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1.
NO emission from non-premixed MILD combustion of biogas-syngas mixtures in opposed jet configuration
《International Journal of Hydrogen Energy》2021,46(75):37641-37655
In this paper NO emission from MILD combustion of the mixture biogas-syngas is deeply elucidated, five NO routes were considered, specifically: thermal, prompt, NNH, N2O and reburning. Several operating conditions are studied namely: fuel mixture composition, oxygen concentration in the oxidizer and injection velocity or strain rate. Biogas is modeled by a mixture of methane and carbon dioxide; while, syngas is considered to be composed by hydrogen and carbon monoxide, this gives a fuel mixture of CH4/CO2/H2/CO. Volume of methane and hydrogen are varied alternatively from 0 to 50% in fuel mixture. Oxidizer is composed by O2/N2 mixture where oxygen volume is increased from 4 to 21%. Finally, injection strain rate is varied from apparition to vanishment of combustion. Atmospheric pressure is considered with constant fuel and oxidizer injection temperatures of 300 K and 1200 K respectively. Chemical kinetics of such complicated system is handled by a composed mechanism from the USC C1–C4 and the Gri 2.11 N-sub mechanism. It is found that under MILD regime, temperature intervals and levels are enhanced by hydrogen compared to methane. Furthermore, temperature levels keep relatively low which guarantees MILD regime. Contrariwise, when oxygen increases in oxidizer, temperature grows up rapidly and the MILD regime disappears. However, if strain rate augments, temperature shows a steep increase then reduces monotonically. It is observed that for low methane volume in the fuel mixture, NNH route dominates NO production. Whereas, when CH4 increases, the prompt route is enhanced and exceeds NNH one at a methane volume of 12%. When hydrogen increases, prompt and NNH routes are enhanced with a domination of the prompt route until 44% of hydrogen volume. Oxygen increasing in the oxidizer improves thermal mechanism which surpasses prompt one at 17% of oxygen volume and governs NO production. Globally, the third most important route in NO production is the reburning one which is enhanced by all parameters except strain rate. 相似文献
2.
This paper reported a numerical study on the NOx emission characteristics of opposed-jet syngas diffusion flames. A narrowband radiation model was coupled to the OPPDIF program, which used detailed chemical kinetics and thermal and transport properties to enable the study of 1-D counterflow syngas diffusion flames with flame radiation. The effects of syngas composition, pressure and dilution gases on the NOx emission of H2/CO synthetic mixture flames were examined. The analyses of detailed flame structures, chemical kinetics, and nitrogen reaction pathways indicate NOx are formed through Zeldovich (or thermal), NNH and N2O routes both in the hydrogen-lean and hydrogen-rich syngas flames at normal pressure. Zeldovich route is the main NO formation route. Therefore, the hydrogen-rich syngas flames produce more NO due to higher flame temperatures compared to that for hydrogen-lean syngas flames. Although NNH and N2O routes also are the primary NO formation paths, a large amount of N2 will be reformed from NNH and N2O species. For hydrogen-rich syngas flames, the NO formation from NNH and N2O routes are lesser, where NO can be dissipated through the reactions of NH + NO → N2 + OH and NH + NO → N2O + H more actively. At a rather low pressure (0.01 atm), NNH-intermediate route is the only formation path of NO. Increasing pressure then enhances NO formation reactions, especially through Zeldovich mechanisms. However, at higher pressures (5–10 atm), NO is then converted back to N2 through reversed N2O route for hydrogen-lean syngas flames, and through NNH as well for hydrogen-rich syngas flames. In addition, the dilution effects from CO2, H2O, and N2 on NO emissions for H2/CO syngas flames were studied. The hydrogen-lean syngas flames with H2O dilution have the lowest NO production rate among them, due to a reduced reaction rate of NNH + O → NH + NO. But for hydrogen-rich syngas flames with CO2 dilution, the flame temperatures decrease significantly, which leads to a reduction of NO formation from Zeldovich route. 相似文献
3.
