Release Rule Of Alkali And Alkaline Earth Metals During Coal Combustion In O2/CO2

The release of alkali and alkaline earth metals(AAEM) in coal may cause problems to combustion furnace during coal combustion, such as corrosion of heater tubes and fouling. AAEM also contribute much to the formation of particulate matter. So it is important to know the release rules of AAEM during coal combustion to develop clean coal technology. Oxy-fuel combustion of coal is one of the most promising methods to produce a stream of concentrated CO2ready for sequestration. Compared with conventional combustion technology, the conditions of oxy-fuel combustion are quite different, and this will leads to changes of AAEM release rules. The aim of this paper is to investigate the influence of specific atmosphere of oxy-fuel combustion on the release rules of AAEM as well as the release mechanism of AAEM.In the study, the fixed-bed reactor coupled with ion chromatography was used to study the release rules of AAEM. XRD was also used to explore the transformation mechanism of sodium in coal during coal combustion. The results show that:the release rates of Na, K and Mg increased with increase of temperature during YZ coal gasification in CO2, however, the release rate of Ca was nearly unchanged. Soluble AAEM was the resource of those released AAEM under the experimental conditions (300℃-800℃). The reason of lower release rate of Mg and Ca is that the percents of soluble Ca and Mg are much lower than those of Na and K. The release rules of AAEM during Yima coal gasification are as same as those of Yanzhou.Temperature and oxygen concentration are the main factors that influence release rules of AAEM during oxy-fuel combustion. The increase of temperature and oxygen concentration can enhance the reactivity of coal, and the release rates of soluble AAEM increase. The release rates of AAEM during combustion are higher than those of gasification due to the higher reaction rate of combustion. Compared with conventional combustion, it is found that the effect of CO2, which is main atmosphere for oxy-fuel combustion, on the release rates of AAEM is much smaller. At lower temperature, the release rate of Na is a little bit lower due to the carbonization reaction. However, with temperature increases the effect of CO2on the release rate of Na disappears, and the release rates for both oxy-fuel combustion and conventional combustion are almost the same.SO2and steam can enhance the release of Na and K during oxy-fuel combustion of Yanzhou coal. The effect of steam is especially obvious. The release rate of Na increased by20%in the presence of10%(v%) steam under the condition of5%(v%) O2at400℃. The reactivity of coal can be enhanced by inducing SO2and steam in the reactor. The presence of high steam content even can change the reaction form of AAEM in coal, i.e. from solid-solid reaction to solid-liquid reaction. These result in the increase of release rates for Na and K. On the other hand, sulfation reaction may take place in the presence of SO2, and the release rates of Mg and Ca decreased due to higher stability of sulfate.The results of XRD analysis indicate that the transformation of Na in coal undergoes the formation of sodium sulfite and sodium sulfate with the increase of temperature during oxy-fuel combustion in the presence of SO2. Three crystalline forms of sodium sulfate exist in ash indicates that the conversion of crystalline occurred during combustion.