Surface Of Char Particles Research Articles

Micro-scale study of mineral transformation and coverability at single coal char particle surface during high-temperature gasification process

During the high-temperature coal gasification process, mineral transformation is essential to the formation of residual carbon and ash/slag discharge. The current work investigated the mineral transformation and coverability of the single coal char particle with various ash content during gasification, as well as established the correlation between the ash layer and carbon conversion. Three stages of the gasification process at 1350 °C were defined by the quadratic difference method based on the interaction between the char and the liquid slag, and the two critical points that distinguish the three stages were between 15%–20% and 71%–90% respectively. Further investigation showed that the melting behavior of coal char during gasification was affected by the inhomogeneous distribution of elements. Ca showed a tendency to migrate from the unmolten area to the molten area. The migration behavior was attributed to the eutectic formation between silica-aluminate in the molten area and calcium salts in the unmolten area, leading to a reduction in melting point and gradual melting of minerals in the unmolten area. Eventually, a homogeneous gehlenite phase was formed and covered on the residual char surface. Besides, the relationship between liquid slag coverage and carbon conversion was established, and for SF, YL, and XLT chars, the maximum coverage of liquid product occurs at the conversion of 0.86, 0.78, and 0.66, respectively. This paper provides an in-depth understanding of the slag melting behavior and residual carbon formation in the entrained gasifiers.

Volatile potassium transfer behavior in carbon matrix and its effect on char gasification

Coal catalytic gasification is one of the effective ways to produce natural gas from coal. However, high cost and low recovery efficiency of catalyst increase the cost of catalytic gasification, which has become a restrictive factor for large-scale promotion of coal catalytic gasification. Therefore, the study on the migration behavior of potassium-based catalysts in the gasification process is of great significance for catalysts recovery. To understand the volatilization behavior between potassium and carbon matrix, the experiments were carried out in high temperature stage microscope (HTSM) and fixed-bed reactor. The migration of volatile potassium between particles was analyzed by SEM-EDS, while the chemical fraction of potassium species was determined by AAS, and the surface elements of the samples was investigated by XPS. The results showed that potassium would volatile at 765-770℃, C/K≈6.2–9.5(wt%), which would be adsorbed by the other char particles, leading to an increasing of the content of soluble potassium and ion-exchanged potassium, promoting carbon conversion in a limited effect and a decreasing of hydroxyl functional groups on the char particles. Meanwhile, the potassium content was proportional to the migration distance, the migration distance was about 720–800 μm at maximum under HTSM. The migration of potassium at high temperature caused the formation of liquid phase around the surface of char particles, which would prevent the contact between the gasifier agent and char particles and increase mass transfer resistance, thus preventing the increase of particle carbon conversion. In addition, no obvious liquid phase formation occurred at 800 °C in SFA-K, indicating that the formation of potassium-containing liquid phase and the volatilization of potassium at 800 °C were directly related to the carbon in the char.

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.

Residual gasification char applied to tar reforming in a pilot-scale gasifier: Performance and evolution of char properties for perspective cascade uses

The solid residue of gasification, or gasification char, is in many cases considered as a waste material inevitably generated during the operation of gasifiers. However, it is a good candidate to substitute synthetic catalysts for tar reforming and it is a carbon-rich material with a good potential for carbon negative applications as biochar. In this work, a wood-derived gasification char was tested for the upgrading of producer gas generated from pilot-scale gasification of wheat straw. The effect of a char bed in combination with partial oxidation was assessed under different conditions. The presence of the char bed markedly reduced the total tar concentration, but the bed material suffered deactivation. Partial oxidation prolonged the activity of the char bed by promoting reforming reactions at the surface of char particles and further reduced the tar content below 1 g/Nm3. Post-experimental characterization of the spent chars revealed a decreased surface area, a more ordered carbon phase and an increased resistance to thermal degradation. The results show that under optimized conditions, gasification char is a promising substrate for tar reforming and the upgrading of producer gas to syngas. Moreover, the changes in the char suggested possible cascade uses of the spent material for carbon sequestration.

