Book - School of Science and Technology

Miscellaneous burner equipment 293Water cooleddoorFire tubesFlueLargecombustionchamberDuct from doormounted frameFigure 10.27 Straw burning boiler (Ranco)350 450 900 and weighs about 18 kg and the large cylindrical sizes some of whichmeasure up to 2 m in diameter and weigh upwards of 250 kg. Moisture content is quotedat 17% and the net CV appears to be about 12 MJ/kg.The original ranges of boilers were designed to burn the small bales and had ratings ofbetween 20 and 100 kW based upon a 4 hour loading cycle, the combustion chamber of a100 kW unit, for instance, being large enough to accept seven small bales. Later modelsdesigned to burn the larger cylindrical bales are rated at between 100 and 250 kW basedupon a 6 hour loading cycle. The loading doors for all sizes are, of course, extremely largeand seem to be arranged in most cases for water cooling.To overcome loading difficulties which, with the larger bales, must be considerable,automatic feed systems are available. These use hydraulic power to divide the bales intosmaller compressed portions and to provide a piston feed to the boiler via a conveyortube. This tube is fitted with a water drenching system and a spring-return fire guillotine.Fluidised bed boilersThe dramatic effect upon the use of low grade fuel and upon boiler design, which it isexpected may result from progress in the development in fluidised bed firing, has yet to befully appreciated. Whereas, for instance, it is current practice to halve the rated output ofa given boiler when it is converted to firing by solid fuel from either gas or oil, thisderating would not be necessary if the solid fuel firing were via a fluidised bed.The concept of combustion within a fluidised bed is so different from conventionalpractice that it deserves further explanation of the principles involved. If a bed of sand, ora similar inert material such as a crushed refractory, be mounted over a plenum box then,when a critical air velocity is reached through the bed, it will behave very much as if itwere boiling, with the bed particles mixing rapidly throughout the depth. If such a bed beheated to a temperature of say 750 C and fuel be then added, combustion will be selfsustainingand the entire bed will become incandescent. The principles are illustrated inFigure 10.28.