Single Char Particle Combustion Research Articles

Hydrodynamic resolved simulation of a char particle combustion by immersed boundary-lattice Boltzmann method

A Lattice-Boltzmann model coupled with boundary method is proposed for the hydrodynamic resolved simulation of a single char particle combustion. A pure lattice Boltzmann scheme with multi distribution functions is presented for this low Mach reactive flows. Real varying thermodynamic and transport properties are considered, and the fluid density can bear significant change depending on the varying temperature and species concentrations. The recently proposed boundary-thickening based direct forcing-immersed boundary method is extended to implement the boundary conditions of velocity, temperature, and species concentrations at the char particle surface. Two heterogeneous reactions at particle surface and one homogeneous reaction in fluid are adopted to describe the combustion. The Stefan flow near the particle surface caused by the heterogeneous reactions is also considered. A satisfactory agreement can be found between the present simulation results and the previous experimental and numerical results. Three flame modes of a char particle combustion and the transition mechanisms between them are investigated. Furthermore, the effect of hydrodynamic interaction and the oxygen concentration in O2/CO2 atmosphere on particle combustion behavior are explored in detail. The present work establishes the foundation for the efficient simulations of particle combustion with size change and motion.

Experimental study of a single char particle combustion characteristics in a fluidized bed under O2/H2O condition

Oxy-steam combustion is a potential new route for oxy-fuel combustion with carbon capture from coal-fired power plants. In the present work, the combustion behavior of single char particles were investigated in a transparent fluidized bed combustor under different operating conditions (i.e., gas atmosphere, oxygen concentration, coal rank, location, fluidization number, particle size, and bed temperature). Both pre-calibrated two-color pryrometry and a flexible thermocouple were used to measure the char particle temperature in the combustion tests. Results indicated that the pore structure of the char generated in H2O atmosphere was better than that generated in CO2 and N2 atmospheres. As expected, with increase of oxygen concentration, the burnout time (tb) decreased, and the particle temperature (Tp) increased. The sequence of burnout times for different rank coal char particles was: anthracite > bituminous coal > lignite. Interestingly, comparing O2/CO2 and O2/N2 combustion, a shorter tb and a lower Tp of char could be achieved simultaneously in O2/H2O combustion, regardless of location and oxygen concentration. Furthermore, the increase of fluidization number strengthened the mass and heat transfer between the char and the environment, thereby reducing the tb and Tp of char. With increasing of particle size, the Tp slightly decreased, the tb increased markedly, and the gasification reactions became more and more significant. As the bed temperature increased, the gasification rate increased exponentially, and the mass transfer coefficient increased gradually.

Coupling of CFD-DEM and reaction model for 3D fluidized beds

In this paper, a three-dimensional (3D) numerical model coupling of CFD-DEM with chemical reaction has been developed to study the combustion process in lab-scale bubbling fluidized beds. The gas phase is modeled with k-ε turbulent model and the solid phase is modeled with discrete element method (DEM). The dense gas-solid flow, heat transfer, and comprehensive chemical reaction are simultaneously considered. The reactive submodel includes the fuel pyrolysis, combustion of char and volatiles (CO, H2, CH4, tar), formation and reduction of gaseous pollutants (NO, N2O, and SO2).The established model is first validated by the simulation of the combustion of single char particle in a quasi-3D fluidized bed. The flow patterns, distributions of particle temperature and combustion gas compositions are comparatively analyzed. The heat released by char combustion raises the temperature of surrounding particles, and meanwhile, CO and CO2 are generated at the same position as char particle. The variations of temperature, char particle diameter, and CO2 concentration with time are compared with experimental data, showing good agreements. Based on model validation, the simulations are performed to study the coal combustion in a 3D cylindrical fluidized bed. The computational domain contains about 110,000 spherical particles. The general flow pattern, distributions of gas and solid temperature, and concentrations of gas compositions inside the reactor are obtained. The effects of excess air coefficients on the particle temperature and gaseous pollutants emissions are also analyzed. The results show that the particle temperature rises rapidly in the initial stage and then tends to be a stable value, accompanied by the oscillation throughout the evolution. An increase in excess air coefficient causes the increase of emissions of NO, N2O, and SO2. The current numerical model presents a promising way to predict the flow behaviors and reactive characteristics for dense particle reaction system at both macro- and meso-scales.

