Simulating Combustion of Natural Gas.

AIM:

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. 

THEORY:

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.  The bond energies in the fuel play only a minor role, since they are similar to those in the combustion products; e.g., the sum of the bond energies of CH4 is nearly the same as that of CO2.

The heat of combustion is approximately −418 kJ per mole of O2used up in the combustion reaction, and can be estimated from the elemental composition of the fuel.   Uncatalyzed combustion in air requires relatively high temperatures. 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).

Thus, the produced smoke is usually toxic and contains unburned or partially oxidized products. Any combustion at high temperatures in atmospheric air, which is 78 percent nitrogen, will also create small amounts of several nitrogen oxides, commonly referred to as NOx, since the combustion of nitrogen is thermodynamically favored at high, but not low temperatures. Since burning is rarely clean, flue gas cleaning or catalytic converters may be required by law. Fires occur naturally, ignited by lightning strikes or by volcanic products.

Combustion (fire) was the first controlled chemical reaction discovered by humans, in the form of campfires and bonfires, and continues to be the main method to produce energy for humanity. Usually, the fuel is carbon, hydrocarbons, or more complicated mixtures such as wood that contains partially oxidized hydrocarbons. The thermal energy produced from combustion of either fossil fuels such as coal or oil, or from renewable fuels such as firewood, is harvested for diverse uses such as cooking, production of electricity or industrial or domestic heating. Combustion is also currently the only reaction used to power rockets. Combustion is also used to destroy (incinerate) waste, both nonhazardous and hazardous.   Oxidants for combustion have high oxidation potential and include atmospheric or pure oxygen, chlorine, fluorine, chlorine trifluoride, nitrous oxide and nitric acid. For instance, hydrogen burns in chlorine to form hydrogen chloride with the liberation of heat and light characteristic of combustion. Although usually not catalyzed, combustion can be catalyzed by platinum or vanadium, as in the contact process.  

Complete & Incomplete Combustion:  

Complete:

In complete combustion, the reactant burns in oxygen and produces a limited number of products. When a hydrocarbon burns in oxygen, the reaction will primarily yield carbon dioxide and water. When elements are burned, the products are primarily the most common oxides. Carbon will yield carbon dioxide, sulfur will yield sulfur dioxide, and iron will yield iron(III) oxide. Nitrogen is not considered to be a combustible substance when oxygen is the oxidant. Still, small amounts of various nitrogen oxides (commonly designated NOx species) form when the air is the oxidative.

 

Combustion is not necessarily favorable to the maximum degree of oxidation, and it can be temperature-dependent. For example, sulfur trioxide is not produced quantitatively by the combustion of sulfur. NOx species appear in significant amounts above about 2,800 °F (1,540 °C), and more is produced at higher temperatures. The amount of NOx is also a function of oxygen excess.

 

In most industrial applications and in fires, air is the source of oxygen (O2). In the air, each mole of oxygen is mixed with approximately 3.71 mol of nitrogen. Nitrogen does not take part in combustion, but at high temperatures some nitrogen will be converted to NOx (mostly NO, with much smaller amounts of NO2). On the other hand, when there is insufficient oxygen to combust the fuel completely, some fuel carbon is converted to carbon monoxide, and some of the hydrogens remain unreacted. A complete set of equations for the combustion of a hydrocarbon in the air, therefore, requires an additional calculation for the distribution of oxygen between the carbon and hydrogen in the fuel.

 

The amount of air required for complete combustion to take place is known as pure air. However, in practice, the air used is 2-3x that of pure air.

Incomplete:           

Incomplete combustion will occur when there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. It also happens when the combustion is quenched by a heat sink, such as a solid surface or flame trap. As is the case with complete combustion, water is produced by incomplete combustion; however, carbon, carbon monoxide, and hydroxide are produced instead of carbon dioxide.

 
For most fuels, such as diesel oil, coal, or wood, pyrolysis occurs before combustion. In incomplete combustion, products of pyrolysis remain unburnt and contaminate the smoke with noxious particulate matter and gases. Partially oxidized compounds are also a concern; partial oxidation of ethanol can produce harmful acetaldehyde, and carbon can produce toxic carbon monoxide.

