Oxy-combustion of Low-Volatility Liquid Fuel with High Water Content

Abstract

Following our previous studies of swirl-stabilized spray combustion of fuels that have a high water content, in this work, we investigate low-volatility fuels, specifically glycerol. Results show that, to obtain a self-sustained flame, the concentration of glycerol in water is much higher than that of ethanol (a high-volatility fuel) in water. However, even for glycerol concentrations as high as 60 wt %, the self-sustained flame is lifted from the nozzle and not attached, as in the ethanol case. The calculated stoichiometric adiabatic flame temperature (Tad) for 60 wt % glycerol is 2602 °C, whereas for 15 wt % ethanol (which is a very stable flame), it is only 673 °C. The two systems differ in that ethanol is more volatile than water, whereas glycerol is less. Thus, in the former case, the fuel is preferentially vaporized, whereas in the latter case, water is preferentially vaporized. Thus, flame stabilization is particularly challenging for fuels where the volatility is less than that of water when the water content of the spray is high. To assist flame stability for glycerol–water solutions, tert-butanol and ethanol were added in low concentrations. Experimental results revealed that stable attached flames can be obtained for 30 wt % glycerol in water when 8.3 wt % tert-butanol (B8.3/G30) or 10 wt % ethanol (E10/G30) is added under oxy-fired conditions. Both tert-butanol and ethanol have been shown to be particularly volatile in water when present at low concentrations because of their high vapor pressures and very high activity coefficients. The flame stability for B8.3/G30 and E10/G30 was characterized under 100 and 85% swirl flow conditions. Temperature measurements were conducted for four stable flames, and temperature contours were generated. For the stable flames, a hot zone exists in the near burner region, because of the preferential vaporization of the high-volatility fuel component, which provides heat to robustly vaporize the fuel and stabilize the flame.