Ceria-zirconia solid solution has been prepared by the urea grind combustion and citric acid sol-gel methods for catalytic applications as oxygen storage/release materials in this study. The properties and oxygen storage/release capacities of samples with different Zr contents were characterized and evaluated by X-ray diffraction (XRD), N2 adsorption, scanning electron microscopy (SEM), Raman spectroscopy, and in-situ COCO2 looping test. The results demonstrate that the samples prepared by two methods are all of excellent lattice [O] release/storage properties and maintain good long-term cycle stability. But the preparation method significantly impacts the homogeneity of samples related to their redox properties and the content of Zr over 20%, which greatly changed the properties of ceria-zirconia solid solutions and caused their changing of crystalline symmetry from cubic to tetragonal. The samples prepared by citric acid sol-gel method are of more homogeneous particle sizes and higher specific surface areas than that by urea grind combustion method, which is benefit to the oxygen release rather than oxygen storage. The bulk oxygen amount migrated to surface increases with the increasing Zr content, however, the amount of lattice oxygen migration decreases when Zr content is over 20%. When Zr content is 20%, the differences of storage/release capacities from two different preparation methods are reduced at high temperature in the long-term loop reaction.Oxygen storage and release circularly occurred through the Ce4+/Ce3+ redox couple, and the effects of Ce/Zr ratio and preparation method on the oxygen storage and release capacities of ceria-zirconia solid solution had been highlighted.Download high-res image (164KB)Download full-size image

•DSC was introduced to determine the intrinsic reaction of brown coal oxidation at low temperature.•The optimal test conditions of coal oxidation at low-temperature were 1.0 °C min− 1 heating rate and 5 mg coal mass.•The exothermic characteristics of the intrinsic reaction can be well expressed by subtracting DSC curve in N2 from air.•Activation energy obtained from DSC-sub curve can better reflect intrinsic oxidation reaction.The oxidation of coal at low-temperature involves a series of physical and chemical processes and many parallel reactions. But the intrinsic oxidation reaction between O2 and coal is the main source responsible for the self-heating and spontaneous combustion of brown coal. In this research, differential scanning calorimetry (DSC) was introduced to determine the intrinsic reaction of Ximeng brown coal oxidation at low temperature. The heat evolution of the intrinsic reaction after eliminating the evaporation of water and thermal decomposition of inner oxygen-containing functional groups was obtained by subtracting the DSC curve in N2 from the DSC curve in air. It is considered that the intrinsic reactions between coal and oxygen could be divided into three stages, including the slow oxidation, accelerated oxidation and rapid oxidation stages. Compared with DSC-air curve, the DSC-sub curve based on the subtracting results elucidated the exothermic characteristics of intrinsic oxidation reaction in each stage more clearly. In addition, DSC-sub curve reduced the experimental errors inborn from the heating rate and the sample mass, so it had more practical application value than DSC-air curves. Activation energies obtained from DSC-sub curves can better reflect intrinsic oxidation reaction and be used as important indicators for the evaluation of coal spontaneous combustion tendency.

•Two different rank coals with high organic sulfur content were pretreated by PCE extraction.•Change of sulfur forms and their transformations during pyrolysis were investigated by combined GC-FPD, Raman and XAS.•Changes of sulfur forms in coals and chars before and after PCE extraction are related to the rank of coal.•PCE extraction is more suitable for the desulfurization of low rank coal.•The sulfur is unevenly distributed on the surface and in bulk of coal.Two different rank coals with high organic sulfur content were selected and pretreated with tetrachloroethylene (PCE) under ultrasonic field. The effects of pretreating process on sulfur forms in two coals have been investigated by chemical analysis and sulfur K-edge X-ray absorption spectroscopy (S-XAS). The transformation behaviors of sulfur forms during pyrolysis of raw and PCE-treated coals in the fixed bed reactor with temperature-programmed apparatus were studied by gas chromatography (GC) and S-XAS. Results show that sulfur removal degree of Coal A and Coal D are 29.7% and 24.1%, respectively; and the removal of organic sulfur are dominant, accounting for 87.1% and 66.1% of total removal of sulfur from Coal A and Coal D. The sulfur compositions in coals obviously change before and after PCE extraction, and these changes appear to be unevenly distributed on the surface and in the bulk of coal. The effects of PCE extraction on the release of sulfur-containing gases are different for two coals, which are dependent on the sulfur forms in the coals, as well as the chemical structure in coals. The sulfur forms retained in the chars are significantly different for low rank Coal A before and after PCE extraction, but there is little change for high rank Coal D.

