Under co-combustion conditions coarse waste/biomass particles often fall down together with bottom ash increasing appreciably its carbon content. For this reason MAGALDI Ricerche e Brevetti in its past experience has registered the occurrence of extensive ash post combustion on the dry ash belt conveyor MAC (Magaldi ash cooler). In the present work a model is presented that describes the fate of ashes over their falldown from the boiler and extraction by the belt-conveyor. The model takes into account that the size distribution of ash particles falling from the furnace on the belt may be rather broad and polydisperse and that when particles reach the belt conveyor they build up a granular bed. Therefore thermal processes are considered at two distinct length- and time-scales: at the macroscale the granular bed exchanges heat with the surrounding atmosphere and with the belt itself; at the microscale ash particles of different size and temperature exchange heat among themselves. The rate of change of the carbon content in the particles is computed for each particle size and at any position in the moving granular bed. Equations are solved according to the method of characteristics using an optimal discretization of the spatio-temporal computational domain. Model results confirm that considerable carbon conversion can be achieved on the belt. Control of the belt velocity and of the cooling air as a means to optimize the process are discussed.
Extraction and postcombustion of bottom ash from utility boilers
O Senneca;
2011
Abstract
Under co-combustion conditions coarse waste/biomass particles often fall down together with bottom ash increasing appreciably its carbon content. For this reason MAGALDI Ricerche e Brevetti in its past experience has registered the occurrence of extensive ash post combustion on the dry ash belt conveyor MAC (Magaldi ash cooler). In the present work a model is presented that describes the fate of ashes over their falldown from the boiler and extraction by the belt-conveyor. The model takes into account that the size distribution of ash particles falling from the furnace on the belt may be rather broad and polydisperse and that when particles reach the belt conveyor they build up a granular bed. Therefore thermal processes are considered at two distinct length- and time-scales: at the macroscale the granular bed exchanges heat with the surrounding atmosphere and with the belt itself; at the microscale ash particles of different size and temperature exchange heat among themselves. The rate of change of the carbon content in the particles is computed for each particle size and at any position in the moving granular bed. Equations are solved according to the method of characteristics using an optimal discretization of the spatio-temporal computational domain. Model results confirm that considerable carbon conversion can be achieved on the belt. Control of the belt velocity and of the cooling air as a means to optimize the process are discussed.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
