Effect of iron enrichment with GIC or FeCl3 on the pore structure and reactivity of coking coal

The effect of a Lewis acid addition to a coking coal on the porosity and reactivity towards steam of the resulting iron enriched coal chars are studied. GIC (FeCl₃ graphite intercalation compound) or free FeCl₃ are used as iron containing additives. Coal iron enrichment was performed using either directly FeCl₃ in vapour phase, or by mixing of coal and additives in decaline or by common grinding of coal and additives under argon. Iron enriched coals were carbonized at 750°C (heating RATE = 5°C min) and activation made with pure steam at 800°C to a burn-off off of 50 wt%. The pore structures of coal chars before and after activation were evaluated on the basis of CO₂ and C₆H₆ sorption at 25°C. A significant development of the microporosity is observed in the iron enriched char before activation and its steam reactivity is also increased. After activation, BET surface area values are increased in presence of iron, and porosity is mainly microporous.     

Experimental comparison of biomass chars with other catalysts for tar reduction

In this paper the potential of using biomass char as a catalyst for tar reduction is discussed. Biomass char is compared with other known catalysts used for tar conversion. Model tar compounds, phenol and naphthalene, were used to test char and other catalysts. Tests were carried out in a fixed bed tubular reactor at a temperature range of 700–900 °C under atmospheric pressure and a gas residence time in the empty catalyst bed of 0.3 s. Biomass chars are compared with calcined dolomite, olivine, used fluid catalytic cracking (FCC) catalyst, biomass ash and commercial nickel catalyst. The conversion of naphthalene and phenol over these catalysts was carried out in the atmosphere of CO₂ and steam. At 900 °C, the conversion of phenol was dominated by thermal cracking whereas naphthalene conversion was dominated by catalytic conversion. Biomass chars gave the highest naphthalene conversion among the low cost catalysts used for tar removal. Further, biomass char is produced continuously during the gasification process, while the other catalysts undergo deactivation. A simple first order kinetic model is used to describe the naphthalene conversion with biomass char.     

Effect of micropores diffusion on kinetics of CH4 decomposition over a wood-derived carbon catalyst

In order to optimise hydrogen production from biomass gasification, catalytic conversion of methane contained in a surrogate biomass syngas (CH₄ 14%; CO 19%; CO₂ 14%; H₂ 16%; H₂O 30%; N₂ 7%) is investigated over a fixed bed of porous wood char as a function of temperature (800–1000 °C) and space time (1.6–6.2 min g L⁻¹). Determination of Thiele modulus evidences a change of kinetic regime from chemically- to diffusion-controlled when the temperature increases; this finding is particularly relevant when porous chars having an average pore width of 1 nm are used as catalysts. Mass diffusion transfers are accounted for by a model introducing an internal effectiveness factor. Knudsen diffusion in micropores is shown to limit the conversion rate of methane per unit mass of catalyst, and explains why such a rate is not proportional to the BET surface area, especially when the latter is higher than typically 300 m²/g. It is concluded that diffusion limitations in micropores should be taken into account, otherwise underestimated activation energy and intrinsic kinetic constant are obtained in some experimental conditions.     

Changes in char reactivity and structure during the gasification of a Victorian brown coal: Comparison between gasification in O2 and CO2

Char reactivity is an important factor influencing the efficiency of a gasification process. As a low-rank fuel, Victorian brown coal with high gasification reactivity is especially suitable for use with gasification-based technologies. In this study, a Victorian brown coal was gasified at 800 °C in a fluidised-bed/fixed-bed reactor. Two different gasifying agents were used, which were 4000 ppm O₂ balanced with argon and pure CO₂. The chars produced at different gasification conversion levels were further analysed with a thermogravimetric analyser (TGA) at 400 °C in air for their reactivities. The structural features of these chars were also characterised with FT-Raman/IR spectroscopy. The contents of alkali and alkaline earth metallic species in these chars were quantified. The reactivities of the chars prepared from the gasification in pure CO₂ at 800 °C were of a much higher magnitude than those obtained for the chars prepared from the gasification in 4000 ppm O₂ also at 800 °C. Even though both atmospheres (i.e. 4000 ppm O₂ and pure CO₂) are oxidising conditions, the results indicate that the reaction mechanisms for the gasification of brown coal char at 800 °C in these two gasifying atmospheres are different. FT-Raman/IR results showed that the char structure has been changed drastically during the gasification process.     

