Cold Condition Experimental Study Of Ash-Disposal System By Ice-Pellets In Boiler Hearth

In China the main fuel of power plants is coal which contains high ash content and sulfur content and has low ash fusion temperature. In the case that in the furnace the local atmosphere is reductive or the load is too high or the air dynamics is disordered, slagging will be easily induced. Worsely, it will lead to high temperature erosion on the heating surfaces. Slagging in the furnace affects the heat transfer efficiency of the heating surfaces and causes the decreasing in the thermal efficiency of the boiler. In the operation process, ash-disposal (or slag-disposal) becomes a critical problem.The available ash-disposal systems have respective features. Water jet flow ash-disposal device shows good ash-disposal effect for furnaces that slagging is mild, but the water jet flow may exert repeated heat shock on the heating metal surfaces which causes the heat fatigue of tube walls. In the ash-disposal period the much water consumption causes the decreasing in the boiler load and the thermal efficiency. Compressed air or steam ash-disposal devices are commonly equipped with long length-variable structure which causes poor reliability under hot conditions. The effect of ash-disposal is not ideal either. Steam ash-disposal device may sometimes cause the erosion and ash clogging on the low temperature heating surfaces at the end of the boiler. For all the above-mentioned systems, there exist dead zones that ash-disposal devices do not work.Based on ice pellet launching system, a new ash-disposal technique, i.e. ice pellet ash-disposal technique, is discussed. The principle is that ice pellets are mixed with high speed air jet flow and accelerated to very large momentum which are then used to remove the ash in the furnace. Experimental investigation on particle launching and scatter is performed. On the basis of the experimental results, a numerical model for simulating the particle scatter is built and solved. The particle scatter is then determined.The experimental and numerical results show a few conclusions. For the particle launching system used in the present experiment, a balance pipe between the air pipe and the particle bin is necessary to make sure that the system can work well. A peak value of particle velocity at the exit of the converging nozzle is observed which means that there is an optimum tube length at which the launching speed at the exit of the nozzle is maximized. The particle spray probability at the exit of the nozzle meets the two-peak distribution. For a specified particle spray distance, the relation between the particle scatter probability and the scatter radius agrees to the Rosin-Rammler rule. When the spray distance is less than 1.5 m, the particle scatter is affected by the air jet flow significantly and the exerted force is very complex so that it is not appropriate to simulate the process by using the exit of the nozzle as the initial point. Good simulation results are obtained by adopting the position at 1.5 m distance from the exit of the nozzle as the initial point. The values of r₀ and m show the same increasing trends. Based on the analysis of the simulating results, the relation between r₀ and d (the spray distance) is achieved.Due to the limitation of the lab, the experiments are not fully implemented. The mathematical model is developed based on some simplifications which are to some extent different from the real process. So more experimental work needs to be done to validate the model or modify it on the basis of the equations presented in this thesis.