The Nature of Biofuels: First-, Second — and Third-generation Biofuels

The alternative energy sources are derived from biological material and it is these sources that are the main focus of the book. Recently the use of biological materials to provide a source of energy that is renewable and can mitigate carbon dioxide accu­mulation has attracted considerable attention (Chum and Overend, 2001; Hamelinck et al., 2004; Cockroft and Kelly, 2006).

The range of biofuels that can be produced is listed below and includes biofuels that are being used at present and others which are still at the development stage. Biological-based fuels can be solid, liquid and gaseous, and the physical state of the fuel greatly influences the way it is used. It is the developmental stage that has been used to divide biofuels into first-, second- and third-generation biofuels (Fig. 4.1). Those biofuels currently used and produced in large quantities are the first-generation biofuels. The biofuels that have been produced but technical difficulties and high costs have delayed their application on a large scale are the second generation. The third-generation biofuels are those which are still at the research and development stage.

Solid fuels:

• Biomass.

• Wastes.

Gaseous fuels:

• Methane (biogas).

• Hydrogen.

• Dimethyl ether (DME).

Liquid fuels:

• Methanol (FT origin).

• Ethanol.

• Biobutanol.

• Synthetic petrol (FT origin).

• Synthetic diesel (FT origin).

• Biodiesel (esters).

• Biodiesel (bio-oil).

• Biodiesel (plant and microalgal hydrocarbons).

• Biodiesel (microalgal oils).

The first-generation biofuels are represented by biomass, biogas, biodiesel and etha­nol. Biomass is not included in Fig. 4.1 as it is mainly combusted or co-fired with coal to produce electricity. However, biomass in the form of wastes and lignocellulose can

also be converted into biogas, diesel, petrol, methanol and dimethyl ether using gas­ification (Fig. 4.1).

The first-generation biofuels are produced from energy crops such sugarcane, sugarbeet, maize, wheat, rapeseed, soybean and sunflower. However, to completely replace the fossil fuels gas, petrol and diesel large areas of land will be required. Hence, there is not enough land to grow sufficient energy crops without competing with food crops for land. For these reasons second — and third-generation biofuels are under development (Fig. 4.1). The second-generation biofuels will be produced from ligno — cellulose biomass and wastes which have much better yields per hectare as the whole of the harvested plant will be used. The higher yields will mean that second-generation biofuel production will compete less with food crops. The direct production of hydro­gen and extraction of oil for biodiesel from microalgae are third-generation biofuels which will not compete with food crops. Microalgae can be grown on non-agricultural land or in marine conditions and because they are some 50-100 times more productive than biofuel crops much less land will be required (Chisti, 2007).

Therefore, it is essential that second — and third-generation biofuels are developed as first-generation biofuels can only realistically supply 5% of the fuels required.