WATER TREATMENT CHEMICALS

Supply water for HRSGs in combined cycle gas turbine power plants contains impurities that can impair the operation of the HRSG, and lead ultimately to permanent damage to the HRSG and other system equipment including turbines, pumps, condensers, and piping. These impurities can be in the form of minerals, gases such as oxygen and CO₂, various other chemicals, and microbiological organisms.  The results of these impurities can be corrosion and deposition of scale within the HRSG and elsewhere. Oxygen attack and carbonic acid (H₂CO₃) attack are common causes of corrosion. Deposition of carbonates is a cause of scale buildup. The purpose of water filtration and chemical treatment is to prevent corrosion and scale buildup.

Water treatment for any given HRSG installation is a complex process that reflects many factors including the quality of the raw water, the operating pressure and temperature of the HRSG, the application of the HRSG, and the metallurgy of all system components. Usually the water treatment program consists of complex blends of multiple and carefully balanced chemicals and catalysts. For larger systems, intermittent or continuous on-line monitoring of water conditions for conductivity, pH, ORP, and dissolved oxygen are performed to track the effectiveness of water treatment processes.

Oxygen reduction through the use of oxygen scavengers such as sodium sulfite has been a common practice for many years. However, water treatment regimes are different for high pressure boilers as compared to low pressure boilers. As an example, increasing numbers of high pressure utility boilers make use of oxygenated treatment (OT) to minimize corrosion within the boiler by using small amounts of carefully controlled and injected oxygen to promote development of a passivation layer consisting of hematite.  This practice is widely used in Europe, and increasingly in the United States among utility power producers.  Also, increasing use of reverse osmosis filtration systems is changing requirements for chemicals for large utility boilers, and a growing number of lower pressure industrial process boilers as well.

The following table provides a list of common boiler water treatment chemicals, by function.

Biocides

Oxidizing Biocides

 Oxidizing biocides kill organisms by destroying the cellular structure of the organism.  Advantages include rapid action, relatively low unit cost, and relatively low dosage requirements. Also, usage has been promoted by an OSHA determination that oxidizing biocides are effective in limiting the growth of the legionella bacteria in cooling systems. Disadvantages include an undesirable decrease in cooling water pH, environmental effects, and possible interference with other water treatment chemicals. Despite the disadvantages, oxidizing biocides remain the largest class of biocide in use. 

 Non-Oxidizing Biocides

Non-oxidizing biocides kill organisms by interfering with the metabolic processes of the organism. They are seldom used as a sole treatment regime, and usually are used in combination with a primary oxidizing treatment. Advantages include lesser interference with other treatment chemicals and minimal impact on cooling water pH. Disadvantages include relatively long contact time, and relatively high unit dosage requirements and high unit cost.  Common non-oxidizing biocides include quaternary amines, isothiazoline, glutaraldehyde and DBNPA.

Other Biocide Options

Ultraviolet radiation (UV) is a non-chemical alternative that is being increasingly used for water disinfection. 

Coagulants & Flocculants

Coagulants and flocculants are used to remove suspended solids from liquids.  The clarification process involves two stages; coagulation and flocculation.

The first stage (coagulation) promotes the agglomeration of tiny particles into larger microscopic particles or "microfloc" using a coagulant. The coagulants act to neutralize the negative (repulsive) charge on the suspended particles such as clay or organic matter, thereby permitting particle collisions and agglomeration.

The second stage involves the addition of a flocculant to form larger visible "macrofloc" that readily settle out of solution.

There are two types of coagulants and flocculants in common usage: inorganic and organic (polymer). Widely used inorganic coagulants include aluminum sulfate (Alum), and ferric chloride. Widely used organic flocculants include polyDADMAC, and non-ionic, cationic, and anionic PAM.

Corrosion Inhibitors

Corrosion inhibitors typically perform in one or more of four basic protection modes: oxygen scavenging; surface passivation; film formation, or acid neutralization. Many more chemical groups are available than listed below, and most treatment regimes include combinations or blends of chemicals rather than a single chemical.

 Oxygen Scavengers

Oxygen scavengers are chemicals that act to remove free oxygen held in solution. Oxygen scavengers include sodium sulfite (Na₂SO₃), hydrazine (N₂H₄), bisulfite, metabisulfite, ammonium bisulfite, dithionite salts, quanidines, oximes, activated aldehydes, and polyhydroxyl compounds. The most common oxygen scavenger across most major industries is sodium sulfite due to low cost and overall effectiveness.  Sodium sulfite does have limitations at high temperatures and pressures and is not used on high-pressure boilers beyond 35 bar (500 psi). The following excerpted text from the De-aerator Manufacturers Group of the American Boiler Manufacturers Association (ABMA) provides important guidance for boiler applications.

