Page 45 - Advances in Eco-Fuels for a Sustainable Environment

22 Advances in Eco-Fuels for a Sustainable Environment

Table 2.5 Carbohydrate content of different algal species [15]

Microalgal species Total carbohydrates (% per dry weight)
Arthrospira platensis 40.8
Nannochloropsis sp. 56.8
Dunaliella tertiolecta 50.6
Dunaliella salina Teod 69.7
Galdileria partita Sentz 50.1
Cosmarium sp. 58.4
Nostoc sp. 52.3
Chlorella vulgaris IAMC-534 37.0 (starch)
C. vulgaris CCAP 211/11B 55.0
C. vulgaris P12 41.0 (starch)
C. vulgaris FSP-E 55.0 (starch)
Chlamydomonas reinhardtii UTEX 90 60.0
C. reinhardtii IAM C-238 55.0 (starch)
Chlorococcum humicola 32.5
Chlorococcum sp. TISTR 8583 26.0 (starch)
S. acutiformis TISTR 8495 16.4 (starch)
S. obliquus CNW-N 51.8
Tetraselmis sp. CS-362 26.0

Another favorable property of this feedstock is the diversity of growth possibilities,
including:
Open ponds, in which algae are grown within a pond at the open air. These are simple and
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low-cost, but less efficient than others. Furthermore, external organisms could contaminate
the pond and potentially damage or kill the algae.
Closed-loop systems, similar to open ponds, but not exposed to the atmosphere and using a
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CO 2 source before ever released into the atmosphere.
Photobioreactors, the most advanced and most difficult systems to implement, with high
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capital costs but maximum advantages in terms of process yield and control.
These systems imply that algae could be grown almost anywhere, if ambient temper-
atures are warm enough, and that no farmland need be converted. Furthermore, algae
can be grown in wastewater, with secondary benefits as they can help treat municipal
sewage without taking up additional land. All these factors make algae easier to cul-
tivate than any other traditional biofuel crop [14].
Algae have only one major downside in that they require large amounts of water
and nutrients (N and P) to grow. Even though closed-loop and photobioreactor sys-
tems have been used in arid and desert settings, algae need huge amounts of water.
Their nutrient needs are such that the alternative production of fertilizers to meet them
would produce more GHG emissions than those that would be saved using algal-based
biofuel. Industrial N production needs, in fact, 19.3kWh/kg N produced by the Haber-
Bosch process, and P production from phosphate rock requires 2.11kWh/kg P. While
this is not an issue if the alternative is nutrient removal from wastewater, which actu-
ally causes additional GHG emissions, it could be such if the foreseen alternative is