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det.ermlroed.. C....OOn e.xlsu In.number of fort:M eacll of whldt probably bas 1110 0... 1IlO1ectib.c- _Isht. The Ialest II'Ive$tl-For the commercial production of heat it is essential thatthe combustible elements have a strong and ready affinity foroxygen. Carbon and hydrogen which are by far the most im-portant of combustible elements encountered in the commonfuels meet this requirement admirably. These occur either ina free or combined state in all fuels, liquid, solid and gaseous.The combustible elements and the compounds in which theyappear in any of the fuels used for commercial heat generationare given in Table 1. This table gives the symbols of theclements and their compounds which occur in combustion worktogether with their molecular weights. rt also includes the non-combustible elements and compounds, a knowledge of which isnecessary in the obtaining and application of combustion data.AIRAs we find in nature the combustible matter for tbe generationof heat, so (rom the same source we obtain, in the oxygen of theair, the necessary supporter of combustion.Atmospheric air is a mechanical mixture-as distinguished[rom a chemical compound--of oxygen. nitrogen, and slightamounts of carbon dioxide, water vapor, argon and otherinert gases. For engineering purposes the carbon monoxideand the inert gases are ordinarily included with the nitrogen andof the slightly ~rarying proportions of oxygen and nitrogen givenby different authorities the generally accepted ~ralues are :8,WeichtPet e-,0, 20•91 23.15N. 79.0<) 76.85The oxygen with its strong affinity for the combustible con•stituents of the fuel, under the proper conditions of temperaturewhich will be discussed hereafter. separates itself from itsmechanical union with nitrogen and enters into chemical combi-nation with. tbe available combustible, thus fulfilling its functionin the promotion of combustion. The nitrogen serves no purposein combustion and in fact is a source of direct loss in that itabsorbs heat in its passage through the furnace and carries offa portion of such heat in leaving the boiler; further, as a uselessconstituent it necessitates in the design of the furnace. boiler and flue, space for its accommodation,
which, were it possibleor practicable to supply oxygen alone to the fuel, would notbe required.The combination of oxygen with the combustible elements andcompounds is, as stated, in accordance with fixed laws. Consideredas a chemical reaction the manner of such combination is simpleand may be readily computed from the molecular weights givenin Table J. Assuming complete combustion and that the exactamount of oxygen required is supplied and utilized in combination,these reactions and the resulting combinations are as given inTable 2.TABLE 2CHEMICAL REACTIONS OF COMlJUSTlONCo,nbuulbl. Sut......nn 1l.M/;llnLlCaThen (to CO) 2C+ 0t_ZCOCarbon (to CO,) 2C+ZO,_2CO,Carbon Monoxide: 2CO+ O._2CO,Hydrogen 2H.+ O,_2H.OSulphur (to 50,1 5+ 0,-50,Sulphur (10 S0.) 25+3°._25° ..Me:t.hane CH..+20,_CO, +2H,OAcetylene zC.H.+SO,-4CO.+2H.OElhylene C.H..+30._2CO,+2H,OElhane 2C,H.+,O,-4CO.+6H,OHydrogen Sulphide 2H,S+30,_ 21:1,0+250.It is important to note from this tahle that carbon may enterinto combination with oxygen to form two compounds, carbonmonoxide and carbon dioxide. In burning to carbon monoxide,carbon has not combined with all of the oxygen with which it iscapable of entering into combination and is not therefore com•pletelyoxidized. In burning to carbon dioxide it has combinedwith all of the oxygen possible and oxidization is complete. Carbonmonoxide may unite with an additional amount of oxygen to formcarbon dioxide and in this way the carbon of the original com•bination may become completely oxidized. The (act that carbonmay enter into these two combinations with oxygen is of thegreatest importance in furnace efficiency and will be discussedhereafter at greater length in the consideration of the heat ofcombustion and air supply.Before discussing in detai.l the effects of supplying oxygenfor combustion in excess of the requisite amount or of supplyingless than the amount required, the other important factor ofcombustion, viz., temperature, should be considered.The speed of combustion is, as stated, dependent upon theaffinity of the combustible matter for oxygen and the conditionsunder which combustion takes place. The chief of these con-ditions is that of temperature. The mere fact that oxygen isbrougbt into the presence of a combustible substance does not ofnecessity mean that combustion will follow.Every combustible substance has a temperature called itsignition temperature to which it must be brought before it willunite in chemical combination with oxygen and below which suchcombination will not take place; and this ignition temperaturemust exist with oxygen present or there will be no combustion.The ignition temperature of different combustible substancesvaries greatly. These temperatures for various fuels and for thecombustible constituents of the fuels used in boiler practise aregiven in Table 3.TA.BLE 3IGNITION TEMPERATURESCombuulble Sllbll<UlceMolClCIlI •• IL"idoo Temp",..,ullnSymbol ocr"'" Tllh,,,,,htitSulphur.• • •• • • • • • • S. ''0 Fixed Carbon-Dituminous Coal . • • . • . 766Fixed Carbon-Semi-bituminoul Coal• • '7°Fixed Ca.rbon-Anthracile Coal • • .• . 9'5Acetylene • • • • • • • CtHt 000Ethane • • • • • • • CtH. .= Ethylene • • • • • •..• • • • CtH.. 102:::1Hydrogen • • • • • • • • H. 1130Mllthane • • • • • • • • • eH, 12:02Ca.rbon MonOldde • • • • • • • eo 1 ZIOIt is of interest to note that the temperature of ignition ofthe gases of a coal vary from each other (see flame) and areconsiderably higher than the ignition temperature of the fixedcarbon of the coal. The ignition temperature of coal is theignition temperatllre of its fixed carbon content, since the gaseous constituents are ordinarily distilled off, though not ignited. beforesuch temperature is attained.When combustion has started, the heat evolved in theoxidization of the combustible matter will maintain under properconditions sufficiently high temperatures for further ignition.DENSITY, WEIGHT AND VOLUME OFGASESIN the computation of combustion data it is frequently neces-sary to know the density, weight and volume of air and ofthe various gases encountered in commercial practise.The density of a gas (commonly expressed by the symbol 6)which is ordinarily referred to that of air as standard,