•The K2CO3-catalyzed char gasification occurs fast in a vertically blown reactor.•The diffusion effect can be eliminated thoroughly in this reactor.•Elevating steam pressure substantially promotes the catalytic gasification rate.•An overall kinetic equation with consistency of parameters is proposed.The K2CO3-catalyzed steam gasification of ash-free coal char was conducted in a pressurized and vertically blown reactor which was also characterized as a differential reactor. At a typical condition (total pressure, 0.5 MPa; steam partial pressure, 0.15 MPa; temperature, 750 °C; catalyst loading, 10%; superficial gas velocity, 55 cm s− 1), the gasification proceeded so fast that it was completed within 5 min. The dependence of either instantaneous gasification rate or average gasification rate on pressure was found to follow the Langmuir-Hinshelwood rate equation and the nth order rate equation, but the parameters for either rate equation varied with the carbon conversion. For a given coal char with a fixed catalyst loading, an overall kinetic model with the constancy of parameters is proposed to predict the gasification rate as a function of steam partial pressure, temperature and carbon conversion. While the elevation of steam partial pressure promoted the methanation mainly via the gas-char reaction, the yield of CH4 was negligible. The gas composition was not in the equilibrium of water gas shift reaction.

•A rapid hydrogasification char/lignite blend is upgraded by hydrothermal treatment.•Some interactions occur between two types of fuels during hydrothermal treatment.•The treated blend has a much better slurryability than the original blend.•The treated blend slurry has an improved rheological feature.The char produced from the rapid hydrogasification (RH char) has a high carbon content, a low volatile matter content and a high calorific value, whereas lignite has high contents of oxygen and volatile matter and a low calorific value. These two carbonaceous fuels are complementary to each other for making CWS, but both of them are poor in slurryability. Hydrothermal treatment (HTT) is consequently used to improve the slurryability of the blend. HTT experiments were carried out in a stirring autoclave at 150–320 °C with a holding time of 30 min. Two types of feedstock were found to have some interactive effects during HTT on changing the elemental composition, acidic functional groups, particle size distribution, particle morphology and water holding capacity in or of the treated blend. The slurryability of the blend is appreciably improved by HTT, and the extent of improvement increases with the temperature increasing till 300 °C but not to a higher temperature. The reasons for the slurryability changed after HTT are discussed.

•Catalytic steam gasification of JY coal char (ash-rich) and AF char (ash-free) is studied.•The relative catalytic activities of Li, Na and K carbonates depend on minerals in char.•Impregnating Ca(OH)2 to JY coal facilitates the gasification rate for the three alkali catalysts.•Only the binary K2CO3/Ca(OH)2 catalyst has a better activity for gasification of AF char.•There are some different catalytic synergies between Ca(OH)2 and each alkali carbonate.The order of catalytic activity of three alkali carbonates (Li2CO3, Na2CO3, and K2CO3) towards the steam gasification of coal char varies depending on the presence and absence of the acidic mineral species and their enrichment in coal. In the gasification of an ash-rich coal char, Li2CO3 exhibits the severest catalytic deactivation among three alkali carbonates, whereas Na2CO3 is the most resistant to the deactivation. The impregnation of Ca(OH)2 to the ash-rich coal substantially increases the rate of char gasification for all three alkali carbonates as a consequence of the inhibited catalytic deactivation. In the gasification of an ash-free coal char, however, only the binary K2CO3/Ca(OH)2 catalyst shows a synergic effect on promoting the gasification rate, whereas the Li2CO3/Ca(OH)2 catalyst and the Na2CO3/Ca(OH)2 catalyst have a worse catalytic activity than their respective alkali carbonates alone. The mechanisms of the concerted effects between alkali carbonates and Ca(OH)2 on the gasification of the ash-rich char and the ash-free char are discussed.

•Rapid hydrogasification chars (RH chars) were enriched with micropores and capillary pores.•RH chars had high holding capacities of water, particularly capillary water.•RH chars showed the very poor slurryability and unique rheological behavior.•An applicable water slurry was prepared by blending RH chars with a raw coal.•An interactive effect between RH char and raw coal could promote the slurryability of blend.The purpose of this work is first to investigate the slurryability of two chars produced from the rapid hydrogasification of two bituminous coals (YY coal and FG coal) in a pilot-scale entrained flow gasifier (abbreviated as RH char). Results showed that both RH chars had very poor slurryability mainly owing to the microporous and capillary structures of particles and the high water holding capacity, in sharp contrast to the properties of the parent coals and the char obtained from a common slow pyrolysis. The work thereafter aims at making a commercially usable water slurry by using the blends of RH char and YY coal. The slurry containing an adjusted solid blend of 60% total solid weight loading with 0.5% naphthalene sulfonate formaldehyde dispersant showed all satisfactory performances in the apparent viscosity, fluidity and static dispersion stability. Furthermore, it was found that the slurryability could be improved by an interactive effect between RH char and raw coal, which depended on the blending pairs and blending proportions.