Correlation of methane uptake with microporosity and surface area of chemically activated carbons

Two series of activated carbon discs have been prepared by chemical activation of olive stones with ZnCl₂ and H₃PO₄. Some of the carbons have been post-treated in order to modify their porous texture and/or surface chemical composition. All carbons have been characterized by adsorption of N₂ (−196 °C) and CO₂ (0 °C) and immersion calorimetry into dichloromethane. The volume of methane adsorbed at 25 °C and 3.5 MPa is proportional to the surface area deduced from immersion calorimetry into dichloromethane. Consequently, it is possible to estimate, using a single experiment, the possibility of using activated carbons for the storage of natural gas. On the other hand, the methane uptake can be also correlated to the volume of micropores, provided by the adsorption of N₂ at −196 °C and CO₂ at 0 °C, although the correlations is not as good. Only carbons slightly activated, with low surface area and microporosity below around 0.6 nm, do not adjust the above correlations because they adsorb more methane than the expected, the effect of chemical nature of the carbon surface being almost negligible.     

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.     

Catalytic gasification of char from co-pyrolysis of coal and biomass

The catalytic gasification of char from co-pyrolysis of coal and wheat straw was studied. Alkali metal salts, especially potassium salts, are considered as effective catalysts for carbon gasification by steam and CO₂, while too expensive for industry application. The herbaceous type of biomass, which has a high content of potassium, may be used as an inexpensive source of catalyst by co-processing with coal. The reactivity of chars from co-pyrolysis of coal and straw was experimentally examined. The chars were prepared in a spout-entrained reactor with different ratios of coal to straw. The gasification characteristics of chars were measured by thermogravimetric analysis (TGA). The co-pyrolysis chars revealed higher gasification reactivity than that of char from coal, especially at high level of carbon conversion. The influence of the alkali in the char and the pyrolysis temperature on the reactivity of co-pyrolysis char was investigated. The experimental results show that the co-pyrolysis char prepared at 750 °C have the highest alkali concentration and reactivity.     

Modification of the pyrolysis/carbonization of PPTA polymer by intermediate isothermal treatments

Pyrolysis/carbonization of poly (p-phenylene terephtalamide) (PPTA) was investigated, studying the possibility of modifying the pyrolysis/carbonization behavior and hence the carbon yield by introducing intermediate isothermal treatments. Thermogravimetric analysis (TG) was used to establish the main degradation steps of the material. It showed that the yield of solid residue at 950 °C increases by more than 15 wt.% by introducing an isothermal step at 500 °C for at least 50 min. Intermediate decomposition products at different temperatures/times of PPTA decomposition were characterized by X-ray diffraction (XRD), elemental microanalysis and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). XRD results showed that carbonization progresses during the isothermal step, so that the material is degraded loosing its crystallinity in a continuous way. DRIFTS spectra showed that PPTA undergoes a rupture of polymeric chains during the isothermal stage enhancing aryl nitrile formation. This favors crosslinking reactions that take place with increasing temperature, yielding a solid residue with a higher nitrogen content and higher char yield.     

Preparation and characterization of chars and activated carbons from Saskatchewan lignite

Saskatchewan lignite was used as a precursor to prepare carbonaceous adsorbents for use as SO₂ adsorbent from flue gases. The lignite was carbonized producing char in a fixed bed microreactor system at different temperatures from 350 to 550 ^°C in nitrogen atmosphere. The chars obtained at 475 ^°C for 120 min exhibited the highest micropore surface area (136 m²/g) and volume (0.062 cm³/g) and the smallest median pore diameter (∼ 0.7 nm). Carbon dioxide and steam were used as activating agents. Activation of char at optimum conditions of 650–675 ^°C for 15 min with carbon dioxide and steam resulted in a further increase in micropore surface area (220 and 186 m²/g for CO₂ and steam, respectively) and volume (0.090 and 0.085 cm³/g for CO₂ and steam, respectively). The yield of char was 64 wt.%, while the yields of activated carbon were 60 and 57 wt.% for CO₂ and steam activation, respectively; all based on the mass of original lignite.     

