Getting Hydrogen - How?

Green hydrogen means electrolysis, but there are other more efficient ways!

Fuel cells are designed to generate electricity from hydrogen. This requires a corresponding purity of the gas, which is defined in the XYZ standard.

In practice, there are several ways to obtain the appropriate hydrogen. However, some of these have serious disadvantages or difficulties that make economic use very difficult. Here I try to simplify the sometimes very complex interrelationships so that investment decision-makers can get a feel for the challenges.

Let's get started!

Electrolyzers

Electrolysers appear to be the simplest: take water and electricity and you get hydrogen and oxygen. But this splitting alone costs more than 45 kWh of electricity per kg of hydrogen!

However, you need about 8 kg of water per kg of hydrogen and for most stacks it should also be distilled water.

In regions with a lot of sunshine, you usually have a problem with enough water. Seawater must first be desalinated at great energy expense...

But then pure hydrogen gas is available!

Reforming

The classic process is reforming via various process steps with temperatures of up to 950°C and various catalysts. It is stated that plants from 250 to over 100,000 m3/h are built and operated, although the focus here is more on plants from 20,000 m3/h.

An energy comparison with electrolysis would be interesting. Would anyone like to post in the comments?

Methane pyrolysis

The aim of methane pyrolysis is to split (usually fossil) methane into hydrogen and carbon. The splitting energy is only a fifth of that of electrolysis, which is a positive result. Unfortunately, the methane is normally fossil, because the biogas plants themselves cannot supply the required quantities in the long term.

The cracking process produces very fine powder (#plasmagasification), which presents certain difficulties in handling, or carbon contaminated with metal (#methane pyrolysis), the further use of which still needs to be clarified.

Thermal cracking of solids

There are various ways of producing syngas, including

• #Gasification of #biomass
• #Pyrolysis of used #tires
• #Thermolysis of #plastics

The main problem is that syngas (I prefer to call it process gas) is a wild mixture of gases and elements that have their own special characteristics:

• Longer-chain hydrocarbons release carbon during cracking, which has to be deposited somewhere and usually settles in places where it interferes.
• Sulphur is a catalyst poison which must be removed before catalytic conversion of the process gas or use in fuel cells.
• The same applies to the halogens chlorine (#PVC) and fluorine (#PTFE)

The big challenge is how I can get the hydrogen out of such a gas in the desired purity.

Pressure Swing Adsorption (#PSA)

The gas is compressed to a pressure of mainly 6 - 40 bar and then purified in several stages. These processes are recommended if a correspondingly high concentration of hydrogen is already present. This is usually > 90%. However, the process gas from the cracking processes rarely contains more than 80%.

Technically, these processes are sufficient, but compressing the gas costs a lot of energy. At higher pressures even more energy than the actual cracking process itself, which has a significant negative impact on economic efficiency.

Membrane for biogas

A calculation with a normal membrane, which is also used as standard in biogas plants, shows astonishing results:

• The required inlet pressure of 6 bar is just about acceptable in terms of energy.
• The separation efficiency of methane + carbon monoxide versus carbon dioxide is also very good. Unfortunately, the hydrogen migrates out with the carbon dioxide on the same side

Now I need another separation process to separate hydrogen and carbon dioxide, which means further investment and energy requirements...

Palladium Membranes

These #palladium membranes typically operate at 300 - 350 °C and pressures of 30 - 50 bar. This means we

• need plenty of energy for compression
• a sulphur-free gas (ppb !)
• and compressors that are allowed to operate in this temperature range (in accordance with safety regulations).

Quantum membranes

These membranes use a special effect in which only the hydrogen proton travels through the membrane. However, this in turn requires an impulse:

• Overpressure: With the classic disadvantage: energy consumption
• Negative pressure: In addition to the energy consumption for the vacuum generator, negative pressure is (almost) another knock-out criterion for pure hydrogen.
• Temperature: A catalytic reaction heats the gas and breaks down any water present, allowing the hydrogen to diffuse through the membrane.
• Electrical voltage: This already works at ambient pressure and requires comparatively little power. It should also be very robust in relation to the other gas components.

Chemical storage

Another approach is chemical storage. During discharge, oxygen is removed from a vapor stream by a chemical reaction so that hydrogen can also be released under higher pressure.

When loading, the oxygen must now be removed from this storage tank. In the original case, hydrogen is introduced, which then combines to form water. This would then be a "hydrogen storage tank".

However, loading can now also be carried out using hydrocarbon-containing gases. the important thing is to remove the oxygen.

The interesting thing about the storage tank is that it can be "charged" with process gas at ambient pressure and then "discharged" from the hydrogen under pressure, so that the hydrogen would be pre-compressed at 10 - 20 bar, for example.

For example, a thermal source could be used for a relatively simple pressure build-up (steam generator) to replace electrical compressor work.

The disadvantage is that this process gas would then lose a large part of its calorific value.

The right decision...

The following must now be considered in the context of an installation

• Do I only need hydrogen or do I want to use part of the process gas for my own power supply?
• What degree of purity do I need for the hydrogen? For a fuel cell? For a hardening furnace or metal smelter?
• Do I want to regulate the hydrogen content in the process gas with a controlled hydrogen separation system?
• What order of magnitude am I thinking about? A decentralized solution for local value creation/safety or a large-scale plant?

Final

If one of the possible providers has recognized himself and would like to come out with his offer, please feel free to do so in the comments! ;-)