Effects of char content and simple additives on biomass pyrolysis oil droplet combustion

Abstract

Over the past few years, a laminar-flow, isolated-droplet combustion facility has been used to evaluate the fundamental combustion properties of biomass pyrolysis oils. This work has focused on characterizing oils produced from various feedstocks with a small-scale, fast-ablative vortex reactor coupled with various hot-gas filtration techniques. Recently, combustion of an oak oil produced in a large-scale, entrained fluidized bed has been characterized. This oil exhibits microexplosions very early in the combustion process, presumably as a consequence of the significant loading of small char particles in the oil. Backlit stroboscopic imaging of the burning droplets reveals that the oak oil microexplosions are characterized by the formation of a cellular network of vapor bubbles that ultimately contracts and results in the formation of coke particles. In addition, combustion experiments have been performed on a low-char poplar oil with small amounts of water and methanol additives. These simple additives have been suggested as an inexpensive means of reducing the viscosity and rate of chemical aging of biomass oils. Stroboscopic imaging of the droplets of these mixtures reveals a dramatic effect of both additives on the microexplosion process. However, long-exposure photographs of the complete combustion history suggest that the addition of methanol does not improve the shattering effect of the microexplosion and hence the total droplet burnout time. The addition of water delays the occurrence of microexplosion but improves its effectiveness, resulting in a small reduction in the droplet burnout time. Addition of both methanol and water yields the shortest burnout time.