Char Combustion Kinetics Research Articles

외부 및 내부 확산을 적용한 인도네시아 석탄촤의 연소 반응율 분석

본 연구의 목적은 국내 화력발전소에서 사용중인 인도네시아 석탄촤의 연소반응율을 연구하는 것이다. 석탄촤의 반응율은 External, Internal effectiveness factor를 고려하여 입자 내부 및 외부확산을 정량적으로 정리하였으며, Random pore model을 사용하여 탄소변환율에 따른 입자내부비표면적의 변화를 반영하였다. 가열 및 측정이 가능한 WHR(Wire Heating Reactor)를 활용하여 반응시간을 측정함으로써 석탄촤의 반응율을 도출하였고 BET(Brunauer-Emmett-Teller) 및 TGA(Thermo-Gravimetric Analysis) 장비를 활용하여 석탄촤의 물리적인 특성인 내부비표면적과 Random pore model의 구조변수(<TEX>${\Psi}$</TEX>)를 측정하였다. 석탄 종에 따른 활성화에너지 및 빈도인자를 도출하기 위해 아역청탄인 BARAMULTI, ENERGYMAN, AGM탄을 사용하였다. 본 연구 결과에서 External, Internal effectiveness factors를 통해 확산에 따른 kinetics를 비교한 결과 외부 확산 보다 내부 확산의 영향이 지배적임을 확인하였다. 최종적으로 내부 및 외부 확산에 대한 영향을 고려한 3종의 석탄촤 Intrinsic kinetics의 활성화에너지는 110~118 kJ/mol의 값을 보였다. The experiment was designed to compare the char combustion kinetics of pulverized Indonesia coals commonly utilized in Korea power plants. The reaction rate of coal char has been formulated using the external and internal effectiveness factors to describe the diffusion effect quantitatively. The Random Pore Model (RPM) was used for applying internal specific surface area as a function of carbon conversion ratio. Reaction rate was obtained from reaction time using the Wire Heating Reactor (WHR) which can heat and measure the char particle temperature at the same time. BET and TGA were used to obtain physical properties such as internal specific surface area and structural parameter. Three kinds of Indonesia Sub-bituminous coals "BARAMULTI, ENERGYMAN, AGM" were used in order to derive the activation energy and pre-exponential factor. The results of this study showed that the effect of internal diffusion than that of external diffusion is the dominant as comparison of kinetics was reflected in external and internal effectiveness factors. For three kinds of coal char, finally, activation energy of intrinsic kinetics indicates 110~118 kJ/mol.

Intrinsic reaction kinetics of coal char combustion by direct measurement of ignition temperature

A wire heating reactor that can use a synchronized experimental method was developed to obtain the intrinsic kinetics of large coal char particles ranging in size from 0.4 to 1 mm. This synchronization system consists of three parts: a thermocouple wire for both heating and direct measurement of the particle temperature, a photodetector sensor for determining ignition/burnout points by measuring the intensity of luminous emission from burning particles, and a high-speed camera–long-distance microscope for observing and recording the movement of luminous zone directly. Coal char ignition was found to begin at a spot on the particle's external surface and then moved across the entire particle. Moreover, the ignition point determined according to the minimum of dT/dt is a spot point and not a full growth point. The ignition temperature of the spot point rises as the particle diameter increases. A spot ignition model, which describes the ignition in terms of the internal conduction and external/internal oxygen diffusion, was then developed to evaluate the intrinsic kinetics and predict the ignition temperature of the coal char. Internal conduction was found to be important in large coal char particles because its effect becomes greater than that of oxygen diffusion as the particle diameter increases. In addition, the intrinsic kinetics of coal char obtained from the spot ignition model for two types of coal does not differ significantly from the results of previous investigators.

