Securing added life and revenue from idle assets through repowering is a common topic within the power industry as coal-fired plants continue to be decommissioned worldwide. While those conversations largely revolve around the benefits and what site characteristics would make an ideal repower project, few delve into the details — specifically the importance of commissioning.
Repowering a conventional coal-fired facility entails replacing the conventional boiler with one or more combustion turbine generators (CTG). The hot exhaust from the CTGs is then directed into a heat recovery steam generator (HRSG), where the energy in the exhaust is converted to high-pressure steam. That steam is then used to power the existing steam turbine. The term combined-cycle power plant comes from the fact that the repowered facility uses two thermodynamic cycles (Brayton and Rankine) to generate electricity. The net benefit is greater power output while increasing thermal efficiency.
Because a repower uses both new and existing equipment, no two projects are alike. The unique nature of a repowering project is well-served by involving an experienced commissioning team from concept to completion. Converting a coal-fired facility often requires blending over 40-year-old equipment with modern technologies. Successfully integrating these components requires an innovative approach to solve the numerous issues that will undoubtedly arise. By having commissioning specialists work with the engineering team throughout the design process, they can provide a unique perspective in identifying and resolving these issues. Such perspective can mitigate project risks early, resulting in a project that can be recommissioned in a safe, smooth and efficient manner.
Although the gas turbine/HRSG components of a repowered facility are critical, the auxiliary equipment is sometimes overlooked. This oversight can often prove to be the Achilles’ heel of a repowering project. Commissioning specialists perform realistic condition assessments of these seemingly minor assets, determining what can be reused, what requires modifications and what should be replaced. Through this assessment, these activities can be integrated into the project schedule early enough to minimize the impact that long lead times will have on the facility's return to service.
The restart of the newly configured plant will require many of the same unique activities that were performed when the facility was new, such as piping flushes, steam blows and chemical cleaning. When the commissioning professionals get involved early, they can work with the engineering team to incorporate the appropriate jumpers and access points in the design, eliminating the need to perform long and expensive field modifications.
Commissioning also considers what resources beyond routine operations might be needed during the restart process and what existing equipment can be modified to support the restart effort. For example, since feedwater heaters are not reused in a conversion to combined-cycle operation, the large drains from these heaters into the condenser are often simply welded closed. By modifying one or more of these welded caps to incorporate a blind flange, the hotwell can be used to store demineralized water for use during the steam blows that will take place as part of the restart effort.
The HRSGs will also require a chemical cleaning before they can be placed in service. Involving the commissioning team early in the site modeling process can see that adequate planning is done to efficiently execute the cleaning operation.
Repowering necessitates major modifications to the steam piping as well. This piping must be cleaned by conducting steam blows before being placed in service. To prevent steam from entering the steam turbine during steam blows, special blow kits are installed in all the turbine stop valves to prevent emissions. These blow kits are provided by the steam turbine’s original equipment manufacturer (OEM) for new purchases but are often unavailable for older units. Typically these kits must be remanufactured using the OEM-supplied drawings for the four-decades-old units. Early procurement of these kits will allow the steam blow operation to be performed on time.
As part of the steam blow planning process, a clear cleanliness criterion should be established. This usually comes from the steam turbine OEM, but since the steam turbine already exists the standard for acceptance may not be well-defined. When drafting the steam blow plan, this criterion should be included, as well as who will be the party that decides when that criterion has been reached.
Although most repowered facilities have an experienced operations team, the repowered facility will require a very different operating methodology. By incorporating the operations team in the recommissioning effort early on, team members gain invaluable experience operating the newly repowered plant. Frequent shutdowns or trips due to inexperienced operators can increase the number of required equipment inspections and overhauls, negatively impacting unit availability.
To help provide a seamless handoff to operations, commissioning teams incorporate plant personnel in the recommissioning process as soon as the operations team is available. This allows the operations group to spend time with the vendor technical specialists for both the gas turbines and the HRSGs. Doing so maximizes on-the-job training and helps operations teams adopt the new operating philosophy of the repowered plant.
Early integration of commissioning is vital to mitigate risks and deliver a reliable repowered plant. Early discussions among the engineer, owner and equipment OEMs can be moderated by the commissioning team to minimize any negative project impacts later in the project. Experienced firms — like Burns & McDonnell, which has worked on repower projects for more than 30 years — understand the importance of the details that commissioning will encounter and have background knowledge to determine optimal solutions.
Cooperative Energy partnered with Burns & McDonnell to nearly triple unit generation capacity with a heat recovery repower approach, becoming the most efficient coal-to-gas repower project in North America.