Research On The Motion And Heat Transfer Characteristics Of Slag In Rolling-Cylinder Type Slag-cooler

With the maximization of Circulating Fluidized Bed(CFB) Boiler, the normal operation of slag cooler is directly related to the continuous, reliable ,safe and economic boiler operation. The rolling-cylinder type slag-cooler is widely used as it is adaptable, simple and reliable. At present, the design and production of the rolling-cylinder type slag-cooler is mainly by experiences, lack of appropriate theoretical guidance and scientific information, and there were many problems. The research on the slag motion and heat transfer characteristic in rolling-cylinder type slag-cooler has important significance to improve the ability and safe operation of slag cooler. Based on the support of National Key Technology R&D Program in the 11th Five year Plan of china--Reseach on 600MWe Supercritical Circulating Fluidized Bed Boiler Auxiliary System", the research on the slag motion and heat transfer characteristic in rolling-cylinder type slag-cooler were taken. The aims of the study are to improve the operating performance, reliability and economy of slag cooler, and to provide technical support for the development or application of supercritical CFB boiler's slag cooler.The observed and testing experiments for the motion characteristics in the rolling-cylinder type slag-cooler by color tracer technique were taken. Based on the models in rotary kiln, the one-dimensional motion model in rolling-cylinder type slag-cooler drum was established and was validated by experiments. The slag mixing was weak both at the slipping and the slumping state, but it is better at rolling state. The mean residence time of the slag in the cooler is inversely proportional to the rotational speed, and increased as the slag amout, longth of the cooler or spiral angle of the fins increasing. The cross-mixing in the cooler is enhanced as the rotational speed increasing or the slag amout decreasing.With the heat transfer apparatus, the heat transfer characteristics between the slag and internal wall of the cooler were studied. The one-dimensional heat transfer model in rolling-cylinder type slag-cooler drum was established, and then both quasi 3-d heat transfer model and single-particle heat transfer model were also established. It was shown that the heat transfer coefficient increased firstly then decreased and finally increased again, as the average partical size smaller. The heat transfer coefficient could be enhanced by increasing the rotational speed, slag amout or temperature, and it increased littler with slag amout increasing, when the slag amout is large. The output increases linearly with speed increasing, and the slag outlet temperature increases too. Both the output and slag outlet temperature increased with slag amout or the spiral angle of the fin increasing. There is no effect on output by the length of the cooler, but the slag outlet temperature decreases with the longth increasing.It shows that the temperature gradient exist near wall of the internal cooler, and the core temperature is consistent, the quantity of heat is mainly released from the slag near the wall. And nearer the entrance higher the temperature.In addition,as the slag particle size increasing or the residence time reducing, the effect of slag initial temperature and porosity on heat transfer is enlarged, and the outlet slag temperature exhibits an exponential increase with the size increasing.