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1.
In the present study, the air turbulator, which is a part of a nonpremixed burner, is investigated numerically in terms of its effects on the diffusion methane flame structure and NOX emissions. A computational fluid dynamics (CFD) code was used for the numerical analysis. At first, four experiments were conducted using natural gas fuel. In the experimental studies, the excess air ratio was taken constant as 1.2, while the fuel consumption rate was changed between 22 and 51 Nm3/h. After the experimental studies, the CFD studies were carried out. Pure methane was taken as fuel for the simulations. The nonpremixed combustion model with the steady laminar flamelet model (SFM) approach was used in the combustion analyses. Methane‐air extinction mechanism with 17 species and 58 reactions was used for the simulations. The results obtained from the CFD studies were confronted with the measurements of the flue gas emissions in the experimental studies. Then, a modified burner head was analysed numerically for the different air turbulator blade numbers and angles. The CFD results show that increasing the air turbulator blade number and angle causes the thermal NO emissions to be reduced in the flue gas by making the flame in the combustion chamber more uniform than the original case. This new flame structure provides better mixing of the fuel and combustion air. Thus, the diffusion flame structure in the combustion chamber takes the form of the partially premixed flame structure. The maximum reduction in the thermal NO emissions in the flue gas is achieved at 38% according to the original case. 相似文献
2.
《International Journal of Hydrogen Energy》2020,45(10):5971-5986
In the present study, the effects of hydrogen enrichment of methane are investigated numerically from the diffusion flame structure and emissions aspect. Fluent code is utilised as the simulation tool. In the first part of the study, four experiments were conducted using natural gas as fuel. A non-premixed burner and a back-pressure boiler were utilised as the experimental setup. The natural gas fuel consumption rate was changed between 22 Nm3/h and 51 Nm3/h. After the experimental studies, the numerical simulations were performed. The non-premixed combustion model with the steady laminar flamelet model (SFM) approach was used for the calculations. The methane-air extinction mechanism was utilised for the calculation of the chemical species. The numerical results were verified with the experimental results in terms of the flue gas emissions and flue gas temperature values. In the second part of the study, four different hydrogen-enriched methane combustion cases were simulated using the same methane-air extinction mechanism, which included the hydrogen oxidation mechanism as a sub mechanism. The same energy input (432 kW) was supplied into the boiler for all the studied cases. The obtained results show that the hydrogen addition to methane significantly change the diffusion flame structure in the combustion chamber. The hydrogen-enriched flames become broader and shorter with respect to the pure methane flame. This provides better mixing of the reactants and combustion products in the flame regions due to the use of a back-pressure boiler. In this way, the maximum flame temperature values and thermal NO emissions are reduced in the combustion chamber, when the hydrogen addition ratio is less than 15% by mass. The maximum temperature value is calculated as 2030 K for the case with 15% hydrogen addition ratio by mass, while it is 2050 K for the case without hydrogen enrichment. Therefore, it is determined that the hydrogen-enriched methane combustion in a back-pressure combustion chamber has the potential of reducing both the carbon and thermal NO emissions. 相似文献
3.
The analysis of local entropy generation and exergy loss was performed in a turbulent non-premixed H2-enriched CH4–air bluff-body flame. Detailed chemical kinetic, transport properties, and turbulence-chemistry interaction were taken into account in using laminar flamelet model for the simulation of combustion process via an in-house, finite volume code. The analysis was based on local entropy generation calculation. Results showed that thermal conduction made the most contribution to entropy generation followed by chemical reaction and mass diffusion, while the contribution of viscous dissipation was negligible. Entropy generation resulting from thermal conduction occurs in a large volume of the domain, while entropy generation resulting from chemical reaction and mass diffusion occurs only near the bluff surface. The effect of H2 addition to fuel and air preheating on the entropy generation rate was investigated. It was observed that entropy generation and exergy loss were decreased by H2 addition, mainly due to a decrease in the chemical reaction component of entropy generation, while entropy generation resulting from thermal conduction slightly increased and entropy generation resulting from mass diffusion remained almost constant. Entropy generation resulting from heat conduction by preheating combustion air decreased, while entropy generation resulting from chemical reaction and mass diffusion remained almost constant. The decrease of thermal conduction contribution in entropy generation is so significant that, by preheating air up to 750 K in the case of pure CH4, chemical reaction becomes the main source of irreversibility. These investigations show that H2 addition and preheating the combustion air both lead to the improvement of the second law efficiency, although the second law efficiency is more sensitive to flame structure and air temperature. 相似文献
4.
