Book - School of Science and Technology

The CO 2 present in the combustion products, with 25% excess air would be:Combustion products, by volume0:9m 3 of methanecombines with 0:9 2 ˆ 1:8m 3 of O 2produces 1:8(79:1/20:9) ˆ 6:8m 3 of N 2produces 0:9 1 ˆ 0:9m 3 of CO 20:05 m 3 of ethanecombines with 0:05 3:5 ˆ 0:175 m 3 of O 2produces 0:175(79:1/20:9) ˆ 0:66 m 3 of N 2produces 0:05 2 ˆ 0:1m 3 of CO 20:01 m 3 of propanecombines with 0:01 5 ˆ 0:05 m 3 of O 2produces 0:05(79:1/20:9) ˆ 0:19 m 3 of N 2produces 0:01 3 ˆ 0:03 m 3 of CO 2Thus, volume of CO 2 in products (m 3 )ˆ 0:005 ‡ (0:9 ‡ 0:1 ‡ 0:03) ˆ 1:035and volume of N 2 in products (m 3 )ˆ 0:035 ‡ (6:8 ‡ 0:66 ‡ 0:19) ˆ 7:685and excess air volume (m 3 )ˆ 0:25[7:685/(79:1/100)] ˆ 2:429Thus,CO 2 ˆ 1:035(1:035 ‡ 7:685 ‡ 2:429) ˆ 9:3%Chimneys 323ChimneysThe purpose of a chimney is to provide a means whereby the products of combustion maybe exhausted in such a manner as either to avoid pollution or, at worst, to provide meansof dispersion such that pollution is diluted to an acceptable level. Obviously, the nature ofthe pollutants will vary with the nature of the fuel used. Particulate matter may be carriedover in some cases, acid vapours in others and in some the discharge may be no more thana thermal carrier-plume.Chimney design is a task which may involve a number of disciplines in the solution ofproblems related to the aesthetic, structural and functional aspects. Very many chimneysare concealed for much of their height within the massing of a tall building but thisapproach to concealment may lead to local difficulties as a result of lee-side downwash orvortex effects. It is not possible to discuss these matters here and, in any event, they arebest resolved through model tests in a wind tunnel using techniques which are now wellestablished.There are certain principles applying to all chimneys, regardless of fuel type, which arenow recognised as being fundamental to good design. These are:. The velocity of the gases through the stack should be as high as possible commensuratewith the draught available.