Cool fuel for hypersonic aircraft

Keep an eye on the seat-back display during the first 20 minutes or so after takeoff, and you’ll see that as your flight climbs, the air temperature plummets. At altitudes near 10.5 km (roughly 35,000 feet), the outside temperature hovers near –50 °C. For a common commercial plane traveling around 920 km/hour, that bitter-cold air is enough to cool the exterior of an aircraft as it cuts through the sky. An ultrafast military jet traveling at more than three times that speed, however, creates such friction with the surrounding air that those frigid temperatures don’t do enough to cool the aircraft and prevent possible damage to equipment.

The solution? “Use the fuel as the primary coolant,” says Christopher E. Bunker, a fuels specialist at the Air Force Research Laboratory (AFRL) at Wright-Patterson Air Force Base in Ohio. “It sounds crazy, like putting gasoline in your radiator,” he acknowledges.

But running cold, yet flammable, jet fuel through heat exchangers—in much the same way coolants like ethylene glycol flow through a car radiator to carry heat away from a car engine—is old hat. That kind of “conventional” jet cooling worked for aircraft such as the SR-71 Blackbird, a now-retired U.S. Air Force reconnaissance jet that flew for 30-some years and reached top speeds of just over Mach 3 (more than 3,500 km/hour). Its fuel cooled the aircraft, then went on to burn, propelling the jet in the usual way once the fuel had heated up.

But that cooling strategy still does not provide sufficient thermal protection for aircraft bulleting at hypersonic speeds, generally defined as greater than Mach 5 (more than 6,100 km/hour). Research vehicles operated by the U.S. Air Force have reached that velocity in speed tests for brief periods.

For sustained flight at those speeds, which is a goal of the U.S. and other countries, a more advanced fuel is needed. Scientists believe that a hydrocarbon fuel that additionally sucks up heat via heat-absorbing, or endothermic, chemical reactions might do the trick.

The basic idea is to use a heat exchanger to transfer heat from the parts that need to be cooled to a hydrocarbon fluid that starts out cold. As fuels engineer Matthew J. DeWitt of the University of Dayton Research Institute explains, when the fuel is used in this way, it grows hotter and eventually reaches a temperature that triggers endothermic chemical reactions such as dehydrogenation and cracking, in which hydrocarbons break into simpler units.

At that point, the fuel continues functioning as a heat sink but does not continue to climb in temperature. Rather, the absorbed heat chemically converts the starting compound to other, simpler hydrocarbon products that can be used as fuel for propulsion. Analogously, transferring energy to liquid water causes the water temperature to rise until it reaches the boiling point. The water can continue absorbing energy at that temperature, but the energy input does not cause a further temperature rise; it instead drives evaporation.

The concept of endothermic fuels isn’t new. In fact, one of the most frequently cited papers in this field dates back to the 1960s (Ind. Eng. Chem. Prod. Res. Dev. 1966, DOI: 10.1021/i360017a018). But no one is flying a jet that’s cooled this way today. Despite the long research track record, many scientists continue trying to design endothermic fuels that will work as envisioned. Those scientists include several at AFRL whose work is supported by the U.S. Air Force, as well as many working for air forces in other countries. Some of them, including DeWitt, were on hand at last month’s ACS national meeting in New Orleans, where they presented their latest investigations of hydrocarbon fuels for hypersonic flight. These researchers are studying jet fuels and simpler model compounds to determine how much energy they absorb by undergoing cracking or dehydrogenation reactions under intense conditions encountered in jet fuel lines. They are also examining the fuel worthiness of the products and searching for catalysts that can guide reactants along preferred chemical routes.

To design better endothermic fuels, DeWitt and coworkers specifically want to know whether the shapes of molecular fuel components affect performance. So they compared a commercial mixture of normal alkanes in the C₁₀–C₁₅ range with a commercial mixture of highly branched alkanes in the same molecular weight range. These mixtures serve as surrogates that are simpler than but similar in molecular weight to those found in Jet Propellant 7 (JP-7), which fueled the SR-71 Blackbird and other military aircraft.

To evaluate the fuels, the researchers fed them through pressurized reactors as they ramped the temperature up to roughly 700 °C to mimic jet fuel-line conditions. They monitored the fluids’ flow rates, temperature rises, energy uptake, and other parameters. Then they used gas chromatography/mass spectrometry to analyze the products formed by thermal decomposition in the absence of a catalyst.

Among other observations, the Dayton researchers found that at temperatures exceeding approximately 500 °C, both mixtures do, indeed, function as endothermic fuels. The group determined that in addition to providing conventional cooling, which causes their temperatures to rise, the mixtures each provided additional cooling—an endothermic heat-sink bonus—of more than 30%, with the straight-chain mixture slightly edging out the branched one.

The researchers also probed the accumulation of a layer of carbonaceous material known as coke. This film, which often forms as a result of endothermic hydrocarbon reactions, has a bad reputation because it gunks up surfaces. In a working fuel system, for example, it could block flow through fuel lines, plug fuel injectors, and possibly lead to engine failure.

The analysis showed that neither fuel sample produces much coke at relatively low temperatures. But as the temperature climbs and starts activating chemical reactions, coke starts to build up. The study also showed that the compounds differ in their tendencies to form coke as temperatures rise. For normal, straight-chained alkanes, the onset comes earlier and builds gradually. In contrast, branched alkanes delay awhile, then start making copious quantities of the unwanted stuff, likely through reactions of alkenes and aromatics that form polycyclic aromatic coke precursors. The results confirm that the components of JP-7 can undergo heat-absorbing reactions but underscore the complexity of this cooling strategy: A class of compounds that is a little better at cooling may be more troublesome when it comes to forming coke. And the tendency to form coke can change quickly with increasing temperature.

