Wet Compression

Magazine of the Siemens Power Generat ion Group

Wet Compression: Gas Turbine Power Output Enhancement for Peak-Load Demand

Reprint from Power Journal International

Author: Elliot Smith

Power for Generations Siemens Power Generation

GAS TU R B I N ES

Upgrading gas turbines with a Wet Compression System significantly enhances power output, increases efficiency and reduces NOx emissions.

Real Power Boosters Wet Compression Systems can enhance gas turbine power output by 20 percent and more when it benefits most: for example, for increased earnings during peak load demand or for restoring power loss during hot days. BY ELLIOTT SMITH

as turbine modernization and upgrading will increasingly be the key to maintaining the competitiveness of power producers in a growing fleet of gas turbines in a deregulated market. One of the upgrade tech-

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nologies is Wet Compression, a method of safely injecting volumes of water into the compressor for continuous operation with significant performance gain. This technology has been cooperatively developed by Siemens Westinghouse and The Dow Chemical Company. Wet Com-

pression Systems have now been applied to nine W501-series gas turbines with the lead unit having accumulated 25,000 hours of system operation. System performance has been demonstrated in field operation and power increases in the range of about 10 to 25 percent have been attained.

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GAS TU R B I N ES

Examples of Power Gains using Wet Compression and Evaporative Coolers 30

Power increase

MW 25

Wet Compression Evaporative cooling

20 15 10 5

Louisiana Louisiana New Mexico

New Mexico

Mexico*

Illinois

Louisiana Louisiana

Power plants equipped with W501A, W501D5 or W501D5A gas turbines * no evaporative coolers

Since late 1999, several Wet Compression Systems have been installed in power plants and have been successfully used to increase plant output, generating significant revenues for plant operators. One example is the W501D5A unit at Springfield Station in Illinois operated by City Water, Light and Power.

Power Augmentation during Compressor Wash et Compression evolved out of the field experience with compressor water wash used on gas turbines. Through a combination of circumstances, personnel at The Dow Chemical Company, which operates its own power plants, came to recognize that a power gain was produced through the addition of compressor wash water to the compressor. This prompted an investigation into the possibility of optimizing the power enhancement, by adding greater amounts of water to the compressor. Dow’s development work led to its conception and first implementation of the Wet Compression technology. It was at

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this point that the former Westinghouse (now Siemens Westinghouse Power Corporation) joined Dow in further development. The first, full-scale version of the system was demonstrated in 1995, showing a power output increase of greater than 20 percent when operating downstream of an existing evaporative cooling system. Today, the lead engine (a W501A) has accumulated 25,000 hours of operation with the Wet Compression System in essentially continuous service, averaging in excess of 20 percent augmentation over that period. Since the first full-scale implementation, the spray nozzles, rack layout, con-

trols, and water supply systems have continued to be refined to improve system reliability and durability. The greatest improvements have been demonstrated in system control and on the ability to minimize casing distortion, which is a critical aspect on more modern gas turbines as blade clearances have been reduced to optimize compressor and turbine efficiency. Siemens Westinghouse is a licensee of Dow’s patents and has successfully continued with its own development of Wet Compression technology. In fact, Dow has recently purchased Siemens Westinghouse-designed systems for its turbine fleet.

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0 Plant site in:

Wet Compression improves power capacity through a combination of evaporative cooling, overspray of water and a reduction in compressor work from the intercooling effect the water produces as it is vaporized in the compressor. These effects increase compressor efficiency and help to enhance mass flow through the engine. Key design elements of a Wet Compression System include: • water injection spray rack layout, • spray rack location, • nozzle design, • water carryover protection devices, • optimization of turbine cooling circuits, and • modification of turbine control logic. The layout and location of the spray racks are established based upon the configuration of the inlet system for each site. This serves to optimize the droplet size and nozzle flow rates required to safely produce the increase in power output. The water carryover protection devices are typically water vapor traps installed in the compressor bleed piping. These traps prevent water droplets from entering the turbine cooling circuits. When water is injected into the compressor inlet, a change occurs in the work distribution of the compressor. The matching of compressor bleed pressures and turbine pressures is affected, and this requires adjustments to be made to the turbine cooling circuits. The changes can be made through the use of an automated flow control system or by adjustment of fixed orifices in the cooling circuits. The automated flow control system allows performance to be optimized when operating in either a dry or Wet Compression mode of operation. Modifications to the existing turbine control system must be made to include additional logic for system response to engine operating conditions such as alarm functions or functions related to trip the Wet Compression System. Fast Installation, Quick Amortization The system can be designed and installed within about 16 weeks with the installation typically being performed

during a regular outage. It can provide a power boost when plant operators can benefit most, when market demands are at their highest levels. When considering peak market rates in the U. S. from the summers of 1998 and 1999, a Wet Compression System can provide a very quick return on investment, but also for more moderate peak prices it can typically pay for itself within the first year of operation.

