The application of homogeneous reactor models to examine the conversion of Polycyclic Aromatic Hydrocarbons in biomass derived fuel gases

The removal of tar from biomass derived fuel gases is considered to be a bottleneck in the widespread application of biomass gasification. Tar is made up of Polycyclic Aromatic Hydrocarbons (PAHs) consisting of fused aromatic rings. Past research has proven the possibility to remove the PAH species naphthalene from producer gas at high temperature by means of introducing a limited amount of oxidizer. This process, which also can be regarded as partialcombustion, is not yet fully understood. By conducting a fundamental study of the mechanisms involved it will be possible to optimize the process conditions and the reactor geometry. In the work presented here it is examined if partial combustion can be described by homogeneous reactors. The PAH species representing the tar content of the producer gas is naphthalene (in correspondence with the executed experiments). Chemical equilibrium calculations indicate that, if time scales approach infinity, it is possible to convert naphthalene to lighter hydrocarbons at smallvalues of the equivalence ratio, lambda. For lambda <0.2, soot is produced. The influence of chemical kinetics is examined with Perfectly Stirred Reactor (PSR) calculations. The results reveal that the time scales involved in the conversion of naphthalene at small values of the equivalence ratio (0 <lambda <0.2) are considerate. This leads to the conclusion that the process of tar cracking by partial combustion cannot be described by homogeneous reactors alone. The experimental results are not fully explained by the presented results. This indicates that heterogeneous effects should be taken into account. This requires full 2D simulations in which molecular transport is included

A priori testing of flamelet generated manifolds for turbulent partially premixed methane/air flames

To reduce high computational cost associated with simulations of reacting flows chemistry tabulation methods like the Flamelet Generated Manifold (FGM) method are commonly used. However, H2, CO and OH predictions in RANS and LES simulations using the FGM (or a similar) method usually show a substantial deviation from measurements. The goal of this study is to assess the accuracy of low-dimensional FGM databases for the prediction of these species in turbulent, partially-premixed reacting flows. It will be examined to what extent turbulent, partially-premixed jet flames can be described by FGM databases based on premixed or counterflow diffusion flamelets and to what extent the chosen molecular transport model for the flamelet influences the accuracy of species mass fraction predictions in CFD-simulations. For LES and RANS applications a model that accounts for subgrid fluctuations has to be added introducing additional errors in numerical results. A priori analysis of FGM databases enables the exclusion of numerical errors (scheme accuracy, convergence) that occur in CFD simulations as well as the exclusion of errors originating from subgrid modeling assumptions in LES and RANS. Four different FGM databases are compared for H2O, H2, CO, CO2 and OH predictions in Sandia Flames C to F. Species mass fractions will be compared to measurements directly and conditioned on mixture fraction. Special attention is paid to the representation of experimentally observed differential diffusion effects by FGM databases

Computational study on the stability of lean CH4-air and H2-CH4-air laminar premixed flames

Recently, Shoshin et al. [1] reported measurements on blow-off limits for methane-air and hydrogen-methaneair flames stabilized on metallic rods finding a so-called "anomalous" blow-off behaviour of hydrogen-methaneair flames with certain hydrogen content. It is well known that lean methane-air and hydrogen-methane-air flames have characteristics that differ substantially owing to preferential diffusion effects. In this study, two-dimensional simulations of steady, rod-stabilized, inverted, lean, methaneair and hydrogen-methane-air premixed laminar flames are performed to further investigate the stability and blowoff characteristics of such flames. The simulations are carried out with complex chemistry and non-unity Lewis transport. For the hydrogen-methane-air flames, mixtures with a 40% (molar based) hydrogen content in the fuel are considered. Six cases for different values of equivalence ratio, , and mean inlet velocity, V , of the premixed mixture are studied. The conditions for all the cases are summarized in Table 1. In what follows, the governing equations are provided, the burner and computational setup are described and the numerical results are discussed

Computational study on the stability of lean CH4-air and H2-CH4-air laminar premixed flames

Recently, Shoshin et al. [1] reported measurements on blow-off limits for methane-air and hydrogen-methaneair flames stabilized on metallic rods finding a so-called "anomalous" blow-off behaviour of hydrogen-methaneair flames with certain hydrogen content. It is well known that lean methane-air and hydrogen-methane-air flames have characteristics that differ substantially owing to preferential diffusion effects. In this study, two-dimensional simulations of steady, rod-stabilized, inverted, lean, methaneair and hydrogen-methane-air premixed laminar flames are performed to further investigate the stability and blowoff characteristics of such flames. The simulations are carried out with complex chemistry and non-unity Lewis transport. For the hydrogen-methane-air flames, mixtures with a 40% (molar based) hydrogen content in the fuel are considered. Six cases for different values of equivalence ratio, , and mean inlet velocity, V , of the premixed mixture are studied. The conditions for all the cases are summarized in Table 1. In what follows, the governing equations are provided, the burner and computational setup are described and the numerical results are discussed

Numerical determination of iron dust laminar flame speeds with the counter-flow twin-flame technique

Iron dust counter-flow flames have been studied with the low-Mach-number combustion approximation. The model considers full coupling between the two phases, including particle/droplet drag. The dispersed phase flow strain relations are derived in the Stokes regime (Reynolds number much smaller than unity). The importance of solving a particle flow strain model is demonstrated by comparing three different cases: a free unstrained flame, a counter-flow flame where slip effects are neglected and a counter-flow flame where slip effects are included. All three cases show preferential diffusion effects, due to the lack of diffusion of iron in the fuel mixture, e.g. DFe,m= 0. The preferential diffusion effect causes a peak in the fuel equivalence ratio in the preheat zone. On the burned side, the combined effect of strain and preferential diffusion shows a decrease in fuel equivalence ratio. Inertia effects, which are only at play in the counter-flow case with slip, counteract this effect and result in an increase of the fuel equivalence ratio on the burned side. A laminar flame speed analysis is performed and a recommendation is given on how to experimentally determine the flame speed in a counter-flow set-up. Novelty &amp; Significance We introduce a novel model to include particle flow strain in a dispersed counter-flow set-up. For the first time, the impact of particle flow strain on the flame structure of iron dust is studied with a one-dimensional (1D) model. Two major effects that modify the flame structure and burning velocity are identified: preferential diffusion and inertia of the particles. Preferential diffusion effects are found to be always present in (iron) dust flames. Inertia effects play a role in the counter-flow case with slip. Due to the inertia of the particles, the particle flow strain is lower than the gas flow strain. As a consequence, higher particle concentrations are reached compared to the other cases. Furthermore, it is shown that each particle size experiences a different particle flow strain rate, which is important when doing experiments as it implies that the PSD at the flame front will be different than at the inlet.</p

Investigation of mass and energy coupling between soot particles and gas species in modelling counterflow diffusion flames

A numerical model is developed aiming at investigating soot formation in ethylene counterflow diffusion flames at atmospheric pressure. In order to assess modeling limitations the mass and energy coupling between soot solid particles and gas-phase species are investigated in detail. A semi-empirical two equation model based on acetylene as the soot precursor is chosen for predicting soot mass fraction and number density. For the solid-phase the model describes particle nucleation, surface growth and oxidation. For the gas-phase a detailed kinetic mechanism is considered. Additionally, the effect of considering gas and soot radiation heat losses is evaluated in the optically thin limit approximation. The results show that for soot volume fractions higher than a certain threshold value the formation of the solid particles begins to significantly influence the gas-phase composition and temperature. The results also show that the inclusion of radiant heat losses decreases this influence. Keywords: Combustion, Soot model, Coupling effect, Counterflow flame