Page last updated at 09:42 GMT, Tuesday, 11 August 2009 10:42 UK

How aviation can clean up its act

Professor Fred Dryer
Fred Dryer

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.

Aircraft landing
Bio-derived fuels must be fully compatible with petroleum fuels, particularly for aircraft applications, because of the international nature of the aviation industry

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%.

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

In selecting bio-based transport fuels, two criteria stand out.

Arable land (Getty Images)
Demand for biofuels could lead to land being taken from food production

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.

Rainforest destruction in Indonesia
Forest clearing for biofuels results in a vast volume of CO2 being released

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.

Flying aeroplane (Getty Images)
The aviation industry has been singled out for criticism by environmentalists

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

Do you agree with Professor Dryer? Can aviation be made sustainable? What fuel sources can help deliver non-fossil fuel options? Or do we simply need to curtail the number of flights we take?

All forms of travel should reflect their environmental impact through associated costs. The aviation industry needs to be realistic in dealing with the pollution produced by aircraft; in this partcular instance CO2. The cost of all flights should include some funding toward mitigation measures, which might, admittedly, see the end of cheap air travel. This in itself would be no bad thing, as the proliferation of air travel, and the concomitant need for structural upgrades to airports, is unsustainable. Efficiency measures can only go so far - there has to be a major re-think over air travel to try and reduce it's negative environmental impacts. And while we're at it, we also need to address unregulated ocean shipping.
Ironspider, Airstrip One

The basic question is to act on 'primary' and 'secondary' properly. Present definition of 'primary' is to support the existing patterns and to seek some alternative for the natural part which is 'secondary' or less important. The so called 'secondary' part is gaining importance at such a time, when the potential of environment and biosphere has severely damaged. The short term gains have been converted into the long term setbacks. There is no doubt about the technologies what Prof Dryer is emphasizing. The biomass and coal based liquid fuels and CCS is most promising for the future. But, it would be better if the major part of the technologies like CCS is used to clean the previous damages done to the environment. Up to a certain extent, use of biomass and coal based liquid fuels is good because the land use would be changed from the 'food crop to bio fuel crop'. In my opinion, the best diversion of the land use is 'from food crop to forest'. Forest may be like 'agro forestry', what Miranda Spitteler has highlighted in the previous article or 'simple forest' to act as carbon sinks and to save the biodiversity.
Sanjay Singh Thakur, Indore,India

In 1966 when I was fourteen the reported world population was 3.2 Billion people. 43 years later, that's now, the world population stands at 6.7 Billion, a little more than doubled in that time and I'm now 57. I could live to be 43 years older than I am now and I'd have achieved the ripe old age of 100 and the world population would be approximately 13.4 Billion. Overpopulation is THE problem to be solved concurrent with reduction of greenhouse gas emissions. Since the likelihood of that happening is about zero, enjoy your day today because the future looks extremely bleak.
Gary, Alameda, California, USA

This is all just nonsense. "Global warming" and "climate change" are all just things to make the governments more money by increasing tax. Since the dawn of time, CO2 has been emitted and always will be. Singling out air travel aswell is just stupid when there are bigger sectors firing out more CO2 than this. Once again this is down to taxation. It's all down to greed. I don't think we should be made to feel guilty about something that happens naturally anyway. Remember, the United Kingdom used to be a tropical climate and also went through the ice age. Climates change. Blaming human activity is just nonsense and greed.
Craig, Edinburgh, Scotland

