Role of CO2 in the CH4 oxidation and H2 formation during fuel-rich combustion in O2/CO2 environments

The effect of CO₂ reactivity on CH₄ oxidation and H₂ formation in fuel-rich O₂/CO₂ combustion where the concentrations of reactants were high was studied by a CH₄ flat flame experiment, detailed chemical analysis, and a pulverized coal combustion experiment. In the CH₄ flat flame experiment, the residual CH₄ and formed H₂ in fuel-rich O₂/CO₂ combustion were significantly lower than those formed in air combustion, whereas the amount of CO formed in fuel-rich O₂/CO₂ combustion was noticeably higher than that in air. In addition to this experiment, calculations were performed using CHEMKIN-PRO. They generally agreed with the experimental results and showed that CO₂ reactivity, mainly expressed by the reaction CO₂ + H → CO + OH (R1), caused the differences between air and O₂/CO₂ combustion under fuel-rich condition. R1 was able to advance without oxygen. And, OH radicals were more active than H radicals in the hydrocarbon oxidation in the specific temperature range. It was shown that the role of CO₂ was to advance CH₄ oxidation during fuel-rich O₂/CO₂ combustion. Under fuel-rich combustion, H₂ was mainly produced when the hydrocarbon reacted with H radicals. However, the hydrocarbon also reacted with the OH radicals, leading to H₂O production. In fact, these hydrocarbon reactions were competitive. With increasing H/OH ratio, H₂ formed more easily; however, CO₂ reactivity reduced the H/OH ratio by converting H to OH. Moreover, the OH radicals reacted with H₂, whereas the H radicals did not reduce H₂. It was shown that OH radicals formed by CO₂ reactivity were not suitable for H₂ formation. As for pulverized coal combustion, the tendencies of CH₄, CO, and H₂ formation in pulverized coal combustion were almost the same as those in the CH₄ flat flame.     

HDMR correlations for the laminar burning velocity of premixed CH4/H2/O2/N2 mixtures

Correlations for the laminar burning velocity of premixed CH₄/H₂/O₂/N₂ mixtures were developed using the method of High Dimensional Model Representation (HDMR). Based on experiment data over a wide range of conditions reported in the literature, two types of HDMR correlation (i.e. global and piecewise HDMR correlations) were obtained. The performance of these correlations was assessed through comparison with experimental results and the correlation reported in the literature. The laminar burning velocity predicted by the piecewise HDMR correlations was shown to agree very well with those from experiments. Therefore, the piecewise HDMR correlations can be used as an effective replacement for the full chemical mechanism when the prediction of the laminar burning velocity is needed in certain combustion modeling.     

Laminar burning velocities and flame stability analysis of H2/CO/air mixtures with dilution of N2 and CO2

Experimental measurement of the laminar burning velocities of H₂/CO/air mixtures and equimolar H₂/CO mixtures diluted with N₂ and CO₂ up to 60% and 20% by volume, respectively, were conducted at different equivalence ratios and conditions near to the sea level, 0.95 atm and 303 ± 2 K. Flames were generated using contoured slot-type nozzle burners and Schlieren images were used to determine the laminar burning velocity with the angle method. Numerical calculations were also conducted using the most recent detailed reaction mechanisms for comparison with the present experimental results. Additionally, a study was conducted to analyze the flame stability phenomenology that was found in the present experiments. The increase in the N₂ and CO₂ dilution fractions considerably reduced the laminar burning velocity due to the decrease in heat release and increase in heat capacity. At the same dilution fractions this effect was higher for the case of CO₂ due to its higher heat capacity and dissociation effects during combustion. Flame instabilities were observed at lean conditions. While the presence of CO in the fuel mixture tends to stabilize the flame, H₂ has a destabilizing effect which is the most dominant. A higher N₂ and CO₂ dilution fraction increased the range of equivalence ratios where unstable flames were obtained due to the increase in the thermal-diffusive instabilities.     

Experimental and numerical study on the effect of composition on laminar burning velocities of H2/CO/N2/CO2/air mixtures

Experimental and numerical study on laminar burning velocity of H₂/CO/N₂/CO₂/air mixtures was conducted by using a constant volume bomb and Chemkin package. Good agreement between experimental measurements and numerical calculations by using USCII Mech is achieved. Diffusional-thermal instability is enhanced but hydrodynamic instability is insensitive to the increase of hydrogen fraction in fuel mixtures. For mixtures with different hydrogen fractions, the adiabatic flame temperature is not the dominant influencing factor while high thermal diffusivity of hydrogen obviously enhances the laminar burning velocity. Laminar burning velocities increase with increasing hydrogen fraction and equivalence ratio (0.4–1.0). This is mainly due to the high reactivity of H₂ leading to high production rate of H and OH radicals. Reactions  and  play the dominant role in the production of H radical for mixtures with high hydrogen fraction, and reaction R31 plays the dominant role for mixtures with low hydrogen fraction.     

