•LTA can effectively elucidate transformation of sodium and calcium species in DCL.•Calcium species are easier to be reserved in DCLR than sodium species.•Sodium species in coals are difficult to be detected by XRD technique.•Temperature has significant effects on transformation of AS-Ca in DCL.Lignite, a suitable feedstock for direct coal liquefaction (DCL), is usually rich in sodium and calcium species. To better understand chemical transformation of inherent sodium and calcium species during DCL, two lignites rich in sodium and calcium species were liquefied in this work. The results show that both Australia lignite (AU) and Hami lignite (HM) are preferred raw materials for DCL. After DCL, mineral matters were obviously enriched in residues (DCLR). Under all the temperatures, retention ratios of sodium species were lower than those of calcium species. One obvious transformation of calcium species was reflected in changes of organic-bound calcium species (AS-Ca) and hydrochloric acid-insoluble calcium species (HIS-Ca) in HM. During direct liquefaction of HM, the amount of AS-Ca decreased significantly and the amount of HIS-Ca increased sharply. XRD analyses of low temperature ashes of DCLR confirmed that when temperatures reached above 400 °C, kaoline in HM gradually decomposed and came to react with AS-Ca, which resulted in formation of HIS-Ca. Due to the low ash content, transformation of calcium species in AU showed unobvious tendency and only CaCO3 was detected in DCLR.

In this study, significant effects of ionic catalysis on the formation of H2 and CO during the steam gasification process of cellulose are revealed. The energy of the C–H bonds of cellulose can be remarkably reduced by Na+ and OH− ions produced by the dissociation of NaOH, enabling dehydrogenation of cellulose at low temperature. Dehydrogenation of cellulose is evidently affected by the concentration of Na+ and OH− ions that cellulose can come into contact with. Higher concentrations of Na+ and OH− ions can reduce the initial dehydrogenation temperature of cellulose to lower than 403 K. The production of CO increases after this remarkable dehydrogenation of cellulose, which indicates that the C–O bonds of cellulose are prone to forming CO by pyrolysis.

The new catalyst, Al2O3·Na2O·xH2O/NaOH/Al(OH)3, was made by means of hydrolyzation and hydration of sodium aluminum oxide (Al2O3·Na2O). Hydrogen and hydrogen-rich gas were produced through the reaction of cellulose with the catalyst and steam. In order to avoid production of tar, the gasification temperature is controlled at ≤673 K. The temperature of producing hydrogen is controlled at about 473–623 K. The conversion degree of hydrogen from cellulose at about 473–673 K could come up to 59.63%. The production of hydrogen-rich gas was set at about 673 K. The gasification residue could be used as material for combustion. Al2O3·Na2O could be regenerated from the byproducts Al2O3 and Na2CO3 produced in the combustion process. The catalyst could be re-prepared from the regenerative Al2O3·Na2O.

Pyrolysis of sawdust and its three components (cellulose, hemicellulose and lignin) were performed in a thermogravimetric analyzer (TGA92) under syngas and hydrogen. The effect of different heating rates (5, 10, 15 and 20 °C/min) on the pyrolysis of these samples were examined. The pyrolysis tests of the synthesized samples (a mixture of the three components with different ratios) were also done under syngas. The distributed activation energy model (DAEM) was used to study the pyrolysis kinetics. It is found that syngas could replace hydrogen in hydropyrolysis process of biomass. Among the three components, hemicellulose would be the easiest one to be pyrolyzed and then would be cellulose, while lignin would be the most difficult one. Heating rate could not only affect the temperature at which the highest weight loss rate reached, but also affect the maximum value of weight loss rate. Both lignin and hemicellulose used in the experiments could affect the pyrolysis characteristic of cellulose while they could not affect each other obviously in the pyrolysis process. Values of k0 (frequency factor) change very greatly with different E (activation energy) values. The E values of sawdust range from 161.9 to 202.3 kJ/mol, which is within the range of activation energy values for cellulose, hemicellulose and lignin.

