Carbon fibers with high performance are critically important materials for various industries, especially pitch-based carbon fibers including isotropic pitch-based and mesophase pitch-based are applied widely in the fields of aerospace, defense, sports, electronics, environmental protection, etc. Therefore, their development and fabrication have been a research focus in recent years. Carbon fiber production is a complicated process including raw material pretreatment, pitch preparation, melt-spinning, stabilization, carbonization, and graphitization. Each step has a significant influence on the properties and performance of the final carbon fiber product, and thus should be controlled effectively for preparing carbon fiber with desirable performance. This review presents a systematic and critical review of the preparation mechanism, influence factors, operational conditions, and various properties of the carbon fiber product, which was clarified along the master line of the step-by-step preparation of pitch-based carbon fibers. Besides, the current issues of carbon fiber production and its potential industrialization, which is the main challenge for the broad application of pitch-based carbon fibers, are also discussed in detail in this review.

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.

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.