Analysis of entropy generation in a hydrogen-enriched turbulent non-premixed flame

The analysis of local entropy generation and exergy loss was performed in a turbulent non-premixed H₂-enriched CH₄–air bluff-body flame. Detailed chemical kinetic, transport properties, and turbulence-chemistry interaction were taken into account in using laminar flamelet model for the simulation of combustion process via an in-house, finite volume code. The analysis was based on local entropy generation calculation. Results showed that thermal conduction made the most contribution to entropy generation followed by chemical reaction and mass diffusion, while the contribution of viscous dissipation was negligible. Entropy generation resulting from thermal conduction occurs in a large volume of the domain, while entropy generation resulting from chemical reaction and mass diffusion occurs only near the bluff surface. The effect of H₂ addition to fuel and air preheating on the entropy generation rate was investigated. It was observed that entropy generation and exergy loss were decreased by H₂ addition, mainly due to a decrease in the chemical reaction component of entropy generation, while entropy generation resulting from thermal conduction slightly increased and entropy generation resulting from mass diffusion remained almost constant. Entropy generation resulting from heat conduction by preheating combustion air decreased, while entropy generation resulting from chemical reaction and mass diffusion remained almost constant. The decrease of thermal conduction contribution in entropy generation is so significant that, by preheating air up to 750 K in the case of pure CH₄, chemical reaction becomes the main source of irreversibility. These investigations show that H₂ addition and preheating the combustion air both lead to the improvement of the second law efficiency, although the second law efficiency is more sensitive to flame structure and air temperature.     

Analysis of entropy generation distribution in micro-combustors with baffles

This study presents the analysis of entropy generation distribution in H₂/air premixed flame in micro-combustors with baffles. The numerical simulation of combustion is performed with the help of Ansys Fluent code. The entropy generation rates are derived from entropy transport equation and calculated based on the numerical results. The entropy generation caused by various irreversible processes such as chemical reaction, thermal conduction and mass diffusion are studied in micro-combustors with baffles. The effects of the height of the baffles, H₂/air mass flow rate and equivalence ratio are investigated. It is found that a higher baffle will lead to more entropy generation and relatively larger destruction of available energy. The exergy efficiency decreases significantly when the H₂/air mass flow rate is increased. The lean and rich H₂/air mixture shows an obvious lower entropy generation rate and higher exergy efficiency than the stoichiometric mixture.     

Investigation of dilution effects on partially premixed swirling syngas flames using a LES-LEM approach

The dilution effects of CO₂ and H₂O on partially premixed swirling syngas flames are investigated with the large eddy simulation (LES) method. The linear-eddy model (LEM) is employed to directly resolve the unclosed molecular diffusion, scalar mixing and chemical reaction processes occurring at subgrid scale level using their specific length and time scales instead of modelling, which makes the LES-LEM approach quite attractive for hydrogen fuel combustion as the obviously different diffusion and reaction characteristics of H₂ and H compared to other species in the syngas mixture. Firstly, adding CO₂ into the fuel stream can significantly decrease the flame temperature during the partially premixed combustion. The concentration of H and OH radicals decreases upon CO₂ dilution and thus the chemical reaction processes are modified. Compared with CO₂, H₂O is less effective in changing the temperature field because of the chemical effects of H₂O. The simultaneous addition of H₂O and CO₂ as dilution gases with volume ratio 1:1 into the fuel stream is also conducted to identify the effects of H₂O and CO₂ on partially premixed combustion dynamics by comparing with single H₂O and CO₂ cases. The obtained results are expected to provide helpful information for the design and operation of gas turbine combustion systems with syngas fuels.     

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.     

