The knowledge of the reactivity of pulverised coal in oxy-fuel (O₂/CO₂) conditions is important for both new and retrofit oxy-fuel combustors. While a lot of data exists for pulverized coal combustion in conventional air (O₂/N₂) conditions, very scarce data is available for reactivity under oxy-fuel conditions. Also, the reactivity data for combustion in O₂/N₂ conditions is not applicable for O₂/CO₂ conditions due to major differences in the properties of the bulk gases namely N₂ and CO₂. Fuel particle reactivity may be different in O₂/CO₂ atmospheres when compared to the reactivity in O₂/N₂ atmospheres at the same level of O₂. A number of reasons have been identified to cause these differences: (1). The char particle temperature is an important factor that determines the combustion rate of char, especially when burning in Regime-I or Regime-II conditions . The higher specific heat of CO₂ in comparison to N₂ causes comparatively lower gas temperatures and therefore reduced fuel particle temperatures during oxy-fuel combustion in comparison to combustion in air ; (2). Combustion conditions in practical pulverised coal combustors lead to char combustion being controlled by diffusion of O₂ through the external gas boundary layer at very high temperatures (Regime III) or limited by a combination of reaction kinetics and diffusion through the porous char at moderately high temperatures (Regime-II) . The lower diffusivity of O₂ in CO₂ in comparison to the diffusivity in N₂ affects the transport of O₂ to the surface of the particle leading to reduced char combustion rates in Regime-II and Regime-III conditions ; (3). The char-CO₂ gasification reaction may increase the reactivity of char in O₂/CO₂ conditions due to the high concentrations of CO₂. However, the effect on burnout depends on other parameters, namely, particle temperature, pressure, and char properties. For example, the endothermic gasification reaction will reduce the char burning temperature. In summary, the rates of combustion in oxy-fuel combustion may differ from air conditions, with identified mechanisms consistent with both improved and worse rates and burnouts at similar conditions.
1st Oxyfuel Combustion Conference. 1st Oxyfuel Combustion Conference: Book of Abstracts (Cottbus, Germany 8-11 September, 2009)