MILD combustion for hydrogen and syngas at elevated pressures

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.     

NO mechanisms of syngas MILD combustion diluted with N2, CO2, and H2O

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., X_O2 < 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.     

Effects of hydrogen addition on structure and NO formation of highly CO-Rich syngas counterflow nonpremixed flames under MILD combustion regime

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.     

Second stage porous media burner for syngas enrichment

The syngas production from hydrocarbons by porous media combustion (or conventional gasification) processes has been intensively and extensively studied due its calorific value and its applications in the energy sector. However, the syngas produced in a first stage gasifier can still have concentrations of light hydrocarbons (e.g., methane) which can be post-processing for further enrichment of hydrogen and carbon monoxide. The present work numerically investigates the performance of a second stage porous media burner to enrich the syngas content, mainly hydrogen and carbon monoxide. A one-dimensional model based on a two-temperature approximation is implemented, based on the PREMIX code, and supported by CHEMKIN and GRI-MECH 3.0 database and routines. From hybrid porous bed reactor experiments, five types of mixtures in the equivalence ratio range between 0.4 and 2.4 are tested: pure methane as baseline, pure Eucalyptus nitens syngas, pure Pinus radiata syngas, and two mixtures of methane with each of the biomass syngas in equal volume quantities, as transition points. The results obtained show that in the enriched syngas, in comparison to the first content after the first stage reactor, the concentrations of hydrogen and carbon monoxide increase, due to the partial oxidation of methane, however part of the hydrogen is consumed in the process. The intermediate species present in methane processing for pure syngas mixtures have broader reaction zones and in lower concentrations compared to the use of pure methane. For equivalence ratios greater than 1.9, pure syngas mixtures show higher conversion efficiencies compared to the baseline. At the equivalence ratio of 2.4, the pure syngas from Eucalyptus nitens and Pinus radiata has an energy return over energy invested (EROI) of 58.27% and 53.95%, respectively, and a maximum hydrogen and carbon monoxide yields of 31.66% and 48.40%, respectively. In the case of the Pinus radiata, the outlet syngas concentrations of the hydrogen and carbon monoxide on dry basis expose an increment about two times in comparison to the initial concentrations.     

Effects of hydrogen addition on combustion characteristics of a methane fueled MILD model combustor

Combined with need of the carbon emissions, the feasibility of Moderate or Intense Low-oxygen Dilution (MILD) combustion fueled with hydrogen/methane blends needs to be investigated. This paper discusses the pollutant emissions, the stable operating range and the flame morphology for a jet-induced MILD model combustor. The hydrogen/methane volume ratios range 0:10 to 5:5. The NO_x emissions are less than 5 ppm@15%O₂ when the hydrogen content is less than 50% by volume in the atmospheric conditions. The calculation using chemical reactor network (CRN) model demonstrates that the effect of heat loss on NO_x emissions increases as the adiabatic combustion temperature increases, which is consistent with the experimental results. The maximum OH1 signal intensity increased at higher hydrogen content, especially when the hydrogen content exceeds 30% by volume. Due to the increase in turbulent burning velocity and the enhancement in the reaction intensity, the reaction zones shrink with increasing hydrogen content. In addition, with increasing hydrogen content, the stable operation range of the combustor becomes narrower, and the stable combustion is not maintained when the hydrogen content exceeds 50% by volume. The findings of the paper help to further understand the effect of hydrogen content on the formation of MILD combustion in the jet-induced combustor.     

Effects of hydrogen addition to methane on the thermal and ignition delay characteristics of fuel-air,oxygen-enriched and oxy-fuel MILD combustion

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.     

Hydrogen and syngas production from propane and polyethylene

Hybrid filtration combustion of propane in a porous medium composed of aleatory polyethylene pellets and alumina spheres is studied to examine the suitability of the concept for hydrogen and syngas production. Temperature, velocity, and chemical products of the combustion waves were recorded experimentally in the range of equivalence ratios from stoichiometry (φ = 1.0) to φ = 1.65. The model predictions for combustion in inert porous media using GRI 3.0 reaction mechanism are in good qualitative agreement with experimental data. Hydrogen, carbon monoxide, methane and carbon dioxide are dominant combustion products for upstream sub-adiabatic temperatures recorder in all the range of equivalence ratios studied. The maximum hydrogen and carbon monoxide yields are close to 48% and 89%, respectively.     

