Dependence of carbon dioxide sorption on the petrographic composition of bituminous coals from the Czech part of the Upper Silesian Basin, Czech Republic

The effect of the rank and of the maceral composition of bituminous coal on carbon dioxide sorption capacity was studied on the basis of samples from two coal mines (Darkov, ?SM) from the Czech part of the Upper Silesian Basin. The samples from the two mines cover a small but very significant section of coalification within the transition zone between high-volatile bituminous A coal and medium-volatile bituminous coal, where porosity and sorption properties pass through their minima. The coal porous system was characterized by the micropore volume evaluated using the sorption isotherm of carbon dioxide and the volumes of meso-, macro- and coarse pores were determined by high-pressure mercury porosimetry. The micropore fraction in the coal porous system ranged between 53% and 75%. It was particularly high in coals with high vitrinite content, namely collotelinite, and also in coals with high inertinite content. The carbon dioxide sorption capacity was determined from the carbon dioxide sorption isotherms measured using a gravimetric sorption analyzer at 298 K until a relative pressure of 0.015 p/p_s, and was interpreted by characteristic parameters of the Dubinin and Langmuir equations. It was found that the adsorbed amount of CO₂ in the ?SM coal increases with the content of vitrinite and collotelinite, whereas no increase or only a slight increase was observed for the Darkov coal. The tendency of adsorption capacity to depend on maceral composition, and also to some extent on coalification, observed for the ?SM coal, may be related to higher microporosity due to the coalification process or oxidative processes leading to the formation of pseudovitrinite.     

The influence of sorption processes on gas stresses leading to the coal and gas outburst in the laboratory conditions

The seepage of gas through coal and in particular through coal briquettes has already been a quite well known phenomenon. This also refers to its influence on certain initial conditions of coal and gas outbursts such as threshold values of so called gas stresses. In turn a detailed research on the influence of sorption processes on these conditions has been neglected until now. There is, however, an opinion that (fast) desorption is crucial for rock and gas outburst initiation and continuance, although many examples contradict such a thesis. The goal of the studies described here was to investigate a part of this ambiguity, namely the influence of sorption processes on gas stresses leading to coal and gas outbursts carried out in laboratory conditions.A series of laboratory experiments concerning provoking coal and gas outbursts was conducted. Coal briquettes and two gases: nitrogen and carbon dioxide were used. The experimental data was computed out into the gas stresses and the most important information was extracted. The obtained results showed that in experiments with nitrogen slightly higher gas stresses and thus more intensive provoking were needed in order to initiate outburst than in experiments with carbon dioxide. It indicates that sorption may be the factor that promotes outbursts. Comparative analysis implied also that for a given level of gas stresses the more sorptive the gas was, the longer it took to initiate an outburst. Moreover, in the experiments with nitrogen the global maximum of gas stresses occurred deeper inside of the briquette for any given time than in experiments with carbon dioxide. All the found differences were weak, though easily noticeable.     

Catalytically Assisted Self-Propagating High-Temperature Synthesis of Tantalum Carbide Powders

The use of gas-phase iodine and carbon dioxide as transport agents in the tantalum/carbon/tantalum carbide combustion synthesis system has been examined to determine the effects of transport agents on product composition and microstructure. Two tantalum reactant particle sizes, a range of transport agent concentrations, and total pressures were studied. The effects of the combustion conditions on product morphology and composition were evaluated using scanning electron microscopy, nitrogen adsorption (specific surface area), and X-ray diffraction analyses. The results of the investigation indicate that the presence of the iodine vapor and carbon dioxide significantly enhances the combustion synthesis process, leading to higher conversion efficiencies and influencing product microstructure. The results are discussed in the context of gas-phase and solid-phase transport models.     

Molecular probe chromatography of bituminous coal

The properties of some bituminous coals and an anthracite have been studied by “Molecular Probe Chromatography” using selected probes. Irrespective of the rank of coal, some behaviour common to all has been found. Following the sorption of air components and carbon dioxide at ambient temperature, interesting flow-disturbed peaks have been observed. Unlike the behaviour observed on a freshly packed column, both the retention and peak asymmetry of carbon dioxide increase upon column conditioning. The probe molecules, irrespective of their chemical identity, are sorbed on the surface by adsorption alone, up to 175°C. The probes are retained in the column both by specific and non-specific interaction forces and the contribution of the former to retention predominates in low rank coal. The selectivity of carbon dioxide at ambient temperature, and the retention of water and methanol at 175°C are dependent on the rank of coal. This dependence is similar to the porosity—rank relationship of coal. The permeability of bituminous coal decreases progressively with rank.     

