May 3, 2021
Hydrogen was thrust into the spotlight as a promising clean energy source by President George W. Bush in his 2003 State of the Union address. President Bush touted the potential for a “hydrogen economy” that would greatly slow the release of carbon dioxide into the atmosphere. Since then, billions of dollars have been invested in an attempt to realize this vision.
Hydrogen’s appeal is obvious. When hydrogen is combusted in an engine or consumed in a fuel cell, it combines with oxygen to form water. Thus, a car running on hydrogen is primarily emitting water vapor as a waste product.
Hydrogen’s Dirty Secret
However, whether hydrogen truly has a low carbon footprint hinges on how the hydrogen is produced.
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Although hydrogen is the most abundant element in the universe, it is almost exclusively tied up in various compounds. Hydrogen gas is reactive, and thus there do not exist deposits of hydrogen that can be exploited.
The vast majority of the world’s commercial hydrogen — over 95% by most estimates — is produced using the steam methane reforming process (SMR). In this process, natural gas is reacted with steam at an elevated temperature to produce carbon monoxide and hydrogen (which is synthesis gas, or simply syngas). A subsequent reaction — the water gas shift reaction — then reacts additional steam with the carbon monoxide to produce additional hydrogen and carbon dioxide.
As I explained in last year’s article Estimating The Carbon Footprint Of Hydrogen Production, the carbon footprint of hydrogen production via SMR is high. In fact, more carbon is generated in the production of hydrogen via SMR than if you simply burned the methane used to make the hydrogen.
So, why do we make hydrogen using this method? It has historically been the cheapest method of large scale hydrogen production.
50 Shades of Hydrogen
Some have attempted to classify hydrogen production using a color scheme. Grey hydrogen denotes hydrogen produced from fossil fuels, such as via the SMR process. Thus, most of the world’s hydrogen production is grey.
However, it is possible to capture the carbon dioxide produced in this process. The carbon can then be sequestered or otherwise used for other purposes. This lowers the carbon footprint, and can result in the subsequent hydrogen being classified as “blue hydrogen.”
Blue hydrogen is produced using non-renewable resources, but it meets the threshold of a low carbon footprint. Depending on the process, blue hydrogen can be produced from fossil fuels, but it can also be produced from nuclear power.
Green hydrogen meets the low-carbon threshold, but it is produced using renewable resources. For example, electricity from solar power can be used to electrolyze water into its constituents, hydrogen and water. Renewable production of hydrogen is the idealized vision of the hydrogen economy, but there are some obstacles that have thus far kept this vision from being realized.
The biggest issue with green hydrogen is the cost. It simply isn’t yet cost effective enough to produce hydrogen using intermittent renewables. It could become cost effective if the renewable supply is overbuilt, and hydrogen production only takes place when there is excess electricity being produced. However, that means that all of the associated hydrogen production equipment is only being utilized a small fraction of the time.
Because of the low capacity factor of renewables, the subsequent capital costs of the hydrogen equipment drive the price quite high per unit of mass of hydrogen produced. Current estimates put green hydrogen production at roughly twice the cost of hydrogen production via SMR, but with a carbon footprint that is about 80% lower. Costs are expected to come down, but it will be challenging because of the intermittency.
The Nuclear Option
This is where nuclear power can make a huge impact. A hydrogen economy will require a massive increase in hydrogen production. That means scalable options. Hydrogen can be produced from nuclear power in a scalable fashion in two different ways.
First is simply using nuclear power to produce electricity, which is then used to electrolyze water. This would be the same process as that used to produce green hydrogen, except in this case, it would utilize nuclear power at a capacity factor of 90% instead of renewables at 20% to 40% capacity factor. That, in turn, drives down the cost of hydrogen production.
A 2020 paper in Applied Energy estimated the carbon footprint of hydrogen production via a number of different methods, and concluded that hydrogen production via nuclear electricity has a comparable carbon footprint to hydrogen produced by renewables.
However, the cost via this route is still high. It has been estimated to be comparable in cost to the renewables route. The primary reason is that electrolysis isn’t especially efficient. Generally about 20% of the power used to produce hydrogen from electrolysis is utilized in the process. Or, to put it another way, for a given input of electricity you only get 0.8 equivalent units of hydrogen back out.
Although that’s not terrible, there is an even cheaper way to produce hydrogen from nuclear power. Instead of using electricity, methane — with its four hydrogen atoms — can be thermally decomposed to carbon and hydrogen. This is a high-temperature process called thermal decomposition of methane (TDM) or simply methane pyrolysis.
In the non-catalytic process, methane pyrolysis occurs only at temperatures above 1100–1200 °C. That implies a steep energy requirement. However, catalysts can reduce the temperature requirement. Nickel catalysts have been shown to work effectively in a temperature range of 500–700 °C, while iron catalysts have shown good success at 700–900 °C.
There are two primary factors here that can give hydrogen production costs that are competitive with SMR. The first is that even though carbon is produced in this process, it is pure solid carbon, or so-called black carbon.
Solid carbon is used in all sorts of applications, hence capturing and utilizing this stream would be significantly simpler than capturing and sequestering carbon dioxide. One of the promising emerging applications is for carbon black to be potentially used to produce carbon-fiber, a valuable alternative to the strongest industrial materials available today, essentially as a free by-product.
The second factor is that these temperatures are available in nuclear power plants, and to an even greater degree, in advanced nuclear reactor technologies. So-called Generation-IV nuclear technologies provide much higher temperature operation – ranging from 500 to 1,000o C – and hence can provide heat directly to an industrial process, rather than converting heat to electricity and suffering thermal efficiency losses in the bargain.
“Advanced high-temperature nuclear systems like Terrestrial Energy’s IMSR can provide much of the energy required in the form of lower cost heat,” said Dr. David LeBlanc, Chief Technology Officer of nuclear technology developer, Terrestrial Energy.
“A key advantage of methane pyrolysis is that it requires the lowest energy input to create hydrogen, almost 8 times lower than by electrolysis of water. With methane pyrolysis powered by high-temperature nuclear, the energy output of the hydrogen produced is several times higher than for the low temperature electrolysis process. This is how a hydrogen economy can possibly be enabled at a global scale.”
If the world is serious about developing a massive clean hydrogen economy, hydrogen from nuclear power has to be given serious consideration. It is the only low-carbon option that is deployable at large scale, and that operates with a high capacity factor.