Coal Reactivity Research Articles

Hydrogen rich gas production through co-gasification of low rank coal, flotation concentrates and municipal refuse derived fuel

In the paper a novel approach to thermochemical utilization of low rank coal, flotation concentrates and municipal refuse derived fuels was presented. The economic attractiveness of low rank coals and flotation concentrates is limited and that is why they are commonly stored at excavation heaps causing additional costs and the risk of endogenous fires occurrence. One of the crucial parameters determining the attractiveness and usability of a fuel in the gasification process is its reactivity. In the study several low rank coals, flotation concentrates and municipal refuse derived fuels were tested in terms of their reactivity in the process of steam gasification. The reactivity of low rank coal and flotation concentrates at 50% of carbon conversion, R50, varied between 1.46·10−4 and 2.39·10−4 s−1, whereas the maximum reactivity, Rmax, from 3.28·10−4 to 4.62·10−4 s−1. Advanced mathematical models were developed to investigate the similarities and dissimilarities between the studied fuels as well as the relationships between the physical and chemical parameters and the reactivities of fuel chars in steam gasification. On this basis, a low rank coal was selected and blended with 20%w/w of municipal refuse derived fuel in co-gasification experiments. The aim of the research was to utilize the low rank coal characterized by the lowest reactivities (R50 and Rmax of 1.46·10−4 and 3.28·10−4 s−1, respectively) in steam co-gasification to hydrogen-rich gas with an alternative fuel in a fixed bed reactor at the temperature of 800 °C. The selected low rank coal was blended with 20%w/w of municipal refuse derived and the resulting fuel yielded the average concentration of hydrogen in the produced gas of 58.99%vol.

Dynamic migration mechanism of organic oxygen in Fugu coal pyrolysis by large-scale ReaxFF molecular dynamics

Organic oxygen accounts for essential constituents of the chemical coal structure and makes significant contributions to the reactivity of coal in pyrolysis. The dynamic migration mechanism of organic oxygen during coal pyrolysis was explored by heat-up ReaxFF molecular dynamics simulations. A multicomponent molecular model for Fugu subbituminous coal was simulated in the temperature range of 300−2500 K with a heating rate of 1 K/ps using the GMD-Reax software. The difference between the pyrolysis products yields and their oxygen content indicates that organic oxygen in Fugu coal migrates preferentially into gas-phase oxygen during pyrolysis. Molecules of CO2, H2O, CH2O, and CO are identified as major O-containing gas products and their generation sequence shows consistency with reported experimental observations. CH2O molecules are produced mainly from cleavage of alkyl-alkyl ethers and their secondary cracking reactions will lead to their decrease trend and contribute partially to the rapid production of CHO radicals at high temperatures. The phenolics and carbonyl groups are found as the dominant functionalities in asphaltene and tar. The cleavage of alkyl-aryl and aryl-aryl ethers contributes to the production of phenolics that transform into carbonyl groups at higher temperatures through isomerization of aromatic rings. The decrease of phenolics and increase of carbonyl groups in the fast pyrolysis stage should be attributed mainly to the structure transformation reactions rather than migration among different pyrolyzate lumps of gas, tar, and asphaltene. This work suggests that ReaxFF MD simulation is useful for exploring the comprehensive dynamic migration mechanism of organic oxygen in coal pyrolysis.

Interaction and kinetic analysis of co-gasification of bituminous coal with walnut shell under CO2 atmosphere: Effect of inorganics and carbon structures

In this study, co-gasification behaviors and synergistic interaction of bituminous coal and walnut shell under CO2 atmosphere were investigated by using a thermogravimetric analyzer. The evolution of physicochemical properties of samples was characterized with an inductively coupled plasma emission spectrometer, an X-ray fluorescence spectrometer, and an X-ray diffractometer. The results indicated that the addition of walnut shell significantly enhanced the gasification reactivity of coal, and coal/biomass blend ratio of 7:3 showed an intense gasification. Moreover, a significant co-gasification interaction was observed in the char gasification stage and demonstrated that a high coal/biomass blend ratio induced the formation of amorphous carbons and further accelerated the co-gasification reaction. Meanwhile, the active inorganics were severely released as 30 wt% bituminous coal added to walnut shell, causing a deteriorative gasification reactivity. Furthermore, three kinetic models illustrated that the coal/biomass blend ratio of 7:3 required a minimum activation energy to initialize gasification. The findings in this work would be beneficial for revealing the co-gasification mechanism of biomass and coal, and provide a useful basis on their multi-scale applications.