EVOLUTION LIGNITE MESOPORE STRUCTURE DURING DRYING. EFFECT OF TEMPERATURE AND HEATING TIME

The knowledge of the intrinsic pore structure of coals is significant in elucidating the kinetics of mass transport and chemical reaction that leads to design of more efficient coal combustion and conversion equipment. The results of pore structure studies of Greek lignite are reported in this work. Isothermal drying of Greek lignite samples, under vacuum, caused mesopore structure evolution despite the severe (∼50%) particle size contraction due to heating. Mesopore volume and surface area were increased as the drying temperature was raised to 200°C while further drying up to 250°C caused a mesopore volume and surface area decrease. Lignite drying at 100°C for up to 3 h resulted in a monotonic increase of the mesopore structure properties while heating for a longer period i.e., 6 h, despite a slight increase of weight loss, caused pore volume and surface area reduction. Nitrogen sorption (77 K) hysteresis data obtained for partially dried samples have been processed to deduce BET surface area and pore size distributions (PSD) by using both the Roberts and a new method based on a Corrugated Pore Structure Model (CPSM-nitrogen) methods. The latter method was applied successfully in hysteresis loop simulations and predicted pore surface areas consistent with the respective BET values. Bimodal PSD have been detected with one peak at 3 nm and the second at 20 nm while surface area varied over the range 2.98-5.30 m²/g. Dry Greek lignite has shown a higher mesopore volume than that of several American and Canadian coals of varying rank. Mesopore volume distribution of dry Greek lignite, obtained from nitrogen sorption data, agree well with those deduced from mercury penetration data corrected for coal compressibility.     

Coal properties and their influence on air activation

This research sought to determine how the properties of a raw coal influence the degree to which it may be activated. Twelve coals were analyzed using thermogravimetry, mass spectroscopy, mercury porosimetry, nitrogen sorption and infrared spectroscopy, and activated in air, water-saturated helium and carbon dioxide. The greatest amounts of surface area using air activation were generated for bituminous coals. To investigate possible reasons for the sample-to-sample variation in the amount of surface area generated, a spline-fitting program was used to generate a curve that encloses the surface areas generated for all samples. The differences between measured surface areas and surface areas predicted by the spline-fitting program were determined. Statistical analyses of twenty-two predictor variables suggest that the percent vitrinite present accounts for nearly 73% of the deviation from predicted surface area. The % liptinite + % ash predicts 86% of the deviation and the inclusion of two additional variables, the % pyrite and % organic S, appear capable of predicting nearly 99% of the difference between measured and predicted surface areas. Increasing amounts of vitrinite and pyrite appear to increase the surface area generated, while increasing amounts of liptinite, ash and organic S appear to decrease the surface area generated. Pyrite may act as a catalyst during activation.     

Preparation and properties of sorbents from the coals of Mongolia

The textural and sorption properties of sorbents prepared by the thermal steam activation of the semicokes of brown and black coals from nine deposits of Mongolia were studied. Their properties were compared with the properties of sorbents prepared from Borodino brown coal and Kuznetsk black coal. A common extremal dependence of the volume of mesopores and the specific surface area on combustion losses was found: a maximum specific surface area of 600–700 m²/g was reached at a combustion loss of 50–60%. Among the Mongolian samples, the sorbents obtained from the brown coals of the Baganuur, Bagankhai, and Shivee Ovoo deposits exhibited the highest surface areas and sorption capacities for iodine. Nearly linear dependence of sorption capacity on specific surface area was found. The sorbents prepared from Borodino brown coals under identical conditions were characterized by a higher degree of combustion losses because of the increased concentrations of catalytically active calcium compounds in them.     

Gas sorption and transport in syndiotactic polystyrene with nanoporous crystalline phase

In this contribution we analyse sorption and transport of several gases in semicrystalline syndiotactic polystyrene with nanoporous crystalline δ form. Investigation was performed on amorphous samples and on samples characterized by different degrees of crystallinity. Sorption isotherms of carbon dioxide, nitrogen and oxygen in the crystalline phase have been determined starting from experimental results obtained for semicrystalline and amorphous samples. Corresponding isosteric heats of sorption were evaluated for the crystalline and amorphous phase. Permeation tests were also performed to gather information on mass transport properties of semicrystalline samples, evaluating average diffusivities of carbon dioxide and oxygen, in the limit of small concentrations as function of degree of crystallinity.     

