Monday, October 09, 2006

Ethanol from Wheat Straw Can only Work by Curbing Car Use.

The Royal Society of Chemistry has published its "Policy Bulletin" (Issue No. 4, Autumn 2006) which carries an article entitled "Growing Energy" and is about "Biofuels". They note that in January, President Bush pledged to make plant derived ethanol cost-competitive by 2012, and that in 2005 the U.K. Transport Secretary Alistair Darling announced the "Renewable Transport Fuels Obligation", which requires that 5% of all U.K. transport fuel will come from a renewable source by 2010 (just under three years!). If we used bioethanol (which I have calculated in previous postings - "Bioethanol: The Math" - to be the best bet, over biodiesel or biohydrogen), that would require turning-over around 6,300 square kilometers (km*2) of arable land for the purpose, or 10% of the total arable area of the U.K. Thus we would need to compromise a sizable quantity of our food production just to produce 5% of our fuel. How is that tiny amount going to make any difference either to breaking our dependency on imported fossil fuels or reducing our CO2 emissions? In short, it isn't.
The RSC article talks about converting waste products from existing agriculture to ethanol, for example wheat straw. This sounds like a perfect solution, but begs the question, could this work on any significant scale? Let's look at the math:

In the U.K., some 2 million hectares (20,000 km*2) of arable land is used to grow wheat, and another 1.1 million hectares (11,000 km*2) to grow barley. From a typical wheat crop is obtained 5,420 kg of grain per hectare plus 7,050 kg of wheat straw. Although the process is still under development, since various procedures are required to break down the complex cellulose (lignocelluloisic) materials into fermentable sugars, it is thought that 230 kg of ethanol might be produced per tonne of wheat straw.

Therefore, (assuming a best case scenario and combining the area of wheat and barley production) we have a potential production of:

3.1 x 10*6 hectares x 7.050 tonnes x 0.23 tonnes = 5,026,650 tonnes of ethanol. Now, we are trying to substitute for the current 54 million tonnes (oil equivalent) of imported fuel that we currently use for transportation (12 million tonnes of that, or nearly a quarter for aviation!). It is not a matter of a straight division, since I have worked out before, that ethanol only packs 71% of the energy punch of petrol (gasoline), and so this quantity is equivalent to:

0.71 x 5,020,650 = 3,548,627 tonnes of oil (equivalent, since the crude oil is refined into gasoline). Dividing out the millions, and rounding out, this would provide:

3.55/54 = 6.5% of current transportation fuel. Now, it takes energy to make energy, and while estimates vary, they are in the range that it takes 0.7 to 1.3 barrels of fossil fuel or natural gas equivalent to produce one barrel of ethanol. This is a U.S. based figure, which accounts for the fact that America's ethanol comes from corn and corn must be intensively fertilized (natural gas), irrigated (natural gas), transported by truck or rail to a processing plant, processed, then transported again to distant blending facilities all over the United States. However, all our agriculture also depends on artificial fertilizers and hence, on natural gas to make them.

I will, however, ignore this matter and take provision of the 6.5% of the U.K.'s transportation fuel from "scrap" - i.e. without growing a special crop to provide it - from ethanol at face value. At first sight, the figure seems rather feeble, and so it is. There is no way we can produce enough ethanol to match our current level of fuel use, either using biomass waste or without compromising our food production. On the other hand, if we move to systems of energy efficiency: living in localised communities, which would cut fuel demand by 90%, then 6.5% of that remaining 10% begins to look significant. I am, after all reassured that survival is possible for the U.K. in terms of intrinsic fuel supplies, but only given a paradigm shift (to use that hackneyed phrase, which in this context is true) in the way we live our lives. Otherwise we can neither break our dependency on imported fuels nor meet the government's targets to reduce CO2 emissions.


biofuelsimon said...

good to see someone who’s maths is a bit sharper than my rough and ready calculations of a couple of days ago.(

The amount of energy that can be produced using conventional crops is very small and as you say takes energy to produce. But I guess that it would help replace more marginal oil from tar-sands and the like. So if these marginal sources take more energy to extract usable gasoline from the feed than tractable materials like crude, the environmental benefits would be greater than the 6.5% of transportation fuel. Of course that assumes that it will always be cheaper to use bioethanol than gasoline from tar-sands.

I’m pretty sceptical about using crops, even from the seed to the root. If you took tye world’s annual supply of edible oils and converted all the wheat the US will grow this year, you’d be lucky to get 25-30% of the volume, let alone calories you’d need for a total replacement of crude.

MCrab said...

Hi Chris,

You state:

"since I have worked out before, that ethanol only packs 71% of the energy punch of petrol"

Comparing the energy densities of petrol and ethanol can be misleading. Ethanol has a higher octane number than petrol, enabling engines to be run at higher compression ratios. In a vehicle specifically optimised for ethanol the engine will run at higher efficiencies, allowing similar miles per gallon to petrol despite its lower energy density.

energybalance said...

Hi mcrab,
and thank for that important piece of information. If that is so, that ethanol and gasoline can be considered as equal energy sources in terms of miles-per-gallon, using suitably optimised engines, then we might provide in fact nearer 9% of our current fuel requirements using ethanol from "waste" biomass. This is still a relatively small amount, but if it becomes 9% of 10% (by cutting fuel use overall by 90%), we are almost there! I feel somewhat reassured! Chris.

energybalance said...

Hi biofuelsimon,
that's a nice blog you have there - I will continue to drop by, now I know about it! You may well be right, that there could be considerable environmental benefits over the catastophe that extracting tar-sands may prove to be. However, by living in localised communities we could cut dramatically our fuel use (more so in the U.S. given the more dispersed pattern of living than is the case say in Europe - i.e. greater distances to get to work etc.) and noting mcrab's comment (below) with specificaly designed ethanol-burning engines, we could then get the majority of our (remaining) fuel using bioethanol produced from scrap biomass! Chris.