Book - School of Science and Technology

320 Combustion and chimneysTable 12.1, the mass of water produced is 36/4 ˆ 9 times the mass of hydrogen burnt, itfollows that the latent heat within the uncondensed vapour may be calculated as:Latent heat re 15 C ˆ 2450 kJ/kgThus CV g CV n ˆ H(9 2450)/(100 1000)ˆ (0:221 H) MJ/kgSince the heat liberated as a result of the oxidation of various elements is known, acalculation may be made using a suitable analysis to derive a theoretical calorific value.For solid fuels in particular, the result of such a calculation will lack accuracy as a resultof the presence of a variety of extraneous elements or impurities. Practical evaluations arethus made using a calorimeter.Chimney lossThe chimney loss, as the name suggests, is a summation of all those losses which haveaccumulated by the time that the products of combustion reach the chimney. They aredirectly proportional to the mass gas flow, to the temperature at which discharge from theboiler occurs and to the specific heat capacity of the gases. Further, the total includes alsothe latent and sensible heat content of water vapour arising from the combustion ofhydrogen. The inter-relation of the various components of the total is complex and it isconvenient to make use of diagrams as Figures 12.1±12.3 which are abridged versions ofthose included in the Guide Section C5.Sample calculationsSolid or liquid fuelsA single example common to both these fuels may be taken to introduce the use of Table12.1 by assuming that a sample has, by mass, the following ultimate analysis:3030Chimney loss % of gross C.V.252015102CO 8%2CO 10%2CO 12%2CO 14%2CO 16%Chimney loss % of gross C.V.252015102CO 6%2CO 8%2CO 10%2CO 12%2CO 14%5100 150 200 250 300 350Flue gas temperatureleaving boiler above ambient KFigure 12.1 Chimney loss for coal firing5100 150 200 250 300 350Flue gas temperatureleaving boiler above ambient KFigure 12.2 Chimney loss for oil firing