Emissions in combustion of lean methane-air and biomass-air mixtures supported by primary hot burned gas in a multi-stage gas turbine combustor

Thermal reaction of lean to ultra-lean premixed mixtures supported by the hot burned gas from the upstream stage can be used for obtaining a better trade-off between ultra-low-NO_x and high combustion efficiency over a wide range of operations of a gas turbine. A three-stage model combustor designed based on this concept is being developed for a biomass gas-fueled regenerative cycle 10 kW micro-gas turbine. Tubular flame combustion is used for the primary stage and mixtures of lean to ultra-lean compositions are injected into the cross-flowing hot burned gas from the up-stream stage in the secondary and tertiary stages. The emissions and combustion characteristics are evaluated with methane and simulated biomass gas of different CO₂ contents at atmospheric pressure and inlet air temperatures up to 700 K. In the experiments, the fuel flow for the secondary mixture was gradually increased while maintaining the fuel flow to the primary stage in the two-stage combustion mode and the fuel flow for the tertiary mixture was gradually increased while maintaining the fuel flows to the primary and secondary stages in the three-stage combustion mode. The combustor exit NO_x concentration corrected to 15% O₂ remained at or slightly lower than the level that was achieved at the start of fuel staging as far as the injected mixture was leaner than the primary mixture and NO_x emissions in the 10 ppm level were achieved at gas temperatures less than 1700 K. In contrast, the NO _x concentration increase steeply with equivalence ratio in non-staged combustion mode. The reaction of the injected mixture was completion when the reaction zone temperature was higher than 1500 K regardless of inlet air temperature and equivalence ratio of the primary stage. The results show that, the multi-stage combustion where mixtures of ultra-lean to lean compositions are injected into the hot burned gas from the up-stream stage achieved low-NO _x emissions and high combustion efficiency over a wide range of overall equivalence ratios or combustor exit gas temperatures. It is found that the CO₂ in the simulated biomass gas suppresses the NO formation by slowing down the progress of combustion reaction, especially at high temperature conditions.