By Siân Green

German utility Kraftwerke Mainz-Wiesbaden is commissioning a new gas fired combined cycle power plant, designed to meet its power, heat and process steam load. With a net efficiency of over 58 per cent and a high fuel utilization factor, the plant will be one of the cleanest and most efficient power plants in Germany.

In 1998, German utility Kraftwerke Mainz-Wiesbaden made a decision to construct a 400 MW combined cycle cogeneration plant to help meet its current and future needs. In March 2001, this new plant was officially inaugurated and is currently in its final commissioning phase before commercial operation starts.

Located 40 km from Frankfurt, Germany, the new power plant supplies power to the local distribution grid as well as district heat to the local area and process steam to local industries. It is equipped with advanced gas turbine technology that will enable it to reach a combined cycle efficiency of over 58 per cent, and the simultaneous extraction of process steam and district heat means the plant will have a high fuel utilization factor of 80 per cent.

Owned and operated by Kraftwerke Mainz-Wiesbaden, the new plant meets the high standards of efficiency and environmental performance demanded in today’s power industry. It will operate on natural gas and replace three coal-fired units at the site, thus helping to reduce emissions of sulphur dioxide (SO2) in the local area.

An economic decision?

The existing coal-fired power plant at the Mainz-Wiesbaden site was commissioned between 1958 and 1964 and consists of three 100 MW units. It supplies power and district heating to the surrounding area but is ageing and inefficient.

Figure 1. The power plant is currently in its final commissioning phase
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In 1997, Kraftwerke Mainz-Wiesbaden therefore began to examine the possibility of constructing a new power plant to replace this capacity.

A gas-fired power plant is also in operation at the Mainz-Wiesbaden site, producing power and process steam for the local area, but is not large enough to meet all of Kraftwerke Mainz-Wiesbaden’s load by itself. A new power plant was therefore required to replace the coal units, and Kraftwerke Mainz-Wiesbaden initially considered constructing a new coal-fired plant.

However, after carrying out an analysis of the situation, Kraftwerke Mainz-Wiesbaden found that the cost and lead-time for new coal build were prohibitive, and decided that a gas fired combined cycle cogeneration plant was a more cost-effective option. The new combined cycle plant will therefore operate alongside the existing gas-fired power plant, while the coal fired plant closed in 2000.

Figure 2. The 400 MW plant meets high efficiency standards
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The existing gas fired power plant consists of an 85 MW ABB GT13D gas turbine, a 270 MW steam turbine and an auxiliary gas fired boiler. This plant was commissioned in 1976 and primarily produces power, but also provides process steam when needed.

Kraftwerke Mainz-Wiesbaden brought in two new shareholders at the beginning of the project: HEAG, with 22 per cent; and EVO, with 11 per cent. The existing shareholders, Stadtwerke Mainz and Stadtwerke Wiesbaden, reduced their shares from 50 per cent each to 33 per cent each.

HEAG and Stadtwerke Mainz recently signed an agreement with a number of other local municipal utilities to merge their operations and assets – a sign of the high level of consolidation taking place in the German electricity market at the moment, brought on by competition and low electricity prices.

Interestingly, Kraftwerke Mainz-Wiesbaden made the decision to build the new power plant just weeks before Germany liberalized its electricity market. The company has stated that if it had been able to forecast how much electricity prices in Germany would drop after liberalization, it would not have decided to build the new plant; instead it would have closed down its Mainz-Wiesbaden site and bought power on the wholesale market.

Figure 3. The plant will produce power and provide district heating and processed steam for local industries
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As well as supplying heat and power to the Mainz-Wiesbaden area, Kraftwerke Mainz-Wiesbaden also owns and operates the local high pressure natural gas distribution grid. In May 1998 the utility reached agreement with Siemens AG’s Power Generation group (Siemens PG) to develop and construct the new plant on a turnkey basis. Site installation began in January 1999 and mechanical erection began three months later.

Under the contract, Siemens PG was responsible for engineering, procurement and construction of the plant, which consists of one gas turbine in a multi-shaft arrangement, a heat recovery steam generator (HRSG), one steam turbine and an auxiliary boiler. Babcock Borsig Power-owned NEM supplied the HRSG under contract to Siemens PG, while Technip supplied the balance of plant equipment.

