Just when you think you can praise hydrogen as a squeaky-clean replacement for fossil fuels, someone has to spoil the scene by asking pesky questions about pollution.
Today, we think of coal, oil, and gas as "dirty" energy sources because they release heat-trapping carbon dioxide when you burn them, thereby contributing to global warming. Sometimes coal also contains sulfur, which makes nasty, lake-sterilizing sulfuric acid rain. One potential solution: Hydrogen, which makes nothing but benign water vapor when burned.
I've long been a fan of hydrogen, and have blogged in favor of it here . But nothing is perfect (except, perhaps, that classic guitar solo in "Blue Sky" from the Allman Brothers' Eat A Peach), and I'm now wondering about one possible blemish on hydrogen's otherwise shiny facade.
The problem isn't exactly with hydrogen itself, but with the machines we might burn it in. In order to ignite the hydrogen in a typical eco-vehicle's internal combustion engine, you need a spark--as in spark plugs. Such sparks provide the energy needed to oxidize the H2 into H2O, but there's a lot more than hydrogen and oxygen in the atmosphere. We normally think "oxygen" when we draw a breath of air, but nitrogen makes up 78% of it.
We don't use N2 gas for anything directly; it simply visits and exits our lungs as inert filler. But combine it with oxygen in the fiery cylinder of an internal combustion engine and you get "NOx" compounds such as nitrate, which forms nitric acid in raindrops. Such nitrogen fixation happens naturally, whenever lightning sizzles a strip of air. But we're doing it on a huge scale now, in vehicles whose spark-plug explosions amount to miniature lightning strikes .
This isn't all bad. Nitrogen fixation is how nitrogen gas becomes biologically useful, and at least a third of the protein in our bodies is built around artificially fixed nitrogen atoms. To make fertilizer, for example, factories blast hydrogen and nitrogen with electricity to form ammonia, an important plant food that eventually enters our own bodies through the global food web. Without this human-driven fixation, there simply wouldn't be enough active nitrogen available to keep so many billions of people alive.
But you can also have too much of a good thing. Here in the Adirondack Mountains of upstate New York, many of our 3,000 lakes and ponds are still chronically acidified by nitrogen-rich emissions from coal-fired power plants and from the millions of cars and trucks upwind of us in the mid-continent. Ecologists also worry that our forests and lakes face nutritional imbalances from airborne nitrogen overloads.
Hydrogen certainly has its detractors. Some call it a deceptive savior because you have to burn fossil fuels in order to produce it, meaning that the electricity needed for splitting water into its hydrogen and oxygen components is likely to come from coal-fired power plants (though it can be derived from clean sources of electricity, including hydro dams, solar hydrolysis, and the geothermal power plants that Iceland is now counting on to support a hydrogen-based transportation system).
Some detractors also say that the water vapor from hydrogen combustion is itself a greenhouse gas, but they overlook the vast amounts of water that evaporate from the oceans day and night without parboiling us. Water vapor eventually condenses and falls out of the atmosphere as rain or snow rather than remaining aloft for thousands of years as CO2 does, so the climatic effects of hydrogen-car emissions should be minimal.
So here's my pesky question. I assume that hydrogen vehicles would be nowhere near as harmful as today's gasoline-guzzlers when it comes to air pollution, if only because they don't release CO2. But if we just re-tool our engines to accept hydrogen rather than fossil fuels, won't we still be polluting the air, waters, and woods with nitrogen oxides?
There are ways to reduce NOx production from hydrogen-driven vehicles, or so I'm told by experts who know a lot more about this sort of thing than I do. Catalytic converters help, for example, as do certain engine designs. But I've not yet heard much discussion of this problem in the sustainable energy community, which tends to treat hydrogen like a flawless panacea.
Back when sulfuric and nitric acids in rain (aka acid rain) was still the primary environmental issue of note, media attention helped to pass clean-air legislation that reduced acidic sulfur emissions from coal-fired power plants. Now that global warming occupies center stage, it's easy to forget that the nitric half of the old acid rain problem was left largely unsolved.
So let's remember that there's more to air pollution than CO2 alone, and take advantage of the coming energy-transition to stop acid rain as well. Then when we claim to have found some clean alternatives to carbon-based fuels, perhaps we'll be telling the truth.
Update from the author:
For those of you who are following this string of comments, please note:
1. The focus of this mostly pro-hydrogen piece is a question about NOx emissions from internal combustions engines
2. Although many hydrogen proponents support the use of technology other than internal combustion engines in a future hydrogen economy, the proposed use of such engines for hydrogen-burning is widely discussed on the web and in the technical literature, including "International Journal of Hydrogen Energy" and "Renewable Energy" (a Google search of "hydrogen internal combustion engine emissions" or similar wording yields an abundance of hits)
3. As noted in the article, the NOx emissions may indeed be reduced by converters and engine designs (hence much of the engineering research mentioned above). But they are not totally prevented and can therefore represent a pollution problem.
4. Nitric acid pollution from internal combustion engines is an ongoing problem here in the northeast, hence my concern.
Thanks for your interest!
[Image by Flickr user Siemens PLM Software ]
Curt Stager is an ecologist, paleoclimatologist, and science journalist with a Ph.D. in biology and geology from Duke University. His new book is DEEP FUTURE: The Next 100,000 Years of Life on Earth  (St. Martin's Press, March 2011).