Devolatilization Rate Research Articles

Modelling of sulfur retention in circulating fluidized bed combustors

The effects of operating conditions (linear gas velocity, Ca S molar ratio, type of coal and limestone, etc.) on sulfur retention efficiency in a circulating fluidized bed combustor were studied. The operating conditions affect the sulfur retention differently. To explain this, a mathematical model was developed. To determine axial voidage profiles, the model uses a modified exponential decay hydrodynamic model, which divides the bed into a dense region at the bottom of the bed and a dilute region above. In the dilute region a core-annulus structure with solid dispersion from core to annulus is considered. The SO 2 retention rate is considered to depend on the bed height through the SO 2 profiles generated by the coal devolatilization and char combustion rates, which change with the bed height. The model gives good predictions of the effect of the operating variables on sulfur retention in a CFB pilot plant of 20 cm i.d. and 6.5 m height, burning two different lignites and an anthracite with four types of limestone.

Kinetics Rate of Devolatilization of Pulverized Coal Ignited by Pulsed Laser.

Combustion behaviors of pulverized coal particles suspended in a laminar upwind flow. A single pulsed YAG laser was used to heat coal particles at heating rates of l05-106 K/s. The rate constant of devolatilization of the bituminous coal was obtained by observing the light emission from the volatile matter combustion flame in a temperature range between l000 and 1200 K. The light intensity profiles of coal particles had two peaks and that of activated carbon showed only one peak. This indicated that two types of reaction proceeded in the devolatilization process of coal particles. The rate constant of devolatilization determined in this study agreed with the higher value among the published values determined using furnaces designed to yield heating rates of lO4-1O5 K/s.

95/04166 Flammable refrigerants — practical experience of these substances based on the example of propane

Coal is an important energy resource for meeting the future demand for electricity, as coal reserves are much more abundant than those of other fossil fuels. In this study, the percolation model, which can account for swelling due to devolatilization and ash agglomeration, is applied to particulate matter formation process in coal combustion, and the effects of coal properties, ambient temperature, ambient pressure and initial coal size on the characteristics of a burning coal particle are studied. The devolatilization rate of coal is given by the first-order reaction model with FLASHCHAIN® model [Niksa, S., Combust. Flame, 100, (1995) 384–394.]. The characteristics of a burning coal particle are investigated under the atmospheric and high pressure conditions. The results show that in the atmospheric pressure condition, the characteristics of the burning coal particle obtained by the percolation model are in general agreement with the experimental data. The particle diameter of Newlands coal with higher fuel ratio and ash content is larger than that of Plateau coal in the char-combustion-dominant process. As the ambient temperature increases, the particle diameter becomes small in the early stage of the char-combustion-dominant process, but becomes large afterward. The porosity in the char-combustion-dominant process decreases with decreasing the initial coal size. It is also observed that the effect of ambient pressure is prominent in the char-combustion-dominant process. The particle diameter and porosity in the pressurized condition are greater than those in the atmospheric pressure condition. These behaviors can be explained by the interaction between char reaction and ash agglomeration.

Hydrocarbon composition of sapropelite hydrogenates

Platinum filament pyrolyzers achieve very high temperature and heating rate and can provide useful parameters for practical applications in combustion, pyrolysis and gasification processes. The critical use of an experimental instrument is necessary to provide reliable data. In this work, a commercial pyrolyzer (CDS Pyroprobe 2000) is characterized to obtain a correspondence between the nominal and the effective operating conditions. This is the basis for the modeling estimation of the effective thermal history of the sample during each experimental run. The experimental results obtained performing the devolatilization of coals, biomass and waste fuels using the pyrolyzer are compared with those obtained in a conventional thermogravimetric balance, to evaluate the effects of extremely different operating conditions. The amount of volatile released programming the most severe thermal conditions using the pyrolyzer (thus in conditions more similar to large-scale plants) differs significantly from that of thermogravimetric runs. Global kinetics are obtained fitting the experimental results and using the thermal history of the sample from the model results. They depend strongly on the conditions used for the devolatilization. Global kinetics obtained in the thermogravimetric balance runs (low heating rate) overestimate the rate of devolatilization in the pyrolyzer (high heating rate).