Namsu Kim Yongmo Kim Mohammad Nazri Mohd Jaafar Muhammad Roslan Rahim Mazlan Said 《International Journal of Hydrogen Energy》2021,46(17):10518-10534
The present study has numerically investigated the Moderate or Intense Low oxygen Dilution (MILD) combustion regime, combustion processes and NO formation characteristics of the highly CO-rich syngas counterflow nonpremixed flames. To realistically predict the flame properties of the highly CO-rich syngas, the chemistry is represented by the modified GRI 3.0 mechanism. Computations are performed to precisely analyze the flame structure, NO formation rate, and EINO of each NO sub-mechanism. Numerical results reveal that the hydrogen enrichment and oxygen augmentation substantially influence the NO emission characteristics and the dominant NO production route in the CO-rich syngas nonpremixed flames under MILD and high temperature combustion regimes. It is found that the most dominant NO production routes are the NNH path for the lowest oxygen level (3%) and the thermal mechanism for the highest O2 condition (21%). For the intermediate oxygen level (9%), the most dominant NO production routes are the NNH route for the hydrogen fraction up to 5%, the CO2 path for the hydrogen fraction range from 5% to 10% and the thermal mechanism for the hydrogen fraction higher than 10%, respectively. To evaluate the contribution of the specific reaction on EINO the sensitivity coefficients are precisely analyzed for NO formation processes with the dominance of NNH/CO2/Thermal mechanism under the highly CO-rich syngas flames. 相似文献
4.
Nan DingRajat Arora Michael NorconkSeong-Young Lee 《International Journal of Hydrogen Energy》2011,36(4):3222-3231
In recent years, research efforts have been channeled to explore the use of environmentally-friendly clean fuel in lean-premixed combustion so that it is vital to understand fundamental knowledge of combustion and emissions characteristics for an advanced gas turbine combustor design. The current study investigates the extinction limits and emission formations of dry syngas (50% H2-50% CO), moist syngas (40% H2-40% CO-20% H2O), and impure syngas containing 5% CH4. A counterflow flame configuration was numerically investigated to understand extinction and emission characteristics at the lean-premixed combustion condition by varying dilution levels (N2, CO2 and H2O) at different pressures and syngas compositions. By increasing dilution and varying syngas composition and maintaining a constant strain rate in the flame, numerical simulation showed among diluents considered: CO2 diluted flame has the same extinction limit in moist syngas as in dry syngas but a higher extinction temperature; H2O presence in the fuel mixture decreases the extinction limit of N2 diluted flame but still increases the flame extinction temperature; impure syngas with CH4 extends the flame extinction limit but has no effect on flame temperature in CO2 diluted flame; for diluted moist syngas, extinction limit is increased at higher pressure with the larger extinction temperature; for different compositions of syngas, higher CO concentration leads to higher NO emission. This study enables to provide insight into reaction mechanisms involved in flame extinction and emission through the addition of diluents at ambient and high pressure. 相似文献
5.
Xudong Jiang Pengfei Li Junjun Guo Fan Hu Feifei Wang Jianchun Mi Zhaohui Liu 《International Journal of Hydrogen Energy》2018,43(17):8534-8557
This study systematically investigates the detailed mechanism of nitrogen oxides (NOx) in CH4 and CH4/H2 jet flames with O2/CO2 hot coflow. After comprehensive validation of the modeling by experiments of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]; the effects of CO2 replacement of N2, mass fraction of oxygen in the coflow (YO2), and mass fraction of hydrogen in the fuel jet (YH2) on NO formation and destruction are investigated in detail. For methane oxy-fuel combustion, the NNH route is found to control the NO formation at YO2 ≤ 3%, while both NNH and N2O-intermediate routes dominate the NO production at 3% < YO2 < 10%. When YO2 ≥ 10%, NO is obtained mainly from thermal mechanism. Moreover, in the oxy-combustion of methane and hydrogen fuel blends with YO2 = 3%, with hydrogen addition the contribution of the NNH and prompt routes increases, while that of the N2O-intermediate route decreases. Furthermore, the chemical effect of CO2 is significant in reducing NO in both oxy-combustion of methane with YO2 ≤ 3% and combustion of methane and hydrogen fuel blends with YH2 ≤ 10%. 相似文献
6.