Char Structure and Combustion Kinetics of a Phenolic-Impregnated Honeycomb Material

The combustion behavior of a honeycomb material used for the core in sandwich panels, consisting of aramid fiber sheets (Nomex) dipped in a phenolic resin, is studied. Char, produced from pyrolysis of thick samples at 950 K, preserves the honeycomb structure, but the total volume is highly reduced in spite of an increase in the wall thickness. Following softening, the more internal fibers lose their shape, giving rise to a highly porous swelled char. Instead, the more external ones, trapped by the resin char, still show their original shape but with evident shrinkage. Thermogravimetric curves in air are used to formulate a combustion mechanism. It consists of three reactions for the stage of oxidative decomposition of the honeycomb material and one reaction for combustion of the resulting char. Activation energies for the various steps vary in a narrow range of high values (190–256 kJ/mol), testifying low reactivity.

Combustion Kinetics of Secondary Biomass Chars in the Kinetic Regime

In this study, the oxidation of secondary biomass chars, generated from evaporation/decomposition of four bio-oil samples (BTG, Dynamotive, Ensyn, Pyrovac), is investigated under a kinetic control....

A Comparison of Combustion of Coal Chars in O2/CO2 and O2/N2 Mixtures - Isothermal TGA Studies

Two chars prepared from parent coals of a high volatile bituminous coal and an anthracite coal were subjected to isothermal combustion tests in a thermogravimetric analyser (TGA). The chars were burned in mixtures of O2/CO2 and O2/N2 with O2 concentrations of 3%, 6%, 10%, 21% and 30% and combustion temperature ranging from 723 K to 1163 K. Experimental results show that replacing the inert nitrogen gas in the oxidizer with CO2 has little influence on the measured reactivity of coal chars under conditions of the experiments. The measured reactivity of each char was found to be approximately first order (0.88 – 0.90) to the concentration of O2 in the O2/CO2 mixtures. As char combustion tests were conducted under relatively high temperatures conditions, internal and external effectiveness factors were estimated in order to derive combustion kinetics of both chars. The values of activation energy for both chars, obtained by taking account of both the internal and external effectiveness factors (i.e. with correction), were very much in line with the literature data. The activation energy of the bituminous coal char combustion in O2/CO2 was found to be 96323 (without correction) – 108893 (with correction) kJ/kmol within the temperature range of 723 – 873 K, whereas the activation energy of the anthracite char was found to be 125716 (without correction) – 140486 (with correction) kJ/kmol within the temperature range of 773 – 973 K.

Characterization and Combustion Kinetics of Chars Obtained from Loquat Stones

This paper describes the production of chars from loquat stones in a tube furnace at carbonization temperatures of 600°C, 700°C, and 800°C in a nitrogen flow at a heating rate of 10°C/min. The physical and chemical structures of the stones and chars were investigated by FTIR spectrometer, X-ray diffractometer, and SEM, as well as the combustion reaction and kinetic parameters based on the TG-DTG graphs obtained by thermogravimetric analysis. The combustion reaction was carried out at a heating rate 10°C/min and an oxygen flow rate of 50 ml/min up to 700°C. The kinetic parameters were calculated using the methods by Arrhenius; Coats and Redfern; Horowitz and Metzger; and Ingraham and Marier.

A single particle model of the fluidized bed combustion of a char particle with a coherent ash skeleton: Application to granulated sewage sludge

A single particle model of the fluidized bed combustion and of the parallel course of attrition of a fuel particle characterized by a coherent ash skeleton is presented. The model combines a shrinking core feature to represent carbon combustion with a shrinking particle feature to represent attrition of the carbon-depleted ash layer. Moreover, the establishment of non-uniform temperature profiles within the particle as a consequence of the resistance exerted to thermal conduction from the unreacted core across the ash layer is taken into account. The inherent stiffness of the model has been dealt with by developing a solution procedure that proved to be efficient and not time-consuming. The model has been applied to the description of the fluidized bed combustion of pelletized sewage sludge. Experiments consisting of the fluidized bed combustion of single pellets in a bench scale reactor have been purposely carried out with the aim of determining the values of selected model parameters and of enabling model validation. The experiments confirmed the relevance of the formation of a coherent ash skeleton to combustion and attrition. In particular, it appeared that attrition departs significantly from the combustion-assisted attrition pattern typical of fuels characterized by incoherent ash. Model results highlight the role of the coherent ash skeleton to the establishment of oxygen concentration and temperature fields within the particle. The interplay of the different processes that contribute to the apparent kinetics of char combustion, namely boundary layer and intraparticle diffusion of oxygen and heterogeneous reaction between oxygen and carbon at the core surface, is analyzed and discussed. The relevance of model variables to the time–temperature history of the particle is assessed.

