Document 8VVoMarNVm5LB9M2kzXz6ydka

American Society oj Heating and Ventilating Engineers Guide, 1935,:. Petroleum Coke The sale of petroleum coke for domestic furnaces has been small and is gen confined to the Middle West. They vary in the amount of volatile matter they cont^-- but all have the common property of a very low ash content, which necessitates^?' use of refractory pieces to protect the grates from being burned. *"! COMBUSTION OF COAL AND COKE In order to obtain perfect combustion a definite amount of air is r quired for each pound of fuel fired. A deficiency of air supply will resu in combustible products passing to the stack unburned. An excess of a absorbs heat from the products of combustion and results in a greater la of sensible heat to the stack. Total Air Required The theoretical amount of air required per pound of fuel for perfect combustion is dependent upon the analysis of the fuel; however, for practical purposes the air supply required for different grades of fuel may roughly be taken as follows: Pounds of Air per Pound' of Fuel as Fired Anthracite Coxa Semi-Bituminous Bituminous 9.6 11-2 11.2 10.3 Lignite 6.2 Chapter 15--Solid Fuels ' of about 50 per cent over the theoretical amount is conpractice under usual operating conditions, nt of excess air, based upon the laws of combustion, can be ,-l.fr.'Ll its relation to the percentage of C02 (carbon dioxide) in ^^Sldrts of combustion. This relationship is shown by the curves high and low volatile coals and for coke. In hand-fired furlong pgriods between firings the combustion goes through a *?^;W1each period and the quantity of excess air present varies. SecindoC/ Air ISfTdivision of the total into primary and secondary air necessary to ^rce the same rate of burning and the same excess air depends on a -T^ber 0f factors which include size of fuel, depth of fuel bed and Jrdieter of fire pot. The ratio of the secondary to the primary air Vhkses with decrease in the size of the fuel pieces, with increase in the nth of the fuel bed, and with increase in the area of the fire pot; the rltio also increases with increase in rate of burning. ^ Size of the fuel is a very important factor in fixing the quantity of '^ohdary 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 a larger draft above the fuel bed, which thus draws in more secondary air for the same slot openings. In !miteof this, a small size fuel requires a larger opening of the door slots; -Tor a certain size for each fuel no slot opening is required, and for larger -sizes too much excess air gets through the fuel bed.; : ltis impossible to establish a single rule for the correct slot opening for 'ah types and sizes of fuels and for all rates of burning. Furthermore, the size of slot opening is dependent on whether the ashpit damper is open -:Cr.'closed. It is better to have too much than too little secondary air; 2tHe,opening is too small if there is a puff of flame when the firing door is .opened. yV/Fjig. 3 taken from the U. S. Bureau of Mines, Report of Investigations .'^fo')2980, shows the relationship of the slot opening, for a domestic fur nace, to the size of coke and the rate of burning'; these openings are with Ahe;ashpit damper wide open, and would be less if the available draft ^permits of it being partly closed. The same values are satisfactory for ^anthracite. . Bituminous, coals require a large amount of secondary air during the -period.subsequent to a firing, to consume the gases and. to reduce the {slpoxe. The smoke produced is a good indicator, and that opening is best ' which reduces the smoke to a minimum. Too much secondary air will . cpollthe gases below the ignition point, and prove harmful instead of :jtd}feneficia]. . following suggestions will be helpful: co^ weather, with high combustion rates, the secondary air damper should be all the time. Y vt-'ry mild weather, with a very low combustion rate, the secondary air damper ^should, be closed all the time. . 245