Speciation of the inorganic components in brown coal

Coal is an important energy resource for meeting the future demand for electricity, as coal reserves are much more abundant than those of other fossil fuels. In this study, the percolation model, which can account for swelling due to devolatilization and ash agglomeration, is applied to particulate matter formation process in coal combustion, and the effects of coal properties, ambient temperature, ambient pressure and initial coal size on the characteristics of a burning coal particle are studied. The devolatilization rate of coal is given by the first-order reaction model with FLASHCHAIN® model [Niksa, S., Combust. Flame, 100, (1995) 384–394.]. The characteristics of a burning coal particle are investigated under the atmospheric and high pressure conditions. The results show that in the atmospheric pressure condition, the characteristics of the burning coal particle obtained by the percolation model are in general agreement with the experimental data. The particle diameter of Newlands coal with higher fuel ratio and ash content is larger than that of Plateau coal in the char-combustion-dominant process. As the ambient temperature increases, the particle diameter becomes small in the early stage of the char-combustion-dominant process, but becomes large afterward. The porosity in the char-combustion-dominant process decreases with decreasing the initial coal size. It is also observed that the effect of ambient pressure is prominent in the char-combustion-dominant process. The particle diameter and porosity in the pressurized condition are greater than those in the atmospheric pressure condition. These behaviors can be explained by the interaction between char reaction and ash agglomeration.

The influence of burner injection mode on pulverized coal and biomass co-fired flames

A new dual fuel burner designed for the co-firing of waste-derived solid fuels (e.g., biomass, refused-derived fuel, sewage sludge) with pulverized coal in practical combustors was evaluated through trials undertaken in a 0.5 MW down-fired furnace. A new mathematical procedure was also constructed that accounts for multimode combustion of these fuels. It includes the influence of the heating and devolatilization rates of each fuel on the effective stoichiometry of the volatiles in the combustion domain depending on their respective particle trajectories. Results included for sawdust-coal flames, show the sognificant effect of co-firing ratio and fuel injection mode on flame ignition, combustion aerodynamics, and nitric oxide emissions. Predicted indices of the coal devolatilization rate along the particle trajectories emphasize the influence of the faster devolatilization and ignition of the sawdust on coal combustion in the near burner region. When the sawdust particles are injected through the center of the burner, surrounded by an annular coal jet, they immediately ignite thereby enhancing the combustion intensity of the coal within the internal recirculation zone. This injection mode leads to a subsequent reduction in the nitric oxide formation along with a higher combustion efficiency as compared with a flame where the sawdust and coal injection positions are reversed. An optimum co-firing ratio in which the sawdust provided 30% of the total heat input was found to exhibit the maximum particle burnout and minimum nitric oxide emissions. Co-firing results obtained for a lower reactivity and higher nitrogen content fuel (pulverized sewage sludge) as compared with sawdust, show that the fuel injection mode had a marginal effect on burnout and NO emissions. The sawdust and sewage sludge co-firing results emphasize the need to consider both the reactivity and nitrogen content of the fuel prior to selecting an injection mode.

Modeling of coal pyrolsis kinetics

AbstractFour different coals were instantaneously introduced into a laboratory‐scale pyrolysis chamber at various temperatures, and their develatilization profiles were analyzed with respect to kinetics. The experimental setup consisted of a gas chromatograph (GC) with a pyrolysis unit, an empty column, and a detector. Detector signals were digitalized and stored for further handling. Normally, 1,024 values were sampled at 250 Hz. For deconvolving the true instantaneous devolatilization from the observed detector response, the residence time distribution was compensated for.An empirical model approach is developed to correlate the total devolatilization rates when instantaneously introducing a particle in a constant temperature environment. The model is also extended to pyrolysis application.

Heat transfer and kinetics of lignite devolatilization in wire-mesh apparatus

A model of lignite devolatilization in a wire-mesh apparatus is proposed in this paper. The model couples the heat transfer between grid and particle with a multiple-reaction kinetic model. The kinetic parameters are derived from the experiments, in which two Chinese lignites are devolatized. The model predictions of heating-rate and particle-size dependence of particle temperature, devolatilization rate and weight loss are presented. An index is suggested to assess the cumulative effect of heat transfer limitation on devolatilization. The model reveals that under a typical condition (particle size 50 μm, heating rate < 1000°Cs −1), the heat transfer limitation could be negligible. However, with further increasing particle size and heating rate, the heat transfer limitation seems to become somewhat important.

