Of the various forms of biofuels, the 4th generation appears to be the least clearly defined; encompassing, essentially, all the means of obtaining biofuels not previously defined by the prior three generations. However, the current selection of techniques is broadly confined to the following:
- Microorganism manipulation
- Pyrolysis
- Gasification
- Genetic manipulation of feedstocks
The distinction between first generation and fourth generation production of biofuels is primarily in the nature of the organism utilised. An example is production of biofuels by hi-jacking the amino-acid processes in E-Coli bacteria through genetic manipulation. A paper on the subject can be found here: http://www.nature.com/nature/journal/v451/n7174/pdf/nature06450.pdf
Pyrolysis
This is the process of separating chemicals through application of heat in an endothermic process. At present the process is limited to production of bio-oil. The process is, in lay terms, the heating of a material in the absence of oxygen. There are variations on the basic process where the material is either heated in a vacuum (referred to as Vacuum Pyrolysis) or flash heated to a much higher temperature (referred to as Flash Pyrolysis) and a combination of the two (referred to as Flash-Vacuum-Pyrolysis).
The obvious problem with this method is the requirement for a powerful and controlled source of thermal energy, which will have to either consume the fuel produced and reduce the efficiency of the process of rely on an external source of fuel and remove the carbon-neutrality of the process.
Expansion of mentioned terms and explanation of the remaining terms to follow.
Links for reference and citations:
http://en.wikipedia.org/wiki/Gasification
http://en.wikipedia.org/wiki/Pyrolysis
http://en.wikipedia.org/wiki/Gasification
http://en.wikipedia.org/wiki/Butanol_fuel
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