Combustion kinetics of coal chars in oxygen-enriched environments

Oxygen-enhanced and oxygen-fired pulverized coal combustion is actively being investigated to achieve emission reductions and reductions in flue gas cleanup costs, as well as for coal-bed methane and enhanced oil recovery applications. To fully understand the results of pilot scale tests and to accurately predict scale-up performance through CFD modeling, accurate rate expressions are needed to describe coal char combustion under these unconventional combustion conditions. In the work reported here, the combustion rates of two pulverized coal chars have been measured in both conventional and oxygen-enriched atmospheres. A combustion-driven entrained flow reactor equipped with an optical particle-sizing pyrometry diagnostic and a rapid-quench sampling probe has been used for this investigation. Highvale subbituminous coal and a high-volatile eastern United States bituminous coal have been investigated, over oxygen concentrations ranging from 6 to 36 mol% and gas temperatures of 1320–1800 K. The results from these experiments demonstrate that pulverized coal char particles burn under increasing kinetic control in elevated oxygen environments, despite their higher burning rates in these environments. Empirical fits to the data have been successfully performed over the entire range of oxygen concentrations using a single-film oxidation model. Both a simple nth-order Arrhenius expression and an nth-order Langmuir–Hinshelwood kinetic equation provide good fits to the data. Local fits of the nth-order Arrhenius expression to the oxygen-enriched and oxygen-depleted data produce lower residuals in comparison to fits of the entire dataset. These fits demonstrate that the apparent reaction order varies from 0.1 under near-diffusion-limit oxygen-depleted conditions to 0.5 under oxygen-enriched conditions. Burnout predictions show good agreement with measurements. Predicted char particle temperatures tend to be low for combustion in oxygen-depleted environments.

Global Kinetics of Wood Char Devolatilization and Combustion

Thermogravimetric curves in air of wood chars obtained from different species and conventional or fast pyrolysis show a low-temperature shoulder (devolatilization) followed by a high-temperature peak (combustion). A n-order global reaction provides a very poor description of the differential curves and, in agreement with previous literature, requires relatively low activation energies (114.5 and 167 kJ/mol for conventional and fast pyrolysis chars, respectively). The combination with an additional first-order reaction for the devolatilization stage produces accurate predictions of both integral and differential curves. It also gives rise to much higher activation energies of the combustion reaction (183 or 229 kJ/mol, depending on the severity of the pyrolysis conditions). The activation energy of the devolatilization reaction also increases with the severity of the pyrolysis conditions (from 114.5 to 218.5 kJ/mol), whereas the differences in the char reactivity deriving from the wood species can be taken into account by preexponential factors and order of the combustion reaction. The use of two first-order reactions for the devolatilization stage does not improve significantly the accuracy of the predictions.

A Thermogravimetric Study of Nonfossil Solid Fuels. 2. Oxidative Pyrolysis and Char Combustion

The study addresses the competition between the course of purely pyrolytic processes and heterogeneous oxidation during oxidative pyrolysis of nonconventional fuels. This feature is one key, together with volatile matter flammability and effectiveness of oxygen transport to the particle, to establish whether flaming or glowing ignition of the fuel takes place. A selection of six nonconventional high volatile solid fuels is considered in the study, namely two plastics (polyethylene and poly(ethylene terephthalate)), two lignocellulosic materials (Robinia Pseudoacacia and waste wood), and two rubber-derived materials (scrap tires and ebonite). The analysis is based on the comparison of the behavior of fuels when heated in a thermogravimetric apparatus under inert and oxidizing atmospheres. Further analysis is directed to assess the heterogeneous combustion kinetics of chars obtained by pre-pyrolyzing in inert conditions samples of the fuels. The study is complementary to a companion paper (Senneca, O.; Chirone, R.; Masi, S.; Salatino, P. A Energy Fuels 2002, 16, 653) where pyrolysis in nitrogen of the same fuels has been addressed. Results indicate that heterogeneous oxidation and pyrolytic processes play different roles depending on the nature of the fuel. A general feature of all fuels tested is that burnoff cannot be simply described as sequential reaction paths corresponding to purely thermal degradation and heterogeneous oxidation. Synergistic effects between these processes are significant and need to be taken into account.

