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.     

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.     

Hydrogen enrichment of methane and syngas for MILD combustion

Moderate or Intense Low-oxygen Dilution (MILD) combustion is a technology with important characteristics such as significant low emission and high-efficiency combustion. The hydrogen enrichment of conventional fuels is also of interest due to its favorable characteristics, such as low carbon-containing pollutants, high reaction intensity, high flammability, and thus fuel usage flexibility. In this study, the effects of adding hydrogen to methane and syngas fuels have been investigated under conditions of MILD combustion through numerical simulation of a well-set-up MILD burner. The Reynolds-Averaged Navier-Stokes (RANS) approach is adopted along the Eddy Dissipation Concept (EDC) combustion model with two different chemical mechanisms. Molecular diffusion is modeled using the differential diffusion approach. The effects of oxidizer dilution and fuel jet Reynolds number on the reactive flow field have been studied. Results show that with an increase in hydrogen portion of the fuel mixtures, the volume of the high-temperature region of combustion field increases whereas a reduction of oxidizer oxygen content leads to more proximity to the MILD condition. Increasing the fuel jet Reynolds number will result in an expansion of the combustion zone and shifting of this region in the axial direction. Predictions revealed that the methane flame is more sensitive to the oxidizer dilution and fuel jet Reynolds number than syngas. Moreover, enrichment of fuel with hydrogen seems to be better for acquiring condition of the MILD combustion for syngas rather than methane. Indeed, syngas shows more sensitivity to hydrogen enrichment than methane, which makes hydrogen a good additive to syngas in terms of MILD condition benefits.     

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 (NO_x) 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 (Y_O2), and mass fraction of hydrogen in the fuel jet (Y_H2) 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 Y_O2 ≤ 3%, while both NNH and N₂O-intermediate routes dominate the NO production at 3% < Y_O2 < 10%. When Y_O2 ≥ 10%, NO is obtained mainly from thermal mechanism. Moreover, in the oxy-combustion of methane and hydrogen fuel blends with Y_O2 = 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 Y_O2 ≤ 3% and combustion of methane and hydrogen fuel blends with Y_H2 ≤ 10%.     

Numerical and experimental analysis of NO emissions from a lab-scale burner fed with hydrogen-enriched fuels and operating in MILD combustion

An experimental and computational investigation of a lab-scale burner, which can operate in both flame and MILD combustion conditions and is fed with methane and a methane/hydrogen mixture (hydrogen content of 60% by vol.), is carried out. The modelling results indicate the need of a proper turbulence/chemistry interaction treatment and rather detailed kinetic mechanisms to capture MILD combustion features, especially in presence of hydrogen. Despite these difficulties, Computational Fluid Dynamics results to be very useful, as for instance it allows evaluating the internal recirculation degree in the burner, a parameter which is otherwise difficult to be determined. Moreover the model helps interpreting experimental evidences: for instance the modelling results indicate that in presence of hydrogen the NNH and N₂O intermediate routes are the dominant formation pathways for the MILD combustion conditions investigated.     

Effect of the combustion model and kinetic mechanism on the MILD combustion in an industrial burner fed with hydrogen enriched fuels

A numerical and experimental investigation of a burner operating in MILD combustion regime and fed with methane and methane-hydrogen mixtures (with hydrogen content up to 20% by wt.) is presented. Numerical simulations are performed with two different combustion models, i.e. the ED/FR and EDC models, and three kinetic mechanisms, i.e. global, DRM-19 and GRI-3.0. Moreover, the influence of molecular diffusion on the predictions is assessed. Results evidence the need of a detailed chemistry approach, especially with H₂, to capture the volumetric features of MILD combustion. The inclusion of molecular diffusion influences the prediction of H₂ distribution; however, the effects on the temperature field and on the major species are negligible for the present MILD combustion system. A simple NO formation mechanism based on the thermal and prompt routes is found to provide NO emissions in relatively good agreement with experimental observations only when applied on temperature fields obtained with the EDC model and detailed chemistry.     

