Mercury 50 introduces high efficiency recuperation to stationary gas turbines22 February 1998
A 4.2 MWe gas turbine generator set with a thermal efficiency of 40.5 per cent, over 30 per cent higher than current gas turbines in this size class, has been introduced to the stationary generating set market by Solar Turbines in San Diego, California. Using an advanced but well proven recuperator design packaged for optimal performance and simple servicing, the Mercury 50 was developed under the US DOE ATS programme. The first unit is to be installed by Rochelle Municipal Utilities near Chicago for use in economic dispatch mode.
The Mercury 50 gas turbine is the result of a $164.8 million, five year co-operative effort with the US Department of Energy (DOE) as one of four main streams in the ATS (advanced turbine systems) programme. The key component – the primary surface recuperator – has been under development by Solar Turbines and Caterpillar Inc., its parent company, since the early 1970s.
The resulting product has now been launched into the market at two recent venues – the Power Gen International '97 conference in Dallas, Texas, and the US DOE ATS Annual Program Review meeting in Morgantown, West Virginia.
Recuperated cycles have been used with gas turbines in the past with dubious success – mainly in the petrochemical industry. In general, bulky shell and tube or plate-fin heat exchangers were added to standard machine designs using elaborate and cumbersome piping with little effort to optimise the thermal cycle.
Limited performance improvement was gained at the cost of poor thermal transient response, thermal cracking and other mechanical performance problems. The Mercury 50 is the first attempt to design a gas turbine arrangement specifically to work with an advanced recuperator, starting from a clean sheet. The turbine, and particularly the innovative compressor, is a remarkable demonstration of state of the art turbine technology development.
The result is an electrical efficiency of 40.2 per cent, single figure NOx emissions, and an extraordinarily compact but readily serviceable power plant. Its power output rating is 4.2 MWe continuous duty at ISO conditions.
The single shaft gas turbine, running at 14 179 r/min, drives either 60 Hz or 50 Hz generators via an epicyclic gearbox driven from the compressor end of the turbine. The layout of the turbine/recuperator arrangement has been effectively adapted for optimum simplification of the gas flow paths between the compressor, recuperator, combustor and turbine. The flow path has been organised to naturally follow the flow of the recuperator.
Compressor discharge is at the end of the machine in the same plane as the inlet header to the recuperator. The inlet to the combustor is also situated at the end of the machine in the same plane as the recuperator discharge header.
The turbine exhausts upwards through the recuperator at the centre of the engine. This has necessitated the reversal of the normal layout of the compressor and turbine to provide a greatly simplified and streamlined flow path.
This configuration requires the design of a centre frame structure to interconnect the turbine and compressor housings in place of the hot strut design used in most gas turbine assemblies. The load carrying members of the centre frame are located in a cool environment out of the turbine hot gas flow path, thus minimizing axial extension of the turbine case due to thermal growth and allowing tight blade tip clearances to be maintained.
A high degree of modularity allows each of the major sub-system including the combustor system, the turbine, recuperator, gearbox or generator to be changed independently in the field in a single shift without the need to replace the entire engine.
Great efforts have been made to reduce ancillary equipment power consumption, which typically takes as much as eight per cent of the power output. By using a double-helical epicyclic gearbox for the generator drive and variable frequency AC motors for lubricating oil pumps and fans instead of mechanical drive from the turbine, an additional one per cent increase in efficiency is obtained.
With an exhaust temperature of 368°C and a mass flow of 61 200 kg/h, there is substantial scope for combined heat and power operation, and supplementary firing would increase performance in this kind of service.
Working in concert with Dr John Adamczyk of the NASA Lewis research centre in Cleveland, Ohio, Solar have developed the ACE (advanced component efficiency) compressor using the latest three dimensional flow blading design codes and modelling techniques. The techniques used in the Mercury 50 design were first used for the redesign of the Mars T15000 compressor in 1993.
The ACE design, it is claimed, benefits from 3-dimensional wide chord airfoils that are lightly loaded, resulting in a 40 per cent reduction in the number of airfoils for a given pressure rise.
The Mercury 50 compressor has a pressure ratio of 9.1:1 from a ten-stage compressor which makes for increased flexibility of fuel supply – the minimum pressure requirement is only 12 bar.
A variable inlet guide vane in the first stage is followed by stages of variable guide vanes that are ganged together and controlled as a unit for optimum compressor control across the operating load range.
The compressor design was validated at the Compressor Research Facility of the Wright Patterson Air Force Base in Dayton, Ohio. During the testing, which was completed in July 1997, the performance was fully mapped and compared to Mercury 50 design goals as well as the current compressors. The results were reported to validate the design goals and showed an efficiency gain of over two points over Solar's current compressors.
Solar PSRs have now accumulated well in excess of 1.5 million operating hours without encountering much of the incipient problems reported by other recuperator technologies. The design is inherently resistant to low cycle fatigue failure because the clamped air cell structure allows the assembly to flex freely to relieve stresses rather than concentrating stresses at the weld locations.
