Coal Particle Ignition Temperatures And Combustion Characteristics Of Different Rank Coals In O₂/CO₂ Environments

Coal remains as the predominant primary energy source for electrical industry in China for a long time. Oxy-fuel combustion with flue gas recirculation, is one of the most promising techniques for carbon dioxide capture and sequestration (CCS) and other pollutant emissions. Therefore, it is of academic and practical importance to inverstigate coal particle ignition and combustion characteristics of different rank coals in O₂/CO₂ environment.First of all, the pyrolysis and combustion experiments in N₂ and CO₂ environments are carried out with thermogravimetric analyser (TGA) for different rank coals. Compared to pyrolysis process in N₂ environment, coal pyrolysis process in CO₂ environment can be divided into three stages: moisture evaporation, volatile matters release and char-CO₂ gasification in higher temperature zone. With the increasing oxygen concentration, the combustion TG-DTG curves shift to lower temperature zone, maximum mass loss rates increase, coal particle ignition and burnout temperatures decrease. Accordingly, the comprehensive combustion property index increases. Moreover, the ignition temperature and burnout temperature of PRB sub-bituminous coal are obviously lower than those of Utah bituminous coal and Illinois bituminous coal under the same experimental conditions.Secondly, the coal particle ignition temperatures are measured in drop tube furnace at variant combustion environments and oxygen concentration for different rank coals. The authors proposes a quantitative method to identify the coal particle ignition mode through the O₂ and NO concentration measurements by the flue gas analyser and measure the corresponding ignition temperature by the separating flue gas temperature profiles. The experimental results show that the coal particle igniton temperature in 21%O₂/79%CO₂ environment is higher than that of 21%O₂/79%N₂ environment ranging from 60 to 130℃for different rank coals. The coal particle ignition temperature decreases with the increasing oxygen concentration in O₂/CO₂ environment. Moreover, the coal particle ignition temperature can match that in 21%O₂/79%N₂ environments when the oxygen concentration increases in O₂/CO₂ environments to 26-41% for different rank coal. The igniton order for different rank coal is Utah coal> Illinois coal> PRB coal in 21%O₂/79%N₂ environment and in 21%O₂/79%CO₂ environment. The results also indicate that the coal particle ignition temperatures measured by drop tube furnace are almost 300-400℃higher than those determined by TGA, which approaches to the ignition temperatures in real boilers.Lastly, the coal particle combustion experiments are conducted in drop tube furnace for Shenghua bituminous coal and Zhungeer lignite. The results show that the coal combustion rate in O₂/CO₂ environment is lower than that in O₂/N₂ environment at the same oxygen concentration. Accordingly, the unburnt carbon in fly ash is higher and the NO emission concentration decreases.