How much of this makes sense? Are biofuels really a solution to the world's energy needs, or are they as I gently suggested in a recent talk , largely `snake oil', the wonderful cure for all ills which doesn't quite work?
Hydrocarbon resources are ultimately limited, and most of them are controlled by unpleasant and unreliable regimes.
Nuclear is pretty good bet for countries with effective safety standards, but hysterical politicians have made it hard to develop. Fusion, despites billions spent in research, is pie in the sky and probably always will be.
Hydropower is limited in availability, geothermal is even more so. Wind and wave are becoming more plausible, but still have a long way to go to become seriously competetive without subsidies. Solar is a probably a good way of keeping cool, e.g. running air conditioning in hot countries, but pretty useless for keeping warm in cold ones.
Furthermore, nothing other than chemical fuels as yet provide portable energy sources adequate for the kind of transport which societies now expect. Biofuels are chemical fuels and so can, in principle, fill this bill.
So biofuels are necessary, and so growing them has to make sense. However, this answer has to be qualified by consideration of what kind of fuel to grow, how to use it and where to grow it.
In 2005 the UK  used, reducing all energy sources to million tonnes of oil equivalent (Mte), 247 Mte, approximately in the following forms:
Definitive energy balances for biofuel production are thin on the ground. However a report with estimates for biodiesel and ethanol in the UK was produced by Levington Agriculture  for BAFBO, an organisation representing producers of oilseeds and biofuels. A comparison of this and other estimates may be found in .
A hectare of arable land used to produce biodiesel would yield about 54 GJ worth of fuel, or about 74 GJ if used for grain based ethanol. So as biodiesel, one would get 1.28 te of oil equivalent. This is a bit less than the 1.5 te of biodiesel which the report predicts.
To quickly put things in context, the total land area of the UK is 241,590 km2 or about 25 Mha. This includes cities and mountaintops, but if all of it could be used to grow biodiesel, it would still produce less than one fifth of the UK fossil fuel consumption.
In fact the total amount of arable land in the UK is about 5.5 Mha. If we put all of this down to biodiesel, we would replace at best 8.25 Mte of oil, less than 10% of our total oil consumption! Ethanol yields are somewhat higher, but it has been estimated that growing grain for ethanol on all this land would provide only the equivalent of about one third of the UK's current gasoline (petrol) requirements. It seems unlikely that more than, say 20% of agricultural land could be divertied to fuel use, so we are looking less than 1% of the national energy requirements.
Furthermore, these are the gross energy figures. The energy required to produce and process the crop is more than 50% of the energy available from biodiesel and nearly 90% for ethanol - so in practice nothing like this proportion of our energy requirements would be replaced.
We can immediately draw some conclusions.
Consider the figures from  for ethanol from wheat. For each 1 te of grain we get 0.276 te of ethanol. The calorific value of the grain is about 17 MJ/kg, so if the grain were burned it would produce 17GJ of energy. However, the energy value of the ethanol is only 8.3GJ, a loss of more than 50% without even taking into account the processing costs! In fact the energy value of the wheat straw is 10.9GJ, greater than that of the ethanol. If the entire plant were used as solid fuel it would yield nearly 28GJ of energy instead of 8.3GJ and the production cost would be substantially less, so the net energy yield would be 4 to 5 times greater.
Plant products are normally solids. It is therefore most efficient to use them in this form unless transforming them leads to increased efficiency elsewhere. In this case it is by no means obvious that i.c. engines running on ethanol will be more efficient than, say, power stations fuelled with straw.
The conclusion here is that biofuel crops have to a) allow the whole plant to be used and b) involve minimum additional processing.
A possible exception to this might be gasification. Existing well-establish coal gasification technology can be used for biomass on both large and small  scale. The gas could be used in a gas turbine which does have a higher thermodynamic efficiency than a steam plant
Although it is perfectly possible either to use straight vegetable oil  (called SVO or pure plant oil, PPO) after only physical processing in slightly modified diesel engines, or to process it along with petroleum oils in existing refineries, a more complex process is being promoted by tax breaks.
