Decomposition Of Oxygen-containing Functional Groups Research Articles

Study on secondary oxidation characteristics of coal gangue at different pyrolysis rank

In order to further investigate the spontaneous combustion characteristics of coal gangue (CG) hills, this paper explored the potential connection between the spontaneous combustion characteristics of CG and its pyrolysis rank. Though the method of gas chromatography (GC) and mass spectrometry (MS), the temperature programmed experiments of CG samples with different pyrolysis ranks were carried out. The variation of index gas emission behavior and the oxidation kinetics of CG after high-temperature pyrolysis were investigated under the condition of heating from 30 ℃ to 900 ℃. The results showed that there is a certain connection between the rank of pyrolysis of CG and its emission behavior and the thermodynamic parameter. The energy required for the combustion of samples after pre-pyrolysis were significantly higher than the raw sample. With the increase of the rank of pyrolysis, due to the decomposition of oxygen-containing functional groups in the process of pyrolysis, a large amount of volatiles were released, the CG in the combustion process of the emission of index gas and its oxygen consumption rate gradually reduced, the rapid oxidation stage shifted to the high temperature direction, and the combustion interval was also greatly shortened. The increase of the rank of pyrolysis will greatly reduce the thermal value and spontaneous combustion risk of the CG. Therefore, the CG hill with low rank of pyrolysis is more likely to cause spontaneous combustion, with higher risk and greater pollution. The results of this paper have important guiding significance for exploring the spontaneous combustion characteristics and management of CG hills.

Correlations of composition and structural characteristics between pyrolysis tar and soluble organic species of Baoqing lignite

During pyrolysis, the intrinsic organic species in coals play an important role on the formation of tar but the relationship between them has not been elucidated in detail. In this study, the composition and structural characteristics of pyrolysis tars and soluble portions (SPs) via thermal dissolution from Baoqing lignite were analyzed with Fourier transform infrared spectroscopy (FTIR), gas chromatograph/mass spectrometer (GC/MS) and quadrupole/Orbitrap mass spectrometer (Orbitrap MS), and the formation mechanisms of main components in pyrolysis tars were proposed. FTIR analyses showed that both the SPs and tars are rich in -OH, CO, aromatic CC, C-O, aliphatic and aromatic C-H functional groups, suggesting they are partially similar in composition and structure. GC/MS and Orbitrap MS analyses indicated that a part of soluble organic species was directly volatilized into the tars, while others were suffered from complex reactions during pyrolysis, such as the cleavage of bridged bonds, the decomposition of oxygen-containing functional groups and the condensation of aromatics. Compared with the SPs, the higher contents of CH and O1 classes but lower contents of O2-O6 classes in the tars demonstrated that the soluble organic species are partially or completely deoxygenated during pyrolysis.

Towards understanding of internal mechanism of coal reactivity enhancement after thermal decomposition at low temperature

Coal after thermal decomposition has high reactivity and can easily self-heating and self-ignite. The spontaneous combustion problem of thermally decomposed coal has been widely concerned, especially in the unsealing of closed fire area, the magmatic erosion area of coal and the storage of low-rank coal after drying. Related studies mainly explain the phenomenon from the perspective of physical pore and moisture, the effect of the chemical decomposition process on the subsequent spontaneous combustion process of coal is worthy of further study. In this paper, the room temperature oxidation of thermal decomposed coal was first carried out. The physical and chemical parameters of coal before and after the decomposition process were than investigated by modern analytical instruments such as scanning electron microscope (SEM), nitrogen adsorption instrument, X-ray diffraction (XRD), infrared spectrometer (FTIR), X-ray photoelectron spectroscopy (XPS) and Electronic paramagnetic resonance (EPR). Results show that a large number of active sites are generated and accumulated in the thermally decomposed coal, and these sites can rapidly oxidize at room temperature, leading to an increase in coal temperature. Changes in physical structures indicate that thermal damage at high temperatures results in the development of cracks and pores. Variations of chemical structures manifest that the coal decomposition process leads to the decomposition of oxygen-containing functional groups and the increase of active alkyl radicals. It is revealed that the thermal decomposition of oxygen-containing functional groups in coal produces a large number of reactive alkyl radicals, and the oxidation of these radicals is the internal reason for the high reactivity and self-heating of thermally decomposed coal. The development of physical structures such as pores and cracks provide a wider oxygen transport channel for the oxidation of the active sites. This paper is of great significance in revealing the spontaneous combustion mechanism of thermally decomposed coal and helping the development of high-efficiency chemical retardant.

