Woodland Generation Station, Unit 2
Modesto Irrigation District
83-MW, 1×1, LM6000 combined cycle located in Modesto, Calif
Plant manager: Paul Mercer
Challenge. The Modesto Irrigation District (MID) Woodland Generation Station Unit 2, an LM6000 combined cycle gas turbine commissioned in 2003, has long faced operational challenges in meeting its stack NOx and ammonia slip limits during transitions between fired and unfired operation. These challenges required operational modifications to always stay in emissions compliance.
Solution. SCR Solutions LLC conducted a detailed inspection and analysis using advanced computational fluid dynamics (CFD) modeling to evaluate the existing SCR and AIG performance. The CFD model revealed severe distribution imbalances, with ammonia concentration too high along the top and bottom of the duct and too low along the side walls, leading to ineffective NOx reduction and high local ammonia slip. The original AIG also featured large flow bypass zones due to poor mechanical design.
It was determined that a complete AIG redesign was required. The new best practice retrofit included:
- Increasing number of AIG lances from 24 to 50
- Increasing number of injection holes by 2x
- Adding top and bottom AIG lances to address dead flow zones
- Installing integral mixing plates on each lance
- Incorporating side wall fairings to eliminate bypass flow
- Extending upper and lower flow baffles to enhance flow uniformity
Engineering efforts included laser scanning of the system to compensate for incomplete OEM drawings and developing a precise 3D model for fabrication. All materials were fabricated in New Jersey and shipped to the Modesto site. Installation was completed on an aggressive schedule, with demolition of the old AIG and installation of the new system.
Results. The retrofit yielded substantial performance improvements. Fig 1 shows a before and after comparison of the ammonia distribution at the SCR catalyst face. The original design produced high ammonia concentrations at the top and bottom of the catalyst bed.
In addition, there were low ammonia concentrations along the side walls. These poor distributions created poor SCR performance. The modified design greatly improved the distributions of ammonia, both along the top and bottom and also along the side walls.
After the AIG was installed, MID ran a stack test to confirm the stack NOx, ammonia slip and ammonia flow (Fig 2). The table below shows a comparison of the before and after results where ammonia slip and consumption was reduced by 83% and 16% respectively.
As a result of the retrofit, the following were the benefits realized:
- Environmental Compliance. Consistent NOx and ammonia slip compliance across all operating modes.
- Operating Cost Savings. Reduction in ammonia consumption translates to tangible annual savings.
- Catalyst Longevity. Improved distribution reduces localized ammonia slip and extends catalyst life.
- Improved Plant Flexibility. Reliable NOx control across varying loads and during transitions between fired/unfired modes.
- Fast-track Execution. Project engineered, fabricated, and installed within ~12 weeks total, minimizing downtime.
Project participants:
Paul Mercer, plant manager
Richard Behler, generation supervisor
Michael J Miller, environmental compliance specialist