The coal ash obtained at 815 °C under oxidizing atmosphere was further treated at 1300 °C and 1400 °C under reducing atmosphere. The resultant ashes were examined by XRD, SEM/EDX and FTIR. The results show that the residence time of coal ash at high temperatures has considerable influences on the compositions of coal ash and little effect on the amounts of unburned carbon. The amorphous phase of mineral matters increases with the increasing temperature. The FTIR peaks due to presence of different functional groups of minerals support the findings of XRD, and supply additional information of amorphous phase which cannot be detected in XRD. The ash samples generated from a fixed bed reactor during char gasification were also studied with FTIR. The temperatures of char preparation are responsible for the different transformation of minerals during high temperature gasification.

The sulfur transformation during pyrolysis and gasification of Shenhua direct liquefaction residue was studied and the release of H2S and COS during the process was examined. For comparison, the sulfur transfer of Shenhua coal during pyrolysis and that of pyrolyzed char during gasification were also studied. The residue was pyrolyzed at 10 °C /min to 950 °C. During pyrolysis about 33.6% of sulfur was removed from the residue, among which 32.1% was formed H2S in gas and 1.5% was transferred into tar, 66.4% of the sulfur was remained in residue char. Compared with coal, the residue has generated more H2S due to presence of Fe1−xS which was enriched in residue during liquefaction process. There is a few COS produced at 400–500 °C during pyrolysis of coal, but it was not detected form pyrolysis of the residue. During CO2 gasification, compared with pyrolysis and steam gasification, there are more COS and less H2S formation, because CO could react with sulfide to form COS. During steam gasification only H2S was produced and no COS detected, because H2 has stronger reducibility to form H2S than CO. After steam gasification no sulfur was detected in the gasification residue. The XRD patterns show after steam gasification, only Fe3O4 is remained in the gasification residue. This indicates that the catalyst added during the liquefaction of coal completely reacted with steam, resulting in the formation of H2 and Fe3O4.

Two Chinese coals, Liuzhi high pyrite coal with high ash content (LZ) and Zunyi high organic sulfur coal (ZY), were pyrolyzed in a fixed-bed reactor under nitrogen and hydrogen at temperature ranging from 400 to 700 °C. The effects of heat rate, temperature and gas atmosphere on sulfur transformation and sulfur uneven distribution were examined by XPS combined with traditional sulfur analysis method. The ratio of surface S to bulk S is used to describe the uneven distribution of sulfurs. It is found that oxygen is rich on the surface, while S in the bulk. The increasing ratio of surface S to bulk S with increasing temperature clearly indicates the sulfur transfer from the bulk to the char surface during pyrolysis. The ratios are higher at all temperatures studied for ZY coal than for LZ coal, which may be related to the higher ash content in LZ coal. The ratio of surface S to bulk S increases with increasing heating rate for LZ coal, while it decreases for ZY coal. In the presence of H2, the S on the surface is much lower than that under N2 and surface S in sulfidic, thiophenic and sulfoxide forms is totally disappeared for LZ coal at various temperatures and heating rates, while the surface S in thiophenic and sulfoxide forms is not totally disappeared for ZY coal, which may be related to the high rank of ZY coal. The ratio of surface S to bulk S decreases before 600 °C with increasing temperature for both coals in the presence of H2, showing that gaseous H2 can easily react with the surface S to form H2S, while above 600 °C it increases because the supply of H2 cannot match the rate of formation of HS free radicals at high temperature.

