Organic solid waste (OSW) contains many renewable materials. The pyrolysis and gasification of OSW can realize resource utilization, and its products can be used for methanation reaction to produce synthetic natural gas in the specific reactor. In order to understand the dynamic characteristics of the reactor, a three-dimensional numerical model has been established by the method of Computational Fluid Dynamics (CFD). Along the height of the reactor, the particle distribution in the bed becomes thinner and the mean solid volume fraction decreases from 4.18% to 0.37%. Meanwhile, the pressure fluctuation range decreased from 398.76 Pa at the entrance to a much lower value of 74.47 Pa at the exit. In this simulation, three parameters of gas inlet velocity, operating temperature and solid particle diameter are changed to explore their influences on gas-solid multiphase flow. The results show that gas velocity has a great influence on particle distribution. When the gas inlet velocity decreases from 6.51 to 1.98 m/s, the minimum height that particles can reach decreases from 169 to 100 mm. Additionally, as the operating temperature increases, the particle holdup inside the reactor changes from 0.843% to 0.700%. This indicates that the particle residence time reduces, which is not conducive to the follow-up reaction. Moreover, with the increase of particle size, the fluctuation range of the pressure at the bottom of the reactor increases, and its standard deviation increases from 55.34 to 1266.37 Pa.

With the rapid development of the economy, people’s living standards have improved significantly. However, the amount of solid waste in the process of life and production is also increasing. It is estimated that by the end of the 21st Century, the daily output of global solid waste will exceed 11 million tons. Among them, the proportion of organic solid waste can reach 46% [

There are many ways to deal with the OSW, including traditional methods such as sanitary landfill, incineration and composting. Some researches have also proposed some new technologies to treat the OSW and produce renewable materials and high-value-added chemicals. For example, the OSW can be converted into inert materials to make insulating plates [

The demand for natural gas in the Asian market continues to grow strongly, which will inevitably require a large increase in imports [

Compared with the fixed bed reactor, the fluidized bed reactor can strengthen the catalytic reaction, mass transfer and heat transfer, bringing about higher CH_{4} output and lower bed temperature [_{2}/CO ratio of inlet gas, operating pressure and temperature on fluidization quality and CO conversion were studied [_{2}O generated in the methanation process, in order to shift the balance to the direction of methane generation. Liu et al. [

During experimental researches, it is time-consuming and expensive to establish a reaction device composed of reaction systems, measurement systems and control systems. Moreover, the addition of temperature and pressure measuring devices is likely to destroy the flow field in the reactor, which makes it difficult to observe the multiphase flow characteristics. The details of temperature and concentration distribution in the reactor can be obtained by the Computational Fluid Dynamics (CFD) simulation, which is conducive to process simplification, optimization and amplification [_{4} [

In order to facilitate the exploration of complex gas-solid flow and mass transfer mechanisms in the fluidized bed reactor, CFD technology has been used in this study. The spatiotemporal statistical results of particle distribution and pressure change under different operating conditions have been obtained. The main contributions of this simulation are (a) the successful construction of a comprehensive model of a three-dimensional fluidized bed reactor to simulate the gas-solid multiphase flow behaviors during the methanation process, (b) in-depth investigation into the hydrodynamic Mechanism between pyrolysis and gasification syngas and catalyst particles, and (c) insight into the influences of important operating variables on the internal multiphase characteristics of the methanation reactor.

The calculation area is divided into several parts. The regular cylindrical area adopts the hexahedral grid (such as the vertical reactor furnace and outlet of gas-solid mixed phase), and the tetrahedral grid is generated in the irregular area (such as the connecting part between the solid inlet and reactor). Using the combination of the two grid division methods cannot only simplify the division process, but also improve the grid quality. Considering the requirements of grid accuracy and subsequent calculation speed, the calculation domain of 670733 elements is finally determined for this simulation. The grid division of some computing domains is shown in

The continuity equations are as follows [

gas phase:

solid phase:

where

The momentum equations for the gas phase and solid phase are given by [

gas phase:

where

For different solid volume fraction distribution,

where,

And

In

In momentum equations, the expression of gas stress tensor

For the solid phase, the expression of its stress tensor

where

where

and

where

In

The objective of this simulation is to investigate the flow characteristics of the gas phase and solid phase in the reactor, so the Eulerian-Eulerian multiphase model is used to simulate the gas-solid two-phase flow in the reactor. Other models used in the simulation process are shown in