Combined solar energy and combustion of hydrogen-based fuels under MILD conditions

This work presents the stability and performance characteristics of a Hybrid Solar Receiver Combustor operating in the Moderate or Intense Low oxygen Dilution (MILD) combustion regime. The device was operated at 12-kW_th in two different modes of operation, i.e. combustion-only (MILD) and mixed (combustion and solar introduced into the device simultaneously), using natural gas (NG), liquefied petroleum gas (LPG), hydrogen (H₂), NG/H₂ and LPG/H₂ blends. A 5-kW_el xenon-arc lamp was used to simulate the concentrated solar radiation introduced into the device. The influence of the mode of operation and fuel composition on the combustion stability, thermal efficiency, energy balance, pollutant emissions, heat losses and distribution of heat flux within the receiver are presented for a range of values of the heat extraction. It was found that MILD combustion can be successfully stabilised within the HSRC over a broad range of operating conditions and fuel type, and in mixed operations, with low CO (for carbon-based fuels) and NO_x emissions. The addition of H₂ and/or concentrated solar radiation to the MILD process was found to increase its stability limits. Mixed and combustion-only operations showed similar performance, regardless of the fuel type, providing further evidence that the fuel flow rate can be used dynamically to compensate for variability in the solar resource. Also, the heat extracted from the heat exchanger and the specific fuel consumption were found to increase and decrease, respectively, by adding H₂ to the system for both modes of operation, showing that hydrogen addition is beneficial. The numerical analysis revealed that the higher performance with H₂ is attributable to a higher radiative heat transfer rate than for NG and LPG under MILD conditions.     

Stability characteristics of non-premixed turbulent jet flames of hydrogen and syngas blends with coaxial air

The stability characteristics of attached hydrogen (H₂) and syngas (H₂/CO) turbulent jet flames with coaxial air were studied experimentally. The flame stability was investigated by varying the fuel and air stream velocities. Effects of the coaxial nozzle diameter, fuel nozzle lip thickness and syngas fuel composition are addressed in detail. The detachment stability limit of the syngas single jet flame was found to decrease with increasing amount of carbon monoxide in the fuel. For jet flames with coaxial air, the critical coaxial air velocity leading to flame detachment first increases with increasing fuel jet velocity and subsequently decreases. This non-monotonic trend appears for all syngas composition herein investigated (50/50 → 100/0% H₂/CO). OH^∗ chemiluminescence imaging was performed to qualitatively identify the mechanisms responsible for the flame detachment. For all fuel compositions, local extinction close to the burner rim is observed at lower fuel velocities (ascending stability limit), while local flame extinction downstream of the burner rim is observed at higher fuel velocities (descending stability limit). Extrema of the non-monotonic trends appear to be identical when the nozzle fuel velocity is normalized by the critical fuel velocity obtained for the single jet cases.     

Assessment of steady and unsteady flamelet models for MILD combustion modeling

Moderate or intense low-oxygen dilution (MILD) combustion is a novel combustion technology with the comparable chemical and turbulent mixing timescales. In the most of literatures, relatively expensive volume-based models are recommended for this combustion regime while this regime is not completely idealized homogenized reactor and nor strong flamelet like. This paper is focused on the assessment of the lower cost, flamelet approach for MILD condition. In this way, simplifying JHC burner of Dally et al. is considered to model using RANS approach. The effects of inclusion of unity versus non-unity Lewis numbers, radiation heat transfer, and various scalar dissipation rates are evaluated. Results show that the flamelet model may not be totally rejected for the MILD condition because it could still capture flame characteristics relatively acceptable. Choosing an appropriate scalar dissipation rate value is discussed for single flamelet and higher values are recommended by O₂ increment. Moreover, considering non-unity Lewis numbers improved species concentration at the jet centerline for low O₂ levels. Furthermore, the unsteady flamelet model with radiation had a better prediction for NO_x especially for higher O₂ Levels.     

