Numerical studies on the combustion properties of char particle clusters

Particle clustering is an important phenomenon in dense particle–gas two-phase flow. One of the key problems worth studying is the reacting properties of particle clusters in coal particle combustion process in the dense particle region. In this paper, a two-dimensional mathematical model for the char cluster combustion in airflow field is established. This char cluster consists of several individual particles. The comprehensive model includes mass, momentum, and energy conservation equations for both gas and particle phases. Detailed results regarding velocity vector, mass component, and temperature distributions inside and around the cluster are obtained. The micro-scale mass and heat transfer occurred inside and around the char cluster are revealed. By contrastively studying the stable combustion of char particle clusters consisting of different particles, the combustion properties of char clusters in various particle concentrations are presented and discussed.     

The combustion of large particles of char in bubbling fluidized beds: The dependence of Sherwood number and the rate of burning on particle diameter

Particles of char derived from a variety of fuels (e.g., biomass, sewage sludge, coal, or graphite), with diameters in excess of , burn in fluidized bed combustors containing smaller particles of, e.g., sand, such that the rate is controlled by the diffusion both of O₂ to the burning solid and of the products CO and CO₂ away from it into the particulate phase. It is therefore important to characterize these mass transfer processes accurately. Measurements of the burning rate of char particles made from sewage sludge suggest that the Sherwood number, Sh, increases linearly with the diameter of the fuel particle, dchar (for ). This linear dependence of Sh on dchar is expected from the basic equation Sh=2εmf(1+dchar/2δdiff)/τ, provided the thickness of the boundary layer for mass transfer, δdiff, is constant in the region of interest (). Such a dependence is not seen in the empirical equations currently used and based on the Frössling expression. It is found here that for chars made from sewage sludge (for ), the thickness of the boundary layer for mass transfer in a fluidized bed, δdiff, is less than that predicted by empirical correlations based on the Frössling expression. In fact, δdiff is not more than the diameter of the fluidized sand particles. Finally, the experiments in this study indicate that models based on surface renewal theory should be rejected for a fluidized bed, because they give unrealistically short contact times for packets of fluidized particles at the surface of a burning sphere. The result is the new correlation

     

Laminar flow with combustion in inert porous media

This work presents one-dimensional numerical results for combustion of an air/methane mixture in inert porous media using laminar and radiation models. Comparisons with experimental data are reported. The burner is composed by a preheating section followed by a combustion region. Macroscopic equations for mass, momentum and energy are obtained based on the volume average concept. Distinct energy equations are considered for the porous burner and the flowing gas. The numerical technique employed for discretizing the governing equations was the control volume method with a boundary-fitted non-orthogonal coordinate system. The SIMPLE algorithm was used to relax the entire equation set. Inlet velocity, excess air, porosity and solid-to-fluid thermal conductivity ratio were varied in order to investigate their effect on temperature profiles. Results indicate that higher inlet velocities result in higher gas temperatures, following a similar trend observed in the experimental data used for comparisons. Burning of mixtures close to the stoichiometric conditions also increased temperatures, as expected. Increasing the thermal conductivity of the preheating section reduced peak temperature in the combustion region. The use of porous material with very high thermal conductivity on the combustion region did not affect significantly temperature levels in the combustion section.     

Analysis and modeling of char particle combustion with heat and multicomponent mass transfer

A char combustion model suitable for a large-scale boiler/gasifier simulation, which considers the variation of physical quantities in the radial direction of char particles, is developed and examined. The structural evolution within particles is formulated using the basic concept of the random pore model while simultaneously considering particle shrinkage. To reduce the computational cost, a new approximate analytical boundary condition is applied to the particle surface, which is approximately derived from the Stefan–Maxwell equations. The boundary condition showed reasonably good agreement with direct numerical integration with a fine grid resolution by the finite difference method under arbitrary conditions. The model was applied to combustion in a drop tube furnace and showed qualitatively good agreement with experiments, including for the burnout behavior in the late stages. It is revealed that the profiles of the oxygen mole fraction, conversion, and combustion rate have considerably different characteristics in small and large particles. This means that a model that considers one total conversion for each particle is insufficient to describe the state of particles. Since our char combustion model requires only one fitting parameter, which is determined from information on the internal geometry of char particles, it is useful for performing numerical simulations.     

