Thermal performance analysis of a syngas-fuelled hybrid solar receiver combustor operated in the MILD combustion regime

Abstract

This article examines the overall performance of a hybrid solar receiver combustor when operated under the moderate or intense low oxygen dilution (MILD) combustion mode and contrast it with performance under the solar-only mode. A three-dimensional computational fluid dynamics model, coupled with analytical model data, is utilised to investigate the influence of fuel type (syngas, natural gas and tail gas) on the thermal efficiency, heat transfer mechanisms and heat flux distribution within the cavity. It was found that irrespective of the fuel type, similar thermal performance characteristics can be achieved under the MILD combustion mode and the solar only mode of operation (given a cavity of sufficient length and an appropriate arrangement of the heat transfer fluid (HTF) pipes). Nonetheless, it was found that the type of fuel influences significantly the rate of radiative heat transfer and the ratio of radiative to convective heat transfer rates, and so the configuration must be optimised for each type of fuel. The total heat absorbed by the HTF pipes and the thermal efficiency are predicted to be higher for the fuelled-syngas MILD hybrid solar receiver combustor case than for the CH4-fuelled and a simulated tail gas-fuelled MILD cases (where the tail gas is the by-product from a Fischer–Tropsch process) owing to a greater rate of radiative heat transfer (the latter increases with the H2 concentration in the syngas). Also, the level of dilution of the fuel stream was found to significantly influence the thermal performance of the device. In addition, the calculated distribution of heat flux to the receiver pipes was found to be significantly different under the solar and combustion modes of operation, which needs to be considered in the selection of design strategies and materials.