Char Particle Temperature Research Articles

Quantifying the effect of CO2 gasification on pulverized coal char oxy-fuel combustion

Previous research has provided strong evidence that CO2 and H2O gasification reactions can provide non-negligible contributions to the consumption rates of pulverized coal (pc) char during combustion, particularly in oxy-fuel environments. Fully quantifying the contribution of these gasification reactions has proven to be difficult, due to the dearth of knowledge of gasification rates at the elevated particle temperatures associated with typical pc char combustion processes, as well as the complex interaction of oxidation and gasification reactions. Gasification reactions tend to become more important at higher char particle temperatures (because of their high activation energy) and they tend to reduce pc oxidation due to their endothermicity (i.e. cooling effect). The work reported here attempts to quantify the influence of the gasification reaction of CO2 in a rigorous manner by combining experimental measurements of the particle temperatures and consumption rates of size-classified pc char particles in tailored oxy-fuel environments with simulations from a detailed reacting porous particle model. The results demonstrate that a specific gasification reaction rate relative to the oxidation rate (within an accuracy of approximately +/- 20% of the pre-exponential value), is consistent with the experimentally measured char particle temperatures and burnout rates in oxy-fuel combustion environments. Conversely, the results also show, in agreement with past calculations, that it is extremely difficult to construct a set of kinetics that does not substantially overpredict particle temperature increase in strongly oxygen-enriched N2 environments. This latter result is believed to result from deficiencies in standard oxidation mechanisms that fail to account for falloff in char oxidation rates at high temperatures.

Pressurized oxy-fuel combustion of a char particle in the fluidized bed combustor

Pressurized oxy-fuel combustion of coal in fluidized bed (FB) holds the potential to realize low-cost CO2 capture. However, the fundamental study in this manner is still rare due to the difficult access to the pressurized oxy-FB combustion tests. In this work, the experimental study of single char combustion was firstly conducted in a visualized pressurized FB combustor under various operating conditions. Then an experimentally verified particle-scale char combustion model was developed to reveal the dependence of char combustion on parameters. Results showed that the char conversion was accelerated with the increase of pressure, mainly due to the high oxygen diffusion and char gasification. The gasification played a non-negligible role in pressurized oxy-fuel combustion, especially under high oxygen concentration and bed temperature. Increasing oxygen concentration and bed temperature not only promotes the char oxidation rate and particle temperature, but also increases the gasification rate and the share of char conversion via gasification, resulting in shortening the burnout time of char. In addition, a higher fluidization number lowered both the burnout time and peak temperature of char particle, due to the simultaneous improvement of mass and heat transfer. The influences of char size and fluidization number on char gasification conversion ratio are very weak. In addition, the quantitative analysis of the influence of different operating parameters on the combustion process was obtained by model sensitivity analysis.

Investigations on the emissivity of burning coal char particles: Influence of particle temperature and composition of reaction atmosphere

Radiative properties of burning char particles are one of the key parameters for heat transfer in pulverized fuel boilers. Among the parameters which are suspected to change emissivity of burning char surfaces, particle temperature and the reaction atmosphere (air-fired vs. oxy-fuel) are two factors needing investigation. An experimental campaign is presented which studies the effect of both factors in the spectral range from 1.25 µm to 5.5 µm. An in-flight spectrometer test rig measuring thermal radiation emitted by single coal particles was employed. Particle temperature was adjusted by varying the oxygen content in the oxy-fuel atmosphere between 15 and 40%. Average char particle temperatures increase from 1952 to 2582 K with increasing oxygen content. The emissivity drops from ε = 0.6 to ε = 0.26 (λ = 2.4–5.5 µm) and ε = 0.33 to ε = 0.24 (λ = 1.25–2.25 µm) with increasing temperature. The influence of the reaction atmosphere was investigated by performing experiments in a N2/O2 atmosphere and comparing to the results to data from an oxy-fuel atmosphere, both containing 20% of oxygen. The difference εoxy-fuel-εN2/O2 is in the range of 0.1 and 0.2 for λ = 1.25–2.25 µm and λ = 2.4–5.5 µm. Effects of burnout and char structure are discussed to evaluate the results.