《International Journal of Hydrogen Energy》2022,47(35):15896-15908
Syngas is a promising alternative fuel for stationary power generation due to cleaner combustion than convectional fossil fuels. During the gasification processes, the by-products of CO2, H2O, or N2 may be present in the syngas mixture to control the flame temperature and emissions. Several studies indicated that syngas with dilutions is capable of reducing pollutant emissions such as NOx emissions. This work applied a numerical model of opposed-jet diffusion fames to explore the dilution effects on NOx formation and differentiate the inert effect, thermal/diffusion effect, chemical effect, and radiation effect from CO2, H2O, or N2 dilutions. The numerical study was performed by a revised OPPDIF program coupling with narrowband radiation model and detail chemical mechanism. The dilution effects on NOx formation were analyzed by comparing the realistic and hypothetical cases. Regardless the diluent types, the inert effect is the main cause to reduce NO production, followed by chemical effect and radiation effect. The thermal/diffusion effect may promote NO formation because the preferential diffusion due to different diffusivities between diluents and syngas magnifies the reaction rate locally. CO2 dilution reduces NO by radiation effect at low strain rate, and contributes NO reduction by chemical effect at high strain rate. At the same dilution percentage, CO2 dilution reduces NO production the most, followed by H2O and N2. Besides the thermal/diffusion effect, the chemical effect of H2O enhances NO production through thermal route and reburn route. 相似文献
7.
The present study provides an extensive and detailed numerical analysis of NOx chemical kinetics in low calorific value H2/CO syngas flames utilizing predictions by five chemical kinetic mechanisms available out of which four deal with H2/CO while the fifth mechanism (GRI 3.0) additionally accounts for hydrocarbon chemistry. Comparison of predicted axial NO profiles in premixed flat flames with measurements at 1 bar, 3.05 bar and 9.15 bar shows considerably large quantitative differences among the various mechanisms. However, at each pressure, the quantitative reaction path diagrams show similar NO formation pathways for most of the mechanisms. Interestingly, in counterflow diffusion flames, the quantitative reaction path diagrams and sensitivity analyses using the various mechanisms reveal major differences in the NO formation pathways and reaction rates of important reactions. The NNH and N2O intermediate pathways are found to be the major contributors for NO formation in all the reaction mechanisms except GRI 3.0 in syngas diffusion flames. The GRI 3.0 mechanism is observed to predict prompt NO pathway as the major contributing pathway to NO formation. This is attributed to prediction of a large concentration of CH radical by the GRI 3.0 as opposed to a relatively negligible value predicted by all other mechanisms. Also, the back-conversion of NNH into N2O at lower pressures (2–4 bar) was uniquely observed for one of the five mechanisms. The net reaction rates and peak flame temperatures are used to correlate and explain the differences observed in the peak [NO] at different pressures. This study identifies key reactions needing assessment and also highlights the need for experimental data in syngas diffusion flames in order to assess and optimize H2/CO and nitrogen chemistry. 相似文献
8.
This paper used the opposed-flow flame model and GRI 3.0 mechanism to investigate NO emission characteristics of H2-rich and H2-lean syngas under diffusion and premixed conditions, respectively, and analyzed influences of adding H2O, CO2 and N2 on NO formation from the standpoint of thermodynamics and reaction kinetics. For diffusion flames, thermal route is the dominant pathway to produce NO, and adding N2, H2O and CO2 shows a decreasing manner in lowering NO emission. The phenomenon above is more obvious for H2-rich syngas because it has higher flame temperature. For premixed flames, adding CO2 causes higher NO concentration than adding H2O, because adding CO2 produces more O radical, which promotes formation of NO through NNH + O = NH + NO, NH + O = NO + H and reversed N + NO = N2 + O. And in burnout gas, thermal route is the dominant way for NO formation. Under this paper's conditions, adding N2 increases the formation source of NO as well as decreases the flame temperature, and it reduces the NO formation as a whole. In addition, for H2-lean syngas and H2-rich syngas with CO2 as the diluent, N + CO2 = NO + CO plays as an important role in thermal route of NO formation. 相似文献
9.
