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Products and solutions, 2007-01-29, 11:00 AM

Cogeneration regeneration

A powerful performance in which all concerned put in a concerted effort: inside only 13 months, the combined heat-and-power plant in Würzburg, Germany, was given a new gas and steam turbine system under conditions of ongoing operation. On behalf of Würzburg’s municipal services department, the new equipment was supplied by a consortium consisting of Alstom, Siemens, and ThyssenKrupp Xervon Energy. The latter, heading the consortium, also organized many of the trades and services required during the assembly and installation phases.

Around one-half of the contract value for the rebuilding of plant was the responsibility of ThyssenKrupp Xervon Energy. This company not only supplied the new waste-heater boiler along with the auxiliary firing system and forced-draft fan but also handled the dismantling of all piece of equipment, the supply and installation of all the waste-gas flues, the smokestacks, pipework, pumps and fittings including their insulation, all of which comprised the specified tasks of this industrial services provider specialized in the power plant sector. Another responsibility was the on-time construction of a new 20 kV/110 kV transformer station.

Siemens AG Power Generation supplied the complete instrumentation and electrical engineering systems for the gas and team turbine plant and the transformer station. Siemens Industrial Turbomachinery AB (previously, Alstom Power Industrial Turbines) supplied the heart of the new system, an SGT 800 gas turbine with an output of 40 to 50 MW. This is capable of burning natural gas in dry low-emission combustion chambers at a temperature of over 1,200 °C, and with very low CO and NOx emissions. The waste gas developed is then routed to the free power turbine which uses part of the energy for driving the gas turbine generator. The waste gases, still 540 °C hot, are used by the boiler unit for generating steam.

Two boilers, one housing

To maximize the efficiency, Xervon Energy installed a dual-pressure boiler (natural cycle). Its generous water volume enables close adjustment to volatile and rapid load fluctuations. The boiler has a high- and a low-pressure steam system. The maximum steam output in the HP section is 125 t/h at a hot-steam temperature of 515 °C and an exit pressure of 73 bar absolute. This is achieved in waste-heat operating conditions with auxiliary firing as well as in forced-draft operation. In the LP section, dry-saturated steam at 5 bar is produced at around 12 t/h. The special feature of the LP section: if for periods of time LP steam is not needed, the unit can be filled up to 15 bar and reoperated when required.

The boiler structure is mounted to a supporting grid to enable unimpeded expansion in all directions, without the damage potentially caused by any obstructions. Another specialty: all the heating surfaces are vertically positioned, with full water drainage and air venting via the collectors. The surfaces themselves are made up of 28 prefabricated modules.

Made-to-measure insulation

The ThyssenKrupp Xervon Energy specialists fitted the boiler walls and the gas turbine ducting with interior heat insulation. In view of such factors as interior temperature, outside temperature, heat conductivity and insulating thickness, they opted for a multilayer lining. For this purpose, various types of insulating material in pad or mat format are anchored to the sheet metal jacket with the aid of pins made of stainless steel. Whereas such a type of fastening proved impossible due for instance to complex conduit structures, the insulators resorted to a stacking system by which bans of ceramic fiber 610 mm wide, 2.54 mm thick and at a depth equivalent to the total insulating thickness (mostly 250 mm) are bonded to expanded metal.

Firing system for flexibility

With natural gas-fired duct-type burners mounted between the gas turbine and the boiler, the operator has the option of running the system flexibly in a variety of modes. The five duct burners with a total firing capacity of 50 MW are able to raise the temperature of the exhaust gas flow in the upstream turbine to 900 °C which, in turn, allows high-pressure steam generation in the boiler to rise to 125 t/h. Should for some reason or other the gas turbine not be operable, the additional firing system goes into main firing mode. A forced-draft fan (designed for 380,000 Nm³/h) then delivers via a duct system the air necessary for the combustion process.

New smokestack

Also new is the power plant’s smokestack engineered and erected by Xervon Energy and designed to replace the previous 104-m tall collector stack. It consists of three individual, two-part, double-walled, freestanding and hence self-supporting steel stacks. For optical reasons, all three have the same support tube diameter of four meters. The diameter and wall thickness of the internal flues, in contrast, vary, depending on their function. The 50-m tall stacks are mounted in clearances of 7.50 meters and a height of around 20 m on the roof of the 43-m long turbine shop whose steel frame construction has been accordingly engineered.

Built in only 13 months

So much as to the engineering specifications of the cogeneration plant which, for well over one year now, has been delivering power and heat to the city of Würzburg, and very efficiently so. For much lower emissions and upgraded efficiency, the plant covers up to two-thirds of the electricity needs and the entire district heating requirements of around 130,000 people living in and around Würzburg.

