Effects On Ash Formation Of Coal Particle Properties In Fluidized Bed Combustion

Ash formation behaviors of coal partice have great influences on the boiler design and operation for a fluidized bed boiler. The coal particles with differenct properties behave differently on ash formation. Unfortunately, the coal particle properties may be varied form time to time enven for a given coal type. This thesis focuses on the research of effect of particle properties on ash formation.Firstly, a review on the effect of the coal properties on ash formation during f ludized bed combustion is completed. Yima bituminous coal sample is divided into several ranks with different density and size by the ZnCl2 solution floatation and the size sieving processes. Experiments on a bench-scale fluidized bed combustor for every coal rank are completed in different operation conditions. Based on the measurements results, effects of particle density and size on the ash formation behaviors such as ash size distribution, fly ash mass fraction, carbon content in the bottom ash and fly ash are obtained. In additional, effect of burning residence time are discussed.The experimental results indicate that: (1) There are different characteristic particles in the same type of coal. With increasing of coal particle density, the content of ash, volatile matter, C, H, N and 0 and the quantity of heat decreas. (2)As far as the low density particles with low ash content are concerned, there is no bottom ash left after a long term combustion in the fluidized bed. The mode of combustion is "single combusting surface" mode. On the contrary, the high density coal with the high ash content produces large amount of bottom ash and the mode is mainly "two combusting surface" mode. (3)At the condition of the same combustion interval and particle size, with the increasing of coal density, the proportion of bottom ash increases. And the carbon content of fly ash decreases. The proportion of the particle left in the original particle size rank increases distinctly, while the fine particle proportion decreases dramatically. And the burn exhaust ratio decreases. (4)With the increasing of partcle density, the extent of the impact of the particle size distribution to the distribution of bottom ash size graduallyABSTRACTincreases.A simple mathematical model is established based on the single particle coal combustion mode and experimental results. The experimental results of burning for 5min and 20min are matched. The prediction of buring for SOmin is made. The comparison and analysis of model results and experimental results are made.At last, the thesis introduces a combustion experiment carried out on a 0. 5MW circulating f luidized bed combustion test rig. The characteristic of ash formation is discussed through the analysis of experimental results including the ash particle distribution, the propotion of bottom ash and fly ash, ash carbon content and combustion efficency. Some suggestions are provided on the boiler design.