Much discussion on biofuels circles around bioethanol, as I mentioned most recently in the posting "U.S. May Need to Import Corn", since this is provides a better crop-fuel yield than say, biodiesel and certainly biohydrogen. However, there is a new kid on the block and that is biobutanol. Converting sugars to butanol by fermentation is not new at all, and the ABE process dates back to 1916, when Chaim Weizmann, a student of Louis Pasteur, developed a means for thus making a mixture, of acetone, butanol and ethanol, from which the acetone was used to make cordite - an explosive used extensively in the First World War. However, for every pound of acetone, two pounds of butanol were produced, and it was not until the 1920's and 30's that butanol became widely used in the manufacture of paints for cars and of synthetic rubber to the extent that the acetone was a mere by-product. Weizmann was to become the first President of Israel, and the Weizmann Institute, internationally renowned for its scientific research, is named after him.
The essential science behind biobutanol production is that for producing biohydrogen, as I discussed some while ago, noting that it is a fantasy to suppose that sufficient H2 could be so produced to replace the current levels of fuel used in the U.K. to power its transportation infrastructure. However, the ideal reaction in this regard, namely producing a maximum yield of hydrogen, also produces butyric acid, in a first step, which may then be reduced to butanol, if an appropriate co-bacterium is present to do so. (When the intention is to maximise the yield of H2, a second competing reaction, which produces double the amount of H2 plus acetic acid as the organic counter-product is the most desirable of the two). A batch-process has now been developed which uses Clostridium tyrobutyricum to produce the butyric acid and then Clostridium acetobutylicum to turn this into butanol. Look up the trademarks "Butyl-Fuel" and "Freedom Fuel" and you will find various information about this patented process. I may be missing something, and perhaps the process is more complex that a simple conversion of sugar into butanol plus H2, but the 2.5 gallons per bushel yield claimed, seems to me to imply a yield of 117%, since I would reckon a maximum of 2.14 gallons from the 35 pounds of sugar contained in one bushel of corn. A second figure they give is that a yield of 42% of butanol is obtained based on glucose, which is about 100% yield on the reaction overall. As I say, I may be missing something (patents never disclose all the details do they?), but I am not aware of 100% yield ever being obtained from a fermentation process - e.g. for ethanol somewhere in the range 55% - 77% is typical.
It was intially further confusing to me in that the output is reckoned in U.S. gallons (Avoirdupois) e.g. 3.785 litres, as opposed to U.K. (Imperial) gallons which are equal to 4.56 litres.
Similarly, it is quoted that 2.5 gallons of ethanol can be obtained per bushel of corn (containing 35 pounds of sugar) can be obtained. Let's check that one...
C6H12O6 (mono-saccharide e.g. glucose)--> 2 CH3CH2OH (ethanol) + 2CO2.
(92/180) x (35 pounds/2.2 pounds/kg) = 8.13 kg of ethanol. The specific gravity of ethanol is 0.789 kg/litre and so we have 8.13/0.789 = 10.30 liters of it, for yield of 100%. Dividing by 3.785 litres/gallon (U.S.) we get 2.723 gallons, and so the yield is 2.5/2.723 x 100 = 92%, which seems very high!
If anyone can add anything here to clear up why the quoted yields of butanol or ethanol should be so large, or if I have made a mistake I should like to know what it is!
Anyway, let's accept that we can get 100% yield from glucose (or other sugars), and work the potential butanol production for the U.K. in terms of sugar crops (since we don't grow corn on a scale of proportion as the U.S. does - our traditional crop is wheat, but sugar beet grows well in the climate here). The processes of fermenting sugar to butanol can be represented:
C6H12O6 --> CH3CH2CH2COOH + 2CO2 + 2H2; then
CH3CH2CH2COOH (butyric acid) --> CH3CH2CH2CH2OH (butanol).
If both steps were to go to 100% completion then we expect a yield of (74/180) the ratio of the molecular weights of butanol and glucose, multiplied by the quantity of glucose used. 74/180 gives 41.1%, close to the 42% claimed. So let's (with an element of dubiousness) assume this is the yield than is to be expected.
One tonne of glucose would yield 0.411 tonnes (411 kg). Since it is reckoned that a crop of sugar beet can yield about 19 tonnes per hectare, each hectare would produce 19 x 0.411 = 7.81 tonnes of butanol. The density of butanol is 0.808 kg/litre and so this would occupy a volume of 7.81/0.808 = 9,666 litres/ha.
From the relative enthalpies of combustion of butanol and gasoline we may deduce that the energy punch of ethanol is 92% that of gasoline. However, as with ethanol, an engine can be tuned to burn the fuel at higher efficiency, and so to keep the business simple, lets assume we can compare the two fuels 1:1.
Hence we need to replace 57 million tonnes of oil equivalent in terms of current fuel by butanol. As I have pointed out before, standard internal combustion engines only get about 14% of the total energy that the fuel contains out as miles on the road, and hybrid e.g. Prius vehicles can achieve 42%, so we might deduce that that figure could be cut to a third, making a "mere" 19 million tonnes of butanol we would need to replace with biobutanol. So the crop to provide this would have to be grown on 19 x 10*6/7.81 = 2.433 x 10*6 hectares of land, which is 243,278 km*2, or about the same area as the total U.K. mainland. If we used all our arable land, which is just 65,000 km*2 we could supply 26.7% or about a quarter. (With standard gas-guzzling engines it would be about 9%, or less than one tenth). N.B. the latter figures only apply if we grow no food at all and convert all agriculture over to biobutanol production.
There is a butanol farm in Norfolk which will supply 70 million litres of butanol by 2010, which sounds a lot, and it is, but then we use an awful lot of fuel! Indeed, this would need to be grown on 70 x 10*6/9,666 = 7,242 hectares of land = 72.4 km*2 of arable land. So, how much would it produce as a proportional substitute for the current requirement? Well, the current requirement amounts to: 57 x 10*6 x 1000/0.808 = 70.5 billion litres. Hence 70 million/70.5 billion x 100 = 0.1%. If Prius hybrid engines were universally installed, that rises to a mere 0.3% of the total. Even if blended 5:95 in existing fuels, it amounts to 2% (or 6%) of the total, and a blend of this dilution would make so difference whatsoever to curbing CO2 emissions or ensuring security of fuel supplies. Forget it! Localise communities and cut vehicle use, and grow food instead. Other means must be found to provide for what remaining transport requirement we have following re-localisation, mainly in the form of synthetic oil from coal-liquefaction and electricity-driven transport systems operating over relatively small areas. The only biofuel worth bothering with is bioethanol, and only then on a small scale. If wheat-grass and other agricultural waste can be converted into ethanol, that is a bonus since it avoids any compromise of food production. If there are to be serious amounts of fuel available, post peak-oil, they will necessarily stem from coal, and for that to happen, "many" coal-liquefaction plants must be built from scratch!