Decarbonisation Technology - February 2024 Issue

Technologies for ultrapure water generation Electrolyser feed water must be prepared in a similar way to boiler feed water. Many thermal power plants withdraw river water or seawater and purify that raw water prior to it being vaporised and introduced as the working fluid for the steam turbines. The higher the pressure of operation, the lower the permitted levels of impurities. However, ultrapure water for electrolysis must be even more pure than boiler water prepared for 30 bar steam generation operation. “ Reverse osmosis is a core technology for the final stages of freshwater purification and seawater desalination ” In the early stages of water purification, various filtration processes are used (see Table 2 ). Initially, a rough metal grate can be used to trap fish, leaves, logs, or other debris, such as floating plastic waste. After this, additives may be used to flocculate dissolved metals. For example, aluminium sulphate addition is commonly used. Pre-chlorination can support the flocculation of heavy metals. A multimedia or sand bed filter can then be used to trap the flocculated particles. It will also remove sludge, algae mud, and entrained sand from the raw water. This filter must be backwashed periodically, and the resultant

water should be purified prior to discharge to the drain, sea or river. An activated carbon filter can then be used to trap the chlorine, which would corrode membranes that are used in subsequent water purification processes and also damage the PEM electrolyser membrane. Chlorine might have been used in the pre-chlorination stage. It could also have been dosed to mains water to avoid microbial contamination. The activated carbon filter will also remove dissolved organic material. Spent activated carbon filter cartridges are not regenerated in situ, they must be replaced. Often, the cartridges are exchanged by a service provider. The saturated activated carbon is generally incinerated. Following the activated carbon filter, water softening can be used to remove calcium and magnesium ions in hard water areas. These are especially damaging to PEM electrolyser membrane electrode assembly catalysts. Reverse osmosis (RO) is a core technology for the final stages of freshwater purification and seawater desalination. To purify fresh water, low-pressure reverse osmosis (LPRO) can be used. It operates at around 15 bar. The power consumption of an LPRO plant is in the order of 10 kWh per cubic metre of water purified. Between 70 and 85% of the water is recovered. For seawater reverse osmosis (SWRO), a

Separation process for water purication

Mechanical screen

Multimedia granular lter

Microltration (MF)

Ultraltration (UF)

Nanoltration (NF)

Reverse osmosis (RO)

Feed component

Water

Monovalent ions

Dissolved organic substances Divalent/ multivalent ions

Viruses

Bacteria, protozoa, suspended solids

Sand, mud, algae and slime

Large debris

Table 2 Separation process for water purification