Hydrogen: The Not-So-Green Fuel of the Future

Fukushima Hydrogen Energy Research Field (FH2R) uses a 20 MW array of solar PV panels supplemented by grid electricity to power a 10 MW-class electrolyser. It can produce, store, and supply up to 1200 Nm³ of Hydrogen per hour. N refers to Normal specific temperature and pressure conditions.
Image credit: NEDO

Hydrogen is seen by many as the ideal ‘green’ replacement for natural gas (methane) as a source of energy. Politicians believe that it will be completely pollution-free to produce, store, transport, and burn. The science says otherwise.

All the colours of Hydrogen

For a seemingly ‘invisible’ (colourless) gas, Hydrogen is frequently described in terms of colour. In fact, the colour tag is used to indicate the extraction process used, and the nature of any environmentally harmful by-products released as a result. Let’s start with the ‘worst’ colour group, all produced by a process called Steam-Reforming, and where the by-products are not captured and stored.

Steam Reforming

This is the most popular method for extracting Hydrogen from ‘fossil fuels’ such as natural gas, coal and lignite. Methane is the feedstock of choice because it’s widely available and is the cleanest in terms of polluting by-products. Essentially, steam reforming is a two-stage process involving two catalytic chemical reactions, plus a third stage to physically separate the resulting gases of H₂ and CO₂.

Firstly, superheated (700–1100°C) steam and methane are passed over a nickel or platinum catalyst which causes the following reaction to take place. This is the basic Steam Reforming stage:

CH₄ + H₂O → CO + 3H₂ - Endothermic reaction for which heat needs to be applied.

Then the gas is passed over an iron oxide catalyst at a much lower temperature (about 360°C):

CO + H₂O → CO₂ + H₂ - Exothermic reaction that produces heat.

This second stage, called a Water-Gas Shift Reaction (WGSR), releases more Hydrogen, and converts Carbon Monoxide to the less nasty Carbon Dioxide.

Finally, a process known as Pressure Swing Adsorption (PSA) separates the H₂ gas from the CO₂ gas.

The PSA process has been used at an industrial scale over many years for separating gasses that happen to be mixed together. It works using the principle of ‘adsorption’ where molecules of a gas under high pressure will ‘stick’ to the surface of a solid adsorbent material. When the pressure is released, the gas will ‘unstick’ from the surface. A particular material type will have an affinity for a particular gas, so in this case we select one that traps CO₂. The H₂/CO₂ mixture from the second stage is forced at high pressure through a tank lined with the adsorbent material. The gas emerging from the tank output will be almost pure H₂ until the adsorbent material becomes ‘saturated’ with CO₂. At that point, the input is cut off, and the output diverted into a second, identical tank. The rapid drop in pressure releases all the CO₂ from the first tank’s adsorbent. This CO₂, together with residual H₂ exits from tank two unmolested by its adsorbent because the high-pressure has gone. The tanks then swap roles, tank two taking the next charge of high-pressure gas. Hence the name: Pressure Swing Adsorption.

Up until it was established that man-made Carbon Dioxide was bringing about serious climate change, steam reforming was seen as the only cost-effective and environmentally-friendly way of obtaining ‘pure’ Hydrogen gas. It isn’t ideal, because some contaminants such as sulphur compounds remain which can ‘poison’ the catalyst of a Hydrogen fuel cell. Gas for such an application needs to be passed through a ‘desulphurisation’ process before steam reforming.

Grey Hydrogen

H₂ produced by steam reforming of Methane where the CO₂ is not captured and stored is known as Grey Hydrogen. Gas from Coal and Lignite can also be steam reformed into Black and Brown Hydrogen respectively.

Most H₂ produced today is ‘grey’, not ‘green’, because of the CO₂ by-product problem. And it is a big problem. The UK government sees H₂ as the fuel of the future, replacing natural gas for domestic heating, at least in the short term. Using grey H₂ for this purpose is completely pointless as there is obviously no environmental value or financial saving in separating the CO₂, dumping it in the atmosphere, then burning the H₂. The only way this works is with Carbon Capture and Storage (CCS), turning Grey Hydrogen into Blue Hydrogen. And indeed, that’s the ‘official’ plan.

