The preparation of active carbons from natural materials for use in gas storage

The repetitive treatment of monolithic pellets of coconut shell chars and peach pit chars with an air stream at relatively low temperatures, 450 K, followed by heating the samples to an elevated temperature in an inert atmosphere, around 1120 K, resulted in considerable weight loss. The surface of the char is most likely covered with surface oxide groups at low temperatures which are desorbed as CO and CO₂ upon heating. Associated surface area increases indicate a selective activation of the char with each cycle from low to high temperature. The resulting changes in the macropore volumes of the pellets were minimized, evidence of a selective activation procedure. The activated chars show an increase in adsorption of methane on a per volume basis suggesting this cyclic activation method may be beneficial when attempting to increase storage capacity in a system where limited space is available.     

Computer synthesis of char and its characterization

A mimetic method based on Monte Carlo simulation is proposed to generate a molecular model of char. This char model consists of crystalline and amorphous phases, which are heated and cooled during the simulation of the carbonization process. Resultant char shows irregular shape and interconnected pores whose properties depend on the percentage of non-organized carbon and the carbonization temperature. These chars were characterized with Monte Carlo integration techniques to obtain the pore size distribution, pore volume, solid density and surface area. These were then compared with the experimental data of two chars, longan seed derived char and coconut shell derived char. The results show that the molecular model captures the trend of the properties with carbonization temperature for both experimental chars.     

The significance of heating rate on char yield and char properties in the pyrolysis of cellulose

The carbonization of powdered cellulose was investigated in the temperature range 200–950°C by measuring weight loss, carbon and hydrogen content, BET-adsorption of nitrogen and carbon dioxide, mercury penetration and particle-size distribution. Evidence is presented in support of a kinetic model according to which cellulose decomposition is controlled by dehydration at low temperature and by cleavage/scission at high temperature. Increased char yield and lower $\text{O}\text{C}$ ratio at low heating rate, as well as kinetic investigations into the effect of potential catalysts, support this model. The difference in reaction mechanism according to the heating rate appeared to influence the char properties considerably. Yield in micropore volume and surface area of slowly carbonized cellulose is up to four times larger than that of rapidly heated cellulose. Mercury pore volume, density and particle diameter depend on the heating rate, also. By adsorption of various gases, differences in relative size of the pore openings of different chars can be discerned. Micropore volumes measured with carbon dioxide were as much as seventy times larger than the corresponding volume measured with nitrogen. Thus, it is possible to obtain chars with molecular sieve properties by simple pyrolysis heating schemes.     

Microstructural variations of lignite,subbituminous and bituminous coals and their high temperature chars

Microstructure of a North Dakota lignite, a Washington subbituminous and a New Mexico bituminous coal and their chars produced by devolatilization in nitrogen at 1000 to 1300°C was investigated in this work using the CO₂ adsorption method conducted at 25°C. For each coal and char, specific surface area, micropore volume, micropore surface area, mean equivalent radius of micropores and characteristic energy of adsorption, as well as micropore volume distribution, were determined, and their variations with devolatilization temperature studied and interpreted. It was found that, overall, specific surface areas, micropore volumes and micropore surface areas of chars decreased monotonically as devolatilization temperature was raised from 1000 to 1300°C, although most of these values were much larger than that of their parent coals. The micropore volume distributions of the three coals and their high temperature chars were interpreted and found to provide an interesting insight into the micro structural variations of these coals and chars.     

Microporosity and mesoporosity of PPTA-derived carbons. Effect of PPTA thermal pretreatment

Isothermal treatments of the polyaramid fiber, [poly(p-phenylene terephthalamide)] (PPTA) in an inert atmosphere below its decomposition temperature are known to induce an important increase in char yield and modify the chemical composition and some other properties of the resulting chars. The objective of this work was to study the effect of this isothermal stage on the porous texture of chars and activated carbon fibers (ACFs) produced from PPTA. To this end, chars and ACFs were prepared by PPTA pyrolysis to 850 °C followed by CO₂ activation at 800 °C to various burn-offs (BOs), introducing or not an intermediate isothermal pre-treatment under the conditions (500 °C, 200 min) known to lead to a maximum increase in char yield. The porosity characteristics of the resulting chars and ACFs were comparatively investigated by adsorption of CO₂ (0 °C), and N₂ (−196 °C). The isothermal stage led to a char with enhanced micropore volume and wider micropores. The ACFs prepared from this char exhibited larger amounts of wide micropores and mesopores than those prepared from PPTA pyrolyzed at a constant heating rate.     

