Simulating Combustion of Natural Gas.

Aim –

Perform a combustion simulation on the combustor model using methane as fuel.

Objectives -

Part 1: Perform a combustion simulation on the combustor model and plot the variation of the mass fraction of the different species in the simulation using line probes at different locations of the combustor as shown in Fig. You need to plot for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation.

Part 2: As you must have observed from the above simulation, the Nox and soot are getting formed at the outlet of the combustor. Such formation has harmful effects on the environment and humans. The stringent government norms also demand the least formation of Nox and soot and to satisfy those requirements, you need to check the effect of adding the water in the fuel.

In this part, you need to add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results. It is necessary to provide line plots and contours to prove your claim.

Theory –

1. Combustion: 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. Since a flame only appears when components undergoing combustion vapourize, combustion does not always result in fire, but when it does, a flame is a distinctive sign of the event. The heat from a flame may be enough energy to make the reaction self-sustaining, even though the activation energy must be overcome to initiate combustion (such as when using a lit match to start a fire). 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 CO2 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:

2. Chemical Kinetics: The area of physical chemistry that deals with comprehending the speeds of chemical processes is known as chemical kinetics, sometimes known as reaction kinetics. Contrast that with thermodynamics, which focuses on the direction in which a process takes place but says nothing about the rate of that process. Chemical kinetics is the study of how experimental conditions affect a chemical reaction's rate and reveal details about its mechanism and transition states. It also involves the development of mathematical models that can be used to represent the properties of a chemical reaction.
3. Chemical Reaction Model: Physical knowledge is transformed into a mathematical formulation via chemical reaction models, which may then be used to simulate real-world chemical engineering problems computationally. The versatility of studying chemical processes under various circumstances is made possible by computer simulation. Convection, diffusion, and reaction source conservation equations for each species involved in the reaction must be solved when modelling it.

Series transport equation-

Ri is the net rate of production of species i by chemical reaction and Si is the rate of creation by addition from the dispersed phase and the user-defined source. Ji is the diffusion flux of species i, which arises due to concentration gradients and differs in both laminar and turbulent flows. In turbulent flows, computational fluid dynamics also considers the effects of turbulent diffusivity. The net source of chemical species i due to reaction, Ri which appeared as the source term in the species transport equation is computed as the sum of the reaction sources over the NR reactions among the species.

Reaction Rate Models –

1. Laminar Finite Rate Model: The Arrhenius formulas are used by the laminar finite rate model to compute the chemical source terms, and turbulence fluctuations are not taken into account. Due to the very non-linear Arrhenius chemical kinetics, this model produces a correct answer for laminar flames but an inaccurate one for turbulent flames, where turbulence has a significant impact on the rates of chemical reactions. For combustion with minor turbulence variations, such as supersonic flames, this model may, nonetheless, be accurate.
2. Eddy Dissipation Model: A turbulent-chemistry reaction model, the eddy dissipation model is based on the work of Magnussen and Hjertager. The majority of fuels burn quickly, and turbulence mixing regulates the reaction's overall rate of speed. Turbulence slowly blends the fuel and oxidizer into the reaction zones of the non-premixed flames, where they burn rapidly. In premixed flames, the turbulence gradually combines the hot products and cool reactants in the reaction zones, where the reaction proceeds quickly. When this occurs, the combustion is considered to be mixing-limited, and it is safe to ignore the intricate and frequently unknowable chemical dynamics. According to this model, the large-eddy mixing time scale controls the chemical reaction. Anytime there is turbulence in the flow, combustion begins. The combustion can begin without an igniting source. The reactant is assumed to start burning as soon as it enters the computation model, which is a weakness of this model because in reality the reactant needs some time to reach the ignition temperature to start the combustion. This type of model is valid for non-premixed combustion.

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: Chemists and chemical engineers can better comprehend and characterise chemical processes including food degradation, microbial development, stratospheric ozone decomposition, and the chemistry of biological systems using the mathematical models that describe chemical reaction kinetics. These models can also be used to optimise product yield, more effectively segregate products, and get rid of by-products that are bad for the environment in chemical reactor design or modification. Kinetic models can be used to determine the temperature and pressure at which the largest yield of heavy hydrocarbons into petrol will occur when performing catalytic cracking of heavy hydrocarbons into petrol and light gas, for instance.

As a result of solving ordinary differential equations (ODEs) and curve-fitting, modelling in specialised software is frequently used to validate and investigate chemical kinetics.

4. Mass Fraction : A mass fraction can also be stated as a percentage by mass with a denominator of 100. The mole fraction (percentage by moles, mol%) and volume fraction (percentage by volume, vol%) are two more ways to indicate the composition of a mixture in a dimensionless size.

The term "mass fraction" can also refer to the proportion of an element's mass to the overall mass of a sample when the prevalences of interest are those of specific chemical elements rather than compounds or other substances. A mass per cent composition can be used as an alternative phrase in these instances. The empirical formula or chemical formula of a compound can be used to determine the mass fraction of each element in the compound.

