Activated carbon fiber derived from wasted coal liquefaction residual for CO2 capture.

Wasted coal liquefaction residual was used to synthesize activated carbon fibers (ACFs) for CO2 capture, and the properties of the developed ACFs were optimized by adjusting the activation conditions, including the reaction temperature and soaking time. The yield, element distribution, pore structure, composition, functional group, morphology, and adsorption capacity of the as-synthesized ACFs were characterized by various apparatuses. In addition, static and dynamic adsorption experiments were conducted to investigate the adsorption capacity of CO2 in flue gas. The results revealed that the synthesized ACFs are mainly composed of carbon, accounting for more than 90% of the total elements. The specific surface area, pore volume, and pore width distribution of the prepared ACFs were optimized by changing the activation conditions, and ACFs with a specific surface area higher than 1400 m2/g were successfully developed by activation at 950 for 3 h. The amount of micropores occupied more than 90% of the total pore volume. The pore width distribution dominated by micropores is beneficial for CO2 adsorption since the diameter of CO2 is 0.33 nm. From FTIR and XPS analysis, it is found that the main structure of ACFs is a carbon skeleton composed of polycyclic aromatic hydrocarbons with a small number of oxygen-containing functional groups. The adsorption isotherm of ACFs for CO2 conforms to the Langmuir model, indicating that the adsorption process of CO2 by ACFs can be attributed to monolayer adsorption. Both the specific surface area and oxygen-containing functional groups have crucial effects on the adsorption capacity of CO2. The dynamic adsorption experiment determined that ACFs-920-3 had the highest adsorption capacity for CO2 in flue gas, and adsorption equilibrium was achieved after 7 min of adsorption. The adsorption process of CO2 in flue gas by the as-synthesized ACFs fits well with the pseudosecond kinetic model. The CO2 adsorption capacity of the obtained ACFs remained unchanged after 10 cycles of adsorption. A high-value-added route for synthesizing ACFs for CO2 capture using CLR as a raw material was developed.

Quaternary ammonium salts targeted regulate the surface charge distribution of activated carbon: A study of their binding modes and modification effects.

Activated carbon (AC) is negatively charged in aqueous solution, which seriously restricts its application range. Quaternary ammonium salt as a common cationic surfactant was utilized to modify the surface charge distribution of materials. The evolution of the surface charge distribution of AC modified by benzalkonium chloride (BAC), diallyl dimethyl ammonium chloride (DDA) and 3-chloro-2-hydroxypropyl tri-methyl ammonium chloride (CTA) was investigated. Results showed that the surface charge of AC modified by CTA does not change significantly. BAC has a high molecular weight, low surface electrostatic potential and large steric hindrance due to its hydrophobic long-chain alkyl. The diffusion of BAC molecules from solution to AC changed its charge distribution. But these molecules were difficult to combine with AC surface, and most of them were adsorbed into the pores of AC to form aggregates, resulting in a significant reduction in the surface area. BAC modified AC could enhance the adsorption capacity of F- in aqueous solution through electrostatic attraction, but the improvement effect was limited due to the reduction of surface area, and the maximum adsorption capacity was only increased from 1.18 to 3.31 mg/g. DDA has a small molecular weight and high surface electrostatic potential and easily binds to the surface of AC. Some CC bonds in DDA combined with the ionized hydrogen ions derived from phenolic hydroxyl groups in AC to form carbonium-ions. Then, they could react with AC to form ether bonds, causing DDA to be closely bonded with the surface of AC. DDA realizes the targeted regulation of the surface charge distribution of AC, it has little effect on the porous structure of AC. The modified AC still maintained strong adsorption capacity, and the maximum adsorption capacity for F- was 54.98 mg/g. Meanwhile, a large number of zeolites were loaded on the modified AC and formed coating structures.

Backwashing behavior and hydrodynamic performances of granular activated carbon blends.

Ozone-biological activated carbon (O3-BAC) process has been proved to be an efficient and cost-effective technology in advanced treatment of drinking water. However, O3-BAC raises strict requirements in adsorption, hydrodynamic and regeneration performances, which one single activated carbon could hardly all-sided meet. Blending activated carbons seems to be an appropriate and economically feasible method to deal with the issue. Thus, the uniformity and stability of activated carbon blends during water treatment, especially in backwashing process are of great importance. In this paper, cyclic experiments of downward adsorption and upward backwash on 11 typical commercial granular coal-based activated carbons and their blends were carried out in column test. Hydrodynamic performances such as bed expansion rate and bed pressure drop were measured. The uniformity and stability of activated carbon blends were investigated by determining iodine number of samples collected from different heights of activated carbon bed. Then, both traditional regression methods and back-propagation neural network model were utilized to predict superficial velocity at 30% bed expansion rate and maximum bed pressure drop of activated carbon blends. The results indicate that water backwashing process has no effect on the composition proportion of activated carbon blends, and slightly changes the particle distribution of activated carbon bed regarding pore structure and adsorption capacity. A three-layer back-propagation neural network model for superficial velocity at 30% bed expansion rate yields mean relative errors of 2.17%, which is much lower than that given by traditional regression methods such as 5.53% (weighted average), 4.08% (linear) and 4.06% (polynomial). Moreover, the back-propagation neural network model for maximum bed pressure drop yields mean relative errors of 1.37%, which is much lower than that given by traditional regression methods such as 4.31% (weighted average), 4.28% (linear) and 4.22% (polynomial). The non-linear relationships can be accurately identified by the back-propagation neural network model.

