Similar to second-generation biofuels, so-called third-generation biofuels are pro­duced from non-edible specially engineered low-cost, high-energy and entirely renewable crops such as algae (Chris ti 2007). These are capable of generating more energy per acre than conventional crops and can also be grown on land and in water that is not suitable for food production. Fourth-generation biofuels use genetically modified crops (Table 2). The conversion process in this case is similar to that employed for second — and third-generation biofuels, but involves an addi­tional step where the carbon content in the fuel is oxidized by processes such as oxy-fuel combustion (Gray et al. 2007). The CO2 released is then absorbed and stored in oil and gas fields or saline aquifers (ZEP-EBTP 2012).

A distinction often used in favour of third — and fourth-generation biofuels is that they are produced from carbon neutral or negative biomass. However, as Centi et al. (2012) note, this has not yet been proved empirically, while Gasparatos et al.

(2012) point out that the technologies involved are still in their infancy. In the light of these uncertainties, this chapter focusses on first — and second-generation biofu­els, more so given that many environmental aspects of third — and fourth-generation biofuels hold true for second-generation fuels.