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Gas reactions of carbon
... Char gasification is heterogeneous in nature. Gasification rate depends on various factors such as chemical processes, mass and heat transfer, impurities in the carbon, and nature of heat treatment prior to gasification [2]. Non-catalytic gasification is virtually non-existent as all carbon sources have inorganic impurities, which act as catalysts. ...
... The ultimate analysis is presented on dry ash free basis. The gas-carbon reactions are largely dependent on the rate controlling step or steps as the reaction orders, activation energies and specific reaction rates are affected by it [2]. At lower temperatures, the reaction rate is low enough to make sure that the overall process is controlled by chemical process: includes chemisorption of the reactants, elementary reactions and desorption of products. ...
... This indicates that the gasification reaction was no longer chemically controlled. This temperature falls into transition zone between the chemically controlled and diffusion controlled process [2]. At this temperature the reaction rate is controlled by both chemical processes and mass transfer in the pores. ...
CO2 gasification of Victorian (Morwell) brown coal char was studied using a thermogravimetric analyser (TG). Gasification kinetics of demineralised, Ca-loaded, and Fe-loaded Morwell char were also studied. The grain model and random pore model were used to fit the gasification data. The random pore model fitted the experimental data better than the grain model. The activation energy was 189.05 kJ mol−1 for the CO2 gasification of Morwell coal char. With 2 % Ca loading, the activation energy increased to 204.53 kJ mol−1 due to lowering of the surface area. However, an order of magnitude increase in the pre-exponential factor indicated an increase in active reaction sites for the 2 % Ca-loaded sample, resulting in a net increase in gasification rate. 5 % Ca loading and 2 % Fe loading proved to be less effective in increasing the gasification rate. Analysis of the TG outlet gas also proved the effectiveness of 2 % Ca loading as a gasification catalyst.
... Il est généralement admis que la désorption du monoxyde de carbone est l'étape cinétiquement déterminante (Ergun 1956;Walker et al. 1959). Dans le cadre de cette hypothèse, le mécanisme suivant (le plus utilisé) a entre autres été proposé : ...
... Ce mécanisme est limité par la formation des complexes C(O) à la surface réactive du matériau et l'effet inhibiteur du CO se traduit par une recombinaison avec des complexes C(O) en CO 2 . En revanche, le CO ne se chimisorbe pas sur les sites actifs (Strickland-Constable 1950;Walker et al. 1959). ...
... En outre, Walker et al. (1959) ont défini un modèle théorique de régimes de gazéification des matériaux carbonés en traçant le logarithme de la vitesse de gazéification en fonction de l'inverse de la température pour traduire ces changements observés sur la cinétique avec l'augmentation de la température. ...
The decommissioning of French gas cooled nuclear reactors (UNGG), all arrested since 1994, will generate 23,000 tons of graphite waste classified Low Level and Long Lived and notably containing 14C. The aim of this thesis is to study a new method for selective extraction of this radionuclide by CO2 gasification.The multiscale organization of virgin and irradiated graphite has been studied by a coupling between microspectrometry Raman and transmission electron microscopy. With the neutron fluence, the structure degrades and the nanostructure can be greatly changed. In extreme cases, the lamellar nanostructure nuclear graphite has become nanoporous. Furthermore, these damages are systematically heterogeneous. An orientation effect of "crystallites", shown experimentally by ion implantation, could be a cause of these heterogeneities.This study also showed that from a specific fluence, there is an important development of nanoporous zones coinciding with a dramatic 14C concentration increase. This radionuclide could be preferentially concentrated in the nanoporous areas which are potentially more reactive than the remaining laminar areas which could be less rich in 14CThis process by CO2 gasification was firstly tested on "analogous" non-radioactive materials (mechanically milled graphite). These tests confirmed, for temperatures between 950 and 1000 °C, the selective and complete elimination of nanoporous areas.Tests were then carried out on graphite waste from Saint-Laurent-des-Eaux A2 and G2 reactors. The results are promising with notably the quarter of 14C inventory extracted for a weight loss of only few percent. Up to 68 % of 14C inventory was extracted, but with an important gasification. Thus, this treatment could allow extracting selectively a share of 14C inventory (mobile or linked to nanoporous areas) and allows imagining alternative scenarios for graphite waste managing.
... Below 400 o C, graphite oxidation is negligible. The prevailing oxidation modes are categorized based on temperature as follows: [23][24][25][26] Mode ( ...
... For example, at 800 o C and 0.1 atm, the relative rates for the C-O 2 (5b), C-H 2 O (5f), C-CO 2 (5d) and C-H 2 (5g) reactions are 10 5 , 3, 1 and 3 x 10 -3 , respectively. 24 Homogeneous reactions of gaseous species are: 23 Reactions (6a) and (6b) occur in the boundary layer and the bulk multi-species gas mixture. The first reaction competes for oxygen, and the second competes for water vapor in the bulk gas. ...
... Where 13) and (14) have been developed assuming a constant diffusion coefficient and either zero-order (n = 0) or first-order (n = 1) reaction. 18,24 Approximate asymptotic solutions are also available for the intermediate order reactions (0 < n < 1) and for the general case of Langmuir-Hinshelwood rate equations for i R ′ ′ . The maximum oxidation rate, max T R occurs when the reactions in the volume pores involve the total internal surface. ...
A massive air or steam ingress in High Temperature Reactors (HTRs) nominally operating at 600-950 o C is a design-basis accident requiring the development and validation of graphite oxidation and erosion models to examine the impact on the potential fission products release and the integrity of the graphite core and reflector blocks. Nuclear graphite is of many types with similarities but also differences in the microstructure, volume porosity, impurities, type and size of filler coke particles, graphitization and heat treatment temperatures, and the thermal and physical properties. These as well as the temperature, types and partial pressures of oxidants affects the prevailing oxidation mode and kinetics of the oxidation processes of graphite in HTRs. This paper reviews the graphite crystalline structure, the fabrication procedures, characteristics, chemical kinetics and modes of oxidation of nuclear graphite for future model developments.
... Because the steam−carbon reaction occurs through the same basic mechanism as the Boudouard reaction, it seemed likely that it would also be accelerated due to similar microwave effects. 2 Equilibrium constants for the steam−carbon reaction system were determined at five different power settings that produced graphite surface temperatures from 764 to 997 K. The distribution of species at equilibrium, as a percent of the total composition, is shown in Figure 4 for each of the temperatures. ...
... The WGS reaction is generally considered an integral part of the carbon steam reaction, which accounts for production of CO 2 in the product gas (reaction 2). 2 The reaction is weakly exothermic, and, unlike the other reactions in the steam−carbon system, carbon is not a reactant. Instead, the reaction is thought to be catalyzed by the carbon, and, under conditions typical of the steam−carbon reaction, equilibrium is rapidly attained. ...
... Instead, the reaction is thought to be catalyzed by the carbon, and, under conditions typical of the steam−carbon reaction, equilibrium is rapidly attained. 2 Among the gas−carbon reactions, however, the WGS reaction is not an independent reaction but can be written as the difference between the steam−carbon and Boudouard reactions (i.e., reactions 1−3) so that the equilibrium constants are simply related, (K sc )(K Bou ) −1 = K WGS . Thermodynamically, this means that the enthalpy of the reaction is the difference in enthalpies, ΔH wgs = ΔH sc − ΔH Bou , and, similarly, the entropy is ΔS wgs = ΔS sc − ΔS Bou . ...
A series of heterogeneous catalyst materials possessing good microwave absorption properties were investigated for their activity as oxidation catalysts under microwave irradiation. These catalysts, a series of nanoscale magnetic spinel oxides of the composition MCr2O4 (M = Cu, Co, Fe), were irradiated in aqueous methanol solution (1:1 MeOH:H2O v:v). This resulted in rapid conversion of methanol to formaldehyde, directly generating aqueous formalin solutions. The catalytic reaction occurred under relatively mild conditions (1 atm O2, 60 °C), with irradiation times of 80 min converting 24.5%, 17.7%, and 13.2% of the available methanol to formaldehyde by the Cu, Fe, and Co chromite spinel catalysts, respectively. Importantly, reactions run under identical conditions of concentration, time, and temperature using traditional convective heating yielded dramatically lower amounts of conversions; specifically, 1.0% and 0.21% conversions were observed with Cu and Co spinels, and no observable thermal products were obtained from the Fe spinels. This work provides a clear demonstration that microwave-driven catalysis can yield enhanced reactivity and can afford new catalytic pathways.
