Furnace Combustion Process Simulations And Experimental Investigations Of Ultra-Supercritical Boiler Of Wall Tangential-firing Combustion

Coal-fired boilers are usually designed according to the type of coal to be used. Boiler type, structure, burners and pulverizing system all depend on the coal combusted. In China, coal burned changes frequently and is of poor quality, so it is not possible to guarantee the type of coal entering power plants. As-fired coal changes accordingly. And because of some reasons to do with the combustion method of large scale utility boilers, it often causes some boiler-related safety, economic and environmental issues.Following the increasing capacity of utility boilers, the traditional corner tangential fired boiler has suffered from various problems. To solve these problems, the wall tangential fired boiler, i.e., where the burners are on the boilers walls, not in the corners, has been utilized. This solves some of the problems of the corner fired boiler, such as poor rigidity, slagging and thermal differentials. Researching wall type tangential fired boilers therefore has significant practical and theoretical engineering importance.Firstly, a ultra-supercritical boiler plant plans to use a mix of lean coal and bituminous coal as the design coal. Because the ignition and burnout characteristics of the two coals in the mixture differ, the combustion and slagging characteristics are possibly quite complex. Further work needs to be done to see if the mix ratio is optimal and to see if the ignition, stable combustion, burnout and slag prevention needs can be met.Therefore it is necessary to analyze the coal mixture and carry out combustion experiments. This is to provide data for optimal boiler design and safe and economic operations, i.e., to guarantee safe boiler operation and to satisfy strict environmental requirements. A thermo gravimetric analyzer (TGA) is used to analyze the ignition and stable combustion characteristics of the coal mixture used in the supercritical boiler. Analysis and evaluation is carried out on different levels on the ignition, stable combustion, burnout, slagging, fouling and viscosity characteristics of the coal mixture. Fuzzy mathematic method is used for predictive analysis of the best coal mixture ratio. Additionally, the combustion characteristics of the coal mixture provide preliminary suggestions for choice of boiler equipment parts and operating parameters. From the adaptive analysis of boilers to the coal type and the combustion characteristics of the coal mixture, the conclusion reached is that wall tangential firing is the best choice.Secondly, using one 600MW wall tangential fired supercritical boiler as an example, value simulation of the combustion process inside the furnace can be developed. Values simulated are particle movement, temperature, speed and thermal differentials inside the furnace and at the furnace outlet. The value simulation shows that the pulverized particles ignited easily, had good stable combustion characteristics, good radiation firmness, good penetration ability, low sulphur content in flyash, flow stream not easily diverted or form slag, tangential diameter slightly smaller, furnace thermal difference slightly smaller, thermal load in the furnace was uniform, and thermal differential at furnace outlet very small.Following on, this document next uses a similar 600MW unit corner tangential fired boiler as an example. It looked at furnace combustion simulated values of corner tangential fired boilers and compared them to those of wall fired tangential boilers. The results showed the corner fired tangential boiler imaginary circle diameter is larger than the wall fired boiler and the thermal differential inside the furnace is also larger. For the wall fired boiler, flue gas flow is more stable, temperature dispersion is more uniform, which lowers flue gas temperature differential at the furnace outlet, which is beneficial for safe operation. Wall fired tangential boilers allow for a large space at the burner outlets, so the air flow will not easily be affected by the water wall, and therefore helps prevent slagging on the water wall. The extent to which the coal/air mixture is affected by cooling from the water wall is less in a wall fired boiler than in a corner fired boiler. This improves ignition capability of the coal/air mixture and increase stability at low load. Simulated results and onsite testing results are similar. Results show wall tangential fired boilers have better safety and economic characteristics.Then, an optimizing test was carried out on the wall fired tangential boiler. Particle fineness was adjusted to within a reasonable scope based on actual operating situation of the pulverizers and boiler combustion status. Under loads of 600MW,450MW and 350MW, by adjusting flow of secondary air, operating oxygen content was changed. Boiler efficiency and auxiliary power consumption was also measured. This was to compare flue gas thermal heat loss, incomplete combustion particles in the flyash and auxiliary power factor under different oxygen conditions. Thus the best oxygen condition could be found. The optimal air/coal ratio under conditions of stable pulverizer outlet was found by adjusting primary air flow, measuring boiler efficiency and primary air fan power, as well observing combustion conditions around the burners. The optimal secondary air flow was found by adjusting the opening of each secondary air damper, which altered the ratio of excess air and each level of secondary air, and by measuring each part of boiler thermal losses. Results of this combustion experiment can be used as a guide for safe and economic operation of wall fired tangential boilers.Finally, from onsite measuring and safety tests, it could be seen that temperature distribution in the flue gas duct at the furnace outlet in the horizontal direction on the flue gas side was more even for wall fired tangential boiler than for corner fired boilers. Adjustments on the combustion side maintained thermal absorption differentials between the platens at a lower level. This not only lowered steam temperature differentials in the platens which can lower the corresponding temperature in the pipes, but also, because the thermal load of the flue gas dropped, the temperature of the pipe walls also dropped, leading to a double positive effect. In large scale boilers, wall fired tangential boilers have the value of replacing corner fired boilers. This report is helpful in providing guidance for promotion of wall fired tangential boilers.