XPS study and physico-chemical properties of nitrogen-enriched microporous activated carbon from high volatile bituminous coal

N-enriched microporous active carbons of different physico-chemical parameters have been obtained from high volatile bituminous coal subjected to the processes of ammoxidation, carbonisation and activation in different sequences. Ammoxidation was performed by a mixture of ammonia and air at the ratio 1:3 (flow ratio 250 ml/min:750 ml/min) at 350 °C, at each stage of production i.e. that of precursor, carbonisate and active carbon. Ammoxidation performed at the stage of demineralised coal or carbonisate has been shown to lead to a significant nitrogen enrichment and to have beneficial effect on the porous structure of the carbon during activation, allowing obtaining samples of the surface area of 2600-2800 m²/g and pore volume 1.29-1.60 cm³/g to be obtained with the yield of about 50%. The amount of nitrogen introduced into the carbon structure was found to depend on the sequence of the processes applied. The greatest amount of nitrogen was introduced for the processes in the sequence carbonisation → activation → ammoxidation. The introduction of nitrogen at the stage of active carbon leads to a reduction in the surface area and lowering of its sorption capacity. From the XPS study, ammoxidation introduces nitrogen mainly in the form of imines, amines, amides, N-5 and N-6, irrespective of the processing stage at which it is applied.     

Effects of catalyst addition to coal on CO2 adsorption kinetics of coal and char

The surface area of Illinois No. 6 coal impregnated with a series of carbon gasification catalysts was measured by carbon dioxide adsorption before and after pyrolysis. Chars were prepared by pyrolysing the coal in flowing nitrogen at a low controlled rate. The adsorbate uptake was described by a logarithmic time dependence. The amounts of CO₂ adsorbed at a fixed pressure within 1 minute and at maximum coverage were used to describe the surface area associated with micropores larger than 0.5 nm in diameter and with the total surface area, respectively. Catalyst addition decreased the surface area of coal accessible to CO₂ in all cases. After pyrolysis, some of the chars showed a considerable increase, others a massive decrease, in their adsorption capacity.     

Ignition and combustion of single coal and char particles: A quantitative differential approach

A quantitative differential technique for studying the coal particle combustion process and particularly the ignition step was developed. The approach is based on the continuous and simultaneous analysis of the carbon monoxide and carbon dioxide produced when a captive single coal particle is burnt after injection into an isothermal flow reactor swept with a preheated oxygen—nitrogen mixture. To check the results obtained with the new approach, the intensity of the light generated during the particle combustion was also registered. A microcomputer controlled and executed the data collection process in the millisecond time frame. Experiments were performed burning single particles from the 850–1000μm sieve fraction of a Wyoming subbituminous coal in air, at five gas temperature levels ranging from 928 to 1283 K. The gas product curves show the ignition mechanism is defined by the relative rates of volatiles evolution from the particle and oxygen diffusion to the particle. Initial peaks in the rate of formation of carbon monoxide or carbon dioxide allow the determination of the pyrolysis time during combustion. The occurrence of heterogeneous and homogeneous ignition for particles of the same coal was detected using both techniques: gas analysis and light intensity. A transition was detected in the ignition mechanism, from heterogeneous to homogeneous, when the reactor temperature was increased. Even though the light intensity technique is simpler, the results demonstrate that the approach developed in this work has more sensitivity, particularly in the transition zone. An additional advantage of the gas analysis approach is that the total carbon in the original particle and its variation with time can be calculated by integration of the gas product curves. This information provides a method for estimating the fraction of carbon released from the particle as volatile matter during the ignition. After ignition, the variation in the mass of carbon with time can be used to test different combustion mechanisms. As expected, the results show that at the higher temperatures used, the particle burn-off proceeds under external diffusion control. The total combustion times, as obtained from the gas product curves, show good correlation with the results obtained from light intensity measurements.     

