Getting rid of kerosene-based aviation fuels will be one of the most difficult tasks to accomplish in a zero-carbon world. Aviation is responsible for only 2% of the global volume of emissions and 12% of those generated by the transport sector, but the switch to electric motors for airplanes is a much more complicated process than it is for cars and trains. .
The aviation fuel we currently use – the most common type being the one known as Jet A-1 – has several advantages. Its energy density is very high, for each kilogram of weight it contains 42.8 megajoules; a value slightly lower than that of gasoline, but unlike the latter it can remain in the liquid state up to 47 C below zero and is more convenient from the point of view of cost, loss by evaporation because it is recorded at high altitudes and of the risk of it catching fire when handling it. Credible rivals still do not appear on the horizon. Batteries that have enough capacity to carry hundreds of people on intercontinental flights are still science fiction stuff, and it will take some time before we can see liquid hydrogen-powered wide-body airplanes. What we need is fuel equivalent to kerosene but derived from plant material or organic waste.
Such an aeronautical biofuel would release, during the combustion process, a carbonic quantity equal to that which is absorbed by plants during their growth. The feasibility of the idea has been demonstrated: since 2007, test flights made using mixtures of Jet A-1 and biofuel have revealed that they can constitute an adequate alternative, at least for part of the flights, for modern aviation.
Since then, some 150,000 flights have been made using blends of this type, but only five of the major international airports offer a regular distribution of biofuel (Oslo, Stavanger, Stockholm Brisbane and Los Angeles), while its availability at the other airports is only sporadic. The extent of the biofuel consumption of the largest company United Airlines perfectly conveys the discouraging proportions of this, albeit necessary, replacement work: the contract with a biofuel supplier provides for the supply of a quantity equal to 2% of the total fuel consumed annually by the company. True, airlines today are incredibly frugal: they burn about 50% less fuel per passenger-kilometer than they consumed in 1960. But the fuel savings have been canceled out by the continued expansion of the aviation industry, which has increased. its overall annual consumption to exceed 250 million tons globally.
To meet such a fuel requirement, using mainly biofuel, we would have to go beyond organic waste and take advantage of oil-rich, seasonal (corn, soybean, rapeseed) or perennial (such as oil palm) plants, whose cultivation would require large plots. of land and would lead to environmental problems. Oil crops in temperate climates have a relatively low yield: with an average yield of 0.4 tons of aviation biofuel per hectare of soybean land, the United States would have to cultivate 125 million hectares – a territory larger than that of Texas, California and Pennsylvania combined, or slightly larger than the surface of South Africa – to meet their need for fuel. Four times the 31 million hectares that the country used in 2019 for the cultivation of soybeans. Even the best-yielding plant – the oil palm, which can provide an average of 4 tons of aircraft biofuel per hectare – would still require the exploitation of more than 60 million hectares of tropical forest in order to meet global demand for aircraft fuel. It would mean quadruple the land currently used for palm oil cultivation, causing the release of carbon dioxide accumulated during the natural growth of vegetation into the atmosphere.
But why occupy vast plots of land when biofuel can be obtained from oil-rich algae? Intensive and large-scale cultivation of algae would require a relatively small area and provide a very abundant harvest. However, ExxonMobil’s experience shows us how difficult it would be to increase the size of crops enough to produce tens of millions of tons of aircraft biofuel each year. In 2009, Exxon, in collaboration with Craig Venter’s Synthetic Genomics, set out to achieve a similar goal, but in 2013, after spending more than $ 100 million, it concluded that the obstacles were too great and decided to return to focus on pure long-term research.
As always, energy replacement would be an easier goal to achieve if we just reduced waste. For instance, flying less. But considerable growth in air traffic is expected, particularly in Asia. Get used to the unmistakable smell of aviation kerosene, it will keep us company for a long time.
Taken from the book by Vaclav Smil, “Numbers don’t lie”
Source: Il Blog di Beppe Grillo by beppegrillo.it.
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