Power Engineering

Afton Combined Cycle with Hybrid Cooling

 

A cooling system that may be applicable at any plant site facing water constraints.

 

By Jack Groves and Todd Krankkala, Power Engineers; and Greg Nugent, Public Service Co. of New Mexico

The Afton Generating Station near Las Cruces, N.M., is a 225 MW-class 1x1 combined cycle project owned and operated by Public Service Co. of New Mexico (PNM). The preliminary design efforts moved through five distinct configurations to reach the final design. The facility represents the state of the art in combined cycles, including unique parallel path water conserving hybrid cooling system. This cooling system is applicable for consideration at all plant sites facing water constraints.

The project was first conceived in 2001 as a 100 percent wet-cooled 550 MW 2x2x1 Frame 7FA unit. This prompted PNM to procure the gas turbines and steam turbine from General Electric and construct one simple cycle turbine as Phase I. In 2005, PNM completed a re-assessment of the optimal resource mix for its customers. As a result, Afton was redefined as a 225 MW-class critically needed 1x1x1 combined cycle asset for Phase II. The reassessment also concluded that the original inventoried steam turbine was not ideally suited for the proposed Phase II configuration. To meet the project schedule, gray market steam cycle equipment was located and purchased in late 2005. Commercial operation was achieved in late 2007.

PNM executive management during the detailed design phase implemented an Environmental Sustainability Policy which included a mandate to substantially reduce cooling water consumptive use at future power generation facilities. The design team elected to reconfigure the cooling system from a 100 percent wet tower to a hybrid parallel cooling system, with a target of consuming a maximum 600 acre-feet (AF) of water as compared to a projected wet system consumptive use of 1,750 AF. This met Environmental Sustainability Policy requirements.

A second goal, beyond water conservation, was to sustain minimal performance impacts during high ambient temperature operation. Such impacts often are a common penalty associated with air cooled-only condensers in the desert Southwest.

The project was confronted with many challenges including:

  • Incorporating gray market equipment into an existing plant site
  • Compressing and overlapping engineering and construction activities
  • Weather-related delays
  • Market volatility (pricing and delivery) for labor, materials and equipment
  • Final operational shakeout.

 

This article offiers a brief history of the evolution of Afton and a summary of lessons learned. We include emphasis on the hybrid cooling system which conserves 60 percent of the water as compared to a wet tower, with minimal performance impacts during high ambient temperature operation. One hundred percent equivalent availability monthly periods are common as unit operations mature and cooling system operations are being refined and enhanced on a number of fronts.

The Afton Combined Cycle Project fired up in 2007 for commercial operation and represents a high level of power plant development for its market setting. Afton applies industry-standard gas-fired combustion turbine equipment with a water-thrifty hybrid cooling system and delivers 225 MW of baseload and mid-merit power service with NOx emissions at 3.5 parts per million (ppm) or below.

The Afton Plant would seem like an obvious match of technology to need. In fact, Afton followed an evolutionary path over two decades of head-scratching and boardroom discussions, from initial development as a peaking unit to its ultimate form and configuration.

In 2001, the project was first conceived as a 550 MW 2x2x1 Frame 7FA unit with wet cooling. PNM moved at that time to procure the gas turbines and steam turbine from GE and decided to install one simple cycle 7FA as Phase I and place the rest of the equipment in inventory. In 2005, PNM redefined Afton Phase II as a 225 MW 1x1x1 combined cycle. As a result, PNM determined that the steam turbine in inventory was not ideally suited for the revised Phase II plant layout. In 2005, PNM located and bought gray market replacement equipment. PNM also took bids for the combined cycle balance of plant equipment, including GEA’s Parallel Condensing System (PAC). This system was identified by PNM because of its potential to significantly reduce plant water consumption.

 

Selecting a Site

 

The Afton site was first identified in 1991 by Energy Southwest Inc. (ESI), a New Mexico developer/consultant. ESI developed the site for peaking and to meet part of the City of Las Cruces’ generation portfolio. The proposed power plant use was found to be compatible with the neighborhood; an adjoining El Paso Natural Gas (EPNG) compression station already has three gas-fired GE Frame 5s for mechanical drives. EPNG also had production water wells and industrial-use water rights suitable for a Frame 7.

The site is at 4,220 feet, ideal for emission disbursement characteristics. Beneath the site is a large, proven groundwater resource of suitable quality for power generation use in aquifers of various depths up to 1,500 feet. On a conventional siting scale of 1to 10, the site is ranked a 9. Its sole deficit is a lack of five-mile proximity to a major freeway.

The Afton Generating Station evolved through five different design configurations.

Original Configuration: The project was initially defined by ESI as one natural-gas-fired combustion turbine (CT) linked to a 1x1 GE Frame 7FA/A10 combined cycle plant at a copper mine at Morenci, Ariz. Afton and the Morenci combined cycle were to be operated in conjunction to serve this industrial load. Afton was intended also to provide peaking service for the City of Las Cruces.

