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4 Jan, 2008 12:25 pm
Making hydrogen from sugar is not feasible on any sensible scale, since to do so would require using the world's arable land in competition with food production, and the scarcity of platinum limits severely the number of currently oil-fuelled vehicles that could be replaced by fuel cell driven alternatives. e.g. In 30 years only about 7% of the current 600 million road vehicles might be so replaced, by when world oil production will have fallen to 50 - 70% of current levels according to a Hubbert Peak analysis. Using biomass is a better option since arable land is not necessarily commandeered to produce it and biomass-to-liquids methods offer a source of diesel from initially produced hydrogen which is then handled as a fuel in liquid form, thus avoiding the need to install an unfeasible new infrastructure to handle gaseous hydrogen. However, it is most unlikely that oil-derived fuels can be replaced in this way on a scale comparable to our current consumption of them, meaning that levels of transportation will fall, resulting in a partial relocalisation of global society into small communities supplied e.g. by local farms.
For example, if PEM (proton exchange membrane) fuel cells are really the answer to running cars (and planes?) without oil-based fuels, the kinetic barrier is the rate at which platinum might be produced for their fabrication. Around 150 tonnes of "new" platinum is extracted annually, which amounts to around 3 million vehicles worth, and that is if all of it were turned-over to this purpose, i.e. no jewelry, scientific apparatus or catalytic convertors to keep the existing fleet of oil-powered cars running clean. This should be compared with an approximately 600 million vehicles on the world's roads, and hence in 30 years a mere 7% of that total could be so provided, by when world crude oil supplies will be vastly reduced. (A Hubbert peak analysis suggests to perhaps just 50 -70% of current levels).
How might we produce hydrogen to put into fuel cells anyway? Since most of the world's hydrogen is currently made from natural gas by steam-reforming, this merely places an additional burden of demand on this resource, and so the ideal would be to make H2 by sustainable methods instead. One such suggestion is to ferment sugar into "biohydrogen", and I was recently berated for stressing the point that if the U.K. were to make its hydrogen this way, it would require more than the nation's total arable land to grow the sugar crop.
"Haven't you heard of trade, dummy?" was the general theme. "I thought you Brits were a nation of mariners!" We were, and also a nation of engineers, hence it should not be beyond our wit to fathom the machinations of implementing a hydrogen economy, and probably the sums have already been done in Whitehall, which is why no serious efforts have been made in this regard here, nor anywhere else for that matter.
If my critic is right and we can simply buy all that sugar in from elsewhere, how much arable land would it take to grow enough sugar to run the world's transportation on biohydrogen made from it? Roughly 30% of the Earth's surface is land and around one tenth of that is arable. This makes a grand total of 14.9 million square kilometres. We may deduce that to grow sufficient sugar from cane or beet would require 34.4 million km^2 of arable land to substitute for the entire world's oil requirement to fuel transport (clearly not feasible) and more than half of it, or 8.8 million km^2 just to keep the U.S. mobile. Unfeasible though these numbers are per se, they must be further regarded against recent estimates that the Earth can only support about 3 billion people, or half the present human population, in the absence of fertilizers etc. and a system of modern agriculture based on oil and natural gas. It should be noted too, that this population is predicted to rise to around 9 billion by 2050, but how can it, when many currently producing wells of oil and gas will be running out by then?
It makes sense to avoid using arable land to make biofuels, be that hydrogen, ethanol, biodiesel or anything else altogether, since we will need all that available area, and more, to grow food. Alternatives are biomass-to-liquids (BTL) technologies, in which biomass is employed to produce H2 in the form of syngas (a mixture of H2 and CO), and this is then turned into diesel using Fischer-Tropsch catalysts, mostly based on cobalt, similar to those used in indirect coal-to-liquids (CTL) methods, also via syngas. Either biomass or coal can provide the carbon component of the final fuel, but only biomass is renewable. Another advantage of using biomass is that arable land need not be used to grow it and e.g. sustainably managed forests, trees that are planted and harvested according to a managed programme, can provide large quantities of biomass. Other chaff, husks etc. from normal crop production ans sewage and other animal waste might also be included.