Biomass char particle surface area and porosity dynamics during gasification

Experimental measurements of industrially relevant biomass char particles (12.5 mm and smaller) undergoing kinetics-limited gasification reactions with CO2 and H2O reveal how porosity and surface area change with char conversion. Reactant concentrations range from 0 to 90% CO2 and 0–50% H2O, including mixtures of both reactants. Reactor temperatures range from 1150 °C to 1350 °C. Particle types include wood (poplar), an herbaceous energy crop (switch grass), and an agricultural residue (corn stover). The data include measurements of particle size, shape, and mass as essentially continuous functions of time combined with discrete measurements of surface area and porosity and provide a robust data set for comparison.Theoretical predictions of porosity from standard models do not adequately describe the experimental trends. Total surface area increases slightly with conversion, with most of the increase in the largest pores but most of the surface area in the small pores. Porosity also increases uniformly with conversion·H2O, CO2 and blended gasification environments produce qualitatively similar surface area and porosity data over wide ranges of reactant concentrations when plotted against conversion. Char particle diameters slightly decrease during these kinetically controlled reactions, in part because of sintering and in part because the reaction is endothermic and creates a decreasing particle temperature gradient toward the particle center. Therefore, the particle center reacts more slowly than the comparatively warmer char surface. An abrupt change in char diameter occurs at conversions greater than 99.5% associated with residual ash sintering. SEM images qualitatively confirm the quantitative measurements and show that the biomass microstructure remains easily recognizable through essentially the entire conversion process.The primary conclusions indicate that (1) the vascular structure appears to have a major role in particle reactivity, is preserved during essentially the entire char conversion history, and is not captured by BET or similar analysis techniques, (2) the pore models developed for coal and other chars do not describe and cannot be adjusted to reasonably describe the development of surface area observed in these experiments, and (3) the great majority of the surface area is in the nanopores which do not change at all during conversion and appear to have no influence on char reactivity. The larger pores, and especially the vascular structure, primarily contribute to reactive surface area.

Effects of reaction condition on NO emission characteristic, surface behavior and microstructure of demineralized char during O2/H2O combustion process

Surface behavior and structural characteristics of char particles generally play a vital role in affecting char NO emission characteristics during combustion process. A typical bituminous Shenhua from northwest China was employed for demineralization in this research. The demineralized coal without catalytic interference of alkali metal salts was employed as the experimental sample. Moreover, by means of the utilization of isothermal combustion test, Temperature Programmed Desorption (TPD) and Raman spectrometer, the conversion ratio of nitrogen content in char to NO, as well as the evolution of char surface behavior and micro-structure in the process of O2/H2O combustion under different conditions were determined. With the increase of reaction temperature (Tr) and O2 concentration, more surface active sites (Cf), oxygen containing functional groups (C(O)) and small aromatic ring structures generated on char particle surface inhibiting the emission of NO during O2/H2O combustion process. Under identical reaction conditions (Tr, O2 and H2O concentration), the partially oxidized chars with moderate burnout degree (Xc ≈ 0.3) had the largest amount of Cf on particle surface. On the other hand, the amount of Cf and small aromatic ring structures first increased and then decreased as the H2O concentration enhanced. When the Cf amount and the value of I(Gr+VL+Vr)/ID reached the maximum at the H2O concentration of approximately 8.5 vol%, the conversion ratio of char-N to NO reached the minimum value. Meanwhile, the increase of H2O concentration initially accelerated the NO reduction rate and this positive effects gradually diminished when the H2O concentration exceeded the critical value (8.5 vol%). Thus, the optimal combustion condition for char combustion with the lowest NO conversion ratio in this study was: 1473 K, 30% O2 and 8.5 vol% H2O.

Numerical investigation of different effects of carbon dioxide properties and carbon monoxide oxidation on char particle combustion in actual and fictitious O2/CO2 environments

The overall and individual effects of carbon dioxide properties and carbon monoxide oxidation on char combustion were numerically simulated with a continuous-film model in different O2/CO2, O2/N2 and O2/Ar environments, and different CO2 physicochemical properties were artificially changed to distinguish the individual effect on char particle temperature and combustion rate. The results show that the char combustion rate in 21%O2/79%CO2 environment is slightly higher than that in air environment because of two opposite effects of higher char gasification reaction rate with high concentration CO2 and lower char oxidation rate with O2 resulting from lower particle temperature. At the same time, the CO flame front in 21%O2/79%CO2 environment is farther away from char particle surface than that in 21%O2/79%N2 environment because of lower diffusion coefficient of oxygen in CO2 environment although the gas temperature is lower. Furthermore, the net effect of molar heat capacity of CO2 on char combustion rate decreases and the net effect of char gasification reaction with CO2 on char combustion rate distinctly increases with the increase of ambient gas temperature and O2 mole concentration.

