TwinPac turbines power German cogen industrial facility

Europe`s first TwinPac gas turbine application at Solvay Alkali`s Rheinberg chemical plant increases capacity, reduces emissions and boosts system efficiency

By Axel Emde, GHH Borsig Turbomaschinen GmbH

Ranier Bru, Solvay Alkali GmbH

Solvay Alkali GmbH`s Rheinberg plant in Germany produces several industrial commodities, including salt-based chemical soda ash, chlorine and polyvinylchloride. Energy costs for the production of these materials are very high, resulting in high demand for both electrical energy and process steam. As a result, the plant`s management decided to install a gas turbine-based cogeneration plant to take over part of the baseload requirements for process heat and electric power.

More power

Solvay teamed with GHH Borsig to develop the overall concept for the new system. The design incorporated Europe`s first FT8 TwinPac gas turbine set, supplied by GHH Borsig Turbomaschinen GmbH. The gas turbines are paired with two downstream waste heat recovery steam generators (HRSG), thus replacing an obsolete coal-fired boiler that produced 100 t/h of steam. The coal-fired boiler had been in service since 1961 and its operating permit had expired after approximately 260,000 service hours.

The maximum electric power provided by the gas turbine package is 54 MW at site conditions. Shutdown of the original coal-fired boiler reduced electrical energy output from the backpressure steam turbines by about 14 MW. The additional 40 MW in capacity allows the plant to meet steam production requirements at a more economical plant utilization factor.

Solvay demanded that both operating costs and capital outlay for the project be optimized according to the technical input requirements. The project team selected aeroderivative gas turbines as the best choice for this combined heat and power generation application. With exhaust temperatures around 450 C, aeroderivatives are particularly well suited to cogeneration processes requiring hot water or process steam at temperatures less than 400 C. The two aeroderivative gas turbines in the plant are each rated for an electric power capacity of 27 MW and are directly connected to a centrally located synchronous generator.

The FT8 gas turbine consists of a gas generator and a free-running power turbine. The gas generator is adapted from the Pratt & Whitney JT8D-219. The JT8D is used in the Boeing 727 and 737 aircraft as well as in the McDonnell Douglas MD80. Because of its two-spool configuration and low bypass ratio, this aeroderivative engine is well suited for industrialization, where the engine fan stage is replaced by adjustable inlet guide vanes and two additional compressor stages. The engine components have been extensively field tested, with more than 400 million flying hours accumulated on over 14,000 engines.

The gas generator has a low-pressure (LP) and a high-pressure (HP) shaft. A two-stage LP turbine drives an eight-stage LP compressor. The HP turbine is on a common shaft with a seven-stage HP compressor. The two-shaft concept of the gas generator, the adjustable inlet guide vanes and the adjustable stator vanes on the first two stages of the gas turbine compressor provide good part-load efficiency.

Both power turbine and gas generator run in anti-friction bearings and are fed with synthetic lube oil from a common oil system. For power generation with 50 or 60 Hz grid frequencies, four-stage power turbine designs are available with design speeds of 3,000 or 3,600 rpm for clockwise and counterclockwise rotation (for the direct drive of synchronous generators). The negative impacts of a gearbox on investment costs, efficiency and availability are thus avoided.

Overrunning clutches

Overrunning clutches from SSS-Gears are installed on both sides of the generator to permit the gas turbines to operate independently, thus providing maximum flexibility. The clutches incorporate synchronized self-switching–hence the term SSS–for more effective operation. When the input speed matches the output speed, the teeth of the clutch are automatically engaged axially with precise alignment of tooth and flute. The driving torque is then transmitted from the gas turbine to the generator.

Disengagement of the clutch occurs when the input speed drops below the output speed. Engagement or disengagement occurs without torque transmission. As a result, there is virtually no wear.

The basic design and functionality of the SSS clutches are illustrated in Photo 3. Clutches at the Rheinberg plant are seated in their own casings, which are mounted to the generator base frame. They are fed with mineral lube oil from the generator oil system. When engaged, the power loss per clutch is only 50 kW.

When fitted with overrunning clutches, the TwinPac system can function as a synchronous condenser. In this operating mode, the generator is uncoupled from the driver and provides reactive power to the electricity grid. Mechanical losses are overcome by tapping electric power from the grid. However, this feature is not currently used at the Solvay plant.

Heat recovery

Each gas turbine is fitted with a HRSG without supplementary firing. These horizontal, self-supporting natural convection boilers, provided by EVT Neumark, are made up of five modules: inlet duct, superheater module, evaporator module (including boiler drum), hot-water economizer module and outlet duct. The HRSGs are rated for a gas turbine starting ramp rate of approximately 10 MW/minute after synchronization. Operation is without bypass, thereby reducing the capital outlay without any limitation on functionality.

