South Africa is shaping up to become a world leader in green hydrogen technology within the next five years.
Clean energy technologies, including renewable energy and their deployment, is central to the country’s efforts to move from high to low carbon emissions aligned with the Paris Climate Agreement.
In this article, we’ll explore the role of reverse osmosis in producing green hydrogen.
Green hydrogen is an alternative energy source that can be used as fuel or converted into electricity and heat. Green hydrogen is significant in achieving the goals of the Paris Climate Agreement.
Water lies at the heart of green energy production. It is the central feedstock for the production of green hydrogen in an electrolyser. There are different types of electrolysers, but they all have one thing in common – they require ultrapure water.
To get water to the ultrapure quality required for the electrolyser, the feed water must be treated in a water treatment plant designed according to the raw water quality and the volume of water to be fed into the process.
Green hydrogen and the Paris Climate Agreement
The United Nations has declared that “climate change is a global emergency that goes beyond national borders.”
In December 2015, world leaders reached a breakthrough in developing the Paris Climate Agreement during the UN Climate Change Conference (COP21), setting long-term goals to guide all nations to combat global warming and its impact.
One of the goals is to substantially reduce global greenhouse gas emissions. To limit the global temperature increase in this century to 2℃ while pursuing efforts to limit the increase even further to 1.5℃.
Alternative energy sources play an important role in the decarbonisation of the global economy and in climate protection.
Why hydrogen?
Hydrogen is a simple, natural element made of one proton and one electron, stable in the diatomic form H₂. Hydrogen, under natural conditions, is in the gaseous state. It is tasteless, colourless and odourless.
The low density of hydrogen gives it two characteristics:
- It is not found free in nature but is associated with other elements, most commonly in water and hydrocarbon
- It can retain and provide energy making it an energy vector, and a useful energy source
Hydrogen is obtained through various production processes. The cleanest method of hydrogen production uses an electrical current to separate hydrogen from oxygen in water, a process known as electrolysis.
What is green hydrogen?
When the electricity used for electrolysis is obtained from a renewable source, and hydrogen is produced without the emission of carbon dioxide into the atmosphere, then the hydrogen energy produced is green hydrogen.
As mentioned above, water is the central feedstock for the production of green hydrogen in an electrolyser. And all electrolysers require ultrapure water.
Various international organisations have set the quality standards for purified water as a feed source for electrolysis:
- American Society for Testing and Materials (ASTM) standard D1193 2011
- International Organisation for Standardisation ISO 3696
- Clinical and Laboratory Standards Institute (CLSI NCCLS)
ASTM standard D1193 2011 Type I quality as a feed to the hydrogen electrolyser is tabulated (Table I).
Table I: The ASTM D1193 2011 Type I quality specification of electrolyser feed water
| Parameter | Unit | Specification |
|---|---|---|
| Maximum electrical conductivity | µS/cm @ 25°C | 0,056 |
| Maximum TOC | µg/L | 10 |
| Maximum Sodium | µg/L | 1 |
| Maximum Silica | µg/L | 3 |
| Maximum Chlorine | µg/L | 1 |
The use of freshwater as a source material for hydrogen production is unsustainable because of its scarcity. Seawater is abundant but the high total dissolved solid loading of this source adds to the complexity of the water treatment process.
The extent of feed water treatment, choice and configuration of equipment is determined by the incoming and final product water quality.
Designing water treatment plants that are fit for purpose
Watercare Innovations’ design philosophy for water treatment plants is to consistently and cost-effectively produce quality water that is fit for purpose. Our objectives are to:
- maximise product water recovery
- increase plant life span
- ensure feed water quality that meets predetermined specifications
- optimise the consumption of reagents
Our water treatment plants for ASTM D1193 2011 Type I ultrapure water are designed with pre-treatment processes to produce reverse osmosis (RO) feed water with turbidity less than 1NTU and salt density index (SDI) of not more than 4.
The water plant configurations vary depending on the quality of the raw feed water.
Figure 1: Various types of reverse osmosis pre-treatment filtration methods
Our water treatment plants will typically comprise:
- Pre-treatment; and
- Reverse osmosis
1. Pre-treatment
1.1 Sand Filtration
Sand filtration removes suspended solids, colloids and microbial biomass. In the case where feed water has high microbial contamination, prior disinfection is applied to eliminate the risk of microbes passing to the treatment membranes where they will form a biofilm and cause membrane fouling.
Bank of sand filters to remove suspended solids and improve water turbidity
1.2 Granular Activated Carbon (GAC)
GAC filters are employed where the feed water contains organic compounds required to be removed. Activated carbon also reduces oxidants in the water.
Granular activated carbon filters for the removal of organic components and the reduction of oxidants.
1.3 Ion Exchange
Softening is the removal of hardness ions achieved through ion exchange. Selective ion exchange also effectively removes unwanted metals and minerals such as iron, manganese and strontium.
Ion exchange for softening and the removal of other metals and minerals.
1.4 Membrane Filtration
Ultrafiltration or nanofiltration has the effect of polishing the pre-treated water before passing it to the RO units. Particles between 0,001 and 0,01 micrometres are removed through these membranes. These membranes also filter viruses and other sub-microscopic microorganisms.
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- Bank of ultrafilters
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- Bank of nanofilters
2. Reverse Osmosis
Reverse osmosis (RO) is a membrane technology that removes highly soluble salinity from water. The objective of this treatment stage of the water treatment plant is to produce water with a conductivity of 0,056 µS/cm, ideal for optimum electrolysis to produce green hydrogen.
The RO process forces water containing dissolved salts through a semi-permeable membrane, creating two streams – the permeate or purified product water and the brine stream containing the retained salts.
It may be necessary to recirculate the permeate through the RO membrane, or pass it through a second stage, to achieve the required conductivity.
2.1 High-Pressure Pumping Unit
High-pressure pumping is required to overcome the natural osmotic pressure that causes water to diffuse through a semi-permeable membrane from an area of high osmotic pressure to an area of low osmotic pressure created by the imbalance in solute concentrations on either side of the membrane. The pump specification is also determined by the required permeate flow rates.
High-pressure pumping unit
2.2 RO membrane unit
The membrane configuration and design are determined by:
- the quality of the feed water (feed water silt density index)
- the operating parameters of the plant (permeate recovery, flux, delta p)
- the concentration polarisation across the membrane
Reverse osmosis unit
2.3 Instrumentation and control system
Continuous advancements in information and technology systems enable the water treatment plant, and the RO process, to operate seamlessly with minimal operational risks.
Instrumentation and control are designed to streamline monitoring, reporting, and the physical control of water treatment systems.
Instrumentation and control
2.4 Cleaning unit
Optimising membrane life requires automated “cleaning in place” (CIP) of the RO membranes. Normalised data tracking is a sensitive forecaster of decreasing membrane efficiency, while normalised data trending is used to trigger the CIP process.
Normalised permeate flow rate, normalised salt rejection and normalised pressure drop are important CIP triggers.
RO membrane cleaning unit
2.5 Storage and distribution
The production of green hydrogen is characterised by the need to guarantee a continuous supply of ultrapure water to the production process.
This motivates the installation of redundant systems and excess capacity to avoid stoppages in the hydrogen production process in the event of failure, breakdown, or maintenance of any of the equipment that makes up the water treatment plant.
Treated water storage tanks
Conclusion
The production of hydrogen as an alternative energy source is positioned in the market as a booming sector characterised by demanding and restrictive water quality requirements.
As a specialist water treatment company, Watercare Innovations is perfectly positioned to meet the water needs associated with green energy production, specifically the needs required for green hydrogen production.
