Coal, Emissions

Design Features Enhance Operating Flexibility of Sentinel Energy Project

Issue 1 and Volume 118.

The Sentinel Energy Project looking south toward Mt Jacinto.

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By Thomas Mastronarde, Gemma Power Systems LLC; Mark McDaniels, Competitive Power Ventures Holdings LLC; and Val Madden, Mott MacDonald, Inc

Competitive Power Venture’s (CPV) Sentinel Energy Project is an 800-MW natural gas-fired peaking power plant featuring eight General Electric Company (GE) LMS 100 aero-derivative combustion turbines. This state-of-the-art project is the world’s largest facility utilizing GE’s intercooled aero-derivative combustion turbine and is located adjacent to the nexus of high voltage transmission facilities and high wind area in the vicinity of 3,000 wind turbines northeast of Palm Springs, California.

Because of the plant’s unique operating demands – capacity anywhere from 50 MW to 800 MW, flat efficiency curve, rapid start and load change profile and stringent emissions requirements – design constraints on balance-of-plant support systems posed unique challenges to the project implementation team.

These challenges included SCR/CO catalyst designs for rapid emissions compliance in severe seismic conditions, a variable gas supply pressure, a Zero Liquid Discharge system capable of accommodating highly variable load conditions, and site congestion and construction in a high-wind, high-temperature environment. The project has undergone unit-by-unit performance tests and complete facility tests and is in full commercial operation. This paper will review plant features, performance enhancements and test results associated with this unique peaking power plant that integrates intermittent renewable energy resources with Southern California’s growing energy demands.

Description of the Project

The CPV Sentinel Energy Project is a nominal 800-MW natural gas-fired peaking power plant. This energy project was originally conceived and developed to fulfill the need for efficient, fast-response, peaking electricity in Southern California, in an area that already has significant renewable energy resources. It is expected that the project will support further expansion and integration of renewable energy resources into the utility network serving Southern California.

Specifically, CPV Sentinel provides peak power on demand by capitalizing on the rapid start and ramp rate capability of the GE LMS100 combustion turbines. Additionally, the plant operates over an exceptionally wide range of dispatch loads (from 50 MW to 800 MW) and provides a variety of ancillary services (e.g. spinning- and non-spinning reserve, regulation up/down, etc.) to help stabilize the grid and support intermittent renewable power sources.

The power plant is located just west of the community of Desert Hot Springs, Calif. and about five miles north of Palm Springs. Construction in this area must meet high seismic design factors, as the project is located near several active major faults. The location is in an area designated as a special wind area for design purposes and has been extensively developed as a wind generation resource over the past twenty years. The plant is sited among a cluster of 3,000 wind turbines located east of the ridgeline of the San Gorgonio Pass (Figure 1).

The 800-MW Sentinel Energy Project
The 800-MW Sentinel Energy Project is equipped with eight aero-derivative gas turbines and can meet a wide range of dispatch loads ranging from 50 to 800 MW. Surrounded by 3,000 wind turbines near Palm Springs, the project was built to accommodate a growing amount of renewable power in Southern California.

The Sentinel Energy Project is adjacent to Southern California Edison’s Devers Substation, one of Southern California’s largest high voltage transmission substations, serving multiple high-voltage transmission lines radiating from the substation (Figure 2). The plant produces power from pipeline natural gas with eight GE LMS100 aero-derivative combustion turbines and is the world’s largest facility using this unique intercooled combustion turbine technology.

Aerial View of Sentinel Energy Project with Devers Substation in the background.
Aerial View of Sentinel Energy Project

The mission and the operating characteristics of the CPV Sentinel Energy Project are out of the ordinary for a plant of this size. This paper will discuss how the facility capitalizes on the unique operating characteristics of the GE LMS100, the operational challenges that dictated specific design features of the overall plant and adjustments in the installation and construction concept to accommodate the severe desert environment at the plant location. Enhancements to the performance of BOP equipment to achieve exceptional plant efficiency over the entire operating range from 50 MW to 800 MW will be reviewed. Results from testing of certain key operating parameters will be presented.

GE’s LMS100 Aero-derivative Turbine

Each LMS100 installed at the Sentinel Energy Project is nominally rated at 100 MW and is fueled with pipeline-quality natural gas. The fuel gas pressure required is above what would be typically provided on high pressure gas pipelines, requiring a fuel gas booster compressor at most installations.

