Page 18 - Biofuels for a More Sustainable Future

Biofuels technologies: An overview of feedstocks, processes, and technologies 11

biofuels production line) or metabolically engineered algae (with high oil
contents, increased carbon entrapment ability, and improved cultivation,
harvesting, and fermentation processes) (thus improving the third generation
production) (Dutta et al., 2014). While algae have commonly been recog-
nized for its high oil contents, the exact parameters depend on the respective
algae strains. Botryococcus braunii, Chaetoceros calcitrans, Chlorella species, Iso-
chrysis galbana, Nannochloropsis, Schizochytrium limacinum and Scenedesmus spe-
cies have been analyzed in the literature so far for their applicability and
suitability for biofuels production (Chisti, 2007; Rodolfi et al., 2008; Singh
and Gu, 2010). It has been found that the fast growing algae (e.g., Spirulina)
have low oil content, while algae strains high in lipid contents are charac-
terized by slower growth rates. Thus introducing new technologies like
metabolic engineering for accelerated growth of algae biomass or increased
lipid contents can result in faster commercialization and improved economic
feasibility of fourth generation biofuels (Singh and Gu, 2010). Nanotechnol-
ogy could also be applied in algae fuel production to increase efficiency of
algae biomass and decrease production costs, thus making it a cost-
competitive addition to the biofuel market (Ziolkowska, 2018).
The fourth generation biofuels is distinguished from other biofuels pro-
duction technologies also by the fact that in most cases they represent a com-
bination of different technologies, for example, sustainable energy
production (biofuels) and capturing and storing CO 2 emissions. Biomass
absorbing CO 2 during its growth is manufactured into biofuel by means
of the same or similar processes as second generation biofuels. The difference
between the fourth generation biofuels compared to the second and third
generation production is that the former captures CO 2 emissions at all stages
of the biofuels production process by means of oxy-fuel combustion
(Oh et al., 2018; Sher et al., 2018). Oxy-fuel combustion is a process utiliz-
ing oxygen (rather than air) for combustion yielding flue gas CO 2 and water
(Markewitz et al., 2012). While the process is more effective in generating
CO 2 stream of a higher concentration (the mass and volume are reduced by
about 75%), making it more suitable for carbon sequestration, the economic
problem occurs mainly at the initial stage of separating oxygen from the air
and using it for combustion. The process requires high energy inputs; nearly
15% of production of a coal-fired power station can be consumed for this
process (University of Edinburgh, n.d.), which can ultimately drive up pro-
duction costs and make the final process economically infeasible. Even
though currently still not competitive, oxy-fuel combustion has been stud-
ied as a potential alternative in combination with biofuels production. For