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452 Refining Biomass Residues for Sustainable Energy and Bioproducts

in reusing the waste and producing valuable products. Above all, this technology is
believed to support economic growth and environmental sustainability. However,
the operational cost of marine biorefineries still remains too expensive to consider
them as a viable option for biofuel production and other related applications.
Hence, biorefinery system should be designed with minimal investments, less
energy usage, recycling options, easy separation, wide range of raw material avail-
ability, and transportation.

20.2 Biofuel generations

Based on the biomass used, biofuels can be categorized into first, second, and third
generations that determine the flexibility and potential of the biofuel industry.
Different generations of biofuel with their characteristics are shown in Table 20.1.
Due to the ethical problems faced by first- and second-generation biofuels, the
economic concern has shifted from terrestrial to marine biomass for the biorefinery
process (Gouveia, 2011).

20.3 Sources of biofuel

20.3.1 Algae
Algae are usually aquatic oxygen-emitting photosynthetic organisms with the
simple structure of no roots, stems, or leaves. Since they don’t fit under a single
monophyletic group, they cannot be easily defined. They exist as a group of
ubiquitous, but individual species, thus occupying specific habitats. Algae exist in
various forms, such as a few of the algae attached to plant substrates; few possess
motility behavior like animals; few are suspended in water; few grow loosely in
trees, soil, and animals; and some form symbiotic association with other organisms
(e.g., lichens, corals). Microalgae lack complex multicellular structures with great
variation in their internal cell structure. The blue-green algae or cyanobacteria have
a prokaryotic cell structure that closely resembles bacteria.
The demands for the biofuels lead to the search for feedstocks of micro and
macroalgae for their production. Algae consist of 40% of lipids, which helps in
rapid conservation of biofuel, thus making the process environment friendly and
cost-effective (John et al., 2011). In addition, the cultivation of algae is also a
relatively simple and easy technique. So this method has become globally attractive
(Miao and Wu, 2004). Algal biomass can be classified as sugars, proteins, and
triglycerides, which can be converted into various valuable products, such as
pharmaceutical products, food, feedstocks, enzymes, and pigments along with
biofuel (bioethanol, biodiesel, biomethanol). Owing to these advantages, petroleum
refinery can be partially replaced by biorefinery. Therefore this section deals with
different algal resources involved in biofuel production.