Experimental study of a single char particle combustion characteristics in a fluidized bed under O2/H2O condition

Oxy-steam combustion is a potential new route for oxy-fuel combustion with carbon capture from coal-fired power plants. In the present work, the combustion behavior of single char particles were investigated in a transparent fluidized bed combustor under different operating conditions (i.e., gas atmosphere, oxygen concentration, coal rank, location, fluidization number, particle size, and bed temperature). Both pre-calibrated two-color pryrometry and a flexible thermocouple were used to measure the char particle temperature in the combustion tests. Results indicated that the pore structure of the char generated in H2O atmosphere was better than that generated in CO2 and N2 atmospheres. As expected, with increase of oxygen concentration, the burnout time (tb) decreased, and the particle temperature (Tp) increased. The sequence of burnout times for different rank coal char particles was: anthracite > bituminous coal > lignite. Interestingly, comparing O2/CO2 and O2/N2 combustion, a shorter tb and a lower Tp of char could be achieved simultaneously in O2/H2O combustion, regardless of location and oxygen concentration. Furthermore, the increase of fluidization number strengthened the mass and heat transfer between the char and the environment, thereby reducing the tb and Tp of char. With increasing of particle size, the Tp slightly decreased, the tb increased markedly, and the gasification reactions became more and more significant. As the bed temperature increased, the gasification rate increased exponentially, and the mass transfer coefficient increased gradually.

Study of temperature, apparent spectral emissivity, and soot loading of a single burning coal particle using hyper-spectral imaging technique

A hyper-spectral imaging device that can simultaneously capture three-dimensional data information of spectra, space, and time is employed to experimentally investigate the combustion behavior of a single coal particle. Three types of single pulverized coal particles are injected and burned on a Hencken flat-flame burner, and their flames are captured by a high-speed camera and the hyper-spectral imaging device. The camera observations demonstrate that the three single coal particles undergo particle heat-up, volatile release and combustion, and volatile and char oxidation during the combustion process. Furthermore, the image captured by the hyper-spectral imaging device records the spectral and spatial history of the single burning coal particles throughout the combustion process. Then, the time–space temperature and apparent spectral emissivity of the single burning char particles are calculated from the time–space spectra radiative intensity obtained by the hyper-spectral imaging device. The gray characteristic exhibited by the spectral emissivity of the three single burning char particles in the longer wavelength range demonstrates the feasibility of calculating the temperature using the two-color method. Finally, the soot temperature and volume fraction distribution in the volatile flames are obtained to investigate the formation of soot during the combustion of a single coal particle. The results show that an elongated tail-like soot cloud is formed around the two single bituminous coal particles during the combustion process, but a nearly spherical soot cloud is formed around the single lignite coal particle. Moreover, the range of soot volume fractions in the three envelope volatile flames is comparable to the order of magnitude of the values measured by other researchers.

Measurement and theoretical prediction of char temperature oscillation during fluidized bed combustion

There is experimental evidence of oscillations of the char particle temperature during combustion in a fluidized bed (FB), resulting from the movement of the char throughout the bed. However, in most theoretical FB combustion studies the char particle is assumed to always stay in the emulsion phase, and existing models do not take into account the movement of the char particle explicitly. Moreover, it is difficult to quantify the magnitude and frequency of these temperature oscillations with the common measurement techniques employed in FB (thermocouple and pyrometry with optical probe). In this work, the combustion of single char particles (8 mm) from beech wood and sub-bituminous coal is carried out in a 2-dimensional FB made of quartz, using two O2 concentrations (11 and 21%v) in N2. The time-evolution of the temperature and the size of the char in the different phases are estimated by the analysis of images resulting from a new method combining pyrometry with readings from a digital camera. It is found that the combustion temperature oscillates in hundredths of seconds with an amplitude varying from 10 to 100 °C, resulting from the movement of a particle between the emulsion, bubble and splash phases. The amplitude increases with higher O2 concentration and smaller char-particle size. A combustion model is developed using the experimental characterization of the movement of the char particle through the bed as input. The temperature and burnout time predicted by the model compare well (within 15 %) with measurements obtained from this work and from literature.

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.

