Raw Coal Research Articles

Pyrolysis kinetics and reaction mechanisms of coal slime for cleaner energy

Pyrolysis has been demonstrated to be promising for the conversion of coal slime (CS) to fuels and high value-added chemicals, which could achieve resource utilization of CS. For the in-depth understanding of the pyrolysis mechanism of CS, in this study, the pyrolysis kinetics and pyrolytic products distribution were investigated by TG-FTIR and Py-GC/MS and compared with those of raw coal (RC). The results demonstrated that the pyrolysis of CS comprises three reaction stages, with the decomposition of volatiles as the main stage (180–680 °C). Using the master plot method, the pyrolysis kinetic mechanism of RC and CS were predicted to be f(α) = (1 – α)4.6 and f(α) = (1 – α)5.4, respectively. The enrichment of inorganic minerals significantly increased the order of the CS pyrolysis reaction, indicating that minerals play an important role in the pyrolysis chemical reaction. Compared to that of RC, the alkene content of CS pyrolysis products decreased significantly, while the contents of CO2 and aromatics increased, indicating that the minerals in CS promote the polymerization and cyclization of alkene, as well as the formation of macrocyclic aromatic hydrocarbons in CS tar. Those consequences also determined that inorganic matter in CS considerably affects pyrolysis, which in turn affects the whole pyrolysis process and tar quality. Therefore, in this study, theoretical and practical guidance on the efficient thermochemical conversion and utilization of CS is provided.

Mechanism of aliphatic hydrogen on the caking property of 1/3 coking coal during rapid preheating

It is of significance to reveal the mechanism of rapid preheating to enhance the caking property of coking coal and expand high-quality coking coal resources. In this study, a 1/3 coking coal was preheated at two low and one rapid rates, then raw coal and heat-treated samples were comprehensively analyzed. The results revealed that the aliphatic H in coal formed volatiles at the early stage of pyrolysis (≤ 380 °C) under low-rate preheating. In the thermoplastic stage, the bonding phase was confirmed to be formed by a part of aliphatic H. The decreases amount of volatile and hydrogen contents and H/C and O/C atomic ratios of rapidly preheated coals were less than those of low-rate preheated coals, indicating that its thermal decomposition reaction degree was lower. This was considered to be the basis for keeping G values of rapidly preheated coals from decreasing. The aliphatic H concentration and CH2/CH3 ratios of rapidly preheated coals were higher and lower than those of raw coal respectively, which contributed to the fact that their G values were basically close to or higher (∼349–401 °C) than that of raw coal. Rapid preheating weakened the consumption of aliphatic H by volatiles formed during low-rate heating. Moreover, it forced weak bonds in the molecular of coal to break, such as bridge bonds and intermolecular forces that connect structural units. The breaking of bridge bonds between alkyl side chains or cycloalkanes and aromatic groups enhanced hydrogen supply. The weakening of intermolecular force resulted in loose macromolecular network structure and enhanced molecular dynamic properties. These evolutions of coal molecular structure strengthened the combination of hydrogen and free radicals and generated more bonding phases. Finally, a mechanism which provided a new explanation for the rapid preheating to alter the molecules structure and enhance the caking property of coking coal was proposed, and it will be significant valuable to enlarge the utilization of weakly-caking coal in coking process.

Control mechanism of small organic molecules on methane adsorption capacity of coal

Small organic molecules have a significant influence on methane adsorption of different rank coals. To explore the control mechanism of small organic molecules on methane adsorption capacity of coal, in this paper, based on the relative content of small organic molecules in different rank coals obtained by extraction experiment, the complex models of raw and residual coal with different ranks were constructed. The influence of small organic molecules on the methane adsorption of coal was analyzed by molecular simulation calculation. The control mechanism of small organic molecules on the methane adsorption capacity of coal was further discussed. The results showed that for the low-rank coal with high content of small organic molecules, small organic molecules mainly play the role of blocking and separating so that separate the larger coal pores into more micropores, thereby the methane adsorption capacity of coal pores is increased. For the middle-rank coal with the same high content of small organic molecules, micropores are closed by small organic molecules, so that its methane adsorption capacity is reduced. For the high-rank coal, due to its low content of small organic molecules, few pores are blocked or closed, that the existence of small organic molecules has no significant effect on its methane adsorption capacity. The research results help to reveal the microscopic mechanism of methane adsorption of coal, which has great significance to the development of coalbed seam gas and mine gas disaster prevention.