An industrial burner operating in the MILD combustion regime through internal recirculation of exhaust gases has been characterized numerically. To develop a self-sufficient numerical model of the burner, two subroutines are coupled to the CFD solver to model the air preheater section and heat losses from the burner through radiation. The resulting model is validated against experimental data on species concentration and temperature. A 3-dimensional CFD model of the burner is compared to an axisymmetric model, which allows considerable computational saving, but neglects some important burner features such as the presence of recirculation windows. Errors associated with the axisymmetric model are evaluated and discussed, as well as possible simplified procedures for engineering purposes. Modifications of the burner geometry are investigated numerically and suggested in order to enhance its performances. Such modifications are aimed at improving exhaust gases recirculation which is driven by the inlet air jet momentum. The burner is found to produce only 30 ppmv of NO when operating in MILD combustion mode. For the same air preheating the NO emissions would be of approximately 1000 ppmv in flame combustion mode. It is also shown that the burner ensures more homogeneous temperature distribution in the outer surfaces with respect to flame operation, and this is attractive for burners used in furnaces devoted to materials' thermal treatment processes. The effect of air excess on the combustion regime is also discussed. 相似文献
5.
To fundamentally elucidate the requirement for an inherently safe technique of rapidly mixed type tubular flame combustion, experiments have been made to investigate (1) the mixing process of fuel and oxidizer, and (2) the appearances of methane flames under various oxygen mole fractions. Three optically accessible quartz burners of different slit widths were made for measuring the mixing layer thickness with a PIV system. Under various rates of flow of the oxidizer to the fuel, a boundary layer type flow is recognized to dominate the mixing of fuel and oxidizer around the exit of the injection slit, namely the mixing layer thickness is inversely proportional to the square root of mean injection velocity. Using two stainless steel burners, combustion tests were conducted with the oxidizers of oxygen/air mixtures. To quantitatively investigate the requirement for tubular flame establishment, the Damköhler number, which is the ratio of characteristic mixing time to characteristic chemical reaction time, has been discussed in detail. The mixing time was calculated according to estimated mixing layer thickness, while the chemical reaction time was computed with the Chemkin code. The Damköhler number has proved to be a useful measure for success/failure of tubular flame combustion. When the Damköhler number is larger than unity, chemical reaction starts before complete fuel/air mixing and the tubular flame fails to be established; when the Damköhler number is much smaller than unity, the fuel and the oxidizer are completely mixed before the onset of reaction, resulting in successful tubular flame combustion. The results confirm our hypothesis in a previous study. Furthermore, based on the concept of Damköhler number, the minimum flow rate to achieve the tubular flame combustion could be estimated. 相似文献
6.
《International Journal of Hydrogen Energy》2022,47(24):12317-12337
In the present study, the pure ammonia combustion in a model combustor is performed to seek ammonia-fueled applications. To this aim, effects of the oxygen enrichment with an oxygen concentration of 100% in the oxidizer on flame characteristics, temperature profiles and NO profiles during the ammonia combustion were evaluated in terms of excess air/oxygen coefficients. Furthermore, in order to better understand the effect of an oxygen of 100% usage under the oxy-ammonia combustion conditions, the air-ammonia combustion has been studied as well and their results are compared and discussed each other. According to the results predicted, the oxidizer with an oxygen content of %100 provides better flame stability in the case of pure ammonia combustion. The most stable flame for oxy-ammonia combustion can be achieved when the excess oxygen coefficient is 1.0 or 1.2. Furthermore, the minimum NO levels emerge under the fuel-rich condition. Temperature and NO emissions decrease considerably under the air-ammonia combustion. However, except the fuel-rich conditions, flame stabilities are not satisfactory due to ammonia's flame speed under the air-ammonia combustion. Moreover, the air-ammonia combustion under the fuel-rich condition seems as a good option for obtaining the lowest NO levels. On the other hand, the oxy-enrichment condition is thought as a promising method for pure ammonia combustion provided that NO emissions should be optimized by using NO reduction methods. 相似文献
7.
8.
