Numerical investigation of diluent influence on flame extinction limits and emission characteristic of lean-premixed H_2-CO (syngas) flames

摘要

In recent years, research efforts have been channeled to explore the use of environmentally-friendly clean fuel in lean-premixed combustion so that it is vital to understand fundamental knowledge of combustion and emissions characteristics for an advanced gas turbine combustor design. The current study investigates the extinction limits and emission formations of dry syngas (50% H_2-50% CO), moist syngas (40% H_2-40% CO-20% H_2O), and impure syngas containing 5% CH_4. A counterflow flame configuration was numerically investigated to understand extinction and emission characteristics at the lean-premixed combustion condition by varying dilution levels (N_2, CO_2 and H_2O) at different pressures and syngas compositions. By increasing dilution and varying syngas composition and maintaining a constant strain rate in the flame, numerical simulation showed among diluents considered: CO_2 diluted flame has the same extinction limit in moist syngas as in dry syngas but a higher extinction temperature;H_2O presence in the fuel mixture decreases the extinction limit of N_2 diluted flame but still increases the flame extinction temperature;impure syngas with CH_4 extends the flame extinction limit but has no effect on flame temperature in CO_2 diluted flame;for diluted moist syngas, extinction limit is increased at higher pressure with the larger extinction temperature;for different compositions of syngas, higher CO concentration leads to higher NO emission. This study enables to provide insight into reaction mechanisms involved in flame extinction and emission through the addition of diluents at ambient and high pressure.