Modeling on the Effect of Heat Exchanger Submersion on Controlling Spontaneous Combustion in A Coal Pile

Spontaneous combustion of coal has been well-known as a problem faced by coal industries, especially in storing and trans-shipping processes. The negative impacts of this phenomenon have led to several hazardous incidents and degrading product quality. Several methods have been researched to minimize the impacts; one of the proposed ways is immersing heat exchangers inside the coal stockpile. An experiment was conducted to analyze the cooling effect of an immersed simple heat exchanger made of a copper coil. By varying the number of windings, the experiment showed a significant decrease in pile temperature due to the immersed heat exchanger. This work continues exploring the possibility of applying the method by observing and analyzing the simulation model. COMSOL Multiphysics was used to model the physics phenomena that occur within the coal reactor. The effect of the heat exchanger surface area was studied from the model to observe the heat propagation within the coal reactor. The vast reach of heat propagation from the heat exchanger through the coal pile on the simulation was promisingly showing that this method was useful to limit the occurrence of spontaneous fire in coal piles.

Stress Sensitivity for the Occurrence of Coalbed Gas Outbursts: A Reactive Force Field Molecular Dynamics Study

Coalbed gas outburst processes commonly occur in ductile shear zones and release gas quantities that far exceed the maximum gas adsorption capacity of coal seams. Although outburst accidents pose a serious safety hazard, this process has long remained poorly understood at a molecular level. In this work, we use reactive force field molecular dynamics simulations to analyze the influence of stress on the mechanochemical decomposition reactions and products of coal macromolecular models. The results indicate that a maximum of reaction amounts occurs upon changing the external force for the mechanochemical reactivity of bituminous coal under working conditions. This implies that gas generation readily occurs over a suitable stress-sensitive range, which can be used to qualitatively explain why coalbed gas outbursts typically occur in ductile shear zones. When a force is too large, it can cause brittle fracture and is less likely to initiate mechanochemical reactions that generate gas, whereas a too small of a force is insufficient to trigger chemical changes of the coal macromolecular structure. The anisotropic properties of coal must also be considered in terms of mechanochemical reactions. Coal molecules can generate different kinds of gases when driven by external forces in different orientations. The simulation results provide important insights into coalbed gas outburst characteristics and the anisotropic nature of coal. Only by comprehensively considering stress factors will it be possible to understand the occurring conditions of coalbed gas outbursts.

COMPARATIVE STUDY OF VARIOUS APPROACHES IN ANALYSIS OF TGA DATA OF DEVOLATILIZATION OF HIGH ASH INDIAN COAL

ABSTRACT Thermo gravimetric analysis (TGA) has been regularly employed for the estimation of devolatilization rates and measurement of char reactivity of coal. In general, TGA studies may be classified as isothermal or non-isothermal. In the first case temperature of the sample is kept constant whereas in case of latter the rate of temperature increase is kept constant. The current research work deals with the non-isothermal TGA study and kinetic analysis of three typical pulverized high-ash Indian coal samples each weighing around 15 mg. TGA experiments were carried out for four different heating rates (5, 10, 15, 20 K/min) and weight loss curves have been obtained. Out of the various methods proposed to analyze the TGA data, the following methods have been evaluated i) Friedman method, ii) Flynn-Wall-Ozawa method (FWO), iii) Kissinger method, iv) Kissinger-Akahira-Sunose (KAS) method, and v) Miura-Maki method. Activation energy and frequency factor to be used in the Arrhenius equation is calculated using all the above methods. The most accurate method for analyzing TGA data for high-ash Indian coal is FWO as per studies carried out in this research work.

