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Peaking power relief

By Charlotte Gliddon-Bush

Wärtsilä recently won a contract to install its PeakingPlus units at a PG&E plant in Colorado, USA. When the plant becomes operational in April 2002, it will be the largest in the US to use natural gas-powered reciprocating engines.

The deregulation process in the USA has led to high price spikes with utilities paying premium prices for power during peak periods. Wärtsilä NSD has designed a new breed of pre-engineered and prepackaged gas-fired power plants to help utilities meet this challenge, and the company recently received its largest order for the equipment.

The 18-cylinder 34SG engine has an operating speed of 720 r/min. The 34 cm cylinders are configured in a V-block design
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PG&E National Energy Group has contracted Wärtsilä for the installation of a turnkey 110 MW PeakingPlus plant near the city of Plains End, Colorado. The plant will be based upon 20 Wärtsilä 18V34SG natural gas engines, and will be the largest plant in the US to use natural gas-powered reciprocating engines when it comes on line in April 2002.

PG&E contacted Wärtsilä in June of this year to be the engineering, procurement and construction (EPC) provider for the PeakingPlus plant. When the plant becomes operational on April 1, 2002 it will supply power to Public Services Colorado under a ten year power purchase agreement. The plant will operate for up to 6000 hours per year, with ten months notice to proceed.

Thomas M. Carbone, president of Wärtsilä North America said: “Distributed power generation with PeakingPlus will help Plains End maintain its high level of service and low rates for electrical services, while mitigating the risks of power shortfalls.”

Peaking needs

PeakingPlus was introduced in October 2000 and is a line of modular, preconstructed, 5.7 MW power units that can be slotted together to give a power output that matches the user’s needs. The units allow users to generate power when electricity demand and price are at their highest, and were designed to address the unique peak and intermediate energy requirements of the US market.

Kelly Speakes, marketing director at Wärtsilä North America expanded on the need for this product: “PeakingPlus was developed to address the emerging intermediate peaking needs of the market, which could be between $20 to $100 per MW, 1500-4000 hours per year. The emission constraints of traditional peaking equipment [diesel standbys cannot operate over 500 hours], the transmission constraints of a central plant, and the increasing need for intermediate peaking power led us to develop the value-priced, pre-engineered modular package. It is optimized for environmentally advanced operation and low variable cost through high efficiencies at high/hot/dry areas.”

PeakingPlus has been specifically tailored to meet the needs of the US market
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Carbone expanded: “The phenomenal demand for PeakingPlus in the US is driven by what’s happening in the deregulated electricity market – a shortage of power, unreliable supply, and price volatility. Many industrial and municipal users are seeking greater control over their power consumption.”

PeakingPlus represents a timely solution to easing the energy crisis in the US, because the units are modular and use natural gas fired reciprocating engines in place of the gas turbines favoured by many US power plant operators. In addition to this, the units are also preconstructed so they only have to be assembled on-site, which allows PeakingPlus to be delivered and installed within ten months – less than half the time required for many gas power plants sold today. Wärtsilä believes that as the modules are constructed in a controlled manufacturing environment, it reduces costs and improves consistency.

PeakingPlus engines

The PeakingPlus units use Wärtsilä’s 18V34SG engines, which are the biggest lean-burn spark-ignited gas engines in the Wärtsilä range. The 34SG engine is a four-stroke engine that works according to the Otto process and the lean-burn principle. The engine has ported gas admission and a prechamber with spark plug for ignition.

The 18-cylinder engine has an operating speed of 720 r/min and the 34 cm cylinders are configured in a V-block design. The engines give 44.6 per cent efficiency while producing low emissions achieved by the design components, consisting of:

  • A lean gas mixture for clean combustion
  • Individual combustion control and monitoring, providing even load on all cylinders
  • Stable combustion, ensured by a high energy ignition system and pre-combustion chamber
  • Self-learning and self-adjustable functions in the control system.

The air-fuel ratio in the 34SG engine is very high and is uniform throughout the cylinders due to the premixing of fuel and air before introduction into the cylinders. There is more air present in the cylinders than is needed for complete combustion. Maximum temperatures and subsequent NOx formation are therefore low as the same specific heat quantity released by combustion is used to heat up a larger mass of air. This feature of the lean-burn principle allows the 34SG engine to comply with the most stringent NOx legislation.

The Plains End plant will use 20 Wärtsilä 18V34SG gas engines to produce 110 MW of peaking power
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The spark plug used to ignite the lean air-fuel mixture is located in the prechamber, giving a high-energy ignition source for the main fuel charge in the cylinder. The ignition module communicates with the main control module to determine global ignition timing and ensures reliability and efficiency in each cylinder.

The spark plug has been specially developed for a long life span and to withstand the high temperatures and pressures caused by the engine.

This design ensures:

  • Reliable and powerful ignition
  • High combustion efficiency and stability
  • Extended spark plug life
  • Low NOx emissions.

The natural gas has to pass through a gas regulating unit, which includes filter, pressure regulators, shut-off valves and ventilating valves, before it is supplied to the engine. The external pressure regulator regulates the gas pressure to the correct value under different loads, however, the maximum pressure needed is not more than 4 bar under full load.

Common pipes run along the engine to supply it with gas, continuing through individual feed pipes to each main gas admission valve located on each cylinder head. There are two common pipes per bank, one for the main and one for the prechamber gas supply. A filter is placed before every gas admission valve to prevent particles from entering the valve.

Cooling system

One of the most appealing aspects of the PeakingPlus design is that it does not need a continuous source of water due to its closed-loop-cooling system. The cooling system is split into two separate circuits – high temperature and low temperature. The cylinder liner and the cylinder head temperatures are controlled through the high temperature circuit which is also connected to the high temperature part of the double-stage air cooler.

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The low temperature circuit serves the low temperature part of the charge air cooler and the lube oil cooler. Both of the high temperature and low temperature water pumps are engine driven. The closed-loop-cooling system works in a similar way to a car’s water pump as it utilizes a stored supply.

Control system

The Wärtsilä Engine Control System (WECS) is the ‘brain’ of the whole engine. The system controls the entire combustion process and monitors and controls the gas admission valves to allow the correct amount of gas into each cylinder.

WECS also individually controls each cylinder to ensure operation at the correct air-fuel ratio and with the correct timing of the ignition to ensure the best efficiency and lowest emissions. The stable and controlled combustion also contributes to less mechanical and thermal load on engine components.

The WECS control system also monitors engine load, speed and cylinder exhaust temperatures.

Proven success

There will be six employees on site at Plains End for daily maintenance and operations up through to major maintenance intervals, and remote monitoring will be conducted from Wärtsilä’s service centre in Ft Lauderdale. Wärtsilä will operate the plant when it comes online in April 2002.

Wärtsilä believes that the PeakingPlus concept will prove popular in the US for three main reasons:

  • The engines are more efficient than many aeroderivative engines in the same size bracket (100 MW).
  • PeakingPlus can operate at higher elevations and higher ambient temperatures where a lot of gas turbines will de-rate.
  • PeakingPlus does not require a constant source of water due to its closed loop cooling system, which is essential for some states where there are premium prices for water.

Commenting on the PeakingPlus project at Plains End, Carbone said: “This deal is a clear validation that PeakingPlus offers tremendous benefits over other power generation solutions. PeakingPlus was chosen for its fast delivery time and ability to operate at peak efficiency even at high altitudes – two criteria that many of the gas turbine plants sold today cannot match.”