Biomass Combustion CFD Simulation, ANSYS Fluent Training

Biomass

Energy production using the process of burning fossil fuels in the world has caused problems; Disadvantages of these fossil fuels include non-renewability, high cost, high pollution, and the production of carbon dioxide and highly toxic substances. Hence, the world has moved toward replacing these fossil fuels with clean, accessible, renewable energy sources such as wind, solar, sea waves, geothermal energy, and biomass.

Therefore, the use of biomass as an alternative energy source has been welcomed in terms of environmental, economic, and ease of use. In fact, biomass is a renewable energy source made from biomass. Wood and forest waste, livestock waste, municipal and industrial waste, sewage, and agricultural products and plant wastes such as wheat, corn, potatoes, sugarcane, beets, etc. are among the renewable sources of biomass.

In fact, it is possible to convert sugars and cellulose and other substances in agricultural waste into clean fuels by performing special chemical reactions. In general, synthesis gas is a term used to describe a set of processes that produce a gas that contains hydrogen and carbon monoxide.

Biomass Gasification

The process of gasification is defined as the conversion of biomass raw materials into synthesis gas; Thus, the combustion process for solid fuel at high temperatures occurs at a temperature of about 1000 K, which leads to the production of synthetic gas. The gas is mainly a combination of carbon monoxide, methane, carbon dioxide, hydrogen, water vapor, nitrogen, and some particles of hydrocarbons, tar, and ash. The synthesis gas from this technology can be used to generate thermal energy and generate electricity. This gas has a lower thermal value than natural gas but has the ability to ignite.

To produce these gases, we use a gasifier reactor; So that in this gasifier, the biomass material reacts with the gas-producing agent. Gasifiers include two general categories called packed bed and fluidized bed. In packed bed gasifiers, the gas-producing agent, usually air, enters the chamber slowly from below or above the reactor. In fluidized bed reactors, solids float in the reactor space due to turbulence inside the reactor, which is responsible for causing the turbulence (air, oxygen, vapor, or a combination of these gases).

Biomass Gasification

This gasification process consists of four main stages, including drying, pyrolysis, combustion, and gasification. During the drying stage, the moisture of the biomass is taken at temperatures above 100 C and the water content in it is converted to steam. In the pyrolysis phase, in a reaction in the absence of oxygen, the gases escaping from the solid carbon are released and leave the residue, which includes coal and steam, and decomposes into more materials (CH4, H2, CO2, CO, H2O). During the combustion phase, the air, including oxygen and water vapor, reacts with the carbonized solid fuel, producing more H2 and CO2. Finally, during the gasification stage, a number of high-temperature chemical reactions occur between 400 K and 1500 K; In this way, the semi-burning coal reacts with steam and carbon dioxide to eventually convert the biomass into the final synthesis gas.

The following figure shows a view of a gasifier and the combustion process inside the gasifier.

Project Description

The present problem simulates the biomass combustion process inside a gasifier chamber by ANSYS Fluent software. The material used for combustion is biomass, which reacts with the oxidizer. It is a biomass substance made from wheat straw that reacts with oxygen to produce synthetic gas as a healthy fuel. Since in this simulation, the combustion reaction between biomass and oxidant occurs and various gas species are involved in the process as reactants or products, the Species Transport model has been used, and by activating the reaction within it, a Non-Premixed type reaction model has been used.

In the present model, fuel containing biomass and air enters the chamber from two separate inlets from the upper area and creates a mass of materials including ash and semi-combustible coal in the lower part of the chamber. Finally, the resulting gas is discharged from the outlet at the bottom of the chamber to the next stage, entering the relevant boiler to create the combustion process. Also, the incoming fuel must enter the chamber as discrete particles; This means that the injection of this substance into the chamber is defined based on the Lagrangian view. Therefore, the Discrete Phase Model has been used. Also, since in the combustion process, there is radiant heat energy from the flames, the Radiation model is defined.

Geometry & Mesh

The present 2-ِD model is drawn using Gambit software. The geometry of the model is related to a gasifier chamber with a nozzle structure in the middle of its body; Thus, two narrow, small-diameter tubes on either side of the chamber, are located as the air inlet duct. The other section with a larger diameter for injecting biomass at the top of the chamber and a synthesis gas outlet duct is located at the bottom of the chamber. The figure below shows a view of the geometry.

The meshing of the present model has been done using Gambit software. The mesh type is unstructured and the element number is 1108. The figure below shows an overview of the mesh.

