Char Burnoff Research Articles

Investigation of ammonia/steam activation for the scalable production of high-surface area nitrogen-containing activated carbons

Ammonia/steam activation was investigated using a gaseous mixture of reductive ammonia and oxidative steam as the activator in an effort to develop an industrially feasible method to produce nitrogen-containing activated carbons. The results show that the ammonia/steam mixture can activate charcoals in the temperature range of 850–950 °C and produce nitrogen-containing activated carbon with a surface area >2000 m2/g and nitrogen content of ∼5% at 950 °C. Moreover, it was found that employing an equation of the natural exponential function, y=ae−bx+c (a b and c are the constants), can quantitatively describe the evolution of the increased surface area, micropore volume and iodine adsorption capacity of the activated carbon material per gram of gasified char as a function of the char burn-off in the activation process. Analysis of the X-ray diffraction patterns of the activated chars further revealed that the activation proceeds via the chemical gasification of the carbon atoms in the amorphous regions and, subsequently, the graphite-like crystallite regions. The former and latter stages are responsible to the formation of a majority of the micropores and mesopores, respectively. In short, ammonia/steam activation is an efficient and cost-effective method to manufacture nitrogen-containing activated carbon materials on a large scale.

Modeling of gasification reaction to produce activated carbon from pistachio shells in a spouted bed

A one-dimensional mathematical model has been developed to predict behavior of a spouted bed reactor during the gasification of char obtained from pyrolysis of pistachio shells leading to the production of activated carbons. The pores overlapping phenomenon is considered in the model with no auxiliary empirical correlation used. This model is capable of the prediction of BET surface area of the produced activated carbons besides the gas-solid hydrodynamics and distribution of gas and solid temperature and concentration in the bed. The results revealed that the one-dimensional model well predicts values for char burn-off, BET surface area, and bed temperature in gasification reactions in spouted bed reactor. This is due to the excellent mixing and high circulation rates of the materials inside a spouted bed which reduces radial distributions.

Atomistic simulation of coal char isothermal oxy-fuel combustion: Char reactivity and behavior

Here a new automated and inexpensive atomic simulation approach including sequential oxygen diffusion and simplified reaction was created to explore the char reactivity and combustion behavior during oxy-fuel combustion. Two large-scale (>40,000 carbon atoms) coal chars (higher- and lower-porosity chars) that differ primarily in pore volume and pore size distribution were evaluated against graphite with similar number of carbon atoms. The structural transformations, total atoms, reactive atoms, their location, conversion, rate (number of atoms reacting), size of the char, and apparent density were captured. Post analysis evaluated the total porosity, micro/mesoporosity volume, and atomic surface area. The higher porosity char burn-off was ∼38% more rapid than the graphite (number of steps), similar to the lower porosity char (∼33% faster). The peak reactivity followed the maximum atomic surface area (with a delay of 5–10% conversion) than decreased—as there were fewer reactive carbons. Porosity volume increases with conversion until 50–70% where the porosity declines. For both chars the microporosity contribution increased until 30–35% conversion then the pores either grow or coalesce into the mesoporous classification. The chars burning modes are initially changing density (pore growth accompany mass loss) until ∼50% conversion when the mode became a combination of changing density and shrinking core. Graphite was less reactive and followed a shrinking core consistent with the simulation restrictions. This approach is likely to aid in the exploration of the influence of char structure and conversion conditions upon reactivity and char behavior in combustion and gasification.

Combustion Characteristics of Biomass Charcoals Produced at Different Carbonization Conditions: A Kinetic Study

The combustion properties of spruce chars and spruce forest residue chars were studied in the kinetic regime by a series of thermogravimetric analysis (TGA) experiments. The work aimed at establishing how the pressure of the char preparation affects the reactivity with oxygen. Parts of the chars were prepared from a thin layer of biomass in inert gas flow at atmospheric pressure and 0.8 MPa. Other chars were formed in a pressurized reactor by a flash carbonization method [Antal, M. J., Jr.; Mochidzuki, K.; Paredes, L. S. Flash carbonization of biomass. Ind. Eng. Chem. Res. 2003, 42 (16), 3690−3699, DOI: 10.1021/ie0301839]. Despite the differences in the preparation, remarkable similarities were observed in the combustion behavior of the samples. The kinetics of the char burnoff was described by assuming three partial reactions. A total of 18 experiments at three different temperature programs were evaluated by the method of least squares to obtain dependable kinetic model variants. A common activation ene...

