Week 10 - Simulating Combustion of Natural Gas.

Aim

Simulating Combustion of Natural Gas with different setups and comparing the results for pollution.

plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation for :-

1) H2O = 0%

2) H2O = 5%

3) H2O = 10%

4) H2O = 20%

5) H2O = 30%

Introduction

Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually, atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion does not always result in fire, because a flame is only visible when substances undergoing combustion vapourise, but when it does, a flame is a characteristic indicator of the reaction. While the activation energy must be overcome to initiate combustion (e.g., using a lit match to light a fire), the heat from a flame may provide enough energy to make the reaction self-sustaining. Combustion is often a complicated sequence of elementary radical reactions. Solid fuels, such as wood and coal, first undergo endothermic pyrolysis to produce gaseous fuels whose combustion then supplies the heat required to produce more of them. Combustion is often hot enough that incandescent light in the form of either glowing or a flame is produced. A simple example can be seen in the combustion of hydrogen and oxygen into water vapour, a reaction commonly used to fuel rocket engines. This reaction releases 242 kJ/mol of heat and reduces the enthalpy accordingly (at constant temperature and pressure):

2H₂(g) + O₂(g) → 2H2O(g)

Combustion of an organic fuel in air is always exothermic because the double bond in O₂ is much weaker than other double bonds or pairs of single bonds, and therefore the formation of the stronger bonds in the combustion products CO
2 and H2O results in the release of energy.

Stoichiometric combustion of a hydrocarbon in oxygen:

Generally, the chemical equation for stoichiometric combustion of a hydrocarbon in oxygen is:

Where,

For example, the stoichiometric burning of propane in oxygen is

2. Chemical kinetics:

Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is to be contrasted with thermodynamics, which deals with the direction in which a process occurs but in itself tells nothing about its rate. Chemical kinetics includes investigations of how experimental conditions influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition states, as well as the construction of mathematical models that also can describe the characteristics of a chemical reaction.

3. Chemical reaction model:

Chemical reaction models transform physical knowledge into a mathematical formulation that can be utilized in computational simulation of practical problems in chemical engineering. Computer simulation provides the flexibility to study chemical processes under a wide range of conditions. Modelling a chemical reaction involves solving conservation equations describing convection, diffusion, and reaction source for each component species.

Theory

Factors affecting reaction rate:

1. Nature of reactants

2. Physical state

3. Surface area of solid-state

4. Concentration

5. Temperature

6. Catalysts

7. Pressure

8. Absorption of light

Applications:

The mathematical models that describe chemical reaction kinetics provide chemists and chemical engineers with tools to better understand and describe chemical processes such as food decomposition, microorganism growth, stratospheric ozone decomposition, and the chemistry of biological systems. These models can also be used in the design or modification of chemical reactors to optimize product yield, more efficiently separate products, and eliminate environmentally harmful by-products. When performing catalytic cracking of heavy hydrocarbons into gasoline and light gas, for example, kinetic models can be used to find the temperature and pressure at which the highest yield of heavy hydrocarbons into gasoline will occur.

Chemical Kinetics is frequently validated and explored through modelling in specialized packages as a function of ordinary differential equation-solving (ODE-solving) and curve-fitting.

Soot formations:

Soot forms when hydrocarbon fuels such as oil, natural gas, and wood are burned. Soot particles form when gaseous molecules are heated to high temperatures, and they don't easily turn back into gaseous molecules the way water droplets do when they are heated. Soot as an airborne contaminant in the environment has many different sources, all of which are results of some form of pyrolysis. They include soot from coal-burning, internal-combustion engines, power-plant boilers, hog-fuel boilers, ship boilers, central steam-heat boilers, waste incineration, local field burning, house fires, forest fires, fireplaces, and furnaces. These exterior sources also contribute to the indoor environment sources such as smoking of plant matter, cooking, oil lamps, candles, quartz/halogen bulbs with settled dust, fireplaces, exhaust emissions from vehicles, and defective furnaces.

Solving & Modelling approach
I have soved through the stages mentioned above with the help of videos shown and guidance of the technical staff.

The process fluesnt fluid flow will be done in following these 5 steps:-

1) Geometry - The geometry of the model will be formed where the actual flow will flow/interfere

2)Mesh - The meshing of the volume is done whoose parameters are set by default with help of 'Boundary Box'

3)Setup - The initial and boundary conditions or parameters and the model choosen are given to the model.

4) Solutions - Based on conditions and requirements the result is calculated with the help of model choosen

5) Results - The final results are appeared here from where the investigation of simulation is done.

Step 0) Download and import the model available

Step 1) Prepare geometry of our system

1.1) Use SPLIT PLAN feature under design to select the right quater of model

1.2) Copy and Paste the face we need for simulation

Step 2) Mesh of geo

2.1) Generate mesh to our model as given below providing SIZING

2.2) Assign names to the faces

Pre-processing and solver setting

Step 3) Setup the system's parameters

3.1) Give physics as given below

3.2) Model the SPECIES

3.3) Give model for NOX & SOOT

3.4) Give the bounary conditions parameters to the inlet and outlet

3.5) Hybrid Initialise the system

3.6) provide all contours required

3.7) Create animation for contours

3.8) Calculate the results

Step 4) Represent the result from CFD-Post

4.1) Give the surface variable color parameter

4.2) Locate 4 lines

4.3) Insert chart and create for required parameters

Result

Following results are obtained from our simulation

Parameters	0% H2O	5% H2O	10% H2O	20% H2O	30% H2O
Temperature
NOx
CH4
H2O
O2

Conclusion

Following points can be concluded from our simulation's results:-

• In cases where the water is added with fuel in respect of 5% to 30% there is reduction of NOx by certain amount. Hence adding the water with fuel is considerably decreasing the NOx emissions from the exhaust products.
• Similar to NOx, it can be seen that the mass fraction of CO2 is also decreasing as water percentage is increasing. The Mass fraction of N2 is almost constant for all the cases which means the flame instability for the burner is almost constant for all the cases of combustion.
• In case of soot formation, it can see that, mass fraction of soot is decreasing for case 1 and 2 but for case 3 and 4 it has increased. After checking the Mass fraction of soot contour for all the cases, it is seen that from case 3 and 4 contour of mass fraction of soot, the formation of soot is nearer toward the outlet of burner which is common practice of soot to deposited at the outlet.
• From the whole simulation it concluded that providing water with fuel will reduce the harmful exhaust emission such as NOx and Soot.