Characterization of porous structure of coal char from a single devolatilized coal particle: Coal combustion in a fluidized bed

The combustion characteristics of coal char are highly dependent on initial pore structure of devolatilized char as well as on the structural evolution during the combustion of char. The development of pore structure also throws light on the mechanism of the combustion process. In the present work evolution of pore structure of partially burnt coal char of Indian origin has been investigated experimentally in a batch-fluidized bed and analyzed. The BET surface area, micropore surface area and porosity of char at various levels of carbon burn-off have been determined. Experimental specific surface area has been found to agree well with theoretical prediction using random pore model. Modified random pore model is used to determine the active surface area. Char combustion mechanism based on shrinking unreacted core and shrinking reacted core models are delineated during the course of reaction at various bed temperatures. This is substantiated with the proportional representation of ash and carbon matrix in scanning electron microscope images. It is also concluded that in the present investigation the mean pore size is much smaller and hence the Knudsen diffusion predominates. Analysis based on similar experimental observations and models for pore structure evolution to investigate char combustion reaction regime has not been reported in literature.     

Control of carbon dioxide emissions from a power plant (and use in enhanced oil recovery)

The design of a compact, environmentally acceptable, carbon dioxide-diluted, coal-oxygen-fired power plant is described. The plant releases no combustion products to the atmosphere. The oxygen for combustion is separated in an air liquefaction plant and the effluent nitrogen is available for use in oil well production. Recycled carbon dioxide mixed with oxygen replaces the nitrogen for the combustion of coal in the burners. The carbon dioxide produced is used in enhanced oil recovery operations and injected into spent wells and excavated salt cavities for long-term storage. The recovery of CO₂ from a coal-burning power plant by this method appears to have the lowest energy expenditure and the lowest by-product cost compared to alternative removal and recovery processes.     

The devolatilisation of millimetre sized coal particles at high heating rate: the influence of pressure on the structure and reactivity of the char

Most studies on the influence of pressure on the combustion of coal particles have shown that for a constant oxygen concentration, an increase of pressure leads to a decrease of combustion rate. Among the different phenomena, which can explain this behaviour, the influence of the devolatilisation pressure on the structure and reactivity of the char formed may be important. The aim of this paper was to obtain a quantitative characterisation of the physical and chemical structure of chars formed during pyrolysis under a large range of pressure. Experiments of single coal particle pyrolysis were conducted in a laser reactor with pressure ranging from 0.014 to 2.1 MPa in a nitrogen atmosphere. As expected, an increase of pressure lead to a decrease of the volatile matter yield, which can be related to the secondary reactions of volatile matter. A characterisation of the char was performed by gas adsorption methods: nitrogen adsorption, carbon dioxide adsorption and active surface area (ASA) measurement. True and apparent densities, porosities and swelling of the particles were also investigated. Although the volatile matter yield decreases, the porosity and the swelling of the char increases with increasing pyrolysis pressure. We observed an increase in surface area and microporosity with increasing pressures up to 0.6 MPa. The ASA surface also increases in this temperature range, but the ratio of ASA to CO₂ surfaces shows that the intrinsic reactivity of the surface decreases with increasing pyrolysis pressure.     

Identification of the mechanism of coal char particle combustion by porous structure characterization

Two limiting models—the shell progressive mechanism and the homogeneous mechanism—can describe combustion of a single coal particle. Some information about the real mechanism can be obtained from investigation of the porous structure development during combustion. Using the principles of gas adsorption and mercury penetration, the porous structure of a partially combusted particle was estimated. Experiments were carried out in an equipment by applying the thermogravimetric method and using a single devolatilized coal particle. The inlet concentration of oxygen was 5 and 15 mol%. The initial temperature of combustion was in a range from 450 to 800 °C. The mechanism of coal char particle combustion depends on the initial temperature and the inlet concentration of oxygen. At low temperature and low inlet concentration of oxygen, the rate of principal chemical reactions is comparable with the rate of diffusional transport of oxygen inside the particle. Combustion is governed by the diffusion mechanism. This is evident from the values of the specific surface area of pores and proportional representation of individual pore types. At higher temperatures and low inlet concentrations of oxygen, combustion proceeds by the shell progressive mechanism. The specific surface area is lower in comparison with the previous case. There is a sharp interface between the particle core and the ash shell. The core exhibits a higher value of specific surface area than in the case of a non-combusted coal char particle. This fact can be explained by the consecutive reaction of carbon dioxide with carbon in the core of the particle. The rate of this reaction is sufficiently high at temperatures above 800 °C.     

