We would need an area of land more than twice the size of the U.K. to grow enough sugar-crops to replace our current demand for liquid petroleum fuels by bio-hydrogen, and hence the concept is utterly preposterous. As I stressed in a previous posting "The Hydrogen Economy - How Economic is it?" substituting hydrogen for hydrocarbons (oil) to fuel our nation's current transportation requirements is a task of such considerable magnitude that it probably will never be accomplished. I made my arguments in terms of the MW (Mega-Watt) generating capacity that would be required in simplistic terms of energy conversion, ignoring details of the losses that are inevitably incurred in converting one form of energy to another, an irrefutable consequence of the Second Law of Thermodynamics. When such losses are factored-in, the demand on generating supply becomes even worse than the 61 Sizewell B capacity reactors or the 180,000 2 MW wind turbines that would be needed, respectively, to produce sufficient hydrogen from the electrolysis of water to replace the equivalent of 54.2 million tonnes of oil that fuel internal combustion engines each year in the U.K. alone; 12 million tonnes of it being burnt by the aviation industry. In all likelihood, we would need nearer 120 new Sizewell B's or 360,000 2 MW turbines, allowing for a 50% loss: 70% loss in the electrolysis step and another 70% in the "combustion" of hydrogen in fuel cells, the overall conversion from "tank to wheel".
This leaves us with the question: are there any alternative - ideally sustainable, renewable - sources, that might generate the quantities of hydrogen required to meet this task? One potential source is "Bio-hydrogen", as made from fermenting sugar, and quite a sizeable number of academic research groups are drawing considerable research funding for projects that purport to develop methods that will lead to the production of hydrogen from renewable sources - essentially from the fermentation of sugar into hydrogen, CO2 and acetic acid (vinegar). This may be expressed by the shorthand equation:
C6H12O6 (glucose) --> 2C2H4O2 + 2CO2 + 4H2.
It is instantly apparent that one CO2 molecule is produced for every two hydrogen molecules (H2) and so, but it is assumed that the growth of next-year's sugar-crop will consume this amount of CO2 and, ignoring the additional CO2 emissions produced from its cultivation/harvesting/processing etc., the process may be thought of as "carbon neutral", to coin a phrase. Crop cultivation to make "biofuels" is not neutral in fact, but is thought to consume about 20-40% of the total CO2 produced in production/combustion of the fuel. Notwithstanding, if the glucose is formed from fermenting crops that are grown on our own shores, there remains the appeal of security of supply. The overriding criteria, however, are exactly how much hydrogen would we need to produce and what level of resource - arable land, fermentation vessels etc. - might this mean. I will concentrate on the basic aspect of hydrogen production, in the form of a simple calculation. In the following, a term such as 10*6 means "ten raised to the power six", i.e. one million; put another way, it is the number one written with six noughts after it.
The U.K. uses 54 million tonnes of oil to supply its transportation = 54 x 10*6 x 42 x 10*9 = 2.268 x 10*18 Joules of energy.
The heat of combustion of hydrogen = 285.83 kJ/mole, and so we would require 7.93 x 10*12 moles of H2 = 1.78 x 10*14 litres of H2 = 1.78 x 10*11 m*3 (cubic metres) of H2.
Assuming 100% efficiency, by the conversion of glucose according to the reaction:
C6H12O6 + 2H2O --> 2C2H4O2 + 2CO2 + 4H2, 0.5 m*3 of H2 would be produced per Kg of sugar. However, the process becomes thermodynamically less favourable as the H2 concentration increases, which must therefore be continuously flushed from the digester.
The process is not so simple, in fact, and the following reaction also occurs:
C6H12O6 --> C4H8O2 (butyric acid) + 2CO2 = 2H2, which is clearly less effective in terms of its hydrogen yield. It is also far worse in terms of CO2 production (now four times that produced from natural gas reforming). Experimentally, it is found that the latter reaction competes with the ideal one to the extent of about 3:1 and a maximum yield of 0.18 m*3 of H2 is obtained per Kg of sugar, not 0.5 m*3. It may be deduced that overall, the process is occurring with 60% efficiency.
Now, back to the scale of H2 production. As noted, we need to make 1.78 x 10*11 m*3 of it, which according to the above, would require 9.88 x 10*8 tonnes of sugar. Under favourable circumstances, it is possible to produce 19.1 tonnes of sugar (sucrose; I am assuming that this can be converted quantitatively to glucose) and so the cultivation would demand 51,727,749 hectares, or about 520,000 km*2. This is just for the U.K., and is in fact just over twice the entire land area of mainland Britain, to grow it.
Just for fun, let's work out what the total digester volume would be. To ferment 1 Kg of glucose requires 154 litres (about 34 gallons), given typical concentrations under which the process is most efficient. So, we need 154 x 9.88 x 10*11 Kg = 1.52 x 10*14 litres = 1.52 x 10*11 m*3 = 152 cubic kilometers. This would have to be fresh-water, so it couldn't simply be withdrawn from the sea. I note that the entire volume of fresh water available in the U.K. is reckoned to be 2,465 m*3 per person per year, which for a population of 60 million, amounts to 148 km*3, and so even if we used the nation's entire supply of freshwater for this purpose alone, we still couldn't entirely fill our bio-hydrogen fermenting digesters.
There would also be lake-sized quantities of butyric acid (essence of sweat) and acetic acid (raw vinegar acid) to cope with, around 300 million tonnes and 120 million tonnes of each, respectively, which is more than enough to fill Lake Windermere... and all this would have to be done each and every year for the U.K. alone. It just doesn't add-up, and neither do many other putative technologies to replace the world's declining oil supply, as we shall see in these postings.
I am updating this (16-12-07), as I am recently aware that the idea of biomass to liquids conversion is being taken seriously in Europe. In effect, plant material is converted to syngas (a mixture of H2 + CO), and then to liquid hydrocarbon fuels using Fischer-Tropsch catalysis, similar to that used for some forms of coal-liquefaction. This is probably the only way that significant amounts of fuel from petroleum can be replaced. It also means that the hydrogen is used to make combustible liquid fuels and therefore we don't need the huge number of PEM fuel-cells that the "Hydrogen economy" as commonly referred to would require; a genuine problem since there is insufficient platinum available to make enough of them to run 600 million vehicles.
[There are other research-stage approaches to use enzymes to break-down lignocellulose from wood and other plant material into sugars that can be fermented into ethanol]. It is predicted that this will be functional on a wide-scale by 2020. We shall see, but just using sugar and fermentation looks like a nonstarter! I hope this does work, as the world will be pretty short of oil by then... at any reasonable price, that is!
"Creating value from renewable materials. A strategy for non-food crops and uses. Two year report." Defra, November 2006.