Second Configuration: ESI’s power sales contract with the mine was placed on a long delay. Meanwhile, the state and federal governments were proposing a deregulated wholesale market for Exempt Wholesale Generators (EWG). The mine’s combined cycle project was placed on hold and the focus shifted to Afton, which was re-configured as a gas-fired wet-cooled 1x1 GE 7FA-based 225 MW-class plant. The target sales market shifted to potential service to wholesale loads.

Enter PNM: PNM studied numerous development and siting options and determined that the Afton project conception would serve its needs. PNM bought Afton from ESI and reconfigured it as a full duct-fired, wet-cooled 2X1 GE Frame 7 FA-based 550 MW-class combined cycle. (PNM also studied a 2x2 configuration for Afton, but it proved less efficient and more expensive.) ESI/PNM commissioned design work and bought the key prime movers – two 7 FA gas turbines and one D-11 steam turbine – in a strong seller’s market.

Ground Is Broken: The fourth configuration, and the first to achieve a material presence on the site, was a dual-fuel GE 7FA simple cycle fitted with a bypass stack and damper to readily support a future expansion into 1x1 combined cycle. PNM’s other inventoried turbines and ancillaries originally destined for Afton were assigned to other projects or sold.

The Arrival of Steam: Configuration five called for a heat recovery steam generator (HRSG) equipped for duct firing and a GE A10 steam turbine. This combination allows the plant to achieve the much lower heat rate of a combined cycle and elevates Afton to a higher-value generator within PNM’s system. Configuration five also incorporates an innovative and spectacularly effective water-conserving hybrid heat rejection system.

 

Design Basis and Timeline

 

PNM’s development of the combined cycle expansion at Afton needed to comply with its Environmental Sustainability Policy, adopted in 2003, which outlines goals to reduce water consumption, air emissions and waste streams. Consequently, PNM decided that configuration five would use a parallel (water/air) cooling system to reduce water consumption over the original full wet cooling design. PNM also added a wastewater treatment plant. The parallel cooling system was critical to making the best use of the plant in a wide range of operating conditions. Specifically, the parallel system was selected to optimize output during summer peak periods. To serve the wet side of the parallel system, PNM had 555 acre-feet per year of water rights.

Because of the goal to serve the 2007 summer peak, PNM decided early in the project to procure key equipment and materials–in particular, alloy piping, structural steel and wiring–well in advance of final design. Schedule also drove some of the major equipment selection and procurement events.

The Afton Combined Cycle conversion was completed amid an array of supply and construction challenges beyond the control of the parties involved. The combined cycle development stage took place neatly within a period when materials costs were rising with eye-popping speed: steel, copper, and concrete generally doubled and in some cases more than tripled. Escalating costs, driven by shortages, were matched by increasing delivery intervals, causing the right side of the project schedule to grow fiendishly complicated.

 

Parallel Cooling– Selection and Design

 

The parallel cooling system–which offers a blend of a wet tower and an ACC system for

condensing service–was chosen for its ability, via its wet tower, to provide better cooling and higher output in the summer versus a 100 percent air cooled system. The benefit of the dry ACC side of the blend is to reduce overall water consumption when compared to a full-time wet cooling tower.

The system was specified by PNM and its engineering consultants and bid as a complete system. The supplier’s scope included the wet surface condenser with air removal skid and the cooling tower, plus the air-cooled condenser and ducting from the steam turbine exhaust flange to the ACC. The Afton plant design is for a thermal duty split between the wet condenser and air-cooled condenser in the range of 40/60 (wet/dry) at an ambient condition of 70 F. This split was selected based on heat balances run by Power Engineers using operating hour assumptions provided by PNM. The Power Engineers design criteria called for a water consumptive use of 600 acre-feet per year (AFY). However, only 555 AFY was available for purchase at the time, so plant operations were adjusted to meet this limit.

At ambient 98 F, the thermal duty split is closer to 45/55 (wet/dry). The design operating point for the parallel cooling system was set at 98 F, 16 percent RH with a steam flow of 594,981 lb/hr and at a maximum steam turbine backpressure of 5” HgA. When conditions are hotter than this, duct firing is reduced to lower steam flow to maintain the STG backpressure at or below this level. (The steam turbine alarms at 5.5” HgA and trips at 7.5” HgA.)

PNM and Power Engineers selected GEA as the supplier for the parallel cooling system, which consists of the following:

  • 10-cell air cooled condenser with two-speed fans
  • Two-cell cooling tower with two-speed fans
  • Deaerating surface condenser
  • Air removal system.

 

Noncondensable gases are extracted from the exhaust steam using both liquid ring vacuum pumps and steam jet ejectors. Condensate from the surface condenser and ACC is accumulated in a deaerator/condensate tank.

