A biofuel is difficult to define. Most of the fossil fuels we use are biological in nature. Perhaps we have to say that a biofuel is one that does not add to the stock of total carbon dioxide in the atmosphere. These are plant forms that, typically, remove carbon dioxide from the atmosphere, and give up the same amount when burnt. Naturally, all the other fossil fuels have done the same, but we are talking about a time scale of one or two years for the biofuels, whereas the fossil fuels can only be considered on a time scale measured in millions of years.
The biofuels are therefore considered to be "CO2 neutral", not adding to the carbon dioxide level in the atmosphere. The type of biofuel used will depend on a number of factors, chief amongst them being the available feedstock and the energy that can be used locally.
Biodiesel was probably the first of the alternative fuels to really become known to the public. The great advantage of biodiesel is that it can be used in existing vehicles with little or no adaptation necessary. Biodiesel is, naturally, a compromise for this reason, but still balances positively on the energy scales. There are energy plants available that will produce a higher yield in kWh per area, but the simplicity of having a fuel that is fully compatible with present fuel and engine technology makes it very attractive.
Cars running on BioEthanol, which is produced from agricultural crops, sugar cane or bio-mass, are governed by the same law of physics as those using gasoline. That means both emit CO2, as an inevitable consequence of the combustion process. But there is a crucial difference: burning ethanol, in effect, recycles the CO2 because it has already been removed from the atmosphere by photosynthesis during the natural growth process. In contrast, the use of gasoline or diesel injects into the atmosphere additional new quantities of CO2 which have lain fixed underground in oil deposits for millions of years.
Biogas is becoming increasingly interesting as an alternative to natural gas. It is especially useful that the composition is practically identical, so the same burners can be used for both fuels. Biogas can be produced from plant or animal waste, or a combination of both. There are many different methods used dependent on the starting material and quantity involved. A mixtrue of both has proven to be the best method. The animal waste produces the nitrogen needed for growth of the bacteria and the vegetable waste supplies most ofthe carbon and hydrogen necessary.
Biomass can be a practicable alternative for small district heating schemes in rural areas. Traditional biomass is wood residue and excess straw from agriculture being burned to provide heat or power. There are also gasification plants that produce a gas composed mainly of carbon monoxide and hydrogen from plant material. This has the advantage of being capable of transportation by pipeline or being filled into cylinders for distribution. Pyrolyis, as it is known, is being investigated in many countries presently.
Pyrolysis of Biomass
Pyrolysis of biomass is used to produce a mixture of three combustible products from biomass: tar, gas and coke are formed in varying proportions. After cleaning the gas can be used to drive turbines or gas motors. The tar is also suitable for the plastics industry and the coke can also be burned in the conventional way.
Landfill gas analyzer
The landfill gas analyzer is similar to a standard flue gas analyser, but capable of measuring methane and carbon dioxide directly. There are many landfill sites in use still, which all produce gas naturally. More advanced models of landfill gas analyzer will also be capable of measuring the products of combustion.
Landfill sites are now being used for the commercial production of methane in many areas instead of simply flaring the gas for safety reasons. Methane is produced in commercially viable quantities for many years after a landfill site has been closed. Nevertheless, there are still many landfill sites where the gas is being wasted. This source will dry up in time to come, since many countires are now finally emphasising the separation of waste and recycling, but there is gas for the next twenty years in the landfill sites presently existing.
Measurements in biogas
Measurement of the concentrations of carbon dioxide and methane in biogas has produced interesting errors, probably due to the difference in size of the molecules. These factors require consideration when biogas is measured before combustion. Commercial use of biogas makes knowledge of the composition and heating value essential.
Although not a detailed description of how to build a methane digester, this is a good explanation of the working principle. The methane digester is a plant to produce methane in the form of biogas from plant and animal waste. Such systems are common in certain countries, such as India, but sorely neglected in others, although the raw material is available everywhere.
Biodigester construction in the Philippines
A specific section about a methane digester in use in the Philippines constructed with the very kind co-operation of the designer and builder, Gerry Baron. This section is updated when new information comes in about the biogas project. Biogas is one of those matters where results do not come immediately. It can take a few weeks for changes to take effect, so the changes will be sporadic, but they will come.
Renewable energy - A fund of historical and current information on biofuels. Please be patient, the server loads very slowly!
Biogas.ch - very informative site with discussion group in German, French and English.
The Environment Site Site containg plenty of environmental information and a discussion forum.