Low-temperature Pyrolysis Of Coal Research Articles

Staged evolutionary features of the aromatic structure in high volatile A bituminous coal (hvAb) during gold tube pyrolysis experiments

Low-temperature pyrolysis of coal is a crucial step in the coal thermal conversion process and involves very complex physical and chemical reactions that can have different effects on the coal's structure. The thermal evolution behavior and transformation mechanism of the coal microstructure are not yet fully understood, which also limits the efficient utilization of coal to a certain extent. The aromatic structural features (including size, molecular ordering, nematic symmetry, stacking, and curvature) of the char produced from low-temperature pyrolysis of high volatile A bituminous coal (hvAb) from the Xutuan coal mine, China, were quantitatively assessed via high-resolution transmission electron microscopy (HRTEM) image processing and analytical techniques. The thermal transformation process and the mechanisms controlling it were explored. The results show that, except for the unexpected slight growth of aromatic sheets at 440 °C, the lower pyrolysis temperature (< 521 °C) contributed weakly to their size growth, whereas at higher temperatures (561–600 °C), it significantly increased their size. The aromatic molecular ordering tended to gradually change in three stages: increasing between 340 and 440 °C, decreasing between 440 and 521 °C and increasing again between 521 and 600 °C. The nematic symmetry strength of aromatic fringes also followed a similar pattern with temperature at different scales. Additionally, in addition to a very minor development trend at 440 °C, the stacking did not significantly change at temperatures below 521 °C but developed appreciably further with increasing temperatures at 561–600 °C; however, the average spacing of the stacks did not appear to be significantly reduced at all temperatures. The curvature of the aromatic sheets also varied in different temperature stages, i.e., initially slightly increasing (340–380 °C), then gradually decreasing (380–480 °C), later increasing again (480–521 °C), and eventually decreasing (521–600 °C). The properties of the chemical composition and structure of the initial coal play important roles in the thermal reaction behavior, and the physical and chemical reactions that dominate at the different temperature stages may be responsible for such wiggly trends in the evolution of the aromatic structure. Notably, the properties of the mesophase (approximately 440 °C) strongly influence the subsequent structural transformation. These findings could provide useful information for the microstructure–property relationships and preparation of coal-based carbon materials.

Low-temperature pyrolysis of coal with determination of physicochemical properties of coal and its thermal decomposition products

Low-temperature pyrolysis of coal with determination of physicochemical properties of coal and its thermal decomposition products

Acoustic emission (AE)–based study on low-temperature pyrolysis characteristics of long flame coal

Investigating crack formation and propagation in the low-temperature pyrolysis of coal is beneficial for optimising the pyrolysis process and improving the coal utilisation rate. In this study, the low-temperature pyrolysis processes of long flame coal at different temperatures (150–450 ℃) were monitored by acoustic emission (AE) technology. In addition, the effects of temperature and holding time on crack development were also analysed. The results demonstrated that 300 ℃ was a threshold for depicting low-temperature pyrolysis characteristics of long flame coal. The AE signals were weak before 300 ℃ and improved with an increase in temperature. Next, the AE signals increased significantly and were negatively correlated with the temperature. According to the generation development model proposed in this paper, the increase in temperature led to the rapid development of the primary cracks before 300 ℃, which were then slowed down. Notably, the secondary cracks began to expand after 300 ℃. The AE b-values reflected different crack development modes at 350 ~ 450 ℃. The increasing pyrolysis temperature and holding time were conducive to the development of secondary cracks. The rapid decrease and then sudden disappearance of b-values could be regarded as the criterion of macro failure of coal samples.

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.

Effects of iron catalyst and atmosphere on sulfur transformation during pressurized low-temperature pyrolysis of Baishihu coal

A vitrinite-rich low rank coal, Baishihu (BSH) coal with moderate sulfur content was treated by dehydration and crushing. The treated samples were pyrolyzed in an alloy tubular reactor under 2 MPa. Influence of iron catalyst and atmosphere on sulfur transformation during pressurized low-temperature coal pyrolysis was investigated. Molecular composition of sulfur compounds in tar was characterized by gas chromatography with sulfur chemiluminescence detector (GC-SCD) combined with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Sulfur K-edge XANES was used to study sulfur molecular structure after pyrolysis. Sulfur compounds in BSH coal are predominantly S1 class species in branch chain of the coal. Elemental sulfur in catalyst enters the tar and forms mercaptan or thioether compounds during pyrolysis. Iron catalyst promotes activation of hydrogen atoms in coal and contributes to hydrogenation saturation and cracking of aromatic sulfide in tar. The catalyst preferentially captures H2S to increase content of pyrite in char and inhibits formation of sulfate. Under H2 atmosphere, significant decrease of thiophene compounds is observed with catalyst coupled with decrease of sulfoxide compounds.