Correlation effects between turbulence and the conversion rate of pulverized char particles

The effect of turbulence on heterogeneous reactions on the surface of char particles embedded in a turbulent oxidizer, consisting of oxygen and carbon-dioxide, is in this work studied numerically. It is shown that for a small Damköhler number (Da), which is the ratio between a turbulent and a chemical time scale, the char conversion rates are somewhat increased by the turbulence. This is found to be due to the increased mass transfer rate to the char particle surface that is caused by the turbulence-induced relative velocity between the char and the oxidizer. For large Damköhler numbers, however, the char conversion rate is strongly reduced due to particle clustering. This reduction is explained by the fact that when particles are clustered in densely populated particle clusters, the transfer of oxygen to the particles in the centre of the clusters is reduced since the oxygen is consumed by the particles closer to the external surface of the cluster. At the same time, high concentrations of oxygen exist in the voids between the particle clusters. This oxygen cannot take part in the conversion of the char until it is transported to the char surface. The effects of turbulence on the heterogeneous reaction rates are furthermore modelled based on Direct Numerical Simulation (DNS) data for a simplified reacting gas particle system.

Oxyfuel Combustion – Experimental Investigation of Sorption Effects on Coal Char

Abstract The DFG Collaborative Research Center SFB/TRR129 “Oxyflame” is devoted to developing a fundamental understanding of effects relevant for the combustion of solid fuels in oxyfuel atmospheres. 17 complementary research projects at RWTH Aachen, TU Darmstadt, and Ruhr-Universitat Bochum both numerically and experimentally deal with various aspects on scales reaching from single particles to complete combustion chambers. As part of this framework, this project experimentally addresses the impact of sorption effects on the surface of chars and of the diffusion into pores. Since the adsorption of different components of oxyfuel atmospheres is selective and since the composition of a boundary layer on the surface of char particles is relevant for equilibria and dynamics of heterogeneous reactions during char burnout, selectivities of sorption and effective diffusion coefficients are considered important inputs for the advancement of particle-burnout models. This paper describes results of measurements of the sorption of main components of oxyfuel atmospheres (CO 2 and O 2 ) on char particles. Selectivities are derived from the experimental results. The extrapolation of selectivities to higher temperatures, which is essential for their application in burnout models, is discussed.

Effect of temperature on gasification performance and sodium transformation of Zhundong coal

The effect of temperature (between 850°C and 1050°C) on gasification performance and sodium transformation characteristics of a high-sodium Zhundong coal was investigated using a novel design of fluidized bed gasifier. The agglomeration tendency of Zhundong coal was predicted using the ternary phase diagram of Na2O-SiO2-Al2O3, and ways of inhibiting agglomeration were proposed. For the catalytic effect of sodium in coal ash, Zhundong coal shows good gasification performance; the concentration of CH4 is about 1% and changes slightly with an increase in temperature. Zhundong coal ash is located in the carnegieite (NaAlSiO4) region of the ternary phase diagram system, which is a high melting temperature zone where agglomeration does not easily occur. The sodium-containing compound detected in the 950°C, 1000°C, and 1050°C bottom chars and fly ashes is NaAlSiO4. With increase in temperature, the sodium retention ratio first decreases but then increases. At higher temperature, more sodium is retained as NaAlSiO4 and therefore less is released. The enrichments of calcium and sodium on particle surface were observed by EDX analysis. The formed high melting point compounds of NaAlSiO4 and CaS may adhere to the surface of char particles and inhibit the release of sodium at higher temperature. Agglomeration and defluidization phenomena were not found in any experiment. The experimental results are in agreement with the prediction based on the ternary phase diagram analysis. Avoiding the use of silica as bed material, enhancing mixing and inducing the formation of the high melting point sodium-containing compound NaAlSiO4 are essential to inhibit agglomeration and defluidization. The experimental and thermodynamic calculation methods are useful in identifying conditions suitable for the successful use of high sodium coal during fluidized bed gasification.

