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Efficiency is one of the major objectives when designing energy systems. Irreversibilities of combustion processes can be characterized by analyzing entropy generation which is proportional to exergy destruction. In this paper, entropy generation is investigated in turbulent non-premixed counter-flow syngas flames at a high strain rate over a wide range of hydrogen percentage (H2/CO molar fraction from 0.4 to 2.0). The aim is to define the most efficient syngas composition to reduce irreversibilities. Irreversibilities involved in NO formation process are also examined. RANS (Reynolds Averaged Navier Stokes) technique including k-e turbulence model is used for the flow field estimation. Flame structure is calculated using SLFM (Steady Laminar Flamelet Model) and EPFM (Eulerian Particle Flamelet Model) is applied for NOx predictions. Total entropy generation rate accounts for chemical, heat conduction, mixing and viscous effects. Computational results show that the total volumetric entropy generation decreases with H2 enrichment as well as its different contributing effects. Chemical effect is dominant, followed by heat conduction and mixing effects. Viscous effect is negligible. The maximum of both thermal and prompt NO formation routes are influenced by the three main entropy generation modes, with the predominance of the chemical effect. At high strain rates, H2-rich syngas flames are efficient in regards to irreversibilities and NO emissions reduction.

This paper reports a numerical simulation of turbulent non-premixed counter-flow syngas flames structure and NO emissions at a high strain rate with a special focus on mixing. The analysis is conducted over a wide range of hydrogen percentage (H2/CO ratio between 0.4 and 2.0) and operating pressure (from 1 to 10 atm). Numerical model is based on RANS (Reynolds Averaged Navier–Stokes) technique including k-e turbulence model. SLFM (Steady laminar flamelet model) is used for flame structure calculations and EPFM (Eulerian Particle Flamelet Model) is applied for NOx predictions. Mixing is described by mixture fraction and its variance. Radiation effects are also considered. Computational results show an improvement of mixing with hydrogen enrichment and ambient pressure rise. Maximum flame temperature decreases with H2 addition and increases with pressure. NO levels decrease towards hydrogen-rich syngas flames and increase with pressure. Zeldovich route is found to be the main NO formation path in the operating conditions considered.

The aim of the present study is to investigate the enhancement of syngas combustion using the promising oxygen-enrichment technology, with a particular attention on optimal operating conditions in regard to NOx emissions. For this purpose, a numerical study is conducted on a syngas counter-flow diffusion flame, using air enriched with oxygen as the oxidizer. The oxygen concentration ranges from 21% to 30% by volume. Two syngas compositions are considered with H2/CO rates equal to 0.25 and 4, respectively. Flame structure is characterized by solving flamelet equations with the consideration of radiation. The chemical reaction mechanism used is GRI 3.0. Computational results showed that oxygen addition increases the flame temperature and intensifies the radiative heat transfer. It also considerably extends flammability limits allowing stable flames at high values of the scalar dissipation rates and for lean syngas composition. NOx formation is substantially increased with oxygen increment, and Zeldovich mechanism is found to be the main route of NOx formation. H2-lean syngas flames produce less NOx at low scalar dissipation while H2-rich syngas flames NOx emissions are low at high scalar dissipation rates.

This paper reports a numerical investigation of syngas flame structure and NO reaction pathways over a wide range of operating conditions (H 2 /CO ratio between 0.4 and 2.4, scalar dissipation rate from equilibrium to extinction and ambient pressure from 1 to 10 atm) in mixture fraction space. An analysis of optimal operation conditions for syngas combustion in regard to NO index emissions is also provided. Flame structure is characterized by solving flamelet equations with the consideration of radiation. The chemical reaction mechanism adopted is GRI-Mech 3.0. The computational predictions showed that flame temperature exhibits a peak at an intermediate scalar dissipation rate for a given value of H 2 /CO ratio. From hydrogen-lean syngas to hydrogen-rich syngas fuels, maximum flame temperature increases for scalar dissipation rate values lower than the intermediate value whereas decreases at higher values. Zeldovich route is found to be the main NO formation route and its contribution to the NO production continually increases with the increase of hydrogen content and pressure. Hydrogen-rich syngas flames produce more NO at lower scalar dissipation rates while NO levels increase towards hydrogen-lean syngas flames at higher scalar dissipation rates.