Effectiveness of K2CO3 and Ni as catalysts in steam gasification

The catalytic steam gasification of 34 coals ranging from anthracite to peat was conducted in a thermobalance at 1023 K. When the difference in the reactivities of coals with and without catalyst is taken as a measure of the catalyst efficacy, that of K₂CO₃ was found to be extremely large and almost independent of coal rank. The effectiveness of Ni depends on the coal type to a great extent and it was very large for several low rank coals. SEM observation and EDAX analysis showed that the K₂CO₃ catalyst was finely dispersed over the char and such a good dispersion was common for all the coals examined. The dispersion state of Ni catalysts depends on the coal type, Ni catalysts were well dispersed over low rank coal chars which showed high reactivity in the Ni-catalysed gasification.     

Voltammetry and electrolysis of coal slurries and H-coal liquids

The voltammetric and electrolytic behaviour of several coal slurries and H-coal liquids was examined in both aqueous and organic solvents. Most aqueous coal samples yielded anodic currents starting a approximately + 0.4 V versus SCE while current was observed only at more positive potentials in dimethylsulphoxide and acetonitrile. The electroactive species always appeared to consist largely of material extracted from the coal matrix. The extract from the aqueous coal slurries contained significant amounts of iron, and the oxidation of Fe²⁺ to Fe³⁺ was found to be largely responsible for the anodic currents at + 0.4 V. Constant potential electrolysis of the aqueous coal slurries indicated that the onset of coal oxidation occurred at an applied potential difference of approximately 1.0 V and again appeared to be related directly to the oxidation of Fe²⁺ leached from the coals.     

Low temperature oxidation of coals: Effects of pore structure and coal composition

The low-temperature oxidation of five coals, ranging in rank from subbituminous to anthracite, was studied in the temperature range 30–250 °C, and the reaction kinetics were elucidated. The reaction rates were independent of particle diameter <1 mm. The orders of reaction for CO₂ and CO formation were 0.50 and 0.54, respectively, with respect to oxygen. Activation energies of 51.5–59.4 kJ mol^?1 were obtained for the CO₂ and CO formation reactions. The rates of formation of CO₂ and CO were correlated to the internal surface area and the oxygen contents of the coals. It was found that pores having radii >100 Å, and the oxygen-containing groups which decompose to CO₂ and/or CO, were playing important roles in low-temperature oxidation of coals.     

The characterization of surface properties and steam reactivities of two Spanish coals of high ash content

Studies were made on two Spanish coals of high as content (a semi-anthracite and a high volatile bituminous coal) and on the coals after heat treatment and on acid demineralisation in HCl and HF. X-ray diffraction revealed that the mineral matter content of the coals included quartz, siderite and aluminosilicate minerals; siderite and the aluminosilicates were decomposed by heat treatment. Mineral matter content was substantially reduced by acid treatment, but the metallic element content, as revealed by EDAX, remained similar to that of the raw coals. N₂ and CO₂ adsorption isotherms and mercury porosimetry show that the coals contain mainly micropores and macropores, the semi-anthracite having the greater microporosity. For bituminous coal, macro- and micro-porosity increase substantially upon heat treatment; for anthracite there is a smaller increase in macroporosity and a decrease in microporosity. Microporosity in both coals is unaffected by acid demineralisation, but macroporosity is increased. The steam reactivity of the bituminous coal char is greater than that of the anthracite coal char, and demineralisation of the coals increases steam reactivity. N₂ surface areas and steam reactivities for both coal chars follow trends with parent coal rank previously established for US coals.     

Effect of coal type on the flash pyrolysis of various coals

The effect of coal rank on the product distribution pattern was investigated in Ar at 1037 K with a Curie-point pyrolyser, using seventeen coals ranging from lignite to anthracite. The yields of inorganic gases, hydrocarbon gases, light hydrocarbon liquids and tar were determined and correlated with the structural parameters of the coals. The yields of CO₂ were found to have a linear relationship with the carboxylic group content of the coals. The amounts of light hydrocarbon gases correlated well with the aliphatic hydrocarbon content of the coals.     

Ignition characteristics of coal blends in an entrained flow furnace

Ignition tests were carried out on blends of three coals of different rank - subbituminous, high volatile and low volatile bituminous - in two entrained flow reactors. The ignition temperatures were determined from the gas evolution profiles (CO, CO₂, NO, O₂), while the mechanism of ignition was elucidated from these profiles and corroborated by high-speed video recording. Under the experimental conditions of high carbon loading, clear interactive effects were observed for all the blends. Ignition of the lower rank coals (subbituminous, high volatile bituminous) enhanced the ignition of the higher rank coal (low volatile bituminous) in the blends. The ignition temperatures of the blends of the low rank coals (subbituminous-high volatile bituminous) were additive. However, for the rest of the blends the ignition temperatures were always closer to the lower rank coal in the blend.     

