Transient heating of a bio-oil droplet within the pre-explosion stage

Abstract

The study is concerned with a suitable heating model for the prediction of the pre-explosion time of a single bio-oil droplet. Diffusion and distillation limit models of the internal droplet liquid transport are considered.. The diffusion limit model only allows diffusion, while the distillation limit model implies the existence of an infinitely fast transport rate, therefore it represents the fastest possible transport limit. It is most likely that after the droplet ignition, the water and the lighter fuel fractions evaporate and burns firstly at almost constant droplet temperature in accordance with the distillation limit model. The increase of the liquid viscosity leads to domination of the diffusion limit model. According to the diffusion limit model, shortly after initiation of gasification the droplet surface becomes more concentrated of high-boiling point components, so the droplet surface reaches very high temperature, while the droplet core has a higher concentration of low-boiling components, accumulating a substantial amount of heat at temperatures near the superheat limit. Low thermal diffusivity values of bio-oil allow simplification of the solution of the equations that govern the diffusion limit model. This, in turns, allows estimating of the effective value of the bio-oils superheat limit and prediction of the droplets pre-explosion lifetime. Introduction Bio-oil a liquid fuel produced by the pyrolysis of biomass. It is potential alternative liquid fuel source for both power generation and transport. In general, bio-oils are combustible, but not flammable due to the high content of moisture and non-volatile components that require high energy for ignition. Bio-oils are multi-component mixtures comprised of different size molecules derived primarily from depolymerisation and fragmentation reactions of three key biomass building blocks: cellulose, hemicellulose, and lignin. Moisture content of biooil varies over a wide range (15-35%) depending on the feedstock and process conditions [1]. The presence of moisture can promote the onset of micro-explosions in the bio-oil droplets [2]. These micro-explosions can have an important impact on the combustion behaviour of bio-oils. Wornat et al. [3] observed a multistep process: ignition, quiescent burning emitting blue radiation, droplet microexplosion, disruptive sooty burning of droplet fragments emitting bright yellow radiation, formation and burnout of cenosphere particles. A recent scale analysis [4] employed a radiant heating of a water bubble combustible membrane model. However, a further development of this model requires more specific details of droplet combustion behaviour to be revealed. Classical models Consider a fuel droplet that contains two components of different volatility. With the contact between atomised droplets and hot air, heat is transferred to the droplet by convection from the air and by radiation from the feedback of the flame, and converted to latent heat during liquid evaporation. The vapours are transported into the air by convection through the boundary layer that surrounds each droplet. The initially uniformly mixed liquid components separate into a core with expanding radius and growing pressure, and a membrane (shell). The less volatile component covers the surface because of the finite diffusion velocity of the liquid [5]. Core liquid enters a metastable state in which the temperature rises above the usual boiling temperature and an instantaneous transition of the liquid into a vapour phase via the formation of miniscule bubbles occurs [6]. Hallet et al. [7] reported that initially the temperature of a multi-component droplet remains practically constant because the mixture is thermally dominated by evaporation of water and other volatile components (i.e. alcohols and acid groups). Then, the droplet temperature increases sharply and the pyrolysis of the residual factions begins. At the end of the transient heating period, the droplet begins to vibrate with intensity decreasing in time, evidently hindered by viscosity, until it degenerates via a micro-explosion. Hallet et al. [7] expected that the internal boiling could be accompanied by superheating of the liquid, although this was not experimentally verified. According to the classical model of Law [8, 9] the energy conservation equation at the droplet surface is