Circulating Fluidised Bed Combustion (CFBC) Boiler - Sub-Critical
The CETHAR CFBC boiler operates on the circulating bed principle. At high fluidizing velocities (4 to 6 m/s) part of the bed material becomes entrained and is carried through the combustion chamber along with the flue gases. The coarser entrained particles are separated in a water-cooled ‘hot loop’ cyclone and returned via a loop seal back to the bed. Finest fly ash is carried through the boiler convection sections and separated from the flue gases in the dust collection system.
Fuel is fed into the fluidized bed via volumetric or gravimetric feeders either through the front wall and / or rear wall. Combustion takes place in the bed at about 870 - 900°C at full load. The bed material is formed from the fuel ash (and sand in some cases). Due to the large heat capacity of the bed, combustion is stable and no supporting fuels are required, provided the fuel's heating value is sufficient to raise the combustion air and the fuel above its ignition temperature. The intense turbulence ensures good mixing and combustion of the fuel. High heat transfer is obtained through the circulating material, which is approximately proportional to the load. Hence, the boiler has good response over a wide range of loads with a relatively little excess air levels.
The Primary air (taken from atmosphere) is supplied by the centrifugal fan. Air, preheated in air preheater is sent to the air box. In the air box, it is blown through the grid to fluidize the bed and provide combustion air. Primary air fan is controlled via fan IGV / Multi louver damper / VFD control / hydraulic coupling to maintain the required airflow, and is measured by the aerofoil provided in the duct. There is a minimum requirement of primary air flow to fluidize the bed and prevent flow of the material through the nozzles. Hence, at low loads the primary airflow remains constant and does not vary with load.
Secondary air (also taken from the atmosphere) is supplied by centrifugal fan and preheated in the air preheater. Secondary air is brought in at specified levels from the grid. Secondary air aids in achieving "staged" combustion and completes the combustion of the fuel without the formation of excessive NOx. The secondary airflow is measured with an aerofoil provided in the duct. The secondary air fan is controlled with IGV / Multi louver damper / VFD control / hydraulic coupling to maintain the required airflow.
The secondary airflow is measured with an aerofoil in the discharge duct before the air heater. The secondary air fan is controlled with IGV / Multi louver damper / VFD control / hydraulic coupling according to the pressure down stream of the fan.
High-pressure air is used for fluidizing the circulating bed material in the loop seal. There are two high-pressure blowers; one is always on stand-by duty.
A tubular, multi-pass air preheater system with air inside tube design is offered at the downstream of the economizer. The air preheater preheats the combustion air that goes into the furnace. There are separate sections for the primary and secondary air to get heated up. The tubular system offers an advantage of negligible air-gas leakage. The air inside tubes design also facilitates an easy tube cleaning system in case of burning fuels with some sticky characteristics.
After the air preheater, the finer fly ash is transported to the dust collection system (ESP or bag filter) where the fly ash is separated. The gas is drawn through a centrifugal type Induced Draught (ID) fan, after which the flue gas is discharged to the atmosphere via the chimney (stack).
The boiler is with a balanced draft system and the ID fan is controlled according to the pressure at the top of the combustor. Flue gas temperatures and pressure drops are measured throughout the system mainly to indicate the operating condition of the boiler. Similarly flue gas is analyzed for O2 to indicate the efficiency of combustion process.
O2 is kept ~3.5 % to ensure proper combustion and to fulfill the emission requirements