Description | Model | Reference |
---|---|---|

Turbulence model | Standard |
[ |

Turbulence multiphase model | Dispersed | [ |

Granular viscosity | Gidaspow | [ |

Granular bulk viscosity | Lun et al. | [ |

Drag coefficient | Gidaspow | [ |

In the process of simulation, the gas entering the reactor is a mixture of H_{2}, CO and N_{2}, with a mole fraction ratio of 6:2:2. The material of the solid inlet is granular with a density of 1000 kg/m^{3}. The velocity of the solid inlet is 0.04 m/s, the volume fraction is 0.30 and the packing limit is 0.63. Meanwhile, the gas with a velocity of 0.04 m/s is also set at the solid inlet to transport solid particles. Under the reference case, the inlet gas velocity is equal to 4.25 m/s, the particle diameter equals 0.04 mm and the operating temperature is 673 K. Different operating parameters have been set to study the flow behavior in the methanation reactor. The variables are inlet gas velocity, operating temperature and particle diameter, as presented in

Inlet gas velocity (m/s) | Operating temperature (K) | Particle diameter (mm) |
---|---|---|

1.99-3.12-4.25-5.38-6.51 | 673 | 0.04 |

4.25 | 473-573-673-773-873 | 0.04 |

4.25 | 673 | 0.04-0.06-0.08-0.10-0.12 |

Both the gas inlet and solid inlet are set as the velocity inlet, the mixture outlet is set as the pressure outlet, and the reactor wall boundary condition is set as the adiabatic and no slip wall [^{−3} s while the maximum number of iterations per step is set at 5.

Because the simulation finally reaches a quasi-stable operation where the physical quantity will fluctuate up and down within a certain range. Therefore, it is necessary to calculate the mean value to observe the distribution law of pressure, volume fraction, etc. The mean value is defined as follows:

where

The probability is defined as:

where

Grid independence verification is the basic requirement of CFD simulation, which requires the resolution of the divided grid to describe the details of the flow structure. If the quality of the grid is coarse, the local flow structure information will be lost during the particle transmission. On the contrary, the fine grid will considerably increase the time and cost of calculation. Under the cases of three different grid numbers shown in

Serial number | 1 | 2 | 3 |
---|---|---|---|

Number of grids | 495992 | 670733 | 1156295 |

The syngas enters from the gas inlet at the bottom of the reactor while the particles are continuously sent from the solid inlet which is at the side of the reactor 170 mm away from the bottom. In this study, the planes with different heights of the methanation reactor are selected as research objects.

It can be concluded from the curves in

The volume fractions of solid particles counted on the different vertical crosses are illustrated in

Under the quasi-stable condition, the gas velocity distribution at the cross surfaces of different heights is shown in

Once adding the solid particles, the pressure distribution of different inlet gas velocities is exhibited in

In order to observe the variations of solid particles in the reactor, the curves of particle volume fraction as revealed in

If the inlet gas velocity is too fast or too slow, the particle volume fraction in the reactor will fluctuate violently, and a large fluctuation means blockage. If the blockage is serious, and then the internal pressure fluctuates violently, the stable operation of the reactor will be affected. Compared with

In terms of the collective trend, the particle volume fraction increases linearly with the increase of time at first, then decreases for a very short time, and follows steady fluctuations. By comparing the time nodes of the first peak at different speeds, there are some interesting discoveries. Under the conditions of high inlet gas velocities, they reach a peak at the same time. While the gas velocities are slow, the time nodes to reach the peak postpone with the velocities decreasing. From the analysis of the volume fraction value corresponding to the extreme point under each gas velocity, the smaller the gas velocity, the larger the corresponding value. In other words, there are more solid particles in the reactor.

Through the above analysis, taking the operation stability of the device and the heat and mass transfer requirements of the reaction into account, it is considered that the gas velocity of 4.25 m/s is the most appropriate.

According to Bernoulli equation:

where

Based on the above analysis, we further consider the second factor. In order to explore the influences of operating temperature on the particle volume fraction in the reactor, the curves in

When the temperature rises from 473 to 873 K, the fluctuation range of particle volume fraction decreases gradually. Under the high temperature operating conditions, this trend is more obvious. A high operating temperature will keep the reactor operating under stable condition. By calculating the volume fraction of solid particles in the reactor, we find that the increase in temperature strictly reduces the values, which are 0.843%, 0.835%, 0.786%, 0.760% and 0.700%, respectively. The above observations indicate that with the increase in temperature, the ability of gas to carry solid particles will enhance. Thus, more solid particles will be carried away from the reactor, and hence the outflow process of particles will become smooth. Considering the opposite effects of viscosity and density on the gas carrying capacity, we can conclude that viscosity is the main factor relative to density. The larger the particle volume fraction in the reactor, the more solid particles and the greater the internal pressure will be. This changing trend is consistent with that shown in

Therefore, it can be concluded that the second factor plays the dominant role in affecting the pressure distribution. In the dilute phase, there are few particles, and the influence on pressure is also very weak, which will offset the influence caused by the change in gas density. Thus, the pressure distribution in this region is almost the same. A large number of particles gather in the dense phase, and its influence on the pressure distribution is far greater than that of the gas density. For this reason, the changing trend of

