By Douglas J. Smith,IEng
Small Gas Turbines from 5 to 65 MW are in Demand for Cogeneration, Peak Shaving and Distributed generation by industrial self generators and electric generating companies. Economically, the capital cost for constructing a gas turbine power plant is significantly less than that of a coal-fired plant of similar size.
GE LM6000PA gas turbines converted from 50 hertz to 60 hertz. Photograph courtesy of GE Aeroderivative and Package Services.
According to the U.S. Department of Energy (DOE), gas-fired power plants of 40-150 MW, using gas turbines, can be constructed for $200/kW. However, as demand increases for smaller gas turbine power plants of three to ten MW, there is the possibility for decreased costs from the economies of mass production, says DOE.
Although the potential for low cost gas-fired gas turbine distributed generation using very small gas turbines is promising they are unlikely to replace the large conventional coal-fired units. However, the availability of small-scale gas turbine plants will certainly change the competitive structure of the electric supply industry.
A major concern for operators of gas-fired power plants is the cost of natural gas which is less predicable today than it was a few years ago. According to some analysts, a 30 percent increase in gas prices could increase the cost of the electricity generated by 19 percent while the same price increase for coal would only increase electricity costs by 9 percent.
Research and Development
In 1992, the U.S. Department of Energy initiated the Advanced Turbine Systems (ATS) program for developing and producing the 21st century’s gas turbines. Under this program, DOE, in conjunction with major gas turbine manufacturers, are in the process of developing high efficiency, ultra-clean gas turbines that are expected to break through the operating temperature limits of today’s technologies.
Gas turbines under development in this program are:
- Simple-cycle industrial gas turbines from 5 to 15 MW for distributed generation, industrial and cogeneration applications.
- Gas turbine combined-cycle systems for larger base-load central station electric power plants.
DOE expects the new industrial-sized gas turbines will achieve efficiency improvements of at least 15 percent (LHV). Besides being more efficient, the new gas turbines will emit less nitrogen oxides, carbon dioxide and unburned carbon. The gas turbines are being designed to be fuel flexible. In addition to burning natural gas, they will also be able to burn coal-derived gas or gas from renewable biomass.
67 MW gas turbine at Rokeby Generation Station with ice storage tank in background.
A more recent DOE program is the next generation turbine (NGT) systems program. This, a follow-up program to the ATS program, will demonstrate the application of ATS designs for 30-200 MW industrial and utility electric power plants. DOE’s goal is to have a demonstration ATS by 2002 and a commercially available system by 2005.
The major thrust of the NGT program will be the application of gas turbines greater than 30 MW, operating between 4000 and 6000 hours per year. Gas turbines being developed for this market size would have to have rapid start capability from cold to full power in 10 minutes without any significant reduction in their useful life. In addition, the gas turbines should be highly efficient at full and part load operations.
Rolls-Royce’s involvement in the NGT program stems from a joint venture with Northrop-Grumman, to develop a power-flexible high efficiency gas turbine for the U.S. Navy. The gas turbine, designated WR21, combines heat exchanger inter-cooling, recuperation and a variable power turbine
In order for the WR21 technology to meet the criteria of the NGT program, Rolls-Royce proposes to:
- Use the core of an existing gas turbine production assembly.
- Increase the mass flow capacity so as to raise the power output to 40-50 MW.
According to Rolls-Royce, the thrust of their NGT program is to provide a 40+ MW plant for re-powering and distributed generation application by 2005.
Overcoming Gas Turbine Shortage
Because gas turbine generators were in short supply for summer 2000 operation, a Georgia power plant purchased three 50 Hz machines that were originally installed in a Argentinean merchant power plant. Under a fast-track schedule, GE Aeroderivative and Package Services (GEAPS) were awarded a contract to convert the machines from 50 Hz to 60 Hz.