Continuous lignite char combustion in a spouted bed

A simulation model of continuous lignite char combustion in a spouted bed has been developed to predict bed oxygen concentrations, bed particle size distribution, bed carbon loading, mean diameter of bed char, and the fractional combustion in spout, annulus, and fountain. The approach involves taking into account the spouted bed hydrodynamics, a burning law for individual particles, and the combines mass balances for bed char and oxidant in the three typical regions. The predicted results for various operating conditions are compared with some experimental data.     

The dynamics of in-situ combustion fronts in porous media

The sustained propagation of combustion fronts in porous media is a necessary condition for the success of in situ combustion for oil recovery. Compared to other recovery methods, in situ combustion involves the complexity of exothermic reactions and temperature-dependent chemical kinetics. In the presence of heat losses, the possibility of ignition and extinction also exists. In this paper, we address some of these issues by studying the properties of forward combustion fronts propagating at a constant velocity in the presence of heat losses. We extend the analytical method used in smoldering combustion [7], to derive expressions for temperature and concentration profiles and the velocity of the combustion front, under both adiabatic and non-adiabatic conditions. Heat losses are assumed to be relatively weak and they are expressed using two modes: 1) a convective type, using an overall heat transfer coefficient; and, 2) a conductive type, for heat transfer by transverse conduction to infinitely large surrounding formations. In their presence we derive multiple steady-state solutions with stable low and high temperature branches, and an unstable intermediate branch. Conditions for self-sustaining front propagation are investigated as a function of injection and reservoir properties. The extinction threshold is expressed in terms of the system properties. An explicit expression is also obtained for the effective heat transfer coefficient in terms of the reservoir thickness and the front propagation speed. This coefficient is not only dependent on the thermal properties of the porous medium but also on the front dynamics.     

A consistent flamelet formulation for a reacting char particle considering curvature effects

In the present work, the combustion of a single char particle in quiescent and convective environments is investigated numerically. Fully resolved CFD calculations are carried out considering heterogeneous reactions at the particle surface and detailed homogeneous reactions in the gas phase. Unity and non-unity Lewis number diffusion modeling approaches are employed and compared to each other. The flame shape of the particle in a quiescent atmosphere shows full symmetry whereas the particle in the convective environment exhibits a stagnation region upstream of the particle and a wake region downstream of the particle. The detailed CFD results are used to analyze the flame structure around the char particle and corresponding flamelet simulations are carried out. For the presently investigated case, curvature effects of mixture fraction, species and temperature are found to be significant in almost all the cases. These curvature effects correspond to diffusion tangential to iso-surfaces of mixture fraction. To describe these processes, new extended flamelet equations are derived. The individual terms in the flamelet equations are analyzed for both the quiescent and the convective environment based on the CFD data and the results confirm the importance of tangential diffusion. Except for the quiescent environment and unity Lewis numbers, curvature cannot be neglected for the investigated char combustion case. For all other cases, significant differences between the standard flamelet model and the detailed CFD results are found. On the other hand, applying the extended flamelet equations yields very good agreement with the CFD results.     

