June 1, 2009

Focus on Organics in Steam

David G. Daniels

What Is Natural Organic Matter?

Natural organic matter consists primarily of large, complex organic molecules that are the by-product of a living organism. NOM can be humic acids from decaying plant material or polysaccharides created by bacteria in the water.

If the water source is treated effluent from a sewage treatment plant or a source of potable water, it’s likely that the water is chlorinated before it reaches the plant. Chlorinated organic compounds are a particular concern, because they can pass through water treatment equipment only to break down in the boiler, where the amount of chloride in the steam will increase the potential for corrosion on the turbine. These halo-organic compounds present a clear risk to plant equipment.

Documented damage to steam cycle equipment directly attributed to organic compounds is associated with various sources of NOM, some of which have included halogenated organics. Damage has included stress corrosion cracking of turbine rotors, corrosion of carbon steel anchor strips at the base of the turbine blade in the final rows of a low-pressure (LP) turbine, tube failures in a steam generator (chlorinated organic compounds), and corrosion in the LP evaporator tubing in a heat-recovery steam generator.

Many OEM makeup water guidelines give hard limits, such as 300 ppb total organic carbon (TOC). This kind of hard limit is not helpful in solving many of the water treatment challenges facing the power industry. First, the TOC value does not differentiate between different sources of organic compounds. It is important to know the particular source and type of organic compound, as some pretreatment equipment is more effective at removing specific types of organic compounds and less effective at removing others. For example, one utility in the Netherlands saw a jump in cation conductivity when its makeup water source changed, introducing more polysaccharides into the water that were not effectively removed by its pretreatment equipment.

For some units, 300 ppb may be far too high, particularly where there is potential for the TOC to contain chlorinated or sulfonated compounds. For another unit, the pretreatment equipment required to meet these low limits may be extensive and of questionable benefit.

Many water treatment experts are of the opinion that no single limit on organics in makeup water should be applied to all units because many factors determine how much of the organic loading of a steam cycle presents an unacceptable risk. Critical questions to ask before determining an limit include these: What is the source of the organic compounds? What are the operating conditions (temperature and pressure) of the unit? Is it baseloaded or cycling? Will the steam be condensed in the steam cycle, or will most of it go to the steam host? What is the potential for other ions, such as chloride or sulfate, to be associated with the organic compounds? These and similar questions should be answered prior to deciding how and to what extent makeup water needs to be treated for organic removal.

Not only can NOM directly affect a plant, but it also can create problems in equipment, which in turn creates many operations problems. Organic molecules have a long history of fouling anion resins, resulting in shorter run lengths and silica leakage. Cation resins can also be affected by NOM. Organic compounds absorbed onto cation resin have been shown to cause degradation of the resin that results in the release of a different organic contaminant, namely polystyrenesulfonates. So, although the NOM may not contain sulfonated compounds, it may cause increased levels of sulfate in the steam.

Conventional wisdom assumed that NOM would quickly break down into carboxylic acids such as acetic acid and carbon dioxide. Recent testing performed with humic acid at 2,600 psig and 1,022F found that NOM also formed significant percentages of larger and more complex organic molecules in the boiler, even after 48 hours of exposures at these temperatures and pressures. It’s likely that the presence of oxygen in the steam cycle as well as the configuration and recirculation rate on the boiler have a significant effect on the compounds formed and their longevity. Results are likely to be site-specific and may even depend on current operating conditions.