•Sulfur forms in coal and its thermal transformation were well correlated by combined GC-FPD, XRF and XAS•S-containing gases during coal pyrolysis are related to the sulfur species and their inter-conversion•Sulfur species in raw coals and chars were validated by S and Fe K-edge XAS•Sulfur transformation during coal pyrolysis is closely related to the sulfur form and mineral contents•Sulfur transformation during coal pyrolysis is very different for coals with different ranks.Sulfur transformations during pyrolysis of four high sulfur coals with different ranks are studied by measuring the release of S-containing gaseous products using online gas chromatography with a flame photometric detector and analyzing the sulfur forms in raw coals and chars using sulfur K-edge X-ray absorption spectroscopy. Results show that the sulfur forms in different coals are different and they behave differently during pyrolysis. More active disulfide and sulfide in lower rank coal can be decomposed and released as gas products below 500 °C; while a more complex thiophenic structure in the higher rank coal is difficult to decompose even at 1000 °C. Decomposition of pyrite in coal plays an important role in sulfur transformation during pyrolysis of coals rich in pyritic-sulfur. Pyrite begins to convert to FeS above 300 °C; while FeS will also react with nascent char or volatiles above 600 °C. The inter-conversions among sulfur species in solid play a dominant role in the transformation of sulfur forms above 600 °C. The minerals in coal are valuable in understanding conversions of sulfate and sulfide. The sulfur retention in solid form is correlated well with the releasing trend of H2S and COS during the coal pyrolysis.

•A two-stage fixed-bed pyrolysis reactor was used to study feedstock interactions.•Strong impacts of blending observed on tar and gas yields.•Corncob volatiles interacting with lignite was the most significant interaction.•Potassium content and char chemical structure impacted char reactivity.•More work required to resolve exact mechanisms of interaction.Interactions between feedstocks during pyrolysis of coal–biomass blends, and their impacts on product distribution and conversion behavior during utilization, need to be understood if we are to successfully develop systems for the effective co-utilization of biomass and coal. A novel two-stage fixed-bed reactor containing three quartz tubes was designed and used in this study to investigate these interactions, with particular emphasis on the impact on product distribution and char properties. The results show that interactions exist during co-pyrolysis of corncob and lignite blends, which increase overall tar yields and decrease overall gas yields compared with those obtained from pyrolysis of unblended feedstocks, with the interactions between corncob volatiles and lignite playing a dominant role. It is also shown that interactions between chars and volatiles can impact on the reactivity of chars to O2. Consistent with our expectations, there is a link between potassium content in char and its reactivity; however, it is found that for lignite chars in particular the potential impact of potassium is dominated by the changes of char chemical structure. The effect of potassium content is the opposite to that of char chemical structure, leading to significant complexity in determining the net result of any co-pyrolysis interactions on char reactivity. This highlights the importance of understanding these complex interactions to develop the industrial scale co-gasification systems and adjust the distributions of products.

AlCl3/silica gel catalysts were prepared by inclosed grafting method in the autoclave and their catalytic activities for alkylation reaction of thiophene with olefins in benzene were studied. The gas chromatograph and chromatography–mass spectroscopy were used to quantitatively determine the thiophene in benzene and qualitatively measure the reaction products, respectively. XPS, SEM, EDX and N2 adsorption techniques were used to characterize the physical and chemical properties of catalyst samples. The results show that AlCl3 can be effectively anchored by reacting with the silanol groups on silica gel surface to form the Six(OH)y − aOaAlCl(3 − a) species and the prepared AlCl3/silica gel catalyst possesses the significant activity for the alkylation reaction of thiophene and olefin. The preparation conditions, such as mass ratio of AlCl3/silica gel support, grafting temperature and time, and thermal treatment temperature of silica gel, have an important role on the content and distribution of active component over silica gel support, and thus affect the catalytic activity. The optimum preparation conditions of AlCl3/silica gel catalyst are that AlCl3 is grafted on silica gel support, which has been thermally treated at 400 °C for 3 h, and at 180 °C for 4 h with the mass ratio of 0.20 for AlCl3/silica gel support. The loading ratio of AlCl3 on silica gel can reach about 64 wt.% and the optimal conversion rate of thiophene in benzene is 94.23% when the ratio of liquid to catalyst is 20 mL/g.•Preparation method of AlCl3/silica gel catalyst by closed grafting was explored.•Catalyst shows high alkylation activity of thiophene and olefins in benzene.•Grafting manner between AlCl3 and silica gel support was discussed in detail.•Alkylation products of thiophene and olefins were analyzed by results of GC/MS.