Evolution of Chemical Component and Physical Properties of Datong Long-Flame Coal After Superheated Water Vapor Pyrolysis

ABSTRACT Datong long-flame coal from China was pyrolysis with superheated water vapor at 300°C to 536°C. Chemical and physical changes of long flame coal, including composition, weight loss, pore fracture structure, density, and water absorption were tested, and the qualitative and quantitative relationships among these changes were analyzed. superheated water vapor pyrolysis removes moisture and volatile components from long flame coals and increases the fixed carbon content. After superheated water vapor pyrolysis, the O/C and H/C atomic ratios of long flame coal decreased due to the decomposition of oxygen-containing functional groups and chain aliphatic hydrocarbons. superheated water vapor pyrolysis increased the porosity of long flame coal by material removal and matrix fracture. Material removal occurs throughout the pyrolysis process but matrix fracture occurs only at the initial stage of the conversion of solid organic matter to oil and gas products. The expansion of pore fractures in long flame coal caused a significant decrease in apparent density and an increase in water absorption. Quantitative analysis revealed the forced water absorption of long flame coal was quadratically polynomial with respect to porosity.

Effect of the pore structure of coal-based activated carbon and hydrogen addition on methane decomposition for the preparation of carbon nanotubes

Activated carbon with a hierarchical micro/mesoporous carbon structure was synthesized using Shenmu coal as the raw material, which was impregnated with KOH (K) in different K/C ratios before carbonization at various temperatures. The obtained material was loaded with nickel and then employed as catalyst for the catalytic methane decomposition (CMD) to prepare carbon nanotubes. Using a constant nickel loading, the effects of the pore structure of the different activated carbon materials and hydrogen addition on the preparation of carbon nanotubes were investigated. The results show that the coal-based activated carbon with a micropore content of 0.8, synthesized at a carbonization temperature of 850 °C and a K/C ratio of 2, is the most suitable carrier for the methane cracking catalyst to produce carbon nanotubes. The diameter of the prepared carbon nanotubes is in the range of 50–120 nm with a length of 5–15 μm. The addition of hydrogen can prevent the high-temperature decomposition of oxygen-containing functional groups, and the activity of the catalyst can be maintained by inhibiting carbon deposition. Furthermore, the hydrogen present in the CMD process can promote the formation of carbon nanotubes with larger diameters (50–350 nm) and shapes. The nanotubes are more curved, and their length is reduced to 5–11 μm. Finally, the formation mechanism of carbon nanomaterials is discussed according to the morphological changes of the products after hydrogen addition and the existing theories. In conclusion, appropriate amounts of oxygen-containing functional groups and larger micropore volumes facilitate the pyrolysis of coal-based activated carbon catalysts in the preparation of carbon nanotubes.

Kinetic Analysis of the Generation of Active Sites During the Low-Temperature Pyrolysis of Coal

ABSTRACT Decomposition of oxygen-containing functional groups simultaneously produces CO and CO2, accompanied by the generation of active sites. Two different methods were applied in the study to speculate the kinetics of gas generation and the formation of active sites in coal pyrolysis. The evolution of CO and CO2 gas generation during isothermal pyrolysis under different temperatures was first analyzed, and the generation of CO and CO2 in the pyrolysis process was divided into three stages: the rapid decline stage, slow decline stage, and stabilization stage. In the early stage of the pyrolysis reaction, functional groups exhibited a rapid decomposition rate and a high conversion rate (), which rapidly decreased the gas concentrations. With increasing pyrolysis time, the decomposition of functional groups declined with a low conversion rate () and showed a steady reaction rate in the later stage. On this basis, a method to calculate the activation energy during pyrolysis based on the data of gases generated in a steady period with a continuous cooling temperature was proposed. Activation energies of 70.12–148.39 kJ/mol for the CO formation and 64.47–127.38 kJ/mol for the CO2 formation were obtained from four different coals. In addition, a thermogravimetric analyzer was used to explore the apparent activation energies in the decomposition process via Coats-Redfern (C-R) method; activation energies of 39.33–126.65 kJ/mol and pre-exponential factors of 1.31–5.15E+08 s−1 were obtained for different coals. The results revealed that the CO had a greater generation activation energy than CO2, and a higher metamorphism degree implies a higher decomposition activation energy, which indicates that the decomposition of carboxyl groups and low-rank coals result in higher active sites.