Dilution effects analysis on NOx emissions of opposed-jet H2/CO syngas diffusion flames

Syngas is a promising alternative fuel for stationary power generation due to cleaner combustion than convectional fossil fuels. During the gasification processes, the by-products of CO₂, H₂O, or N₂ may be present in the syngas mixture to control the flame temperature and emissions. Several studies indicated that syngas with dilutions is capable of reducing pollutant emissions such as NO_x emissions. This work applied a numerical model of opposed-jet diffusion fames to explore the dilution effects on NO_x formation and differentiate the inert effect, thermal/diffusion effect, chemical effect, and radiation effect from CO₂, H₂O, or N₂ dilutions. The numerical study was performed by a revised OPPDIF program coupling with narrowband radiation model and detail chemical mechanism. The dilution effects on NO_x formation were analyzed by comparing the realistic and hypothetical cases. Regardless the diluent types, the inert effect is the main cause to reduce NO production, followed by chemical effect and radiation effect. The thermal/diffusion effect may promote NO formation because the preferential diffusion due to different diffusivities between diluents and syngas magnifies the reaction rate locally. CO₂ dilution reduces NO by radiation effect at low strain rate, and contributes NO reduction by chemical effect at high strain rate. At the same dilution percentage, CO₂ dilution reduces NO production the most, followed by H₂O and N₂. Besides the thermal/diffusion effect, the chemical effect of H₂O enhances NO production through thermal route and reburn route.     

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.     

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.     

Thermal field under the effect of the chemical reaction of a direct internal reforming solid oxide fuel cell DIR-SOFC

The effects of direct internal reforming in a fuel cell solid oxide (SOFC) on thermal fields are studied by mathematical modeling. This study presents the thermal fields of a standard fuel cell (Ni-YSZ/YSZ/LSM) anode supported. This study is also made in the perpendicular plane at the flow of gases. The fuel cell is powered by air and fuel, CH₄, H₂, CO₂, CO and H₂O hence the birth of the phenomenon of direct internal reforming (DIR-SOFC). It is based on reforming chemical reactions, steam reforming reaction and water–gas shift reaction. The main purpose of this work is the visualization of temperature fields under the influence of global chemical reactions and the confirmation of the thermal behavior of this chemical reaction. The thermal fields are obtained by a computer program (FORTRAN).     

Hydrogen-enriched non-premixed jet flames: Compositional structures with near-wall effects

The species concentrations of non-premixed hydrogen and syngas flames were examined using results obtained from direct numerical simulation technique with flamelet generated manifold chemistry. Flames with pure H₂ and H₂/CO mixtures are discussed for an impinging jet flame configuration. Single-point data analyses are presented illustrating the effects of fuel composition on species concentrations. In general, scatterplots of all species show the effects of fuel variability on the flame compositional structures. The behaviours of major combustion products and key radicals species indicate the effects of CO concentration on the 2/CO syngas combustion. In particular, high concentration of CO tends to induce local extinction in the 2/CO flames in which critical chemical reactions of the fuel mixture such as CO + OH become important. The unsteady fluctuations of species profiles in the wall jet region characterise the complexity of the distributions of compositional structures in the near-wall region with respect to the effects of CO concentration on the combustion of hydrogen-enriched fuels.     

Computed flammability limits of opposed-jet H2/CO syngas diffusion flames

Extensive computations were made to determine the flammability limits of opposed-jet H₂/CO syngas diffusion flames from high stretched blowoff to low stretched quenching. Results from the U-shape extinction boundaries indicate the minimum hydrogen concentrations for H₂/CO syngas to be combustible are larger towards both ends of high strain and low strain rates. The most flammable strain rate is near one s⁻¹ where syngas diffusion flames exist with minimum 0.002% hydrogen content. The critical oxygen percentage (or limiting oxygen index) below which no diffusion flames could exist for any strain rate was found to be 4.7% for the equal-molar syngas fuels (H₂/CO = 1), and the critical oxygen percentage is lower for syngas mixture with higher hydrogen content. The flammability maps were also constructed with strain rates and pressures or dilution gases percentages as the coordinates. By adding dilution gases such as CO₂, H₂O, and N₂ to make the syngas non-flammable, besides the inert effect from the diluents, the chemical effect of H₂O contributes to higher flame temperature, while the radiation effect of H₂O and CO₂ plays an important role in the flame extinction at low strain rates.     

