Influence of carbonization conditions on the gasification of acacia and eucalyptus wood chars by carbon dioxide

Gasification rates of cubic shaped acacia and eucalyptus wood chars were measured thermogravimetrically in a carbon dioxide atmosphere at temperatures in the range 810–960 °C. The effects of wood species and carbonization conditions, such as temperature, heating rate and soaking time, were determined. Both reactivity and the activation energy for the gasification of wood chars were found to be strongly influenced by the carbonization conditions employed during their preparation and wood type. The reactivities of both the acacia and eucalyptus wood chars decreased with increasing preparation temperature; while the activation energy for their gasification increased. Slow carbonization (heating rate: 4 °C min⁻¹) led to the production of wood chars having lower reactivities and higher activation energies than those of the wood chars prepared under rapid carbonization (heating rate: 30 °C min⁻¹) at the same temperature. With increasing soaking time, at carbonization temperatures of 800 and 1000 °C, the reactivity of resulting wood chars was reduced. The results also show that the reactivities of acacia wood chars are higher than those of similarly prepared eucalyptus wood chars.     

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     

Influences of carbon structure on the reactivities of lignite char reacting with CO2 and NO

Raw and demineralized lignite samples were pyrolyzed from 773 to 1673 K to generate chars. The chars were characterized with Raman spectroscopy for the structure evolution. The reactivities of the chars reacting with CO₂ and NO were measured with thermogravimetric analysis. The derived reactivity indexes were correlated with the treatment temperature and the Raman structural parameters to demonstrate the applicability of Raman spectroscopy for evaluation of the reactivities of char CO₂ gasification and char-NO reaction. It was found that char microstructure evolution with the treatment temperature could be represented by Raman band area ratios. I_D1/I_G and I_G/I_ALL represented the evolution of the ordered carbon structure while the combination of I_D3/(I_G + I_D2 + I_D3) reflected the evolution of the amorphous carbon structure of the lignite chars with increasing the treatment temperature from 773 to 1673 K. Reactivity indexes of the demineralized chars reacting with both CO₂ and NO were found to increase with increasing the treatment temperature, implying that the structure ordering did result in the losses of the reactivities. Higher reactivities of the non-demineralized chars indicated the catalytic role of inorganic matter in the reactions with both gases. I_D1/I_G and I_G/I_ALL had good linear correlations with the reactivities particularly of the demineralized chars if considering the structure evolution behaviors at lower and higher temperatures, respectively. I_D3/(I_G + I_D2 + I_D3) was found to have fairly good linear correlations with the reactivity indexes of the lignite chars generated over the whole temperature range.     

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.     

Nanostructures of pyrolytic carbon from a polyacetylene thin film

Pyrolysis of a polyacetylene thin film has been performed in order to carbonize at temperatures of 500 to 1000 °C in vacuum. A trans-polyacetylene thin film was synthesized using a Ziegler-Natta catalyst. A black char below 20% in weight of the original PA film remained after pyrolysis. Structural properties and morphology of the black chars were investigated using Raman scattering spectrum, X-ray diffraction measurements, and scanning and transmission electron microscopy. Dehydrogenation and carbonization of the PA film were almost finished at a pyrolysis temperature of 800 °C. However, hollow spherical or elliptical nano-particles of tens of nanometers in size, which are composed of graphite structure, were included in the black chars obtained at all pyrolysis temperatures in this work. The formation mechanism of a graphite crystal in nanometer size from a PA crystal was discussed.     

Activated carbon fiber cloths as electrodes for high performance electric double layer capacitors

Activated carbon fiber cloth (ACFC) electrodes with high double layer capacitance and good rate capability were prepared from polyacrylonitrile (PAN) fabrics by optimizing the carbonization temperature prior to CO₂ activation. The carbonization temperature has a marked effect on both the pore structure and the electrochemical performances of the ACFCs. Moderate carbonization at 600 °C results in higher specific surface area and larger pore size, and hence higher capacitance and better rate capability. The specific capacitance of the ACFCs in 6 mol L⁻¹ KOH aqueous solution can be as high as 208 F g⁻¹. It remains 129 F g⁻¹ as the current density increases to 10 000 mA g⁻¹.     