Volatile Organic Compounds Adsorption Capacities of Zeolite/Activated Carbon Composites Formed by Electrostatic Self-Assembly.

Activated carbon (AC) is a broad-spectrum adsorbent but is flammable and has low adsorption capacities for polar and/or high-boiling volatile organic compounds (VOCs), while zeolites exhibit high thermal stability but poor adsorption of macromolecular and nonpolar VOCs. In this study, zeolite/AC composites were synthesized with the aim of obtaining broad-spectrum, efficient, and safe adsorbents for VOCs. Dimethyldiallylammonium chloride (DDA)-modified AC was used as a carrier for an in situ hydrothermal reaction enabling assembly with zeolites due to electrostatic attraction. Interface models were constructed for their phases, which revealed the binding force and simulated the binding process. The adsorption and flame resistance of the composites were evaluated. The results showed that DDA effectively modified AC to give it a long-lasting positive charge in solutions. High-silicon and pure-silicon zeolites exhibited low negative charges or were even neutral; it was difficult to combine with the modified AC via electrostatic attractions. Instead, LTA zeolites with high aluminum contents and negative charges were used, and the seed-induction method was used. Ethanol and ultrasonic dispersion were used to prevent agglomeration of the seeds and modified AC powder, so they were self-assembled electrostatically. Moreover, the crystallization time was extended and composites with high zeolite loadings were successfully prepared. According to the model calculation, the binding energy between the zeolite and AC before and after the DDA modification were 324.97 and 1076.46 kcal mol-1, respectively, and the distance between them was shortened by 2.7 Å after DDA treatment. As a result, AC and zeolite combined more closely and exhibited a stronger binding energy. The adsorption capacity for highly polar dichloromethane was improved by zeolite loading on the AC, and the bed penetration time was doubled. However, impregnation with inorganic sodium enhanced the reactivities of the organic components in the composite, and the ignition point was slightly reduced. Furthermore, the electrostatic self-assembly method can expand to prepare the LTA zeolite/columnar AC composite from shaped AC, greatly improving its application prospects.

Preparation and characterization of three-dimensional hierarchical porous carbon from low-rank coal by hydrothermal carbonization for efficient iodine removal.

Low-rank coal, such as Shengli lignite (SL) and Datong bitumite (DT), has abundant reserves and is low in cost. Due to its high moisture content, abundant oxygen-containing groups, high ash content and low calorific value, low-rank coal is mainly used in a low-cost method of direct combustion. For better value-added utilization of SL and DT, a novel strategy has been developed for the preparation of oxygen-rich hierarchical porous carbons (HPCs) by hydrothermal carbonization (HTC), followed by steam activation. In this paper, firstly, the physical and chemical properties of SL and DT were improved by HTC pretreatment, bringing them closer to high rank coal. Then, the effects of HTC pretreatment and activation temperature on the properties of the HPCs were investigated in detail. The results show that the HPCs have mainly microporous structures (the microporosity of 200-SLHPC-900 is 79.58%) based on the N2 adsorption-desorption isotherm analysis and exhibit a higher specific surface area (SSA) and larger pore volume (25.02% and 2.69% improvement for 200-SLHPC-900; 4.93% and 14.25% increase for 200-DTHPC-900, respectively) after HTC pretreatment. The two types of HPCs also present good adsorption performance. The iodine adsorption value of lignite-based HPC presents an increase of 13.72% from 503 mg g-1 to 572 mg g-1, while the value of bitumite-based HPC increases up to 924 mg g-1. A preliminary additional HTC step is therefore an effective method by which to promote the performance of low-rank coal based porous carbon. The process of hydrothermal carbonization and steam activation is a cost-effective and environmentally-friendly preparation method, which omits the use of a chemical activator and reduces the step of alkaline waste liquid discharge compared with the route of carbonization and chemical activation.

Transformation of alkali and alkaline earth metals during the preparation of activated carbon from Zhundong high-alkali coal.

