Hydrogen: a primer

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What is it, how do we make it, and what is it good for

Recently a reader (I have a Reader!) wanted to know more about hydrogen. I provided a quick answer, offering to dig a little deeper if useful. So here goes. There will be a bit of basic chemistry which I hope I have simplified sufficiently. Ultimately every bit of analysis here relates to energy: where does it come from, what does it cost, and what are the greenhouse gas emission implications; and generating energy often involves the chemistry of combustion. After all, 75% of total world greenhouse gas emissions are related to energy production and use.

Hydrogen is the name of an atom, with the symbol H, as well as the name of a molecule made up of two hydrogen atoms, H2. Similarly, two oxygen atoms O pair up to make an oxygen molecule O2. Hydrogen, like a lot of things, will burn in an oxygen atmosphere. The chemists call this process “oxidation”, because the stuff being burned is broken up into smaller units which combine with the oxygen in the air. Combustion, or oxidation, releases a lot of heat, which is why it is useful. So oxidising hydrogen looks like this:

2 H₂ + O₂ → 2 H₂O

This simply says that 2 molecules of hydrogen H₂, containing a total of 4 atoms of hydrogen, combine with a molecule of oxygen O₂, containing two atoms of oxygen, to make two molecules of H₂O, also known as water. (Both sides of this equation contain four hydrogen atoms and two oxygen atoms, so it is balanced – we haven’t created or destroyed any atoms. This is a basic concept called conservation of mass.)

The interesting thing about this is that there is no carbon anywhere in the process, so there is no opportunity to oxidise carbon to carbon dioxide (CO₂); the only emission is water vapour. (Combustion in air, which contains nitrogen as well as oxygen, is a bit more complicated, but we’ll leave that aspect for another day.) From a greenhouse gas perspective, this appears to be a winner: heat without CO₂ generation.

But the real question is where the hydrogen comes from. Production and transportation of hydrogen both require energy, potentially creating a GHG impact.

Hydrogen: how is it made

There are several ways of obtaining hydrogen, which isn’t just lying around – it tends to react with oxygen pretty quickly and turn to water. It is also a key part of fossil fuels. The two major approaches to making pure hydrogen molecules are to break up the water molecule that we just made above, or to break up a hydrocarbon molecule of some sort. We’ll start with the hydrocarbon path.

Natural gas is the simplest hydrocarbon, being made up of a carbon atom and 4 hydrogen atoms: CH₄. This molecule is also known as methane. Separating hydrogen from methane is done with two reactions, the first called steam reforming (see the Wikipedia article for more information, click here):

CH₄ + H₂O → CO + 3 H₂

In this reaction, methane and water (in the form of steam) combine to generate one molecule of carbon monoxide (CO) and three hydrogen molecules. The carbon monoxide is a nuisance, so more water is added (this is called the water gas shift reaction):

CO + H₂O → CO₂ + H₂

So overall we’ve used two water molecules and a methane molecule to make four hydrogen molecules and one molecule of carbon dioxide (CO₂). While the chemists will scream that this is not an accurate representation of what happens at the molecular level, the following illustrates the overall combined mass balance of the two reactions:

CH₄ + 2 H₂O → CO₂ + 4 H₂

If you are burning hydrogen made using this way, your tailpipe or smokestack emissions may consist only of water vapour, but you have generated greenhouse gas emissions (one carbon dioxide emitted for every four hydrogen molecules generated) somewhere else.

It is worth noting that similar processes can be applied to most carbon-containing materials, such as wood chips, oil, coal, etc. (For those interested, look up gasification and the Fischer-Tropsch process.) If the material comes from plants, for example wood chips, the process is presumed to be greenhouse gas neutral, as the CO₂ emitted is presumed to have been pulled out of the air when the plant grew; the assumption is that we are not putting new carbon (from fossil sources) into the atmosphere, but simply returning carbon to the atmosphere that was there until the seedling took root. This assumption comes with a certain amount of hand waving and deserves a deeper dive in a future article.

It is also worth noting that unlike combustion, which generates heat, this reaction, like baking a cake, requires heat: there is an energy cost to this approach.