... Because the steam−carbon reaction occurs through the same basic mechanism as the Boudouard reaction, it seemed likely that it would also be accelerated due to similar microwave effects. 2 Equilibrium constants for the steam−carbon reaction system were determined at five different power settings that produced graphite surface temperatures from 764 to 997 K. The distribution of species at equilibrium, as a percent of the total composition, is shown in Figure 4 for each of the temperatures. ...
... The WGS reaction is generally considered an integral part of the carbon steam reaction, which accounts for production of CO 2 in the product gas (reaction 2). 2 The reaction is weakly exothermic, and, unlike the other reactions in the steam−carbon system, carbon is not a reactant. Instead, the reaction is thought to be catalyzed by the carbon, and, under conditions typical of the steam−carbon reaction, equilibrium is rapidly attained. ...
... Instead, the reaction is thought to be catalyzed by the carbon, and, under conditions typical of the steam−carbon reaction, equilibrium is rapidly attained. 2 Among the gas−carbon reactions, however, the WGS reaction is not an independent reaction but can be written as the difference between the steam−carbon and Boudouard reactions (i.e., reactions 1−3) so that the equilibrium constants are simply related, (K sc )(K Bou ) −1 = K WGS . Thermodynamically, this means that the enthalpy of the reaction is the difference in enthalpies, ΔH wgs = ΔH sc − ΔH Bou , and, similarly, the entropy is ΔS wgs = ΔS sc − ΔS Bou . ...
The steam−carbon reaction, which is the essential reaction of the gasification processes of carbon-based feed stocks (e.g., coal and biomass), produces synthesis gas (H 2 + CO), a synthetically flexible, environmentally benign energy source. The reaction is very endothermic, which mandates high temperatures and a large expenditure of energy to drive the reaction. We have found that using microwave irradiation to selectively heat the carbon leads to dramatically different observed thermodynamics for the reaction. From measurement of the equilibrium constants as a function of temperature, the enthalpy of the reaction under microwave radiation was found to become significantly more exothermic, dropping from 144.2 kJ/mol at the median reaction temperature of 880 K to 15.2 kJ/mol under microwave irradiation. The reaction conditions under which the steam− carbon reaction was run, and under which the equilibrium measurements were determined, consisted of three other reactions that came to equilibrium. These reactions were the Boudouard reaction, which is the reaction of CO 2 with carbon to form CO; the water−gas shift reaction, where CO and water react to form H 2 and CO 2 ; and the carbon−hydrogen reaction, which generates methane from the reaction of H 2 with carbon. We determined the equilibrium constants and thermodynamic parameters for all of these reactions. The Boudouard reaction, which is also strongly endothermic, was found to be more exothermic under microwave radiation (180.2 kJ/mol (thermal) and 27.0 kJ/mol (MW)). The water−gas shift reaction became more endothermic (−36.0 kJ/mol (thermal) and −11.4 kJ/mol (MW)). The carbon−hydrogen reaction also underwent an endothermic shift, from −79.7 to −9.1 kJ/mol. From the associated equilibrium expressions and the equilibrium constants for the steam−carbon reaction system, the mole fractions of the system components under thermal and microwave conditions were estimated. The effect of the microwave radiation was to change the position of the equilibrium so that the temperature at which H 2 was at a maximum dropped from 643 °C in the conventional thermal reaction to 213 °C in the microwave. Notwithstanding the predicted temperature shift, there was an observable threshold below which microwaves could not produce products. In our system, the minimum energy at which H 2 appeared was 373 °C (30 W), while the temperature at which equilibrium could be established in a reasonable period of time (100 min) was 491 °C (100 W). ■ INTRODUCTION The reaction between superheated steam and carbon to produce synthesis gas (reaction 1) is part of the general category of gasification reactions used to obtain hydrogen from coal and other carbon-rich sources. 1,2 Gasification reactions typically occur at temperatures ≥700 °C depending on the carbon source, while industrial processes, such as coal gasification, run at much higher temperatures (>1000 °C). These high temperatures are required to drive the endothermic components of the primary reactions and to obtain useful reaction velocities. 2
... This indicates that the oxidation rate of iron controls the formation rate of CO from the graphite. In water-rich atmospheres, CO is further oxidized to CO 2 as indicated in previous studies (Walker et al., 1959; von Fredersdorff and Elliott, 1963). Formation of ammonia from gaseous nitrogen and hydrogen with metal catalysts is known as the Haber or Haber–Bosh process as shown in the following reaction (Larson and Dodge, 1923; Larson, 1924): ...
... Carbon monoxide was the major carbon-bearing product at 800 °C and above (Fig. 4b). This result is consistent with the ratio of CO to CO 2 calculated using equilibrium constants of the reactions between carbon and water, as well as between CO and H 2 O (Walker et al., 1959; von Fredersdorff and Elliott, 1963). According to previous studies and the equilibrium constants of reactions among carbon, hydrogen, and oxygen, CH 4 is expected to be the major carbon-bearing product below $500 °C (Walker et al., 1959; von Fredersdorff and Elliott, 1963; Schaefer and Fegley, 2010). ...
... This result is consistent with the ratio of CO to CO 2 calculated using equilibrium constants of the reactions between carbon and water, as well as between CO and H 2 O (Walker et al., 1959; von Fredersdorff and Elliott, 1963). According to previous studies and the equilibrium constants of reactions among carbon, hydrogen, and oxygen, CH 4 is expected to be the major carbon-bearing product below $500 °C (Walker et al., 1959; von Fredersdorff and Elliott, 1963; Schaefer and Fegley, 2010). However, CH 4 was not detected in the present experiments even at the lowest temperature (400 °C). ...
... The values of activation energy, E, and preexponential factor, Z, are found to be in the range from 33 to 40 kcal/mol and from 5.5 X 10 9 to 4.5 X 10 11 I min atm''•, respectively. Kinetic parameters for the oxidation of carbon have been well known to be strongly affected by the form of carbon, the nature of the impurities present and experimental conditions (7,8). However, the majority of the results under various experimental conditions have shown that the order of reaction with respect to the oxygen partial pressure lies between zero and one with activation energy in the range of about 35 to 45 kcal/mol (7,8,9). ...
... Kinetic parameters for the oxidation of carbon have been well known to be strongly affected by the form of carbon, the nature of the impurities present and experimental conditions (7,8). However, the majority of the results under various experimental conditions have shown that the order of reaction with respect to the oxygen partial pressure lies between zero and one with activation energy in the range of about 35 to 45 kcal/mol (7,8,9). Kinetic parameters observed for the oxidation reaction of the tobacco char in step II also are within the range of those for the oxidation of carbon. ...