Thermal conversion, kinetics, thermodynamics and empirical optimization of combustion performance of coal gasification fine ash in oxygen-enriched atmosphere

Coal gasification fine ash (CGFA) has the characteristics of high carbon content, fine-grained, and sizeable annual output, causing environmental pollution and energy waste. Oxygen-enriched combustion is an effective way to solve the CGFA problem. This paper used thermogravimetric to study the effect of thermal conversion, kinetic analysis, and empirical optimization on the combustion process of CGFA in oxygen-enriched atmosphere. The results show that oxygen-enriched combustion of raw coal and CGFA improve ignition, burnout performance, combustion stability, and reduced activation energy. As the O2 concentration increased from 30 % to 70 %, the activation energy of CGFA increased, but the improvement in combustion performance was not significant. A two-factor analysis of variance was carried out using a generalized linear model to obtain the effects of oxygen concentration, heating rate, and temperature variations on the combustion properties of CGFA, showing that atmosphere, heating rate and temperature variations all had significant effects on the combustion performance of CGFA, but atmosphere and heating rate had a more substantial impact on the combustion performance of CGFA. The combustion performance of raw coal under 30 %O2/70 %Ar atmosphere was better than that of CGFA, and the oxygen-enriched combustion (30 %O2/70 %Ar) performance of CGFA was comparable to that of CGFA blended with 60 % bituminous coal in air atmosphere. A kinetic model was developed to describe the oxygen-enriched combustion process (R2 > 0.92). The oxygen-enriched conditions of CGFA allow for efficient and clean combustion.

A Study on China coal Price forecasting based on CEEMDAN-GWO-CatBoost hybrid forecasting model under Carbon Neutral Target

The emission peak and carbon neutrality targets pose a great challenge to carbon emission reduction in the coal industry, and the coal industry will face an all-around deep adjustment. The forecast of coal price is crucial for reducing carbon emissions in the coal industry in an orderly manner under the premise of ensuring national energy security. The volatility and instability of coal prices are a result of multiple influencing factors, making it very difficult to make accurate predictions of coal price changes. We propose in this paper an innovative hybrid forecasting method (CEEMDAN-GWO-CatBoost) for forecasting coal price indexes by combining machine learning models, feature selections, data decomposition, and model interpretation. By combining high forecasting accuracy with good interpretability, this method fills a gap in the field of coal price forecasting. Initially, we examine the factors that influence coal prices from five angles: Supply, demand, macroeconomic factors, freight costs, and substitutes; and we employ Spearman correlation analysis to reduce the complexity of the attribute set and devise a coal price forecasting index system. Secondly, the CEEMDAN method is used to decompose the raw coal price index data into seven intrinsic modal functions and one residual term in order to weaken the volatility of the data caused by complex factors. Next, the CatBoost model hyperparameters are optimized using the Grey Wolf Optimizer algorithm, while the coal price data is fed into the combined forecasting model. Lastly, the SHAP interpretation method is introduced for studying the important indicators affecting coal prices. The experimental results show that the combined CEEMDAN-GWO-CatBoost forecasting model proposed in this paper has significantly better forecasting performance than other comparative models, and the SHAP method employed in this study identifies the macroeconomic environment, freight costs, and coal import volume as significant factors affecting coal prices. As part of the contribution of this paper, specific recommendations are made to the government regarding the formulation of a regulatory policy for the coal industry in the context of carbon neutrality based on the findings of this research.

