Condensate Oxygen Control In A Combined Cycle System Without A Conventional Deaerator - Test Results
ABSTRACT
An improved efficiency combined cycle power plant was provided by Bechtel for the Gilroy Energy Company. This enhancement of cycle efficiency was achieved by eliminating all feedwater heating in order to extract the maximum amount of heat from the Heat Recovery Steam Generator flue gas, resulting in a feedwater temperature of 110° F. A new approach to oxygen control was necessary because the system requires makeup water flow varying from 25 to 85% of condenser outlet flow without the use of a conventional feedwater deaerator. This project confirmed that 02 levels can be controlled to 0.005 cc/liter under severe makeup conditions without chemical additives or the use of a conventional deaerator.
One essential component of this unique design is the combined vacuum deaerator/condenser unit. A second major aspect of the design involves the enhanced venting system. Limited testing prior to startup and operation of the Gilroy Energy power plant indicated that oxygen in the boiler feedwater could be controlled satisfactorily with a total system approach which included both the vacuum deaerator and enhanced venting. Subsequently, further testing of a vacuum deaerator has yielded 02 levels at or below the design requirements of 0.005 cc/liter. Additionally, field test data of 02 levels from the Gilroy Energy plant are presented to verify the successful operation of the condensate oxygen control portion of this cogeneration unit.
PROBLEMS WITH DISSOLVED OXYGEN IN CONDENSERS
With the advent of two-shift and cycling operation of base load power plants, the problem of effective removal of noncondensible gases has increasingly been addressed by both condenser designers and utility end users. Power plants exporting steam for district heating (as well as cogeneration plants) are faced with stringent deaeration problems inherent in introducing large amounts of makeup water saturated with oxygen into the condenser. Any plant requiring deaeration of makeup water exceeding 3 - 5% of the total condenser flow rate (or that is required to remove oxygen from stored condensate on daily restart) must find a method of eliminating oxygen from the water other than introducing it over the condenser tubes.