September 15, 2006

Organics in the boiler and steam: Good or bad?

David Daniels

Oxygen scavengers

Plants with copper-alloy feedwater heaters must use oxygen scavengers or reducing agents to achieve a passive layer of cuprous oxide on their copper tubing. Here again, there are a variety of chemical options ranging from eschewing carbon to using large organic molecules.

Hydrazine is the scavenger of choice for power plants, particularly those sensitive to organic compounds or their byproducts in steam and condensate. The molecule contains no carbon and is an aggressive scavenger and passivator of iron oxide. For years, carbohydrazide (which contains only one carbon atom per molecule) was marketed by Nalco Chemical Co. as ElimiNO_x; it is now available as a generic. At feedwater temperatures, one molecule of carbohydrazide forms two hydrazine molecules and one molecule of CO₂. As mentioned earlier, CO₂ does not affect the pH of first condensate in the turbine, although it does increase the cation conductivity of feedwater.

Other volatile oxygen scavengers in use are larger organic molecules such as DEHA (N,N,-diethylhydroxylamine) and methyl-ethyl ketoxime (both with 4 carbon atoms per molecule), and hydroquinone and erythorbate (d-isoascorbic acid), both weighing in at 6 carbons per molecule. The amount of these chemicals needed to achieve the desired oxygen reduction and copper passivity varies by chemical and application. What all the carbon-based scavengers have in common, however, is that they break down (either by reacting with iron oxide or under heat and pressure) to smaller organic molecules and CO₂. Obviously, the more of these chemicals you add to your steam cycle and the more carbon they have in them, the higher the concentrations of by-products in the steam and feedwater will be. If there are any corrosion ramifications of the presence of organic acids in the steam, the plant with the highest concentrations will be the first to see them. This is one of the reasons that many plants have gone back to hydrazine or at least carbohydrazide as their oxygen scavenger of choice and try to add only what is needed to keep the copper passivated.

Polymerized dispersants

Dispersants were among the first chemicals used to treat boiler water. Modern, acrylate-based boiler treatment polymers now have been available for more than 20 years. These long-chain organic molecules are still primarily used by industrial boilers and boilers that operate at pressures lower than those of the typical utility boiler. In an industrial plant, it may be very difficult to keep the amount of iron corrosion product that returns with the condensate to a desirable level. Such cases may call for a dispersant.

At stand-alone power plants, it is difficult to justify the use of dispersants. Their purpose is to disperse corrosion products and contamination long enough to be carried out with boiler blowdown. However, at plants that treat their makeup supply with RO/mixed-bed ion exchange or continuous deionization, the makeup water supply is consistently excellent. In these cases, the primary source of potential contamination is the condenser. If the condenser is tight, there is very little contamination entering the boiler and therefore little reason to open the blowdown. Without significant blowdown, the dispersant ends up circulating in the boiler for a long time. It is important to remember that the use of a dispersant also requires the use of the continuous blowdown to remove dispersed iron. Accordingly, the use of any high-molecular-weight organic molecule, such as a dispersant, in a system with high-quality condensate should be considered carefully.