Study Of Raw Meal Decomposition, Pulverized Coal Combustion And NO_X Control For Cement Precalciner System

Now, the precalciner product technology has been dominant in cement plants with rapid development of cement industry. However, there exist some questions which need to be solved and improved in the precalciner product technology. The characteristics of raw meal decomposition and pulverized coal combustion can provide important parameters for the design, operation and development of precalciner. At the same time, developing low NOX technology and low NOX equipment have become the inexorable trend with strict NOX emission standards. Therefore, it is significant to study the kinetics of raw meal decomposition and pulverized coal combustion as well as NOX formation mechanism and control technology for cement precalciner product system.Advance in kinetics of raw meal decomposition and pulverized coal combustion as well as NOX formation mechanism and control technology in the world was analyzed in this dissertation. It showed that how to use simple and effective models to predict the conversion ratios of raw meal decomposition and pulverized coal combustion was still an important question in cement industry. In addition, it also showed that a lot of foreign researchers paid more attention to the NOX control technology but less on mechanisms of NOX formation, especially for cement rotary kiln. Therefore, kinetics of raw meal decomposition and pulverized coal combustion as well as NOX formation mechanism and control technology for cement precalciner product system were further studied in current work with combination methods of theoretical analysis, simulated experiments and CFD simulation. This dissertation has carried on researches on the several following respects mainly:Thermal decomposition of several kinds of raw meals and limestone were investigated under pure N2 atmosphere with thermo-gravimetre(TG). The data processing method of TG experiments was improved. Moreover, it was proved that the data processing method of three methods combination was better for selecting the most probable mechanism. In terms of method improved, reactive mechanisms and kinetic parameters of thermal decomposition of cement raw meal and limestone were obtained. The results indicated the shrinking cylinder model with surface reaction rate controlling mechanism was the best model fitting the experiment data. Besides, it was also found E (activation energy) and A (pre-exponential factor) varied regularly for different mechanism functions, heating rates and sources.A suit of simulated experiment equipment, experimental procedure and data processing method were established in this dissertation. A high temperature gas-solid suspension reactor was used to simulate the decomposition of cement raw meal. Moreover, the reliable data have been obtained after the measured results were revised by transfer function. Various models were applied to the experiment results and it was found that the shrinking cylinder model with surface reaction rate controlling mechanism is the best model fitting the experiment data. According to the model, the kinetic parameters were obtained. At the same time, the decomposition kinetics of raw meals was investigated under various conditions (different temperature, pressure, atmosphere and particle size) in high temperature gas-solid suspension reactor. It was found that experiments in high temperature gas-solid suspension reactor can represent better the real decomposition process of raw meal in industrial precalciner.The characteristics and reaction kinetics of several kinds of coal used in cement industry were investigated by means of theoretic analysis and experiment study in high temperature gas-solid suspension reactor. The influences of reaction conditions (species, temperature and particle size) on characteristics of coal were studied in this paper. The best models of coal char combustion fitting the experiment data were obtained. The calculating method of couple kinetics of coal and raw meal was discussed theoretically in detail. In addition, interaction between coal char combustion and raw meal calcinations has been investigated by experiments with simultaneous injection. The results showed that the interaction existed obviously in certain coal and raw meal. Therefore, it is necessary to study deeply the interaction mechanism and kinetics between coal and raw meal.Based on theory analysis of low-NOx burner, a type of four-channel low-NOx burner used in the head of rotary kiln was introduced to decrease the emission of NO in cement industry. In order to analyze the effects of different mass flow rate of the four channels of low-NOx burner, according to geometric and flow similarity, a cold test was put into effect and the outlet flow fields of the four-channel low-NOx burner were measured with the PIV (Particle Image Velocimetry) technology. By the experiments, the appropriate air distribution schemes were presented for the type of four-channel low-NOx burner.The chemical and physical processes of clinker formation were analyzed in detail. On this basis, a heat flux function was introduced to take account of the thermal effect of clinker formation. Combining with models of gas-solid two-phase flow, heat and mass transfer, pulverized coal combustion and NO formation, a set of mathematical models for a full-scale cement rotary kiln were established. In terms of commercial CFD code (FLUENT), the distribution of gas velocity, gas temperature and gas components, especially NO in cement rotary kiln were obtained by numerical simulation of 3000t/d rotary kiln with four-channel burner. Besides, mechanisms of NO formation and reduction in cement rotary kiln were deeply analyzed in this dissertation. The results indicated that thermal-NO played an important role in NO formation. Fuel-NO was mainly formed in combustion zone, while thermal-NO in high temperature sintering zone. Furthermore, formation process of fuel-NO and thermal-NO interacted on each other. The means of NO reduction for cement rotary kiln should be focused on decreasing gas peak temperature properly and controlling the O2 concentration in sintering zone.