Book - School of Science and Technology

Combustion processes 319with any sulphur trioxide present to form sulphuric acid. This occurs when temperaturesfall much below about 130 C.In order to reduce the prospects of corrosion resultant upon presence of sulphurtrioxide, practice is to design for discharge temperatures of 250±270 C (some 240 Kaverage above the surrounding atmosphere). Hence, the combustion products hold aconsiderable amount of sensible heat in addition to the latent heat within the uncondensedwater vapour which has been referred to previously. This sensible heat may, inturn, be quantified as being proportional to the mass of the total products including anyexcess air carried over which serves to re-emphasise the importance of setting andmaintaining an optimum quantity of excess.Flue gas analysisCarbon being one of the principal components of most practical fuels, knowledge of theproportion of carbon dioxide present in the products provides a useful indication ofcompleteness of combustion. Dilution of these products with excess air leads to a paralleldecrease in the proportion of carbon dioxide present and thus measurement of the CO 2content provides a relatively simple criterion which indicates not only the degree to whichthe combustion reactions have been completed but also the proportion of excess airadmitted.Taking the simplest case of pure carbon as an example, it may be seen from Table 12.1that, for the ideal state of complete combustion, the CO 2 content of the products byvolume is:CO 2 ˆ 100 (3:67/44)=[(3:67/44) ‡ (8:84/28)]ˆ 8:3/(0:083 ‡ 0:317) ˆ 21%( max )If 50% excess air were admitted, the CO 2 content would then be:CO 2 ˆ 8:3/[0:4 ‡ 0:5(2:67/32 ‡ 0:317)]ˆ 8:3/(0:4 ‡ 0:2) ˆ 13:8%In the case of plants larger than those within the scope of this present chapter, it isnecessary to be aware not only of the carbon dioxide content of the products of combustionbut also that of other components such as oxygen, carbon monoxide and the sulphuroxides, etc.Calorific valueThe calorific value of a fuel is the quantity of heat energy released as a result of thecomplete combustion of unit mass. As described in Chapter 10 (p. 261), two values arenormally quoted, the gross or higher and the net or lower. For ease of reference, these aredefined here again: the gross calorific value includes the latent heat within any watervapour formed as a result of the combustion of hydrogen and the net calorific valueexcludes this constituent.It is not normally practicable to recover this latent heat by condensing the water vapourand thus the net calorific value is the more useful figure except in the case of condensingboilers where the gross value is the more meaningful. Since, again as may be seen from