratio (S/F) on performance and stability characteristics of hybrid processes of solar and MILD combustion of H2. A laboratory-scale MILD Hybrid Solar Receiver Combustor was operated at 8-kWth capacity under MILD combustion and in the mixed-mode (MILD plus solar energy simultaneously). An 18-kWe three-lamp metal-halide solar simulator and the combustion of pure hydrogen were used as energy sources. The global combustion performance and stability limits for each mode of operation are reported for different levels of heat extraction and S/F values in the range 5–25%. It was found that similar thermal performance can be achieved for both modes across a wide range of conditions, together with steady operation in response to transients, indicating for the first time that MILD combustion can be used to efficiently compensate for variability in the solar resource, reduce thermal stresses and guarantees constant output. Steady solar-MILD operations retain similar features of conventional MILD processes (nearly-zero emissions, thermal field uniformity) even at relatively high S/F ratio. The global combustion characteristics, performance and stability limits are found to correlate with S/F in the mixed mode, while the operability region for which steady MILD processes can occur was found to increase significantly by adding high-flux concentrated solar radiation to the combustion process and by increasing S/F.