Agglomeration during Fluidized Bed Combustion and Gasification of Fuels

The use of production waste, primarily various kinds of biomass, for generating heat and electricity is an important trend in solving the energy saving problem. By applying the fluidized bed technology, it is possible to incinerate biomass, including waste from agricultural and some industrial production facilities, in boilers. It is shown that bed agglomeration, a phenomenon that may cause the bed material to become sintered, the temperature inside the bed to become nonuniform, and fluidization to become degraded and even to stop, is one of essential problems associated with biomass combustion in a fluidized bed with conventional bed material (sand). Foreign investigations aimed at clarifying the main agglomeration mechanisms are analyzed. It is shown that the Ca/(K + Na), (Na + K)/(Ca + Mg), and Ca/P ratios, the presence of chlorine in the fuel, and the combustion temperature are the key factors facilitating the fluidized bed material to become agglomerated during biomass combustion. The results from investigations of agglomeration processes carried out in the industry-grade boiler installed at the combined heat and power plant (CHPP) no. 3 of the Arkhangelsk Pulp and Paper Mill (PPM) are presented. The effect that the alkali metals contained in biomass ash have on the bed agglomeration processes and on the agglomeration initiation temperature is considered. It is shown that the bed agglomeration also depends on the phosphorus, sulfur, and chlorine compounds in ash. Certain heavy metals may also participate in the bed agglomeration processes during the combustion of phosphorus-containing biomasses and their mixtures. The basic methods for predicting the onset of agglomeration and for controlling its occurrence are outlined. Significant prospects of preventing agglomeration are associated with using alternative bed materials. The results obtained from investigations into the microstructure and chemical composition of cake particles produced in the course of fragmentation during rapid wood pyrolysis that have been carried out at the All-Russia Thermal Engineering Institute are presented.