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Coal catalytic hydrogasification (CCHG) is a straightforward approach for producing CH4, which shows advantages over the mature coal-to-CH4 technologies from the perspectives of CH4 yield, thermal efficiency, and CO2 emission. The core of CCHG is to make carbon in coal convert into CH4 efficiently with a catalyst. In the past decades, intensive research has been devoted to catalytic hydrogasification of model carbon (pitch coke, activated carbon, coal char). However, the chemical process of CCHG is still not well understood because the coal structure is more complicated, and CCHG is a combination of coal catalytic hydropyrolysis and coal char catalytic hydrogasification. This review seeks to shed light on the catalytic process of raw coal during CCHG. The configuration of suitable catalysts, operating conditions, and feedstocks for tailoring CH4 formation were identified, and the underlying mechanisms were elucidated. Based on these results, the CCHG process was evaluated, emphasizing pollutant emissions, energy efficiency, and reactor design. Furthermore, the opportunities and strategic approaches for CCHG under the restraint of carbon neutrality were highlighted by considering the penetration of "green" H2, biomass, and CO2 into CCHG. Preliminary investigations from our laboratories demonstrated that the integrated CCHG and biomass/CO2 hydrogenation process could perform as an emerging pathway for boosting CH4 production by consuming fewer fossil fuels, fulfilling the context of green manufacturing. This work not only provides systematic knowledge of CCHG but also helps to guide the efficient hydrogenation of other carbonaceous resources such as biomass, CO2, and coal-derived wastes. [ABSTRACT FROM AUTHOR]

The electrochemical CO2 reduction to CH4 is a promising approach for producing highly specific combustion fuel but has relatively poor selectivity and activity at high‐current‐density electrolysis. In this work, ultrathin CuGaO2 nanosheets with highly exposed single‐interlayered Cu edges are synthesized via an induced anisotropic growth strategy. Density functional theory calculations indicate that the exposed single‐interlayered Cu(I) edges on the (001) surface of CuGaO2 present a high‐density of single‐atomic Cu sites, which feature excellent CO2 electroreduction catalytic activity toward CH4. The CuGaO2 nanosheet catalysts exhibit efficient and stable CO2‐to‐CH4 electroreduction with Faradaic efficiency (FECH4) of 71.7% at a high current density of –1 A cm−2, corresponding to a superior CH4 partial current density of 717 ± 33 mA cm−2. This work suggests an attractive design strategy for tuning both the crystal facets and Cu–Cu distance to promote the CH4 electrosynthesis at high‐current‐density CO2 reduction. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new strategy of biomass/coal co-hydrogasification was proposed for CH 4 production. • Mutual interactions between corn stalks (CS) and cobalt-containing coal revealed. • CS helps to boosting CH 4 yield to 1.38 Nm3 CH 4 /kg-coal, while releasing less CO 2. • CS modulates coal structure evolution and Co-C interaction to promote CH 4 formation. • CS showed similar promoting behavior on Fe/Co/Ni-catalyzed hydrogasification. Coal catalytic hydrogasification (CCHG) is a straightforward approach for producing substituted natural gas with a high CH 4 yield and thermal efficiency. To reduce fossil fuel consumption for CH 4 manufacturing, this work proposed a integrated process combining CCHG and biomass hydrogasification. Using a pressurized fluidized bed, the effect of corn stalks (CS) on CCHG was carefully analyzed in terms of product formation behavior, catalysis process, and feedstock adaptability. Experimental results showed that CS benefits CCHG in terms of reactivity, CH 4 production, and CO 2 emissions. Specifically, adding 30 wt% CS resulted in a maximum CH 4 formation rate of 94.9 ml/g·min, production capacity of 1.38 Nm3 CH 4 /kg coal, and CO 2 emissions of 26.37 g/mol CH 4 , which were 2.25, 1.29, and 0.69-folds that of cobalt-catalyzed hydrogasification alone, respectively. In the rapid pyrolysis stage, CS volatiles promoted Co dispersion and restrained coal structure ordering by mediating the interactions between cobalt catalysts and coal, favoring the subsequent hydrogasification. Meanwhile, the cobalt-containing char catalyzed methanation of CS-pyrolyzed volatiles, increasing CH 4 formation while decreasing CO, CO 2 , C 2 -C 3 , and tar yields. In the gasification stage, Ca and Mg compounds in biomass ash gradually promoted the catalytic hydrogasification of coal char via potential Lewis basic-acid interactions. These mutual behaviors of CS and catalyst-containing coal were found to be universal in hydrogasification and helped strengthen cheaper but less active Fe/Ni catalysts. This work will provide theoretical guidance for future research on the production of CH 4 with a low carbon footprint via co-hydrogasification of biomass and coal. [ABSTRACT FROM AUTHOR]