Understanding of physicochemical properties and formation mechanisms of fine particular matter generated from Canadian coal combustion

A systematic study on the fine particular matter emissions from coal combustion of four kinds of coals of different ranks (lignite, sub-bituminous, bituminous, coking coal), including three typical Canadian coals and a Chinese coal have been presented. The results show that a tri-modal mass size distribution (coarse, fine and ultrafine mode) was commonly identified, and it was confirmed by size-segregated elemental distributions. The tri-modal distribution is of temperature and coal ranks independence. Coarse particles (Da∼5μm) presented as round-shaped sphere with smooth surface. Both round-shaped particles and irregular-shaped particles were observed in fine particles (Da∼0.5μm). Clusters of nano-particles (Da∼0.05μm) were identified in the ultrafine mode. The concentration of fine particles (Da<1μm) generated from the TECK coal is the highest (about 150mg/Nm3), while fine particles (Da<10μm) generated from Boundary coal (about 580mg/Nm3) is highest among the four coals. Higher volatile content in coals gives priority for enhancing the formation of fine particles. Excluded minerals would mostly fragmented to form ash residual with irregular shapes, while part of them would experience coalescence individually to form round-shaped particles with size larger than 10μm. Included minerals adhered firmly on the surface of char particles contribute greater in the irregular-shaped particles in fine mode, while the shedding included minerals from char particles should take the major responsibility for the round shaped particles in fine mode.

Clarifying the influence of moisture on the ignition and combustion of wet Victorian brown coal in air-firing and oxy-fuel modes: Part 2: Contribution of gasification reaction to char oxidation rate

This paper is the second part of the study to clarify the influence of moisture in Victorian brown coal oxy-fuel combustion, with a focus on char oxidation and gasification reaction through experimental and modelling efforts. An in-situ high-speed two-colour pyrometer with the wavelength band of 0.85–1.05μm was employed to measure particle temperature in the flat flame burner reactor. The combustion stage of carbon particle was simulated for the transient phenomena with particle heating and radiative heat transfer. The multiple surface reaction single-film approach, including char–O2, char–CO2 and char–steam reactions, was employed and the contribution of individual reactions to carbon consumption rate was determined via matching with the measured particle temperature. Irrespective of the initial moisture content, the extent of char–steam gasification reaction was found to account for ~15% in the air-firing case. This reaction was mainly triggered by the external steam in the reactor, rather than the inherent moisture that resided preferentially as volatile cloud on char particle surface. The combined effect of both char–CO2 and char–steam gasification was significant in oxy-fuel combustion mode, especially for the wet coal. In the oxy-21 case, these two reactions have a total extent of around 8% and 18% on the burning char surface of dried and wet coal, respectively. The char–CO2 gasification is insignificant, because the char particle temperature was low. Increasing the oxygen percentage to 31% in CO2 enhanced the total extent of these two gasification reactions to 28%, based on the mass of total carbon. Such an extent is comparable with the literature. However, the steam gasification rate for brown coal char was far higher, ~26% relative to ~10% for high-rank bituminous coal reported in the literature. This substantiates the strong steam gasification reactivity of Victorian brown coal char. The contribution of inherent moisture to char–steam gasification reaction is crucial in the combustion of wet coal in the oxy-21 case, accounting for ~10%. This is due to the long residence of the unevaporated steam as a thick cloud on the char surface. Increasing the oxygen concentration in CO2 enhanced the char–O2 reaction, the release rate of volatiles and inherent moisture. Therefore, the char–steam reaction caused by the inherent moisture within coal matrix was minimised and eventually diminished in the oxy-31 case.

Effect of O2/CO2 Ratio on Fuel-NOx Formation in Oxy-Coal Combustion

The purpose of this study is to clarify the fundamental features of NOx formation in oxy-coal combustion. The pure effects of various factors related to oxygen concentration, flame temperature and flame shape on NOx formation are investigated by experimental and theoretical analysis, resulting in the following conclusions: (1) In oxy-coal combustion (i.e., CO2/O2 atmosphere), the emission of fuel-NOx increases with O2 concentration. (2) The flame shape changes from a detached flame to an attached flame with the increase in flame temperature. (3) During oxy-coal combustion, the gasification reaction between CO2 and carbon on the surface of char particles occurs more vigorously than during combustion in air. (4) The single particle model for estimating NOx emission is applicable to the detached flame. This implies that in the case of the detached flame the pulverized coal burns with a structure of premixed flame.

