The Chemistry of Boilers
Introduction
There is a nexus between water and energy. In fact, just like the question of who came first, the egg or the chicken, you can ask the question:
Which came first: energy or water?
To produce purified water you need energy and to produce energy you need purified water. The boilers in a power station need ultra pure demineralised water and clean, filtered water for cooling. Hence, every power station needs a specialised chemistry professional like your truly. The missing link here is steel. The technology to produce boilers and steel mechanical machines was essential to the production of electricity and steam power during the Industrial Revolution.
Below I list the most important topics in the chemistry and water treatment of boilers.
Important topics in the chemistry and water treatment of boilers
The chemistry of boilers is really simple if you understand some main concepts that fall into three major chemistry categories:
Water Chemistry/Steam Analysis and Prevention of Scale/Deposits and Corrosion
- You need demineralized make up water because any chlorides in the water will corrode the steel. Normally RO and EDI/MBX are used for this.
Water Impurities and pH
- Any silica in the water will carry over with the steam and deposit on the blades of the steam turbine
- Any hardness in the water will deposit on the walls of the heaters as scale, the pipe bundles that are exposed to fire in the boiler. The same concept applies in Nuclear power stations but the source of heat there is the nuclear fission reaction there is no fire as in coal or gas fired power stations.
- Any organics in the water need to be removed otherwise they decompose under high temperatures to corrosive CO2.
- In Reducing boiler regimes, Dissolved oxygen needs to be removed otherwise it corrodes the steel. For this a deaerator is used or membrane degassing.
- pH about 9.5 is best for minimal corrosion. Look at the Steel corrosion curve. Excurions of high or low pH must be avoided at all costs.
- Phosphate hideout occurs when the boiler load is changed. The solubility of Phosphate changes under different temperatures and it can hide or come out of hideout resulting in unexplicable pH excursions. Best to keep the boiler at constant load.
ORP: Oxidation Reduction Potential
- A reducing environment in the boiler achieved with zero DO (Dissolved Oxygen) forms magnetite protective layers inside the boiler.
- An oxidizing environment, which is used in ultra high pressure critical boilers forms a protective hard hematite layer but the water must be deeply demineralized otherwise under deposit corrosion will occur. These kind of boilers normally have a condensate polishing plant.
Efficiency Monitoring
- The heat exchange between the condenser/cooling water and the condensing steam needs to be monitored and any excursions in conductivity means there is a leak in the tube bundle. For more information cooling water treatment, visit this page.
Boiler Chemicals
- Chemicals are added to the feed-water to maintain a high pH in the water part normally sodium phosphate is dosed. I have an interesting page on chemicals used in water treatment.
- Ammonia is normally dosed to the steam section to maintain a high pH. Ammonia is volatile.
Monitoring
- The main monitoring parameters in the demin water and water steam cycle are: pH, conductivity, cation conductivity, Silica, Phosphate, TOC, Ammonia, DO before and after the deaerator or membrane degasser. The main monitoring parameters in the Main Cooling Water system are: pH, TDS/Conductivity/ORP and Chlorine levels, Bacterial Levels and the LSI.
- Low pressure boilers are sometimes treated with dispersants or anti-scalants to prevent deposits but this is the wrong approach. It is better to work with ultra pure water. Sometimes filming amines are used to prevent corrosion in the steam parts of the boiler but this is chemical charlatanism.
- The cycles of concentration or CoC occur as a result of evaporation and water loss in the boiler which leads to concentration of TDS (Total Dissolved Solids). TDS is blown-down in the steam drum and fresh demin water is added.
I hope you have found this concise description useful. There are many books written about this. But I like to simplify stuff.
Boilers are fascinating mega machines in which water, energy, steel and chemistry come into fascinating play.
Always remember that energy and mass are interchangeable… E = mC^2. This is the secret of Enlightenment. You have to look deep into your soul, your consciousness and you will find The Source of Everything.
Chemical Dosing and Sampling Analysis Suppliers
One of the most professional chemical dosing suppliers that i have worked with in the Power Station sector is German Prominent Dosiertechnik.
As far as sampling and analysis is involved, SWAN Analytical from Switzerland are very professional.
Auxilliary Boiler Systems
All power stations have main and closed cooling water systems. For the science of cooling water treatment, take a look at this prezzy.
Clean Coal power stations have FGD (Flue Gas Desulfurization) systems in which CaCO3 is crushed into a slurry that reacts with SO2 in the flue gas in a large absorber. CaSO4 is formed as a by product which can be recovered.
Zero-Carbon power stations have CCUS systems that capture CO2 from the flue gas. We have a presentation on CCUS that you can view/download here.
Pre-Commissioning Boiler Cleaning/Preservation
Pre-commission boiler cleaning involves thoroughly flushing and cleaning the pipework system to remove debris, biofilm, oil, and grease, ensuring the system is ready for commissioning and long-term optimal performance. Normally caustic surfactants are used as chemicals. A corrosion inhibitor is sometimes added.
After cleaning, and if the boiler is not ready for commissioning it is sometimes preserved. There are two ways to preserve a boiler: dry preservation and wet preservation.
Professional Standards
For professional standards on Water Steam Cycle chemistry refer to the publications of the VGB and Electric Power Research Institute. The European Union’s Pressure Equipment Directive (PED) 2014/68/EU sets standards for the design, manufacture, and conformity assessment of pressure equipment with a maximum allowable pressure above 0.5 bar, ensuring safe and free movement of products within the EU.
Look at my water-steam cycle presentation to familiarise yourself further with these concepts. You will find some useful diagrams there.
For more information or a consultation, please contact us at the following email address:
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