UBM Technology | Is hydrogen really green?

This is the most popular method of extracting hydrogen from “fossil fuels” such as natural gas, coal and lignite. Methane is the feedstock of choice because it is 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 produced H _and CO _gases .

First, superheated (700–1100°C) steam and methane are passed over a nickel or platinum catalyst, where the following reaction occurs:

CH ₄ + H ₂ O → CO + 3H ₂

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

CO + H ₂ O → CO ₂ + H ₂

The second stage, called the water gas shift reaction (WGSR), releases more hydrogen and converts the carbon monoxide into less nasty carbon dioxide.

Finally, a process called pressure swing adsorption (PSA) separates the H _gas from the CO _gas .

The PSA process has been used on an industrial scale for many years to separate gases that are mixed together. Its working principle is "adsorption". Gas molecules under high pressure will "stick" to the surface of solid adsorption materials. When the pressure is released, the gas will "break away" from the surface.

Until it was established that man-made carbon dioxide was causing severe climate change, steam reforming was viewed as the only cost-effective and environmentally friendly way to obtain "pure" hydrogen. This is not ideal because some contaminants, such as sulfur compounds, are still present and may "poison" the hydrogen fuel cell's catalyst. Gases used for such applications undergo a "desulfurization" process before steam reforming.

_{The H2} produced by steam reforming of methane (where the CO2 _is not captured and stored) is called gray hydrogen. Gases from coal and lignite can also be steam reformed into black and brown hydrogen respectively.

Due to CO _{by -product issues, most H} produced today is "grey" rather than "green". this is a big problem. The UK government sees H2 _as the fuel of the future, replacing natural gas for home heating, at least in the short term. It makes absolutely no sense to use gray hydrogen for this purpose, as there is clearly no environmental value or economic savings in separating the _CO2 and emitting it into the atmosphere, then burning the H2 _. The only effective way is to use carbon capture and storage (CCS) to turn gray hydrogen into blue hydrogen.

_{CCS sounds simple enough: take "waste" CO2} from some industrial process (such as electricity generation) , process it in some way, convert it into a form suitable for permanent storage, and then store it. As early as 2017, the UK government saw an opportunity for the UK to become a "world leader" in the technology. Although the government canceled funding for CCS projects in 2015, this was made clear in the Clean Growth Strategy document they released that year. However, the 2017 document does make it clear that there are doubts about the economic viability of CCS. The government has since continued to insist that CCS is vital for the UK to meet its net zero carbon commitments. Just in 2023, the British government released a new report: Carbon Capture, Utilization and Storage (CCUS), reaffirming its commitment to carbon capture principles. Hopes are high that the private sector will fund a massive investment program to “decarbonise” the UK.

Governments are under increasing pressure to re-prioritize CCUS investments over cleaning up fossil fuel power plants and focus limited resources on capturing carbon from processes for which there are no "green" alternatives, such as making cement. Climate scientists have also written an open letter to the Prime Minister criticizing plans to open new gas/oil fields that rely on CCUS to deliver zero-carbon products. While the government trumpets how "vital" CCUS is to achieving net zero carbon targets, politicians may be realizing:

• The technology is in its infancy and there are no climate-scale plants operating anywhere in the world. Ironically, most existing large-scale installations are designed to provide CO2 _for enhanced oil recovery (EOR).

• CCUS will be limited by available storage capacity. Not every country has a readily available supply of empty oil and gas reservoirs.

• Regardless of the technology, the monetary cost of carbon capture operations can be prohibitive for any country.

Climate scientists are now generally opposed to CCUS because they see it as a way for countries to stick to fossil fuel generation instead of investing in "renewable energy." Governments would also object to it because of the possible costs.

By reforming natural gas steam into gray or blue hydrogen, you can do three things with it:

• Burned in a modified natural gas boiler to release stored energy as heat, or

• The stored energy is converted into electricity in a fuel cell.

• Save it for future use.

For the reasons stated above, the first option is sensible only when blue hydrogen is available. Considering the possible technical difficulties and huge costs of CCUS, relying on blue hydrogen is also not a good idea. There's also the issue of polluting gases being produced when H2 _burns in the air - another counterintuitive point. This is true in the pure oxygen environment of the laboratory, whereas the real atmosphere is air composed of 78% nitrogen and only 21% oxygen. The reaction produces some nitrogen oxides (NO _x ), which are very toxic although they are small in quantity. Also since the volumetric energy density of natural gas is approximately five times that of _{H2 , converting a boiler to use H2} while maintaining heat output will involve increasing the gas pressure and/or using higher capacity supply pipes. Increased air pressure may cause leaks with unfortunate consequences (see Leakage below).

The second option is the way to go. Fuel cell technology is already well established, for example as a portable power source for vehicles such as manned spacecraft.

A third option is to store the H2 _in a tank for later use. On the surface, this brings us back to the difficulty of storing captured CO2 _, but there is a key difference: CO2 _storage must be permanent, while H2 _only needs to be temporarily contained.

Large-scale production of "clean" H2 by steam reforming natural gas _still leaves us with the potentially intractable problem of preventing unwanted CO2 from entering the atmosphere _. There is another method, which involves separating the hydrogen gas from the oxygen in the water directly through electrolysis: if the electrolyte is pure water, the only by-product is oxygen (O2 ₎ , which can be safely vented to the atmosphere.

One drawback is that it requires a lot of electricity, which must come from renewable sources. Another is that using pure water results in a slow reaction. Compounds such as sodium chloride (NaCl) are added to the actual electrolytic cells to improve performance, so that not only harmless oxygen is produced, but also chlorine (Cl ₂ ) and sodium hydroxide (NaOH). Fortunately, these two by-products can be used in many industrial processes and are already in mass production using tried and tested electrolysis technology.

It is already possible to store large amounts of natural gas at relatively low cost. There's an unavoidable caveat: While H2 _is not a "greenhouse gas" itself, when it enters the upper atmosphere it can interfere with the chemical reactions that typically result in methane (which, by contrast, is a very Harmful gases) decompose quickly. In other words, the presence of _H in the atmosphere extends the lifetime of the harmful greenhouse gas. No one is really sure whether this will have a serious impact on future global warming. H2 is able to pass through conventional seals because its molecules are much smaller than natural gas _. For this reason, NASA recently ran into a lot of trouble with fuel leaks ahead of the launch of its Artemis 1 lunar mission. Just imagine how difficult it would be to try to seal all the holes and cracks in a huge underground rock cavern.