An experimental and computational investigation of a lab-scale burner, which can operate in both flame and MILD combustion conditions and is fed with methane and a methane/hydrogen mixture (hydrogen content of 60% by vol.), is carried out. The modelling results indicate the need of a proper turbulence/chemistry interaction treatment and rather detailed kinetic mechanisms to capture MILD combustion features, especially in presence of hydrogen. Despite these difficulties, Computational Fluid Dynamics results to be very useful, as for instance it allows evaluating the internal recirculation degree in the burner, a parameter which is otherwise difficult to be determined. Moreover the model helps interpreting experimental evidences: for instance the modelling results indicate that in presence of hydrogen the NNH and N2O intermediate routes are the dominant formation pathways for the MILD combustion conditions investigated. 相似文献
9.
Experimental study of a domestic boiler using hydrogen methane blend and fuel-rich staged combustion
《International Journal of Hydrogen Energy》2021,46(75):37628-37640
Storing excess wind and solar energy in the form of hydrogen injected into the natural gas grid is one of the main ingredients of the energy transition. This hydrogen injection has an impact on emissions and the performance of user equipment. The present work reports on an experimental study of the combustion of methane-hydrogen mixture with fuel rich transverse staggered injection. The 15 kW domestic boiler used was equipped with 16 burners (2 × 8). The aim of this work is to better understand the simultaneous effects of fuel rich staged combustion (Ø = 2.0 ÷ 4.0), hydrogen blending of methane (0 ÷ 45%) on pollutant emissions and efficiency while maintaining a compromise between high power, fuel economy and low emissions. The results show that NOx, CO2 emissions decrease, CO and CxHy emission values increase and thermal efficiency values decrease with increasing hydrogen percentage and fuel rich staggering combustion. 相似文献
10.
Although many detailed chemical reaction mechanisms, skeletal mechanisms and reduced mechanisms are available in the literature to modeling the natural gas, they are computational expensive, required high power computing especially for three dimensional complex geometries with intense meshes. For example, though the DRM19 reduced mechanism does not include NO and NO2 species, it includes 19 species and 84 reactions. On the other hand, Eddy Dissipation combustion model in which the overall rate of reaction is mainly controlled by turbulent mixing can be utilized as a practical approach for fast burning and fast reaction fuels such as natural gas. Unlike fossil fuels, hydrogen is a renewable energy and quite clean in terms of carbon monoxide and carbon dioxide emissions. However, numerical and experimental studies on hydrogen combustion in burners are very restricted. In this study, the combustion of natural gas in an industrial low swirl burner–boiler system has been experimentally investigated. The results obtained from the experimental setup have been utilized as boundary conditions for CFD simulations. With the use of Eddy Dissipation method, methane-air-2-step reaction mechanism is used for modeling of natural gas as methane gas and the reaction mechanism has been modified for natural gas considering the natural gas properties to reveal the similarities and differences of both fuels in modeling. In addition, the combustion performances of natural gas with the use of full and periodic models, which are geometric models of the burner–boiler pair, are compared. Moreover, in order to reveal the effect of the hydrogen-enriched natural gas and pure hydrogen on the performance of low swirl burner–boiler considering the combustion emissions, four various gas contents (thermal load ratio: 75%NG + 25%H2, 50%NG + 50%H2, 25%NG + 75%H2, 100%H2) at the same thermal load have been investigated. The turbulent flames of the industrial low swirl burner have been studied numerically using ANSYS Fluent 16.0 for the solution of governing equations. The results obtained in this study show that with the utilizing Eddy Dissipation method, natural gas can be modeled as methane gas with well-known methane-air-2step reaction mechanism or as natural gas with modified methane-air-2step reaction mechanism with approximate results. Additionally, the use of periodic boundary condition, which enables studying with 1/4 of geometric model, gives satisfactory results with less number of meshes when compared to the full model. Furthermore, in the case of using hydrogen-enriched natural gas or pure hydrogen instead of natural gas as the fuel, the combustion emissions of the burner–boiler such as CO and CO2 are remarkably decreasing compared to the natural gas. However, the NOx emissions are significantly increasing especially due to thermal NO. 相似文献
11.
Guang Tang Pengfei Jin Yulei Bao Wai Siong Chai Lei Zhou 《International Journal of Hydrogen Energy》2021,46(39):20765-20776
In this study, a specially designed premixed combustion chamber system for ammonia-hydrogen and methane-air laminar premixed flames is introduced and the combustion limits of ammonia-hydrogen and methane-air flames are explored. The measurements obtained the blow-out limits (mixed methane: 400–700 mL/min, mixed hydrogen: 200–700 mL/min), mixing gas lean limit characteristics (mixed methane: 0–82%, mixed hydrogen: 0–37%) and lean/rich combustion characteristics (mixed methane: ? = 0.6–1.9, mixed hydrogen: ? = 0.9–3.2) of the flames. The results show that the ammonia-hydrogen-air flame has a smaller lower blow-out limit, mixing gas ratio, lean combustion limit and higher rich combustion limit, thereby proving the advantages of hydrogen as an effective additive in the combustion performance of ammonia fuel. In addition, the experiments show that increasing the initial temperature of the premixed gas can expand the lean/rich combustion limits of both the ammonia-hydrogen and ammonia-methane flames. 相似文献
12.