Calcination and thermal degradation mechanisms of triblock copolymer template in SBA-15 materials

The calcination under air and degradation under inert atmosphere of as made SBA-15 surfactant templated mesostructured silica materials were studied using a combination of N₂ sorption at −196 °C, mass spectrometry (MS) monitored temperature programmed oxidation and degradation, thermogravimetric analysis (TGA), ¹³C MAS NMR and Fourier transform infrared (FTIR) spectroscopy. The characterization of the materials treated at different temperatures under oxidative and inert atmospheres indicated that both processes follow stepwise mechanisms. SBA-15 materials exhibit three families of pores: primary main mesopores, complementary intrawall mesopores (>2 nm) and intrawall micropores (<2 nm). Under oxidative atmosphere, the primary mesopores and the larger framework intrawall pores are first emptied below 200 °C with the production of volatile organic compounds (VOCs). This step is followed by an oxidation of the PEO chains from the intrawall micropores (<2 nm) producing CO₂ by combustion. Under inert atmosphere, the degradation of the organic template also begins first in the primary mesopores. However, an increase in the pore diameter up to 550 °C indicates that the complete liberation of primary mesopores is much slower than for calcination under air and occurs simultaneously with the removal of the PEO chains occluded within framework micropores.     

Evaluation of structural features of chars from pyrolysis of biomass of different particle sizes

The structural features of chars derived from pyrolysis of mallee wood of different particle sizes in a novel fluidized-bed/fixed-bed reactor have been investigated. Raman spectroscopy was used for structural evaluation of chars. Spectra were curve-fitted with 10 Gaussian bands representing typical structural features of the chars. The temperature had a significant influence on the evolution of char structure and thus the total Raman peak area between 800 and 1800 cm^− 1 is seen to decrease significantly with increasing pyrolysis temperature for all chars. On the other hand, the ratio I_D/I_{(Gr + Vl + Vr)} between the band intensities of condensed aromatic ring systems (> 6 rings) and amorphous char structures with small aromatic ring (3-5 rings) systems is seen to increase with increasing temperature. The particle size of biomass has a great role in char structure at fast heating rate (> 1000 °C/s) pyrolysis although it has no effect on char structure at slow heating rate pyrolysis (0.17 °C/s). However, in the bigger biomass particle, the structure of char prepared under fast heating rate pyrolysis is similar to that of the structure of char prepared under slow heating rate pyrolysis.     

Chemically activated fungi-based porous carbons for hydrogen storage

A set of porous carbons has been prepared by chemical activation of various fungi-based chars with KOH. The resulting carbon materials have high surface areas (1600–2500 m²/g) and pore volumes (0.80–1.56 cm³/g), regardless of the char precursors. The porosities mainly derived from micropores in activated carbons strongly depend on the activation parameters (temperature and KOH amount). All activated carbons have uniform micropores with pore size of 0.8–0.9 nm, but some have a second set of micropores (1.3–1.4 nm pore size), further broadened to 1.9–2.1 nm as a result of increasing either the activation temperature to 750 °C or KOH/char mass ratio to 5/1. These fungi-based porous carbons achieve an excellent H₂ uptake of up to 2.4 wt% at 1 bar and −196 °C, being in agreement with results from other porous carbonaceous adsorbents reported in the literature. At high pressure (ca. 35 bar), the saturated H₂ uptake reaches 4.2–4.7 wt% at −196 °C for these fungi-based porous carbons. The results imply a great potential of these fungi-based porous carbons as H₂ on-board storage media.     

