Boiler for Unit 5

The boiler on generating unit five is a Sulzer mono-pipe vertical rig with additional circulation in the evaporator and once-through reheating.

It is 96m tall and enclosed up to the height of 49m but bare and with stronger thermal insulation after that point. The active part of the boiler is shaped like a prism that narrows into a cone at the bottom. A membrane of gas-tight welded pipes of the evaporator constitutes the walls of the furnace. The furnace chamber reaches up to the height of 49m above which the pipe work of the convective part of the boiler is located. The evaporator tubing constitutes the walls of the furnace chamber, above which the wall superheater is located; next is the tubing of the final superheater, then the secondary reheater, the primary reheater and the economizer. In 1999 a device for collecting the excess heat was installed to lower the temperature of the flue gasses and this heat is used for the district water heating station.

The feed water, previously heated up to 257.5 ˚C enters the boiler via two parallel mains leading to the inlet header of the economiser, which is fitted inside the membrane walls containing the flue gasses. The two-compartment counter-flow economiser composed of horizontally pendent tube banks, the water is heated to 311 ˚C and flows into the outlet header, through the membrane wall via forked manifolds that join two pipes into a single main. The thus heated water joins the hot water from the separator in the mixing manifold and is pumped into the main circular inlet header at the bottom of the evaporator that is linked with further individual inlet headers.

The evaporator is also the outer wall of the furnace chamber comprises 1,298 vertically pendent tubes welded together into a membrane wall. Towards the top of the furnace chamber, 276 tubes separate from the front and rear wall and serve as supports for the convectional section of the boiler. Their number rises as they fork into 552 tubes at the final superheater. The supporting tubes are hung on the supporting structure of the boiler and serve at the same time as the roof of the boiler. From here the mixture of water and steam travels to the outlet headers of the front and rear wall. Above the furnace chamber the division and the diameter of the membrane wall tubes increases, reducing the number of tubes by half with forked headers. The mixture of water and steam flows from the outlet headers at the top of the evaporator to the separator. Here the water is eliminated and pumped back via the circulation pumps into the evaporator.

The saturated steam from the separator enters the primary superheater which is located on the walls of the furnace between the burners and the exhaust shafts dries up and starts to overheat. In the secondary superheater, fitted in the convective compartment and in the final superheater whose banks are hung over the furnace the steam overheats to a final temperature of 540 °C at a pressure of 184.4 bars.

There are two spraywater injectors installed between the superheaters to regulate the fresh steam temperature.
From the final superheater the steam exits into four outlet headers, the mixing pipes and two mixing headers correct any differences in steam temperature in the individual mains. The steam is led from the boiler by two fresh steam pipes and flows over the valves onto the high-pressure turbine where it expands; the pressure drops to 42 bars and the temperature to 335 °C.

The steam from the turbine then flows back to the boiler for reheating. In the three-section primary reheater, located under the economiser, and the secondary reheater, which is a combination of convective and irradiating transfer, the steam heats back up to 545 °C at a pressure of 41.3 bar.

A spraywater injector is fitted to regulate the steam temperature. The reheated steam flows in two mains, through the valves and onto the medium pressure turbine where it expands partially. After exiting the medium pressure turbine it directly enters the low-pressure turbine and after expanding turns into water in the condenser. From here, it is pumped back to the boiler.

Additionally large tube heating surfaces are fitted with the intent of lowering the temperature of the flue gasses as the temperature was too high and did not allow desulphurization. Primary water is then heated and in the secondary cycle heats the main condenser or the circulation water for district heating station two. With this cogeneration system, we produce 45MW of heat energy at a full load of the boiler.