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Carbonation of fly ash in oxy-fuel CFB combustion

机译：氧燃料CFB燃烧中飞灰的碳化

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Oxy-fuel combustion of fossil fuel is one of the most promising methods to produce a stream of concentrated CO_2 ready for sequestration. Oxy-fuel FBC (fluidized bed combustion) can use limestone as a sorbent for in situ capture of sulphur dioxide. Limestone will not calcine to CaO under typical oxy-fuel circulating FBC (CFBC) operating temperatures because of the high CO_2 partial pressures. However, for some fuels, such as anthracites and petroleum cokes, the typical combustion temperature is above 900 ℃. At CO_2 concentrations of 80-85% (typical of oxy-fuel CFBC conditions with flue gas recycle) limestone still calcines, but when the ash cools to the calcination temperature, carbonation of fly ash deposited on cool surfaces may occur. This phenomenon has the potential to cause fouling of the heat transfer surfaces in the back end of the boiler, and to create serious operational difficulties. In this study, fly ash generated in a utility CFBC boiler was carbonated in a thermogravimetric analyzer (TGA) under conditions expected in an oxy-fuel CFBC. The temperature range investigated was from 250 to 800 ℃ with CO_2 concentration set at 80% and H_2O concentrations at 0%, 8% and 15%, and the rate and the extent of the carbonation reaction were determined. Both temperature and H_2O concentrations played important roles in determining the reaction rate and extent of carbonation. The results also showed that, in different temperature ranges, the carbonation of fly ash displayed different characteristics: in the range 400 ℃ < T ≤ 800 ℃, the higher the temperature the higher the CaO-to-carbonate conversion ratio. The presence of H_2O in the gas phase always resulted in higher CaO conversion ratio than that obtainable without H_2O. For T ≤ 400 ℃, no fly ash carbonation occurred without the presence of H_2O in the gas phase. However, on water vapour addition, carbonation was observed, even at 250 ℃. For T ≤ 300 ℃, small amounts of Ca(OH)_2 were found in the final product alongside CaCO_3. Here, the carbonation mechanism is discussed and the apparent activation energy for the overall reaction determined.
机译：化石燃料的含氧燃料燃烧是产生可被封存的浓缩CO_2物流的最有前途的方法之一。含氧燃料的FBC（流化床燃烧）可以使用石灰石作为吸附剂，以原位捕获二氧化硫。由于高的CO_2分压，在典型的含氧燃料循环FBC（CFBC）工作温度下，石灰石不会煅烧成CaO。但是，对于某些燃料，例如无烟煤和石油焦，典型的燃烧温度高于900℃。在80_25％的CO_2浓度下（典型的含烟气CFBC条件和烟气再循环），石灰石仍然会煅烧，但是当灰分冷却至煅烧温度时，可能会沉积在冷表面上的飞灰碳化。这种现象有可能引起锅炉后端传热表面结垢，并造成严重的操作困难。在这项研究中，在热重分析仪（TGA）中，在预期的含氧燃料CFBC中，将公用CFBC锅炉中产生的粉煤灰碳酸盐化。研究的温度范围为250至800℃，CO_2浓度设定为80％，H_2O浓度设定为0％，8％和15％，并确定了碳酸化反应的速率和程度。温度和H_2O浓度在确定反应速率和碳酸化程度方面都起着重要作用。结果还表明，在不同的温度范围内，粉煤灰的碳化表现出不同的特性：在400℃

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