Char structural ordering during pyrolysis and combustion and its influence on char reactivity

In the present study, two processes, thermal treatment and oxidation, were separated for a fundamental study of structural evolution during pyrolysis and combustion, as well as for the study of the influence of such evolution on char reactivity. Chars were prepared at different temperatures and heating rates from a size-graded low volatile bituminous coal. The reactivity of resultant chars was measured in Kinetic Regime I using a fixed bed reactor. The structure of fresh and partly burnt chars was characterized using quantitative XRD analysis (QXRDA), high-resolution TEM (HRTEM), high-resolution FESEM, and multi-point gas adsorption.Both QXRDA and HRTEM observations show that char structure becomes more ordered with increasing pyrolysis temperature and decreasing heating rate. Char structure was also investigated as a function of char burnoff. The QXRDA results show that the amorphous concentration of char decreases during combustion while the aromaticity and average crystallite size of char increase. As a result, char structure becomes more ordered during combustion. This is in agreement with HRTEM observations. Due to the low reaction temperature (about 673 K), which is much lower than that for char preparation (1473 K), it was believed that oxidation, instead of thermal effect, contributed to the structural ordering observed during combustion. The structural parameters obtained from QXRDA were then correlated to char reactivity. Structural ordering was found to be responsible for char deactivation during thermal treatment and oxidation. Since the amorphous concentration and aromaticity of char are two strongest indicators of char reactivity, a structural disorder index, DOI, was defined based on them to describe char structural evolution, and further correlated to char reactivity.     

Preparation and Phosphine Adsorption of Activated Carbon Prepared from Walnut Shells by KOH Chemical Activation

Walnut-shell activated carbons (WSACs) with high surface area and predominant micropore development were prepared by KOH chemical activation. The effects of carbonization temperature, activation temperature, and ratio of KOH to chars on the pore development of WSACs and PH₃ adsorption performance of the modified walnut-shell activated carbons (MWSACs) were studied. Criteria for determining the optimum preparation conditions were pore development of WSACs and PH₃ breakthrough adsorption capacity of MWSAC adsorbents. The result shows that the optimum preparation conditions are a carbonization temperature of 700°C, an activation temperature of 700°C, and a mass ratio of 3. The BET surface area and the micropore volume of the optimal WASC are 1636m²/g and 0.641cm³/g, respectively. The micropore volume percentage of WSAC plays an important role in PH₃ adsorption when there is a slight difference in BET surface areas. High-surface-area WSACs with predominant micropores are suitable for PH₃ adsorption removal. The MWSAC adsorbent owns the biggest PH₃ breakthrough adsorption capacity (284.12mg/g) due to the biggest specific surface area, total pore volume, and micropore volume percentage. The MWSAC adsorbent will be a potential adsorbent for PH₃ adsorption removal from yellow phosphorus tail gas.     

TPR法研究煤焦—CO_2气化反应──（Ⅰ）碳化条件和掺加金属催化剂对气化反应性的影响

改变碳化条件和掺加金属催化剂，制备了十几个煤焦样品。用ＸＲＤ测定了未载金属煤焦的碳微晶尺寸。用ＴＰＲ法测定了上述煤焦的ＣＯ_２气化反应性，用Ｆｒｅｅｍａｎ－Ｃａｒｒｏｌｌ方法计算得到了反应活化能Ｅ和指前因子Ａ。结果表明：碳化条件越剧烈，煤焦的气化活性越低。掺加不同量的钙催化剂，煤焦的气化活性得到不同程度的提高，同时ＤＴＧ曲线有明显的变化。这些现象可从碳化条件对煤焦气化活性的影响，以及钙对煤焦ＣＯ_２气化的催化作用等方面来解释。     

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.     

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.     

Effect of preparation conditions on the properties of a coal-derived activated carbon honeycomb monolith

Activated carbon honeycomb (ACH) monoliths were prepared by extruding of a mixture of bituminous coal and organic additives and subsequent carbonization and steam activation. Preparation parameters that were varied were carbonization temperature and activation time. The carbonization conditions were 500, 650 and 800 °C for 1 h and the steam activation conditions were 850 °C for 2, 4 and 6 h. The monoliths at various states were characterized by SEM, XRD, nitrogen adsorption and compression test. It was found that carbonization temperature has significant effects on pore size distribution and mechanical strength of ACH monoliths. The ACH monoliths prepared from high carbonization temperatures exhibited lower values of the BET surface area and total pore volume and higher value of the mechanical strength than those of the ACH monoliths prepared from low carbonization temperatures. This was attributed to the effect of high temperature carbonization that results in the formation of relatively less defective structures.     

