Numerical Simulations and Experiments of Ignition of Solid Particles in a Laminar Burner: Effects of Slip Velocity and Particle Swelling

Antonio Attili,Jan Köser,Christoph Schumann,Andreas Dreizler,Christopher Geschwindner,Sima Farazi,Tao Li,Benjamin Böhm,Heinz Pitsch,Pooria Farmand

doi:10.1007/s10494-020-00222-1

Antonio Attili, Jan Köser + Show 8 more

Open Access

https://doi.org/10.1007/s10494-020-00222-1

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Abstract

Ignition and combustion of pulverized solid fuel is investigated in a laminar burner. The two-dimensional OH radical field is measured in the experiments, providing information on the first onset of ignition and a detailed characterization of the flame structure for the single particle. In addition, particle velocity and diameter are tracked in time in the experiments. Simulations are carried out with a Lagrangian point-particle approach fully coupled with an Eulerian solver for the gas-phase, which includes detailed chemistry and transport. The numerical simulation results are compared with the experimental measurements in order to investigate the ignition characteristics. The effect of the slip velocity, i.e. the initial velocity difference between the gas-phase and the particle, is investigated numerically. For increasing slip velocity, the ignition delay time decreases. For large slip velocities, the decrease in ignition delay time is found to saturate to a value which is about 40% smaller than the ignition delay time at zero slip velocity. Performing a simulation neglecting the dependency of the Nusselt number on the slip velocity, it is found that this dependency does not play a role. On the contrary, it is found that the decrease of ignition delay time induced by the slip velocity is due to modifications of the temperature field around the particle. In particular, the low-temperature fluid related to the energy sink due to particle heating is transported away from the particle position when the slip velocity is non-zero; therefore, the particle is exposed to larger temperatures. Finally, the effect of particle swell is investigated using a model for the particle swelling based on the CPD framework. With this model, we observed negligible differences in ignition delay time compared to the case in which swelling is not included. This is related to the negligible swelling predicted by this model before ignition. However, this is inconsistent with the experimental measurements of particle diameter, showing a significant increase of diameter even before ignition. In further simulations, the measured swelling was directly prescribed, using an analytical fit at the given conditions. With this approach, it is found that the inclusion of swelling reduces the ignition delay time by about 20% for small particles while it is negligible for large particles.

Highlights

Oxy-fuel combustion combined with solid fuels is a promising approach for carbon capture and storage (CCS) to sequestrate and reduce carbon dioxide (CO2 ) emissions
In order to assess the effect of particle swelling on the ignition delay time, a series of simulations is performed with the swelling model included in the Chemical Percolation Devolatilization (CPD) and described in
Experiments and numerical simulations, based on a Lagrangian point particle framework, were performed for coal particle ignition in a laminar reactor to investigate the effect of initial particle diameter, initial slip velocity, and particle swelling on the ignition delay time

Summary

Introduction

Oxy-fuel combustion combined with solid fuels is a promising approach for carbon capture and storage (CCS) to sequestrate and reduce carbon dioxide (CO2 ) emissions. Numerical simulations for single (Farazi et al 2019a) and multiple particles (Farazi et al 2019b) have been performed employing the chemical percolation devolatilization (CPD) model (Grant et al 1989) in a fully coupled Eulerian–Lagrangian pointparticle numerical framework to investigate a number of aspects including the sensitivity of ignition delay time to temperature, gas composition, and particle number density. The ignition of solid particles is analyzed comparing detailed experimental measurements Köser et al (2019) of the spatial distribution of OH around the particles with highly resolved numerical simulation employing the Chemical Percolation Devolatilization (CPD), finite rate chemistry, and a Lagrangian point-particle approach for the dispersed phase. It is worth noting that using a point-particle approximation, while not providing the full details of a particle resolved method, allows to analyze and validate the different phenomena and models in the same framework that is typically employed in more complicate settings, such as turbulent and multiple particle flows, which cannot be simulated by the particle resolved method

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