As wind and solar renewable energy technologies continue to evolve and mature, fossil-fired power plants, and particularly coal-fired units, will continue to fade away. With the loss of coal-fired plants, there is reduced need in the power industry for makeup water treatment, steam generation chemistry, cooling water treatment and wastewater discharge cleanup expertise.
However, traditional fossil-fueled power plants represent just a fraction of the many industries around the globe that generate steam. Furthermore, other less-carbon-intensive energy production methods continue to evolve, where water/steam chemistry — and cooling water and wastewater treatment — are and will remain critical. These include:
- Cogeneration plants at many heavy industries.
- Combined cycle power plants that will in part, or eventually in whole, operate with hydrogen as the fuel.
- Biomass-fired power plants and biorefineries.
- Small modular nuclear plants.
- Fusion plants sometime in the distant future.
Yet, as many industry experts can attest from experience, often the plant staff focuses on process chemistry and engineering to the detriment of water and steam systems. That is until a failure occurs that shuts down part or all of the plant, or much worse, injures personnel. Some upsets may also place the plant in jeopardy regarding environmental regulations.
This four-part series will examine modern water treatment and chemistry control/monitoring technologies to assist current and future plant employees with complex issues, where even seemingly slight upsets may cause major difficulties. The discussion will frequently have a holistic flavor, as often a treatment method or operational issue is not confined to one system but can influence multiple processes. This first installment will consider makeup water treatment.
A sometimes-overlooked unit operation: Makeup water treatment
Two recent Water Technology articles have examined particular aspects of makeup water treatment1,2. However, neither completely delved into two very important aspects of the process, particularly the second item listed below:
- The choice and complexity of the makeup system is quite variable depending upon the steam generator(s) served. High-pressure units such as those for power production require high-purity makeup, which rely on sophisticated treatment methods. On the other hand, simpler makeup methods are often sufficient for lower-pressure industrial boilers.
- Even though low-pressure boilers can tolerate lower-quality makeup water, industry experts have seen many instances of insufficient makeup system operation and maintenance that allow poor-quality water to enter boilers and cause scaling and corrosion. (Impurities in condensate return can cause similar issues, which will be covered in a later installment of this series.)
The remainder of this article briefly touches on the first item, but primarily focuses on the second.
Makeup water treatment for high-pressure power generators
Impurities in high-pressure boilers can cause deposition and corrosion, and can potentially carry over with steam to induce fouling and corrosion in superheaters, reheaters and turbines. Proper boiler water/steam chemistry control requires high-purity makeup, well-designed condensate/feedwater chemical treatment programs, comprehensive on-line analytical monitoring around the circuit and judicious use of blowdown to maintain boiler water contaminant concentrations within limits, without wasting energy and water. Regarding makeup system effluent purity, Electric Power Research Institute (EPRI) guidelines recommend3:
- Specific Conductivity (S.C.): <0.1 µS/cm
- Sodium: <2 parts-per-billion (ppb)
- Silica: <10 ppb
Guidelines from the International Association for the Properties of Water and Steam (IAPWS) replace sodium with conductivity after cation exchange (CACE) to calculate the influence of carbon dioxide ingress to makeup, which often occurs in vented storage tanks. IAPWS’ recommended CACE limit is 0.1 µS/cm4.