The air cells are constructed from 0.1 mm thick 347 stainless steel formed into an undulating corrugated pattern which maximizes the primary surface area in contact with the hot exhaust gases on one side and the compressor discharge air on the other. Pairs of these sheets are welded together around the circumference to form air cells.
There are no internal welds or joints within the air cell. Layers of these cells are clamped together with clamping bars and the assembly is welded to the intake and discharge headers.
As well as being inherently resistant to low cycle fatigue failure, high cycle fatigue is also minimized due to the damping characteristics of the clamped design. The stacking of the cells presents multiple friction interfaces for energy absorption. This latter characteristic also provides sufficient sound attenuation to eliminate the need for an additional silencing device and avoids the resulting pressure drop.
The modules, which are claimed to give over 90 per cent effectiveness with only moderate pressure drop, are manufactured in Solar's newly automated production facility located in Channel view, Texas.
The two-stage design of the Mercury 50 turbine was selected because of its inherent cost advantages. The extra cost of the additional cooling required for a third stage would have negated the resulting performance advantage. Another consideration is that it becomes increasingly important to minimize the use of cooling air since the trend towards lean, premixed combustion to minimize NOx emission has made the availability of cooling air a critical commodity.
The turbine rotor inlet temperature (TRIT) is fairly high at 1163°C, and the fully cooled first stage turbine is highly loaded. The second stage incorporates cooled vanes and uncooled, shrouded blades.
A novel leading edge cooling scheme known as vortex cooling has been used in the first stage blades. This involves the use of swirled cooling flow to the leading edge cooling circuit. This technique is seen as having considerable improvement potential without incurring the performance penalties associated with showerhead cooling which would otherwise be required.
Of particular interest in the Mercury 50 are the new alloys and materials that have been introduced into the turbine. The filmcooled first stage nozzles are made from MAR-M-247, while the uncooled second stage vanes are constructed from equiaxed forms of the same material.
For the unshrouded first stage blades, a third generation single crystal alloy, Cannon - Muskegon's CMSX-10, has been selected. The blades will be mounted on a Waspaloy disk using fir tree root fixing modified to take improved disk-post cooling.
The second stage blades are Solar's first shrouded design. These are also made from equiaxed MAR-M-247. Dispensing with cooling in the second stage blades resulted in requirement for the disk beyond the properties of currently used materials. A powdered metal forging of Udimet 720 was selected. The fine grain structure of the rim makes it suitable for the higher rim operating temperatures while still maintaining sufficient low-cycle fatigue strength at the hub.
The combustion system of the Mercury 50 is designed to accommodate ultra-lean premix (ULP) or catalytic-type combustors. ULP annular burner design developed from Solar's existing SoLoNox combustion system, with the new feature of a diverter valve, will direct air flow either through the combuster, or through a bypass circuit to a point downstream of the combustor. This will provide the necessary function of regulating flow through the combustor at part load.
The metallic combustor liner will be protected by a plasma sprayed thermal barrier coating in the first machines. The company is now working on the development of ceramic combustor linings which should be adopted during the product's market life.
A catalytic combustion system using a catalyst bed developed by Catalytica is being worked on as an alternative when this approach attains operational status. Since in the recuperated turbine cycle the combustor temperature is generally higher than the catalyst bed auto-ignition temperature, the need for a preburner to bring the catalyst up to operating temperature is precluded.
Single figure NOx levels are expected from both combustion systems, however, the initial Mercury 50 field units will have an introductory emission level of 25 ppmv on natural gas.
Rochelle field test
The site for the first field test of the Mercury 50, now approved by the US DOE, is a municipal utility group with a service area of some 100 square miles in North Central Illinois, 120 km from Chicago, entirely surrounded by Commonwealth Edison country. The area has one radial transmission connection to the Com-Ed grid.
The host operator will be Rochelle Municipal Utilities. Rochelle currently purchases 95 to 98 per cent of its power and operates a number of reciprocating engines as peak shavers. The utility will utilise the Mercury 50 gas turbine generator set in economic dispatch mode to reduce its cost of power and to improve system reliability.
The new Mercury 50 gas turbine unit will be located proximate to the Rochelle Food Processing and Distribution Centre, which accommodates Hormel, Kraft Foods and Erie Foods International in the food processing business, Del Monte Corp. in the canned food distribution business, and also Americold and Total Logistics Control in the frozen food distribution business.
Other industries in the location include office supplies dealer Avery Dennison, electrical parts supplier Eaton Corp. in addition to can manufacturing company Silgon Container Corp.
General manager Ray Schwartz describes the utility's power supply policy as:purchase 95 to 98 per cent of power several rate schedules of firm power purchased load following with general purpose (GP) power generation assets used for economic dispatch generate when GP power cost exceeds in-house asset costs total output in 1996 = 179 284 000 kWh.
The new Mercury 50 is expected to see between 4000 to 6000 hours per year under normal operating conditions. Mercury 50 design features which minimize efficiency degradation when running at part load or when ambient conditions change are likely to play an important role in the economic dispatch of the prototype unit.
TablesTable 1. General specifications of the Mercury 50 gas turbine Table 2. Typical heat recovery performance