Carbon Capture and Storage

CCS sounds so simple: take ‘waste’ CO₂ from some industrial process (e.g. power generation), treat it in some way to turn it into a form suitable for permanent storage, and then, well, store it. Back in 2017 the UK government saw the opportunity for the UK to become a ‘world leader’ in the technology. This was made clear in their document: “Clean Growth Strategy” [1] published that year, despite the withdrawal of government funding for CCS projects in 2015. However, the 2017 document did make it clear that there were doubts about the economic viability of CCS. Since then, the government has continued to insist that CCS is vital for the UK to achieve its net-zero carbon commitments. As recently as 2023, the UK government issued a new report: “Carbon Capture, Usage and Storage” [2] re-affirming its commitment to the principle of carbon capture; now with ‘Usage’ added alongside Storage. Hopes are pinned on the private sector funding what would have to be a massive investment programme to ‘decarbonise’ the UK.

CCUS Case Study 1: At the time of writing, Enfinium has bid to add an £800m CCUS capability to their domestic waste incinerators on the old Ferrybridge coal-fired power station site. Currently, they generate about 170MW of electricity for the grid, and will ‘capture’ over 1 million tonnes of CO₂ annually once the CCUS plant is up and running. The plan includes a pipeline to the exhausted North Sea gas fields or worked-out underground salt caverns. At least, that’s the aspiration: so far, they’re still thinking about a small experimental plant. So, at the moment the new Ferrybridge plant may be reducing the amount of waste going to landfill, but until its CCUS plant is available the UK carbon footprint is increased by 1 MTPA (MegaTonne Per Annum).

CCUS Case Study 2: The Porthos Hub is a CCUS project in the Netherlands to collect CO₂ from industry in the Rotterdam port area and pipe it to an identified ‘empty’ gas field under the North Sea. The aim is to push 2.5 MTPA of CO₂ into this undersea reservoir which has a capacity of 37 million tonnes. In other words, it will be full in 15 years. Obviously further storage capacity will need to be obtained well before then. It would be funny if it weren’t so serious, but unless a ‘miracle’ new disposal technology is discovered in the next decade, the 20^th century worries about North Sea oil/gas fields emptying will turn into 21^st century fears of them filling up. Currently, the Netherlands’ carbon footprint stands at around 180 MTPA which needs to be more than halved by 2030. Construction is set to begin this year (2024), with operations starting in 2026. The concept of grouping CCUS into Hubs to reduce costs is covered at length in this McKinsey article: “The world needs to capture, use, and store gigatons of CO2: Where and how?” [3].

There is pressure mounting for the government to re-prioritise CCUS investment away from cleaning up fossil fuel power plants and concentrate our limited resources on capturing carbon from processes for which there is no ‘green’ alternative, e.g. making cement. And I might add, turning blue Hydrogen into green until the latter can be produced in quantity directly. A recent research report: “Curb your Enthusiasm. Bridging the gap between the UK's CCUS targets and reality” [4], explains what they say is the need for this change of direction in detail. Climate Scientists have also written an open letter [5] to the Prime Minister criticising the plans for opening new gas/oil fields which rely on CCUS to deliver a zero-carbon product. They needn’t worry though; despite all the rhetoric from the government about how ‘vital’ CCUS is to meeting net-zero carbon targets, politicians are fully aware that (I hope):

• The technology is in its infancy and there are no climate-scale plants operating anywhere in the world. Ironically, most large-scale installations that do exist were built to provide CO₂ for Enhanced Oil Recovery (EOR) – an example of ‘Usage’, the U in CCUS.
• As alluded to in Case Study 2, CCUS will be limited by the available storage capacity. Not every country has a convenient supply of empty oil/gas reservoirs available either.
• Whatever the technology, the monetary cost of carbon-capture operations that will halt the rise in levels of atmospheric greenhouse gas, let alone reduce them, is likely to be prohibitive for any country. Such spending would be open-ended and what happens if it doesn’t succeed in limiting climate-change?

Climate scientists are generally against CCUS now because they see it as a way for countries to hang on to fossil fuel power generation instead of investing in ‘renewables’; governments will turn against it because of the likely cost.

Burning Hydrogen

Having steam-reformed natural gas to Grey or Blue Hydrogen, there are three things you can do with it:

• Burn it in an adapted natural gas boiler to release the stored energy as heat, or
• Convert the stored energy to electricity in a Fuel Cell.
• Store it for future use.

The first option is only sensible when Blue Hydrogen is available, for the reason indicated above. Given the likely technical difficulties and immense cost associated with CCUS, relying on Blue Hydrogen is not looking to be such a good idea either. There is also the issue of polluting gasses being produced when H₂ burns in air. It’s often said that when H₂ burns all you get is heat and water. This is true in the laboratory experiment where pure O₂ is used, not air which is 78% Nitrogen and only 21% Oxygen. The heat released by the reaction creates some oxides of Nitrogen (NOx), and although the quantity is small, they are very poisonous. Natural gas has about five times greater energy density by volume than H₂. Converting a boiler to use H₂ while maintaining the heat output will involve increasing gas pressure and/or using higher-capacity supply pipes. Increased gas pressure may cause leakage which could have unfortunate repercussions (see Leaks below).

