18-01-2013, 03:07 PM
Circulating Fluidised Bed Combustion (CFBC)
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INTRODUCTION
When an evenly distributed air or gas is passed upward through a finely divided bed of solid particles such as sand supported on a fine mesh, the particles are undisturbed at low velocity. As air velocity is gradually increased, a stage is reached when the individual particles are suspended in the air stream – the bed is called “fluidized”.
With further increase in air velocity, there is bubble formation, vigorous turbulence, rapid mixing and formation of dense defined bed surface. The bed of solid particles exhibits the properties of a boiling liquid and assumes the appearance of a fluid – “bubbling fluidized bed”.
Fluidization depends largely on the particle size and the air velocity. The mean solids velocity increases at a slower rate than does the gas velocity, as illustrated in Figure 6.2. The difference between the mean solid velocity and mean gas velocity is called as slip velocity. Maximum slip velocity between the solids and the gas is desirable for good heat transfer and intimate contact.
Types of Fluidised Bed Combustion Boilers
There are three basic types of fluidised bed combustion boilers:
1. Atmospheric classic Fluidised Bed Combustion System (AFBC)
2. Atmospheric circulating (fast) Fluidised Bed Combustion system(CFBC)
3. Pressurised Fluidised Bed Combustion System (PFBC)
Circulating Fluidised Bed Combustion (CFBC)
This CFBC technology utilizes the fluidized bed principle in which crushed (6 –12 mm size) fuel and limestone are injected into the furnace or combustor. The particles are suspended in a stream of upwardly flowing air (60-70% of the total air), which enters the bottom of the furnace through air distribution nozzles. The fluidising velocity in circulating beds ranges from 3.7 to 9 m/sec. The balance of combustion air is admitted above the bottom of the furnace as secondary air.
Pressurised Fluid Bed Combustion
Pressurised Fluidised Bed Combustion (PFBC) is a variation of fluid bed technology that is meant for large-scale coal burning applications. In PFBC, the bed vessel is operated at pressure 16 kg/cm2.
The off-gas from the fluidized bed combustor drives the gas turbine. The steam turbine is driven by steam raised in tubes immersed in the fluidized bed..
The PFBC system can be used for cogeneration or combined cycle power generation. By combining the gas and steam turbines in this way, electricity is generated more efficiently than in conventional system. The overall conversion efficiency is higher by 5% to 8%.
DISTRIBUTOR PLATES
In a bubbling fluidized bed (BFB) the grid plate is used in the furnace alone, but in a circulating fluidized bed, distributor plates are required in three different locations: the lower furnace, the loop-seal, and the external heat exchanger. Owing to its relatively low fluidization velocity (0.5 to 1.7 m/sec), a bubbling fluidized bed needs a much larger grate area for a given energy output. The distributor plate required for this type of bed is therefore much larger than that required by a circulating fluidized bed that operates with a higher fluidizing velocity of 4to 6 m/sec.
The primary function of a distributor plate is to distribute the fluidizing air uniformly across the cross section of the bed. This uniformity should be maintained under all operating conditions of the boiler. For a large boiler furnace this could be a serious problem.