Gasification performance of torrefied Timothy hay and spruce wood chars in a CO2 environment

Timothy hay abundantly available in New Brunswick, Canada, is mostly used for animal feed and bedding. Upgrading biomass using Torrefaction method can offer benefits in its waste management, energy density and energy conversion efficiency. Temperature and residence time play an important role in the torrefaction process. Meanwhile, CO₂ gasification is also a promising thermochemical conversion process due to its potential to reduce net GHG emissions and tune syngas composition. This study investigates the impact of torrefaction parameters on isothermal and non-isothermal CO₂ gasification of Timothy hay and spruce chars. Timothy hay chars exhibited higher CO₂ gasification reactivity than chars from spruce. The physicochemical properties analysis indicated that higher reactivity of Timothy hay char was mainly attributed to the high amount of alkali and alkaline earth metal (AAEM) content, relatively large BET surface area, a high number ofactive sites, and a low crystalline index. Moreover, in both experimental cases, char derived through a high heating rate and high residence time conditions exhibited improved gasification performance, which was attributed to the generation of large amounts of AAEM (Ca and K) and high specific surface area. Co-gasification results during non-isothermal processes under CO₂ showed the presence of larger interactions in coal char/Timothy hay char blends than that of coal char/spruce char blends. For both experimental conditions, interactions were enhanced once the char prepared from high heating rate and high residence time was gasified with coal char. Thus, the proposed approach is a sustainable way of conversion of Timothy hay under CO₂ environment.     

Additives assisted catalytic cyclo-dehydration of diethylene glycol in near-critical water

The additives assisted the cyclo-dehydration of diethylene glycol (DEG) reaction was studied in near-critical water (NCW). Zinc chloride (ZnCl₂) and sodium carbonate (Na₂CO₃) were selected to investigate their effects on the cyclo-dehydration of diol. The influences of reaction temperature, time, pressure, reactant/water ratio (r/w) and the concentration of additives on the product yield of the cyclo-dehydration of DEG were examined. The results showed that the final obtained product was primarily 1,4-dioxane, resulting from the cyclo-dehydration of DEG in NCW. The yield of 1,4-dioxane was only 9.84 wt.% in pure water by reacting at 340 °C for 240 min, but the maximum yield of 1,4-dioxane could reach as high as 50.89 wt.% in the solution of 0.50 wt.% ZnCl₂ at 340 °C for 120 min, and the conversion of DEG was 91.94 wt.%. The complete conversion of DEG was obtained in 1.00 wt.% ZnCl₂ at 340 °C for 120 min, but the yield of 1,4-dioxane was only 10.18 wt.%. In the case of Na₂CO₃, it did not have significant promotion effect on the cyclo-dehydration of DEG reaction. All these experimental results demonstrated that ZnCl₂ had the positive effects on the dehydration of DEG while Na₂CO₃ depressed the reaction. Based on these results, a possible reaction mechanism and pathway was proposed in NCW.     

Reactivity of Australian coal-derived chars to carbon dioxide

The reactivities to CO₂ of four chars derived from Australian coals at 610 °C, were measured thermogravimetrically. Reaction rates in 100% CO₂ (total pressure, 101 kPa) varied from 0.026 mg h^?1 mg^?1 at 803 °C for char derived from a Lithgow coal to 6.3 mg h^?1 mg^?1 at 968 °C for a Millmerran coal char. Activation energies for the four chars were in the range 219–233 kJ mol^?1. The results show that for Lithgow (Hartley Vale) coal char, reactivity increases with CO₂ concentration and decreasing particle size. The apparent reaction order for this char with respect to CO₂ concentration was found to be 0.7. For different chars, reactivity is inversely proportional to the rank of the parent coal. No general correlation has been established between total mineral content (ash) and char reactivity.     

