Experiment And Numerical Simulation Of Biomass Combustion And Heat Transfer Enhancement

Compared with coal, oil and other traditional fossil energy, biomass energy is not only renewable clean energy but also great potential for development. With the development of the material compression molding technology and combustion technology, biomass energy pays people’s increasing attention. Particularly in the vast rural areas, as the main source of ordinary energy, biomass energy plays an important role. Due to the formation of different conditions and processes, the composition and structure of biomass and coal is different, which determines the combustion equipment is different, especially in the structure design and the air supply. Currently the mainly application of biomass is direct combustion, there are many problems in the combustion process, which the most serious problem is heating surface deposition, not only hinder the heat transfer process, but also have a serious impact on boiler efficiency.In this paper I design a furnace on the basis of gasification combustion. The fuel goes into the combustion chamber top-down. The air is forced into chamber from the top. The new fuel subject to the burning fuel layer, being preheated, dried, then igniting. Firstly, I test the industry and elemental analyzes and calorific value of corn straw, cotton stalk and wood pellet fuel. Secondly, I use thermocouples measure the temperature of grate and furance, and flue gas analyzer measure the CO concentration of the flue gas. I research the effect of grate area heat load, furnace volume heat load, moisture content and excess air ratio on the combustion process and chose the suitable grate area heat load, furnace volume heat load and excess air ratio.On the gasification burner, I research the effect of ash deposition on the heat transfer process, the result shows that the fouling resistance of deposition leads to deterioration of the heat transfer, which makes the heat transfer coefficient significantly reduced. Take the dirt growth rate of the heat transfer surface as a indicator, analysis the factors which affect the deposition, including fuel type, particle size and flue gas velocity. Then I summarize the effect of ash deposition on the heat transfer.For the problem of ash deposition and heat exchange shortage, I use the rotating-twisted-tape to enhance heat transfer. A three-dimensional fluid flow model of rotating twisted tape is presented by CFD technology. I analysis the temperature field and velocity field, the results show that the fluid in twisted tape tube is three-dimensional spiral flow. In the same cross section, the axial velocity, the tangential velocity and the radial velocity are higher than the plane tube; The twisted tape reduces the average temperature of the fluid section, and strengthen heat transfer efficient. According to the simulation results, I analysis the relationship between heat transfer coefficient, drag coefficient and Reynolds number, Taking the flow and heat transfer performance as a indicator, select the best structure size of the twisted tape.Finally1research the heat transfer process of rotating twisted-tape-insert tube, the result shows that twisted tape increases the heat transfer coefficient by35.9%. The rotating twisted tape has the ability to remove dirt on the heat transfer surface, after cleaning the heat transfer coefficient increased by approximately20%, which reduces ash deposition on the heat transfer surface and improve the utilization of thermal energy.