Kinetic Behavior of Torrefied Biomass in an Oxidative Environment

The combustion of four torrefied wood samples and their feedstocks (birch and spruce) was studied at slow heating programs, under well-defined conditions by thermogravimetry (TGA). Particularly low sample masses were employed to avoid the self-heating of the samples because of the huge reaction heat of the combustion. Linear, modulated, and constant reaction rate (CRR) temperature programs were employed in the TGA experiments in gas flows of 5 and 20% O2. In this way, the kinetics was based on a wide range of experimental conditions. The ratio of the highest and lowest peak maxima was around 50 in the experiments used for the kinetic evaluation. A recent kinetic model by Várhegyi et al. (Várhegyi, G.; Sebestyén, Z.; Czégény, Z.; Lezsovits, F.; Könczöl, S.Energy Fuels 2012, 26, 1323−1335) was employed with modifications. This model consists of two devolatilization reactions and a successive char burnoff reaction. The cellulose decomposition in the presence of oxygen has a self-accelerating (autocatalytic) kinetics. The decomposition of the non-cellulosic parts of the biomass was described by a distributed activation model. The char burnoff was approximated by power-law (n-order) kinetics. Each of these reactions has its own dependence upon the oxygen concentration that was expressed by power-law kinetics too. The complexity of the applied model reflects the complexity of the studied materials. The model contained 15 unknown parameters for a given biomass. Part of these parameters could be assumed common for the six samples without a substantial worsening of the fit quality. This approach increased the average experimental information for an unknown parameter by a factor of 2 and revealed the similarities in the behavior of the different samples.

Combustion Kinetics of Biomass Materials in the Kinetic Regime

Wheat straw, willow from an energy plantation, and municipal sewage sludge were studied by thermogravimetry at linear and nonlinear heating programs in gas flows containing 4 and 20% oxygen. A kinetic scheme of successive devolatilization and char burnoff reactions was assumed. A distributed activation energy model (DAEM) was assumed for the devolatilization with a Gaussian distribution and a constant pre-exponential factor. The burnoff of the forming char was approximated by first-order kinetics with respect to the amount of char. The dependence of the reactions upon the oxygen concentration was described by power functions. This model gave a suitable description for the wheat straw and sewage sludge. An additional partial reaction with accelerating kinetics was needed for describing the oxidative cellulose pyrolysis in the willow sample. The evaluations were carried out by the method of least squares. 9-17 model parameters were determined from ten experiments for each sample. Good fit quality and reasonable kinetic parameters were obtained. Test evaluations revealed that the first-order kinetics, with respect to the amount of char is an adequate model; the assumptions of more complex char burnoff submodels did not led to notable improvements. The replacement of the DAEM devolatilization by simpler n-order kinetics gave inadequate performance. Earlier works with simpler models and linear temperature programs showed that the successive mechanism can be well-approximated by parallel reactions. Such approximations proved to be viable in the present case, too.

Thermogravimetric Analysis of Tobacco Combustion Assuming DAEM Devolatilization and Empirical Char-Burnoff Kinetics

Two blends of tobacco (Virginia and Burley) were studied by thermogravimetry at linear and nonlinear heating programs in gas flows containing 2, 4, and 9% oxygen. A kinetic scheme of successive devolatilization and char-burnoff reactions was assumed. A distributed activation energy model was assumed for devolatilization with a Gaussian distribution and a constant preexponential factor. The evaluations were also carried out using nonconstant preexponential factors that depended on the activation energy. The char burnoff was described by first-order kinetics and by an empirical model that took into account the change of the reactivity with conversion of the sample. The dependence of the rate of combustion on the oxygen concentration was approximated by a power function. Series of 15 and 30 experiments were used for the determination of the model parameters by simultaneous least-squares evaluation of the experiments. The considerations, methods, and results can also be used in the fields of biomass gasificat...