In this paper, thermal analyses (thermogravimetry, TG: derivative thermogravimetry, DTG: differential scanning calorimetry, DSC) of the tobacco char left after pyrolysis of tobacco shreds were carried out in a nitrogen atmosphere containing given amounts of oxygen under a linear heating or an isothermal condition. The principal object of the present work is to obtain apparent kinetic parameters useful in predicting the overall rate of the tobacco char - oxygen reaction. The TG-DTG-DSC curves obtained under linear heating conditions showed that the oxidation process of the tobacco char apparently consisted of two main steps ( I and II), which were independent of each other. Assuming that a uniform reaction model based on reaction rate - determined processes could be applied to the tobacco char - oxygen reactions in both steps, the oxidation rates for both steps were determined. The results showed that the oxidation rates of both steps could be expressed as:
The values of activation energies, E, and pre-exponential factors, Z, for the oxidation reactions in steps I and II were found to be 19-21, 33-40 kcal/mol and 5.9 × 10
Nomenclature
As reactant (solid phase, tobacco char)
E activation energy [kcal mol
po2,∞ ambient oxygen partial pressure [atm, 1 atm = 1.013 × 10
R gas constant [ 1.98 cal mol
T temperature [K]
t time [min]
W weight loss of tobacco char up to time, t, or temperature, T [mg]
Wc weight loss of tobacco char at completion of reaction [mg]
Z pre-exponential factor [min
a fraction of As decomposed at time, t, defined by a = W/W
... As a result, the gaseous reactant can be evenly distributed throughout the particle and have a concentration identical to the bulk gas stream [2,5]. In other words, the solid reactant homogeneously converts in the porous particle, and the reaction follows the uniform reaction model [11]. For this case, the reaction rate can be expressed simply in terms of , and the gaseous reactant concentration in the solid particle and is obtained [5] by integrating ...
... When the intrinsic reactivity of the solid in a porous particle is not low, gaseous reactant can be consumed before penetrating deeply into the particle. Thus, the reaction typically takes place in a narrow zone close to the external surface of the particle, and the reaction front moves towards the core, similar to the shrinking particle model for non-porous particles [2,11]. When the intrinsic reactivity of the solid is at an intermediate level, the major difference between porous and non-porous particles is that the reaction occurs at a macroscopic sharp interface for non-porous particles, while the reaction zone is diffuse for porous particles [2]. ...
This chapter focuses on non‐catalytic gas–solid reactions in fluidized beds and considers solids participating in non‐catalytic gas–solid reactions. The performance of a fluidized bed reactor for gas–solid reactions with a continuous flow of solids greatly depends on three factors: characteristic reaction time for single particles in the reactor environment, residence time distribution of particles in the bed, and spatial variation of gas composition in the reactor. The chapter combines these factors for the reliable prediction of reactor performance and/or successful design of a relevant fluidized bed reactor. It discusses reaction models for porous particles and non‐porous particles and the type of reaction. The chapter illustrates the procedure to be followed in predicting the performance of a gas–solid fluidized bed reactor and discusses the thermal conversion of solid fuels in fluidized bed reactor.
... Despite the early success [7,8,[21][22][23][24] of the LH kinetic model for graphite oxidation by Thermodynamics requires that endothermic processes become more probable with the increase of temperature, but does not provide information on the actual rates of processes along various pathways. In reality, the competing adsorption of H 2 O and H 2 on graphite surface and desorption of oxidation products occur through a cascade of reaction steps in parallel or in series. ...
... It is proposed that this new Boltzmann-enhanced LH model (BLH) can serve as a more reliable basis of the new computer codes[19] for modeling chronic oxidation behavior of particular grades of nuclear graphite during long term exposure in HTGR and VHTR reactors. The pitfalls of the Langmuir-Hinshelwood model A comprehensive review of the basic assumptions used for derivation of the LH kinetic model was offered by Walker et al.[8]. The following non-linear reaction rate equation was reported to fit numerous experimental data from early literature on carbon (graphite) oxidation by water vapor or steam (see reaction Scheme I above). ...
Four grades of nuclear graphite were oxidized in helium with traces of moisture and hydrogen in order to evaluate the effects of slow oxidation by moisture on graphite components in high temperature gas cooled reactors. Kinetic analysis showed that the Langmuir-Hinshelwood (LH) model cannot consistently reproduce all results. In particular, at high temperatures and water partial pressures, oxidation was always faster than the LH model predicts. It was also found empirically that the apparent reaction order for water has a sigmoid-type variation with temperature which follows the integral Boltzmann distribution function. This suggests deviations from the LH model are apparently caused by activation with temperature of graphite reactive sites, which is probably rooted in specific structural and electronic properties of graphite. A semi-global kinetic model was proposed, whereby the classical LH model was modified with a temperature-dependent reaction order for water. This new Boltzmann-enhanced Langmuir-Hinshelwood (BLH) model consistently predicts oxidation rates over large ranges of temperature (800–1100 °C) and partial pressures of water (3–1200 Pa) and hydrogen (0–300 Pa). The BLH model can be used for modeling chronic oxidation of graphite components during life-time operation in high- and very high temperature advanced nuclear reactors.
... A large amount of data exists on solid carbon oxidation [11][12][13][14][15], and the specific behavior of diesel particulate formation and oxidation has been studied [7,[16][17][18][19][20][21][22][23], yet remains an active area of research. One important generalization about global oxidation rate that has arisen from previous studies is the three-zone model for carbon particle burnout [24][25][26][27], which takes into account the concurrent processes of boundary layer oxygen diffusion, intra-particle oxygen diffusion, and surface adsorption and chemical reaction. Work by investigators such as Essenhigh, Walker, and Smith [14,24,[28][29][30] has firmly established the existence of three distinct regimes in which chemical reaction or one of the diffusion processes limits the overall reaction rate. ...
... One important generalization about global oxidation rate that has arisen from previous studies is the three-zone model for carbon particle burnout [24][25][26][27], which takes into account the concurrent processes of boundary layer oxygen diffusion, intra-particle oxygen diffusion, and surface adsorption and chemical reaction. Work by investigators such as Essenhigh, Walker, and Smith [14,24,[28][29][30] has firmly established the existence of three distinct regimes in which chemical reaction or one of the diffusion processes limits the overall reaction rate. This theory has been widely accepted and used to interpret data in the coal and char literature for over 30 years. ...
The NO2 oxidation kinetics and burning mode for diesel particulate from light-duty and medium-duty engines fueled with either ultra low sulfur diesel or soy methyl ester biodiesel blends have been investigated and are shown to be significantly different from oxidation by O2. Oxidation kinetics were measured using a flow-through packed bed microreactor for temperature programmed reactions and isothermal differential pulsed oxidation reactions. The burning mode was evaluated using the same reactor system for flowing BET specific surface area measurements and HR-TEM with fringe analysis to evaluate the nanostructure of the nascent and partially oxidized particulates. The low activation energy measured, specific surface area progression with extent of oxidation, HR-TEM images and difference plots of fringe length and tortuosity paint a consistent picture of higher reactivity for NO2, which reacts indiscriminately immediately upon contact with the surface, leading to the Zone I or shrinking core type oxidation. In comparison, O2 oxidation is shown to have relatively lower reactivity, preferentially attacking highly curved lamella, which are more reactive due to bond strain, and short lamella, which have a higher proportion of more reactive edge sites. This preferential oxidation leads to Zone II type oxidation, where solid phase diffusion of oxygen via pores contributes significantly to slowing the overall oxidation rate, by comparison.
... This reaction, called the Boudouard reaction after its discoverer, has been known since 1905 and is one of the equilibria that takes place during the gasification of coal and other carbon-rich sources. 8 The reaction is highly endothermic; as such, the equilibrium lies far to the left, with CO 2 being the favored product. However, the free energy of the formation of CO 2 is relatively insensitive to temperature, while the entropy is positive; at high temperatures (>700°C is typically cited), the free energy change becomes negative, making CO formation progressively more favored. ...
... 9 For this reason, the reaction only plays a significant role in high-temperature (>900°C) gasification and smelting processes. 8,10 The contemporary appeal of this reaction is that it potentially represents a means of CO 2 remediation by converting it to more synthetically flexible CO. Its use has been proposed as part of "clean coal" schemes that convert the CO 2 product gas to CO, which can then be used to produce hydrogen via the water−gas shift reaction or hydrocarbons through the Fischer−Trøpsch process. ...
... This reaction, called the Boudouard reaction after its discoverer, has been known since 1905 and is one of the equilibria that takes place during the gasification of coal and other carbon-rich sources. 8 The reaction is highly endothermic; as such, the equilibrium lies far to the left, with CO 2 being the favored product. However, the free energy of the formation of CO 2 is relatively insensitive to temperature, while the entropy is positive; at high temperatures (>700°C is typically cited), the free energy change becomes negative, making CO formation progressively more favored. ...