Model for coal oxidation kinetics. 2. An effectiveness factor interpretation

Rate expressions are developed to describe the oxidation of coal in a fixed-bed reactor under conditions of significant diffusional effects. The model defines individual effectiveness factors for carbonic gas formation, oxygen deposition, and water production, to modify a chemical model derived previously. Experiments carried out on seven coals ranging in rank from lignite to bituminous to anthracite. All the coals exhibited particle size effects and the results fit the model in most cases to within ±10% both in the rate of overall oxidation and with respect to the individual reaction rates for several products. Activation energies and effectiveness factors were obtained for each coal and for each reaction. The effluent gas $\text{CO}{}_2\text{CO}$ ratio was found to be virtually constant for all five bituminous coals, being independent of reaction temperature, coal particle size, and oxygen partial pressure.     

Correlation of gasification rates of various coals measured by a rapid heating method in a steam atmosphere at relatively low temperatures

Twenty-five kinds of coals (carbon content on dry ash-free basis, C[%], ranges from 65.0 to 92.8%) were pyrolysed and gasified simultaneously by use of a rapid heating method (heating rate ≈ 1600 K min⁻¹) in steam at temperatures between 750 and 850 °C to clarify the factors which control the gasification rates of various coals. The relationships were examined in detail between the reactivity of each coal, represented by the initial gasification rate − r_cm0, and various properties such as pore surface area of char, ultimate and proximate analyses of coal, reflectance of coal, contents of metals in char, and the amount of oxygen trapped by char. For gasification at 800 °C, the relation between − r_cm0 and the carbon content C[%] changed abruptly at C ≈ 75–80%. For higher rank coals (C > 75–80%), − r_cm0 was rather small and was well correlated by C[%]. On the other hand, the plot of − r_cm0versus C % scattered largely for the lower rank coals (C < 75–80%). For these coals, the rate of CO₂ formation was much greater than that of CO formation, and was almost proportional to − r_cm0. The CO₂ formation reaction is known to be catalysed by alkali or alkaline earth metals such as Na, K and Ca. Then the reactivities of lower rank coals were supposed to be controlled mainly by the catalytic effect of the minerals in the coal.     

In-situ temperature-depth profile evaluation of South African unmineable coal seams to determine CO2 sequestration potential

This study aimed to investigate the sorption behaviour of South African coal seams with relation to the effect of temperature during CO₂ sequestration. The excess adsorption isotherms of CO₂ adsorption were undertaken using a high-pressure volumetric system for four coals of different coal rank (denoted by Somkele [SK], anthracite KZN [AN], Tshikondeni [TD], and Syferfontein [SF]). The volumetric pressure step method was conducted at increments of system temperature of 35, 45, 55, and 65°C for pure CO₂ adsorption at incremental pressures up to 93 bar. The results showed that high temperatures have a very significant negative effect on the amount of CO₂ adsorbed on the coal samples. The high-rank coal samples (SK and AN) demonstrated elevated CO₂ adsorption capacity across all tested temperatures due to their high vitrinite content. The medium-rank coals (TD and SF) exhibited comparatively lower CO₂ adsorption capacity, attributed to the presence of adsorption hindrances such as higher ash content and volatile and mineral matter. The isosteric heat of adsorption revealed an increasing trend with coverage for all coal samples, with higher rank coals displaying greater slopes. The determined range of the isosteric heat of adsorption, spanning from 10 to 59 kJ/mol, indicated that the adsorption process is primarily of a physisorption nature. Three theoretical models (Langmuir, Freundlich, and Temkin) were evaluated and fitted to the sorption experimental data. The Temkin model exhibited superior fitting compared to the Langmuir and Freundlich isotherms. The Temkin isotherm parameters suggest that the adsorption of CO₂ onto coal is a physisorption process.     

Preparation of active carbons from coal: Part III: Activation of char

Active carbons with a burn-off of 52% have been prepared from four coals of different rank and origin after preoxidation to different degrees at 543 and 473 K, and further carbonization at 1123 K. The activation has been carried out with CO₂ at 1123 K at two flow rates viz. 7 cm³ min⁻¹ and 500 cm³ min⁻¹. Active carbons have also been prepared from a preoxidized coal by activation to different degrees of burn-off between 10 and 80%. The effect of the degree of oxidation, the flow rate of the activating gas and the extent of burn-off on the porous structure development of active carbons has been examined. The active carbons prepared from unoxidized coal have poor textural characteristics (porosity, N₂ and CO₂ surface area). Nevertheless, the textural characteristics are enhanced as the degree of preoxidation of the coal is increased. The low flow rate of CO₂ (activating gas) produces active carbons with a better microporous character. The degree of activation (the extent of burn-off) of the carbon determines the porous structure of the active carbon. At low degrees of burn-off (less than 50%) the product is largely microporous. At higher degrees of burn-off between 35–65% the product has a mixed porous structure and contains all types of pores. Active carbons with a given textural character can be obtained by controlling the degree of oxidation of coal and the degree of activation of the carbonized material.     