NO formation characteristics and fuel-nitrogen transformation mechanism during co-firing of low-volatile carbon-based solid fuels with bituminous coal

Co-firing of low-volatile carbon-based solid fuels with bituminous coal is an encouraging approach to realize the large-scale clean utilization of pyrolyzed and gasified semi-cokes. However, the interactions on co-combustion characteristics between bituminous coal and semi-cokes have yet to be fully understood, while less work has aimed at fuel-nitrogen migration mechanisms without the influence of thermal-NO generation. Here, an experimental investigation of bituminous coal blended with pyrolyzed semi-coke and/or gasified semi-coke in O2/Ar atmosphere was performed focusing on NO yield, burnout performance and fuel-nitrogen migration mechanisms. The characterization and reactivity of bituminous coal and semi-cokes were revealed by thermalgravimetric analyzer, etc. The results indicate that the promotion effects of bituminous coal on combustion of semi-coke are enhanced with the enlarged difference between volatile contents of various samples. The char-nitrogen of semi-cokes are mainly converted to NO firstly, and then might be converted to HCN and N2 by pyrolysis products HCN, NH3 and CHi. The method of staged feeding could lessen the interactions between semi-cokes and bituminous coal, which could significantly inhibit the conversion of fuel-nitrogen to NO compared with the strategy of out-furnace blending. The present research can offer guidance for co-firing of low-volatile semi-cokes with bituminous coal in power stations.

Analysis of Coal Gasification Reactivity, Kinetics, and Mechanism with Iron-Based Catalyst from Coal Liquefaction.

In this work, the effect of an iron-based catalyst from coal liquefaction on coal gasification was studied. Two catalyst loading methods and three catalyst loading contents were taken into consideration. Besides, the carbon structure, surface morphology, and element distribution of coal char and gasified semi-char were investigated, and the interactions between the catalyst and internal minerals of coal were studied. The results showed that the coal char prepared by wet impregnation had higher reactivity than that prepared by a dry mixing method. From the perspective of improving the coal reactivity, the optimal addition method should be wet impregnation with a 2% catalyst. The model-free and model-fitting methods were applied to study the catalytic gasification kinetics. The iron-based catalyst would be broken during wet impregnation, and the catalyst fragments could stick to the surface of coal char, resulting in higher reactivity. The graphitization of char increased with the addition of the iron-based catalyst. This can imply that the carbon structure cannot effectively represent the gasification reactivity in the presence of the iron-based catalyst. The Iron-based catalyst can accelerate the gasification rate alone and can also provide higher catalytic activity with the internal minerals of coal.

Thermal behavior of Mongolian low-rank coals during pyrolysis

Low temperature pyrolysis behavior of four low-rank coals from the biggest deposits of Mongolia were studied using a fixed-bed reactor and Gray-King retort. The coal samples and resultant chars were analyzed using Fourier transform infrared spectroscopy (FTIR), 13C nuclear magnetic resonance (13C NMR) to explore the structure changes and thermogravimetric analyzer coupled with mass spectrometry (TG-MS) was used to investigate the pyrolysis reactivity of coal during thermal decomposition process. The results showed that the aliphatic structure in coal was significantly reduced and the carboxyl groups almost completely decomposed during pyrolysis process at 600 °C. The tar yield of Gray-king test is higher than that obtained from the fixed-bed reactor. The gaseous evolution measured by TG-MS during pyrolysis showed that the maximum peaks of evolution of CH4, H2, C2H2, C2H5, CO, and CO2 gases were well corresponded to the maximum rate of weight loss in the range 428–445 °C.

Experimental development of approaches to reduce the slagging and corrosive activity of salty coal

The problems of reducing the slagging ability and corrosiveness of coal with a high content of low-melting salts (the so-called salty coal (SC) in the processes of its combustion) are considered. Salty coal is considered to be the coal, the ash of which contains Na2O>2 %. The object of study is the salty coal of Donbass and ways of solving the problems of its use. The influence of low-melting salts on the formation of ash deposits and the development of corrosion of the metal surface during the combustion of salty coal from different fields was determined.A noticeable decrease in the slagging ability and corrosiveness of the test coal was noted during the removal of salts by water extraction. The composition of corrosive compounds (oxides Fe2O3, Fe3O4 and iron sulfide FeS) has been determined, formed during the combustion of native coal, and their absence in the case of desalinated coal.Artificial fuel mixtures produced from more reactive salty and conventional low reactive coal have been studied. To create a mixed fuel, long-flame salty coal (low stage of metamorphism) from the Northern Donbas and unsalty lean coal (high stage of metamorphism) from Kuzbas were used. A significant deviation (to 9 %) was noted for the release of ash during the combustion of mixtures from the additivity, indicating a chemical interaction between the mineral components of the mixture. The formation of new refractory mineral phases of ash (nepheline, ultramarine, combeite) during the combustion of composite fuel from coals of different metamorphism and salinity was established.The obtained results will be useful in the development of recommendations for the preparation of model fuel mixtures and their accident-free combustion in industrial boiler units. Experimental data on the determination of new mineral compounds in the case of composite fuels can be used to create a general theory of slagging in the combustion of salty coal of different origins