Water adsorption in activated carbons with different burn-offs and its analysis using a cluster model

This work presents the behavior of water adsorption in the activated carbons with different porous structure derived by varying the level of char burn-off. Water adsorption isotherms of activated carbons prepared from longan seed at three different burn-offs (19, 26 and 60%) were measured gravimetrically. These obtained carbons were dif- ferent in terms of their pore size distribution and also the surface functional group properties by showing an increasing of total pore volume and the concentration of surface functional groups with increasing in the burn-off level. The water adsorption isotherms showed that the behavior and amount of water uptake could be divided into three consecutive ranges of relative pressure, 0.0-0.3, 0.3-0.7 and 0.7-0.94, corresponding to adsorption in ultramicropores, supermi- cropores, and mesopores, respectively. The isotherm data were simulated by a cluster model proposed by Do and Do. The correlation was found to be satisfactory over the entire range of relative pressure only with the lowest burn-off carbon which contained mainly micropores. For higher burn-off carbons, which showed increasing proportions of mes- opores, the model needed to be modified by increasing the cluster size of the adsorbed water molecules from 5 to 20 for adsorption at relative pressures greater than about 0.7.

Porous properties of activated carbon produced from Eucalyptus and Wattle wood by carbon dioxide activation

This work focused on the preparation of activated carbon from eucalyptus and wattle wood by physical activation with CO2. The preparation process consisted of carbonization of the wood samples under the flow of N2 at 400°C and 60 min followed by activating the derived chars with CO2. The activation temperature was varied from 600 to 900°C and activation time from 60 to 300 min, giving char burn-off in the range of 20/2-83%. The effect of CO2 concentration during activation was also studied. The porous properties of the resultant activated carbons were characterized based on the analysis of N2 adsorption isotherms at −196°C. Experimental results showed that surface area, micropore volume and total pore volume of the activated carbon increased with the increase in activation time and temperature with temperature exerting the larger effect. The activated carbons produced from eucalyptus and wattle wood had the BET surface area ranging from 460 to 1,490 m2/g and 430 to 1,030 m2/g, respectively. The optimum activation conditions that gave the maximum in surface area and total pore volume occurred at 900°C and 60 min for eucalyptus and 800°C and 300 min for wattle wood. Under the conditions tested, the obtained activated carbons were dominated with micropore structure (∼80% of total pore volume).

A carbon activation model with application to longan seed char gasification

In this paper a new structural model is presented to describe the evolution of porosity of char during the gasification process. The model assumes the char structure to be composed of bundles of parallel graphite layers, and the reactivities of each layer with the gasification agent are assumed to be different to represent the different degree of heterogeneity of each layer (i.e. each layer will react with the gasification agent at a different rate). It is this difference in the reactivity that allows micropores to be created during the course of gasification. This simple structural model enables the evolution of pore volume, pore geometrical surface area and the pore size distribution to be described with respect to the extent of char burn-off. The model is tested against the experimental data of gasification of longan seed-derived char with carbon dioxide and it is found that the agreement between the model and the data is reasonably satisfactory, especially the evolution of surface area and pore volume with burn-off.

Char structural ordering during pyrolysis and combustion and its influence on char reactivity

In the present study, two processes, thermal treatment and oxidation, were separated for a fundamental study of structural evolution during pyrolysis and combustion, as well as for the study of the influence of such evolution on char reactivity. Chars were prepared at different temperatures and heating rates from a size-graded low volatile bituminous coal. The reactivity of resultant chars was measured in Kinetic Regime I using a fixed bed reactor. The structure of fresh and partly burnt chars was characterized using quantitative XRD analysis (QXRDA), high-resolution TEM (HRTEM), high-resolution FESEM, and multi-point gas adsorption. Both QXRDA and HRTEM observations show that char structure becomes more ordered with increasing pyrolysis temperature and decreasing heating rate. Char structure was also investigated as a function of char burnoff. The QXRDA results show that the amorphous concentration of char decreases during combustion while the aromaticity and average crystallite size of char increase. As a result, char structure becomes more ordered during combustion. This is in agreement with HRTEM observations. Due to the low reaction temperature (about 673 K), which is much lower than that for char preparation (1473 K), it was believed that oxidation, instead of thermal effect, contributed to the structural ordering observed during combustion. The structural parameters obtained from QXRDA were then correlated to char reactivity. Structural ordering was found to be responsible for char deactivation during thermal treatment and oxidation. Since the amorphous concentration and aromaticity of char are two strongest indicators of char reactivity, a structural disorder index, DOI, was defined based on them to describe char structural evolution, and further correlated to char reactivity.

