A carbon activation model with application to longan seed char gasification

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

Role of active sites in the steam activation of high unburned carbon fly ashes

Previous studies on carbon gasification have not included high unburned carbon content fly ashes, and therefore it remains unclear why not all fly ash carbon samples are equally suitable for activation. The concentration of active sites is well known to influence carbon gasification reactions. Therefore, the objective of this work was to investigate the effect of the concentration of active sites on the behavior of fly ash carbon samples upon steam activation. Six fly ash carbons were selected to produce activated carbons using steam at 850 °C. The concentration of active sites was determined by non-dispersive infrared analysis (NDIR), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). XRD analyses were also conducted to determine the crystallite size. It was observed that the concentration of active sites played a more significant effect on the surface areas of activated carbons in the carbon burn-off zone of >60%. Statistical analysis was used to relate the surface areas of activated carbon variances with carbon burn-off levels.     

Steam and carbon dioxide activation of Eucalyptus globulus charcoal

The partial gasification of Eucalyptus globulus charcoal, using carbon dioxide and steam as activating agents is studied. The influence of some preparation variables such as temperature and reaction time on textural, morphological and acid–base properties are given. The results obtained show that both activating agents produce microporous activated carbons and that a large increase in meso- and macroporosity is observed when steam and high burn-off are used. It has been found that these activated carbons have weak acid sites and the effect of the burn-off on the acid strength depends on the activating agent. Thus, the acid strength of these sites decreases with the burn-off when steam is used, but no effect is observed when carbon dioxide is used.     

Gasification of carbonaceous particulates

The reactions of soot particulates and carbons with three oxidising agents expected in the exhaust gases from a diesel engine have been studied. Nitrogen dioxide has been found to be an efficient gasifying agent for carbons, carbon oxides and nitric oxide being produced. The reaction with diesel particulates commences above c. 150°C: with graphite, gasification became significant only above c. 550°C. The presence of oil on the particulates does not affect gasification. © 1998 SCI.     

Pore structure of activated carbons prepared by carbon dioxide and steam activation at different temperatures from extracted rockrose

The influence of the activation temperature on the pore structure of granular activated carbons prepared from rockrose (Cistus ladaniferus L.), extracted previously into petroleum ether, is comparatively studied. The preparation was carried out by pyrolysis of a char in nitrogen and its subsequent activation by carbon dioxide and steam (flow of water controlled to generate the same mol number per minute of water as well as carbon dioxide/nitrogen) at 700-950°C to 40% burn-off. The techniques applied to study the pore structure were: pycnometry (mercury, helium), adsorption (carbon dioxide, 298 K; nitrogen, 77 K), mercury porosimetry and scanning electron microscopy. The preparation by steam activation, especially at 700°C, yields activated carbons showing a total pore volume larger than those prepared by carbon dioxide activation. The pore structures present the greatest differences when the activations are carried out between 700 and 850°C and closer at higher temperatures. At high temperatures, the decrease of differences in pore development caused by carbon dioxide or steam is attributed to an external burn-off. The micropore structure of each activated carbon is mainly formed by wide micropores. At the lowest activation temperatures, especially at 700°C, steam develops the mesoporosity much more than carbon dioxide. At 950°C, a similar reduction of pore volume in the macropore range occurs.     

Rheological behavior and thermal properties of pitch/poly(vinyl chloride) blends

The effect of adding poly(vinyl chloride) (PVC) and coke filler on the rheological behavior and thermal properties of a coal tar pitch was investigated with a view to developing an appropriate viscoelastic binder for the injection molding of graphite components. Dynamic mechanical analysis revealed that the pitch formed compatible blends with PVC featuring a single glass transition temperature (Tg) intermediate to the two parent Tg’s. Adding PVC to the pitch increased melt viscosity substantially and resulted in strong shear thinning behavior at high PVC addition levels. Adding coke powder as filler increased the melt viscosity even further and enhanced shear thinning trends. Pyrolysis conducted in a nitrogen atmosphere revealed interactions between the PVC and pitch degradation pathways: the blends underwent significant thermal decomposition at lower temperatures but showed enhanced carbon yields at high temperatures. Pyrolytic carbon yield at 1000 °C was further improved by a heat treatment (temperature scanned to 400 °C) in air or oxygen. However, carbon yield decreased with addition of PVC. In addition, the degree of ordering attained following a 1 h heat treatment at 2400 °C also decreased with increasing PVC content.     

