October 2014 - Water - Wastewater Treatment Chemicals Insights 35

Lots of New Challenges for Power Plant Chemicals – Was the Hot Topic on September 25

The level of knowledge demonstrated by the speakers relative to the challenges of preventing corrosion and malfunction of gas turbine combined cycle power plants which are cycling hundreds of times per year and may be using poor quality source water, makes a persuasive case for continuing expert advice. Chemicals are a big expense. Failure to maintain the right chemistry is even more expensive. The average combined cycle power plant does not have the staffing of a large nuclear plant. So it pays to contract with experts who can monitor the chemistry programs.

The recorded webinar yesterday is available to you as a Utility E-Alert subscriber. It is also available free of charge as part of two free systems for power plants around the world. One knowledge system deals with all the gas turbine issues and another with all the coal issues. Power plants are encouraged to use the systems for decision making but also to use the websites and recordings for training programs. The session yesterday will be valuable for both purposes.

Overview of Power Plant Chemicals

Bob McIlvaine reviewed some of the challenges:

Use of municipal treated wastewater (Chemtreat article posted)
Rapid cycling (reviewed in monitoring session earlier)
Air Cooled Condenser and iron transport (reviewed in monitoring recording) Steam Generator - Continuous Analyses
Effluent standards (CCR and Effluent - Continuous Analyses)
Zero liquid discharge

MIOX Summary - Tom Muilenberg

The Mixed oxidant system is providing savings in terms of cooling tower chemical usage and power plant output.

Ø The NIPSCO power plant achieved cleaner condensers, saving ^~$640,000/yr.

Ø Proprietary biocides could not control biofilm in 90,000 ton tower. Visible biofilm buildup in the condenser.

Ø Mixed oxidant chemistry eradicated the biofilm. Replaced the biocide regime.

Ø Paul Schrock, NIPSCO Senior Chemist stated, "Reducing our treatment regimen.down to a single mixed oxidant product generated on site has resulted in substantial treatment chemical and labor cost savings."

HDR Summary - Colleen Layman

Colleen Layman advised that chemistry limits developed for traditional industrial watertube boilers are not generally applicable to HRSGs.
Chemistry is unique due to:
- Multiple pressure circuits in one casing
- Complex flow patterns
- Rapid cycling or fast start needs
- Integration of cycles
- Multitude of design options
Selection of operating chemistry

o Program needs to factor in unit specific design and operational aspects

Cycling and Fast Start Considerations
Cycling service long associated with phosphate hideout problems
Recommend considering AVT programs for cycling service + polishers
Where aux boilers are used for holding vacuum, warming, etc.
- Treatment program should match main cycle
Fast start designs gaining in popularity
- OEM chemistry requirements may be quite different than standard fare
- No tolerance for old school chemistry holds
- High purity system maintenance a priority
Challenges with ACCs
Difficulties in cleaning
Large surface area promotes iron transport particularly during restarts & load changes
Air in-leakage potential
Prone to FAC
Consider installing permanent condensate polishers
- Precoat vs. Deep Bed
Supplemental HP feedwater pH adjustment

Use of amine blends with different vapor/liquid distribution ratios

Kiewit Summary - Brad Buecker

Brad answered some of the most important questions facing the industry.

"Bob McIlvaine and his staff posed a set of questions to me that are very important regarding HRSG water/steam chemistry and prevention of corrosion and fouling in steam generators. Even seemingly minor issues have been known to cause failures that cost power producers millions of dollars and in some cases even claimed lives. This is the ultimate cost. Each question and a straight forward answer follows:

Question #1

Are chemical treatment methods available to reduce flow-accelerated corrosion (FAC) in HRSGs?

Answer #1

Yes. For starters unless the feedwater system contains copper alloys (virtually non-existent in HRSGs) do not use an oxygen scavenger/reducing agent.
Second, keeping the pH elevated in a mid 9 range or even a bit higher, particularly in the LP circuit, will help with single-phase FAC.
The situation is more complicated for two-phase FAC.
Please feel free to contact me directly for a more detailed discussion of FAC. (brad.buecker@kiewit.com)

Question #2

With fast start HRSGs and constant cycling, what chemical additions will counter some of the negative consequences of this operating mode?