Ce–Fe mixed metal oxide sorbent C2F3B850 was prepared by using cerium oxide and red mud (an iron oxide waste from a steel company). The ability of C2F3B850 (CeO2/red mud = 0.85) was evaluated over a fixed-bed reactor in a simulated coal-derived gas. It was found that the main active components of sorbent C2F3B850 were cerium and iron oxides, the addition of red mud in CeO2-based sorbent could decrease sulfurization temperature, and 500 °C is the best temperature for sulfurization in the range of 500–700 °C. After eight successive sulfurization–regeneration cycles over C2F3B850, the regenerated sorbent still performed well with no apparent deterioration. The average breakthrough sulfur capacity corresponding to H2S concentrations below 50 ppmv is 9.97 (g S/100 g sorbent). Sorbent C2F3B850 during successive sulfurization–regeneration cycles was characterized by means of X-ray photoelectron spectroscopy, X-ray diffraction, and nitrogen adsorption–desorption techniques. The results reveal that the C2F3B850 sorbent with good durability, high efficiency, and high mechanical strength is suitable for desulfurization of hot coal-based gas in the chemical industry.

•Combined study of S transformation during pyrolysis by GC-FPD and XAS.•S-containing gases during pyrolysis monitored by GC-FPD.•S in raw coals and chars from pyrolysis analyzed by S K-edge XAS.•S release in inertinite-rich coals is closely related to inertinite and mineral contents.Sulfur transformation of inertinite-rich coals, which were sampled from three Western China coal mines, Xinjiang Hami (HM), Ningxia Lingwu (LW) and Shendong (SD), during pyrolysis is studied through measuring the release of H2S and COS gases by gas chromatography with flame photometric detector and through analyzing the sulfur forms in raw coals and chars from pyrolysis by X-ray absorption spectroscopy (XAS). It is revealed that the transformation of sulfur during coal pyrolysis is closely linked with coal properties, such as the vitrinite/inertinite ratio, alkaline mineral contents (especially calcium compounds) and H/C atomic ratio for three inertinite-rich coals. Comparisons are performed with a coal sample taken from Pingsuo (PS) coal mine located in North China, of which the properties are significantly different. The maximal release temperature of sulfur-containing gases for the pyrolysis of inertinite-rich coal is higher than that of the PS coal. The release of the S-containing gases in inertinite-rich coals has a maximal temperature interval around 600 °C and this is associated with the conversions of inorganic sulfur species, such as pyrite transforming to FeS observed by the XAS in chars of these coals. During the process of pyrolysis, the organic sulfur compounds in inertinite-rich coal can be oxidized to form sulfoxide-like species due to the decomposition of oxygen-containing function groups in the coal matrix, but the active sulfur in PS coal can react with fresh char to form relatively stable thiophene structures. The formation of COS during the pyrolysis of inertinite-rich coals is mainly due to secondary reactions between H2S with CO and/or CO2.

Two iron oxide based sorbents, TG-1 and TG-F, with high desulfurization efficiency, were selected for simultaneous removal of H2S and Hg from simulated syngas. Our evaluation tests were carried out using a fixed bed reactor at different temperatures and in ambient atmosphere. The different activities for the simultaneous removal of H2S and Hg between TG-1 and TG-F or TG-1-S (denoted as such after uptake of H2S) were compared. The results show that the two iron oxide based sorbents can capture Hg effectively from simulated syngas. The preferred temperature for Hg removal using the TG-F and TG-1 sorbents are 60–120 °C and 100–140 °C, respectively. The Hg absorption capacity of TG-1 is higher than that of TG-F under the same conditions. CO and H2 in the feed gas have negligible effect on the efficiency of Hg removal. H2S is favoured for the removal of Hg over iron-based sorbents and it was found that the influence of H2S concentration on the Hg capacity of the TG-1 and TG-F sorbents are different. It is found that the main active components of the two sorbents during the reaction are different. After several desulfurization cycles, the TG-1-S sorbent has a high efficiency for Hg removal from the simulated syngas.

