SIMULATION AND DIAGNOSTICS OF THE EXTERNAL AND INTERNAL CIRCULATION FLOWS OF SOLIDS IN A BIOMASS FIRED CFB BOILER

摘要

Ash related operational problems are very common in biomass fired boilers. Biomass naturally contains both sodium chloride and potassium chloride and theses compounds lower the melting temperature of the ash which may cause large operational problems with agglomeration and defluidization in Circulating Fluidized Bed (CFB) boilers. The number of biomass fired CFB-boilers for combined heat and power (CFfP) production in the Scandinavian market is growing due to their good combustion efficiency, fuel flexibility and low emissions. The power companies are asking for a method to calculate the internal and external circulation flows of solids in the boiler and an accurate diagnostic method to detect initial agglomeration in order to be able to prevent the problem of defluidization that leads to large costs and loss of revenue when the boiler has to be shut down for cleaning. Two heat and mass balance based models have been developed in order to calculate the fuel flow and the internal and external solids circulation flows in a CFB boiler with internal heat exchangers (INTREX). The solids circulation model is divided into three parts: cyclone, combustion and INTREX chambers. Measurements used in the calculation are from commissioning tests on CFB-boiler 5 at MalarEnergi in Vasteras, Sweden. The boiler was manufactured by Foster Wheeler OY in Finland and has a thermal heat output of 157 MW. The external solids flow at 100 % load, with and without air humidification, is 215 kg/s and the internal solids circulation is 93 kg/s. The external solids circulation flow at 60 % load is 30 kg/s and the internal solids circulation flow is 486 kg/s. At 60% load, there is no data available for validation, which means that this is more an estimate then a calculation. The calculated internal flow of solids is very sensitive to changes in the total heat flow in the INTREX chamber caused by agglomeration or combustion, whereas the external solids flow is not affected. Hence initial agglomeration and combustion can be detected. A simulated agglomeration in the INTREX chambers by decreasing the total heat flow by 1 %. led to a decrease in the internal solids circulation flow by 11%. A simulation of combustion in the INTREX chamber of 0.5 kg/s of fuel entering the chamber corresponds to an increase in the total heat flow of 22 % and a decrease in the calculated internal mass flow of 16 %. The potential for using this method of diagnostics for detecting initial agglomeration is very promising.