Oxy-fuel Combustion Of Methane Research Articles

Investigation of oxy-fuel combustion for methane and acid gas in a diffusion flame

Co-combustion of methane (CH4) and acid gas (AG) is required to sustain the temperature in Claus reaction furnace. In this study, oxy-fuel combustion of methane and acid gas has been experimentally studied in a diffusion flame. Three equivalence ratios (ER = 1.0, 1.5, 2.0) and CH4-addition ratios (CH4/AG = 0.3, 0.5, 0.7) were examined and the flame was interpreted by analyzing the distributions of the temperature and species concentration along central axial. CH4-AG diffusion flame could be classified into three sections namely initial reaction, oxidation and complex reaction sections. Competitive oxidation of CH4 and H2S was noted in the first section wherein H2S was preferred and both were mainly proceeding decomposition and partial oxidation. SO2 was formed at oxidation section together with obvious presence of H2 and CO. However, H2 and CO were inclined to be sustained under fuel rich condition in the complex reaction section. Reducing ER and increasing CH4/AG contributed to higher temperature, H2S and CH4 oxidation and CO2 reactivity. Hence a growing trend for CH4 and AG to convert into H2, CO and SO2 could be witnessed. And this factor enhanced the generation of CS2 and COS in the flame inner core by interactions of CH4 and CO2 with sulfur species. COS was formed through the interactions of CO and CO2 with sulfur species. The CS2 production directly relied on reaction of CH4 with sulfur species. The concentration of COS was greater than CS2 since CS2 was probably inhibited due to the presence of H2. COS and CS2 could be consumed by further oxidation or other complex reactions.

Effects of Reaction Mechanisms and Differential Diffusion in Oxy-Fuel Combustion Including Liquid Water Dilution

The influence of chemistry and differential diffusion transport modeling on methane oxy-fuel combustion is analyzed considering different diluent characteristics. Analyses are conducted in terms of numerical simulations using a detailed description of the chemistry. Herein, different reaction mechanisms are employed to represent the combustion of methane. Simulations were performed with the computational fluid dynamics (CFD) code CHEM1D following different numerical setups, freely propagating flame, counter flow flame, and propagating flame in droplet mist reactors. The employed method is validated against experimental data and simulation results available in the literature. While the counter-flow flame reactor is exclusively used in the validation stage, different scenarios have been established for propagating flame simulations, as in single- or two-phase flow configuration. These comprehend variations in diluent compositions, reaction mechanisms, and different models to account for diffusion transport. Conducted investigations show that the choice for a specific reaction mechanism can interfere with computed flame speed values, which may agree or deviate from experimental observations. The achieved outcomes from these investigations indicate that the so-called GRI 3.0 mechanism is the best option for general application purposes, as a good balance is found between accuracy and computational efforts. However, in cases where more detailed information and accuracy are required, the CRECK C1-C3 mechanism demonstrated to be the best choice from the evaluated mechanisms. Additionally, the results clearly indicate that commonly applied simplifications to general flame modeling as the unitary Lewis number and mixture averaged approach strongly interfere with the computation of flame propagation speed values for single- and two-phase flows. While the application of unitary Lewis number approach is limited to certain conditions, the mixture averaged approach demonstrated a good agreement with the complex model for flame speed computations in the various tested scenarios. Such an outcome is not limited to oxy-fuel applications, but are straightly extensible to oxy-steam and air-blown combustion.