随机孔模型研究煤焦O2/CO2燃烧动力学特征

在热重分析仪上对焦作无烟煤焦和云浮烟煤焦O₂/CO₂条件下燃烧特性进行研究。确定在不同温度下不同煤焦O₂/CO₂燃烧的特征。利用随机孔模型(RPM)表征两种煤焦反应速率与碳转化率的关系,同时与未反应缩核模型(Model Ⅰ)和混合模型(Model Ⅱ)的拟合结果进行比较。研究表明,在不同反应条件下,随机孔模型具有最佳的拟合效果,相关系数都在0.986以上。比较RPM、ModelⅠ和Model Ⅱ计算结果发现,焦作无烟煤焦的O₂/CO₂等温燃烧的活化能比云浮烟煤焦的高,且同一煤种燃烧反应温度越高反应速率常数越大。由于随机孔模型的结构参数ψ可以很好地表现孔结构变化对煤焦燃烧反应的影响,因此随机孔模型能更加准确地描述煤焦O₂/CO₂燃烧特征。     

Co-carbonization of coking coal with K2CO3: structure of the resulting chars

Oakdale coking coal was co-carbonized with up to 30 wt% of K₂CO₃ to 900 °C. The resulting chars were examined for optical texture and morphology by scanning electron microscopy. No changes in optical texture were observed with additions of < 1 wt% K₂CO₃. Increased additions created an isotropic, non-fusing layer of char around the particles and this prevented the formation of a coherent coke. The size of the remaining anisotropy was also reduced, some char fragments being composed of isotropic carbon. Severe fissuring occurred in the particles of char, causing fragmentation; this was presumably due to diffusion of potassium into the char structure. X-ray studies indicated increased peak half-widths of (002) diffractions for the isotropic carbon.     

Evolution of biomass char structure during oxidation in O2 as revealed with FT-Raman spectroscopy

FT-Raman spectroscopy has been used to identify structural features and evaluate the structural evolution of biomass chars during gasification with air. Chars prepared from the pyrolysis of a cane trash sample with a fast particle heating rate in a novel fluidised-bed/fixed-bed reactor at 500, 700 and 900 °C were oxidised at 400 °C in air in a TGA. The data derived from the spectral deconvolution of Fourier Transform — Raman spectra suggest that the 500 °C char showed very different structural features after pyrolysis and during oxidation from the 700 and 900 °C chars, while the differences between the latter two chars were small. Preferential consumption by O₂ of smaller aromatic rings and structures of somewhat aliphatic characteristics left the char more enriched with larger aromatic ring systems. The changes in char structure are in agreement with the observed reactivity measured in O₂ in a thermogravimetric analyser.     

Effect of carbonization temperature on the yield and porosity of char produced from palm shell

Several batches of chars were prepared from palm shell by carbonization in a flow of nitrogen using a fixed‐bed reactor. Palm shell was carbonized at temperatures of 500, 600, 700, 800 and 900 °C for 1 h to study the effects of carbonization temperature on char yield and its porosity. The prepared chars were characterized for the micropore volume using CO₂ adsorption while the meso‐ and macropore volumes were analyzed using a mercury porosimeter. The char yield was around 25% and is comparable with yields reported from other lignocellulosic materials. The results show that carbonization temperature has a significant effect on the micro‐ and mesopore volumes. However, it has negligible effect on the macropore volume. © 2001 Society of Chemical Industry     

Control of pore development during CO2 and steam activation of olive stones

Several series of activated carbons were prepared from olive stones by means of carbonization followed by activation with carbon dioxide, water steam and a mixture of them, under different experimental conditions. The changes in porosity of the original char during activation were studied by adsorption of N₂ at 77 K, CO₂ at 273 K and Hg porosimetry. The study was carried out covering a wide range of burn-off (19–83%) using activation times of 20–120 min, and temperatures between 650 and 950 °C. It is shown quantitatively how the individual factors influence the development of microporosity. It was found that in general terms, increasing activation produces a continuous increase in the volume of micropores and mesopores. However, this development occurs in a different proportion whether CO₂ or steam are used: while CO₂ produces narrow micropores on the carbons and widens them as time is increased, steam yields pores of all the sizes from the early stages of the process. The simultaneous use of these two activating agents resulted positive at times higher than 1 h, since it yielded carbons with higher volumes of pores.