Influence of CO2, CH4, and initial temperature on H2/CO laminar flame speed

The objective of this study is to investigate the impact of syngas composition by varying the H₂/CO ratio (1:3, 1:1, and 3:1 by volume), the CO₂ dilution (0%–40%), and methane addition (0%–40%) on laminar flame speed. Thus, laminar flame speeds of premixed syngas–air mixtures were measured for different equivalence ratios (0.8–2.2) and inlet temperatures (295–450 K) using the Bunsen-burner method. It was found that laminar flame speed increases with increasing H₂/CO ratio, while CO₂ dilution or CH₄ addition decreased it. The location of the maximum flame speed shifts to richer mixtures with decreasing H₂/CO ratio, while it shifts to leaner mixtures with the addition of CH₄ due to its inherent slower flame speed. The location of the maximum flame speed is also shifted towards leaner mixtures with the addition of CO₂ due to the preponderance of the reduction of the adiabatic flame temperature with increasing dilution. Comparison between experimental and numerical results shows a better agreement using a modified mechanism derived from GRI-Mech 3.0. A correlation, based on the experimental results, is proposed to calculate the laminar flame speed over a wide range of equivalence ratios, inlet temperatures, and fuel content.     

The effects of composition on burning velocity and nitric oxide formation in laminar premixed flames of CH4 + H2 + O2 + N2

Experimental measurements of adiabatic burning velocity and NO formation in (CH₄ + H₂) + (O₂ + N₂) flames are presented. The hydrogen content in the fuel was varied from 0 to 35% and the oxygen content in the air from 20.9 to 16%. Nonstretched flames were stabilized on a perforated plate burner at 1 atm. The heat flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of methane + hydrogen + nitrogen + oxygen mixtures were found in satisfactory agreement with the modeling. The NO concentrations in these flames were measured in the burnt gases at a fixed distance from the burner using probe sampling. In lean flames, enrichment by hydrogen has little effect on [NO], while in rich flames, the concentration of nitric oxide decreases significantly. Dilution by nitrogen decreases [NO] at any equivalence ratio. Numerical predictions and trends were found in good agreement with the experiments. Different responses of stretched and nonstretched flames to enrichment by hydrogen are demonstrated and discussed.     

The effects of hydrogen addition on Fenimore NO formation in low-pressure,fuel-rich-premixed,burner-stabilized CH4/O2/N2 flames

We investigate the effects of hydrogen addition on Fenimore NO formation in fuel-rich, low-pressure burner-stabilized CH₄/O₂/N₂ flames. Towards this end, axial profiles of temperature and mole fractions of CH and NO are measured using laser-induced fluorescence (LIF). The experiments are performed at equivalent ratios of 1.3 and 1.5, using 0.25 mole fraction of hydrogen in the fuel, while varying the mass flux through the burner. The results are compared with those reported previously for burner-stabilized CH₄/O₂/N₂ flames. The increased burning velocity caused by hydrogen addition is seen to result in a lower flame temperature as compared to methane flame stabilized at the same mass flux. This increase in burner stabilization upon hydrogen addition results in significantly lower CH mole fractions at φ = 1.3, but appears to have little effect on the CH profile at φ = 1.5. In addition, the results show that not only the maximum flame temperature is reduced upon hydrogen addition, but the local gas temperature in the region of the CH profile is lowered as well. The measured NO mole fractions are seen to decrease substantially for both equivalence ratios. Analysis of the factors responsible for Fenimore NO formation shows the reduction in temperature in the flame front to be the major factor in the decrease in NO mole fraction, with a significant contribution from the decrease in CH mole fraction at φ = 1.3. At φ = 1.5, the results suggest that the lower flame temperature upon hydrogen addition further retards the conversion of residual fixed-nitrogen species to NO under these rich conditions as compared to the equivalent methane flames.     