Predicting chemical species in spark-ignition engines

The unsteady motion of chemical species in the intake and exhaust ducts of spark-ignition internal combustion (I.C.) engines is studied numerically by employing a finite-difference-based engine simulation code. The time-dependent mass fractions of six dominant products (CO₂, H₂O, CO, H₂, O₂, N₂) combined with five additional minor species (H, NO, O, OH, N) are computed in the combustion chamber and tracked throughout the breathing system for wide-open-throttle as well as part-load operating conditions. The property calculations employ the NASA database to determine the composition. The effect of the number of product species on the engine performance and dynamic quantities, including pressure and temperature in the breathing system, is also investigated.     

结构化载氧体颗粒化学链燃烧内扩散影响模拟分析

采用有限速率模型对CuO/Al_2O_3结构化载氧体颗粒化学链燃烧进行了数值模拟,考察了不同结构化载氧体颗粒模型和进气速度对载氧体化学反应内扩散的影响。结果表明,相比于球形载氧体,环形结构拉西环载氧体内扩散更为容易,反应快,温升高;相比于全活性载氧体,1/4活性和半活性载氧体气体扩散到活性物质距离短,反应更快;增加进气速度能够加速颗粒内扩散,改善反应速率。     

Hydrogenoxygen steam generator for a closed hydrogen combustion cycle

The paper deals with the problems of hydrogen combustion in an oxygen environment to produce high-temperature steam to be used in electricity generation at various power stations including nuclear power plants (NPP). For example, the use of H₂/O₂ steam generator within a hydrogen energy complex may allow increasing the NPP power and efficiency under operating conditions due to hydrogen steam superheating of the main working fluid in a steam-turbine unit. In addition, the use of the hydrogen energy complex may allow adapting NPP to variable electric load schedules with the increasing share of such power stations as well as developing environmentally friendly technologies for electricity generation. In the paper, a new solution to the problem of the effective and safe use of hydrogen energy at NPP with a hydrogen energy complex has been proposed.Technical solutions to hydrogen combustion in an oxygen environment using direct injection of cooling water or water steam into combustion products may have a significant weakness, namely the “quenching” phenomenon occurring during water/water steam injection resulting in the recombination efficiency decrease during the cooling of combustion products which is reflected in the increased proportion of non-condensable gases. In this case, the supply of such mixture to the steam-power cycle may be unsafe, as it could result in the increased concentration of unburned hydrogen in the steam turbine flow path. In the paper, a closed hydrogen cycle along with the hydrogen steam superheating system on its basis has been proposed to solve this problem. The closed-circuit system of hydrogen combustion preventing hydrogen permeation into the working fluid of a steam cycle completely as well as ensuring its full oxidation due to some excess of circulating oxygen has been investigated by the authors.Two types of H₂/O₂ combustion chambers for the system of safe hydrogen steam superheating in NPP cycle by using the closed-circuit system of hydrogen combustion in an oxygen environment have been considered in the study. The required parameters of H₂/O₂ steam generator with regard to operating temperature conditions as well as the power range of H₂/O₂ steam generators with the proposed combustion chamber construction design have been determined by mathematical modeling of the combustion and heat-mass-exchange processes.     

Experimental study on superheated steam generation by the reaction of high humidity hydrogen and oxygen in a model internal combustion steam generator

Internal combustion steam cycle (ICSC) is a novel steam power cycle using hydrogen as an energy carrier to produce superheated steam. High humidity hydrogen produced during fast hydrogen production process is directly used to produce superheated steam by combusting with stoichiometric oxygen without hydrogen storage. The ICSC efficiency is greatly affected by the content of non-condensable gas in superheated steam. In the present study, superheated steam generation by high humidity hydrogen was investigated in a model internal combustion steam generator. Effects of H₂O/H₂ molar ratio of humid hydrogen and velocity ratio of humid hydrogen to oxygen on non-condensable gas content, combustion efficiency, and mixing rate were evaluated. The results showed that the critical H₂O/H₂ ratio for the humid hydrogen humidity limit was 2.8. With increasing velocity ratio, mixing rate and combustion efficiency increased under the same H₂O/H₂ ratio. The H₂O/H₂ reaction rate monotonously decreased as the H₂O/H₂ ratio increased from 1.0 to 2.5, while the mixing rate increased along with the velocity ratio. The combustion efficiency initially increased and subsequently decreased, and the peak value was reached at a H₂O/H₂ ratio of 1.75. This result indicated that the humid H₂-O₂ combustion was controlled by diffusion under H₂O/H₂ ratios of 1.0 to 1.75, but turned to be controlled by chemical kinetics when the H₂O/H₂ ratio ranged between 1.75 and 2.5.     

