A Study of Combustion Characteristics of Gasified Coal Fuel

Abstract

The development of integrated, coal-gasification combined cycle (IGCC) systems provides cost-effective and environmentally sound options for meeting future coal-utilizing power generation needs in the world. The Japanese government and the Electric Power Industries in Japan promoted research and development of an IGCC system. We have being working on developing a low-NOx combustion technology used in gas turbine combustors for IGCC. Each gaseous fuel produced from some raw materials contained CO and H2 as the main combustible components, and a small amount of CH4. Compositions and calorific values of gasified coal fuels varied widely depending on raw materials and gasifier types. Gaseous fuel, produced in various gasifiers, has a calorific value of 4–13 MJ/m3, which is about one-tenth to one-third that of natural gas. The flame temperatures of fuels increase as the fuel calorific value rises. When the fuel calorific value rises 8 MJ/m3 or higher, the flame temperature is higher than that of natural gas, and so NOx production from nitrogen fixation is expected to increase significantly. Also, some gasified coal fuels contain fuel nitrogen, such as ammonia, if the hot/dry type gas cleaning system is employed. These factors affect the combustion characteristics of the gasified coal fuel. In this paper, we clarified the influence of gasified coal fuel properties on NOx and CO emissions through experiments using a small diffusion burner and through numerical analysis based on reaction kinetics. The main results were as follows: 1 NH3 conversion to NOx increases with increasing CH4 concentration in gaseous fuel. 2 If gaseous fuel contains CH4, there will be some specific equivalence ratio in the primary combustion zone for the minimum NH3 conversion to NOx in the two-staged combustion. 3 Its specific equivalence ratio in the primary combustion zone increases with decreasing CH4 concentration in gaseous fuel. 4 If the fuel contains a small percent of CH4, there is no influence of the CO/H2 molar ratio in the fuel on the conversion rate of NH3 to NOx, while there is an influence in the case where fuel contains no CH4. The conversion rate increases with rises in the CO/H2 molar ratio. 5 As the pressure increases, the conversion rate of NH3 to NOx slightly decreases and the CO emission declines significantly.