Numerical study on the auto-ignition characteristics of methane oxy-fuel combustion highly diluted by CO2

The methane oxy-fuel combustion highly diluted by CO2 at supercritical region has the best potential for its near-zero emission characteristics and high thermal efficiency, while its auto-ignition characteristic is still not clear. In current investigation, a model with detailed chemical kinetic mechanism was introduced to research its ignition delay times (IDs). Results showed that the fold points in low-temperature and high-pressure region is due to CH3O2 which only involved in Aramco-Mech 1.3. When temperature increases, the inhibition of the termination reactions plays a dominate role at low pressure, while the promotion of the branching reactions contributes more to the acceleration of ignition at high pressure. Five dominant pathways and their variations were clarified to depict the inhibition effect of the ratio of CO2/O2. Based on the newly proposed combustion mode, it was found that temperature dependence of the nonnegligible CO2 collision effect is opposite under low and high pressure with full-oxy combustion mode. This investigation extended the research region to higher pressure and comprehensively studied the relevant factors on the auto-ignition which not only provides theoretical reference for the chemical kinetics study of methane, but also contributes to the further researches of EGR, SCF and near-zero emissions technologies.

Comprehensive kinetic study of combustion technologies for low environmental impact: MILD and OXY-fuel combustion of methane

The development of processes with near-zero emissions such as MILD, flameless, and OXY-fuel combustion are of great interest in various energy scenarios. The assessment and design of new burners to meet energy demands and pollution reduction strongly depends on an accurate description of the chemistry involved in the combustion process. The main outcome of this study is the collection and review of a vast amount of experimental data on MILD and OXY-fuel combustion of methane that have been reported in recent years, together with a thorough kinetic analysis to identify aspects of the mechanism that requires further revision. The CRECK core model presented here is developed upon the Aramco 2.0 mechanism [1], and further extends the validation objectives to MILD and OXY fuel combustion conditions. The aim of this work is not only the mechanism validation but also a better understanding of the combustion characteristics and critical reaction pathways in MILD, flameless, and OXY-fuel combustion.

Evaluation, development, and validation of a new reduced mechanism for methane oxy-fuel combustion

The chemical kinetics under oxy-fuel combustion is significantly different from that of conventional air-combustion due to the effect of the high CO2 concentration. Although previous studies have made substantial achievement in reaction mechanisms for air-combustion, their performance under oxy-fuel conditions is still unknown. This study proposes a new 22-species, 19-step reduced mechanism for methane oxy-fuel combustion, developed using comprehensive mechanism evaluation, reduction, and validation methods. First, through quantitative error evaluation against a large experimental data set, for the first time we find that USC-Mech II obtains the best overall predictions among seven detailed combustion mechanisms in oxy-fuel conditions, particularly for the prediction of CO concentration. This detailed mechanism is then thoroughly simplified (including both skeletal and time-scale reduction) with error control under both atmospheric and pressurized oxy-fuel conditions. The obtained reduced mechanism is systematically validated using the detailed mechanism and the relative errors are found to be less than 10%. Relative to other mechanisms, this specially developed reduced mechanism for oxy-fuel combustion not only has minimal species, but also significantly improves the prediction of CO formation. The chemical influence of CO2 under oxy-fuel conditions is further discussed to identify dominant elementary reactions for CO formation, which is important for future development of methane oxy-fuel combustion.

Novel design of integrated gasification combined cycle (IGCC) power plants with CO2 capture

Abstract To develop the new integrated gasification combined cycle (IGCC) power plants with CO2 capture, first the integrated intermittent chemical looping air separation (IICLAS) unit is added to improve the coal gasification and carries out the oxy-fuel combustion, second the CO2 compression and sequestration (CCS) unit is added to achieve deep reductions in emissions of CO2 to the atmosphere. Compared to the conventional cryogenic air separation (CAS) unit, the IICLAS unit continuously produces the high-purity oxygen without the solid looping cycles and it also generates the additional electricity by virtue of the heating jacket for the oxy-fuel combustion of methane (OCM) and the waste heat recovery mechanism. A few comparisons show that the IICLAS-based IGCC + CCS power system (Design 2) can effectively increase 12.2% of the gross power compared to the CAS-based IGCC + CCS power system (Design 1). For the dual IICLAS-based IGCC + CCS power system (Design 3), the OCM units are removed and the flue gas is split to reduce the circulated steam/water. Finally, Design 3 can ensure the lowest oxygen supply rate by 50.67 kg/s, the highest net efficiency by 40.25%, and the largest KCC factor by 0.58.

