The depletion of world rock phosphate reserves will restrict the amount of food that can be grown across the world, 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 produce2. If a peak in worldwide production of rock phosphate will occur within the next few decades, this will restrict the amount of food that the world will be able to produce in the future, against a rising number of mouths to feed. Additionally, there is a consensus of analytical opinion that we are also close to the peak in world oil production. One proposed solution to the latter problem is to substitute oil-based fuels by biofuels, although this matter 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 in 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 (high compression ratio) engine to recover more of its energy in terms of tank to wheels miles, otherwise it could deliver only about 70% of the energy contained petrol, or diesel, weight for weight. In order to obviate the competition between fuel and food crops, it has been proposed to grow algae from which to make biodiesel27. Some strains of algae28 can produce 50% of their weight of oil, which may be 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 hectare29 is thought possible from algae, grown in ponds of equivalent area. Since the ponds can, in principle, be placed anywhere, there is no compromise over arable land, unlike fuel/food crops. Some algae grow well on salt-water too which avoids diverting increasingly precious freshwater from our normal uses for it, as opposed to the case for land-based crops which require enormous quantities of freshwater.
Diesel engines are more efficient27 by about 40%, in terms of tank to wheels miles, than petrol-fuelled spark-ignition engines, and so, in the spirit of energy efficiency, we shall assume that all vehicles are converted to run on diesel. This means that a total of around 40 million tonnes/year of diesel would need to be substituted for. To produce an equivalent amount of algal biodiesel, and assuming a yield of 40 tonnes/hectare (the lower end of the 40─90 t/ha range29), the algae ponds would require a total area of 10,000 km2. Ideally, the algae plants could be integrated with fossil fuelled power plants, to absorb CO2 from smokestacks by photosynthesis, driven only by the flux of natural sunlight. However, for the 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. Two different methods are now used to estimate how much phosphate would be needed to grow this amount of algae: (1) to fuel the UK and (2) to fuel the world:
(1) An analysis of dried Chlorella [http://en.wikipedia.org/wiki/Chlorella} is taken as illustrative, which contains 895 mg of elemental phosphorus per 100 g of algae.
UK Case: To make 40 million tonnes of diesel would require 80 million tonnes of algae (assuming that 50% of it is oil and this can be converted 100% to diesel).
The amount of "phosphate" in the algae is 0.895 x (95/31) = 2.74 %. (The Formula Weight, FW of PO43- is 95, while that of P is 31).
Hence this much algae would contain: 80 million x 0.0274 = 2.19 million tonnes of phosphate. By initially assuming the chemical composition of the rock to be that of fluorapatite, Ca5(PO4)3F, FW 504, we can conclude that this amount of PO43- is contained in 3.87 million tonnes of it. However, actual mined rock phosphate is a more impure material than this, and that normally used for fertilizer production is reckoned to contain 29─34% P2O5 (to be compared with 42.3% as may be deduced from the chemical formula of pure fluorapatite, above). Following the calculations so far for the quantity of PO43- involved, and by using the ratio of FW for 0.5 P2O5/PO43- (71/95), we may conclude that there are (71/95) x 2.19 million = 1.64 million tonnes of P2O5 contained in this amount of PO43-. Taking the range average of 31.5% for the mineral P concentration, reckoned as P2O5, this would accord with 5.20 million tonnes of actual “rock phosphate”, yielding a conversion factor of 1.34 up from the value reckoned for pure fluorapatite. We may use the latter in the remaining calculations.
World Case: The world produces and consumes 30 billion barrels of oil a year, of which 70% is used for transportation (assumed). Since 1 tonne of oil is contained in 7.3 barrels, this equals 30 x 109/7.3 = 4.1 x 109 tonnes and 70% of that = 2.88 x 109 tonnes of oil for transportation.
Assuming that 50% of it is water, we would require twice the deduced mass of algae = 5.76 x 109 tonnes of it, which contains: 5.76 x 109 x 0.0274 = 158 million tonnes of phosphate. As before, by taking the chemical composition of the material as fluorapatite, Ca5(PO4)3F, FW 504, this amount of "phosphate" is contained in 279 million tonnes. Applying the factor of 1.34 as arrived at above, to account for the typical degree of impurity in the mineral, we obtain a requirement of 374 million tonnes of actual mined rock phosphate.
(2) To provide an independent estimate of these figures, we may note that growth of this algae is efficient in a medium containing a concentration of 0.03─0.06% phosphorus. To avoid being alarmist, we may use the lower part of the range, i.e 0.03% P. "Ponds" for growing algae vary in depth from 0.3─1.5 m, but we now assume a depth of 0.3 m.
UK Case: assuming (vide supra) that producing 40 million tonnes of oil (assumed equal to the final amount of diesel, to simplify the illustration) would need a pond/tank area of 10,000 km2. 10,000 km2 = 1,000,000 ha and at a depth of 0.3 m, this amounts to a volume of: 1,000,000 x (1 x 104 m2/ha) x 0.3 m = 3 x 109 m3.
A concentration of 0.03 % P = 0.092% phosphate, and so each m3 (assuming a density of 1t/m3) of volume contains 0.092/100 = 9.2 x 10-4 tonnes (920 grams) of phosphate. Therefore, we need:
3 x 109 x 9.2 x 10-4 = 2.76 million tonnes of phosphate, which is in reasonable accord with the amount of PO43- taken-up by the algae (2.19 million tonnes), as deduced above. This corresponds to 4.87 million tonnes of Ca5(PO4)3F, and if the 1.34 “impurity factor” is applied, to 6.53 million tonnes of rock phosphate.