Release characteristics of AAEMs and physicochemical structural evolution of char during rapid coal pyrolysis

Rapid pyrolysis experiment of Zhundong coal was conducted in a high-frequency furnace to investigate release characteristics of alkali and alkaline earth metals (AAEMs) and its correlation with changes of physical and chemical properties of char. Residence time and atmosphere during pyrolysis were important process parameters that affected the migration characteristics of AAEMs and physicochemical structural evolution of char. Experimental results show that the release of AAEMs in coal char increases over time. The maximum release rate of Na, Mg and Ca is 61.05%, 64.47%, and 44.01%, respectively. Promoting effect of CO2 on release of Na mainly occurs in the initial stage of rapid pyrolysis, nevertheless the promoting effect on release of Mg and Ca mainly is in the middle and late stage. CO2 accelerates release of AAEMs by promoting release of volatiles, facilitating decomposition of oxygen-containing functional groups and aliphatic functional groups, and promoting formation of cracks on surface of coal char.

Carbon consumption and adsorption-regeneration of H2S on activated carbon for coke oven flue gas purification.

Carbon consumption of activated carbon varies with sulfur-containing products. In this work, differential thermogravimetric (DTG), electron paramagnetic resonance (ESR), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD) we re used to reveal the adsorption-regeneration process of H2S and the effect of adsorption products on carbon consumption. The results show that H2S reacts with the C=C bond to form the C-S bond as an intermediate state, followed by the formation of elemental sulfur. It directly sublimates at approximately 380 °C, about 30 °C higher than the decomposition temperature of H2SO4. In the thermal regeneration process, the elemental sulfur in the form of monoclinic sulfur (S8) first breaks into infinitely long chain molecules (S∞) and then into small molecules, finally into sulfur vapor. The desorption of elemental sulfur consumes less oxygen and carbon functional groups, reducing the chemical carbon consumption by 59.8% than H2SO4; moreover, the compressive strength reduces less due to its slight effect on the disordered graphitic structure. H2S also reacts with the C=O bond to form H2SO3 or H2SO4. The desorption of H2SO3 does not require carbon consumption. The decomposition of H2SO4 needs to react with the C=C bond to release SO2, CO2, and CO, and the compressive strength of activated carbon significantly decreases. The carbon consumption originates from two aspects; the one from the regeneration of sulfur-containing products is more than twice the other one from the decomposition of oxygen-containing functional groups.

Enhanced pyrolysis of palm kernel shell wastes to bio-based chemicals and syngas using red mud as an additive

In the present study, the red mud (RM, a waste from aluminium industry) was employed to catalyze the pyrolysis of palm kernel shell (PKS, a waste from palm oil generation industry) to produce bio-chemicals and syngas using a bench scale fixed bed reactor. The composition of the achieving bio-oil was determined by a GC/MS analyzer. The results show that RM could promote the conversion of PKS biochar and bio-oil to non-condensable gases during the pyrolysis process of PKS. The addition of RM enhanced the decomposition of oxygen-containing functional groups and double bonds in side chains on benzene rings, resulting in the formation of phenol and aromatics accounting for 85.61 area% in bio-oil. Moreover, the total energy-based yield of gaseous and liquid products reached up to 87% at 800 °C with a PKS/RM mass ratio of 1:1.8. The steam gasification reactivity of PKS biochar was also significantly improved with the addition of RM. The findings imply that RM is a promising additive that allows the utilization of PKS waste riched in lignin component to generate valuable products via pyrolysis.