The combustion process of a four-stroke optically accessible single cylinder Port Fuel Injection spark ignition (PFI SI) engine was experimentally investigated. It was fueled with two methane/hydrogen blends. The in-cylinder pressure and the related data were analyzed as indicators of the combustion quality. 2D-digital imaging measurements were performed to evaluate the flame propagation. UV–visible spectroscopy allows to characterize the combustion by means of the detection of OH* and CH*. The exhaust was characterized using conventional analyzers. For the methane/hydrogen blends the indicated data suggests an increase of the thermal efficiency and a decrease of the combustion duration with the increase of the hydrogen fraction. The optical results highlight a more homogeneous mixture that increases the combustion reaction rate and provides a more uniform and rapid flame propagation. On the other hand, high NOx emissions were measured likely because of the higher combustion temperature due to hydrogen addition. 相似文献
13.
14.
Haitao Zhang Fengyu Li Gaofeng Wang Qifu Huang 《Energy Sources, Part A: Recovery, Utilization, and Environmental Effects》2018,40(11):1366-1373
Stabilization and autoignition mechanisms of lifted flames have been widely investigated to improve combustion efficiency and safety of combustion equipment. This paper focuses on liftoff behavior and combustion characteristic of methane and propane flames under various coflow conditions in a coflow burner. Unlike the case of free jet flame in ambient air, the different tendencies of liftoff height changes with jet velocity for both methane and propane flames in vitiated coflow illustrate a transition from conventional combustion to Moderate & Intense Low Oxygen Dilution (MILD) combustion. Flame temperature difference with radial position measured by primary spectrum pyrometry proves the transition regime. 相似文献
15.
《International Journal of Hydrogen Energy》2021,46(76):38106-38118
In this study, combustion and emission characteristics of methane mixed with steam (CH4/H2O) and the products of methane reforming with steam (CO/H2/H2O) were compared. Four fuel compositions were analysed: CH4+H2O, CH4+2H2O, and products of complete methane reforming in these mixtures, respectively. A comparison was carried out through the numerical model created via Ansys Fluent 2019 R2. A combustion process was simulated using a non-premixed combustion model, standard k-ϵ turbulence model and P-1 radiation model. The combustor heat capacity for interrelated fuel compositions was kept constant due to air preheating before combustion. The inlet air temperature was varied to gain a better insight into the combustion behaviour at elevated temperatures. The effect of steam addition on the emission characteristics and flame temperatures was also evaluated. NOx formation was assessed on the outlet of the combustion zone. The obtained results indicate that syngas has a higher combustion temperature than methane (in the same combustor heat capacity) and therefore emitted 27% more NOx comparing to methane combustion. With the air inlet temperature increment, the pollutant concentration difference between the two cases decreased. Steam addition to fuel inlet resulted in lesser emissions both for methane and syngas by 57% and 28%, respectively. In summary, syngas combustion occurred at higher temperature and produced more NOx emissions in all cases considered. 相似文献
16.
《International Journal of Hydrogen Energy》2022,47(26):13143-13156
To understand the fundamental mechanisms of NO formation in natural gas-diesel dual fuel combustion, a numerical study on NO formation in laminar counterflow methane (CH4)/n-heptane (n-C7H16) dual fuel flames is conducted. The results reveal that the flame structure and NO formation vary with the fuel equivalence ratio. For a given n-C7H16/air mixture, the NO emission index decreases with increasing the equivalence ratio of the CH4/air mixture (φ(CH4/air)). The NO formation route analysis suggests that the prompt and thermal routes dominate the NO formation. The increase in φ(CH4/air) causes the decrease in the contribution of the prompt route to overall NO formation. NO formation by prompt route is mainly caused by rich n-C7H16 combustion. As φ(CH4/air) increases, the mole fractions of the radicals (OH, O and H) related to CH formation in the reaction zone of rich n-C7H16/air flame branch are decreased, which reduces the formation of NO by prompt route. 相似文献
17.
