Modeling and CFD simulation of heat transfer process coupled with Unmixed Combustion for the application of generating superheated steam

Abstract

• Unmixed combustion (UMC) process is investigated for generating supherheated steam. • A design of steam superheater integrated with UMC based reactor system is proposed. • A model of the proposed system is developed and CFD simulations are performed. • Superheated steam of desired temperature (1 atm) is obtained with sustained UMC operation. • Generation of LP and MP superheated steam using UMC was also found to be feasible. Superheated steam is produced in a super-heater using the energy of hot flue gases generated in the premixed combustion and it requires a larger surface area. Recently, unmixed combustion (UMC), an environmentally better process to premixed combustion, a variant of chemical looping combustion (CLC), has been investigated successfully for transferring heat to air, with a relatively compact system. Hence to address the limitations of the size of a super-heater and environmental aspects, it is decided to investigate the application of UMC for generating superheated steam. With this objective, the existing design of UMC based heat transfer system was modified for heating saturated steam to a desired temperature on a continuous basis. The modified system with Cu-based Oxygen Storage and Release Material (OSRM) was investigated using modeling and simulation study. A 2-D computational fluid dynamics (CFD) model was developed and simulated using COMSOL TM . Oxidation and reduction cycles of UMC were studied separately and the study was restricted to laminar steam flow conditions. Results of the representative cases showed that the superheated steam of desired bulk temperature of 650 – 720 K at atmospheric pressure and heat flux of 13,000 – 15000 W/m 2 at the interface can be obtained with sustained combustion (the bed temperatures between 723 and 873 K) in both the cycles. The effect of four important operating parameters viz. inlet steam velocity, inlet reactive gas velocity and concentration, metal loading, was also investigated and optimum ranges were suggested to achieve the maximum possible heat transfer without affecting the bed operation. The simulation study was then extended to generate low and medium pressure superheated steams and obtained results have indicated that it is still possible to generate superheated steam at desired temperatures using UMC.