Char Particle Size Research Articles

Film-pore diffusion model for the fixed-bed sorption of copper and cadmium ions onto bone char

The sorption of copper and cadmium ions onto bone char in single component systems has been studied using fixed-bed column adsorbers. The effects of solution flowrate, initial metal ion concentration and bone char particle size have been studied. A film-pore diffusion model has been developed to predict the fixed-bed breakthrough curves for the two metal ions. A sensitivity analysis has been carried out to investigate the influence of the external mass transfer coefficient (film resistance), the effective diffusion coefficient (pore diffusion) and the solid phase loading capacity. It is found that under the experimental conditions employed in the study, film diffusional resistance was low and the Biot numbers were relatively high. Furthermore, a constant effective pore diffusivity was not sufficient to correlate the breakthrough curves accurately and a variable dependent effective diffusivity was required; suggesting a possible contribution from surface diffusion. Since the metal ion–bone char systems take a long time to reach equilibrium, the solid phase loading capacity, as predicted by the “best-fit” equilibrium isotherm, was not suitable for use in the diffusional mass transport model and the mass balance solid phase loading was utilised instead.

Coupling thermal deactivation with oxidation for predicting the combustion of a solid fuel

A thermal annealing model has been coupled with a detailed single particle model for char oxidation to predict the burning of solid fuels in combustion facilities. The combined model is tested against reported laminar flow reactor burning profiles of several coal chars at the intermediate stages of combustion and for a single coal char also at the later stages of combustion. The model shows for all coals a transition from combustion controlled by oxygen bulk diffusion at low char conversions to kinetically controlled combustion at high conversions. The impact of annealing on char reactivity is strongest at the early stages of combustion, and when the simultaneous annealing of the char is not accounted for, shorter burning times are obtained. In many cases, the experimental burning profiles of the coal chars can only be explained by taking into account the presence of fragments being released at the periphery of the particle. This type of fragmentation appears to be the prime means of particle shrinking and is most accentuated at higher oxygen concentrations in the reactor. The model was able to predict reasonably well the unburned carbon fraction of a single coal char at the latest stages of combustion (up to ∼98%). The model relies on reasonable estimates of the temperature and oxygen profiles in the reactor, as well as the char density, particle size, porosity and initial reactivity (e.g., at room temperature) of the non-annealed fuel. The latter is particularly important for determining burnout at the final stages of combustion. This can be determined by simply combining the char’s reactivity (extracted from a single TGA experiment of the fuel involving pyrolysis at high temperature and subsequent char oxidation), with the proposed annealing model.

Ca ion-exchanged coal char as H 2S sorbent

H 2S removal using Ca ion-exchanged coal char was studied in a fixed bed reactor at a temperature of 900°C. Yallourn, an Australian brown coal was used for ion exchange. The CaO obtained from the pyrolysis of Ca ion-exchanged coal was finely dispersed in the char and much more reactive toward H 2S than that from limestone. Ca ion-exchanged coal char showed high H 2S capture and almost no emission of H 2S was observed in the earlier reaction stage. At a reaction time of 100 min, the S/Ca atomic ratio in Ca ion-exchanged coal char increased to 1.2. This was due to H 2S capture capacity of both the coal char itself and the CaO distributed in the char. The sorption capacity of char was determined as 1.5 mmol-H 2S/g-char. The desulfurization characteristics of calcined limestone strongly depended on the particle size, whereas those of Ca ion-exchanged coal char was almost independent of the char particle size.

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.

98/00135 Mixing-heating apparatus for manufacture of coal briquets

Char made from narrow size fractions of lignite in a fluidized bed was briquetted at 140 MPa with various binder tars produced from the lignite. The raw briquettes were cured in air at 200 °C and then carbonized, usually at 925 °C. The tensile strength of the resulting formed coke could be as much as three times that of the best metallurgical coke, or as low as 2–3 MPa, depending on the type of binder used, or even a factor of ten lower than this if the curing stage was omitted. Curing is thus a crucial stage. Distinct optima of binder percentage and char particle size were found, giving maximum briquette strengths (≈ 13 MPa). A corresponding ‘strength surface’ suggested four factors associated with weakness of a briquette. Binders produced by air-blowing rather than nitrogen-blowing gave stronger briquettes. Fluidized bed tar was greatly ‘improved’ in this way, but Gray-King tar only slightly so, probably because the former contained more generally reactive species. Cured-only briquettes were usually surprisingly strong and carbonization increased this strength ≈ 1.5 times. Curing is considered to ‘fix’ the binder; carbonization changes the chemical nature of the ‘cement’ between the char surfaces but does not appreciably alter its spatial distribution. The carbonized briquette thus has substantially the same structural flaws as were present in the cured state and so has a strength closely correlated with that achieved merely by curing.

