Aviation's big problem - Can flying really be sustainable?

The aviation industry - a rapidly expanding sector - is looking for ways to secure its fuel supplies without increasing greenhouse gas emissions, says Fred Dryer. In the Green Room this week, he outlines some of the options available to deliver these goals.

There is wide agreement that to mitigate climate change, global greenhouse gas emissions must be reduced to 50% of 2005 levels by 2050, with industrialised countries cutting their emissions by 80%.

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In order to achieve this goal, large increases in both energy efficiency and renewable energy will be required.

Biofuel alternatives to petroleum are getting much attention for both ground and air transportation.

Combustion of fuels made from biomass (plant matter) recycles carbon dioxide extracted during photosynthesis back to the atmosphere, making the fuels nearly “carbon-neutral” if only modest amounts of fossil fuel are used to produce them.

However, many biofuel options will be greatly constrained by resource scarcity and cost.

Using food biomass, such as rape, corn or soybeans, creates food/fuel conflicts.

Opening up land to compensate for the reduced food output leads to large greenhouse gas (GHG) emissions during land clearing and tilling that offset reductions achieved in subsequent cropping.

Oil-producing crops, such as jatropha and camelina, can be grown on non-agricultural land but factors such as land availability and rainfall limit how much can actually be produced.

Approaches using algae are attracting interest but costs are very high, and - as for camelina and jatropha - half or more of the harvested biomass ends up in by-products.

Syn of the times

First, technologies are needed that maximise liquid fuel production and carbon mitigation from scarce biomass supplies.

Second, bio-derived products must be fully compatible with petroleum fuels.

That’s particularly true for aircraft applications, because of the international nature of the aviation industry and because petroleum fuels will be widely used for decades.

Our NetJets-sponsored research at Princeton University is seeking to identify technologies for displacing petroleum and reducing GHG emissions from transportation, both air and ground.

NetJets, the world’s largest provider of corporate jet services, is seeking ways to reduce its carbon footprint substantially: its customers expect it, and the long-term future of its business depends on it.

Our analyses show that the most promising options involve borrowing technologies that are already used with coal, along with carbon capture and storage (CCS) technology that is being developed to enable continuing fossil fuel use under a carbon policy constraint.

Superclean “synfuels”, otherwise known as synthetic fuels, can be made from coal via commercial processes that begin with gasification.

But the GHG emission rate for production and use of these fuels is about double that of petroleum fuels.

However, about half of the carbon is released at the plant as a stream of undiluted CO2, so CCS can be pursued at relatively low cost, reducing emissions to about the level of petroleum fuels - a lot better, but not nearly good enough for a carbon-constrained world.

What if synfuels were made from biomass via gasification?

One advantage is that all biomaterials (not just speciality crops) can be processed, which increases the effectiveness of using scarce biomass resources.

As for some other biofuels, the overall process of making and burning the liquid fuels would be nearly “carbon neutral”.

However, under a carbon policy it would often be worthwhile to include CCS, making the overall GHG emission rate strongly negative - thereby greatly enhancing the carbon mitigation potential of scarce biomass supplies.

Such biofuels are not competitive with crude oil-derived products today but are likely to be very competitive at carbon prices that may be typical in 20 years’ time.

Zero sum

An option that can be pursued now involves co-processing biomass with coal in similar gasification plants with CCS.

Co-processing about 40% biomass and 60% coal provides liquid fuels with a net-zero GHG emissions rate.

Moreover, the amount of liquid fuel produced per unit of biomass is twice as much or more than most other biomass options.

Our analysis suggests that fuels from such systems will be competitive with crude oil-derived products, even with zero pricing on GHG emissions, when the crude oil price is $100 per barrel - a figure that is expected to be typical once the global economy recovers.

The technology is ready to be deployed now in plants that co-process about 10% biomass and inject the captured CO2 into mature oil fields to coax out more crude oil.

The world’s first synthetic fuel plant with CCS - the US Dakota Gasification Project that produces synthetic natural gas (not liquid fuels) from coal - has been capturing a million tonnes of CO2 annually since 2000 and transporting it 300km by pipeline into Canada, where it is used for enhanced oil recovery.

Careful monitoring has shown that after repeated re-injection, the CO2 ultimately remains stored underground.

Our research has identified a credible bio-based liquid fuel supply strategy for solving the carbon problem for transportation, even with limited biomass supplies and without abandoning crude oil.

The key concepts are gasification-based liquid fuels and CCS for both biomass and coal.

Coal/biomass-to-liquids technologies would be deployed first in coal-rich countries, to be followed by biomass-to-liquids technologies in coal-poor but biomass-rich regions.

Fred Dryer is professor of mechanical and aerospace engineering at Princeton University, US

The Green Room is a series of opinion articles on environmental topics running weekly on the BBC News website

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