 
The designs of combustion devices can improve the quality of combustion, such as burners and internal combustion engines. Further improvements are achievable by catalytic after-burning devices (such as catalytic converters) or by the simple partial return of the exhaust gases into the combustion process. Such devices are required by environmental legislation for cars in most countries. They may be necessary to enable large combustion devices, such as thermal power stations, to reach legal emission standards.


The degree of combustion can be measured and analyzed with test equipment. HVAC contractors, firemen and engineers use combustion analyzers to test the efficiency of a burner during the combustion process. Also, the efficiency of an internal combustion engine can be measured in this way, and some U.S. states and local municipalities use combustion analysis to define and rate the efficiency of vehicles on the road today.

Incomplete combustion produced carbon monoxide:    

Carbon monoxide is one of the products from incomplete combustion. Carbon is released in the normal incomplete combustion reaction, forming soot and dust. Since carbon monoxide is a poisonous gas, complete combustion is preferable, as carbon monoxide may also lead to respiratory troubles when breathed since it takes the place of oxygen and combines with hemoglobin.

Problems associated with incomplete combustion:

Environmental problems:

These oxides combine with water and oxygen in the atmosphere, creating nitric acid and sulfuric acids, which return to Earth's surface as acid deposition, or "acid rain." Acid deposition harms aquatic organisms and kills trees. Due to its formation of certain nutrients that are less available to plants such as calcium and phosphorus, it reduces the productivity of the ecosystem and farms. An additional problem associated with nitrogen oxides is that they, along with hydrocarbon pollutants, contribute to the formation of tropospheric ozone, a major component of smog.

Human health problems:

Breathing carbon monoxide causes headache, dizziness, vomiting, and nausea. If carbon monoxide levels are high enough, humans become unconscious or die. Exposure to moderate and high levels of carbon monoxide over long periods is positively correlated with risk of heart disease. People who survive severe carbon monoxide poisoning may suffer long-term health problems. Carbon monoxide from air is absorbed in the lung which then binds with hemoglobin in human's red blood cells. This would reduce the capacity of red blood cells to carry oxygen throughout the body.

Applications:

Humans have been making practical use of combustion for millennia. Cooking food and heating homes have long been two major applications of the combustion reaction. With the development of the steam engine by Denis Papin, Thomas Savery, Thomas Newcomen, and others at the beginning of the eighteenth century, however, a new use for combustion was found: performing work. Those first engines employed the combustion of some material, usually coal, to produce heat that was used to boil water. The steam produced was then able to move pistons and drive machinery. That concept is essentially the same one used today to operate fossil-fueled electrical power plants. Before long, inventors found ways to use steam engines in transportation, especially in railroad engines and steam ships. However, it was not until the discovery of a new type of fuel—gasoline and its chemical relatives—and a new type of engine—the internal combustion engine—that the modern face of transportation was achieved. Today, most forms of transportation depend on the combustion of a hydrocarbon fuel such as gasoline, kerosene, or diesel oil to produce the energy that drives pistons and moves the vehicles on which modern society depends. When considering how fuels are burned during the combustion process, "stationary" and "explosive" flames are treated as two distinct types of combustion. In stationary combustion, as generally seen in gas or oil burners, the mixture of fuel and oxidizer flows toward the flame at a proper speed to maintain the position of the flame. The fuel can be either premixed with air or introduced separately into the combustion region. An explosive flame, on the other hand, occurs in a homogeneous mixture of fuel and air in which the flame moves rapidly through the combustible mixture. Burning in the cylinder of a gasoline engine belongs to this category. Overall, both chemical and physical processes are combined in combustion, and the dominant process depends on very diverse burning conditions.

General Application Heads:

In heating devices In explosives In Internal-Combustion Engines In Rocket Propulsion In Chemical Reactions

Possible Types of Combustion Simulations:

Based on Mixing:

Non-Premixed Combustion ( Direct / Late Injection)

Premixed Combustion (Carburator)

Partially Premixed Combustion

Based on Phase:

Fluid Phase (Volumetric Reactions)

Wall (Surface Reactions)

Particles (Surface Reactions)

Porous Region (After Treatment System)

Basics of Combustion Simulations:

For combustion simulation, equations are solved as given below:

Mass

Momentum

Energy

Species

Transport

Equations ( Source Term is computed using Combustion Model)

We can give the basic equation for combustion as given below:

Non-premixed Combustion:

In non-premixed combustion, fuel and oxidizer enter the reaction zone in distinct streams. This is in contrast to premixed systems, in which reactants are mixed at the molecular level before burning.