The effects of the particle diameter on the pressure and volume fraction are shown in

From the previous analysis, we can know that the change in particle volume fraction indicates internal blockage to a certain extent. Under working conditions of large particle diameters, the lower straight pipe is more likely to be blocked. In addition, the particle volume fraction in the reactor under the different quasi-stable conditions is 0.786%, 0.901%, 1.211%, 1.239% and 0.989%, respectively. In other words, the mean particle volume fraction in the reactor increases at the beginning and then decreases. Under conditions of small particle diameters, the reactor operates smoothly so that the mean particle volume fractions can effectively reflect the residence time of particles. When the values of particle volume fraction are high, the CO methanation reaction is very thorough and the CH_{4} selectivity is very high as well [

The above analysis will draw some useful conclusions. Choosing particles with large diameters will lead to two hidden dangers. On the one hand, the solid back-flow seems to occur, which easily blocks the reactor and destroys its normal operation. On the other hand, the internal pressure fluctuation increases significantly, and sometimes negative pressure appears, which will further threaten the stability of the internal operation of the reactor. When choosing the smaller particles, they can be easy to gather together and take away from the reactor. Besides, there are a small amount of particles in the reactor, which is very unfavorable for the subsequent reaction. Considering the influences of the above factors, when the particle diameter is about 0.06 mm, it is the most suitable for the methanation reaction.

This work is conducive to achieving the full utilization of the OSW and converting it into green energy. Thus, a three-dimensional model of the fluidized bed methanation reactor is established. The comprehensive model exhibits a good prediction of the behaviors of multiphase flow under different working conditions. From the simulation results, the following conclusions can be drawn:

1) In the area above the solid inlet, the gas phase and the solid phase are fully mixed in the vertical direction. The fluctuation amplitudes of pressure and particle volume gradually decrease, from 398.76 to 74.47 Pa and from 0.08 to 0.01, alternatively. Disturbed by the solid particles, the variation range of gas velocity near the solid inlet is very wide while its range becomes narrower and narrower with the increase in height. This shows the full development of the multiphase flow.

2) The influences of the inlet gas velocity are complex in terms of the particle volume fraction, the fluctuation amplitude of pressure, the location of the blockage, and the residence time of particles. Under the conditions of high gas velocities, the turning at the upper part of the reactor is easy to be blocked, resulting in unstable operation. When the inlet gas velocity decreases, the minimum height that catalyst particles can reach decreases from 169 to 100 mm, which is easy to block the straight pipe at the lower part of the reactor.

3) The operating temperature changes the density and viscosity of the gas, which will affect the gas carrying capacity. And viscosity plays the dominant role. Under the conditions of high temperatures, the increase of the viscosity makes the particles evenly taken away from the reactor, and the particle volume fraction decreases to 0.007. On the other hand, at a lower temperature, the gas carrying capacity is too poor to keep stable operation, and hence the particle volume fraction in the reactor will become larger.

4) The particle size affects the back mixing and aggregation of particles. With the increase of particle diameter from 0.04 to 0.12 mm, the pressure fluctuation range increases from 250 to 20000 Pa. A particle diameter greater than 0.10 mm will lead to abnormal operation because of the serious back-mixing, which is unfavorable to the device. In addition, a decrease in the particle diameter contributes to the loss of particles in the reactor as a result of agglomeration.

Drag coefficient [-]

Diameter of particles [mm]

Gravity [m/s^{2}]

Radial distribution function [-]

Stress tensor invariant [-]

Pressure, particle volume fraction

Conductivity of granular temperature [W/(m·K)]

Mean value of physical quantity i, [Pa] or [m/s]

Instantaneous value of the physical quantity i, [Pa] or [m/s]

Number of values in the range of [a, b] [-]

Total number of values of parameters [-]

Pressure [Pa]

Probability [-]

Pressure of solid phase [Pa]

Reynolds number [-]

Time [s]

Temperature [K]

Gas velocity [m/s]

Solid velocity [m/s]

Volume fraction of gas phase [-]

Volume fraction of solid phase [-]

Momentum exchange coefficient between gas and solid [-]

Density of gas phase [kg/m^{3}]

Density of solid phase [kg/m^{3}]

Stress tensor of gas phase [Pa]

Stress tensor of solid phase [Pa]

Shear viscosity of gas phase [Pa·s]

Shear viscosity of solid phase [Pa·s]

Bulk viscosity of particles [Pa·s]

Collision dissipation of energy [kg/(m·s^{3})]

Interphase energy exchange [kg/(m·s^{3})]

Constant

Granular temperature [m^{2}/s^{2}]

This work was supported by the National Key Research and Development Program of China [Grant No. 2019YFC1906802].

The authors declare that they have no conflicts of interest to report regarding the present study.

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