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The ignition of a stream of coal particles under oxy-fuel atmosphere is studied numerically in a laminar flow. Devolatilization of coal particles is modeled with the chemical percolation devolatilization (CPD) method coupled with a detailed chemistry model for homogeneous reactions. The particle and gas phase interactions are modeled with a fully coupled Euler-Lagrange code. A parametric study is performed to investigate the influence of particle number density, gas temperature, and velocity on ignition. We found that increasing particle number density delays the onset of ignition. Delayed ignition in denser streams is due to the lower gas temperature, which is caused by the higher energy required for particle heating. In addition, increasing particle number density leads to a more continuous and narrower flame front. Particle heating and ignition induce velocity variations in the gas phase through the coupling with particles. In the denser stream, velocity variations become significant and compromise the validity of a constant velocity assumption that is usually made in computing ignition delay time from the observed ignition location. It is also found that high particle slip velocities lead to a locally low volatile concentration and low temperature, and consequently increase ignition delay time.

The ignition and combustion of coal particles are investigated numerically under conventional and oxy-fuel atmospheres. Devolatilization is computed using the chemical percolation devolatilization (CPD) model. The CPD model is coupled with a Lagrangian particle tracking method in the framework of a multiphysics, multiscale Navier–Stokes solver. Combustion in the gas phase is described using finite rate chemistry. The numerical results for ignition are compared with available experimental data and a remarkably good agreement is observed. The effect on flame ignition of the different phases characterizing the release of volatile gases is assessed. These different phases manifest themselves in two distinct peaks in the devolatilization rate and it is observed that ignition can occur during the first volatile release or on the onset of the second, depending on the particle size and gas temperature. It is found that an increase of ignition delay time in oxy-atmosphere compared to the air case is related to the depletion of radicals that react with the abundant carbon dioxide of the oxy-atmosphere, while the increased heat capacity of the mixture does not play a role.

Flow, turbulence and combustion (2020). doi:10.1007/s10494-020-00222-1, Published by Springer Science + Business Media B.V., Dordrecht [u.a.]

The aim of this work is to study spatially and chemically resolved particle combustion cases to understand chemical and laminar transport processes and to support model development. In the present study, the combustion process of a single char particle located in air or oxy-fuel atmosphere composed of oxygen, carbon dioxide, and steam is investigated. Char burnout is represented in highly resolved numerical simulations including a detailed description of the surface and the gas phase chemistry. At the solid-gas interface, heat and mass fluxes due to the surface reactions involving carbon oxidation and gasification are considered. The model is validated based on experimental results for char burnout phase in a flat flame burner. We perform a comprehensive set of fully resolved reactive 2-D simulations by varying particle size, relative velocity, diluent, and oxygen composition in the surrounding gas. The simulation results are discussed regarding the CO 2 and N 2 content of the atmosphere highlighting the effects of oxy-fuel combustion. Furthermore, the impact of the particle flow motion on the flame that forms around the char particle is investigated by varying relative Reynolds number with particle size and relative slip velocity.

Transient combustion behavior of char particle arrays in oxy-fuel atmosphere is analyzed with a resolved-particle two-dimensional simulation framework, considering both heterogenous and homogeneous chemical reactions. A range of various parameters is considered, such as particle distance, oxygen levels in the incoming flow, particle Reynolds number, and the particle arrangement. The analysis shows a large dependence on the distance between the particles and the location of each particle within the array. While particles facing the incoming flow show the highest burning efficiency, this efficiency drops as consecutive rows are considered. This effect is stronger for arrays, where the distance between the consecutive rows is less than three particle diameters. Within the range of particle Reynolds numbers considered in this study ( 1 ⩽ Re ⩽ 8 ), higher Reynolds numbers result in more efficient burning. The arrangement of the particles inside the array has a mild influence on the combustion behavior. As arrays with lower particle number densities are considered, the influence of the arrangement of the particle array diminishes. Finally, considering the temperature and the oxygen mass fraction around all the simulated particles in the parameter space provides insight towards the global relation between these variables, highlighting possible modeling directions.