CFD simulation of biomass thermal conversion under air/oxy-fuel conditions in a reciprocating grate boiler

Abstract

A comprehensive 3D CFD model has been developed for analyzing thermal conversion of woody biomass fuel in a large reciprocating grate boiler under air/oxy-fuel condition. A number of sub-models representing solid conversion processes such as drying, devolatilisation and char combustion, gas phase flow and chemical reactions, solid and gas phase interaction through heat and mass transfer and bed movement is included in the model by user defined subroutines. The model has been validated by simulating a moving grate furnace from literature and comparing results with this reference experimental study. In this work, wet woody biomass fuel combustion is investigated under different oxy-fuel condition. Three different oxy-fuel cases have been investigated with 25% O2 (OF 25), 27% O2 (OF 27) and 30% O2 (OF 30) concentration by volume in the feed oxidizer gas with CO2 as recycled flue gas. Furnace operating condition is varied by changing air mass flow and inlet oxidizer gas composition, but all other operating conditions and furnace thermal load were kept constant. The effect of O2/recycled flue gas (CO2) ratios on flame temperature distribution, species concentration, char burnout and fuel consumption have been studied in detail and substantial differences were noticed comparing with the air-fired case. The numerical prediction showed that, gas temperature profile OF 25 case is closer to the referenced air fired combustion. The gas flame temperature has increased with O2 concentration rise, reaching maximum temperature at OF 30 case. CO2 concentration was increased to almost three times than air fired case due to use of CO2 in the feed gas. Increased CO2 concentration in the furnace has augmented char combustion rate and fuel consumption. Maximum fuel consumption was observed for OF 30 case where the fuel bed distribution over the grate was found minimum. Therefore, biomass bed combustion under oxy-fuel condition can provide better flame temperature and char burning to improve combustion condition.