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1.
The NO mechanism under the moderate or intense low-oxygen dilution (MILD) combustion of syngas has not been systematically examined. This paper investigates the NO mechanism in the syngas MILD regime under the dilution of N2, CO2, and H2O through counterflow combustion simulation. The syngas reaction mechanism and the counterflow combustion simulation are comprehensively validated under different CO/H2 ratios and strain rates. The effects of oxygen volume fraction, CO/H2 ratio, pressure, strain rate, and dilution atmosphere are systematically investigated. For all the MILD cases, the contribution of the prompt and NO-reburning routes to the overall NO emission is less than 0.1% due to the lack of CH4 in fuel. At atmospheric pressure, the thermal route only accounts for less than 20% of the total NO emission because of the low reaction temperature. Moreover, at atmospheric pressure, the contribution of the NNH route to NO emission is always larger than 55% in the N2 atmosphere. The N2O-intermediate route is enhanced in CO2 and H2O atmospheres due to the increased third-body effects of CO2 and H2O through the reaction N2 + O (+M) ? N2O (+M). Especially in the H2O atmosphere, the N2O-intermediate route contributes to 60% NO at most. NO production is reduced with increasing CO/H2 ratio or pressure, mainly due to decreased NO formation from the NNH route. Importantly, a high reaction temperature and low NO emission are simultaneously achieved at high pressure. To minimize NO emission, the reactions should be operated at high values of CO/H2 ratios (i.e., >4) and pressures (e.g., P > 10 atm), low oxygen volume fractions (e.g., XO2 < 15%), and using H2O as a diluent. This study provides a new fundamental understanding of the NO mechanism of syngas MILD combustion in N2, CO2, and H2O atmospheres.  相似文献   

2.
The present study investigated the effect of adding hydrogen to methane on the thermal characteristics and ignition delay in methane-air, oxygen-enriched and oxy-fuel MILD combustion. For this purpose, numerical simulation of MILD furnace is performed by k-ε turbulence, modified EDC combustion, and DO radiation models. Additionally, a well stirred reactor (WSR) analysis alongside with CFD simulations is used for getting the better insight of combustion process and numerical results. The results show that H2 addition to CH4 provides a more uniform temperature field with higher peak and average temperatures under a similar oxidizer atmosphere. Also, more ignition delay time (IDT) obtained by the replacement of CO2 with N2, can be compensated by consideration of H2 in the fuel composition. This study implies that the use of H2 as an additive to methane is an effective strategy for conversion of methane-air to oxy-fuel combustion system with almost identical thermal and ignition characteristics.  相似文献   

3.
As gas recirculation constitutes a fundamental condition for the realization of MILD combustion, it is necessary to determine gas recirculation ratio before designing MILD combustor. MILD combustion model with gas recir- culation was used in this simulation work to evaluate the effect of fuel type and pressure on threshold gas recir- culation ratio of MILD mode. Ignition delay time is also an important design parameter for gas turbine combustor, this parameter is kinetically studied to analyze the effect of pressure on MILD mixture ignition. Threshold gas re- circulation ratio of hydrogen MILD combustion changes slightly and is nearly equal to that of 10 MJ/Nm3 syngas in the pressure range of 1-19 atm, under the conditions of 298 K fresh reactant temperature and 1373 K exhaust gas temperature, indicating that MILD regime is fuel flexible. Ignition delay calculation results show that pres- sure has a negative effect on ignition delay time of 10 MJ/Nm3 syngas MILD mixture, because OH mole fraction in MILD mixture drops down as pressure increases, resulting in the delay of the oxidation process.  相似文献   