The devolatilisation of millimetre sized coal particles at high heating rate: the influence of pressure on the structure and reactivity of the char

Most studies on the influence of pressure on the combustion of coal particles have shown that for a constant oxygen concentration, an increase of pressure leads to a decrease of combustion rate. Among the different phenomena, which can explain this behaviour, the influence of the devolatilisation pressure on the structure and reactivity of the char formed may be important. The aim of this paper was to obtain a quantitative characterisation of the physical and chemical structure of chars formed during pyrolysis under a large range of pressure. Experiments of single coal particle pyrolysis were conducted in a laser reactor with pressure ranging from 0.014 to 2.1 MPa in a nitrogen atmosphere. As expected, an increase of pressure lead to a decrease of the volatile matter yield, which can be related to the secondary reactions of volatile matter. A characterisation of the char was performed by gas adsorption methods: nitrogen adsorption, carbon dioxide adsorption and active surface area (ASA) measurement. True and apparent densities, porosities and swelling of the particles were also investigated. Although the volatile matter yield decreases, the porosity and the swelling of the char increases with increasing pyrolysis pressure. We observed an increase in surface area and microporosity with increasing pressures up to 0.6 MPa. The ASA surface also increases in this temperature range, but the ratio of ASA to CO₂ surfaces shows that the intrinsic reactivity of the surface decreases with increasing pyrolysis pressure.     

Structural ordering of coal char during heat treatment and its impact on reactivity

The effect of heat treatment on the structure of an Australian semi-anthracite char was studied in detail in the 850-1150°C temperature range using XRD, HRTEM, and electrical resistivity techniques. It was found that the carbon crystallite size in the char does not change significantly during heat treatment in the temperature range studied, for both the raw coal and its ash-free derivative obtained by acid treatment. However, the fraction of the organized carbon in the raw coal chars, determined by XRD, increased with increase of heat treatment time and temperature, while that for the ash-free coal chars remained almost unchanged. This suggests the occurrence of catalytic ordering during heat treatment, supported by the observation that the electrical resistivity of the raw coal chars decreased with heat treatment, while that of the ash-free coal chars did not vary significantly. Further confirmatory evidence was provided by high resolution transmission electron micrographs depicting well-organized carbon layers surrounding iron particles. It is also found that the fraction of organized carbon does not reach unity, but attains an apparent equilibrium value that increases with increase in temperature, providing an apparent heat of ordering of 71.7 kJ mol⁻¹ in the temperature range studied. Good temperature-independent correlation was found between the electrical resistivity and the organized carbon fraction, indicating that electrical resistivity is indeed structure sensitive. Good correlation was also found between the electrical resistivity and the reactivity of coal char. All these results strongly suggest that the thermal deactivation is the result of a crystallite-perfecting process, which is effectively catalyzed by the inorganic matter in the coal char. Based on kinetic interpretation of the data it is concluded that the process is diffusion controlled, most likely involving transport of iron in the inter-crystallite nanospaces in the temperature range studied. The activation energy of this transport process is found to be very low, at about 11.8 kJ mol⁻¹, which is corroborated by model-free correlation of the temporal variation of organized carbon fraction as well as electrical resistivity data using the superposition method, and is suggestive of surface transport of iron.     

Hydrotreatment of heavy oil from a direct coal liquefaction process on sulfided Ni-W/SBA-15 catalysts

A series of Ni-W catalysts supported on SBA-15 with different pore sizes were prepared by incipient wetness impregnation method and characterized by N₂ adsorption-desorption and X-ray diffraction. The hydrogenation of heavy oil (distillation temperature: 320-340 °C) derived from the direct coal liquefaction process using Shengli coal in the presence of sulfided Ni-W/SBA-15 catalysts with different pore sizes were evaluated at 400 °C and initial H₂ pressure of 5.0 MPa. The results showed that the catalyst preparation method and the pore size of the support had a significant influence on the Ni/W crystallite size, hydrodenitrogenation (HDN) and hydrodearomatization (HDA) activities of coal-derived heavy oil. The larger pore could cause the Ni-W/SBA-15 to form larger Ni-W crystallite. The catalysts with largest pore in the range studied displayed highest HDN and HDA activities for upgrading of the coal-derived heavy oil.     

