Coconut power and rocket planes

by Jeff Hardy

The aviation industry has been under environmental spotlight for quite some time now. It’s a rapidly growing industry and as it expands so does its carbon dioxide footprint. Two tools can be brought to bear on aviation to reduce its footprint, behavioural change and technological innovation. Since I don’t want to turn this blog into a rant about travel choices I think my best bet is to stick to technology.

Very crudely speaking, the way I see it is that there are three technological options for reducing the carbon emissions for aeroplanes, weight, design and fuels. Here I’m going to concentrate mainly on fuels but it is worth briefly discussing the other two as they are equally important.

I’m going to pick on the Boeing 787 to demonstrate how advances in lightweight materials and engine design can lead to lower carbon emissions. The 787 is constructed 50% from composite materials (carbon fibre reinforced plastics) leading to significant weight reductions. Some versions of the 787 will be powered by advanced Rolls Royce Trent 1000 engines which are very efficient and quiet engines. Combined these measures mean that some versions of the 787 will burn almost 30 per cent less fuel than previous generation airliners.

Whilst there plenty of room for further advances in reducing weight and innovative design, the inescapable fact is planes are currently fuelled by fossil fuel derived kerosene. So what are the options here? Well, if you believe a group of gifted and talented youngsters I taught when at the University of York green chemistry group then the answer is as follows. Take a standard passenger plane. Fill the back half with cows. Feed the cows a diet guaranteed to produce copious methane. Use the methane to power the plane. Provide passengers with gas masks and free milk. Inspired thinking, but perhaps not so practical.

More realistically there appear to be two options for fuels – biofuels and hydrogen. Both off these have been recently trialled with some early success.

In the case of biofuels, Virgin flew a jumbo jet between London's Heathrow and Amsterdam with one engine being fed enough biofuel to provide about 20% of its power. The biofuel was derived from Brazilian babassu nuts and coconuts. The key problem with using fuels derived from natural oils, according to Virgin, is that there exists the possibility that they could freeze at the low temperature at high altitude (for reference note how olive oil goes cloudy and viscous in cold weather). Not an insignificant problem it would appear.

A potential way around this is to make the biofuel in a different way. Biomass derived kerosene can made by converting biomass to synthesis gas (a mixture of carbon monoxide and hydrogen) by a process called gasification and converting the synthesis gas to kerosene through the Fischer-Tropsch process. I’d be happy to go into the chemistry of this if anyone is interested. The advantage of this route is that the kerosene produced is quite similar to that already used and thus should be compatible. In fact Airbus has successfully tested a fuel based on the similar gas to liquid technology, where natural gas is used instead of biomass as feedstock.

It is important that biofuels are derived from sustainable sources and that they have minimal carbon emissions across their whole life cycle. If the biofuel falls down on either of these criteria then it is difficult to see the advantage in its application. This has made the headlines recently in relation to the Renewable Transport Fuels Obligation.
Looking more to the future, you may have seen that Boeing have successfully tested the first manned, hydrogen-powered plane in Spain. The plane, powered by a hybrid battery and fuel cell system developed by UK firm Intelligent Energy, flew for around 20 minutes and landed safely. It’s unlikely that this technology will be suitable for powering commercial passenger aircraft, but it may be capable of providing a secondary source of energy.

This doesn’t rule out hydrogen as a potential aviation fuel in the future, far from it if you believe the claims of the European Space Agency. They are proposing a hydrogen fuelled supersonic passenger jet plane potentially capable of up to Mach 8 – blimey! Concorde on a good day managed a sluggish Mach 2. The plane could be capable of flying from Brussels to Sydney in 4.6 hours – that’s barely time to get comfy. The so called A2 is based on a special engine technology named Scimitar which seems to be described as a rocket engine with a turbo booster! It’s all rather exciting, but I think someway off so I should probably calm down a little.

For now it appears that the introduction of the A380 and the 787 may achieve some savings in carbon dioxide emissions per passenger (assuming they have a full quotient of passengers). However, it appears that in the short term, the only mechanism to reduce the environmental impact of flying is for people to fly less…