5th Asia-Pacific Conference on Combustion, The University of Adelaide, Adelaide, Australia 17-20 July 2005 Analysis of the Pre -Conditions for Micro -Explosions of Bio -Oil Droplets V. Stamatov, D. R. Honnery, J. Soria Laboratory for Turbulence Research in Aerospace & Combustion Department of Mechanical Engineering Monash University, VIC 3800, Australia Abstract Results from a mathematical modelling of the process of explosive boiling of bio-oil droplets reveal that at otherwise identical conditions larger droplets require longer time to explode. An increase of the volumetric heating source leads to a reduction of the pre-explosion time. Droplets that contain more moisture require longer time to explode, possibly because of the larger thermal inertia of water compared with that of the solid residues. According to the model, explosion is impossible at low levels of the volumetric heating source. Solids suspended in the liquid core can act as heterogeneous nucleation sites and can lower the transition temperature of the liquid into a vapour phase. Since bio-oil contains suspended solid particulates, a redefinition of the transition limit as a function of the droplet fluid composition and the presence of solid nucleation sites is required. 1 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. 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. Water in bio-oil can present in two forms: (i) as a separate liquid phase known as pyroligneous liquor; and (ii) mixed with other components at molecular level. Most of the pyroligneous liquor can be removed by heating the bio-oil in a water bath under vacuum. This process is known as purification of bio-oil. The presence of moisture (15-35%, [1]) 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. 2 Physical model of explosive boiling of bio - oil droplets 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 [3]. 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. A condition for explosion is satisfied if the energy absorbed per unit volume during the exposure to the heat flux is greater than the pressure due to the surface tension of the matter surrounding the vapour bubble [4]. Once the explosion condition is satisfied, vapour bubbles explode out of the confines of the droplet forming an aerosol of fuel vapour and non-evaporated residues. This process is also known as a secondary atomisation of the droplet. 3 Transport models Hallet et al. [5] 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. It can be assumed that the pre-explosion stage of droplet combustion consists of two periods: an initial droplet heating dominant period of slow gasification, followed by vigorous gasification with almost no further droplet heating and nearly constant droplet surface temperature. Two extreme rates of liquid-phase transport can be considered. The slowest limit only allows diffusion, which is always present. It is referred as the diffusion limit [6]. Alternatively, if an infinitely fast transport rate exists, such that the special variations are always uniformised, the limit is referred as the distillation limit. 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. Initiation of homogeneous nucleation with extremely rapid rate of gasification nucleation will initiate at locations where the temperature exceeds the local concentration-weighted limit of superheat. 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, which corresponds to the distillation limit model. With increasing in liquid viscosity, the droplet combustion shifts and it might be assumed that the diffusion limit model dominates. High viscosity of bio-oils [7] and limited internal liquid circulation lead to reduction of the period dominated by the distillation limit model. 59