The project to the value of €55 million, was completed inside only 13 months, which goes to show how perfectly it was planned. Operator, planners, public authorities, the consortium and its subcontractors worked in close unison to fulfill the ambitious schedule. And all this despite obstacles and constraints: a lack of space, ongoing power plant operation, and the critical character of the traffic situation around the construction site (two Main river bridges, the river itself, and the customs authorities as external parameters) called for the ultimate in logistic skills.

The location for the new gas turbine plant and the transformer station was the coal storage site used until then and the meanwhile outdated boiler unit K4 (dating back to 1966). This latter had to be dismantled and the coal regularly delivered by truck so that the existing power plant components could remain operational, without any disruptions, throughout the construction phase. In all, it was only in the closing switchover stage that a total shutdown proved necessary.

Around 30 heavy haulages within tight time windows delivered the plant components to the site where they were unloaded and brought into position by a high-capacity crane. There were virtually no possibilities for any intermediary storage. Throughout the project, the delivery, escape, rescue routes, the site equipment, the material storage areas and the containers accommodating the site workers were repeatedly adapted and readjusted to new conditions.

Exacting, exciting one-off challenges

“Thanks to the skills of the site personnel, there were no hold-ups in the progress of the project and there was no need to compromise in terms of health and safety or the quality of the exacting work,” is how Dr. Ing. E. Rejek from the Richter Power Plant Engineering office assesses the project. Praise to be proud of.

ThyssenKrupp Xervon Energy employed on site three highly experienced multi-trade supervisors to oversea and organize the work of up to 120 craftsmen skilled in a wide variety of trades. The jobs included such repeated chores like boiler assembly, pipework construction, insulation, pump and fittings installation — jobs that Xervon Energy as a specialist in power plant construction handles through its own staff.

Other chores included, for example, the disassembly of the boiler and other plant components. The old 104-m collector stack built from reinforced concrete with internal flues also had to be torn down. Because of the lack of space, the latter were sawn and the individual segments (maximum 11 tonnes) carried away by a building crane. For this specific purpose, Xervon Energy commissioned an external specialty firm.

The foundation (21.60 m long, 7.2 m wide, and 1.90 m tall) to which all the new gas turbine modules are anchored, was likewise built under the supervision of Xervon. The same is true of the concrete structure and the electric/plumbing system for the new transformer station (a reinforced concrete building 43 m long, 14 m wide, and 20 m tall). The new building is designed so that the large transformers on the ground floor will still do their job in the event of 300-year record river floods.

Although the services provided in Würzburg was multi-faceted, they were nonetheless all provided from a single-source responsibility. For future such power plant projects it should be even easier to assemble a complete package thanks to the close ties with ThyssenKrupp Xervon and access to the expertise assimilated in this part of the group.

Background info

<u>The Würzburg cogeneration facility:</u>
Since 1954, the Würzburg CCPP cogeneration plant has been providing power and district heating to the town of Würzburg and the surrounding region. The newest €55-million investment in the CCPP is the biggest ever made by Würzburg municipal services, enabling it to step into the 21st century in terms of energy generation. The switchover to natural gas as a prime source of energy means that the plant is using a fossil-type fuel with the lowest emissions. Despite its superior efficiency, the new plant emits annually up to 120,000 t less greenhouse carbon dioxide.

<u>The CCPP process:</u>
Burning coal, fuel oil or natural gas in a boiler produces steam which is routed at high pressure (72 bar) and temperature (515 °C) to a steam turbine. Here, the steam decompresses and drives the turbine. A generator connected to the turbine generates electricity. The almost completely decompressed steam exits the turbine and is sent at 3 bar and 180 °C into the district heating network which routes the steam to the consumer. In the process, the remaining energy is extracted from the steam for the purpose of producing hot water. This production of thermal energy, district-heating steam and electricity in a single process is termed power-heat cogeneration. The efficiency of the process can be much improved by combining steam and gas turbines. In this process, natural gas is burnt in a gas turbine. The resultant hot gases drive a turbine which, in turn, drives a generator for outputting electricity. And now comes the critical step: the hot waste gases from the gas turbine are not emitted into the atmosphere but routed to a boiler that produces steam without burning any additional fuel. The downstream steam turbine process remains unchanged and continues to generate electricity while disconnected from the district-heating function. This combination of gas turbine and steam turbine is termed a Combined Cycle Power Plant (CCPP).

Source: Heizkraftwerk Würzburg GmbH

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