Fe2O3/γ-Al2O3, PdO/γ-Al2O3, and PdO–Fe2O3/γ-Al2O3 sorbents were prepared using the pore volume impregnation method. Experiments to study the removal of Hg and H2S from simulated syngas were carried out using a conventional flow-type packed-bed reactor system over the temperature range of 100–300 °C. Sorbents before and after the adsorption of Hg and H2S were analyzed by Raman spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The experimental results show that the bimetal oxide PdO–Fe2O3/γ-Al2O3 sorbent can simultaneously remove Hg and H2S in syngas and the operation temperature can be as high as 200–300 °C. The high efficiency of Hg removal by PdO/γ-Al2O3 and H2S removal by Fe2O3/γ-Al2O3 enhances the ability of PdO–Fe2O3/γ-Al2O3 to remove Hg and H2S simultaneously at a relatively high temperature. H2 and CO can enhance the efficiency of the removal of Hg over the PdO–Fe2O3/γ-Al2O3 sorbent at 250 °C, but there is apparently no influence on the removal of H2S. On the basis of the analysis of the different capture performances for Hg and H2S between PdO/γ-Al2O3 and PdO–Fe2O3/γ-Al2O3, there are two possible mechanisms for the capture of Hg over bimetal oxides, namely, by formation of HgS or HgO and Pd–Hg amalgam. The reactions of Hg (adsorbed on the surface of the sorbent) with Pd (formed by reduction of PdO) and Sad [formed by the reaction of H2S with lattice oxygen in Fe2O3 (3H2S + Fe2O3 → 2FeS + Sad + 3H2O)] are the dominant factors.

Series sorbents of Cu, Zn, Ni, Ce and Ag metal components supported on γ-Al2O3 carrier for removing thiophene from benzene were prepared by conventional and ultrasound-assisted incipient-wetness impregnation method. The static adsorption experiments were carried out in the thiophene-benzene solution with thiophene concentration of 500 mg/L. The results show that the desulfurization activity of all γ-Al2O3 sorbents modified by different metal components obviously increase, among which the sorbent modified by silver nitrate has the best performance. The active components of sorbents from Cu, Zn, Ni, Ce nitrates loaded on γ-Al2O3 carrier are their oxides. Besides Ag2O, the products of silver nitrate thermal decomposition in sorbent prepared still have Ag0 and Ag–O–Al species. The assistant ultrasound in the process of sorbent preparation can not only shorten the impregnation time, but also enrich the pore structure of sorbent and improve the size and distribution of the Ag species, which is favorable to the removal of thiophene from benzene. The desulfurization capacity of sorbent changes with the Ag content loaded. The sorbent with 15 % quality content of Ag prepared by ultrasound-assisted impregnation method has the highest desulfurization efficiency. It could reduce the thiophene concentration to 1.7 mg/L from 500 mg/L at room temperature and ambient pressure, with the desulfurization efficiency of more than 99 %, when the ratio of sorbent to solution was 1:4 (g/mL).

A new preparation method of zinc oxide (ZnO) sorbents for removing H2S from hot coal gas was explored. High-pressure impregnation with semi-coke as the support and zinc nitrate as the active component precursor was used to prepare Zn-based sorbents (HZnSC). The desulfurization activity of the sorbents was studied in a fixed-bed reactor under the simulated coal gas mixture of CO (33 vol%), H2 (39 vol %), H2O (5 vol %), H2S (300 ppmv), and N2 (balance) at 300–550 °C. The results show that the desulfurization activity of the HZnSC sorbents is higher than that of the sorbents prepared using atmospheric pressure impregnation. The surface area of the semi-coke expanded from 16.65 to 272.59 m2/g. Nanoscale ZnO was acquired, and the active component was uniformly dispersed on the support with the high-pressure impregnation method, which afforded H2S removal from hot coal gas. HZn20SC (prepared with 20 wt % zinc nitrate solution) reduced H2S from 300 to less than 0.1 ppmv at 300–550 °C, and its sulfur capacity was 4.5 g of S/100 g of sorbent at 500 °C. It also maintained good heat stability during three desulfurization/regeneration cycles. The findings suggest that semi-coke with a low surface area can be used as a support of sorbents to remove H2S from hot coal gas using high-pressure impregnation.

