The depletion of world rock phosphate reserves will restrict the amount of food that can be grown, a situation that can only be compounded by the production of biofuels, including the potential large-scale generation of diesel from algae. The world population has risen to its present number of 7 billion in consequence of cheap fertilizers, pesticides and energy sources, particularly oil. Almost all modern farming has been engineered to depend on phosphate fertilizers, and those made from natural gas, e.g. ammonium nitrate, and on oil to run tractors etc. and to distribute the final produce. A peak in worldwide production of rock phosphate is expected by 2030, which lends fears over how much food the world will be able to grow in the future, against a rising number of mouths to feed [1]. Consensus of analytical opinion is that we are close to the peak in world oil production too.
One proposed solution to the latter problem is to substitute oil-based fuels by biofuels, although this is not as straightforward as is often presented. In addition to the simple fact that growing fuel-crops must inevitably compete for limited arable land on which to grow food-crops, there are vital differences in the properties of biofuels, e.g. biodiesel and bioethanol, from conventional hydrocarbon fuels such as petrol and diesel, which will necessitate the adaptation of engine-designs to use them, for example in regard to viscosity at low temperatures, e.g. in planes flying in the frigidity of the troposphere. Raw ethanol needs to be burned in a specially adapted engine to recover more of its energy in terms of tank to wheels miles, otherwise it could deliver only about 70% of the "kick" of petrol, pound for pound.
In order to obviate the competition between fuel and food crops, it has been proposed to grow algae to make biodiesel from. Some strains of algae can produce 50% of their weight of oil, which is transesterified into biodiesel in the same way that plant oils are. Compared to e.g. rapeseed which might yield a tonne of biodiesel per hectare, or 8 tonnes from palm-oil, perhaps 40 - 90 tonnes per hectare is thought possible from algae [2], grown in ponds of equivalent area. Since the ponds can in principle be placed anywhere, there is no need to use arable land for them. Some algae grow well on salt-water too which avoids diverting increasingly precious freshwater from normal uses, as is the case for growing crops which require enormous quantities of freshwater.
The algae route sounds almost too good to be true. Having set-up these ponds, albeit on a large scale, i.e. they would need an area of 10,000 km^2 (at 40 t/ha) to produce 40 million tonnes of diesel, which is enough to match the UK's transportation demand for fuel if all vehicles were run on diesel-engines [the latter are more efficient in terms of tank to wheels miles by about 40% than petrol-fuelled spark-ignition engines], one could ideally have them to absorb CO2 from smokestacks (thus simultaneously solving another little problem) by photosynthesis, driven only by the flux of natural sunlight. The premise is basically true; however, for algae to grow, vital nutrients are also required, as a simple elemental analysis of dried algae will confirm. Phosphorus, though present in under 1% of that total mass, is one such vital ingredient, without which algal growth is negligible. I have used two different methods of calculation to estimate how much phosphate would be needed to grow enough algae, first to fuel the UK and then to fuel the world:
(1) I have taken as illustrative the analysis of dried Chlorella [2], which contains 895 mg of elemental phosphorus per 100 g of algae.
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