Experimental study on effects of combustion atmosphere and coal char on NO2 reduction under oxy-fuel condition

Nitrogen oxides (NO_x) as the principal air pollutants are mainly from the combustion of fossil fuels. Oxy-fuel combustion is a promising clean coal technology, by which carbon dioxide (CO₂) can be captured in large-scale and NO_x emission can be reduced significantly. The formation of nitrogen dioxide (NO₂) in oxy-fuel combustion exceeds that under traditional air condition. However, the specific studies on NO₂ chemistry under oxy-fuel condition are still insufficient and the functional mechanisms of minerals and combustion atmosphere on NO₂ reduction have yet to be fully understood. The objective of present study is to experimentally clarify the effects of combustion atmosphere and coal char on NO₂ reduction in oxy-fuel combustion using a fixed-bed reactor. Experimental results showed that the decomposition of NO₂ had a strong temperature dependence and the NO₂ reduction rate showed a positive variation with temperature. The strength of catalytic activity in NO₂ reduction to nitric oxide (NO) was Fe₂O₃ > MgO > CaO > Al₂O₃ > Na₂CO₃ > K₂CO₃ > SiO₂. In addition, the increased concentrations of carbon monoxide (CO) and CO₂ could promote the reduction of NO₂, while the low content of CO₂ only established a slight impact on NO₂ reduction. However, the increase of oxygen (O₂) concentration displayed an inhibition effect on NO₂ reduction to a certain extent. The variation of atmosphere in oxy-fuel combustion generated a substantial influence on the creation and reduction of NO₂. The char prepared in lower temperature exhibited a higher promotion effect on the consumption of NO₂. Higher contents of fixed carbon and basic oxides had more obvious stimulation effects on NO₂ reduction. Fixed carbon had a superior activity in NO₂ reduction than ash. The kinetic analysis indicated that high content of CO and the presence of char could reduce the apparent activation energy of NO₂ reduction. The present study can be helpful to improve the understanding of NO₂ chemistry in oxy-fuel combustion.     

Numerical prediction of the transport and pyrolysis in the interior and surrounding of dry and wet wood log

The numerical simulation of the pyrolysis process of a dry and wet birch wood log in a cylindrical heating chamber is preformed. The model includes the flow inside and outside the porous wood log and accounts for convective, conductive and radiative heat transfer. A two-step pyrolysis reaction scheme is used to model the conversion from wood to tar, gas and char. The results of the simulations compare well with the authors experimental data which are presented in terms of radial temperature distribution and mass reduction, for both dry and wet cases. Our transient simulations provide us with the detailed flow field inside and outside the wood log. It clearly shows not only the existence but also the structure of the pyrolysis gas plumes leaving the wood. These plumes have only been visualised experimentally by few authors [Brackmann C et al. Optical and mass spectroscopy study of the pyrolysis gas of wood particles. Appl Spectros 2003;57(2):216–22, [12]] without any quantitative measurements and the present investigation gives a realistic estimation that we presently use to evaluate its impact on the heat and mass transfer, and on the momentum balance and the particle dispersion in a near future work. The gas plumes have a maximum velocity magnitude ranging between 0.1 and 0.2 m s⁻¹ and vanish when all the wood gas is produced. It is shown that increasing the convective flow around the wood log do not significantly modify the pyrolysis gas plume structure and seems to have small effect on the overall heating and the pyrolysis process which are mainly controlled by the thermal radiation from the hot surrounding walls.     

Syngas production in hybrid filtration combustion

Rich and ultrarich combustion of butane inside porous media composed of aleatory wood pellets and alumina spheres is studied experimentally to evaluate the suitability of the concept for syngas production. Temperature, velocity, and chemical products of the combustion waves were recorded experimentally at a range of equivalence ratios from stoichiometry (φ = 1.0) to φ = 2.6. It is observed that hydrogen and carbon monoxide are dominant partial oxidation products for ultrarich hybrid combustion waves of butane and wood pellets. Syngas yield in hybrid filtration combustion is found to be essentially higher than for butane filtration combustion in an inert porous medium.     