Study on the generation of active sites during low-temperature pyrolysis of coal and its influence on coal spontaneous combustion

The coal after low-temperature pyrolysis is more prone to self-heating and spontaneous combustion. To study the original heat source, this paper conducted sequentially isothermal pyrolysis, room-temperature oxidation and low-temperature re-oxidation on four different coal samples. Formation laws of active sites were studied through gas emission rules during isothermal pyrolysis of coal. Besides, the influence of pyrolyzed coal at low temperatures on spontaneous combustion in following mining process was stimulated by the subsequent room-temperature oxidation and low-temperature re-oxidation of the coal. Experimental results reveal that during low-temperature pyrolysis of coal samples, oxygen-containing functional groups are decomposed. They produce numerous active sites which are stabilized in anaerobic conditions. These sites are so active that they can be oxidized at room temperature to release massive CO and CO2 and huge heat. Therefore, the viewpoint of room-temperature oxidation of active sites is proposed. The exothermic oxidation of active sites at room temperature is the initial heat source of self-heating and spontaneous combustion of the pyrolyzed coal. Furthermore, room-temperature oxidation of the pyrolyzed coal presents object proof of direct burn-off reaction, which is of great significance to the interpretation of coal spontaneous combustion mechanism.

Semi-coke powder filtration experiments using a dual layer granular bed filter

The separation of semi-coke powders from tars and gases during coal pyrolysis is of crucial importance to the coal chemical industry. This work describes an experimental study on semi-coke powder filtration using an experimental dual layer granular bed filter with an inner diameter of 100 mm. With an initial filtration velocity of 0.2 m/s, the dual layer granular bed filter had a filtration efficiency of 99.943% and a pressure drop across the filter of only 1456 Pa. When the initial filtration velocity was increased to 0.25 m/s, the filtration efficiency was 99.937% and the pressure drop was 1834 Pa. Our results indicated that the dual layer granular bed filter we developed and tested showed significant promise as a high-temperature dust collector in low-temperature coal pyrolysis.

Study on oxidation and gas release of active sites after low-temperature pyrolysis of coal

The low-temperature pyrolysis of oxygen-containing functional groups in coal plays a vital role in coal spontaneous combustion. This paper conducted isothermal oxidation experiments on raw and pyrolyzed coal samples to investigate the influence of the content of oxygen-containing functional groups, pyrolysis temperature, oxidation temperature, particle size and pyrolysis-oxidation times on the oxidation characteristics of pyrolyzed coal samples. It attributes the generation of massive active sites that are oxidized once exposed to O2 to the pyrolysis of oxygen-containing functional groups in coal. In addition, the oxidation of active sites releases such gaseous products as CO and CO2 in large quantity and producing oxygen-containing functional groups. For a coal sample with higher content of oxygen-containing functional groups and pyrolysis temperature and smaller particle size, the yield of active sites is greater. The formation law of gaseous products during isothermal oxidation after pyrolysis satisfies the exponential attenuation function P = A + Be−t/K1 + Ce−t/K2, suggesting the two reaction processes during the oxidation of pyrolyzed coal, that is, the oxidation of active sites after coal pyrolysis and the subsequent thermal decomposition with the generation of functional groups. The activation energy required for the former is far lower than that for the latter, and the oxidation of active sites can occur at very low temperatures (below 9 °C). The results are highly applicable to the study on the mechanism of self-heating and spontaneous combustion of coal.

A portable, continuous system for mercury speciation in flue gas and process gases

During fuel combustion, mercury in the form of Hg0 and Hg2+ is emitted to the atmosphere. Effective reduction of mercury emission requires the application of speciation systems for emission control and research. To study the speciation of mercury in flue gas a portable and continuous system was constructed. The speciation system is based on the wet method of Hg2+ transformation to Hg0. The main part of the system is the Nippon Instrument Corporation EMP-2 WLE-8 set. In order to operate in flue gas from coal burning the Nippon Instrument Corporation set was equipped with the following devices: fly ash filter, steel probe, transfer line, and the tee connector. The portable, continuous system was successfully tested in the laboratory and during the industrial tests in Poland and the Czech Republic. The described system was also used to determine mercury in the process gases (low-temperature pyrolysis of coal). The results of mercury speciation in flue gas were compared with Durag HM 1400 TRX system.

