OCR Text |
Show I. Introduction Coal-water-sluny (CWS) fuelled small scale atmospheric fluidized bed combustors (FBC) have been considered for the last decade as a potential replacement to oil and electrical heating. A number of problems had to be resolved before it could successfully compete with the existing technologies. These problems were grouped in two categories: - Fundamental problems: Gaseous (NOx, SCh, CO) and particulate emission, combustion efficiency, - practical operating problem: such as, ash collection, control, etc. To address the fundamental problems, injection of secondary swirling air into the freeboard was proposed. Instead of feeding the total combustion air through the distributor plate at the bottom of the bed, as in most FBC boilers, the combustion air was to be supplied in two streams. These were: the primary fluidizing air feeding to the lower combustion chamber (the bubbling bed), and the secondary tangential air injection from the wall into the freeboard. In this configuration, the primary air fluidizes and entrains small fuel particles into the freeboard for continued combustion, while large amounts of secondary air is introduced tangentially to form a swirling, high shear flow field This results in a substantial increase in gaS turbulence level and, the residence time distribution of the solids. Consequently, the combustion efficiency and heat transfer to the cooling swfaces are improved while the emission levels drop considerably. The following program has been undertaken: 1. Cold Flow Model Experiments Flowfield diagnostics were carried out to investigate the particle trajectories in the freeboard This enabled us to detennine the effect of swirl on particleparticle and particle-gas mixing patterns in the freeboard. The results of this investigation were then used as a basis for the next stage (hot) experimental program. 2. Actual Combustion Experiments Investigation of the effect of swirling secondary flow on the control and reduction of nitrogen oxides (NOX> and other gaseous emissions as well as on the particulate carry over was conducted. 2 |