Experimental, Computational, and Chemical Kinetic Analysis to Compare the Flame Structure of Methane-Air with Biogas–H2–Air

We studied the effects of increasing pressure and adding carbon dioxide, hydrogen and nitrogen to Methane-air mixture on premixed laminar burning velocity and NO formation in experimentally and numerically methods. Equivalence ratio was considered within 0.7 to 1.3 for initial pressure between 0.1 to 0.5 MPa and initial temperature was separately considered 298 K. Mole fractions of carbon dioxide, hydrogen and nitrogen were regarded in mixture from 0 to 0.2. Heat flux method was used for measurement of burning velocities of Methane-air mixtures diluted with CO2 and N2. Experimental results were compared to the calculations using a detailed chemical kinetic scheme (GRI-MECH 3.0). The results in atmosphere pressure for Methane-air mixture were calculated and compared with the results of literature. Results were in good agreement with published data in the literature. Then, by adding carbon dioxide and nitrogen to Methaneair mixture, we witnessed that laminar burning velocity was decreased, whereas by increasing hydrogen, the laminar burning velocity was increased. Finally, the results showed that by increasing the pressure, the premixed laminar burning velocity decreased for all mixtures, and NO formation indicates considerable increase, whereas the laminar flame thickness decreases.

Spherically expanding flames propagating at constant pressure were employed to determine the laminar burning velocity and flammability characteristics of biogas-air mixtures in premixed combustion to uncover the fundamental flame propagation characteristics of a new alternative and renewable fuel. The results are compared with those from a methane-air flame. Biogas is a sustainable and renewable fuel that is produced in digestion facilities. The composition of biogas discussed in this paper consists of 66.4% methane, 30.6% carbon dioxide and 3% nitrogen. Burning velocity was measured at various equivalence ratios (ϕ) using a photographic technique in a high pressure fan-stirred bomb, the initial condition being at room temperature and atmospheric pressure. The flame for methane-air mixtures propagates from ϕ=0.6 till ϕ=1.3. The flame at ϕ ≥ 1.4 does not propagate because the combustion reaction is quenched by the larger mass of fuel. At ϕ≤0.5, it does not propagate as well since the heat of reaction is insufficient to burn the mixtures. The flame for biogas–air mixtures propagates in a narrower range, that is from ϕ=0.6 to ϕ=1.2. Different from the methane flame, the biogas flame does not propagate at ϕ≥1.3 because the heat absorbed by inhibitors strengthens the quenching effect by the larger mass of fuel. As in the methane flame, the biogas flame at ϕ≤0.5 does not propagate. This shows that the effect of inhibitors in extremely lean mixtures is small. Compared to a methane-air mixture, the flammability characteristic (flammable region) of biogas becomes narrower in the presence of inhibitors (carbon dioxide and nitrogen) and the presence of inhibitors causes a reduction in the laminar burning velocity. The inhibitor gases work more effectively at rich mixtures because the rich biogas-air mixtures have a higher fraction of carbon dioxide and nitrogen components compared to the lean biogas-air mixtures.

Laminar burning velocity, emissions, and flame structure of dimethyl ether-hydrogen air mixtures

Effect of hydrogen addition on laminar burning velocity of CH4/DME mixtures by heat flux method and kinetic modeling

In a future sustainable society, hydrogen is likely to play an important role as an energy carrier. In an EET-project called Greening of Gas (VG2) the transition path from pure natural gas towards the use of mixtures containing more and more hydrogen is investigated. The research carried out at the TU/e is focused on the safety of burner devices. A crucial parameter for the safety of burner devices is the laminar burning velocity. In this thesis the laminar burning velocity of methane-hydrogen mixtures is experimentally determined and compared to numerical data using several combustion reactionmechanisms. An asymptotic theory for stoichiometric methane hydrogen flames is presented. This theory is validated with the experimental and numerical data. To measure the laminar burning velocity accurately the heat flux burner is used, which is developed previously at TU/e. Based on the earlier works of van Maaren and Bosschaart the heat flux method is further analysed in this thesis. This analysis results in a better understanding of several aspects of themethod. For example it is shown that the influence of the heating jacket is negligible when using a temperature difference of at least 30 K between the unburnt gas temperature and the temperature of the heating jacket should be maintained. Furthermore, it is not likely that the burner surface influences the heat flux experiments in the presented measurement range. However when higher unburnt gas temperatures will be used this influence should be regarded. In the present research, three sets of laminar adiabatic burning velocities have been measured and presented using 95% confidence error intervals. The first set consists of hydrogen-oxygen-nitrogen mixtures at various fuel equivalence ratios and several nitrogen dilutions. The second set of measurements deals with methane-hydrogen-air mixtures at various fuel equivalence ratios and hydrogen contents up to 40%. The last set of measurements show a glimpse towards gas turbine situations. Here the unburnt gas temperature is increased from298 K up to 420 K for methane-hydrogen-airmixtures. The laminar burning velocity measurement data of hydrogen-oxygen-nitrogen mixtures, show significant differences with experimental results of other authors. This discrepancy is probably related to the non-linear stretch correction performed by them. The differences between the combustion reaction mechanisms and the heat flux data show significant differences in the performance of the methane based combustion reaction mechanisms in the case of hydrogen-oxygen-nitrogenmixtures. Especially the commonly used GRI-mechanism deviates from the experimental data. Remarkably the performance of the methane based SKG03 mechanism is comparable or even better compared to hydrogen based mechanisms for fuel lean flames to slightly rich hydrogen-oxygennitrogen flames. Generally, the hydrogen based kinetic mechanisms perform quite well for the investigated parameter range; especially the Konnov mechanism. When comparing the measurements of the laminar burning velocities at ambient conditions as well as increased unburnt gas temperatures of methane-hydrogen-air mixtures with numerical combustion mechanisms it is shown that both the SKG03 mechanism and the GRI-mechanism perform very well. Experimental data of the laminar burning velocities of methane-air flames show that the measurements of Bosschaart give comparable results with the present measurements. Regrettably experimental data of methanehydrogen- air flames is scarce; the data of Halter et al. show comparable results. In order to get more insight in the basic properties describing methane-hydrogen-air flames, the asymptotic theory of Peters and Williams for stoichiometric methane-air flames is adapted to stoichiometricmethane-hydrogen-air flames. This theory is validated both with experiments performed using the heat flux burner and numerical simulations using CHEM1D. With this theory for stoichiometric flames the laminar burning velocity as a function of the hydrogen content can be predicted qualitatively even for higher pressures and temperatures. The resulting equations show that the driving force for the increase in burning velocity of a methane-hydrogen flame is the increase in temperature difference between the inner layer temperature and the adiabatic flame temperature.