Effects of the oxidation temperature on the structure and properties of polyacrylonitrile‐based activated carbon hollow fiber

Polyacrylonitrile (PAN) hollow fibers were pretreated with ammonium dibasic phosphate, oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of the oxidation temperature of the PAN hollow fiber precursor on the microstructure, specific surface, pore size distribution, and adsorption properties of PAN‐based activated carbon hollow fiber (PAN‐ACHF) were studied. When PAN hollow fibers were oxidized at 270°C, because of drastic oxidation, chain scission occurred, and the number of pores within and on the surface of the resultant PAN‐ACHF increased, but the pores were just in the thinner region of the skin of PAN‐ACHF. The surface area of PAN‐ACHF reached a maximum when the oxidation temperature was 270°C. The adsorption ratios to creatinine were all higher than 90% at all oxidation temperatures, and the adsorption ratio to VB₁₂ reached a maximum (97%) at 230°C. The dominant pore sizes of the mesopores in PAN‐ACHF ranged from 2 to 5 nm. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 203–207, 2005     

Thermal Regeneration of Activated Carbons Exhausted with Phenol Compound

Abstract

Two methods, pyrolysis and steam regeneration, were used to investigate the regeneration efficiencies of two types of activated carbons that were exhausted by the phenol compound in the liquid phase. NaOH titration and temperature program deposition indicated that the two carbons exhibit different surface acidities, oxygen group distributions, and oxygen populations on the carbon surface. After treating by pyrolysis and steam regeneration, the surface recovery of spent carbons gradually increased with the regeneration temperature. Pyrolysis at 700?900°C cannot completely remove all phenol molecules from the spent carbons, thereby having a lower surface recovery than steam regeneration. Comparing the two methods, steam regeneration is an efficient method in not only recovering the porosity, but also enlarging and deepening the pore structure of carbons. In comparison with the two carbons, the higher oxygen population on the carbon surface would decrease the surface recoveries and cause a more serious carbon weight loss. This could be attributed to the fact that a greater amount of surface groups would promote the possibility of chemisorbed fraction, thus leaving more degradation species in the pores. This study would shed one light on how oxygen groups existed on carbon surface act as cracking sites in diminishing the removal efficiency.     

Influence of activation temperature on the properties of polyacrylonitrile-based activated carbon hollow fiber

In this work, polyacrylonitrile hollow fiber was oxidized, carbonized, and activated by carbon dioxide into activated hollow carbon fiber. The effects of the activation temperature on the characteristics of the resulting activated hollow carbon fiber, including the mechanical properties, the surface area, and pore size distribution, were studied. The results show that by activating for 40 min at 800°C, the mechanical properties was better, the surface area was larger, and the pore size was distributed in three ranges. Higher activation temperature led to the decrease in the mechanical strength, the increase in the burn-off degree of the surface, the reduce of the portion of micropores, and the greatly broadening the pore size distribution. Lower activation temperature can only produce pleading on the surface of the fiber instead of open pores, due to the milder attack of CO₂. Therefore, the characteristics of the activated hollow carbon fiber can be controlled by the activation temperature. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1331–1336, 1998     

Preparation and characterization of activated carbons for SO<Subscript>2</Subscript> adsorption from Taixi anthracite by physical activation with steam

Taixi anthracite was used as a precursor to prepare activated carbons (AC) for SO₂ adsorption from flue gas. In this work the activated carbons were prepared by physical activation with steam. Specifically, the effects of activation temperature and burn-off degree on the physico-chemical properties of the resulting AC samples were comparatively studied. The different types of pore volumes, pore size distributions and surface chemistries of the activated carbons on the SO₂ adsorption were also analyzed. The results show that the increasing burn-off leads to samples with continuous evolution of all types of pores except ultramicropore. The ultramicropore volume increases to a maximum of 0.169 cm³/g at around 50% burn-off and then decreases for 850 °C activation. At higher activation temperature, the micropore volume decreases and the mesopore structure develops to a certain extent. For all the resulting AC samples, the quantities of the basic surface sites always appear much higher than the amount of the acidic sites. The activated carbon prepared with higher micropore volume, smaller median pore diameter and higher quantities of the basic surface sites represents better SO₂ sorption property.     

