Lv Wei 1, Li Yan Dong 1, Li Rui Yang 2Liu Jianhua 3Shao Haijiang3
(1. School of Mechanical and Power Engineering, Harbin University of Science and Technology, Harbin 150080, Heilongjiang, China; 2. School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, Heilongjiang; 3. Lanxi Thermoelectric Co., Ltd., Lanxi 321100,
Abstract: According to the gas-solid two-phase flow theory, the mathematical model of heat and mass transfer of biomass fuel particles during drying in a horizontal straight tube was established by analyzing the characteristics of the drying process of biomass fuel particles. The numerical method was used to solve the model, Experiments were carried out to verify the effects of initial moisture content, inlet air temperature and feed volume on biomass drying were tested and analyzed respectively.
0 Preface
Biomass energy is second only to coal, oil, natural gas, the fourth largest energy source, accounting for about 14% of global total energy consumption[1]Biomass energy accounts for about 33% of primary energy in our country, the second largest energy source next to coal[2].
Development and utilization of biomass crop straws is an important way to utilize renewable energy, but the freshly harvested biomass straws have high water content, easily lead to their storage deterioration and difficult to catch fire and stabilize combustion in the direct combustion of the stratified combustion boiler. These problems can be effectively solved by injecting a certain proportion of the dried biomass fuels which have been grinded in the upper part of the grate, and the problems of biomass drying are gradually exposed in its large-scale utilization and have begun to become the major factors that affect the large-scale combustion and utilization of crop straws A serious problem.To achieve a large-scale stratified combustion and utilization of stalks with high water content, it is of practical significance to carry out effective drying studies on some of the biomass straw particles that have been pulverized (crushed and mostly granular).
Airflow drying is an efficient, continuous fluidization drying method, which is characterized by: ① short drying time; ② gas-solid two-phase contact is full, conducive to heat and mass transfer; ③ simple equipment, wide adaptability [3- 5]In this paper, the mathematical model of biomass fuel drying process based on gas - solid two - phase flow theory is used to simulate the airflow drying process of wet biomass fuel, and the conclusions are obtained through experimental verification and comparison. It provides reference for the design and performance analysis of airflow dryer and provides valuable guidance for the actual drying process of biomass power generation encouraged by the state.
1 model assumptions
Based on the characteristics of air-drying, in order to facilitate mathematical calculations, air-drying to make some reasonable assumptions[6- 8]:
1) the material is isotropic uniform sphere;
2) the initial temperature and moisture content of the material is evenly distributed;
3) Volume shrinkage of material during the drying process is negligible;
4) The moisture diffuses from the inside of the material to the surface, and the evaporation takes place only on the surface.
5) convection heat exchange between the hot air and the surface of the material, and then to the heat conduction inside the material;
6) Drying pipe insulation.
2 Mathematical model[9- 10]
3 model solution and experimental verification
Through computer programming, combined with physical properties of materials, air and water vapor characteristics of parameters, given the initial parameters under the conditions of the main program using the subroutine call (using ode45 function [11]) To solve the system of equations, get the biomass fuel moisture content, air moisture content and other parameters with the tube length change numerical solution, and draw the relevant curve.
As can be seen from Figure 2, there is some deviation between the experimental value and the calculated value, but basically coincide. The reasons for the deviation are as follows: ① The model is based on the above assumptions; ② The measurement results of the biomass fuel moisture content are affected by the materials taken, measurement methods, etc. Therefore, the numerical simulation will show some differences, so the simulation of drying can reflect the development trend of the airflow drying process and the real situation that the particle humidity changes in the drying tube.
4 results analysis
As can be seen in Figure 3, the airflow drying process can be broadly divided into two stages, where the humidity of the material and the air varies greatly during the initial stage of the drying process due to the relative velocity and temperature difference between the solid particles and the gas flow rate Large and broken materials can be better dispersed in the air flow, the entire surface area of the material can be used as an effective drying area. At the same time as the air flow on the material dispersion and stirring effect, so that the evaporation surface constantly updated .Therefore, the solid particles and hot air In this stage, the drying process has been carried out quite fully, so at this stage the temperature of the solid particles and the hot air temperature decreases with the length of the drying tube to reduce its value Then the drying process goes into relaxation period. With the increase of solid particle velocity and temperature and the decrease of hot air temperature and velocity, the driving force of heat and mass transfer between hot air and solid particles decreases. To ease.
5 factors affecting the drying analysis
5.1 initial moisture content of the drying effect
The air drying at a temperature of 140 ° C and a feed rate of 1.5 kg / min was carried out at different initial water contents of 56% ~ 43%, and the results are shown in Fig. 4. It can be seen that the drying is mainly concentrated in the front section of the drying pipe 1 ~ 2m stage drying faster, after drying more slowly in the case of the same length of the drying tube can be seen through the comparison, the higher the initial moisture content to achieve the more difficult to dry the actual drying operation, you can pre-drying the desired material in Does not affect the use of the case of the necessary drying, in order to achieve the desired drying effect faster.
5.2 air temperature on the drying effect
In the test bed feed rate of 1.5kg / min, the initial water content of 50% under the conditions of the air at different inlet temperatures (100 ~ 150 ℃) biomass straw fuel gas stream drying test, the test results shown in Figure 5. By the curve It can be seen that with the increase of air temperature, the drying rate will also increase and the drying effect will be better.This is because the increase of air temperature increases the temperature difference between gas and solid phases, and the heat and mass transfer between the two Exacerbated with the increase of air temperature, biomass straw particles surface moisture evaporation speed, the internal moisture diffusion speed also accelerated, the final result is the drying rate .This shows that increasing the air inlet temperature is conducive to the progress of drying.
5.3 wet material mass flow on the drying effect
Drying temperature of 140 ℃, the initial moisture content of the material was 50%, changing the mass flow rate of the material under test conditions.The test results shown in Figure 6. Wet material mass flow rate change is the drying process of the gas-solid two-phase As can be seen from the figure, as the material flow rate decreases, ie, the gas-solid ratio increases, the particle drying rate along the length of the drying tube increases, the moisture content of the particles through the dryer outlet significantly reduced. Decreasing the flow rate means more hot air is used as a medium to remove moisture from the material and the amount of water vapor that can be contained in the air is correspondingly increased.However, an excessively high gas / solid ratio results in an excessively high exhaust gas temperature, The heat energy of the air can not be effectively used, resulting in energy waste.And, a higher gas-solid ratio, can also lead to air flow and particles were dried too fast, making the particles in the drying tube shorter residence time, can also cause heat can not be sufficient Be used; so the appropriate gas-solid ratio, is conducive to improving the drying effect.
6 Conclusion
Through the drying test, the simulation results are in good agreement with the experimental results, and the dry mathematical model is correct. The model can simulate and predict the heat and mass transfer of the whole drying process well, and carry out experimental research and analysis on the factors affecting the drying. Power generation in the actual drying problems provide a valuable reference.
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