Investigation On Mechanism Of OXY-Fuel Coal Combustion In Fluidized Beds

Coal would still be the predominant fuel in Chinese power generations in the foreseeable future. However, coal used in power plants represents a main source of CO2 emissions. Developing CO2 capture technologies in coal-fired power plants has became a very important strategic research project in China. Among various carbon capture technologies, O2/CO2 combustion (also known as oxy-fuel combustion) is one of the most competitive technologies due to its technical feasibility and economic advantages, where fuel is burnt with oxygen and recycled flue gas. The O2/CO2 fluidized bed (FB) combustion technology combines advantages of O2/CO2 combustion and FB combustion. Investigation on the mechanism of O2/CO2 FB combustion is significant for the development and commercialization of this technology.The overall objectives of the research presented in this thesis are to understand and reveal the combustion behavior and mechanism of coal combustion in FBs when N2 is replaced by CO2. To these ends, tools for experiment and modeling are performed in this thesis. The results obtained are summarized as follow:(1) A new transparent and electrically heated FB was developed and built. A transparent quartz glass was served as the front wall of the FB, allowing using optical measurements and overcoming design flaws of the traditional transparent FB with the small window where the whole conversion process of coal particles could not be observed. The new designed FB could provide a large number of reliable experimental data, which can be not only used to obtain combustion behavior, but also to establish and validate coal combustion model.(2) Devolatilization of typical Chinese coal particles (anthracite, bituminous, sub-bituminous, lignite) was studied in the FB under O2/CO2 and O2/N2 conditions, and a detailed one-dimensional transient model was developed to describe the devolatilization of a coal particle in FBs. The experimental and simulated results indicate that the devolatilization of a fuel particle of a few mm in size in an FB is controlled by heat transfer, and is influenced by both internal and external processes. Compared to O2/N2 atmosphere, the transition of ignition mode and the decreased heat rate during the last stages of devolatilization in O2/CO2 atmosphere are due to the lower O2 diffusion rate in CO2 than in N2. The lower temperature of the volatiles’flame and the longer ignition delay observed in O2/CO2 atmosphere than in O2/N2 atmosphere as a consequence of the higher specific heat capacity of CO2. Most of heat released from the homogeneous combustion is lost to the gas flow, explaining why the rate of particle heating is hardly affected by the flame. Under the same combustion conditions, the different devolatilization behaviors of the tested fuels are a consequence of the particle properties.(3) The experiments of Chinese char conversion were carried out in the FB in O2/N2 and O2/CO2 atmospheres, and a one-dimensional and transient char combustion model was developed to study the effect of CO2 on the combustion. The results indicate that oxidation of a few mm sized char particle in an FB is controlled by the diffusion of O2 in both O2/CO2 and O2/N2 atmosphere. The lower O2 diffusion rate in CO2 is the reason for the longer burnout time and lower peak temperature found using O2/CO2. The CO2 gasification reaction is controlled by the chemical kinetics. The contribution of the gasification reaction to the overall consumption of carbon (Xgas) in the char increases with the bed temperature. Under the bed temperature of commercial FB combustor conditions, Xgas is very limited less than 7%. When the bed temperature is attained at 1220 K, Xgas could attain 14% and cannot be ignored. The diffusivity of O2, porosity and density of the fuel particle are the main factors influencing the combustion characteristics. The conversion of char under the conditions investigated could be reasonable well described by a non-isothermal sharp-interface model, which could greatly simplify the computational fluid dynamics (CFD) simulation of an FB combustor.(4) An evaluation was made to compare the differences in the combustion behavior (ignition delay time, volatiles’flame temperature, devolatilization time, char temperature and burnout time) of sub-bituminous coal in pulverized coal (PC) and FB conditions, when the atmosphere is shifted from air-firing to oxy-firing. The ignition of coal is delayed, the volatiles’flame and char temperature are decreased, the burn out time is increased in both PCs and FBs, and the devolatilization time is increased in PCs when shifting from O2/N2 to O2/CO2. However, the oxy-fuel combustion mechanisms are not fully the same:regardless of the different heat and mass transfer behavior of coal particle in PCs and FBs, the low diffusivity of O2 and high specific heat capacity of CO2 play a significant role during the conversion of char in FBs, however, PCs operate at high temperature, besides those factors, the effect of CO2 gasification reaction on the char conversion can not be ignored. To achieve the similar burnout times in an oxy-fuel atmosphere as in air, about 25% and 30% O2 concentrations are needed in FBs and in PCs, respectively.