The catalytic steam gasification of chars from various sources by K2CO3

Gasification of a char prepared from hydrocracked residuum was compared with the gasification of chars prepared from bituminous and sub-bituminous Canadian coals, wood and graphite. Each material was mixed with 10 mass per cent K₂CO₃ and pyrolyzed up to 900°C. The yield of char was inversely proportional to the amount of volatile matter in the original material. The char prepared from hydrocracked residuum was different from the others. The other chars all followed zero-order gasification kinetics. Gasification of char prepared from the residuum was first-order in the solid. The development of a liquid phase during the pyrolysis of the residuum to char may explain this difference. The gasification rate of the char. from residuum was slower than the rates with the two coal chars and the wood char, but faster than the gasification rate of graphite. A combination of transient experiments and X-ray photoelectron spectroscopic (XPS) measurements indicated that hydrogen was formed almost instantaneously when steam reacted with the char. XPS spectra at liquid nitrogen temperature indicated that during gasification the formation of carbon oxygen bonds proceeded in the following sequence: COH, CO and CO.     

Influence of mineral matter in coal on decomposition of NO over coal chars and emission of NO during char combustion

NO-char reaction and char combustion in the presence and absence of mineral matter were studied in a quartz fixed bed reactor. Eight chars were prepared in a fluidized bed at 950 °C from four Chinese coals that were directly carbonized without pretreatment or were first deashed before carbonization. The decomposition of NO over these coal-derived chars was studied in Ar, CO/Ar and O₂/Ar atmospheres, respectively. The results show that NO is more easily reduced on chars from the raw coals than on their corresponding deashed coal chars. Mineral matter affects the enhancement both of CO and O₂ on the reduction of NO over coal chars. Alkali metal Na in mineral matter remarkably catalyzes NO-char reaction, while Fe promotes NO reduction with CO significantly. The effect of mineral matter on the emission of NO during char combustion was also investigated. The results show that the mineral constituents with catalytic activities for NO-char reaction result in the decrease of NO emission, whereas mineral constituents without catalytic activities lead to the increase of NO emission. Correlation between the effects of mineral matter on NO-char reaction and NO emission during char combustion was also discussed.     

Hydrogen production by methane cracking over different coal chars

Hydrogen production by methane cracking over a bed of different coal chars has been studied using a fixed bed reactor system operating at atmospheric pressure and 1123 K. The chars were prepared by pyrolysing four parent coals of different ranks, namely, Jincheng anthracite, Binxian bituminous coal, Xiaolongtan lignite and Shengli lignite, in nitrogen in the same fixed bed reactor operating at different pyrolysis temperatures and times. Hydrogen was the only gas-phase product detected with a GC during methane cracking. Both methane conversion and hydrogen yield decreased with increasing time on stream and pyrolysis temperature. The lower the coal rank, the greater the catalytic effect of the char. While the Shengli lignite char achieved the highest methane conversion and hydrogen yield in methane cracking amongst all chars prepared at pyrolysis temperature of 1173 K for 30 min, a higher catalytic activity was observed for the Xiaolongtan lignite char prepared at 973 K, indicating the importance of the nature of char surfaces. The catalytic activity of the coal chars were reduced by the carbon deposition. The coal chars had legible faces and sharp apertures before being subjected to methane cracking. The surfaces and pores of coal chars were covered with carbon deposits produced by methane cracking as evident in the SEM images. The results of BET surfaces areas of the coal chars revealed that the presence of micropores in the chars was not an exclusive reason for the catalytic effect of the chars in methane cracking.     

褐煤及其干馏半焦的微孔结构分析

用CO_2吸附法于298K下研究了大雁褐煤、黄县褐煤及其干馏半焦的微孔结构特性。用由Dubinin-Astakhov方程导出的关系式计算了所研究样品的微孔孔径分布和微孔有效表面积S_(micro),讨论了干馏温度对半焦的微孔孔容及平均当量半径的影响。     

Enhanced reactivity of metal-impregnated peat char and semi-coke during gas phase hydrogenolysis

Methane formation in the reaction between peat-derived chars and hydrogen was studied using peat semi-coke and doped chars. The latter were prepared by ion exchange of the initial material prior to pyrolysis. Impregnation with relatively large amounts of finely dispersed transition metals (up to ≈15 wt%) results in significant enhancement of methane production. Devolatilization prior to hydrogenolysis exerts a marked influence on the behaviour of the char due to its effect on the concentration of surface oxygen species. The lower the devolatilization temperature, the higher the methane yield. Two distinct peaks are present during the methane formation: a low-temperature peak involving the presence of CO and a high-temperature peak related to the direct reaction between H₂ and the char. The conversion profiles could be well approximated by a two-parameter model.     