Initial stages of the devolatilization of pulverized-coal in a turbulent jet

The initial stages of the devolatilization of three United Kingdom pulverized coals have been studied in a coal-laden turbulent air jet emerging into the combustion gas environment produced by a methane-air flat flame burner. The temperature range provided by these flames was between 1670 and 1870 K and the corresponding particle heating rate was about 10 5 K/s. Data are reported for major gas species concentrations, particle weight losses, and nitrogen released, which quantify the effects of gas temperature, particle size, and coal type. Overall, the results show that the rate of devolatilization increases with temperature, significantly decreases with particle size and that the composition of the volatiles released is a strong function of the structure of the parent coal. The fuel nitrogen release is correlated to the total volatile yield. The formation of fuel-NO depends on the extent of the nitrogen release and the coal particle combustion mechanism. The turbulence intensity has an important effect on the extent of devolatilization; hence the total weight losses detected in the present experiment exceed those found under nonturbulent conditions.

Structure of laminar premixed carbonmethane-air flames and ultrafine coal combustion

Premixtures of graphite-methane-air have been burned in a flat laminar flame at a pressure of 0.16 atm. A fluidized bed was employed to entrain graphite particles of 4 μm diameter. The extended reaction zone enabled reliable measurements to be obtained of species concentrations, gas velocities, and temperatures. The measurements to a large degree validated the predictions of a one-dimensional mathematical model of the combustion. The model couples detailed carbon particle and gas-phase chemical kinetics, as well as radiative energy exchanges. Some limitations arise because the experimental flame is not truly one dimensional and also because of uncertainties in the chemical kinetics of the richer mixtures. Allowance for radiation improves the accuracy of the predictions, although its influence is not great. Earlier predictions that active radicals in the gas-phase catalyze the char oxidation are supported. The general validity of the model is the springboard for a related model for ultrafine coal combustion. This retains the coupled kinetics and radiation, but adds a coal devolatilization rate to the kinetic scheme and assumes the volatiles to consist entirely of methane, which mixes rapidly with the surrounding gas. Despite these restrictive assumptions, the model reveals important aspects of coal flame combustion and predicts a laminar burning velocity close to that measured.

Global rates of devolatilization for various coal types

This modeling study reports global rate expressions for transient weight loss and yields of tar and gas for coals across the rank spectrum. A novel kinetic analysis demonstrates that the Distributed Activation Energy Model (DAEM) reliability correlates transient weight loss at various heating rates, and makes reasonably accurate extrapolations for pulverized coal (p.c.) firing conditions. This performance motivates solutions of the DAEM for arbitrary thermal histories, including approximate closed-form solutions for transient weight loss along linear temperature ramps and exponential temperature histories. Our analysis also rigorously defines the rate constants in single first order reactions that give the same devolatilization rate as the DAEM at every instant in any thermal history. For devolatilization during heatup at constant rates, frequency factors increase by a factor of 6 for every order-of-magnitude increase in heating rate, while activation energies remain constant. Throughout convective heating, however, both rate parameters decrease continuously, so that straight lines on Arrhenius diagrams do not represent rate constants for coal devolatilization. Rate variations for different coal ranks are modest by comparison. For ranks from lignite to high-volatile bituminous, rates decrease by up to a factor of 3. Low volatility coals are much more resistant to thermal decomposition; they devolatilize at rates up to 5 times slower than all other lower ranks. Correlations for different coal types from FLASH2, a more detailed devolatilization model, describe separate gas and tar evolution rates, and replace hypothetical ultimate yield parameters by accurate predictions for ultimate tar and gas yields.

The prediction of pulverized greek lignite combustion in axisymmetric furnaces

A general numerical method for calculating pulverized coal combustion in axisymmetric geometries is utilized, aiming at the optimization of combustion model constants for Greek lignite. The two-equation, “ k-ϵ” turbulence model is used for the gas phase, whereas a stochastic approach, based on the lagrangian technique, is used for the particulate phase. The method employs a “constant devolatilization rate” model for both the volatiles and the moisture of the fuel, a char combustion model, a relation of the “eddy-dissipation” family for the reaction rate of the gaseous mixture, and the “nonequilibrium diffusion” radiation model. The maximum permissible rate of devolatilization and the preexponential factor of the char oxidation reaction rate are optimized in order to match available experimental data for Greek lignite; these parameters were found to influence considerably, albeit in a different fashion, the combustion process in cylindrical furnaces operating on pulverized lignite. Finally, the liftoff characteristics of the lignite flame were also predicted using a temperature criterions, as well as a flame stability model.