Scalar gradient behaviour in MILD combustion

The results of three-dimensional Direct Numerical Simulation (DNS) of Moderate, Intense Low-oxygen Dilution (MILD) and conventional premixed turbulent combustion conducted using a skeletal mechanism including the effects of non-unity Lewis numbers and temperature dependent transport properties are analysed to investigate combustion characteristics using scalar gradient information. The DNS data is also used to synthesise laser induced fluorescence (LIF) signals of OH, CH₂O, and CHO. These signals are analysed to verify if they can be used to study turbulent MILD combustion and it has been observed that at least two (OH and CH₂O) LIF signals are required since the OH increase across the reaction zone is smaller in MILD combustion compared to premixed combustion. The scalar gradient PDFs conditioned on the reaction rate obtained from the DNS data and synthesised LIF signals suggests a strong gradient in the direction normal to the MILD reaction zone with moderate reaction rate implying flamelet combustion. However, the PDF of the normal gradient is as broad as for the tangential gradient when the reaction rate is high. This suggests a non-flamelet behaviour, which is due to interaction of reaction zones. The analysis of the conditional PDFs for the premixed case confirms the expected behaviour of scalar gradient in flamelet combustion. It has been shown that the LIF signals synthesised using 2D slices of DNS data also provide very similar insights. These results demonstrate that the so-called flameless combustion is not an idealised homogeneous reactive mixture but has common features of conventional combustion while containing distinctive characteristics.     

Coal dust gasification in the filtration combustion mode with syngas production

A new method for the gasification of fine solid fuel was proposed and worked out, by partial oxidation in a flow of gaseous oxidant with filtration of the suspended fuel through an inert porous matrix. In this case, the solid fuel gasification was carrying out similar to the filtration combustion of gases. The gasification of fine solid coal allows one to produce a combustible gas rich in H₂ and CO was studied. A possibility of pulverized coal gasification in a fixed bed reactor with production of gaseous products containing up to 13% by volume of hydrogen and carbon monoxide was shown experimentally.     

常温空气非对称射流MILD燃烧特性实验研究

比较了不同的燃烧器结构后设计了非对称射流燃烧器,并通过实验研究了非对称射流燃烧器的射流速度、喷嘴角度对常温空气MILD燃烧的影响。结果表明:丙烷流量在0.4~0.8m3/h、空气流量在11~22m3/h都可以达到MILD燃烧状态;增大射流速度,减小喷嘴角度可以使温度峰值降低,温度分布更均匀,MILD燃烧更稳定,效果更好,NOx排放量大大减小,达到了"近零排放"。     

NOx formation in post-flame gases from syngas/air combustion at atmospheric pressure

Species concentration measurements specifically those associated with nitrogen oxides (NO_x) can act as important validation targets for developing kinetic models to predict NO_x emissions under syngas combustion accurately. In the present study, premixed combustion of syngas/air mixtures, with equivalence ratio (Φ) from 0.5 to 1.0 and H₂/CO ratio from 0.25 to 1.0 was conducted in a McKenna burner operating at atmospheric pressure. Temperature and NO_x concentrations were measured in the post-combustion zone. For a given H₂/CO ratio, increasing the equivalence ratio from lean to stoichiometric resulted in an increase in NO and decrease in NO₂ concentration near the flame. Increasing the H₂/CO ratio led to a decrease in the temperature as well as the NO concentration near the flame. Based on the axial profiles above the burner, NO concentration increases right above the flame while NO₂ concentration decreases through NO₂-NO conversion reactions according to the path flux analysis. In addition, the present experiments were operated in the laminar region where multidimensional transport effects play significant roles. In order to account for the radial and axial diffusive and convective coupling to chemical kinetics in laminar flow, a multidimensional model was developed to simulate the post-combustion species and temperature distribution. The measurements were compared against both multidimensional computational fluid dynamics (CFD) simulations and one-dimensional burner-stabilized flame simulations. The multidimensional model predictions resulted in a better agreement with the measurements, clearly highlighting the effect of multidimensional transport.     

Hydrogen and/or syngas production by combined steam and dry reforming of methane on nickel catalysts

Two alumina supported Ni catalysts with pore sizes of 5.4 nm and 9 nm were synthetized, characterized and tested in the Combined Steam and Dry Reforming of Methane (CSDRM) for the production of hydrogen rich gases or syngas. The reaction mixture was designed to simulate the composition of real clean biogas, the addition of water being made in order to have molar ratios of H₂O:CO₂ corresponding to 2.5:1, 7.5:1 and 12.5:1. Structural and functional characterization of catalysts revealed that Ni/Al₂O₃ with larger pore size shows better characteristics: higher surface area, lower Ni crystallite sizes, higher proportion of stronger catalytic sites for hydrogen adsorption, and higher capacity to adsorb CO₂. At all studied temperatures, for a CH₄:CO₂:H₂O molar ratio of 1:0.48:1.2, a (H₂+CO) mixture with H₂:CO ratio around 2.5 is obtained. For the production of hydrogen rich gases, the optimum conditions are: CH₄:CO₂:H₂O = 1:0.48:6.1 and 600 °C. No catalyst deactivation was observed after 24 h time on stream for both studied catalysts, and no carbon deposition was revealed on the used catalysts surface regardless the reaction conditions.     