Effect of O2/CO2 Ratio on Fuel-NOx Formation in Oxy-coal Combustion

The purpose of this study is to clarify the fundamental features of NOx formation in oxy-coal combustion. The pure effects of various factors related to oxygen concentration, flame temperature and flame pattern on NOx formation are investigated by experimental and theoretical analysis, resulting in the following conclusions: (1) In oxy-coal combustion (CO2/O2 atmosphere), the emission of fuel-NOx increases with O2 concentration. (2) The flame pattern changes from a detached flame to an attached flame with the increase in flame temperature. (3) During oxy-coal combustion, the gasification reaction between CO2 and carbon on the surface of char particles occurs more vigorously than during combustion in air. (4) The single particle model for estimating NOx emission is applicable to the detached flame pattern. This implies that in the case of the detached flame the pulverized coal burns with a structure of premixed flame.

A new technique for the measurement of the product CO/CO 2 ratio at the surface of char particles burning in a fluidized bed

A new experimental technique is proposed to measure the product CO/CO 2 ratio at the surface of spherical char particles during fluidized bed combustion. It is based on the measurement of the burning rate of a single char particle under low oxygen concentration conditions and on the use of an accurate prediction of the particle Sherwood number. This technique was applied to spherical char particles obtained from a bituminous coal which is characterized by a low attrition and fragmentation propensity. The product CO/CO 2 ratio was measured at a bed temperature of 850 °C and at a fluidization velocity of 0.3 m/s in a lab-scale apparatus operated with a bed of 0.5–0.6 mm sand. The char particle size was varied between 2 and 7 mm and the inlet oxygen concentration between 0.1% and 2.0%. Results showed that under the experimental conditions investigated carbon was mostly oxidized to CO 2 within the particle boundary layer, with a maximum fraction of carbon escaping as CO of 10–20% at the lowest oxygen concentrations and largest particle sizes.

石炭ガス化過程におけるチャーの粒度分布および比重の変化における挙動解明とモデル化

The entrained flow coal gasifiers, which are adapted for almost all IGCC technologies, are operated under the high-temperature and pressurized condition. It is necessary to clarify the coal behavior in such a condition for the design of the gasifier and the estimation of the gasifier's performance. In this work, the changes of char morphology, such as particle size distributions and bulk density, in the coal pyrolysis or the char gasification reaction were examined using a pressurized drop tube furnace facility (PDTF). Though a lot of pulverized coal particles have swelled by the rapid pyrolysis, the tendency was shown that the initial pyrolysis reaction of a coal with a high fuel ratio was controlled by pressure and the swelling was also suppressed. In the char gasification process, the change of the char particle diameter was few in the early stage of the gasification, because the surface was eroded by the reaction and the pore grew in the surface of char particles. The structure of char particles became fragile and the particles fragmented into smaller parts in the middle and latter stages of the gasification. The numerical model of particle size distributions and particle density of gasified char was proposed to explain the shrinkage and the fragmentation of char particles during gasification, and the validity of the model was shown.

Examination of catalytic roles of inherent metallic species in steam reforming of nascent volatiles from the rapid pyrolysis of a brown coal

In-situ steam reforming of tar from the rapid pyrolysis of a Victorian brown coal was studied, employing a single-stage drop-tube reactor and a particular type of two-stage reactor, in which the nascent tar underwent steam reforming and thermal cracking in the presence and absence of nascent char particles, respectively. Na was the most abundant inherent metallic species contained in the coal, and a significant proportion of Na (60–80%) was volatilized during the pyrolysis. However, the Na dispersed in the vapor phase seemed to have no significant catalytic effect on the steam reforming. Na, and/or Ca remaining on the surface of char particles were responsible for rapid and extensive steam reforming of the nascent tar into gases, resulting in tar yield decrease by nearly 90%. The presence of steam alone was effective for suppressing soot formation from the tar vapor by approx. 80%, but in the absence of char particles containing metallic species, the addition of steam led to an increase in the yield of poly-nuclear aromatics.

