# Guide Solid Fuels Combustion and Gasification: Modeling, Simulation, and Equipment Operation

Such a considerable amount of unburned fuel leads to a decrease in the gasification efficiency and requires measures for its further processing and utilization. There are technical difficulties hindering the widespread adoption of the technology, for example, 1 The number of operating parameters that have an effect on the gasifier operation exceeds This circumstance complicates optimization of its operating conditions. Fine gas cleaning makes the technology much more expensive. In this case, the tangential air supply is used, which provides good mixing of pyrolysis gases with oxidizer, and, consequently, a high speed and complete decomposition of tar.

Such an organization of the process makes it possible to gasify char not only by the entrained flow of incomplete oxidation products of pyrolysis gas obtained in the first stage but also by heated air blow in the process countercurrent with respect to the fuel motion.

A block diagram of the plant for multistage gasification. This paper presents the results of the research into the first stage of the multistage gasification, that is, allothermal wood pyrolysis in a screw reactor. In the last years, there has been continuous evolution of the calculation models of pyrolysis plants. The research presented in [ 17 ] shows a mathematical description of the pyrolysis process. The authors devised a calculation model for the plant designed by them. The model consists of a set of stages of drying, thermal decomposition, interphase transitions, and cooling.

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This method allowed the common model to be decomposed into submodels suitable for practical implementation that have different degrees of detail and respective form of equations ordinary differential or in partial differential. Change in the temperature of gas phase during its passing through the fuel bed was determined from the following equation:. Solving Eq. Moisture content in a fuel particle was determined by the authors [ 17 ] from the following expression:.

## solid fuels combustion and gasification - modeling simulation and equipment operation

Solving Eqs. The only assumption in their model was the absence of nonuniform distribution of parameters in the cross section. To verify their model, the authors of [ 17 ] made an experimental bench since in their case, as in many other studies, verification of the constructed mathematical models is based on the unique experimental data. This complicates a comparative analysis of the proposed approaches to the selection of the best description of the processes that occur in similar plants. A more thorough study on partial gasification, on the basis of a numerical modeling and application of thermogravimetrical analysis, is presented in the recent publications [ 18 , 19 ].

Thus, a good correspondence was achieved between the calculated data and the experimentally measured process of fuel conversion. The model in [ 21 ] included the following equations:. The analysis of the numerical modeling results that was carried out by the authors allowed them to make a conclusion on the effect of heat generated in the gasification process on 3d temperature fields and gas velocities inside the furnace.

A thorough analysis of mathematical models was presented in [ 20 ]. Large sizes of the plants do not allow us to construct highly detailed aerodynamic models. Therefore, special attention is paid to the development of various closing relationships, in particular the equations of kinetics of chemical reactions. As in the majority of other studies, these authors apply the relationships of the form. One of the main parts in each similar research is the determination of a required list of conditions that determine the relationship between the reaction rates and parameters of state, for example, temperature.

The authors of [ 21 ] properly note that such a method can only be applied to certain ranges of temperatures, whereas for high values above K it gives absolutely unreal estimates of the rate of solid fuel drying. It is possible to conclude that the application of Eqs. Also, the authors of [ 21 ] indicate that the CFD models that refer to a continuous porous medium are very attractive in terms of their possible implementation.

The screw represents a spiral with a wall, 3 mm thick, which is wound on a hollow shaft. The interior space of the pyrolyzer with solid fuel and formed products is separated from heating gases by a wall, 5 mm thick. The screw represents a geometrically complex structure. There are 11 complete spiral turns with a pitch of mm on the hollow steel shaft. An external diameter of the spiral made up mm, and the shaft diameter was In the calculation, we took into account the body of the screw, shaft, and external coating.

All the objects are constructed from cylinders. The construction of all these objects caused the emergence of surfaces and regions of small size at the point of their intersection. The total number of components in the calculation mesh equals N. Optimal distribution and sizes of mesh cells were calculated using the Comsol Multiphysics Software on the basis of corresponding algorithms that ensure good convergence for the differential equations applied. The calculation mesh includes different number of cells:.

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For gas part, it includes hot gas from internal combustion engine and gas inside the screw above the porous medium. For the others, it includes solid volumetric parts such as screw, spiral, coating, and so on. The Comsol Multiphysics Software is used as a simulation environment.

Heat exchange in the process of pyrolysis is simulated considering physical properties porosity, permeability, etc. In the equation, the changes in masses Q br correspond to the gas masses formed in the course of the solid fuel decomposition. Then, the equation of continuity has the following form:. The equation of a diffusion barrier for the plane separating the porous solid body from the gas space in the pyrolyzer is set as follows:. The calculated fields of temperature, velocities, densities of media, streamlines, and other derivative data were obtained by using the mathematical model of heat and mass transfer processes in the screw pyrolyzers by using the Comsol Multiphysics software.

The calculation example demonstrates a sufficiently high degree of fuel heating and an adequate pattern of gas flow over the porous medium around the screw. The example of calculations performed to determine the fields of temperatures and the motion velocities of pyrolysis products. The parametric calculations can be made on the stable calculation model allowing the variation of parameters in the solution, that is, boundary conditions, over a sufficiently wide range.

The developed calculation model of the screw pyrolyzer was tested on the basis of the following parametric studies:.

## solid fuels combustion and gasification - modeling simulation and equipment operation

The other variants of parametric studies are quite possible and seem to be the least problematic from the viewpoint of numerical solution stability, since the current tasks in solving the numerical problems are aimed first of all at the achievement of stability to the initial and boundary conditions. Distribution of the temperature and its gradient in the pyrolyzer space for different heating gas flow rates. Temperature distribution in the pyrolyzer space for different assigned values of heating gas temperature. The parametric calculations with the changing heating gas flow rate reveal an asymptote in the solutions of temperature distribution; as far as starting with 0.

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This result enables the supposition about the existing optimal ratio between the working parameters of the developed stepwise gasification scheme in the forthcoming optimization study. As a result, the efficiency of the heating process and the uniformity of temperature fields are secured, avoiding the zones with irregular reaction rates. The color indicates the calculated temperature values. Markedly distinct velocity fields and as a consequence the diffusion coefficients characterizing transfer of the gas phase from the porous mass to the free space can be formed for different operating conditions.

The effect of heating gas temperature on the diffusion processes of formed gas yield was analyzed. The calculations show a negligible effect of the boundary conditions on motion of the media in the region modeled by the diffusion thin bed. This fact suggests feasibility of using the constant coefficients to describe gas transition from the porous medium to the space free for gas flow in the screw reactor. Thereby, the general distribution type of values in the velocity field did not change. The validity of applying the stationary thermophysical dependences to the determination of tree properties in the pyrolysis process was analyzed.

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