In a previous blog, I covered one of the molecules mentioned in the same breath with the energy transition or climate goals, namelyCO2. This time we will cover another molecule that is frequently in the news: hydrogen.

Hydrogen (H2) (english: hydrogen and not waterdust which I heard someone say once...), is the lightest molecule known and consists of two hydrogen atoms, each with one proton and one electron. It is an odorless and odorless gas at room temperature and atmospheric pressure. Because it is so light, hydrogen gas has a low energy density per unit volume (liter orm3) under atmospheric conditions. But per unit of weight, hydrogen does have a high energy density: 1 kg of hydrogen contains about 120 megajoules (MJ) of energy compared to 45 MJ for 1 kg of natural gas.

A hydrogen molecule consists of two protons and two electrons

Colored hydrogen

You can read about how hydrogen can be made in the blog Hydrogen: available in all the colors of the rainbow.

By far the majority is made now and for the time being via Steam Methane Reforming (SMR) in a high-temperature reactor. Natural gas is often used as a feedstock.

CH4 + H2O <-> CO + 3 H2 

The thermodynamic equilibrium shifts to the right (more hydrogen) at higher temperatures.

To increase the amount of hydrogen, two additional steps take place: the high and low temperature shift reactions.

CO + H2O <-> CO2 + H2

This is split because the thermodynamic equilibrium toward the hydrogen side is optimal at lower temperature, but the reaction rate (kinetics) actually benefits from higher temperature.
TheCO2 formed is absorbed with an amine solution under low temperature and high pressure and stripped back from the amine in a stripper column under low pressure and higher temperature. The clean amine(lean amine) is then used again in the absorber to reabsorbCO2. The released, concentrated,CO2 can ideally be reprocessed and reused, stored underground(CCS) or released to the atmosphere. The latter, of course, is highly undesirable becauseCO2 is a strong greenhouse gas. The Porthos project being built in Rotterdam will make the route fromCO2 storage to underground storage a lot more realistic.

The storage of hydrogen

Since hydrogen has such a low energy density per unit volume, efficient storage will require greatly increasing its density. This can be done by storing it under high pressure (~ 700 bar) or liquefying it and storing it under cold (cryogenic) temperatures (~ -270 ˚C). Both ways, of course, again cost energy.

Liquefaction of hydrogen

It is very easy to talk in the media about liquefying hydrogen, in order to use it in road or marine transportation, for example. But there are some snags in this process.

The nucleus of the hydrogen atom (the proton) is not stationary, but rotates. If in a hydrogen molecule the protons rotate in the same direction, this is called ortho-hydrogen. If they rotate in the opposite direction, this is called para-hydrogen. At room temperature, the ratio of ortho/para hydrogen is about 25/75. At the temperatures where hydrogen liquefies and is stored, say -273 ˚C the ratio (the thermodynamic equilibrium) is >99% para. And now the adders...if you liquefy hydrogen you get a mixture that has approximately the composition of hydrogen at room temperature. But at the low temperature, the molecules will over time, spontaneously, go to their equilibrium state at that temperature. So from ortho to (almost only) para. This is an exothermic reaction that thus releases heat. And that heat causes much of the hydrogen to vaporize again and thus be lost.

Ortho (top image) and para (bottom image) hydrogen

So work to do...

In a liquid hydrogen plant, the hydrogen is cooled in several steps and converted from ortho to para over a catalyst. A remarkable phenomenon because a catalyst in a chemical reaction affects the electrons in the outer electron shells. But this catalyst thus acts physically and targets the nucleus of the atom. During such a step, the heat of reaction is then dissipated and thus nature is given a hand to arrive at liquid hydrogen that is at equilibrium concentration and thus evaporation is reduced to a minimum amount.

How much hydrogen is needed?

Currently the total Dutch hydrogen demand per day is ~6000 tons. But this can quickly become more as we move to a hydrogen economy.

The largest green hydrogen plant in the world is being built at the Maasvlakte and has a capacity of ~60 tons/day. So this is still only 1% of the daily requirement. Furthermore, only one liquid hydrogen plant is currently operational in the Netherlands with a capacity of ~6 tons/day. So if we want to liquefy that first green hydrogen, we already need 10 such plants. Every beginning is difficult, but I do put into perspective how easily these things are talked about. Time for action!

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