... 9 For this reason, the reaction only plays a significant role in high-temperature (>900°C) gasification and smelting processes. 8,10 The contemporary appeal of this reaction is that it potentially represents a means of CO 2 remediation by converting it to more synthetically flexible CO. Its use has been proposed as part of "clean coal" schemes that convert the CO 2 product gas to CO, which can then be used to produce hydrogen via the water−gas shift reaction or hydrocarbons through the Fischer−Trøpsch process. ...
The Boudouard reaction, which is the reaction of carbon and carbon dioxide to produce carbon monoxide, represents a simple and straightforward method for the remediation of carbon dioxide in the environment through reduction: CO 2 (g) + C(s) ⇌ 2CO. However, due to the large positive enthalpy, typically reported to be 172 kJ/mol under standard conditions at 298 K, the equilibrium does not favor CO production until temperatures >700 °C, when the entropic term, −TΔS, begins to dominate and the free energy becomes negative. We have found that, under microwave irradiation to selectively heat the carbon, dramatically different thermodynamics for the reaction are observed. During kinetic studies of the reaction under conditions of flowing CO 2 , the apparent activation energy dropped from 118.4 kJ/mol under conventional convective heating to 38.5 kJ/mol under microwave irradiation. From measurement of the equilibrium constants as a function of temperature, the enthalpy of the reaction dropped from 183.3 kJ/mol at ∼1100 K to 33.4 kJ/mol at the same temperature under microwave irradiation. This changes the position of the equilibrium so that the temperature at which CO becomes the major product drops from 643 °C in the conventional thermal reaction to 213 °C in the microwave. The observed reduction in the apparent enthalpy of the microwave driven reaction, compared to what is determined for the thermal reaction from standard heats of formation, can be thought of as arising from additional energy being put into the carbon by the microwaves, effectively increasing its apparent standard enthalpy. Mechanistically, it is hypothesized that the enhanced reactivity arises from the interaction of CO 2 with the steady-state concentration of electron−hole pairs that are present at the surface of the carbon due to the space-charge mechanism, by which microwaves are known to heat carbon. Such a mechanism is unique to microwave-induced heating and, given the effect it has on the thermodynamics of the Boudouard reaction, suggests that its use may yield energy savings in driving the general class of gas−carbon reactions.
... Regarding porous media combustion, extensive research studies have considered porous combustion of coal and biomass par-ticles in the past several decades. The combustion of char was classified into three regimes (Regime I, II, and III), depending on the penetration depth of the reactants into a porous solid [19,20] . In Regime I, particle mass reduction occurs at low temperature. ...
Relatively low temperature (500 °C) combustion has been applied to shale rocks collected from Lianmuqing, Xinjiang Province, China, to improve the permeability of shale without pore structure change caused by mineral decomposition. The shale rocks were firstly grinded into small particles, then burned in a furnace at a constant temperature of 500 °C for 5 min, 15 min, and 30 min, respectively. A cylindrical shale sample was also subjected to the combustion experiment. It was found that thermal cracking occurred along the height of the shale as combustion propagated from bottom to top. From the low-temperature nitrogen adsorption test, it was found that the pore diameters of shale samples were in the range of 2–50 nm, which were less than the mean free path of an oxygen molecule. Thus, the diffusion of gas inside the shale was Knudsen diffusion and the Knudsen number (Kn) was between 8 and 25. Moreover, the mean diameter of shale pores and the effective diffusion coefficient increased with increasing oxidization time. Whereas the surface area decreased after combustion, the diameter of the shale particles remained constant. So the density of the shale decreased with increase of combustion time. According to the porous media combustion model, the oxidization of shale particles was considered to be in Regime I, which is under kinetic control at isothermal combustion condition (500 °C). Furthermore, the effective diffusion coefficient was in a range of 3 × 10⁻⁶–6 × 10⁻⁶ m²/s. It increased with increasing combustion duration, especially during the first five minutes. The experiment results showed that low temperature combustion can effectively improve shale permeability to facilitate gas extraction from shale reservoir.
... In Powder Technology, DOI: 10.1016/j.powtec.2017.09.037 Available online: Oct 5 th , 2017 13 the zone between these two temperature ranges, pore diffusion plays the most important role (Walker et al., 1959;Gray et al., 1976). Smith (1982Smith ( , 1971) assumed a first order reaction for coal char and developed a two-resistance reaction model assuming that mass transfer and surface reaction take place simultaneously, while Schmal et al. (1981) showed that, for temperature ranges above 900 ᵒC, the shrinking core model gives a better representation of the experimental data. ...
The population balance model (PBM) and the governing computational fluid dynamics (CFD) equations were solved numerically using the finite size domain complete set of trial functions method of moments (FCMOM) approach. The overall objective of this study was to demonstrate the effect of property variation of solid particles in the simulation, design, and scale up of fluidized bed processes. In this study, the process of coal char gasification in a fluidized bed at elevated temperature in the presence of hydrogen and steam as gasification media was simulated. The coupling of coal char size variations, hydrodynamics of the system, and heterogeneous gasification reaction rate was included in the simulation. The heterogeneous gasification reaction was modeled based on the available experimental data using the shrinking core model (assuming surface reaction is the limiting step). The simulation results showed that the effect of particle property variations (e.g., size) during a process such as gasification of coal char in a fluidized bed is significant in the modeling and design of such processes.
... The temperature required to gasify these deposits at a reasonable rate varies with the type of gas, the structure and reactivity of the carbon or coke, and the activity of the catalyst. Walker and co-workers [302] reported the following order (and relative magnitudes) for rates of uncatalyzed gasification at 10 kN/m 3 and 800 °C: O2 (105) > H2O (3) > CO2 (1) > H2 (3 × 10 −3 ). However, this activity pattern does not apply in general for other conditions and for catalyzed reactions [1]. ...
Catalyst deactivation, the loss over time of catalytic activity and/or selectivity, is a problem of great and continuing concern in the practice of industrial catalytic processes. Costs to industry for catalyst replacement and process shutdown total tens of billions of dollars per year. [...]
... It has been reported that heat treatment of GO in CO ambient results in the removal of oxygen with simultaneous healing of the defects in graphitic network [45]. In the present case, the reaction of CO with GO produces CO 2 , which may diffuse towards graphite and produce CO by Boudouard reaction [46] at 1000 C, thus establishing a selfsustained cyclic process, which may be responsible for the prolonged and effective reduction of GO and restoration of the graphitic network. It may also be mentioned that during the cooling of the furnace, the local equilibrium of Boudouard reaction is likely to shift towards the formation of CO 2 and C, the latter being active carbon may further function as a reducing cum healing agent. ...
... The temperature required to gasify these deposits at a reasonable rate varies with the type of gas, the structure and reactivity of the carbon or coke, and the activity of the catalyst. Walker and co-workers [302] reported the following order (and relative magnitudes) for rates of uncatalyzed gasification at 10 kN/m 3 and 800 °C: O2 (105) > H2O (3) > CO2 (1) > H2 (3 × 10 −3 ). However, this activity pattern does not apply in general for other conditions and for catalyzed reactions [1]. ...
Deactivation of heterogeneous catalysts is a ubiquitous problem that causes loss of catalytic rate with time. This review on deactivation and regeneration of heterogeneous catalysts classifies deactivation by type (chemical, thermal, and mechanical) and by mechanism (poisoning, fouling, thermal degradation, vapor formation, vapor-solid and solid-solid reactions, and attrition/crushing). The key features and considerations for each of these deactivation types is reviewed in detail with reference to the latest literature reports in these areas. Two case studies on the deactivation mechanisms of catalysts used for cobalt Fischer-Tropsch and selective catalytic reduction are considered to provide additional depth in the topics of sintering, coking, poisoning, and fouling. Regeneration considerations and options are also briefly discussed for each deactivation mechanism.
