Next Generation Energy is managed and written entirely by Seed editors and expert guest bloggers. Read more about them Following harvest of a grain crop, stalks or straw remain. The bulk of those materials are comprised of the sugar polymers cellulose and hemicellulose, which can be converted to liquid transportation fuel. Seems like something for nothing, remove "trash" from the field to make energy. One estimate suggests that in the US over 400 million dry tons (2.67 Mb pdf file) of crop residues are available annually. This is an appreciable proportion of the one billion tons of biomass required to replace 30% of the current US petroleum consumption. There is good reason to look to dedicated energy crops rather than removing crop residues because of the non-sustainable potential. Lets first consider what happens to corn stover, for example, if it is not removed from the field.
Agricultural residue, such as this corn stover, can serve
as a source for biomass. Photo: NREL
The material as it lay on top of the soil provides a buffer for wind shear and raindrop impact. Gradually, soil organisms decompose it adding organic matter to the soil, which improves water-holding capacity while the remaining residue cover inhibits evaporation. Break down of the residues also returns valuable nutrients to the soil. In short, the residues help to retain the soil and improve its physical and chemical properties.
The presence of residues can have a deleterious effect. The ground cover can exacerbate cool moist conditions and cause poor germination. They can also provide a place for pathogens to over winter providing a whole heap of inoculum for the next crop.
While the effects of removing crop residues on soil conditions are somewhat generalizable, the effects on crop yield are more site-specific. Areas with low slopes and high yields may tolerate residue removal better than areas with low yielding crops in warmer and more humid areas, for example. In general, approximately 30% of US corn stover can be removed and remain sustainable, but this is an average value. There is extreme site specificity that is influenced by local wind erosion, soil moisture, and crop choice and yield.
The first cellulosic ethanol plant (Iogen Corp, Ottawa Canada), processes 40 tons of wheat straw per day, so crop residues offer a very practical source of bioenergy, but as we will see soon enough, a diverse and sustainable array of feedstocks will supply our liquid transportation fuel.
Comments
Thank you for publishing this. I have often thought that the only way ethanol from corn makes sense is if you you use the stalks not the grain.
Here's my request for future discussion:
We produce an awful lot of urban and suburban "agricultural waste." Every fall when I clean up the yard and end up with a mountain of leaves I aways think, "damn, I wish I could turn this stuff to gasoline!" There's about ten times as much as I can compost with my four little bins, and when I imagine this multiplied by a million other yards around the country I wonder if this would not be a significant, if seasonal, source of cellulosic (sp.?) feedstock. Add to that the other yard wastes - grass clippings that people should but don't compost, the poison ivy, bittersweet, and other woody weeds that homeowners wage war against, etc. There are are other invasive plants like japanese knotweed that grow in massive patches along roadsides that could be harvested with a mower and added the pile. How difficult would it be to set up the infrastructure to collect all this stuff and send it to the ...distillery? rendering plant? What do you call it?
Posted by: Big Cat | August 11, 2008 10:45 AM
It's a stretch to say that a plant that uses 40 TPD of straw proves that cellulosic ethanol is a viable economic concept. The Tavda plant in Russia used close to 200 TPD of wood to make cellulosic ethanol, but it was an environmental catastrophe of the first order. The Iogen website is pretty, but it's short of all details on conversion efficiency and economics. The footprint of this plant looks substantial for something that produces maybe 2000 gallons of ethanol per day.
We're supposed to be heading towards 20 billion gallons per year of cellulosic alcohol by 2021 in the US. I think the GTL route hold more promise to deliver high volume and economics for a liquid cellulosic fuel compared to the alcohol schemes. I recently read Ben Thorp's assessment of competing cellulosic liquid fuel strategies ("Compelling Case for Integrated Biorefineries"), and found some process information on Iogen. The assessment of Iogen: about 71 gal/T ethanol yield, with a capital effectiveness of $11 per gallon/year (about 5x higher than a corn starch ethanol plant). So to generate 20 billion gallons per year of cellulosic ethanol by this route, we'd need 280 million dry tons of biomass and a capital investment of around $300 billion. The best of the GTL routes ("Range") is about half the capital, with about 30% higher conversion efficiency.
The last problem with these liquid cellulosic schemes is the one that plagued Tavda: conversion efficiency. Even the best of them only promises 30% conversion of dry biomass to liquid fuel. The other 70% HAS to be used efficiently for something. We can't dump it into the air and rivers the old-fashioned Soviet way. Not on a scale of 200 million tons a year.
Posted by: tom quick | August 11, 2008 11:48 AM
As far as yard waste goes, the main problem is always collection. Set up a collection system and you can just dump it all into an industrial composting system to make fertilizer, which is generally a more efficient use than alcohol. That's sort of the general problem with cellulose biofuel -- there are all sorts of ways you can use excess biomass, and production of alcohol isn't really the most efficient of them. About the only biomass that's best used for fuel is stuff that's otherwise a biohazard.
Posted by: Anthony | August 11, 2008 1:37 PM