The intercooled LMS100 combustion turbine employs two sections of air compressors. Air from the first compressor section is discharged to an external heat exchanger, where the air is passed through a water-cooled heat exchanger and returned to the second compressor section at a lower temperature (Figure 3). Intercooling boosts generating capacity and provides the highest efficiency available in a simple cycle combustion turbine.

The Intercooled LMS100® Aero-derivative Combustion Turbine. Courtesy General Electric
Intercooled LMS100® Aero-derivative Combustion Turbine

The LMS100 features a starting protocol that achieves full output in ten minutes from the command to start. The 10-minute time period includes a combustion path purge period, internal checks on all permissives to allow admission of fuel, ignition of fuel, acceleration to synchronous speed, generator breaker closure, and time to accelerate to full load.

The LMS100 is capable of rapid upward and downward load ramps within the operating range of minimum load (50 percent) and base load (100 percent). This load following capability supports grid stabilization in areas of significant intermittent renewable energy sources, such as wind turbines and solar photovoltaic installations.

Each LMS100 at the Sentinel Energy Project also includes an inlet air evaporative cooler for improved hot weather performance, minimizing the drop-off in the power output curve in a dry, desert environment.

Key Features of Balance-of-Plant Facilities

When considered as complete plant, the Sentinel Energy Project delivers a net output to the transmission substation consisting of power generated by the LMS100 combustion turbine generators minus the station use and parasitic power loads used by the balance-of-plant (BOP) auxiliary equipment to support the Units. One of the goals of the detail design phase was to minimize the use of auxiliary power in the facility. The curve in Figure 4 shows the Facility Net Power Output for a range of one to eight units in operation at base load over a range of ambient temperatures likely to be experienced at the site. This chart includes the impact of the supporting BOP auxiliary power consumption and all parasitic loads including Generator Step-Up Transformer losses.

fig. 4

The BOP equipment and systems facilities that support the eight LMS100s have distinctive features to promote the most favorable operating and environmental footprint for the facility: During the plant’s detailed design phase, an effort was made to optimize the overall plant efficiency by minimizing the auxiliary power consumption in each unit’s auxiliaries and the BOP systems; notably, the exhaust catalyst system and the fuel gas booster compressor system. The aim was to reduce auxiliary power consumption not only at full load with eight units in operation, but also to achieve the minimum possible auxiliary load with only one unit in operation.

Steps taken to minimize plant auxiliary power loads for the gas booster compressors and the exhaust catalyst system are described here:

Fuel Gas Booster Compressors

The facility includes a central natural gas booster compressor station that is capable of providing a fuel gas flow to support any number of LMS100s in operation (from 1 to 8), with a minimum pipeline gas supply pressure of 350 psig. The compressor station includes multiple gas compressors, including spare capacity. The historical pipeline supply pressure ranges from 550 to 620 psig.

Many other LMS100 installations use a single dedicated centrifugal gas compressor for boosting supply pressure to the machine. A centrifugal compressor is a constant volume machine and has the disadvantage of requiring recirculation to control excess flow and pressure when pipeline delivery pressure is above the minimum design point. Variable volume reciprocating compressors were selected for the Sentinel Energy Project to reduce excess recirculation of fuel gas at normal pipeline supply pressures. Fuel gas compressor output flow rate can be matched to the full range of gas flow demand by use of automatic suction head-end unloaders on each gas compressor.

Unloading the suction head valves reduces both volume compressed and power consumption. With all eight units in operation, the auxiliary power savings from the use of reciprocating compressors is in the range of 1,000 to 3,000 kW.

Vaporization of Aqueous Ammonia for NOx Control

The original plant concept included electrically heated vaporizers to support the rapid warming of the ammonia injection system required to support achievement of 2.5 ppmvd (corrected to 15 percent O2) of NOx at the stack within 25 minutes of startup. When requested during the procurement phase for the exhaust systems, one supplier for the exhaust catalyst system was able to demonstrate that a hot gas recirculation system to vaporize ammonia was not only more efficient, but could meet the limited startup time for emissions compliance. With all eight units in operation, utilization of hot exhaust gas to vaporize ammonia resulted in auxiliary power savings of 1,200 kW.