... Among the gas-phase reactions of carbon materials [1,2], the reaction with nitric oxide has gained increasing importance for both fundamental and practical reasons. The removal of NO x from stationary and mobile sources, as well as smog elimination, could benefit from its wider applicability. ...
Cognizant of the key experimental facts from studies of carbonaceous solids ranging from soot to graphite, we performed a quantum chemistry study of the interaction of NO monomer or dimer with one or more zigzag sites. Thermodynamic and kinetic results were used to examine two alternative mechanisms proposed in the literature, and to compare them with the graphene–O2 reaction mechanism. The chemisorption stoichiometry similarities are striking; but the differences, especially regarding the intermediate role of N2O, have important practical implications. Monomer chemisorption on an isolated site is a dead-end and temporarily inhibiting process, similar to that of formation of a stable C–O surface complex in the graphene–O2 reaction. When two sites are available, successive monomer adsorption eventually leads to N2O formation subsequent to parallel reorientation of the first NO molecule. If three contiguous sites are available, N2 and CO are the principal products. Chemisorption of the dimer provides a straightforward path to N2 and CO2 when one site is available and to N2 and CO when two sites are available. The formation of N2O is also feasible in this case, both during adsorption and desorption; in the adsorption phase it is very sensitive to the details of the electron pairing processes.
... It is well known, that gasification with steam as gasification agent result in higher reaction rates than gasification with CO 2 [5]. If steam is the primary gasification agent instead of CO 2 the temperature in the gasifier can therefore be lowered. ...
The two-stage biomass gasification process was developed at the Technical University of Denmark (DTU). The process is a unique biomass gasfication process: It combine stable unmanned operation, high coldgas efficiency (above 95%) and low tar content in the gas (<5 mg/Nm3), [1]. With use of modern gas engines the electric efficiency will exceed 35% and the total efficiency will exceed 100%. COWI and DTU have a long tradition of corporation within development of biomass gasification, especially together with various industrial partners [2]. In order to mature and demonstrate the two-stage technology COWI and DTU corporate regarding upscale and further process development. In order to upscale to plants of 1MWe and above the two-stage gasification process is modified [3]. The upscaleable concept, which both offer low tar and high efficiency can be designed as both moving (fixed) beds and fluidbeds. The first two industrial partners are now involved in the upscale and further development of the two-stage technology. Weiss A/S regarding the moving bed and Babcock&Wilcox Voelund regarding fluid bed technology.
... In particular, CO 2 char gasification reactivity is important in gasifiers since it is not only one of the main char gasification reactions but also the lowest reaction rate in char reaction of entrained gasifier. The temperatures at which different physical processes dominate the observed reaction rate have been described in terms of three zones ( Fig. 1(a)) [7,16]: zone I (at low temperature), in which intrinsic chemical reactivity alone controls the particle reaction rate, zone III (at very high temperature), in which bulk diffusion to the particle surface controls the overall particle reaction rate; and zone II, in which the combined effects of pore diffusion and intrinsic chemical reactivity control the overall particle reaction rate. In the case of zone I and II, char gasification reactivity is also affected by pore structure ( Fig. 1(b)) [17,18]. ...
... Studies of intrinsic char reactivity have revealed information regarding the mechanisms of char−gas reactions (e.g., see ref 24), aspects of kinetic descriptions of char gasification systems, 25 and how these kinetics are affected by product gas concentrations 26 and increased partial pressures of reactants. 27,28 Some studies have begun to investigate the interactions between individual reactions and reactants at a reacting char surface. ...
Many factors affect the gasification reactivity of chars. These include physical structure (e.g., surface area and pore structure), catalytically active inorganic species, and chemical structure (the extent of ring condensation of the carbonaceous matrix and the nature of the functional groups that comprise the char structure). This work uses Raman spectroscopy to investigate the relationships between the char chemical structure and intrinsic reactivity. Chars made from an Australian bituminous coal and a Chinese lignite at temperatures of 900 and 1100 °C were characterized in terms of their intrinsic reactivities with CO2 and H2O (separately). Unreacted and partially reacted chars were characterized using Raman spectroscopy and gas adsorption to determine chemical structure indicators and surface area, respectively. The ratio of small to large aromatic rings decreased with increasing carbon conversion for all chars in all reactants, and for the bituminous coal char, this was related to the reactivity behavior as a function of conversion. While reaction seemed to affect the structure of the lignite chars in a similar manner, this did not influence the intrinsic reactivity of the lignite chars to the same extents, perhaps because of the significant influence of catalytic activity of inorganic species.
... The actual burnout of the char will follow processes dictated by both the reactivity of the char as well as the mass transfer characteristics in the boiler. Porous solid–gas reactions are often characterized by three reaction regimes: kinetic control (sometimes called Zone I), pore diffusion control (Zone II), and film diffusion control (Zone III), with the shorthand zone nomenclature adopted from the work of Walker et al. [90]. It was once commonly assumed that the temperatures of combustion, and thus rates, are sufficiently high in pulverized coal combustors that the combustion mostly takes place under Zone III conditions. ...
The control of mercury in the air emissions from coal-fired power plants is an ongoing challenge. The native unburned carbons in fly ash can capture varying amounts of Hg depending upon the temperature and composition of the flue gas at the air pollution control device, with Hg capture increasing with a decrease in temperature; the amount of carbon in the fly ash, with Hg capture increasing with an increase in carbon; and the form of the carbon and the consequent surface area of the carbon, with Hg capture increasing with an increase in surface area. The latter is influenced by the rank of the feed coal, with carbons derived from the combustion of low-rank coals having a greater surface area than carbons from bituminous- and anthracite-rank coals.The chemistry of the feed coal and the resulting composition of the flue gas enhances Hg capture by fly ash carbons. This is particularly evident in the correlation of feed coal Cl content to Hg oxidation to HgCl2, enhancing Hg capture. Acid gases, including HCl and H2SO4 (at small concentrations) and the combination of HC1 and NO2, in the flue gas can enhance the oxidation of Hg.In this presentation, we discuss the transport of Hg through the boiler and pollution-control systems, the mechanisms of Hg oxidation, and the parameters controlling Hg capture by coal-derived fly ash carbons.
... The rate constants were determined for the binary systems CO 2 /CO and H 2 O/H 2 at pressures of up to 70 bar. Note that squared and hydrogasification terms contained in Muehlen's original work have been neglected in Eq. (2) since they have been shown to be 3-5 orders of magnitude smaller at the high temperatures characteristic of an EFG [33,34]. The work conducted by Muehlen et al. is of especial significance. ...
Coal–CO2 slurry feed has been suggested as an attractive alternative to coal–water slurry feed for single-stage, entrained-flow gasifiers. Previous work demonstrated the system-level advantages of gasification-based plants equipped with CO2 capture and CO2 slurry feed, under the assumption that carbon conversion remains unchanged. However, gasification in carbon dioxide has been observed to be slower than that in steam. In view of this, the impact of CO2 slurry feeding on gasification kinetics and ultimately on carbon conversion and oxygen consumption in a pressurized, single-stage entrained-flow gasifier processing bituminous coal is studied here using a 1-D reduced order model. Results show that the CO2 gasification reaction plays a dominant role in char conversion when the feeding system is CO2 slurry, increasing the CO content in the products by up to a factor of two. CO inhibition of the gasification reaction and a higher degree of internal mass transport limitations lead to an up to 60% slower gasification rate, when compared to a system based on coal–water slurry. Accordingly, a gasifier with CO2 slurry feed has 15% less oxygen consumption but a 7%-point lower carbon conversion for a given reactor outlet temperature. The gasifier outlet temperature must be raised by 90 K in order to achieve the same conversion as in a water slurry-fed reactor; the peak reactor temperature increases by 220 K as a result. Net oxygen savings of 8% are estimated for a system with a CO2 slurry-fed gasifier relative to one with water slurry and the same level of conversion.
