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This article or section includes content from Original:Biogas and liquid biofuels by Practical Action. Used with permission.

Biofuel is a type of fuel which is made using animal or plant-based resources which are regenerated quickly. Biofuels are hence different from fossil fuels as although fossil fuels too are created trough anaerobic decomposition of buried dead organisms, this process takes a very long time (exceeding millions of years). This means that -unlike fossil fuels- biofuels are a form of closed-end recycling, whereby the waste product goes directly into production of the fuel. Like fossil fuels, biofuels are a form of indirect solar energy.


Biomass residues can be used as is (solid biomass) or converted into various non-solid fuel forms. These fuels are referred to as biogas and liquid biofuels. The aim of this conversion process is to improve the quality, specific energy content, transportability, etc., of the raw biomass source or to capture gases which are naturally produced as biomass is micro biologically degraded or when biomass is partially combusted. Biogas is a well-established fuel for cooking and lighting in a number of countries, whilst a major motivating factor in the development of liquid biofuels has been the drive to replace petroleum fuels. In this fact sheet we will be looking at some of these fuels, their applications and the conversion technologies used to derive them.

In Europe and the United States, as well as in several developing countries, there is a move toward cultivating energy crops specifically for the production of biomass as a fuel. The potential for energy production from biomass throughout the world is enormous and as fossil-based fuels become scarcer and more expensive, as carbon emission levels are becoming of greater concern and as people realise the benefits of developing integrated energy supply options, then biomass could begin to realise its full potential as an energy source.

Biomass energy and the environment

There are two areas of environmental concern when considering using biomass as a form of energy. Firstly, there is the issue of land degradation and deforestation. This concern can be addressed by proper management of sustainable energy crops. Although much of the biomass requirement for energy production can be met through utilising residues from the food industry (=second generation biofuels), from agriculture or from commercial activity, careful planning of energy cropping is required to prevent undue stress on the environment.

The large growth in the use of biofuels has promoted large scale mono-crop feedstock production and associated problems (increased pests/diseases, ...). The use of a variety of crops and possibly different methods of agriculture (ie techniques as companion planting, IPM ...) can reduce these problems.

The use of crops that are native to the region (see Oil_crops.png) can also provide part of the answer. In addtition to this, the exact place (and the current use of the location -ie food production, CO2 already locked in the soil, ...) where the crops are planted also matters. According to Wouter Achten of KU Leuven, biofuel-crops are best planted in CO2-poor soils and which are currently not used for agriculture. The first is for obvious reasons: by requiring the farmer to fertilise the soil with CO2 he locks away part of the CO2 in the atmosphere. The downside however is that extra fertilisation (and thus an increased cost) is required. The second is for less obvious reasons: if the land is used for agriculture, the crops that were planted need to be relocated. This could mean that there is an extra CO2-cost in transport (crops need to be transported further). This is known as ILUC.

Localised decentralised biofuel production from feedstock grown using sustainable agricultural practices been shown to offer part of a sustainable energy portfolio. A good example is for example rapeseed. This crop creates both biofuel (oil) as animal feed (the rest of the plant).

With the recent global call to reduce carbon dioxide emissions, there is a strong case for promoting the use of sustainable biomass-to-energy technologies worldwide. Using modern technology, enormous reductions can be made in carbon dioxide emissions, particularly if liquid biofuels are used to replace their fossil-based equivalents. In fact, if biomass energy production is done on a sustainable basis, there is little net carbon dioxide addition to the environment.

There are other environmental concerns related to each fuel that need to be kept in mind, such as toxic emissions and production of tars and soots.[1][2][3][4][5]

Advantages and disadvantages

Biofuels are not made from petroleum; not purchasing petroleum products allows you to avoid supporting business practices such as oil drilling that are harmful to the environment and human rights.

Pollution is any byproduct that cannot be fed back into the closed-end system. For biofuels (except for biohydrogen), this includes particulates and unburnt hydrocarbons (smoke), oxides of nitrogen, carbon monoxide, and a few others. These are typically much lower level than when fossil fuels are combusted, but they remain a problem, particularly for the human health (ie may cause respiratory problems, certain cancers, ...).

Zero-emissions fuels do not have this problem, yet are more difficult to use in practice, and are also more expensive.

Note that what is pollution for one technology may be the biofuel in another. For example, if wood is heated anaerobically (with limited oxygen), it produces carbon monoxide, which is normally considered a pollutant, but if collected, can be burnt as a biofuel.[6]

Types of biofuel

First generation biofuels

'First-generation (or conventional) biofuels' are biofuels made from substances in crops (ie sugar, starch, and vegetable oil) that can be used for human consumption. Due to this, the production of fuel from these crops effectively creates problems in regards to the global food production.[7][8]

Solid biofuels

See wood, sawdust, grass trimmings, charcoal, agricultural waste, and dried manure.


