Numerical study of the three-folded effect of steam dilution on hydrogen ignition in a RCM with detailed kinetics

Moderate or Intense Low-oxygen Dilution (MILD) regime is a promising candidate for combustion future, since it meets the nowadays requested criteria for fuel flexibility, efficiency and inhibition of pollutant formation, such as NOx and soot. This regime is obtained through preheating of the oxidizer flow, and a specific internal aerodynamic of the burner in conjunction with high velocity inlet, which is responsible for flue gases recirculation and turbulent mixing. This results in a localized reduction of O2, and a strongly diluted fuel mixture, leading to delayed ignition and to a homogeneous as well as distributed reaction zone. In the last years, hydrogen has attracted great attention as Energy Carrier for its storage opportunity and the absence of the pollutant (CO2, SOx and UHC) among its products. Its importance will increase within the next years. Its usage as an enrichment for methane, has been investigated for MILD condition in a Jet in hot coflow burner. In particular, A. Parente et al. [4] concluded that the hydrogen addition leads to complex oxidation behaviors, which requests detailed kinetics for a full phenomenon description. In fact, MILD combustion is characterized by a low Damköhler numbers regime, and the presence of a relevant amount of diluent make the mixing and the chemistry time scales overlap. For this reason, the kinetic mechanism, which were validated using conventional combustion data, usually accomplish a non-accurate estimation for these, conditions. According to Koroglu et al., diluents like CO2, and H2O exert a three-folded effect on the system, namely thermal (like N2 does), indirect and direct participation to single kinetic steps, as a collider and a reactant, respectively. However, we are far away from having a clear insight into the role of such species in MILD combustion, especially for Ignition delay time, one of the most important kinetic parameters in MILD combustion, along with the maximum temperature. Different experimental studies faced the H2/Oxidizer/Steam mixtures combustion in canonical reactors, namely: Wang et al. and Vasu et al. using a shock tube reactor, while Das et al., and Donohoe et al.. Recently, Shareh et al. studied the three-folded effect of CO2 dilution on methane flame speed for oxyfule combustion performing a fake species analysis (FSA). The aim of this work is to understand what is the steam dilution driving effect, for hydrogen ignition using the latter FSA approach for high Temperatures

Kinetic modeling of soot formation in premixed burner-stabilized stagnation ethylene flames at heavily sooting condition

A detailed kinetic mechanism of soot formation and oxidation is revised and extended to include temperature-dependent collision efficiencies. The collision efficiency for various particle size is studied and compared with experimental data and molecular dynamics simulations for the PAH dimerization where the experimental data are not available. This revised kinetic model is validated in comparison with the premixed burner-stabilized stagnation ethylene flames at heavily sooting conditions. The results showed that quasi-one-dimensional numerical simulations can capture the flame structure and predict soot formation quite satisfactorily. The predicted particle size distribution function (PSDF) is in reasonable agreement with experimental results, but the model only partially reproduces the distinct separation between nucleation and coagulation modes observed experimentally. This leads to some discrepancies in the prediction of soot number density, while the predicted soot volume fraction, which is dominated by the large particles of the PSDF, is in generally good agreement with the experimental data. There is an overestimation of the initial soot volume fraction in the flame region close to the burner, which is a consequence of the over-prediction of the amount of young particles. Therefore, the prediction of PAHs formation and their condensation on soot, which controls the nucleation rate, will require further attention. The comparison between the temperature-dependent model and the model neglecting the temperature dependency showed that the temperature-dependent model could improve the prediction of soot number density, which is controlled by small particles