... However, the time it takes to reach a specific weight loss depends on temperature, and hence the mode of oxidation (Fig. 1), the oxygen partial pressure, and the type of graphite. Below 400 @BULLET C, graphite gasification is negligible, but at higher temperatures (Fig. 1), it progressively proceeds through 3 basic modes (Walker et al., 1959; Fuller and Okoh, 1997; Xiaowei et al., 2004; El-Genk and Tournier, 2012), which depend on temperature: Mode (a) (low temperatures): in which graphite gasification is solely controlled by the kinetics of the elementary chemical reactions . In this mode, graphite gasification occurs mostly within open volume pores, and thus does not change the outside dimensions, but progressively increases the size of open pores and access to previously closed pores (Fig. 1a). ...
This paper introduces a chemical kinetics model and compares its calculations with reported measurements of weight loss and total gasification rate for different NBG-18 nuclear graphite specimens in experiments performed at 876–1226 K. Results show that the gasification rate is chemical-kinetics limited at low and intermediate temperatures and diffusion-limited at high temperatures. At high temperatures, the model calculates the diffusion velocity of oxygen through the boundary layer using a developed correlation for Reynolds numbers of 0.006–1000. The agreement of the calculations with reported measurements of the total gasification rate and transient weight loss confirms the soundness of the chemical kinetics approach and validates the developed model and the multi-parameter optimization algorithm for determining the chemical kinetics parameters, based on reported measurements. These parameters are the values and Gaussian-like distributions of the specific activation energies for oxygen adsorption and desorption of CO, the specific activation energy for desorption of CO2, the initial surface area of free active sites and the rate constants for the four elementary chemical reactions in the model. The performed parametric analyses for NBG-18 nuclear graphite specimens investigated the effects of temperature and oxygen partial pressure on total gasification rate and production rates of CO and CO2 gases, for wide ranges of temperatures and oxygen partial pressures.
This article was renamed after peer-review as: Oyarzún-Aravena, A.M., Gottschalk-Ojeda, C., Moya-Barría, I., and Vallejos-Burgos, F., Edge type effect in the gasification mechanism of graphene clusters with H2O and/or CO2: armchair vs. zigzag. Carbon, 2022. 193: p. 412-427.
https://doi.org/10.1016/j.carbon.2022.02.048
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Amorphous calcium lignosulphonate (CaLS), a by-product of the paper industry, was evaluated as an adhesive to bind demineralised bituminous coal fines in various weight percentage ratios. Sequential acid (HCl, HF, HCl) leaching was used to reduce mineral matter content in the coal fines. The pyrolysis and gasification of the samples were investigated in laboratory-scale experiments to determine the effect of the CaLS addition on the pyrolysis and gasification reactivities. Coal-CaLS blends consisting of 5, 10, and 15% calcium lignosulphonate were prepared, compressed into pellets, and characterised using proximate, X-ray diffraction, and X-ray fluorescence analyses. An increase in binder concentration increases the mechanical strength of the pellets due to the increased interparticle contact area. The relative coal gasification reaction reactivity (1/Tmax and 0.5/T50), which was determined from the derivative thermogravimetry curves, increases with CaLS addition. The experimental results revealed that these wastes (coal fines and calcium lignosulphonate) could be utilised in thermochemical processes. Utilising these wastes will reduce the environmentally unfriendly volumes of amorphous calcium lignosulphonate and coal fines particles.
Fe is an inexpensive and safe catalyst for gasification, but the corresponding catalytic mechanism, in particular the role of the interaction between Fe and C, has not been theoretically described. In this study, DFT is used to search for possible CO2 gasification paths at the two char edges (armchair edge and zigzag edge) catalyzed by Fe. The optimal reaction path is determined by path energy analysis, and the catalytic effect of Fe at the armchair edge is more obvious. Because of its strong interaction with C, Fe has an obvious catalytic effect on gasification adsorption and desorption. In contrast to alkali metals and alkaline earth metals, Fe yields unique catalytic pathways, and its active intermediate is more likely to exist in the form of C-Fe-CO. The catalytic mechanism of Fe is explained by electron wave function information theory: In the adsorption process, Fe can easily adsorb CO2 and bond with the C in CO2 through the electrostatic charge and d orbital. In the desorption process, Fe can destroy the aromaticity of the char edge, weakening the bonds of the carbon ring. The active d electrons of Fe can enter the antibonding orbital of the C–C bond to promote breaking, thereby promoting CO desorption.
Extraction residue (ER) is, in this study, considered as an insoluble portion derived from a novel extraction process with direct coal liquefaction residue, which is the byproduct from 1000 kton/y direct coal liquefaction and oil production process in Ordos, China. ER mostly consists of unreacted coal and minerals, spent catalyst, and solvent. It is considered as a hazardous waste in China. Gasification is a promising way to employ ER, as the carbon resource contained in it could be adequately utilized, so strongly reducing the environmental impact of this material. In this paper, a kinetic study is carried out concerning CO2 gasification (in thermobalance) of ER char, coal char, and their blends. The results show that, at temperatures ranging from 1123 to 1323 K, the reactivity of ER char is much larger than that of coal char, and the reactivity of ER/coal char blends increases when the ER fraction increases, consistently resulting in between the reactivities shown by ER char and coal char alone. The partial pressure of CO2 promotes the reactivity of all samples. BET surface area resulted three times larger for ER char vs coal char, while pore surface area and pore volume were four times larger. Moreover, the relevant parameters of the random pore model for CO2 gasification of ER char, coal char, and their blend #2 (with 20% ER) were obtained; the activation energy resulted 210.6, 240.5, and 232.8 kJ/mol, and the reaction order 0.30, 0.45, and 0.40, respectively.
The oxidation behavior of matrix‐grade graphite in air‐ or steam‐ingress accident scenarios is of great interest for high‐temperature gas reactors (HTGRs). In this study, the microstructures of two variants of matrix‐grade graphite based on the German A3‐3 and A3‐27 formulations were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy, and correlated to oxidation behavior observed through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Through TEM imaging and selected area electron diffraction (SAED), a higher volume fraction of partially graphitized carbon was identified in the A3‐3 type graphite than in the A3‐27 type. This structure is believed to have contributed to the accelerated oxidation exhibited by A3‐3 in the chemical reaction‐controlled oxidation regime.
Gasification is a thermal process involving the interaction between physical and chemical processes. Thus, it is necessary to understand the operating conditions in the design and operation. The optimum conversion of the solid fuel to the desired gas will depend heavily on the configuration, size and selection of gasifier operating condition. Optimum operating conditions can be obtained through research on existing plants or testing on a particular equipment. In addition, the optimum conditions can also be obtained through mathematical modeling. Although it cannot provide very a ccurate results, this method can provide clues about the quantitative effects of design, raw materials and other operating parameters. Representation of the gasification process in the form of mathematical equations can also help understand the importance of the effect of operating parameters on the performance of a gasifier and can provide a good understanding of the mechanisms that occur. Gasification involves mass transfer events and chemical reactions. The kinetic equations constructed in the gasification process are mostly based on heterogeneous (solid-gas) reactions whereas in gasification reactions and there is also a homogeneous reaction (gases). It is therefore necessary to prepare a kinetic calculation involving heterogeneous reactions and homogeneous reactions. Random Pore Model was first proposed by Bhatia and Perlmutter. The model uses the assumption that char consists of cylindrical pores with a random size distribution. The rate of heterogeneous reactions, which take place on the contact surface of solid and reactant gas, are dominated by the inner surface area of the pores. The diameter of the pores also influences the transportation of the reactant gases into the solid and the resultant gases out of the solid. The homogeneous reactions are those among gas species; both volatile gases and gasification agent gases reactions include water gas shift reaction. The developed model for the steam gasification of solid chars has been solved under the operating temperatures of 600°C, 700°C and 800°C as used in this experiment. The model has been solved in MATLAB software using a numerical technique.
