The Physical Basis for Biofuels

Biofuels are ultimately based on the ability of photosynthetic organisms to use solar irradiation for the conversion of CO2 into glucose (C6H12O6) and subsequently into biomass; the overall reaction for the conversion into glucose usually being:

6CO2 + 6H2O ^ C6H12O6 + 6O2

Some photosynthetic bacteria may not produce oxygen but give off elemental sul­phur.

In practice, only part of incident solar radiation is captured by plants. And of the solar irradiation captured by plants, only a part (approximately 43-45% of radia­tion in the visible part of the spectrum for land plants) is photosynthetically active (Sinclair and Muchow 1999; Vasudevan and Briggs 2008).

The synthesis of glucose is powered by light reactions generating NADPH, ATP and O2. Thereafter, the reactions can proceed in the dark. In these reactions, collec­tively known as the Calvin cycle, ATP, NADPH and CO2 are converted into glucose, NADP+, ADP and phosphate.

The first enzyme of the Calvin cycle is ribulose bisphosphate carboxylase. As ribulose bisphosphate carboxylase is sensitive to oxygen, photorespiration is impor­tant to protect the enzyme. When CO2 levels in the atmosphere increase, protec­tion by photorespiration can be reduced. At the present atmospheric concentration of CO2, in most plants, photorespiration leads to the release of up to about 50% of the CO2 originally fixed by photosynthesis. These plants are called C3 plants. This name is linked to the first product of photosynthesis that contains 3 C atoms, 3-phosphoglyceric acid. All large trees are C3 plants (Heaton et al. 2008). More re­cently in the evolution of terrestrial plants, a retrofit to the Calvin cycle has emerged that reduces the need for photorespiration. The plants having such a retrofit are called C4 plants. This name is again linked to the first product(s) of photosynthesis that are organic acids with 4 C atoms. Examples of C3 terrestrial plants relevant to biofuels are wheat, rapeseed, soybean, sunflower, eucalyptus, sugar beet, potato, poplar, coconut, cassava, cotton and Jatropha, while examples of C4 plants are sugar cane, corn (maize), switchgrass, sorghum, millet, and Miscanthus.

Natural C4 species tend to be better adapted to relatively warm climates than C3 species. However, breeding and selection have changed the temperature response in a number of C3 and C4 species. Thus, there are now C3 species that do optimally in relatively warm climates (e. g. cotton) and C4 species, such as corn varieties which have been well adapted to temperate climates (El Bassam 1998). The reduced need for photorespiration in C4 species is reflected in a higher maximum theoretical effi­ciency in the conversion of solar irradiation into biomass.

For C4 plants on land at the present concentration of CO2, the maximum theoret­ical efficiency is estimated at 5.5-6.7% and for C3 plants on land at 3.3-4.6% (Hall 1982; El Bassam 1998; Kheshgi et al. 2000; Heaton et al. 2008). For algae, a theo­retical efficiency varying between 5.5 and 11.6% has been suggested (Heaton et al. 2008; Vasudevan and Briggs 2008). Actual efficiencies in commercial cultivation are much lower, as will be discussed in Chap. 2. Most transport biofuels are derived from photosynthetic organisms, though there is also a limited supply of biofuels derived from animals (based on, for example, yellow grease and animal meal).