Aug 17 2021

Blue Hydrogen Is Not Green

As we explore our options to reduce the CO2 produced by our energy, transportation, industry, and agricultural sectors, hydrogen remains a point of discussion. Hydrogen had a brief moment in the sun about 20 years ago, when the dominant talk was about the “coming hydrogen economy” which never came. Technical hurdles got in the way. For example, researchers never cracked the problem of safely storing large amounts of hydrogen, so while it has a high specific energy (energy per mass), it has a relatively low energy density (energy per volume) which limits its applications.

Still, hydrogen has persisted, waiting in the wings for its moment to reappear. I have written recently about using hydrogen fuel for aircraft, it remains the best fuel option for rockets, it may be ideal for large vehicles like trains and trucks. Hydrogen also has the potential to be a reasonably good energy storage medium, but certainly far from the best. Batteries, for example, have a round trip efficiency (the amount of energy you get back after storing then releasing energy) of between 70-95%. Hydrogen has a round trip efficiency of 47%. This is why, by the way, I think battery electric vehicles will ultimately win out against hydrogen fuel cells. If you use the heat generated by the burning of hydrogen then you can boost this efficiency to 66%, still short of even the worst battery. But still, for grid storage a hydrogen infrastructure may be practical.

Hydrogen also has lots of industrial and agricultural uses. It is a high-energy molecule, which can be fed into chemical reactions to make fertilizer and other useful products. It can also be used to make high energy density fuel to replace fossil fuel.

There is insignificant free hydrogen on Earth, which is why hydrogen is not a source of energy. It is an energy storage medium because you can expend energy to make it from hydrogen-containing compounds. This brings me to the focus of this article – where do we currently get our hydrogen from? First let me describe what is currently the optimal source of hydrogen: water. Water is H2O, and you can electrolyze water to separate it into its two component gases, hydrogen and oxygen. The hydrogen can then be burned back with oxygen to generate energy and water, without any other waste product. It’s a neat closed system with the only significant carbon footprint coming from the energy used to electrolyze the water.

Currently only about 5% of the world’s supply of hydrogen comes from this process. This is because we lack a scalable, efficient and cost effective method for doing so. This is why there is so much research into so-called “artificial leaf” technology. If we could efficiently convert water into hydrogen and oxygen, let’s say using sunlight directly or powered by photovoltaics, in a scalable process this would be a tremendous boost to the hydrogen economy. Plentiful cheap hydrogen would transform the energy sector and many industries. But we aren’t there yet.

So where does the other 95% of the world’s supply of hydrogen come from? It comes mostly from methane – fossil fuel. And the process to make hydrogen from methane releases CO2 and is energy intensive. While this remains true, hydrogen should not be considered a green fuel. In fact, it is worse than just burning fossil fuel.

Hydrogen made from methane using the steam-methane reforming reaction, which produces H2 and carbon monoxide, which is then combined with water to make CO2 and more H2:

Steam-methane reforming reaction
CH4 + H2O (+ heat) → CO + 3H2

Water-gas shift reaction
CO + H2O → CO2 + H2 (+ small amount of heat)

This is a hugely inefficient process. You have to heat water into steam, something you can use to directly make electricity with a turbine. The final reaction also releases CO2. If you just burned the methane for energy the ultimate release of CO2 would be less. The result is so-called “gray” hydrogen, because of the CO2 release. However, this CO2 can be captured and sequestered, in which case the resulting hydrogen is called “blue” hydrogen. Blue hydrogen sounds good, and it is touted as an environmentally friendly energy source (although not as good as green hydrogen, which comes from electrolyzing water). However, in the linked recent study the researchers found:

Perhaps surprisingly, the greenhouse gas footprint of blue hydrogen is more than 20% greater than burning natural gas or coal for heat and some 60% greater than burning diesel oil for heat, again with our default assumptions.

What are those default assumptions? All methane infrastructure has some leakage. Methane itself is a more powerful greenhouse gas than CO2 over the short term, say 20 years. The authors write:

For our default assumptions (3.5% emission rate of methane from natural gas and a 20-year global warming potential), total carbon dioxide equivalent emissions for blue hydrogen are only 9%-12% less than for gray hydrogen.

About 95% of the world’s hydrogen is gray hydrogen, and the proposed solution to the high carbon footprint of gray hydrogen is to capture the CO2, producing the blue hydrogen. But this only reduces the carbon footprint by 9-12%, and is still worse than just burning the methane directly. Essentially, as long as hydrogen is sourced from methane or other fossil fuels (gray or blue) it should be considered a fossil fuel itself. This is a deal-killer for the hydrogen economy. Essentially we are propping up the nascent hydrogen infrastructure with the dirty little secret that most hydrogen is, in fact, just another form of fossil fuel, and not a particularly good one. The only real advantage is that, at the point of its use it does not release pollution. This is good for keeping smog out of cities and residential areas, but does nothing for the world’s total carbon footprint.

The only hope for the (eventually) coming hydrogen economy is if we develop a scalable artificial leaf process. We need a way to mass produce green hydrogen in an energy-efficient way. There is also another option for this – Gen IV nuclear power plants. Because some Gen IV designs run hot enough to electrolyze water, you can build in hydrogen production into the plant design. The hydrogen would be automatic energy storage, and could feed many other high-energy processes. For example, we also make nitrogen fertilizer from fossil fuel. The Haber-Bosch process fixes nitrogen from the atmosphere by combining it with hydrogen to form ammonia (NH3). That hydrogen comes from methane. But if we had an ample supply of green hydrogen, we could eliminate methane from the process and make “green” nitrogen fertilizer.

This is the kind of win-win technological advance we need to reach our climate goals. We can do this now, if we really wanted to.

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