The proven impact of combustion chamber deposits, CCD, on advanced compression ignition combustion strategies has steered researchers toward the development of thermal barrier coatings, TBC, which can mimic CCD benefits on combustion efficiency and operational range expansion. However, recent work based on statistical thermodynamics has indicated that inter-molecular radiation during the combustion event may subject the combustion chamber walls to non-negligible radiation heat transfer, regardless of the relatively low soot formation within the well-mixed and lean charge. Additionally, application of thermal barrier coatings within multi-mode engines provides an opportunity for exposure of the TBC to non-negligable soot radiation. In the present paper, the impact of radiation heat transfer on combustion chamber deposits is studied. The morphological construction of the combustion chamber deposit layer is shown to be partially transparent to radiation heat transfer, drawing corollaries with ceramic-based thermal barrier coatings. Additional experimentation eliminates the optical transparency of CCD to reveal an “effective radiation penetration depth” facilitated by open surface porosity. The effective radiation penetration depth of combustion chamber deposits and a magnesium zirconate TBC are then established. The impacts of CCD/TBC radiation penetration and transparency on the time varying surface temperature trend are revealed through an analytical investigation. With only a portion of the total heat transfer attributed to radiation, the surface temperature swing of the thermal barrier is impacted, altering the projected impacts of the thermal barrier coating on advanced compression ignition operation. When a viable radiation component is present in-situ, enabling shallow penetration of radiation heat flux into a TBC enhances the TBC surface temperature swing, which can amplify the TBC efficiency benefits.
Graphite is a key component in designs of current and future nuclear reactors whose in-service lifetimes are dependent upon the mechanical performance of the graphite. Irradiation damage from fast neutrons creates lattice defects which have a dynamic effect on the microstructure and mechanical properties of graphite. Transmission electron microscopy (TEM) can offer real-time monitoring of the dynamic atomic-level response of graphite subjected to irradiation; however, conventional TEM specimen-preparation techniques, such as argon ion milling itself, damage the graphite specimen and introduce lattice defects. It is impossible to distinguish these defects from the ones created by electron or neutron irradiation. To ensure that TEM specimens are artifact-free, a new oxidation-based technique has been developed. Bulk nuclear grades of graphite (IG-110 and NBG-18) and highly oriented pyrolytic graphite (HOPG) were initially mechanically thinned to ∼60 μm. Discs 3 mm in diameter were then oxidized at temperatures between 575 °C and 625 °C in oxidizing gasses using a new jet-polisher-like set-up in order to achieve optimal oxidation conditions to create self-supporting electron-transparent TEM specimens. The quality of these oxidized specimens were established using optical and electron microscopy. Samples oxidized at 575 °C exhibited large areas of electron transparency and the corresponding lattice imaging showed no apparent damage to the graphite lattice.
This study has investigated the laboratory scale thermal oxidation of nuclear graphite, as a proof-of-concept for the treatment and decommissioning of reactor cores on a larger industrial scale. If showed to be effective, this technology could have promising international significance with a considerable impact on the nuclear waste management problem currently facing many countries worldwide. The use of thermal treatment of such graphite waste is seen as advantageous since it will decouple the need for an operational Geological Disposal Facility (GDF). Particulate samples of Magnox Reactor Pile Grade-A (PGA) graphite, were oxidised in both air and 60% O2, over the temperature range 400–1200°C. Oxidation rates were found to increase with temperature, with a particular rise between 700–800°C, suggesting a change in oxidation mechanism. A second increase in oxidation rate was observed between 1000–1200°C and was found to correspond to a large increase in the CO/CO2 ratio, as confirmed through gas analysis. Increasing the oxidant flow rate gave a linear increase in oxidation rate, up to a certain point, and maximum rates of 23.3 and 69.6 mg / min for air and 60% O2 respectively were achieved at a flow of 250 ml / min and temperature of 1000°C. These promising results show that large-scale thermal treatment could be a potential option for the decommissioning of graphite cores, although the design of the plant would need careful consideration in order to achieve optimum efficiency and throughput.
The reaction between carbon and nitric oxide is as complex as that of carbon with O2. Its importance has grown tremendously over the past several decades, as society struggles to prevent acid rain and smog formation through NOx reduction. Use of carbon as a convenient reducing agent (e.g., 2C+2NO=C+CO2+N2), or even better as a catalyst (2C+2NO=O2+N2), has been hampered by a lack of understanding of key details of the reaction mechanism. Here we use computational quantum chemistry to explore similarities and differences with respect to the much better understood, both experimentally and theoretically, carbon combustion reaction. We emphasize the adsorption process on zigzag and armchair sites of graphene, keeping in mind that, once the carbon-oxygen surface complexes are formed, the similarities between the two reactions are likely to increase (e.g., 2C(O)=C+CO2).
Biomass—forestry and agricultural residues—can locally be directly utilized for combustion. However, conversion of these solid materials to gaseous products is of great importance to make transport of energy practicable.
The aim of this paper is to remind briefly the state of knowledge in the carbon gasification in O2, CO2 and H2O as it emerges from the papers presented at the international conference “La combustion du carbone” held in Nancy (1949). Analysis of the experimental results and of their interpretation is made with reference to the main topics of which most are still discussed today, but sometimes with other final (practical) objectives. Many fundamental concepts and especially the carefully described kinetic features of combustion at very high temperatures are far from being obsolete. On the other hand, deficiencies in carbons characterization and in mass transfer information prevent the then research workers to check experimentally many suspected facts or phenomena. This paper is hoped to help avoiding oblivion of valuable works and to point out to young researchers the continuous, but slow progress in this field of science.
The chemical vapor deposition of silica from tetraethoxysilane (TEOS) was studied on a model carbon substrate (V3G graphitized carbon black) with the goal of a better understanding of the carbon-silica interface. Deposition was carried out in a static high vacuum system at 848 K and a starting pressure in the range from 0.015 T to 0.15 T. Deposition occured preferentially on the carbon active sites accessible to the reactant molecule. The reactivity of the substrate toward silica deposition was seen through many layers of deposit indicating an epitaxial type of growth. The active sites on the carbon surface were found to have a higher intrinsic reactivity toward TEOS pyrolysis and silica deposition than either the silica deposit or other silica surfaces. The silica surfaces in turn were found to be more reactive than the carbon basal plane.
The phenomenon of sulfur loss during the calcining of petroleum cokes and the subsequent impact on anode quality has been widely reported in the literature. However, the fact that these same petroleum cokes undergo a further extensive heat treatment during anode baking, which can also lead to sulfur loss, has not been as well documented. Although anodes are subjected to lower maximum temperatures during baking than are petroleum cokes during calcining, the exposure time in the baking operation is much greater. In this study, the combined effect of baking furnace time and temperature have been examined and found, under certain conditions, to result in anode desulfurization leading to poorer anode quality. © 2013 The Minerals, Metals and Materials Society. Published 2013 by John Wiley and Sons, Inc.
The capture and sequestration of CO2 generated from largescale stationary power plants is considered to be one of the leading technologies that could potentially have a significant impact on reducing greenhouse emissions. Among these emerging technologies, the oxy-fuel combustion is a near-zero emission technology that can be adapted to both new and existing pulverized coal-fired power stations. The goal of this work is to make a comparative study on char structural characteristics (including char yield, swelling ratio, BET surface area, pore distribution, morphology) and reactivity during conventional air and oxy-fuel combustion. Specific experimental designs include two series. One is carried out in pure N2 and CO2 (pyrolysis experiments), and another is prepared in N2+5%O2 and CO2+5%O2. Coal samples included raw coal, low density fraction coal and medium density fraction coal in all experiments. The present study is a further effort to extend our knowledge about physical and chemical structural characteristics and reactivity of char in the presence of high concentration CO2. Combustion and pyrolysis of a density fractionated China coal at drop tube furnace yielded the following conclusions. Compared to oxy-chars obtained under pure CO2 atmosphere, the swelling ratios of char obtained in pure N2 atmosphere are higher. When adding 5% O 2, experimental results are completely different with those of the pyrolysis experiment. In comparison with the oxy-chars obtained under CO 2 + 5%O2 atmosphere, the swelling ratios of the char obtained in N2 + 5%O2 atmosphere are lower. In the pyrolysis experiment, the BET surfaces Area of the oxy-chars are about ten to twenty times as much as chars. When adding 5% O2, the BET surfaces Area of the oxy-chars are about two to four times as much as chars. During pyrolysis experiment, the total pore volumes of the oxy-chars obtained under pure CO2 are larger in comparison with chars obtained in pure N 2. The pore distributions of the oxy-chars are mainly mesopore and the chars have macropore mostly. The micropore and mesopore volume of oxy-chars are about ten times as much as chars separately and the macropore volume of oxy-chars are about two times as much as chars. When adding 5% O2, the total pore volumes of the oxy-chars obtained under CO2 + 5%O 2 are also larger in comparison with chars obtained in N2 + 5%O2. In the pyrolysis experiment, the reactivity indexes of oxy-chars obtained under pure CO2 are lower in comparison with chars obtained in pure N2. When adding 5% O2, no significant differences can be observed, which may be due to the ordering of carbon microcrystalline structure. In pure CO2 atmosphere, the oxy-chars have a highly order polycrystalline structure, but chars obtained at pure N 2 atmosphere have a highly disordered carbonaceous structure. © Tsinghua University Press, Beijing and Springer-Verlag Berlin Heidelberg 2012.