Evaporation of multicomponent fuel droplets in buoyancy driven convection

In this work, the evaporation process of multicomponent fuel droplets is analyzed, both from an experimental and numerical point of view. The droplets are hanged on a thin thermocouple against gravity and evaporated in natural convection regime, following the process by means of high speed shadowgraphs. The experimental analyses were performed hierarchically, starting from pure components (n-dodecane and n-hexadecane), then moving to their mixtures. The numerical modeling is performed with the DropletSMOKE++ code, a comprehensive CFD framework for the simulation of 3D evaporating droplets under gravity conditions. The numerical results present a good agreement with the experimental data, especially if compared with the same cased modeled in microgravity conditions. The difference evaporation rate is analyzed as well as the different surface temperature, highlighting the important role of internal and external convection on the evaporation process

Analysis of acetic acid gas phase reactivity: Rate constant estimation and kinetic simulations

The gas phase reactivity of acetic acid was investigated combining first principle calculations with kinetic simulations. Rate constants for the unimolecular decomposition of acetic acid were determined integrating the 1D master equation over a Potential Energy Surface (PES) investigated at the M06-2X/aug-cc-pVTZ level. Energies were computed at the CCSD(T)/aug-cc-pVTZ level using a basis set size correction factor determined at the DF-MP2/aug-cc-pVQZ level. Three decomposition channels were considered: CO2+ CH4, CH2CO + H2O, and CH3+ COOH. Rate constants were computed in the 700-2100 K and 0.1-100 atm temperature and pressure ranges. The simulations show that the reaction is in fall off above 1200 K at pressures smaller than 10 atm. Successively, the PESs for acetic acid H-abstraction by H, OH, OOH, O2, and CH3were investigated at the same level of theory. Rate constants were computed accounting explicitly for the formation of entrance and exit van der Waals wells and their collisional stabilization. Energy barriers were determined at the CASPT2 level for H-abstraction by OH of the acidic H, since it has a strong multireference character. The calculated rate constant is in good agreement with experiments and supports the experimental finding that at low temperatures it is pressure dependent. The calculated rate constants were used to update the POLIMI kinetic model and to simulate the pyrolysis and combustion of acetic acid. It was found that acetic acid decomposition and the formation of its direct decomposition products can be reasonably predicted. The formation of secondary products, such as H2and C2hydrocarbons, is underpredicted. This suggests that reaction routes not incorporated in the model may be active. Some hypotheses are formulated on which these may be

A comprehensive CFD model for the biomass pyrolysis

The present work addresses the study of the pyrolysis of biomass particle, with the aim to improve the comprehensive mathematical model of the thermochemical processes involving solids decomposition. A new CFD model for the biomass pyrolysis was developed at the particle scale in order to properly describe the relative role of reaction kinetics and transport phenomena. The model is able to solve the Navier-Stokes equations for both the gas and solid porous phase. The code employs the open-source OpenFOAM® framework to effectively manage the computational meshes and the discretization of fundamental governing equations. The mathematical algorithm is based on the PIMPLE method for transient solver and exploit the operator-splitting technique that allows the separation of the transport and the reactive term in order to handle complex computational geometries minimizing the computational effort. The model was tested with experimental data for both reactive and non-reactive conditions. The code is able to provide correct information about temperature distribution within the particle, gas, tar and char formation rates

Detailed kinetics of pyrolysis and combustion of catechol and guaiacol, as reference components of bio-Oil from biomass

Fast biomass pyrolysis is an effective process to produce bio-oils thus allowing to partially replace nonrenewable fossil fuels. Bio-oils are complex mixtures with a great amount of large oxygenated organic species, such as substituted phenolic components. Although experimental and kinetic modeling studies of phenol and anisole pyrolysis and combustion are available in the literature, only a minor attention has been devoted to kinetic mechanisms of substituted phenolic species, such as catechol and guaiacol. Multiple substitutions on aromatic ring can originate proximity effects and thus significantly modify bond energies, consequently affecting reaction pathways. Careful evaluations of bond dissociation energies and reference kinetic parameters, based on theoretical computations, are first performed. Guaiacol and catechol pyrolysis and combustion reactions are then compared with the corresponding phenol and anisole mechanisms. This kinetic study allows to identify some preliminary rate rules useful to validate a detailed kinetic mechanism of bio-oil pyrolysis and combustion

