10 Technology Quarterly |Aviation The EconomistJune 1st 2019
C
ompared withelectricity generation (44%), road transport
(17%) and even cement-making (4%), aviation, at about 2%, is
not a huge source of man-made greenhouse-gas emissions. But it
is a source, and it is growing fast. The International Civil Aviation
Organisation, an arm of the un, forecasts that such emissions
could rise between three- and seven-fold by 2050 if nothing sub-
stantial is done. More efficient engines (see chart) and the intro-
duction of a certain amount of electrification will help, but will
only cut into this growth rate, not reverse it. To do that would re-
quire the fuel itself to be made “carbon-neutral”.
That is not the same as carbon-free. The chemistry of what went
into an aircraft’s fuel tank would change little, if at all. Rather, the
idea would be to borrow the carbon in the fuel from the air, in the
form of carbon dioxide. Then, add energy
and hydrogen and remove the oxygen to
turn it into appropriate hydrocarbons, and
release the energy added in the same way
that the energy in conventional fuel is re-
leased—by burning the fuel in a turbojet
that then turns a turbofan or a propeller. As
long as the added energy was not itself de-
rived from fossil fuels, this would not add
to the natural stock of atmospheric CO 2.
Though plans exist to build machines
which would suck the carbon for this pro-
cess directly out of the air, doing so on an
industrial scale would be a heroic endeav-
our. Such carbon-capturing machines al-
ready exist in nature, though. They are
called plants.
Chemically, there are at least half a doz-
en ways of turning plant matter into avia-
tion fuel. Two stand out. One is called hefa(Hydroprocessed Esters and Fatty Acids). The other is the Fischer-
Tropsch process.
The raw material for hefais plant oils. One of the attractions of
this is that the oils in question can be cast-offs, like used cooking
oil, that would otherwise be thrown away. At small scale, hefacan
therefore rely on recycled waste products as its raw material. Plant
oils and their derivatives (the esters and fatty acids in the process’s
name) are similar to the hydrocarbon molecules in petroleum-
based aircraft fuel, but need to be stripped of their oxygen atoms to
become identical. hefadoes that by getting them to react with hy-
drogen, in the presence of a catalyst. The oxygen atoms are carried
away either in water molecules or in molecules of carbon monox-
ide or carbon dioxide, depending on the details of the process.
Just how green hefais depends on the source of the hydrogen.
Ideally, it would come from the electrolysis of water, the electricity
involved having, in its turn, been generated by some fossil-fuel-
free method such as solar, wind or nuclear power. Unfortunately,
the main source of industrial hydrogen at the moment is steam
reformation, a two-stage operation in which methane and steam
react together to make hydrogen and carbon dioxide.
The Fischer-Tropsch process is a well-established set of chemi-
cal reactions (it was invented in 1925) that have, in the past, been
used to convert both coal and natural gas into liquid fuels. It takes
carbon-rich material from whatever source and reacts it with
steam in a manner identical to the first stage of steam reformation.
This produces a mixture of hydrogen and carbon monoxide
known as syngas, which can be further reacted, using appropriate
catalysts, to produce hydrocarbon molecules of the desired size. To
make aviation fuel, for example, those molecules should have be-
tween eight and 16 carbon atoms in them.
Both of these approaches work chemically. But they also have
carbon footprints of their own, and therefore reduce global warm-
ing by different amounts. According to a report published in 2017
by Imperial College, London, preparing and burning aviation fuel
made by hefafrom used cooking oil yields a 69% saving of carbon-
dioxide emissions compared with those created by refining and
burning aviation fuel made from petroleum. Used cooking oil is,
however, in finite supply. Start with fresh oil, as would be needed if
a significant fraction of aviation fuel were to be made this way, and
the saving drops to 20-54%. For the Fischer-Tropsch process, using
fast-growing grasses known as energy crops, the saving is 85-90%,
rising to 95% if leftover wood from forestry is the feedstock.
On the face of things, then, the Fischer-Tropsch route looks the
better one. It may also be the cheaper. For hefa, the price of the
vegetable oil alone, unless it is waste, already exceeds the cost of
petroleum-based aviation fuel. And waste oil is a drop in the ocean
of the raw materials that would be required for biofuels to make a
dent in CO 2 emissions. Energy crops are a
lot cheaper than that. The Fischer-Tropsch
process does, however, require enormous
capital investment in the necessary plant.
In this regard hefais cheaper.
Unfortunately, if they are to meet a sig-
nificant fraction of the demand for avia-
tion fuel, both of these methods will re-
quire a lot of land to grow their raw
materials. But this could change. In partic-
ular, there are hopes that the new field of
synthetic biology will come up with ways
of generating esters and fatty acids in fast-
growing micro-organisms—or even ar-
range for those micro-organisms to syn-
thesise the relevant hydrocarbons directly.
That would reboot everyone’s calculations,
and have ramifications far beyond the field
of aviation. For the moment, though, the
outlook for bio-aviation fuel is glum. 7Smoking grass
Aviation biofuel is a nice idea, but is unlikely to fly for nowAviation and the environmentGetting thereSource:IATAGlobal passenger flights, CO2 emissions
kg per revenue passenger kilometre00.050.100.150.200.251990 95 2000 05 10 15 17