These include ethanol and methanol. See Alcohols as fuel.

Biodiesel and green diesel

See biodiesel

Pure Plant Oil and Waste Vegetable Oil

See vegetable oil as fuel and, Waste vegetable oil, Pure plant oil as fuel and Waste vegetable oil


See biogas


See syngas

Wood gas

See wood gas

Second generation biofuels

'Second generation biofuels' are biofuels produced from made from substances in crops (ie cellulose) that can not be used for human consumption. Unlike first generation biofuels they do not create problems in regards to the global food production.

Cellulosic ethanol

Wood can be converted to ethanol yet at present (2007 D.C.) it is not yet a economicly viable method.[9]

Algae fuel

See algaculture


see Biohydrogen




Fischer-Tropsch diesel

Biohydrogen diesel

Mixed alcohols

Wood diesel


With most biofuels the incompatibility with available engines provides an additional barrier to the adoption as reliable operation requires expensive engine modifications. 'Flexi-fuel' engines are available in some regions, commonly spark ignition engines able to run straight petrol(US-gas) or petrol/ethanol blends. Additives (bio ethers) can be applied to fuels to improve their performance.

Use in heat engines

It is possible to use biofuels in several heat engines, including internal combustion engines (diesel, gasoline) and Stirling engines. Reliability and performance of the engine will depend on:

  • biofuel material compatibility - the compatability of fuel system and engine components to the fuel
  • engine parameters: such as fuel delivery or spark timing, being optimised for the given fuel
  • a suitable maintenance regime

Use in piston steam engines

Piston steam engines can be run on both solid as liquid and all gas-based biofuels. They however do need an additional energy conversion (fuel to steam) meaning there is some additional energy loss. They are also a lot less efficient than steam turbines.

Use in IC engines (diesel engines)

It is possible to use a wide range of liquid biofuels in a diesel engine, most commonly lipid based biofuels are used either in their pure form, plant oil, or transesterified as biodiesel. Diesel engine fuel delivery can be altered to suit the fuel. See also:

Use in IC engines (gasoline engine)

Some liquid biofuels as ethanol can be used, oil-based biofuels can't be used though. Gases can also be used (ie wood gas (if filtered), biohydrogen, biogas and pure methane) See

Use in Stirling engines

Stirling engines can use a wide range of biofuels, both liquid biofuels (oils, ethanol, ...), solid biofuels (ie wood, seeds, ...) and gas-based biofuels (ie wood gas (if filtered), biohydrogen, biogas, pure methane )

Use in steam and fuel-powered turbines

Fuel-powered turbines can be run on liquid biofuels as oils, ethanol, ... aswell as some gas-based biofuels (ie biohydrogen, methane). Gas-based biofuels as wood gas and biogas are potentially also possible, but could give problems with fouling (due to tar, ...) Steam turbines (bladed-rotor, Tesla, ...) can run on all biofuels (solid, liquid, and gas-based biofuels). Fouling isn't a problem here (as opposed to fuel-powered turbines) as the heater chamber is generally seperated from the chamber housing the turbine blades. Steam turbines however do require an additional energy conversion (fuel to steam) meaning there is some additional energy loss. The incineration of the fuel can btw be done using a pulse jet engine to increase efficiency, and to decrease fouling in this seperate heating chamber (although it isn't a big problem) even more.


  1. Anderson, T., Doig, A., Rees, D. and Khennas, S., Rural Energy Services: A handbook for sustainable energy development. ITDG Publishing, 1999.
  2. Ravindranath, N. H. and Hall, D. O., Biomass, Energy and the Environment: A Developing Country Perspective from India. Oxford University Press, 1995.
  3. Karekezi, S. and Ranja, T., Renewable Energy Technologies in Africa. AFREPEN, 1997.
  4. Kristoferson L. A., and Bokalders V., Renewable Energy Technologies - their application in developing countries. ITDG Publishing, 1991.
  5. Johansen, T.B. et al, Renewable Energy Sources for Fuels and Electricity. Island Press, Washington D.C., 1993.
  6. Biofuel
  7. Jean Ziegler calling first generation biofuels a crime against humanity
  8. Issues relating to first and some second generation biofuels
  9. Cellulosic Ethanol: One Molecule Could Cure Our Addiction to Oil, Evan Ratliff, Wired Magazine October 24, 2007

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