Today’s world might run on fossil fuel, but many people predict that hydrogen will fuel the future–in cars, houses, and countless handheld electronic devices. Hydrogen-powered fuel cells (SN: 9/7/02, p. 155: Pocket Sockets) can generate electricity much more efficiently than fossil fuel can and without spewing polluting byproducts such as nitrous oxides, which contribute to smog, and carbon dioxide, the most prevalent gas behind global warming.
“All you do is generate water,” says engineer Bruce E. Logan of the Pennsylvania State University in State College. “Who can argue with water coming out of tailpipes?”
Yet there’s a big cloud hanging over this sunny image of the fossil-fuel-free future: The main source of hydrogen at the moment is the hydrocarbon molecules in fossil fuel. That has to change, says Logan. Not only does the use of fossil fuel for making hydrogen create pollution, but fossil fuel eventually will run out.
“Right now, we can produce hydrogen,” says Logan. “Can we do it with a sustainable method? No.”
That’s why Logan and others are trying to find alternative sources of hydrogen. Among these are renewable fuels, such as crops, agricultural detritus, and factory wastewater. Some researchers are even turning to dirt containing hydrogen-generating microbes. The success of the search could well determine whether hydrogen’s promise as the clean fuel of the future will be fully realized.
Hydrogen world
The first and simplest element on the periodic table, hydrogen is colorless, odorless, and tasteless. It’s the most common element in the galaxy, but frustratingly difficult to make on Earth without using fossil fuel.
Nature is rich in hydrogen. It turns up throughout animal and plant tissue and fossil fuel, but breaking the element free is generally difficult. Water, for example, can split into hydrogen and oxygen when electricity passes through it. Unfortunately, on large scales, this seemingly straightforward process isn’t yet economical. “And we are far, far, far away from it,” says chemical engineer Jens Rostrup-Nielsen at Haldor Topsoe in Lyngby, Denmark.
“Of course, the ideal would be to split water, but you need energy to split water, and where do you get the energy from?” says Rostrup-Nielsen. “Today, no doubt, the most economic way of producing hydrogen is from fossil fuels.”
Producers generate some 45 million metric tons of hydrogen globally each year from fossil fuel. Almost half of this hydrogen goes to making ammonia, NH3, a major component of fertilizer and a familiar ingredient in household cleaners. Refineries use the second largest chunk of hydrogen for chemical processes such as removing sulfur from gasoline and converting heavy hydrocarbons into gasoline and diesel fuel. Food producers use a small percentage, adding hydrogen to some edible oils in a process called hydrogenation.
To make hydrogen, Haldor Topsoe and other companies usually employ a method called steam reforming. Vaporized fossil fuels, primarily natural gas, mix with steam at high pressures and temperatures with assistance from a nickel-based catalyst. The reforming technique yields hydrogen, but it also gives off carbon monoxide and carbon dioxide, the primary greenhouse gas.
Such hydrogen generation from fossil fuel is the first step toward a new hydrogen economy, says Rostrup-Nielsen.
Logan explains that although this approach still generates the pollution people are trying to avoid, those gases are released in a potentially more manageable way–in the reforming plant rather than in millions of mobile car engines.
Nonetheless, shedding the habit of fossil fuel entirely is the only way a wholesale shift to hydrogen will work in the long term, Logan says.
One approach to this goal is to apply steam-reforming methods to alternative renewable materials, says Esteban Chornet, who works at the National Renewable Energy Laboratory in Golden, Colo. Such materials might be derived from crops. Other scientists are experimenting with ponds of algae that use sunlight-driven reactions to make hydrogen (SN: 2/26/00, p. 134: Power plants: Algae churn out hydrogen). Yet others are considering innovative ways of electrolyzing water for large-scale hydrogen generation.
Logan thinks that converting biological waste, such as the sugar and starch in candy- or soda-factory wastewater, is a good way to go. Chemical engineer James A. Dumesic of the University of Wisconsin–Madison is focusing on the byproducts of his state’s corn, cheese, and paper production to make hydrogen.
Not only do these biomass-conversion schemes turn trash into a valuable product, but the researchers say there’s another plus: Any carbon dioxide released in the processes could be soaked right back up by the planting of new crops to provide the needed biomass.
A biomass strategy of hydrogen generation could be a useful intermediate step between the current fossil fuel method and the dream of efficient water splitting, says Rostrup-Nielsen. Still, any realistic contender for hydrogen generation must first knock out the reforming of fossil fuel as the cheapest and most efficient process, says Chornet.
That’s not going to be easy.
Waste to fuel
About 5 years ago, Logan was taking a walk in his town, State College, thinking about new research projects for his environmental engineering lab. He realized that working on an alternative method of hydrogen generation appealed to him. The work not only involves the kind of science he knows a lot about, he says, but it could help solve the world’s energy conundrum.
