27-03-2014, 04:16 PM
Coal Combustion System
Combustion System .ppt (Size: 946 KB / Downloads: 34)
Combustion of fuels
Solid Fuel (Coal):
Volatile matter is released over a temperature of 250 - 900 degree Centigrade.
The volatile matter is first ignited.
The coal particle upon releasing the volatile matter become a char. The char slowly burns out.
Burner Arrangement
In a tangentially fired boiler, four windboxes ) are arranged, one at each corner of the furnace.The coal, oil and gas burners are sandwiched between air compartments.
The fuel and combustion air streams from these compartments are directed tangentially to an imaginary circle at the centre of the furnace. This creates a turbulent vortex motion of the fuel, air and hot gases which promotes mixing, ignition energy availability and thus combustion efficiency.
Combustion Air Distribution
The Combustion air, referred to as Secondary Air, is provided from FD Fans. A portion of secondary air called `Fuel Air', is admitted immediately around the burners (annular space around the oil/gas burners) into the furnace. The rest of the secondary air , called 'Auxiliary Air' , is admitted through the auxiliary air nozzles and end air nozzles. The quantity of secondary air (fuel air + auxiliary air) is dictated by boiler load and controlled by FD Fan inlet guide vane regulation.
The burners are tiltable +/- 30o about horizontal, simultaneously at all elevations and corners. This shifts the flame zone across the furnace height and enables control over steam temperature.
OVERFIRE AIR
Overfire air is introduced into the furnace tangentially through two additional air compartments, termed as overfire air ports, designed as vertical extensions of the corner windboxes. The overfire air ports are sized to handle 15 percent of total windbox air flow.
The proven success of overfire air as a supplement to the tangential firing in limiting NOx formation during coal combustion lies in the fact that this technique inhibits formation of both fuel NOx and thermal NOx as an oxygen deficient environment is established in the primary combustion zone.
At design levels of overfire, a 20 to 30% reduction in NOx formation is achieved.