Carbonization of Plant Residues Decreased their Capability of Reducing Hexavalent Chromium in Soils

The mechanism of Cr(VI) immobilization in soils by organic substances are not well understood. In the present study, two crop residues (maize stalk and peanut shell) and their biochars obtained at various pyrolysis temperatures were prepared to investigate their influences on the immobilization of Cr(VI) in two contaminated soils via an incubation test and a bioassay. The results showed that Cr(VI) immobilization in soils was ascribed to dominant reduction (52–99%) coupled with secondary surface adsorption (1.2–48%) by organic substances. The maximum reduction capacity (Ym, 238 mg/kg) was found in an acidic brown soil with maize stalk amendment, secondly, with peanut shell amendment (Ym, 231 mg/kg). Their biochars presented a weaker capacity in reducing Cr(VI) to Cr(III) of soils than raw crop residues since carbonization accelerated the decomposition of oxygen-containing functional groups of organic substances. Soil properties, mainly soil pH, were negatively related to Cr(VI) reduction in soils remarkably, while concomitant anions in soils mainly the phosphate could compete with Cr(VI) for surfaces of soil particles and decrease Cr(VI) adsorption and subsequent reduction. The bioassay in pots with wheat seedlings further validated that maize stalk was a better organic substance for Cr(VI) immobilization and subsequent decrease of its bioavailability in contaminated soils than its biochar according to the results of wheat biomass and Cr contents in shoots and roots. Accordingly, to develop a cost-effective method for immobilizing Cr(VI) in contaminated soils, the raw maize stalk is more advantageous than the carbonaceous counterparts because no pyrolysis is required for the application.

Room temperature oxidation of active sites in coal under multi-factor conditions and corresponding reaction mechanism

The thermal decomposition of oxygen-containing functional groups can produce active sites that accelerate coal spontaneous combustion. To investigate the room temperature oxidation of the active sites in coal by varying multiple experimental conditions (e.g., pyrolysis temperature, pyrolysis duration, oxygen concentration, and pyrolysis-oxidation times), pyrolysis-room temperature oxidation experiments and corresponding infrared spectroscopy experiments were carried out. First, by analysing the evolution of gas generation and the coal core temperature as a function of time during the room temperature oxidation of coal samples, it is found that the active sites oxidize more intensely under the conditions of higher the pyrolysis temperature, longer the pyrolysis duration and higher oxygen concentration. Then, the continuous pyrolysis-room temperature oxidation experiment and the changes in the chemical structure prove that the oxygen-containing functional groups can be converted into the active sites and vice versa. In addition, the concentration of the gaseous products evolved in each room temperature experiment shows that the amount of CO2 produced is higher than that of CO, and the molar ratio of CO2 to CO is approximately 3.3:1, which was found by using a statistical method and indicated that the two gas products are derived from different reaction pathways of the identical substance. Finally, a possible microscopic reaction mechanism of the room temperature oxidation of the active sites is proposed. The research results are of great value not only for revealing the coal spontaneous combustion mechanism, but also for preventing the spontaneous combustion of heat-invaded coal in the subsequent mining process.

Insight into the chemical reaction process of coal self-heating after N2 drying

It is generally considered that changes in physical rather than chemical structures are responsible for the increase in the spontaneous combustion tendency of coal after inert drying. In this paper, an isothermal flow reactor was combined with FTIR and ESR techniques to study the room temperature oxidation of coal samples under the effects of drying temperatures, numbers of drying-oxidation cycles and anaerobic heating temperatures. Macro-physical parameters such as gaseous generation, oxygen consumption, coal core temperature and micro-chemical structural parameters such as functional groups and free radicals were examined. After coal drying, the thermal decomposition of oxygen-containing functional groups generates free radical active sites, which can be oxidized and are exothermic at room temperature, accompanied by the generation of large amounts of CO and CO2 gaseous products. In addition, the active sites and oxygen-containing functional groups can be transformed into one another under certain conditions. The results indicate that the inherent reason responsible for self-heating and uncontrolled spontaneous combustion of N2-dried coal is the room temperature oxidation of the active sites, especially the room temperature oxidation of the secondary active sites generated by thermal decomposition of the oxygen-containing functional groups. The results presented in this research have applications to reveal the chemical reaction mechanism determining why lignite is more prone to spontaneous combustion after inert drying.