The [formula omitted] product ratio for a porous char particle within an incipiently fluidized bed: a numerical study

By comparing results generated from an advanced single-particle model with those obtained from experiment, we find reaction does occur within the boundary layer of a stationary char resident within the emulsion phase of a fluidized bed, with the extent of reaction diminishing as superficial gas velocity increases. Comparison also shows that the CO-O2, C-O2and C-CO2 reactions all affect the particle related COCO2 product ratio, with the influence of each varying with temperature. These observations help explain the peculiar product ratio/temperature profile recently reported by Linjewile and Agarwal (1995, Fuel74, 5) and suggest, for the first time, a relationship between the product ratio and char particle size. The model results show that boundary layer reaction influences the heat transfer coefficient associated with a char that is burning within a fluidized bed, and that any correlations which do not explicitly account for this may only serve as an approximation.

96/06654 National strategy statement on natural refrigerants (NRs)

Char made from narrow size fractions of lignite in a fluidized bed was briquetted at 140 MPa with various binder tars produced from the lignite. The raw briquettes were cured in air at 200 °C and then carbonized, usually at 925 °C. The tensile strength of the resulting formed coke could be as much as three times that of the best metallurgical coke, or as low as 2–3 MPa, depending on the type of binder used, or even a factor of ten lower than this if the curing stage was omitted. Curing is thus a crucial stage. Distinct optima of binder percentage and char particle size were found, giving maximum briquette strengths (≈ 13 MPa). A corresponding ‘strength surface’ suggested four factors associated with weakness of a briquette. Binders produced by air-blowing rather than nitrogen-blowing gave stronger briquettes. Fluidized bed tar was greatly ‘improved’ in this way, but Gray-King tar only slightly so, probably because the former contained more generally reactive species. Cured-only briquettes were usually surprisingly strong and carbonization increased this strength ≈ 1.5 times. Curing is considered to ‘fix’ the binder; carbonization changes the chemical nature of the ‘cement’ between the char surfaces but does not appreciably alter its spatial distribution. The carbonized briquette thus has substantially the same structural flaws as were present in the cured state and so has a strength closely correlated with that achieved merely by curing.

Coal gasification in a stirred bed reactor

Two Canadian non-caking coal chars of millimetre particle size were gasified in a stirred bed reactor at atmospheric pressure using steam-nitrogen mixtures. Experimental results showed that the reaction atmosphere changed with carbon conversion and the unit operated as a typical integral reactor. Some properties of the partially reacted chars were measured. The effects of gas superficial velocity, char particle size, steam partial pressure and temperature on conversion were examined. The role of transport resistance was assessed and empirical kinetic models were tested.

Simulation of secondary fragmentation during fluidized bed combustion of char particles

A computer program has been developed in order to simulate the secondary fragmentation process during fluidized bed combustion of a population of char particles. The simulation intends to predict the evolution of char particle size distribution during combustion. It is based on two statistical functions, one which accounts for the probability density that a particle of a given size breaks into fragments and the other for the size distribution on the basis of the number of fragments generated by the breaking up of the particle. The parameters involved in the simulation have been obtained by minimizing the difference between theoretical predictions and experimental results. The latter were obtained by image analysis of partially burnt samples of anthracite char ( d 0 = 1,98 and 3.50 mm, 750 < T b < 950 °C) and graphite particles ( d 0 = 1.98 mm, 850 < T b < 950 °C) in a laboratory-scale fluidized bed reactor (inner diameter 5 mm).

The effect of agitation on the char combustion characteristics of large coal particles

To simulate combustion behaviour in a pilot-scale combustor, a laboratory-scale agitated combustor has been developed which allows burnout and combustion rates for large coal particles to be measured. Particle sizes up to 25 mm can be studied, typical of those used in fixed-bed retorts. Char burnout data were obtained to supplement previously published volatiles release data. A power law relationship between char burnout time, t b, and particle size, d p, given by t b = k d d p n is proposed, with k d and n as correlation constants. The data were also used to correlate char combustion rates, r c, for single particles, in the form r c = k r/ d p n′ .

