Influence of Water Vapor on the Direct Sulfation of Limestone under Simulated Oxy-fuel Fluidized-Bed Combustion (FBC) Conditions

Oxy-fuel combustion of fossil fuel is one of the most promising methods for producing electricity, together with a stream of concentrated CO2 ready for sequestration. Oxy-fuel fluidized-bed combustion (FBC) can also use limestone as a sorbent for in situ capture of sulfur dioxide. However, although a limited number of studies have been performed on sulfation of limestone under oxy-fuel combustion conditions, there are still a number of important but unanswered questions. Here, the effect of water vapor on the sulfation of limestone was studied, because it has not been examined in detail in previous sulfation studies and past studies on direct sulfation of limestone in FBC either did not explore the influence of H2O or did so under unrealistic conditions for oxy-fuel FBC. The purpose of this study is to identify the effect of water vapor on direct sulfation of limestone under simulated oxy-fuel circulating FBC (CFBC) conditions. Direct sulfation of three limestones was conducted in a thermogravimetric analyzer (TGA) apparatus at 800 and 850 degrees C. The limestone particle sizes used were 75-125, 125-150, 150-250, and 250-425 yin, and tests were carried out in a synthetic flue gas atmosphere, consisting of 80% CO2, 15%, 10% or 0% H2O, 4% O-2, 5000 ppm SO2, and balance N-2. Water always improved limestone sulfation, especially at 850 C. In addition, for some limestones, such as Kelly Rock (Nova Scotia, Canada), when the reaction gas contained no H2O, the calcium conversion ratio was higher at 800 degrees C than at 850 degrees C. However, when the reaction gas contained 10% H2O, the conversion ratio and the sulfation reaction rate were always higher at 850 degrees C than at 800 degrees C. Because coal-fired boiler flue gases always contain water vapor, the role played by H2O in the limestone sulfation reaction should always be considered in future studies.