Examination of CO, CO2 and active sites formation during isothermal pyrolysis of coal at low temperatures

The thermal decomposition of oxygen-containing functional groups produces CO, CO2 and active sites that accelerate coal spontaneous combustion. Study on this process is of great significance for research on the mechanism of coal spontaneous combustion. In this paper, research was made on the dependency relationship between gas production and time under the condition of different coal samples, particle sizes and pyrolysis temperatures. Results reveal that the time-dependent generation rate of active sites follows an exponential compound function P=A+Be-t/K1+Ce-t/K2 during isothermal pyrolysis of coal. Besides, infrared spectrum experiments were also carried out to study changes in oxygen-containing functional groups of raw coal, pyrolyzed coal and room temperature oxidized coal after pyrolysis. The results prove that when coal reaches a certain temperature, oxygen-containing functional groups will adsorb heat and undergo decomposition to produce active sites and a few gas products. Then, active sites are oxidized, generating oxygen-containing functional groups and releasing both massive heat and gas products. Therefore, the whole reaction is a continuous cyclic process where combined effects of the two reactions cause rapid accumulation of heat, which triggers uncontrollable coal spontaneous combustion. The results from this research will be helpful to inhibit the occurrence of coal spontaneous combustion by reducing the generation and oxidation of active sites.

Effect of solvent and atmosphere on product distribution, hydrogen consumption and coal structural change during preheating stage in direct coal liquefaction

In order to study the necessity of hydrogen source at preheating stage of direct coal liquefaction, product distribution, hydrogen consumption, and coal structural change of Hami sub-bituminous coal (HM) with 1-methylnaphthalene (1-MN) and tetralin respectively were investigated under hydrogen and nitrogen atmosphere. The results show that oil could be generated from coal itself with 1-MN under N2 and the yield increases with temperature rising at the range of 200–350°C, whereas only 4.94% oil is obtained at 350°C. Moreover, volatiles and H/C ratios of pretreated coal decrease dramatically above 250°C. At 350°C, preheating with gas phase H2 in 1-MN promotes oil yield to 10.42%. In the presence of tetralin, oil yields of 28.55% and 26.15% are achieved under N2 and H2 at 350°C, respectively. Approximately same total hydrogen consumption is observed under N2 and H2 with tetralin, and the hydrogen consumption from tetralin is higher than that from H2. With the addition of hydrogen sources, aromaticity of the residual solid decreases and reactivity increases. Decomposition of oxygen-containing functional groups like COOH and CO is not affected by the preheating solvent and atmosphere but relies on temperature. In addition, pretreated solid samples achieve increasing oil yield and conversion in the subsequent liquefaction with their rising thermal reactivity and H/C ratios.

Heat effect of the oxygen-containing functional groups in coal during spontaneous combustion processes

The heat generated from the reactions of oxygen-containing functional groups in coal has been studied using organic chemistry and quantum chemistry analysis methods. Structural models of the oxygen-containing functional groups in coal were established and used to analyze the reactions of these groups during spontaneous combustion. The Gibbs free energy and enthalpy changes associated with these reactions were determined using quantum mechanical analysis, and the results indicated that the dehydration and dehydrogenation reactions of the hydroxyl groups in coal were only mildly exothermic, whereas reactions involving the oxidation of the carbonyl groups were much more exothermic. In contrast, reactions resulting in the generation of carbon monoxide were endothermic. By comparing the heat release characteristics of the reactions of the oxygen-containing functional groups with the levels of oxygen consumption and gas production, as well as temperature profiles of the reactions, it was concluded that the decomposition of oxygen-containing functional groups is critical to the production of heat during the initial stages of the spontaneous combustion of coal and that oxidation does not occur during this period. These results also explain why the temperature of coal rises slowly during the initial stages of its spontaneous combustion.

Effect of microwave irradiation on the propensity for spontaneous combustion of Inner Mongolia lignite

The propensity for spontaneous combustion of Inner Mongolia lignite irradiated in microwave irradiation was investigated. The functional groups on the lignite surface were obtained by FTIR and XPS spectra, and the pore structures of irradiated lignite were measured using N2 adsorption/desorption method. The propensity for spontaneous combustion of lignite was classified using the crossing-point method. The irradiation treatment in microwave resulted in a prominent decrease in moisture content, decomposition of oxygen-containing functional groups, breakage of long alkyl side-chains, and increase in aromatic carbons. The quality of lignite developed towards high-rank coals after microwave irradiation. Microwave irradiation showed significant effects on the pore structures of lignite; the specific surface area and total pore volume of the irradiated lignite initially increased and then significantly decreased. A short-term microwave irradiation resulted in an increase the spontaneous combustion propensity of lignite. However, when the irradiation time extended, the propensity for spontaneous combustion was reduced because of the significant change in chemical composition and drastic decomposition of the pore structures.

