Friday, September 22, 2006

Biofuels - a Comparison of Practicality.

In order to break the dependency of the West on imported oil, principally from the Middle East, alternative and indigenous sources particularly of fuel are urgently being sought. For example, in the U.K., according to the DTI (Department of Trade and Industry) figures for 2003, 67.4 million tonnes of petroleum were used in that year in total, of which the lion's share, 54 million tonnes was used for transportation. Hydrogen is very frequently spoken of as a possible replacement for gasoline, but the case is far less simple than a straightforward substitution of one for another. Oil is a primary fuel, and can be dug out of the ground, along with gas and coal. Hydrogen on the other hand, must be made - using some primary source - as indeed electricity must, and hence both are "energy carriers" rather than fuels. Hydrogen is a problematic material in many ways. It has a very low boiling point and is a very "light" material. Hence, to achieve a workable fuel/tank weight ratio (at least 6%), it must be liquefied, rather than simply compressed, and this requires considerable energy - probably half the energy overall that will be extracted from the hydrogen itself, given the low well-to-tank efficiency in its production (i.e. most routes to hydrogen manufacture are fairly inefficient compared to the 88% efficiency incurred in extracting petroleum from oil-wells or gas from gas-deposits). The situation can be improved by storing the gas in zeolites (see recent posting), but these also need to be kept cold, or the gas is rapidly released, thus eliminating any real advantage of this technology. Hydrogen also has the habit of rendering metals brittle, and hence e.g. steel tanks and pipes used in a putative hydrogen-delivery infrastructure dangerous and leaky!
Most of the world's hydrogen is manufactured from natural gas (methane) by reforming. I have described this before, but it involves reacting methane with steam at high temperatures, when the oxygen from the water extracts the carbon atom from a methane molecule, leaving behind free hydrogen. The resulting carbon monoxide can go on and remove an oxygen atom from another water molecule, thus releasing yet more hydrogen. Most of the hydrogen is used to make ammonia by combining it with nitrogen in the Haber process, for manufacture of fertilizers, and so overall most of the world's food production depends on natural gas, supplies of which in the U.K. and the U.S. are diminishing, and food-production too will depend increasingly on imports of gas.
Therefore, making hydrogen may be worthwhile on a number of counts. Biohydrogen can be made from fermenting sugar, which in an ironic cycle of logic, requires chemical fertilizers (made from natural gas) to grow it. I concluded in the previous two postings that the amount of sugar needed to supply enough hydrogen to replace the 54 million tonnes of oil used to run transport, would vastly exceed the area of arable land in the U.K., even if the implicitly huge hydrogen infrastructure could be implemented.
However, another thought occurred to me. Hydrogen is not the only product of the sugar fermentation process required to generate it, since huge quantities of butyric acid and acetic acid must be produced simultaneously. I now estimate exactly how much of these materials are indeed produced and whether they might be themselves used as a fuel, rather than requiring wholesale disposal, and being wasted.

Burning one mole of butyric acid produces 521.87 kilocalories = 2181.42 kilojoules (kJ) of heat. Similarly, one mole of acetic acid would provide 873.70 kJ of heat.

We have calculated that to make enough H2 to substitute for the 54 million tonnes of oil (equivalent, since it is refined into other fuel fractions) requires the fermentation of 9.94 x 10*8 tonnes of sugar (C6H12O6).

Each tonne ferments as 0.75 tonnes x 58% x 88/180 = 0.213 tonnes of butyric acid; and 0.25 tonnes x 58% x (2 x 60)/180 = 0.097 tonnes of acetic acid. (88 and 60 are the molecular weights of butyric and acetic acids respectively).

Hence the process produces: 0.213 x 9.94 x 10*8 = 2.12 x 10*8 tonnes of butyric acid and
0.097 x 9.94 x 10*8 = 9.64 x 10*7 tonnes of acetic acid. The energy produced by burning these materials may be estimated as follows:

Butyric Acid: (10*6/88) x 2181.42 kJ = 24.79 Gigajoules (GJ), which is equivalent to:
2.12 x 10*8 x (24.79/42) = 125.1 x 10*6 tonnes oil (equivalent).

Acetic Acid: (10*6/60) x 873.70 kJ = 14.56 GJ, which is equivalent to:
9.64 x 10*7 x (14.56/42) = 33.4 million tonnes of oil.

So, out of our 9.94 x 10*8 tonnes of C6H12O6, we get the equivalent of 125.1 million tonnes (butyric acid) + 33.4 million tonnes (acetic acid) + 54 million tonnes (H2) = 212.5 million tonnes of oil, in total.

Hence, we actually need 9.94 x 10*8 x (54/212.5) = 2.53 x 10*8 tonnes C6H12O6 to provide 54 million tonnes oil equivalent of combined fuels. Grown on 2.53 x 10*8/16.53 = 15,281,000 hectares = 153,000 km*2 from sugar cane, or 132,000 km*2 from sugar beet. However, the fermentation vessels would still need to be filled with just over 30 cubic kilometers (km*3) of water, which is 20% of the entire U.K. freshwater capacity, which is already under pressure of supply for drinking, washing and for commerce. Fixing the leaky pipes would stem much of this shortfall, and so perhaps an additional demand could be met if the delivery infrastructure were shored-up!

This figure may be compared with 125,000 km*2 required to grow enough sugar to produce the 76.4 million tonnes of ethanol necessary to stand-in for 54 million tonnes of oil. So, bioethanol scores best in terms of requiring somewhat less land, although growing this amount would still use twice the available arable land area of the U.K. - so no more food production, and we can still only meet half the demand!! However, the amount of water required to run the process is only 2.4 km*3 which is "possible".

As a matter of interest, I note that in an early posting "Biofuels - how practical are they" I quoted that a yield of 2 tonnes of biodiesel/hectare could be obtained, and so 54 x 10*6/2 = 27 x 10*6 ha = 270,000 km*2 of land would be required to meet that fuel requirement (i.e. about twice as bad as for the other potential fuels produced by fermentation).

Hence, I conclude that all these schemes are unworkable on the full scale, without cutting the demand to be met in the first place. To meet a thus reduced scale, bioethanol seems to be the best bet. Hydrogen has all kinds of problems, and using the vile fermentation by-products of butyric acid (essence of sweat) and acetic (raw vinegar) acid would not only be extremely unpleasant (imagine how the world would smell!) but would not get past any health and safety regulations. The latter process is also highly demanding in terms of its water requirements, far more so than ethanol production. It seems that comparatively small quantities of biodiesel might be produced as a precious chemical feedstock, rather than as a fuel, to substitute for some of the (67.4 - 54) = 13.4 million tonnes of petroleum that is imported for use in industry.

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