Arsenic, lead and mercury are the major hazardous trace elements contained in coals. During coal combustion these trace elements release and emit to the atmosphere and cause harmful effects to the human beings and the environment. This paper is to study the fate of arsenic, lead and mercury in Chinese Yima coal by sub-critical water treatment. The experiments were performed in a semi-continuous apparatus. Demineralization, float-sink experiment and sequential chemical extraction were carried out in order to examine the transformation of occurrence mode of the three elements. The results show that with increasing temperature and residence time, the removal of As, Pb and Hg increases. At 410 °C, 15 MPa, 30 min, with water flow rate of 450 ml/h, the removal of mercury and arsenic is about 100% and 57%, respectively. It is difficult to remove lead by sub-critical water extraction. Arsenic and lead in Yima coal are associated not only with pyrite, but also with monosulfides and sulfates, other minerals including silicates, aluminum silicates etc. and organic matters. Mercury is mainly associated with pyrite (about 75%) by adsorption, and is also associated with other minerals such as clays, silicates, aluminum silicates etc. During sub-critical water treatment, arsenic associated with carbonates, sulfates and monosulfide, pyrite and other minerals all decrease, and that associated with organic matters increase. Only a little lead in Yima coal is removed by this process. Mercury associated with pyrite and other minerals decreases, and that associated with sulfates and monosulfides disappears after sub-critical water extraction.

Liquefaction of sawdust under syngas was performed in an autoclave and the effects of temperature, initial syngas pressure and reaction time on the product distribution of sawdust liquefaction were studied. The results using different solvents and atmospheres were also compared by product distribution and analyses of GC-MS, TG, IR and GPC. It was found that hydrogen donor solvent showed remarkable effect than either non-hydrogen donor solvent or without presence of solvent and its hydrogenation ability was much higher than gaseous hydrogen. Among various atmospheres H2 displayed higher activity than syngas and both of them were better than Ar and CO, while CO did not give the favorable influence. With increasing temperature and reaction time (10–30 min) the oil yield increases, while less effect with increasing initial syngas pressure. The thermal decomposition of sawdust to form preasphaltene and asphaltene (PA + A) is a fast step, while longer reaction time is necessary for conversion of PA + A to oil as the 2nd step. The results also indicated that syngas can replace hydrogen in sawdust liquefaction.

The transformation behavior of mercury in two Chinese coals (WJP and HYS coal) during sub-critical water treatments was studied in a semi-continuous apparatus. Float–sink method, demineralization and sequential chemical extraction were used to study the occurrence mode of mercury in raw coals and extracted samples. The results show that with increasing temperature, pressure, water flow and extraction time, the removal of mercury increases. During sub-critical water treatment, the content of mercury associated with sulfates, monosulfides, disulfides, organic material, and insoluble forms decreases. The removal efficiency of mercury is almost 100% at 410 °C, 15 MPa, 0.58 l/h, and 60 min for HYS coal and 96.7% at 380 °C for WJP coal. Under the same temperature and pressure the mercury removal through pyrolysis is less than that through sub-critical water extraction which is an efficient method to remove most mercury from coal.

Temperature programmed combustion of a Chinese brown coal before and after demineralization was carried out in a tubular quartz fixed bed reactor. The evolution of NOx during coal combustion has been investigated with an aim of figuring out the influence of indigenous mineral matter in coal as well as the loaded alkali and alkaline earth metals (Na, Ca). The results show that the mineral matter in the coal suppresses the conversion of fuel-N to NOx, and at the same time, significantly promotes the coal combustion process. The added Na exhibits high catalytic activity for the coal combustion and NOx reduction. In the case of Ca, its effect on the combustion of coal is ignorable but the emission level of NOx increases distinctively due to the presence of Ca. The formation of N–Ca intermediates that preferentially direct the conversion of fuel-N towards NOx during coal combustion is proposed and discussed.

Effect of iron and calcium compounds on nitrogen distribution during pyrolysis of carbazole char up to 950 °C has been studied in a quartz fixed-bed reactor. It was found that impregnated iron and calcium compounds facilitate the carbazole nitrogen to turn into N2, the nitrogen remaining in char and the amount of evolved HCN and NH3 are lower than those without additives. The impregnated iron sharply decreases the evolution of NH3; it also suppresses the HCN formation during pyrolysis. The impregnated calcium also shows some effects on reduction of NH3 and HCN, but its catalytic effect is lower than that of iron. To investigate the super catalytic effects on NH3 reduction, iron was introduced into different supporters. It was found that iron loaded in different supporters show quite different catalytic effects on NH3 reduction, which means that supporters strongly affect the catalytic effects of iron.

