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Diesels offer hot Competition

Diesels offer hot Competition

Testing of a pilot project has begun which will demonstrate the feasibility of operating a diesel engine in a combined cycle configuration. The goal is to deliver 55 per cent electrical efficiency for small to medium sized plants.

With the growing wave of privatization and deregulation, small and medium sized electricity users are increasingly exploring the possibilities for self-production. These users can choose between gas turbine and diesel engine based plants. The electrical efficiency of a diesel engine based plant is some ten per cent higher and costs are similar at about 30-50 MW.

Gas turbine based plants, however, have traditionally enjoyed the advantage of being able to recover exhaust heat for use in combined cycle operation. Diesel engine manufacturer, Wärtsilä NSD is trying to negate this advantage through the development of a diesel combined cycle plant near Vaasa, Finland.

WPPP project

The Wasa Pilot Power Plant (WPPP) is located at a site which has other power plants for supplying electricity and district heating to Vaasa and surrounding towns. It is owned by Wärtsilä NSD (49.9 per cent), Etelä-Pohjanmaan Voima (19.9 per cent), Vaasan Sähkö Oy (19.9 per cent) and ABB Oy (10.3 per cent).

The aim of the WPPP is to achieve a thermal efficiency of 53 per cent. Today`s best diesel plants achieve about 46 per cent.

This will be achieved by the combination of three main project components:

•A newly developed large diesel engine (Wärtsilä 12V64)

•Diesel combined cycle steam turbine and steam generation (utilizing heat from engine cooling and exhaust gases)

•Hot combustion (rearranging the heat balance to achieve more efficient steam generation).

To maintain high efficiency on a low turndown ratio, a Wärtsilä 12V46 was used in combination with the 12V64 engine. This gives a plant output of 6 MWe to 38 MWe with good efficiency even at low load.

Exhaust heat from each engine is sent to two dual-pressure boilers, one for each engine, supplied by Aalborg. The steam turbine used is an ABB V45A with an output of 5 MWe operating on steam pressures of 20 bar and 4.2 bar.

Each engine has a deNOx system, designed by Haldor Topsøe, located between the superheater and boiler to attain optimum SCR reactor operating temperatures. There is also one common deSOx system to enable the use of heavy fuel oil. The process flow diagram is shown in Figure 1.

Engine design

The Wärtsilä 64, the world`s largest medium speed engine, was first introduced in 1996. The prototype was an in-line engine which was systematically followed by a Vee-model. Assembly of this new 12-cylinder prime-mover was completed at the end of 1997 and testing began in March this year. Key technical data is shown in Table 2.

The designed engine shaft efficiency is achieved through a number of features:

•Large bore size

•Efficient uncooled turbochargers

•Design which allows high maximum pressure and compression ratio

•Possibility to vary fuel injection timing as a function of speed, load and ambient conditions.

Compared to other engines, the Wärtsilä 64 runs at a relatively high speed in order to reduce the alternator cost. High alternator speed has a positive influence on the quality of electrical current. Higher average piston speed is accomplished by the use of a proven box-design piston and state-of-the-art liner technology with an anti-polishing ring. This ensures trouble-free running without bore polishing even with a maximum cylinder pressure of more than 200 bar.

The large engine unit size has many benefits for power generation:

•Minimum number of installed cylinders and lower maintenance costs

•Larger turbocharger size with higher efficiency

•Smaller number of exhaust boilers for a given plant output in HRSG applications.

The large engine size called for new assembly and manufacturing techniques to make casting of the engine block and final transportation of the engine possible. Wärtsilä 64 V engines are manufactured as separate macro-modules each weighing less than 150 t - an international limit for road transport.

Engine auxiliary systems were carefully matched for power generation needs. A special feature of the engine is that all pumps are engine driven, which reduces pipe work and elastic joints, thus enhancing reliability and reducing parasitic losses of the plant.

A complete lubricating oil system with automatic filters is mounted on the engine. The efficiency of the pumps is improved for a wide operating range. In addition to this the power plant lube oil, together with the second and third stages of the charge air cooler, are cooled by district heat water. This helps to maximize the overall plant efficiency.

The engine also has some interesting features in its fluid system for improved reliability. As the engine is almost pipeless, there is minimum risk of joint leakage. Less piping also helps to simplify installation, and makes for a more compact engine design.

Hot combustion concept

As a prime mover, a diesel engine has a very high simple cycle efficiency. This means the recoverable waste heat that can be used for electricity generation in a bottoming cycle is low.

The key area in the research and development work has been the reduction of the low-temperature cooling losses while increasing the heat content of exhaust gas. The project goal is an electrical efficiency of 55 per cent in plants larger than 100 MW.

The redistribution of energy flows was achieved through:

•Efficient isolation of combustion chamber components

•Reduction of scavenging and combustion air excess

•Increasing the cylinder cooling water temperature.

Operation at elevated temperatures calls for several innovative elements in the design of critical components. This is particularly true of those components that are subject to increased thermal load and corrosive attack. Composite-type structures have been used to reduce the volume of hot components and thus the total cost of the high-cost material. The new components comprise the piston, cylinder liner, cylinder head with valves, and the necessary changes in auxiliary equipment.

The piston has a composite design with a totally uncooled piston top made of a special super alloy which has high thermal stability. Elimination of top oil cooling reduces cooling losses and oil deterioration in the piston.

The cylinder liner has a special insulating ring inserted in the combustion chamber part of the cylinder. The insert also acts as a liner anti-polishing ring.