Reactive structures and NOx emissions of methane/hydrogen mixtures in flameless combustion

Methane/hydrogen combustion represents a concrete solution for the energy scenario to come. Indeed, the addition of hydrogen into the natural gas pipeline is one of the solutions foreseen to reduce CO₂ emissions. Nevertheless, the replacement of methane by hydrogen will enhance the reactivity of the system, increasing NOx emissions. To overcome this issue, non-conventional combustion technologies, such as flameless combustion represent an attractive solution. This study aims to improve our understanding of the behaviour of methane/hydrogen blends under flameless conditions by means of experiments and simulations. Several experimental campaigns were conducted to test fuel flexibility for different methane/hydrogen blends, varying the injector geometries, equivalence ratio and dilution degree. It was found that a progressive addition of hydrogen in methane enhanced the combustion features, reducing the ignition delay time and loosing progressively the flameless behaviour of the furnace. Reducing the air injector diameter or increasing the fuel lance length were found to be efficient techniques to reduce the maximum temperature of the system and NOx emissions in the exhausts, reaching values below 30 ppm for pure hydrogen. MILD conditions were achieved up to 75%H₂ in molar fraction, with no visible flame structures. Additionally, RANS-based simulations were also conducted to shed further light on the effect of adding hydrogen into the fuel blend. A sensitivity study was conducted for three different fuel blends: pure methane, an equimolar blend and pure hydrogen. The effect of chemistry detail, mixing models, radiation modeling and turbulence models on in-flame temperatures and NOx emissions was also studied. In particular, it was found that the usage of detailed chemistry for NOx, coupled with an adjustment of the PaSR model, filled the gap between experiments and predictions. Finally, a brute-force sensitivity revealed that NNH is the most important route for NOx production.     

Numerical investigation on NO formation in laminar counterflow methane/n-heptane dual fuel flames

To understand the fundamental mechanisms of NO formation in natural gas-diesel dual fuel combustion, a numerical study on NO formation in laminar counterflow methane (CH₄)/n-heptane (n-C₇H₁₆) dual fuel flames is conducted. The results reveal that the flame structure and NO formation vary with the fuel equivalence ratio. For a given n-C₇H₁₆/air mixture, the NO emission index decreases with increasing the equivalence ratio of the CH₄/air mixture (φ(CH₄/air)). The NO formation route analysis suggests that the prompt and thermal routes dominate the NO formation. The increase in φ(CH₄/air) causes the decrease in the contribution of the prompt route to overall NO formation. NO formation by prompt route is mainly caused by rich n-C₇H₁₆ combustion. As φ(CH₄/air) increases, the mole fractions of the radicals (OH, O and H) related to CH formation in the reaction zone of rich n-C₇H₁₆/air flame branch are decreased, which reduces the formation of NO by prompt route.     

Hydrogen utilization as a fuel: hydrogen-blending effects in flame structure and NO emission behaviour of CH4–air flame

Hydrogen-blending effects in flame structure and NO emission behaviour are numerically studied with detailed chemistry in methane–air counterflow diffusion flames. The composition of fuel is systematically changed from pure methane to the blending fuel of methane–hydrogen through H₂ molar addition up to 30%. Flame structure, which can be described representatively as a fuel consumption layer and a H₂–CO consumption layer, is shown to be changed considerably in hydrogen-blending methane flames, compared to pure methane flames. The differences are displayed through maximum flame temperature, the overlap of fuel and oxygen, and the behaviours of the production rates of major species. Hydrogen-blending into hydrocarbon fuel can be a promising technology to reduce both the CO and CO₂ emissions supposing that NO_x emission should be reduced through some technologies in industrial burners. These drastic changes of flame structure affect NO emission behaviour considerably. The changes of thermal NO and prompt NO are also provided according to hydrogen-blending. Importantly contributing reaction steps to prompt NO are addressed in pure methane and hydrogen-blending methane flames. Copyright © 2006 John Wiley & Sons, Ltd.     

Effect of hydrogen on hydrogen–methane turbulent non-premixed flame under MILD condition

Energy crises and the preservation of the global environment are placed man in a dilemma. To deal with these problems, finding new sources of fuel and developing efficient and environmentally friendly energy utilization technologies are essential. Hydrogen containing fuels and combustion under condition of the moderate or intense low-oxygen dilution (MILD) are good choices to replace the traditional ones. In this numerical study, the turbulent non-premixed CH₄+H₂ jet flame issuing into a hot and diluted co-flow air is considered to emulate the combustion of hydrogen containing fuels under MILD conditions. This flame is related to the experimental condition of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]. In general, the modelling is carried out using the EDC model, to describe turbulence–chemistry interaction, and the DRM-22 reduced mechanism and the GRI2.11 full mechanism to represent the chemical reactions of H₂/methane jet flame. The effect of hydrogen content of fuel on flame structure for two co-flow oxygen levels is studied by considering three fuel mixtures, 5%H₂+95%CH₄, 10%H₂+90%CH₄ and 20% H₂+80%CH₄(by mass). In this study, distribution of species concentrations, mixture fraction, strain rate, flame entrainment, turbulent kinetic energy decay and temperature are investigated. Results show that the hydrogen addition to methane leads to improve mixing, increase in turbulent kinetic energy decay along the flame axis, increase in flame entrainment, higher reaction intensities and increase in mixture ignitability and rate of heat release.     

