ZG Industries offers a wide range of worldclass Circulating Fluidised Bed Combustion Boilers. A modern concept in steam generators, these state-of-the-art
systems are designed and manufactured for a wide range up to about 500 TPH.
These boilers have multi-fuel firing capacity, high thermal efficiency, low emissions of SOx & NOx and can effectively utilise high ash coals, lignites, washery rejects, petcoke, etc.
The fuel and the cold fly ash recirculated to the fluidised bed is well mixed with the hot bed material resulting in a uniform temperature distribution in the bed. This is achieved by a higher turbulence caused by introducing fluidising air in smaller bed dimensions.
Unlike the Bubbling AFBC boilers the erosion prone submerged heating surfaces are dispensed with. The recirculated cold fly ash takes over the cooling of the fluidised bed. It is in the convection pass that the heat taken is transferred to the convectional heating surfaces. The boiler has a tower-type arrangement. The first boiler pass is formed by water-cooled, gas-tight membrane tube walls.
They are part of the evaporator system and are designed for natural circulation. The lower section of the first boiler pass consists of the combustion chamber with the fluidised bed and the freeboard above. The upper section is made up of a convection heating surfaces namely superheater, evaporator and part economiser. This first boiler pass is top supported allowing easy expansion downwards. The second boiler pass has remaining part of the economiser heating surface and the tubular airheater. The elutriated fly ash is separated from the flue gases in cyclone separators located between the boiler first and second pass, at a temperature of around 400 C.
It is recirculated into the combustion chamber via a siphon system which serves as a seal. The heated combustion air from airheater is passed through the air nozzles into the fluidised bed as primary air, and above the fluidised bed into the freeboard as secondary/tertiary air. Combustion takes place within an optimum temperature of 800 C to 950 C.
The flue gases are conducted to the chimney via an ESP and induced draught fan. The preparation of fuel is relatively simple. The fuel is crushed to a size of apprx. 8 mm. No grinding of coal is necessary. The fuel is fed into the fluidised bed together with recirculated fly ash. Limestone can be added to the fuel to capture sulphur, if required, to meet stringent SOx emission values. The ash from the fluidiesd bed, the cyclone and the ESP is conveyed pneumatically to the main ash silo, keeping the plant clean of ash.
The CFB boiler is designed with fluidising velocities up to 4.5m/s, generating a high turbulence and resulting in a good mixing of hot bed material with fuel and recirculated fly ash. The height of the bed is kept constant by removal of the produced bed ash as a function of the differential pressure between the airbox and freeboard. In the freeboard (the upper part of the combustion chamber) the fine fuel particles with a size below 0.4 mm that have been elutriated from the fluidised bed and part of the volatile matter are burnt.
This post-combustion releases approx. 40% of the total combustion heat and then is transferred to the flue gas or, by way of radiation to the cooled combustion chamber walls. The amply dimensioned freeboard height guarantees a mean residence time of flue gases of at least 4 seconds. The correspondingly long residence time of elutriated fuel particles has its decisive share in the high degree of combustion and desulphurization efficiency.
The recirculated fly ash quantity is adjusted to maintain a temperature of the fluidised bed of about 850 C. This quantity of fly ash recirculation suffices to ensure the burn-out of fines and the capture of sulphur in the freeboard at optimal consumption of limestone.