Book - School of Science and Technology

294 Boilers and firing equipmentIgnitionburnerFuelsupplySand bedPlenumFan (off)Fan (on)Figure 10.28 Principles of fluidised bed combustionHeat exchange within a combustion chamber provided with a fluidised bed is extremelycomplex, including radiant and convective transfer to the surrounding surfaces and directtransfer to any surfaces immersed in the bed, the latter being particularly significant.What is important, however, is that combustion is highly efficient and almost completewith minimal ash residuals.InstrumentationThe theoretical background to analysis of the products of combustion, as a measure of the`efficiency' of the combustion process, will be dealt with in Chapter 12. There is nowavailable a number of sophisticated instruments which enable the necessary measurementsto be taken but, for site use, the more robust and simple devices of the past maycontinue to be of value.Flue gas analysisThe standard apparatus is the Orsat with which a sample of the gas is first dried to removewater vapour and is then subjected to an adsorption process by three liquids in turn. Thefirst is a solution of caustic soda which adsorbs CO 2 and the second an alkaline solutionof pyrogallic acid plus caustic soda which adsorbs O 2 . The third is a solution of cuprouschloride in hydrochloric acid which reacts with CO. Measuring burettes enable theproportion of gas adsorbed in each case to be assessed with comparative ease and thevolumetric content of the whole established.Flue gas temperatureGases at boiler exit are measured as to temperature by means of either pyrometers or hightemperature thermometers which latter may be either nitrogen filled, mercury-in-glass,mercury-in-steel or some form of thermocouple. A variety of devices has been developedto ensure that the gas flow sensed is representative.