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Published online 20 February 2008 | Nature 451, 880-883 (2008) | doi:10.1038/451880a
News Feature
Energy: Not your father's biofuels
If biofuels are to help the fight against climate change, they have to be made from more appropriate materials and in better ways. Jeff Tollefson asks what innovation can do to improve the outlook.
Biotechnology has changed the way that drugs are discovered, designed and, often, made. It has spread new capabilities across the farms of much of the world, sometimes amid much controversy.
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I was surprised by the lack of mention of Searchinger and Tilman's recent Science papers on the enormous breakeven times of biofuels. We unfortunately don't have decades to solve our greenhouse gas emissions problem. Biofuels will also have a tough time supplying anything but a small fraction of our energy because plants are so inefficient. Fundamentals of Renewable Energy Processes gives the solar energy to ethanol conversion efficiency of sugarcane (one of the best) as 0.13%. If you compare this to the 30% of a Stirling dish, you've got a factor of 231. Worse, ethanol, butanol, etc. are converted into mechanical energy far less efficiently (much less than half) than the output from a Stirling dish (heat engines are subject to the Carnot limit after all). All of this translates into enormous differences in land area to create the same amount of work. In my opinion, the land area requires should be a primary criteria for evaluating renewable fuels.
There is talk in this article of growing sources of cellulose on “marginal land†or with no fertilizer inputs. However, unless you recycle the wastes from the energy production process back to the soil the cellulose was grown in it will eventually become depleted for e.g. phosphate – and growth of plants that even tolerate low phosphate levels will become restricted. This is not a small problem since a Hubbert linearization analysis of mineral phosphate extraction predicts that this resource is 75% depleted! ( See www.energybulletin.net/33164.html and www.energybulletin.net/40300.html ) Phosphate extraction from lower quality sources would require greater energy inputs. This means that, if any phosphate for fertilizer is available in 10 years time, it will be exceedingly expensive. The looming phosphate shortage has enormous implications for biofuels and, more importantly for the continuation of western industrial agriculture.
The article failed to discuss that there is one cellulosic feedstock that is readily available and at present has negative environmental value, the 2 billion tons of straw available world wide. Straw eventually becomes carbon dioxide without providing value, and at the same time temporarily binds nutrients, causing farmers to overfertilize. Straw also harbors pathogens requiring fungicide use which is why it used to be burnt in the field. The use of this byproduct would not necessitate putting new land in cultivation. The major problem with all the crops being used as cellulosic feedstocks is that they have not been domesticated for use as biofuel feedstocks. The main problem is lignin, which prevents metabolism of cellulosics by the wonderful microorganisms being developed (as described by the author). All the cellulosics described must be pretreated with heat and acid before the microbes can be used. Reducing and modifying lignin by genetic engineering of the cellulosic crops, whether switchgrass, Miscanthus, or straw from crops, can substantially reduce the amount of pre-treatment, as has already been demonstrated in the pulp and paper industry in transgenically modified trees. The potential uses of biotechnology for developing biofuel feedstocks is described in a recent review: “Transgenics are imperative for biofuel crops. Plant Science 174: 246-263 (2008).â€