Research On Computer Simulation Methodology And Operation State Simulation Testing For Tunnel Kilns

Tunnel kiln is a kind of large nonstandard thermo-technical equipment that is widely used for product firing in many inorganic nonmetal industries. In order to increase the product quality and reduce the energy consumption it is necessary to make efforts on the research of firing schedule, structure design and control strategy. However, the on-spot test and the physical modeling experiment are limited to some extents because of the complicated structure and mechanism, excessive operating procedure parameters and bulky volume. In this paper, a gas-fired tunnel kiln with direct heating of sanitary ware is studied based on the computer simulation theory and methods. A lot of simulation is carried out for operation states of the tunnel kiln. The effects on the output parameters such as the smoke exhaustion capacity and the firing schedule, including the temperature and pressure in the kiln, of operating control variables are investigated.The Methodology of computer simulation. The main work includes choosing the theory and method of simulation, collecting and analysing the data and information, selecting the programming language or the general simulation system, modeling, programming, modifying models, and so on. There are three kinds of kiln system models. The structure model represents interrelations of subsystems. The general mathematic model describes general mathematic and logical relationships between the system (or a subsystem) and its descriptive variables. The simulation model is one accepted by a computer. The computer simulation system of tunnel kilns generally consists of subsystems, such as the database, the data analysis, the dynamic simulation and the CAD, etc. It can be used to simulate a tunnel kiln under real or ideal conditions to solve the existing problems so as to provide theories for the design and control of kilns.The operation state simulation testing. The relations among the elements of actual tunnel kiln system are analyzed. The structure model, the general mathematical model (including heat transfer calculation, gas flow resistance, the governing equations of gas flow and heat transfer) and the simulation model (including selection of general software, selection of calculation zone, control of mesh, calculation of resource term in fluid equations and the simplification treatment in firing setup) are constructed. Different simulations are arranged by the orthogonal experiment design method. The correlation of variables is analyzed and the model is amended. The benchmark operating point is determined by combining the field test data and the simulation results. Then, operation states of the tunnel kiln are simulated near the benchmark point. The regularity and the mechanism in the tunnel kiln, that is the influence of each control variables on the temperature, the pressure, the exhaust smoke capacity, etc, are investigated.The operating regularity. A operating state of a tunnel kiln is determined by 15 control variables that include the total exhaust smoke pressure, the hot air extraction pressure of the end cooling section, the hot air extraction pressure of the sharply-cooling section, the gas flow resistance of the article-loading channel zone, and so on. Thus, the operating output parameters such as the temperature and pressure in a kiln, the exhaust smoke capacity, the hot air extraction capacity and the air overflow capacity at the entry and exit can be expressed as functions of these 15operating variables. The system can be treated as linear near the benchmark point.The simulation result treatment. The computer simulation is based on the theoretical model because the production record and the physical model experiment data can not meet the requirements. The modeling information of a tunnel kiln can be extracted from the combination of simulation results, the production records and the physical model experimental data. An experiential model is thus derived which is used for the on-spot simulation and the real-time control.The normal operating state. The reasonable flow of gas is the basis of normal operation for a tunnel kiln and the system reaches the equilibrium point when there is no gas flow between the heating section and the cooling one. Under the normal operating condition, the equilibrium point should deflect a little from the heating section. In other words, there should be some cold air flowing from the fast-cooling section to the firing one. The equilibrium state will be broken and the firing system cannot maintain any more when the deflection of operating point reaches a critical value. The rate and extent of deterioration for the operating state can be reduced when the kiln doors is sealed.The pressure schedule in a tunnel kiln is determined by its structure configuration. The pressure curve is the external appearance of gas flow in a kiln. This parameter not only can reasonably represent the gas flow but also can be easily measured. Different structure configuration leads to different pressure curve. The characteristics of structure should be taken into account when the pressure schedule in a kiln is referred. The pressure difference at cross section is decided by the layout and the structure of ventilation setups, exhaust smoke holes and burners in a kiln. The static pressure and the kinetic pressure of gas play a more important role than the geometric pressure in a kiln ventilated by fan.The adjustment of temperature curve in a tunnel kiln. The reasonable flow of gas is the precondition of temperature adjustment in a kiln. As an accumulation consequence of heat transfer between the gas and the articles the variety of temperature in a kiln is a continuous process. The temperature difference of cross section in a tunnel kiln can be reduced by setting the flame channel in kiln cars, the mixing gas curtain at the preheat section and the distributed air ejection. It is necessary to take the reasonable flow of gas in a kiln into account when the temperature and the pressure are adjusted by using exhaust smoke holes.Other improvements. The performance of tunnel kiln can be improved further by following steps:Maintain a appropriate distance among setups whose technologic conditions are very different.Reduce the total pressure in a tunnel kiln by reducing the total exhaust smoke pressure, the cooling air pressure of sharply-cooling section and the cooling air pressure of last cooling section.Prevent the smoke from flowing backwards by adjusting the air ejection proportion or increasing the air ejection capacity of the first few air muzzles at sharply-cooling section.