The prediction of char combustion kinetics at high temperature

The prediction of char combustion kinetics at high temperature

The prediction of char combustion kinetics at high temperature

Abstract The optimisation of numerical methods applied to the description of char combustion behaviour is required in order to achieve a good prediction of the amount of unburned carbon in fly ash from pulverised coal fired boilers. A question that arises is whether it is necessary to take into account the distribution of the coal properties and reactivity and how to describe them in such a simplified way in order to be handled in a simulation code. The reliability of a simple model for char heterogeneous oxidation describing the burning of single char particles has been assessed through the use of available experimental data for a high volatile bituminous coal in a pilot plant, under single stage and staged combustion conditions. Experiments in an electrodynamic chamber effected on the char particles collected in the pilot plant, showed the heterogeneity of particles properties, evaluated by fitting the temperature-time histories of single burning particles. The model was later used to predict kinetics data at high heating rate to be used in comprehensive code: the results of the prediction provide a reactivity distribution expressed in the form of a pre-exponential factor as a function of particle size.

Combustion and gasification kinetics of pyrolysis chars from waste and biomass

The combustion and gasification of chars from medium temperature pyrolysis of municipal waste, electronic scrap, wood and straw has been investigated in view to potential technical applications. Reaction rate measurements with oxygen and carbon dioxide have been carried out at lower temperatures in the chemical regime, using a thermobalance, a differential flow reactor and a fluidised bed of sand. These chars from waste and biomass belong to the most reactive technical carbon materials. The major reason is the porous and highly disordered carbon structure, catalysts play a minor role. In char incineration tests in a small rotary kiln facility, complete burnout could be attained in 1h at temperatures down to ≈600°C. Use of oxygen-enriched air improves burnout and drastically reduces the flue gas volume and the specific emissions per ton of waste.

98/03191 The influence of pressure on char combustion kinetics

98/03191 The influence of pressure on char combustion kinetics

A Kinetic Model of Carbon Burnout in Pulverized Coal Combustion

The degree of carbon burnout is an important operating characteristic of full-scale suspension-fired coal combustion systems. Prediction of carbon loss requires special char combustion kinetics valid through the very high conversions targeted in industry (typically >99.5%), and valid for a wide range of particle temperature histories occurring in full-scale furnaces. The present paper presents high-temperature kinetic data for five coal chars in the form of time-resolved burning profiles that include the late stages of combustion. The paper then describes the development and validation of the Carbon Burnout Kinetic Model (CBK), a coal-general kinetics package that is specifically designed to predict the total extent of carbon burnout and ultimate fly ash carbon content for prescribed temperature/oxygen histories typical of pulverized coal combustion systems. The model combines the single-film treatment of char oxidation with quantitative descriptions of thermal annealing, statistical kinetics, statistical densities, and ash inhibition in the late stages of combustion. In agreement with experimental observations, the CBK model predicts (1) low reactivities for unburned carbon residues extracted from commercial ash samples, (2) reactivity loss in the late stages of laboratory combustion, (3) the observed sensitivity of char reactivity to high-temperature heat treatment on second and subsecond time scales, and (4) the global reaction inhibition by mineral matter in the late stages of combustion observed in single-particle imaging studies. The model ascribes these various char deactivation phenomena to the combined effects of thermal annealing, ash inhibition, and the preferential consumption of more reactive particles (statistical kinetics), the relative contributions of which vary greatly with combustion conditions.