《International Journal of Hydrogen Energy》2023,48(79):30909-30929
Moderate or Intense Low-oxygen Dilution (MILD) combustion of a syngas fuel under air-fuel, oxygen-enhanced, and oxy-fuel condition are numerically studied with using counterflow diffusion flame. Fuel composition, temperature of oxidant (Tox), and oxygen mole fraction (XO2) are selected as the main parameters. Fake species (FCO2) with the same CO2 physical properties is used for separation the physical and chemical effects of replacing CO2 with N2. According to the results, under the high preheating temperatures, the chemical effect of changing the oxidant composition from N2 to CO2 is the main reason of the changes in flame structure, ignition delay time (IDT) and heat release rate (HRR) while physical differences play a more prominent role in the low preheating temperature MILD combustion. In all XO2, the physical and chemical effects of replacing CO2 with N2 have almost the same role on the maximum flame temperature. The results of IDT expressed that chemical discrepancies of CO2 and N2 play a key role on IDT enhancement by increasing CO2 in the oxidant composition. The sensitivity analysis of CH2O for variations of Tox and XO2 shows that reactions R54, R56, R58, and R101 are the main responsible of lower HRR and higher IDT by moving from air-syngas to oxy-fuel MILD combustion. 相似文献
10.
The NO formation characteristics and reaction pathways of opposed-jet H2/CO syngas diffusion flames were analyzed with a revised OPPDIF program which coupled a narrowband radiation model with detailed chemical kinetics in this work. The effects of strain rates ranging from 0.1 to 1000 s?1 and diluents including CO2, H2O and N2 on NO production rates were investigated for three typical syngas compositions. The numerical results demonstrated that NO is produced primary through NNH-intermediate route and thermal route at high strain rates, where the reaction of NH + O = NO + H (R51) also become more active. Near the strain rate of 10 s?1, the flame temperature is the highest and thermal route is the dominant NO formation route, but NO would be consumed by reburn route where NO is converted to NH through HNO, especially for H2-rich syngas. At low strain rates, radiative heat loss results in a lower flame temperature and further reduce NO formation, while the reaction of N + CO2 = NO + CO (R140) become more important, especially for CO-rich syngas. With the diluents, NO production rates decreased with increasing dilution percentages. When the flame temperature is very high as the thermal route is dominant near strain rate of 10 s?1, CO2 dilution makes flame temperature and NO production rate the lowest. Toward both lower and higher strain rates, adding H2O is more effective in reducing NO because R140 and NNH-intermediate route are suppressed the most by H2O dilution respectively. 相似文献
11.
This paper reports a numerical study on the combustion and extinction characteristics of opposed-jet syngas diffusion flames. A model of one-dimensional counterflow syngas diffusion flames was constructed with constant strain rate formulations, which used detailed chemical kinetics and thermal and transport properties with flame radiation calculated by statistic narrowband radiation model. Detailed flame structures, species production rates and net reaction rates of key chemical reaction steps were analyzed. The effects of syngas compositions, dilution gases and pressures on the flame structures and extinction limits of H2/CO synthetic mixture flames were discussed. Results indicate the flame structures and flame extinction are impacted by the compositions of syngas mixture significantly. From H2-enriched syngas to CO-enriched syngas fuels, the dominant chain reactions are shifting from OH + H2→H + H2O for H2O production to OH + CO→H + CO2 for CO2 production through the key chain-branching reaction of H + O2→O + OH. Flame temperature increases with increasing hydrogen content and pressure, but the flame thickness is decreased with pressure. Besides, the study of the dilution effects from CO2, N2, and H2O, showed the maximum flame temperature is decreased the most with CO2 as the dilution gas, while CO-enriched syngas flames with H2O dilution has highest maximum flame temperature when extinction occurs due to the competitions of chemical effect and radiation effect. Finally, extinction limits were obtained with minimum hydrogen percentage as the index at different pressures, which provides a fundamental understanding of syngas combustion and applications. 相似文献
12.
