Microexplosive combustion behavior of blended soybean oil and butanol droplets

Experiments were conducted to examine the combustion characteristics of refined, bleached and deodorized soybean oil and butanol blends. Single droplets were suspended on microfibers in a combustion chamber at atmospheric pressure and ignited with a hot wire. Ignition characteristics and burning behaviors including burning to completion, burning with microexplosion and incomplete combustion were analyzed for initial concentrations ranging from 25-75% butanol. Droplet size and temperature measurements were analyzed throughout the droplet lifetimes. Relative concentrations prior and during combustion were estimated. Temperature measurements at ignition and during combustion were analyzed. The addition of butanol significantly lowered the ignition temperature. All mixtures studied ignited similarly to pure butanol droplets. The results were consistent with closed-cup flashpoint temperatures of butanol-soybean oil blends. A three-staged burn including a microexplosion was observed for all mixed droplets. Burning characteristics suggest a diffusion limited gasification mechanism, which has been previously linked to bi-component droplets with high volatility differentials. Microexplosions occur as a result of the more volatile component trapped within the droplet superheating at flame shrinkage. As butanol decreased to near equi-volume concentrations the microexplosions occurred earlier in the combustion process leading to higher concentrations of butanol trapped within the droplet at flame shrinkage. Therefore, equi-volume mixtures exhibited microexplosions with the greatest intensity. Blends of near equal concentrations by volume proved to exhibit the most favorable combustion characteristics. Bu40 exhibited the most violent microexplosions of all mixtures studied.