Characterization of gasification reactivity of peat char in pressurized conditions: Effect of product gas inhibition and inorganic material

The characteristic effects of the main operating parameters in pressurized fluidized bed gasification, such as pressure and product gas composition, and of inorganic material on peat char gasification reactivity were studied. The measurements were carried out isothermally in a pressurized thermobalance at temperatures between 1023 and 1233 K and pressures up to 1.5 MPa. Steam and carbon dioxide, both pure and mixed with the product gases H 2 and CO, were used as reaction agents. The reactivity was expressed in the form of instantaneous gasification rate vs. char burnoff. The steam gasification rate of peat char was only slightly higher than the CO 2 gasification rate, and the gasification rates decreased at increasing pressure in both steam and carbon dioxide. The steam gasification rate of the peat char increased to a maximum before decreasing with increasing char burnoff. In carbon dioxide, the main trend was that the gasification rate slightly increased with the burnoff. Demineralization of the peat decreased the reactivity and also removed the negative burnoff behaviour observed in steam gasification. The presence of the product gases H 2 and CO inhibited the gasification of the peat char almost entirely.

The impact of fragmentation on char conversion during pulverized coal combustion

Synthetic char particles with controlled macroporosity are used to study char fragmentation behavior during pulverized coal combustion. The chars are burned in an atmospheric laminar flow reactor that permits the control of gas temperature and composition. Char particles are extracted from the reactor at selected residence times and characterized for extent of mass loss and particle size distribution. Chars of 23% and 36% porosity are subjected to environments containing 12 mol % O2 at nominally 1500 K. Number distributions show a large increase in the numbers of small particles during devolatilization, the higher porosity char exhibiting the larger increase. Results show that char burn off increases with porosity at any given residence time, demonstrating an impact of fragmentation on char burn off. The data indicate that both particle diameter and apparent density decrease during burn off and support power-law relations between char particle mass, apparent density, and diameter. A particle population balance model is developed and used to characterize the type of fragmentation that occurs during char oxidation and to quantify the rates of fragmentation events. The model allows for attrition-type behavior (in which only fines are produced), breakage-type behavior (in which particles break into two or three smaller particles), and percolation-type behavior (in which particles fragment into a distribution of smaller-size particles). Calculations using the model indicate that fragmentation during burn off is percolative in nature and char burning rate parameters determined from mass loss, size, and temperature measurements are too high if account is not made for the effects of particle fragmentation. Calculations also suggest that fragmentation during develatilization is percolative in nature and the extent of fragmentation increases with coal volatile yield. Fragmentation rates during devolatilization are estimated to be as high as five times the fragmentation rates during char oxidation.

An Evaluation of the Oxygen Chemisorption Capacity of Mild Gasification Char at Various Burnoff Levels

Abstract Our previous results addressed the reactivity of coal chars prepared at mild and severe conditions. The active site concentration of coal chars as determined by oxygen chemisorption capacity is known to serve as a predictive parameter to reflect the reactivity of these chars. However, it is not certain whether the unoccupied active surface area at a given conversion can effectively describe the corresponding burnoff rate. In this study, changes in the oxygen chemisorption capacity of char as a function of burnolT are determined in a thermogravimetric analysis system for low-lemperaturc (500°C) chars. The oxygen chemisorption capacity undergoes changes as a function of char burnoff. The results demonstrate that the changes in oxygen chemisorption capacity does not follow the same trend as the reactivity profile. suggesting that the stable carbon-oxygen complex (present on the surface but not measured during chemisorption) may play a key role in determining the overall gasification reactivity. The changes in the oxygen chemisorption capacity as a function of burnoff can be attributed to three competing mechanisms: (a) formation of stable C-O complexes, (b) opening of previously closed micropores, and finally, (c) loss of micropores because of pore coalescence and burnoff at a high level of oxidation. This study also demonstrates that the reactivity data can be normalized by any reference time (between 10- to 70-percent burnoff). Therefore, arbitrarily defined reference times used in the literature do not necessarily signify parameters of fundamental importance.