King County South Plant Power Cogeneration (Phase 1)

City of Seattle, 
Washington

King County South Plant Power Cogeneration (Phase 1)

Service Type: 
Project Highlights: 
  • Evaluation of two technical alternatives that best meet the King County Wastewater Treatment Division’s (WTD) objectives for cogeneration power supply – an 8-MW combustion turbines alternative and a commercial-scale power plant. The 8-MW alternative was ultimately recommended.
  • Design and construction phase services for an 8-MW plant as a dual-gas turbine generator-based, combined cycle cogeneration plant.
  • Facilities were constructed and brought online in 2007.
King County South Plant Aerial
Our team consisted of many of the same individuals that worked with King County to provide design of an 8-MW cogeneration facility at the South Treatment Plant.

Evaluation

Potential cogeneration/standby power systems for King County Department of Natural Resources and Parks’ South Treatment Plant have been reviewed several times in recent years. While wastewater treatment plant digesters produce gas that can potentially be used for cogeneration, the primary reason for investigating cogeneration options has been a concern about power quality, and, more recently, vulnerability to electricity price fluctuations. The South Plant is served by a dual-feed system to satisfy the plant’s National Pollution Discharge Elimination System (NPDES) permit requirement for redundant power.

R.W. Beck, along with Carollo, conducted an updated evaluation of cogeneration, which included:
• A peer review of previous studies and support for an evaluation of a proposal by Puget Sound Energy.
• Refinement of alternatives; evaluation of economic and non-economic factors; and assistance with selecting a preferred technical alternative(s).
• Advice on future power market trends and risks.
• An evaluation of project delivery approaches and assistance in identifying a preferred project delivery strategy or strategies.
• Preparation of a preliminary project implementation plan.

The review and evaluation of alternatives identified two technical alternatives that best meet the King County Wastewater Treatment Division’s (WTD) objectives for the project – an 8-MW combustion turbines alternative and a commercial-scale power plant. These two alternatives represent very different approaches to providing power at the South Plant.

While the commercial-scale power plant alternative appears attractive from a number of perspectives, it would represent a significant change for the South Plant and the WTD, which has not historically been involved in the commercial power generation business. In addition, the actual terms offered by a developer have not been thoroughly tested in the market. On the other hand, the 8-MW combined cycle gas turbine alternative would offer challenges to the WTD’s operations staff, who have no experience operating such a facility.

In light of these issues, the R.W. Beck/Carollo report recommended that King County pursue a “parallel path” development strategy prior to the final identification of a preferred alternative.

Preliminary Design

Previous cogeneration facility evaluation work performed by R.W. Beck and Carollo identified two technical alternatives that best meet the King County Wastewater Treatment Division’s (WTD) objectives for this project—an 8-MW combustion turbines alternative and a commercial-scale power plant. These two alternatives represent very different approaches to providing power at the South Plant. This follow-on project consists of three phases:

Phase 1 - Select the best option and develop contracting options for the 8-MW option.
Phase 2 - Recommend implementation of the selected option and contracting method.
Phase 3 - Project implementation.

In the Phase 1 work, the R.W. Beck/Carollo team developed information to allow the WTD to decide between the two substantially different courses of action. For the 8-MW cogeneration plant, work included developing the preliminary design documentation. For the commercial power plant, the purpose of the work was to result in “real-world” input and offers from developers. The team completed much of the work for these two options in parallel. Certain evaluations—defining plant interface requirements and permitting constraints—constituted a coordinated effort in order to save time and money.

The Phase 1 work was completed in September 2002. The Phase 2 work, final implementation of the recommended 8-MW cogeneration plant option was completed in August 2003. Phase 3 work was completed in 2004, and the project was constructed and is currently operational.

The team designed the 8-MW cogeneration plant as a dual-gas turbine generator-based, combined cycle cogeneration plant. Two 3.5-MW gas turbine generating units were prepurchased and construction documents finalized to install these generating units along with all required support equipment into a new 8-MW cogeneration plant.

Electrical work for this project consisted of prepurchasing 13.09 kV switchgear lineup for the cogeneration facility, and design of all required plant electrical system modifications to integrate the cogeneration system into the plant's electrical distribution system as well as to interconnect with the local utility, Puget Sound Energy, 115 kV plant main switchgear.
Instrumentation work for this project consisted of prepurchasing a complete SCADA system for the cogeneration facility, and design of all required cogeneration plant instrumentation and controls.

The plant was constructed and brought online in 2007.