The vegetable oil is reacted with methanol to produce methyl ester, see e.g. . This is about one third of the molecular weight of the oil and can be used in an unmodified engine. In the reaction below R has typically 20 carbons.
C3H5(COOR)3 + 3 CH3OH = 3 CH3COOR + C3H5(OH)3The byproduct, glycerol, C3H5(OH)3, is a potentially valuable chemical used in the food, cosmetic and pharmaceutical industries. However, it is not a bulk chemical like methanol or diesel fuel. In fact the US market (2000) for glycerol was around 200 kte/yr  and was less than installed capacity. Byproduct glycerol from 5% of the UK diesel market alone would provide an additional 150 kte/yr. Excess glycerol from European biodiesel production by 2000 had already dropped US by up to 45% in the late 1990s.
At present the UK has about 1.9 Mha of wheat and 1 Mha of barley . The straw from this is essentially a waste product. More than 40%  is currently ploughed in as it has some minor value as a fertiliser. Taking the the heat value of wheat straw  to be applicable also to barley suggests that the UK has at the present the potential to produce (1.9 x 97.5) = 185 million GJ of energy from waste straw. This corresponds to 4.4Mte of oil equivalent to which could be added the straw from 0.58Mha of current rape production, at 60 GJ/ha, a further 0.8Mte, giving a total of 5.2Mte is equivalent to nearly 10% of the energy used in transportation without reducing the land area available for food production.
Of course, there are other issues. Straw is produced over a limited period, although conveniently this is in the autumn, before the winter peak of energy demand, it has to be transported, stored and processed, and the technology of efficient combustors developed. At least one coal burning power station, in Denmark , has successfully `co-fired' straw. The last of these problems would appear to already be solved.
There would seem to be no reason why, e.g. in Scotland Cockenzie power station, convenient to the agricultural are of East Lothian, could not start a program of straw burning more-or-less straight away.
Provided a strategy for using them directly in power generation can be developed, these are possible energy sources to replace fossil fuels. Some have a further advantage in that they will grow on marginal land unsuitable for grain production. Furthermore, these are plants which have largely evolved naturally. Applying modern plant breeding methods, especially genetic modification, could increase yields still further. One might hope that public hysteria about GM plants will be assuaged if they are to be used for fuel rather than food.
Power generation companies are already carrying out a significant number of tests with biomass fuels, normally co-fired with coal . In the UK, Drax have tested co-firing with willow .
The above calculation for the total energy available from wheat on a per hectare basis, with a yield of 8.96 te of grain per ha, implies an energy yield of 6.2 te oil equivalent per hectare.
Assume that genetic modification of selected varieties of fuel crop could increase this by about 50% to around 9 te/ha, and that land equivalent to 30% of the current UK arable total were to be used for fuel crops. This could be comprised of 10% set-aside, 10% diversion and 10% of additional marginal land suitable for fuel but not food.
We could produce about 0.3 x 5.5 x 9 = 14.8 Mte of fuel crops. Add to this the energy available from straw and other agricultural byproducts and a figure of 20Mte is plausible.
This could either replace about two thirds of the coal used in power generation or two thirds of the oil and gas.
Replacing coal, the worst fuel from a CO2 emission standpoint, would result in a significant reduction in the UK's emissions, providing politicians and concerned members of the public with a warm green glow and meeting everyone's `targets' for greehouse gas reduction. The practical significance of of this, given that China will be continuing to add several times this amount every year is perhaps questionable.
Replacing oil and gas would free these fuels for use in transportation. This would be both ecomomically and environmentally more efficient than making liquid fuels from biomass using any presently available technology.
Most work has focussed on conversion of cellulose to ethanol. This would enable all of a plant rather than just its sugar or starch content, to be converted. Efficiencies are still too low for this to be a realistic process.
