Anode half-cell
Cathode half-cell
Cathode half-cell
Anode half-cell
H O
+
H
O
H
Repeat cells to build stack
Bipolar plate
Bipolar plate
H O
Membrane
Porous transport layer (PTL) anode
Porous transport layer (PTL) cathode
Anode
Cathode
Membrane electrode assembly (MEA)
Figure 4 PEM electrolyser stack architecture and critical components
higher operating pressure of around 80 bar is used to achieve desalination. Due to the higher- pressure operation, the power draw is around four times that of the LPRO system. SWRO is a common desalination process and has largely replaced less efficient thermal desalination. However, where waste heat from power generation can be used for desalination, thermal desalination techniques using mechanical vapour recovery to upgrade steam can be the most economic option. Polishing, degassing, and sterilisation After LPRO to purify the fresh water, intermediate buffer storage is generally implemented. As water is withdrawn from the storage, it may flow through an electro deionisation (EDI) plant for polishing. Here, the power requirement is similar to LPRO at 10 kWh/m³ of water. The concentrate discharge from the EDI plant can be fed to the LPRO inlet to recover water. EDI is a mature technology that has been applied to create demineralised water for supercritical steam generation, for example in concentrated solar plants. It is also used to purify water for direct injection to gas turbines for NOx reduction. Many petrochemical, pharmaceutical, and semiconductor facilities also rely on EDI for ultrapure water supply. After the EDI process, microbial contamination
can be removed. Post-chlorination is not a suitable technology due to the incompatibility of chlorine with the sensitive electrolyser components, such as the polymer electrolyte membrane that is at the heart of the membrane electrode assembly (MEA) in a PEM electrolyser. Therefore, a UV steriliser lamp is generally used to kill any bacteria that may be present. Following the UV lamp, an ultrafilter traps the dead bacteria. The power requirement for the ultrafilter may be around 3 kWh/m³ of water. The ultrafilter is periodically backwashed to clear the filter media cake. As with other backwash waters, this waste must be treated prior to discharge. Degassing of the pure water may also be required. Dissolved oxygen, nitrogen, and carbon dioxide (CO₂) gases will always be present in the feed water. CO₂ is unwelcome in the electrolyser since it is broken down through electrolysis to form carbon monoxide (CO). This will be an impurity in the hydrogen, making the product unsuitable for fuel cell applications. At this stage, the ultrapure water is finally ready to be introduced to the electrolyser water or lye recirculation circuit.
Stephen B Harrison sbh@sbh4.de
www.decarbonisationtechnology.com
27
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