The oxyacetylene torch facility is used to measure the ablation rates of graphite and the surface temperatures of different aerospace materials. The free-stream flame environment is characterized as a function of flame chemistry for heat flux, pO2, and flow velocity. Measured ablation rates for graphite increase as a function of increasing heat flux and pO2, which are validated by applying an oxygen diffusion based model. The model uses experimentally measured values for temperature, pO2, and gas velocity in order to confirm torch testing results are reliable and reproducible. Surface temperatures of ultra-high temperature ceramic composites are measured as a function of increasing heat flux and show an enthalpic cooling effect on the flame during oxidation testing.
SYNOPSIS This paper outlines the principles involved in the regeneration of activated carbon for renewed use in the recovery of precious metals or the purification of water. While chemical methods are effective in restoring the activity of spent activated carbon containing only single or defined adsorbates, thermal methods have to be used when the spent carbon is loaded with a heterogeneous mixture of adsorbates such as those normally present in industrial process streams or effluents. The steps involved in thermal regeneration are described, together with the operating conditions that can be controlled to give the required degree of ectivation. Finally, an account is given of the structural changes that occur in a new or spent activated carbon during thermal treatment. SAMEV A TTING Hierdie referaat bespreek in hooftrekke die beginsels betrokke by die regenerering van geaktiveerde koolstof vir hernieude gebruik in the herwinning van edelmetale of die suiwering van water. Terwyl chemiese metodes doel-treffend is om die aktiwiteit van uitgewerkte koolstof met net enkele of omskrewe adsorbate te herstel, moat termiese metodes gebruik word wanneer die uitgewerkte koolstof gelaai is met 'n heterogene mengsel van adsorbate BOOS die wat gewoonlik in nywerhe,idsprosesstrome of -uitvloeisels aanwesig is. Die stappe betrokke by termiese regenerasie word beskryf, tesame met die bedryfstoestande wat beheer kan word om die vereiste graad van aktivering te gee. Ten slotte word die strukturele veranderinge bespreek wat tydens die termiese behandeling in 'n nuwe of uitgewerkte koolstof plaasvind. Introduction The economic feasibility of processes using granular activated carbon for the recovery of precious metals or the purification of water and waste-water is contingent upon re-use of the carbon in multiple adsorption-regeneration cycles. Different techniques can be used to restore the activity of a spent (exhausted) activated carbon.
This paper deals with the thermal behaviour of 36Cl in nuclear graphite used in UNGG French reactors (graphite moderated and CO2 cooled reactors). Implanted 37Cl simulates 36Cl displaced from its original structural site by recoil. The implanted nuclear graphite samples were annealed in the 200–1600 °C temperature range and the 6–50 h time range. Structural modifications were followed by Raman microspectrometry. 37Cl concentration depth profile evolution was determined by Secondary Ion Mass Spectrometry or by Rutherford Backscattering Spectrometry, depending on the implantation fluence. This study shows a correlation between the reordering of the graphite structure with annealing temperature and the chlorine release. It evidences also two distinct chlorine release steps with different kinetics (a rapid one followed by a much slower one) and suggests the presence of two different chlorine trapping sites. A low energy site at edge surface of crystallites (or coherent domains) and a high energy one located inside the crystallites (or coherent domains) for which temperatures higher than 1300 °C are required to allow chlorine removal.
We propose a novel and simple method to uniformly deposit a thin carbon layer on the pore walls in mesoporous silica (MPS) by the chemical vapor deposition (CVD) at as low a temperature as 600 °C. Four types of MPSs (FSM16, SBA15, TMPS16 and MCM41) were trimethylsilylated and then the silylated MPSs were subjected to the CVD using acetylene. Thanks to the silylation, pyrolytic carbon deposition was significantly enhanced, although only little carbon deposition occurred on the non-silylated MPSs. It is found from the analyses of the former three types of MPSs that the carbon was deposited solely on the mesopore surface as a very thin layer and thereby the carbon-coated MPSs still keep a high surface area and a large pore volume with their mesopore structures intact. The decomposition of the trimethylsilyl groups during the CVD resulted in the formation of Si radicals on the silica surface and their catalysis can explain the observed uniform carbon coating, i.e., the radicals catalyze the trimerization of acetylene to form benzene and induce the carbonization on the silica surface. This mechanism allows the carbonization to occur only on the silica surface and the uniform carbon deposition was achieved as a result.
A new thermochemical process for (Fischer–Tropsch) FT fuels and electricity coproduction based on steam hydrogasification is addressed and evaluated in this study. The core parts include (Steam Hydrogasification Reactor) SHR, (Steam Methane Reformer) SMR and (Fisher–Tropsch Reactor) FTR. A key feature of SHR is the enhanced conversion of carbon into methane at high steam environment with hydrogen and no need for catalyst or the use of oxygen. Facilities utilizing bituminous coal for coproduction of FT fuels and electricity with carbon dioxide sequestration are designed in detail. Cases with design capacity of either 400 or 4000 TPD (Tonne Per Day) (dry basis) are investigated with process modeling and cost estimation. A cash flow analysis is performed to determine the fuels (Production Cost) PC. The analysis shows that the 400 TPD case due to a FT fuels PC of 5.99 /gallon and 2.27 $/gallon diesel equivalent at overall carbon capture ratio of 65% and 90%, respectively. Prospective commercial economics benefits with increasing plant size and improvements from large-scale demonstration efforts on steam hydrogasification.
Chars prepared from potassium-exchanged carboxy methyl cellulose at several heat treatment temperatures (HTTs) were gasified in air isothermally at selected gasification temperatures (GTs) in the range 633–893K to investigate the catalytic effectiveness of potassium species. The chars displayed a noticeable jump in gasification rate at a particular gasification temperature (called jump temperature, Tj). The magnitude of jump was much less than that reported for copper and nickel catalysis, but comparable with that for calcium catalysis. Increase in HTT caused a decrease in the jump temperature of chars in contrast with the increase observed in copper, nickel and calcium catalysis; also the magnitude of jump did not decrease, but remained unaltered, on increasing HTT. The different behavior of potassium catalysis is correlated to a change in the chemical state of potassium at higher HTT. The results reveal the dependence of jump phenomenon on chemical state and dispersion of catalyst in the char.
Isothermal and non-isothermal thermogravimetric experiments (TG) with real and synthetic (Printex U) soot were performed at
different O2 concentrations (5–22%O2/N2), sample masses (0.5–10 mg), heating (5–20 °C min−1) and flow rates (80–100 mL min−1). The significance of the experimental and calculation uncertainties (i.e. experimental parameter dependencies, calculation
method and mass transfer limitations), which are related to TG for the extraction of chemical kinetics, was explored. Finally,
an intrinsic kinetic equation for soot oxidation is proposed.
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