Document gvadvmG23ez8V0GBEoaLQr63

160 Chapter 8 1945 Guide by poor'mhring of- thefuelandairr It is considered good practicer under -- usual operating conditions, to supply from 25 to 50 per cent excess air, dependent upon the fuel utilized. SECONDARY AIR When bituminous coal is hand-fired in a furnace the volatile matter in the fuel distills off leaving coke on the grate. The product of combustion of the coke is CO% and under certain conditions some CO may arise from the bed. The combustion of the volatile matter and the CO may amount Combustion and Fuels ;______________________________________________________________ ____________ __ ____ _ This process _of_ combustion is illustrated in Fig. 21. The free.oxygen of the air passes through the grate and the ash above it and burns the carbonin the lower 3 or 4 in. of the fuel'bed forming carbon dioxide. This layer noted as the oxidizing zone is indicated by the symbols C0% and Ot. Some of the carbon dioxide of the oxidizing zone is reduced to carbon monoxide in the upper layer of the fuel bed noted as the reducing zone and indicated by the symbols COt and CO. The gases leaving, the fuel bed are mainly carbon monoxide, carbon dioxide, nitrogen and very, little free oxygen. Free oxygen is admitted through the firing. door in an attempt to burn carbon monoxide and the volatile combustible distilled from the freshly fired fuel. The division of the total into primary and secondary air necessary to produce the same rate of burning and the same excess air depends on a number of factors which include size and type of fuel, depth of fuel bed, and size of fire-pot. Size of the fuel is a very important factor in fixing the quantity of 0 20 40 GO 80 (00 120 140 ' 180 Excess AIR, PER CENT Fig. 1. Relation Between CO, and Excess Air in Gases of Combustion to the liberation of from 40 to 60 per cent of the heat in the fuel in the combustion space over the fuel bed. The air that passes through the fuel bed is called primary air and the air that is admitted over the fuel bed in order to burn the volatile matter and CO is called secondary air. Table 4. Representative Maximum CO, Value Fuel Coke......................................................................... Anthracite.......................................... Bituminous Coal................................................. No. 2 Fuel Oil........................................................ No. 6 Fuel Oil...................... Natural Gas........................................................... Coke Oven Gas.......................................... Theoretical CO* 21.00 20.20 18.20 15.00 16.50 12.00 11.00 CO* Usually Attained In Practice 12-14 12-14 13 10.5 13.5 9.7 8.5 Fig 2. Combustion of Fuel in a Hand-Fired Furnace * secondary air required for non-caking coals.. With caking coals it is not so important because small pieces fuse together and form large lumps. Fortunately a smaller size fuel gives more resistance to air flow through the fuel bed and thus automatically causes a larger draft above the fuel bed, which draws in more secondary air through the same slot openings. In spite of this, a small size fuel requires a larger opening of the door slots; for a certain size for each fuel no slot opening is required, and for larger sizes too much excess air gets through the fuel bed. It is impossible to establish a single rule for the correct slot opening for all types and sizes of fuels and for all rates of burning. Furthermore, the effect of slot opening is dependent on whether the ashpit damper is open or dosed. . Bituminous coals require a large amount of secondary air during the period subsequent to a firing in order to consume the gases and to reduce the smoke. The smoke produced is a good indicator, and that opening is best which reduces the smoke to a minimum. Too much secondary air will cool the gases below the ignition point, and prove harmful instead of beneficial. Secondary air that enters the combustion chamber too far removed from the zone of combustion will also be harmful, for the oxygen in the `Hand Firing Soft Coal Under Power Plant Boilers (C7. S. Bureau of Mines Technical Paper No. 80).