• The numerical study of CH* chemiluminescence in methane inverse diffusion flame was studied. • The effect of formation and quenching reactions on the CH* distribution was analyzed. • The diffusion transport of CH* in the peak reaction zone was discussed. • CH* can characterize the local heat release rate on the fuel side. Flame optical diagnostic method based on CH* chemiluminescence is often used in the combustion process monitoring. In this work, a numerical study of CH* chemiluminescence was conducted for the laminar methane inverse diffusion flame with a range of global oxygen/fuel equivalence ratios. CH* chemiluminescence is mainly distributed in the upstream of the inverse diffusion flame and the CH* concentration increases with the global oxygen/fuel equivalence ratio. CH* is generated by C 2 H, which mainly comes from the reactions OH + C 2 H 2 <=>C 2 H + H 2 O and H + C 2 H 2 <=>C 2 H + H 2. In addition, most CH* is quenched by collisions with H 2 O, CO, CO 2 and H 2 , and the CH* collision quench is enhanced with increasing global oxygen/fuel equivalence ratios. The CH* generated in the peak reaction zone will diffuse to the fuel and oxidizer sides due to the mass diffusion and the thermal diffusion. Moreover, the correlation between CH* and local heat release rate distribution is explored and CH* can characterize the local heat release rate on the fuel side. [ABSTRACT FROM AUTHOR]

Highlights • High coal conversion and CH 4 yield could be simultaneously achieved in CCHG. • Co-Ca catalyst triggered catalytic coal depolymerization and char hydrogasification. • Individual Co and Ca had inferior catalytic activity towards coal hydrogasification. • Ca retarded Co sintering, and more importantly initiated graphite carbon conversion. • CH 4 and tar yield could be moderated through varying catalyst loading. Abstract Coal catalytic hydrogasification (CCHG) was carried out in a lab-scale pressurized fluidized bed to study the cobalt-calcium (Co-Ca) catalyst on the gross distribution of products by changing the addition amount of the catalyst. The CH 4 formation behaviors and the structure properties of gasified coal char were analyzed to elucidate the role of Co and Ca in the dual catalyst system. Results show that 90.0 wt% of carbon conversion and 77.3 wt% of CH 4 yield could be simultaneously achieved in 30 min with the help of 5%Co-1%Ca catalyst through hydrogasification at 850 °C and 3 MPa. Co-Ca dual catalyst triggered coal catalytic hydropyrolysis and coal char catalytic hydrogasification in succession. However, neither individual Co nor Ca had profound catalytic activity towards the condensed aromatic rings in coal structure. Co alone was prone to agglomerate during CCHG, resulting in the inactivation and blockage of the micropore structures. The addition of Ca retarded Co sintering and more importantly mediated Co-C interaction, making Co penetrating into the interior of coal matrix to accelerate the carbon conversion. The catalytic process of CCHG was summarized and the probable interaction between Co-Ca-C was discussed. The findings in this work will provide the theory foundation for the design of the catalyst in the pressurized fluidized bed gasifier. [ABSTRACT FROM AUTHOR]

The characterization of the heat release rate is of great importance for studying the combustion process. In this work, the correlation between heat release rate and OH*, CH* chemiluminescence in methane inverse diffusion flame is explored with a numerical simulation over a wide range of oxygen/fuel equivalence ratios and methane flow rates. It is found that the flame heat release rate is mainly related to the formation and consumption of the species OH, C 2 H 2 , CH 3 , CH 4 , CO, CO 2 , H, H 2 O and O. The ground state OH concentration gradient is correlated with the heat release rate distribution, and the peak location of the gradient in the ground state OH concentration aligns with the peak location of the heat release rate. The outline of the OH* distribution is consistent with the profile of the maximum of the OH concentration gradient. CH* is used to indicate the main distribution of the heat release rate, and the outline of the OH* distribution coincides with the outline of the heat release rate. • The numerical study of OH*, CH* chemiluminescence and heat release rate is studied. • The effect of species and elementary reactions on heat release rate is analyzed. • The outline of the OH* distribution coincides with the outline of the heat release rate. • CH* can characterize the main distribution of the heat release rate. [ABSTRACT FROM AUTHOR]