Detailed investigation of NO mechanism in non-premixed oxy-fuel jet flames with CH4/H2 fuel blends

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%.

Oxy-fuel combustion of methane in a swirl tubular flame burner under various oxygen contents: Operation limits and combustion instability

Oxy-fuel combustion of methane was conducted in a rapidly mixed tubular flame burner, as oxygen content (mole fraction) was varied from 0.25 to 1.0. The fuel and oxidizer were injected from individual slits, and the slit widths were varied to promote mixing. Temporal variation of flame structure and acoustic pressure fluctuations were simultaneously measured by using a high speed video camera and a microphone respectively to examine unsteady combustion. Detailed results show that when the oxygen mole fraction was below 0.5, a steady flame can be established in a wide range from lean to rich; above 0.6, combustion-driven oscillation occurred by increasing the equivalence ratio to around 1.0. By reducing fuel slit width to half of the oxidizer slit and assigning CO2 to both fuel and oxidizer slits to maintain the same, high injection velocities, steady tubular combustion range was much expanded. However, when the oxygen mole fraction exceeded 0.7, steady combustion evolved into periodic oscillations by gradually increasing equivalence ratio; around stoichiometry the flame became steady, and thereafter evolved into unsteady state in the rich condition. Based on pressure fluctuation measurements and frequency spectral analyses, high frequency (around 2000Hz) combustion-driven oscillation was observed. This study shed lights on the operation limits of steady oxy-methane tubular combustion under various oxygen contents. For combustion under ultrahigh oxygen contents (0.7–1.0), the inlet width should be much thinner, and the fuel and oxidizer streams be maintained at the same, high injection velocities to achieve steady tubular flame around the stoichiometry.

Novel design of chemical looping air separation process for generating electricity and oxygen

The continuous- and batch-types chemical looping air separation (CLAS) processes are developed by using Aspen Plus® software. Through the heat integration of CLAS systems and oxy-fuel combustion of methane (OCM), new continuous- and batch-types of stand-alone electricity/oxygen production systems are denoted as Designs 1 and 2, respectively. The redox performances of both CLAS processes regarding Cu-, Mn-, and Co-based oxygen carriers with different ratios of inert binders are evaluated in terms of power/oxygen production ratios and specific CO2 emissions. A few comparisons show that (i) Design 1 using Cu- or Mn-based oxygen carriers at prescribed temperatures and normal pressures of oxidation and reduction can ensure almost zero CO2 emissions if the outlet 97 vol% of CO2 can be captured, (ii) Design 2 using Cu-based oxygen carrier with 83 wt% MgAl2O4 at a pressure of 8 atm and a temperature of 1070 °C during oxidation ensures almost zero CO2 emissions as well as the highest power production ratio, and (iii) Design 2 using Mn-based oxygen carrier with 37 wt% MgAl2O4 at a pressure of 15 atm and a temperature of 930 °C during oxidation ensures almost zero CO2 emissions as well as the largest oxygen production ratio.

The chemical mechanism of the effect of CO2 on the temperature in methane oxy-fuel combustion

