Oil shortages and price rises are once again stimulating interest in the search for alternative fuels for aerospace


Fuel is a necessary evil to the aerospace industry, particularly the airlines. It is increasingly expensive, it generates environmentally harmful waste products and even causes accidents. And ultimately, the supply of crude oil that now provides the life blood of the industry will run out.

Fuel crises have traditionally provided the incentive for improvements in efficiency, reduced emissions and the driving down of dependency on volatile fuel supplies. By contrast, the search for direct or quasi-alternatives to conventional fossil-based JET-A and A1 has never been so consistent. Tests of a promising hydrogen-powered aircraft engine were conducted in the USA during the late 1950s on a Martin B-57, and various attempts to develop viable, full-scale hydrogen-based fuel systems were pushed during the 1980s and 1990s in the former Soviet Union and latterly in Germany.

Tupolev developed and flew a liquid hydrogen-powered testbed derivative of the Tu-154, dubbed the Tu-155, in 1988 and planned a fully-fledged passenger version called the Tu-156. One of the Tu-155's three conventional engines was replaced by a hydrogen-fuelled Kuznetsov NK-88 engine, a modified version of the aircraft's standard NK-8s. The tests proved so encouraging that Soviet and German government officials signed a pact in May 1990 to develop the technology for a liquid methane or hydrogen-powered Airbus A300 demonstrator by 1996. The project, like so many others, fell victim to funding and technical problems, compounded by the break-up of the Soviet Union.

Now NASA's Revolutionary Aeropropulsion Concepts Programme is considering an alternative fuels research effort, ostensibly for high mach airbreathing propulsion, but with potential for future subsonic applications. "We're putting a lot of thought behind it," says Chi Ming Lee, chief of the combustion branch at NASA's Glenn Research Center in Ohio. "There will be an emphasis on cleaner combustion combining 'designer fuels' with advanced combustor concepts." The scheme is aimed at developing a new fuel, or fuels, from hydrogen, methane, ethanol and methanol.

The ultimate scheme would see today's petroleum companies producing "designer" fuels to order. "We would say, order a fuel for low emissions or one for high thrust," says Lee. NASA's approach, should funding be granted, will be a step-by-step programme beginning with fundamental research into the thermodynamic properties of each candidate fuel. "First we will find the ones with the best structure. Then we will work with the petroleum companies to produce some for us to use in the laboratory. We will identify if the fuel is safe, and whether it has sufficiently low enough emissions. Once we confirm that, we will work with the engine makers to take it further."

Lee concedes the idea is not new, but says the recent leap in oil prices is starting to focus attention on the alternative fuel debate once again. "The crude oil price has been so low for such a long time, but with it going to $35-$40 per barrel we are starting to have some contact with the petroleum companies. They want to collaborate with us," he adds. With the bulk of NASA's on-going work on reduced emissions, the focus of the project will still be aimed at ensuring lower levels of nitrous oxides (NOx), reduced carbon dioxide (CO²) and "aerosols in particular" in this case, says Lee. "We have a good idea of what we want to achieve, but it will definitely be a trial and error process."

NASA estimates it will be as long as three years from the start of the programme before the ideal fuel structure is confirmed.

Using hydrogen

After this initial phase, it will take another year of laboratory-scale work before the results will be available to take to the industrial trial level. "Then it will take around a further three years with them to develop the fuel," says Lee. Although the prospect of a seven or eight year project may seem overly long, Lee adds, "That's nothing: it took the American Petroleum Institute [API] 20 years to approve an additive".

Aside from generations of earlier research work on cryogenic fuels for aircraft, another NASA project likely to benefit the new programme is the Zero CO² emissions (ZCET) programme for gas turbines. Like the Revolutionary Aeropropulsion Concepts programme, the basic aim of ZCET is to reduce emissions and improve efficiency, rather than specifically to develop alternative fuels. The use of hydrogen is therefore seen as more of a means to an end, rather than the main aim of the project. Hydrogen, after all, has several major disadvantages including the cost of manufacturing and storing it, the high volume it occupies and the safety problems associated with its use. High costs could be tackled by improvements in infrastructure as the use of hydrogen fuel spreads, but volume and safety problems are not so easily solved. Liquid hydrogen boils at -253°C (-513°F) and any leak or depressurisation results in almost instantaneous vapourisation. Even at extremely low concentrations, the vapour is highly inflammable and can be ignited by the smallest of sparks, creating the need for elaborate fire precautions.

Despite this, NASA ZCET hydrogen combustion manager Tim Smith says the advantages make it an attractive target. Hydrogen produces water as a by-product of combustion and does not generate CO², the main "greenhouse" gas targeted for reduction by the Kyoto protocol. Hydrogen also has high reaction rates, high heating value and is available in unlimited amounts. The three-year ZCET effort is therefore focused on developing hydrogen as both a fuel and as the basis for fuel cells. "We're looking at the Boeing 777-type scale of use for liquid hydrogen, and all sorts of packaging issues that go with that," says Smith.

The first focus for the fuel element of the ZCET research effort is the development of advanced fuel injectors that provide quick mixing, low emissions and high performance. "Hydrogen burns really easily, that's never a problem," says Smith. "But because it burns so easily, the temperatures are high and - as NOx is a function of temperature - hydrogen and air combustion can still produce significant levels of NOx. We could make a great hydrogen burner, but if we are putting out more emissions then it defeats the purpose."

