June 1, 2009

Focus on Organics in Steam

David G. Daniels

Polishers as a Source of Organics in Feedwater

Condensate polishers are the plant’s first and best defense against small amounts of contamination from a weeping condenser tube or demineralizer upset. However, we now know they can also be the source of organic contamination. Full-flow condensate polishers are the rule in nuclear power plants, fossil-fired supercritical plants, and many high-pressure drum units.

Leachable organic compound from new cation resins include monomers of styrene and residuals of organic solvents. As resins age, they leach functional groups from the resin. For cation resins, this includes sulfonated monomers. These can break down in the boiler or steam cycle to produce sulfate. Anion resins tend to leach aliphatic amines, which may be observed only as increased cation conductivity.

New resins leach out higher concentrations of organic compounds than well-used resins. The organics left over from the manufacturing process have a far greater potential for damage. They not only have the capacity to contaminate the process, but they will also foul other resins that contact them.

For example, compounds that leach out of new cation resins will stick to and foul anion resins. Therefore, new cation resins should be regenerated and rinsed separately before they are mixed with anion resins. One expert suggests that new cation resin be regenerated, exhausted with brine, and then double regenerated before it comes into contact with the new anion resin.

Another method is to soak new separated cation and anion resins in warm (120F) demineralized water for four hours or more to remove leachable organic compounds. Each time the strong acid and strong base resins are regenerated, the bead shrinks in the presence of the regenerant. As the resin is rinsed and put into service, the bead swells again. Each shrinking and swelling cycle acts to squeeze some additional level of leachable organic material out of the resin. Extended rinse times may be required not only to reach the desired conductivity but also to remove the leachable organic compounds.

One area where amines may have a detrimental effect is on the polisher resins. Polishers remove acetic acid and other organic breakdown products, but they may also contribute to contamination problems.

Ethanol amine has been a problem with some polisher resins, causing contamination, particularly if the condensate temperatures are high. Carboxylic acids that are absorbed by the cation resin are exposed to sulfuric acid during regeneration, become sulfonated, and be released when the resin is in service. Similarly, organic acids on anion polisher resins can absorb sodium during regeneration and release it during operation. Every organic compound, whether it is an amine or a declumping agent, should be assessed prior to use to see if it will have a detrimental effect on the polisher resins.

To Manage Organics, Know Your Water and Treatment Chemicals

In the future, as power plants contend with poorer quality raw water as source water for their demineralizers, attention to NOM will gain importance. This source of organics has the potential to do damage in the steam cycle, particularly where the organic compounds include halogens such as chlorine. Fortunately, there are treatment technologies capable of removing these organic compounds. Each plant needs to know not only the level of TOC in the raw water but also where those organic compounds originate. This is particularly important when raw water sources are changed. Often, simple steps can be taken to reduce the organic loading on a unit.

Any water treatment chemicals normally added to the steam cycle present a level of risk to the equipment if they are misapplied or overused. With the substantial history of good performance, it would be a shame to summarily dismiss neutralizing amines just because they contain carbon. Properly evaluated and applied, amines and other carbon-based treatment chemicals have the potential to benefit the steam turbine and other plant equipment.

—David G. Daniels (david_daniels@mmengineering.com) is a principal of M&M Engineering Associates and a contributing editor to POWER.