Coal

Repowering a Shuttered Plant: A Feasible Alternative for Decommissioning

Issue 4 and Volume 119.

The Hunlock site’s ash pond was closed and new transformers were added to the switchyard. Photo courtesy: Stantec
Two LM6000 CTGs were each paired with one HRSG in the gas conversion. Photo courtesy: Stantec

As regulatory restrictions have increased and renewable and cleaner sources of energy have become more available, smaller, less-efficient coal fired power plants are being forced to close.

The consequences of these closures remove a source of electrical power, reduce the number of available jobs, and reduce the economic well-being of the local community. A number of plant closings have led to either shutting the plant down and selling off their assets or leaving them as is (mothballed or simply shuttered). But there are other alternatives, including transforming the plant into one that uses another energy source – particularly shifting from coal to natural gas. Repowering can be both a practical and economical solution for the owner, the electric grid and the community.

Why Repower?

Reusing a former power plant for a new type of power generation can work well for many reasons:

  1. The community was built around power generation. Many traditional power plants are in rural locations, close to the fuel source they relied on. The economy of the surrounding communities depends on the plant for employment, purchases of goods and services, and financial support. Replacing that source of jobs and income with a similar use can maintain stability, not to mention take advantage of the community’s existing acceptance of a power-generation-related use.
  2. The locations of most plants aren’t ideal for commercial or retail-based uses. With their often rural locations around small communities, power plant sites aren’t typically in high-traffic areas suited for commercial redevelopment. While there are exceptions, in most cases a more industrial type of use is a better fit. In addition, reusing a power production site for any sort of non-industrial purpose may take a significant amount of environmental remediation, requiring more time, money, and risk than repurposing the site for another power use.
  3. Permitting is generally easier to obtain for a power-related use. Given the former power generation use of a site, permitting for the site has historically centered on power generation uses. Developing a new, cleaner, power source will be more easily accepted by the surrounding community, and using an existing facility that is producing cleaner energy will help the compliance requirements for the permitting process.
  4. The infrastructure for power use is already in place. With the steam turbine, switch yards, intake structures, access, grid connections, and other power plant infrastructure already on site, the costs of reenergizing the site as a new type of power plant will be significantly lower than those needed to redevelop the site.

A Case Study: Hunlock Power Station

With those benefits in mind, one coal plant in Pennsylvania recently went through this type of conversion after a lengthy evaluation of whether repowering would work for the site.

By 2008, the 44-MW coal-and-oil-fired Hunlock Power Station had been operating for over 50 years and was facing some significant environmental upgrades to be able to continue functioning under changing regulations. The plant was originally constructed as a coal-fired power plant with a steam turbine generator. The original coal-fired units included ash storage and handling and a steam turbine generator with condenser, pumps, and intake structure, generating 44MW. Four 66-kv transmission lines connected the plant to the grid. With so much useable equipment available, the idea of modifying the site for another type of power generation did seem feasible. The owner, UGI, decided to engage an engineering consultant to assess the idea and develop a concept for how it could work.

With so much of a power plant’s site already dedicated to industrial equipment and infrastructure, finding a new industrial use for the site was feasible. Photo courtesy: Stantec

The resulting plan reused as much of the existing infrastructure as possible to replace the coal-fired generation with new natural gas generation. However, the team had a number of challenges to address before determining if repowering the plant truly was a feasible alternative.

