What’s holding up use of hydrogen as a fuel?
In a recent “Ask Charlie” blog posting at www.controleng.com , Senior Editor Charlie Masi addressed one of the burning questions on many minds: With everyone touting the detractions of ethanol production, what’s delaying our use of hydrogen as an alternative fuel?
According to Masi, the problem is that hydrogen is simply not a fuel. It is so reactive that it is practically unavailable on Earth in its free form (H 2 ). It appears naturally only in oxidized form — bound with other elements, such as oxygen (water), carbon (hydrocarbons, such as methane), or both (carbohydrates, such as alcohol), as well as a host of other organic and inorganic compounds of no interest as potential fuels.
It takes energy input to reduce naturally occurring hydrogen to free form, which can then be re-oxidized to get the energy back. H 2 , therefore, is an energy storage medium, not an energy source. That fact, of course, is most obvious when looking at electrolysis.
As for extracting free hydrogen from methane using the steam methane reforming (SMR) process — that takes energy, too. In a sense, the syngas it produces is a way station in the reaction of burning methane. Overall, you start by adding heat to break methane molecules (CH 4 ) into free carbon and hydrogen. You get that energy back when you allow those to react with available oxygen. The resulting energy is the heat obtained by binding free carbon into carbon dioxide and free hydrogen into water, minus the heat needed to break up the methane in the first place and the (often large) energy losses along the way.
If you just watch the hydrogen part of the reaction, it looks very clean. Add a little heat to get free hydrogen, then burn it to make water. Unfortunately, you still have carbon left over, which you then have to do something with. That something will eventually result in oxidizing it to CO 2 .
Energetically and environmentally, you’d have been better off to just burn the methane directly, since you waste heat at every process step (second law of thermodynamics). Besides, simple kinetic theory of gases shows that methane, which can also be burned in internal combustion engines, has a higher energy density (energy available per unit volume) than H 2 stored at the same temperature and pressure.
What’s holding up progress is that its makes more economic and environmental sense to burn carbon-based fuels directly than to add the H 2 -storage step. H 2 will only become a good choice when fossil fuels become sufficiently scarce, and if at that time it is the best choice for storing and delivering energy derived from other sources (such as nuclear) that can’t be operated in a vehicle.
To view this and other “Ask Charlie” blog postings, visit www.controleng.com , click on the Blogs tab, and select the “Ask Charlie” blog. Or submit your question..