Experimental investigations of H2 and H2-enriched syngas flame radiation properties have been conducted through spectroscopic and DFCD (Digital Flame Colour Discrimination) techniques. A spectrograph was employed to quantify the emission profile of H2-based flames in the UV–visible spectral domain. The OH* emission was found to be the strongest in reactants with highest amount of H2. Further addition of CO and/or CO2 resulted in the reduction of OH* intensity with the addition of CO2 causing greater radical intensity-loss than that of CO. The decrease in OH* intensity is accompanied by a corresponding increase in the CO–O* broadband continuum in the short-wavelength domain of the visible spectrum. Such reduction of OH* along with increase in CO–O* intensity can be related to the endothermic reaction mechanism of CO + OH => CO2 + H, which describes the role of CO/CO2 addition in H2-enriched syngas flames. Comparison with direct imaging results provided additional credence to the effect of temperature reduction as flames with CO and/or CO2 additions resembled colourations closer to typical bluish premixed hydrocarbon flame. The employment of DFCD processing effectively characterised different syngas combustion conditions by combining aspects of digital flame colour intensities with spatial combustion distributions. This colour signal quantification method was shown to yield useful characterisation of H2-based flames, similar to the use of OH* chemiluminescence intensity variation from spectrometry. Also, DCFD analysis was able to depict the variances between the burning of different syngas gaseous constituents. Thus, useful image-based parameters related to the H2-based combustion can be derived and potentially applied as a practical monitoring and characterisation mean for syngas combustion. 相似文献
13.
Jianfei Xi Ye Yuan Weiben Yang Xianpeng Zhang Shengfu Wang 《Energy Sources, Part A: Recovery, Utilization, and Environmental Effects》2018,40(7):821-829
The effect of CO2/N2/CH4 dilution on NO formation in laminar coflow H2/CO syngas diffusion flames was experimentally and numerically investigated. The results reveal that the NO emission index increases with H2/CO mole ratio. In all cases, CO2/N2/CH4 dilution can reduce the peak temperature of syngas flame and have the ability to reduce peak flame temperature is decreased in the following order: CO2>N2>CH4. CO2/N2 dilution reduces the NO formation in syngas flame while CH4 dilution promotes the NO formation. Besides, the dilution of CO2/N2/CH4 can reduce the peak mole fraction of OH and its variations with H2/CO mole ratio and dilution ratio show the same trend as the peak flame temperature variations. The height of the flame with CO2 and N2 dilution increases with dilution ratio. The flame with CH4 dilution becomes higher and wider with the increase of dilution ratio. 相似文献
14.
《International Journal of Hydrogen Energy》2019,44(33):18505-18516
Demand for the clean and sustainable energy encourages the research and development in the efficient production and utilisation of syngas for low-carbon power and heating/cooling applications. However, diversity in the chemical composition of syngas, resulting due to its flexible production process and feedstock, often poses a significant challenge for the design and operation of an effective combustion system. To address this, the research presented in this paper is particularly focused on an in-depth understanding of the heat generation and emission formation of syngas/producer gas flames with an effect of the fuel compositions. The heat generated by flame not only depends on the flame temperature but also on the chemistry heat release of fuel and flame dimension. The study reports that the syngas/producer gas with a low H2:CO maximises the heat generation, nevertheless the higher emission rate of CO2 is inevitable. The generated heat flux at H2:CO = 3:1, 1:1, and 1:3 is found to be 222, 432 and 538 W m-2 respectively. At the same amount of heat generated, H2 concentration in fuel dominates the emission of NOx. The addition of CH4 into the syngas/producer gas with H2:CO = 1:1 also increases the heat generation significantly (e.g. 614 W m-2 at 20%) while decreases the emission formation. In contrast, adding 20% CO2 and N2 to the syngas/producer gas composition reduces the heat generation from 432 W m-2 to 364 and 290 W m-2, respectively. The role of CO2 on this aspect, which is weaker than N2, thus suggests CO2 is preferable than N2. Along with the study, the significant role of CO2 on the radiation of heat and the reduction of emission are examined. 相似文献
15.