Multiscale thermal behavioral characterization of spontaneous combustion of pre-oxidized coal with different air exposure time

To comprehensively investigate the multi-scale characteristics of coal spontaneous combustion, in situ infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) were utilized. The samples were pre-oxidized at 70 °C for 2, 4, 6, and 8 h (the raw coal and above samples were named R0, R2, R4, R6, and R8), then the secondary oxidation experiments were carried out. The results showed that the contents of –OH, –C=O, and –C=C- increased with the pre-oxidation time. The aromatic structure of pre-oxidized coal is more susceptible to oxygen erosion during secondary oxidation. The migration and transformation of –OH, –C=O are the underlying causes of water evaporation and gas desorption during low-temperature oxidation. The spontaneous combustion behaviors in stage I and stage II have different reaction mechanisms and kinetic characteristics. In light of Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS), the apparent activation energy of stage I is R0>R2>R4>R6>R8, and stage II is R6>R8>R2>R4>R0. Long pre-oxidation time in stage I will inhibit coal spontaneous combustion, while the opposite is true in stage II. By the Pearson correlation analysis, –OH is the key structure affecting the low-temperature endothermic stage of coal, revealing the correlation between the multi-scale thermal behavior of pre-oxidized coal spontaneous combustion.

Comparative Study on the Seepage Characteristics of Gas-Containing Briquette and Raw Coal in Complete Stress-Strain Process.

In this study, triaxial compression and seepage tests were conducted on briquette and raw coal samples using a coal rock mechanics-seepage triaxial test system (TAWD-2000) to obtain the complete stress–strain curves of the two samples under certain conditions. On this basis, the different damage forms of the two coal samples and the effect of their deformation and damage on their permeability were analyzed from the perspective of fine-scale damage mechanics. Moreover, the sensitivity of permeability to external variables and the suddenness of coal and gas outbursts were discussed. The results show that the compressive strength of raw coal is 27.1 MPa and the compressive strength of briquette is 17.3 MPa, the complete stress–strain curves of the two coal samples can be divided into four stages and show a good correspondence to the permeability–axial strain curves. Since briquette and raw coal have different structural properties, they present different damage mechanisms under load, thus showing great diversity in the permeability-axial strain curve, especially in the damage stage. The deformation affects the seepage characteristics of briquette mainly in the latter two stages, while it affects raw coal throughout the test. The four stages of the complete stress–strain seepage test of raw coal can well explain the four stages of coal and gas outburst process, i.e., preparation, initiation, development, and termination. Hence, the law of coal permeability to gas variation can be utilized for the coal and gas outburst prediction and forecast. The research results are valuable for exploring the real law of gas migration in coal seams.

Study on Permeability and Flame Retardancy of Coal Aerosol Atomized by Ultrasonic Wave

In order to analyze the permeability and particle size distribution of atomized aerosol in different coal particle sizes and to explore the inhibition performance of an atomized inhibitor in different atomization times and positions, the following conclusions are obtained by ultrasonic atomization device: The results of permeability experiment show that the particle size of aerosol decreases with the increase of atomization frequency. In addition, with the particle size of the coal sample decreasing from 5 cm to 0.6 cm, the penetration ratio of aerosol in coal decreases from 64.2% to 15.5% due to the interception of large particle size water mist by dense coal. The aerosol with a particle size of 2~4 μm has better permeability in crushed coal. The flame retardant experiment shows that compared with the raw coal, the aliphatic hydrocarbon of the atomized coal decreases by 75.9%, the aromatic hydrocarbon decreases by 57.5%, and the flame retardant effect is good. Meanwhile, the extension of solution atomization time will further enhance the flame retardant ability of ultrasonic atomized aerosol.

Deformation and seepage properties of raw coal during the simultaneous loading and unloading of axial stress and confining pressure

The mining of coal resources causes the stress on the front and both sides of the working face to be in a complex loading and unloading process, and the gas contained in the coal body migrates, which can cause gasdynamic disasters in severe cases. Therefore, this study designed a three-dimensional stress path based on the in situ stress distribution characteristics of the No. 2461 working face and conducted the experiment on the deformation and seepage characteristics of raw coal under simultaneous unloading (loading) axial stress and loading (unloading) confining pressure. It is considered that in the process of loading axial stress, the volumetric strain increases gradually with the increase in effective stress, and the permeability decreases exponentially with the increase in effective stress. In the process of simultaneous unloading axial stress and loading confining pressure, the volumetric strain of coal sample increases with the increase in effective stress, and the permeability decreases exponentially with the increase in effective stress. However, the processes of simultaneous loading axial stress and unloading confining pressure are opposite, and the variation curves of the two do not overlap, indicating that the coal sample undergoes elastic-plastic deformation after cyclic loading, the permeability cannot be restored to the initial state, and the volumetric strain further expands. The permeability damage rate and maximum permeability damage rate are used to evaluate the recovery degree and reduction range of coal samples' permeability. The volume expansion rate and maximum volume expansion rate are used to evaluate the expansion degree and increase range of coal samples' volume.

