What kind of wonder fuel would it take to conveniently and inexpensively power a quiet, environmentally benign automobile engine? How about . . . gasoline?
Perhaps the biggest reason for the continued survival of gasoline-powered engines, despite all the pollution they cause, is simple human inertia. Why, after all, would anyone buy an electric car or use an alternative fuel when 100,000 gas stations line American roads? But this may soon change. A team of engineers has developed a system to power an electric vehicle cleanly and silently and as conveniently as with gasoline. That’s because the engineers have found a way to generate electricity from the chemical breakdown of gasoline.
To a certain extent, the internal combustion engine also putters along via a chemical reaction—air and fuel ignite when exposed to a spark. The heated gases expand against pistons mechanically linked to the vehicle’s wheels. But after losses to wasted heat, mechanical friction, and incomplete burning, a mere 15 to 20 percent of the possible energy in the fuel ends up moving an automobile.
Battery-powered cars tend to be much more efficient but require several hours of recharging after they run down instead of a few minutes at a fuel pump. To get around this problem, many researchers are working on various sorts of fuel cells, which are essentially batteries with a fuel source—usually hydrogen—that continually recharges the battery. nasa has used them for decades. In a typical fuel cell electrons stripped from hydrogen provide electricity. The fuel cell’s chief waste product is water vapor, produced when the hydrogen reacts with oxygen.
But fuel cells have their own problems. The cells typically contain expensive metal catalysts to pull electrons off hydrogen, and the process can be gummed up by contaminants. Perhaps the biggest drawback is that existing fuel cells can work only with a few fuels, such as hydrogen and methane, both of which are difficult to store and dangerous to handle.
Back in 1991, Jeffrey Bentley, a mechanical engineer working for the consulting firm Arthur D. Little in Cambridge, Massachusetts, surveyed the state of research on hydrogen-powered fuel cells. What would be ideal, he realized, would be to run a fuel cell with gasoline, a liquid that’s easy to find, easy to use, and cheaper per gallon than beer. Bentley still hasn’t found a way to do that, but he and his colleagues have done the next best thing—they’ve found a way to make hydrogen from gasoline and use the hydrogen to power a fuel cell.
Petroleum, says Bentley, makes a pretty good hydrogen storage tank. Gasoline molecules have lots of hydrogen atoms bound to rings and chains of carbon. So if you start with a typical molecule of gasoline with 14 hydrogen atoms on its 6 carbons, all you have to do is mate 12 oxygen atoms to the carbons and—voilà—you get a mixture of carbon dioxide exhaust and enough hydrogen to run the fuel cell. Instead of the smog-producing nitrogen oxides, ozone, and particulates that bellow out of the standard tailpipe after combustion, Bentley’s dream was to convert gasoline into CO2, water, and power.
But there are a wide variety of ways to go wrong, Bentley says. If you just heat gasoline up with a bunch of catalysts, you get goo. Or you can use too much of your fuel just running the process. It took years to perfect. Bentley found that by carefully heating gasoline in a steam-filled, fuel-rich, oxygen-poor environment, he could tear off hydrogen atoms from the petroleum. In a conventional engine, droplets of fuel burn up in an oxygen-rich environment. But in Bentley’s device the oxygen atoms are so scarce that they don’t burn but only bind to the carbons in the gasoline, creating a mixture of simple gases like carbon dioxide, carbon monoxide, and hydrogen, plus some compounds of sulfur, which commonly contaminates petroleum. The hydrogen gets pumped into a standard fuel cell to power the car. The sulfur can be scrubbed out and the CO2 is inert, but the carbon monoxide is pure trouble—if it gets into the fuel cell it gloms onto the electrodes that separate the hydrogen from its electron, rendering the whole cell useless.
Fortunately, researchers at Los Alamos National Laboratory in New Mexico had been studying the same problem and came up with a solution. Much as a catalytic converter destroys unburned fuel in exhaust, the Los Alamos technology uses beds of heated metal catalysts to get the carbon monoxide to react with other gases to form carbon dioxide. The device, called a preferential oxidizer, has to be designed carefully or it winds up burning lots of hydrogen or even generating more carbon monoxide.
This gets complicated for an automotive application because you have to do this at different flow rates, different mixtures, and different temperatures, says Nick Vanderborgh, a chemical engineer at Los Alamos. I think many researchers can do what we did under steady-state conditions, but the trick is to maintain the reaction under all these complex conditions. When the gas stream runs through the oxidizers, carbon monoxide concentrations drop to less than 20 parts per million, well below the level that poisons the fuel cell. The mixture of hydrogen-rich gases can then safely pass into the fuel cell to power the car.
In fact, the result pretty much matches Bentley’s original dream: from gasoline, this system generates mostly water, carbon dioxide, and electricity. Of course, carbon dioxide is a greenhouse gas—but per gallon of fuel, this system doesn’t put out much more CO2 than conventional engines. And despite energy used up in running the Los Alamos oxidizer, Bentley says the hydrogen pulled from gasoline can provide twice as much power as burning gasoline in a conventional engine. This means a car could get about 80 miles per gallon of gas, resulting in a net cut in greenhouse emissions. What’s more, the system can handle alternative fuels such as alcohol and methane without any adjustments, making the transition from a fossil fuel to a renewable one much less traumatic for car-addicted Americans.
Even before this technology was unveiled, automakers in the United States and Germany were building prototype fuel-cell-powered vehicles with an eye toward starting production sometime in the next decade. Now that motorists can top off their fuel-cell tanks with gasoline rather than methane or hydrogen, the switch to fuel cells may come much more quickly.
