The Droplet Burning Characteristics of Algae-Derived Renewable Diesel, Conventional #2 Diesel, and Their Mixtures

Bio-derived fuels have received significant attention for their potential to reduce the consumption of petroleum-based liquid fuels, either through blending or direct use. Bio-feedstocks that employ algae, in particular heterotrophic microalgae, which convert sustainable plant sugars into renewable oils are especially attractive because the sugar that feeds this process can come from many sources — from sugarcane to corn, and even waste biomass, also known as cellulosic sugars. The microalgae grow in the dark and transforms sugar into nearly any oil type for almost any purpose anywhere, all while drastically compressing production time, from months and years to a matter of days. Much of the work in this area has focused on fuel production technologies. Little research has been reported on the combustion performance of algae-derived fuels, with most of the effort being directed to more system-level studies associated with combustion in engines. In this paper, we report the results of experiments that address some more fundamental multiphase combustion characteristics of algae-derived fuels relevant for spray combustion, namely a configuration involving a single isolated burning droplet. Experimental conditions are created that promote near spherical symmetry such that the gas flow arises primarily through the evaporation process (i.e., stationary droplets are ignited by spark discharge in stagnant air in the standard atmosphere and the droplet burning history is recorded in a free-fall facility that minimizes the influence of buoyant convection). The combustion symmetry that results, in which the droplet and flame are concentric spheres, facilitates the understanding of the combustion process while providing useful validation data for basic models of droplet burning that assume one-dimensional gas transport. Experiments were performed using algae-derived renewable diesel, and its performance was compared to #2 diesel fuel and a mixture of algal renewable diesel/#2 diesel (0.5 v/v). Additionally, the results of detailed chemical analysis are reported where it is shown that the composition of the algae-based diesel that was employed in the experiments was comprised of a complex mixture of aromatics and normal alkanes. The highly sooting propensity of these components resulted in droplet flames being luminous and producing soot during the burning history. A comparison of the flame brightness suggests that the sooting propensities are in the order of #2 diesel > renewable diesel #2 diesel blend > algae renewable diesel, which is consistent with observations of the sooting dynamics from back-lit droplet images. In spite of this difference in sooting propensities, algal renewable diesel droplets were found to have burning rates that are very close to #2 diesel and the mixture. Furthermore, the relative position of the flame to the droplet was almost indistinguishable for the fuels examined. These results suggest that algae renewable diesel could potentially be considered a drop-in replacement for conventional diesel fuel, or at the least serve as a useful additive to reduce the consumption of petroleum-based #2 diesel fuel.