Highlights • Coal catalytic pyrolysis and hydrogasification was analyzed in a fluidized bed. • Catalytic pyrolysis atmosphere greatly affects subsequent char reactivity. • Catalytic hydropyrolysis yields char with disordered structure and more volatiles. • Union of Co-Ca catalyst and H 2 rapidly fracture C C bonds in coal structure. • Catalytic cleavage of C C bonds dominates coal catalytic hydrogasification. Abstract Coal pyrolysis, hydropyrolysis and hydrogasification were performed in a lab-scale pressurized fluidized bed with or without cobalt-calcium bimetallic catalyst (Co-Ca) to preliminarily discern the role of hydropyrolysis, Co-Ca catalyst and H 2 in the course of coal catalytic hydrogasification (CCHG). The hydrogasification reactivity and chemical structure of the coal chars generated after hydropyrolysis were also analyzed by means of pressurized thermogravimetric analyzer (P-TGA), FT-Raman and FT-IR, etc. Results indicate that Co-Ca catalyst possesses catalytic depolymerization and hydrogenation effect on coal during hydropyrolysis, which resulted in the promoted yield of CH 4 , tar and the high hydrogasification reactivity of the generated char. Co-Ca catalyst alone and H 2 alone are hardly to fracture the C C bonds in condensed aromatic rings of coal, but the union of them yields a superior bonds cleavage ability. The catalytic cleavage of C C bonds is the crucial step of CCHG at the temperature above 800 °C with the hydrogen pressure above 1 MPa. Moreover, the CCHG process over Co-Ca catalyst is discussed and the probable pathways of the catalytic cleavage of C C bonds are proposed. [ABSTRACT FROM AUTHOR]

The catalytic hydrogasification of the sub-bituminous coal was carried out in a lab-scale pressurized fluidized bed with the Co-Ca, Ni-Ca and Fe-Ca as catalysts at 850 °C and 3 MPa. The effect of different catalysts on the characteristics of gasification products was investigated, and the behavior of the catalysts was also explored by means of the X-ray diffraction (XRD), FT-Raman, Brunauer–Emmett–Teller (BET), etc. Experiment results showed that all the catalysts promoted the carbon conversion in the coal catalytic hydrogasification (CCHG), and the catalytic activity was in the order: 5%Co-1%Ca > 5%Ni-1%Ca > 5%Fe-1%Ca. Compared with the raw coal hydrogasification, the carbon conversion increased from 43.4 wt.% to 91.3 wt.%, and the CH 4 yield increased from 23.7 wt.% to 77.3 wt.% within 30 min after adding the 5%Co-1%Ca catalyst into the coal. Co-Ca and Ni-Ca possessed catalytic effect on both processes of pyrolysis of coal and hydrogasification of coal char in CCHG, by which the graphitization of the coal was suppressed and methane formation rate was significantly accelerated. Fe/Co/Ni-Ca could penetrate into the interior of coal during CCHG, making the catalytic production of CH 4 conduct in the pore structures. The activity difference of the catalysts was owing to the different ability of rupturing the amorphous C C bonds in coal structure. The incomplete carbon conversion of the 5%Co-1%Ca loaded coal was due to the agglomeration of the catalyst and the blockage of the reactive sites by the sintered catalyst. This work will provide a straightforward method and reference data for the further industrial-scale production of 1.2 N m 3 CH 4 /kg–coal from CCHG by using pressured fluidized bed reactor. [ABSTRACT FROM AUTHOR]

• The numerical study of OH* chemiluminescence in methane inverse diffusion flames was studied. • The transition of OH* core region distribution was discussed. • The effect of formation and quenching reactions on OH* distribution was analyzed. • OH* diffused from core region to both sides in pure diffusion zone. The OH* chemiluminescence is closely related to flame properties in the study of combustion diagnosis. In this work, numerical investigation of the OH* reaction mechanism in methane inverse diffusion flames for evaluating the OH* chemiluminescence is presented. The two-dimensional distribution of OH* emission intensity in laminar inverse diffusion flames was obtained by CCD imaging system and agreed well with the numerical simulation results. By changing oxygen/fuel equivalence ratios, the effect of elementary reactions on OH* chemiluminescence was analyzed. The results show that the OH* distribution depends upon the formation reactions, quenching reactions and diffusion characteristics. The OH* is generated by R1 (H + O + M <=> OH* + M) and R2 (CH + O 2 <=> OH* + CO). The trend of R1 distribution is consistent with the OH* molar concentration distribution at different oxygen/fuel equivalence ratios. Although the R1 reaction rate is strongest at the root of flame, the OH* molar concentration is relatively low due to the strong quenching reactions, where the OH* is mainly quenched with H 2 O and O 2. In the pure diffusion zone, the generation and destruction reaction rate of OH* is unbalanced. At the root of the flame, OH* diffuses from the net generation zone to both sides. [ABSTRACT FROM AUTHOR]