Coal Combustion in the Raceway and Tuyere of a Blast Furnace.

To achieve a pulverized coal rate of more than 200 kg/t, a technology is needed by which the combustion zone in front of tuyeres can be properly controlled in terms of the combustibility of pulverized coal. It is, especially, necessary to quantify the combustion behavior inside the tuyeres and raceway. Experiments were conducted on off-line models with and without coke to investigate the pyrolysis and combustion behavior of pulverized coal in order to analyze the influence of blast conditions and coal properties on their combustibility.(1) The final volatile yield of pulverized coal increases with the rise in blast temperature, and rise in O2 content accelerates the devolatilization rate.(2) With the progress of pyrolysis, the pulverized coal became spherical and a large number of pores formed in the particles. Thus the reactivity of char is estimated to be higher than that of coke.(3) Rising blast temperature is effective to improve the combustion efficiency of pulverized coal in the raceway, presumably a little effective to improve with the addition of steam. O2 enrichment at constant air ratio requires enhancement of the gas and particle mixture.(4) Q-factor that determined the final volatile yield can be shown by the blast temperature and C content in coal, and devolatilization rate parameters can also be learned by C content in coal. A practical equation representing the devolatilization rate inside tuyeres with various kinds of coal are presented.

Volatiles Release Rates and Temperatures during Large Particle Refuse Derived Fuel—Municipal Solid Waste Devolatilization

Abstract Practices to minimize pollutants from municipal solid waste combustion devices include preparation steps to provide relatively uniform feed particles. This is the rationale behind commercially available densified refuse-derived fuel (d-RDF). However the solid phase devolatilization rate, the composition and rate of production of reaction products and pollutants, and the influence of waste feed properties on these processes have not been quantified. Devolatilization experiments were performed on single, well-characterized d-RDF particles under carefully controlled conditions of practical importance. Intrapar-ticle temperature histories, gas evolution rates and compositions were measured. The results presented here quantify the simultaneous effects of both initial moisture and particle heating rate on the intraparticle temperature field which controls the extent of local reaction. It appears moisture does not strongly alter the overall production of both gases and condensibles (tars) from the devol...

Model predictions of the extent of heterogeneous coal combustion during rapid evolution of volatiles

Two relatively simple, single-particle coal combustion models are developed and used to predict the extent of heterogeneous combustion during rapid devolatilization. The first model uses the flame sheet approximation while the second employs finite gas-phase reaction rates. Both models predict that small particles (< 10 μm diameter) burn heterogeneously and large particles (> 100 μm diameter) burn homogeneously, but heterogeneous mass loss is severely underestimated by the simpler flame-sheet model in comparison to the more realistic finite-rate-chemistry model for intermediate sized particles typical of a utility grind. Small particle size, high oxygen concentration and low devolatilization rate, in that order of importance, yield more extensive heterogeneous combustion, while temperature, volatiles composition, gas-phase kinetics, and surface reaction rates have negligible impact.

Devolatilization of large coal particles under fluidized bed conditions

The rate of devolatilization of various sizes of coal particles is an important parameter in the design and interpretation of the performance of fluidized bed combustors. Combined with data on char combustion kinetics, bed mixing and residence times of the gaseous and particulate phases, devolatilization data are a primary input to combustion models. It is proposed here that the phenomenon of volatiles release from a coal can be represented by a single reaction rate equation of the nth order Arrhenius type: dV dt = k (V ∞ − V) n , where n is the order of reaction, not zero, k is the devolatilization rate constant, V is the fractional amount of volatiles evolved to time t, and V ∞ is the fractional amount of volatiles evolved as t → ∞. A group of nine coals ranging in type from semianthracite to subbituminous, having particle sizes from 1.4 to 29 mm were devolatilized under simulated fluidized bed combustion conditions. Order of reaction and devolatilization rate constants were calculated for each coal type and particle size combination and a relationship between rate and size determined. The order of the devolatilization reaction was constant for each coal over the size range examined, ranging between 0.4 and 0.6 for the various coals. A relationship to estimate total devolatilization time of various sizes of coal particle was derived from the nth order rate expression and shown to be of similar form to a previously published semi-empirical relationship, τ = K d N . The rate relationships are used to estimate the coal particle size below which substantial freeboard volatiles will occur.