Oxy-fuel combustion is considered to be a promising technology for the abatement of CO2. To study the chemical effects of CO2 on the combustion temperature, we have adopted counter-flow jets of methane versus heated air and an O2/CO2 mixture within a wide range of O2/CO2 volume ratios. An artificial material X, which is defined as such that it has exactly the same thermochemical, transport, and radiation properties as the real CO2 and is chemically inert, was adopted to separate the physical and chemical effects of CO2. The comparisons among air combustion, the oxy-X combustion and oxy-fuel combustion show that the chemical effects of CO2 reduce the combustion temperature in oxy-fuel combustion, and they are comparable to the physical effects of CO2 in an atmosphere of 36% O2/64% CO2. The heat release rates of R99(OH+CO⇔H+CO2) and R153(CH2(S)+CO2⇔CO+CH2O) are the most heavily impacted by CO2 in the range of the mole fraction of O2 from 20% to 28%. When the mole fraction of O2 increases from 28% to 32%, the effects of CO2 on elementary reactions offset each other. The heat release rates of R84(OH+H2⇔H+H2O) and R38(H+O2⇔O+OH) are the most heavily influenced by CO2 in the mole fraction of O2 from 32% to 36%. The heat release rates of R287(OH+HO2⇔O2+H2O) and R84 are significantly impacted by CO2 in the range of the mole fraction of O2 from 36% to 50%. The key reasons to the combustion temperature during oxy-fuel combustion are that R38 and R35(H+O2+H2O⇔HO2+H2O) in the mole fraction of O2 from 20% to 24% and from 26% to 50%, respectively.

Characteristics of Oxyfuel and Air–Fuel Combustion in an Industrial Water Tube Boiler

The characteristics of oxyfuel combustion and air–fuel combustion in the furnace of a typical industrial water tube boiler using methane as the operating fuel are investigated. Two oxyfuel cases are considered. The analysis is conducted for two oxyfuel cases that correspond to 21% O2 and 29% O2 in the oxidizer mixture (O2 + CO2). A renormalized group (RNG) turbulence model and the eddy dissipation model are utilized in the present work to provide the turbulence characteristics and the production rate of species. The solution of the radiative transfer equation was obtained using the discrete ordinates radiation model. The set of governing equations and the boundary conditions are solved numerically using Fluent computational fluid dynamics code considering a single-step reaction kinetics model for methane–oxyfuel combustion. Comparison of both oxyfuel combustion and air–fuel combustion indicates that the temperature levels are reduced in oxyfuel combustion. The results show that the temperature levels are greatly reduced as the percentage of recirculated CO2 is increased. It is concluded that the flame propagation speed in the CO2 environment is lower than that in N2. It is found that the natural gas and oxygen consumption rates are slower in oxyfuel combustion relative to air–fuel combustion. Heat transfer from the burnt gases to the water jacket along the different surfaces of the furnace is calculated. It is shown that the energy absorbed is much lower in the case of oxyfuel combustion along all surfaces except for the end part of the furnace close to the furnace rear wall. However, the same performance of the methane-oxy-flames is expected by increasing the oxygen concentration slightly above 29%.

Experimental investigation on the effect of O 2 and CO 2 on burning rates during oxyfuel combustion of methane

For combustion in an CO 2/O 2 atmosphere, known as oxy-combustion, altered combustion rates have to be expected. Previous investigations reported that this can be explained not only with the different thermophysical and radiative properties, but also by the fact that CO 2 participates directly in the chemical reactions. This paper presents an experimental study on oxyfuel methane combustion with the aim to investigate the importance of the chemical effects of high CO 2 concentrations. Experiments have been carried out in a 25 kW furnace for flameless combustion which provides the possibilities to achieve stable combustion of methane within a wide range of oxygen concentrations in the CO 2/O 2 mixture at constant reactor temperature. This allows to focus on the chemical effects of CO 2 by keeping the remaining factors affecting the combustion rate constant. Four different oxidizer mixtures (CO 2/O 2 and N 2/O 2 both with 21 vol% and 18 vol% O 2) have been studied by detailed in-furnace measurements for flue gas compositions and temperature. In case of combustion in N 2/O 2 atmosphere, the CO profiles obtained for different O 2 concentrations overlap thus demonstrating that changing the O 2 concentration did not affect combustion rates, with keeping the temperature constant. In case of combustion in CO 2/O 2 atmosphere, the CO concentrations obtained were much higher than those in N 2/O 2 atmosphere. In contrast to N 2/O 2, the O 2 concentrations had a significant impact on the production and consumption rates of CO in oxyfuel combustion. The results obtained in this work demonstrated that by elimination of the influence of: molar heat capacity, CO 2 dissociation, and thermal radiation, it can be estimated that the observed effects of high CO 2 concentrations on combustion rates can be attributed to its participation in the chemical reactions. An increase of O 2 in oxyfuel led to a reduction of this impact, however, further investigations on the exact mechanism are necessary.