Low temperature pyrolysis properties and kinetics of non-coking coal in Chinese western coals

Chinese western low-rank coals known as for its favorable characteristics play a key role in the production of coal oil. In this work, based on the changes in element occurrence during low-temperature pyrolysis of coal, the kinetic and thermodynamic characteristics of coal at 400–500 °C temperatures with 10 °C/min heat rate were studied, which resulted in the drastically cracking of aliphatic groups. However, a significant amount of oxygen-containing functional groups were also found during coal pyrolysis at low temperature followed pseudo-first-order kinetics. According to the low-temperature thermal kinetics analysis of different coals, the activation energy of coking coal is higher than that of non-coking coals, and the activation of non-coking coals is relatively low under the same pyrolysis conditions. The objective of this work is to provide theoretical data about the pyrolysis characteristics of non-coking coal at low temperature and increase the non-coking coal proportion in coking proceeding.

Energy analysis for low-rank coal based process system to co-produce semicoke, syngas and light oil

The low temperature coal pyrolysis technology and the atmospheric and vacuum tar distillation were combined to establish a low-rank coal based process system, which can co-produce semicoke, syngas and light oil. And then the simulation models of key units and whole system were also developed. As the calculation results, the lignite with 41.0% moisture can be converted to semicoke, syngas and light oil. Their yields are 42.31%, 8.47% and 4.10%, respectively. The distribution shows that the energy consumption and the exergy loss of drying unit are all the largest with 323.1 kW and 300.1 kW, and those of pyrolysis unit rank the second with 196.2 kW and 131.4 kW. Based on a graphic illustration of energy analysis, the reasons of energy consumption and depreciation were explained. The energy grades of products are increased or decreased at the cost of energy consumption and depreciation. A heat integrated co-production system was proposed to assess the energy saving potential of the original system. The energy consumption and the exergy loss for the whole system have been reduced by 14.9% and 10.9%, and the heat integration effect of drying unit has a relatively larger influence on that of the whole system.

Distribution of Sulfur Forms in Low Temperature Pyrolysis of Coal

The distribution of sulfur forms in the products of low temperature pyrolysis of Carboniferous high sulfur coal from Northwest China was investigated. The typical method of Gray-King assay was used to carry out the low temperature pyrolysis experiments. GC-MS analysis was used to investigate the composition of sulfur compounds in the coal tar. The results show that sulfur mainly remained in the semi-coke and accounted for 80.97% of the total sulfur. Pyrites decomposed and transformed into sulfates and organic sulfur. 5 sulfur containing compounds were detected in the coal tar and they are dibenzothiophene, benzonaphthothiophene and their substituted homologs.

99/01199 Analysis and use of liquid derivatives from low-temperature coal pyrolysis : Banak-Tabkowska, J. Karbo-EnergoChem.-Ekol., 1998, 43, (3), 124–129. (In Polish)

99/01199 Analysis and use of liquid derivatives from low-temperature coal pyrolysis : Banak-Tabkowska, J. Karbo-EnergoChem.-Ekol., 1998, 43, (3), 124–129. (In Polish)

Release of volatile sulfur compounds during low temperature pyrolysis of coal

The behaviour of sulfur structures in coal during low temperature pyrolysis has been studied. Nine low rank coals with high organic and pyritic sulfur contents were heated in a swept fixed bed reactor (N 2) up to 850 °C, and the evolved sulfur compounds were determined by sulfide electrode (H 2S) and Fourier transform infrared (FT-i.r.) spectroscopy (SO 2, COS). Evolution of H 2S as a function of temperature passes through two peaks between 500–560 °C and 630–700 °C, related to the decomposition of organic and pyritic sulfur, respectively. Assignment of organic sulfur structures and pyritic sulfur to the two observed peaks of H 2S evolution were tested using float-sink fractions with different mineral matter and pyrite contents (0.3–33 wt%). An important role of coal organic matter in pyrite decomposition to H 2S was found. The evolution of COS with pyrolysis temperature, followed a trend similar to that for H 2S. The evolved SO 2 was related to the decomposition of iron sulfate from weathering of pyrite, but also with the presence of oxidation reactions during pyrolysis. The amount of sulfur removed from coal by pyrolysis varied from 35 to 64% of the total sulfur content for the assayed samples. Pyrolysis temperatures below 700 °C were sufficient to obtain desulfurization levels close to 90% of the total amount of sulfur which can be removed by pyrolysis.