I've read a lot of interesting comments and I will make a few of my own. Bio-fuels for both aviation and ground transportation are not only viable but solves other issues in the process. First... Bio-fuels do not need feedstock that contributes to destroying rain-forest or feedstock that competes in the food industry. Algae is a feedstock that can be grown anywhere, does not compete in the food industry, the bio-mass once oil has been extracted can be used in dyes as well as for livestock feed. The process is a closed loop system that grows continually and has year round harvesting. Algae is approximately 10x better at producing oil than any terrestrial crop... approx. 10,000 gallons per acre. To top it off... CO2 can be siphoned off factories and plants and pumped into algae growth systems which utilize the photosynthesis process and will use the CO2 to speed up the algae growth process. There is typically acres of unwanted or unused land that is close to factories where algae production systems would thrive! If your interested in this and other advanced technologies and alternative energy solutions visit
A Henderson, Rowlett, TX

some years ago I worked briefly on a bio fuels concept in India where the gov has a very far sighted policy in that food crops may not be used for bio-fuels. As a consequence there is a great deal of interest in Jatrophya, an oil seed bearing shrub, and other Tree Bourne Oilseeds (TBO). One of the problems though with bio fuels and aviation is solidification at low temp.. bio-fuels are not particularly fluid at low temps, some vegetable oils solidify. However one seed oil I know of has I believe some possibilities, the horse chestnut or conker tree.. the nut of which is rich in oil, has a natural 'antifreeze' component and its production is not only already extensive but takes place in woodland not arable farm land... correctly identified and sourced an alternative fuel could be created that makes aviation the most environmentally friendly of them all.. now that would be a turn up for the books!
malcolm mcewen, jersey u.k.

All this talk of stopping the airline industry and banning cars is totally not realistic. It simply wont happen. Also, no one has really mentioned the use of aquatic algae as fuel. The Americam Aquatic Species Program of the 1990s proved its viability albeit with open ponds. It seems that for a genuinely reliable process closed bio-reactor systems are needed. These algae grow rapidly in salty water on land that has no agricultural use. So they dont compete for water supplies or land that is needed for food production. The extracted fats become hydrocarbon fuels and the remaining sludge can be burnt in a power station boiler. The ash goes back as nutrient for algae production. The issue is cost - it wont be cheap. But the ASP stated in the late 1990s - 100 billion dollars of investment would allow USA to become independent from middle East petroleum for transport fuels. How much has already been spent in Iraq pretty much to bolster the USA's "long term" petroleum supply? I suggest it's not about the money its about the will to actually do something.
Dave, Midlands, UK

We need to cut back on air travel as much as possible. Even if we can achieve negative carbon dioxide emissions with CCS and biofuels, according to the Royal Commission on Environmental Pollution, aircraft emissions at higher altitudes have between 3 to 5 times more global warming impact than at ground level. More research is needed to clarify this.
Jean Aldous, Bury St Edmunds

Blah blah blah; another - thoroughly charming and I am sure very professional - eminent scientist with his head too deeply buried in the pages to stand back and see the bigger picture; This is yet another "Oh, fix this little technical CO2 glitch, then it's back to business as usual, ripping up the planet"; well, we've run out of planet ! CO2 is only one of many symptoms; the fundamental problem is too many people, consuming too much stuff. That's the problem Mr eminent scientist ! So you build a "zero carbon" aeroplane; so what ? Where are you going to fly if the remote place you are heading for can't support the demands of the people already living there ? We've run out of planet; we are currently ripping it up 2 - 3 times faster than it can cope, and until we fix that fundamental underlying problem; this tinkering with the symptoms is so much deluded puff. These guys are supposed to be eminent, highly qualified, scientists; the brightest of the brave . . . . But even Homer Simpson never took this long to have his "Doah !" moment ?? How much longer are these guys going to take till they twig it. Or perhaps they are simply being paid to think about how to keep the airline industry going on more year, rather than solving the problem, and how to get us out of the mess we are in. Pity; we could do with a few eminent scientists working on that problem. Well, it's been 16 years now and they still haven't figured it out; you've got 20 years left. Enjoy it; take flight round the world and take a last look ! Cheers Steven
steven walker, Penzance

