Experimental Study On Cement Clinker Cogeneration In Pulverized Coal Combustion Boilers Of Power Plants

Coal is the most important energy source in China. To date, over 70 percent of energy consumption still stems from coal combustion. Therefore, large amounts of solid wastes are generated from thermal power plants every year. In 2001, for instance, the newly generated ashes amounted to 160 million tons. Although researchers have studied the comprehensive utilization of ashes for decades and have found many ways to use these solid wastes, it is a problem that there are still a lot of ashes left which cannot be used and must be put into landfills, which require large area of land and may pollute the surroundings. The difficulty is that the amount of ashes generated in thermal power plants is too large to be completely utilized. In China, for example, the ashes that can be utilized are less than 60 percent.Rotary kilns, whose disadvantage is the low heating rate of raw materials, are generally used in cement clinker calcination. The development direction of cement clinker calcinations is to calcine at about 1300C with raw materials in suspended or fluidized state. In this way, raw materials can finish heating, decomposing and calcining nearly at the same time, for the hating rate of particles is very high. It is well known that suspended-bed and fluidized-bed combustion are widely applied in the boilers of power plants. Hence, in this dissertation the idea is put forward to calcine cement clinker in PCC(Pulverized Coal Combustion) boilers, so that electric power and cement can be produced together.To testify the above idea, a serial of groping experiments were done on a two-stage polyphase reaction setup, which was designed and put up by myself. In the experiments, the experimental setup worked at 1300'C, and the material particles could stay at high temperature for about 7s. Through the experiments, it is found that some cement minerals, such as dicalcium silicate(2CaO SiO2) and calcium sulfoaluminate(3CaO 3Al2O3 CaSO4), etc, can generate when coal and lime are grinded together and burnt in suspended state. The best range of added CaO is about 63-68, expressed in the form of KSt. The experiments with CaCO3 replacing CaO were also done in the dissertation, but the results were not exciting because of the high amount of free CaO. In all the experiments, tricalcium cilicate(3CaO SiO2) had not been found all through. Then it can be concluded that co-generating cement clinker in PCC boilers is feasible, yet the product may not be the common silicate cement clinker.This dissertation roundly studied the effect of calcination temperature, calcination time, material coherent condition, material grinding process, and coal component on the formation of cement clinker minerals while calcining in suspended state. To form cement clinker minerals, the temperature above 1200C is enough. For the Changguang coal selected in the experiments, the best range is between 1250~1350C. With higher temperature, usually, more 2CaO SiO2 and less 2CaO Al2O3 SiO2 can be generated. While the calcination time of materials varies between l~7s, the product performancechanges little. However, when the material coherent condition is improved, the material particles can get more chances of collision, so the solid phase reactions can proceed more deeply and the product quality can be better. In addition, by grinding together, the coal and CaO can form agglomeration particles, which can promote the solid-solid reactions. Comparing the experiment result of Yanzhou coal with that of Changguang coal, it is found that coals with high sulfur are more suitable to form cement-like product. Through all the experiments, the practice base has been founded for larger-scale experiments and production on real PCC boilers.The sample, calcined after grinding Changguang coal and CaO together, was quantitatively measured by X-ray diffraction (XRD) analysis. The result shows that in the sample about 75 percent are hydraulic minerals, most of which are '-2CaO S1O2 and -2CaO SiO2, the left is 3CaO 3A12O3 CaSO4, and that the other 25 percent, mainly 2CaO Al2O3 SiO2, hav...