DIRECT FIRING SYSTEM PRIMARY AIR FAN PULVERISER BURNERS
Exhaust gas cyclone Bag filterPulveriser BURNERS INDIRECT FIRING SYSTEM
The Process of Combustion involves threestages:(a) Ignition(b) Combustion Stability(c) Completion of CombustionIn Combustion process, ignition occurs invapour phase. Solid and liquid fuels get ignitedfrom their vapours.
Combustion of fuelsSolid 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 matterbecome a char. The char slowly burns out.
Causes of Ash Deposition• Reduced heat transfer,• Physical damage to pressure parts• Erosion and corrosion of pressure parts• Slagging• Fouling Post -Combustion Performance due to Ash• Dry Ash Disposal• Fly Ash Disposal problems Power station performance• Capacity• Heat rate• Maintenance• Availability
Burner Arrangement In a tangentially fired boiler, fourwindboxes ) are arranged, one at each corner ofthe furnace.The coal, oil and gas burners aresandwiched between air compartments. The fuel and combustion air streams fromthese compartments are directed tangentially toan imaginary circle at the centre of the furnace.This creates a turbulent vortex motion of the fuel,air and hot gases which promotes mixing, ignitionenergy availability and thus combustionefficiency.
Combustion Air Distribution The Combustion air, referred to as SecondaryAir, is provided from FD Fans. A portion ofsecondary air called `Fuel Air, is admittedimmediately around the burners (annular spacearound the oil/gas burners) into the furnace. Therest of the secondary air , called Auxiliary Air , isadmitted through the auxiliary air nozzles and endair nozzles. The quantity of secondary air (fuel air+ auxiliary air) is dictated by boiler load andcontrolled by FD Fan inlet guide vane regulation.The burners are tiltable +/- 30o about horizontal,simultaneously at all elevations and corners. Thisshifts the flame zone across the furnace height andenables control over steam temperature.
Emerging trends in Fuel firing system Design1. Burners shall be designed to operate on multiple fuels from LCV gases , Liquid fuels to solid fuel or in combination of these fuels .2. Less polluting burners - Technology development3 .Larger guarantee periods for high ash coals - Material selection, Improved design features4. New gadgets like thermal analysers, on line coal flow measurement and control thro coal air dampers etc.,5. Micro processor based on line measurement and control of secondary air admission to individual burners.6. Computer simulations using software tools viz., ANSYS, CAEPIPE, STADD and CFD.7. Virtual assembly using CAD tools in design office before taking
COAL - AIR BALANCING IN FUEL PIPING Un balance in Coal- air flow in to the furnace results inuneven heat release , uneven distribution of excess airand unpredictable Nox formation1. Selection of Orifices for Coal air two phase flowregime.2. Provision of on -line adjustment dampers in coal airflow path.3.On-line measurement of coal-air flow usingmicrowave techniques4. Continuous measurement using dirty pitot- tube
OVERFIRE AIR Overfire air is introduced into the furnacetangentially through two additional air compartments,termed as overfire air ports, designed as vertical extensionsof the corner windboxes. The overfire air ports are sized tohandle 15 percent of total windbox air flow. The proven success of overfire air as a supplementto the tangential firing in limiting NOx formation duringcoal combustion lies in the fact that this technique inhibitsformation of both fuel NOx and thermal NOx as an oxygendeficient environment is established in the primarycombustion zone. At design levels of overfire, a 20 to 30% reduction inNOx formation is achieved.
NOX DEPENDENCE ON OVERFIRE AIR FLOW 350 300 250 OFA VS NONO (PPM @ 3% O2) 200 150 EXCESS O2 (%) VS NO 100 50 0 0 2 4 6 8 10 12 14 16 OVERFIRE AIR(%)