4.
The growing concern about pollutant emissions and depletion of fossil fuels has been a strong motivator for the development of cleaner and more efficient combustion strategies, such as the gasification of coal, biomass or waste, which have increased the interest in using a new type of fuels, mainly composed of CH4, H2, CO and CO2.These new fuels, commonly called syngas, display a wide range of compositions, which affects their combustion characteristics and, in some cases, are more prone to instabilities or flashback. Since flame properties have been demonstrated to be strongly related to equivalence ratio, a precise measurement of the flame stoichiometry is a key pre-requisite for combustion optimization and prevention of unstable regimes. In particular, chemiluminescence emission from flames has been largely tested for stoichiometry monitoring for methane flames, but its use in syngas flames has been far less studied. Consequently, the main goal of this work is analyzing the effect of fuel composition on the chemiluminescence vs. equivalence ratio curves for different fuel blends, as a first approach for a wide range of syngas compositions. The experimental results revealed that the ratio OH*/CH*, which had been widely demonstrated to be the best option for methane, may not be suitable for monitoring with certain fuels, such as those with a high percent of hydrogen. Alternatively, other signals, in particular the ratio OH*/CO2*, appear as viable stoichiometry indicators in those cases.The analysis was also completed by numerical predictions with CHEMKIN. The comparisons of calculations with different flame models and experimental data reveals differences in the chemiluminescence vs. equivalence ratio curves for the different combustion regimes, depending on the range of the equivalence ratio ranges and fuel compositions. This finding, which confirms previous observations for a much narrower range of fuels, could have important practical consequences for the application of the technique in real combustors.  相似文献   

5.
《能源学会志》2020,93(6):2388-2398
This paper studied the effect of high temperature (up to 1873K) and separated combustion mode (volatile combustion and char combustion are separated) on SO2 release characteristics during pulverized coal combustion under O2/CO2 atmosphere. Coal combustion experiments were conducted at different combustion environment temperatures utilizing a high temperature fixed-bed setup. The results show that as temperature rises, the SO2 release curve is transformed from a single-peak process to a double-peak process. In separated combustion, temperature has little effect on the volatile-SO2 (SO2 released during volatile combustion) but brings about a significant effect on char-SO2 (SO2 released during char combustion). Char-SO2 release amount and the ratio of it to fuel-SO2 release amount (total SO2 released during coal combustion) increase with temperature rising. The increase of temperature leads to a dramatic decreasing of sulphur mass fixed in the ash and causes SO2 release amount to rise when temperature is lower than 1573 K. Separated combustion causes a higher SO2 release amount than coupled combustion (the same as conventional combustion, volatile combustion and char combustion are simultaneous). Thermochemistry equilibrium composition calculation results show that alkali metals and alkaline-earth metals are significant in sulphur retention. CaSO4 and Na2SO4 are the main sulphates at high temperatures.  相似文献   

6.
The combination of oxy-fuel and blended-coal combustion may be one of these effective methods to both reduce CO2 emissions and improve energy utilization efficiency in coal-fired power stations. The aim of this study is to investigate oxy-fuel combustion interactions of blended coals under different conditions using a thermo-gravimetric analyzer. The results show that compared with those in an O2/N2 mixture, the promotive and inhibitive effect and the comprehensive interactions are considerably weaker in an O2/CO2 mixture. In the O2/CO2 mixture, both increasing the O2 concentration and decreasing the particle size result in decreasing the promotive effect but increasing the inhibitive effect and the comprehensive interactions, which increase the non-additive combustion characteristics. Enhancement of the heating rate increases the promotive effect but decreases the inhibitive effect and the comprehensive interactions, which weaken the non-additive combustion characteristics. Of these factors, the effects of the oxygen concentration and heating rate on comprehensive interactions are greater than that of particle size. This study provides useful information for the design and optimization of thermo-chemical conversion systems of coal blends in the O2/CO2 atmosphere.  相似文献   

