Production of Biofuels from Microalgae (Lit review 2)

Production of Biofuels from Microalgae

            Due to the depletion of our oil supply and knowing its reaching its peak, as well as the global warming from Carbon Dioxide emissions has led to means of looking at other resources. This includes the infamous biofuels, mainly produced from plant biomass. This includes biodiesel from plants of an oil seed that includes soy beans, oil palm rape seed and others. Also, ethanol can be taken from starch plants like corn, wheat, and sugar cane. This on the other hand leads into the common argument of ‘food vs. fuel’ as well as limitations of availability with land and water. However, algae have been proposed as an alternative for biodiesel as well as sugars for fermentation to ethanol using algae’s plant lipids. Now the question to be asked is; will algae be a feasible resource when looking at a large-scale production?

            The current issue with algae produced biofuel is that the scale required is to much, as well as the economical approach is just hard to lean toward. This is an issue regarding both a biological and engineering challenge for large scale production. The aim to this article is to highlight some key biological and engineering challenges caused by large scale biofuel plants for algae as well as possible ways to fix them.

            To begin, we must dive into the many required steps in the production of biofuels from microalgae. If one must produce this biofuel, they need lipid-rich algal biomass. Then once harvesting the biomass that was grown from the medium, a dewatering process is initiated. Once possibly drying the biomass, extraction of the lipids and sugars is done. This allows the conversion of lipids and sugars to biofuels.

            A key step in production of these algal biofuels is harvesting and dewatering of the algal biomass before extraction. This leads into one of the first major challenges. High cost of harvesting and dewatering sets back the development of commercially viable microalgae-based biofuels. Also for a suitable biofuel production to be something as a resource, we must look at certain key points. The production must be able to process very large volumes, be highly reliable, and most important, cost effective. The problem with this is that microalgae vary immensely in the properties that affect harvesting. This includes things such as size, surface charge, resistance and so on. This makes the recovery process having to be specifically designed to whatever the species microalgae and its growth system. Not to mention the amount of water that is in need for production as well as the disposal issues of the soon to be nutrient-containing medium byproduct. To show the amounts of water in need look at chart below. 

            The Harvesting, thickening, and dewatering of microalgae has been vastly reviewed by people like Shelef, Moraine, Mohn, and Molina Grima which all came to the same results. Key properties of microalgae that had influenced their separations were shape, size, specific weight, and surface charge. Filamentous algae such as Spirulina were harvested using filtration, but most of the considered algae’s that fit the characteristics in the sentence above were unicellular and too small for filtration. They tried centrifuging but that’s at too small of a scale to be an effective biofuel, as well as Sedimentation to be an idea that turned out to be too slow. Thus, the most commonly considered process is flocculation that is then followed by a process called flotation or settling to be the first step. To flocculate, the need to recycle the water back into the growth system without pretreatment. Cost is substantial for flocculates that could handle this. Basically, the production of a relatively low-value product such as these biodiesel, flocculants and the energy cost to run the system is astronomical. To have an effective system, you would need a layout shown below.

            If one wants to use biofuels generated by the production of microalgae, then harvesting and extraction of biomass, as well as efficient conversions of lipids to fuel to be at a large scale and low cost. Is this feasible? No, not yet at least. However, to achieve significant production of algal biofuels to partially replace fossil fuels, we would have to optimize all the unit processes in the figure listed above.

Fon Sing, S., Isdepsky, A., Borowitzka, M.A. et al. Mitig Adapt Strateg Glob Change (2013) 18: 47. doi:10.1007/s11027-011-9294-x