World Case: To meet the requirements of the entire world would demand the substitution by algae fuel of 2.88 x 109 tonnes of crude oil, and an installation to produce this amount would occupy an area of 2.88 x 109/40 t/ha = 7.20 x 107 ha of land.
7.2 x 107 ha x (104 m2/ha) = 7.2 x 1011 m2 and at a depth of 0.3 m the ponds would occupy a volume = 2.16 x 1011 m3. Assuming a density of 1 tonne = 1 m3, and a concentration of PO43- = 0.092%, we need:
2.16 x 1011 x 0.092/100 = 1.99 x 108 tonnes of PO43-, i.e. 199 million tonnes. This corresponds to 352 million tonnes of Ca5(PO4)3F, or (x1.34) 472 million tonnes of actual mined rock phosphate.
This is also in acceptable accord with the figure deduced from the mass of algae, accepting that not all of the P would be withdrawn from solution during the algal growth.
Rock phosphate production10 in 2011 amounted to 191 million tonnes, against which must be compared the 472 million tonnes we have estimated would be needed to grow sufficient algae to fuel the world with algal biodiesel. Since food production is already being thought compromised by rock phosphate resource depletion, finding such a significant additional quantity is probably impossible. Currently, the U.S. produces less than 30 million tonnes of rock phosphate annually, but would require 104 million tonnes of the material to produce 22% of the world's total algal diesel, in accord with its current "share" of world petroleum-based fuel. Hence, for the U.S., security of fuel supply could not be met by algae-to-diesel production using even all its indigenous rock phosphate output, and significant imports would still be needed. This is in addition to the amount of the mineral necessary to maintain agriculture. In principle, phosphate could be recycled from one batch of algae to the next, but how exactly this might be done remains a matter of some deliberation, e.g. the algae could be dried and burned30, and the phosphate extracted from the resulting “ash”, or the algae could be converted to methane in a biodigester31,32, releasing phosphate and other nutrients in the process. Clearly there are engineering and energy costs attendant to any and all such schemes and none has been adopted as yet.
I remain optimistic over algal diesel, but clearly if it is to be implemented on a serious scale its phosphorus has to come from elsewhere than mineral rock phosphate. There are regions of the sea that are relatively high in phosphates and could in principle be concentrated to the desired amount to grow algae, especially as salinity is not necessarily a problem. Recycling phosphorus from manure and other kinds of plant and animal waste appears to be the only means to maintain agriculture at its present level beyond the peak for rock phosphate, and certainly if additionally, algae are to be produced in earnest. In principle too, the phosphorus content of the algal-waste left after the oil-extraction process could be recycled into growing the next batch of algae. These are all likely to be energy-intensive processes, however, requiring "fuel" of some kind, in their own right. A recent study33 concluded that growing algae could become cost-effective if it is combined with environmental clean-up strategies, namely sewage wastewater treatment and reducing CO2 emissions from smokestacks of fossil-fuelled power stations or cement factories. This combination appears very attractive, since the impacts of releasing nitrogen and phosphorus into the environment and also those of greenhouse gases might be mitigated, while conserving precious N-P nutrient and simultaneously producing a material that can replace crude oil as a fuel feedstock.
It is salutary that there remains a competition between growing crops for fuel, and those for food, even if not directly in terms of land, for the fertilizers that both depend upon. There is a dissonance in that apparently clean, renewable sources of biofuels, in reality depend on inputs of a mined and finite reserve, which not only is growing more scarce, but its recovery leaves radioactive waste as a legacy (phosphogypsum)18. In principle, at least, recycling phosphorus from one batch to the next, in algae production, might be done more readily than for land-based crops. Along with the higher relative areal yields that are possible with algae, this might alleviate the rising demand for phosphate rock. The above, however, illustrates the complex and interconnected nature of, indeed Nature, which as any stressed chain, will ultimately converge its forces onto the weakest link in the "it takes energy to extract energy" sequence. It seems quite clear that with food production already stressed, the production of (algal) biofuels will never be accomplished on a scale anywhere close to matching current world petroleum fuel use (> 20 billion barrels/annum)27. Thus, the days of a society based around personalized transport powered by liquid hydrocarbon fuels are numbered. We must reconsider too our methods of farming2, to reduce inputs of fertilisers, pesticides and fuel. Freshwater supplies are also at issue2, in the complex transition to a more localised age that uses its resources much more efficiently.
(2) Rhodes, C.J. (2012), Sci. Prog. 95, 203.
(27) Rhodes, C.J. (2008) Sci. Prog., 91, 317.
(28) Rhodes, C.J. (2009) Sci. Prog., 92, 39.
(29) Rhodes, C.J. (2012) Making Fuel from Algae: Identifying Fact Amid Fiction, in Gordon, R. and Seckbach, J. Eds. The Science of Algal Fuels: Phycology, Geology, Biophotonics, Genomics and Nanotechnology. Dordrecht, Springer, p177.
(30) Knoshaug, E.P. and Darzins, A. (2011) Chem. Engin. Prog., March, 37.
(31) Sialve, B. et al. (2009) Biotech. Adv. 27, 409.
(32) Heaven, S. et. al. (2011) Biotech. Adv., 29, 164.
(33) Clarens, A.F. et al. (2010) Environ. Sci. Technol., 44, 1813.
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