Effect of pore diffusion on reactivity of lump coke

The gasification of fine-grained and lump coke in CO₂ atmosphere was measured in a laboratory reactor. Owing to the effect of pore diffusion, the reactivity of lump coke at 1050 °C was found to be considerably lower than that of fine-grained coke at otherwise the same conditions. The diffusion effect is more significant for cokes produced by a stamping method than for cokes from top charges. To express the gasification of lump coke, a simplified mathematical model considering the effect of pore diffusion on gasification rate was adopted. Kinetic parameters were determined from experiments with fine-grained coke, while effective diffusivity was evaluated from experiments with a single coke particle. The model fits experimental data of lump coke well. The relation between reactivity and textural characteristics was also examined: the correlation between effective diffusivity and the fraction of largest pore volume was found.     

Gasification of oil sand coke: Review

The production of synthetic crude from the tar sands in Western Canada has been steadily increasing. Most of the delayed coke produced by Suncor is combusted on site, whereas all fluid coke produced by Syncrude is stockpiled. The database on the chemical and physical properties of the oil sand coke, including the composition and fusion properties of the mineral matter, has been established. The reactivity of the coke was determined by oxygen chemisorption, fixed bed and fluid bed bench scale gasification and pilot plant gasification. The reactivity of the oil sand coke for gasification is rather low and comparable to high rank coals, such as anthracite. Slurrability tests revealed that a solid concentration in water, approaching 70 wt.%, can be achieved. Gasification is the front runner among clean technologies for the conversion of carbonaceous solids to useful products. Several commercial gasifiers are available to cover the wide range of severity. Because of the low reactivity of oil sands coke, high severity conditions are required to achieve high gasification conversion. Such conditions can be attained in entrained bed gasifiers. Gasifiers employing both dry and slurry feeding systems are suitable. A high efficiency, low SO_x and NO_x emissions, as well as a low solid waste production are among the key advantages of the gasification technology compared with the competing technologies. Commercial gasification of oil sands coke is delayed because of the availability of natural gas on the site of the upgrading plants. Potential for the transportation of the oil sand coke to USA for electricity generation using the integrated gasification combined-cycle (IGCC) technology was evaluated.     

Laboratory-scale coal reactivity testing for entrained gasification

A relatively simple and rapid micro-gasification test has been developed for measuring gasification reactivities of carbonaceous materials under conditions which are more or less representative of an entrained gasification process, such as the Shell coal gasification process. Coal particles of < 100 μm are heated within a few seconds to a predetermined temperature level of 1000–2000 °C, which is subsequently maintained. Gasification is carried out with either CO₂ or H₂O. It is shown that gasification reactivity increases with decreasing coal rank. The CO₂ and H₂O gasification reactions of lignite, bituminous coal and fluid petroleum coke are probably controlled by diffusion at temperatures 1300–1400 °C. Below these temperatures, the CO₂ gasification reaction has an activation energy of about 100 kJ mol^?1 for lignite and 220–230 kJ mol^?1 for bituminous coals and fluid petroleum coke. The activation energies for H₂O gasification are about 100 kJ mol^?1 for lignite, 290–360 kJ mol^?1 for bituminous coals and about 200 kJ mol^?1 for fluid petroleum coke. Relative ranking of feedstocks with the micro-gasification test is in general agreement with 6 t/d plant results.     

微型流化床反应分析及其对煤焦气化动力学的应用

在概述最新研发的微型流化床反应分析(micro-fluidized bed reaction analysis,MFBRA)方法与应用的基础上,应用该方法进一步研究了半焦-CO₂、半焦-水蒸气等温气化反应动力学,并与热重分析(thermogravimetric analyzer,TGA)求取的气化反应动力学数据比较。在最小化气体扩散的实验条件下,利用MFBRA和TGA测定求算的半焦-CO₂、半焦-水蒸气气化反应在受反应动力学控制的低温段的活化能非常接近,说明了MFBRA对等温气化反应分析的适用性和可靠性。实验研究还发现:半焦-CO₂、半焦-水蒸气气化反应在MFBRA中受反应动力学控制的温度范围较在TGA中明显宽,且在具有明显扩散影响的高温段通过MFBRA测定的半焦-CO₂气化反应表观活化能明显大于利用TGA测定的值,表明在MFBRA中受到的气体扩散抑制效应较小。     

Potassium-catalyzed steam gasification of petroleum coke for H2 production: Reactivity, selectivity and gas release