Answer #2

I have worked with my friend Dan Dixon of Lincoln Electric System on this issue, and we co-authored an article for Power Engineering on the subject.
Keep oxygen out of the system during shutdowns. The best method is nitrogen blanketing. Nitrogen generators are available that can do a great job in producing 99.9-plus percent N₂.
Remove oxygen from makeup water. Membrane systems are available that can reduce water saturated with oxygen to low ppb levels.

Question #3

· If the plant has an ACC rather than a water-cooled condenser and the condensate iron content is much higher, how can this problem be solved?

Answer #3

· Install a full-flow particulate filter in the condensate line. Most of the iron generated from corrosion in ACCs is particulate in nature.

I purchased one of these units at a coal-fired plant where I worked to remove iron particulates following boiler chemical cleanings. The equipment paid for itself several times over after the first use.
The original system had filters with 7-micron pore size, but plant personnel found that 10-micron worked just as well.

Question #4

What are the water chemistry issues facing GTCC operators that are unique to this type of power generation?

Answer #4

Several major issues are facing plant operators, some of which transcend many industries.
Dealing with less-than-pristine raw water sources. Municipal wastewater treatment plant effluent is one example.
Discarding the idea that an oxygen scavenger/reducing agent is needed for condensate/feedwater treatment.
Handling the different chemistry regimes in multi-pressure HRSGs.
New requirements for cooling tower chemical treatment.
Dealing with increasingly stringent wastewater discharge guidelines.

Question #5

What are the chemical treatment needs if zero liquid discharge (ZLD) technology is mandated?

Answer #5

ZLD is a complex subject, not to be taken lightly.
One method gaining popularity is treatment of the discharge with membrane technologies to greatly reduce the volume. Even so, a waste stream still remains. Methods, all of which can be problematic, to deal with the final waste stream include:
Evaporation ponds
Deep-well disposal
Truck the liquid off-site.
Thermal evaporation/crystallization with solids disposed in an approved landfill

Question #6

Treated wastewater often contains much higher concentrations of ammonia, phosphorus, organics, and suspended solids than fresh water. These impurities can cause induce excessive microbiological fouling in cooling systems, can carry over into the wastewater stream, and can be problematic for makeup water systems.

Answer #6

Treated wastewater often contains much higher concentrations of ammonia, phosphorus, organics, and suspended solids than fresh water. These impurities can cause induce excessive microbiological fouling in cooling systems, can carry over into the wastewater stream, and can be problematic for makeup water systems.
Makeup water clarification and solids precipitation may be a requirement to treat these streams.
Selection of an alternative to chlorine (bleach) such as chlorine dioxide may be necessary.
Cooling tower sidestream filtration is never really a bad idea.

Question #7

How can plugging of combustion turbine inlet air fogging nozzles be prevented, and what is necessary to prevent introduction of contaminants to a combustion turbine?

Answer #7

Any water injected ahead of or into a combustion turbine for cooling, NO_x control, or power augmentation must be very high-purity, i.e., a stream from the makeup water system. Impurities can be very harmful to combustion turbine blades and rotors in the extremely harsh environment. Far more often than fogging systems I see evaporative coolers used for inlet cooling. I also have assisted a group that developed an ammonia-based chilling system for inlet air cooling (and heating in the winter). It eliminates the issue of poor water possibly being inducted into the turbine.

You can view and listen to the recording at Power Plant Water Treatment Chemicals 100 minutes.
You will need to enter your name and email address after clicking link to view recording.

Presentations follow:

Power Plant Chemicals Webinar - Hot Topic Hour September 25, 2014

The level of knowledge demonstrated by the speakers relative to the challenges of preventing corrosion and malfunction of gas turbine combined cycle plants which are cycling hundreds of times per year and may be using poor quality source water makes a persuasive case for continuing expert advice.