A NASA scientist is confident that within five years commercial aircraft could be powered using a type of biofuel derived from saltwater plants, or halophytes, grown in desert areas and irrigated using sea water. While the concept may sound far-fetched, engine manufacturer General Electric says it is following developments in this area "with interest", and a major oil company, which prefers to remain anonymous, says it is considering the idea to see how much benefit it has.
Dennis Bushnell, chief scientist at NASA's Langley Research Center in Virginia, says 22 countries are carrying out small experimental activities into the cultivation of halophytes for use in food production, although he admits "nobody is doing this type of biomass for aircraft" at this time. Nevertheless, Bushnell sees "no stoppers" to augmenting halophyte-derived biomass to produce biofuels capable of powering aircraft.
"This is far from evolutionary, it's just outside people's radar screens and the usual human reaction to this is to say that it's impossible," says Bushnell. "What's nice about biofuel is that it can use the existing infrastructure used by the oil companies and can be available much sooner than hydrogen, which would require changes to infrastructure and is, therefore, much further into the future."
Fuels such as biodiesel can be produced from biomass ranging from cow manure to wood chips. The advantage of developing biofuel from halophytes as opposed to other types of biomass is that saltwater plants are not dependent on fresh water, which is in increasingly short supply, and can instead be irrigated using plentiful sea water supplies. Bushnell notes that, following irrigation, the salt from the sea water "should leach back into the ocean" without causing problems to agriculture.
Suitable areas around the world for cultivating halophytes include the Sahara desert, Western Australia, south-west USA, parts of the Middle East and parts of Peru. Scientists claim that an area smaller than the Sahara desert could yield enough biomass to replace the world's fossil fuel requirements.
Furthering the case for halophyte production, Bushnell says that, as these plants are grown in the desert, they will produce a cooler, wetter land surface, which could lead to rainfall in areas of the world where rainwater is in short supply.
GE Aviation manager of advanced combustor engineering Timothy Held believes some progress can be made within five years on testing biofuels derived from halophytes for use in commercial aircraft engines, but he says that the entire process of developing and producing the fuel will take longer. "It seems plausible that some amount of suitable fuel could be made available for testing purposes in the five-year timeframe," he says.
"However, the steps of establishing suitability for use in flight gas turbines, obtaining approval from the engine manufacturers, incorporation of the new fuel into a specification and developing large-scale production capacity are quite time-consuming," Held says. While biodiesel has been used by GE to operate marine gas turbines, it is not suitable for aircraft engines because of its poor low-temperature properties, but he believes a fuel derived from bio-oil by conversion to a paraffin-based product has a significant chance of becoming a viable aviation fuel.
NASA's Bushnell believes the argument in favour of biofuels for aviation is being reinvigorated by "the incipient demise of cheap oil" and increasing evidence of global warming due to the burning of fossil fuels. "This is the only easy solution I know of, both in terms of economics and timescale, and we do not need major capital investments to do this. It is definitely worth a serious look," he says.
Source: Flight International
