June 1, 2009

Focus on Organics in Steam

David G. Daniels

Organic Feedwater Treatment Chemicals

Currently, the only approved chemicals for boiler and feedwater treatment in the EPRI Cycle Chemistry Guidelines for Fossil Plants are:

• Ammonia for feedwater pH control.

• Hydrazine (N₂ H₄) for control of the oxidation-reduction potential in mixed metallurgy units. (All ferrous units typically do not use hydrazine.)

• Sodium hydroxide and trisodium phosphate in the boiler water of drum boilers for units using phosphate or caustic treatments.

None of the EPRI guidelines describes suggested chemicals used for water treatment that contain carbon. EPRI and all the major turbine manufacturers rely on cation conductivity as the most reliable indication of the presence of part per billion levels of chlorides and sulfates in steam. In addition, for the large 2,600-psig conventional fossil-fired power plants with consistent high-purity demineralized water and all-ferrous metallurgy, non-carbon-based chemicals have proven to be very successful at maintaining a passive oxide layer in the boiler and preventing corrosion on the turbine.

However, more and more generating stations do not fit this mold. Combined-cycle power plants with air-cooled condensers turn to organic amines to provide a higher pH condensate to prevent corrosion of the air-cooled condenser tubing. There are still many units that have copper-alloy feedwater heaters that will not be replacing their heaters in the near future. These units have long benefitted from the use of amines to protect the copper metallurgy. Amines may also be needed in plants that see some level of NOM in their makeup water to provide an alkaline pH boost to the first condensate in the turbine.

Remember that over the more than 60 years of using neutralizing amines in all types of industrial and utility boilers, to date, no turbine failures can be tied directly to the use of these organic feedwater chemicals. The record includes an extensive history of amine use in pressurized water reactor nuclear steam generators, where amines successfully reduce the rate of two-phase flow-accelerated corrosion.

A variety of neutralizing amines also are used to raise the pH of feedwater and steam condensate. The two most important characteristics of these amines are their basicity and the volatility of the amine. The basicity of the amine determines how much will be required to raise the pH of the feedwater to the desired level. This in turn determines the effect of the amine on cation conductivity. A high basicity amine will be able to produce the desired feedwater pH with less chemical and less cation conductivity. The volatility of the amine determines at what point in the steam cycle the amine will begin to condense.

If protection of the first condensate in the turbine were the desired outcome, then a low-volatility amine would be preferred. If a plant has an extensive network of steam heating coils or sends steam across the fence and recovers and reuses the process condensate, a high-volatility amine is preferred so that the amine will stay in the vapor until it condenses at the end of the pipe. Often, more than one amine is required to cover a plant’s needs. Blends of two and three different amines are common.