Measurement and theoretical prediction of char temperature oscillation during fluidized bed combustion

There is experimental evidence of oscillations of the char particle temperature during combustion in a fluidized bed (FB), resulting from the movement of the char throughout the bed. However, in most theoretical FB combustion studies the char particle is assumed to always stay in the emulsion phase, and existing models do not take into account the movement of the char particle explicitly. Moreover, it is difficult to quantify the magnitude and frequency of these temperature oscillations with the common measurement techniques employed in FB (thermocouple and pyrometry with optical probe). In this work, the combustion of single char particles (8 mm) from beech wood and sub-bituminous coal is carried out in a 2-dimensional FB made of quartz, using two O2 concentrations (11 and 21%v) in N2. The time-evolution of the temperature and the size of the char in the different phases are estimated by the analysis of images resulting from a new method combining pyrometry with readings from a digital camera. It is found that the combustion temperature oscillates in hundredths of seconds with an amplitude varying from 10 to 100 °C, resulting from the movement of a particle between the emulsion, bubble and splash phases. The amplitude increases with higher O2 concentration and smaller char-particle size. A combustion model is developed using the experimental characterization of the movement of the char particle through the bed as input. The temperature and burnout time predicted by the model compare well (within 15 %) with measurements obtained from this work and from literature.

Fully resolved simulations of single char particle combustion using a ghost‐cell immersed boundary method

A novel ghost‐cell immersed boundary method for fully resolved simulation of char particle combustion has been developed. The boundary conditions at the solid particle surface, such as velocity, temperature, density, and chemical species concentration, are well enforced through the present method. Two semiglobal heterogeneous reactions and one homogeneous reaction are used to describe the chemical reactions in the domain, and the Stefan flow caused by the heterogeneous reactions is considered. A satisfactory agreement can be found between the present simulation results and experimental data in the literature. The method is then used to investigate the combustion property of a char particle and the interaction between CO2 gasification and O2 oxidation. Furthermore, combustion effect on the exchange of mass, momentum and energy between gas‐ and solid‐ phase is explored. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2851–2863, 2018

Resolved simulations of single char particle combustion in a laminar flow field

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 CO2 and N2 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.

Numerical simulation method and experimental validation of a single char particle combustion model in bulk space

Numerical simulation method and experimental validation of a single char particle combustion model in bulk space

Combustion of single char particles from Victorian brown coal under oxy-fuel fluidized bed conditions

Combustion performance of individual and large spherical single char particles of 5–6mm diameter during oxy-fuel fluidized bed (Oxy-FB) combustion is investigated. These char particles were prepared from briquetted Victorian brown coal. In these experiments, a thin thermocouple was inserted inside each char particle to monitor the particle temperature during the experiment. Particle temperature was observed to be higher compared to the bed temperature which varied between 880°C and 925°C. Up to 48°C difference was noticed between the char particle temperature and the bed temperature using 15% steam in oxy-fuel combustion atmosphere. A kinetic model on char combustion was developed to calculate the combustion time, carbon consumption rate and the particle diameter as a function of time in an atmosphere consisting of O2, CO2 and steam. It was evident that boundary-layer oxygen diffusion controlled the carbon combustion rate during oxy-fuel fluidized bed combustion of these brown coal char particles. The gasification reactions were almost negligible for the feed gases and temperature range applicable for Oxy-FB combustion.

Biomass Char Nitrogen Oxidation—Single Particle Model

In this study, the release of NO during the combustion of single char particles of spruce bark was investigated. Modeled NO release rates were compared to experimentally determined NO rates from a single particle reactor of quartz glass. The operating conditions were between 1073 and 1323 K with 3–19 vol % O2 and with 0% NO in the surrounding gas. The NO release rates from the single particles were determined from measured molar fractions of NO at the outlet from the reactor system. In the model, the char-N is assumed to be oxidized proportionally to the char-C oxidation rate, which is measured. The formed NO is assumed to be reduced as a function of the local NO concentration inside the particle. The NO concentration profile inside the char particle is numerically calculated by taking into account the transport of NO inside and outside the particle and the reduction rates of NO inside the char particle. The modeled NO release rates were in good agreement with the measurements. The results show that the fractional conversion of char-N to NO increases as functions of decreasing particle size and conversion. The most likely explanation for this is that the formed NO diffuses away at an increasing rate as the particle decreases in size. Further, a suggested analytical model for the fractional conversion of char-N to NO is tested.

