Document 85JvG4pNrxyOVooY2BwpaEMYZ

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 100 ( Or - TM ) Per cent excess air = --------------------- --- ' Nt X 0.264 - (o, - (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 COz 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 COz attainable varies. Representative-values for perfect com bustion of several fuels are given in Table 4. CHAPTER 8. COMBUSTION AND FUELS Coke.......................... Anthracite---- --------Bituminous coal----- Oil..:..... ..........-.....-- Natural gas........... .. Coke oven gas--.... Table 4. Maximum COt Values Fuel Peb Cent COt 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. 22. 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 COz and Oz- 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 COz and CO. The gases leaving the fuel Fig. 1. ,Relation Between CO 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 COz and under certain conditions some <00 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 Fig 2. Combustion of Fuel in a Hand-Fired Furnace', bed are mainly carbon monoxide, carbon dioxide, nitrogen and very little free oxygen. Free oxygen is admitted through the firing door to bum 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. The ratio of the secondary to the primary air in creases with decrease in the size of the fuel pieces, with increase in the depth of the fuel bed, and with increase in the area of the fire^pot; the ratio also increases with increase in rate of burning. 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 From Bureau of Mines Technical Paper No. 80. 171 '