Potassium-catalyzed steam gasification of petroleum coke for H₂ production was performed using a laboratory fixed-bed reaction system with an on-line quadruple mass spectrometer. The gasification reactivity, gasification selectivity and gas release for the catalytic gasification were investigated, compared with the non-catalytic gasification. The catalytic gasification could not only effectively promote these reactions (the water-carbon reaction, the water-gas shift reaction and the methane-steam reforming reaction), but also elevate greatly the gasification selectivity towards CO₂ (a high gasification selectivity towards CO₂ meant a high H₂ production). A quantitative calculation method for the gasification selectivity towards CO and CO₂ was proposed to further understand the catalytic behaviors of catalysts. In the case of catalytic gasification, the gasification temperature had opposite effects on the gasification reactivity and the gasification selectivity towards CO₂, suggesting that there existed an optimum gasification temperature (about 750 °C) for H₂ production from the potassium-catalyzed steam gasification of petroleum coke. In addition, petroleum coke could be feasibly utilized as the feedstocks for the catalytic steam gasification to produce gases with high H₂ (55.5-60.4%) and virtually no CH₄ (below 0.1%).     

CFD simulations of mass transfer from Taylor bubbles rising in circular capillaries

Computational Fluid Dynamics (CFD) is used to investigate mass transfer from Taylor bubbles to the liquid phase in circular capillaries. The liquid phase volumetric mass transfer coefficient k_La was determined from CFD simulations of Taylor bubbles in upflow, using periodic boundary conditions. The separate influences of the bubble rise velocity, unit cell length, film thickness, film length, and liquid diffusivity on k_La were investigated for capillaries of 1.5, 2 and diameter. The mass transfer from the Taylor bubble is the sum of the contributions of the two bubble caps, and the film surrounding the bubble. The Higbie penetration model is used to describe the mass transfer from the two hemispherical caps. The unsteady-state diffusion model of Pigford is used to describe the mass transfer to the downward flowing liquid film. The developed model for k_La is in good agreement with the CFD simulated values, and provides a practical method for estimating mass transfer coefficients in monolith reactors.     

Catalytic gasification of metallurgical coke by carbon dioxide using potassium salts

The catalytic gasification of metallurgical coke by carbon dioxide with potassium salts is studied from the view-point of the catalytic activity of the salts, the optical texture of the coke and its reactivity, monitoring weight losses and the detail of topographical change induced in the coke surface by gasification. The activity of the catalyst and the reactivity of the coke were found to be dependent upon the anion of the salt and optical texture of the coke. Potassium metal may be produced in situ in the gasification reactions and has a significant role in the mechanism of catalytic gasification. The reactivity of a coke and its performance in a blast furnace must be associated both with extents of gasification and development of pits and fissures which could promote a weakening of the coke.     

Catalytic graphitization of a phenolic resin carbon by nickel (~100 μm): Selective gasification of three resultant components as studied by SEM

A phenolic resin carbon containing nickel particles, ~100 μm diam., was graphitized to 2773K for 1 h. This treatment created within the specimen three structural components known as graphitic (G), turbostratic (T_s) and matrix (A). The graphitized carbon was oxidized by carbon dioxide, HNO₃-H₂SO₄ mixed acid and Simon's reagent and topographical changes were monitored by SEM. The CO₂ gasified preferentially the T_s-component to produce fissures mainly at the interface with the A- andG-component. The mixed acid reacted preferentially with the G- andT_s-components, the Simon's reagent reacting significantly only with the G-component. The mineral matter of metallurgical coke can act as a graphitization catalyst similar to nickel particles. Hence, the development of structural components within the metallurgical coke followed by gasification by carbon dioxide may cause weakening of the coke at the interfaces of components.     

Gasification of heavy fuel oils: A thermochemical equilibrium approach

Combustion of heavy fuel oils is a major source of production of particulate emissions and ash, as well as considerable volumes of SO_x and NO_x. Gasification is a technologically advanced and environmentally friendly process of disposing heavy fuel oils by converting them into clean combustible gas products. Thermochemical equilibrium modeling is the basis of an original numerical method implemented in this study to predict the performance of a heavy fuel oil gasifier. The model combines both the chemical and thermodynamic equilibriums of the global gasification reaction in order to predict the final syngas species distribution. Having obtained the composition of the produced syngas, various characteristics of the gasification process can be determined; they include the H₂:CO ratio, process temperature, and heating value of the produced syngas, as well as the cold gas efficiency and carbon conversion efficiency of the process. The influence of the equivalence ratio, oxygen enrichment (the amount of oxygen available in the gasification agent), and pressure on the gasification characteristics is analyzed. The results of simulations are compared with reported experimental measurements through which the numerical model is validated. The detailed investigation performed in the course of this study reveals that the heavy oil gasification is a feasible process that can be utilized to generate a syngas for various industrial applications.     