An experimental study of mild flameless combustion of methane/hydrogen mixtures

This paper presents an experimental study of mild flameless combustion regime applied to methane/hydrogen mixtures in a laboratory-scale pilot furnace with or without air preheating. Results show that mild flameless combustion regime is achieved from pure methane to pure hydrogen whatever the CH₄/H₂ proportion. The main reaction zone remains lifted from the burner exit, in the mixing layer of fuel and air jets ensuring a large dilution correlated to low NOx emissions whereas CO₂ concentrations obviously decrease with hydrogen proportion. A decrease of NOx emissions is measured for larger quantity of hydrogen due mainly to the decrease of prompt NO formation. Without air preheating, a slight increase of the excess air ratio is required to control CO emissions. For pure hydrogen fuel without air preheating, mild flameless combustion regime leads to operating conditions close to a "zero emission furnace", with ultra-low NOx emissions and without any carbonated species emissions.     

Counterflow diffusion flame of hydrogen-enriched biogas under MILD oxy-fuel condition

Biogases are commonly found renewable fuels. Meanwhile they are difficult to be economically utilized because their low calorific values are very small and the induced costs of upgrading are expensive. To overcome the above deficiencies, in this paper we discuss the feasibility to utilize biogases under the MILD oxy-fuel operation recently proposed by the present authors. A popularly used counterflow configuration is adopted as the research prototype in this work. The effects of (1) the preheated temperature of the oxidizer mixtures, (2) the oxygen concentration in the oxidizer flow and (3) the hydrogen concentration in the fuel mixtures on the reaction structure of biogas under the new combustion condition are investigated with the aid of the lattice Boltzmann method (LBM). Through numerical simulation, it is found that the MILD oxy-fuel combustion fueled by biogas can be sustained even with relatively low preheated temperature of the oxidizer, extremely highly diluted oxygen concentration in the oxidizer flow and little hydrogen addition in the fuel mixtures, which provide a solid theoretical basis to develop a novel scheme to respond to the challenge caused by CO₂ emissions. Moreover, our discoveries imply the breakdown of the popularly used flamelet approach and emphasize the urgency to develop new turbulent combustion models for this novel combustion strategy.     

Comparative study of flame structures and NOx emission characteristics in fuel injection recirculation and fuel gas recirculation combustion system

A numerical study with momentum‐balanced boundary conditions has been conducted to grasp the chemical effects of added CO₂ to fuel‐ and oxidizer‐sides on flame structure and NO emission behaviour in H₂–O₂ diffusion flames with varying flame location. A reaction mechanism is proposed to show better agreements with experimental results in CO₂‐added hydrogen flames. Oxidizer‐side dilution results in significantly higher flame temperatures and NO emission. Flame location is dramatically changed due to high diffusivity of hydrogen according to variation of the composition of fuel‐ and oxidizer‐sides. This affects flame structure and NO emission considerably especially the chemical effects of added CO₂. The present work also displays separately thermal contribution and prompt NO emission due to the chemical effects caused by thermal dissociation of added CO₂ in NO emission behaviour. It is found that flame temperature and the flame location affect the contribution of thermal and prompt NO due to chemical effects considerably in NO emission behaviour. Copyright © 2004 John Wiley & Sons, Ltd.     

Effect of fuel composition on jet flames in a heated and diluted oxidant stream

The role of hydrogen addition on the structure of the Moderate or Intense Low oxygen Dilution (MILD) combustion regime is examined using a combination of experimental techniques and laminar flame calculations. Laser diagnostic imaging is used to simultaneously reveal the in situ distribution of the hydroxyl radical (OH), formaldehyde (H₂CO), and temperature using the Jet in Hot Coflow (JHC) burner. The fuels considered are natural gas, ethylene, and LPG (each diluted with hydrogen 1:1 by volume). Hydrogen addition to the primary fuel was found necessary to stabilise the flames. Further to the role of hydrogen in the stabilisation of the flames, hydrogen addition also leads to the reaction zone exhibiting similar structure for different primary fuel types. The independence of the reaction zone structure with hydrogen addition suggests that a wide variety of fuels may be usable for achieving MILD combustion.     