Catalytic CO2 reforming of CH4 over Cr-promoted Ni/char for H2 production

The objective of the study is to investigate the catalytic performance of Cr-promoted Ni/char in CO₂ reforming of CH₄ at 850 °C. The char obtained from the pyrolysis of a long-flame coal at 1000 °C was used as the support. The catalysts were prepared by incipient wetness impregnation methods with different metal precursor doping sequence. The characterization of the composite catalysts was evaluated by XRD, XPS, SEM-EDS, TEM, H₂-TPR, CO₂-TPD, CH₄-TPSR, and CO₂-TPO. The results indicate that the catalyst prepared by co-impregnation of Ni and Cr possess higher activity than those by sequential impregnation. The optimal loading of Cr on 5 wt% Ni/char is 7.8 wt‰. Moreover, the molar feed ratio of CH₄/CO₂ has a considerable effect on both the stability and the activity of Cr–Ni/char. The main effect of Cr is the great enhance of the adsorption to CO₂. It is interesting that the conversions of CH₄ and CO₂ over Cr-promoted Ni/char and Ni/char decrease initially, following by a steady rise as the reaction proceeds with time-on-stream (TOS). In addition, cyclic tests were conducted and no distinct deterioration in the catalytic performance of the catalysts was observed. On the basis of the obtained results, nickel carbide was speculated to be the active species which was formed during the CO₂ reforming of CH₄ reaction.     

NOx formation and reduction mechanisms in staged O2/CO2 combustion

The purpose of this study was to investigate the NO_x formation and reduction mechanisms in staged O₂/CO₂ combustion and in air combustion. A flat CH₄ flame doped with NH₃ for fuel-N was formed over the honeycomb, and NO_x formation characteristics were investigated. In addition, chemiluminescence of OH^* distribution was measured, and CHEMKIN-PRO was used to investigate the detailed NO_x reduction mechanism. In general, the NO_x conversion ratio decreases with decreasing primary O₂/CH₄ ratio, whereas NH₃ and HCN, which are easily converted to NO_x in the presence of O₂, increases rapidly. Therefore, a suitable primary O₂/CH₄ ratio exists in the staged combustion. Our experiments showed the primary O₂/CH₄ ratio, which gave the minimum fixed nitrogen compounds in O₂/CO₂ combustion, was lower than in air combustion. The NO_x conversion ratio in O₂/CO₂ combustion was lower than in air combustion by 40% in suitable staged combustion. This could be explained by high CO₂ concentrations in the O₂/CO₂ combustion. It was shown that abundant OH radicals were formed in O₂/CO₂ combustion through the CO₂ + H → CO + OH, experimentally and numerically. OH radicals produced H and O radicals through H₂ + OH → H + H₂O and O₂ + H → OH + O, because a mass of hydrogen source exists in the CH₄ flame. O and OH radicals formed in the fuel-rich region enhanced the oxidation of NH₃ and HCN. NO_x formed by the oxidation of NH₃ and HCN was converted to N₂ because the oxidation occurred in the fuel-rich region where the NO_x reduction effect was high. In fact, the oxidation of NH₃ and HCN in the fuel-rich region was preferable to remaining NH₃ and HCN before secondary O₂ injection in the staged combustion. A significant reduction in NO_x emission could be achieved by staged combustion in O₂/CO₂ combustion.     

Experimental estimate of the laminar burning velocity of iso-octane in oxygen-enriched and CO2-diluted air

Oxygen-enriched combustion is of great interest for industrial applications, since membrane separation technology can be used. The objective of this work is to provide unique data on laminar burning velocity, a key parameter in real combustion development, for the oxygen-enriched combustion of an iso-octane/air mixture for various dilution (by air or CO₂) cases. Experiments were carried out in a stainless steel combustion chamber at atmospheric pressure and 373 K. The iso-octane was mixed with a mixture of O₂, CO_2, and N₂. The volume fraction of O₂ was varied from 21% to 29% and CO₂ was varied from 0% to 28%. The classical shadowgraphy technique was used to detect the reaction zone in order to deduce the un-stretched burning velocity, using a nonlinear methodology. All the experimental data were compared with the numerical results obtained with chemical kinetic schemes available in the literature. For further experimental investigations, a correlation is proposed to predict laminar burning velocity as a function of the quantity of O₂ and CO₂ in the gas mixture. Finally, analytical and experimental data concerning Markstein length are discussed.     

