PROJECT: Twin 335 MW Units Complete Controls Tuning Program

These twin large fossil fueled generating units are each rated at 335 MW with cross compound GE turbine/generators and steam supplied by Combustion Engineering controlled circulation boilers rated at 2,490,000 lbs/hr at 2450 pisg, 1050F superheat, and 1000F reheat. The boiler design is somewhat non-traditional and looks upside down in architecture. The boiler utilizes tangentially fired burners located near the upper structures of the furnace area and a major portion of steam temperature control is performed by "tilting" the burner mechanisms to direct the flame path. In addition, this is a pressurized furnace design with only forced draft fans providing combustion air and gas recirculation fans.

An independent power producer (IPP) purchased this facility from Southern Cal Edison in the 1998 era of power plant sales. Shortly after the purchase, a major NOx reduction upgrade was installed to lower emission levels with the use of a Selective Catalytic Reaction (SCR) draft modifications. Additionally, the Bailey Net90 (aka: ABB DCS) system had been managed by Southern Cal Edison engineers but that expertise was lost with the plant sales. Subsequently, no formal control system tuning program had thus been implemented since the sale of the plant resulting in significant process changes without compensation within the control system.

For a number of years, plant operators manually dealt with many unstable combustion issues during load changes and numerous other unit stability problems with load, boiler pressure, steam flow, and steam temperatures. It was very common for the stack to "smoke" even with the use of natural gas as their fuel supply during a load change.

Engineering services were brought in to conduct a full control system assessment, instrumentation maintenance including all final control elements, and complete unit tuning program. The goals of the project were to stabilize the process, increase the load ramp rate, improve efficiency, reduce NOx excess incidents, and eliminate the combustion smoking safety issue.

Due to an immediate improvement in all of the goals achieved on the first unit, engineering services were continued to conduct a duplicate program on the sister unit and achieved similar results.