Makeup water for the wet cooling system comes from raw water wells on the Afton site. The well water is warm (80-90 F) and is cooled prior to treatment. The water treatment system delivers demineralized water for makeup to the condensate tank; reject water from the water treatment skid is dispatched to the wastewater treatment plant.

Treated and reclaimed permeate from the wastewater treatment is sent to the wet tower to serve as makeup water. Reject water from the wastewater treatment system is sent to evaporation ponds on site.

The GEA PAC System functions in several modes, depending on ambient temperature. The system’s design basis is for an ambient 98 F and a turbine backpressure of 5” HgA. At 98 F, the system is designed so that all ACC fans run at high speed, both cooling tower fans run at high speed and circulating water passes through the surface condenser at full flow. As ambient drops, the ACC fans operate in varying regimes to maintain design backpressure. Fans can be turned off or run in low-speed or high-speed settings. Control functionality for these functions resides in the Afton plant’s distributed control system logic. PNMs operators have discovered that in automatic control mode, the ACC fans tend to cycle between high and low speed too frequently. Because of this, operators are currently controlling ACC fans manually.

In winter the wet tower fans can be turned off, at least from the perspective of backpressure. PNM’s current practice is to run one circulating water pump during the winter. The reason for this is that GEA recommends maintaining some water flow through the surface condenser tubing to prevent fouling. It may be feasible to divert auxiliary cooling water for this purpose if the auxiliary pumps can maintain the required minimum flow through the condenser.

In January 2008, one ACC fan saw catastrophic failure of five of its six blades. Inspection of the other nine fans showed that 80 percent of the blades had cracks along the leading edge. GEA and fan supplier Howden replaced the six-bladed fans with seven-blade fans, changed the pitch angle of the blades from 12.8° to 12.3° and also used blades made in a different process. Howden also mounted a strain gauge on the root of one blade and is observing fan stress against fan/motor rpm and ambient wind speeds to determine if wind conditions were involved in the blade failure. The replacement blades have not experienced any noticeable degradation or failures.

The NCG extraction system includes vacuum pumps and steam jet ejectors. In the early

period of operation, the ejectors did not perform optimally during start-up conditions when steam pressure is building. So the ejector nozzles were replaced to provide a wider range of effective operation to cover this condition. This has remedied the problem.

 

Cooling System Performance

 

Though the Afton DCS allows automatic control of ACC fan speed operation, PNM’s

operators prefer to operate the ACC manually during some temperature conditions to keep the fans from cycling excessively between high and low speeds. Table 1 shows actual Afton Combined Cycle plant availability during 2008, its first full year of operation.

Click to Enlarge

Some lessons learned:

The schedule was severely compressed by rightward drift in the start date and some cost and design efficiencies were sacrificed. Mobilization of construction was breathtakingly early: four weeks after the full release for engineering activity. Long lead times and escalating bulk material costs squeezed the engineering effort mightily, and some materials were procured based on preliminary information. In some cases, the plant design was driven by construction requirements; in a more measured schedule setting, additional efficiencies in value-engineered design could be realized.

Although gray market opportunities offered Afton expedited deliveries and potentially lower pricing, there is a cost. The buyer may be forced to a configuration not ideally suited to the site or the performance requirements of the plant. At Afton, the gray market HRSG’s piping connections were on the wrong side of the HRSG for the ideal layout; the team elected to use them rather than modify the HRSG. Since the HRSG and turbine were designed for a different site and different conditions, Power Engineers and PNM spent additional time studying the impacts of the differences on Afton. Grey market equipment drawings and specifications (or the lack thereof) need to be carefully reviewed, and this activity should be accounted for in the scope and schedule.

Afton’s wastewater treatment plant has seen several process upsets triggered by flows greater than anticipated. These high flows resulted from the cycling operation of the power plant. This problem was solved by providing additional residence time for wastewater streams upstream of the clarifiers.

Afton has proven an excellent site for a power plant. The plant is well matched to its regional grid, is a sound asset for its owner and is a good neighbor. The project went through a dizzying series cycle of embodiments while morphing from an audacious merchant plant concept to a utility jurisdictional asset, and acknowledging both external and internal requirements which shaped the project through its formative stages and determined its final form.

Afton’s NOx emissions are notably low, at 3.5 ppm or less. This brings Afton to a tie for lowest power plant emissions in New Mexico. Although Afton is a combined cycle plant in a hot region, it meets PNM’s Environmental Sustainability Policy, including the policy’s guidelines for water conservation. Afton’s hybrid cooling systems has shown itself to be effective and reliable. Canny operational practices, fan blade improvements and water treatment improvements have been incorporated at Afton to make the plant work even better.

Afton, because of its adventurous application of a parallel cooling system to save water and increase output during hot weather, is an excellent model for similar applications elsewhere. It seems inevitable that, with growing pressure for water conservation and increased plant efficiency, similar hybrid cooling systems may find homes elsewhere.

 

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