Temperature-programmed Studies of Coke Resistant Ni Catalyst for Carbon Dioxide Reforming of Methane

Argon glow discharge plasma was applied for drying the impregnated Ni/SiO₂ catalyst instead of the drying thermally. Such plasma treated catalyst significantly inhibits the coke formation from methane decomposition. The methane-derived carbon shows an improved reactivity against CO₂. This can result in a better balance of coke formation and gasification by CO₂ when the plasma-treated Ni/SiO₂ catalyst is used for the CO₂ reforming of methane.     

Petroleum coke conversion behavior in catalyst-assisted chemical looping combustion

Efficiently using petroleum coke as fuel and reducing carbon emission meanwhile have become attractive in oil processing industry. The paper is focused on the application of Chemical Looping Combustion (CLC) with petroleum coke, with the purpose of investigating its combustion performance and effects of potassium. Some experiments were performed in a laboratory scale fluidized bed facility with a natural manganese-based oxygen carrier. Experimental results indicated that the coke conversion is very sensitive to reaction temperature. The present natural manganese-based oxygen carrier decorated by K has little effect on the improvement of coke conversion. XRD, SEM–EDX, and H₂-TPR were adopted to characterize the reacted oxygen carrier samples. After being decorated by K, the oxygen carrier's capacity of transferring oxygen was decreased. A calcination temperature above the melting point of K₂CO₃ (891 °C) shows better oxygen transfer reactivity in comparison to the one calcined at a lower temperature. The natural oxygen carrier used in the work has a high content of Si, which can easily react with K to form K(FeSi₂O₆). Further, irrespective of reaction temperature, the coke conversion can be significantly enhanced by decorating the coke with K, with a demonstration of remarkably shorter reaction time, faster average coke gasification rate and higher average carbon conversion rate.     

Mechanism of coke deposition on the surface of molybdenite

The formation of coke is studied by SEM/TEM on the basal plane of a thin (ca. 500 Å) single-crystal film of MoS₂. From exposure to benzene at 550 and 700 °C, which is exothermic in decomposition, carbon islands are formed on the face opposite to the exposure face; whereas for the endothermic decomposition of methane (at 700 °C), carbon islands are formed on the exposure face. This result provides a direct evidence for the carbon diffusion mechanism for coke deposition in which a temperature gradient is the driving force for carbon diffusion. It is demonstrated that the formation and growth of thin (perhaps monolayer) carbon islands can be studied by gold decoration/TEM. A gold-decoration/TEM study of coke formation of benzene on MoS₂ show that screw dislocations are not preferred sites for carbon deposition, and that steps on the basal plane are likely the active sites for decomposition of benzene.     

Dry gas cleaning process by adsorption of H2S into activated cokes in gasification of carbon resources

Gasification of carbon resources including biomass and coal is one of promising energy production technologies. The R&D on effective and convenient gas cleaning processes for removal of contaminants as well as high efficient reliable gasifiers is essential for industrial application in broad fields. In this study, a dry process of synthesis gas cleaning by adsorption of H₂S into activated cokes was proposed as a candidate of desulfurization technologies in gasification. The H₂S adsorption performance of activated coke produced from coal, which are used industrially for de-SO_x and de-NO_x, was evaluated by the thermogravimetric analyses and the adsorption examination in a fixed bed under the atmospheric and high pressures. Activated coke was not only the most active at about 423 K for the H₂S adsorption rate but also regenerative over 573 K by H₂S desorption with a sufficient rate under an inert gas flow of nitrogen. The H₂S adsorption performance of the activated coke was not inhibited by the co-existence of CO₂ or COS but enhanced rather by the co-existence. The adsorbent was promisingly active for both H₂S and COS adsorption as well. These behaviors suggest that the activated coke are available for simultaneous desulfurization of H₂S and COS. The H₂S breakthrough examination in the fixed bed revealed that it was possible to remove H₂S to lower level than 1 ppm for a long time depending on the residence time of gas flow in the bed. When the adsorption operation was carried out under high pressures up to 0.6 MPa, the regeneration of activated coke by H₂S desorption took place under the pressure reduced to the atmosphere. As the results, it was implied that the present activated coke could be applicable to the desulfurization process in coal gasification.     

