The most critical assumption on cellulosic biofuels: yields
My most critical assumption with cellulosic biofuels is on land efficiency: tons of biomass per acre, and hence gallons of fuel produced per acre, and more accurately, miles driven per acre. I believe biomass yields per acre will multiply by two to four times from today’s norms.
The lack of genetic optimization and research on cultural practices, harvesting, storage, and transport with would-be energy crops — miscanthus, sorghum, switchgrass, and others — means that there is significant potential for improvement. The application of advanced breeding methods like genetic engineering and marker-assisted breeding, limiting water usage through drought resistant crops, and large-scale application of biotechnology (i.e., optimizing the process by which plants conduct photosynthesis, or reducing stress-based yield losses) will also contribute to increased yields with fewer inputs.
More importantly, different energy crops are likely to be optimal for different climates — jatropha makes sense on degraded Indian land, but not in the American Midwest. Rather than a single dominant energy crop, we are likely to see a variety of feedstocks that allow specialization to local conditions, mixes, and needs, while mitigating the risks.
Some reported examples and datapoints of biomass yields speak to the reasonableness of our estimates of yields between 18-24 tons per acre by 2030 (e.g., Prof. Lee Lynd at Dartmouth):
- Miscanthus averaged 16.5 dry tons per acre per year, where switchgrass averaged 4.6 at 3 Illinois sites, with data taken over 3 years. Research in Europe notes yields ranging up to 16 dry tons per acre (PDF).
- Sugarcane ventures in Brazil (Allelyx is using GMO techniques, Canavalis is using more traditional plant breeding) are breeding energy cane that will likely result in a yield of 25 dry tons per acre/year of harvestable biomass. Similar progress is being made by USDA sugarcane geneticists in Louisiana.
- Megaflora Corp. has measured productivities of 28 dry tons per acre per year from crossing North American hardwoods with the paulownia tree in North Carolina. Similar progress is being made by USDA sugarcane geneticists in Louisiana.
- Anagenesis Corp claims of their trees, “one acre can yield 48x times as much ethanol as an acre of corn.”
- DOE estimates (PDF) suggest that collecting existing biomass with only a small change in agricultural practices could generate 1.3 billion dry tons of biomass in the U.S. (most of our biomass needs) and still be able to meet all food, feed, and export demands. This would be an alternative scenario to get biomass without energy crops.
- According to Prof. Mark Holtzapple at Texas A&M, high-yield sorghum can be grown in 35 U.S. states and produce yields as high as 25 dry tons per acre/year with low water usage.
- Researchers at Texas A&M have developed new “freakishly tall sorghum plants” that reach heights of nearly 20 feet — more than double the height of regular sorghum and yielding double the amount of crop per acre. They use little water and have been bred to prevent flowering (thus trapping more energy), and can be grown on marginal crop lands.
A wide variety of crops have potential as feedstocks for cellulosic ethanol. Bical notes: “The criteria for the ideal energy crop are high dry matter yield, perennial growth, and efficient use of nitrogen, water, other resources, and pest and disease resistance.” The previously cited Univ. of Illinois study compared corn, short-rotation coppice, and miscanthus versus a set of idealized criteria for energy crops and found miscanthus (and by extension, other C4 photosynthetic grasses) to meet most of the requirements (PDF, or see charts below). Of particular interest to me is miscanthus that “partitions nutrients back to the roots in the fall just before harvesting.” I figure crops that provided (and survived) energy for mammals in the prairies can now provide energy for humans!
Many of the advantages of miscanthus are also applicable to some of the other proposed feedstocks. The new, higher-yielding strains of sorghum developed at Texas A&M use less water than conventional sorghum (making them more drought-resistant) and are sterile (not flowering prevents the escape of energy). Their 20-feet height means that yields have effectively doubled. The table below (from Ceres) highlights the advantages and disadvantages of various feedstocks — however, it is notable that most noncellulosic sources (example, vegetable oils) would fail on the vast majority of the criteria.
Long canopy duration
Nutrients recycled to roots
Low crop inputs
Low fossil fuel inputs
Adapted to marginal land
Minimal pests/plant diseases
|Non-invasive or sterile||■||■||■||■||■|
|High water-use efficiency||■||■||■||■|
|Planted by seed||■||■|
|Harvest first year||■|
Examples abound of people in action on energy crops. Ceres has been attacking the problems from a multitude of angles, and is utilizing biotechnology in combination with better crop practices (such as those highlighted earlier).
Firstly, they are attempting to increase the usable land available, by working on crops that can deal with problems such as drought tolerance (and recovery), heat tolerance, salt tolerance, and even cold germination. They are also working on increasing yields with plants that have shorter flowering times, greater photosynthetic efficiency, and greater shade tolerance.
Additionally, they are attempting to reduce the costs per acre by increasing the efficiency of nitrogen utilization, improving the efficiency of photosynthesis with lower nitrogen usage, increasing the biomass present in the root of the plant, and reducing costs through enzyme production while working to increase the gallons per acre that result from various feedstocks.They are also proposing better agronomy techniques like polycultivation (plots of monoculture crops interleaved together) as opposed to a polyculture (mixed crop cocktails).
As a whole, the company is developing genetically modified, commercial energy crops, and expects to have proprietary commercial varieties ready for market in two to three years and transgenic varieties in five to seven years. There are others with similar efforts.
I have highlighted some of the feedstocks that (I believe) are likely to meet feedstock needs, but there are many other potential sources not yet researched (or discovered!). In time, some feedstocks may prove to be more efficient than others, but local needs and transportation costs mean that cellulosic biofuels (utilizing local feedstocks) can be produced in many locations in the U.S. and worldwide.
The innovation ecosystem will ensure that, over time, new ideas will continue to be developed — the better ideas will persist as more and more intelligent people, resources, and capital join the field, and the best ideas will eventually rise to the top. Of course, traditional oil interests will continue to fight this trend with the hundreds of billions of dollars at their disposal, especially the national oil companies that own 80 percent of the world’s oil resources.
There is plenty of biomass available (computed here for the U.S., but similar calculations are possible for other world geographies). Biomass from energy crops can replace oil while improving traditional agriculture and biodiversity while reducing needs for chemicals and water for both the energy crops and the row crops that we use today. Far from being a “food versus fuel” battle, as many tunnel-vision critics have imagined, biomass-based income may be one of the few fundamental economic tools we may have to solve poverty issues in Africa.
Of course, biofuels can be produced as defined above, or we can produce biomass on land from cut-down rainforests. They can be done well or done poorly. It behooves us to regulate each biofuels facility and qualify its feedstock sources as being eco-qualified (a LEEDS-like rating for each biofuels factory).
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