Cooling Hot Days in New Mexico outhwestern Public Service’s power generating asset includes two W501D5A gas turbine units at its Cunningham Station, near Hobbs, New Mexico, at 3,800 feet elevation. The naturalgas-fired units are used for simple-cycle peaking. Conventional evaporative coolers in the compressor inlet duct were originally installed to help maintain power output for hot days with low humidity. This method can produce up to 8 percent more power above the dry operating condition. In 1995, SPS decided to add the more advanced Siemens Westinghouse technology of spraying water into the front end of the compressor to boost power output. These Wet Compression Systems have significantly improved machine performance. For example, during very hot days with 101°F (38 °C) ambient temperature and 14 percent relative humidity • output can be increased by an additional 15 percent while • heat rate can be reduced by 1.5 to 3 percent.

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Performance Enhancements from Wet Compression Increased Power Output Wet Compression enables an increase in power production capacity for any ambient conditions where icing is not an issue (i.e., greater than 50°F (10°C) compressor inlet temperature). Because Wet Compression acts through several mechanisms, power gains are more reliable and much higher than those from conventional evaporative cooling or fogging systems. These systems cool the inlet air by evaporating

Typical Wet Compression Power Augmentation Curve for a W501D5A Gas Turbine 150 MW 145 140 135

Power output

no evaporative cooler and no Wet Compression

with Wet Compression (downstream of evaporative cooler) with evaporative cooler (assumes 50% relative humidity)

130 125 120

(ISO) 115 110 105 100 20

25 30 -5

0

35

40 5

45

50 10

55

60 15

65

70 20

75 80 85 90 95 100 105 Compressor inlet temperature °F 25 30 35 40 Compressor inlet temperature °C

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The Wet Compression System

Because Wet Compression Systems act through several mechanisms, their power gains are more reliable and much higher than those of conventional evaporative cooling systems.

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GAS TU R B I N ES

water via a porous media or by injecting water mist into the air stream up to 100 % saturation. The efficiency of these techniques strongly decreases with increasing relative humidity of the air. One additional benefit of Wet Compression is that it can work with conventional evaporative coolers or in place of them. In units that do not already have evaporative cooling systems, the Wet Compression System can provide both evaporative cooling and compressor intercooling. In a Mexico project, only a Wet

Compression System was installed: Three of the fifteen MW provided by the Wet Compression System can be attributed to evaporative cooling effects, while the remaining twelve MW were gained from the overspray effects and compressor intercooling.

mainly as a result of the compressor intercooling effect. This represents a very significant difference to direct combustor water injection for NOx control or power augmentation, which tends to increase heat rate as heat is lost in vaporizing the water.

Heat Rate Improvement

Nitrogen Oxide Reduction

In recent applications to W501A, W501D5, and W501D5A engines, the Wet Compression System has decreased overall heat rates more than 1.5 percent,

Application of Wet Compression Systems to W501D5A and W501D5 engines equipped with conventional combustion systems has shown that NOx emissions can be reduced by 20 to 40 percent of dry uncontrolled emission levels when machine output is not strongly increased, or be kept well within the original levels even while realizing significant capacity increases. The reduction of NOx emissions is attributed to the increased moisture content and decreased compressor discharge temperature. For engines equipped with Dry Low-NOx (DLN) combustion systems, the use of Wet Compression understandably has a lesser impact.

How Wet Compression Works sing Wet Compression enhances the overall performance of a gas turbine through three mecha-

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nisms: • Evaporative cooling of the inlet air stream if the air is not already saturated: Cooler air means reduced work required to compress the inlet air, which is translated into higher engine efficiency. • Intercooling of the compressor: As the air gets hotter in the front stages of the compressor, water is rapidly evaporated which effectively cools the air. Again, this results in a reduction in the amount of

work required to compress the working fluid, allowing more power to drive the generator. Since the compressor absorbs more than half of the power from the turbine, this improvement alone accounts for a significant improvement in power and efficiency. • Increased mass flow through the engine: This is due to the “overspray” of water into the compressor inlet and to the additional amount of fuel that is used to raise the temperature of the combustion air to maintain the turbine design inlet temperatures.

Spray rack of the Wet Compression System: Demineralized water is injected directly into the compressor inlet duct with optimized droplet size and nozzle flow rates.

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Increased Steam Production in Combined-Cycle Applications The increased mass flow through the gas turbine and increased specific heat of the exhaust gas due to the additional water vapor present can benefit in combined-cycle applications also. A 2 to 3 percent increase in steam production can be achieved with Wet Compression for use as process steam, for additional power generation, or for supporting less effi■ cient operating equipment.

Elliott Smith is a Senior Marketing Engineer for Gas Turbine Plant Modernizations and Upgrades which includes development, sales & marketing, and implementation of Wet Compression Systems.

This article appeared in: Power Journal April 2000, pages 29–32 © copyright 2000 by Siemens AG Power Generation This reprint is published by Siemens AG Power Generation Freyeslebenstraße 1 91058 Erlangen, Germany e-mail: [email protected] www.pg.siemens.com Siemens Westinghouse Power Corporation The Quadrangle 4400 Alafaya Trail Orlando, FL 3 28 26-23 99, USA www.siemenswestinghouse.com

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