The two improvements cited above, coupled with smaller incremental improvements in a variety of other BOP equipment and systems, allows the facility to operate over a range of output from 50 MW to 800 MW with the percentage of auxiliary power consumption in relation to gross power generated relatively unaffected by the number of units dispatched, as illustrated in Figure 5.

fig. 5

Facility Net Output and Efficiency

The facility can achieve any dispatch output from 50 MW to 800 MW (nominal) by operating the last unit dispatched at part load with the other units at base load.

Figure 6 illustrates the possible unit load combinations available to address the full range of Facility Net Output available for dispatch. It can be observed that any value of net facility output between 50 MW and 800 MW can be achieved by running Units at a variety of combinations of units at base load and part load.

fig. 6

The facility features a flat efficiency profile over an output range of 100 MW to 800 MW, if each unit that is on-line is dispatched at base load. As illustrated in the upper curve of Figure 7, the Facility Net Efficiency on an LHV basis ranges from 41.6 percent to 42.1 percent. If the nits are dispatched in some combination of part load and base load, the lower bound of the facility efficiency curve follows the lower curve in the Figure 7. If more than two units are on-line, the facility efficiency ranges from approximately 40 percent to 41.5 percent if some of the units are at part load.

fig. 7

Station Dispatch and Control

The original design concept of the facility was to have one net electrical meter for the entire station, which would be dispatched and controlled as a single entity capable of producing 50 to 800 MW as described above. During the construction phase, the automatic generation control of individual units became an operating requirement. As a result, revenue meters were installed on each of the eight units and the required communication and control hardware was added as well. Real time net metering calculations are performed and the results are transmitted to Southern California Edison (SCE) and the California Independent System Operator (CAISO). This equipment permits each unit to be dispatched and controlled independent of the others.

The Generation Management Systems (GMS) group of SCE has primary control of the units, but GMS is able to remotely transfer control of some or all of the units to CAISO. Therefore, at any one time it is possible that some units may be under the control of SCE and others may be under the control of CAISO. To dispatch on-line, GMS will request start of a unit to be performed by the facility operators. After the unit is on-line and operating in Automatic Generation Control (AGC) mode, GMS will assume MW output control of the unit. If CAISO requests AGC control of the unit, GMS will enable dispatch authority to CAISO by means of electronic communications. If CAISO signals that the unit is no longer needed, GMS can take back control of the unit, or order a dispatch off-line.

Wastewater Recovery and Re-use

The facility was required to be designed as a zero liquid discharge plant. Individual single-cell wet cooling towers and inlet air evaporative coolers produce wastewater from blowdown of the respective circulating water basins. Evaporative cooler blowdown for each unit is recovered and re-used in the cooling tower basin for each unit.

Early design specifications on the zero liquid discharge system (ZLD) called for a combination of reverse osmosis membranes and thermal brine concentration and crystallization equipment to produce a solid waste from the common plant cooling tower blowdown stream and other minor wastewater drain flows. It was recognized that proper sizing of the ZLD process equipment posed a problem if directly coupled to the generation of power. The ZLD system would have been required to accommodate a wide range of wastewater flow rates (with an effective turndown of 16 to 1) resulting from intermittent operation of a variable number of units in peaking service.

Decoupling Zero Liquid Discharge System from Plant Operations

A simplified process concept was developed and a Facility Wastewater Storage Tank, with approximately four days of storage capacity under maximum dispatch conditions, was added to serve as a buffer. Blowdown from each cooling tower is sent to a common wastewater collection system and stored for processing in the Wastewater Storage Tank. During periods of low dispatch, the Wastewater Storage Tank can hold the volume produced by many weeks of intermittent operation.

The ZLD system was reconfigured as a single large brine concentrator and a single crystallizer, without reverse osmosis equipment. Thermal energy to evaporate and concentrate the brine is supplied by vapor compressors. The system recovers more than 99 percent of the wastewater processed as clean water and returns the effluent to the plant’s Raw Water Storage Tank. Dry solids are recovered from the crystallizer effluent slurry in a belt filter press. If the generating units are offline, the contents of a full Wastewater Storage Tank can be processed in about five days. If the generating units are fully dispatched during day-time hours under the normal operating profile, the ZLD system has the capacity to process the full wastewater production on a daily basis. The simplified system is energy efficient and uses about 80 kW per thousand gallons processed, equivalent to about 3 percent of the energy required to evaporate water with a direct heating thermal process.