It is right to address the issues of Aviation due to the rate at which it is growing if not the total emissions when compared to other industries such as cement manufacture, electricity or road transport. The biggest challenge is changing public expectations from one over ever increasing consumption to one where reducing consumption is the trend. It is difficult to envisage the complete abandonment of flying, but a dramatic increase in cost and removal of short haul would be very valuable. A system of personal carbon credits would seem an effective way of engaging people with the carbon cost of given activities and allow them the freedom to chose how they will 'spend' their emissions be it through flying, owning a more polluting car or some other activity. One area missed by the article is that of radiative forcing. The emissions from a flight have two or three times greater warming impact than the same emissions at ground level and this should be accounted for in all carbon accounting including trading. There is growing consensus the climate change, peak oil, population growth and food production are problems that are converging into a 'perfect storm' to break in the coming decades. The change on attitudes that has occurred over the recent years is encouraging but the changes need to be delivered and quickly if we are going to have a habitable planet for future generations.
T Rotheray, Solihull, UK

What about growing these crops on polluted brown lands? Would that work? It would kill two birds with one stone: rehability polluted environments and provide some relief to aviation industry costs. Does anyone study the amount of polluted land there is? What about desert blooming plants that do not consume much water? There has to be some alternative.
Laura, New York, NY

One important problem with biofuels that is glossed over in this article is habitat destruction. A move towards biofuels would be devastating for rainforests in particular. Mass land clearance for biofuels may solve our CO2 problems but as the Earth would only consist of us, pigeons and rats, I'm not sure I want to survive.
Chris, Bristol

The basic premise is flawed.. The future will be decided well before 2050 as far as global warming is concerned and I think the vast majority of scientists and those who follow the findings of research would concur. The usefulness of lighter than air flight should be explored instead of insisting on finding alternative fuel sources for high octane dependent jet aircraft. If combined with a new understanding by all nations to allow and support clear passage of flights and to address subsequent security concerns there really isn't any limit to the usefulness of flight. Hospitals could move where needed; food and supplies could travel and shade could be generated. Time is the key ingredient not in enough supply. It's time to stop thinking as so many have in the past that mankind isn't capable of turning the course of history by acting together in a unified manner. All other things are not remaining the same and it's time to stop thinking small as though they were.
Dale Lanan, Longmont, Colorado, USA

Any bio fuel replacement will mean food costs rise and the poor will be faced with food pverty issues. The irony of government policy to 'democratise' aviation as they say as they don't just want the rich to fly (even in the face of overwhelming evidence that this is the case) would be laughable. I believe what we are seeing here is a last gasp of a broken mode of transport - jets will go the way of the chariot, short haul first, its inconceivable that short haul flights will exist in a world of energy challenges. Electrified rail will be the mode we all take for european vacations - jest only for long haul plus 4 hour flight time.
Christian Ball, London, UK

Graham Ellis, well said, this article is far too one-sided and does not consider the energy intensity of CCS, not to mention gasification. The aviation industry needs to be allowed to grow (with some limits on short-distance travel), but with strict requirements on new aircraft efficiencies (e.g.: the new 787 is 20% more fuel efficient than its predecessor, the 767) and with a great push towards the use of biofuels. Biofuels are seemingly the only near-term answer to reducing emissions from commercial aviation and have been shown to work on flying test-bed engines. However, the amount of land required to fuel the entire aviation industry with biofuels is immense and a new generation of far-more-productive biofuel crops/algae will need to be developed before we can answer the problem effectively. In the long-term, other technologies such as fuel cells, or even hydrogen-fuelled scramjets could be the answer, but these technologies are a long way from being applicable to commercial aviation.
Nick, London

The central point of this article and the readers' comments is that aviation is among the worst polluting forms of travel. In fact, shipping is much, much worse, and is growing at a faster rate. Perhaps the enthusiasm and ideas presented on this page should be directed at shipping, especially as little attempt has been made as yet to reduce pollution from shipping.
David, Cheshire