• High yields of CH 4 and liquid hydrocarbons were achieved in a catalytic fluidized bed. • Calcium (Ca) as an additive promoted cobalt (Co) dispersion and Co-coal interaction. • CaO triggered cobalt-catalyzed depolymerization of coal and hydrogenation of inert carbon. • Role of temperature and pressure in catalytic hydrogasification of coal were revealed. • Analyzing catalysis process and reaction kinetics to optimize coal hydrogasification. Catalytic hydrogasification of pulverized coal was performed using a pressurized fluidized bed to investigate the effect of experimental variables including catalyst additives, temperature (600–850 °C) and hydrogen (H 2) pressure (0.6–3.0 MPa) on the catalytic behavior of cobalt (Co) and formation profiles of target products, i.e., methane (CH 4) and liquid hydrocarbons (HCL). The experimental results suggest that calcium (Ca) as an additive demonstrated a superior promoting effect on Co by mediating the Co-coal interaction, which initiated the catalytic depolymerization and hydrogenation of unreactive carbon. The temperature above 750 °C stimulated the diffusion of Co-Ca catalyst in the coal structure, and enhanced the coal catalytic hydrogasification by lowering the activation energy. However, the elevated temperature decreased the HCL yield due to the intensified hydrocracking. The elevation of H 2 pressure strengthened the Co-Ca-Coal-H 2 interaction, which conduced to the depolymerization and hydrogenation effect of Co-Ca catalyst, and improved the yield of CH 4 and HCL. Based on the experimental results, a two-stage pressurized fluidized bed reactor scheme was proposed to couple the coal catalytic hydropyrolysis with char catalytic hydrogasification. Staged hydrogenation of coal was realized, and a yield of 74.2 wt% for gaseous hydrocarbons (CH 4 and C 2 -C 3) and 3.36 wt% of HCL (BTX, PCX, and naphthalene) were obtained. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fluidized bed hydrogasification of coals with diverse properties. • Carbon structure and sulfur content in coal affect reactivity significantly. • Blending low rank coal minimized caking propensity and promoted hydrogasification. • CaO migrated adjacent to Co to retard poisoning and promote catalytic activity. • Effect of coal properties on hydrogasification mechanism revealed. Coal catalytic hydrogasification is an efficient approach to produce CH 4. This study investigated hydrogasification characteristic of coal with diverse properties (caking propensity, high rank and sulfur content) in a pressurized fluidized bed. The approaches to realize high CH 4 yield and stable fluidization were explored. Experimental results showed that carbon structure and sulfur content in coal were the key factors influencing hydrogasification reactivity. The mechanically mixed CaO/CaCO 3 migrated into the interior of coal particle to enhance the activity of Co towards graphite carbon, and capture sulfur adsorbing on Co surface. The caking propensity of coal brought little resistance to the activity of catalyst, whereas, caused serious agglomerating phenomenon. The blending of caking coal with non-caking, low rank coals induced the active hydrogen spill-over and the volatiles-catalyst-coal interactions, which not only in-situ minimized the agglomeration propensity, but also enhanced the overall hydrogasification reactivity. Furthermore, the mechanism of hydrogasification was proposed by taking account of coal property, to broadening the application range of hydrogasification and producing CH 4 efficiently (~80.0 wt% in 1.0 h). [ABSTRACT FROM AUTHOR]

• The effect of calcium hydrogasification was further studied by analyze the evolution of catalysts. • Calcium could promote the reduction of metal iron as well as carburization reaction. • Negative correlation between VCH4 and ratio of Fe3C to Fe was found. • The hydrogenation of Fe3C to form methane is the rate determining step. The evolution of Fe and Fe-Ca catalysts during char catalytic hydrogasification was investigated in a pressurized fixed-bed reactor. The effects of Fe and Fe-Ca catalysts on CH 4 yield and CH 4 formation rate were studied. The change of the catalysts and char was also characterized by means of the transmission electron microscopy (TEM), X-ray diffraction (XRD), Temperature-programmed reduction (TPR), etc. The relationship between the CH 4 formation rate and the ratio of Fe 3 C to Fe was discussed to identify the rate determining step, as well as possible reaction mechanism. The results show that the role of calcium on char catalytic hydrogasification is not only dispersion of iron-group metals and desulphurization but also promotes the reduction of metal iron as well as carburization reaction. As for Fe-Ca catalyzed case, a negative correlation between V CH4 and ratio of Fe 3 C to Fe was found during hydrogasification. The larger the proportion of Fe 3 C is, the slower the methane formation rate becomes. These results indicate that iron carbide is a stable intermediate product of catalytic hydrogasification. The hydrogenation of iron carbide to form methane is the rate determining step for Fe catalyzed hydrogasification. [ABSTRACT FROM AUTHOR]