The effect of using thermocouples on the char particle combustion in a fluidized bed reactor

The char temperature during combustion in a fluidized bed (FB) is often measured by thermocouples due to simplicity, and because it is assumed that the thermocouple effect on the movement of the char is negligible. It is also accepted that the combustion temperature of a char particle fluidized with an embedded thermocouple is similar to a freely fluidized particle. However, few publications have given evidence of this fact. In the present article this question is dealt with by comparing the evolution of conversion and temperature of fluidized char particles with and without an embedded thermocouple. Char from beech wood, sub-bituminous and bituminous coal of an initial diameter of 10mm is burnt in two laboratory FBs; one 2-dimensional made of quartz with rectangular cross-section allowing visual observation and temperature tracking of the particles, and the other being 3-dimensional with 50mm internal diameter. The surface temperature of the char particles is followed by a recently developed visual technique, based on pyrometry coupled with a digital camera. The consumption of char is evaluated by sampling particles from the bed at different stages of conversion, analyzing the density and size of the particle. It is found that, due to the greater resistance of the particle-thermocouple to the drag of the bed, the time that the char particle is in the bubble phase is 40% longer when the thermocouple is used, leading to higher combustion rate and temperature, and consequently, shorter burnout time. Moreover, the rotation of the char particle is restricted, provoking a non-homogeneous consumption of char, which enhances the size reduction of the particle along the direction perpendicular to the insertion of the thermocouple. Overall, these findings suggest that conclusions about char conversion by temperature measurements using thermocouples should be made with caution, since significant error might be committed under some operating conditions.

Measurement on the Surface Temperature of Dispersed Chars in a Flat-Flame Burner Using Modified RGB Pyrometry

A new spectral-emittance method to modify RGB pyrometry is developed for the surface temperature measurements of dispersed chars in a Hencken flat-flame burner. Nikon D300s, with calibrated spectral response, was used as the detector for the measurement. First, we calculated an original lookup table for ratio–temperature of the camera on the basis of the blackbody assumption. Second, 420/440 two-color pyrometry using an intensified charge-coupled device (ICCD) combined with two adjacent bandpass filters centered at 420 and 440 nm was adopted here for a verified comparison to RGB pyrometry. On the basis of the normalized spectral emission distribution of char particles in the visible light region (390–710 nm) acquired by an Ocean2000 spectrometer, a calibrated normalized spectral emissivity model of burning chars was built. Then, the lookup table of the Nikon camera was further modified by this calibrated spectral emissivity model, enabling better predictions on char particle temperatures in cases of all d...

Kinetic modeling of the formation and growth of inorganic nano-particles during pulverized coal char combustion in O2/N2 and O2/CO2 atmospheres

In the formation of nano-particles during coal char combustion, the vaporization of inorganic components in char and the subsequent homogeneous particle nucleation, heterogeneous condensation, coagulation, and coalescence play decisive roles. However, conventional measurements cannot provide detailed information on the dynamics of nano-particle formation and evolution. In this work, a sophisticated intrinsic char kinetics model that considers ash effects (including ash film formation, ash dilution, and ash vaporization acting in tandem), both oxidation and gasification by CO2 and H2O, homogeneous particle nucleation, heterogeneous vapor condensation, coagulation, and coalescence mechanisms is developed and used to compare the temporal evolution of the number and size of nano-particles during coal char particle combustion as a function of char particle size, ash content, and oxygen content in O2/N2 and O2/CO2 atmospheres. Based on comparisons with measurements of char particle temperature, carbon conversion, mineral vaporization, and mean size of nano-particles at various residence times, the model can accurately predict the transient combustion of pulverized coal char particles and nano-particle formation and growth. Model results show that in either O2/N2 or O2/CO2 atmospheres, the char combustion temperature has a dominant effect on the formation and growth of nano-particles. High char burning temperatures result in a high mineral vaporization rate within the char particle, and subsequent high nucleation and condensation rate, and consequently more and larger nano-particles. As a result, high oxygen content, low ash content, and small sized char particles, all of which promotes high local char burning temperatures, yield more nano-particles and shift the nano-particle size distribution to larger sizes. In comparison to combustion in O2/N2, both the number density and size of the nano-particles formed in O2/CO2 are lower. Unlike condensation, which contributes to particle growth until the vapor molecules are fully consumed, nucleation ceases during the last stage of char combustion.

Performance Evaluation of South African Coals under Oxy-Fuel Combustion in a Fluidized Bed Reactor

In this study, the experimental results of the oxy-fluidized combustion of three different South African coals (sub-bituminous) are presented. The coal samples were denoted Coal A, B, and C. Three combustion atmospheres—air, oxy (21% O2/79% CO2), and oxy (30% O2/70% CO2)—were studied. A total of 18 tests were conducted in a bubbling fluidized bed reactor at 850 and 925 °C. The results obtained showed that the highest carbon burnout was obtained at 30% O2/CO2, followed by air, and last at 21% O2/CO2. Coals A and B had a higher carbon burnout than Coal C. There was a marginal difference of 1% in the carbon burnout for both Coals A and B in all the conditions except at 21% O2/CO2 at 925 °C, when the carbon burnout of Coal A was 5% higher than that of Coal B. Carbon burnout in Coal C was about 20% lower than those of Coals A and B. Carbon burnout had a good correlation with the char particle temperature at all the combustion conditions studied for the three coals. The carbon burnout only had a good correlatio...