We can't/won't face up to the problem, there are too many people in the world. Arable land is a diminishing quantity for food/fuel production as a result. In the West, capitalism can only function by increasing demand, so this has to change, consumption has to decrease not increase. We are all at a crossroads, and our future is by no means assured. We are straining to find a silver bullet solution to our energy needs to allow "business as usual" to prevail.Sadly it will not happen.
colin mountford, Brisbane Australia

Surely the simple fact that Carbon Dioxide is NOT the evil pollutant should make all the hot air about aviation fuel a pointless exercise in yet another set of the Emperor's Clothes. Where is the factual data that proves CO2 is causing anything outside normal climate variation? Biofuels are a red herring and a waste of valuable food resources. Less media hype would help.
Brian Johnson, Farnham Surrey UK

One commonaly ignored plant which is ideal for use in biofuel production is hemp. the strain of hemp used has no narcotic properties. Hemp has a three-month cycle, and it can even be processed into all sorts of fibres which can replace cotton and timber. it is also a very hardy plant requiring minimal activity to cultivate. why arent we using this wonderful plant? because logging is big business in america and they wouldnt want to disrupt that.
B Strickland, Melbourne, Australia

The one subject that hardly ever gets mentioned and no one seems to want to discuss, is the problem that there are six billion of us on the planet and our population continues to sky rocket. Reducing our carbon foot print is great, but if our population continues to grow it will negate any savings.
P Fleming, Nashville, TN

The only biofuel with real potential to reduce the rate at which we change our climate is made from an algae that blooms on sewage. Every other energy crop either causes habitat destruction, or takes land away from food crops. Europe is subsidising completely the wrong technologies. And, of course, airlines that claim they'll fly you anywhere from Stansted for ten pounds aren't paying the real cost of fuel. The market price, perhaps, but not the real cost. Our children will have to pay that.
Jon Blackwell, Hull, UK

There is an alternative source of fuel, which is particularly relevant to aviation, but is also applicable to transport in general. That is organic fuel (petrol, diesel, etc) synthesised from carbon dioxide and hydrogen. This is still at the research stage, and so is not widely known about. Of course, this fuel would be expensive, as it requires the production of hydrogen. (High temperature electrolysis can reduce the electrical requirement, and the thermodynamic losses incurred in converting heat to electricity. However, the energy budget would remain high.) Because of the high cost, this type of fuel synthesis would not become economically viable until the alternatives - petroleum oil, and then coal-to-oil synthesis - are exhausted. Once this happens, the primary sources of energy will be renewable, so the cycle of fuel synthesis and combustion would be carbon-neutral. As well as providing a long term source of fuel, this process could address other problems. In the medium term, when petroleum oil has run out, coal-to-oil synthesis will become the next cheapest option. Hard economic reality means that this will become the fuel of choice for many countries and industries. Unfortunately, this will approximately double carbon dioxide emissions, making the need for carbon capture even greater. The prospect of being able to use carbon dioxide as a feedstock for future fuel synthesis will provide an economic incentive for its capture and storage, as soon as the technology becomes feasible. As the carbon dioxide would be captured and stored for decades, rather than centuries or millenia, the volume required to be stored would be less, and sites would not be limited to those with good long-term stability. The use of synthetic fuel would at least reduce the need for bio-fuel production. This would reduce the potential for conflict with food production, particularly if the required production volume was within the capacity of crop residues (cellulose conversion). As synthetic fuel will be expensive, in order that society can afford it, we will need to reduce demand - not just for fuel, but for all our uses of energy. The era of cheap energy is an accidental consequence of the (short-term)availability of fossil fuels. When they run out, we can still have a technologically advanced society, with the energy it requires, providing we use that energy wisely.
David Johnston, Newcastle upon Tyne, UK