水煤浆气化原料的成浆性研究

在实验室条件下研究了从低煤化度烟煤到高煤化度无烟煤,以及石油焦等不同气化原料煤的成浆性.为提高低煤化度烟煤的成浆浓度,在保证其混合原料灰熔融特征温度满足液态排渣前提下,将低煤化度烟煤与一种或两种煤化度较高的煤或者石油焦配比,考察了它们的成浆性.结果表明,煤化度适中的QD煤单独制浆浓度达到70%,黏度536mPa.s,流动性为A;通过不同煤种的级配,三种原料配合的料浆浓度为62%时,黏度在340mPa.s～550mPa.s之间,可以获得符合液态排渣气化要求的混合料水煤浆,扩大了气化原料来源.     

低阶煤在干燥氧气下低温氧化过程的机理研究

采用pulse calorimeter仪器,对低阶煤在干燥氧气下低温氧化过程进行了研究．实验结果表明,低阶煤在干燥氧气下的氧化反应热随着温度的上升而增加,并得到了与Arrhenius公式相同的动力学方程以及反应过程的活化能．提出了干燥氧气下低阶煤低温氧化过程的反应历程为：氧在煤粒表面吸附并生成氧自由基、氧自由基与煤发生氧化反应生成CO2（或CO）、CO2（或CO）由煤粒表面向本体扩散等三步过程．并通过计算和模拟,证明了该反应机理的正确性．     

Reactivities of 34 coals under steam gasification

The reactivities of 34 coal chars of varying rank with H₂O have been determined to examine the effect of coal rank on the gasification rate of coal char. The reactivities of chars derived from caking coals and anthracites (carbon content > 78 wt%, daf) were very small compared with those from non-caking (lower-rank) coals. The reactivities of low-rank chars do not correlate with the carbon content of the parent coals. To clarify which factor is more important in determining the reactivity, the evolution of CO and CO₂ from char, the moisture content of char and the amount of exchangeable cations were determined for these low-rank coals or their chars. These values were considered to represent the amount of active carbon sties, the porosity and the catalysis by inherent mineral matters, respectively. It was concluded that the amount of surface active sites and/or the amount of exchangeable Ca and Na control the reactivity of low-rank chars in H₂O.     

Effect of pressure on gasification reactivity of three Chinese coals with different ranks

The gasification reactivities of three kinds of different coal ranks (Huolinhe lignite, Shenmu bituminous coal, and Jincheng anthracite) with CO₂ and H₂O was carried out on a self-made pressurized fixed-bed reactor at increased pressures (up to 1.0 MPa). The physicochemical characteristics of the chars at various levels of carbon conversion were studied via scanning electron microscopy (SEM), X-ray diffraction (XRD), and BET surface area. Results show that the char gasification reactivity increases with increasing partial pressure. The gasification reaction is controlled by pore diffusion, the rate decreases with increasing total system pressure, and under chemical kinetic control there is no pressure dependence. In general, gasification rates decrease for coals of progressively higher rank. The experimental results could be well described by the shrinking core model for three chars during steam and CO₂ gasification. The values of reaction order n with steam were 0.49, 0.46, 0.43, respectively. Meanwhile, the values of reaction order n with CO₂ were 0.31, 0.28, 0.26, respectively. With the coal rank increasing, the pressure order m is higher, the activation energies increase slightly with steam, and the activation energy with CO₂ increases noticeably. As the carbon conversion increases, the degree of graphitization is enhanced. The surface area of the gasified char increases rapidly with the progress of gasification and peaks at about 40% of char gasification.     