Secondary oxidation of crushed coal based on free radicals and active groups

The secondary oxidation of active groups was studied for crushed coals with various particle sizes. Then, free radicals of these crushed coals were comparatively analyzed after primary and secondary oxidation. The secondary oxidation of active structures on the coal surface was studied by combining the theories of free radicals and functional groups. The coal oxidation reactivity was stronger in Particle 5#. Secondary oxidation had a stronger effect on the reactivity of high-rank coal. A relatively stable aliphatic chain structure was formed during the secondary oxidation of low-rank Coal 1#. The aliphatic hydrocarbon structure was relatively active in high-rank Coals 2# and 3#. Primary oxidation can improve the oxidizability of the aliphatic hydrocarbon structure and facilitate its secondary oxidation process. The OH group in Coal 1# possesses stronger oxidation reactivity and was consumed rapidly during secondary oxidation. In contrast, the consumption and generation rate of the OH group tended to reach a balance in Coal 2# and Coal 3#. CO and CO groups in Coal 1# experienced relatively violent secondary oxidation. The two groups in Coal 2# and Coal 3# experienced relatively gentle secondary oxidation.

Study on Reactivity and Synergy Behavior of Cogasification between Biomass Char and Coal Char

The cogasification reactivity of Shenmu bituminous coal (SM) char and corn straw (CS) char in CO2 was evaluated by a fixed-bed/fluidized-bed reactor and mass spectrometry. The effects of blend rati...

Effect of coal rank, oxygen level and particle size on oxidation reactivity of typical Chinese coals

The changeable and deteriorating coal quality in air staging mode threats the safe and economic operation of the boiler. Non-isothermal TGA experiment was performed with varying heating rates (5, 10, 15 and 20 K/min) on six Chinese coals at 20 % O2 and on the lean coal with varying O2 (5%, 10 %, 20 % and 40 %). Isoconversional method is adopted to determine the activation energy (Eα) and frequency factor (Aα). Influence of particle size at varying oxygen concentration is compared between lean coal and bituminous coal at 20 K/min. The characteristic temperature (T0.1, T0.5 and T0.9) increases as the coal rank increases and oxygen level decreases. The slope of the line (characteristic temperature vs. O2) increases from 1.6 to 3.6 as the conversion increases from 0.1 to 0.9, which means the oxygen diffusion has an increasingly important role as the oxidation proceeds. As O2 decreases, the effect of particle size reduction on combustion improvement of low volatile coal is increasingly significant than that of high volatile coal. The smaller economic fineness of coal particle can be established for low volatile coal in low NOx combustion mode compared with that for high volatile coal.

A brief review of CO2 utilization for alkali carbonate gasification and biomass/coal co-gasification: Reactivity, products and process

Carbon gasification in molten alkali carbonate salts over CO2 is a process to produce gaseous fuels with mitigating the greenhouse effect. Alkali carbonate salts can facilitate Boudouard reaction, which is difficult to occur even at high reaction temperature if the catalyst is absent. Alkali carbonate allows easily carbon−CO2 contact by changing the reaction interface from gas-solid to gas-liquid. This review focused on CO2 utilization for alkali carbonate gasification and biomass/coal co-gasification. CO2 gasification of biomass and coal with the alkali carbonate salts enhances the carbon conversion rate and CO yield. Furthermore, alkali carbonate salts gasification of biomass and coal could reduce reaction temperature, activation energy, and tar formation in the product gas. The factors that influence the carbon conversion rate were discussed, such as reaction temperature, CO2 concentration, alkali carbonate addition, and char feeding mode. On the other hand, blending coal with biomass is considered as potential fuel base for CO2 co-gasification and displays an advantage for improving the coal char reactivity. The alkali and alkaline earth metals (AAEMs) present in biomass ash acted as a natural catalyst to increase the coal reactivity and CO production. The catalytic effect in the co-gasification enhances by raising the percentage of biomass in the blend. In contrast, the inhibiting effect in co-gasification could be attributed to silica and alumina reactions with AAEMs to form aluminosilicates complexes.