97/02126 Combustion kinetics of coal chars in a fluidized bed

Char combustion has been studied in an electrically heated, 78 mm diameter laboratory fluidized bed combustor. The parent coals were lignite, brown coal and anthracite. The experiments were carried out at bed temperatures of 500--800 C, superficial velocities of 0.44--0.53 m/s and particle diameter of 5--15 mm. Bed temperature and O{sub 2}, CO and CO{sub 2} concentrations were continuously measured during combustion. Char particle temperature was calculated using Baskakov heat transfer correlation. Adopting shrinking particle combustion model, kinetic parameters were estimated. Burning times were estimated, based on the determined kinetic parameters and the particle temperatures, and compared with char burning times based on the off gas analysis.

The influence of pressure on char combustion kinetics

Combustion experiments in a fixed-bed reactor were performed at 0.2, 0.6, and 1.0 MPa for temperatures between 850 and 1200 K with Westerholt bituminous coal char sized in the range 90–106 μ m. The rate constants were first determined with the so-called “ n th-order” overall expression of the combustion rate. Then, a simplified heterogeneous reaction mechanism including a two-stage adsorption-desorption process was proposed to understand the influence of the total pressure on the adsorption and desorption reactions. Using the technique of sudden removal of oxygen, it was found that the total pressure has no effect on the desorption rate constants. Because the combustion is controlled by the adsorption of oxygen, the adsorption rate constant appeared very similar to the overall rate constant. Increasing total pressure up to 0.6 MPa was found to decrease the rate constants if the oxygen “concentration” is expressed as the oxygen partial pressure ( r=k P O2 n ). If the oxygen concentration is expressed as the oxygen mole fraction ( r=k y O 2 n ) there is only a weak effect of the total pressure on the rate constant. Another effect of a pressure increase is to decrease the transition temperature from a kinetics-controlled regime to a regime co-controlled by pore diffusion. To clarify the effect of pressure on combustion, a Langmuir adsorption isotherm was chosen and the adsorption constant was determined at 0.2, 0.6, and 1.0 MPa. It was then found that the Langmuir adsorption isotherm is particularly suitable when working at elevated pressure.

95/04998 Experimental investigation of char combustion kinetics - CO/CO 2 ratio during combustion

95/04998 Experimental investigation of char combustion kinetics - CO/CO 2 ratio during combustion

Residual carbon from pulverized coal fired boilers: 1. Size distribution and combustion reactivity

The amount of residual, or unburned, carbon in fly ash is an important concern in the design and operation of pulverized coal fired boilers. Char oxidation is the slowest step in the coal combustion process, and the rate at which this heterogeneous reaction proceeds has an important effect on the degree of carbon burnout. There is an extensive literature on char combustion kinetics based on data in the early and intermediate stages of carbon conversion. A critical fundamental question is whether the small fraction of the fuel carbon that passes unreacted through a boiler is representative of the char during the main portion of the combustion process. This article addresses that question through a detailed characterization of eight carbon-containing fly ash samples acquired from commercial scale combustion systems. The fly ash characterization included measurement of joint carbon-size distribution and determination of the combustion reactivity of the residual carbon. To minimize mineral matter interactions in the reactivity tests, the technique of incipient fluidization was developed for separation of carbon-rich extracts (∼75 wt% carbon) from the inorganic portion of the fly ash. Reactivity measurements were made at 1400–1800 K to represent conditions in pulverized coal fired boilers. Measurements were also made at 700–1100 K to minimize transport effects and to isolate the influence of char chemistry and microstructure. In both temperature regimes, the residual carbon extracts were significantly less reactive than chars extracted from a laboratory scale laminar flow reactor in the early to intermediate stages of combustion. It is concluded that the boiler environment deactivates chars, making high carbon burnout more difficult to achieve than is predicted by existing char combustion kinetic models that were developed from data on laboratory chars. Finally, the results are used to discuss potential char deactivation mechanisms, both thermal and oxidative, in coal fired boilers.

The combustion characteristics of char from pulverized bagasse

A 90–125 μm sample of pulverized bagasse was charred in an ASTM volatile matter test, and the char combustion kinetics were evaluated by DTG. In the temperature range 450–550 °C, the combustion reaction occurred in regime II, with a Thiele modulus ranging from 470 to 3100. The activation energy was 180 kJ mol −1 and the reaction order with respect to oxygen was 0.65. These kinetic parameters are similar to those of low-rank coal.