Low Load Operational Flexibility for Siemens G Class Gas Turbines :
Pratyush Nag*, David Little, Damien Teehan, Kris Wetzl, David Elwood, Siemens Power Generation Inc, 4400 Alafaya Trail, Orlando, FL 32826

Abstract
The US gas turbine (GT) power generation market has seen significant volatility in recent years. The trend is likely to continue given the changing environmental conditions: climatic changes, natural gas prices, nuclear and coal power generation. This volatility has required many gas turbine operators, who normally operated on a continuous basis (base load) to operate in an intermittent dispatch mode which has caused some operators to frequently shut down their units. This frequent cycling of units increases start-up and maintenance costs. It would be beneficial to these plants to operate at lower loads when power demand is low and ramp up to higher loads asdemand increases.

A key issue in operating at lower loads is an increase in Carbon Monoxide (CO) emissions. When the engines are base loaded, the combustion system operates at high firing temperatures and most of the CO is oxidized to Carbon Dioxide (CO2). But at part loads, when the firing temperature is lower, the CO to CO2 oxidation reaction is quenched by the cool regions near the walls of the combustion liner. This results in increased CO emissions at low loads. In order to provide greater operational flexibility to the G-class gas turbine operators, Siemens has developed an upgrade for the engine system designed to allow the gas turbine to operate at some lower loads while maintaining emissions within set limits.

This developmental effort culminated in the recent installation and successful testing of the upgrade product at one of the Siemens G-class operating plant sites. The plant was previously operating between 70% and 100% of GT base load while maintaining their emissions limits. Now, with this upgrade product installed, they have operated as low as 32% of base load while still maintaining emissions within set limits. The plant has continued to operate in this mode as it undergoes further product validation. Some of the key items being evaluated as part of the validation are hot gas path and Heat Recovery Steam Generator (HRSG) components. Initial evaluation has been positive and further inspections and data analysis will continue through the validation phase. Upon successful completion of the validation phase, the product is expected to be rolled out to the other G-class engine sites.

Introduction

The W501G gas turbine design concept was driven by the changing power market. Between 1993 and 1995, the power market was moving towards deregulation and replacing aging base load plants, such as coal-based power plants. The market was demanding clean and highly efficient combined cycle plants. Fears of deregulation in the North American electricity market caused prospective plant buyers to turn to cleaner plants with shorter installation and commissioning times compared to traditional coal plants which had a six year lead time to permit and build. The belief at that time was that the new high-efficiency, low emissions, clean fuel plants would be the economic and environmental choice and displace coal plants. Because the W501G had a high efficiency (58% in CC application compared to the typical 54-55%efficiency CC power plant), it was intended to be operated primarily at base load. In the late 1990’s there was an increased demand for electric power and a relatively low price for natural gas, about $2.50/MMBtu, causing an increased demand for gas turbines for simple and combined cycle operation. By 2002, the demand for power was subsiding and some areas were over capacity. Natural gas had increased to above $6/MMBtu and the price for electricity had decreased causing the gas turbine combined cycle plants to be operated at only 30% average capacity (higher utilization for advanced frames including the W501G fleet). The increased natural gas price has caused combined cycle plants to move lower in the dispatch order forcing them to operate in cycling duty mode. In this environment, the amount of time a merchant plant can operate profitably may be significantly reduced.

In response to the change in market demand for more cyclic operation design changes and improvements are continually being incorporated into the W501G engine. Customer feedback through user groups, direct feedback, and the analysis of data collected via Power Diagnostics® allows Siemens to focus on development programs that are directly aligned with customer requirements.

The W501G adaptability to cyclic/flexible operation was further improved by enhancements incorporated over the last several years which include the following:

  • Combustor basket design improvements to reduce emissions, improve reliability
    and increase the time between inspection intervals
  • Steam cooled transition improvements to reduce metal temperatures and extend
    inspection intervals based on both hours and starts
  • Improved compressor and turbine sealing for increased performance and re-
    duced emissions
  • Optimized cooling on the first four rows of turbine airfoils
  • Optimized rotor cooling air temperature to enhance operational flexibility
  • Enhanced turbine disk material, which is utilized in the existing Siemens V-fleet
  • Redesigned exhaust system to improve performance and service life Improved starting reliability and reduced capital costs from changing the starting motor to a Static Frequency Converter (SFC)
  • Optimized and simplified gas turbine and plant controls to improve the engines operational flexibility and starting reliability.
Figure 1: Siemens W501G Gas Turbine
Figure 1: Siemens W501G Gas Turbine

Market drivers for low load operational flexibility

Since the beginning of the power market’s liberalization in the mid 1990’s, the power plant business has been changing. Today, power plant operators find themselves in a more challenging market environment with the presence of strong competition, higher fluctuation of fuel prices, and many without long term power purchase agreements. Despite these new challenges, the market liberalization also presents new business opportunities such as the utilization of market price fluctuations for operation and maintenance optimization, participation in ancillary service markets, and short term trading. All of these opportunities can contribute to significantly improved operating margins. By knowing how to approach these opportunities, an operator can in some cases achieve higher profits when compared to a long term power purchase agreement.

The changed market conditions have an influence on the operating profile of virtually every power plant. Combined cycle power plants often do not strictly operate in a base load regime running 8,000 hours per year. Many units are operating in a daily start-stop regime with some units starting up to twice a day. In this market environment, an economic model that incorporates only a certain amount of base load hours with fixed power revenues will not describe the full picture. Additional earnings from the above mentioned market opportunities would not be considered. To be more accurate, an extended approach for evaluating a cycling plant with high flexibility is necessary. Key parameters for operational flexibility are, for example, start-up time, standby operation and shut-down time.

From the mid-1990’s to 2000 there was a steady reduction in the US electric power reserve margins. It was believed that deregulation and cleaner, more efficient combined cycle plants would replace aging base load generation stations, such as nuclear and coal-based plants. Increasing demand for electricity and high electricity prices caused a surge in new orders for both simple cycle and combined cycle plants. The result was an increase in total generated electricity capacity and reserve margins in all U.S. regions, as well as a decrease in CC capacity factor (defined as the ratio of actual generation to the total possible generation over a time period). The capacity factor reduction forced operators to operate in peaking and intermediate modes rather than in the planned base load, thus increasing demand for cyclic operation capability.

Demand growth, economic dispatchability and operational flexibility are key factors that determine the electricity-generating plant’s ability to improve its dispatch rate (i.e. the order in which it is dispatched as demand for electric power increases during the day). Due to current overcapacity and the increase in reserve margins, the units that excel in economic dispatchability and operational flexibility will dispatch before other competing units. The dispatch order is determined by the unit’s variable production cost (VPC). Fuel cost and variable Operation and Maintenance (O&M) cost are used to calculate VPC. Small changes in VPC can significantly affect the unit’s dispatch ranking. Fuel cost is directly impacted by the gas turbine’s efficiency, thus increased efficiency improves not only the revenue per megawatt hour but also the unit’s total dispatch hours. Reduced O&M costs will also lower VPC, improve dispatchability and increase net cash flow. Units that are operationally flexible and can load follow, cycle on and off more economically which will allow improved dispatchability and a competitive advantage in the current market. Design improvements have been made in the Siemens fleet of SGT6-5000F (formerly called W501F), intended to help enhance the units’ dispatchability, increase efficiency and lower life cycle costs (hence reduced VPC), and improve operational flexibility.

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