Numerical simulation of mass transfer in circulating drops

Numerical simulations of mass transfer are performed for a circulating liquid drop with applications in liquid–liquid extraction. Simulation parameters are chosen for a multi-component ternary system acetone–methanol–benzene. The drop circulation pattern is estimated via a truncated Galerkin representation of the drop streamfunction. Fickian diffusivities for multi-component mass transfer are obtained via Maxwell–Stefan theory with thermodynamic corrections. The advection–diffusion equations governing mass transfer are solved via two distinct numerical methods: a finite difference scheme (using the alternating direction implicit method) and a finite element scheme. Good agreement was obtained between both schemes. Simulation results are presented for a Reynolds number (Re=30) and for a selection of Peclet numbers (Pe=100, 1000 and 10 000, thereby giving insight into the effects of increasing Peclet number). The numerical simulations of the full advection–diffusion equations are compared against predictions of a rigid drop model (i.e. without circulation) and also against predictions of a semi-analytical boundary layer model developed by Uribe-Ramirez and Korchinsky. Results for bulk mass fractions reveal that the rigid drop model predictions evolve too slowly, while the boundary layer model predictions evolve much more quickly than the numerical simulations. Advection–diffusion simulation results for the evolution of mass fractions at selected individual locations in the drop show that points on streamlines nearest to the drop surface and/or drop axis evolve fastest, while those closest to the drop internal stagnation point evolve slowest. Corroborated by contour plots of component concentrations throughout the drop at selected times, this supports a picture whereby mass fractions become roughly uniform along individual streamlines, but mass is transferred diffusively from streamline to streamline.     

Reactivity of petroleum coke to carbon dioxide between 1030 and 1180 K

Measurements were made of the reaction rate of three sizes (2.9, 0.9 and 0.22 mm) of petroleum-coke particles with carbon dioxide over the temperature range 1018–1178 K, and at carbon dioxide partial pressures between 26 and 118 kPa. A limited number of similar measurements were made on samples of a commercial aluminium-smelting anode, an experimental anode, and AGKSP graphite. The materials were all reacted under conditions of chemical rate control alone: there were no rate limitations due to transport processes without or within the carbon particles. The order of the rate with respect to carbon dioxide concentration was found to be close to 0.6 for the petroleum coke and anode carbons, and between 0.6 and 0.8 for the graphite. Activation energies in the range 203–237 kJ/mol were found for petroleum coke; 187–237 kJ/mol for electrode carbon; and 293 kJ/mol for the graphite. For the petroleum coke, the order was found to be constant up to 45% burn-off and the activation energy essentially constant between 21 and 45% burn-off. The reactivity ?_s, based on unit pore surface area of the petroleum coke at a carbon dioxide pressure of 101 kPa, can be represented by: $\text{?}{}_{\mathrm{s}}\text{= α exp [?}\text{E}\text{(RT)}\text{]}$. For the 2.9 and 0.9 mm particles, α = 6.1 /sx 10⁶ g/m² min and E = 215 kJ/mol; for the 0.22 mm particles the respective values are 1.8 /sx 10⁷ and 222. The reactivity ? of the commercial electrode on a weight basis was within the range of those of the coke and experimental electrode. For AGKSP graphite, values of ?_s were close to those found by Walker and Raats¹⁴.     

A reaction kinetic study of CO<Subscript>2</Subscript> gasification of petroleum coke,coals and mixture

Characteristics of Char-CO₂ gasification were compared in the temperature range of 1,100–1,400 °C using a thermogravimetric analyzer (TGA) for petroleum coke, coal chars and mixed fuels (Petroleum coke/coal ratios: 0, 0.25, 0.5, 0.75, 1). The results showed that reaction time decreased with increasing gasification temperature, BET surface area and alkali index of coal. Mixed fuels composed of petroleum coke/coal exhibited reduced activation energies. Modified volumetric reaction model and shrinking core model might be suitably matched with experimental data depending on coal type and petroleum coke/coal ratio. Rate equations were suggested by selecting gas-solid reaction rate models for each sample that could simulate CO₂ gasification behavior.     