Generation of fine chars from Australian black coals in pressurized fluidized bed combustion

Five Australian black coals were characterized and the most significant mechanisms for the in-bed generation of fine char particles were studied under pressurized fluidized bed combustion (PFBC) conditions of 1.6 MPa, 850°C, U = 0.9 m/s, 7% O2, and coal particle size 4 to 4.75 mm in a novel batch-fed, bench-scale pressurized fluidized bed of 1.3 mm silica sand particles. The generation of elutriable fine char particles in our PFBC tests was attributed to attrition, fragmentation, and combustion residues of in-bed char particles. Both primary fragmentation and secondary fragmentation mainly produced large particles, which remained in the bed, but also generated some elutriable small pieces, contributing to carbon elutriation from PFBC. However, attrition was the dominant contributor to the generation of elutriable fine char particles. The specific char attrition rates were approximately constant during the early stage of burnout (for larger char particles > ∼2 mm), because of the stable development of detachable asperities on the external surface of char particles, when combustion was controlled by the external mass transfer of oxygen. The specific attrition rates increased at the later stage of burnout, because of the more effective intraparticle diffusion of oxygen into the pores of smaller particles and the development of porous structure within the surface layer, giving a higher specific attrition rate.

Particle/Turbulence Interactions, Mass Transfer and Gas/Solid Chemistry in a CFBC Riser

Gas/solid chemistry in the upper, dilute region of a circulating fluidised bed combustor (CFBC) riser is addressed. The limitations of turbulent mixing are illustrated by the example of the heterogeneous NO/CO/char reaction, relevant in CFB combustion of nitrogen-containing solid fuels. The mass transfer of the gaseous reactants to the char surface is determined, and how the conversion is influenced by the degree of mixing of the multiphase system by turbulent dispersion. Particle/turbulence interactions are taken into account by a (Lagrangian) frequency spectrum of velocity fluctuations, which determine the dispersion of the char particles described here with the Tchen–Hinze model. Chars from solid fuels characterised by fuel ratio (FR) ranging from 0.1 (wood) via 0.5 (peat) and 1 (coal A) to 2 (coal B) were considered. The effective rate of the NO/CO/char reaction is determined as a function of the size and type of the char particle, temperature, particle concentration, reactor dimensions and fluidization velocity, at atmospheric pressure. It was found that for this case the effective gas/solid conversion rate in the upper, dilute region of the riser is much lower than the gas/solid chemistry, mainly due to mass transfer limitations for char particles with sizes of typically 300 μm. The concentration of NO at the char particle surface is only a few % of that in the bulk gas phase. Strong influences were found for particle size and temperature, whilst the fluidisation velocity and the reactor size have only a small influence. It is concluded that for a typical CFB riser, for particles larger than approx. 20 μm, mass transfer has a stronger influence on the heterogeneous NO/CO/char reduction mechanism than the ``unmixedness'' due to particle eddy dispersion limitations. It is recommended that this or a similar approach to turbulent dispersive mixing is implemented in CFD codes when these are used for boiler and furnace calculations.

Combustion of a porous char particle in an incipiently fluidized bed

A simple single particle model has been developed for the combustion of carbon. The heterogeneous reactions of carbon oxidation and gasification are assumed to take place inside the porous char, while the homogeneous oxidation of carbon monoxide is assumed to occur in the film surrounding the burning particle. Depending on the extent of consumption of oxygen by the primary product CO, oxygen may or may not reach the char surface. Hence, the formulation leads to a single film porous particle model in the case where oxygen reaches the particle surface, and a double film porous particle model when oxygen does not reach the char particle surface. Analytical expressions are derived for the concentration profiles for the different gaseous species. The application of the model is illustrated through calculations relevant for combustion of carbon in an incipiently fluidized bed. These simulations, then, reflect the combustion behavior in the dense phase of a bubbling fluidized bed. The model successfully predicts the experimental observation of two extrema in CO/CO 2 product ratio at the surface of a burning particle. Parametric studies were conducted to assess the effect of char and inert particle sizes, moisture content and the bulk species concentration on the CO/CO 2 product ratio, as well as the average rates of species consumption (or generation). The results show that char gasification reaction influences the carbon consumption behavior significantly at high temperatures. The CO/CO 2 product ratio at the surface increases—as a consequence of a decrease in the net film oxidation rate—with an increase in the size of bed particles. The CO/CO 2 product ratio is also enhanced by a decrease in char size and gas phase moisture content. The temperature corresponding to a minimum in CO/CO 2 ratio increases in a similar fashion. An increase in particle porosity and specific internal surface area led to a decrease in this temperature. The model results also permit identification of the three regimes of combustion.