Production, Application and Development

A. H. Scragg

Biofuels are energy sources derived from biological materials, which distinguishes them from other non-fossil fuel energy sources such as wind and wave energy. Biofu­els can be solid, liquid or gaseous, and all three forms of energy are sustainable and renewable because they are produced from plants and animals, and therefore can be replaced in a short time span. In contrast, fossil fuels have taken from 10 to 100 million years to produce and what we are burning is ancient solar energy. In addition, the energy derived from plant material should be intrinsically carbon-neutral, as the carbon accumulated in the plants by the fixation of carbon dioxide in photosynthesis is released when the material is burnt.

At present it is clear that the supply of fossil fuels is finite and a time can be envis­aged when the supplies of fossil fuels become scarce or even run out. Also the burning of fossil fuels releases additional carbon dioxide into the atmosphere over and above that released in the normal carbon cycle. The accumulation of carbon dioxide in the atmosphere appears to be the major cause of global warming. The consensus suggests that the long-term effects of global warming will be severe, with drastic changes in climate and sea levels. At the same time, modern society requires increasing amounts of energy, most of which is obtained from fossil fuels. Thus, mankind is almost totally reliant on fossil fuels to provide electricity, heating/cooling and transport fuels. This reliance can be seen in the effects on countries when oil supplies are interrupted by wars, embargos and strikes. In 2008, the world suffered from rapid rises in oil prices which affected the price of many commodities.

Alternative energy supplies are needed, therefore, to provide both power and fuel for transport. The possible energy sources available are very diverse and include hydroelectric, nuclear, wind, biological materials and many others. Whatever energy source is used, it should be sustainable and as carbon-neutral as possible. Biofuels encompass the contribution that biological materials may make to energy supply and in particular liquid fuels for transport. Solid biofuels, principally biomass, have been used for thousands of years to provide heat and for cooking, and are used at present to generate electricity and in combined heat and power systems. The gaseous biofuel methane is produced by the anaerobic digestion of sewage, in landfills and is also used for electricity generation and for heat and power systems. It is the liquid biofuels that will be used to replace the fossil fuels petrol and diesel, and they have attracted much attention. At present liquid biofuels can be divided into first-, second — and third-generation biofuels. The first-generation biofuels consist of ethanol which is used to either supplement or replace petrol and biodiesel, as a replacement for diesel. Ethanol is produced from either sugar or starch which is extracted from crops such as sugarcane, sugarbeet, wheat and maize and can save 30-80% of greenhouse gas emissions when compared with petrol. Biodiesel is produced from plant oils and animal fats and can save between 44 and 70% greenhouse gases compared with diesel. At first glance this situation appears ideal but there are problems with first — generation biofuel production and supply. One problem is the amount of energy that is used to produce and convert the crops into biofuels. Another problem is the amount of biofuels needed to replace fossil transport fuels. In the UK, in 2006 19,918 million t of petrol and 23,989 million t of diesel were used, that is a total of 43,907 million t. To supply this tonnage is a formidable task. For example, to supply the diesel required in the UK using the oil-seed crop rapeseed, 113% of the agricultural land would be needed. This is clearly not possible and even at modest levels of diesel replacement the biofuel crops would compete with food crops. It is this feature that has brought forward many objections to biofuels, and they have been blamed for some food shortages; however, in reality food prices are influenced by a number of factors. Converting sensitive lands such as rainforests to grow biofuel crops has also engendered justifiable resistance. In addition, crops such as wheat and other starch — containing crops require considerable processing and energy input, and when inves­tigated by life-cycle analysis show only marginal gains in energy.

However, the resistance to biofuels need not be the case as the first-generation biofuels bioethanol and biodiesel were only intended to be used as a 5% addition to fossil fuels to comply with the EU directives, and to show that fossil fuels could be replaced. It was clear that any more would compromise food crops. It is the second — and third-generation biofuels that should replace the bulk of the transport fossil fuels. The second-generation biofuels are ethanol, produced directly from lignocellulose, and the gasification of lignocellulose and waste organic materials producing petrol, diesel, methanol and dimethyl ether. Lignocellulose and organic wastes are available in large quantities and their use does not compromise food crops. Lignocellulose is often the discarded portion of food crops such as straw. The third-generation biofuels are hydrogen, produced either by the gasification of lignocellulose or directly by microalgae, and biodiesel produced from oil accumulated by microalgae. These second — and third-generation biofuels should not compromise food crops, but to bring these fuels into production will require both research and investment. To stop the use of land areas, such as the rainforests, for first-generation biofuels will prob­ably require legislation.