A review on single bubble gas–liquid mass transfer

It is common to empirically correlate volumetric mass transfer coefficient k_La for predicting gas–liquid mass transfer in industrial applications, and the investigation of single bubble mass transfer is crucial for a detailed understanding of mass transfer mechanism. In this work, experiments, models and simulations based on the experimental results were highlighted to elucidate the mass transfer between single bubbles and ambient liquid. The experimental setups, measurement methods, the mass transfer of single bubbles in the Newtonian and the non-Newtonian liquid, models derived from the concept of eddy diffusion, the extension of Whitman's, Higbie's and Danckwerts' models, or dimensionless numbers, and simulation methods on turbulence, gas–liquid partition methods and mass transfer source term determination are introduced and commented on. Although people have a great knowledge on mass transfer between single bubbles and ambient liquid in single conditions, it is still insufficient when facing complex liquid conditions or some phenomena such as turbulence, contamination or non-Newtonian behavior. Additional studies on single bubbles are required for experiments and models in various liquid conditions in future.     

Thermogravimetric investigations in prediction of coking behaviour and coke properties derived from inertinite rich coals

The coal quality, temperature, pressure, heating rate, various processes and reactor type affect coking behaviour of coal and resulting coke properties. Several petrographic and chemical methods were proposed earlier for prediction of coking behaviour of coals. Inertinite rich coal samples (I^mmf>30 vol%) having different petrographic compositions were selected for thermogravimetric investigations (DTA, DTG and TGA) and their coking behaviour was studied. The petrographic build up, micro-structural properties (porosity and cell wall thickness) and mechanical strength of the resulted cokes were also investigated. ΔH and ΔH_max (the main endothermic area of heat absorption and fast absorbing main endothermic area, respectively) were distinguished in DTA curves. ΔA and ΔA_max (the main decomposition area and fast disintegrating main decomposition area) under DTG curves were identified. ΔH_max/ΔA_max shows good correlation with Roga's indices (RI, caking properties) as well as with petrographic caking ratio (PCR). The coarse mosaic content of cokes seem to depend on LΔT_max/ΔT_max (ratio of weight loss during fast decomposing main reaction to temperature difference) under DTG. L^mΔT/ΔT (ratio of weight loss during main decomposing reaction to temperature difference) under DTG exhibits correlation with p₁ (mean pore size) and t₁ (mean cell wall thickness) of cokes. ΔA_max/(L^mΔT_max) also indicates good relationship with mechanical strength of cokes. (L^mΔT_A−T_B)/(L^mΔT) (i.e. ratio of weight loss during main endothermic reaction under DTA to weight loss during main decomposing reaction) appears to have relationship with DD (compactness) of cokes. The course of main endothermic reaction/main decomposition reaction under DTA, DTG and TGA seems to govern coking behaviour and the resulting coke strength, which in turn is controlled by microlithotypes.     

Mass transfer in sparged and stirred reactors: influence of carbon particles and electrolyte

Mass transfer in multiphase systems is one of the most studied topics in chemical engineering. However, in three-phase systems containing small particles, the mechanisms playing a role in the increased rate of mass transfer compared to two-phase systems without particles, are still not clear. Therefore, mass transfer measurements were carried out in a 2D slurry bubble column reactor , a stirred tank reactor with a flat gas-liquid interface, and in a stirred tank reactor with a gas inducing impeller. The rate of mass transfer in these reactors was investigated with various concentrations of active carbon particles (average particle size of ), with electrolyte (sodium gluconate), and with combinations of these. In the bubble column, high-speed video recordings were captured from which the bubble size distribution and the specific bubble area were determined. In this way, the specific mass transfer area a_gl was determined separately from the mass transfer coefficient k_l. Mechanisms proposed in literature to describe mass transfer and mass transfer enhancement in stirred tank reactors and bubble columns are compared. It is shown that the increased rates of mass transfer in the 2D bubble column and in the stirred tank reactor with the gas inducing impeller are completely caused by an increased gas-liquid interfacial area upon addition of carbon particles and electrolyte. It is suggested that an increased level of turbulence at the gas-liquid interface caused by carbon particles accounts for a smaller effective boundary layer thickness and an enhancement of mass transfer in the flat gas-liquid surface stirred tank reactor. However, for the carbon particles used in this study, it is rather unlikely that mass transfer enhancement takes place due to the well-known shuttle or grazing effect.