Figuring out, as the Dayton group did, what happens to fuels when they get hot enough to undergo cracking in the absence of a catalyst is key to designing a viable endothermic fuel system. But with the right catalyst in hand, engineers may be able to steer the cracking process along a reaction path that increases cooling and generates better fuel products or ones less prone to coking.

That’s what motivated University of Virginia catalysis specialist Robert J. Davis and coworkers to study zeolite Y and its effect on the pyrolysis of JP-10. Zeolite Y is a porous aluminosilicate catalyst used commercially for petroleum cracking. JP-10, exo-tetrahydrodicyclopentadiene, is a high-energy, nearly single-component hydrocarbon fuel used in specialty jet-propulsion applications. Because of its compositional simplicity, JP-10 is often used in combustion research.

The Virginia group studied JP-10’s cracking chemistry with and without a zeolite Y catalyst under a wide range of intense reaction conditions. In the absence of the catalyst, thermal cracking converted JP-10 to numerous products, the predominant ones being cyclopentadiene and cyclopentene. In the presence of the catalyst, however, the fuel reacted to form naphthalene and substituted indenes. The catalyst also slightly decreased JP-10’s endothermic cooling capacity and lowered the cracking temperature by 210 °C (AIAA J. 2017, DOI: 10.2514/1.J056432). In effect, the catalyst protects the fuel by preventing it from reaching the temperature at which hard-to-control thermal reactions take off.

A large fraction of the research on endothermic fuels published nowadays in chemistry journals comes from scientists in China. Guozhu Liu and coworkers at Tianjin University also studied the effect of zeolites on JP-10 cracking. Rather than studying zeolite Y, the Tianjin researchers focused on an acidic aluminosilicate catalyst called HZSM-5 and sought to control the cracking process by fine-tuning the structure and composition of the zeolite.

The team used a customized synthesis procedure to grow nanosheets of HZSM-5 with Si/Al values of 25 and 50 (dubbed ZNS-25 and ZNS-50, respectively). In reactor tests, the researchers compared the nanosheet zeolites with conventional bulk zeolites with the same composition.

Under comparable reaction conditions, the nanosheets were more active at converting JP-10 than their bulk counterparts, as were the zeolites with the higher aluminum content—making ZNS-25 the best of the bunch. Boosting fuel conversion means driving more endothermic reactions, which in principle should provide more endothermic cooling (Energy Fuels 2017, DOI: 10.1021/acs.energyfuels.7b02397).

The explanation is that nanostructuring shortens the length of the zeolite channels and exposes more surface area for cracking. Shortening the channels enhances diffusion of reactants and products, especially ones the size of JP-10. And boosting the aluminum content increases the number of catalytically active acidic sites, which in the case of the nanosheets reside on the surface where they are exposed and accessible to reactants.

Others have gone a step further in studying HZSM-5 as a potential catalyst for endothermic fuels. Yu-Hao Yeh, Raymond J. Gorte, and coworkers at the University of Pennsylvania designed a reactor to measure heat flows as n-hexane reacted under intense conditions found in ultrafast aircraft on the surface of HZSM-5 and its zinc-enriched counterpart. The zinc-doped form is known to catalyze aromatizations. The results tell a cautionary tale.

The researchers found that when the reactor is operated in a way that converts only a small fraction of hexane, the reactions on HZSM-5 are only mildly endothermic. At conversions above 50%, the situation is even worse: Not only is there no cooling benefit, but the reactions are actually exothermic, releasing heat.

Like a helpful crossing guard at a confusing multiway intersection, Zn-HZSM-5 does a better job than the zinc-less form at guiding the reaction down an endothermic path. The zinc-driven reactions are endothermic for conversions of less than 70%. At higher conversion rates, the reactions become exothermic (Ind. Eng. Chem. Res. 2017, DOI: 10.1021/acs.iecr.7b01006).

The Penn team’s analysis shows that the endothermicity observed with Zn-HZSM-5 at low hexane conversions is likely due to formation of benzene, toluene, and xylene. High conversion conditions probably lead to higher-molecular-weight species, which alter the heat flow.

Clearly n-hexane isn’t the right fuel for endothermic cooling, but its chemistry spells for catalyst designers the main take-home message: Pay attention to product distribution, not just fuel conversion.

The bulk of research in endothermic fuels focuses on cracking chemistry typically mediated by zeolites. But there are other promising options. Dehydrogenations, for example, can also soak up a lot of heat, and platinum catalysts drive those reactions enthusiastically. The problem is, platinum’s activity tends to fall quickly as coke accumulates and blocks the catalysts’ active sites.

To understand why that happens and how to prevent it, Scott L. Anderson of the University of Utah and Anastassia N. Alexandrova of the University of California, Los Angeles, decided to focus on a model system: ethylene dehydrogenation on size-selected platinum clusters containing just a few atoms. The researchers chose that system because it could be analyzed in great detail computationally and they had experience making and probing the clusters experimentally.

The team knew from earlier work that Pt₇ clusters are more active in this reaction than clusters with four or eight platinum atoms but that Pt₇ deactivates more quickly than the others via coke buildup. So the researchers experimented by modifying the clusters with boron or tin. These tweaks weakened the clusters’ affinity for ethylene and reduced the catalyst’s propensity to become fouled with coke. It follows then that a well-designed catalyst could dehydrogenate an alkane fuel and release the alkene product before it undergoes subsequent coke-forming dehydrogenations (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b05894).

For more than half a century, researchers have searched for ways to cleverly exploit the thermodynamic properties of a hydrocarbon fuel so that it can double as a coolant for hypersonic aircraft. The idea that an onboard fuel could mitigate the extreme heat buildup associated with ultrafast aviation is tantalizing. The concept is simple. Taming the complex chemistry is another story entirely.