Book - School of Science and Technology

Chapter 12Combustion and chimneysAs an extension to the understanding of fuel characteristics and of the methods used forstorage, handling and burning, some knowledge of the chemistry of combustion is useful.In addition, this has a bearing upon the selection of and design for the chimneys necessaryto disperse the products of combustion. The text which follows is no more than anintroduction to the subject, with particular reference to those fuels which have been notedin the preceding chapter as being commonly used for plant of the scale necessary to serviceindividual buildings or relatively compact groups of buildings.Fuels encountered in practice consist of carbon, hydrogen and oxygen combined toform relatively complex mixtures or compounds; the hydrogen may be uncombined andwill be found present in the free state in most gaseous fuels. In addition, there aregenerally small quantities of sulphur, nitrogen and ± in the case of most solid and liquidfuels ± ash. The moisture content of solid fuel takes two forms, the superficial or freemoisture resulting from either pit-head water-screening or subsequent open-air storage,and the inherent or air-dried content.Combustion processesThe combustion of fuel is an oxidation process which is accompanied by the liberation ofheat energy. The reactions occur as the carbon, hydrogen and, when present, sulphurcombine with the oxygen in the air supplied. These reactions can take place only at arelatively high temperature, known as the ignition temperature, which varies between400 C and 700 C according to the fuel. If an adequate supply of air has been madeavailable, then the carbon will burn completely to form carbon dioxide but, when combustionis incomplete due to shortage of air, carbon monoxide will be formed. The hydrogenwill burn to form water vapour and any sulphur present will burn to produce sulphurdioxide which may perhaps later combine with more oxygen to form sulphur trioxide.On average, by mass, air contains 23.21% oxygen and 75.81% nitrogen, plus traces ofargon, helium, krypton and other inert gases: without sensible error, therefore, theproportions may be considered as 23.2% oxygen and 76.8% incombustible. By volume,the proportions are 20.9% oxygen and 79.1% incombustible. The various elementspreviously mentioned combine with oxygen in proportion to their respective molecularweights which are: oxygen (O 2 ˆ 32), carbon (C ˆ 12), hydrogen (H 2 ˆ 2), nitrogen(N 2 ˆ 28) and sulphur (S ˆ 32). Table 12.1 lists the fundamental combustion equations.