Improved combustion performance of waste-fired FB-boilers -The influence of the dynamics of the bed on the air-/fuel interaction

One of the key benefits of fluidized bed combustion is that the bed – through mixing of fuel and air and accumulated heat – facilitates combustion at low stochiometri and with low emissions.

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Even so, it is not unusual that waste-fired FB-boilers are operated at 6-8% oxygen that corresponds to 30-40% higher flows of gas than theoretically needed. In addition to that and in comparison to grate furnaces, FB-boiler can cause high pressure drop losses because of the fluidization of the bottom bed, which in turn are associated with high costs for power (fans). This work aims therefore at increasing the knowledge for how the dynamics of the bed affects the air and fuel mixture. Methods to explain and characterize the phenomenon have been derived within this work showing:

Distribution of air in a bed for various cases and the influence of pressure drop, bed height and fluidization velocity

A semi-empiric method to calculate an even bubble distribution

The relation between fluidization and fuel distribution for various fluidization flows and fuels

Dispersion rates for various fuels

Volatilization rates for waste in relation to biomass

The result can be useful when optimizing units, for instance through finding as low pressure drops as possible with an even bubble distribution, low risk for sintering and unwanted emissions.  The work has thereby reached its ultimate goal of increasing the generic knowledge about waste combustion in FB-boiler.