The explosion process of multi-component gas mixture is extremely complex and may cause serious disaster effects. The safety issue concerning explosion of multi-component gas mixture is urgent to be investigated on account of its wide range of applications. In current work, series of experiments were performed in a 20 L spherical explosion vessel at initial conditions of 1 atm and 293 K, involving methane–hydrogen/air mixtures. The proportion of hydrogen in fuels varied from 0% to 100%. It was observed that peak temperature is always behind the peak pressure in arrival time whatever the fuel equivalence is. Experimental values of peak overpressure are lower than adiabatic ones due to heat loss. It was also founded that the hydrogen addition can raise explosion pressure and temperature in experiment but slightly decrease that in adiabatic condition, and both the increase in experiment and the decrease in adiabatic show a linear correlation versus the proportion of hydrogen. Hence the deviation between the experimental results and the adiabatic results decreases as the hydrogen proportion rises. Moreover, the positive effect of hydrogen addition on (dp/dt)max is very slight at low hydrogen proportion, while the effect becomes much more pronounced at higher hydrogen contents, showing an exponential growth. For each fuel composition throughout all experiments, the peak overpressure, peak temperature and (dp/dt)max concerning fuel equivalence ratios of 0.6, 1 and 1.5 follow a same rule: Ф = 1 is the highest, followed by Ф = 1.5 and Ф = 0.6. Finally, the MIEs of gaseous methane–hydrogen/air mixtures at a fuel equivalence ratio of 1.5 were measured as a function of hydrogen proportion. It shows a sharp decrease as the fraction of hydrogen in fuel rises, from 118 mJ for methane–air to 0.12 mJ for hydrogen–air. It is also observed that the MIE of multi-component gas mixtures can be approximately figured as the linear weighted sum of the MIE of each component; the weighting factor is respectively the volume fraction of each component. This can be considered as a universal method to obtain the MIE for a specific multi-component gas. 相似文献
18.
Zhao Pinghui Chen Yiliang Liu Minghou Ding Min Zhang Genxuan 《Frontiers of Energy and Power Engineering in China》2007,1(2):233-238
Premixed combustion in porous media differs substantially from combustion in free space. The interphase heat transfer between
a gas mixture and a porous medium becomes dominant in the premixed combustion process. In this paper, the premixed combustion
of CH4/air mixture in a porous medium is numerically simulated with a laminar combustion model. Radiative heat transfer in solids
and convective heat transfer between the gas and the solid is especially studied. A smaller detailed reaction mechanism is
also used and the results can show good prediction for many combustion phenomena.
Translated from Journal of Combustion Science and Technology, 2006, 12(1): 46–50 [译自: 燃烧科学与技术] 相似文献
19.
Yunfei Yan Weimin Tang Li Zhang Xin Zhang Lixiang Niu Ke Liu Junchen Zhu 《International Journal of Hydrogen Energy》2014
This paper focuses on investigating that the influence of O2, CO2 and H2O on characteristics of autothermal reforming of methane in micro premix chamber on Ni catalysts. In addition, the effect of catalytic wall temperature on autothermal reforming reaction of methane under a certain ratio of CH4/CO2/H2O/O2 is simulated. The results indicate that appropriately increasing O2 concentration can increase the conversion efficiency of CH4, so does adding CO2 or H2O. The positive effect of O2, CO2 and H2O is more pronounced at the higher temperature. The temperature range of 650–750 K is the important transitional region in the reactions of CH4/O2, CH4/H2O and CH4/CO2. It also gives a guide to the available range of parameters in the high efficiency reforming process of micro-reactor. 相似文献
20.
The effect of surfaces on the extent of high pressure horizontal unignited jets of hydrogen and methane is studied using computer fluid dynamics simulations performed with FLACS Hydrogen. Results for constant flow rate through a 6.35 mm diameter pressure relief Device (PRD) orifice from 100 barg, 250 barg, 400 barg, 550 barg and 700 barg compressed gas systems are presented for both horizontal hydrogen and methane jets. To quantify the effect of a horizontal surface on the jet, the jet exit is positioned at various heights above the ground ranging from 0.1 m to 10 m. Free jet simulations are performed for comparison purposes. Also, for cross-validation purposes, a number of cases for 100 barg releases were simulated using proprietary models developed for hydrogen within commercial CFD software PHOENICS. It is found that the presence of a surface and its proximity to the jet centreline result in a pronounced increase in the extent of the flammable cloud compared to a free jet. 相似文献