Report Finale, FUNDAMENTAL BEHAVIOUR OF BFO IN COMBUSTION - EU Contract JOR3-CT97-0253 "BIO FUEL OIL: Upgrading by Hot Filtration and Novel Physical Methods"

Objective of the task was to highlight, in a lab scale environment, the combustion fundamentals of droplets composed of pure pyrolysis oil, as well as of BFO-based emulsions and mixtures. As the utilisation of such fuels was foreseen in boilers and in Diesel engines, experiments were performed at both ambient and high-pressure. Two single-droplet combustion chambers were developed to study evaporation-combustion of single droplets up to 100 bar. Droplets composed of different fuels were suspended to a thermocouple or to a quartz fibre. Quartz windows permitted the optical access to the chambers. Both laser and halogen light sources were used to illuminate droplets during vaporisation, before ignition. The thermal behaviour exhibited by droplets was followed by means of high-speed digital imaging. About two thousands of frames were collected for each test. The acquisition velocity ranged between 400 and 4000 frames/s. Eight different fuels were examined for a total of about five hundred tests. Experiments were carried out on a pure pine pyrolysis oil, two emulsions of forest residue oil (10% and 30%) in #2 diesel oil, an emulsion of 30% of pine oil in #2 diesel oil, a mixture of pine oil (30%) in diglyme. Reference tests were carried out on commercial Italian Diesel oil, on diglyme and on hexadecane. Diglyme was chosen for the good ignition property (cetane number 110-130) and for the very low sooting tendency; n-hexadecane (or "cetane") was tested because it is used in the standard procedures to evaluate ignition characteristics of Diesel engine fuels. The pressure ranged between 1 and 60 bar, the size of droplets between 400 ?m and 1100 ?m. All fuels were easy to ignite at normal and high pressure. In the investigated experimental conditions, the environment temperature had a significant influence on the ignition delay: this was due mainly to the relevance of the physics of the process (heating and vaporisation of the fuel) respect to the chemistry in the pre-ignition phase. Also the influence of different heating rates on combustion phenomenology was investigated. The main effect resulted on droplet evaporation rate and swelling phenomena. The temperature of the chamber and the heating ramp influenced instead only marginally after-ignition processes. Increasing the pressure, the swelling phenomena - typical of pure pyrolysis oils - exhibited a reduced intensity and completely disappeared at pressures higher than 20 bar. Mixture and emulsion droplets showed less marked swelling at normal pressure and the phenomenon completely disappeared already at 10 bar. However, strong bubbling and foaming were observed. The temporal sequence heating-swelling-vaporisation/bubbling/foaming-ignition was strongly dependent on the properties of the "carrier" fuel used in the preparation of the mixture or emulsions. After ignition, both mixture and emulsions droplets underwent a diameter reduction that could be approximated with a D2 law. Some difficulties arose for pure oil droplets due to the strong swelling that prevented a correct determination of the drop diameter. The droplets burning time (normalised respect to the drop diameter) decreased with pressure for all the samples. The liquid burning rate was around 1 mm2/sec at normal pressure, increased to about 1.5 mm2/sec at 12 bar and to 2 mm2/sec at 45 bar. All the samples, even if to a different extent, evidenced liquid-phase pyrolysis and the formation of a cenosphere during the last part of the liquid combustion. However, the formation of a solid residual, peculiar of pure oil droplets, was reduced in the case of emulsions and mixtures. Interesting enough, these samples gave rise to a smaller solid residual respect to what could be expected from the BFO content in the parent droplet. This was due to the different combustion properties of emulsions and mixtures when compared to pure BFOs. The other main parameter influencing cenosphere formation was, of course, the BFO quality and composition. The synergy between BFO quality and combustion properties can result in very surprising effects: in practice no solid residual was formed by the emulsion made with 30% pine oil in #2 diesel oil. The pressure also played a role in the structure and shape of the residual since it forced the droplet/cenosphere surface in opposite direction respect to the internal pressure generated by the liquid vaporised in the droplet interior. At intermediate pressures, around 10 bar, the balancing between these forces produced "cocoon" shaped cenospheres when pure BFO was used. Pressure had a minor influence on cenosphere formation when emulsions and mixtures were used. This could be explained by considering their low content of oil and, hence, of "swelling" agent. An increase of the residual size with pressure was observed at 45 bar for the samples containing the higher oil concentration. In the examined experimental conditions, the combustion of residuals from all samples was diffusion limited. The cenosphere burning rate was, hence, determined essentially by the particle surface extension.