An experimental study on the evolution of aggregate structure in coals of different ranks by in situ X-ray diffractometry

The XRD profiles of raw coal.

Fabrication of hydrophobic, electrically conductive and flame-resistant carbon aerogels by pyrolysis of regenerated cellulose aerogels.

In this paper, we reported miscellaneous carbon aerogels prepared by pyrolysis of regenerated cellulose aerogels that were fabricated by dissolution in a mild NaOH/PEG solution, freeze-thaw treatment, regeneration, and freeze drying. The as-prepared carbon aerogels were subsequently characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), nitrogen adsorption measurements, X-ray diffraction (XRD), Raman spectroscopy, and water contact angle (WCA) tests. The results showed that the carbon aerogels with pore diameters of 1-60 nm maintained interconnected three-dimensional (3D) network after the pyrolysis, and showed type-IV adsorption isotherm. The pyrolysis process leaded to the decomposition of oxygen-containing functional groups, the destruction of cellulose crystalline structure, and the formation of highly disordered amorphous graphite. Moreover, the carbon aerogels also had strong hydrophobicity, electrical conductivity and flame retardance, which held great potential in the fields of waterproof, electronic devices and fireproofing.

On the role of the residual iron growth catalyst in the gasification of multi-walled carbon nanotubes with carbon dioxide

The gasification of carbon with CO2 was applied to examine the role of the residual iron growth catalyst in multi-walled carbon nanotubes (CNTs), which were pre-treated either by refluxing in nitric acid at 120°C or by nitric acid vapor at 200°C. Temperature-programmed desorption (TPD) and surface reaction (TPSR) experiments were performed in He and CO2, respectively. The Fe nanoparticles were retained after the treatment in HNO3 vapor, whereas the liquid HNO3 treatment was able to remove the accessible residual Fe catalyst. The exposed Fe nanoparticles were found to catalyze the gasification of CNTs with CO2 according to the reverse Boudouard reaction C+CO2=2CO. In case of the CNTs pretreated in HNO3 vapor, evolving CO2 formed due to the decomposition of oxygen-containing functional groups during the TPD experiments was fully converted above 750°C into desorbing CO, and the addition of 2000ppm CO2 in the feed gas during the TPSR experiments resulted in full conversion at 1000°C. X-ray photoelectron spectroscopy studies show that the treatment in HNO3 vapor at 200°C favors the formation of oxygen species doubly bound to carbon (CO groups). During the TPSR experiments, CO2 as a weak oxidant partially oxidized the CNTs leading to the formation of CO groups, and a much higher amount of these groups was detected on HNO3 vapor-treated CNTs with residual Fe catalyst. Their presence suggests that CO groups are reaction intermediates of the CNT gasification with CO2, which is considered an effective test reaction for the presence of residual catalytically active nanoparticles.

Modified activated carbon with an enhanced nitrobenzene adsorption capacity

To enhance nitrobenzene removal from aqueous solution, commercial activated carbon (AC) was modified by oxidation with HNO3 followed by heat treatment in a nitrogen atmosphere. The modification process introduced oxygen-containing functional groups and changed the charge properties of the AC surface. The effects of surface chemical properties and pore structure on the adsorption of nitrobenzene by the AC were investigated through kinetics and equilibrium isotherms. HNO3 oxidation modified the surface chemical properties and increased the number of acidic oxygen-containing surface groups, but had an almost negligible effect on the pore structure. Subsequent heat treatment caused decomposition of oxygen-containing functional groups on the AC surface, increased the pH point of zero charge (pHPZC) and increased the number of mesopores. The maximum adsorption capacity achieved by the modified AC was 1,443.53 mg g−1, 3.33 times of that unmodified AC. HNO3 oxidation combined with heat treatment was an effective method for the preparation of AC with high nitrobenzene adsorption capacity and fast adsorption kinetics. An appropriate pHPZC, amount of surface oxygen groups and the presence of well-developed mesopores together with micropores were the main reasons for the high nitrobenzene adsorption capacity of the modified AC.