Examples of non-premixed combustion include methane combustion, pulverized coal furnaces, and diesel (compression) internal-combustion engines.

Considerations: Here we are taking some considerations regarding non premixed combustion simulations, which are as given below:  

Diffusion:

Turbulent Diffusion:

Diffusion is combination of Laminar and Turbulent Diffusion. For engineering applications, Turbulent Diffusion is so high, that we can ignore the Laminar Diffusion. Typically, for IC Engines, Gas Turbines, Large Burners,

Turbulent Diffusion >> Laminar Diffusion

So, we use Fick’s Law for “Dilute Diffusion”. With help of “Dilute Approximation”, we are looking at diffusion coefficients of each and every species. In which how they diffuses into entire mixture, instead of diffusion coefficient from species to species. Generally, when the flow is turbulent, we should adapt the “Dilute Approximation”.

Combustion Source Calculation:

Finite Rate Eddy Dissipation Model:

IT is combination of “Finite Rate Chemistry” model and “Eddy Dissipation” Model. Where “Ignition” is going to take place, and “Ignition Delay” is important, this model would ideally choose “Finite Rate Chemistry”. And where Turbulence has major role than reaction rate is going to predicted by “Eddy Dissipation” model. So, we are applying both models. Based upon the flow condition, whichever rate is going to be the slowest, solver will pick that.

___________________________________________________________________________________________________ ANALYSIS:

Here we will perform simulation for combustion in 2D geometry in two parts. In Part_1, we will perform simulation on a combustor model and get the mass fraction of different species like CO2, H2O, CH4, N2, O2, NOX AND SHOOT formation. In Part_2, we will add water in the fuel from 5% to 30% by mole and observe the results of NOX emissions and SHOOT Formation.

For Part_2, we will do a parametric study.


For Fuel, Use Methane-Air-2step as a mixture. (IT allows to add water in the fuel or air) For Soot Formation, use one-step Soot Model.

  ___________________________________________________________________________________________________GEOMETRY:

                                                         FIG 1:FULL GEOMETRY

Here we can see the 3D geometry. If we run simulation for entire 3D model, then it will cost a lot in terms of time. So instead of 3D, we will perform simulation on 2D geometry, which is Axis-symmetric.

For 2D geometry, we have to cut 3D models in two parts, and extract 2D geometry of the quadrant portion. And for that, we can do it as given below. Also Get X-Y plane and cut the entire model (all three components) using “Split Body” command as shown above. Remaining Half portion can be cut out by X-Z plane as given below:

Then copy all face from X-Y Plane and Paste it in New Design. Make sure that we paste it only on X-Y Plane. After pasting, we can merge all faces by using “combine” command and can make it a single surface. So the 2D geometry can be ready for further process as given below:

                                                       FIG 2:FLUID VOLUME CREATION
Final 2D geometry will look like given below:

                                                   DIMESNIONAL SETUP OF 2D GEOMETRY
  ___________________________________________________________________________________________________PROCEDURE:  

After opening Fluent, browse Geometry named as “2D_Combustion”, which has been saved as above. After successfully attatching geometry, we can cross verify if it is the 2D geometry or not. Here geometry 2D. So, the assumption is that flow is Axis symmetric. Any change along the radial direction is 0. Rate of change of Velocity, Temperature and Pressure with respect to is 0, where  is radial direction. This is called Axis symmetric approximation.

