Stability of spray combustion for alcohols heavily diluted with water for applications in direct steam generation

摘要

The feasibility of burning fuels that are heavily diluted with water was evaluated. This concept could potentially be utilized to directly generate steam for a variety of applications, including power generation. Water-soluble fuels, either ethanol or 1-propanol, were diluted with water and pumped into a swirl-stabilized burner and the blow-off limits were obtained to identify the stability of the flames. The blow-off limit was defined as the lowest O_2 concentration when a flame could exist under a given oxidizer flow rate. For the present experimental system, stability maps were determined as a function of fuel concentration and oxidizer flow rate for different nozzle types and the two alcohols. Contours of temperature and overall fuel mass fraction in the droplets were obtained for an ethanol flame burning 15 wt% ethanol in an oxidizer of 50% O_2 and 50% N_2 at a flow rate of 20 L/min. The results show that the preferential vaporization of ethanol over water plays an important role in enhancing flame stability. In addition, a model of bi-component droplet vaporization was employed for both the diffusion limit and distillation limit modes of liquid mass transfer. The results confirmed the preferential vaporization of ethanol and revealed that it is necessary to account for the non-ideal solution behavior of the alcohol-water mixture to appropriately model the vaporization process. The model also demonstrated that for this system droplet vaporization was better characterized by the distillation limit mode of droplet vaporization than the diffusion limit mode.