The preparation of activated carbon (AC) is a promising approach for the efficient utilization of Zhundong high-alkali coal. The volatilization and release of alkali and alkaline earth metal (AAEM) species can be effectively inhibited by using a lower operating temperature and a carbon matrix. However, the long time of the pyrolysis and activation process may promote the release of the AAEM from the coal during the process. Therefore, it is necessary to explore the transformation of AAEM during the preparation of AC from Zhundong high-alkali coal, and the cleanness of this process is evaluated accurately. In this study, the evolution of AAEM, distribution, and chemical speciation is characterized before and after the preparation of AC from the coal, and then thermodynamic calculations were performed using FactSage to simulate the transformation of AAEM in the coupled process of pyrolysis and activation. The results showed that in the process of AC preparation, the AAEM species inside the carbon matrix moved towards the surface of the AC with the aid of released volatiles and the activation reaction. Some Na and K species were released due to their weak binding with the carbon matrix and this resulted in the loss of Na and K content, whereas Mg and Ca were closely combined with the carbon matrix and were enriched in the AC. Furthermore, the defects and amorphous structure of the AC prepared with H2O activation were more than that of the AC prepared with CO2 activation, which meant that more of the AAEM species were exposed to the high temperature environment. As a result, the loss of AAEM content in the AC with H2O activation was higher than that in the AC with CO2 activation. In this process, a small amount of highly volatile and corrosive AAEM was produced, and the release of volatile matter and the consumption of the carbon matrix were the main factors for the AAEM loss. Therefore, the preparation of AC from Zhundong high-alkali coal is a viable method for its clean use.

Mechanism of the evolution of pore structure during the preparation of activated carbon from Zhundong high-alkali coal based on gas-solid diffusion and activation reactions.

Zhundong coal can significantly reduce the preparation temperature of activated carbon (AC) due to the high contents of alkali and alkaline earth metals (AAEMs) present in it. Moreover, because of its lower operating temperature and the presence of carbon matrix, Zhundong coal can effectively inhibit the release of AAEM during the preparation of AC. For these reasons, the preparation of AC from Zhundong coal is a promising approach for the clean utilization of Zhundong coal. Accordingly, this study was aimed to investigate optimum conditions for the preparation of AC from Zhundong coal. For this purpose, at first, Raman spectroscopy was used to determine the conditions for an optimal carbonization process using a coal sample; then, the evolution of the pore structure of AC under different conditions was examined by small-angle X-ray scattering (SAXS) and the N2 adsorption analyser. Furthermore, environmental scanning electron microscopy (ESEM) was performed to analyze the surface morphology of AC. Finally, by dividing the activation process into gas-solid diffusion and activation reactions, a mechanism for the evolution of pore structure during the preparation of AC was proposed. The results showed that the char with an amorphous structure and less graphite-like carbon, which was obtained by heating Zhundong coal from room temperature to 600 °C at 5 °C min-1 under the protection of N2 and then maintaining it at this temperature for 60 min, is suitable for the subsequent activation process. At low temperatures, the diffusion of H2O was dominant in the activation process, and the weak gas-solid reaction resulted in poor development of the pore structure; on the other hand, the CO2 activation reaction mainly occurred on the surface of the char due to the poor diffusion of CO2, and then, the produced pores could improve the diffusion of CO2; this led to significant development of the pore structure. With an increase in temperature, the H2O diffusion reaction was enhanced, and the pore structure of AC was completely developed; however, the diffusion of CO2 reduced with an enhancement in the CO2 activation reaction, leading to the consumption of carbon matrix by CO2 gasification instead of pore formation by the CO2 activation reaction. Therefore, proper utilization of the unique characteristics of H2O and CO2 during pore formation is important to control the activation process.

Effects of Bromination-Dehydrobromination on the Microstructure of Isotropic Pitch Precursors for Carbon Fibers.

In this work, isotropic pitch precursors are synthesized by the bromination-debromination method with ethylene bottom oil (EO) as the raw material and bromine as the initiator for pitch formation and condensation reactions. The aggregation structure, molecular weight distribution, and molecular structure of isotropic pitch precursors are characterized by thermal mechanical analyzer (TMA), MALDI TOF-MS, and 13C NMR, respectively, for revealing the mechanism of synthesis of isotropic pitch precursors. The results show that at low bromine concentrations, polycyclic aromatic hydrocarbons (PAHs) were mainly ordered in cross-linked structures by bromination-debromination through substitution reactions of side chains. The condensed reactivity can be improved by the effect of bromine, meaning that condensation reaction was aggravated by the method of bromination-dehydrobromination. In the presence of excess bromine, the cross-linked stereo structure of PAHs changed to the planar structure of condensed PAHs, which was not conducive to the subsequent spinning and preparation of carbon fibers.