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Designing gas turbines for Chinese CHP

Designing gas turbines for Chinese CHP

Both low emissions and high efficiency are becoming increasingly important for gas turbines in China. MAN Diesel & Turbo explains why its 6″7 MW class MGT6000 product family has been successful in the Chinese market, where the gas turbines are used for combined heat and power applications

Small gas turbines in the power range up to 20 MW have been used for years as electricity generators for peak power demand as well as for baseload-operated stations mostly in combined heat and power (CHP) applications, where besides power demand steam and/or hot water is required for industrial purposes or district heating. Short installation times and high flexibility in transient operating conditions combined with very low emissions are some of the features which are criteria to select gas turbines for power generation applications. In addition, such small gas turbines are also widely used as mechanical drivers for compressor applications in upstream as well as in midstream markets.

Some years ago, MAN has introduced the 6″7 MW class MGT6000 product family into the market. This product family is very successful in the Chinese market where the gas turbines are used for high efficiency CHP applications combined with very low emissions.

Design and performance

The MGT6000 single-shaft version is a robust gas turbine with cast casings, horizontally split at the compressor section to allow top or bottom casing removal for complete rotor access to compressor blades and vanes. Power output is at the cold compressor end with engine shaft speed reduced by a load reduction gearbox. The air intake casing is located at the front end of the gas generator and supports the front of the compressor section. The front-end radial/thrust bearing arrangement is located within the middle of the inlet casing. This feature provides easy access to the bearing. The auxiliary gearbox is directly connected to the front flange of the air inlet casing and houses the main lube oil mechanical pump. An electric starting motor drives through the gearbox to the compressor shaft.

The 11-stage axial compressor is designed for a pressure ratio of 15. Its construction allows for simplified installation and overhaul activities. The inlet guide vane and three downstream rows of stator vanes are adjustable to optimize the aerodynamic behaviour and enhance the stability margin at part load operation. This also allows high exhaust gas temperature for excellent part load CHP efficiency by varying part-load airflow.

The six individual Advanced Can Combustion (ACC) chambers are evenly distributed along the circumference, inclined relative to the centreline of the shaft and flanged to a single piece inter stage casing. This means simplifying inspection and service activities, as both the combustion chambers and the transition ducts may be installed or easily removed from the complete machine. A simple borescope access allows the inspection of the entire combustion chamber, the transition duct and the first row of turbine stator vanes.

MGT6000 gas turbine
Credit: MAN Diesel & Turbo

The combustion chamber design contains a vortex-stabilized burner section within the combustor can, followed by a transition duct leading into the first stage turbine nozzle.

The combustors are of modular design, easily accessed, removable and exchangeable in the field. The burner has a swirl chamber and two different gas fuel injectors which are operated in diffusion mode (pilot gas), in pre-mix mode, or in the combined diffusion and pre-mix mode depending on load. With this design, emissions below 10 ppm can be achieved over a wide power range.

As with the compressor rotor, extensive access for borescope inspection is easy for the entire combustor section and for the turbine nozzles and blades. Ignition of the flame is through individual high-energy torch igniters, one for each combustor can.

The turbine is a three-stage axial flow design. Like the compressor rotor, the three turbine discs are assembled and held in place with Hirth couplings and centre tie-bolt.

The first two turbine stator vanes and the rotor blades are internally cooled. The tip area of the first stage rotor blades is provided with a rubbing edge to reduce tip clearance losses. The second row blades have contacting pre-loaded tip shrouds.

The gas turbines are delivered as pre-mounted units on a steel base frame, complete with lubrication and auxiliary systems as well as enclosure, inlet and exhaust system modules, engine and driven equipment controls, fire detection and suppression system. Such pre-assembled packages help to minimize erection and engine installation work at site.

For customer support (e.g., for maintenance purposes or remote diagnostics), the control system is equipped with a remote data transmission interface. This allows the transmission of data for analysis, diagnostics and maintenance support or to allow remote monitoring for the operator.

The MGT6000 power generation unit has been designed to meet international requirements for power generation and is able to compensate for grid instabilities. Due to ongoing changes worldwide in electrical power generation structures, the technical requirements for power generation units are also changing. A very robust design with regard to grid failures, disturbances and instabilities is required, which means that the machine needs to stay in operation in many of these cases.

This results in a design which allows rapid load changes while the machine remains in operation ” even in case of a sudden power drop of 100 per cent. The MGT6000 power generation package has been tested in order to prove its capability to meet the requirements regarding the dynamic and static behavior for power generation units operated in an electrical grid or in island mode. As part of the test, sudden load changes, e.g. a 100 per cent power drop or a 60 per cent load increase in less than
50 milliseconds, were applied and successfully handled without initiating shutdowns and without exceeding the allowed frequency and voltage variation limits.

Therefore the MGT6000 power generation unit is able to fulfil increasingly stringent requirements with regard to dynamic behaviour in case of grid failures, availability and reliability.

Distributed generation with CHP

CHP technologies are nowadays proven, reliable and cost-effective systems focusing on decentralized power generation where heat recovery is used to produce steam, hot water and/or cooling for a wide range of industries like food processing, breweries, automotive, agriculture and pulp and paper, among others.

