Case study: Biomass power

Renewable electricity is generated from decomposing solid waste.

By Robert R. Jones Jr., PE, LEED AP; and Leslie Fernandez, PE, LEED AP, JBA Cons September 26, 2016

The Apex Regional Landfill north of Las Vegas generates renewable electricity from methane captured from decomposing solid waste. The facility uses two Solar 5.6-MW combustion turbine generators (CTG) to produce enough electricity to power more than 6,000 homes.

The engineering, procurement, and construction contractor for the generating plant was DCO Energy of Mays Landing, N.J., which specializes in renewable energy projects. The electrical systems were designed by JBA Consulting Engineers and the mechanical systems by Concord Atlantic Engineering of Voorhees Township, N.J. The plant is operated by DCO Energy. DCO Energy has a 20-year contract to supply electricity to the local utility, NV Energy.

Landfill gas is composed of approximately 50% methane and 50% carbon dioxide along with trace amounts of other gases including siloxanes, which are compounds containing silicon and oxygen. To prepare the landfill gas for combustion within the CTGs, it needs to be processed. The first step in the process is to pass the gas through a hydrogen sulfide-removal process, then blowers are used to increase the gas pressure enough to pipe it about a mile from the landfill up to the generating plant. It next goes through a chiller skid to condense and remove moisture, then through a compression stage, a siloxane-removal process, a carbon polisher to remove additional siloxanes (as needed), and then another compression stage to raise its pressure to about 270 psi.

After the gas is combusted, it passes through a selective catalytic reduction (SCR) system, which combines nitrogen oxide (NOX) in the exhaust gases with vaporized ammonia to produce nitrogen and water. Without this step, the NOX would turn into nitric acid, the main ingredient of acid rain. Apex is one of the few landfill generating projects to include SCR, making it one of the world’s cleanest generating plants.

Utility upgrades

The existing utility’s 12.47-kV overhead power lines between the existing facilities and the nearest transmission lines needed to be upgraded to increase transmission capacity from the Apex site. Approximately one mile of existing overhead cabling was replaced with 954-kcmil ASCR (Aluminum conductor steel-reinforced) cable while maintaining power to the existing facility. The utility had an intensive permit process for the new overhead line. Although the utility was only upgrading the existing overhead line, the line crosses over U.S. Bureau of Land Management (BLM) land, a Union Pacific Railroad spur, and Interstate 15, which is under Nevada Department of Transportation jurisdiction. Each agency had its own approval process and environmental requirements.

Finally, the utility installed a 15-kV switchyard within the Apex landfill site that provides a means to isolate and disconnect the generation site service from the existing customer utility service.

Customer utility interface

Just outside the utility 15-kV switchyard is the generation facility point of interconnect and service switchgear. It contains the code-required service disconnect as well as overcurrent and ground-fault protection. The utility additionally required a visible switch-position window at the service-meter equipment.

From the service switchgear, approximately 1.25 miles of new 12.47-kV overhead lines to the new generating plant were installed. At the generation plant, the utility installed a remote terminal unit (RTU) interface with connections to the plant supervisory control and data acquisition (SCADA) system, protection relays, generation meter, and input/output connections for the plant main breaker for status and remote operations. This RTU communicated to the utility control system located more than 6.5 miles away via fiber-optic communication lines.

The utility required the owner to have the following protective devices installed on the main breaker and coordinated with the upstream substation relay:

  • Over/under frequency relay (81)
  • Under/over voltage relay (27/59)
  • Time-overcurrent relay (51)
  • Synchronize-check relay (25)
  • Reverse-power relay (32).

Lastly, the utility required the following items from the plant SCADA system:

  • Gas turbine 1: kW, kVA, kVAR, generator automatic voltage regulator (AVR)
  • Gas turbine 2: kW, kVA, kVAR, generator AVR
  • Gas turbine breaker 1: open/close status, tripped lockout, transfer trip, breaker failure
  • Gas turbine breaker 2: open/close status, tripped lockout, transfer trip, breaker failure
  • Transfer trip communication fault alarm
  • Generation meter: kW, kVA, kVAR
  • Plant net meter: kW, kVA, kVAR.

Robert R. Jones Jr. is the associate director of electrical for the Las Vegas office at JBA Consulting Engineers. He has experience in multiple market sectors including hospitality, commercial, medical, and government projects.

Leslie Fernandez is senior project engineer, electrical at JBA Consulting Engineers. He specializes in renewable energy systems and complex medium and high voltage distribution systems.