7.
A deep understanding of the ignition characteristics of syngas in O2/CO2 and O2/H2O atmospheres is essential for the application of oxy-syngas combustion. In the present work, ignition properties of a syngas with a typical H2-to-CO ratio (1:2) in O2/N2, O2/CO2 and O2/H2O atmospheres were investigated numerically. The ignition temperatures were determined by a 1-D model of a micro flow reactor with a controlled wall temperature profile, demonstrating that CO2 and H2O can lead to an increase in the ignition temperature compared to N2, and the increase is more pronounced for the O2/H2O atmosphere. The analysis manifests that CO2 and H2O can suppress OH production at the region with relatively lower wall temperature by promoting R10: H + O2(+M) = HO2+(M) to compete with R11: H + HO2 = 2OH for H radical. Moreover, the direct reaction effect (directly take part in reactions as reactants) and third-body effect of CO2 and H2O on ignition temperature were numerically isolated by adopting artificial species. The computation results reveal that the increase in ignition temperature mainly results from the enhanced reaction rate of R10 by the third-body effects of CO2 and H2O.  相似文献   

8.
The mechanisms of formation and destruction of NO in MILD combustion of CH4/H2 fuels blends are investigated both experimentally and numerically. Experiments are carried out at a lab-scale furnace with the mass fraction of hydrogen in fuel ranging from 0% to 15%; furnace temperature, extracted heat and exhaust NOx emissions are measured. Detailed chemical kinetics calculations utilizing computational fluid dynamics (CFD) and well-stirred reactor (WSR) are performed to better analyze and isolate the different mechanisms.  相似文献   

9.
Green ammonia is a candidate fuel to decarbonise shipping and other industries. However, ammonia features a lower reactivity compared to conventional fuels and is therefore difficult to burn. To resolve this issue, thermo-catalytic cracking of ammonia using waste heat is often employed to produce NH3/H2/N2 blends as fuel. However, on-site operational variations in this process can become sources of uncertainty in the fuel composition, causing randomness of the flame's physicochemical properties and challenging flame stability. In the present work, a surrogate model is built using the polynomial chaos expansion (PCE) method to investigate the impact of fuel composition variability on combustion characteristics at different operating conditions. Impacts of 1.5% deviation in the fuel composition on the flame properties for different initial pressures (Pi) and unburnt fuel temperatures (Tu) are investigated for a wide range of equivalence ratios covering lean and rich mixtures. The uncertainty effects defined by the coefficient of variation (COV) fluctuate for equivalence ratios greater than 1.1, while no fluctuation is observed in COV for near stoichiometric combustion conditions. It is shown that H2 variation in the fuel blend has the strongest effect (over 80%) on the uncertainty of all investigated physicochemical properties of the flame. The least affected property is the adiabatic flame temperature with variations of about 2.5% in richer fuel conditions. The results further show that preheating of the reactants can significantly reduce the COV of laminar flame speed. The consequences of these uncertainties upon different combustion technologies are then discussed and it is argued that moderate and intense low oxygen dilution (MILD) and colourless distributed combustion (CDC) technology may remain resilient.  相似文献   

10.
The presence of appreciable molecular oxygen at the location of maximum temperature has been observed in non-premixed oxygen-enhanced combustion (OEC) processes, specifically in flames having a high stoichiometric mixture fraction (Zst) produced with diluted fuel and oxygen-enrichment. For conventional fuel-air flames, key features of the flame are consistent with the flame sheet approximation (FSA). In particular, the depletion of O2 at the location of maximum temperature predicted by the FSA correlates well with the near-zero O2 concentration measured at this location for conventional fuel-air flames. In contradistinction, computational analysis with detailed kinetics demonstrates that for OEC flames at high Zst: (1) there is an appreciable concentration of O2 at the location of maximum temperature and (2) the maximum temperature is not coincident with the location of global stoichiometry, O2 depletion, or maximum heat release. We investigate these phenomena computationally in three non-premixed ethylene flames at low, moderate, and high Zst, but with equivalent adiabatic flame temperatures. Results demonstrate that the location of O2 depletion occurs in the vicinity of global stoichiometry for flames of any Zst and that the presence of appreciable O2 at the location of maximum temperature for high Zst flames is caused by a shift in the location of maximum temperature relative to the location of O2 depletion. This shifting is attributed to: (1) finite-rate multi-step chemistry resulting in exothermic heat release that is displaced from the location of O2 depletion and (2) the relative location of the heat release region with respect to the fuel and oxidizer boundaries in mixture fraction space. A method of superposition involving a variation of the flame sheet approximation with two heat sources is shown to be sufficient in explaining this phenomenon.  相似文献   

