Oxygen Scavenger Chemistries

As has been discussed at length in this book, it is almost always necessary to remove all traces of oxygen in the boiler plant system to protect 

Where oxygen removal is required, this includes the eradication of 02 from  

The level of DO in water is primarily a function of temperature and pressure, but salt concentration (or TDS) and some other parameters can influence oxygen solubility, and some variation may occur. Nevertheless, a good approximation is shown in Table 11.1. 

Smaller or lower pressure boilers usually do not have mechanical deaeration and thus employ only a chemical reactant for DO removal, which can be expensive if the FW temperature is low and less than totally effective if the feedlines are very short. Treatment is by the direct injection of a suitable oxygen-scavenging chemical into the FW system. The feed rate requirement is based on combining a quantity proportional to the level of oxygen present, plus a further amount that contributes to a BW reserve. 

Ideally, the oxygen scavenger is added continuously rather than periodically shot-dosed where batch tanks are employed, there should not be more than one day s supply of oxygen scavenger in the tank. The tank contents should not be agitated any longer than necessary, to avoid aerating the mix. 

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Carbohydrazide is a derivative of hydrazine that hydrolyzes in the boiler to produce hydrazine, although CHZ is an oxygen scavenger in its own right. Other derivations of this oxygen-scavenging chemistry now exist, including polyacrylic hydrazide. 

All large boiler plants and many smaller units employ mechanical deaerators to remove oxygen. In addition, various oxygen-scavenging chemicals are employed as consumable treatments such as sodium sulfite-bisulfite, hydrazine, various novel chemistry organics (hydrazine replacements), and certain tannins. 

Here, the use of inhibitor formulations having a less dramatic effect on TDS (such as certain tannins) may be extremely beneficial. Formulations are available that are based on tannin chemistry and contain blends that act as oxygen scavengers and metal passivators, with additional sludge dispersant and antifoam properties. 

NOTE This chart assumes oxygen scavenger, polymer, and other necessary water chemistry controls are in place. If the FW is fully softened, there is no benefit in using this program. Employ a phosphate-cycle or alternative program. 

NOTE This chart assumes oxygen scavenger, polymer, and other necessary water chemistry controls are in place. 

Tannin chemistry is complex and tannin blends (composed of singletree species tannins mixed together) offer a number of useful water treatment functions. Apart from oxygen-scavenging abilities, they act as passivating agents, film-formers, sequestrants, and polymeric sludge dispersants (for more details, see Section 10.3). 

Alkalinity boosters, which are vitally necessary to enable carbonate, phosphate, balanced polymer (polymer plus phosphate or chelant), and some other program types to function, are perhaps best described as conjunctional treatments, whereas oxygen scavengers, antifoams, and condensate line corrosion inhibitors are adjuncts. Programs such as phosphate-tannin mixtures are sometimes described as adjunct treatments. The chemistries and applications of various types of conjunctional treatments and adjuncts are described in this chapter. 

Reardon, P. A. Kelly, J. A. New Oxygen Scavengers and Their Chemistry Under Hydrothermal Conditions. Nalco/NACE International Corrosion Forum, 86. NACE International, USA, March 1986. 

Attempts are made to minimize corrosion by controlling the chemistry of the coolant water. Adding 7LiOH raises the pH to 8. One can use oxygen scavengers such as hydrazine to reduce the oxygen concentration. 

The chemistry of the feedwater to the steam generators is precisely controlled by demineralization, de-aeration, oxygen scavenging, and pH control. A blowdown system is provided for each steam generator that allows impurities collected in the steam generators to be removed to prevent their accumulation and possible long-term corrosive effects. In some reactors, the blowdown is collected and recirculated. 

Reardon, P.A. New oxygen scavengers and their chemistry under hydrothermal conditions. Corrosion 86, Paper No. 175 NACE Houston, TX, 1986. 

Wettasinghe M, Shahidi F. Scavenging of reactive-oxygen species and DPPH free radicals by extracts of borage and evening primrose meals. Food Chemistry. 2000 70 17-26. 

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