Gasifier CFD Simulation

To simulate the present model, several assumptions are considered, which are:

• The solver is pressure-based.
• the simulation is steady-state.
• the gravity effect is ignored.

The following is a summary of the steps for defining the problem and its solution:

Models (gasifier)
k-epsilon	Viscous model
standard	k-epsilon model
standard wall function	near-wall treatment
non-premixed combustion			Species model
chemical equilibrium	state relation
non-adiabatic	energy treatment
P1			Radiation model
on			Discrete phase model
Interaction with continuous phase	interaction
unsteady particle tracking	particle treatment
two-way turbulence coupling	physical
wheat straw	injection material
combusting	Injection type
on			Energy
Boundary conditions (gasifier)
Mass flow inlet	Inlet type
0.0007955 kg.s-1	mass flow rate	air
300 K	temperature
1	Internal emissivity
0	soot mass fraction
0	mean mixture fraction
0	mixture fraction variance
scape	discrete phase BC type
0.00011661 kg.s-1	mass flow rate	fuel
300 K	temperature
1	Internal emissivity
0	soot mass fraction
1	mean mixture fraction
0	mixture fraction variance
scape	discrete phase BC type
Pressure outlet	Outlet type
0 Pa	gauge pressure	gas outlet
1	Internal emissivity
0	soot mass fraction
0	mean mixture fraction
0	mixture fraction variance
scape	discrete phase BC type
wall			Walls type
stationary wall	wall motion	wall
0 W.m-2	heat flux
1	Internal emissivity
reflect	discrete phase BC type
Solution Methods (gasifier)
SIMPLE	Pressure-velocity coupling
second-order	pressure		Spatial discretization
second-order upwind	momentum
second-order upwind	energy
first-order upwind	turbulent kinetic energy
first-order upwind	turbulent dissipation rate
second-order upwind	soot
second-order upwind	discrete ordinates
second-order upwind	mean mixture fraction
second-order upwind	mixture fraction variance
Initialization (gasifier)
Hybrid	Initialization method

Radiation

P1 Model

In the present model, the P1 model is used because the process is related to combustion and the thickness of its optical layer is high and radiation is emitted in the presence of fuel and gas particles. It is also assumed that the internal diffusion coefficient at the inlet sections of the injected fuel and air and the wall of the gasifier chamber is equal to one.

Species Transport

Non-Premixed Model

In the present simulation, since the inlets of air and the fuel are separated, and the fuel and oxidizer do not combine before entering the inner space of the chamber, the reaction is defined as non-premixed. Also, energy behavior is non-adiabatic. A PDF (probability density function) file has been imported to define the non-premixed combustion and to define a mixture consisting of biomass and air and other gaseous species in Fluent software. This file defines a set of thermodynamic data based on the final and approximate analysis of the tested biomass materials.

The gaseous species used in the non-premixed combustion reaction include CH4 fuel with a mass ratio of 1 and oxidizers including N2 with a mass ratio of 0.78992 and O2 with a mass ratio of 0.21008. The operating pressure of the non-premixed combustion reaction is 101325 Pascal and the rich flammability limit of the fuel flow is 0.1. Also, the inlet temperature of the fuel and reactants is 300 K.

Discrete Phase Model (DPM)

Combusting Injection

In the present model, the discrete phase model (DPM) is used because the fuel is injected particle by particle into the internal space of the gasifier chamber in a discrete state to perform the combustion process. This type of discrete phase is used to interact with the continuous phase, and particle tracking is performed with a time step of 0.001s. Also, the physical state of the discrete phase is a two-way turbulent coupling.

In the present model, the injected substance is a biomass called wheat straw that is injected into the inner space of the chamber as a surface. The injection of biomass enters the chamber in the form of a combustion reaction, and therefore we define the combustion mode as combusting. This injection process results in combustion at a rate of 0.5 m.s-1 and a total flow rate of 0.00012 kg.s-1 and a temperature of 300 K at a time of 10,000 s.

Gasifier Results

After the solution process, we obtain two-dimensional contours related to pressure, temperature, velocity, density, methane mass fraction, carbon-oxide mass fraction, carbon-monoxide mass fraction, oxygen mass fraction, nitrogen mass fraction, water vapor mass fraction, as well as two-dimensional pathlines and two-dimensional velocity vectors.

You can obtain Geometry & Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.