The second option is the way to go if all the drawbacks of the first are to be avoided. Fuel cell technology is well established – as a source of portable power on vehicles such as manned spacecraft, for example.

The third option is to store the H₂ in tanks for use later. On the face of it, this takes us right back to the difficulty of storing captured CO₂, but with one crucial difference: CO₂ storage must be permanent, and thus limitless, while H₂ requires only temporary accommodation.

Green Hydrogen

The digression into CCUS issues was necessary to show how large-scale production of ‘clean’ H₂ by Steam-Reforming natural gas still leaves us with the possibly intractable problem of preventing the unwanted CO₂ getting into the atmosphere. There is another way, involving the separation of H₂ from the O in water (H₂O) directly by means of electrolysis:

2H₂O → 2H₂ + O₂ - An electrical voltage is applied across electrodes submerged in an electrolyte.

If the electrolyte is pure water, the only by-product is Oxygen gas (O₂), which can be safely dumped into the atmosphere. One drawback is that a very great deal of electrical power is needed and that must come from renewable sources: the wind and the Sun. Another is that using pure water results in a slow reaction. Practical electrolysers use electrolytes with added chemical compounds such as Sodium Chloride (NaCl) to boost performance. Unfortunately, instead of harmless Oxygen, two unpleasant by-products, Chlorine (Cl₂) and Sodium Hydroxide (NaOH) are produced. The up-side, is that these two compounds are used in many industrial processes, and are already produced in bulk using tried and tested electrolytic technology. In other words, there is a ready market for the unwanted by-products of Green Hydrogen production.

Electrolysers, Fuel Cells, and Salt Caverns

Industrial Chloralkali electrolysers based on NaCl electrolytes have been around since 1892 and should be considered ‘safe’.  The Fukushima Hydrogen Energy Research Field (FH2R) is a pilot plant that has just started operating in Japan taking electricity from on-site solar panels, creating Hydrogen from a 10MW-class Chloralkali electrolyser, and storing it. Adding fuel-cell technology gives us an end-to-end system which generates ‘green’ electricity, uses it to release H₂ from a water-based electrolyte, stores the H₂ until needed, and then re-generates the electricity (Fig.1).

The FH2R plant represents the first step in scaling up laboratory models to grid-scale systems. It provides an answer to the fundamental question with ‘renewables’: what happens when the wind doesn’t blow, and the Sun doesn’t shine? The stored H₂ provides the back-up. The thing is though, wouldn’t it be a lot easier using the wind/solar power generators to charge huge batteries instead? Yes, it would if battery technology were more advanced so GWh capacities were possible without covering the UK in massive installations containing huge quantities of rare materials such as Lithium. The equivalent energy in gas needs to be stored of course, but that might just be possible using vast underground caverns left behind after Salt has been extracted. The Advanced Clean Energy Storage (ACES) project in the USA is already underway to create underground storage for GWh of mostly H₂-based energy. Research is being carried out to locate salt caverns in the UK for the same purpose.

Leaks

There is much excitement nowadays about the possibilities of creating a ‘Hydrogen economy’, especially as existing underground caverns may make it possible to store sensible amounts of the gas at relatively low cost. There is an inevitable caveat: while H₂ is not a ‘greenhouse gas’ as such, when it gets into the upper atmosphere it interferes with a chemical reaction which normally causes Methane, by contrast a really bad gas, to break down quickly. In other words, the presence of H₂ in the atmosphere extends the life of the damaging greenhouse gas. Nobody is really sure if this will have a serious impact on global warming in the future [6]. H₂ gets past seals because it’s a much smaller molecule than say, natural gas. NASA had a lot of trouble recently with fuel leaks prior to the launch of the Artemis 1 Moon mission for that reason. Just imagine trying to seal up all the holes and cracks in a vast underground rock cavern……

Reports and Documents on the Web

[1] UK Government Clean Growth Strategy

[2] UK Government Carbon Capture, Usage and Storage

[3] McKinsey & Co. The world needs to capture, use, and store gigatons of CO2: Where and how?

[4] Carbon Tracker  Curb your Enthusiasm. Bridging the gap between the UK's CCUS targets and reality

[5] Cambridge Zero Open Letter to Prime Minister Rishi Sunak from Climate Scientists

[6] UK Government Atmospheric implications of increased Hydrogen use

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