In addition to converting the machines to 60 Hz operation, GEAPS uprated the LM6000 machines and installed dual fuel and spray inter-cooling systems. With the conversion and modifications, each gas turbine has an output of 47 MW at ISO conditions. The three modified gas turbines have been installed in an existing Georgia power plant. Each gas turbine is operated as a peaking unit. According to GE, this is the first field conversion they have done from 50 Hz to 60 Hz.
Colorado Springs Utilities (CSU) has also experienced lengthy delivery times for new gas turbines. However, to overcome this, CSU decided to purchase two new General Electric, Frame 6B machines that had been in storage since 1994. The two gas turbines, furnished by Coastal Power Company, were originally sold to Brown & Root for installation at Texas A&M University.
As a result of the Texas project being cancelled, the gas turbines and their generators were placed into storage at GE facilities in New York and Virginia. Although ownership of the turbines changed over the following years, they remained in storage until they were purchased by CSU in December 1998. At that time CSU purchased them for their Nixon Power plant located near Fountain, Colorado. The equipment was purchased without any performance or emission guarantees.
In their bid, Coastal Power quoted a shipment date of December 1998, which supported CSU’s commercial operation of July 1, 1999. Prior to the installation, the gas turbines, load gears and accessory skids were shipped to PRECO Turbine Services of Houston, Texas for inspection and refurbishing.
Refurbishing of the gas turbines required the complete disassembly, rotor removal and the checking of clearances. After disassembly, the turbine buckets, nozzles, shroud block, compressor blades, bearings, and lube oil system were all inspected and cleaned. The turbines, load gears and auxiliary equipment were all found in excellent condition and only required minor repairs. After refurbishing the units were shipped to the Nixon power plant in March 1999.
On July 4, 1999, the units were synchronized and put into commercial operation on July 6, 1999. The project took 6 months from the awarding of the engineering/procurement/construction contract to full power operat ion of the gas turbines. Except for some modifications to the firing sequences, NOx levels below 10 ppm have been routinely achieved.
According to CSU engineers, when an electric utility has a need for new peaking capacity quickly, utilizing gas turbines manufactured for other projects, is a viable option as it can reduce project schedules significantly.
Increasing Gas Turbine Capacity
There are several ways for increasing the performance of gas turbine power plants. Some of the options are: steam/water injection, fuel preheating, installing duct burners, and turbine inlet air cooling. However, inlet air cooling is considered to be one of the most effective ways for increasing capacity of gas turbines.
Because gas turbines are constant flow machines, any change in the mass flow of air through the turbine affects their output. When gas turbines are installed for summer peaking, the ambient temperatures invariably are high. When this happens the mass flow of the air through the machine decreases which in turn reduces the gas turbine’s capacity and efficiency. Invariably, this decrease in capacity occurs when the utility needs for the power the most.
To overcome the loss in capacity from high ambient temperatures, many electric utilities are retrofitting inlet air cooling systems to their gas turbines. One utility that has installed a gas turbine inlet air cooling system is Lincoln Electric Systems (LES), at their Rokeby Generating Station. The plant currently has a single 67 MW gas turbine generator but a second gas turbine unit is under construction.
Lincoln Electric chose an ice storage cooling system manufactured by Mueller Thermal Storage Products of Springfield, MO. The system has been designed to cool ambient air down to 42 F from an ambient temperature of 101.5 F. Although only one gas turbine is currently in operation, the system has been sized to supply cooling air for two gas turbines. By decreasing the inlet air temperature, Lincoln Electric has been able to increase the turbine’s capacity by 14 MW.
The Rokeby inlet air cooling system, installed in May 2000, consists of six ice making units and an ice storage tank. Chilled water from the ice storage tank is pumped to coils in the inlet air stream of the gas turbines. Because ice is only made during off-peak periods, Lincoln Electric is able to operate the plant more efficiently.
Mueller has also installed a similar system at Texaco Cogeneration Company’s San Ramon, California plant. The system, installed on a 31 MW gas turbine, has increased its capacity by 7 MW.