Co-firing NH3 with H2/CO/syngas (SYN) is a promising method to overcome the low reactivity of NH3/air flame. Hence, this study aims to systematically investigate the laminar premixed combustion characteristics of NH3/air flame with various H2/CO/SYN addition loadings (0–40%) using chemical kinetics simulation. The numerical results were obtained based on the Han mechanism which can provide accurate predictions of laminar burning velocities. Results showed that H2 has the greatest effects on increasing laminar burning velocities and net heat release rates of NH3/air flame, followed by SYN and CO. CO has the most significant effects on improving NH3/air adiabatic flame temperatures. The H2/CO/SYN additions can accelerate NH3 decomposition rates and promote the generation of H and NH2 radicals. Furthermore, there is an evident positive linear correlation between the laminar burning velocities and the peak mole fraction of H + NH2 radicals. The reaction NH2 + NH <=> N2H2 + H and NH2 + NO <=> NNH + OH have remarkable positive effects on NH3 combustion. The mole fraction of OH × NH2 radicals positively affects the net heat release rates. Finally, it was discovered that H radicals play an important role in the generation of NO. The H2/CO/SYN additions can reduce the hydrodynamic and diffusional-thermal instabilities of NH3/air flame. The NH3 reaction pathways for NH3–H2/CO/SYN-air flames can be categorized mainly into NH3–NH2–NH–N–N2, NH3–NH2–HNO–NO(?N2O)–N2 and NH3–NH2(?N2H2)–NNH–N2. CO has the greatest influence on the proportions of three NH3 reaction routes. 相似文献
16.
The conversions of fuel-N to NO and N2O during devolatilization and char combustion stages of a single coal particle of 7 mm in diameter were investigated in a laboratory-scale flow tube reactor under oxy-fuel fluidized bed (FB) conditions. The method of isothermal thermo-gravimetric analysis (TGA) combing with the coal properties was proposed to distinguish the devolatilization and char combustion stages of coal combustion. The results show that the char combustion stage plays a dominant role in NO and N2O emissions in oxy-fuel FB combustion. Temperature changes the trade-off between NO and N2O during the two stages. With increasing temperature, the conversion ratios of fuel-N to NO during the two stages increase, and the opposite tendencies are observed for N2O. CO2 inhibits the fuel-N conversions to NO during the two stages but promotes those to N2O. Compared with air combustion, the conversion ratios of fuel-N to NO during the two stages are lower in 21%O2/79%CO2, and those to N2O are higher. At <O2> = 21–50% by volume, the conversion ratios of fuel-N to NO during the two stages reach the maximum values at <O2> = 30% by volume, and those to N2O decrease with increasing O2 concentration. H2O suppresses the fuel-N conversions to NO and N2O during the two stages. A higher coal rank has higher total conversion ratios of fuel-N to NO and N2O. Fuel-N, volatile matter, and fixed carbon contents are the important factors on fuel-N conversions to NO and N2O during the two stages. The results benefit the understanding of NO and N2O emission mechanisms during oxy-fuel FB combustion of coal. 相似文献
17.
《International Journal of Hydrogen Energy》2020,45(6):3785-3794
One-dimensional premixed freely-propagating flames for (CH4+CO2/H2O)/air(79%N2+21%O2) mixtures were modeled using ChemkinⅡ/Premix Code with the detailed mechanism GRI-Mech 3.0. The investigation of the effects of CO2 and steam addition on the H2 intermediate formation and NO emission was conducted at the initial conditions of 1 atm and 398 K. Both physical and chemical effects of CO2, H2O on laminar burning velocities and adiabatic flame temperatures were also analyzed. The calculations show that with the increase of αCO2 and αH2O, both physical and chemical effects of CO2 and H2O result in the reduction of laminar burning velocities (LBVs) and adiabatic flame temperatures (AFTs) in which the chemical effects of CO2 addition are more significantly than H2O. Especially, the chemical effects of steam promote the increase of AFTs and the influence in rich BG65 flames are larger than in methane. With a proper amount of H2O addition, the chemical effects of H2O on the peak concentration of H2 are more significantly than physical at Φ = 1.2. Moreover, CO2, steam and their mixture addition have significant reduction on the NO emission. The most sensitive reaction for the formation of H2 and NO emission were determined. The responsible reactions for H2 formation and NO emission are R84 OH + H2 <=> H + H2O and R240 CH + N2 <=> HCN + N (a prompt routine), respectively. 相似文献
18.
