25-08-2017, 09:32 PM
Boiler Technologies
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Existing and emerging trends
Boiler technology the world over has evolved vastly over the years. From the conventional pulverized coal boilers to fluidised bed combustion technology and multi-fuel firing boilers, the industry has indeed come a long way.
This write-up describes the available and emerging technology options, their benefits and limitations.
CURRENT TECHNOLOGIES
Pulverised fuel boiler
Pulverised fuel boiler is the most commonly used method in thermal power plants, and is based on many decades of experience. Units operate at close to atmospheric pressure, simplifying the passage of materials through the plant.
Most coal-fired power station boilers use pulverised 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 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 oC, 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. Pulverised coal boilers have been built to match steam turbines, which have outputs of between 50 and 1,300 Mwe. 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.
Fluidised bed combustion
Fluidized bed combustion has emerged as a viable alternative and has significant advantages over the conventional firing system and offers multiple benefits. Some of the benefits are compact boiler design, fuel flexibility, higher combustion efficiency and reduced emission of noxious pollutants such as SOx and NOx. The fuels burnt in these boilers include coal, washery rejects, rice husk, bagasse and other agricultural waste. Fluidised bed boilers have a wide capacity range – from 0.5 T per hour to over 100 T per hour.
There are three basic types of fluidized be combustion boilers:
• Atmospheric classic fluidized bed combustion system (AFBC).
• Atmospheric circulating (fast) fluidized bed combustion system (CFBC)
• Pressurised fluidized bed combustion system (PFBC).
AFBC/ Bubbling bed
In AFBC, coal is crushed to a size of 1-10 mm depending on the rank of coal, and type of fuel fed into the combustion chamber. The atmospheric air, which acts as both the fluidisation air and combustion air, is delivered at a pressure and flows through the bed after being preheated by the exhaust flue gases. The velocity of fluidizing air is in the range of 1.2 to 3.7 m per second. The rate at which air is blown through the bed determines the amount of fuel that can be reacted.
Almost all AFBC/ bubbling bed boilers use in-bed evaporator tubes in the bed of limestone, sand and fuel for extracting the heat from the bed to maintain the bed temperature. The bed depth is usually 0.9 m to 1.5 m and the pressure drop averages about 1 inch of water per inch of bed depth. Very little material leaves the bubbling bed-only about 2 to 4 kg of solids are recycled per kg of fuel burnt.
The combustion gases pass over the superheater sections of the boiler, flow past the economiser, the dust collectors and the air preheaters before being exhausted to the atmosphere.
The main feature of atmospheric fluidized bed combustion is the constraint imposed by the relatively narrow temperature range within which the bed must be operated. With coal, there is a risk of clinker formation in the bed if the temperature exceeds 950 oC and loss of combustion efficiency if the temperature falls below 800 oC. For efficient sulphur retention, the temperature should be in the range of 800 – 850 oC.
Features of bubbling bed boilers
Fluidised bed boilers can operate at near-atmospheric or elevated pressure and have these essential features:
• Distribution plate through which air is blown for fluidising,
• Immersed steam-raising or water heating tubes which extract heat directly from the bed,
• Tubes above the bed, which extract heat from hot combustion gas before it enters the flue duct.