Case Studies: Problems Solving in Fluidized Bed Waste Fuel Incineration

Fluidized bed combustion technology has been widely used as the new, flexible, multi-fuel boiler for waste combustion and energy recovery from low-grade fuels. However, problems such as low thermal efficiency, high emissions and bed agglomeration etc., are still encountered in operation of fluidized beds. Valuable experiences were gained from the two case studies recently conducted regarding wastes combustion in industrial-scale fluidized beds. In the first case, the performance of a fluidized bed combustor for energy recovery from oil sludge was evaluated during the commercial trials. Apart from the sludge characterization and bed material analysis, the combustion efficiency, solid flow balance, on-stack emission of CO, SO_x, NO_x were addressed, as well as the fluidization quality. Although the system was operated with good combustion efficiency (>99.9%), sulfur dioxide emission (>1,000ppm) was found to be substantially higher than the allowable discharge limit. It was recommended to increase limestone feed rate in order to meet the SO₂ emission standard and subsequently, installation of a cyclone is suggested to remove the potential significant increase in ash and fine particles. The second case study focused on the bed agglomeration observed in a fluidized bed incinerator where burning blend of three wastes (i.e., carbon soot, biosludge and fuel oil) are involved. To understand the mechanisms and chemistry related, several analytical approaches are employed to identify the bed materials (fresh sand and degrader sand) and clinkers formed from full-scale incinerator tests. The formation of clinker is believed to follow the mechanism of partial melting and/or reactive liquid sintering. The effects of temperature and blending ratio are tested in a muffle furnace. Carbon soot is believed to be more susceptible than the other two fuels. Thermodynamic multi-Phase multi-Component Equilibrium (TPCE) calculations predict that the main low melting point species are predominant under oxidizing condition, suggesting that reducing conditions might be favorable to restrain the bed agglomeration. This study provides valuable information for the better understanding of the chemistry related to clinker formation; it also helps in developing methods for the control and possible elimination of the bed agglomeration problem for the design fuels.