•A kinetic model is proposed to describe the K-catalyzed steam gasification of ash-free char.•The model is strictly formulated based on an active site/intermediate mechanism.•The model fits the profiles of gasification rate over the carbon conversion well.•The model fits the variations of gas composition with carbon conversion well.•The reaction rate constants estimated from modeling are reasonably explained.Experimental results showed that the K2CO3-catalyzed steam gasification of ash-free char had two distinct characteristics of gas evolution. One was a broad maximum of gasification rate versus carbon conversion at a higher carbon conversion as a result of the sustained catalytic activity. Another was the variations of gas composition with carbon conversion arising from the complex reaction pathways. No model was reported to describe both characteristics in the literature. In this work, a new kinetic model was formulated in terms of an active site/intermediate mechanism with an assumption of the variations of effective carbon concentration with carbon conversion. The mechanism involved three potassium species and four reaction pathways for the catalyst cycle and gas release. The model was associated with five parameters including four rate constants and a catalysis factor, and it manifested a capability of predicting accurately the profiles of both gasification rate and gas composition over the entire range of carbon conversion. All four rate constants estimated from the modeling followed the Arrhenius equations with the reasonable activation energies. Moreover, the model could predict the formation, growth and decline of three potassium species during the gasification. The modeling result would deepen the understanding of the chemical processes of K2CO3-catalyzed steam gasification of char.

•XANES was used to examine the sulfur transformations during co-pyrolysis of a calcium-rich lignite and high-sulfur bituminous coal.•Co-pyrolysis synergistically increased the retention of sulfur as CaS and CaSO4.•Co-pyrolysis synergistically promoted the progressive decomposition of pyrite and sulfide.Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy was applied to determine the sulfur forms in a calcium-rich lignite coal (L coal) and a high-sulfur bituminous coal (B coal), and to investigate their transformations during the co-pyrolysis of two coals. Calcium K-edge XANES spectroscopy was used for examining the occurrence modes of calcium in two coals and their transformation during the coal pyrolysis. Sulfur was present in L coal in the form of thioether, thiophene, sulfate and sulfonate, and in B coal in the form of thiophene, pyrite, thioether and sulfate. Most calcium in L coal was organically associated. The co-pyrolysis was found to synergistically facilitate the progressive decomposition of pyrite and also result in the virtually complete elimination of thioether from the char, with the promoted retention of sulfur as CaS and CaSO4. It was evident that the calcium in L coal was an important factor underlying the synergistic effects on the sulfur transformations during the co-pyrolysis.

•Thermal and hydrothermal treatments improved the slurryability of low-rank coals.•Calcium in coal affected the acidic group elimination and the slurryability of coal.•Calcium in coal influenced the dispersion stability of coal-water slurry.•The slurry prepared from treated coal was much less pseudoplastic.The aim of this work is to study the slurryability of two low-rank coals (a Chinese YX lignite and a Chinese ZD sub-bituminous coal) and various upgraded coals or chars obtained by thermal and hydrothermal treatments. Thermal treatment was conducted in an air-isolated atmosphere at temperatures from 145 °C to 700 °C. Hydrothermal treatment of coal was carried out in an autoclave at temperatures from 150 °C to 300 °C with autogenous pressure from 0.8 MPa to 9.2 MPa. The two raw coals exhibited the poor slurryability, and a significant enhancement in the slurryability was achieved by both the thermal and hydrothermal treatments depending on the conditions. A coal or char slurry (CWS) with the solid loading of 59 wt.% was prepared from the YX lignite by the thermal treatment at 400 °C, which had the apparent viscosity of 1208 mPa s and the good stability. The apparent viscosity of CWS was found to be related to but not limited to the elimination of hydrophilic groups by pretreatment. The hydrothermal treatment was more effective to decompose the hydrophilic groups than the thermal treatment for YX lignite, while this was not the case for ZD coal. The enrichment of carboxyl-bound calcium in ZD coal enabled the hydrophilic groups to be more thermally refractory, and this calcium-combining structure appeared to have a negative effect on the slurryability and the dispersion stability. The CWSs prepared from the thermally treated YX coals or chars were less pseudoplastic and less easily subjected to a pseudoplastic change with increasing solid loading than those from the parent coal.