Thermogravimetry (TG), in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) and on-line mass spectrometry (MS) were combined to study the effect of O-methylation and O-acetylation on the hydrogen bonds (HBs) in coal. A new method was proposed to compare the spectra of coal before and after the treatments by calibration of the band of kaolinite in coal. It was shown that both measures could reduce the amount of HBs in coal significantly (particularly the weak HBs), but neither of them could completely eliminate the HBs in coal because of the spatial resistance of three-dimensional structure in coal. It was proved that the stronger the hydrogen bonds formed by –OH groups, the less the extent to which they were reduced. The effect of O-acetylation is stronger than that of O-methylation on the reduction of HBs due to the low penetrability of the solvent used in the later treatment. Compared with the parent coal, both the pyrolysis temperature range and conversion of the treated coal increased, the initial decomposition temperature lowered, the decomposition rate accelerated and the composition of gas products was improved. Among the gases produced from the treated coal with these two methods, the amount of C1–C4 hydrocarbons increased, and CO2 and water reduced during pyrolysis compared with those of the raw coal.

Vitrinite and inertinite were separated by DGC from Chinese Shenmu bituminous coal and the structural characteristics of the macerals, before and after pyrolysis, were analyzed by ultimate analysis, FTIR and 13C NMR. The results showed that vitrinite chars always had higher H and lower C content than inertinite char at the same pyrolysis temperature. The FTIR and 13C NMR indicated that vitrinite had more aliphatic C–H, hydrogen bonding and lower aromaticity. With increasing temperature, the aliphatic C–H decreased, aromatic C–H, aromaticity and Har/Hal ratio increased. At the same temperature, inertinite always had higher Har/Hal ratio than vitrinite, which is consistent with that inertinite had higher aromaticity than vitrinite. And the Har/Hal ratio was also related to the remainder volatile matter. With increasing Har/Hal ratio, the remainder volatile matter in vitrinite and inertinite decreased. The higher aromaticity and Har/Hal ratio and lower H content of the inertinite in all temperature range were correlated with its higher thermal stability and lower volatile yield than vitrinite.

The catalytic effects of Na, Ca and Fe on the formation of HCN and NH3 during pyrolysis of nitrogen-containing model chars and on the emission of NOx during the char combustion have been investigated in a fixed bed reactor. The results show that fuel-type nitrogen is mainly retained in char under the pyrolysis conditions (∼900 °C). The presence of Na favors the transformation of char-nitrogen to volatile-nitrogen at high temperature, but the influence of Ca and Fe is negligible. The NH3:HCN ratio under catalytic pyrolysis conditions is higher than that under non-catalytic pyrolysis conditions. It was also found that the emission of NOx was restrained under catalytic conditions at high combustion temperature, but was favored at low temperature. The conversion of char-N to NOx depended on a number of factors including the properties of char, the types of catalysts and the combustion conditions. The influence of catalysts on the emission levels of NOx reflects the relative importance of the catalytic effect on char-N oxidation and on NO-char reaction during the combustion of nitrogen-containing char.

The catalytic reduction of NO over coal-derived chars and the catalytic effect of Na–Fe on NO emission during char combustion were investigated in a quartz fixed bed reactor. The catalytic characteristics of Na and Fe in the NO–char reaction were studied and compared in detail. The results show that the catalytic activity of Na depends greatly on its loading amount, while the activity of Fe is more sensitive to temperature. Na–Fe composite catalysts were also prepared with chars as support. Synergistic effect was found both in the reduction of NO and the char combustion. The Na–Fe composite catalysts exhibit significantly higher catalytic activity than the mono-metallic catalysts with the same metal loading amount. It is intriguing to note that the effectiveness of the catalysts on reducing NO emission during char combustion is in the same order as that in the NO–char reaction, i.e. the chars with catalysts not only have high activity in NO–char reaction but also emit less NO during their combustion.

