Study on heat transfer between solid heat carrier and biomass particles

Li Zhihe, Cui Xibin, Bai Xueyuan, Yi Weiming, Li Yongjun

Abstract: For the heat transfer law between graduated material particles and ceramic ball solid heat carrier, the convective heat transfer characteristics between ceramic ball heat carrier and gas and the biomass and ceramic ball particles were studied by using the self-made bulk particle heat exchange test bench. The heat transfer characteristics between the two were experimentally studied. The convective heat transfer coefficients of single ceramic spheres and air were analyzed by analytical method and RMC correlation method, respectively, 291.3W/(m 2·°C) and 200.3W/(m 2· °C), the criterion equation for determining the heat transfer of the ceramic ball heat carrier and the biomass particle group is Nu c=176+0.079Re cAnd Nu c=22.97+0.2251Re b, provides a theoretical basis for the study of solid heat carrier heating biomass pyrolysis law.

introduction

The pyrolysis of biomass in the reactor is a complex physical and chemical process, which is affected by the flow of the particle phase, the heat and mass transfer between the gas-solid and multiphase, and the thermochemical reaction kinetics. Mass transfer, and heat and mass transfer affects the thermal cracking process. Therefore, to fully reveal the thermal cracking mechanism of biomass, it is necessary to flow from the particle phase, heat and mass transfer between gas-solid and multiphase, and chemical kinetics of biomass pyrolysis. Comprehensive research. At present, the study of heat transfer involving particles is mostly in fluidized bed and circulating fluidized bed reactors. [1~ 5]However, the research on the mechanism of biomass pyrolysis under high temperature solid heat carrier heating process is rare. Studying the heat transfer law is helpful to the optimization of the process and the scientific understanding of the pyrolysis mechanism.

In this paper, the convective heat transfer characteristics between heat carrier particles and gas (using air instead of pyrolysis gas products) and the heat transfer characteristics between biomass particles and ceramic ball heat carrier were studied by using a self-made bulk particle heat exchange test bench. According to the experimental data, the convective heat transfer coefficient between the single heat carrier particles and the air is analyzed and calculated by analytical method and dimensionless analysis method. The convective heat transfer between the ceramic ball heat carrier and the biomass particle group is analyzed and determined. The heat transfer criterion equation of particle group lays the foundation for the study of solid heat carrier heating biomass pyrolysis law.

1 test bench and materials

The structural schematic diagram of the bulk particle heat exchange test bench used in the study is shown in Figure 1. It mainly includes ceramic ball heat carrier, biomass powder particle feeder, down pipe, particle separation device, and computer temperature detection system. Its working principle is : Ceramic ball heat carrier heated to a preset temperature, quickly placed in the bin of the ceramic ball feeder and temperature controlled, fed from the screw feeder and fed from the heat carrier feeder The ceramic balls flow down the downcomer to cause heat exchange. The mixed particles are separated in the separation device at the lower end of the downcomer, and the ceramic balls and biomass powder fall into different aggregate boxes and use T-type thermocouples for temperature data. Acquisition. The gas temperature is extracted by the exhaust thermocouple at each sampling point of the downcomer.

The down pipe is made of PVC pipe with a length of 1600mm and an inner diameter of 110mm. In order to reduce the heat loss of the pipe wall, a layer of 25mm thick foam insulation material is attached to the inner wall of the pipe, and the outer wall is insulated by the aluminum silicate spray felt. A T-type armored thermocouple was placed at the 100, 400, 800, 1200 and 1500 mm points from the upper nozzle to measure the gas temperature inside the tube.

In the experiment, the ceramic ball is a regular sphere with a diameter of 2 mm, and the biomass is 60-80 mesh corn stover powder.

2 experimental results

The ceramic ball with a temperature of 90 °C has a mass flow rate of 1.0, 1.2, 1.4 kg/min for the convective heat transfer experiment between the ceramic ball heat carrier and the air, and the initial and final heat exchange between the air and the heat carrier in the down tube The temperature experimental data are shown in Table 1. Under the above heat carrier flow rate, the heat transfer between the heat carrier and the biomass particles was carried out at a mass ratio of ceramic balls to biomass of 15:1, 20:1, 25:1, respectively. The heat transfer experimental data of heat carrier particles and biomass particles are shown in Table 2.

