Development of an unmixed combustion (UMC) based water heating system-Modeling and CFD simulation study for the oxidation cycle

Conventional combustion of fuel is used in steam boilers that operate at high temperatures, are big, and produce environmentally harmful NOx and CO2 mixed flue gases. These limitations can be addressed using an unmixed combustion (UMC) process (a variant of chemical looping combustion (CLC)), an alternative to conventional combustion. In UMC, air and fuel are alternately passed over specific metal/metal oxides that undergo oxidation/ reduction reactions and energy released can be used for relevant applications. Recent work has demonstrated that 85-95% of the energy generated in the UMC bed can be radially transferred to air while having sustained combustion (maintaining bed temperatures within 723 K-1173 K). However, when this system was used for heating water, achieving sustained combustion was a challenge. The design was then modified and the operational feasibility of the proposed system to heat water flowing in the laminar range was investigated for the oxidation cycle of UMC by carrying out modeling and simulation studies. A pseudo-homogeneous packed bed reactor model integrated with a non-isothermal pipe water-flow model was developed. It was implemented and simulated using CFD software COMSOL Multiphysics 5.5TM. The results showed that for the considered operating conditions, the water (10 bar(g)) gets heated to saturated water temperature continuously with sustained combustion in most sections of the bed (>92%). The effects of four different parameters are also investigated and analyzed. This system thus has the potential to increase the boiler efficiency, if used as an upstream unit to transform water to saturated water.