Methanol: The New Hydrogen
Hydrogen has been getting plenty of hype as a potential replacement transportation fuel, for cutting carbon dioxide emissions and reducing dependence on fossil fuels. But methanol would be far better than the more reactive and volatile hydrogen, argues George Olah, a chemist and Nobel laureate, in a new book, Beyond Oil and Gas: The Methanol Economy.
Olah notes that methanol, a clean-burning liquid, would require only minor modifications to existing engines and fuel-delivery infrastructure (see “The Methanol Economy”). And manufacturing it could even make use of carbon dioxide, a source of global warming. Methanol’s benefits have long been understood – now recent advances in methanol synthesis and methanol fuel cells could make this fuel even more attractive.
Currently, about 90 percent of the worldwide production of methanol (CH3OH) is derived from methane (CH4), the main component of natural gas. Today’s methods of making methanol have two stages: converting methane into syngas, a mixture of primarily carbon monoxide and hydrogen, and then into methanol. Although these steps have become more efficient over time, the elimination of the syngas step could save money, since it currently accounts for up to 70 percent of the cost of making methanol.
In an effort to eliminate this cost, Olah and his colleagues have explored ways of converting methane directly into methanol. “You take methane and stick in just one oxygen atom,” says Olah, director of the Loker Hydrocarbon Research Institute at the University of Southern California (USC). “Easily said, but not so easily done.” The problem is that methane is chemically inert, and combines readily with oxygen only at high temperatures. A catalyst helps, but commonly used catalysts themselves work only at 300 degrees Celsius or higher. At these temperatures, most of the methanol produced is oxidized to carbon dioxide and water. Indeed, methanol yields from such reactions can be as low as 2 percent.
Recently discovered lower-temperature catalysts offer better yields, says Roy Periana, associate professor of chemistry at USC. Using a platinum-based catalyst dissolved in concentrated sulfuric acid at 200 degrees Celsius, Periana has achieved a methanol yield of more than 70 percent. He’s now looking for less expensive catalysts, and has found some promising ones.
Olah and his colleague Surya Prakash, professor of chemistry at the university, have developed an alternative method for converting methane to methanol, using a halogen such as bromine. In the presence of special catalysts and at less than 250 degrees Celsius, methane reacts with bromine to form methyl bromide (CH3Br) and hydrogen bromide (HBr). Methyl bromide then reacts with water to form methanol. The bromine from the hydrogen bromide can be recovered by reaction with air, and reused.
Making methanol from natural gas – which still involves fossil fuels and increases carbon dioxide in the atmosphere – is just the first step, says Olah. Chemists have long known that methanol can be made by combining carbon dioxide and hydrogen. Such a process requires considerable energy, for example, to harvest the hydrogen from water, but this energy could come from carbon-free sources such as nuclear or wind power. The carbon dioxide could be captured from flue gases, and eventually directly from the atmosphere, he says.
In such a system, the carbon dioxide released by burning methanol would be cancelled out by the carbon dioxide captured to make it. So the process would be carbon neutral, and the methanol produced would be a convenient liquid fuel that could replace petroleum-based fuels. If the carbon dioxide comes from air and the hydrogen from water, this method of making methanol would be like fast photosynthesis: “We don’t have to wait for plant life to slowly convert excess carbon dioxide into hydrocarbons,” Olah says. “We can substitute for Mother Nature.”
Olah emphasizes that the methanol produced in this way would not be a new energy source, but simply a convenient way of storing energy. Its advantage over hydrogen would be the ability to use existing engines and infrastructure with only minor modifications.
In many ways, with its low emissions and an octane rating of 100, methanol is already a better fuel for internal combustion engines than gasoline. A methanol engine can run at a higher compression ratio, and is easier to cool. But methanol has some drawbacks: it has lower vapor pressure than gasoline, which makes engines sluggish on cold starts, and it burns with an invisible flame, which could be a safety hazard, since it would be hard for emergency workers to detect in an accident, for example. To mitigate these problems, methanol today is usually blended with 15 percent gasoline to make a fuel mix known as “M85.”
Methanol is an even better automotive fuel when used in combination with fuel-cell technology, says Paul Erickson, assistant professor in mechanical engineering at the University of California, Davis. Fuel cells, which convert chemical energy directly into electricity, are more efficient than engines that burn fuel. The hydrogen fuel cell, in particular, has been widely proposed as a clean and efficient alternative to gasoline-powered internal combustion engines. Erickson’s laboratory has a functioning hydrogen fuel-cell bus with an onboard reactor that “reforms” methanol to produce hydrogen for its fuel cells. “We completely avoid having to store hydrogen,” Erickson says.
Onboard “reforming,” however, consumes space and energy. In 1993, Prakash, Olah, and a team at the Jet Propulsion Laboratory in Pasadena, CA, jointly invented a fuel cell that runs directly on a mixture of methanol and water. The cell’s positive and negative electrodes are separated by a membrane designed to allow only protons from the methanol to migrate from one electrode to the other. Early versions of this membrane, however, allowed some methanol to get across and react with oxygen at the second electrode, which reduced the voltage of the cell and wasted energy in the form of heat.
In 2001, Prakash and his colleagues developed a new membrane that is both cheaper and more resistant to crossover. With this refinement, the direct methanol fuel cell gives an efficiency of 35 percent, about twice that of an internal combustion engine, but well short of its theoretical efficiency of 97 percent.
The direct methanol fuel cell is currently too expensive to be used in passenger cars. Its high cost comes mainly from the platinum and ruthenium used as catalysts. Prakash and others are developing a variety of approaches to reduce the amount of catalyst needed: making the catalyst more active, increasing its surface area, and using nanoscale methods. When this technology matures, Erickson believes it might replace the hydrogen fuel cell. “An inexpensive, high-power direct methanol fuel cell is the Holy Grail,” he says.
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