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.     

Investigation of colorless distributed combustion regime using a high internal recirculative combustor

This study aims at investigating the combustion characteristics of methane and a hydrogen-rich fuel on distributed regime in a combustor with high internal recirculation as distributed regime can achieve with highly internal entrainment. The model validation was first taken place with the existing experimental results under non-reacting conditions, and it is demonstrated that the mean velocity profiles predicted are in satisfactorily good agreement with the existing data in the combustor. Then, the methane and the hydrogen-rich fuel were consumed at an equivalence ratio of 0.8 and a thermal power of 10 kW under distributed conditions which means that oxygen concentration in the oxidizer is reduced from 21% to 15% along with internal entrainment for each fuel used. The results showed that the velocity magnitudes increased in the combustor with decrease in oxygen concentration because of diluent introduction. Moreover, distributed regime enabled uniformly temperature field inside the combustor together with the ultra-low NO_X values with introducing the diluent. In addition, one can say that the maximum temperature and the outlet NO_X values (1800 K and 27.60 ppm at 21% O₂ – 1450 K and 0.05 ppm at 15% O₂ for methane, 2000 K and 32.47 ppm at 21% O₂ – 1700 K and 0.19 ppm at 15% O₂ for the hydrogen-rich fuel) were relatively higher while combusting the hydrogen-rich fuel compared to those of methane due to the presence of hydrogen. However, It is concluded that distributed regime provided uniformly temperature fields and ultra-low NO_X levels even the hydrogen-rich fuel is used.     

Hydrogen addition effect on NO formation in methane/air lean-premixed flames at elevated pressure

The present study investigates freely propagating methane/hydrogen lean-premixed laminar flames at elevated pressures to understand the hydrogen addition effect of natural gas on the NO formation under the conditions of industrial gas turbine combustors. The detailed chemical kinetic model which was used in the previous study on the NO formation in high pressure methane/air premixed flames was adopted for the present study to analyze NO formation of methane/hydrogen premixed flames. The present mechanism shows good agreement with experimental data for methane/hydrogen mixtures, including ignition delay times, laminar burning velocities, and NO concentration in premixed flames. Hydrogen addition to methane/air mixtures with maintaining methane content leads to the increase of NO concentration in laminar premixed flames due to the higher flame temperature. Methane/hydrogen/argon/air premixed flames are simulated to avoid the flame temperature effect on NO formation over a pressure range of 1–20atm and equivalence ratio of 0.55. Kinetic analyses shows that the N₂O mechanism is important on NO formation for lean flames between the reaction zone and postflame region, and thermal NO is dominant in the postflame zone. The hydrogen addition leads to the increase of NO formation from prompt NO and NNH mechanisms, while NO formation from thermal and N₂O mechanisms are decreased. Additionally, the NO formation in the postflame zone has positive pressure dependencies for thermal NO with an exponent of 0.5. Sensitivity analysis results identify that the initiation reaction step for the thermal NO and the N₂O mechanism related reactions are sensitive to NO formation near the reaction zone.     

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.     

The effects of hydrogen addition on NO formation in atmospheric-pressure, fuel-rich-premixed, burner-stabilized methane, ethane and propane flames

The effects of hydrogen addition on NO formation in fuel-rich, burner-stabilized methane, ethane and propane flames are reported. Profiles of temperature and NO mole fraction were obtained using spontaneous Raman scattering and laser-induced fluorescence (LIF), respectively. Experiments were performed at equivalent ratio of 1.3, with 0 and 0.2 mole fraction of hydrogen in the fuel; and the mass flux through the burner was varied for each mixture. The addition of hydrogen only modestly affects the flame temperature and NO mole fraction. For the vast majority of the flames studied, the temperature and NO decrease by less than 40 K and 20% (relative), respectively, upon hydrogen addition. The decrease in NO fraction is more distinct in methane and propane flames, and more modest for ethane. The comparison of the experimental data obtained for a given fuel in near-adiabatic C_nH_2n+2/H₂/O₂/N₂ and burner-stabilized C_nH_2n+2/Air flames shows that the NO mole fraction at a given mass flux is practically independent of the composition of the oxidizer. Comparison of the experimental profiles with the predictions of one-dimensional flame calculations with detailed chemical mechanisms indicates that the decrease in the Fenimore NO formation with hydrogen addition arises from the concomitant decrease in CH fraction. Analysis of the computational results suggests that the reaction NCN + H → CH + N₂ returns a considerable fraction of NCN back to N₂.