The effect of oxidation on the thermoplastic and coking properties of coal at elevated pressures

The effects of oxidation on coal thermoplastic and coke properties have been investigated using a variety of techniques. The methods used include thermogravimetric analysis, high pressure dilatometry, constant shear rate plastometry and optical anisotropy. The apparent effects of oxidation are strongly influenced by the temperature and time of oxidation as well as the experimental conditions under which the coal carbonization behaviour is studied. Oxidation decreases swelling, the maximum rate of volatile evolution and optical anisotropy except under very mild conditions where usually little or no effect was observed. Oxidation can cause the shape of the dilatation versus pressure curves to change dramatically. This may result in the dilatation of an oxidized coal being higher at elevated pressure than at atmospheric pressure in contrast to the results for the fresh coal. However, under the same experimental conditions, the dilatation of the oxidized coal is either identical within experimental error or less than that of the original fresh coal. Comparison of coals with varying extents of oxidation has shown that an increase in the carbonization pressure tends to decrease the apparent effect of oxidation on the coal swelling and coke optical anisotropy index. There are good correlations between the changes in the swelling, maximum rate of devolatilization and coke optical anisotropy parameters, resulting from oxidation indicating the relationship between these concurrent processes during carbonization. The perturbation of the structure of a coal by oxidation in conjunction with the investigation of the dependence of caking, swelling, devolatilization and coke structure factors on experimental conditions provides a clear insight into the development of coal thermoplasticity and coke structure during carbonization and gasification.

Pyrolysis and volatile combustion of a single large lignite particle

A model for the evolution and combustion of volatiles from a single large lignite particle in a hot oxidizing gas stream is proposed. The analysis is divided into preignition and postignition stages. The transfer of heat both to and through the particle, as well as multireaction pyrolysis kinetics, are considered in order to find the overall devolatilization rate. The model also includes allowance for the effects of volatile species on the flame temperature. Predictions of transient weight, ignition and extinction time delay for a flame supported by the volatiles are in good agreement with the experiments.

Spouted and spout-fluid bed combustors 1: Devolatilization and combustion of coal volatiles

The devolatilization and volatile combustion of a single coal particle in spouted and spout-fluid beds have been studied. The results showed that the flame extinction time increases with the particle diameter, and decreases with the bed temperature. When the bed temperature and the air flow rate were fixed, the operation modes (spouted or spout-fluid bed) showed less effect on the mean flame extinction time. A mathematical model of the spouted bed mode for preignition and postignition periods has also been developed assuming the devolatilization rate to be controlled by heat transfer and multireaction pyrolysis kinetics based on volatile products. Ignition, heat transfer back from the volatile flame to the particle surface, variation in flame temperature, and the hydrodynamics of SB are taken into account. The model predictions, with some adjusting parameters, were in good agreement with experimental results.

Measurements and analysis of temperature histories and size changes for single carbon and coal particles during the early stages of heating and devolatilization

A novel system is described for monitoring rapid changes in particle size and temperature during coal devolatilization at heating rates representative of high-intensity combustion environments. The system incorporates an electrodynamic balance and a pulsed radiation source to isolate and rapidly heat individual particles. A high-speed two-dimensional photodiode array and a single wavelength radiation pyrometer are applied to record changes in particle size and temperature. Dynamics of volatile evolution and particle swelling are also recorded using high-speed cinematography. Measurements of temperature and size changes are reported for carbon spheres and a HVA bituminous coal at heating rates on the order of 10 5 K/s. Measured temperature histories are compared with theoretical estimates of the temperature response of radiatively heated coal and carbon particles. The analysis considers the development of the transient temperature distribution inside the particles by numerical solution of the Fourier equation. Measurements and model predictions for 135-μm-diameter carbon spheres were in excellent agreement using property data correlations commonly applied in modeling coal devolatilization and combustion behavior. Model predictions for coal particles, however, significantly underestimated (on the order of 50%) the observed heating rates for 115-μm-diameter coal particles using the same property correlations. It is concluded that coal particles heat significantly faster than is predicted using commonly employed approaches to model heat transfer with assumptions routinely applied to coal. Potential reasons for this may include inadequate understanding of relevant coal thermodynamic and heat transfer properties as well as failure to account for particle shape factors. Heat transfer analyses employing spherical particle assumptions and commonly used coal property correlations can lead to large errors in predicted temperature histories and associated devolatilization rates.