The term "advanced biofuel" is a bit misleading because any biofuel can be advanced as long as it is made from sustainable feedstock. The definition of a sustainable feedstock is not well developed, but in general, a feedstock is considered sustainable if it:
Is available in large enough quantity to meet a reasonable proportion of our energy demands;
Has a limited impact on greenhouse gas emissions. This is a tough criteria as the impact a feedstock has varies depending the land it is grown on, the fertilizers used, etc.;
Does not have an impact on biodiversity. In other words, it won't lead to an ecological problem like super-pests and it is not so invasive so as to choke off native organisms;
Does not result in major land use changes. This, of course, goes back to the impact the fuel will have on greenhouse gas emissions, but also evaluates its impact on food crops.
As you can see, the criteria are rather loosely defined. They act more as conceptual, qualitative criteria than as quantitative metrics for making definitive decisions about the value of a biofuel. The lines between an advanced and a traditional biofuel are blurred in the sense that a fuel that has limited energy density, but can be grown on arid land and have little impact on greenhouse gas emissions is going to be highly valued despite its poor performance in the first criteria above.
When considering how "advanced" a feedstock is, one must consider water impacts, pesticide residue, fertilizer use and runoff, biodiversity, invasiveness, energy content, impact on food supply, impact on the climate, ease of production, and economic return just to name a few. Despite these vast considerations, a few feedstock sources have risen to the forefront of the investigation into sustainable biofuel.
Lignocelluloses
Lignocelluloses is a derivative of plant biomass that contains cellulose and lignin. Cellulose is the main structural component of plant walls and is often found in algae as well. It is a tough polysaccharide (sugar) that can be hundreds to thousands of glucose (sugar) units long. Lignin is an extremely complex chemical that fills the spaces between cellulose molecules and helps to stiffen the walls of plants.
Lignocelluloses can be broken down into ethanol because it contains carbon, hydrogen, and oxygen. However, doing so is not so easily accomplished. Over the years, scientists have developed a number of ways of producing ethanol from lignocelluloses, but the processes are not particularly economical. As of 2007, a gallon of ethanol produced from cellulose cost roughly U.S. $7/gallon compared to the $1-$3/gallon for ethanol produced from corn.
The benefits of using cellulose for a feedstock derive primarily from the fact that it is usually the leftover, inedible part of crop plants. In other words, we are already producing a large abundance of this feedstock and are simply throwing it away. Estimates put the annual production at around 323 million tons (British tons) in the U.S. alone. This quantity, combined with the use of marginal agricultural land for growing cellulose crops, is enough to substitute for all petroleum imports (though not all petroleum used) in the United States.
In addition to waste agricultural products, paper and other cellulose components make up roughly 70% of all landfill waste. When these decompose, they produce methane gas, which is 21 times more potent as a warming gas than carbon dioxide. So, converting this material to ethanol may have a very positive net environmental impact.
Finally, lignocelluloses yield about 80% more energy than is consumed in growing the plant and converting it to ethanol. This compares very favorably with corn, which yields only 26% more energy. The conversion rate is roughly 4-5 fold, meaning that energy invested in producing ethanol from lignocelluloses gives you 4-5 times more energy than if it were invested into producing ethanol from corn. Estimates suggest that cellulosic ethanol could reduce greenhouse gas emission over the long term by about 115%.