Most combined-cycle power plants add an oxygen scavenger to their condensate & feedwater system. Indeed, 3 of my 4 combined cycle plants used an oxygen scavenger. The exception was a new plant where I set up the boiler water chemistry program. There are several different oxygen scavengers with hydrazine and carbohydrazide being the most commonly used.
Now, do you need to add an oxygen scavenger? What if I told you the use of oxygen scavengers often leads to tube failures in the Heat Recovery Steam Generator (HRSG)?
Most power plant operators were taught oxygen, in a boiler tube, causes corrosion. That statement, by itself, is untrue. While the presence of oxygen in a low pH environment is a recipe for corrosion; it turns out oxygen, in the presence of a high pH, actually prevents corrosion. This occurs because oxygen – with a high pH – produces hematite (Fe2O3) and magnetite (Fe3O4). These produce a tough, corrosion-resistant oxide layer on the HRSG tube surfaces.
Adding an oxygen scavenger changes the boiler water chemistry conditions from oxidizing to reducing. Now, the hematite formed is reduced to magnetite and the magnetite is further reduced, becoming soluble in the boiler water. This results in removing the protective oxide layer with corrosion and thinning of the tubes occurring. There is a flow component and this type of corrosion is known as single-phase Flow Accelerated Corrosion (FAC). In the HRSG, the susceptible areas are the low pressure (LP) and intermediate pressure (IP) section economizers and evaporators. As of the last publicly available EPRI HRSG Chemistry Guidelines, FAC still ranked in the top two for HRSG tube failures.
The Electrical Power Research Institute (EPRI) discourages the use of oxygen scavengers in combined cycle power plants . The exception is where the HRSG contains copper-bearing components. In these rare occurrences, an oxygen scavenger is required. If your plant adds an oxygen scavenger, the addition should be feedback controlled, using an ORP (Oxidation-Reduction Potential) analyzer, and the feed controlled to keep the ORP in the range of -300 to -350 mV. This will reduce the potential for FAC. Dissolved iron levels < 5 ppb are confirmation that minimal FAC is occurring.
Since the vast majority of HRSG’s do not contain copper bearing components, an oxygen scavenger isn’t needed. The boiler water oxygen should be removed by deaeration (Deaerator or LP Drum). The remaining oxygen – under proper pH control – will form hematite and magnetite on the tube surfaces, resulting in a tough oxide layer resistant to corrosion.
So go ahead and ditch your oxygen scavenger!
Combined Cycle Solutions, LLC offers HRSG boiler water chemistry consulting solutions based on EPRI guidelines. My services include complete program building, training, and troubleshooting.