11.
Oxy-steam combustion is a potential new-generation option for CO2 capture and storage. The ignition and combustion characteristics of single coal and biomass particles were investigated in a flow tube reactor in O2/N2 and O2/H2O at various oxygen concentrations. The ignition and combustion processes were recorded using a CCD camera, and the two-color pyrometry was used to estimate the volatile flame temperature and char combustion temperature. In O2/N2 and O2/H2O, coal ignites heterogeneously at <O2> = 21–50%. In O2/N2, biomass ignites homogeneously at <O2> = 21–30%, while it ignites heterogeneously at <O2> = 40–50%. In O2/H2O, biomass ignites homogeneously at <O2> = 21–50%. With increasing oxygen concentration, the ignition delay time, volatile burnout time and char burnout time are decreased, and the volatile flame temperature and char combustion temperature are increased. At a certain oxygen concentration in both atmospheres, the ignition delay time, volatile burnout time and char burnout time of biomass are shorter than those of coal. Moreover, biomass has a higher volatile flame temperature but a lower char combustion temperature than coal. The ignition delay time, volatile burnout time and char burnout time in O2/H2O are lower than those in O2/N2 for coal and biomass. The presence of H2O can improve the combustion rates of coal and biomass. The volatile flame shows a lower temperature in O2/H2O than in O2/N2 at <O2> = 21–50%. The char combustion shows a lower temperature in O2/H2O than in O2/N2 at <O2> = 21–30%, while this behavior is switched at <O2> = 40–50%. The results contribute to the understanding of the ignition and combustion characteristics of coal and biomass in oxy-steam combustion.  相似文献   

12.
An experimental study is conducted to investigate the effect of CO addition on the laminar flame characteristics of H2 and CH4 flames in a constant-volume combustion system. In addition, one-dimensional laminar premixed flame propagation processes at the same conditions are simulated with the update mechanisms. Results show that all mechanisms could well predict the laminar flame speeds of CH4/CO/O2/CO2 mixtures, when ZCO is large. For mixtures with lower CO, the experimental laminar flame speeds are always smaller than the calculated ones with Han mechanism. For mixtures with larger or smaller ZCO2, GRI 3.0, San diego and USC 2.0 mechanisms all overvalue or undervalue the laminar flame speeds. When CO ratio in the CH4/CO blended fuels increases, laminar flame speed firstly increases and then decreases for the CH4/CO/O2/CO2 mixtures. For H2/CO/O2/CO2 mixtures, San diego, Davis and Li mechanisms all undervalue the laminar flame speeds of H2/CO/CO2/CO2 mixtures. Existing models could not well predict the nonlinear trend of the laminar flame speeds, due to complex chemical effects of CO on CH4/CO or H2/CO flames. Then, the detailed thermal, kinetic and diffusive effects of CO addition on the laminar flame speeds are discussed. Kinetic sensitivity coefficient is far larger than thermal and diffusive ones and this indicates CO addition influences laminar flame speeds mainly by the kinetic effect. Based on this, radical pool and sensitivity analysis are conducted for CH4/CO/O2/CO2 and H2/CO/O2/CO2 mixtures. For CH4/CO/O2/CO2 mixtures, elementary reaction R38H + O2 ↔ O + OH and R99 OH + CO ↔ H + CO2 are the most important branching reactions with positive sensitivity coefficients when CO ratio is relative low. As CO content increases in the CH4/CO blended fuel, the oxidation of CO plays a more and more important role. When CO ratio is larger than 0.9, the importance of R99 OH + CO ↔ H + CO2 is far larger than that of R38H + O2 ↔ O + OH. The oxidation of CO dominates the combustion process of CH4/CO/O2/CO2 mixtures. For H2/CO/O2/CO2 mixtures, the most important elementary reaction with positive and negative sensitivity coefficients are R29 CO + OH ↔ CO2 + H and R13H + O2(+M) ↔ HO2(+M) respectively. The sensitivity coefficient of R29 CO + OH ↔ CO2 + H is increasing and then decreasing with the addition of CO in the mixture. Chemical kinetic analysis shows that the chemical effect of CO on the laminar flame propagation of CH4/CO/O2/CO2 and H2/CO/O2/CO2 mixtures could be divided into two stages and the critical CO mole fraction is 0.9.  相似文献   