Effects of particle sizes on transport phenomena in single char combustion

A one-dimensional char combustion model including pore structure effects is used to numerically investigate single char particle combustion for several different types of char samples. Previously, it is expected that small char particles have less combustion time. However, the present work shows that this is true only if the combustion time is defined as that needed for a char particle diameter diminished below a certain value. If the combustion time is defined as time needed for the carbon conversion ratio higher than a certain value, there are optimal particle sizes in a limited combustion period. Just reducing the char particle sizes may not get high carbon conversion ratios. It has also been found that, in general, the larger particles have higher temperatures at the exterior surfaces.

Modeling and countermeasures of combustion characteristics of char particles in CFBC

A mathematical model of single char particle combustion in circulating fluidized bed combustor (CFBC) is developed in this paper. Its numerical solution in operating conditions of CFBC verifies the nature of a phenomenon that the distribution of carbon content of char particles has a peak value versus their diameters. The results show that the temperature of smaller char particle is close to the bed temperature, and there also exits a peak value for the burn-out time of char particles versus their diameters. The countermeasures are presented to improve combustion of fine particles, such as use of the fly-ash recirculation, the hot cyclone, and so on.

97/02181 Single char particle combustion at moderate temperature: effects of ash

97/02181 Single char particle combustion at moderate temperature: effects of ash

A Numerical Analysis of Single Char Particle Heated by a Laser.

An unsteady state boundary layer model was developed in order to analyse the combustion of single char particles in radiation fields. The model is based on the following assumptions, a spherical char particle with small diameter is combusted at atmospheric pressure, the temperature difference between the surface and the inside of the particle is negligible, particle diameter shrinks due to the reaction on the char surface, the surrounding gas is stagnant, but Stefan flow is taken into consideration and bulk gas flow is neglected. The model predicts the particle temperature increase and the distributions of gas temperature and gas components in the boundary layer. The computed distribution of gas component proves clearly that the reaction rate is mainly controlled by oxygen diffusion in the boundary layer except during the initial heating and reaction. Using the simulation model, the effects of particle size, oxygen content in bulk gas and input laser power is discussed. The results predict that smaller particles heat up faster in initial heating and higher laser input has the same effect. Further, it is shown that a higher oxygen content raises the sustained combustion temperature and shortens the combustion period.

Single char particle combustion at moderate temperature: effects of ash

The effects of ash content, ash layer heat and mass transfer on a single granulated char particle (10–18 mm in diameter) combustion in an air stream (12 cm s −1 in cold base) are studied. The transient temperature of various ash content char particle burning in the surrounding gas temperatures of 900 to 1200 K is simulated. Agreement between simulated and experiment results is obtained by adjusting the ash layer diffusion coefficient and heat conductivity, surface emissivity and the reaction rate constant. The reaction rate constant plays an important role in modeling the initial stage of char particle combustion even when the overall rate is ash layer diffusion controlled. It determines the particle heating rate in the initial stage of combustion, and then indirectly influences the peak temperature. The ash layer diffusion resistance affects the rate controlling processes and the pattern of the time-temperature profile. The higher ash content char particle burns with a lower peak temperature and earlier temperature decrease due to the lower ash layer porosity and lower ash layer diffusion coefficient. It is concluded that the high ash particle combustion is controlled by ash layer diffusion except in the initial stage of combustion. As for the lower ash content char particle, it is controlled mainly by reaction at lower ambient temperature and by film diffusion at a higher temperature in the earlier stage. However, in the last stage, it is controlled by ash layer diffusion. The transition occurs when the ash layer is formed and the diffusion resistance is significant, and it is at that time that the particle reaches its peak temperature.