Study on the variations of extractable polycyclic aromatic hydrocarbons in lignite and semi-coke

Low-temperature pyrolysis is one of the most important technologies for the clean and efficient utilization of lignite. Semi-coke, the solid product of this process, has been widely used as both an industrial and household clean fuel. In this study, three lignites from China (EE), Indonesia (YN), and Mongolia (WM) and their semi-cokes obtained through pyrolysis at 550 °C were treated with dichloromethane, and the extracted polycyclic aromatic hydrocarbons (PAHs) were analyzed using a gas chromatograph/mass spectrometer. The results showed that the total concentrations of PAHs in EE, YN, and WM were 125000, 4400, and 5800 ng/g, respectively, and those in their semi-cokes were 4700, 10000, and 5500 ng/g, respectively. The concentrations of alkylated PAHs (aPAHs) were higher than those of parent 16 US EPA priority PAHs (16 PAHs) in lignites, while parent 16 PAHs were more abundant in semi-cokes. Both lignites and semi-cokes were dominated by 2-ring PAHs (naphthalene and alkylated naphthalenes), followed by 3- and 4-ring PAHs. The proportion of 2-ring PAHs in the semi-cokes increased with respect to that in the lignites, and the degree of alkylation of the PAHs decreased. This suggested that the extractable PAHs changed to smaller parent PAHs during the low-temperature pyrolysis. The concentration and composition of PAHs in the mobile phase of the semi-coke are an important part of its structural characteristics and are closely related to the macromolecular structure of raw coal. Thus, this study on PAHs in semi-coke provides relevant information on the structural evolution of coal during pyrolysis and further improves our understanding of clean fuel.

Pollutant emission and attrition performance of low calorific blended coals during co-combustion in fluidized bed

The co-combustion technology in the fluidized bed is usually used for the utilization of low-rank coals. The pollutant emission and attrition of coal particles, which are subject to bed temperature, excess air coefficient, mixture ratio and particle size, cannot be ignored. In this work, a pyrolysis-combustion-fragmentation method was presented to study the attrition and the quantitative fragmentation indexes of different coal blends, followed by the experimental investigation of the blended coal's gaseous pollutant emissions in a fluidized bed combustion reactor. As demonstrated by the experimental results, the emission of N2O decreases with increasing the bed temperature from 800 to 850 °C, while attrition rate increases with the bed temperature. Increasing the excess air coefficient is conducive to reducing CO2 and SO2. Considering the low flue gas emission and the attrition behavior, the mixed ratio of raw coal, coal gangue and coal slime at 20:60:20 may be an appropriate choice for their co-combustion in a fluidized bed reactor. The main reason for the fragmentation of blended-coal particles is the primary fragmentation. This study is useful for optimizing the condition of blended coal’s combustion in CFB boiler.

Insight into the low-temperature oxidation behavior of coal undergone heating treatment in N2

Heat treatment of coal under nitrogen is an effective technology to reduce water content and upgrade lignite, but the subsequent oxidation behavior and spontaneous combustion characteristic of dried coal are unclear, possibly posing a threat to lignite transportation and storage. In this study, raw coal and dried coal at different temperatures were selected and oxidation parameters were first analyzed by temperature-programmed experiment. The carbon oxides emissions and oxygen consumption of dried coals are obviously higher than that of raw coal, and increase with rising dehydration temperature, indicating coal after drying is more prone to oxidation and spontaneous combustion. Then, the physical structure, chemical groups and free radicals of coal were determined by nitrogen adsorption, FTIR and ESR technology. During drying process, gas release and water evaporation increase pore diameter and pore volume of coal, thus improving O2 transport capacity and oxidation rate. The hydroxyl content in coal decreases, but the CO content increases due to the polycondensation and dehydration reaction of hydroxyl groups after high-temperature drying. Meanwhile, carbonyl compounds and carboxyl groups undergo thermal decomposition and generate free radical active sites on the coal surface. The subsequent oxidation of accumulated free radicals not just release large amounts of carbon oxides, but also contribute to the formation of oxygen-containing groups inside coal with the rise of temperature, accelerating coal-oxygen reaction. Therefore, it is proposed that heat treatment expands physical channels, increases the content of CO groups, and enhances the content of active sites, which leads to vigorous oxidation of dried coal and even uncontrolled coal fires.

