Wednesday, September 2, 2009

Sinclair, T. R. (2009) Taking the measure of biofuel limits. American Scientist 97:400-407.

I am enjoying greatly Sinclair's concise treatment of basic plant physiology, biochemistry, and the physical environment in which C3 and C4 crops are grown. It is the height of back-of-the-envelope artistry and clear thinking, which are hallmarks of strong quantitative, empirical biologists.

Sinclair starts with the loaming problem: the US Energy Independence and Security Act (currently) mandates that by 2022, the US should be producing 144 billion barrels of ethanol, roughly 25% percent or one barrel of ethanol for every three barrels of gasoline/diesel. This is the daunting task - it is a shit-load (my word, not his) of ethanol. Sinclair then asks whether the physical limits to plant growth will allow this mandate to be met by growing plants.

Having set up the problem, he goes about describing the elements of the puzzle:
Total annual ethanol production =
g Sugar / MJ of light intercepted by the canopy per day (C3 vs. C4) X
MJ incident light / sq. m. (max vs. average) X
days in the growing season X
grain vs. whole plant harvest X
gal ethanol / tonnes feedstock (corn vs. stalk) X
water use efficiency (C3 vs. C4 in dry vs. humid env.) X
Leaf area / land area (LAI) X
LAI / g nitrogen in tissue (C3 vs. C4) X
g N available in soil

I don't think that the above is a perfect rendering of Sinclair's elucidation, but it is close enough for now.

Sinclair next goes on to describes the sustainability of biomass harvest, in terms of N flux and the pool in the soil. He points out that the
rate of annual change in available soil N (g) =
annual application - harvest - runoff - leaching
+ production(cyanobacteria, thunderstorms)
+ mineralization(dead biomass)
+ N sequestration (perennials only)

Last, he considers land available, pointing out that most fertile land in humid regions is already in production. He concludes cautiously with

"... realistic assessments of the production challenges and costs ahead impose major limits."

This approach is a great companion to the work of Searchinger et al., Fargione et al., and Tilman et al. that have focused on land use and biodiversity issues. Much work lies ahead, and Sinclair has been of great help to me.

1 comment:

  1. Does the article mention anything about the use of algae for biofuel production? The idea has been around for a while, but as far as I know it hasn't gotten much mainstream attention. It's less land intensive and can be utilized on "marginal" land (i.e. deserts), thus the problem of competing with food crops is virtually eliminated. And I believe that algae also produce more biofuel per unit weight than corn.