Extensive computations were made to determine the flammability limits of opposed-jet H2/CO syngas diffusion flames from high stretched blowoff to low stretched quenching. Results from the U-shape extinction boundaries indicate the minimum hydrogen concentrations for H2/CO syngas to be combustible are larger towards both ends of high strain and low strain rates. The most flammable strain rate is near one s−1 where syngas diffusion flames exist with minimum 0.002% hydrogen content. The critical oxygen percentage (or limiting oxygen index) below which no diffusion flames could exist for any strain rate was found to be 4.7% for the equal-molar syngas fuels (H2/CO = 1), and the critical oxygen percentage is lower for syngas mixture with higher hydrogen content. The flammability maps were also constructed with strain rates and pressures or dilution gases percentages as the coordinates. By adding dilution gases such as CO2, H2O, and N2 to make the syngas non-flammable, besides the inert effect from the diluents, the chemical effect of H2O contributes to higher flame temperature, while the radiation effect of H2O and CO2 plays an important role in the flame extinction at low strain rates. 相似文献
19.
《International Journal of Hydrogen Energy》2023,48(11):4503-4515
The paper reports on the results of an experimental study of methane and syngas combustion as well as their co-firing in a bidirectional swirling flow. The results confirmed that the bidirectional flow structure provides a significant decrease in the lean blow-off equivalence ratio as well as that of emissions of main pollutants. The combustion intensification becomes more evident when using syngas is as fuel. The composition of the used syngas is as follows (by volume): H2 - 29.42%; CO - 14.32%; CH4 - 3.8%; N2 - 49.11%; H2O - 3.35%. In this case, the lean blow-off is achieved at ? < 0.1, NOx emission is halved, while CxHy and CO emissions become 20 times less compared to pure methane combustion. However, according to experimental results, the co-combustion of syngas (volume fraction Vsyn = 15%) and methane is the most appropriate fuel utilization mode. It provides blow-off and emission properties similar to those for combustion of pure syngas, whereas energy consumption for its production is much lower. Moreover, unlike hydrocarbon fuel combustion, that of syngas in a bidirectional swirling flow is characterized by the presence of density stratification. This is accompanied by the flame formation at significantly different locations in the combustion chamber at lean and “ultra-lean” modes of operation. Hydrogen combustion most likely to occur in the core region at near-blow-off modes ? < 0.1, whereas normal ‘operating modes in the range 0.2 = ? ≤ 0.4 result in the formation of a conical flame surface where CH4 and CO combustion occurs. These new results with respect to the flame structure as well as blow-off and emission properties make it possible to consider bidirectional vortex combustors for application in modern gas turbine power plants in order to meet the strict environmental and energy requirements. 相似文献
20.
Flameless combustion is a well known measure to reduce NOx emissions in gas combustion but has not yet been fully adapted to pulverised coal combustion. Numerical predictions can provide detailed information on the combustion process thus playing a significant role in understanding the basic mechanisms for pollutant formation. In simulations of conventional pulverised coal combustion the gasification by CO2 or H2O is usually omitted since its overall contribution to char oxidation is negligible compared to the oxidation with O2. In flameless combustion, however, due to the strong recirculation of hot combustion products, primarily CO2 and H2O, and the thereby reduced concentration of O2 in the reaction zone the local partial pressures of CO2 and H2O become significantly higher than that for O2. Therefore, the char reaction with CO2 and H2O is being reconsidered. This paper presents a numerical study on the importance of these reactions on pollutant formation in flameless combustion. The numerical models used have been validated against experimental data. By varying the wall temperature and the burner excess air ratio, different cases have been investigated and the impact of considering gasification on the prediction of NO formation has been assessed. It was found that within the investigated ranges of these parameters the fraction of char being gasified increases up to 35%. This leads to changes in the local gas composition, primarily CO distribution, which in turn influences NO formation predictions. Considering gasification the prediction of NO emission is up to 40% lower than the predicted emissions without gasification reactions being taken into account. 相似文献