Experimental and Quantum Chemical Study on the Inhibition Characteristics of Glutathione to Coal Oxidation at Low Temperature.

In response to the frequent occurrence of coal spontaneous combustion accidents, this paper proposes to use glutathione (GSH) as an inhibitor to inhibit the coal oxidation at low temperature. Based on the gas production of oxidation, thermogravimetric analysis, electron spin resonance, and in situ Fourier infrared transform spectroscopy experiments, it is known that GSH has a good inhibiting effect on lignite, long-flame coal, and fatty coal. The optimal action temperature of GSH is 60–150 °C, which can effectively slow down the weight loss and exothermic process and reduce the gas production of CO and CO2. Compared with the raw coal, the GSH-treated coal samples possess higher crossing point temperature and lower reactive group content. Subsequently, quantum chemical calculations are performed using density functional theory. The results demonstrate that the inhibiting mechanism of GSH is inerting the reactive radicals in coal and converting them into more stable compounds. Meanwhile, the activation energy of the reaction between GSH and each reactive radical is small, and all of them can occur at room temperature and pressure. This study lays the groundwork for future development of inhibitors.

Comparison of oxy-fuel preheated combustion characteristics of high- and low- volatility carbon-based fuels

A comparative evaluation and analysis study of combustion characteristics, main pollutant emission behaviors, and fuel-N conversion paths of high- and low- volatility carbon-based fuels are conducted under O2/CO2 atmospheres in a preheating combustion reactor, i.e., CFB. Compared with conventional air combustion, the oxy combustion with high oxygen concentrations from 21% to 27.3% is studied. The studies show that the net calorific value of the preheated coal gas of semi-coke in O2/CO2 is up to 2 times that in air, and the net calorific value of the preheated coal gas of bituminous coal in O2/CO2 is up to 1.3 times that in air. During the preheating process of semi-coke, CH4 and HCN occupy a discreet proportions of preheating coal gas. Fuel characteristics and preheating atmosphere have no effect on the particle size of preheated coal char. Under an O2/CO2 atmosphere, the highest BET surface area of preheated coal char of bituminous coal is up to 30 times that of raw coal and that of semi-coke is up to 40 times that of raw coal. The carbon structure modification ratio of preheated coal char during the preheating process of bituminous coal is larger than that during the preheating process of semi-coke. With the increase in the concentration of oxygen in the primary stream, the NO emission decreased during the bituminous coal combustion process but increased during the semi-coke combustion process. In addition, the main generation path of NO in the reduction zone in O2/CO2 is NH3-NH2-NH-HNO-NO. In O2/CO2, the total promotion effect of reactions on NO formation in the combustion process of semi-coke preheated fuel is larger than that of bituminous coal.

Study on the microstructure of the symbiosis of coal-based graphene and coal-based graphene quantum dots: preparation and characterization