The aim of this paper is to investigate an approach for inhibiting the interaction of kaolinite with K2CO3 via the pretreatment with calcium additive (Ca(OH)2 or CaCO3). The reactions between kaolinite, calcium additive, and K2CO3 were, therefore, examined by means of thermogravimetry, differential scanning calorimetry, X-ray diffraction analysis, and inductively coupled plasma atomic emission spectrometry. It was found that Ca(OH)2 could interact with kaolinite at low temperature, although the bulk reaction indeed required a temperature of higher than 900 °C to afford a crystalline calcium aluminosilicate product. The addition of Ca(OH)2 to kaolinite even without heat pretreatment was proved to be significantly resistant to the reaction of kaolinite with K2CO3, while CaCO3 showed such a smaller effect. The high-temperature product, calcium aluminosilicate, exhibited an almost complete inactivity for reaction with K2CO3 at a typical temperature of catalytic coal gasification (750 °C).

•Calcium additive synergized with K2CO3 towards gasification of ash-free chars.•A main mechanism for the synergistic effect (SE) was the formation of a eutectic.•Another mechanism for SE was due to the organic binding of calcium on char.The purpose of this work is to investigate the synergistic catalysis between calcium species and K2CO3 for the char gasification. Graphite and four mineral-free chars including an anthracite coal char, a bituminous coal char, a lignite coal char and a pine wood char were gasified at 750 °C in a stream of steam/argon under atmospheric pressure. It was found that each of three calcium species (Ca(OH)2, Ca(CH3COO)2 or Ca(Ac)2, and CaCO3) synergistically promoted the catalytic activity of K2CO3 for the gasification of all samples, and the extent of promotion was dependent on calcium species and carbon or char samples. The formation of a eutectic from the calcium species/K2CO3 catalyst was proved to be an important mechanism for its better catalytic effect. The presence of the organically bound calcium on the char was also likely to be responsible for the promoted catalytic gasification.

K2CO3-catalyzed char gasification with steam was carried out on a laboratory fix-bed reactor and it featured in the preparation of char from a high-sulfur coal by the prior addition of Ca(OH)2 to the coal. This work was focused on the clarification of sulfur transformations during the coal pyrolysis and the char gasification. Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy was employed to determine quantitatively the forms of sulfur in coal and their changes in process. Results showed that the addition of Ca(OH)2 to coal promoted the decomposition of both pyrite and organic sulfur during the charring. In the K2CO3-catalyzed char gasification, the calcium additive enabled more sulfur to be retained eventually as K2SO4 rather than CaSO4. The formation of K2SO4 would be conducive to the recycled use of potassium catalyst.Highlights► A high-sulfur bituminous coal char was gasified with K2CO3 by pre-addition of Ca(OH)2 to coal. ► Sulfur XANES was used to determine sulfur transformations associated with coal pyrolysis and gasification. ► The calcium additive facilitated the decomposition of both inorganic and organic sulfur during coal pyrolysis. ► The calcium additive allowed more sulfur to be retained in the ash as K2SO4 after gasification.

Co-pyrolysis of a lignite coal and a bituminous coal was carried out on a fixed-bed reactor. The lignite was enriched with calcium, and the bituminous coal featured in high sulfur and strong swelling. Experiments were also conducted for the blends using the acid-washed lignite and/or the acid washed bituminous coal to address the influences of calcium in the lignite on the synergistic behaviors. Calcium in the lignite exhibited some aspects of synergy including the catalytic cracking reactions of tar, the retention of sulfur in the char, and the catalyzed polyaromatization and gasification of char. These synergies impacted the differences in the product distribution and gas composition between the co-pyrolytic results and the additive ones. Moreover, there appeared to be a synergistic effect on the cross-linking reaction of volatile matter, resulting in an increase in the char yield irrespective of coal demineralization. The co-pyrolysis was also observed to destroy the swelling of coal. This synergistically increased the tar yield due to less resistant escaping of tar from the intra-particles of coal.Highlights► Co-pyrolysis of a calcium-rich lignite coal and a high-sulfur bituminous coal on a fixed-bed reactor had some aspects of synergistic effect on the product distribution and the gas composition. ► Calcium in the lignite synergistically influenced the catalytic cracking reactions of tar, the retention of sulfur in the char and the catalyzed polyaromatization of char. ► There also existed some non-mineral-related synergies involving the cross-linking of volatile matter and the swelling property of the bituminous coal.