Modeling Non-Premixed Combustion Using Tabulated Kinetics and Different Fame Structure Assumptions

Nowadays, detailed kinetics is necessary for a proper estimation of both flame structure and pollutant formation in compression ignition engines. However, large mechanisms and the need to include turbulence/chemistry interaction introduce significant computational overheads. For this reason, tabulated kinetics is employed as a possible solution to reduce the CPU time even if table discretization is generally limited by memory occupation. In this work the authors applied tabulated homogeneous reactors (HR) in combination with different turbulent-chemistry interaction approaches to model non-premixed turbulent combustion. The proposed methodologies represent good compromises between accuracy, required memory and computational time. The experimental validation was carried out by considering both constant-volume vessel and Diesel engine experiments. First, the ECN Spray A configuration was simulated at different operating conditions and results from different flame structures are compared with experimental data of ignition delay, flame lift-off, heat release rates, radicals and soot distributions. Afterwards, engine simulations were carried out and computed data are validated by cylinder pressure and heat release rate profiles

A first evaluation of butanoic and pentanoic acid oxidation kinetics

International audienceDespite the recent interest in large carboxylic acid oxidation due to their presence in pyrolysis bio-oils, their kinetics of pyrolysis and oxidation has not been experimentally addressed. For the first time, this paper reports a new set of experimental data for the oxidation in a jet-stirred reactor of two high molecular weight carboxylic acids: butanoic (butyric) and pentanoic (valeric) acids. This work was performed at 106.7 kPa (800 Torr) over a range of temperatures from 800 to 1100 K. The experiments were carried out under highly diluted conditions (inlet fuel mole fraction of 0.005) for three equivalence ratios: 0.5, 1 and 2. During this study a wide range of products has been identified and quantified from CO and CO2 to C5 species: 36 for pentanoic acid and 18 for butanoic acid. An interpretative kinetic model has been developed based on a recent theoretical study on the pyrolysis and oxidation of acetic acid (Cavalotti et al. PROCI, 37 (2019) 539-546) and on alkane rate rules (Ranzi et al. Combust. Flame, 162 (2015) 1679-1691). This new kinetic subset has been implemented in the CRECK kinetic framework covering the pyrolysis and oxidation of molecules from syngas up to heavy fuels, including PAHs formation. The mole fractions of fuel and product species were compared with results from model simulations over the experimental temperature range, providing reasonable agreement. A flow rate analysis allowed a better understanding of the most important degradation pathways of these acids, including a small contribution of low-temperature oxidation channels

Mathematical Modeling of Fast Biomass Pyrolysis and Bio-Oil Formation. Note II: Secondary Gas-Phase Reactions and Bio-Oil Formation

This paper summarizes the research activities done at Politecnico di Milano in the field of the detailed kinetic modeling of fast pyrolysis of biomass to produce bio-oil. Note I of this work already discussed biomass characterization and the multistep pyrolysis mechanisms of reference species. The model is able to provide a detailed composition of pyrolysis products and char residue. Different critical steps are involved in this multicomponent, multiphase and multiscale problem. The first complexity relies in biomass characterization. Then, fast pyrolysis process involves detailed kinetic mechanisms, first in the solid phase for the biomass pyrolysis, then in the gas-phase for the secondary reactions of released products. The complexity of these kinetic mechanisms requires strong simplifications, thus chemical lumping procedures are extensively applied. Successive or secondary gas phase reactions of gas and tar components released during the pyrolysis process complement the kinetic model, together with the heterogeneous reactions of residual char. The modeling of fast pyrolysis process requires a comprehensive description of the coupled transport and kinetic processes, both at the particle and the reactor scale. A few examples and comparisons with experimental data validate the reliability of the overall model. Finally, the composition and physical properties of the pyrolysis bio-oil are also discussed, with emphasis on combustion and pollutant emissions