As Shell Aviation celebrates last week's first gas-to-liquid kerosene-powered commercial service, the company and its partners are undertaking a detailed ground- and flight-test evaluation of the environmental benefits GTL can bring and understand what the optimum blend is for mixing with oil-based fuel.
Qatar Airways operated the world's first commercial GTL-fuelled flight between London and Doha on 12 October, with a 50/50 blend of Shell-supplied GTL and oil-based kerosene. The airline aims to begin regular GTL flights from 2012 when Shell's Pearl GTL plant in Qatar is ramping up to full production.
"The availability of a new fuel in our industry is very rare," says Shell Aviation general manager operations/technical and supply Gary Woodward. "We need new fuels to bring a diversity of supply options."
A focus for Shell's research is understanding what local air quality benefits at airports GTL brings due to its cleaner burning characteristics, says Woodward. "The fuel is purely paraffinic - it has no aromatics - so it burns with no soot."
This means it produces less particulate matter, which reduces sulphur dioxide emissions.
Ground testing of a GTL-fuelled Pratt & Whitney engine is under way in the Netherlands and flight tests, using Delft University's fully instrumented P&W Canada JT15D-powered Cessna Citation, are due to start in a couple of months.
These tests will help establish the "sweet spot" for the mix of GTL and oil-based kerosene with respect to emissions, says Woodward. "The blend could be anything from 20% up to 50%," he says, adding that results are expected over the next 12 months.
GTL kerosene has a lower specific density than conventional fuel (around 0.74g/litre against 0.8g/litre) so should provide up to 5% higher energy density per weight.
However, full advantage cannot be taken of this as the global kerosene standard sets a minimum specific density of 0.75g/litre. "We are going to challenge this as we don't believe the lower density has any impact on combustion characteristics," says Shell.