Preparation And Applications Of Carbon Materials From Direct Coal Liquefaction Residue

Direct coal liquefaction residue (CLR) is rich in carbon, ash and sulfur contents, sharing about20-30wt.%of the raw coal used in the direct coal liquefaction process. It is necessary to effectively utilize CLR for the benefits of the process and developing clean and efficient coal technologies. In this study, various CLR based carbons were prepared by KOH activation and used as catalysts for hydrogen production by catalytic methane decomposition (CMD) and as supercapacitor electrodes.Effects of carbon preparation conditions including the weight ratio of KOH/CLR, solvents for mixing KOH and CLR, carbonization procedure and solvents for washing the carbonized samples and CLR compositions, were investigated on the resultant carbon pore structure, catalytic activity and electrochemical performance. The results show that CLR based mesoporous carbons, with the pore sizes centred at3-5nm, can be directly prepared by KOH activation by using the mineral matters contained in the CLR and their salts formed with KOH as the templates.The CLR were separated into five fractions, including oil, asphaltene, preasphaltene, complex carbon matrix and mineral matter, by Soxhlet extraction and demineralization. They share8.0,29.2,12.1,29.0and21.7wt.%of the CLR. As for their effects on the resultant carbons, the organic matter alone is conducive to developing a microporous structure, while the mineral matter plays a positive role on the KOH activation process of the CLR and can serve as the template for mesoporous structure (the mesoporosity up to92%). The resultant mesoporous carbon shows higher and more stable activity for CMD than the microporous. The contents of the impurities(including sulfur and Fe) in the resultant carbon can be down to less than0.2%after KOH activation, carbonization and washing after carbonization. And the CLR-based mesoporous carbon has higher and more stable activity for CMD than commercial coal-based activated carbon, carbon black BP2000and the similar carbons reported in literatures.The relationship between carbon catalytic activities and the textural properties indicates that larger surface area and higher pore volume correspond to higher and more stable catalytic activity. With respect to the decomposition-regeneration cycles, pulsed regeneration is better than the continuous. However, CO and/or CO2will release during the regeneration process, resulting in that the CMD reaction will wipe out its advantage compared with the traditional steam methane reform process. The porous structures of CLR-based carbons can be adjusted and hierarchical porous carbons can be obtained by KOH activation with addition of some additives. It was investigated the effects of three kinds of additives, including different silica materials (SiO2, TEOS, Na2SiO3and SBA-15), metal oxides(AI2O3and MgO) and organic materials (sugar, urea and CTAB), on the resultant carbon pore structure, catalytic performance and electrode capacitance. The results show that different additives correspond to different mechanisms for the carbon porous structures. Some nanoparticles, formed by the reaction of the silica materials (or AI2O3) and KOH, can serve as space fillers of nanopores in the carbonized carbon. So can MgO particles themselves. The gases, produced by the decomposition of the organic additives, can develop and/or widen some pores. Equivatent resistances of the carbon electrodes can be reduced by the optimum dosage of the additive, with the capacitance increased by more than30%. When MgO was used as the additive, the resultant carbon electrode shows excellent capacitive performance, with the capacitance up to186F/g at the scan rate of5mV/s or137F/g at the current density of10A/g and good cycle stability (keeping the capacitance of about118F/g) after6000cycles at200mV/s. When Al2O3was used as the additive, the resultant carbon can serve as the carbon catalyst of CMD for simultaneous production of hydrogen and fibrous carbons, which was reported for the first time. And the produced fibrous carbons show good catalytic activity for CMD.Fe (or Ni)doped carbons can be directly prepared from CLR by KOH activation with addition of Fe(NO3)3(or Ni(NO3)2). The preparation method can utilize the carbon reducibility at high temperature, eliminating hydrogen reduction process required by the traditional synthesis process. When Ni doped carbon was used as the catalyst for CMD, methane conversion increased with the reation time. It was mainly attributed to that the carbon deposits produced by CMD were mixed with small particles of active Ni. So they can serve as the catalysts for CMD, increasing the active Ni utilization.4coals and2oil shales, as carbon precursors with different ash contents, were used to prepare carbons by the methods mentioned for CLR-based carbons. The results show that the method combined with KOH activation and the additive can improve the porous structure and electrochemical performance of the carbons from coal with low ash content, but do harm to those from coal with high ash content. Due to the high ash content, carbons from oil shales had a low yield. But the mineral matters contained in oil shales and their salt nanoparticles formed by the reaction with KOH can serve as the template for mesoporous carbons.