Selective adsorption of greenhouse gases on the residual carbon in lignite coal liquefaction

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.

Energy Calculation and Simulation of Methane Adsorbed by Coal with Different Metamorphic Grades

The law of specific surface energy variation during the adsorption process is an important basis for studying the mechanisms of coal gas adsorption. Based on the theory of adsorption energy, eight coal samples with different ranks were analyzed using an isothermal adsorption experiment at three different temperatures (30, 40, and 50 °C) and six different pressures (0.1, 0.2, 0.6, 1.0, 1.4, and 1.8 MPa). Then, the single-layer adsorption model and multilayer adsorption model were used to calculate the energy variation during the adsorption process. Just like the adsorption capacity, it is clear that the specific surface energy is inversely proportional to temperature and proportional to gas pressure. The energy difference between the single-layer adsorption and the multilayer adsorption calculation is large. Therefore, the adsorption energy was calculated based on the calorific value, and the comparative analysis shows that the specific surface energy based on the multilayer adsorption model can better reflect the gas adsorption capacity than the single-layer adsorption model. The mechanisms of gas adsorption were explored, such as intermolecular force, energy variation, and specific surface area. The adsorption energy was simulated, which indicated that the energy variation is affected by both coal physical properties and internal chemical structure.

Exploring a New Method to Study the Effects of Surface Functional Groups on Adsorption of CO2 and CH4 on Activated Carbons

The commercial coconut shell-activated carbon was modified to change the number of oxygen-containing functional groups. N₂ adsorption/desorption isotherms, Fourier transform infrared (FT-IR), and Boehm titration were adopted to describe the physical and chemical properties of the samples. The adsorption isotherms of CO₂ and CH₄ on both the unmodified and modified samples were measured. To better understand the effects of surface oxygen-containing functional groups on adsorption of CO₂ and CH₄, the overall adsorption could be considered as the result of adsorption within the pores and adsorption onto the oxygen-containing functional groups. Thus, a new way to understand different adsorption mechanisms by calculation was proposed. On the basis of the results, there is a significant correlation between the saturation adsorption capacity of CO₂ and the number of oxygen-containing functional groups, especially carboxyl and hydroxyl. According to the values of enthalpy (-12.2 to -20 kJ/mol), it can be known that the adsorption caused by oxygen-containing functional groups is exothermic and belongs to physisorption. A semiempirical relationship between the variation of the surface oxygen-functional groups and the variation of the adsorbed amount was established. The method proposed in this paper provides a new way to study the effects of surface functional groups on the adsorption of CO₂ and CH₄ and can be even promoted in studying the adsorption mechanism of other adsorbates.