Insight into Relationship between Thermal Dissolution of Low-Rank Coals and Their Subsequent Oxidative Depolymerization

Oxidative depolymerization of low-rank coals is promising for obtaining benzene carboxylic acids (BCAs). However, it is hindered by the low yield of BCAs along with a large number of alphatic acids. Thermal dissolution could modify the physico-chemical structural features of low-rank coals, which is expected to improve the oxidation of LRCs. In this paper, lignite and subbituminous coal were firstly subjected to thermal dissolution with cyclohexane at 250 °C for 2 h. Then, the raw coal and the corresponding thermal insoluble portion (TIP) were oxidized by NaOCl under the same conditions. The residual yields of TIPs oxidation were both lower than those of raw coals oxidation, indicating that TIPs were more easily oxidized than the raw coals. The yield of BCAs obtained by TIPs oxidation was above 19% higher than that from the oxidation of raw coals. Meanwhile, the selectivity of BCAs was improved in the resulting oxidation products from TIPs compared with that from the raw coals. The relationship between BCAs generation and thermal dissolution of low rank coals was investigated by ultimate analysis, Fourier transform infrared spectroscopy, and nitrogen adsorption-desorption analysis. The results suggested that thermal dissolution could enrich aromatic portion in the remaining TIPs, resulting in an increasing of the yield and selectivity of BCAs. Simultaneously, thermal dissolution raised the specific surface area and expanded the looser space structure of TIPS, which were beneficial for the sufficient collision between aromatic structures and oxidant, facilitating the oxidative depolymerization of TIPs. This investigation would provide a novel route for promoting BCAs production by mild oxidative depolymerization of low-rank coals.

Effects of Chemical Properties and Inherent Mineral Matters on Pyrolysis Kinetics of Low-Rank Coals

The kinetics during the pyrolysis process depend on both chemical structure and inherent mineral matters in coal, but normally, only one of these components is investigated in literature. In the present work, four low-rank coals were pyrolyzed in a thermogravimetric analyzer at a heating rate of 10 K/min in a constant nitrogen stream at a temperature up to 900 °C to study the pyrolysis behavior and kinetics. Two of the samples were raw coal (R-YL) and clean coal (C-YL) obtained through the washing process. The results showed that the coal-washing process mainly affected the inorganic part (mineral matters) and structure in coal, which did not largely change the chemical properties. The pyrolysis behavior in primary stage (before 550 °C) was mainly affected by the chemical properties of coal, while the pyrolysis behavior in higher temperature also depended on inherent mineral matters. The kinetics of four coals were obtained using the Coats–Redfern (CR) method with five theoretical models. The difference of E value was almost negligible for R-YL and C-YL, also showing that the coal-washing process did not largely change the chemical properties. The higher frequency factor A for clean coal C-YL showed a more porous structure due to the coal-washing process. The apparent activation energy E in the third stage was affected by the thermodynamic property of inherent minerals.

Utilization of Low-Rank Coals for Producing Syngas to Meet the Future Energy Needs: Technical and Economic Analysis

Increased energy demand in recent decades has resulted in both an energy crisis and carbon emissions. As a result, the development of cleaner fuels has been under the research spotlight. Low-rank coals are geographically dispersed, abundant, and cheap but are not utilized in conventional processes. Syngas can be produced from coal-using gasification which can be used in various chemical engineering applications. In this study, the process model for syngas production from low-rank coal is developed and the effects of various process parameters on syngas composition are evaluated, followed by a technical and economic evaluation. The syngas production rate for the low-rank coal has been evaluated as 25.5 kg/s, and the contribution to H2 and CO production is estimated as 1.59 kg/s and 23.93 kg/s, respectively. The overall syngas production and energy consumed in the process was evaluated as 27.68 kg/GJ, and the CO2 specific emissions were calculated as 0.20 (mol basis) for each unit of syngas production. The results revealed that the syngas production efficiency for low-rank coals can be as high as 50.86%. Furthermore, the economic analysis revealed that the investment and minimum selling prices per tonne of syngas production are EUR 163.92 and EUR 180.31, respectively.