Syngas production from polyethylene and biogas in porous media combustion

The production of syngas from biogas (surrogate of CH₄/CO₂: 55/45 v/v) and polyethylene in a porous media combustion reactor is experimentally studied. The employed setup is novel and has not been studied before. A semi-continuous feed of solid fuel and a constant filtration velocity of the gaseous reactants of 17 cm/s were considered. Temperature, velocity of propagation, and composition of the syngas produced in the combustion waves were registered in a tubular reactor packed with a ceramic foam porous medium and two solid fuel inlets. In the first part of the study, a baseline determined by the filtration combustion of a biogas/air mixture through the ceramic foam at the equivalence ratio ($\mathrm{?}$) range $0.7\le \mathrm{?}\le 1.6$, having transient (upstream and downstream) and stationary combustion wave propagation regimes, is established. In the second part of this work, portions of the ceramic foam in two different separated zones are extracted, leaving space for the semi-continuous supply of polyethylene. In this second part the biogas-air combustion was performed only for $\mathrm{?}=0.8$ and $\mathrm{?}=1.6$. Although the combustion temperature decreased by the presence of polyethylene, it was found that the syngas (both H₂ and CO) yield was larger than for the baseline. The highest degrees of conversion to hydrogen and carbon monoxide was reached under the presence of polyethylene, having 45% and 67% for $\mathrm{?}=0.8$, and 45% and 60% for $\mathrm{?}=1.6$, respectively. These results are very promising and they demonstrate the capabilities of the presented methodology and experimental setup, which should encourage future attempts of applications of the technology.     

Hydrogen production in ultrarich combustion of hydrocarbon fuels in porous media

Rich and ultrarich combustion of methane, ethane, and propane inside inert porous media is studied experimentally and numerically to examine the suitability of the concept for hydrogen production. Temperature, velocities, and chemical products of the combustion waves were recorded experimentally at a range of equivalence ratios from stoichiometry (φ = 1.0) to φ = 2.5, for a filtration velocity of 12 cm/s. Two-temperature numerical model based on comprehensive heat transfer and chemical mechanisms is found to be in a good qualitative agreement with experimental data. Partial oxidation products of methane, ethane, and propane (H₂, CO, and C₂ hydrocarbons) are dominant for ultrarich superadiabatic combustion. The maximum hydrogen yield is close to 50% for all fuels, and carbon monoxide yield is close to 80%.     

Porous media combustion for micro thermophotovoltaic system applications

The micro combustor is the key component of the micro TPV power generator. To obtain high power density and performance efficiency, it is important for a micro combustor to achieve a high and uniform temperature distribution along the wall. In this paper, we compare the performance of a micro cylindrical combustor with and without employing porous media. Results indicate that packing the combustor with porous media can significantly enhance the heat transfer between the high temperature combustion products and the emitter wall. The use of porous media increases the contact area thereby increasing the temperature along the wall of the micro combustor resulting in an increase in its radiation energy. The effects of some parameters on radiation energy of the micro combustor are also highlighted.     

Mechanism and kinetics of sodium release from brown coal char particles during combustion

A model for the release of sodium during the combustion of single Loy Yang brown coal char particles is presented. The model is combined with further analysis of recently published measurements of the release of sodium from single brown coal particles burning in a flat flame to estimate the rate constant for sodium release as a function of burnout time for these experiments. A char combustion and heat transfer model is also used to predict the char burnout behaviour and surface temperature of the particle as a function of time during combustion for each of the experiments. By combining the predicted time–temperature history of the particles with the estimated rate constant for sodium release, an Arrhenius expression for the release of sodium during char combustion is determined as:A full mechanism for sodium release during the various stages of coal combustion is also proposed. Utilising the proposed mechanism, the rate-determining step for sodium release during char combustion is proposed to be the formation of a reduced form of sodium in the char which subsequently leads to the rapid loss of sodium from the particle.     

A review of investigations on liquid fuel combustion in porous inert media

Utilization of a porous medium for combustion of liquid fuels is proved to be a promising approach for future applications. The porous medium burner for liquid fuels is more advantageous than the conventional open spray flame burner for several reasons; these include enhanced evaporation of droplet spray owing to regenerative combustion characteristics, low emission of pollutants, high combustion intensity with moderate turn-down ratio and compactness. This article provides a comprehensive picture of the global scenario of research and developments in combustion of liquid fuels within a porous medium that enable a researcher to determine the direction of further investigation. Accordingly, a glossary of the important terminology, the modeling approach, advances in numerical and experimental works and applications are included. The papers published in standard journals are reviewed and summarized with relevant comments and suggestions for future work.     

