Area: Plant Reliability Improvement Projects
As operators retire coal-fired plants, they will be faced with the challenge of cycling, according to Bill Siegfriedt, project manager at Sargent & Lundy*. Most of the recent combined cycle plants were originally built for base load operation, he said. Yet they find themselves tasked with cycling duty to fit the needs of a grid holding an abundance of variable renewable resources. Unfortunately, many of the design techniques to equip CC plants for cycling duty are not suited for retrofit.

The original intent of most CC plants assumed full load operation and infrequent starts, no quick starts and low capital costs for a short-term payback, said Siegfriedt. Cycling plants, on the other hand, run only on higher demand days, times or seasons, and typically at part load. For plants not designed for this kind of duty, it can lead to premature wear, excessive maintenance and equipment failure, said Siegfriedt. You have to take a close look at the existing plant to find the potential for modifications. However, some features are not suitable for retrofit.

He explained that it is possible to retrofit many older CC sites for reduced load operation, rapid and frequent starts, and high availability. Procedures and control systems can be adapted to assure that a CC plant with multiple GTs will turn down while avoiding inefficient GT part-load modes. Also, conversion to sliding pressure operation can improve efficiency of the steam cycle in part-load situations.

The need for rapid starting should also be addressed. Large heat transfer tubing requires a heavy wall and contains a great deal of water, which requires gradual warm up. A Heat Recovery Steam Generator (HRSG) retrofit, to a faster-heating design, would be financially unattractive. That said it is important to keep the steam cycle warm to speed up starting as much as possible.

This can be done by adding: a stack isolation damper; steam sparging to the HRSG mud drum; and LP condensate recirculation between the evaporator and the economizer. Of course, the ramping of the GTs can be uncoupled from the HRSG warm up requirement by using Inlet Guide Vanes (IGV). These can help match the temperature of the flue gas entering the HRSG to allowable levels.

The steam turbine also should be addressed. Installing attemperation for the main steam and reheat steam will uncouple the steam turbines warm up rate from those of the GTs and the HRSGs. Adding a steam bypass to the condenser will reduce the venting of excess steam and cut down on demineralized water make-up requirements. Some steam will still be vented, so it may also be desirable to increase makeup water treatment capacity. Keeping the combustion turbine and steam turbine warm is a good strategy to be able to better deal with faster or more frequent starts, said Siegfriedt.

Further tips: invest in redundancy so that if one system fails, you can maintain high availability; review trips and start failures to determine what systems or components are causing repeated events; upgrade control algorithms to provide intelligence to operators so they can more reliably follow load patterns; and automate control of the steam bypass to assure accurate dumping from HRSGs to minimize steam pressure swings.