The proponents of this approach counter the GHG argument by proposing a technology called carbon capture, use and sequestration (CCUS) to redirect the CO2 back into the ground. But this technology is an expensive energy hog which doesn’t remove 100% of the carbon dioxide in a gas stream, and which remains to be proven at large scale. The whole objective needs to be a smarter use of energy, and using lots of energy to make more energy is problematic in my view, especially if it isn’t a zero-carbon approach. We also need technologies that work now, as time is getting very short.

The second path involves reversing the combustion process described above. If that process created heat, i.e. energy, reversing it needs energy. Typically this involves a process called electrolysis (you can also look this up on Wikipedia, click here) which requires a lot of electricity. The result is the generation of molecules of oxygen and hydrogen which need to be separated to prevent them combining again:

2 H₂0 →2 H₂ + O₂

This approach generates no carbon dioxide, as long as the power is not generated with fossil fuels; if power is fossil-based, once again we have simply shifted the emissions elsewhere.

Hydrogen: How would we use it if we had a lot of it

Hydrogen is a very dilute gas and needs to be compressed to very high pressures in order to get a reasonable amount of energy into a reasonable sized reservoir, such as a vehicle fuel tank. So beyond the energy required to make it, there is the energy required to compress it.

Furthermore, it is highly explosive. Oxidation as described above happens very quickly and can make a huge mess if there is enough hydrogen available. Think of your barbecue propane tank on steroids, or Google “Hindenburg disaster” for a preview. So safe handling is a major issue. For this reason, I think using hydrogen to fuel vehicles is a major disaster looking for an opportunity to happen; Hollywood movies notwithstanding, a ruptured gasoline tank burns nice and hot but a ruptured pressurised hydrogen tank can fling hot, flaming shrapnel over distances of hundreds of metres, potentially killing or injuring many random bystanders.

Hydrogen is used in the production of a range of petrochemicals and fertilisers. These are non-combustion processes and generate no GHGs, except in the production of any energy required to run the reactions in question; they take place in large industrial sites where occupational health and safety (OH&S) procedures for dealing with high pressure flammable gasses are in place, so we’ll leave these for another day. But I will point out that this will be a problem eventually as the hydrogen used in these processes typically arises as a byproduct of oil refinery operations: if we stop burning gasoline and diesel, which we should, this source of hydrogen may disappear. So these users may eventually need to find new hydrogen sources.

Hydrogen could replace fossil fuels in a variety of industrial processes such as steel or cement making. If these large industrial users have access to green power (hydro, solar or wind), hydrogen production onsite from water begins to make sense. The hydrogen would then be used onsite, with no pipeline or tanker transport required, in situations where industrial-grade OH&S procedures are in place to prevent an explosion.

It has been suggested that hydrogen could be distributed via the existing natural gas pipeline network to homes and factories across the continent, but this requires assurance that every single inch of pipe in the network is made of steel grades that resist hydrogen embrittlement, a condition where hydrogen attacks and weakens steel. And when I say every inch, I mean right to the burner tip in every gas appliance connected to the grid, whether domestic, institutional, commercial or industrial. To my mind, this creates an unacceptable risk to the public unless a local green hydrogen production facility is paired with new pipelines to new developments. 

Hydrogen’s place in a Net Zero world

So there you have it: Hydrogen is best used on large industrial sites, where it can be made onsite from water using green electricity. Onsite storage and use can be regulated through OHSA or other regulatory bodies to eliminate safety risks to the public. The benefits are decarbonising large industrial processes for which there aren’t really any immediately obvious substitutions available today.

Hydrogen use in vehicles is possible but the safety issues are huge. The same is true if we try to put hydrogen into the existing natural gas pipeline network.

Hydrogen from natural gas raises uncomfortable questions around fugitive methane emissions, carbon capture at less than 100% levels, and high levels of complexity, cost and energy demand. In particular, the cost per tonne of CO₂ abated, the so-called carbon index, should be a guide: Could those dollars and energy resources be better invested elsewhere, with greater levels of GHG reductions per dollar spent? And the technology is not yet proven at large scales; arguably we don’t have the time to sit around and hope it all works.

I hope I’ve answered my Reader’s questions; and I would be happy to try to answer yours too! Please write, especially if you feel this outline has been excessively or insufficiently simplistic.