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Boiler for Unit 4

Boiler 5

The Babcock Benson Boiler on Generating Unit Four

A boiler is a device that allows for a relatively economic production of heat from fuel, by creating the necessary conditions for the conversion of the chemical energy contained in the fuel (coal, oil) into thermal energy by burning. The gasses that emerge when the fuel is burned are of very high temperature and transmit the heat onto the water and steam in the pipes.

The water heats to boiling point and then evaporates. By adding further heat, the steam overheats. Boilers produce steam with different pressures and temperatures, depending on the intended purpose. The steam produced in the boiler, with a certain pressure and temperature, is fed to a mechanical device; it can be a steam turbine, a steam engine or a steam hammer. The steam expands in the machine, converting the thermal energy into mechanical energy. In thermal power plants, the steam turns the rotors of the turbines that spin the rotor of the generator, which in turn produces electrical energy.

The Structure of the Babcock-Benson Boiler a.k.a. Supercritical Steam Generator

The boiler is named after its constructor. He put to good use the properties of steam in supercritical conditions: a pressure of 225.6 bars and temperature of 374.2 ˚C, when the heat of vaporization equals zero, so that water changes into steam without a change in volume. This boiler therefore does not need devices for the separation of water from steam. The vaporization process does not begin and end in one section of the tube but occurs along a suitably long “evaporation circuit”.

The boiler is built with welded membrane tube walls that are insulated with mineral wool, like the other parts. The boiler is attached to the supporting frame at a height of 89m and hangs to the ground. The tube system expands because of the heat and in the low point of the furnace chamber cone; the operating heat expands the structure by 400mm.

At a height of 46 metres, the roof of the boiler housing structure joins the frame. The forces that stem from the boiler are partly transferred to the bunker building and partly to the boiler frame. The net weight of the boiler including the supporting structure is 10,000 tonnes. The convective part of the boiler is as wide as the furnace; the depths of the convective channel are reduced according to the depth of the furnace from both sides by altogether approximately 27 percent. This bottleneck is located at the 44m mark and creates the necessary speed for the flue gasses in this area. The membrane tube walls of the convective channel and the belonging pendent tubes for the economisers and superheaters are hung from the ceiling of the boiler frame and extend freely downwards.

The left and right lateral walls of the furnace (evaporator) hang on the respective walls of the convective part (primary superheater). The middle and aft wall of the furnace are suspended on a special “hot strap”, that is diagonally mounted tubes that are heated from the outlet manifold of the evaporator, to prevent tension. A special construction is also used for supporting the cone of the furnace. These are flat supports and they are positioned every 1.5 metres at a height of 45m. Altogether, the boiler has 300,000 metres of tubing and 75,000 welds (50,000 prefabricated and 25,000 welded during assembly on site). The furnace burns lignite dust and has six mills with direct blowing into the appropriate burners at two different heights.

The Water - Steam Cycle

The feedwater pump directs the water through high-pressure preheaters where it is heated up to 257 ˚C (at a rate of 860 tonnes per hour) and on to the feeding header. Here the main conduit splits into two pipes that pass along the sides of the boiler and connect to the inlet header of the economiser. The water enters the header axially from both sides, and the flow continues via a system of tube curtains in the economiser in the direction opposite the flow of the flue gasses. In the last tube curtain, the flow of the furnace gasses and the hot water is parallel. This redirection is made possible by an interceding header. The economiser heats the water up to 257 ˚C (at full load). The transfer of the heat in the economiser, which has a net heating surface of 10,945m2, is convectional.

From the outlet header of the economiser the water flows to the inlet header of the evaporator through the inlet pieces set before the evaporator. The headers are positioned on the left and right side of the furnace cone. The tube banks emerge from the collector winding spirally (at an angle of 10.48°) and constitute the wall of the evaporator wall, wrapping around the furnace. The angle of the tubes is the same on both sides. The water heats up to the evaporating temperature of 375 ˚C while flowing through the evaporator. At the end of the evaporator all the tubes connect into the evaporator header and the outlet headers are located on all four walls of the boiler at a height of 44 metres. The evaporator has a heating surface of 2,220m2. The heat transfer on the tubes is irradiating.

The outlet headers of the evaporator lead the steam to the primary wall super-heater where it is convectively heated. The walls are membrane welded and the spread between the pipes and their diameter are chosen according to the temperature of the flue gasses. In the flue gas temperature zone of over 800 ˚C, the spread between the pipes is of 40mm. At a height of 53 metres, four interceding headers for the primary superheater are fitted. In the flue gas temperature zone between 800 and 400 ˚C, the spread between the pipes is of 80mm. The flow of the flue gasses and the steam are parallel in the case of the primary superheater; its tubes are fitted vertically and are parallel with each other. Under the ceiling of the boiler, which is protected by a metal cover, the four outlet headers are located. The headers are cross-connected into a further two outlet headers that are mounted on the front and rear walls of the boiler.

From these outlet headers the steam is lead via 12 parallel tubes on the front and rear wall of the boiler to two inlet headers to which the supporting tubes are connected. The steam then travels via the supporting tubes to four collecting headers, which are cross-connected with two outlet headers of the primary superheater on the ceiling of the boiler. The steam is then convectively superheated to temperatures above 400 ˚C in these tubes and flows from the outlet header of the primary superheater to the secondary superheater via piping that has spraywater injectors mounted. The two inlet headers of the secondary superheater lead the steam to convective overheating, flowing against the direction of the flue gasses, which are downward. The steam heats up to 498 ˚C here and is collected in two outlet headers.

The steam passes from the outlet headers to the final, high-pressure superheater. The connecting tubes have spraywater injectors installed. From the two inlet headers the steam undergoes convective-irradiating heating up to a temperature of 540 ˚C. The flow of the steam and the flue gasses here is parallel to reduce the median temperature of the lower tube wall, which is most exposed to overheating.  Two headers are installed on the outlet of the tertiary and final superheater, spanning the whole length of the boiler’s front side. These outlet headers take turns receiving the steam from the heating banks to minimise temperature differences. The superheated steam from the outlet headers flows in two pipes that join into a single pipe that leads to the high-pressure part of the turbine. The net heating surface of the high-pressure superheaters is 9,247m2. The fresh steam outlets are connected to a safety station, which protects the high-pressure part of the boiler from an excessive pressure and maintains the pressure during start-up.

After the expansion in the high-pressure part of the turbine (the pressure and temperature drops), the steam is conveyed back to the boiler for reheating. It enters axially into two inlet headers of the primary reheater, in which it is heated and flows in the opposite direction of the flow of the flue gasses. In the primary reheater, located between the economiser and the secondary super-heater the steam heats up from 345 to 477 ˚C. It then passes through two outlet headers of the primary reheater to the inlet header of the secondary reheater. These lines also have spraywater injectors installed.

The steam flows parallel to the flue gasses in the secondary reheater and is heated from 441 to 545 ˚C. The secondary reheater is located between the secondary and final superheaters. The steam exits via two outlet headers and flow via two pipes to the medium pressure part of the turbine. The net heating surface of the two reheaters is 9,325 m2. At 38 metres of height, safety valves are installed on these pipes - two on each - that release steam into the atmosphere in case of excessive pressure.

Simplified schematics of boiler Babcock Benson 860 t/h