Potential Use of Entire Microalgae for Biofuel Application: Experimental Results

Scientific community is today devoting great interest to microalgae sector, as a potential alternative feedstock for biofuels industry. The state of the art of the technology still has several bottlenecks that are limiting the diffusion of this next generation biofuel. One of the main critical phases in biofuel production from microalgae is the extraction and the valorization of the oil or carbohydrates. The energy and the economical balances of this phase, even if debatable, result to be still negative. Alternative pathways are under investigation but solutions are still far from a commercial scale. The work presented in this paper tries to overcome this barrier using the entire microalgae. Many of the most promising microalgae have an average dimension (2-15 ?m) compatible with many fuel injectors today adopted in commercial systems used for energy production. The driving idea of the work is to create a suspension of microalgae in regular fuels and in biofuels, to directly feed the injector. This approach has the advantage of saving the energy related to the extraction and post-processing phases. The chemical physical characteristics of the entire microalgae are suitable to a direct use, as the average LHV is about 18-20 MJ/kg. However, a systematic study of the combustion properties of these biofuels is necessary for evaluating a real potential for their use in commercial combustion systems. A viable way to burn complete algae is to realize slurry composed of dried algae biomass dispersed in combustible oil. Aim of this work is to assess the combustion characteristics of suspensions composed by the microalgae Nannochloropsys diluted in biodiesel and in vegetable oil (rapeseed), respectively. For comparison pure biodiesel and vegetable oil were also tested. The combustion experiments were carried out in a closed single droplet combustion chamber. The chamber is equipped with quartz windows to allow the application of optical techniques. Experiments were performed in controlled conditions of temperature and pressure. Droplets composed of the different fuels were suspended to a bare thermocouple junction (type K, ?wire = 75 ?m). The size of droplets varied between 400 ?m and 1200 ?m. A high resistive coil, positioned below the droplet hanging system, was used to heat and ignite the droplets. Shadowgraphy technique was used to follow the different processes undergone by the droplets: heating, evaporation, ignition and combustion. A high speed digital imaging systems working up to 8000 frames/sec was used to catch the droplet shadowgraphs thus permitting the analysis of the various processes. Average and instantaneous evaporation and combustion rates were inferred. Also the ignition characteristics were inferred by the experimental data. The evaporation and the burning rate, as well as the ignition properties, of suspensions of microalgae were systematically compared with commercial biodiesel and vegetable oil. These data permit to assess how this special biofuel formulation can be applied in practical combustion systems like furnaces, gas turbines and Diesel engines. Finally, the techno-economic analysis of the approach will be discussed in order to evaluate the feasibility of the route on industrial scale.