The ZCET work is focused on flame tube tests which simulate engine combustors. "It's a single can type of system and we're using it on new concepts we have in preliminary design," says Smith. To ensure that no stone is left unturned, NASA is working with combustor manufacturers on these new designs, he adds. The target is a reduction to zero CO2 and a cut in NOx by a factor of three within 10 years. This also includes the overall targets of NASA's much wider Ultra Efficient Engine Technology (UEET) programme which aims to reduce NOx emissions by 70% by 2016 compared against a 1996 baseline standard set by the International Civil Aviation Organisation.

Although Smith believes the emissions targets are achievable, the main challenge is developing combustors that will have to operate at far leaner mixture conditions than in current engines. The design will have to be adapted to reduce the increased chance of lean blow-outs "but the area of biggest concern is flash back where the flame wants to go too far back up the nozzle and burns in places it should not," says Smith. Lee adds: "Our goal is to get the flame tube activities up to TR 3 or 4 [technology readiness level], and our success and budget grows from there."

Work on the fuel cell meanwhile continues in parallel. Hydrogen will be one of the reactants in the cells which is likely to consist of a carbon anode and cathode immersed in an electrolyte. Both electrodes contain platinum, or similar, which will act as a catalyst, breaking down oxygen molecules into atoms as the gas bubbles pass the cathode. Here oxygen will absorb electrons while combining with water to form hydroxide ions. At the same time hydrogen will give up electrons at the anode as it combines with the hydroxide ions to form water. The resulting current flowing between the electrodes will be used to drive a geared fan.

"The fuel cell cannot provide enough thrust for take-off," says Lee, "so we could combine it with a hydrogen powered turbofan for take-off and use the fuel cell for cruise only." Part of the study is addressing the "break point" at which fuel cell technology will be appropriate for future aero engine power levels. "They are still heavy and they produce water, so you have to look at what you do with that," says Smith. The study is evaluating the concept of fuel cells for aircraft ranging in size from "a Cessna to a 737", with the outcome expected to favour its use on a new generation of light and robotic aircraft.

Shorter-term solutions

While NASA wrestles with the difficulties of perfecting hydrogen-based propulsion technology, others foresee shorter term solutions - new supplies of jet fuel from non-conventional sources such as coal, liquified natural gas, renewables, lampblack and "extenders".

Sasol, a South African petroleum company, was forced to look for alternatives during the oil embargoes of the apartheid era and developed a process for producing kerosene from coal deposits. The fuel is refined from a complex refining process which also produces carbon monoxide (CO) and hydrogen. The resulting kerosene is mixed roughly 50:50 with conventionally refined aviation fuel to produce Jet A. Advanced refining techniques have also been developed to distil aviation fuel from liquified natural gas, though the relative cost makes it hard to justify at current costs. "With oil at around $20 to $22 per barrel, it cost about four times as much to produce one gallon of jet fuel from liquified natural gas as it did from conventional crude oil," says Stan Seto, a fuel expert at General Electric Aircraft Engines.

Renewable sources of synthetic jet fuel provide another intriguing possibility, says Seto. One such scheme is the recycling of old tyres to salvage the lampblack, which is almost 100% carbon. "You get rid of the latex that was rubber, and are left with the lampblack that provides an ideal building block for refining fuel. What's more, there are millions of tyres out there," says Seto. Another group, based at Loughborough University in the UK, has carried out feasibility studies into production of synthetic kerosene from wood grain and soybean. The process is similar to the Sasol refining technique, and generates CO and hydrogen as well as the fuel itself. "It works well, but the problem is that you are starting with much lower specific energy levels, which makes it hard to use in aerospace applications," says Seto.

The environmental impact of synthetic kerosene is relatively benign, however, leading to the possibility of further research. The fuel itself is cleaner, emitting less sulphur and NOx when burned. The developers also believe that, because the wood and soybean absorb CO² whilst growing, the fuel does not contribute any more overall CO² to the atmosphere when burned. Wood supplies are another issue, and researchers point to vast forested areas of Russia as one possible source of material for industrial scale synthetic kerosene production.

Another approach being examined is the use of "extenders" to increase volume and reduce emissions. This was originally developed for automotive fuel, and involves additives such as ethanol, derived from corn, or ETBEs (ethyl tertiary butyl ethers), which are again derived from renewable sources. "People thought it was the holy grail when it was first introduced because it killed emissions when it was used. However, it turned out to be a problem in terms of waste," cautions Seto, who sees similar issues ahead where aviation fuel is concerned. Like extenders and other experimental fuels, Seto sees the bulk of near-term alternative fuel technology emerging in the automotive industry first before migrating to aerospace. "It will almost certainly show up in cars and trucks before it ever gets into an aircraft engine. The big focus is on re-developing the energy-based economy away from crude oil, and that's the more logical place to start."

So how long will the oil reserves really last? "A few years ago I heard we only had 20 years left. Now we're probably looking at 50 to 60 years before we start getting into a crunch," says Seto. The API believes the total remaining recoverable US oil may exceed 200 billion barrels, or 70 years' worth at current consumption rates. The US Geological Survey estimates total worldwide oil fields could produce as much as 2.1 trillion barrels of crude oil.

At current rates of consumption, the API says there is a 95% probability that the world's remaining oil resources will last 63 years and a 5% chance it will last 95 years. Although the true figure lies somewhere in the middle, any airline planner will probably agree that the time has come for major research into alternative aviation fuels to begin.

Source: Flight International