In a world of near zero anthropogenic emissions of carbon dioxide, there remains the problem of finding a fuel or energy carrier of sufficiently high energy density that it remains practical to fly a modern jet aeroplane. Commercial aviation is heading towards some 1 billion tonnes of carbon dioxide per annum so doing nothing may not be an option.
Although planes will certainly evolve over the course of the century, the rate of change is likely to be slow and particularly so if a step change in technology is involved. In 100 years of civil aviation there have been two such step changes; the first commercial flights in the 1910s and the shift of the jet engine from the military to the commercial world with the development of the Comet and Boeing 707. The 787 Dreamliner is in many respects a world away from the 707, but in terms of the fuel used it is the same plane; that’s 60 years and there is no sign of the next change.
Unlike domestic vehicles where electricity and batteries offer an alternative, planes will probably still need hydrocarbon fuel for all of this century, perhaps longer. Hydrogen is a possibility but the fuel to volume ratio would change such that this could also mean a radical redesign of the whole shape of the plane (below), which might also entail redesign of other infrastructure such as airport terminals, air bridges and so on. Even the development and first deployment of the double decker A380, something of a step change in terms of shape and size, has taken twenty years and cost Airbus many billions.
For aviation, the simplest approach will probably be the development of a process to produce a look-alike hydrocarbon fuel. The most practical way to approach this problem is via an advanced biofuel route and a few processes are available to fill the need, although scale up of these technologies has yet to take place. But what if the biofuel route also proves problematic – say for reasons related to land use change or perhaps public acceptance in a future period of rising food prices? A few research programmes are looking at synthesising the fuel directly from water and carbon dioxide. This is entirely possible from a chemistry perspective, but it requires lots of energy; at least as much energy as the finished fuel gives when it is used and its molecules are returned to water and carbon dioxide.
Audi has been working on such a project and recently announced the production of the first fuel from their pilot plant (160 litres per day). According to their media release;
The Sunfire [Audi’s technology partner] plant requires carbon dioxide, water, and electricity as raw materials. The carbon dioxide is extracted from the ambient air using direct air capture. In a separate process, an electrolysis unit splits water into hydrogen and oxygen. The hydrogen is then reacted with the carbon dioxide in two chemical processes conducted at 220 degrees Celsius and a pressure of 25 bar to produce an energetic liquid, made up of hydrocarbon compounds, which is called Blue Crude. This conversion process is up to 70 percent efficient. The whole process runs on solar power.
Apart from the front end of the facility where carbon dioxide is reacted with hydrogen to produce synthesis gas (carbon monoxide and hydrogen), the rest of the plant should be very similar to the full scale Pearl Gas to Liquids (GTL) facility that Shell operates in Qatar. In that process, natural gas is converted to synthesis gas which is in turn converted to a mix of longer chain hydrocarbons, including jet fuel (contained within the Audi Blue Crude). The Pearl facility produces about 150,000 bbls/day of hydrocarbon product, so perhaps one hundred such facilities would be required to produce enough jet fuel for the world (this would depend on the yield of suitable jet fuel from the process which produces a range of hydrocarbon products that can be put to many uses). Today there are just a handful of gas-to-liquids plants in operation; Pearl and Oryx in Qatar, Bintulu in Malaysia and Mossel Bay in South Africa (and another in South Africa that uses coal as the starting feedstock). The final conversion uses the Fischer Tropsch process, originally developed about a century ago.
Each of these future “blue crude” facilities would also need a formidable solar array to power it. The calorific content of the fuels is about 45 TJ/kt, so that is the absolute minimum amount of energy required for the conversion facility. However, accounting for efficiency of the process and adding in the energy required for air extraction of carbon dioxide and all the other energy needs of a modern industrial facility, a future process might need up to 100 TJ/kt of energy input. The Pearl GTL produces 19 kt of product per day, so the energy demand to make this from water and carbon dioxide would be 1900 TJ per day, or 700,000 TJ per annum. As such, this requires a nameplate capacity for a solar PV farm of about 60 GW – roughly equal to half the entire installed global solar generating capacity in 2013. A Middle East location such as Qatar receives about 2200 kWh/m² per annum, or 0.00792 TJ/m² and assuming a future solar PV facility that might operate at 35% efficiency (considerably better than commercial facilities today), the solar PV alone would occupy an area of some 250 km² , so perhaps 500 km² or more in total plot area (i.e. 22 kms by 22 kms in size) for the facility.
This is certainly not inconceivable, but it is far larger than any solar PV facilities in operation today; the Topaz solar array in California is on a site 25 square kms in size with a nameplate capacity of 550 MW. It is currently the largest solar farm in the world and produces about 1.1 million MWh per annum (4000 TJ), but the efficiency (23%) is far lower than my future assumption above. At this production rate, 175 Topaz farms would be required to power a refinery with the hydrocarbon output of Pearl GTL. My assumptions represent a packing density of solar PV some four times better than Topaz (i.e. 100 MW/km² vs 22 MW/km²).
All this means that our net zero emissions world needs to see the construction of some 100 large scale hydrocarbon synthesis plants, together with air extraction facilities, hydrogen and carbon monoxide storage for night time operation of the reactors and huge solar arrays. This could meet all the future aviation needs and would also produce lighter and heavier hydrocarbons for various other applications where electricity is not an option (e.g. chemical feedstock, heavy marine fuels). In 2015 money, the investment would certainly run into the trillions of dollars.