碳化法制备酚醛树脂基微滤碳膜

以高含碳热塑性酚醛树脂为原料,通过碳化制备了酚醛树脂基微滤碳膜,考察了碳化条件,包括碳化终温、升温速率、恒温时间和保护气流速对膜的平均孔径、孔径分布和气体透量的影响. 对碳膜进行了CO2活化处理,考察了活化条件对碳膜性能的影响. 用热重分析考察了碳膜碳化失重,用泡点法测量了其孔径分布. 实验结果表明,随着碳化终温的升高,碳膜的平均孔径和气体透量均减小,当碳化终温从650℃升高到950℃时,碳膜的平均孔径从0.61 mm下降到0.54 mm,气体透量从1.84′10-5 mol/(m2×s×Pa)下降到1.14′10-5 mol/(m2×s×Pa). 350℃碳化温度得到的碳膜爆破强度最低,随着碳化温度的升高爆破强度增加. 升温速率、恒温时间和保护气流速对碳膜性能影响不大. 活化导致膜孔径变大,当CO2浓度从12.5%增加到50%时,碳膜的平均孔径从1.54 mm增大到1.96 mm,气体透量从7.0′10-5 mol/(m2×s×Pa)增大到1.68′10-4 mol/(m2×s×Pa).     

Pore development during carbonization of coals

Changes in pore properties during carbonization at a constant heating rate and no external pressure were followed for bituminous and subbituminous coals by porosimetry, sorption and density measurements. Observations were supplemented by scanning electron microscopy and rate of volatilization measurements. All coals exhibited similar devolatilization behaviour and pore structure development, showing maximum micro and macropore volumes around 600 °C, and a partial collapse of pore structure after 800 °C. The micropore structure did not collepse in the case of non-caking coal. Pore volume maxima occur at least 100 °C above the temperature of the devolatilization rate maxima. Some interpretation is provided.     

Effect of pre-carbonization of petroleum cokes on chemical activation process with KOH

The influence of pre-carbonization of petroleum cokes on the properties of the activated carbon precursors and final carbons activated with KOH was investigated by using TG-DTG, FTIR, XRD, and BET techniques. TG-DTG study revealed the decomposition of volatile species and FTIR analysis identified the presence of C-O, C-O-C, C-O-H and some alkyl species on the surface of the precursors. XRD results indicated that a disorder occurred after the carbonization at 500 °C and an order happened gradually with increasing the temperature. The decreases in the BET surface area and N₂ adsorption capacity coincided with the reduction of the functional species on the precursor surface with the increase of the pre-carbonization temperature. These surface functional species were proposed to play a key role in the porosity development during the KOH activation. First, they reacted with KOH to form the intermediate C-O-K species, then the resultants further reacted with the carbon of the precursor, and finally developed the porous structure. As a result, the surface area and the pore size distribution of the activated carbon can be controlled by optimizing the conditions of the pre-carbonization and activation processes.     

炭化条件对小麦秸秆炭棒燃烧性能的影响

针对小麦秸秆成型燃料中挥发分含量较高导致燃烧过程中产烟量较大、热值不高的缺点,将秸秆成型燃料进一步炭化制得秸秆炭棒以改善成型燃料的燃烧性能。考察了炭化条件,包括炭化温度、升温速率和炭化时间对所制得的炭棒燃烧性能的影响。实验结果表明制得的炭棒较成型燃料的热值得到了明显的提高,以5 ℃/min的速率升温至300 ℃并保持1 h条件下制得的炭棒热值可以由炭化之前的14.48 MJ/kg升高至19.08 MJ/kg。该炭棒的密度由1.31 g/cm³降至0.99 g/cm³,炭棒中固定碳质量分数达到32.60%,较成型燃料显著升高;此外挥发分由约60%降至约40%,产烟时间和产烟量显著降低,得率为52.5%,可更好地满足工业及生活的使用需求。     

Changes of porosity during carbonization of bituminous coals: effects due to pores with radii less than 2500 nm

Based on both available reports and the results of experimental studies, the pores in coal chars have been divided into three groups. The first group comprises of pores with radii <7.5 nm, the second one consists of the pores with radii in the range 7.5≤r≤2500 nm and the third group consists of the pores with r>2500 nm. Based on the results of the experimental study conducted for ten Polish bituminous coals, the set of equations describing volume changes of the pores with r≤2500 is derived and discussed for slow carbonization of selected coals up to 1000 °C. The equations describe both the transformation processes of the primary pores and the formation and development of new pores depending on the final temperature of carbonization and properties of the parent coal. These properties include the Gieseler temperatures of softening and resolidification as well as the characteristics of the chemical structure of coal organic matter. The impact of the coal's organic matter contraction and the coal's total mass loss on the pore volume (cm³/g) are also taken into account.