5. Nitrogen Oxide (NOx) emissions: Inflammation of the airways is a side effect of nitrogen dioxide, an irritating gas. Nitric oxide (NO), which is produced when oxygen and nitrogen mix during fuel combustion, is released during fuel combustion. Nitrogen dioxide (NO2) is created when this mixes with oxygen in a further step. At average ambient quantities, nitric oxide is not thought to be harmful to human health, although nitrogen dioxide can be. Oxides of nitrogen (NOx) refers to both nitric oxide and nitrogen dioxide collectively.

In addition to playing a key role in the creation of ground-level ozone and fine particles (PM), which both have negative health impacts, NOx emissions also combine to produce smog and acid rain.

6. Soot Formation: Burning hydrocarbon fuels like oil, natural gas, and wood produces soot. When gaseous molecules are burned to high temperatures, soot particles are created, and unlike heated water droplets, soot does not readily decompose back into gaseous molecules. There are numerous distinct sources of soot in the environment, all of which are products of pyrolysis in some way. They consist of soot from internal combustion engines, coal-burning power plants, hog-fuel boilers, ship boilers, central steam-heat boilers, waste incineration, small-scale field burning, house fires, forest fires, fireplaces, and furnaces. Smoking of plant matter, cooking, oil lamps, candles, quartz/halogen bulbs with accumulated dust, fires, and vehicle exhaust emissions are some examples of outdoor sources that also affect the indoor environment.

Geometry Preparation –

Since the combustor is a 3D model and we are only interested in its 2D geometry, upload the combustor model to SpaceClaim. Due to the axisymmetric constraint, we must divide the body in half and choose the plane XY plane region of geometry.

3D model of the combustor –

Splitting the body –

Select the 2D Plane normal to the Z-axis after splitting the body, then duplicate that geometry to a new design sheet, deleting the old one as we don't need it.

2D geometry -

Mesh Setup -

Generate mesh with capture proximity settings and set the cell gap to 3 cells or as per your requirement and mesh size of 5 mm.

Named Selections –

Fluent Setup –

General Settings:

FLUENT setup and solving the required problem is the part of pre-processing. In this section the different part of setup is discussed in which viscous model setup, boundary conditions and solution method setup are involved. The general setup required for simulation is pressure based steady state approach for problem. The gravity is not enabled for the simulation.

The viscous model setup comprises of information about setting up required flow characteristics for the problem. As state earlier in solution approach of the problem that, flow will be turbulent in nature and species transport equation is need to be solved using eddy dissipation method. In the same context k-epsilon turbulent model with standard wall function are used as input for the flow phenomenon. As per the problem description the velocity of air is just 0.5m/sec whereas fuel velocity is 80m/sec. Considering Reynolds number it is higher to the side and because of K and epsilon are proportional reaction rate ‘omega’ it is required to have k-epsilon model.

 

 After completing the viscous model selection, the species model can be selected from the model of FLUENT in outline view or can be also select from physics tab > models. As the species model turn on, the energy equation will get turn on automatically. Energy model use to determine the temperature generated during the reaction in species transport equation. In species model, species transport option has to be selected followed by volumetric reaction. The mixture properties should be select one at a time. If user has select any other mixture properties model rather than given one, then that setup need to be reset and again whole process has to be followed accordingly. The mixture model which is used for simulation is ‘methane-air-2step’ in which number of volumetric species are six i.e. CO2, H2O, CH4, N2, O2 and CO. As state earlier this model is governed by effect of turbulence hence Eddy dissipation turbulence-chemistry interaction is selected.

 The boundary conditions which are used for this simulation are velocity inlet and pressure outlet with an axis for Axi-symmetric condition. There are two inlets for this simulation. The first velocity inlet is for air which is flowing inside the burner at the speed of 0.5m/sec. The Air is entering at atmospheric pressure inside burner and at 300 kelvin temperature. In species tab of air inlet, it is required to provide information about how much of oxygen is entering in the burner. The 0.23 mass fraction of O2 is coming with air for the reaction purpose.

Similar boundary conditions are taken into account for the fuel inlet, which has a velocity inlet of 80 m/sec. In addition, the fuel is entering at 300 kelvin and atmospheric pressure. The specification of species for the simulation is the most crucial component of the fuel inlet. The mass fraction of Fuel is defined as 1 of CH4 in the species tab of the fuel inflow boundary condition.

Other than this boundary condition, the pressure outlet boundary condition is kept similar and axis should be put as axis as per the defined by FLUENT automatically.

NOx and soot models must be set up similarly to species models. Since it was previously discussed that high temperatures and a rich fuel mixture can cause NOx to be produced, all pathways that can monitor the production of NOx are turned on for the simulation. Similar to this, the one-step soot model is also active, allowing for the tracking of soot formation throughout the combustion process.