Temperature programmed desorption technique to predict the carbon oxygen reaction

The kinetics of the reaction o2 oxygen with a sucrose char particle size: (88 μ<dp<105 μrn) has been studied using a thermogravimetric analyzer (TGA) and a mass spectrometer (MS) to measure weight change and CO and CO2 formation rates during reaction. Experiments were performed to determine the surface oxide formation rate and to determine the mechanism of CO desorption in the temperature range of 762 K to 851 K and for oxygen pressures of 0.04 to 0.3 atm, respectively. When the reaction rate at 30% conversion was used in the Arrhenius plot, an activation energy of 34±3 kcal/mol was obtained and the CO/CO2 ratio was found to increase with increasing reaction temperature. Analysis of the rate of formation of CO and CO2 shows the activation energy for CO formation is greater than for CO2 formation. Temperature programmed desorption (TPD) studies of the surface oxides were made to provide a better understanding of the carbon oxidation process. The activation energy distribution function for desorption was approximately Gaussian and the average activation energy is 55 Kcal/mol for a preexponential factor of 1013 1/sec. The peak of the energy distribution function shifts to higher activation energies for surface complexes formed at higher reaction temperature.

Determination of coal combustion reactivities by burnout time measurements in a batch fluidized bed

A rapid procedure to obtain combustion rate equations useful in the modelling of AFBC boilers is proposed, based on the technique of burnout time measurements in batch fluidized beds. To improve the reliability of the apparent kinetic parameters, the variation of reaction temperature and Sherwood number as the particle shrinks is considered. To avoid phase-exchange resistance, experimental burnout times are obtained under differential conditions. The surface temperature of the particles is considered in the data analysis as a function of the char particle size and the kinetic rate constant. Valid results can be obtained by a simple experimental technique and assumptions in deriving the kinetics and analysing the data. Some uncertainty in the individual values of the activation energy and the pre-exponential factor (especially for lignites) is observed in the fitting process. Nevertheless, the apparent kinetic parameters obtained for five lignites and two anthracites are satisfactory, and ready to use in a global model of an AFBC boiler, in which overall kinetic information is used to define the rate of shrinkage of the fuel particles.

Kinetic model of biomass gasification

Abstract A kinetic model for biomass gasification is developed based on the mechanism of surface reactions. The apparent rate constants are computed by minimizing the differences between experimental data and theoretical results for different residence times and different temperatures. The kinetic model is validated by comparing experimental data with the theoretical results for different equivalence ratios; the simulations agree well with the experimental data. Simulations modeling the influence of char particle size on the time required to achieve 90% carbon conversion agree with the results of tests performed by other investigators. Simulations are performed to evaluate the effects of the following parameters on biomass gasification: (a) type of oxidant, (b) residence time, (c) char particle size, (d) temperature, (e) pressure, (f) equivalence ratio, and (g) moisture.

Char fragmentation and fly ash formation during pulverized-coal combustion

Experimental measurements of char and fly ash size distributions are reported in the size range from approximately 0.5 to 100 μm for three coals, ranging in rank from high-volatile bituminous coal to lignite. These measurements are coupled with a theoretical model of fly ash formation to determine the extent of char fragmentation as a function of initial char particle size. These data reveal several mechanistic aspects of fragmentation. The extent of fragmentation is strongly dependent or size and coal rank. Bituminous coals may form over 100 fragments per char particle at large initial char particle sizes (above 80 μm) and less than 10 at small initial char particle sizes (less than 20 μm). Lignites fragment less extensively, with the number of fragments per original char particle being less than 5 at all particle sizes. These results partially resolve some apparent discrepancies in published studies of fragmentation.

The nature of strength-controlling structural flaws in formed coke

Curing and carbonizing a raw briquette of char gives a formed coke whose tensile strength varies markedly with the (UK Snibston, rank 902 coal-derived) blend of char and binder used to make the briquette. The strength increases considerably with decrease of (input) char particle size despite extensive crushing during compaction, and appreciably but discontinuously with increase of binder percentage. A previous theoretical picture is extended to show that the char particle size indirectly determines the ‘basic’ size of flaws in the final coke, that this basic size can be reduced by (sufficient) binder under pressure acting proportionately as a ‘flaw-size modifier’ and that hairline fractures arising late during briquetting are the principal source of the flaws in question. Such (Griffith) flaws control the tensile strength and three types are found to operate in sequence as binder percentage is increased. Strength control passes from one flaw type to another, giving an interval of constant strength within an otherwise increasing trend of strength and, being contrary to ‘common sense’, this highlights the accepted view that subtle factors such as flaws control the strength of a brittle solid.