Investigating various effects of reformer gas enrichment on a natural gas-fueled HCCI combustion engine

Homogenous charge compression ignition (HCCI) combustion has the potential to work with high thermal efficiency, low fuel consumption, and extremely low NO_x-PM emissions. In this study, zero-dimensional single-zone and quasi-dimensional multi-zone detailed chemical kinetics models were developed to predict and control an HCCI combustion engine fueled with a natural gas and reformer gas (RG) blend. The model was validated through experiments performed with a modified single-cylinder CFR engine. Both models were able to acceptably predict combustion initiation. The result shows that the chemical and thermodynamic effects of RG blending advance the start of combustion (SOC), whereas dilution retards SOC. In addition, the chemical effect was stronger than the dilution effect, which was in turn stronger than the thermal effect. Furthermore, it was found that the strength of the chemical effect was mainly dependent on H₂ content in RG. Moreover, the amount of RG and concentration of species (CO–H₂) were varied across a wide range of values to investigate their effects on the combustion behavior in an HCCI engine. It was found that the H₂ concentration in RG has a more significant effect on SOC at lower RG percentages in comparison with the CO concentration. However, in higher RG percentages, the CO mass concentration becomes more effective than H₂ in altering SOC.     

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.     

The thermodynamics of chemical looping combustion applied to the hydrogen economy

Adoption of the hydrogen economy (HE) is one means by which industrial economies can reduce point source CO₂ emissions. At its simplest, H₂ is generated centrally using a primary energy source to split water; the H₂ is then transmitted to end users, thereby ‘carrying’ energy from the central plant to, say, a motor vehicle. Assuming the primary energy input to drive the system comes from fossil fuels, carbon capture at these plants is required to reduce the specific CO₂ emissions of the system to the minimum. However, an additional thermodynamic advantage of the HE is often ignored, as it facilitates a rise in second law efficiency in the utilisation of fossil fuels. The HE can be viewed as an open-loop, chemical looping combustion (CLC) system, with H₂ as the oxygen carrier. In CLC systems, entropy recirculation leads to a reduction in the reversible reaction temperature; in the HE this results in a rise in the efficiency of both H₂ producing and H₂ consuming devices. In consequence, the second law efficiency of internal combustion engines burning H₂ is increased for a given peak cycle temperature. For fuel cells, with notionally higher thermal efficiency than internal combustion (IC) engines, the percentage gain in second law efficiency is even more pronounced. A process flow analysis allowing for likely irreversibilities shows that combining a CLC plant and a fleet of fuel cells, the overall efficiency of the system equals 40.8%, exceeding the performance of competing fuel powered technologies.     

Effects of equivalence ratios on the normal and inverse diffusion flame of acid gas combustion in the pure oxygen atmosphere

