Week 10 - Simulating Combustion of Natural Gas.

Aim: To perform and study combustion simulation of a non-premixed model.

 

Introduction and Theory: 

Combustion or burning is a high-temperature exothermic redox chemical reaction between a fuel(the reductant) and an oxidant, usually, atmospheric oxygen. Complete combustion is stoichiometric concerning the fuel, where there is no remaining fuel, and ideally, no residual oxidant. Thermodynamically, the chemical equilibrium of combustion in air is overwhelmingly on the side of the products. However, complete combustion is almost impossible to achieve, since the chemical equilibrium is not necessarily reached, or may contain unburnt products such as carbon monoxide, hydrogen, and even carbon(soot or ash). Any combustion at high temperatures in atmospheric air, which is 78% nitrogen, will also create a small amount of nitrogen oxides, commonly referred to as NOx.Thus, the produced smoke is usually toxic and contains unburned or partially oxidized products.

In this project, we will be burning methane in the air inside a cylindrical combustor. Here, the source term is modeled using the eddy-dissipation model where the beginning of ignition(ignition delay) is calculated using a turbulent time scale. This means that the ignition will start depending on when the turbulent quantities reach a limiting value. This is a crude approximation but helps us to visualize the steady-state temperature distribution in the combustion chamber at a faster rate. This type of mixing based model is called a Turbulence Chemistry interaction model.

 

Types of NOx model formation:

1. Thermal NOx

The formation of thermal NOx is determined by a set of highly temperature-dependent chemical reactions known as the extended Zeldovich mechanism. The principal reactions governing the formation of thermal NOx from molecular nitrogen are as follows:

A third reaction has been shown to contribute to the formation of thermal NOx, particularly at near-stoichiometric conditions and in fuel-rich mixtures:

2. Prompt NOx

The presence of a second mechanism leading to NOx formation was first identified by Fenimore and was termed "prompt NOx''. There is good evidence that prompt NOx can be formed in a significant quantity in some combustion environments, such as in low-temperature, fuel-rich conditions and where residence times are short. Surface burners, staged combustion systems, and gas turbines can create such conditions.

At present the prompt NOx contribution to total NOx from stationary combustors is small. However, as NOx emissions are reduced to very low levels by employing new strategies (burner design or furnace geometry modification), the relative importance of the prompt NOx can be expected to increase.

Prompt NOx is most prevalent in rich flames. The actual formation involves a complex series of reactions and many possible intermediate species. The route now accepted is as follows:

3. Fuel NOx

The extent of conversion of fuel nitrogen to NOx is dependent on the local combustion characteristics and the initial concentration of nitrogen-bound compounds. Fuel-bound compounds that contain nitrogen are released into the gas phase when the fuel droplets or particles are heated during the devolatilization stage. From the thermal decomposition of these compounds, (aniline, pyridine, pyrroles, etc.) in the reaction zone, radicals such as HCN, NH, N, CN, and NH can be formed and converted to NOx. The above free radicals (i.e., secondary intermediate nitrogen compounds) are subject to a double competitive reaction path. Below, is a simplified model of the same:

4. N2O Intermediate

Melte and Pratt proposed the first intermediate mechanism for NOx formation from molecular nitrogen (N) via nitrous oxide (NO). Nitrogen enters combustion systems mainly as a component of the combustion and dilution air. Under favorable conditions, which are elevated pressures and oxygen-rich conditions, this intermediate mechanism can contribute as much as 90% of the NOx formed during combustion. This makes it particularly important in equipment such as gas turbines and compression-ignition engines. Because these devices are operated at increasingly low temperatures to prevent NOx formation via the thermal NOx mechanism, the relative importance of the NO-intermediate mechanism is increasing. It has been observed that about 30% of the NOx formed in these systems can be attributed to the NO-intermediate mechanism.

The NO-intermediate mechanism may also be of importance in systems operated in flameless mode (e.g., diluted combustion, flameless combustion, flameless oxidation, and FLOX systems). In a flameless mode, fuel and oxygen are highly diluted in inert gases so that the combustion reactions and resulting heat release are carried out in the diffuse zone. As a consequence, elevated peaks of temperature are avoided, which prevents thermal NOx. Research suggests that the NO-intermediate mechanism may contribute about 90% of the NOx formed in flameless mode and that the remainder can be attributed to the prompt NOx mechanism.

The simplest form of the mechanism takes into account two reversible elementary reactions:

Here, M is a general third body. Because the first reaction involves third bodies, the mechanism is favored at elevated pressures. Both reactions involve the oxygen radical O, which makes the mechanism favored at oxygen-rich conditions.

For the purposes of the current simulation, Thermal NOx and Prompt NOx is suitable.

Pre-Processing

Step1: Importing geometry into SpaceClaim.

The following geometry was imported into SpaceClaim:

Conducting simulation on this entire geometry is expensive and time-consuming. Hence, we make the assumption that the combustion properties are axisymmetric.

Therefore, we extract a 2D plane out of this 3D figure, by using the split body command which cuts the bodies via different planes to obtain a plane of interest, the required plane was copied and pasted to a new design window. The following final design is created:

 

Step 2: Meshing

The following mesh was created:

 

Mesh Properties:

Element size	1 mm
Capture curvature and proximity	Turned On
Mesh method	Quad dominant method
No. of nodes	69217
No. of elements	68377

 

Named selections:

 

Mesh metric:

1. Element quality:

 

2. Aspect ratio:

 

3. Skewness:

 

4. Orthogonal quality:

 

Step 3: Solving using Ansys Fluent.

