Gasification Project Description

The present problem simulates the gasification process inside a gasifier chamber by ANSYS Fluent software. The gasification process is a set of chemical reactions in which a carbon-based substance is converted to carbon monoxide, hydrogen, and carbon dioxide. The product of this process is considered a renewable energy source with many applications in industry.

In this case, a material is defined as fuel, with a density of 1550 kg.m-3 and a specific heat capacity of 1680 j.kg-1.K-1, which is sprayed into the chamber and reacts with water vapor at high temperature. Evaporating water droplets inside the chamber also create water vapor. Therefore, to simulate this model, defining discrete phases within the continuous phase is necessary.

Hence, the discrete phase model (DPM) is used along with the injection process for this model. The fuel stream enters the gasifier chamber from the lower area as stagnant particles with a velocity of 30 m.s-1, a temperature of 393.15 K, and a mass flow rate of 0.2 kg.s-1; While the flow of liquid water in the form of droplets with a velocity of 122 ms-1, temperature 673.15 K, and a flow rate of 0.02 kg.s-1 enters the chamber from the middle area and, under the process of evaporation, turns into water vapor to react with fuel particles.

The entry of these two flows into the chamber leads to a reaction between them. Two conical areas have been used on both sides of these input flows to strengthen and accelerate the reaction process between them. Also, each input flow enters the chamber from two pipes in opposite directions to each other and at an angle of 180 degrees to each other, both of which lead to a vortex flow and thus strengthen the mixing of the two flows.

Also, in the present simulation, it is necessary to define the chemical reactions between the input flows and the production of the desired gaseous species. Therefore, for the mentioned modeling, the model of species transport with volumetric reactions has been used, and to define the type of reactants and the process of chemical reactions, the CHEMKIN mechanism has been used, which includes 5 chemical reactions involving 8 different types of reactants and products.

The reactions related to this reaction pattern are presented in the table below. This model also assumes that radiation heat transfer due to chemical reactions occurs inside the gasifier chamber, and hence, the P1 model is defined for simulation.

This CFD Project is the 2nd episode of the Radiation Model Training Course.

Gasifier Geometry & Mesh

The present 3-D model is designed using Design Modeler software. The geometry of the model consists of a two-piece cylindrical chamber 7.5 m long and 0.8 m in diameter So that the upper part has a length equal to 5 m and the lower part has a length equal to 2.5 m. The lower part consists of two cones on either side of it in opposite directions;

So that the cone in the middle of the chamber has a diameter of 0.6 m and a height of 0.3 m, and the cone at the bottom of the chamber has a diameter of 0.8 m and a height of 0.35 m. There are two pipes for fuel inlet in the lower part of the chamber, in the form of a narrow cylinder with a diameter of 0.1 m but in opposite directions to each other and at an angle of 180 degrees.

The two pipes for liquid water inlet are located in the middle of the chamber, in the form of a narrow cylinder with a diameter of 0.05 m but in opposite directions to each other and at an angle of 180 degrees. The product outlet pipe with a diameter of 0.2 m is placed at the top of the chamber. The figure below shows a view of the geometry.

The meshing of the model has been done using ANSYS Meshing software and the mesh type is unstructured. The element number is 219170. The following figure shows the mesh.

Gasification CFD Simulation

To simulate the present model, several assumptions are considered:

• We perform a pressure-based solver.
• The simulation is steady.
• The gravity effect on the fluid is ignored.

A summary of the defining steps of the problem and its solution is given in the following table:

Results

At the end of the solution process, three-dimensional temperature and velocity counters, three-dimensional velocity vectors, and a particle track are obtained.