TwinPac concept

The basic TwinPac concept was fully implemented at the Solvay plant, with separate auxiliaries, ancillaries and control systems provided for each gas turbine and HRSG. As such, either gas turbine or HRSG can be shut down for maintenance or repair work while the second unit remains operational. When the plant is being operated at half load, one unit will automatically provide total power supply if the other gas turbine is switched off.

The Solvay gas turbine plant achieves high efficiency over a wide load range, working in the load-sharing mode between full load (54 MW) and half load (27 MW). If power requirements drop below half load, one of the gas turbine can be shut down, providing a fundamental advantage over other gas turbines in the 50 MW power class (see figure).

Rheinberg installation

The Rheinberg site provided an excellent infrastructure for a low-cost installation. Already located in the immediate vicinity of the gas turbine facility were a long-distance natural gas pipeline with a pressure of approximately 45 bar, a transformer station and all necessary process steam piping. In a purpose-built station the fuel gas pressure is reduced to 32 bar. The existing water treatment plant was of sufficient capacity to provide fully demineralized water (about 5 m3/hr) to the gas turbines for NOx reduction. Because cooling water was not available on site, the gas turbines and generator are equipped with air-cooled lube oil systems and the generator is also provided with open-circuit direct air cooling.

All components were separately tested and then assembled on site. Pratt & Whitney performed full-load tests on the gas generators at its shops in the United States and SSS-Gears confirmed the functionality of the SSS clutches at its facilities in Great Britain. Engineers conducted integrated testing at Jeumont Industrie`s facilities in France: the generator, fitted with the SSS clutches, was tested by driving the generator from both ends using electric motors. Key evaluation parameters included the overrunning clutch engagement and disengagement processes.

Prior to delivery to the job site, engineers also conducted individual component testing on the control systems and ancillary equipment, including the gas turbine oil units and the water injection systems. During commissioning and subsequent four-week trial, the project team tested and optimized the completely integrated power station.

The gas turbine is integrated into the existing plant. The HRSGs supply a total of 84 t/h of 12 bar steam to the process lines and to a GHH Borsig extraction condensing steam turbine, which produces 10.3 MW. In addition, 200 t/h of feedwater are preheated from 105 C to 170 C.

System control

Operation of the gas turbine unit is simple at Rheinberg. Plant personnel prepared for operation during equipment commissioning and during a special training course. All operations are maintained from the central control room using remote control. Additionally, all operating and control functions are available in the local control building beside the gas turbine. A Woodward Netcon 5000 system provides gas turbine engine control, while a Siemens S5-115U system supplies unit control of the gas turbine ancillaries. Gas turbine and generator data visualization and logging are performed by a Siemens Coros LSB system in the gas turbine control cabinet. The higher-ranking power station control system, Contronic S by Hartmann & Braun, collects gas turbine data from an L2 bus interface for visualization in Solvay Rheinberg`s main control station.

Reduced emissions

The Solvay Rheinberg plant achieved significant pollutant emission reductions by replacing the coal-fired boiler with the TwinPac system. The use of natural gas reduced carbon dioxide emissions by approximately 90,000 t/y. The gas turbines also avoided the equivalent of 370,000 t/y of carbon dioxide that would have been emitted if a coal-fired boiler/steam turbine system had been used to achieve the same capacity upgrade. Nitrogen oxide emissions were roughly halved and particulate and sulfur dioxide emissions were essentially eliminated.

Economics

To reduce capital outlay required for the facility, engineers optimized the plant concept and its arrangement by designing the gas turbine package for outdoor installation on a “greenfield” site. No cost is incurred for a machine house building. Each gas turbine acoustic enclosure has its own crane system for rapid exchange of the gas turbine components during scheduled overhauls. The enclosures maintain noise levels as low as 70 dBA. The gas turbine ancillaries are housed in separate facilities directly linked to the gas turbine enclosures. Particulate control systems for the gas turbines and the generators use two-stage bag filters designed so exchange is possible during full-load operation, reducing plant downtime.

The packaged TwinPac configuration reduces capital outlay in comparison with two individual engines of the same total power rating. Cost savings are mainly obtained by having to install just one generator with ancillaries and unit transformer. By using prefabricated HRSGs, assembly and erection costs are minimized. In the described configuration, the TwinPac system has total investment costs of approximately (US)$510 per kW of installed electric power.

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Above, the gas turbine set at Solvay Alkali`s Rheinberg plant. Below, the FT8 gas turbine, based on the Pratt & Whitney JT8D-219.

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Schematic of the synchronized self-switching clutch used to mate turbine to generator. The TwinPac design, showing gas turbines,

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Author bios

Axel Emde is a project manager for gas and steam turbines with GHH Borsig Turbomaschinen GmbH. He has a degree in mechanical engineering from RWTH Aachen.

Rainer Bru is energy manager at Solvay Alkali GmbH.