This represents a huge improvement over using sugar alone to make hydrogen or ethanol, where most of the overall plant mass is wasted. As an example, sugar cane can produce in excess of 10 tonnes of sugar per hectare, but the entire mass of the crop is over 50 tonnes. If all of that could be used in BTL, the fuel yield would be enhanced markedly. Using BTL diesel, not hydrogen per se, also means that an unfathomable engineering effort to create an entirely new and untested infrastructure, not even begun as yet, is unnecessary, and the problematic fact that there is insufficient platinum to make enough fuel cells to use it is immediately obviated.
BTL diesel can be used, handled and distributed by conventional means of tanks and tankers and fuelling stations. If engines were installed as "diesel engines", an efficiency of 20% might be obtained on a well-to-wheels basis, over nearer 14% for gasoline in spark-ignition engines. It is thought that BTL plants will be running by 2020, but producing nothing near the amount of fuel currently used, as derived from oil.
Clever, ingenious and innovative though all the proposed techno-fixes are, it is the engineering - the kinetics - that is the rate limiting factor in their installation. In the case of BTL, the obvious question is, just how many of these plants would we need and how quickly might they be installed? It takes resources to extract resources, whether they be the huge amounts of gas and water needed to squeeze oil from the Alberta tar-sands, or agricultural expansion and the construction of new BTL plants, and the steel, gas, coal, nuclear and other potential resources to provide the basic materials of construction and their fabrication - from the iron ore to the final shiny installations themselves.
If the world's governments had begun work on oil-alternative technologies 30-odd years ago when OPEC made the political decision to marginally close its oil production-valves by 5% (which caused the price of oil to rise by 400%!), we might have realistic alternatives on-stream now. Sadly, cheap oil returned to the markets and eliminated much of the incentives to exploit these other options and now, 30 years later, our problem is not merely political but geological, and we see the world political map shifting in response to the reality of cheap oil supplies in decline, and how each nation, especially the U.S. which uses one quarter of all the oil produced on Earth, might grab more of what is left in the ground.
The age of cheap oil is quite distinctly over - the cost of a barrel of oil has just broken the $100 barrier - and it is debatable how much of any substitute for it might reasonably be produced, including from renewable sources. Electricity production is, in principle, less problematic, since it can be made from a variety of sources, gas, coal, nuclear and, of course, hydro-electric power which should be fully introduced, since it is one of the cleanest forms of energy, allowing that it is necessary even here to divert and dam rivers, potentially placing demand on water for irrigation, drinking and other purposes and in some cases displacing large populations, but you can't have it all ways.
The real problem is met in continuing to provide liquid fuel for transportation, admitting that railways can be run on coal, as can shipping, but this is not renewable, and even this estimated great resource will run-out eventually. I envisage a mix of technologies, wherein as much as is practicable comes from renewable sources. Solar energy is the ultimate, and it is probably best harvested using photosynthesis, to provide biomass and food rather than photovoltaics etc. which will be difficult to install on a large scale, although there is much to hope for. Nonetheless making most of our electricity from solar, in replacement of gas, coal and nuclear power stations is a tall order.
Since it appears almost impossible that we will substitute our current use of oil-derived transportation entirely by BTL (including ethanol, even if the cellulose-digesting enzyme methods can be commercialised in the near future), there will be a significant reduction in transportation, driven by economics and rising fuel prices, along with a rising price of food (both in terms of running farms and imports) and all other commodities. World trade will be hit hard by higher fuel prices, and the prognosis is not good for developing countries such as China who rely on exporting their goods to eager western consumers.
We will increasingly relocalise into smaller communities, provided ever more by local farms and other businesses, and local economies will replace the global village, in the model of Cuba, who moved to a system of farmers' markets when the Former Soviet Union cut off their fuel supplies as the Communist regime collapsed, and there were issues closer to home to be contended with. The Cubans have survived, and so might we, but our lives will change entirely and forever, as we gear-down to a lower-energy society. The horse and cart and the bicycle should be expected as an integral part of the final energy mix, along with whatever technology can provide.
"Renewables Based Technology," Edited by: J. Dewulf and H. van Langenhove, Wiley, Chichester, 2006.