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Study of Microwave heating process of Coal

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Introduction

In conventional heating process, heat is transferred through thermal conduction and convection from the heating source to the targeted objects. The heating effect is heterogenous and highly dependent on the material properties (Thermal conductivity, specific heat capacity, mass density…), temperature difference across the material and on convection flows. Therefore, the heating rate is often slow since it is transferred through the other mediums before reaching the internal sample and then, a large part of energy will be lost.

In microwave heating process, exposing the targeted sample to electromagnetic radiation in the microwave frequency range, allows its dielectric material property to convert this electromagnetic energy to heat energy. The capacity of each material to absorb microwave power is directly related to the penetration degree of microwaves in it. When absorption occurs, the conversion of EM energy into heat depends on the dielectric loss of the heated material. The heat is dissipated inside the irradiated sample and transfers from the medium to the outside.

Microwave heating has been implemented in various industrial applications because of its advantages: volumetric heating, low processing time and the ability of heating in hazardous environments using a remote source via waveguide energy transfer.

In this study, the numerical coupled electromagnetic and heat transfer equations were successfully solved using HFWorks to investigate the microwave heating process of coal.

Problem description

In recent decades, the microwave oven has not only become an essential heating technology in most kitchens, but also has the potential for various industrial heating applications. MW ovens for domestic and laboratory applications often use working at 2.45 GHz frequency to allow deeper penetration of microwaves into materials and more uniform heating behavior.

The studied model is made of a rectangular microwave oven filled with air and connected to a 2.45 GHz microwave source via a rectangular waveguide operating in the TE10 mode. Near the bottom of the oven, a glass plate is inserted with the cylindrical coal sample. Table 1 and Table 2 provide the global parameters of the model and the used material properties, respectively.  

In order to reduce the computational time, only half of the model was simulated by using a symmetrical geometry shown in Figure 1.

: a)- Schematic illustration and b)-3D-design of the MW heating oven

Figure 1 - a)- Schematic illustration and b)-3D-design of the MW heating oven
 
Table 1 - Dimensions of the studied model
Part Dimension (mm)
Coal sample Radius: 25      Height: 60
Glass plate   Radius: 113.5 Height: 6
Microwave oven Width:267 Depth:270 Height:188
Waveguide Width:50 Depth:78 Height:18
 
Table 2 - Material properties
Material Relative permittivity Dielectric loss tangent Thermal conductivity (W/m. K)
Coal 2.86 0.17 0.189
Glass 2.5 0 0.23
Air 1 0 0.024

Electromagnetic boundary conditions

Wave port: The wave port boundary is applied to the waveguide entry port with transverse electric wave (TE) and fed with a 500-Watt microwave power at 2450 MHz
Imperfect Electric conductor: an IEC boundary condition is applied to the outer oven and waveguide walls using copper metal.
Perfect Magnetic Conductor Symmetry: PMC-Symmetry is used to define the vertical symmetry boundaries of the model.

PMC-Sym Boundary

Figure 2 - PMC-Sym Boundary

Mesh

The meshed model

Figure 3 - The meshed model

Results

The obtained results show that the absorption of microwave excitation power by the coal sample induces a significant redistribution of the electric and magnetic field in the oven air cavity. They form heterogenous electromagnetic distribution with low and high energy zones. Like shown by the next figure at 2.45GHz.

a)- Electric and b)- Magnetic field distribution for P_in=500 Watt

Figure 4 - a)- Electric and b)- Magnetic field distribution for P subscript i n end subscript equals 500 space w a t t
 

After being microwave irradiated, the coal gradually heats up due to the generated losses. The dielectric losses are calculated and illustrated in the figure 5-a: they are negligble in the Glass plate and surrounding air and highest in the center of the heated sample. The total dielectric losses inside the coal are arroud 321 Watt  (64.2% of the excitation power).

The conductor losses induced by the metallic surrounding walls are shown by Fig5.b. They are insignificant compared to dielectric losses and achive only 75 W/m²  for 500 Watt excitation power.

Distribution of a)- Volume and b)- Surface loss densities with P_in=500 Watt

Figure 5 - Distribution of a)- Volume and b)- Surface loss densities with P subscript i n end subscript equals 500 space w a t t
 

A steady state thermal analysis is performed to analyze the thermal behavior of the MW heating process. A convection boundary condition is applied to the surrounding air cavity at an ambient temperature of 25°C and a convection coefficient set to 10 W/m² .C

Only the sample is heated to the maximum temperature of 1.31 E+4 °C which confirms the selective heating characteristic of MW heating.

(a-(b- Temperature distribution at 2.45 GHz and c)-2D plot of temperature behavior inside the coal sample.

Figure 6 - (a-(b- Temperature distribution at 2.45 GHz and c)-2D plot of temperature behavior inside the coal sample.

Conclusion

Microwave heating via cavity has gained wide popularity in many domestic and industrial applications as a secure and selective heating tool. The numerical coupling between electromagnetic and heat transfer analysis allowed to perform the MW power transfer, dielectric loss and thermal behavior of the studied process. HFWorks could be used to investigate thermal stress aspects in a future extended works.

References

[1]. Huang, Jinxin, et al. "Simulation of microwave’s heating effect on coal seam permeability enhancement." International Journal of Mining Science and Technology 29.5 (2019): 785-789.