13.
The effects of different initial temperatures (T = 300–500 K) and different hydrogen volume fractions (5%–20%) on the combustion characteristics of premixed syngas/air flames in rectangular tubes were investigated experimentally. A high-speed camera and pressure sensor were used to obtain flame propagation images and overpressure dynamics. The CHEMKIN-PRO model and GRI Mech 3.0 mechanism were used for simulation. The results show that the flame propagation speed increases with the initial temperature before the flame touches the wall, while the opposite is true after the flame touches the wall. The increase in initial temperature leads to the increase in overpressure rise rate in the early flame propagation process, but the peak overpressure is reduced. The laminar burning velocity (LBV) and adiabatic flame temperature (AFT) increase with increasing initial temperature. The increase in initial temperature makes the peaks of H, O, and OH radicals increase.  相似文献   

14.
15.
Large eddy simulation (LES) method is employed to investigate the effect of the hydrogen content of fuel on the H2/CH4 flame structure under the moderate or intense low-oxygen dilution (MILD) condition. The turbulence–chemistry interaction of the numerically unresolved scales is modelled using the PaSR method, where the full mechanism of GRI-2.11 represents the chemical reactions. The influence of hydrogen concentration on the flame structure is studied using the profiles of temperature, CH2O and OH mass fractions and the diffusion profiles of un-burnt fuel through the flame front. Furthermore, more details are investigated by contours of OH, HCO and CH2O radicals in an area near the nozzle exit zone. Results show that increasing the hydrogen content of fuel reinforces the MILD combustion zone and increases the peak value of the flame temperature and OH mass fraction. This increment also increases the flame thickness and reduces the OH oscillations and diffusion of the un-burnt fuel through the flame front.  相似文献   

16.
In this study hydrogen flames have been attempted in a rapidly mixed tubular flame combustor for the first time, in which fuel and oxidizer are individually and tangentially injected into a cylindrical combustor to avoid flame flash back. Three different cases were designed to examine the effects of fuel and oxidizer feeding method, diluent property, oxygen content and equivalence ratio on the characteristics of hydrogen flame, including the flame structure, lean extinction limit, flame stability and temperature. The results show that by enhancing mixing rate through feeding system, the range of equivalence ratio for steady tubular flame can be much expanded for the N2 diluted mixture, however, at the oxygen content of 0.21 (hydrogen/air) the steady tubular flame is achieved only up to equivalence ratio of 0.5; by decreasing oxygen content, the lean extinction limit slightly increases, and the upper limit for steady tubular flame establishment increases significantly, resulting in an expanded tubular flame range. For CO2 diluted mixture, the stoichiometric combustion has been achieved within oxygen content of 0.1 and 0.25, for which the burned gas temperature is uniformly distributed inside the flame front; as oxygen content is below 0.21, a steady tubular flame can be obtained from the lean to rich limits; and the lean extinction limit increases from 0.17 to 0.4 as oxygen content decreases from 0.21 to 0.1, resulting in a shrunk tubular flame range. Laminar burning velocity, temperature and Damköhler number are calculated to examine the differences between N2 and CO2 diluted combustion as well as the requirement for hydrogen-fueled tubular flame establishment.  相似文献   