Coal-based graphene sheets (GS) and coal-based graphene quantum dots (GQDs) are usually prepared separately. In this paper, symbiosis of coal-based GS and coal-based GQDs was successfully prepared with our proposed preparation method by using three raw coals with different reflectance (collected from Qinshui coalfield, Shanxi Province) as carbon sources. The results showed that coal-based GS and coal-based GQDs can exist stably in the symbiosis and are distributed in different layers, and the GQDs are freely distributed between layers of GS. The average number of GS (N ave) in the three symbiosis is about 7 and the average interlayer spacing (d 002) is about 0.3887 nm. The average diameter of GQDs in the three symbiosis is about 4.255 nm and the average d 002 is about 0.230 nm. The average N ave of the three symbiosis was about 3 and the average d 002 is about 0.361 nm. The morphology and crystal parameters of symbiosis is more similar to that of graphene, the elements are only carbon and oxygen. In the prepared symbiosis, the higher the reflectance of raw coal, the smoother the lattice skeleton and the less vortex-layer structure of GS, and the larger the diameter and the denser the six membered ring of GQDs. The C and O functional groups of the prepared symbionts are similar. The higher the reflectance of coal, the higher the content of C–C/C=C. Under ultraviolet light, the prepared products all emit blue, and the higher the reflectance of coal, the higher the ultraviolet absorption, and the stronger the fluorescence intensity.

Spontaneous combustion characteristics and mechanism of water-immersed and air-dried brown coal

ABSTRACT To reveal the influential laws and mechanism of water immersion on the spontaneous combustion traits of lignite in the low-temperature oxidation stage. Raw coal (HL), water-immersed for 30 days (HL-S30), 90 days (HL-S90) and 150 days (HL-S150), were taken as the research objects. The pore evolution characteristics and functional group migration law of coal were explored by Scanning Electron Microscopy (SEM), Low-temperature Nitrogen Adsorption Test (LNAT) and Fourier Transform Infrared Spectroscopy (FTIR). The low-temperature oxidation procedure was simulated by Programmed Heating Test (PHT), and the gas release characteristics, exothermicity and ultimate spontaneous combustion parameters of the relic coal were compared. These findings demonstrated that the discriminant indexes of spontaneous combustion tendency of four specimens change periodically with temperature, but generally the spontaneous combustion intensity of HL-S30 and HL-S150 is greater than HL, and HL-S90 is the least. The micro reasons for increasing oxidation activity of HL-S30 and HL-S150 are as follows: the number of micropores of water-immersed coal is almost 3 ~ 4 times that of HL, and the pores expanded with immersion which helps to enhance the adsorption of oxygen; the content of broken aromatic hydrocarbon -C = C- reached 50%, the total amount of hydroxy -OH decreased to 2/3 of HL, while the content of carboxyl -COOH and ether-oxygen bond -C-O- of active group doubled which are beneficial to accelerate the process of free radical chain reaction, and promote the gas release and the temperature of residual coal to reach the ignition point. The research findings enrich the basic theory of spontaneous combustion traits of water-impregnated coal, which help reduce the fire accidents and hidden dangers in actual production.

Research on chemical resistance characteristics of water-immersed coal with different metamorphic degrees

In order to reduce the risk of spontaneous combustion of coal left after long-term flooding in the goaf of the mine, in this paper, the inhibitory properties of different inhibitors on two kinds of water-immersed coals with different metamorphic degrees were studied in depth. The experiment selected Pingzhuang brown coal and Shaqu coking coal as research objects. The raw coal and water-immersed coal samples were compared and analyzed by thermogravimetric experiment method and Fourier transform infrared spectroscopy experiment method. The study showed that the activation temperature of brown coal and coking coal decreased by 7.91 and 2.25 °C respectively, and the activation energy decreased by 43.18 kJ/mol and 20.58 kJ/mol respectively. The natural tendency of coal was enhanced after water immersion, and water immersion had a greater impact on low-metamorphic brown coal. After adding four kinds of inhibitors, MgCl2, TEMPO, TPPI and PA to the two water-immersed coals, it was found that TPPI could significantly reduce the heat release rate of water-immersed brown coal, and the reduction value was 10.49 W/mg. The dry cracking temperature of water-immersed brown coal increased by 11.75 °C, and PA greatly increased the combustion activation energy of water-immersed coking coal by 25.77 kJ/mol. Meanwhile, it was found from the microscopic active groups that TTPI increased the content of water-immersed brown coal ether bonds by 4.84%. The absorption peak intensity of oxygen-containing functional groups such as C=O was significantly weakened. Similarly, PA also produced a large number of stable ethers in water-immersed coking coal, whose content increased by 5.21%, and the hydroxyl content decreased most significantly. The decomposition of TPPI into phosphoric acid after heating can inhibit the growth of active groups such as a large number of oxygen-containing functional groups in the water-immersed brown coal, thereby reducing the risk of spontaneous combustion. As a metal chelator, PA can reduce the catalytic effect of metal ions in water-immersed coking coal with fewer active groups, and inhibit coal spontaneous combustion by generating stable metal complexes to increase activation energy. This indicated that TTPI had the best inhibitory effect on water-immersed brown coal, while PA was more suitable for water-immersed coking coal.