  1. Power demand. The study included an analysis to determine if demand for power warranted continued generation. The coal generation was being decommissioned because of the high cost per MWH resulting from the added equipment to comply with regulatory requirements. Historical data was used to project peaking power requirements that Hunlock could support if more economical energy could be produced.
  2. Natural gas supply. Typical construction costs for gas pipelines are approximately $1M per mile and can quickly drive total installed costs higher than economically feasible. The study confirmed a 16″ diameter gas line within a mile of the site that had sufficient pressure and capacity to supply new gas fired GTGs. The cost of a new pipeline to connect to the main gas pipe line was financially acceptable.
  3. Suitability of existing infrastructure. The study examined the condition and applicability of the existing equipment and buildings to support repowering of the plant. The existing STG was reviewed to determine the potential to adjust the operating characteristics to support a new steam source, including its operating pressure, temperature, and steam flow. The review also assessed the type of turbine extraction arrangements and how they could be modified and, if extraction ports are blinded, would there be adequate size for full flow through the back end and into the condenser. The equipment supplier for the STG could also be a factor in how well it would convert.
    The study also determined the requirements for reuse or reworking of the BOP equipment for use in a new CC plant, if support infrastructure like the switchyard, transmission system, or existing building would need upgrades, and if the control systems would require many changes or additional costs. The removal of the existing boilers was also reviewed to see if they had to be removed or if they could remain in place and decommissioned. Factors considered were the presence of hazardous materials and the cost to demolish the boilers versus decommissioning
  4. Permitting requirements. The original plant permits were reviewed for applicability. The team had to determine if the existing permits still applied (perhaps with some modifications) if the use was considered similar enough. In addition, the requirements for new regulations were reviewed, including regulations that were being considered, such as 316b and carbon capture.
  5. Local community support. Local political or social opposition can be difficult to overcome. Due to the long-term site use, the retention of jobs, and the change in fuel type, the community was supportive of the project.

After researching answers to these questions, the team determined a natural gas conversion could work and proposed a design approach that involved adding two gas-fired CTG units that operate with the exiting steam turbine, condenser, water pumps, intake structure, condensate pump, substation, and transmission lines. In this approach, two LM6000 CTGs (a total of 80 MW) were each paired with an HRSG to recover the exhaust energy and convert it to steam. The steam was piped to the steam turbine. The steam turbine was refurbished, extraction ports were modified, no feed water heater was required and the turbine was de-rated.

In addition to a new design for the production equipment, the design included some modifications to the site infrastructure as well. The ash pond was closed (a major environmental consideration), and the existing transformers in the switchyard were removed. Two new transformers were added for the new CTGs and a new transformer was added for the steam turbine.

The design also addressed a few related challenges. The design included a new control system (and updates to the control room) to combine the two new CGT units together with the existing one. At the same time, the design updated the electrical systems for the reused equipment to, again, ensure all the systems were integrated and up to modern specifications for gas-fired systems. It also ensured the plant would comply with the new cooling water intake (316b) standards that were being considered during the design process (since enacted). Because the water source and basic plant size remained the same, it was not considered a new facility and was grandfathered into its previous compliance, avoiding a potentially expensive upgrade.

As a result, the new system now has the capacity to generate approximately 125 MW of electricity – upgrading it from producing coal-generated electricity for roughly 9,750 homes, to providing power for 27,075 homes with clean-burning natural gas. In fact, NOx, SO2, PM2.5, and CO carbon emissions decreased an average of 95% or more compared to the former coal plant emissions, and the total amount of CO2 emitted dropped below 1000 lbs. per MWH. Converting the plant also incurred significantly lower costs than building an entirely new one. Despite initially intending to run the plant as a peaking facility, production has been so reliable and cost effective that it’s now one of the region’s base load electrical power sources.

To Convert or Not to Convert?

For the owner and community surrounding Hunlock, converting the former coal-fired plant to a natural-gas-powered plant combined older, existing technology with new technology and a new fuel source.

The result was a cleaner source of power and new revenue for the owner and community. The keys to this success were the demand requirements, the available source of fuel, the condition of the existing equipment, local support, and the creativity of the owner and the engineering support.

Not every shuttered plant will have all factors align perfectly.

This example demonstrates that repowering a plant is a feasible option in many cases and should be considered as one alternative when decommissioning discussions begin.

By conducting a thorough and honest assessment of a facility’s infrastructure, site, resources, and challenges, power plant owners may find a practical, profitable decommissioning solution that benefits the utility and the community as a whole.