single Droplet Combustion of Biomass Pyrolysis Oils

Abstract

In an investigation of the combustion behavior of biomass-derived liquids, we have performed single droplet experiments with two biomass oils, produced from the pyrolysis of oak and pine. The experiments are conducted at 1600 K on 320 pm diameter droplets introduced into a laminar flow reactor, operating at O2 concentrations of 14-33 mol %. In-situ video imaging of burning droplets reveals that biomass oil droplets undergo several distinct stages of combustion. Initially biomass oil droplets burn quiescently in a blue flame. The broad range of component volatilities and inefficient mass transfer within the viscous biomass oils bring about an abrupt termination of the quiescent stage, however, causing rapid droplet swelling and distortion, followed by a microexplosion. Droplet coalescence follows, and subsequent burning occurs in a faint blue flame with occasional smaller scale bursts of fuel vapor. At the late stages of biomass oil combustion, droplets are accompanied by clouds of soot, produced from gas-phase pyrolysis. Liquid-phase polymerization or pyrolysis of the oxygenate-rich biomass oils leads to the formation of carbonaceous cenospheres, whose burnout signifies the final stage of biomass oil droplet combustion. Oak and pine oils behave similarly during combustion, though differences in their physical properties cause pine oil to show more susceptibility to fragmentation during the microexplosion. Changes in oxygen concentration alter the timing of the events during biomass oil combustion, but not their nature. Comparison of the biomass oils with No. 2 fuel oil reveals vast differences in combustion mechanisms, which are attributable to differences in the physical properties and chemical compositions of the fuels. Despite these differences, the biomass oils and No. 2 fuel oil exhibit surprisingly comparable burning times under the conditions of our experiments.

A model for the evaporation of biomass pyrolysis oil droplets

“…A nitrogen atmosphere was used in the furnace to suppress combustion and ensure pure evaporation, thus more nearly simulating conditions in spray combustion, where droplets evaporate in a vapour cloud rather than burning individually. Other experiments conducted on biomass pyrolysis oils to date have examined single droplet combustion rather than evaporation [1][2][3][4], and hence are not directly comparable with either these experiments or the results of the present model.…”

“…Typically the initial liquid heating period is followed by bubbling and/or "micro-explosions", after which the heavy non-volatile residue remaining pyrolyzes to a highly porous, irregularly-shaped char particle or "cenosphere" [1][2][3][4].…”

“…The chemical composition may change over time owing to "aging", in which light components are polymerized [18,26,27], and phase separation is also possible [6,7]. Studies of the combustion behaviour of single droplets have been performed 5 on free-falling droplets in drop tube furnaces [1][2][3][4] as well on suspended droplets [4], and have established the sequence of combustion events described earlier. Combustion tests have also been performed in gas turbine combustors and Diesel engines [22,28].…”

Effect of char on the combustion process of multicomponent bio-fuel

“…There have been several experimental studies investigating the combustion behavior of pyrolysis oil (Wornat et al, 1994;Calabria et al, 2007;Hou et al, 2013;Lehto et al, 2014;Beran and Axelsson, 2014;Wu and Yang, 2016). Wornat et al (1994) used single droplets (320 lm) from two biomass oils, produced from the pyrolysis of oak and pine.…”

“…Wornat et al (1994) used single droplets (320 lm) from two biomass oils, produced from the pyrolysis of oak and pine. Different stages of the droplet's combustion life time were depicted by in situ images and were explained in detail.…”