17.
《能源学会志》2020,93(5):1883-1891
The reasons for the intrinsic reactivity differences in coal char conversion under an O2/H2O atmosphere compared with that under an O2/CO2 or O2/N2 atmosphere have been investigated in a thermogravimetric analyzer by a simple variable activation energy (SVAE) method combined with an adsorption/desorption reaction mechanism. The results show that only CO2 or H2O chemisorption occurred in the non-isothermal experiments, not gasification; however, the intrinsic reaction rate (IRR) of coal char conversion at the same O2 concentration still increases in an orderly manner under O2/CO2, O2/N2 and O2/H2O atmospheres. This result is due to the different chemisorption mechanisms of CO2 and H2O, namely, the production of C(CO), C(OH) and C(H) from CO2 and H2O chemisorption. At the same O2 concentration, the trends and magnitudes of variable activation energies for coal char combustion under O2/CO2 and O2/N2 atmospheres are similar, while they are very different from those under O2/H2O conditions. Therefore, CO2 has little influence on the reactivity, while H2O changes the reactivity. In addition, according to the developed reaction mechanism, it is concluded that the SVAE method contributes to the characteristic intrinsic reactivity of coal char conversion under different atmospheres.  相似文献   

18.
《能源学会志》2020,93(4):1278-1287
The freely-propagating laminar premixed flames of CH4–N2/CO2/H2O/Ar-O2 mixtures were conducted with the PREMIX code. The effects of the equivalence ratio and various oxidant atmospheres on the basic combustion characteristics were analyzed with the initial pressure and temperature of 1 atm and 398 K, respectively, O2 content in the oxidant of 21%. The chemical reaction mechanism GRI-Mech 3.0 was chosen to determine the effects of the oxidant atmospheres of N2/O2, CO2/O2, H2O/O2, and Ar/O2 on the adiabatic flame temperature, laminar burning velocity, flame structure, free radicals, intermediate species, net heat release rate and specific heat of the fuel/oxidant mixtures. The numerical results show that the maximum adiabatic flame temperatures and laminar burning velocities are at Ar/O2 atmosphere. The mole fractions of CO and H2 increased fastest at CO2/O2 atmosphere and H2O/O2, respectively. The mole fractions of CH3 and H follow the order Ar/O2> N2/O2>H2O/O2>CO2/O2. In addition, for 4 oxidant atmospheres, the peak mole fraction of C2H2 is following the order H2O/O2>Ar/O2>N2/O2>CO2/O2 and the net heat release rate is following the order Ar/O2>N2/O2>H2O/O2>CO2/O2 for all equivalence ratios.  相似文献   

19.
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   

20.
Oxygen-fuel combustion is a promising technology for CO2 emission reduction. The high-temperature entrained flow reactor and high-temperature drop tube furnace were used to analyses the formation and O2/CO2 combustion characteristics of real-environment coal char in high-temperature oxy-fuel conditions. It proposed “inflection point standard” of high-temperature flame method for the preparation of real-environmental oxy-fuel coal char according to the flame method. The results show that the ratios of C=O/C-O and C=O/Car increase in the coal char compared with the raw coals. The trend of C=O/Car in oxy-fuel condition is opposite to that in the inert atmosphere, due to the effect of high-concentration CO2. To achieve the burnout rate similar to air combustion for coal char, with the increase of coal rank, the O2 concentration should be enhanced. The optimal O2 concentration for the oxy-fuel combustion of JC anthracite is 30%, while that of other low-rank coals could be lower than 30%. The combustion characteristic of JC anthracite is with the highest sensitivity to temperature and O2 concentration.  相似文献   

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