Modeling of radiation heat transfer in the dense-bed flow of solid pyrolysis in indirectly heated rotary kilns

This work presents the further development and the validation of the Discrete Ordinates Model for thermal radiation which is implemented in OpenFOAM® for application to packed beds of biomass particles. This radiation model is an important part of a more comprehensive model which simulates the thermal conversion of discrete phase (here for instance wet biomass particles) which flows continuously inside an indirectly heated rotary kiln. The comprehensive Eulerian-Lagrangian model integrates three-dimensional, time-resolved simulation of the essential chemical and physical processes occurring within and in-between the moving bed of particles. This is realized by combining the particle collision models for non-reactive dense flows with models for heat transport, phase change and chemical reaction for multiphase reacting flow in the framework of OpenFOAM®. For the thermal treatment of solid particles, convection and radiation heat transfer methods couple the energy exchange between the reactor wall, gas- and disperse phase. The original implementation of the finite volume Discrete Ordinate Model (fvDOM) valid for a dilute particulate phase neglects the effect of local opacity due to the existence of individual particles. However, in the present application, a dense-packed bed of the particulate phase exists in the reactor. Therefore, in this study, this direction-based radiation model is adjusted for a computational cell with arbitrary particle volume fractions. To validate the results with the present thermal radiation model, first a simple test case with heating the bed of particles from the top of the domain is carried out. A second test relates to a laboratory-scale reactor. The results of the improved fvDOM are compared to the original implementation of OpenFOAM® and the more simple and computationally cheap P-1 radiation model. In general, the P-1 model largely overpredicts the radiative heat transfer while the original fvDOM underpredicts the heat flux by about 15% for the first test case. The improved model delivers results within 1% deviation at the expense of maximum 10% of the increase in the computational time

Development of an Openfoam Solver for Numerical Simulation of Carbonization of Biomasses in Rotary Kilns

Carbonization is a key process to increase the energy density of high moisture-containing biomasses and biogenic wastes and to provide multipurpose raw chemicals for further applications. Steam-assisted carbonization is a kind of slow pyrolysis, in which wet biomass is treated continuously in superheated steam at elevated temperature and atmospheric pressure. Rotary kiln reactors due to their flexibility and easy control of operating conditions are well suited for this process. In this work, a numerical simulation tool based on an Eulerian-Langrangian approach has been developed to simulate the carbonization of biomasses in rotary kiln reactors resolved in time and space by combining existing OpenFOAM features and developing new physical models. This study demonstrates the features of this extended and validated Eulerian-Lagrangian approach for simulating dense particulate multiphase flows in large-scale rotary kiln reactors. The focus is to use the new tool to aid the design of large-scale rotary kiln reactors by performing parameter studies. The simulations of this kind of large-scale reactors require large computational resources on supercomputers. Therefore, a further focus lies in different approaches to reduce the computational effort while keeping the accuracy at an acceptable level. By using the MP-PIC model, computing time increases linearly with the number of biomass particles instead of exponentially with the DPM model. The optimal cell size has been found to be about twice the largest particle diameter. By choosing the optimal domain decomposition method, simulation time can be reduced by a factor of 1/10. Introducing a solver frequency parameter to the DOM radiation model can help to reduce simulation times further by a factor of 1/8 while decreasing the accuracy by only 2%. Parallel scaling tests show good performance with over 1000 CPU cores. These results show that simulations with a total of 40,000 CPU-hours per studied case become feasible proving the developed solver to be an efficient tool for the design of rotary kiln reactors

Economic and environmental assessment of automotive plastic waste end‐of‐life options: Energy recovery versus chemical recycling

Most automotive plastic waste (APW) is landfilled or used in energy recovery as it is unsuitable for high-quality product mechanical recycling. Chemical recycling via pyrolysis offers a pathway toward closing the material loop by handling this heterogeneous waste and providing feedstock for producing virgin plastics. This study compares chemical recycling and energy recovery scenarios for APW regarding climate change impact and cumulative energy demand (CED), assessing potential environmental advantages. In addition, an economic assessment is conducted. In contrast to other studies, the assessments are based on pyrolysis experiments conducted with an actual waste fraction. Mass balances and product composition are reported. The experimental data is combined with literature data for up- and downstream processes for the assessment. Chemical recycling shows a lower net climate change impact (0.57 to 0.64 kg CO2e/kg waste input) and CED (3.38 to 4.41 MJ/kg waste input) than energy recovery (climate change impact: 1.17 to 1.25 kg CO2e/kg waste input; CED: 6.94 to 7.97 MJ/kg waste input), while energy recovery performs better economically (net processing cost of −0.05 to −0.02€/kg waste input) compared to chemical recycling (0.05 to 0.08€/kg waste input). However, chemical recycling keeps carbon in the material cycle contributing to a circular economy and reducing the dependence on fossil feedstocks. Therefore, an increasing circularity of APW through chemical recycling shows a conflict between economic and environmental objectives

An assessment of fluidized bed dynamics with CPFD simulations

The computational particle fluid dynamic (CPFD) method has been used to simulate a laboratory-scale fluidized bed, which has been designed for plastic pyrolysis. The simulations have been performed under cold-mode condition, where only the fluidization of sand particles is considered. The objective of the work is to gain an in-depth understanding of the hydrodynamic behavior of the fluidized bed, which is of particular importance with regard to an efficient mixing and heating of the bed materials as well as the final product yield. The focus of the work is assessing the dynamic behavior of the fluidized bed in terms of the total kinetic energy of all sand particles KS and the bubble frequency fB. For validation of the numerical approach, the calculated pressure drop Δp shows good agreement with measured data. In accordance with measurement and theoretical analysis, Δp increases with the bed inventory mS and remains nearly constant with the bulk gas flow velocity uG. It has been shown that KS increases with uG, which is due to the increased gas flow momentum flux with uG, leading to a reinforced gas-to-solid momentum exchange. The same behavior has been found for the influence of the sand particle mass mS on KS, where KS increases with mS. uG has been found to have a subordinate effect on fB, whereas fB decreases with mS. An increase in the gas temperature TG has led to a decreased KS, while the bed height hB and Δp remain nearly constant. This is due to the decreased density or momentum flux of the gas flow at higher TG. While up-scaling the fluidized bed, KS and fB have found to be strongly increased, whereas uG, Δp and hB were kept constant. The results reveal that it is not sufficient to use solely the general “static” parameters, i.e., hB and Δp, for characterizing hydrodynamic properties of a fluidized bed. In this case, KS and fB represent measures for the available kinetic energy and its fluctuation frequency of the whole fluidized bed system, which are more suitable for assessing the hydrodynamic behavior of the fluidized bed under up-scaled and elevated temperature conditions