Advanced Biofuels are high-energy liquid transportation fuels derived from: low nutrient input/high per acre yield crops; agricultural or forestry waste; or other sustainable biomass feedstocks including algae. The key word is "sustainable."
A technical definition that focuses on lifecycle greenhouse gas emissions is included in the Renewable Fuels Standard of the Energy Independence and Security Act of 2007. Download PDF here.
TITLE II-ENERGY SECURITY THROUGH INCREASED PRODUCTION OF BIOFUELS
Subtitle A-Renewable Fuel Standard
SEC. 201. DEFINITIONS.
(1) DEFINITIONS.-In this section: ...
(B) ADVANCED BIOFUEL.-
(i) IN GENERAL.-The term 'advanced biofuel' means renewable fuel, other than ethanol derived from corn starch, that has lifecycle greenhouse gas emissions, as determined by the Administrator, after notice and opportunity for comment, that are at least 50 percent less than baseline lifecycle greenhouse gas emissions.
(ii) INCLUSIONS.-The types of fuels eligible for consideration as 'advanced biofuel' may include any of the following:
(I) Ethanol derived from cellulose, hemicellulose, or lignin.
(II) Ethanol derived from sugar or starch (other than corn starch).
(III) Ethanol derived from waste material, including crop residue, other vegetative waste material, animal waste, and food waste and yard waste.
(IV) Biomass-based diesel.
(V) Biogas (including landfill gas and sewage waste treatment gas) produced through the conversion of organic matter from renewable biomass.
(VI) Butanol or other alcohols produced through the conversion of organic matter from renewable biomass.
(VII) Other fuel derived from cellulosic biomass.
Generally, both the practical and technical definitions encompass the same universe of potential sustainable, renewable fuels. Both definitions concentrate on the creation and production of the fuels rather than on their use. The most typical uses of advanced biofuels are for transportation, although some may be used in generators to produce electricity and others may eventually replace propane and heating oils.
The general current idea is that advanced biofuels are liquid fuels made from non-food, non-feed sustainably grown feedstocks and agricultural wastes, and, perhaps, municipal wastes. This web site has links to articles about a number of different kinds of feedstocks.
For example, a groups of potential energy crops are perennial grasses--either something like switchgrass or miscanthus or energy canes; or a mixture of perennial grasses. The idea is that these could be grown on marginal lands that are not so good for growing food; that they take little or no fertilizers and should not require irrigation; and, especially if perennial, require no or little tilling. Consider that as the ocean levels rise, in places along the East Coast in particular, some grasses that are salt-tolerant might be developed to be energy grasses and enable farmers in those areas to continue to grow some kind of valuable crops on their lands.
Challenges to achieving the promise of advanced biofuels include: overcoming biomass recalcitrance; addressing logistics of transportation of raw feedstock and finished products; providing fair prices for crops or agricultural residues; tailoring crops and production to specific environments and cultures; etc. Articles accessed through this web site provide more details on each of these subjects.