FLUENT uses the default solution approach for the simulation. The default solution method was set by Fluent depending on the version that was utilised. As it was discovered, the default way of solution method in versions 20 and after is Coupled rather than SIMPLE in earlier versions. For all species, the spatial discretization should be set to a second order upwind function. It is necessary to utilise the same second order upwind function for turbulent kinetic energy and turbulent dissipation rate for improved convergence. The hybrid initialization approach is used to choose the initial value for the calculation of the entire simulation, and the convergence factor is also set to default.

Results –

Except for the subsequent situations when the boundary condition of the fuel inlet is changed, the information from the preceding section is applied to all simulated scenarios. The parameters for the baseline mesh arrangement are left as they were in the boundary condition discussion, and simulation is conducted for 850 iterations. All of the results from FLUENT are set up to be read into the data file amount for the post-processor. The outcomes of the default mesh configuration are listed below:

1. Residuals Plots 

2. Temperature Plots

1. Mass Fraction of CH4

3. Mass Fraction of CO

4. Mass Fraction of CO2

5. Mass Fraction of H2O

6. Mass Fraction of N2

7. Mass Fraction of O2 

8. Mass Fraction of NOx

9. Mass Fraction of Soot

10. Line Plots for Charts

11. Graph for CH4

12. Graph for CO

13. Graph for CO2

14. Graph for H2O

15. Graph for N2

16. Graph for O2

17. Graph for NOx

18. Graph for SOOT 

Case 1 –

5% H2O

The baseline mesh arrangement demonstrates the proportion of NOx and soot produced by the burner. More NOx is found towards the outflow, and soot production occurs close to the air input. The fuel is combined with water vapour that enters from the same inlet point to reduce NOx and soot formation in the burner. In this instance, only 5% of water is added to the fuel and flow at an 80 m/sec rate. The mass percentage of fuel must be reduced as the mass fraction of water in the fuel input increases. In this scenario, the mass fraction of water will be 0.05 and the mass fraction of fuel will be 0.95.

Results –

1. Residuals Plots

2. Temperature Plots 

3. Mass Fraction of CH4

4. Mass Fraction of CO

5. Mass Fraction of CO2

6. Mass Fraction of H2O

7. Mass Fraction of N2

8. Mass Fraction of O2 

9. Mass Fraction of NOx

10. Mass Fraction of Soot

11. Graph for CH4

12. Graph for CO

13. Graph for CO2

14. Graph for H2O

15. Graph for N2

16. Graph for O2

17. Graph for NOx

18. Graph for SOOT

CASE II

10% H2O

The amount of water is again raised by 5% in Case 2. Below Figure provides the mass fraction of the fuel and water.

Results –

1. Residuals Plots

2. Mass Fraction of NOx 

3. Mass Fraction of SOOT

4. Graph for CO

5. Graph for CO2

6. Graph for NOx 

7. Graph for SOOT

CASE III

20% H2O

In Case 3, the water content is once more raised by 10%. The given figure specifies the mass percentage of fuel and water.

1. Residuals Plots

2. Mass Fraction of NOx

3. Mass Fraction of SOOT

4. Graph for CO

5. Graph for CO2

6. Graph for NOx

7. Graph for SOOT

CASE IV

30% H2O

In Case 4, the water content is once more raised by 10%. The given figure specifies the mass percentage of fuel and water.

1. Residuals Plots

2. Mass Fraction of NOx

3. Mass Fraction of SOOT

4. Graph for CO

5. Graph for CO2

6. Graph for NOx

7. Graph for SOOT 

Result Table –

 

Conclusion –

• In baseline mesh arrangement, where the entire amount of CH4 is used as fuel, there is more soot and NOx than in other situations. Additionally, the top temperature is around 2300 K.
• The amount of NOx is reduced by a specific amount in the subsequent cases where water is added to the fuel in proportion to 5% to 30%. The reduction of NOx in instance 1 is roughly 12.34%. The similar reduction in NOx is 26.32% for scenario 2. Case 3 has a NOx reduction of 48.49%, whereas Case 4 has a NOx reduction of approximately 69.13%. Therefore, mixing water with gasoline significantly reduces NOx emissions from exhaust products.
• The result table shows that, similar to NOx, the mass fraction of CO2 is similarly falling when water percentage is rising. Since the mass percentage of N2 is nearly constant in all instances, combustion flame instability for the burner is also nearly constant.
• In the soot generation scenario, it can be seen that the mass fraction of soot is decreasing for cases 1 and 2, but increasing for cases 3 and 4. It is clear from examining the Mass percentage of Soot Contour for each example that soot formation from cases 1 to 4 is out of balance, however case 3 and case 4 show much less soot formation. The development of soot is closer to the burner's exit, which is typical practise for soot to be deposited towards the outlet, as can be observed from the case 3 and case 4 contour of soot mass fraction.
• The simulation as a whole led to the conclusion that mixing fuel and water will cut down on dangerous exhaust emissions like NOx and soot.