Advanced equipment

The multi-shaft arrangement was required in order to meet the district heat and process steam demands of Kraftwerke Mainz-Wiesbaden. The gas turbine unit is a Siemens V94.3A combustion turbine rated at 265 MW connected to a 330 MVA hydrogen-cooled generator with SFC (static frequency converter) and SEE (static excitation system). The generator connects to the local distribution grid via a 350 MVA transformer, and there is also an auxiliary transformer rated at 12 MVA.

Power from both the gas turbine generator and the steam turbine generator is supplied to the local 110 kV distribution grid via a connection owned and operated by Kraftwerke Mainz-Wiesbaden. This grid is connected to the high voltage (220 kV and 380 kV) regional grid network owned and operated by RWE.

Exhaust gas from the gas turbine passes to the NEM HRSG, a triple-pressure, natural circulation unit with a hot reheat section and a last stage condensate reheater. The HRSG inlet temperature is 600&deg:C. Steam conditions in the high pressure (HP) section are: 110 bar/550&deg:C; in the intermediate pressure (IP)/reheat section: 30 bar, 320/560&deg:C; and in the low pressure (LP) section steam conditions are: 5.5 bar/240&deg:C.

Steam passes from the HP section of the HRSG to a Siemens three-pressure condensing steam turbine rated at 140 MW with steam extraction. The steam turbine is connected to a 200 MVA air-cooled generator and power is supplied to the local grid via a 200 MVA unit transformer.

Steam is extracted from the IP section of the steam turbine and passes through a two-stage heat exchanger to produce 130&deg:C steam providing 100 MWth of district heat for the area around Mainz-Wiesbaden. Process steam at 15 bar/320&deg:C is also extracted at 40 t/h for use by local industrial companies, which include a paper mill and a chemical factory.

Cooling water for the power plant is extracted from the nearby Rhine river and brought to the Mainz-Wiesbaden power plant via a 300 m-long underground piping system. The incoming water is cleaned by a mechanical water cleaning system to remove waste and particles.

The plant is also equipped with an NEM-supplied auxiliary boiler which can be fired on either natural gas or oil. This boiler is capable of producing 30 t/h of steam for district heating independently of the combined cycle plant.

World-class standards

The plant control system is a Siemens Teleperm XP. The plant’s control room is located in the control room of the existing gas-fired power plant, to which it is connected via the condensate system.

The Mainz-Wiesbaden combined cycle plant will normally be operated on natural gas supplied by Kraftwerke Mainz-Wiesbaden. Fuel oil will be used as a backup fuel and also as a means of power augmentation when used with high pressure water injection – operating on emulsion, the gas turbine output can reach 281 MWe. Water injection also helps to keep NOx emissions at 150 mg/m3 when operating on fuel oil.

The plant will be operated as a baseload unit, and Kraftwerke Mainz-Wiesbaden will carry out operation and maintenance. NOx emissions while using natural gas will be 70 mg/m3, and have reached as low as 50 mg/m3, SO2 emissions are virtually zero, while net plant efficiency will reach 58.4 per cent.

Future extension

The Mainz-Wiesbaden plant is currently in its final stages of commissioning, which began in December 1999. Ignition of the gas turbine took place in May 2000, and synchronization of the gas turbine generator occurred in May 2000. Steam was first supplied to the steam turbine in June 2000, and the steam turbine generator was synchronized in July 2000.

During the peak construction phase, around 450 personnel were working on the site. This fell to 40 for the commissioning phase, and Kraftwerke Mainz-Wiesbaden will employ 3-4 people per shift during normal commercial operation.

The Mainz-Wiesbaden region has traditionally been an industrial area and before construction began, the site had to undergo a contaminated land clean-up process. This is costly and time-consuming, and had to be repeated for the extension that Kraftwerke Mainz-Wiesbaden is working on.

Kraftwerke Mainz-Wiesbaden recently decided to extend the new plant with the addition of a refuse incinerator. This extension is being built by a consortium of German companies and is currently under construction (civil works).

The new incinerator will be connected to the new combined cycle power plant via the condensate system, supplying 65 t/h of steam into the cold reheat section, boosting the output of the steam turbine by 12 MWe. The incinerator will be commissioned by 2003.