Professor Dryer is answering the wrong question. The right question should be how do we stop the current aviation industry expanding? For example: Any flights of less than 200 miles should be replaced by more fuel efficient land and water transport. Flights of less than 500 miles should be carried out by balloon which even if a little slower is much more energy efficient. Many business flights can be replaced by efficient electronic communication. Electronic conferencing rather than going forwards seems to have regressed over the last twenty years. The amount of air freight should be reduced by expanding local production and consumption.
David Wilson, Wokingham Berkshire

One biofuel that has been barely considered is methanol (wood alcohol)-CH3OH. It was one of the first building blocks for organic chemicals. It is made by the destructive distillation of (waste) biomass and can be burnt directly or turned into petroleum products, syngas etc. Every year plants fix about 100 billion tonnes of atmospheric carbon and every year about the same amount is given back to the atmosphere. Approximately half of this biomass (50 billion t C) is potentially available for use, but at present only about 1.5 billion t C are used for energy purposes. As a comparison the annual burning of fossil fuel produces about 7 billion t C, therefore renewable carbon based fuels could replace a considerable quantity of fossil fuels, including liquid fuels such as jet A1 and diesel. It boils down to cost. If jet A1 was appropriately taxed, then no doubt fuels from biomass could become competitive.
keith Openshaw, Vienna, VA 22180, USA

The answer to the problem is simple - less travelling, both by air and by land. The advent of rapidly-improving technologies like teleconferencing and telepresence is making the need for business travel increasingly unnecessary. Governments should encourage this change by stopping allowing business travel to be claimed as a legitimate expense. In the end, we really need to discriminate between what we want, and what we actually need. Face to face meetings are so 20th century!
John E, Southampton

This story is very one-sided in terms of the technologies being reviewed. Gasification complexes can cost $1 billion for a 5000 barrel per day facility, in a world which uses millions of barrels per day of jet. Just amortizing that capital adds $1.30 per gallon of finished jet - which should be compared with the total production cost of jet today at $1.90/gallon. Carbon sequestration on a coal gasification unit is in itself energy intensive and studies show that we might have to increase fossil fuel usage by 10% in order to create the energy needed to compress and store the CO2 - and in doing that, we are still creating a future problem (ie hiding the CO2 out of sight). What happened to reducing our fossil fuel demand? The long term issue is one of oil becoming scarcer, and renewable sources of energy of all types having to be added to the mix. Food versus fuel is one argument, but it only scratches the surface of the issue. Rotational crops used in fallow years to create biomass for energy can reduces or eliminate competition with food, opening up better solutions energetically than the syngas route. Better air transportation management reduces fuel usage (no stacking over Heathrow). It is pointless to favour one technology at this point or to cite it as being the future - it took 90 years to get the petroleum industry to the efficiency it has today. The aviation biofuels industry is in its infancy with only a few years of development, and I doubt we can pick a winner yet. The reality is that the solutions will vary around the world based upon local climate, biomass availability, government policy, and of course demand.
Graham Ellis, Chicago, USA

The biofuel argument basically will boil down to a conflict between food and fuel requirements. If we take land from Food production, food will get more expensive. The question is are we stupid enough to replace a necessity with a luxury. Using food to fly us will push people on the breadline over it. The message is clear: The party is over....
Andrew Smith, Bedford

Biofuel is worth pursuing as a fuel source for emergency use in the short term but it's unlikely that biofuel will provide more than a fraction of vehicle fuel even for roads let alone aircraft. What proportion someone's one-planet share of energy is needed for even one flight? I bet it's a fair bit.
Louise, Edinburgh Scotland

What happens when you can't fit in any more C02?, It might work short-term but long term, just storing it isn't going to work. Its no different to storing our nuclear waste in salt mines, your just handing the problem to the next generation, and perhaps a much greater problem at that. As to the airline industry, the newer aircraft and engines are vastly superior in terms of fuel consumption so encourage the retirement of older aircraft or make it viable to re-engine them with more fuel efficient (and quieter) engines.
A Broxholme, Hampshire, UK

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