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Additional file1. Table S1: List of primers used for qRT-PCR validation.

Noncovalent interactions have achieved extensive attentions in thermal conversion of low rank coal, but have been little studied in direct coal liquefaction (DCL). In order to investigate strength of noncovalent interactions (hydrogen bonds and π–π stacking) between lignite and DCL solvents, experimental methods and DFT (density function theory) calculations were utilized. Aromatic components in industrial recycled solvents have been selected, including tetralin, 1,2–dihydronaphthalene (DHN), indane, 1–methylnaphthalene, naphthalene, indene and benzene. On one hand, total strength of noncovalent interactions between those aromatic components and lignite were integrally evaluated by swelling ratio determination. On the other hand, Spatial distribution and strength of noncovalent interactions were investigated by RDG (reduced density gradient) analysis, CVB (core–valence bifurcation) index and LOLIPOP (localized orbital locator integrated pi over plane) index, respectively. Overall, addition of DHN, indane and benzene resulted in significant swelling of lignite. Swelling of lignite in benzene and others (DHN and indane) was enhanced by stronger OH–π hydrogen bonds and π–π stacking, respectively. By conjoint study of swelling ratio and DFT calculations, both hydrogen bonds and π–π stacking play crucial roles in swelling of lignite. Adjustment of noncovalent interactions should be taken into account during design of DCL solvents, based on correlations between noncovalent interactions and structural characteristics of aromatic components.

Low-temperature pyrolysis could effectively recover heavy oil from direct coal liquefaction residue (DCLR), and the derived DCLR char (DCLRCH) could be gasified to compensate hydrogen consumption during liquefaction. The staged processing of DCLR, pyrolysis and gasification, is thought to be a promising method for its high-efficient usage. In the work, DCLR was pyrolyzed under different pyrolysis temperatures (300, 400 and 450 °C) in a fixed-bed reactor to prepare DCLRCH, designated as P-300, P-400 and P-450, which were then used to prepare DCLRCH water slurries. The results of FTIR and successive sequential extraction show that the oxygen-containing functional groups and aliphatic chains of DCLR are removed during pyrolysis and the aromaticity of DCLRCH increases with pyrolysis temperature increasing. The dispersant adsorption behavior suggests that the adsorption layer of P-450 is the thinnest among the four samples. The slurryability, rheological behavior and static stability of DCLRCH water slurries (DCLRCHWS) were systematically investigated. Compared with direct coal liquefaction residue water slurry (DCLRWS), the slurryability of DCLRCHWS prepared with P-300 and P-400 is enhanced and their static stability deteriorates further. DCLRWS and DCLRCHWS are all weak shear-thinning fluids. However, the mixture of P-450 and water is not a homogeneous and stable suspension. Under the experimental condition, 400 °C is the optimum pyrolysis temperature of DCLR for maximizing production of high-value components and preparing high-quality feedstock for gasification.

Direct coal liquefaction residue (DCLR) is a main byproduct in direct coal liquefaction process and its clean and high-efficient utilization is important. If DCLR could be gasified for hydrogen production, it will compensate hydrogen consumption during liquefaction and lower the operation cost, which makes DCLR utilization more promising. We proposed to blend DCLR with low-rank coals to prepare DCLR–coal–water slurries (DCLRCWS) as feedstock for gasification. In this work, one DCLR and four low-rank coals were used to prepare the individual DCLR–water slurries (DCLRWS), coal water slurries (CWS) and the mixed DCLRCWS at the weight ratio of 1:1. The slurryability, static stability and rheology of various slurries were investigated. The results show that DCLR and the low-rank coals are complementary in terms of slurry properties and DCLRCWS could meet the requirement of gasification process. Adding 50 wt.% DCLR apparently improves the slurryability of low-rank coals, and the maximum solid loading ( C max ) of DCLRCWS prepared with ZLNR coal and DCLR is about 10% higher than that of the corresponding CWS. The effects of coal properties, dispersant adsorption, zeta potential on preparation of highly loaded slurries were examined in terms of slurryability. Moreover, compared with DCLRWS, DCLRCWS display higher degree of pseudoplasticity and better static stability owing to the addition of low-rank coals.

The effects of coal characteristics on the properties of coal water slurry (CWS) were systemically studied using sixteen Chinese coals of different ranks from lignite to anthracite. The correlation between coal properties and CWS characteristics were investigated by multivariate progressive regression analysis. CWS properties examined in this study included slurryability (concentration of slurry that gives an apparent viscosity at 1200 mPa · s at a shear rate of 28.38 s−1), rheological behavior (represented by a flow index), and static stability (days for formation of soft sediment). Coal properties examined in the study included coal rank, air equilibrium moisture (Mad), maximum moisture holding capacity (MHC), ash content, surface properties, petrographic macerals, pore structure, and adsorption characteristics of dispersants.

Publisher Summary This chapter establishes the available relations between coal slurry ability and coal properties. Both coal slurry ability described by the maximum coal loading of coal water slurry and the slurry rheological behavior are dependent on coal rank. For low-rank coals with its carbon content lower than about 80 wt. % (daf.), their slurries have a low coal loading but have a shear-thinning behavior, for coals with their carbon content higher than 85 wt. %, a high coal loading and a shear-thinning behavior, and for coals with their carbon content in the range of about 80–85 wt. %, an intermediate coal loading and (65–70 wt. %) a shear-thickening behavior that is unfavorable for industrial use. Fine linear relationships are found between the maximum coal loading of slurry and inherent moisture content and the moisture-holding capacity of coal over the whole coal rank.