BEST PRACTICES: Mid-Georgia Cogen

Logic upgrades improve emissions control during transients

Mid Ga Cogen BP

Challenge. This 2 × 1 combined cycle, powered by Siemens 501D5A gas turbines, relies on Nooter/Eriksen HRSGs equipped with SCRs to limit NOx emissions. The reagent is aqueous ammonia.

When the plant was commissioned, the control logic implemented relied on ammonia flow transmitters to hold NOx emissions within the facility’s Title V permit limits. The control-valve logic worked well when the plant was at full load; however, during transients it was unable to maintain emissions at prescribed levels.

Also, control-valve tuning problems often resulted in inefficient use of ammonia. The corrosive nature of aqueous ammonia was conducive to fouling of transmitter sensing lines, causing inaccurate flow readings.

Both conditions required personnel to manually operate the ammonia control valve to hold NOx emissions in compliance. There were several instances when the failure of an ammonia flow transmitter nearly caused the plant to exceed permit limits.

Solution. Plant staff implemented tuning changes to control NOx emissions using the analog output of the CEMS NOx analyzer. Control-valve demand was tuned to account for the time delay that it takes the hot gas to travel through the HRSG to the NOx analyzer probe in the stack and down the sample line to the CEMS analyzer. This tied the emissions control system to the monitoring system, creating a more reliable method for controlling stack discharges.

Results. Since implementing the logic changes, the ammonia control valves have operated in “Auto” with no operator intervention required to deal with logic-related equipment failures. Mid Georgia Cogen also has realized more efficient use of aqueous ammonia by maintaining the control valve NOx set point closer to the Title V permit limit.

Project participant: Timothy Brooks Waddle Jr

Boiler-feed-pump seal-water mods

Challenge. The site relies on two Ingersoll-Dresser boiler-feed pumps (BFP) to supply water to the HRSGs’ HP drums. The pumps depend on cooling water from the condensate system to lubricate and cool their mechanical seals. By original design, the BFP seal water was supplied via a manually operated valve.

Because operators have many duties during shutdown, the seal-water supply was not always isolated as soon as the BFP was secured. With seal water continuing to flow to a secured BFP, the mechanical seal would experience premature failure.

Solution. Plant staff replaced the manually operated seal-water isolation valve with an automated valve equipped with open and closed limit switches. DCS logic was developed for the automated valve to open with a BFP start and close with a BFP stop.

To provide confirmation beyond valve position, a downstream pressure switch was installed with feedback to the DCS. A manual bypass valve also was installed to increase reliability in the event the automated valve failed to open.

Results. By automating the seal-water isolation valves, plant personnel have eliminated premature mechanical seal failures. Plus, BFP reliability has increased because the control room operator has indication of seal-water valve position and confirmation of seal-water supply pressure.

Project participant: Keith Charles

Mid-Georgia Cogen

Owned by Southeast PowerGen LLC

Operated by Consolidated Asset Management Services

300-MW, dual-fuel, 2 × 1 combined-cycle cogeneration facility located in Kathleen, Ga. Plant now owned and operated by Cogentrix.

Plant manager: Keith Charles