Gas-phase transport and entropy generation during transient combustion of single biomass particle in varying oxygen and nitrogen atmospheres

Transient combustion of a single biomass particle in preheated oxygen and nitrogen atmospheres with varying concentration of oxygen is investigated numerically. The simulations are rigorously validated against the existing experimental data. The unsteady temperature and species concentration fields are calculated in the course of transient burning process and the subsequent diffusion of the combustion products into the surrounding gases. These numerical results are further post processed to reveal the temporal rates of unsteady entropy generation by chemical and transport mechanisms in the gaseous phase of the reactive system. The spatio-temporal evolutions of the temperature, major chemical species including CO, CO₂, O₂, H₂ and H₂O, and also the local entropy generations are presented. It is shown that the homogenous combustion of the products of devolatilisation process dominates the temperature and chemical species fields at low concentrations of oxygen. Yet, by oxygen enriching of the atmosphere the post-ignition heterogeneous reactions become increasingly more influential. Analysis of the total entropy generation shows that the chemical entropy is the most significant source of irreversibility and is generated chiefly by the ignition of volatiles. However, thermal entropy continues to be produced well after termination of the particle life time through diffusion of the hot gases. It also indicates that increasing the molar concentration of oxygen above 21% results in considerable increase in the chemical and thermal entropy generation. Nonetheless, further oxygen enrichment has only modest effects upon the thermodynamic irreversibilities of the system.     

Trends in modeling of porous media combustion

Porous media combustion (PMC) has interesting advantages compared with free flame combustion due to higher burning rates, increased power dynamic range, extension of the lean flammability limits, and low emissions of pollutants. Extensive experimental and numerical works were carried out and are still underway, to explore the feasibility of this interesting technology for practical applications. For this purpose, numerical modeling plays a crucial role in the design and development of promising PMC systems. This article provides an exhaustive review of the fundamental aspects and emerging trends in numerical modeling of gas combustion in porous media. The modeling works published to date are reviewed, classified according to their objectives and presented with general conclusions. Numerical modeling of liquid fuel combustion in porous media is excluded.     

Conversions of fuel-N to NO and N2O during devolatilization and char combustion stages of a single coal particle under oxy-fuel fluidized bed conditions

The conversions of fuel-N to NO and N₂O during devolatilization and char combustion stages of a single coal particle of 7 mm in diameter were investigated in a laboratory-scale flow tube reactor under oxy-fuel fluidized bed (FB) conditions. The method of isothermal thermo-gravimetric analysis (TGA) combing with the coal properties was proposed to distinguish the devolatilization and char combustion stages of coal combustion. The results show that the char combustion stage plays a dominant role in NO and N₂O emissions in oxy-fuel FB combustion. Temperature changes the trade-off between NO and N₂O during the two stages. With increasing temperature, the conversion ratios of fuel-N to NO during the two stages increase, and the opposite tendencies are observed for N₂O. CO₂ inhibits the fuel-N conversions to NO during the two stages but promotes those to N₂O. Compared with air combustion, the conversion ratios of fuel-N to NO during the two stages are lower in 21%O₂/79%CO₂, and those to N₂O are higher. At <O₂> = 21–50% by volume, the conversion ratios of fuel-N to NO during the two stages reach the maximum values at <O₂> = 30% by volume, and those to N₂O decrease with increasing O₂ concentration. H₂O suppresses the fuel-N conversions to NO and N₂O during the two stages. A higher coal rank has higher total conversion ratios of fuel-N to NO and N₂O. Fuel-N, volatile matter, and fixed carbon contents are the important factors on fuel-N conversions to NO and N₂O during the two stages. The results benefit the understanding of NO and N₂O emission mechanisms during oxy-fuel FB combustion of coal.