June 15, 2006

Combined Cycle Users’ Group completes another successful year

Dr. Robert Peltier, PE

The third annual meeting of the Combined Cycle Users’ Group (CCUG) was held May 2–4 in Atlanta at Electric Power 2006, in cooperation with the ASME Power Division Combined Cycles Committee and other industry groups. The CCUG’s leadership (Figure 1) drives the group to address issues involving the major components of a combined-cycle plant and their interactions, which are many. With gas prices now dipping below double digits, such issues have never been more important to owners and operators of plants that are now operating on the razor’s edge of profitability and in modes for which they were not designed.

 

 

 

Presentations at the meeting touched on a wide variety of topics. Among the more eclectic was repowering combined-cycle (CC) plants to burn gasified coal. Among the more mundane were selective catalytic reduction (SCR) system maintenance, the effects of cycling heat-recovery steam generators (HRSGs), staff training, and adopting best practices in O&M. Judging from the lively discussions and questions following the presentations (Figure 2), it was clear that operating a power plant is still as much art as science.

 

 

Retrofitting CC plants for syngas

Kajal Mukherjee of WorleyParsons (Figure 3) got the ball rolling with a thought-provoking discussion of the technical requirements for “refueling” an existing CC plant with synthetic gas, or syngas. The consensus among most participants before the meeting was that such refueling is both on the horizon and not particularly challenging. But preconceived notions have a way of evaporating when their holders are faced with the cold facts. The technical challenges inherent in adapting a CC plant to burn syngas are no different.

 

 

Part of Mukherjee’s presentation focused on practical issues, such as shoehorning the vessels and support equipment for the gasification plant into an already cramped site. He also addressed the need to closely consider fuel supply, transportation, storage, and handling concerns if the gasifier is to be placed close to the CC plant. For many smaller CC plants, repowering would be unfeasible if the added gasification plant and its infrastructure end up larger than the power block. Mukherjee also touched on other salient repowering issues, including the need to handle and dispose of waste by-products, which would require a plant to obtain new environmental permits.

According to Mukherjee, if the syngas plant is located elsewhere and the gas is made available locally, the challenges of retrofitting existing equipment still remain. For example, the mass flow of the low-Btu syngas (approximately 300 Btu/scf for an air-blown gasifier) will be substantially greater than that of natural gas. Accordingly, piping, compression requirements, and combustor design issues become paramount, necessitating significant changes and modifications.

Although the additional mass flow may result in a 15% to 20% increase in power output (if the plant’s generator and switchgear are up to the task), the downside is that the HRSG may be unable to handle the increased mass flow unless it was designed to be highly fired in the first place. What’s more, the higher mass flows and the exhaust constituents will almost certainly wreak havoc with the long-term performance of an existing SCR and/or CO catalyst. Additional surface area in the HRSG, an economizer bypass to prevent steaming, and the ability to handle increased metal temperatures to obtain acceptable thermal performance also will be required. Finally, Mukherjee asked, what do you do with all that extra steam you’ll generate? Because the plant’s steam turbine-generator typically will be too small to convert it to electricity, much of the steam will end up in perpetual bypass unless other uses for it are found.

The trickle-down problems resulting from having significantly more exhaust mass flow than the original design makes refueling an existing CC plant even less palatable. For example, changes to steam piping and the ammonia injection grid likely will be necessary. Because more steam flow implies higher water flows, the plant’s makeup system and polishing system, pumps, valves, and the like may prove to be undersized. At the end of the day, repowering a CC plant could be much like renovating an old house: Lurking behind every wall are new “opportunities” to repair and replace equipment at considerable cost. Refueling opportunities may turn out to be limited when the economics are closely examined.

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