Formation of nitric oxide in premixed hydrocarbon flames
The formation was followed by probing flames of ethylene, methane, or propane with N2-O2 mixtures. The known kinetic laws of the well-understood N2-O2-NO system (limited by the rate of the reaction O+N2→N+NO) described the growth of nitric oxide in the post-flame gas, after hydrocarbons were consumed, but could not describe a faster, transient formation of nitric oxide in the primary reaction zone. The transient formation did not occur in hydrogen or carbon monoxide flames, and therefore, may involve an attack of carbon or hydrocarbon radicals on nitrogen molecules (e.g., CH+N2→HCN+N). At atmospheric pressure, the transient formation gave its greatest yield in flames containing 1.3 to 1.4 times the stoichiometric amount of fuel. Its yield increased somewhat with pressure, particularly in fuel-lean flames, so that at 3 atmos pressure the nitric oxide formed quickly in the primary reaction zone was about the same in flames of 0.8 as of 1.4 mixture strength. The transient formation possessed a relatively weak temperature dependence. Extrapolating the measurements, one can suggest that the new mechanism may contribute significantly to the emission of nitrogen oxides from a gas turbine, but the major contribution occurs by the known mechanism of the N2-O2 system.
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A novel method for quantifying variations in NO formation pathways using a sub-mechanism accounting and reaction tracing algorithm
2024, Combustion and FlameThis work presents a new implementation of reaction path analysis that traces molar fluxes attributable to various sub-mechanisms from inception through to their final product. The post-processing procedure involves creating a network of these fluxes and quantifying the contributions made by the specified formation and destruction mechanisms through an iterative process starting from the individual initiation reactions. A key distinction of this approach is that the reaction fluxes are directly correlated to their initiation reaction, which is then followed throughout the entire reaction network. This allows for the distinction of contribution to the formation of a target species from an arbitrary number of sources. The effectiveness of this approach is demonstrated by examining the formation of NO in three different case studies: (1) the impact of equivalence ratio on methane/air premixed counterflow flames, (2) the effect of hydrogen addition on ultra-lean methane/air premixed counter flames, and (3) the influence of pressure elevation in partially premixed methane/air flames. The results are presented in emission indexes, NO production rates throughout the flame, and multidigraphs illustrating the allocated flows between the species. Compared to replicated analysis methods previously used for these case studies, this new approach provides a more comprehensive and detailed analysis for several reasons. Firstly, the method does not require duplicated simulations with modified kinetic mechanisms, but is applied purely post-processing. This avoids the previous problems of needing to re-simulate the flame multiple times and allows for the fully coupled chemistry set. Secondly, the method allows for an arbitrarily defined set of initiation pathways which propagate through all possible routes within the network. This allows for the identification of spatial/temporal variability in the sub-mechanisms and thereby yields more information than the integral values of previous methods.
Novelty and significance statement
Understanding reaction pathways that lead to the formation of pollutants such as NO is an essential aspect in the development of new sustainable and low-emission combustion concepts. However, evaluating the different underlying sub-mechanisms is difficult due to the high degree of interconnectedness and spatial/temporal overlap of the reaction pathways. This work, therefore, presents a new algorithm that can be used to investigate the contributions of an arbitrary number of sub-mechanisms to the formation or decomposition of a species. The results can be expressed not only as integral values, but also as spatially and/or temporally resolved production rates and reaction path figures and graphs, leading to a more comprehensive and detailed analysis. The approach is also applied as a purely post-processing technique, which does not require multiple simulations with different chemical kinetic mechanisms as has previously be applied.
Experimental investigation of hydrogen-producer gas mixtures in an optically accessible SI engine
2024, International Journal of Hydrogen EnergyProducer gas from biomass gasification offers a renewable alternative to fossil fuels. However, its low energy density results in low conversion efficiency in engines. Blending producer gas with higher-ranked fuels such as hydrogen has been proposed to overcome this issue. This study investigates the combustion of artificially made producer gas and hydrogen mixtures in an optical SI engine. The molar fraction of hydrogen in producer gas ranged from 14 to 62%, which simulated additions of hydrogen to a low calorific producer gas. The experiments are conducted at a constant speed and stoichiometric ratio. The spark timing is varied to achieve the highest power for each mixture. Results include data on emissions, thermodynamics, and flame morphology. The molar fraction of 33% hydrogen on producer gas improves the flame morphology of the mixture to resemble that of pure natural gas, while 24–36% was found to be the optimal range for engines initially designed to run on natural gas with lower and UHC emissions.