Ammonia chemistry in oxy-fuel combustion of methane

The oxidation of NH 3 during oxy-fuel combustion of methane, i.e., at high [ CO 2 ] , has been studied in a flow reactor. The experiments covered stoichiometries ranging from fuel rich to very fuel lean and temperatures from 973 to 1773 K. The results have been interpreted in terms of an updated detailed chemical kinetic model. A high CO 2 level enhanced formation of NO under reducing conditions while it inhibited NO under stoichiometric and lean conditions. The detailed chemical kinetic model captured fairly well all the experimental trends. According to the present study, the enhanced CO concentrations and alteration in the amount and partitioning of O/H radicals, rather than direct reactions between N-radicals and CO 2 , are responsible for the effect of a high CO 2 concentration on ammonia conversion. When CO 2 is present as a bulk gas, formation of NO is facilitated by the increased OH/H ratio. Besides, the high CO levels enhance HNCO formation through NH 2 + CO . However, reactions NH 2 + O to form HNO and NH 2 + H to form NH are inhibited due to the reduced concentration of O and H radicals. Instead reactions of NH 2 with species from the hydrocarbon/methylamine pool preserve reactive nitrogen as reduced species. These reactions reduce the NH 2 availability to form NO by other pathways like via HNO or NH and increase the probability of forming N 2 instead of NO.

Reburn Chemistry in Oxy-fuel Combustion of Methane

This work deals with the reburn chemistry in oxy-fuel combustion of methane. It was studied experimentally in an atmospheric-pressure flow reactor under diluted conditions in N2 and CO2, respectively. The experiments covered temperatures from 1173 to 1773 K and stoichiometries ranging from fuel-rich to fuel-lean. The results showed similar NO reduction efficiencies in N2 and CO2 under reducing conditions, while under stoichiometric and fuel-lean conditions, the NO reduction obtained in CO2 was higher than that obtained in N2. The temperature at which CO exhibited its maximum concentration coincided with a sharp increase in NO reduction. This temperature was higher in the experiments with CO2 compared to those in N2. The results were interpreted in terms of an updated detailed chemical kinetic model. The effect of CO2 is to increase the OH/H ratio, lower the overall concentration of the O/H radical pool, and increase the availability of CO. Under reducing conditions, the different ways that CO2 interacts with the nitrogen chemistry largely cancel out and the overall impact is small. Under stoichiometric conditions, the NO reduction is enhanced in CO2 because of the suppression of the O/H radical pool, in particular of atomic oxygen.

Chemical Effects of a High CO2 Concentration in Oxy-Fuel Combustion of Methane

The oxidation of methane in an atmospheric-pressure flow reactor has been studied experimentally under highly diluted conditions in N2 and CO2, respectively. The stoichiometry was varied from fuel-lean to fuel-rich, and the temperatures covered the range 1200–1800 K. The results were interpreted in terms of a detailed chemical kinetic mechanism for hydrocarbon oxidation. On the basis of results of the present study, it can be expected that oxy-fuel combustion will lead to strongly increased CO concentrations in the near-burner region. The CO2 present will compete with O2 for atomic hydrogen and lead to formation of CO through the reaction CO2 + H ⇌ CO + OH. Reactions of CO2 with hydrocarbon radicals may also contribute to CO formation. The most important steps are those of singlet and triplet CH2 with CO2, while other radicals such as CH3 and CH are less important for consuming CO2. The high local CO levels may have implications for near-burner corrosion and slagging, but increased problems with CO emissi...