Heterogeneous N 2O and NO formation from bound nitrogen atoms during coal char combustion

Using a fixed bed, batch combustion method, heterogeneous kinetics of NO and N2O formation from char-nitrogen have been investigated on highly devolatilized coal chars, in the temperature range between 500 and 1300 Kelvin. Using a conventional steady state technique (adsorption/desorption equilibrium conditions), the heterogeneous NO and N2O reduction on bound carbon atoms, have been determined. Both N2O and NO are mainly obtained as desorption products from oxidized (−CN) sites; only a few percent of the released nitrous oxide is produced by reactions involving hydrocyanic acid as a reactant. The fractions of char-nitrogen converted into NO and N2O during char oxidation, are roughly proportional to the degree of char burnout, showing the charabound carbon and nitrogen atoms to be oxidized in proportions almost identical to their respective concentrations in the char. Due to simultaneously occuring heterogeneous reduction of NO and N2O onbound carbon atoms, the final fraction of char-nitrogen converted into NO and N2O, is smaller than one (0.3 to 0.7, depending on char type, oxygen concentration and particle size). The rate constants of the different adsorption and desorption reactions have been determined in the transient state, created by the sudden suppression of the oxygen in the gaseous reactants. The small values of the oxygen covered sites fractions, as well as the first order with respect to the gaseous reactants, indicate that the overall NO and N2O formation is controlled by oxygen adsorption; below 800 K, this adsorption evolves in the kinetic regime; at higher temperature it appears to be controlled by pore diffusivity.

Kinetics of NO reduction by anthracite char in a fluidized bed reactor

This paper describes research work carried out to investigate the kinetics of the reduction of NO by anthracite char in the temperature range 600–930 °C, in a fluidized bed reactor. The experiments were carried out in a 5 cm diameter quartz reactor, using 150 μm mean Sauter diameter silica particles as the fluidizing medium. The conversion of NO was measured at various temperatures, char particle sizes and char concentrations in the bed. The kinetic data were calculated using the conventional two-phase theory of fluidization, considering the CSTR/PFR and PFR/PFR reactor models; the limiting cases of single phase CSTR and PFR reactors were also included for comparison.

Some theoretical aspects of mixing in the British coal twin-bed combustor/pyrolyser

British Coal, in collaboration with Senior Green Ltd., has incorporated a novel twin-bed coal-fired combustor into a 2.8 MWt water tube boiler. The firing system comprises two interconnected shallow fluidized beds operated as a pyrolyser and a char combustor, respectively. Gas temperatures well in excess of bed temperatures are achieved when the hot gas streams from the two beds are mixed in a common chamber. Particular attributes of the system are high combustion intensity (without in-bed tubes), inherently low NOx emission levels and wide turndown range (up to 10: 1). As part of the ongoing R&D programme, mathematical modelling techniques are being applied to provide a sound theoretical understanding of the physical and chemical processes occurring within the system. In order to assess the factors affecting scale-up of the twin-bed unit, Gloucestershire College of Arts and Technology (GLOSCAT), in collaboration with the Coal Research Establishment of British Coal, is currently developing a comprehensive mathematical model of the system. The paper describes one aspect of the system model, namely solids mixing. The application of simple analytic methods has been found useful to describe the process. Assuming uniform conditions in the vertical plane, mixing of coal and sand within a bed has been regarded as a diffusion process, and a system of ordinary differential equations have been established to predict carbon concentration and temperature profiles for each bed. Although a number of simplifying assumptions have been made, the models provide useful sensitivity analyses, allowing an assessment of the factors influencing scale-up, e.g. the effect of mixing rate and char particle size on temperature profiles. Analytic solutions are presented, and the sensitivity analyses are illustrated using standard computer graphics.

Kinetic Model for Simulating the Evolution of Particle Size Distributions During Char Combustion

Abstract The details of a numerical simulation of particle size distribution evolution during ash-free char oxidation are presented. This work represents the first application of the population balance modeling approach to char combustion, and includes the effects of both oxidation and fragmentation. The simulation consists essentially of a transfer function 'hat determines the evolution of the entire char particle size distribution with lime. Determination of the input parameters to the model, and the sensitivity of the results to variations in these parameters are discussed.

Physical properties and oxidation rates of chars from three bituminous coals

Intrinsic oxidation rates of coal chars derived from three different bituminous coals were measured at 500 °C and the effects of char formation temperature, conversion, coal particle size and char particle size on the rates were evaluated. Characterization of the various samples including BET surface areas, mercury porosimetry, mercury and helium densities, heating values and in some cases elemental analyses were carried out to better understand the roles and interactions of the various parameters. Optical microscopy observations were also made to verify assumptions wherever possible. The results show that apparent and intrinsic rates as well as the heating values are reduced with increasing char formation temperature. N 2-BET surface areas increase an order of magnitude with conversion and exhibit maxima in the range of temperatures considered (1000 K to 1600 K). Of similar sized chars derived from different sizes of coal particles, those from the smaller coal size fractions had higher apparent reaction rates.