Steady-state pressure-based solver with Absolute velocity formulation and Axisymmetric 2D space.

Viscous model:

 

Species model:

 

NOx model:

 

Soot model:

 

Boundary Conditions:

Air inlet: Velocity=0.5 m/s, O2 mass fraction =0.23

Fuel inlet: Velocity=80 m/s, CH4, and H2O mole fraction depending on the case.

Pressure Outlet: Gauge pressure = 0 Pa

Axis - Axi-symmetric

Walls - Type: wall

 

CASES:

1. CH4 - mole fraction =1

2.1. CH4 - mole fraction =0.95 H2O - mole fraction =0.05

2.2. CH4 - mole fraction =0.9 H2O - mole fraction =0.1

2.3. CH4 - mole fraction =0.8 H2O - mole fraction =0.2

2.4. CH4 - mole fraction =0.7 H2O - mole fraction =0.3

 

Solution Methods:

 

Solution Controls:

 

Residuals:

Since the residuals have constant crests and troughs we can conclude that the solution has converged.

 

Results:

Case I. CH4, mass fraction =1

Location of line probes :

Line 1 :

Point 1-> x=0.018m y=0m z=0m

Point 2-> x=0.018m y=0.0852m z=0m

Line 2 :

Point 1-> x=0.1m y=0m z=0m

Point 2-> x=0.1m y=0.0852m z=0m

Line 3 :

Point 1-> x=0.2m y=0m z=0m

Point 2-> x=0.2m y=0.0852m z=0m

Line 4 :

Point 1-> x=0.35m y=0m z=0m

Point 2-> x=0.35m y=0.0852m z=0m

Line 5 :

Point 1-> x=0.45m y=0m z=0m

Point 2-> x=0.45m y=0.0852m z=0m

Line 6 :

Point 1-> x=0.6m y=0m z=0m

Point 2-> x=0.6m y=0.0852m z=0m

Line 7 :

Point 1-> x=0.78m y=0m z=0m

Point 2-> x=0.78m y=0.0852m z=0m

 

a) Contour and plot of Co2 mass fraction

 

b) Contour and plot of H20 mass fraction

 

c) Contour and plot of CH4 mass fraction

 

d) Contour and plot of N2 mass fraction

 

e) Contour and plot of O2 mass fraction

 

f) Contour and plot of NOx mass fraction

 

g) Contour and plot of soot mass fraction

 

Case II.

Location of line probes :

Line 1 :

Point 1-> x=0.4m y=0m z=0m

Point 2-> x=0.4m y=0.0852m z=0m

Line 2 :

Point 1-> x=0.45m y=0m z=0m

Point 2-> x=0.45m y=0.0852m z=0m

Line 3 :

Point 1-> x=0.5m y=0m z=0m

Point 2-> x=0.5m y=0.0852m z=0m

Line 4 :

Point 1-> x=0.6m y=0m z=0m

Point 2-> x=0.6m y=0.0852m z=0m

Line 5 :

Point 1-> x=0.75m y=0m z=0m

Point 2-> x=0.75m y=0.0852m z=0m

 

2.1. CH4, mole fraction = 0.95; H2O mole fraction = 0.05

a) Contour and plot of NOx mass fraction

 

b) Contour and plot of soot mass fraction

 

c) Averaged mass fraction:

1. Area average of Mass Fraction of Pollutant No on outlet = 0.000193868

2. Area average of Mass Fraction of Pollutant Soot on outlet = 0.00008462

 

2.2. CH4, mole fraction = 0.9; H2O mole fraction = 0.1

a) Contour and plot of NOx mass fraction

 

b) Contour and plot of soot mass fraction

 

c) Averaged mass fraction:

1. Area average of Mass Fraction of Pollutant No on outlet = 0.000122585

2. Area average of Mass Fraction of Pollutant Soot on outlet = 0.00000918174

 

2.3. CH4, mole fraction = 0.8; H2O mole fraction = 0.2

a) Contour and plot of NOx mass fraction

 

b) Contour and plot of soot mass fraction

 

c) Averaged mass fraction:

1. Area average of Mass Fraction of Pollutant No on outlet = 0.0000701497

2. Area average of Mass Fraction of Pollutant Soot on outlet = 0.000000035626

 

2.4. CH4, mole fraction = 0.7; H2O mole fraction = 0.3

a) Contour and plot of NOx mass fraction

 

b) Contour and plot of soot mass fraction

 

c) Averaged mass fraction:

1. Area average of Mass Fraction of Pollutant No on outlet = 0.000022944

2. Area average of Mass Fraction of Pollutant Soot on outlet = 0.000000000101713

 

Plot depicting how the pollutant concentrations drop with respect to water addition to CH4 fuel:

Conclusion:

1. Diluting fuel with H2O does indeed reduce the formation of harmful pollutants such as NOx and soot at the outlet.
2. The rate of reduction in the concentration of soot is greater than that of NOx when diluting the fuel.
3. Fuel dilution by H2O is a great way to reduce emissions. However, a trade-off must be made between the performance required and the dilution amount.
4. Since soot value decreases with dilution we can say there is less wastage of fuel, i.e. complete combustion occurs.
5. Eddy - dissipation model(Turbulence Chemistry Interaction) helps us to model combustion via turbulence mixing without the presence of an ignition source.