These plants generally have high process-related constant thermal and electrical requirements. In the food industry, process heating uses approximately 30 per cent of total energy, while process cooling and refrigeration demands about 15 per cent of total energy output. Breweries use particularly large amounts of energy because many steps in the brewing process actually involve heating or cooling. In the chemical industry, energy costs account for up to 15 per cent of manufacturing costs on average; in addition, steam is required for chemical processes.

In typical process solutions for CHP applications, a by-pass stack as well as a supplementary firing unit are used. Both features are optional and can be deployed depending on project-specific requirements.

With such CHP configurations, overall system efficiency of up to 90 per cent can be achieved (ISO conditions, zero inlet/exhaust pressure losses) while having very low emissions in a wide range of operational loads. Compared to conventional production of electricity and steam using coal firing, the reduction of CO2 greenhouse gas production is in the range of 100,000 tonnes per year for each MGT6000 CHP unit.

Exchanging a MGT6000
Credit: MAN Diesel & Turbo

But besides high efficiency and low emissions, industrial CHP applications also have to show good return on investment. Considering average operation times of 8000 hours per year and an average cooling time of 2400 hours per year as well as typical gas and electricity prices, static pay back times between two years (district heating application) and 4.5 years (CHP application with power generation, steam and cooling) can be achieved.

MGT gas turbines in China

MAN looks back at a century-long history in China, where the company today operates more than 750 turbomachinery casings. For power generation applications with gas turbines, several MGT gas turbines have been sold so far to China. All of them are operating or are scheduled to operate in CHP applications.

The first four MGT6000 series gas turbines where set into operation in January 2016 in the Anting/Shanghai power station for end customer CSVW (Volkswagen China/Shanghai Motors). This project was executed together with Shanghai Aerospace Energy Co Ltd which took the EPC responsibility and supplied boilers, air intake and exhaust systems as well as generators. In this power station, four boilers generate steam for automobile production lines and hot water. In total, the plant generates approximately 26 MWe, and heat for production. All four CHP lines have operated approximately 35,000 hours in total since commissioning (status as of February 2018). Two CHP units always feed one production grid. As the factory is operating in island mode, the CHP units have to support all of the electrical and thermal load requirements of the production grids according to their direct demand. This leads to daily load changes.

Air pollution is also a severe issue in the Shanghai area. With the installation of the four CHP units, pollutant output is reduced significantly compared to a conventional coal-fired boiler plant. In particular, emissions of NOx, CO and smoke are very low and a major reduction in CO2 greenhouse gas production is achieved.

Air pollution also affects the CHP plants’ operational performance since the gas turbines’ air inlet filter can be clogged by pollution particles. In addition, micro particles also enter the air intake path and form deposits on the rotating and stator parts of the compressor. This leads to reduction of air flow and compressor efficiency. Whereas air intake filters need to be replaced according to their degree of contamination, compressor losses can be recovered to a great extent by washing the compressor via nozzles which are installed in the compressor inlet. A performance recovery of 90 per cent is not uncommon after successful wash of a fouled compressor.

During 2018, further CHP plants will be commissioned in the town of Changsha. The plant will include an MGT6000 gas turbine as well as a THM series gas turbine. Together these CHP sets will generate about 17 MW of power and 35 MW of steam for an industrial park. They will replace existing coal-fired facilities. These power stations will also be built together with Shanghai Aerospace Energy Co Ltd as EPC.

Additional MGT6000 gas turbines will go into operation. Two of them will be used for power generation of approximately 6 MW and steam as thermal power of 13 MW for paper production facilities. These industries will operate more or less continuously.

Another CHP plant will supply power and steam for an industrial park which is producing electroplating and printings. Also, these CHP plants will intensively contribute to the shift from coal-fired to gas-fired power and heat in the People’s Republic.

Maintenance concepts

Economical operation of gas turbine-fired CHP plants calls for preventive maintenance concepts which help to operate the plant with a low number of disturbances.

This concept includes routine maintenance measures conducted by the operating personnel and periodic scheduled maintenance measures, e.g., inspections of the core engine and systems on an annual basis or after 8000 equivalent operating hours. This includes boroscope inspections of the core engine which can be conducted via openings, avoiding the need for disassembly. In addition, lube oil analysis as well as an audit of control system and auxiliary systems are conducted. After 40,000 equivalent operation hours an overhaul of the core engine is foreseen. For this overhaul the core engine is removed from the package and can be replaced by an exchange unit.

During the development of the MGT6000 gas turbine series, special attention was given to the service aspects of the unit. The core turbine can be removed within 48 hours. This exchange time covers the installation of a replacement core engine and depends on project- and site-specific conditions, e.g., agreed work shift models and accessibility of the gas turbine. These maintenance works can be included in different service and maintenance agreement types.

Ulrich Orth, Andreas Spiegel, Detlef Viereck, Marcel Sicker and Sven-Hendrik Wiers
work at MAN Diesel & Turbo in Germany