Reactivity of some halogenated alkanes on 13X molecular sieve

The reactivity of dichlorodifluoromethane, trichlorofluoromethane, dichlorofluoromethane, chlorodifluoromethane, bromotrifluoromethane, and trichlorotrifluoroethane on a 13X Davison molecular sieve at temperatures of 150 and 320 °C has been examined. For each of these, the nature and concentration of the resulting products have been determined, the variation of sorption capacity of the sieve for carbon dioxide has been measured, and the amount of acid products resulting from sweeping by means of nitrogen saturated with water vapor has been estimated. All the halogenomethanes decompose at 320 °C and only trichlorotrifluoroethane and bromotrifluoromethane do not decompose at 150 °C. The main destruction product is always carbon dioxide, except for dichlorofluoromethane and chlorodifluoromethane which give carbon monoxide. When decomposition is extensive, it gives rise to strong acids during the reaction and during sweeping by means of nitrogen saturated with water vapor. The toxics seen are carbon monoxide and phosgene. Sorption capacity is strongly decreased and is not entirely recovered after sweeping by means of nitrogen saturated with water vapor.     

Preparation of modified active carbon from brown coal by ammoxidation

The technology of obtaining active carbon enriched in nitrogen from brown coal is described. The effect of ammoxidation by a mixture of ammonia and air at the ratio 1:3 at 300 and 350 °C, at each stage of the active carbon production has been tested. The amount of nitrogen introduced into the active carbon has been proved to depend on the stage at which ammoxidation was performed. Carbonisation and activation with steam of the samples enriched in nitrogen have been found to lead to a significant decrease in its content and to cause an increase in the stability of the nitrogen groups. The ammoxidation of the active carbon has decreased their surface area, while the ammoxidation and high temperature of activation favour the formation of surface oxide groups of basic character.     

内蒙古霍林河褐煤热力初步改质研究

针对褐煤水分高、易风化破碎、氧化自燃、利用率低等问题,采用热力脱水方法对内蒙古霍林河褐煤进行了实验室改质研究,分析了改质温度、停留时间、原煤粒径对褐煤改质效果的影响。结果表明:改质温度、停留时间对褐煤改质效果影响显著,原煤粒径对褐煤改质影响不明显,当处理温度为300℃,停留时间为20 min时,褐煤改质效果最好;此时,褐煤Mad降低了79.75%,H含量增加了153%,O含量降低了62.47%,C含量提高了72.46%,N含量增加了26.09%,Qgr,ad增加了108.50%,Qnet,M增加了161.86%;褐煤改质过程明显降低了煤中Mad和O含量,并使C含量和发热量大幅提高,改质效果明显,这将为褐煤热力改质的应用奠定基础。     

Effect of moisture on the methane capacity of American coals

An investigation of methane sorption in bituminous coals ranging from low-volatile to high-volatile B has been carried out. Equilibrium sorption isotherms for dry and moist coal were measured at 30 °C and at pressures up to 60 atm. The natural oxygen content of a coal plays a major role in determining its methane capacity. Capacities of high-oxygen coals undergo a much greater reduction when saturated with moisture than do low-oxygen coals. Measurements of adsorbed-water saturation capacities clearly suggest that only adsorbed water affects the equilibrium capacity of a coal for methane; water present in excess of the adsorbed water has no effect on methane sorption. Excellent agreement has been found between the methane sorption data reported here and field measurements of methane emission from coal samples obtained during borehole drillings.     

Gasification rate analysis of coal char with a pressurized drop tube furnace

Two coal chars were gasified with carbon dioxide or steam using a Pressurized Drop Tube Furnace (PDTF) at high temperature and pressurized conditions to simulate the inside of an air-blown two-stage entrained flow coal gasifier. Chars were produced by rapid pyrolysis of pulverized coals using a DTF in a nitrogen gas flow at 1400°C. Gasification temperatures were from 1100 to 1500°C and pressures were from 0.2 to 2 MPa. As a result, the surface area of the gasified char increased rapidly with the progress of gasification up to about six times the size of initial surface area and peaked at about 40% of char gasification. These changes of surface area and reaction rate could be described with a random pore model and a gasification reaction rate equation was derived. Reaction order was 0.73 for gasification of the coal char with carbon dioxide and 0.86 for that with steam. Activation energy was 163 kJ/mol for gasification with carbon dioxide and 214 kJ/mol for that with steam. At high temperature as the reaction rate with carbon dioxide is about 0.03 s⁻¹, the reaction rate of the coal char was controlled by pore diffusion, while that of another coal char was controlled by surface reaction where reaction order was 0.49 and activation energy was 261 kJ/mol.