Sulfur fixation for raw coal with combined microwave irradiation and ultrafine Ca(OH)2 method

Sulfur fixation technology for raw coal can reduce SO2 emission during coal combustion. A method combining microwave irradiation and ultrafine Ca(OH)2 slurry was proposed, where raw coal was pulverized to mix with the slurry, and then treated by microwave irradiation. For anthracite coal (sulfur content = 1.90 wt%) with particle size distributed in the range of 75–180 μm, the treatment fixed 67.98 % of the sulfur in coal ash. Results prove that there is a synergy between microwave irradiation and ultrafine Ca(OH)2 for coal pretreatment, and finer coal particles had higher fixation rate. Under the same treatment condition, the sulfur fixation rate was increased to 74.68 % by adding 1 mol/L NaOH. Applying more NaOH or less Ca(OH)2 both made the fixation rate decrease. The combined treatment was also applied to bituminous coal (sulfur content = 0.79 wt%), and a fixation rate of 87.93 % was obtained. A continuous pretreatment system was developed for practical utilization of the method. Coal treated by the system was made into honeycomb briquette and burnt in a domestic coal stove, where a sulfur fixation rate of 74.97 % was obtained, compared to that of 4.55 % from untreated coal briquette.

Experimental study on prevention of spontaneous combustion of coal by ionic surfactant solution injection in coal seam

The inhibitor liquid used in the fire-extinguishing technology for preventing spontaneous coal combustion is difficult to inject due to the hydrophobicity of coal. To improve the injection effect and enhance the inhibitory properties, a surfactant (SDS) is added to the inhibitor to prepare the inhibitor solution. The flow law of inhibitor solution in coal and the prevention and control mechanism of coal spontaneous combustion were studied by various experimental methods, the extents of inhibitory effects on spontaneous coal combustion are found to differ due to the variations in properties of these liquids. Among them, the SDS and NH4H2PO4 corrosion inhibitor solution configured according to 1:1, the surface tension is significantly lower than that of pure raw coal by 62%, and the heating rate is lower than that of pure raw coal by 39%. The research results show that under the same conditions, the wetting and inhibiting effect of the corrosion inhibitor solution containing SDS on the coal is significantly improved, which provides a theoretical basis for the improvement of the inhibitor's fire-fighting technology.

Enhanced coalbed methane recovery by the modification of coal reservoir under the supercritical CO2 extraction and anaerobic digestion

Coal reservoir is considered as the ideal site for coalbed methane (CBM) occurrence, biomethanation and CO2 sequestration. Four coal samples with different ranks were selected to conduct supercritical CO2 (Sc-CO2) extraction and anaerobic digestion (AD). X-ray photoelectron spectroscopy, mercury intrusion, and isothermal adsorption experiments were performed on the raw coal samples and the remaining coal samples after Sc-CO2 and Sc–CO2–AD to analyze the mechanism of enhanced CBM recovery by reservoir modification. The results showed that coal pore volume and porosity are greatly increased after Sc-CO2 extraction. Meanwhile, the reduction of the oxygen-containing functional groups and the micropore specific surface area determined the weakening of the adsorption capacity of coal. After Sc–CO2–AD, the macropore volume and porosity of coal continued to increase, and the permeability of coal was effectively improved. And the oxygen-containing functional groups and micropore specific surface area continued to decrease, which determined that the adsorption capacity of coal for methane continued to decrease. Sc-CO2 and AD can mutually promote the modification of coal reservoirs. These effects are quite favorable for gas desorption, migration and production during CBM exploitation, which provide powerful supports for CO2 emission reduction, CO2 geological sequestration and CBM production enhancement.