Development of DEM–CFD Simulation of Combustion Flow in ...

Research Paper

Journal of Chemical Engineering of Japan, Vol. 49, No. 5, pp. 425–434, 2016

Development of DEM–CFD Simulation of Combustion Flow in Incinerator with the Representative Particle Model Kenya Kuwagi 1, Toshihiro Takami 1, Azri Bin Alias2, Degang Rong3, Hiroshi Takeda3, Shinichiro Yanase4, Toshinori Kouchi 4, Toru Hyakutake5, Kaoru Yokoyama6, Yoshiyuki Ohara6, Nobuo Takahashi 6 and Noritake Sugitsue6 Department of Mechanical Systems Engineering, Okayama University of Science, 1-1 Ridai-cho, Kita-ku, Okayama-shi, Okayama 700-0005, Japan 2 Faculty of Mechanical Engineering, University Malaysia Pahang, Pekan Campus, 26600 Pahang, Malaysia 3 R-flow Corporation Ltd., Takanashi Building, 1-10-45 Takasago, Soka-shi, Saitama 340-0015, Japan 4 Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-Naka, Kita-ku, Okayama-shi, Okayama 700-8530, Japan 5 Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama-shi, Kanagawa 240-8501, Japan 6 Japan Atomic Energy Agency, 1550, Kagamino-cho, Tomata-gun, Okayama 708-0698, Japan 1

Keywords: Incinerator, DEM–CFD Simulation, Combustion, Waste A simulation code based on the discrete element method (DEM) and computational fluid dynamics (CFD) coupling model was developed to simulate the behavior of radioactive cesium in waste incinerators. The waste lump was represented by particles in the simulation. The energy equation for a mixed gas, diffusion equation for each gas component, as well as the energy, drying, pyrolysis, and combustion equations for each particle were solved in the simulation by adding a combustion model to the standard DEM–CFD coupling model. The particle size of the waste changed as drying, pyrolysis, and combustion progressed. At the end of the combustion process, particle waste became ash, and the number of ash particles was enormous. To avoid an excessive computational load due to the high particle number, a similar assembly model was adopted to reduce the particle number in the calculation. There was a good agreement between the simulation and experimental results for the temperature at the outlet of the furnace and the flue gas composition.

Introduction After the Fukushima Daiichi Nuclear Power Plant accident, radioactive Cs contaminated the environment in a region of eastern Japan. Waste is currently incinerated at general incineration facilities, and as a result, the disposal of waste has become a serious problem. In order to proceed with such an incineration treatment, the behavior of Cs in an incinerator must be understood. Most Cs is volatilized and gasified at the high temperature in an incinerator (>800°C) and then is clustered due to cooling in the region of an economizer ( 0.8

where Z is the combustion degree for a particle, dB1 is the diameter of a representative particle, CO2 is the concentration of oxygen around a particle, rmp0_char_dry is the mass ratio of char to the initial dry solid content in a particle, rmp0_H2O is the initial water content, Kc is the rate of the chemical reaction, and Kd is the mass transfer rate. The rate of the chemical reaction, Kc, for char is given by the following equation (Field et al., 1967):  − Echar  K c = Achar ⋅ exp    RTp 

(21)

where Achar and Echar are the frequency factor and activation energy of the char surface reaction, respectively, for the heterogeneous combustion. 428

The mass transfer rate of oxygen, Kd, is expressed by the following equations: 0.5 Sh = 2 +0.6 ⋅ Sc 0.33Repg

(22)

Kd =

Sh ⋅ DO2 dB2

(23)

Sc =

μ ρDO2

(24)

Repg =

upg d ⋅ρ μ B2

(25)

where upg is the relative velocity of a particle to the gas, and DO2 is the diffusivity of oxygen in the air. The rates of carbon monoxide generation and oxygen gas consumption by the char combustion reaction on the surface of a single particle are given by the following equations: dg char_CO 28 dZ (26) = rmp0_char_dry ⋅ (1 − rmp0_H2O ) ⋅ mp0 ⋅ dt 12 dt dg char_O2 16 dg char_CO =− dt 28 dt

(27)

where gchar_CO and gchar_O2 are the masses of carbon monoxide gas and oxygen gas, respectively, generated by the combustion reaction on the surface. (4) Heat generation by the surface reaction and phase change The heat source term, i.e., the rate of heat generation, is given by the following equation: Qgen = mp0 ⋅ rmp0_H O ⋅ Qev ⋅ 2



dX + mp0 ⋅ (1 − rmp0_H O ) dt

× rmp0_py_dry ⋅ H py ⋅



2

dY dZ  + rmp0_char_dry ⋅ H char ⋅ dt dt 

(28)

where Hpy is the latent heat of the pyrolysis, and Hchar is the heat produced by the char surface combustion reaction. 1.3.5 Change of mass and diameter of the particle Considering the drying, pyrolysis, and combustion processes, the mass of a single particle is given by the following equation: mp = mp0[(1 − X )rmp0_H O +{(1 − Y )rmp0_py_dry 2

+(1 − Z )rmp0 _char_dry + rmp0_ash_dry }(1 − rmp0_H O )] 2

(29)

where rmp0_ash_dry is the mass ratio of the ash to the initial dry solid content. In this simulation, the changes of dB1 and dB2 during combustion were considered. As the combustion process progressed, the diameter, dB1, shrank according to the following equation: 1/3

 6mp  dB1 =    πρp 

(30)

where ρp is the density of the particle. At each of the three stages, i.e., the drying process (X