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 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 favourably 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%.
Jatropha is a type of flowering plant that, oddly enough, has never been fully domesticated. Despite this fact, the plant has found use in a number of applications including land reclamation after oil contamination and as a decorative addition to many gardens. What makes it interesting as a biofuel feedstock is that Jatropha is drought resistant, has few pests, and produce seeds that contain 27 to 40% oil.
The oil from Jatropha can be refined into biodiesel and the leftover can be used as a solid biofuel or as a feedstock for producing syngas. Because it is refined biodiesel and not chemically converted biodiesel, diesel from Jatropha qualifies as "green diesel" and can be used in any standard diesel engine.
The major benefit of Jatropha is that it can grow in places where most other plants would die. This means it will not threaten the world's food supply because it can be grown on land where food crops cannot survive. Unfortunately, the plant does not produce many seeds (where the oil is found) in poor conditions and it turns out to need just as much water and fertilizer as any other crop. Currently, research is being done into how Jatropha might be domesticated or genetically altered to ensure that it produces oil even under harsh conditions.
The other problem with Jatropha may be the fact that it actually increases greenhouse gas emissions. Like any biofuel, how and where Jatropha is planted impacts its overall greenhouse gas emissions. If it is planted on land that contains other species, the potential carbon debt could take decades or even centuries to pay off. In recent years, interest in Jatropha has dwindled as its need for more nutrient rich environments has undermined its initial appeal.
Camelina is another type of flowering plant that produces seeds rich in oil. Like Jatropha, its seeds can contain up to 40% oil that is easily converted into biodiesel and even jet fuel. Camelina is a hardy plant and does well in water-scarce environments. However, like Jatropha it may face potential problems from the fact that it might actually increase greenhouse gas emissions, does not produce as well in dry environments as in wet, and uses nearly twice the land to produce the same quantity of biofuel as Jatropha. Camelina is of interest to the United States Air Force as a potential replacement for up to 50% of their jet fuel with biofuel by 2016.
Algae produce lipid, which is oil, that can be converted into a number of different fuels including biodiesel, ethanol, methanol, butanol, jet fuel, and others. Unlike the refining processes above, the process of converting lipid to fuel requires chemical reactions that produce esters and alcohols. Algae-derived biofuels cannot be used in standard engines because they can erode and damage the seals, gaskets, and lines made of rubber. Specialized rubber is needed if algae-based biofuels are to be used in an internal combustion engine.
One of the biggest benefits of algae compared to the feedstock above is its energy density. It can produce up to 300 times more oil per acre than any conventional crop and has a harvest cycle of 1-10 days, meaning it grows up to 30 times faster than other feedstock. Additionally, algae are highly suited to growth in extreme environments and grow well in places with high salt and dry climates. In other words, algae have almost no impact on the food supply.
At this point, the energy that must be invested to grow enough algae to meet 5% of our energy demands is unsustainable. In other words, more energy must be invested than is recovered in this process when it is carried out on a large scale. On small scales, the situation is not the same and algae produce more energy than is invested into them. So, the key to making algae a sustainable biofuel is to development methods of scaling up production while maintaining the efficiency. At this point, producing 10 billion gallons of biofuel from algae would require 33 billion gallons of water, 6 million metric tons of nitrogen, and up to 2 million metric tons of phosphorus. This is hugely unsustainable.
The major initiative in the future of algal biofuels is genetic engineering of the algae to produce species that can tolerate higher salt, less water, and fewer nutrients. One avenue of research, for instance, is to produce algae that excrete oil so that the organisms need not be harvested and can be grown indefinitely. At this point, harvesting the algae means that they are killed and that a new stock must be grown from a seed stock. This process is far more time and resource intensive than if the lipid could be harvested without damaging the algae.
There is a good deal of research into the ability of various feedstocks to produce biofuels that can replace traditional fossil fuels. The major problems arise from threats to the food supply, large energy investments that may not be recovered, and the potential to worsen global warming. While some approaches are and will be easily adopted, such as converting waste to energy, others are more problematic and will require more investment to determine if they can be brought to fruition or should be abandoned.
With the complexities of biofuel production in mind, it is important that the lesson of Jatropha be carefully considered. Farmers throughout Asian and Africa spent vast sums of money planting Jatropha because it was touted as the "silver bullet" to end energy problems once and for all. Governments promised subsidies and huge investments were made. Unfortunately, the science was not advanced enough to really draw the conclusions that were being drawn. The result was that Jatropha turned out to require more nutrients and land than previously thought. Farmers who planted the crop saw subsidies dry up and they were left holding the bag. Many lost their livelihoods and others lost their farms altogether. The lesson is that the science must come first and it must be rigorous. As the adage goes, "If it seems too good to be true, then it probably is." Science first, investment later.