Film-forming amine product as an alternative to carbohydrazide oxygen scavenger in high pressure boilers

Abstract

Hydrazine has been largely replaced by carbohydrazide (CHZ) as an oxygen scavenger due to safety and health concerns and CHZ is now used in Saline Water Conversion Corporation (SWCC) high pressure boilers. However, the operational problem of phosphate hide-out has become a continuous challenge for the plant operators. Advances in boiler water treatment have shown that effective corrosion control and prevention of scaling can be achieved by using a mixture of film-forming and alkalizing amines and polycarboxylates [Film Forming Amine Product (FFAP)]. With the use of FFAP, carbohydrazide/ammonia treatment of make-up water and phosphate treatment in the drum will not be required. A uniform FFAP formulation was used throughout the test.

The evaluation study was carried out at a boiler of the Yanbu Phase 1 Desalination and Power Plant (Mitsubishi) generating 60 MWh, with make-up water of 15 t h⁻¹ producing 330 t h⁻¹ steam at a pressure of 67 barg and temperature of 480 °C.

The trial provided evidence that FFAP was a good alternative to use of an oxygen scavenger. Changeover from CHZ to FFAP without phosphate addition in the drum was done initially by dosing FFAP from both hydrazine tank and phosphate tank so that pH was maintained to the required values in both feed water and drum water. With the optimal dose rate (0.6 ppm) maintaining FFAP in the range of 0.3–1.0 ppm in feed water, all the key parameters (pH, ammonia and specific conductivity) were within the specified boiler design limits. The average corrosion rates on the water side were low for both CHZ and FFAP treatment (0.009 ± 0.001 mm y⁻¹), however FFAP treated coupons showed much lower corrosion rates compared to CHZ in the steam side (0.0006 ± 0.0003 mm y⁻¹ cf. 0.0075 ± 0.0006 mm y⁻¹).