Document k68gLg7EmMzKBLo3g6JkdwK1O

HEATINC VENTILATING AIR CONDITIONING GUIDE 1944 100 Per cent excess air =------------------------ -------- ' N, X 0.264 - (o, - ^ J (10) In this formula the symbols represent volumetric percentages of the flue gas constituents as determined by analysis. The amount of excess air in its relation to the percentage of COt is shown by the curves in Fig. 1 for several fuels. These are approximate values. It should be noted that in hand-fired furnaces with long periods between firings the combustion goes through a cycle in each period and the quantity of excess air present varies. Due to the different carbon-hydrogen ratios of the different fuels the maximum CO% attainable varies. Representative values for perfect com bustion of several fuels are given in Table 4. CHAPTER 8. COMBUSTION AND FUELS Table 4. Representative Maximum COt Values Fuel .....................- -............ ----......... -....... COtPer Cent 21.0 20.2 18.2 15.5 12.0 11.0 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. This process of combustion is illustrated in Fig. 2l. The free oxygen of the air passes through the grate and the ash above it and burns the carbon in the lower three or four inches of the fuel bed forming carbon dioxide. This layer noted as the oxidizing zone is indicated by the symbols COt and Ot. Some of the carbon dioxide of the oxidizing zone is reduced to carbon Fig. 1. Relation Between COj and Excess Air in Gases of Combustion In considering the factor of excess air it should be noted that a deficien cy of air supply will result in combustible products passing to the stack unburned. An excess of air absorbs heat from the products of combustion and results in a greater loss of sensible heat to the stack. An excess of air is always required, however, to eliminate combustible losses occasioned by poor mixing of the fuel and air. It is considered good practice, 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 COt and under certain conditions some CO may arise from the bed. The combustion of the volatile matter and the CO may amount to the liberation of from 40 to 60 per cent of the heat in the fuel in the combustion space over the fuel bed. 170 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 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. `Hand Firing Soft Coal Under Power Plant Boilers (U. S. Bureau of Mines Technical Paper No. 80). 171