Utilizing preferential vaporization to enhance the stability of spray combustion for high water content fuels

In our previous study on the stability of spray combustion for mixtures of alcohols (ethanol or 1-propanol) and water, the feasibility of burning fuels heavily diluted with water was demonstrated. Direct combustion of high water content fuels provides an approach to extract chemical energy from fuels that intrinsically have high moisture content. Compared to traditional approaches, this approach has the advantage of avoiding the energy intensive dewatering process and allows for carbon capture if oxygen is used as the oxidizer. In our earlier study it was found that the preferential vaporization of alcohols in water can significantly enhance the stability of spray combustion. Due to their high volatility and high activity coefficient in aqueous solution, the alcohols quickly evaporate from the droplets and generate concentrated fuel vapor at the base of the jet. Therefore, a flame can be ignited and stabilize even though the water content of the fuel is high. In this study, we develop a procedure for selecting chemical fuels showing strong preferential vaporization in water. t-Butanol was identified as an excellent candidate based on its physical and chemical properties, including activity coefficient, vapor pressure, heat of vaporization and heat of combustion. Flame stability was evaluated for aqueous solutions of both ethanol and t-butanol using a spray burner where the extent of swirl was adjustable. Under both high swirl intensity and low swirl conditions the flame stability of t-butanol aqueous solutions was better than that of ethanol. The characteristic time for fuel release from a droplet was modeled for both ethanol and t-butanol. The time to release 99% of the fuel from the droplet for t-butanol is much shorter than that for ethanol, which supports the greater flame stability observed for t-butanol in the experiments.