Experimental evaluation of the dry coal deshaling by pneumatic vibrating FGX separator on the CO2 gasification process

SummaryThe dry deshaling process using the FGX method is a modern technique of coal quality enhancing without the use of water. The positive effect of this operation is the increase of calorific value and the removal of contaminants such as sulphur, chlorine, mercury and other heavy metals along with the separated gangue. However, elements such as sodium, potassium or calcium, considered as catalysts in the gasification process, are also partially removed. Consequently, the global effect of the dry deshaling on coal reactivity, being a key parameter determining its suitability for the gasification, is hard to predict. This study aims to determine the impact of dry deshaling of coal by FGX separator on its reactivity. Using TG analysis, 10 samples of coals from Polish mines, before and after the dry deshaling at commercially working FGX separator, were gasified in CO2 atmosphere under non‐isothermal conditions. The effect of deshaling on the basic properties of coals, ash composition and the content of contaminants such as sulphur and mercury, was also determined. The results showed slightly lower reactivity of clean coals. The values of kinetic parameters of gasification reaction were similar, or an increase in the activation energy value was compensated by the higher value of the pre‐exponential factor. The basic properties of coals and the content of sulphur and mercury have changed after the deshaling process, and the magnitude of these changes differed significantly among the samples analysed. The changes in the ash composition were insignificant. These results suggest that FGX technology may be an attractive way of coal preparing for the gasification, resulting in a reduction of by‐products and contaminants without adverse effect on coals reactivity.

Improving the energy efficiency of flaring in pulverized coal-fired boilers at TPPs of the Siberian Federal District by mechanochemical activation of coal

The heat power system of the Siberian Federal District is almost coal-based: energy generation at coal-fired TPPs makes up 87.2%. But due to low reactivity of coals fed to the pulverized coal boilers, expensive high-reactive fuel oil is used to ignite and light the pulverized coal flame, which is a technological and economic problem. To increase the energy efficiency of flame burning of coal, an increase in their reactivity due to mechanochemical activation is considered using the technology developed at IT SB RAS. For several different coals, experimental studies on burning were carried out at a 5 MW pilot setup using mechanochemical activation of coal. These studies showed reliable ignition and stable access to the autothermal coal combustion, which indicates an increase in coal reactivity and the ability to replace fuel oil with coal.

Structural changes and sodium species redistribution of a typical sodium-rich coal during thermal dissolution with aromatic solvents

Sodium-rich coals are widely distributed all over the world and they are considered as suitable materials for thermal dissolution of coals. In this work, to better understand structural changes of sodium-rich coals during thermal dissolution process, TG-FTIR was used to characterize weight loss and evolution of gases released from pyrolysis of a typical sodium-rich coal and extraction residues. Additionally, the occurrence mode of sodium species was determined by sequential chemical extraction method in order to examine sodium species redistribution. The results show that the organic oxygen content of coal was reduced by thermal treatment, which implied that thermal dissolution tended to remove oxygen and upgrade coal. TG-FTIR analysis also revealed that the CO2 content released from pyrolysis of extraction residues was on a lower level than that released from pyrolysis of raw coal. During thermal dissolution, the chemical structures of raw coal got more compact. It suggests that coal reactivity toward further thermal conversion would be weakened. In pyrolysis process, weight loss of samples treated by tetralin was higher compared with those treated by 1-methylnaphthalene, especially at 300 °C. This difference proves that raw coal had been pre-hydrogenated by tetralin even as low as 200–300 °C. Furthermore, tar produced in pyrolysis of samples treated by tetralin was deoxygenated by the residual tetralin and it leaded to decrease of tar-O structures, which influenced release of CO. Besides, it was observed that most sodium species were reserved in the extraction residue, as retention ratios were significantly high. At the same temperature, more exchangeable sodium species were promoted to be transformed into water-soluble ones when tetralin was used, which demonstrated that tetralin had better promotive effects on decomposition of –COO−. Along with decomposition of –COO−, exchangeable sodium species were detached from coal matrix and transformed into other chemical forms.