The low-temperature SCR of NO over rice straw and sewage sludge derived char

Rice straw char and sewage sludge char were applied as catalysts for selective catalytic reduction between 50 and 250 °C using ammonia as the reducing agent. Each char was activated physically, using water vapor, or chemically, using KOH. The characteristics of the prepared catalysts were analyzed through elemental analysis, N₂ adsorption–desorption, FT-IR, NO-TPD, NH₃-TPD, and NO_x removal efficiency. The physically activated chars showed characteristics similar to those of the non-activated chars, whereas the chemically activated chars exhibited increased specific surface areas, pore volumes, NO adsorption capacities, NH₃ adsorption capacities, and oxygen functional group amounts, leading to higher NO_x removal efficiency. When the catalysts were impregnated with 3 wt% manganese, NO_x removal efficiency significantly increased. In particular, the NO_x removal efficiency was highest when the chemically activated chars were impregnated with manganese.     

Influence of carbonization conditions and wood species on carbon dioxide reactivity of resultant wood char powder

Carbon dioxide reactivities of powdered samples of Acacia and Eucalyptus wood chars were measured thermogravimetrically at 900°C and the effects of carbonization conditions (temperature, heating rate and soaking time) and wood species were determined. The results showed that the reactivity decreased with increasing carbonization temperature and soaking time. Chars prepared under rapid carbonization (heating rate: 30°C min⁻¹) were found to be more reactive than the chars produced by slow carbonization (heating rate: 4°C min⁻¹). In comparison to Eucalyptus wood chars, the Acacia wood chars exhibited higher reactivity.     

Physicochemical properties and structural evolutions of gas-phase carbonization chars at high temperatures

The gas-phase carbonization chars from hydrocarbons with low molecular weight (anthracene oil and petroleum ether) were prepared using a drop tube reactor at 1000-1200 °C, and their physicochemical properties and structural evolutions (elemental composition, carbon crystallite structure, surface morphology, pore structure and chemical composition of volatile matters) were mainly investigated. The chars obtained in the high temperature region, which appeared with high C/H atomic ratio and poor carbon crystallite structure far from natural graphite, could be used as high carbonaceous materials. The chars were composed of uniform spherical particles with a continuous pore size distribution. The average pore diameters of the chars were much smaller and in the rage of 5.0-8.7 nm. The increasing carbonization temperature led to an initial increasing and a sequent decreasing of specific surface areas from mico-meso-pores and an increasing of those from meso-macro-pores in the chars. The volatile matters in the chars were composed of an easily-extracted fraction (CS₂-soluble compounds with three to six aromatic rings) and a hard-extracted fraction (CS₂-insoluble compounds with higher aromaticity). The elevated carbonization temperature led to diminish the two volatile fractions. A liquid core formation mechanism was proposed to explain the gas-phase carbonization process of hydrocarbons.     

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).     

Importance of carbon active sites in the gasification of coal chars

A demineralized lignite has been used in a fundamental study of the role of carbon active sites in coal char gasification. The chars were prepared in N₂ under a wide variety of conditions of heating rate (10 K min^?1 to 10⁴ K s^?1), temperature (975–1475 K) and residence time (0.3 s–1 h). Both pyrolysis residence time and temperature have a significant effect on the reactivity of chars in 0.1 MPa air, determined by isothermal thermogravimetric analysis. The chars were characterized in terms of their elemental composition, micropore volume, total and active surface area, and carbon crystallite size. Total surface area, calculated from C0₂ adsorption isotherms at 298 K, was found not to be a relevant reactivity normalization parameter. Oxygen chemisorption capacity at 375 K and 0.1 MPa air was found to be a valid index of char reactivity and, therefore, gives an indication, at least from a relative standpoint, of the concentration of carbon active sites in a char. The commonly observed deactivation of coal chars with increasing severity of pyrolysis conditions was correlated with their active surface areas. The importance of the concept of active sites in gasification reactions is illustrated for carbons of increasing purity and crystallinity including a Saran char, a graphitized carbon black and a spectroscopically pure natural graphite.     