Numerical investigation on separate physicochemical effects of carbon dioxide on coal char combustion in O2/CO2 environments

The thermophysical and chemical effects of CO2 on the combustion characteristics of a single char particle in O2/CO2 environments remain unclear because these effects are intercoupled during the complex process of coal char combustion. A numerical method was proposed to adjust the char particle temperature in an O2/CO2 environment back to that in an O2/N2 environment by replacing an appropriate portion of CO2 with argon because of the molar heat capacity of Ar < N2 < CO2, and to quantitatively isolate the effects of CO2 on bituminous coal char combustion in specially designed combustion environments with the continuous-film model. The numerical result shows that the char particle temperature in a 21% O2/79% CO2 environment is lower by 493 K than that in a 21% O2/79% N2 environment because of the combined effects of lower local gas temperature and endothermic char gasification reaction with CO2. The char particle temperature increases significantly with the oxygen concentration, and the char temperature in a 52.5% O2/47.5% CO2 environment is equivalent to that in air environment. Most importantly, the effects of CO2 on bituminous coal char combustion in O2/CO2 environments are quantitatively isolated with specially designed combustion environments (21% O2/79% N2, 21% O2/11% CO2/68% Ar, 52.5% O2/47.5% CO2 and 52.5% O2/30% CO2/17.5% Ar). The relative contributions of the oxygen concentration, thermal, and chemical effects on the char combustion rate are 82.1%, 11.2%, and 6.7%, respectively, at an ambient gas temperature of 1200 K for bituminous coal char of 91 µm. The results show that the oxygen concentration effect is the most important factor, followed by the thermal effect and the chemical effect of char gasification reaction with CO2.

Effect of typical modeling assumptions for pulverized coal char combustion commonly used in commercial CFD codes

In Computational fluid dynamic (CFD) models of pulverized coal combustion, which require the consideration of complex phenomena, simplified char combustion sub-models have become necessary for computational efficiency. There are several common assumptions embedded in pulverized char combustion modeling when using commercial CFD codes. We sought to quantify uncertainties that arise from these typical char combustion modeling assumptions based on the single-film apparent char kinetic model often used. The modeling assumptions examined include the exception of the Stefan flow effect, the depiction of CO species as the sole char oxidation product and the constant particle size with reaction progress, and the use of questionable rate parameters irrespective of the considered model concept and coal type. The calculated char mass loss and particle temperatures were investigated to evaluate these modeling assumptions.

Combustion of single char particles from Victorian brown coal under oxy-fuel fluidized bed conditions

Combustion performance of individual and large spherical single char particles of 5–6mm diameter during oxy-fuel fluidized bed (Oxy-FB) combustion is investigated. These char particles were prepared from briquetted Victorian brown coal. In these experiments, a thin thermocouple was inserted inside each char particle to monitor the particle temperature during the experiment. Particle temperature was observed to be higher compared to the bed temperature which varied between 880°C and 925°C. Up to 48°C difference was noticed between the char particle temperature and the bed temperature using 15% steam in oxy-fuel combustion atmosphere. A kinetic model on char combustion was developed to calculate the combustion time, carbon consumption rate and the particle diameter as a function of time in an atmosphere consisting of O2, CO2 and steam. It was evident that boundary-layer oxygen diffusion controlled the carbon combustion rate during oxy-fuel fluidized bed combustion of these brown coal char particles. The gasification reactions were almost negligible for the feed gases and temperature range applicable for Oxy-FB combustion.

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.