The structure and pyrolysis characteristics of three inertinite-rich Chinese western coals were researched and compared with one relative vitrinite-rich Chinese middle coal by means of XRD, TG–DTG and fixed-bed reactor. The results show that the atomic ratio O/C, aromaticity factor, even ring condensation number and ring condensation index increase and atomic ratio H/C decreases with increasing inertinite content in coal; inertinite contains more aromatic ring structure than that of vitrinite; the crystallite structure order of coal char increases slightly with increasing heat treatment temperature. The higher inertinite content in coal is, the lower pyrolysis reactivity of coal is at lower temperature, and yet they have obvious second pyrolysis reactivity in higher temperature. The pyrolysis reaction in primarily devolatilisation phase that comes mainly from the decomposition of containing hydrogen function groups and the secondary devolatilisation at high temperature is mainly the decomposition of stable containing oxygen function groups in coal matrix with higher inertinite.

A series of iron-manganese-based sorbents were prepared by co-precipitation and physical mixing method, and used for H2S removal from hot coal gas. The sulfidation tests were carried out in a fixed-bed reactor with space velocity of 2000 h−1(STP). The results show that the suitable addition of manganese oxide in iron-based sorbent can decrease H2S and COS concentration in exit before breakthrough due to its simultaneous reaction capability with H2S and COS. Fe3O4 and MnO are the initial active components in iron-manganese-based sorbent, and FeO and Fe are active components formed by reduction during sulfidation. The crystal phases of iron affect obviously their desulfurization capacity. The reducibility of sorbent changes with the content of MnO in sorbent. S7F3M and S3F7M have bigger sulfur capacities (32.68 and 32.30 gS/100 g total active component), while S5F5M has smaller sulfur capacity (21.92 gS/100 g total active component). S7F3M sorbent has stable sulfidation performance in three sulfidation-regeneration cycles and no apparent structure degradation. The sulfidation performance of iron-manganese-based sorbent is also related with its specific surface area and pore volume.

The sulfidation behaviors of iron-based sorbent with MgO and MgO-TiO2 are studied under different isothermal conditions from 623 to 873 K in a fixed bed reactor. The results of sorbents sulfidation experiments indicate that the sorbents with MgO and TiO2 additives are more attractive than those without additives for desulfurization of hot coal gas. The sulfur capacity (16.17, 18.45, and 19.68 g S/100 g sorbent) of M1F, M3F, and M5F sorbent containing 1, 3, and 5% MgO, respectively, is obviously bigger than that (15.02 g S/100 g sorbent) of M0F without additive. The feasible sulfidation temperature range for M3F sorbent is 773−873 K. The M3F sorbent is optimally regenerated at the temperature of 873 K, under the gas containing 2% oxygen, 15% steam and N2, in the space velocity of 2500 h−1. The sorbent regenerated is also well performed in the second sulfidation (the effective sulfur capacities of 17.98 g S/100 g sorbents and the efficiency of removal sulfur of 99%). The capacity to remove sulfur decreases with steam content increasing in feeding gas from 0 to 10%, but it can restrain the formation of carbon and iron carbide. The addition of TiO2 in sorbent can shift the optimal sulfidation temperature lower. The iron-based sorbent with 3% MgO and 10% TiO2 (MFT) is active to the deep removal of H2S and COS, especially in the temperature range of 673−723 K. The sulfur removal capacity of MFT sorbent is 21.60 g S/100 g sorbent.

Formation of NH3, HCN, NO and NO2 from the gasification of a Chinese coal (Shenmu) in a fluidised-bed/fixed-bed reactor in 4.1% O2 in argon was investigated. The presence of O2 even at 673 or 773 K could lead to the gradual gasification of char to form HCN and NH3. The yields of NH3 and HCN showed maxima at about 773 K, amounting to 40% of coal-N. In the presence of O2, the H radicals required for the formation of NH3 and HCN may be generated from the reactions in the gas phase and within the solid particles. The yields of NOx increased from 773 to 973 K. A non-negligible proportion (∼5%) of coal-N was converted into NO2 at 773 K through homogeneous and heterogeneous reactions. The yield of NO2 diminished with increasing temperature. At 1173 K, the reactions between char-N and H2O (or O/H-containing radicals) formed from the oxidation of char and volatiles led to the formation of NH3 but not HCN. This route of NH3 formation is believed to be an important pathway for the formation of NH3 in an air-blown gasifier.