On the Horizon
A new technology that is nearing the demonstration phase is a 12 MW “cascaded humidified advance turbine (CHAT) plant.” In this system, ambient air is first compressed in an LP compressor. Following the LP compressor the air passes through a water-cooled inter-cooler, a IP compressor, another intercooler and then an HP compressor. After leaving the HP compressor the air is humidified and pre-heated in a saturator. From the saturator, the pre-heated humidified HP air is used by the HP combustor of the gas turbine.
Until recently the CHAT technology has concentrated on 300 MW or larger applications. However, progress has been made in the development of small CHAT plants. A 12 MW CHAT system, using proven gas turbine technology and components from Rolls-Royce Allison and Dresser-Rand, is ready for full-scale operation.
According to the developers, a 12 MW CHAT power plant has more operating flexibility than a simple-cycle gas turbine or combined-cycle gas turbine power plant. At part load, a CHAT plant operates at higher efficiencies and operates at almost constant power, over a range of ambient temperatures (Figures 1 and 2).
With an efficiency of approximately 46.4 percent, good part load performance and expected single digit NOx emissions, a 12 MW CHAT plant is expected to be an attractive option for distributed generation applications. The capital costs for a CHAT plant are estimated to be from $700 to $750/kW.
According to Bechtel Power estimates, the worldwide demand for new capacity from distributed generation, including cogeneration, will be from 30 to 40 GW annually over the next few years. Similarly, Siemens Westinghouse estimates that 40 percent of new worldwide capacity through 2008 will be from some form of distributed generation. The market for small gas turbines looks very promising.
Small gas turbine manufacturers
Some of the manufacturers offering small gas turbines up to 65 MW include Pratt & Whitney Canada, GE Power Systems, Rolls-Royce, ALSTOM Power and Solar Turbines. The smallest gas turbine manufactured by Siemens Westinghouse is just over 60 MW.
In 1999, Solar Turbines, a subsidiary of Caterpillar, introduced the 13.5 MW Titan 130 industrial gas turbine. According to the manufacturer, the single-shaft Titan 130 gas turbine has an efficiency of 33.3 percent at ISO conditions. This is an equivalent heat rate of 10,249 Btu/kWh. The 396,389 lb/h of exhaust gas at 913 F makes this an ideal option for cogeneration, combined-cycle and district heating/cooling application, according to Solar.
Solar’s Titan 130 has a 14-stage axial-flow air compressor with six inlet guide vanes and a single annular combustor. Low NOx burners are available for pollution control. A split compressor case makes maintenance and inspection of the gas turbine easier. At the cold end of the gas turbine is a three-stage turbine rotor. Air cooling is provided for the first and second stage nozzles and the first stage turbine blades.
At POWER-GEN Europe 2000, S&S Energy Products, a unit of GE Power Systems, announced a new product to its LM product line, the 18 MW GE LM 2000. Jay Manning, Vice President of Global Sales, said that rather than starting from scratch the company decided to improve its existing product line by developing an 18 MW machine with an efficiency to 35.5 percent. In addition, the new gas turbine will have 100 percent commonality with GE’s LM2500.
The new gas turbine will have a lower firing temperature than the LM2500. As a result, it will extend the running time for replacing the hot section to 50,000 hours on natural gas and 25,000 when burning liquid fuel. This is approximately twice the length of time between maintenance intervals of the LM2500, reports GE. These new gas turbines, with a thermal efficiency of 35.5 percent, are expected to be commercially available in the first half of 2001.
Rolls-Royce’s first industrial Trent gas turbine was put into commercial operation in September 1996 at a Canadian cogeneration plant. This aeroderivative gas turbine was fitted with a dry low emissions combustion system. When burning natural gas it achieves less than 25 vppm NOx without the need for water or steam injection. However, when burning liquid fuels, water injection is required.
More recently Rolls-Royce has upgraded its 25-31 MW RB211 gas turbine. The upgrading increased the gas turbine’s output to 30.8 MW and its efficiency to almost 40 percent, an increase of 12 percent in capacity and an increase in efficiency of 6 percent from its predecessor.