MESHING:

Create appropriate Mesh for the geometry as given below:

Details: Element Size: 1 mm

Method: All triangle Method

Edge Sizing: Element Size: 0.01 mm

Capture Curvature : Yes Capture Proximity : Yes

Statistics:

No. of nodes: 307467 No. of Elements: 608928

                                                                 FIG 3:MESHED MODEL

NAMED SELECTION

Create named selection for Air_Inlet, Fuel_Inlet, Outlet, Axis and Walls as given below:

                                                      FIG 4:BOUNDARY CONDITIONS

   _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

 SETUP DETAILS:

General Settings:

Analysis type: Steady-state

Solver: Pressure based solver

2D Space: Axis symmetric  

Energy Model: On  

Viscous Model: Model: K-Epsilon K-Epsilon Model: Standard Near Wall Treatment: Standard Wall Functions  

Species Model:

In species model, we can set parameters related to calculations of species transport and combustion.

Model: Species Transport

It enables calculations of multi-species transport. (Either reacting or non-reacting. Depends on the selection of “Reactions”)

Reactions: Volumetric

It contains options related to modeling of reacting flow. By Volumetric, we can enable calculation of reacting flow with finite rate formulation.

Options: 1. Inlet Diffusion                

2. Diffusion Energy Source.         

Inlet diffusion includes diffusion flux of species at inlet. Diffusion energy source includes effect of enthalpy transport due to species diffusion in energy equation

                     FIG 5:SPECIES SETUP IN FLUENT

Mixture Material: methane-air-2step

Turbulent Chemistry Interactions: Eddy-Dissipation  

Species Transport Reactions:

Nox

Shoot

Enable Nox & Shoot Model for Species Transport Reactions as given below:

                                                     FIG 6:NOX AND SOOT ENABLING

IN fuel Species in Nox Model, enable all pathways and select only 5 species. We can see in above image that, CO is not selected. Select only CH4, H2O, O2, and N2 & CO2. In Shoot model, select one-step model as shown in above image.

Boundary Conditions: (For Part_1)   Air_inlet: Velocity Inlet Velocity Magnitude: 0.5 m/s Temperature: 300 k Species Mass Faction: o2 = 0.23

Fuel_inlet: Velocity Inlet Velocity Magnitude: 80 m/s Temperature: 300 k Species Mass Faction: ch4 = 1

Outlet: >Type: Pressure-outlet   >Gauge Total Pressure: 0 Pa   >Back Flow Total Temperature: 300 k   Wall:   >Type: Wall >Wall Motion: Stationary Wall >Shear Condition: No Slip >Temperature: 300 k  

Then create report definition for temperature contours. Initialize with Hybrid initialization. Then create contours for Temperature, co2, ch4, o2, h2o, n2, n2o, no and shoot. Also create Animation for Temperature contours. After completing all procedure as described above, run the simulation for about 1200 to 1500 iterations.

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

Boundary Conditions: (For Part_2)  

Air_inlet: Velocity Inlet

Velocity Magnitude: 0.5 m/s

Temperature: 300 k

Species Mass Faction: o2 = 0.23

                                                FIG 7:AIR INLET B.C DEFINITION

Fuel_inlet: Velocity Inlet

Velocity Magnitude: 80 m/s

Temperature: 300 k

Species Mole Faction: ch4 = parameter_1 = 0.95 Species Mole Faction: ch4 =  parameter_2 = 0.05  

Here, in boundary condition of Part_2, We have changed the setting for fuel inlet.  We have defined parameters for species “ch4” and “h2o” in Species Mole Fraction. We can give initial value for ch4 = 0.95 and for h2o = 0.05. BY this we have added water into fuel by 5 % mole. Also by applying parameter to it, we can get results for different percentage changes in h2o % amount in fuel.
 
Then create report definition for temperature contours, Mass fraction of Nox and Shoot Formation.
Make sure that “Create output Parameter” is enabled. We can see highlighted box in above image. Initialize with Hybrid initialization. Then create contours for Temperature, no, n2o, shoot. After completing all procedure as described above, run the simulation for about 500 iterations.

After completion of simulation, open the parameter section and give different percentage value of ch4 and h20 to add water in required amount. See the image below.

                                    FIG 8:PARAMETRIC STUDY OF COMBUSTION

After input of all necessary parameter, make sure that “Retain” option is enabled. Then select “Update all Design Points”. We can see in above image that, by updating all design points, we can get all results according to input parameters.

  ___________________________________________________________________________________________________ RESULTS:

Now we will see the results of all cases for Part_1 and Part_2 cases one by one.  