The cylinder head also has a composite structure, with a separate uncooled fire deck that is subjected to the major thermal load of the head. The upper part of the cylinder head together with the liner serve as part of the steam production system. The cylinder cooling water temperature is 170 degrees C at the cylinder outlet.

Exhaust gas valves are subjected to increased thermal load due to higher process temperatures. New alloys were therefore used to provide a really heavy-duty exhaust valve. Valve seat durability against hot corrosion is further improved by a separate seat ring cooling system.

Apart from effective isolation of the combustion chamber, the adjustment of all process media flows and temperatures is a very effective way to optimize the heat balance of the engine. In the hot combustion process, both scavenging and combustion air flows are reduced to increase the exhaust gas energy.

Reduction of scavenging air factor, which is achieved by shortening the valve overlapping, also has a positive effect on engine mechanical efficiency. Combustion air excess factor is reduced by bringing down the charge air pressure and by raising the charge air temperature. All of these modifications increase the extractable energy.

The hot combustion principle and hardware was comprehensively tested in both Wärtsilä 46 and 64 laboratory engines prior to manufacture of the first power plant engines. One of the main achievements of the project has been the raising of the exhaust gas temperature by a minimum of 100 degrees C without sacrificing the already high thermal efficiency and engine reliability.

Emission control

Special attention has been paid to the emission control technology at WPPP. It is expected that CO2 emissions from a combined cycle diesel plant will be roughly half that from a conventional coal fired power plant. The sulphur dioxide emissions are lower as a result of higher efficiency. To make use of cheap high sulphur heavy fuels, a deSOx unit has been added. The unit uses ABB`s NID (Novel Integrated DeSOx) process. This uses quicklime (CaO), which is cheap and results in low operating costs.

Disposal of the end product (calcium sulphite + calcium sulphate) is solved by returning the lime to the mine where it is stabilized by fly ash, and used as filling material to prevent mine collapses during excavation.

Diesel combustion results in low hydrocarbon and CO emissions as a result of high combustion temperatures and pressures. In the WPPP this is even more the case due to extreme combustion pressure and elevated compression ratios.

Particulate emissions from diesel engines are traditionally low at steady load and speed as a result of the complete combustion process. However, the ash constituents originating from the fuel and the lubricating oil cannot be destroyed by the combustion process. A major engine development is the anti-polishing ring in the cylinder which has reduced lube oil consumption from about 1 g/kWh to about 0.5 g/kWh. This lowers both lube derived ash particles and operating costs.

The low specific fuel consumption of the diesel combined cycle will further reduce particle emissions from fuel ash. The deSOx unit has filters for separating the treated quicklime from the exhaust and from the diesel particles.

High combustion temperatures and pressures, while ensuring complete combustion, does however oxidize atmospheric nitrogen to NOx. In the engine, NOx is kept low by using a technique which results in less hot zones in the combustion chamber as well as shorter residence times. NOx is further reduced by urea-based SCR reactors.

Operation

The climate in Finland calls for various modes of operation for plants located at the Wasa site. The electricity demand is such that Finland typically needs a high number of peak power plants to cope with demand during winter. This would imply that electricity prices are higher in the winter than in the summer.

The aim of the WPPP was not, however, to build a peak power plant for production during the winter months only. To improve the economy and extend the yearly operation, part of the plant`s heat is sold for district heating. The district heating demand is only about 20 MW in the summer. Another factor is that bigger plants schedule shutdowns during the summer, thus making chp operation also attractive in the summer months.

The WPPP will need to operate in the following modes:

•Winter: The higher electricity price supports engine operation on full load (33 MWe). Steam turbine load is maximized (5 MWe). The remaining low grade heat is sold as district heating

•Spring and autumn: The diesels are mainly on full output. The steam turbine load is varying from full power to zero by reducing the low pressure steam addition and then high pressure steam to minimum turndown ratio, after which the steam turbine is stopped. The district heating is increased correspondingly from 12 MW to 30 MW.

•Summer: Low electricity price does not justify operation unless district heat is produced. The steam turbine is therefore shutdown. The engine load is adjusted to match the district heat demand. This is done by reducing the load on both engines until the smaller engine (12V46) can be shut down, with operation of the 12V64 only giving 22 MWe and 20 MW of heat. When the district heating is further reduced, the 12V64 load can be decreased. For a very small load, the 12V46 can be operated alone. This ensures better than 45 per cent electrical efficiency even on part loads down to 6 MWe.

Commissioning experience

Official startup of the prototype 12V64 took place in March of this year and the entire plant was ready for testing in May 1998.

The plant is now at the midway stage of its test phase. Engine optimization tests are being performed with checks being made of the turbocharger system and thermal loads on different components. Torsional and vibrational measurements of shafts are also being checked. Depending on the results of the tests, optimization should be completed this autumn.

After engine optimization, the auxiliary equipment; boilers; deSOx; deNOx and district heating control systems will be optimized. Emission figures have not yet been obtained since the engines are currently burning light fuel oil. Testing with heavy fuel oil will begin later this month (August).

Full commercial operation of the plant is expected to begin in January 1999. After this, Wärtsilä expects to bring the new 12V64 to the market.

Further reading

"Wasa Pilot Power Plant (WPPP) - diesel combined cylce power plant," by Stefan Gros, Aulis Silvonen, Wärtsilä NSD, Finland. Presented at Power-Gen Europe, Milan, Italy, June 9-11, 1998.

Click here to enlarge image

Figure 1. Wasa pilot plant simplified process flow diagram

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Figure 2. Artist`s impression of the configuration

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Figure 3.The Wärtsilä 12V64

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Figure 4. Hot combustion components

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Figure 5. Variation of district heating demand in Vaasa

Click here to enlarge image

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