Upgrading of Pitch Produced by Mild Gasification of Subbituminous Cal

Alternative sources of anode binder pitch may be required in the future. One potential process for the production of pitch is low-temperature coal pyrolysis or mild gasification. Characterization data for a pitch produced by mild gasification of Powder River Basin subbituminous coal are reported. This pitch has acceptable sulfur, sodium, ash, and beta-resin levels but is shown to be deficient in aromatic carbon content and quinoline insolubles (QI). It also contains high levels of oxygen which leads to a high softening point and low coking value. Several strategies for upgrading this pitch to meet anode binder standards were investigated. These included heat treating, air blowing, solvent extraction, vacuum distillation, and addition of artificial QI

Compositional changes in the mild gasification liquids produced in the presence of calcium compounds

Low-temperature coal pyrolysis liquids produced in the presence or absence of additives (calcium oxide and Genstar, a dolomitic material) were separated by adsorption chromatography. Selected fractions were analyzed using infrared and carbon-13 nuclear magnetic resonance and gas chromatography/mass spectroscopy to determine the thermal reactions of aromatic compounds with these additives. The separation and spectroscopic results showed that the lower and higher phenols were deoxygenated. In addition, calcium oxide induced a variety of reactions including the cracking of alkanes and alkanes and the aromatization of alkanes.

Prediction of sulphur distribution in products during low temperature coal pyrolysis and gasification

During devolatilization of coal, the sulphur present is distributed into solid, liquid, and gaseous products depending on the type and quantity of the coal sulphur and the processing conditions (e.g., temperature, pressure, and heating rate) used. In this study, a series of coals (mainly high volatile bituminous) was devolatilized at a relatively low temperature (500 °C) in a fixed-bed reactor in an inert atmosphere. Primarily bituminous coals were used in this study as these are shown to yield the most liquid product during pyrolysis. The distribution of sulphur in solid, liquid, and gaseous products was monitored. Influence of peak devolatilization temperature on sulphur distribution in products was determined for a high volatile bituminous coal (Pittsburgh No. 8). The sulphur content of the pyrolysis liquids generated at 500 °C correlates well with the total coal sulphur. The total sulphur of the char can be correlated with the pyritic sulphur content of the coal. Total gaseous sulphur content (sum of H 2S and COS) or sulphur content of tar increases with increase in organic sulphur of coal, and relatively good correlations were obtained. An increase in pyrolysis temperature increases the total gaseous sulphur yield at the expense of char sulphur. Based on sulphur distribution data for 32 bituminous coals, equations have been developed to correlate the sulphur yield in products with the total sulphur of the feed coal. The correlations developed in this study are expected to be applicable for predicting the sulphur distribution in products during conversion of bituminous coals using low temperature pyrolysis technology, and to estimate sulphur products in the exit gases and tars when coal is converted in a fixed-bed gasifier in which initial devolatilization occurs at a relatively low temperature.

Use of silicalite molecular sieve for the gas chromatographic determination of permanent gases and volatile hydrocarbons emitted during coal processing

Permanent gases and C 2C 7 hydrocarbons released during low-temperature pyrolysis of coal and during the cleaning of coal by leaching with molten caustic are efficiently separated and determined using gas chromatographic columns packed with silicalite, a hydrophobic molecular sieve. The advantages of using this material for column packing include: (a) permanent gases can be separated efficiently and rapidly; (b) both permanent gases and volatile hydrocarbons can be determined simultaneously using temperature programming; (c) in contrast with conventional molecular sieves, silicalite is not deactivated by adsorption of water or carbon dioxide; (d) silicalite is not degraded by most acidic or corrosive agents; (e) under special conditions, carbon dioxide can be determined in the presence of permanent gases and volatile hydrocarbons; and (f) trace quantities Of C 3C 7 hydrocarbons can be accumulated on the front end of silicalite columns during repetitive, low-temperature determinations of permanent gases, and the composite sample of hydrocarbons can be subsequently eluted and analyzed by raising the column temperature.