Owned by PG&E National Energy Group, and operated by Wärtsilä North America Inc.

Billed as the world’s largest natural gas–fired reciprocating engine generating station, the 111-MW Plains End Power Plant has successfully demonstrated that large distributed generation plants don’t necessarily have to be gas turbine–based. Plains End’s NOx and CO emissions are comparable to those of a plant powered by gas turbines with expensive catalysts, and its net efficiency comes close to that of a combined-cycle plant. One year’s operating experience proves that recip engines can compete with turbines for selected applications.

By Sami Myllyviita, Wärtsilä North America Inc.

In February 2001, Wärtsilä North America Inc. signed a contract with PG&E National Energy Group (NEG) to engineer, procure, construct (EPC), and commission a 111-MW natural gas–fired reciprocating engine power plant for peaking service in the Rocky Mountains above Denver (Figure 1). Plains End now produces electricity under a 10-year power purchase agreement with Minneapolis-based Xcel Energy, which automatically dispatches the plant. Plains End LLC is a wholly owned, indirect subsidiary of NEG, which is headquartered in Bethesda, Md.

1. Plains End.

Construction of the power plant facility started in June 2002, and only 10 months later the plant was ready for commissioning. Wärtsilä was more than just the plant’s EPC contractor; it operates and maintains Plains End for NEG.

Bigger is better

The innovative Plains End facility is the largest natural gas–fired, internal combustion generating installation in North America. It consists of a preengineered, prepackaged string of 20 gas-fired, spark-ignited reciprocating engine/generating sets from Wärtsilä.

The power plant consists of two power blocks, each with 10 generating sets. The blocks are located at the sides of a central complex housing the control room, electrical annex, and workshops. The 20 Wärtsilä 18V34SG units, each rated at 5.7 MW, generate electricity at a voltage of 13.8 kV, which is then converted to a distribution voltage of 230 kV by two step-up transformers in an adjacent substation.

The engine advantage

Beyond their ability to be deployed quickly, reciprocating engines have several important advantages:

• Unlike gas turbines, recip engines demonstrate consistent heat rate and output at mile-high elevations such as Arvada’s—even during hot summer days, when the ambient temperature can approach 104F. During its performance test, Plains End units achieved 44.2% efficiency (LHV) at full load, and 39.7% efficiency (LHV) at 50% load. Those numbers are significantly higher than those of comparable-size gas turbines at the same elevation.
  

• The plant requires little, if any, process water (Figure 2). No water is used for intake air conditioning, and the engines’ cooling circuits reject their heat to the atmosphere through closed-loop radiators rather than cooling towers. This provides both economic and social benefits, because in many parts of the U.S. sufficient water is either unavailable for use by power plants or prohibitively expensive.

2. Water wise.

• The plant can go from a warm standby condition—the normal shutdown state for the engines—to full output in 10 minutes. The plant was actually dispatched in this fashion several times during the first several weeks of commercial operation.
  
• The design permits several start/stop cycles per day, with rapid load increases and reductions, without negative impact on the engines. This allows the scheduling of maintenance inspections to be based on actual operating hours rather than on calculated “equivalent hours” that take into account the number of start-stop cycles, changes in fuels, and operation at overload conditions.
  
• The plant is able to maintain its low heat rate by optimizing the number and load set points of each of the 20 engines. A quantifiable technology benefit is produced by the ability of the genset to be rapidly cycled several times a day without increasing maintenance costs.
  
• The plant requires only 65 psig of natural gas pressure to operate at full load—much lower than that of a gas turbine.
  
• The modular design of Plains End (Figure 3) and similar plants eliminates the need to shut the plant down completely for planned maintenance (Figure 4). Individual gensets can be taken off-line for inspection and maintenance during off-peak demand periods. Wärtsilä’s recommended maintenance schedule includes a major inspection after 64,000 operating hours.
  

3. Plant configuration.

4. Modular advantages.

• The plant can be dispatched remotely with minimum or no operating staff.
  

Meeting environmental concerns

Local authorities were initially worried that Plains End might produce rising columns of exhaust smoke or continuous steam clouds, adding to those from power plants in the Denver metro area. However, the plant’s closed-loop radiators create no visible water vapor plumes. Likewise, its exhaust gases create no visible steam plumes. Nonetheless, the height of the exhaust gas stacks was kept to less than 60 ft to keep the plant’s profile reasonably low.

Plains End’s emissions are very low thanks to the technology employed by the reciprocating engines. They make use of precombustion chambers, individual cylinder temperature control, and lean-burn technology. CO and volatile organic compounds (VOCs) are further controlled by an oxidation catalyst. NOx emissions are controlled with selective catalytic reduction (SCR) technology, using urea as a reagent. At Plains End, stack emission levels have been well below the expected values of 18 ppm CO and 9 ppm NOx (see table, page 84).

Highly reliable and available

Moreover, when a specific plant dispatch mode is selected, the plant follows the AGC-initiated load demand by ramping the plant load up and down at a maximum rate of 12 MW/min between 50% and 100% of full-rated output. This enables Xcel to absorb load swings in its distribution system.

5. Remote control.

In its first 11 months of operation, Plains End achieved an availability of 98.7% while generating more than 154,587 MWh. Over this period, each of its engines operated an average of 1,463 hours.

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