Factors affecting coal particle ignition under oxyfuel combustion atmospheres

A set of 13 coals of different rank has been tested for ignition propensity in a 20-L explosion chamber simulating oxyfuel combustion gas conditions. Their char residues were also analysed thermogravimetrically. The effects of coal type, coal concentration (from 100 to 600 g/m³), O₂ in CO₂ atmospheres (up to 40% v/v) and particle size were investigated.The higher rank coals were significantly more difficult to ignite and mostly required higher energy chemical igniters (1000 or 2500 J) whereas the lower rank coals could be ignited with a 500 J igniter even at low coal dust concentrations.The minimum explosibility limit/ignition concentration in air varied slightly around a value of 200 g/m³, a little higher for low volatile coals and a little lower for high volatile coals.The ignition limit changed significantly, however, with O₂ concentration in CO₂, where coals required more oxygen to ignite. Most coals failed to ignite at all in 21% v/v O₂ in CO₂, but an increase to 30 or 35% v/v O₂ gave ignition patterns similar to those in air. In addition, the minimum ignition concentration decreased with increase in O₂. However, a further increase to 40% v/v O₂ did not generally affect the minimum ignition concentration.Particle size had a non-linear effect on coal ignition. The fine particles (<53 μm) behaved almost identical to the whole coal. However, the larger size fraction (>53 μm) was generally more difficult to ignite and exhibited a much lower weight loss.     

Effect of demineralization and catalyst addition on N2 formation during coal pyrolysis and on char gasification

Six coals with different ranks and different ash contents have been used to study the effect of demineralization on N₂ formation during coal pyrolysis. Chars obtained after pyrolysis have been also gasified with carbon dioxide at 1000 °C to investigate the influence of the demineralization on char gasification reactivity. The pyrolysis results show that the demineralization by acid washing drastically changes N₂ formation profiles and decreases nitrogen conversion to N₂ for low rank coals; on the other hand, the demineralization has little effect on N₂ formation for high rank coals. Addition of 0.5 wt% Fe promotes N₂ formation from the demineralized coals, but the catalytic effect depends on the coal type. It is found that the Fe remarkably promotes N₂ formation from the demineralized low rank coals, but the effect is much smaller for high rank demineralized coals. These observations suggest that the existing state of Fe-containing minerals and added Fe catalyst is important for catalytic N₂ formation during coal pyrolysis. Gasification results show that the demineralization lowers char gasification reactivity not only for low rank coals but also for high rank coals.     

Predictive equations for CO2 emission factors for coal combustion, their applicability in a thermal power plant and subsequent assessment of uncertainty in CO2 estimation

The emission of carbon dioxide varies systematically with the rank and type of coal combusted. Hence use of a single default emission factor proposed by IPCC (Inter Governmental Panel on Climate Change) for entire categories coals may not be appropriate option to obtain a reliable estimate of carbon dioxide emission level or towards the preparation of national carbon dioxide inventory. Even predictive equations developed based on the coals of different origin may not work well with coals of a specific origin. Several linear predictive equations were thus developed separately for coking and non-coking coals of Indian origin for the estimation of carbon dioxide emission utilizing basic coal parameters such as VM, FC, GCV and NCV on different basis. Large numbers of authenticated data set were used for multiple regression analysis and good correlations were obtained. Those equations were also validated with different data sets of Indian coals. Its applicability towards estimation of CO₂ emission from power plant was also studied and uncertainty in CO₂ estimation is revealed. The developed equations may be utilized to get a realistic estimate of carbon dioxide emission with specific cases where Indian coals are mostly used.     

煤快速热解固相和气相产物生成规律

利用能有效避免二次转化反应的高频炉热解装置对3种不同变质程度的煤进行了600～1200℃条件下的快速热解,考察了在煤热解最初阶段焦产率、焦-C产率、热解气产率、热解气4种主要组分H₂、CO、CH₄和CO₂的比例以及热解气热值随煤阶和热解温度的变化规律。结果表明,焦的产率和焦-C的产率均随煤阶的升高而升高,热解气的产率随煤阶的升高而降低;热解温度的提高能显著降低煤焦和焦-C的产率并提高热解气的产率。热解气组分以H₂相似文献   

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.     

Low temperature oxidation of coals of different rank and different sulphur content

Five coals of different rank and of different content of sulphur were subjected to oxidation by peroxyacetic acid (PAA), 5% nitric acid, by oxygen in 0.5N Na₂CO₃ aqueous solution and the air oxidation for 7 days at 125 °C. The processes of oxidation were carried out for coal samples demineralised by the Radmacher method, and additionally for the pyrite-free coal samples. Proximate, elemental and spectral analyses of the initial coals and the products obtained from them were made. The most effective oxidising agents were 5% HNO₃ and PAA. As a result of oxidation, a significant part of the organic components of coal is converted into acid soluble products. Depending on the oxidising agent, the loss of sulphur in the solid oxidation products was different and the highest for the coal samples oxidised with HNO₃ and PAA. Formation of oxidised sulphur species (SO and -SO₂) was detected by IR spectroscopy. FTIR data also provide useful information on evolution of the molecular structure of different rank coals on oxidation, in particular in the carbonyl and aliphatic ranges (1800-1500 and 3500-2800 cm⁻¹).