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