Site Location Construction Challenges

The site location for the Sentinel Energy Project is at times extremely windy, seasonally very hot, and almost always dry. The plant is located in area classed as a “special wind region” by Riverside County. Therefore, construction practices were modified to accommodate these conditions.

Occasional extreme winds required site housekeeping to be rigorously followed at all times, to prevent any loose materials, tools or unsecured structures (including portable toilets) from becoming dangerous flying debris. Water was applied by a dust control tanker truck on essentially a continuous basis. Graded slopes were treated with “tackifier” to diminish the tendency for surface soil to become a dust cloud.

Field welding of pipes or structures was limited by design and by prefabrication. Each pumping system with redundant pumps (raw water, demineralized water, wastewater, aqueous ammonia) was 100 percent shop fabricated as a single skid, with flanged interfaces at the inlet and outlet. In the case of piping runs, spools were shop-welded to the maximum extent possible, and unavoidable field welds were protected by tenting over the joint area.

The assembly of the combustion turbine exhaust systems and stacks posed the greatest potential exposure to difficulties with welding at above-grade elevations in windy conditions. This problem was alleviated by selection of a supplier experienced in providing a unique modular structure designed for 100 percent field-bolted assemblies for structural steel as well as 100 percent gasketed and bolted flue gas path casing joints (Figures 8 and 9).

The exhaust catalyst system (modular bolted casing assemblies)
exhaust catalyst system
The exhaust stack (modular field-bolted assemblies)
exhaust stack

Spells of dry desert heat from May to October are a regular occurrence and required planning and consideration as to the impact on construction scheduling. To avoid issues with day-time limitations on concrete truck transport time and on-site truck waiting time in extreme high temperatures, the delivery and placement of concrete for the large mass foundations was conducted at night. To reduce the stress on craft personnel, the work day in the summer started as early as 5 a.m., in order to reduce the number of hours worked in the hottest part of the day (mid-afternoon and later).

Despite these challenges, the project execution team, which included Competitive Power Ventures, Gemma Power Systems, Mott MacDonald, General Electric and other suppliers, working together toward a common goal, was able to complete the 800 MW Sentinel Energy Project, successfully demonstrating all required operating and performance criteria 103 days ahead of the scheduled substantial completion date.

Performance Achievements

An extensive performance testing program was conducted on each LMS100 unit, and on the entire Facility. The purpose of these tests was to demonstrate that each of the eight GE LMS100 combustion turbines met power output, heat rate, startup time and ramp rate obligations, and that all BOP systems including the exhaust catalyst systems met emissions requirements.

In addition, the testing program demonstrated that the BOP systems supported each LMS100 in meeting their startup time and ramp rate obligations, and delivered all required fuel gas, raw water, demineralized water for combustion NOx control, and ammonia to each unit. The testing program also demonstrated the ability of the ZLD system to process the wastewater produced by the Facility.

Typical performance data are illustrated below for key performance parameters:

  • Achievement of Base Load within 10 minutes after start initiation is shown in Figure 10.
    fig. 10
  • Achievement of CO less than 4 ppmvd within 10 minutes after startup fuel admission is shown in Figure 11.
    fig. 11
  • Achievement of NOx less than 2.5 ppmvd within 25 minutes of startup fuel admission is shown in Figure 12.
    fig. 12
  • Achievement of upward and downward load changes at ramp rates greater than 10 MW per minute are shown in Figures 13 and 14.
    fig. 13
    fig. 14

Summary

The CPV Sentinel Energy Project has proven to be a success in terms of performance, operability and construction. The CPV Sentinel Energy Project uses the most modern peaking technology available, the GE LMS100 aero-derivative combustion turbine, to supply electricity into the Southern California region.

The facility provides summer peaking capacity and backup power to the region’s wind and solar assets within ten minutes of notification. The design features of the facility enable it to provide flexible dispatch, fast start capacity, ranging from 50 MW to 800 MW, with the highest efficiency of any simple cycle peaking plant, while achieving stringent air emissions requirements and operating under zero liquid discharge constraints.

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