Ignition and combustion of single, levitated char particles

The paper focuses on the experimental measurement and model prediction of burnout times and ignition temperatures of single char particles. Experimental temperature-time histories of single, submillimetric char particles are compared with the predictions of a simple model that takes into account all the experimental findings obtained on a model char with an electrodynamic balance (EDB). The particular features of the experimental technique, allow the measurement of the temperature of the surface of a single levitated particle as the high temperature reaction proceeds. The particle is heated to reaction conditions with a laser while the surrounding gases are maintained at room temperature. Particle ignition temperatures and burnout times are easily detected from experimental curves. Even with the high temperature (2500–3000 K) and low characteristic time of the reaction (30–100 ms), reproducible experimental data were obtained, confirming that the EDB is a viable tool for studying high-temperature heterogeneous gas-solid reactions. Experimental results are compared with the predictions of a simple model developed for the description of high temperature combustion of single char particles. Even if particle morphological modifications occuring at high conversion degrees are not accounted for, the model well describes the particle ignition process and predicts with sufficient accuracy ignition temperatures and burnout times.

Combustion of single char particles in a turbulent fluidized bed

AbstractThe combustion of single bituminous char particles (4‐12 mm diameter) was studied in a turbulent fluidized bed operated at 1098 K using air as the fluidising medium. Results indicated that particles burn with constant density following a shrinking sphere model. Burning rates are much higher than those observed in a bubbling fluidized bed. The rate of transfer of oxygen to the particle surface is also higher than that observed in bubbling beds. A model is proposed to calculate the Sherwood numbers of the burning carbon particles. Experimental values of the Sherwood numbers agree well with those predicted from the model.

ON THE DYNAMICS OF THE COMBUSTION OF SINGLE CHAR PARTICLES

Diffusion and reaction models of varying complexity are used to study the stability of the combustion of single char particles. In contrast to past studies which have considered only the reaction of carbon with oxygen, both the heterogeneous reactions of carbon with oxygen and carbon dioxide and the homogeneous oxidation of carbon monoxide are taken into account in the formulation of the mathematical models. Emphasis is placed on the investigation of the feasibility of occurrence of oscillatory combustion. Our results show that high rates of the C-O2 reaction and high concentration of O2 in the ambient favor the occurrence of multiple steady states and oscillatory solutions, but the appearance of oscillatory instability is suppressed by the homogeneous reaction in the gas phase and the presence of CO and CO2 in the ambient. The parametric investigation of the problem reveals, in agreement with the results of past studies, that the heat capacity of the porous solid, the Lewis number, and the thickness of t...

Post-ignition transients in the combustion of single char particles

The temperature-time histories of single char particles burning at temperatures above 1500 K have been measured by two-colour near infrared pyrometry techniques. Two coal chars and a synthetic char consisting of spherical glassy carbon particles of uniform size were used in the experiments. The results indicate that with lean combustion and low-to-medium temperatures, the char particles do not ignite over their whole external surface, but exhibit preferential ignition at specific sites. These ‘hot-spots’ probably involve regions in which the material exhibits strong inhomogeneities, either on or below the surface. Following ignition, the reaction may propagate over the particle surface. At elevated combustor temperatures and/or high oxygen partial pressures, ignition spreads quickly over the entire particle surface and the importance of the hot-spots diminishes. A simple model is used to estimate the rate of growth of a hot-spot under various experimental conditions.

Testing of a model for fluidized bed coal combustors-effect of char combustion model

A system model, developed previously for the modelling of continuous fluidized bed combustors burning coal with wide size distribution was modified to incorporate various single char particle combustion mechanisms. It was then used to investigate the effect of the single char particle combustion mechanism on bed concentration profiles and the combustion efficiency of a pilot scale coal-fired fluidized combustor for which experimental data had previously been reported in the literature. Combustion mechanisms tested were: o i. Two diffusion-controlled heterogeneous reactions at the particle surface; reduction of carbon dioxide and oxidation to carbon monoxide. ii. A single diffusion controlled heterogeneous, reaction at the particle surface; oxidation of carbon to carbon monoxide. iii. A single heterogeneous reaction at the particle surface; oxidation of carbon to carbon monoxide, controlled by both diffusion and chemical reaction. Bed concentration profiles, carryover losses and combustion efficiencies predicted for various excess air levels were compared with available experimental data. Comparisons showed that the predictions of the model with no carbon dioxide reduction at the particle surface are in better agreement with measurements.

Modelling of the Combustion of Single Char Particles

Modelling of the Combustion of Single Char Particles