Roger Cohen's column in Monday's New York Times sent my mind spinning with its portrayal of a global network of airport-based businesses and organizations that might have closer links to airports a country or continent away than with the traditional urban centers for which these facilities are often named. I'm embarrassed to admit that it was the first time I had run across the "Aerotropolis" concept, which has apparently been around since 2000. Its implications are thought-provoking, not least for their impact on energy and the environment.
The term aerotropolis was apparently coined by a professor at the University of North Carolina business school; it's also the title and subject of his new book. It evokes a retro-1920s science fiction vision of gleaming cities connected by flying cylinders, crossed with the gritty reality of the modern airport and its environs. I wasn't surprised to learn that a third of world trade-- though just 1% by weight--moves by air, but the idea of a hospital integrated into an airport in Hyderabad, India, or an entire city in South Korea growing up around the Incheon International Airport was new to me. The possibilities seem endless, though I can't think about them without also considering where the energy to facilitate the implied explosion of air travel and air freight will come from.
A few years ago, I would have said that air travel was even more closely linked to petroleum than are automobiles. That's not because alternative aviation fuels seemed impossible--quite the contrary--but because the aviation world has historically been understandably cautious and conservative about what goes into the engines that power aircraft. From a technical standpoint, jet turbines offer a great deal more fuel flexibility than the internal combustion engines under the hoods of most automobiles. However, while a fuel failure in your car is a major inconvenience, a fuel failure at 30,000 feet is catastrophic. In some respects the alacrity with which the aviation industry has begun to embrace alternative fuels is nearly as big a surprise as the shale gas revolution, and perhaps ultimately as transformative. Airlines and militaries have entered partnerships and set targets for integrating alternative jet fuel into their consumption, and supplies are gradually appearing.
Scale remains an issue. Kerosene-based jet fuel accounted for 7% of US petroleum consumption last year, down from nearly 8.5% a decade ago, as air carriers have transitioned to more efficient aircraft and higher load factors. That's still a big volume, though it turns out to be easier to make suitable kerosene substitutes from a variety of sources, including natural gas, coal and biomass, than to make comparable substitutes for gasoline. Nor does jet fuel produced from camelina seeds, algae, or the gasification and FT-synthesis of bulk biomass, natural gas or even animal fat entail the kind of performance penalties inherent in our primary gasoline alternative, ethanol. Delivering on this potential will require significant investment, but of a magnitude that seems much more achievable than what is required for many other renewable energy goals.
Another important aspect of scale concerns the logistics of gathering enough biomass to produce meaningful quantities of "biojet". The government of Ontario Province just awarded Rentech, Inc., a company with long expertise in gasification and fuel synthesis, a 1.3 million ton-per-year supply of forest waste and other biomass from Canada's Crown Forests, specifically for the production of renewable jet fuel. The proposed facility would produce around 22 million gallons per year of biojet, along with another 11 million gallons of non-jet products. That equates to roughly 1% of Canada's current jet fuel consumption. Canada might have enough forest biomass available to produce a sizable fraction of its jet fuel needs from such sources, but other countries don't, so it's fortunate that alternative jet fuel can be made through so many different pathways.
That's also fortunate for the aerotropolis concept, because without an incremental supply of non-petroleum jet fuel, meeting the energy needs inherent in this idea without dramatic increases in aviation's current approximately 3% share of global greenhouse gas emissions could become a major obstacle within just a few years. With sufficient supplies of renewable and gas-to-liquids jet fuel, the concept might even be able to withstand a peak in global oil output, even if the price of such alternatives seems likely to track that of oil-based jet fuel.
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