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Buzen's avatar

A major factor you don’t mention is the energy density of the fuels you are discussing. Aviation jet fuel or kerosene has an energy density of 43MJ/l which is much higher than ethanol’s 28MJ/l. This means if a turbofan jet engine could run on ethanol (because of lack of lubricants, turbo jet engines cannot use more than 40% ethanol or the engines will seize) it would need 65% more fuel by weight, either reducing range or increasing prices (to make room for extra fuel storage). This means your simple comparisons are not very accurate. Also battery powered passenger flight is now only possible for very short distances with few passengers, again a consequence of the mass of the batteries needed.

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Lewis Cleverdon's avatar

Hannah, there is no reason to assume that the biofuels required to "recarbonize" the global aviation industry must be obtained from farmland. That assumption skews your entire study into ignoring the prime global potential for sustainable liquid fuels.

In reality, prior to the era of subsidized fossil gas (aka "natural gas") the Standard Industry Practice for the production of the versatile liquid fuel methanol [CH3OH] was via feedstock from Native Coppice Forestry for charcoal production, whose hydrocarbon offgasses were reacted to produce methanol. As a leading hydrogen carrier methanol is not only a prime candidate for the feedstock of so-called Sustainable Aviation Fuel (aka synthetic kerosene) it will also serve to some extent as an easily tanked aviation fuel for the Reformer Methanol Fuel Cells that are now in commercial production in Europe, USA, & China.

Since Native Coppice Forestry has traditionally thrived on poor hill lands, its present expansion would logically pose no threat to farmlands. You may recall the 2012 joint study by WRI & WFN of the available land area suitable for afforestation - without taking farmland, native forest or special ecosystems - which identified 1.6Gha.s globally. This is about 152% of the entire global area of arable farmland.

In terms of the methanol yield available from biomass feedstock, the US NREL gave some useful findings (see: https://www.nrel.gov/docs/legosti/old/5570r2.pdf ).

"Estimates of the [USA] biomass resource available for U.S. fuels production average 2.45 billion metric tons per year. One ton of feedstock can be converted to 721 liters (186 gallons) of methanol. As a renewable resource, biomass represents a potentially inexhaustible feedstock supply for methanol production."

With methanol weighing 0.791 kg /litre, the NREL finding indicates a potential output - just from extant US biomass resources - of :- 0.721 m3 x 0.791 t x 2.45Gt feedstock = 1.397Gt /yr methanol output. As the fuel has an LHV of 19.9 GJ /t, the finding points to an energy value of 27.805 EJ in that output. Given that US biomass resources are a small fraction of what 1.6 Gha.s of Native Coppice Forestry could produce, let alone accounting global sustainable biomass resources overall, there is plainly no justification for presenting biofuels as necessarily posing a threat to agriculture.

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