Direct co-liquefaction of vitrinite and inertinite-rich coal fines with plastic waste material

Abstract

Direct coal liquefaction (DCL) stands as a promising process in the realm of energy production. This process is characterized by its ability to break down the complex macromolecular structure of coal using a solvent under moderate conditions. Direct liquefaction aims to yield liquid fuels with a targeted hydrogen-to-carbon ratio, which is achieved through hydrogenation reactions. Conversely, inadequate hydrogen availability can lead to the formation of undesirable residues. The co-liquefaction of coal with other feed materials have been examined extensively. Co-liquefaction involves the simultaneous treatment of various carbonaceous materials, including coal, biomass, and plastic waste, within a shared solvent or reaction medium. This innovative process capitalizes on the synergistic properties of diverse feedstocks, with the primary goal of increasing overall conversion efficiencies and generating valuable liquid products. Central to the success of co-liquefaction is a profound understanding of the intricate interactions between the heterogeneous components during the thermochemical conversion process. In the initial phase of this study, the direct liquefaction of inertinite-rich coal fines was evaluated, alongside a comparison with a vitrinite-rich coal commonly utilized in coal-to-liquid processes. To discern the influence of mineral matter content, demineralization techniques were employed to eliminate the majority of inorganic constituents. The results revealed that the inertinite-rich coal demonstrated reduced liquefaction conversion values, attributed to a lower proportion of reactive macerals and increased levels of inorganic mineral matter. Notably, the liquefaction efficiency exhibited a strong correlation with the abundance of reactive macerals inherent in the parent coals. Interestingly, in the case of the inertinite-rich coal, the presence of inorganic mineral matter hindered the liquefaction process while promoting CO2 gasification reactions of the resultant chars. Furthermore, CO2 gasification experiments allowed for the determination of reactivities and apparent gasification activation energies of the coal chars, liquefaction residue chars, and pre-asphaltene and asphaltene (PAA) chars. Utilizing models such as the random pore model (RPM) and volumetric reaction model (VRM), these calculations shed light on the chemistry, reactivity, and kinetics of different chars during gasification. Overall, this investigation underscores the substantial disparities in liquefaction conversion values, product distribution, and composition, influenced by factors such as maceral composition, inorganic mineral matter content, solvent hydrogen-donor capabilities, and liquefaction reaction temperatures, with implications extending to CO2 gasification reactivity of liquefaction solid residue chars. In the subsequent phase of this study, polypropylene (PP) and low-density polyethylene (LDPE) were subjected to liquefaction experiments utilizing tetralin and benzene as solvents. This investigation delved into the impact of feed material and solvent selection on conversion values and the composition of liquid yields derived from liquefaction processes. Notably, variations in the molecular structure of polypropylene and low-density polyethylene exerted significant influence on conversion outcomes. Tetralin liquefaction experiments conducted at 450°C yielded conversion values of 97.0% for PP and 23.8% for LDPE, whereas benzene-liquefaction resulted in conversion values of 98.5% for PP and 97.5% for LDPE. GC-MS analysis of the liquid fractions derived from benzene liquefaction of PP and LDPE predominantly comprised alkanes and alkenes, indicative of the solvent's hydrogen-donor properties as evidenced by the presence of biphenyl in the liquid fraction. Overall, both PP and LDPE exhibited potential for substantial conversion to liquid products within the temperature range of 425-450°C. Particularly promising were the results from LDPE liquefaction using benzene, which yielded higher conversion values (97.5%) accompanied by a significant fraction of oils rich in C10-C29 alkanes and C10-C19 alkenes. Remarkably, biphenyl constituted a substantial portion of the liquid fraction during the liquefaction of both PP and LDPE using benzene, suggesting that the dehydrogenation of benzene occurred. Further analysis using 1H NMR revealed distinct differences between the liquid yields derived from LDPE and PP, with liquid fraction produced from the liquefaction of LDPE exhibiting a higher propensity for side chain cleavage due to its structural characteristics, resulting in the formation of diverse aliphatic compounds. Conversely, PP, with its methyl branches, demonstrated a tendency for cleavage, yielding smaller molecules or radicals that interacted with the solvent, leading to the generation of various benzene derivatives. In the final phase of this study, co-liquefaction experiments were conducted utilizing a vitrinite-rich coal and inertinite-rich discard coal with polypropylene (PP) and low-density polyethylene (LDPE) as feed materials. Using tetralin, a hydrogen-donor solvent, and benzene, a hydrogen-poor solvent, allowed for a comprehensive assessment of liquefaction efficiency and the quantity and quality of derived products, comparing aromatic solvents with differing hydrogen-donor capabilities. Notably, tetralin co-liquefaction experiments involving the vitrinite-rich coal exhibited the highest carbon conversion values when co-processes with PP and LDPE reaching 83.3 and 80.8%, respectively. Conversely, overall conversion during tetralin liquefaction involving the inertinite-rich coal was lower, attributed to the differences in maceral composition between the coals. The co-liquefaction of the inertinite-rich coal with PP and LDPE showcased conversion values of 76.4 and 72.5%, respectively. Remarkably, experiments involving LDPE revealed significantly higher conversion values compared to predicted values, indicating synergistic effects between coal and plastic materials, influencing the product yields and overall conversion. Molecular structure disparities in the feed materials notably impacted conversion outcomes. GC-MS analysis of the liquid fractions derived from the co-liquefaction experiments of LDPE using benzene as the solvent revealed high yields of alkanes and alkenes, indicative of benzene's hydrogen-donor properties. 1H NMR analysis of the liquid yields derived from the co-liquefaction using benzene revealed an abundant presence of tetralin and its derivatives, with LDPE derived liquids exhibiting higher aliphatic-to-aromatic proton ratios compared to PP. Remarkably, experiments conducted using benzene demonstrated a high presence of aliphatic protons in the liquid fraction. Alongside the formation of biphenyl, as determined using GC-MS, the results indicate that dehydrogenation of benzene occurred. Moreover, polypropylene and low-density polyethylene were found to facilitate hydrogen transfer in the co-liquefaction process, effectively converting inertinite-rich discard coal into liquid products when co-processed. Additionally, solid residue chars derived from inertinite-rich coal blends exhibited higher gasification reactivities compared to those containing vitrinite-rich coal. The results from this study indicate the potential of utilizing inertinite-rich coal fines and plastics such as PP and LDPE in co-liquefaction processes.