Experiments were carried out to investigate the K2CO3-catalyzed gasification performances for varying chars prepared from four coals with prior addition of calcium additive and without calcium additive. Three calcium species, Ca(OH)2, Ca(Ac)2 or Ca(CH3COO)2, and CaCO3, were used in the study. Experiments were also conducted in an attempt to achieve the understanding of the effect of calcium additive on the potassium deactivation, as well as the interactive effect between potassium, calcium and char. It was found that each calcium additive acted as a deterrent to the potassium deactivation and thus promoted the catalytic gasification, and the magnitudes of promotion varied depending on coal and calcium species. Ca(OH)2 and Ca(Ac)2 were more effective to inhibit the potassium deactivation than CaCO3. Furthermore, potassium together with calcium showed a mineral-unrelated synergy towards the char gasification. A bimetallic carbonate, K2Ca(CO3)2, which formed even when the chars prepared from coal with either Ca(OH)2 or Ca(Ac)2 was subsequently mixed with K2CO3, was likely to contribute to the synergistic effect.Highlights► The addition of all three calcium species to coal promoted the K2CO3-catalyzed char gasification. ► Ca(OH)2 and Ca(Ac)2 showed higher effectiveness than CaCO3. ► Calcium additive served to suppress the deactivation reaction of potassium. ► The suppression of deactivation was more pronounced for the clay-richer coals. ► Calcium and potassium also showed a mineral-unrelated synergistic effect.

Pyrolysis of raw rice straw (RS), water-washed rice straw (WRS) and acid-washed rice straw (ARS) was conducted on a fixed bed reactor. A standard detergent method was used to determine the fiber constituents including neutral detergent solute (NDS), hemicellulose, cellulose and lignin in RS, WRS, ARS and their chars. Results showed that water washing removed most alkali metals (K and Na) from RS, and acid washing removed almost all alkali and alkali earth metals (K, Na, Ca and Mg); meanwhile, either washing eluted a portion of NDS but nearly unaffected hemicellulose, cellulose and lignin in RS. Attempts were made to assess the intermingled influences of the internal alkali and alkali earth metals (AAEMs) and the varied organic matter on the pyrolysis behaviors. It was revealed that the AAEMs inherent in RS had multiple impacts on the product distribution, the decomposition characteristics of fiber constituents, and the compositions of all gaseous, liquid, and char products.Highlights► Water and acid washings were used for removal of mineral matter from rice straw. ► Either washing only removed a portion of neutral detergent solute. ► Influences of internal minerals and varied organics on pyrolysis were assessed. ► Decomposition characteristics of fiber constituents in whole biomass were examined.

Slow pyrolysis of pine sawdust was conducted using a gas sweeping fixed-bed reactor for suppressing the secondary reactions of tar vapor. Dependence of the yield and composition of gaseous, liquid and solid products on temperature was examined in the range of temperature between 200 and 700 °C. The study is focused on the elucidation of the product–precursor relationships. It was observed that the degradation of hemicellulose started at 200–300 °C, forming many sorts of liquid products, such as saccharide, furan, carboxylic acid, ketone and aldehyde, with the occurrence of dehydration, decarboxylation and decarbonylation, while cellulose remained unimpaired in its polymerized structure. Cellulose was substantially decomposed at 300–450 °C, leading to a great increase in the yields of liquid and gaseous products, and simultaneously the solid residue became aromatized, which was characteristic of a concentrated lignin structure. The residue was largely decomposed to numerous guaiacols and phenols at 450–700 °C, with the significant formation of CH4, H2, and CO.

Seventeen trace elements in 24 coals from worldwide deposits of differing ranks and sulfur contents were determined with the use of inductively coupled plasma optical emission spectrometry (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), and flow injection (FI) ICP-MS. By examining multiple correlations between each trace element and three major elements, calcium, aluminum, and iron, we have found that thirteen trace elements (Li, Be, V, Cr, Mn, Ni, Cu, Zn, Ga, As, Se, Sr, and Ba) in the coals show significant correspondence. Elements correlating with aluminum are lithium, beryllium, vanadium, chromium, copper, gallium, and selenium; of these elements, vanadium, chromium, and copper also have a relationship with iron. Manganese, strontium and barium are correlated with calcium, while nickel, zinc, and arsenic are correlated with iron. In the geochemical and mineralogical senses, the significant correlation of a trace element with calcium reflects its common association with carbonate minerals for medium- to high-rank coals, while that with aluminum is implicative of the common association with aluminosilicate minerals and that with iron is characteristic of the association with sulfide minerals for high-sulfur coals, and with iron-bearing carbonate and clay minerals for low-sulfur coals. It is observed that most trace elements have more than one common association(s) in the 24 coals.