Numerical analysis of biogas combustion in a lean premixed swirl burner

Abstract

Modern combustion technology focuses on lean premixed burners to achieve high thermal efficiency, wide operating range, and low pollutant emissions for steady-state operation. The corresponding applications range from boilers to gas turbines. The present paper is a preliminary analysis of a turbulent laboratory test burner with 30 kW combustion power by using CFD. The reference fuel was natural gas and four biogases were tested which were modeled as a mixture of CH4, CO2, and H2 in various compositions. Even though the combustion is steady, the steady solution was inappropriate due to the notable presence of unsteady flow structures. Since the combustion in the present case is dominated by volumetric reactions, a coarse boundary layer could be applied near the wall. However, the shear-dominated flow required the use of at least k-ω SST turbulent viscosity model. The transient cases were calculated by using Scale Adaptive Simulation. Among the fuels, natural gas combustion showed flashback due to the bluff body present at the center in the mixing tube inlet. Nevertheless, its extent was low and some central purge air in the real burner will solve this problem. All the flame shapes were V and W, meaning an optimal condition for combustion chamber loading. Even though the overall mass flow rates at the inlet are increasing with the decreasing heating value of the fuel, natural gas combustion showed the highest velocity and temperature in the flow field. Overall, a small hydrogen dilution of the CH4-CO2 containing fuel acted as an excellent combustion stabilizer without flashback or too intense heat release rate. As a consequence, the presently analyzed burner can run on low calorific value fuels without design modifications or exposing locally high thermal load on the combustion chamber. Since it is an initial study, validation and the evaluation of practical relevance will be discussed in subsequent works.