Posted By: Madhu Bafna

June 18, 2016
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Overview

Pulverized fuel boiler is the most commonly used method in thermal power plants, and is based on many decades of experience. These boiler furnaces operate at below and close to atmospheric pressure, simplifying the passage of fuel and flue gases through the boiler plant. Most coal-fired power station boilers use pulverized coal, and many of the larger industrial watertube boilers also use this fuel. This technology is well developed, and there are thousands of units around the world, accounting for well over 90 per cent of coal-fired capacity. The coal is ground (pulverized) to a fine powder in the coal mills so that less than 2 per cent is +300 micro metre (μm) and 70-75 per cent is below 75 microns, for bituminous coal. The pulverized coal is blown with part of the combustion air into the boiler plant through a series of burner nozzles. Secondary and tertiary air may also be added. Combustion takes place at temperatures from 1,300 to 1,700 0C, depending largely on coal grade. Particle residence time in the boiler is typically two to five seconds, and the particles must be small enough for complete combustion to have taken place during this time. This system has many advantages such as the ability to fire varying qualities of coal, quick responses to changes in load, use of high preheat air temperatures, etc. Pulverized coal boilers have been built to match steam turbines, which have outputs of between 50 and 1,300 Mwe (Megawat Electrical). In order to take advantage of the economies of scale, most new units are rated at over 300 Mwe, but there are relatively few really large ones with outputs from a single boiler-turbine combination of over 700 Mwe. This is because of the substantial effects such units have on the distribution system if they should “trip out” for any reason, or be unexpectedly shut down. The PF Boilers are also classified on the basis of the firing i.e. Front fired, Corner fired, Downshot etc. In the case of Front fired boilers the fuel firing is done by the burners attached to the front wall of the boilers. In case of Corner fired boilers, the fuel firing is done from the corners of the furnace. Figure 1 shows the Furnace and the flue gas path in a typical corner fired P.F. Boiler with tilting type of coal burners. The tilting coal burners fire coal inside the furnace from all the four corners, tangential to an imaginary circle in the centre to form a fireball inside the furnace. The flue gas formed in the furnace passes over the pendent superheater and reheater tubes. The flue gas then changes its direction and passes over the horizontal superheater, economizer, airpreheater, ESP and finally is evacuated by the Induced Draft Fans into the chimney. The ash from the combustion chamber, which falls down is collected in the bottom ash hoppers. The ash which is fine & flies with the flue gas is separated by ESP and collected in ESP Hoppers. The air required for complete combustion of fuel (Secondary Air) is sucked from the atmosphere by the FD Fans and discharged to the Air Preheater for heating. This heated air is then supplied to the furnace through the windbox. The Feed water enters the boiler through the economizer tubes provided in the path of the flue gas. The feed water is heated n the economizer and then enters the boiler drum situated outside the furnace at the top of the Boiler. In the Natural circulation boilers, the water from the boiler drum goes to the bottom ring header which s connected to the waterwalls. All the four waterwalls of the furnace receive heat by radiation and water inside the tubes is converted to steam. The water steam separation is done in the boiler drum. The dry and saturated steam from the boiler drum then passes through the primary superheater section in the second pass, then through the pendent and final superheater and finally available at the boiler outlet header. The Boilers also have pendent Reheater tubes in the flue gas path. The exhaust steam from the high pressure turbines is admitted into the reheater and is reheated by the flue gas. The steam is reheated to approximately the same temperature as that of superheater. The reheated steam then goes out of the boiler to the IP & LP Turbine.

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