Part_1: 

FIG 9:RESIDUALS:

FIG 10:TEMPERATURE:

 FIG 11:TEMPERATURE AT THE OUTLET

FIG 12:CONTOURS OF SOOT:

FIG 13:CONTOURS OF CO2 MASS FRACTION:

FIG 14:CONTOURS OF H20 MASS FRACTION

FIG 15:CONTOURS OF CH4 MASS FRACTION

FIG 16:CONTOURS OF NOx MASS FRACTION:

Now we will see the Variation of Mass Fraction of different species at different locations on the combustor.

With help of line probes, we can plot the variation of Mass Fraction at different locations. For that we have to create line probes at different locations. Line probes at different locations from left to right in meter can be created as given below: 

Variation of Mass Fraction of different species at above locations can be given by plots as follows: 

FIG 17: Variation of Mass Fraction of CO2:

FIG 18:Variation of Mass Fraction of H2O:

 FIG 19:Variation of Mass Fraction of CH4:

FIG 20:Variation of Mass Fraction of N2:

FIG 21:Variation of Mass Fraction of O2:

FIG 22:Variation of Mass Fraction of NOx: N2O:

NO:

 FIG 23:Variation of Mass Fraction of Soot:

Part_2:

                                                          FIG 24:CONTOURS OF TEMPERATURE: WATER 10%

                                                         FIG 25:CONTOURS OF LINE PROBES: WATER 10%

                                                     FIG 26:CONTOURS OF LINE PROBES FOR NO: WATER 10%

CASE II: WATER 20%

                                                      FIG 27: TEMPERATURE CONTOUR
 

                                                      FIG 28: LINE PROBES MASS FRACTION OF SOOT CONTOUR

                                                            FIG 29: LINE PROBES FOR NO

CASE III: 30 % WATER

                                                            FIG 30: TEMPERATURE CONTOUR

                                                             FIG 31: LINE PROBES FOR SOOT

                                                              FIG 32:LINE PROBES FOR NO

CASE IV:40% WATER

                                                           FIG 33:TEMPERATURE CONTOUR

                                                                  FIG 34:LINE PROBES FOR SOOT

                                                            FIG 35: LINE PROBE FOR NO MASS FRACTION
  ___________________________________________________________________________________________________Comparison:   Comparison of Mass Fraction of Different species in Part_1:

We can see that mass fraction on O2 increases up to 0.23. After that it decreases slowly. We can assume that most of the O2 has burnt. Also we can see that from the beginning, Ch4 is burning from high at 1 to near 0. But in real there are some unburnt carbon particles always remains. Due to this incomplete combustion of hydrocarbons, generation of Soot or fly Ash happens. We can observe that Co2 and H2o have nearly similar contours. Because both increase at center and decreases gradually, as the temperature becomes steady. We can see there are Nox emissions, which are due to combustion of nitrogen present in the air, which is 78%.Due to that we can see N2 is maximum all over the contours.

Comparison of Temperature & Mass Fraction of Nox and Shoot:

We can observe very small decreases in temperature from CASE_1 to CASE_4 , as we have added water in fuel in each case from 5 % to 30%. Also we can observe small decrease in Nox emissions as we add water from 5 % to 30%. We can see the significant amount of decrease in Soot formation from CASE_1 to CASE_4. Also there is no ignition source for combustion. So, rate of reactions are controlled by turbulence mixing in Eddy dissipation model. So with large eddy mixing time scale, combustion starts when turbulence comes in picture.

  ___________________________________________________________________________________________________CONCLUSION:

As per above observations , we can say that due to adding water to the fuel, helps in decreasing Nox and Shoot Formation. Water addition leads to increase OH radicals that might have a significant impact in Shoot Oxidation and reduce the Shoot formed in Gas Phase. Added water dilutes calories generated by combustion. That decreases combustion temperature. If temperature reduces sufficiently, Nox cannot be form in great concentration. We can see small decrease in temperature. Reducing combustion temperature means avoiding stochiometric ratio. If this ratio is low, it generates lower concentration of thermal Nox. So, with these or various types of technologies, we can reduce Nox emissions and Shoot formation, which helps us to meet regulatory requirements by government. And we can reduce its harmful effects on environment and humans.