Experimental and computational investigation of the laminar burning velocity of hydrogen-enriched biogas

Biogas is an alternative energy source that is sustainable and renewable containing more than 50% CH4 and its biggest impurity or inhibitor is CO2. Demands for replacing fossil fuels require an improved fundamental understanding of its combustion processes. Flammability limits and laminar burning velocities are important characteristics in these processes. Thus, this research focused on the effects of CO2 on biogas flammability limits and laminar burning velocities in spark ignited premixed combustion. Biogas was burned in a spark ignited spherical combustion bomb. Spherically expanding laminar premixed flames, freely propagating from spark ignition in initial, were continuously recorded by a high-speed digital camera. The combustion bomb was filled with biogas-air mixtures at various pressures, CO2 levels and equivalence ratios (ϕ) at ambient temperature. The results were also compared to those of the previous study into inhibitorless biogas (methane) at various pressures and equivalence ratios (ϕ). Either the flammable areas become narrower with increased percentages of carbon dioxide or the pressure become lower. In biogas with 50% CO2 content, there was no biogas flame propagation for any equivalence ratio at reduced pressure (0.5 atm). The results show that the laminar burning velocity at the same equivalence ratio declined in respect with the increased level of CO2. The laminar burning velocities were higher at the same equivalence ratio by reducing the initial pressure.

Effects of hydrogen and carbon dioxide on the laminar burning velocities of methane–air mixtures

Laminar burning velocity of n-butane/Hydrogen/Air mixtures at elevated temperatures

Experimental and numerical study of laminar burning velocity for Diisobutylene+ PRF/TRF mixtures

Flammability limits and flame speed of methane-carbon dioxide-air mixtures have been studied to understand the effect of carbondioxide on the flammability characteristic of biogas. The fuel of biogas discussed in this study was made by mixing gases of methane and carbon dioxide. The carbon dioxide was varied from 0% (by volume) untill reach the flammability limit of the stoikhiometri biogas-air mixtures. The observation was done using a cubic combustion bomb with the dimension of 500 mm x 200 mm x 10 mm with the initial condition being at room temperature and atmospheric pressure. The ignitor was set at the top of combustion bomb, so the flame propagated downward. Base on the observation results, the presence of carbon dioxide in the fuel ofbiogas caused the flammability limits of biogasair mixture narrower. The biogas-air mixture was still flammable with the highest content of carbon dioxide of 62.5 %vol when the mixture was sthoichiometri. Compared to methane-air mixture, the presence of carbon dioxide in biogas caused a reduction in the flame speed. The stoichiometri mixture has the highest flame speed when the carbon dioxide was not present in the fuel. However, when the carbon dioxide was added in the fuel, the rich mixture has the highest flame speed. This is a consequence of the rich biogas-air mixture having a higher fraction of the carbon dioxide components from the fuel compared to the stoichiometri and lean biogas-air mixture. The result also indicated that at the upper limit the flame still propagated downward to closed to the endwall. However, at the lower limit (lean mixtures), the flame did not intend to propagate downward, it was just at the top and propagate sideward.

In the present work, the combustion characteristics like adiabatic flame temperature (AFT) and laminar burning velocity (LBV) of methane (CH 4 ) diluted with carbon dioxide (CO 2 ), representing biogas, is investigated in detail. The laboratory prepared biogas samples containing CH 4 and CO 2 were also enriched with hydrogen (H 2 ) to realize the change in their combustion behaviour. The experiments were conducted on flat flame burners based on heat flux method at 1 bar, 298 K and at stoichiometric and off-stoichiometric conditions. The experimental results were also compared with the numerical predictions of ANSYS Chemkin-Pro ® with full GRI Mech. 3.0 reaction mechanism. The results revealed that the presence of CO 2 in biogas dominates on richer mixtures, i.e., the CO 2 dilution affects the combustion characteristics of richer biogas mixtures more strongly than for leaner or stoichiometric mixtures. The simulated results of hydrogen-enriched biogas showed that the slope of the laminar burning velocity (LBV) curve for biogas containing the highest percentage of CO 2 sharply rises with about 33% H 2 , whereas the slope of the mixture with least CO 2 and more CH 4 , sharply changes its nature around 40% H 2 -enrichment. This signifies that even with a small amount of H 2 in biogas, may be a suitable option to improve its combustion characteristics. Some preliminary correlations for H 2 -enriched biogas were also derived to estimate the effect of H 2 presence in biogas fuels with higher Hydrogen concentrations

Experimental and numerical investigation of combustion characteristics of carbon-free NH3/H2 blends in N2O

Numerical study on laminar burning velocity and ignition delay time of ammonia/methanol mixtures

Effect of inert species on the laminar burning velocity of hydrogen and ethylene