Numerical modeling of heat transfer characteristics in a two-pass oxygen transport reactor for fire tube boilers under oxy-fuel combustion

Highlights: • Increase in boiler thermal load decreases the heat transferred by 2% at 50 bar. • Combustion heat transferred decreases significantly by about 16% at 50 bar. • Total heat transferred to the load slightly decreases as pipe emissivity increases. • The optimum thermal conductivity of the inner pipe was found to be 0.1 W/m-K. Among the available mitigation options for CO₂ emissions, oxy-fuel combustion technology has been gaining significant attention as a promising technology for curtailing the CO₂ emission. Currently the technology is at the demonstration phase on existing coal and gas power plants and some new pilot plants are being tested. In spite of the prospect, to integrate oxy-combustion to fire-tube boilers, heat transfer challenges need to be addressed due to fragility of the membrane at the hostile operating conditions. In the current study, numerical modeling of a two-pass oxygen transport reactor for fire-tube boiler was conducted to investigate the heat transfer characteristics to the saturated water and steam at various operating pressures, emissivities and thermal conductivities. The results indicate that the effect of varying the boiler thermal load resulted in slight decrease in heat transferred by only about 2% at 50 bar, at fixed fuel firing rate. Despite this insignificant increase in the total heat transferred, the combustion component of the total heat transferred decreases significantly by about 16% at 50 bar. The combustion efficiency was also found to decrease by only about 0.03%, which is considered as insignificant. The total heat transferred to the load slightly decreases as the emissivity of pipes increases up to the emissivity of 0.95 beyond which the heat transferred decreased drastically at emissivity of 1 due to the flame quenching. The optimum thermal conductivity of the inner pipe to deliver highest heat to the load by ensuring flame availability, maximum combustion efficiency of 98.58% as well as optimum heat transfer is found to be 0.1 W/m-K. There were no significant variations in heat transferred and combustion efficiency when the emissivity of the membrane was varied between 0.5 and 1.