The Impact of Coal’s Petrographic Composition on Its Suitability for the Gasification Process: The Example of Polish Deposits

In this paper, we discuss the impact of the rank of coal, petrographic composition, and physico-chemical coal properties on the release and composition of syngas during coal gasification in a CO2 atmosphere. This study used humic coals (parabituminous to anthracite) and lithotypes (bright coal and dull coal). Gasification was performed at temperatures between 600 and 1100 °C. It was found that the gas release depends on the temperature and rank of coal, and the reactivity increases with the increasing rank of coal. It was shown that the coal lithotype does not affect the gas composition or the process. Until 900 °C, the most intense processes were observed for higher rank coals. Above 1000 °C, the most reactive coals had a vitrinite reflectance of 0.5–0.6%. It was confirmed that the gasification of low-rank coal should be performed at temperatures above 1000 °C, and the reactivity of coal depends on the petrographic composition and physico-chemical features. It was shown that inertinite has a negative impact on the H2 content; at 950 °C, the increase in H2 depends on the rank of coal and vitrinite content. The physicochemical properties of coal rely on the content of maceral groups and the rank of coal. An improved understanding these relationships will allow the optimal selection of coal for gasification.

Effect of ash removal on structure and pyrolysis/gasification reactivity of a Chinese bituminous coal

In this study, the effect of ash removal on Shenfu bituminous coal was investigated. The coal was pretreated by hydrofluoric acid (HF) pickling, and the raw/pretreated coal chars were prepared at 900 °C in a fixed bed reactor. The structure of coal and char were detected by Fourier transform infrared (FTIR) and Raman spectroscopy. The reactivity was tested in a thermogravimetric analyzer, including coal pyrolysis and char gasification. The reaction kinetics was analyzed through the Coats–Redfern method, master plots, the model-free and model-fitting method. The results show that the HF pickling can remove silicon from coal efficiently, and the macromolecular framework of coal is quite stable according to FTIR. The Raman parameters imply some carbonaceous structure on coal surface changed. For slow pyrolysis of coal, the effect of heating rate is considered. The changes of pyrolysis characteristics and kinetics are insignificant. For char gasification, the reactivity under isothermal and non-isothermal condition are discussed with an emphasis in different residence time of devolatilization process. In kinetic control region (low temperature), the activation energy (Ea) is very close (about 240 kJ/mol) for all chars. With the temperature increases, the reactivity of raw coal char is more easily suffered by diffusion. The random pore model is more suitable for the ash-free coal char, and the char with long residence time has a larger value of structural parameter ψ and smaller value of pre-exponential factor A. The Ea calculated by model-fitting and model-free method were in good agreement.

Investigation of the effect of reactor size on spontaneous combustion properties of coals with different coalification degrees

The spontaneous combustion of coals is a natural phenomenon that causes serious losses in the mining sector as well as environmental problems as a result of greenhouse gases released during spontaneous combustion of coal. In this study, for the first time, the effect of coal reactor size on the spontaneous combustion tendency parameters (crossing point temperature (CPT), average heating rate (AHR), and liability index (IFCC)) of the coal samples having two different ranks (bituminous coal and lignite) have been searched by the use of crossing point method. According to the results obtained from the reactors manufactured in three different sizes (R-1, R-2, and R-3), ignition temperatures of coal samples were 161–172 °C for bituminous coal and 143–175 °C for lignite; average temperature rise was 0.92–1.45 °C min−1 for bituminous coal and 1.47–2.98 °C min−1 for lignite; spontaneous combustion risk index was determined in the range of 5.61–8.88 min−1 for bituminous coal and 8.39–20.79 min−1 for lignite. As the reactor size decreased, spontaneous combustion tendencies of lignite samples increased, and no significant change was observed in bituminous coal samples. It has been found that the effect of reactor size on the spontaneous combustion of coal gives significant results in low-rank coals but has no effect on high-rank coals.