The experimental studies on the effect of equivalence ratios to the acid gas (H₂S and CO₂) combustion in the pure oxygen atmosphere was presented in a coaxial jet double channel burner. Three equivalence ratios (Φ = 0.8, 1.0 and 1.5) are examined to analyze the distribution of the flame temperature and gas composition in the normal and inverse diffusion flame along central axial (R = 0.0) and axial line at 3 mm (R = 0.75) in radial direction. The results revealed that acid gas combustion mainly occurred chemical decomposition of H₂S and oxidation of H₂S and H₂ at R = 0.0, while mainly occurred H₂S and H₂ oxidation at R = 0.75 in the normal diffusion flame. Reducing Φ increased the flame temperature and it is higher at R = 0 than that at R = 0.75 because of heat loss. It also increased the volume fraction of CO, H₂ and COS in the flame combustion area, while decreased downstream reactor because of occurring oxidation. CO was formed by the reaction of CO₂ and H, and H₂ primarily derived from chemical decomposition of H₂S. COS was generated by the reaction of CO₂ with SH, H₂S and S as well as the reaction of CO with SO and SH at R = 0.0, while was mainly formed by the reaction of CO with SO and SH at R = 0.75. H₂S mainly occurred the oxidation in the inverse diffusion flame. The temperature at R = 0.0 was still higher than that at R = 0.75, and it was higher than that in the normal diffusion flame in the combustion area. Increasing Φ promoted the formation of CO, H₂ and COS, and each gas under Φ of 1.5 was higher significantly. The Φ had no significant effect on the distribution of SO₂ compared to the normal diffusion flame, but changed the distribution of CO, H₂ and COS. It can be inferred that the content of CO, H₂ and COS will be more higher under Claus condition in the inverse diffusion flame.     

Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char

For oxy-combustion with flue gas recirculation, elevated levels of CO₂ and steam affect the heat capacity of the gas, radiant transport, and other gas transport properties. A topic of widespread speculation has concerned the effect of gasification reactions of coal char on the char burning rate. To asses the impact of these reactions on the oxy-fuel combustion of pulverized coal char, we computed the char consumption characteristics for a range of CO₂ and H₂O reaction rate coefficients for a 100 μm coal char particle reacting in environments of varying O₂, H₂O, and CO₂ concentrations using the kinetics code SKIPPY (Surface Kinetics in Porous Particles). Results indicate that gasification reactions reduce the char particle temperature significantly (because of the reaction endothermicity) and thereby reduce the rate of char oxidation and the radiant emission from burning char particles. However, the overall effect of the combined steam and CO₂ gasification reactions is to increase the carbon consumption rate by approximately 10% in typical oxy-fuel combustion environments. The gasification reactions have a greater influence on char combustion in oxygen-enriched environments, due to the higher char combustion temperature under these conditions. In addition, the gasification reactions have increasing influence as the gas temperature increases (for a given O₂ concentration) and as the particle size increases. Gasification reactions account for roughly 20% of the carbon consumption in low oxygen conditions, and for about 30% under oxygen-enriched conditions. An increase in the carbon consumption rate and a decrease in particle temperature are also evident under conventional air-blown combustion conditions when the gasification reactions are included in the model.     

Chemical mechanism for high temperature combustion of engine relevant fuels with emphasis on soot precursors

This article presents a chemical mechanism for the high temperature combustion of a wide range of hydrocarbon fuels ranging from methane to iso-octane. The emphasis is placed on developing an accurate model for the formation of soot precursors for realistic fuel surrogates for premixed and diffusion flames. Species like acetylene (C₂H₂), propyne (C₃H₄), propene (C₃H₆), and butadiene (C₄H₆) play a major role in the formation of soot as their decomposition leads to the production of radicals involved in the formation of Polycyclic Aromatic Hydrocarbons (PAH) and the further growth of soot particles. A chemical kinetic mechanism is developed to represent the combustion of these molecules and is validated against a series of experimental data sets including laminar burning velocities and ignition delay times. To correctly predict the formation of soot precursors from the combustion of engine relevant fuels, additional species should be considered. One normal alkane (n-heptane), one ramified alkane (iso-octane), and two aromatics (benzene and toluene) were chosen as chemical species representative of the components typically found in these fuels. A sub-mechanism for the combustion of these four species has been added, and the full mechanism has been further validated. Finally, the mechanism is supplemented with a sub-mechanism for the formation of larger PAH molecules up to cyclo[cd]pyrene. Laminar premixed and counterflow diffusion flames are simulated to assess the ability of the mechanism to predict the formation of soot precursors in flames. The final mechanism contains 149 species and 1651 reactions (forward and backward reactions counted separately). The mechanism is available with thermodynamic and transport properties as supplemental material.