竹炭基高比表面积活性炭电极材料的研究

以竹节为原料，在隔绝空气的条件下，经不同温度炭化处理后与KOH混合，制取竹炭基高比表面积活性炭。考察了炭化温度、KOH与竹炭的质量比、活化温度和活化时间等工艺因素对活性炭收率、微孔结构和吸附性能的影响，探讨了竹炭基高比表面积活性炭作双电层电容器电极时的充放电特性及其比电容与各种因素的关系。研究结果表明，控制适宜的炭化、活化工艺条件可制得双电极比电容达55F／g的竹炭基高比表面积活性炭，由它组装的双电层电容器具有良好的充放电性能和循环性能，但内阻过高，大电流下充放电时电容量下降过大。     

Gasification studies of onakawana lignite

Onakawana lignite was gasified in air, steam and an air + steam mixture in a fixed bed reactor. The extent of devolatilization was determined by pyrolysis in nitrogen. The composition of products, expressed in terms of H₂/CO ratio, was temperature dependent. The ratio decreased with increasing temperature. During steam gasification the ratio decreased from 4.6 to 2.6 when temperature increased from 700° to 990°C. The addition of air to steam resulted in a marked decrease of this ratio. Steam gasification reactivity of chars prepared from Onakawana lignite at 500°C and 800°C were studied in the temperature range of 650°C to 1000°C. The carbon conversion results were fitted into equations describing the continuous and shrinking core models. The char prepared at 500°C was much more reactive than the one prepared at 800°C. Product distribution expressed as the H₂/CO ratio, was favourable in the temperature range. For comparison, the Kentucky #9 coal and chars derived from this coal were used as referee materials. The reactivity of these chars was markedly lower than that of chars derived from Onakawana lignite.     

Influence of coal preoxidation and the relation between char structure and gasification potential

A study was carried out to ascertain the effects of coal preoxidation and carbonization conditions on the structure and relative gasification potential of a series of bituminous coal chars. Chars were prepared from two freshly mined bituminous coals and preoxidized samples derived from them. Carbonization conditions included a wide range of heating rate (0.2–10000K s^?1), temperature (1073–1273 K) and time (0.25–3600 s). Char properties were characterized in terms of analysis of char morphology, surface area, elemental composition, and gasification reactivity in air. Over the range of conditions used, preoxidation substantially reduced coal fluid behaviour and influenced macroscopic char properties (char morphology). Following slow heating (0.2 K s^?1), preoxidized coals yielded chars having higher total surface areas and higher reactivities toward gasification in air than did similar chars prepared from fresh coal. Following rapid heating (10000 K s^?1) and short residence times (0.25 s), chars prepared from preoxidized and fresh coals exhibited similar microstructural and chemical properties (surface area, $\text{C}\text{H}$ ratios, gasification rates). Carbonization time and temperature were found to be the critical parameters influencing char structure and gasification potential.     

Effects of Ca-based additives on desulfurization during coal pyrolysis

The effects of different Ca-based additives on the sulfur removal of coals during pyrolysis up to 900 °C have been studied in a fixed-bed reactor. It was found that Ca(OH)₂ and CaO were quite effective to capture the sulfur-containing gases, 95% of the sulfur evolved from untreated coal was retained in the char by the use of additives. Both the tar yield and the sulfur content of the tar decreased with addition of Ca-based additives. The effect of Ca(OH)₂ was better than that of CaO due to its higher activity, but CaCO₃ had little effect because of its higher decomposition temperature (−900 °C) than the peak temperature range (400–500 °C) of sulfur-containing gases emission. There is remarkable sulfur retention effect with Ca(OH)₂ prepared by impregnation and ultrasonic treatment due to the higher dispersion in coal particles than by simple mechanical mixing. The ultrasonic treatment is the best method with regard to the lowest SO₂ release during the char combustion. XRD results showed that the sulfur captured by Ca-based additives during pyrolysis turned into CaS. FeS detected in pyrolysis char without additives disappeared in chars with additives, which indicated that CaO could react with FeS through solid-solid reaction. When the chars with calcium-additives were burned in fixed bed reactor, they gave out less SO₂ than the raw coal added with same additives. The best total desulfurization efficiency could reach to about 85%.     

The rate of oxidation of char and coal in relation to their tendency to self-heat

As part of a study of self-heating tendencies of Australian coals and chars, the rates of oxidation of fresh and weathered chars and a weathered coal have been measured in dry (fresh char only) and moist air over the temperature range 45 to 94 °C. The oxidation of the weathered materials has an apparent activation energy lying between 63.9 and 69.0 kJ/mol which is independent of their moisture content. However, the rate of oxidation of char increases with increasing moisture content and decreases with increasing carbonization temperature of the parent coal, and with the extent of the char's weathering. Indeed, under adiabatic conditions it is shown that weathering or progressive oxidation, which conforms to the Elovich relation, may largely offset any substantial self-heating of char (or coal) caused by the accelerating effect of temperature.