Lattice Monte Carlo simulation of single coal char particle combustion under oxy–fuel conditions

A descriptive model for isolated char particle combustion under conventional and oxy–fuel conditions was developed. Suggested model is based on the percolation theory and Monte Carlo simulation technique. Char particle was modeled as a square lattice consisting of a large number of small sites. Sites correspond either to fixed carbon, ash, or pore, and they were distributed randomly inside char lattice using percolation concept, at the start of simulation. Random walk model was used to represent gaseous species diffusion through particle pores. Char combustion was modeled using power law Arrhenius model which assumes that reaction rate depends of particle temperature and oxygen partial pressure on particle surface. The main aim of the proposed model was to take into account influence of heterogeneous char particle structure to randomness of the char combustion process. The suggested model’s behavior was validated by qualitative comparison with experimental data obtained in single particle reactor. It was found that simulated combustion time, char burnout and particle temperature values are in good agreement with experimentally determined data. Special emphasis was given to the CO2 gasification reaction influence on char conversion and particle temperature values. Further development of the proposed model with appropriate simplifications would enable its inclusion in comprehensive CFD codes.

Accuracy of the single-film model in the prediction of coal char conversion rates under oxy-fuel and conventional combustion conditions

The accuracy of the single-film model in the calculation of char burning rates and particle temperatures in conventional and oxy-fuel combustion conditions has been investigated taking as a reference the results obtained with a model with detailed gas-phase kinetics. Both models have been applied to subbituminous coal and anthracite char particles in different sizes (60–1000μm), in N2 and CO2-rich atmospheres with a low and a high oxygen concentration. For 120μm char particles, the models’ predictions have been also compared over an ample range of ambient gas temperatures (1373–1823K) and oxygen concentrations (4–50%). High particle diameters, bulk gas temperatures and oxygen concentrations result in CO-flames closer to the particle surface; however, the specific effects of the flame strongly depend on the coal rank and combustion conditions. In general, the single-film model accuracy is quite high in terms of burning rate predictions, except for fine anthracite particles burning at moderate bulk gas temperatures and high O2 concentrations; greater deviations have been found regarding particle temperatures, even in cases where the corresponding combustion rates are very similar. The oxy-combustion of the reactive coal char is found to be faster than its conventional combustion, except for the finest particles; although qualitatively similar, the difference in rates is overestimated if the single-film model is considered.

Microwave-enhanced CO2 gasification of oil palm shell char

CO2 gasification of oil palm shell (OPS) char to produce CO through the Boudouard reaction (C + CO2 ↔ 2CO) was investigated under microwave irradiation. A microwave heating system was developed to carry out the CO2 gasification in a packed bed of OPS char. The influence of char particle size, temperature and gas flow rate on CO2 conversion and CO evolution was considered. It was attempted to improve the reactivity of OPS char in gasification reaction through incorporation of Fe catalyst into the char skeleton. Very promising results were achieved in our experiments, where a CO2 conversion of 99% could be maintained during 60 min microwave-induced gasification of iron-catalyzed char. When similar gasification experiments were performed in conventional electric furnace, the superior performance of microwave over thermal driven reaction was elucidated. The activation energies of 36.0, 74.2 and 247.2 kJ/mol were obtained for catalytic and non-catalytic microwave and thermal heating, respectively.

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.

Effect of CO2 gasification reaction on char particle combustion in oxy-fuel conditions

CO2 gasification of coal char may play an important role in oxy-combustion environments with flue gas recirculation (FGR), but its effect on the overall reaction rate has not been clearly understood. To give clarity to the likely impact of CO2 gasification on the oxy-combustion of pulverized coal chars, burnout simulations of coal char particles were carried out, adopting apparent char reactivity and a single-film model that includes the Stefan flow effect on mass and energy transfer. Three oxygen concentrations (21%, 30%, and 5% O2), representing air, oxy-fuel, and oxygen-deficient combustion environments were simulated. A new experimental approach was used to directly measure the CO2 gasification rate of a subbituminous coal char at high temperatures and atmospheric pressure. The measured gasification rate is somewhat higher than previous measurements. The simulation results show that the endothermic gasification reaction reduces the char particle temperature and thereby reduces the oxidation rates. However, due to the contribution of the direct gasification reaction on carbon consumption, the char burnout time and the carbon consumption were improved. The gasification reaction has a greater influence on the char burnout time and the relative carbon consumption in an oxygen-deficient environment and on the drop of particle temperature in an oxygen-enriched environment (for a given gas temperature). In addition, the influence of the gasification reaction on char combustion increases as the gas temperature increases and as the particle size increases. Further, it was observed that the impact of the gasification reaction is dependent on the presumed kinetic rate, which highlights the importance of using reliable kinetic parameters in simulations. Based on the present results, it is important to include the gasification reaction by CO2 when simulating char combustion in oxy-fuel combustion environments.