Supplemental Chemical Treatment

A good chemical treatment program should also be instituted (in addition to de-aerator treatment) to assure optimum boiler water quality. This program normally includes the use of oxygen scavengers, mainly Sodium Sulfite or Hydrazine, to remove the last traces of oxygen in the boiler feedwater. However, as previously stated, removal of oxygen by use of chemicals alone is expensive and often incomplete. Sodium Sulfite is most often used as the oxygen scavenger because of low cost and ease of handling. The end product of the reaction of Sodium Sulfite with oxygen is Sodium Sulfate. Boiler water Sulfite residual is typically maintained at 30-60 ppm. A downside affect of using Sulfite, however, is that both Sulfite, and the product of its reaction Sulfate, act to increase boiler water TDS, which may effect increased blow down rates. Hydrazine is sometime used instead of Sodium Sulfite. It has the advantages that it volatilizes and does not contribute to boiler water solids. Boiler water Hydrazine residual is typically maintained at 0.1 – 0.5 ppm. Hydrazine will also act to neutralize acidity of the condensate return system. It suffers the disadvantages of being both more expensive and more hazardous to handle than Sulfite, as well as being more difficult to control. In addition, if fed in excess, Hydrazine can form Ammonia which will attack alloys containing copper.

Oxygen scavengers are available in organic and inorganic products. Inorganic chemicals include sodium sulfite, sodium bisulfite, sodium hydroxide, and hydrazine, Organic oxygen scavengers include DEHA and MEKO, among others.

 Passivators (Anodic Inhibitors)

Passivators are chemicals that react with the base metal to promote the development of a corrosion-resistant oxide layer. Passivating corrosion inhibitors include chromates, molybdates, nitrites, orthophosphates, and the organic DEHA. Protection is provided by the isolation of the base metal from the surrounding liquid by the thin oxide layer. Chromates and nitrites have been linked to environmental and health issues and are declining in usage relative to molybdate, orthophosphate, and DEHA.

Film Formation (Precipitating Inhibitors)

Precipitating inhibitors are chemicals that promote the deposition of a thin film on the metal surface to protect the base metal against oxygen attack and acid attack. Phosphates and silicates are among the film-forming chemicals available to provide surface protection for both ferrous and non-ferrous metals, particularly in water distribution piping systems and in cooling water systems. These chemicals can inhibit corrosion in two ways: by increasing the pH of the water, i.e., making the water more basic; and by promoting an insoluble protective film of metallic phosphate or silicate on the surface of the pipe.  They can also retard the creation of scale, which mitigates bio-fouling. Film-forming amines are used in boiler protection applications.

Acid Neutralization

Neutralizing amines are often used in boiler systems to neutralize carbonic acid formed in steam condensate.  The most common neutralizing amines used in boiler protection are morpholine, DEAE, and cyclohexylamine.

Antiscalants & Dispersants

Antiscalants and dispersants are widely used in combination with corrosion inhibitors to prevent deposits on metallic surfaces.  Common inorganic scale includes calcium carbonate (CaCO₃), calcium sulfate (CaSO₄), iron scale, and silicate and phosphate scale.  Antiscalants are used in many industries, but especially in power, oil & gas E&P, refining, pulp & paper, and the water industry. 

Scale prevention usually involves one or more of four basic treatments: lime softening; ion-exchange; acid dosing; or specialty antiscalants.  Specialty antiscalants are gaining favor because of enhanced performance and lower costs and chemical usage, particularly with respect to acid treatment and ion exchange.  Specialty antiscalants work by disrupting the formation of rigid crystal structures and hence the deposition of hard scale. Three mechanisms include: threshold inhibition which keeps soluble salts in solution; crystal modification which promotes the formation of deformed crystals and soft non-adherent scale; and dispersion which imparts a high anionic charge to keep discrete crystal structures apart.

Major classes of anti-scalants and dispersants include phosphonates, polyphosphates, polymer polycarboxylates, and sulphonated polymers.

Defoamers

Defoamers are usually chemical agents that are insoluble in the foaming liquid medium, and that have a low viscosity and surface-affinity that allows dispersion across the liquid/air interface, where they act to rupture bubbles that comprise the foam.  Defoamers can be grouped into several broad classifications including silicone/silica based, oil based, water based, ester based, and surfactants.

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