The kinetics of decomposition of hydrogen bonds in a Chinese lignite was studied using a new method of in-situ diffuse reflectance FT-IR (DRIFT) with pulverized coal samples directly. In addition, a new experimental technique in DRIFT measurement to avoid the condensation of volatile matter at high temperatures was introduced and used in this work. Based on two hypotheses, the single reaction model is used to calculate kinetic parameters for decomposition of hydrogen bonds (except OH-π) in coal heated up to 560°C. The results show that the decomposition of carboxylic acid dimers, OH-N and SH-N follows the second order reaction, while the decomposition of OH-OR2, tightly bound hydroxyl tetramers and self-associated hydroxyls follows the first order reaction. The calculated activation energies of some hydrogen bonds agree well with those obtained with other methods in references. Among the six types of hydrogen bonds studied, the decomposition of carboxylic acid dimers, OH-N, SH-N and tightly bound hydroxyl tetramers can be divided into two stages (230-380°C and 380-500°C), while that of OH-OR2 and self-associated hydroxyl groups can be treated as only one stage. Moreover, the mechanism of decomposition of tightly bound hydrogen bond was suggested based on the comparison of decomposition activation energy of self-associated OH with its bond strength in references.

Some influencing factors on hydrogen production from steam gasification of lignin with Al2O3·Na2O·xH2O/NaOH/Al(OH)3 catalyst in a fixed bed reactor were examined. The results show that the formation rates of hydrogen increase with increasing ratio of Na2O/C and ratio of vapor to lignin as a whole. Higher flow rate of vapor is conducive to inhibit the generation of CO and CO2 at lower temperature. H2O(g) produced by the hydrates contained in the catalyst can bring on more Na+ and OH− ions to be formed by dissociation of NaOH within the catalyst. This leads to more remarkable reduction of the C—H bond energies of lignin. The conversion degree of hydrogen of lignin at 473–973 K reaches 134.94%, which shows that the present catalyst has good catalytic activity on hydrogen production from steam gasification of lignin at low temperature.

The effects of temperatures and atmospheres on sulfur transformation during pyrolysis of Zunyi (ZY) raw coal and its chars were investigated by AP-TPR-MS combined with chemical analysis. The results show that stable organic sulfur is the main sulfur form in ZY coal. Pyrite can be removed under all the atmospheres in the tests, except for nitrogen at 500°C. 1% O2-N2 atmosphere has strong ability to remove the stable organic sulfur in ZY coal, especially at 700°C. Under 1% O2-N2 atmosphere, not only a part of stable organic sulfurs can be decomposed, but also more stable organic sulfur-containing structure can be broken down into less stable one. Syngas and 1% O2-N2 atmospheres have almost the same effect as hydrogen on sulfur transformation at 700°C.

•Transformation and roles of inherent mineral matter in DCL are briefly overviewed.•Pretreatment methods of low-rank coals to enhance the oil yields are proposed.•Challenges in investigating transformation and roles of inherent mineral matter in DCL.Since direct coal liquefaction (DCL) was invented, it has been a promising technology for the efficient utilization of coal resources and the mitigation of the oil crisis. Generally, coals are regarded as mixtures composed of organic matter and mineral matter. The mineral matter content and nature of coal is highly variable and has been proved to significantly influence the thermal behavior of coals in DCL. Therefore, a clear and comprehensive understanding of the transformation and roles of inherent mineral matter in DCL process is really necessary for guiding the oil yield enhancement and moderating the reaction conditions. After direct liquefaction, the mineral matter would be enriched in the residues. Investigations on transformation and roles of inherent mineral matter can be beneficial for the efficient utilization of the residues as well. In view of the great importance of mineral matter to DCL, this paper briefly summarizes the research progress on the transformation and roles of inherent mineral matter in DCL process, and especially focuses on the major mineral matter in coals. As suitable feedstocks for DCL, the low-rank coals are typically rich in exchangeable metallic species (a kind of mineral matter) which can reduce the oil yield in DCL, hence a detailed discussion of transformation and roles of exchangeable metallic species in DCL is given here and two pretreatment methods of low-rank coals to enhance their oil yields are also proposed.