Zn-Mn-Cu/SC(U) sorbent was hydrothermally synthesized by ultrasound-assisted high-pressure impregnation method with semi-coke (SC) as support and the mixed solution of zinc nitrate, manganese nitrate and copper nitrate as active component precursors. The desulfurization performances of hot coal gas on the prepared sorbent at a mid-temperature of 500 °C were tested in fixed-bed reactor. Morphology and pore structure of the prepared sorbent were also characterized by TEM, N2 adsorption/desorption isotherms and XRD. For comparison, the sorbent of Zn-Mn-Cu/SC prepared by conventional high-pressure impregnation was also evaluated and characterized in order to study the effects of ultrasound treatment. Zn-Mn-Cu/SC(U) sorbent prepared by high-pressure impregnation under ultrasound-assisted condition showed a better desulfurization performance than Zn-Mn-Cu/SC. It could remove H2S from 1000×10−6 m3/m3 to 0.1×10−6 m3/m3 at 500 °C and maintained for 12.5 h with the sulfur capacity of 7.74%, in which both the breakthrough time and sulfur capacity were about 32% and 51% higher than those of Zn-Mn-Cu/SC sorbent. The introduction of ultrasound during high-pressure impregnation process greatly improved the morphology and pore structure of the sorbent. The ultrasonic treatment made particle size of active components smaller and made them more evenly disperse on semi-coke support, which provided more opportunities to contact with H2S in coal-based gases. However, there were no any difference in compositions and existing forms of active components on the Zn-Mn-Cu/SC and Zn-Mn-Cu/SC(U) sorbents.Zn-Mn-Cu based sorbents were prepared by the ultrasound-assisted high-pressure impregnation method, with the inexpensive raw semi-coke as support. The influences of ultrasound on fine desulfurization of sorbent have been studied in this paper.Download full-size image

Na2CO3, Li2CO3, and K2CO3 were used as additives to Pingshuo (PS) coal that was subsequently gasified under a CO2 stream. The catalytic gasification of coal samples by CO2 in the presence single or mixed alkali carbonates was investigated by thermogravimetric analysis. The experimental results indicate that the catalytic effect of Li2CO3 is significantly larger than that of Na2CO3 or K2CO3. The catalytic effect of the mixed, bi-metal carbonate containing Li2CO3 and Na2CO3, or Li2CO3, and K2CO3, is related to the composition of the catalyst and the proportion of the two components. The bi-metal carbonates having a mole ratio of 9:1 (Li:X) has the largest catalytic effect for PS coal gasification. A synergistic effect between Li and K, or Na, carbonate appears at temperatures greater than 1300 K. An un-reacted shrinking core model is suitable for kinetic analysis of catalytic gasification of coal samples in the presence of alkali carbonates. It is inappropriate, however, to evaluate the catalytic effect only by the activation energy obtained from the kinetic calculations.

Minerals inherently present in coal and iron-containing compounds externally added have an important influence on the formation and release of HCN-one of the main precursors of NOx during coal pyrolysis. Pyrolysis of raw coal and demineralized coal with different ranks and iron-containing compounds was studied in a fixed bed quartz reactor at temperature programmed in this article. The trend and influencing factors of HCN formation and release during coal pyrolysis were examined. The results showed that the effect of iron-containing compounds in coal on HCN release depended on the coal types, and the iron added by impregnation and precipitation methods played different roles on HCN formation for different coal types. Additionally, the effects of the particle size and the reaction time on HCN formation during pyrolysis of coal with iron were also studied.