Development of a new reduced n-pentane mechanism with NO<inf>X</inf> for combustion simulation at atmospheric pressure
2024, Journal of the Energy InstituteAir-light hydrocarbon mixing gas is a new type of fuel with great potential, and its key component is n-pentane. In this study, a reduced n-pentane mechanism with NOX is developed for combustion simulation at atmospheric pressure, including 81 species and 348 elementary reactions. First, the dominant species and reactions for n-pentane combustion are recognized by the reaction pathway analysis based on the detailed mechanism. Then, a novel multistep reduction strategy with intersection thought is proposed to reduce the detailed mechanism of n-pentane. Ignition delay time and laminar flame speed are separately selected as the target parameters for validation. The two reduced mechanisms are developed by using the directed relation graphs (DRG) method, the directed relation graphs with error propagation (DRGEP) method, and the full species sensitivity analysis (FSSA) method. Species in the intersection mechanism of the above two mechanisms obtained with multistep reduction are treated as important species. Sensitivity analysis (SA) is further performed to remove the uncertain species, combined with the result of reaction pathway analysis, a reduced n-pentane mechanism including 77 species and 336 reactions is obtained. On this basis, a reduced NOX mechanism (4 species and 12 reactions) is incorporated. Then, the key kinetic parameters of the mechanism are optimized and adjusted according to the results of brute force sensitivity analysis. Finally, the proposed mechanism is validated by using the related data of ignition delay times, laminar flame speeds, premixed flame species profiles, and jet-stirred reactor (JSR) species profiles. The results indicate that the proposed mechanism shows good model performance. This study can provide significant assistance for subsequent research on air-light hydrocarbon mixing gas.
CFD study of flameless combustion in a real industrial reheating furnace considering different H<inf>2</inf>/NG blends as fuel
2024, International Journal of Hydrogen EnergyOn the path towards decarbonisation of the steel industry, the use of H/NG blends in furnaces, where high temperatures are needed, is one of the alternatives that needs to be carefully studied. The present paper shows the CFD study carried out for a full-scale reheating furnace burner case. The real operating conditions as well as experimental measurements provided by the furnace operator were used to validate the results and reduce simulation uncertainties. The burner under consideration (2.5 MW) works in flameless mode with natural gas and preheated air (813 K). Starting from this point, another three fuel blends with volumetric percentages of 23% (also known as G222), 50% and 75% of H in natural gas were considered. For this purpose, the open source CFD code OpenFOAM was used, where the novel NE-EDC turbulence-chemistry interaction model was implemented, which has already been successfully validated specifically for flameless combustion in a furnace. The implementation incorporated an enhanced approach for calculating the chemical time-scale, coupled with a specific post-processing solver to predict NO emissions. The study analyses the relative impact of the considered fuel blends on NO formation and flameless regime. The modelling results demonstrated the burner’s capability to operate efficiently with high concentrations of hydrogen, maintaining flameless regime in all cases. This condition ensured uniform temperature distributions and low levels of NO emissions, reaching a maximum value of 86 mg/m. These results indicated the proper functionality of the existing natural gas-based burner with H/NG blends, which was the primary requirement for the conversion process.
Effects of hydrogen blending and exhaust gas recirculation on NO<inf>x</inf> emissions in laminar and turbulent CH<inf>4</inf>/Air flames at 25 bar
2024, International Journal of Hydrogen EnergyIn this work, the effect of Exhaust Gas Recirculation (EGR) on emissions in a //air combustion at 25 bar, at an equivalence ratio = 0.7, is analyzed in laminar and turbulent partially premixed flames. Numerical simulations of twenty premixed and partially premixed counterflow flames with ranging from 0 to 100%, with and without EGR, are carried out. Analysis of NO formation mechanisms shows that the thermal path is the main responsible for the production in each flame, with a contribution of nearly 80%. In laminar flames an increase of leads to an increase in emissions. The addition of the exhaust gas decreases the flame temperature and therefore : each laminar flame shows a reduction of about 60% with the presence of exhaust gas. Four turbulent slot jet flames with a premixed central core and Air/EGR as coflow are studied in a two-dimensional framework using the Reynolds Averaged Navier–Stokes (RANS) approach and the Large Eddy Simulation (LES) methodology, to take into account some flame dynamics, despite the 2D context. Accurate molecular transport properties are considered and, a reduced specifically designed chemical mechanism for methane/hydrogen-air combustion at , consisting of 48 transported species and 465 elementary reactions is adopted. The four turbulent flames were simulated with 0 and 50% hydrogen concentration, with and without EGR. The presence of hydrogen reduces emissions, but at the same time increases NO concentration, the thermal path being the main NO formation mechanism. The use of exhaust gas recirculation leads to NO reduction as in laminar flames. The results obtained in this work show that at high pressure, the hydrogen enrichment of natural gas in the EGR mode leads to lower as well as emissions.
Numerical modelling of a heavy-duty diesel-hydrogen dual-fuel engine with late high pressure hydrogen direct injection and diesel pilot
2024, International Journal of Hydrogen EnergyDirect gaseous fuel injection in internal combustion engines is a potential strategy for improving in-cylinder combustion processes and performance while reducing emissions and increasing hydrogen energy share (HES). Through use of numerical modelling, the current study explores combustion in a compression ignition engine utilising a late compression/early power stroke direct gaseous hydrogen injection ignited by a diesel pilot at up to 99% HES. The combustion process of hydrogen in this type of engine is mapped out and compared to that of the same engine using methane direct injection. Four distinct phases of combustion are found which differ from that of pure diesel operation. Interaction of the injected gas jet with the chamber walls is found to have a considerable impact on performance and emission characteristics and is a factor which needs to be explored in greater detail in future studies. Considerable performance increase and carbon-based emission reductions are identified at up to 99% HES at high load but low load performance greatly deteriorated when 95% HES was exceeded due to a much reduced diesel pilot struggling to ignite the main hydrogen injection.