Development of the 1919 Weizmann process for bacterial conversion of carbohydrates to acetone to produce butanol have been investigated since the 1980s. Recent developments, e.g.  claim good yields and DuPont  claim to have a viable commercial process. Although precise information is hard to come by, it may be that conversion of cellulose using Clostridium bacteria is possible by this route. If so, this is certainly a better propect than ethanol, since butanol may be more readily mixed with gasoline or used as a straight replacement with engine modification. However, it all hinges on the efficiency of conversion of biomass.
Structured plants are very inefficient converters of sunlight to biomass. Much less than 1% of the incident solar energy is released even if the entire plant is used. However, algae, which are much simpler organisms display greater efficiency, and some of them even produce hydrocarbons. Their faster growth rates, and the fact that their production can be essentially continuous rather than seasonal, could make relatively low efficiencies of hydrocarbon production acceptable . Moreover, dried algae are a fine particulate material which could be slurried with liquid fuels and used directly in suitably designed burners or even i.c. engines.
However, present technology does not offer an efficient route to liquid fuels. Much more research and development is needed.
From the standpoint of ecomomics, sugar cane grown for ethanol in e.g. Brazil, appears to make sense. Bioethanol in Brazil is actually sold at a competitive price without any overt subsidies. However, there must be provisos.
If the whole plant is not used then the process is inefficient from either a thermodynamic or environmental viewpoint, or both. Bagasse, the residual plant material from sugar cane, is traditionally burned to provide energy for refining the raw sugar.
What are the long term effects of intensive sugar cane cultivation? There is evidence that it can be severely damaging to the environment.
Is cane cultivation really not competing with rainforest? Even if it isn't now, one must fear that if biofuels become a lucrative industry, it will in the future.
Similar arguments apply to e.g. palm oil production in Malasia and Indonesia. Indeed the developed world's enthusiasm for biofuels may seriously threaten such countries .
IR Richards, `Energy balances in the growth of oilseed rape
for biodiesel and wheat for ethanol", Report for
BAFBO, June 2000.
Until recently, there was a very useful summary of this on the BAFBO web site. It has now disappeared, but here is a partial copy.
 UK Agriculture web site www.ukagriculture.com/index.cfm.
 M Frondel and J Peters, `Biodiesel, a new Oildorado?', Energy Policy 35, 1675-1684, 2007
 DE Ramey, US patent 5,753,474, 1998
 DuPont website www2.dupont.com/Biofuels/en_US/
 JW Ponton, `Biofuels, Solution or Snake Oil?' seminar to IES, University of Edinburgh, 2007.
 The SVO Users and makers Association http://www.bio-power.co.uk/svoa/index.htm
 AP Armstrong et al, `Energy and greehouse gas balance of biofuels for Europe', report 2/02 for CONCAWE, Brussels, 2002 http://www.senternovem.nl/mmfiles/26601_tcm24-124161.pdf
 P Overgaard et al, `Two years operational experience and further development of full scale co-firing of straw', 2nd World Conference on Biomass for Energy, Industry and Climate Protection, Rome, 10-14 May 2004
 WR Livingston, `Advanced biomass co-firing technologies for coal fired boilers', keynote at International Conference on Coal science and Technology, Nottingham, Sept 2007
 MAFF report NF0403 Miscanthus Agronomy: www.defra.gov.uk/farm/crops/industrial/research/reports/rdrep14.pdf
 Drax Power Ltd web site, http://www.power-technology.com/projects/drax/
NERC Knowledge Transfer Project web site: http://www.bluemicrobe.com/home.htm
 `Yield Models for Energy Coppice of Willow and Poplar', Energy crops and biofuels web site: http://forestry.gov.uk/srcsite/infd-5l8hhr
 V Hofman, `Biodiesel Fuel', AE-1240, Feb 2003, http://www.ag.ndsu.edu/pubs/ageng/machine/ae1240w.htm
 R Clift and Y Mulugetta, `A plea for common sense (and biomass)', TCE 24-26, October 2007
 Ankur Scientific, India, sell small scale biomass gasifiers: http://ankurscientific.com/