•Real-time change in aliphatic hydrocarbon was investigated using in-situ FTIR.•Kinetics of change in aliphatic hydrocarbon from different coals was studied.•Reaction regime for change in aliphatic hydrocarbon varies during coal self-heating.•Relationship between aliphatic hydrocarbon and carbon oxides was explored.It is imperative to have an in-depth understanding of the change in methyl and methylene groups during low-temperature oxidation of coal not only for detecting and preventing coal spontaneous combustion in coal mining industry and for reducing emissions of hazardous gases. The Fourier transform infrared (FTIR) spectroscopy equipped with in-situ reactor cell was introduced to investigate the change in the methyl and methylene groups of different classes of coals (lignite, sub-bituminous and bituminous) during their low-temperature oxidation at temperatures below 230 °C. The reaction kinetics of the methyl and methylene groups was obtained from the real-time measurements of the change in aliphatic hydrocarbon content in coal matrix. These results clearly show that the low-temperature oxidation of coal can be separated into three stages with respect to the activation energies for the change in methyl or methylene groups, which indicates that the reaction regime for the change in methyl or methylene occurrence switches during the spontaneous combustion of coal. At the first and second stages the reaction activation energies of the methyl or methylene groups showed significant difference for the three types of coal, and at the third stage the activation energies were very close for all of the coals. These results indicate that the spontaneous combustion propensity of coals can be evaluated on the basis of the kinetic parameters at the first and second stages. Furthermore, the relationship between the changes in methyl and methylene groups and the emission of carbon oxides (CO2 and CO) was also explored on the basis of the experimental findings.

•A set of modified isothermal batch reactors was designed.•Modes for CO2 and CO production during coal oxidation were investigated.•Kinetics of carbon oxide emissions from different coals was studied.•Reaction regime for CO2 and CO emissions varies during coal self-heating.•Relationship between activation energies and oxygenated species was explored.It is imperative to have an in-depth understanding of carbon oxide emission during low-temperature oxidation of coal not only for preventing fires in the coal industry but also for reducing emissions of hazardous gases. A set of modified isothermal batch reactors was designed to investigate the modes and kinetics of carbon oxide emission from different classes of coals (lignite, sub-bituminous coal, and bituminous coal) oxidized at temperatures below 200 °C. Based on the effects of coal mass and oxygen consumption on CO2 and CO emissions during coal oxidation, it was found that carbon oxides are formed from the decomposition of not only surface oxides, caused by the reaction between coal and oxygen, but also inherent oxygen-containing groups in the coal matrix. The CO2 and CO emission modes are dependent on the types of coal. Owing to the linear relationship between the CO2 and CO emissions and the exposure times, the kinetics of carbon oxide emission was studied. These results clearly show that the low-temperature oxidation of coal can be separated into three stages with respect to the activation energies for CO2 or CO emission, and the activation energies of these three stages increased with increasing atmospheric temperature, which indicates that the reaction regime for CO2 and CO emissions switches during the spontaneous combustion of coal. Furthermore, the relationship between the activation energies for carbon oxide emission and the occurrence of oxygenated functional groups was also explored.

•Complex macromolecular matrix of coal was divided into elements C, H, O, S, and N.•Kinetic and thermodynamic characteristics of coal oxidation were studied.•A kinetic compensation effect between K and Ea was observed.•The enthalpies of formation for CO2, CO, and H2O were calculated.•The mechanism of low-temperature of coal was explored.Due to the heterogeneous characteristics of coal constituents, it is difficult to directly apply conventional methods for calculating the kinetic and thermodynamic characteristics of coal oxidation at low temperatures. In this work, the complex macromolecular matrix of coal was divided into elements C, H, O, S, and N, which are all involved in oxidation reactions. Based on the changes in element occurrence during low-temperature oxidation of coal, the kinetic and thermodynamic characteristics of coal oxidation were studied at temperatures below 200 °C. A kinetic study revealed that the changes in element occurrence during coal oxidation at low temperature followed pseudo-first-order kinetics. The activation energies for the changes in element occurrence obtained by using the pseudo-first order kinetic have been found to be very close to those calculated by applying the Coats and Redfern's equation. At a particular temperature, the release of element H showed the highest rate constant (K) and lowest activation energy (Ea) compared with those same values for C and N. A kinetic compensation effect between K and Ea was also observed for the changes in element occurrence. Negative enthalpy (ΔH) values indicated that the changes in S and O occurrence produced heat, while the changes in C, H, and N occurrence were endothermic, having positive ΔH values. The low values of the rate constants and frequency factors suggested the non-spontaneous nature of changes in element occurrence, which was further supported by the negative entropy (ΔS) values and positive Gibb's free energy (ΔG) values associated with the changes in element occurrence. The enthalpies of formation for CO2, CO, and H2O were calculated, and the exothermic nature for the formation of CO2 and H2O was evident given their negative ΔH values. Based on the kinetic and thermodynamic characteristics of low-temperature coal oxidation, the mechanism of coal self-heating was also explored.