Gas Turbine Combustion Technology Reducing Both Fuel-$NOx$ and Thermal-$NOx$ Emissions for Oxygen-Blown IGCC With Hot/Dry Synthetic Gas Cleanup

In order to improve the thermal efficiency of the oxygen-blown integrated gasification combined cycle (IGCC) and to meet stricter environmental restrictions among cost-effective options, a hot/dry synthetic gas cleanup is one of the most hopeful choices. The flame temperature of medium-Btu gasified fuel used in this system is high so that $NOx$ formation by nitrogen fixation results to increase significantly. Additionally, the gasified fuel contains nitrogenous compound, as ammonia, and it produces nitrogen oxides, the fuel $NOx$⁠, in the case of employing the hot/dry gas cleanup. Low $NOx$ combustion technology to reduce both fuel-$NOx$ and thermal-$NOx$ emissions has been required to protect the environment and ensure low cost operations for all kinds of oxygen-blown IGCC. In this paper, we have demonstrated the effectiveness of two-stage combustion and nitrogen injection techniques, and also showed engineering guidelines for the low-$NOx$ combustor design of oxygen-blown gasified, medium-Btu fuels. The main results obtained are as follows: (1) Based on the basic combustion tests using a small diffusion burner, we clarified that the equivalence ratio at the primary combustion zone has to be adjusted according to the fuel conditions, such as methane concentration, $CO∕H2$ molar ratio, and calorific values of gasified fuels in the case of the two-stage combustion method for reducing fuel-$NOx$ emissions. (2) From the combustion tests of the medium-Btu fueled combustor, two-stage combustion with nitrogen direct injection into the combustor results in reductions of $NOx$ emissions to $34ppm$ (corrected at 16% $O2$⁠) or less under the gas turbine operational conditions of 25% load or higher for IGCC in the case where the gasified fuel contains 0.1% methane and $500ppm$ of ammonia. Through nitrogen direct injection, the thermal efficiency of the plant improved by approximately 0.3% (absolute), compared with the case where nitrogen was premixed with gasified fuel. The $CO$ emission concentration decreased drastically, as low as $20ppm$⁠, or combustion efficiency was kept higher than 99.9%. The above results have shown that a two-stage combustion method with nitrogen direct injection is very effective for reducing both fuel-$NOx$ and thermal-$NOx$ emissions at once in IGCC, and it shows the bright prospects for low $NOx$ and stable combustion technology of medium-Btu fuel.