Thermal degradation of polymethacrylic ester containing bisphenol-S

Thermal degradation of polymethacrylic ester containing bisphenol-S, poly(BPS-M), was investigated under nitrogen and air atmosphere at various heating rates. Ozawa's method was used to calculate the kinetic parameters, activation energy, preexponential factor and reaction order. Thermodegradation of the polymer occurs in one or two stages in nitrogen and air, respectively. The temperature at the start of intense degradation (T_start) and the temperature corresponding to a 50% mass loss (T_50%) were found to be 300 and 402°C, respectively, at a heating rate of 10°C min^?1 in nitrogen. Larger sample masses have a larger temperature interval (ΔT) and a greater mass loss (ΔW). The kinetic order of degradation is unity both in nitrogen and air. The direct pyrolysis mass spectrum of the polymer shows one degradation peak. The most important degradation process under inert atmosphere is the loss of carbon dioxide, phenol and sulphur dioxide. A possible mechanism for thermal decomposition of poly(BPS-M) is proposed based on the product analyses.     

Mechanism of selenium sorption by activated carbon

Selenium, along with mercury and halides, represents one of the most volatile trace metallic emissions from coal‐fired combustors and utility boilers. This study investigates the potential of activated carbon in capturing gas phase selenium species in the low temperature range (125°C to 250°C) and elucidates the mechanism of interaction between selenium species and activated carbon. Selenium dioxide is chosen as the representative selenium species and experimental investigations are carried out in a differential bed reactor to illustrate the mechanism of SeO₂ and carbon Interaction, Activated carbons with different structural properties are studied as adsorbents for selenium dioxide capture at low temperature. The capture mechanism is found to involve both physical and chemical adsorption in the low temperature range. At 125°C, about 1.5 wt% of selenium is captured at equillbrium. Carbon surface analyses and XPS studies confirm the presence of both elemental and oxide forms of selenium on the surface suggesting partial reduction of selenium dioxide to elemental selenium at carbon surface.     

Effect of pre-oxidation on reactivity of chars in steam

The effects of pre-oxidation of char from Taiheiyo coal, a non-caking bituminous coal, in the 400–550 °C temperature range on its gasification reactivity with N₂-H₂O at 0.1 MPa (steam partial pressure of 13.2 kPa) have been investigated. The pre-oxidation of char markedly enhances gasification rates at temperatures between 800 and 900 °C. Reactivity is found to parallel the burn-off level during preoxidation at low temperatures (400–430 °C), whereas at relatively high temperatures (480–550 °C), the burn-off level only affects the reactivity slightly. The amount of CO and CO₂ evolved from the preoxidized char by heat treatment is proportional to the burn-off level at low temperatures (400–430 °C), being closely related to the enhancement of the gasification reactivity in steam.     

Steam gasification of German hard coal using alkaline catalysts: Effects of carbon burn-off and ash content

Laboratory-scale experiments were performed on chars from German hard coals with potassium carbonate addition. The steam gasification rate at 4 MPa and 700 °C as a function of the amount of catalyst added is described for a low-and high-ash char. From the burn-off behaviour the reaction order relative to carbon was determined. For the low-ash char a uniform reaction order was found but the high-ash char indicated a complex interaction of catalytic gasification, catalyst deactivation, and the development of the reacting surface.     

Thermal characteristics and pyrolysis of methyl‐di(phenylethynyl)silane resin

Thermal stability of a recently synthesized polymeric methyl‐di(phenylethynyl)silane (MDPES) resin was studied using a number of thermal and spectrometric analytical techniques. The polymer exhibits extremely high thermal stability. Thermogravimetric analysis (TGA) shows that the temperature of 5% weight loss (T_d5) was 615°C and total weight loss at 800°C was 8.9%, in nitrogen atmosphere, while in air, T_d5 was found to be 562°C, and total weight loss at 800°C was found to be 55.8% of the initial weight. Differential thermal degradation (DTG) studies show that the thermal degradation of MDPES resin was single‐stage in air and two‐stage in nitrogen. The thermal degradation kinetics was studied using dynamic TGA, and the apparent activation energies were estimated to be 120.5 and 114.8 kJ/mol in air, respectively, by Kissinger and Coats–Redfern method. The white flaky pyrolysis residue was identified to be silicon dioxide by FTIR and EDS, indicating that the thermal stability of polymer may be enhanced by the formation of a thin silicon dioxide film on the material surface. © 2006 Wiley Periodicals, Inc. J Appl PolymSci 103: 605–610, 2007     