Determination of laminar burning velocities for lean low calorific H2/N2 and H2/CO/N2 gas mixtures

Combustion of lean and ultra-lean synthetic H₂/CO mixtures that are highly diluted in inert gases is of great importance in several fields of technology, particularly in the field of post combustion for combined heat and power (CHP) systems based on fuel cell technology. In this case H₂/CO mixtures occur via hydrocarbon reforming and their complete conversion requires efficient, compact and low emission combustion systems. In order to design such systems, knowledge of global flame properties like the laminar burning velocity, is essential. Using the heat flux burner method, laminar burning velocities were experimentally determined for highly N₂ diluted synthetic H₂ and H₂/CO mixtures with low calorific value, burning with air, at ambient temperature and atmospheric pressure. Furthermore, numerical 1-D simulations were performed, using a series of different chemical reaction mechanisms. These numerical predictions are analysed and compared with the experimental data.     

An oxy-fuel mass-recirculating process for H2 production with CO2 capture by autothermal catalytic oxyforming of methane

A new oxy-fuel H₂ generation process with CO₂ avoidance is provided. The process utilizes mass recirculation of CO and H₂O to the oxyforming reactor. A comparison between non-recirculating and mass-recirculating oxyforming reactor operation is given. Main benefits of mass recirculation are emphasized. The oxyforming reactor is integrated with the H₂ and CO₂ separators, fuel cell and O₂ generator. In the process C/O is equal to 0.5 while C/H determines the temperature level in the reactor. The reaction system includes combustion, steam reforming and water–gas shift reactions. The oxyforming process is found to be mass transport controlled with O₂ as the limiting reactant. It is emphasized that under MR conditions the decomposition of H₂/CO₂ by water–gas shift reaction is suppressed by means of CO/H₂O-enrichment and hence MR conditions allow for higher temperatures beneficial to endothermic steam reforming reaction. Under MR conditions the thermodynamic equilibrium limits are overcome and all reactions are forced to proceed to the completion which enables 100% selectivities to H₂ and CO₂. The effects of operation parameters such as temperature, flow rate, pressure and composition are examined. The derived S-terms enable for the concise interpretation of the effect of pressure on the concentration gradients transverse to the flow. The consistent control algorithm of the oxyforming reactor is provided.     

Production of hydrogen through the carbonation–calcination reaction applied to CH4/CO2 mixtures

The production of hydrogen combined with carbon capture represents a possible option for reducing CO₂ emissions in atmosphere and anthropogenic greenhouse effect. Nowadays the worldwide hydrogen production is based mainly on natural gas reforming, but the attention of the scientific community is focused also on other gas mixtures with significant methane content. In particular mixtures constituted mainly by methane and carbon dioxide are extensively used in energy conversion applications, as they include land-fill gas, digester gas and natural gas. The present paper addresses the development of an innovative system for hydrogen production and CO₂ capture starting from these mixtures. The plant is based on steam methane reforming, coupled with the carbonation and calcination reactions for CO₂ absorption and desorption, respectively. A thermodynamic approach is proposed to investigate the plant performance in relation to the CH₄ content in the feeding gas. The results suggest that, in order to optimize the hydrogen purity and the efficiency, two different methodologies can be adopted involving both the system layout and operating parameters. In particular such methodologies are suitable for a methane content, respectively, higher and lower than 65%.     

CO2/O2环境对柴油着火及燃烧特性的影响

为研究CO₂/O₂环境对柴油着火和燃烧特性的影响,以正庚烷为柴油表征燃料,利用CONVERGE计算了不同CO₂/O₂环境下正庚烷的着火和燃烧过程,并搭建了可视化定容燃烧弹试验平台进行了验证。使用高速摄影机记录了初始温度850 K,初始压力3 MPa,CO₂体积分数分别为35%、40%、50%和60%时正庚烷燃烧的自发光强度,利用CHEMKIN中定容均质反应器分析了CO₂物理和化学作用对着火的影响。研究结果表明：在CO₂体积分数35%时存在爆燃的现象,随着CO₂体积分数增长,着火延迟时间增长,着火位置远离喷嘴,稳态燃烧阶段火焰的长度和宽度也增大,CO₂体积分数在50%～60%之间时火焰自发光强度峰值明显下降;CO₂的物理作用抑制了着火,第三体作用对着火的促进作用大于直接参与反应对着火的抑制作用,造成CO₂的化学作用缩短了着火延迟时间,并且随着CO     

Effects of modifying Ni/Al2O3 catalyst with cobalt on the reforming of CH4 with CO2 and cracking of CH4 reactions

