Book - School of Science and Technology

322 Combustion and chimneysHence, by volume:CO 2 ˆ 100(2:94/44) ˆ 6:68SO 2 ˆ 100(0:04/64) ˆ 0:06N 2 ˆ 100(14:35/28) ˆ 51:25O 2 ˆ 100(1:86/32) ˆ 5:81 63:80Thus,CO 2 ˆ 6:80/63:80 ˆ 10:5%:If the flue gas temperature were to be 300 C with a CO 2 level of 10.5%, the chimney lossin the case of solid fuel firing, as read from Figure 12.1, would be 23.5%.Gaseous fuelsIn this case, although the principles remain exactly the same as before, it is convenient toapproach any calculations by making use of the data for the various hydrocarbon compoundslisted in Table 12.2. Hence, we consider a fuel having the following composition, by volume:methane 90%, ethane 5%, propane 1%, nitrogen 3.5%, carbon dioxide 0.5%.The air required for stoichiometric combustion would be:By volumeMethane 0:9 2 ˆ 1:8Ethane 0:05 3:5 ˆ 0:175Propane 0:01 5 ˆ 0:05; Oxygen required (m 3 /m 3 ) ˆ 2:025Thus air required (m 3 /m 3 ) ˆ 2:025/(20:9/100)ˆ 9:7Table 12.2 Combustion reactions (gases)Requirementskg/kg m 3 /m 3Reaction O 2 Air O 2 AirHeat liberated(MJ/m 3combustible)MethaneCH 4 ‡ 2O 2 ˆ CO 2 ‡ 2H 2 O 4 2 4016 ‡ 64 ˆ 44 ‡ 36 17.24 9.57Ethane2C 2 H 6 ‡ 7O 2 ˆ 4CO 2 ‡ 6H 2 O 3.73 3.5 6960 ‡ 224 ˆ 176 ‡ 108 16.08 16.75Propane2C 3 H 8 ‡ 10O 2 ˆ 6CO 2 ‡ 8H 2 O 3.64 5 9588 ‡ 320 ˆ 264 ‡ 144 15.69 23.92Butane2C 4 H 10 ‡ 13O 2 ˆ 8CO 2 ‡ 10H 2 O 3.59 6.5 121116 ‡ 416 ˆ 352 ‡ 180 15.47 31.1Pentane and aboveC 5 ‡ 3.1 8.3 16413.36 39.7