Corrosion degradation of mullite subject to carbon monoxide atmosphere at 1000oC–1600°C

Thus far, studies on the damage to refractory materials under carbon monoxide atmospheres have mostly concentrated on the effects of carbon deposition, and the testing temperature was always set at approximately 500°C to promote the deposition of carbon. However, this testing temperature is far below the operating temperature of most refractories. In this study, mullite, a widely used high-temperature structural material, was subjected to a carbon monoxide atmosphere at 1000°C-1600°C to investigate its phase and microstructural evolutions. Changes to the grain boundaries were initially observed in mullite specimens treated at 1000°C and 1200°C. After treatment at 1400°C, the specimen surface comprised α-Al₂O₃, a glass phase, and a small amount of mullite. However, treatment at 1600°C resulted in only α-Al₂O₃ and a small amount of glass phase on the surface. Additionally, pores and voids were found in the glass phase on the surface and in the bulk of the specimens treated at 1400°C and 1600°C. This study demonstrated the stability of pure mullite in a carbon monoxide atmosphere and revealed that impurities accelerating generation of the liquid phase in Al₂O₃–SiO₂ system significantly affect the stability of mullite in a carbon monoxide atmosphere.     

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

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

Preparing and characterizing the active carbon produced by steam and carbon dioxide as a heavy oil hydrocracking catalyst support

Active carbon was prepared from Yallourn brown coal char using steam and carbon dioxide activation in a laboratory rotary kiln. The activation rate with steam was faster than that with carbon dioxide. The pore structure of the active carbons was characterized using the nitrogen isotherms at 77 K. The pore volume and specific surface area of the active carbon increased with the carbon burn-off, and compared to carbon dioxide, steam activation produced active carbon that was richer in mesopores by increasing the pore size from micropores to mesopores. The porosity of the active carbons was related to the ability to adsorb maltene, the normal hexane-soluble fraction, in the vacuum residue of petroleum crude. The steam-activated carbon rich in mesopores had a greater ability to adsorb maltene, which consists of large-molecular-weight hydrocarbons.     

Diuron Multilayer Adsorption on Activated Carbon from CO2 Activation of Grape Seeds

Granular activated carbons were obtained from grape seeds by pyrolysis at 600°C and subsequent physical activation with CO₂ (750–900°C, 1–3 h, 25–74% burn-off). The carbon and ash content increased during the activation, reaching values of 79.0% and 11.4%, respectively. Essentially microporous materials with BET surface areas between 380 and 714 m²/g were obtained. The performance of the activated carbon in the adsorption of diuron in aqueous phase was studied within the 15–45°C temperature range. Equilibrium data showed that the maximum uptake increased with temperature from 120 to 470 µmol/g, also evidencing some dependence of the adsorption mechanism on temperature. Data were fitted to five isotherm models [Langmuir, Freundlich, Dubinin–Radushkevich, BET, and GAB (Guggenheim, Anderson, and de Boer)]. Kinetic data were analyzed using first- and second-order rate equations and intraparticle diffusion model. The second-order rate constant values obtained (2.8–13.5 × 10^?3 g/µmol min) showed that the hollow core morphology of the material favors the adsorption kinetics.     

Liquid-phase adsorption of phenol by activated carbons prepared from bituminous coals with different oxygen contents

Activated carbons prepared from two bituminous coals were used to adsorb phenol in aqueous solutions. The major difference between the coal precursors is the oxygen content. The carbon preparation consisted of carbonization of the coals followed by activation in CO ₂ to various extents of burn-off. Experimental results show that the amount of phenol adsorbed generally increases with the BET surface area of the carbon. The carbons prepared from the coal with a higher oxygen content have larger surface areas, and, therefore, exhibit higher capacities for phenol. The surface area of the carbon increases with the extent of carbon burn-off, whereas the increase in the adsorptive capacity due to the increasing burn-off level does not show a linear relationship with the increase in area; the ratio of the capacity to BET surface area is not constant and decreases with the burn-off level. This has been attributed to the accessibility of phenol to the surface being affected by the length of diffusion path, which is an increasing function of the burn-off level or the particle size. The amount of phenol adsorbed decreases with the temperature for these carbons. It was found, according to the Langmuir model, that the adsorption process was significantly affected by the oxygen content in the coal precursors. © 1999 Society of Chemical Industry