“Right now, it’s dirt cheap to reform a fossil fuel into hydrogen,” says Logan. Pursuing an even cheaper, more environmentally friendly method, he’s actually turned to dirt–that is, the soil microbes that can generate hydrogen from sugars and starches.
“These hydrogen-producing bacteria are everywhere,” Logan says. “You go outside, grab a bucket of soil, and they’re there.” Using bacteria to ferment biological waste is not a new idea, but in the June 1 Environmental Science and Technology, Logan and his coworkers from Penn State and the KwangJu Institute of Science and Technology in Korea reported that it’s easier and more efficient to produce hydrogen in this way than other scientists had predicted.
The researchers found they could easily segregate hydrogen-generating bacteria from those that consume hydrogen. When they heated some ordinary soil–taken from a local tomato plot–for 2 hours at a temperature just above water’s boiling point, the hydrogen-consuming microbes died off. However, bacteria that generate hydrogen survived because they can form heat-resistant spores. “You don’t need some specialized bacterium or genetically engineered bacterium in some science professor’s lab,” Logan says.
The researchers then mixed the tomato-plot dirt in an enclosed reactor with sugar water to represent wastewater from a food-production plant. It looked like “dirty river water,” says Logan, but the concoction generated gas that was about 60 percent hydrogen.
Logan and his coworkers also found that similar fermentation experiments done by other research groups probably had unwittingly hindered hydrogen generation. Those researchers had collected hydrogen from their reactors only intermittently rather than continuously as Logan’s group had done. Letting the gas build up seems to suppress hydrogen production, says Logan. Culling it continuously from a reactor yields 43 percent more hydrogen.
Although Logan and his coworkers haven’t yet completed studies on actual wastewaters from food manufacturers, Logan says his team’s preliminary results indicate that common sugar- or starch-bearing wastewaters can be used to generate hydrogen in this rather simple way. What’s more, he says, this kind of biological method–which relies on bacteria and sugar- or starch-rich crops–has an advantage over, say, algae-based production, because it doesn’t require large ponds for collecting the sunlight that drives the hydrogen-generating chemistry.
Technical challenges remain, of course. For example, the researchers need to improve the hydrogen yield of their process, and they need to scale it up for commercial use. Moreover, says Chornet, in any biological process, researchers still must determine whether the input has components that will be toxic to the bacteria or limit their efficiency.
Dumesic and his colleagues use a metal catalyst rather than microorganisms. The Wisconsin researchers were working on a project unrelated to hydrogen generation when they realized that oxygenated hydrocarbons could release hydrogen at more modest temperatures than those required in the steam reforming of fossil fuel. In the Aug. 29 Nature, the team reports that a platinum catalyst on an aluminum oxide base can be used to make hydrogen from glucose. The reaction also produces carbon dioxide and methane, and the latter might be burned to generate more energy, Dumesic suggests.
The method is much like the steam reforming process, but it occurs in liquid water at high pressures and moderate temperatures around 250C, instead of in much hotter vapor.
Although Dumesic and his coworkers used ordinary sugar and other laboratory supplies in their experiments, they hope eventually to turn to agricultural waste materials, such as cheese whey and corn stover. Other Wisconsin industries, including paper manufacturing, also create waste likely to be useful, Dumesic says.
The researchers still need to show that their catalyst works with bone fide sugar-containing fluids, not just model solutions in the laboratory, cautions Chornet.
Moreover, they must prove that the catalyst doesn’t deactivate with extended, real-world use.
The process also needs several improvements if it is to become commercially viable.
For one thing, the glucose quickly decomposes in solution into other products before reacting on the catalyst to produce hydrogen efficiently. Another problem: Platinum-based catalysts don’t come cheap.
To solve these problems, Dumesic and his coworkers are improving the reactor design and identifying more-active metal catalysts.
“Now, I think the challenge is to find better catalysts that are even more active or are based on cheaper components,” Dumesic says.
The new frontier
Demand for hydrogen in the next decade–both for traditional uses, such as making ammonia, and for running fuel cells–is expected to accelerate, says Rostrup-Nielsen. In fact, many car manufacturers already have produced prototype vehicles powered by hydrogen fuel cells. At least in the near future, this thirst for hydrogen will be quenched primarily through the use of fossil fuels.
Nontechnical issues also have a bearing on whether and how soon a hydrogen economy weaned from fossil fuel comes on line. For example, new regulations and cost-cutting legislation, such as tax credits, could help make alternative methods of hydrogen generation more financially attractive to industry.
But when will an appropriate technique be ready for use? If more funding were put into the research, it could come sooner, Logan says. Nonetheless, he predicts, “in the next 10 years or so, I think we’ll have figured out much more efficient and better ways to do this.”
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