Alumina supported nickel (Ni/Al₂O₃), nickel–cobalt (Ni–Co/Al₂O₃) and cobalt (Co/Al₂O₃) catalysts containing 15% metal were synthesized, characterized and tested for the reforming of CH₄ with CO₂ and CH₄ cracking reactions. In the Ni–Co/Al₂O₃ catalysts Ni–Co alloys were detected and the surface metal sites decreased with decrease in Ni:Co ratio. Turnover frequencies of CH₄ were determined for both reactions. The initial turnover frequencies of reforming (TOF_DRM) for Ni–Co/Al₂O₃ were greater than that for Ni/Al₂O₃, which suggested a higher activity of alloy sites. The initial turnover frequencies for cracking (TOF_CRK) did not follow this trend. The highest average TOF_DRM, H₂:CO ratio and TOF_CRK were observed for a catalyst containing a Ni:Co ratio of 3:1. This catalyst also had the maximum carbon deposited during reforming and produced the maximum reactive carbon during cracking. It appeared that carbon was an intermediate product of reforming and the best catalyst was able to most effectively crack CH₄ and oxidize carbon to CO by CO₂.     

Comparative study on the effect of CO2 and H2O dilution on laminar burning characteristics of CO/H2/air mixtures

A study on the effect of CO₂ and H₂O dilution on the laminar burning characteristics of CO/H₂/air mixtures was conducted at elevated pressures using spherically expanding flames and CHEMKIN package. Experimental conditions for the CO₂ and H₂O diluted CO/H₂/air/mixtures of hydrogen fraction in syngas from 0.2 to 0.8 are the pressures from 0.1 to 0.3 MPa, initial temperature of 373 K, with CO₂ or H₂O dilution ratios from 0 to 0.15. Laminar burning velocities of the CO₂ and H₂O diluted CO/H₂/air/mixtures were measured and calculated using the mechanism of Davis et al. and the mechanism of Li et al. Results show that the discrepancy exists between the measured values and the simulated ones using both Davis and Li mechanisms. The discrepancy shows different trends under CO₂ and H₂O dilution. Chemical kinetics analysis indicates that the elementary reaction corresponding to peak ROP of OH consumption for mixtures with CO/H₂ ratio of 20/80 changes from reaction R3 (OH + H₂ = H + H₂O) to R16 (HO₂+H = OH + OH) when CO₂ and H₂O are added. Sensitivity analysis was conducted to find out the dominant reaction when CO₂ and H₂O are added. Laminar burning velocities and kinetics analysis indicate that CO₂ has a stronger chemical effect than H₂O. The intrinsic flame instability is promoted at atmospheric pressure and is suppressed at elevated pressure for the CO₂ and H₂O diluted mixtures. This phenomenon was interpreted with the parameters of the effective Lewis number, thermal expansion ratio, flame thickness and linear theory.     

Catalytic conversion of CH4 and CO2 to synthesis gas on Ni/SiO2 catalysts containing Gd2O3 promoter

A series of Ni/SiO₂ catalysts containing different amounts of Gd₂O₃ promoter was prepared, characterized by H₂-adsorption and XRD, and used for carbon dioxide reforming of methane (CRM) and methane autothermal reforming with CO₂ + O₂ (MATR) in a fluidized-bed reactor. The results of pulse surface reactions showed that Ni/SiO₂ catalysts containing Gd₂O₃ promoter could increase the activity for CH₄ decomposition, and Raman analysis confirmed that reactive carbon species mainly formed on the Ni/SiO₂ catalysts containing Gd₂O₃ promoter. In this work, it was found that methane activation and reforming reactions proceeded according to different mechanisms after Gd₂O₃ addition due to the formation of carbonate species. In addition, Ni/SiO₂ catalysts containing Gd₂O₃ promoter demonstrated higher activity and stability in both CRM and MATR reactions in a fluidized bed reactor than Ni/SiO₂ catalysts without Gd₂O₃ even at a higher space velocity.     

Ni/CeO2 catalysts with high CO2 methanation activity and high CH4 selectivity at low temperatures

CO₂ methanation was performed over 10 wt%Ni/CeO₂, 10 wt%Ni/α-Al₂O₃, 10 wt%Ni/TiO₂, and 10 wt%Ni/MgO, and the effect of support materials on CO₂ conversion and CH₄ selectivity was examined. Catalysts were prepared by a wet impregnation method, and characterized by BET, XRD, H₂-TPR and CO₂-TPD. Ni/CeO₂ showed high CO₂ conversion especially at low temperatures compared to Ni/α-Al₂O₃, and the selectivity to CH₄ was very close to 1. The surface coverage by CO₂-derived species on CeO₂ surface and the partial reduction of CeO₂ surface could result in the high CO₂ conversion over Ni/CeO₂. In addition, superior CO methanation activity over Ni/CeO₂ led to the high CH₄ selectivity.