A computational study of combustion and extinction of opposed-jet syngas diffusion flames

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.     

Detailed numerical simulation of syngas combustion under partially premixed combustion engine conditions

Two-dimensional detailed numerical simulation is performed to study syngas/air combustion under partially premixed combustion (PPC) engine conditions. Detailed chemical kinetics and transport properties are employed in the study. The fuel, a mixture of CO and H₂ with a 1:1 molar ratio, is introduced to the domain at two different instances of time, corresponding to the multiple injection strategy of fuel used in PPC engines. It is found that the ratio of the fuel mass between the second injection and the first injection affects the combustion and emission process greatly; there is a tradeoff between NO emission and CO emission when varying the fuel mass ratio. The ignition zone structures under various fuel mass ratios are examined. A premixed burn region and a diffusion burn region are identified. The premixed burn region ignites first, followed by the ignition of mixtures at the diffusion burn region, and finally a thin diffusion flame is formed to burn out the remaining fuel. NO is produced mainly in the premixed burn region, and later from the diffusion burn region in mixtures close to stoichiometry, whereas unburned CO emission is mainly from the diffusion burn region. An optimization of the fuel mass in the two regions can offer a better tradeoff between NO emission and CO emission. The effects of initial temperature and turbulence on the premixed burn and diffusion burn regions are investigated.     

Numerical study of rich catalytic combustion of syngas

The rich catalytic combustion of syngas/air mixtures over platinum has been investigated numerically in a two-dimensional circular channel using steady simulations and detailed hetero-homogeneous chemistry. The channel dimensions are representative of a catalytic monolith. Simulations have been conducted in the pressure range of 1–10 bar and Φ = 3–5 with varying inlet velocities, residence time, H₂/CO ratio and CH₄ percentage. Detailed kinetic studies including the reaction path diagram (RPD) in a plug flow reactor have also been conducted to understand the kinetic interactions between H₂, CO, and CH₄. It has been observed that the homogeneous reaction rates are significant at higher pressures and cannot be neglected, although they were highly localized. The channel temperature significantly affected the relative conversion of H₂ and CO. The kinetic coupling between H₂ and CO oxidation was studied and the reason for the differential consumption of O₂ by the reactants was addressed by analyzing the reaction pathways. The residence time in the channel affected the species oxidation and four operation regimes were identified. Both the water-gas shift reaction and the reverse water-gas shift reaction were observed under varying conditions of pressure and equivalence ratio. The effect of H₂/CO ratio has also been investigated. The present study shows that rich catalytic combustion of syngas is fundamentally different from lean combustion.     

MILD combustion of hydrogenated biogas under several operating conditions in an opposed jet configuration

MILD combustion of biogas takes its importance firstly from the combustion process that diminishes significantly fuel consumption and reduces emissions and secondly from the use of biogas which is a renewable fuel. In this paper, the influence of several operating conditions (namely biogas composition, hydrogen enrichment and oxidizer dilution) is studied on flame structure and emissions. The investigation is conducted in MILD regime with a special focus on chemical effects of CO₂ in the oxidizer. Opposed jet diffusion combustion configuration is adopted. The combustion kinetics is described by the Gri 3.0 mechanism and the Chemkin code is used to solve the problem.It is found that oxygen reduction has a significant effect on flame temperature and emissions while less sensitivity corresponds to hydrogen enrichment in MILD combustion regime. Temperature and species are considerably reduced by oxygen decrease in the oxidizer and augmented by hydrogen addition to the fuel. The maximum values of temperature and species are not influenced by the composition of the biogas in MILD regime. Blending biogas with hydrogen can be used to sustain MILD combustion at very low oxygen concentration in the fuel.In MILD combustion regime, the chemical effect of CO₂ in the oxidizer stream reduces considerably the flame temperature and species production, except CO which is enhanced. For high amounts of CO₂ in the oxidizer, the chemical effect of CO₂ becomes negligible.