Abstract
Fast biomass pyrolysis is an effective and promising process for high bio-oil yields, and represents one of the front-end technologies to provide alternative, sustainable fuels as a replacement of conventional, fossil-based ones. In this work, the effect of droplet initial diameter on the evaporation and ignition of droplets of crude fast pyrolysis bio-oil (FPBO) and FPBO/ethanol blend (50% vol) at ambient pressure is discussed. The experimental tests were carried out in a closed single droplet combustion chamber equipped with optical accesses, using droplets with a diameter in the range of 0.9–1.4 mm. The collected experimental data show a significant effect of droplet diameter and initial fuel composition on the evaporation and combustion of the droplets. At the same time, 1-dimensional modeling of the evaporation and ignition of different droplets of crude FPBO and its blend with ethanol is performed to understand the complex physical and chemical effects. To this purpose, an 8-component surrogate was adopted, and a skeletal mechanism (170 species and 2659 reactions) was obtained through an established methodology. The comparison of numerical and experimental results shows that the model is able to capture the main features related to the heating phase of the droplet and the effect of fuel composition on droplet temperature and evaporation, particularly the increased reactivity following ethanol addition and the variation of diameter with time. Also, a sensitivity analysis highlighted the reactions controlling the autoignition of the droplets in the different conditions. It was found that the autoignition of pure FPBO droplets is governed by dimethyl furane (DMF), because of its high volatility and in spite of not being the most abundant species. On the other side, ethanol chemistry drives the gas-phase ignition in the case of the blended (50/50 v/v) mixtures, due to its higher volatility and reactivity.
Highlights
As combustion retains a leading role in the world energy scenario,[1] the utilization of renewable energy and the replacement of fossil fuels with alternative sources is one of the priorities for a sustainable development, toward a global reduction of pollutants and greenhouse gas emissions and an improved efficiency
Fast pyrolysis bio-oils (FPBO) are blackbrownish liquids, the use of which in the replacement of fossil fuels could significantly cut down the overall CO2 emissions, when these are analyzed from a life-cycle-analysis perspective
The effect of initial droplet diameter and composition on the autoignition of fast pyrolysis bio-oil droplets (FPBO) and a blend with ethanol (50% v/v) are presented and discussed
Summary
As combustion retains a leading role in the world energy scenario,[1] the utilization of renewable energy and the replacement of fossil fuels with alternative sources is one of the priorities for a sustainable development, toward a global reduction of pollutants and greenhouse gas emissions and an improved efficiency. The high amount of water and oxygenated compounds have a dampening effect on the heating values (15−20 MJ/kg) For all of these reasons, FPBO upgrading, either physical or chemical, is often required,[5] for example, for use as transportation applications. In compression−ignition engines, viscosity must be kept around 10−20 cSt to allow an optimal droplet penetration into the combustion cylinder,[6] while surface tension must be below ∼30 mN/m for both light and heavy fuel oils, and pH must be kept around 7.4 Expectably, upgrading the mixture to obtain transportation fuels results in a non-negligible increase in the cost of the biofuel.[4] On the other side, direct FPBO combustion covers a wide variety of applications
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