3 analysis and discussion

Ceramic ball heat carrier, the heat transfer between the biomass particles and the air in the downcomer belongs to multiphase flow and heat transfer. There is a collision between the particles and the particles, the collision between the particles and the wall, and the convection between the particles and the air. Heat and the heat transfer of the tube wall to the radiation of the particles. The experimental study of particle flow PIV shows [6]: Except for the vicinity of the side wall of the pipeline, the collision probability between the two kinds of particles and the wall surface during the descending process is small, and the thermal conductivity of the thermal insulation material attached to the pipe wall is extremely small, and the collision heat transfer between the particles and the pipe wall can be neglected; The ceramic ball does not collide during the descent process, so it is not necessary to consider the heat transfer and heat transfer between the ceramic ball particles. The particle flow experiment finds that the ceramic ball and the biomass particles fill the entire pipe and block each other, so although there is radiation, However, the heat transfer cancels each other out, so the influence of radiative heat transfer can be ignored. MansooriZ also believes that in dense particle systems, the effect of radiative heat transfer is less than 600 °C, which can be ignored. [7]In this way, only the convective heat transfer of the ceramic ball and the air inside the tube needs to be considered in the analysis. The analysis of convective heat transfer is the analysis and solution of the convective heat transfer coefficient.

3.1 Convective heat transfer coefficient of single heat carrier particles

3.1.1 Analytical method

It is assumed that when the ceramic ball heat carrier exchanges heat with air in the downcomer, the heat released at time tc is all absorbed by the air, then

3.1.2 RMC Association Method

The RMC method was proposed by RanzWE and MarshallWR in 1952. [8], the equation is

Using the above two methods, the convective heat transfer coefficient between the ceramic ball heat carrier and the air calculated according to the experimental data is shown in Table 3. The analytical method is based on the temperature data of the ceramic ball heat carrier and air at the inlet and outlet. Therefore, it can be regarded as the average heat transfer coefficient in the entire downcomer. The RMC method is calculated according to the motion parameters and physical property parameters at the exit of the downcomer, which is the local heat transfer coefficient, so there is a big gap between the two.

3.2 Heat transfer analysis of particle groups

3.2.1 Thermal equilibrium analysis

There is a thermal equilibrium relationship between the ceramic ball, the biomass powder and the air as shown in Fig. 2. In the down tube, after the ceramic ball and the biomass powder exchange heat, the high temperature ceramic ball emits heat and the temperature decreases; the biomass powder absorbs heat. The temperature rises; the air inside the downcomer also absorbs heat and the temperature rises.

3.2.2 Heat transfer criterion equation of ceramic ball heat carrier particle group

The heat transfer in the particle system is very complicated. The heat transfer of the single particle cannot reflect the heat transfer law of the whole particle system. For example, due to the change of the particle motion state at different falling distances, the relative velocity between the particle phase and the gas phase is caused. At the same time, the particles at different spatial locations may affect heat and mass transfer due to temperature and movement speed. Therefore, the heat transfer coefficient and criterion equations analyzed by single particles are not suitable for heat transfer research of particle group systems, and must be utilized. The feature quantity of the particle group system parameters is used to calculate the criterion number. For the heat transfer of the particle group system, the equivalent diameter of the particle group and the characteristic velocity can be used for analysis and calculation.

According to the experimental data in Table 1 and the above-mentioned thermal equilibrium analysis method, the Nusselt number and Reynolds number of the ceramic ball heat carrier particle group under various working conditions are calculated as shown in Table 4.

According to Nuc and Re in Table 4 cThe value of the linear relationship obtained by linear regression using the data processing software Oringin810 is shown in Fig. 3. As seen from Fig. 3, Nu cAnd Re cThe number has a good linear relationship.

3.2.3 Heat transfer criterion equation of biomass particle group

For the treatment of characteristic parameters of biomass particle group, see the literature '9'. According to the characteristic parameters of the biomass particle group, the experimental data in Table 2 and the above analysis method calculate the equivalent diameter of the biomass particle group, the heat transfer coefficient and the criterion equation analysis. The values ​​of the relevant parameters used in it are shown in Table 5.

Nu calculated according to the data in Table 5 cWith Re bAs shown in Table 6.

According to Nu in Table 6 cAnd Re bValue, linear regression yields Nu cAnd Re bThe relationship is shown in Figure 4. As seen in Figure 4, Nu cAnd Re bThe number has a good linear relationship.

4 Conclusion

On the experimental platform of the falling tube bulk particle heat transfer, the convective heat transfer experiment of ceramic ball and air (instead of pyrolysis gas) was carried out with the mass flow rate of ceramic balls of 1.0, 1.2, 1.4 kg/min. The multi-phase heat transfer experiments of ceramic ball heat carrier, biomass powder and air were carried out with mass ratio of ceramic ball to biomass powder of 15:1, 20:1, 25:13 respectively. Analytical method and correlation The convective heat transfer coefficient of single ceramic ball particles and air was analyzed by the method, which was 291.3W/(m 2·°C) and 200.3W/(m 2·°C). The heat balance method is used to analyze the non-dimensional heat transfer criterion equations of ceramic ball heat carrier particle group and biomass particle group respectively. c=176+0.079Re cAnd Nu c=22.97+0.2251Re b, provides the theoretical basis of heat transfer for the study of biomass pyrolysis law.

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