Liquefaction of subbituminous coals under apparently non-hydrogenative conditions

Reactivities of several coals of different ranks have been examined in degrading extractions with aromatic solvents under apparently non-hydrogenative reaction conditions. Pyrene and A240 pitch liquefied the fusible coals in high yields and the slightly-fusible coals in moderate yields, indicating the importance of fusibility in such liquefaction processes. A240-LS pitch is a powerful solvent for slightly-fusible coals. Considerable amounts of pyridine- or THF-soluble fractions were produced especially with A240-LS pitch. A240 pitch is a better solvent than pyrene for some slightly-fusible coals. However, the extent of depolymerization of liquefied coal, pyridine- or THF-solubility, was definitely inferior. Yields of such fractions are higher for lower-rank coals. The mechanism of coal liquefaction under apparently non-hydrogenative conditions is discussed with emphasis on the stabilization of thermal fragments derived from the coal.     

Formation of carbonaceous cenospheres during fluidized-bed combustion of bituminous coals

The discovery of cenospheres in fly ash from fluidized-bed combustors is reported and differences between the fluidized bed and more conventional combustors which are likely to affect cenosphere formation are indicated. Morphological studies and qualitative chemical analyses obtained by electron microscopy/energy dispersive X-ray specttometry (SEM/EDS) techniques show differences in composition between the ‘window’ areas of cenospheres and their ‘skeletons’.     

Model for drying during fluidized-bed combustion of wet low-rank coals

An unsteady state heat conduction model with a convective boundary condition is proposed for the drying of low-rank, high-porosity coals, such as lignites, during fluidized-bed combustion. The drying front is assumed to be the receding surface of a wet core. The solution technique for this moving boundary problem is based on the heat balance integral approach with immobilization of the moving boundary by a change in space variable. The governing cubic equation describing the drying curve in dimensionless form may be solved easily by the Newton—Raphson method. The model predictions are compared with experimental data for Mississippi lignite with excellent agreement. A correlation for estimation of total drying time is proposed. The temperature profiles obtained may be used for the study of the coupled drying and devolatilization in fluidized-bed combustors. The profiles could also be of importance in the study of formation of fissures/cracks in lignites subjected to intense heating conditions encountered during fluidized-bed combustion.     

Pyrolysis—gas chromatography of Australian coals. 1. Victorian brown coal lithotypes

Pyrolysis—gas chromatography (Py—g.c.) has been shown to be a useful technique for characterizing Victorian brown coal lithotypes. The pyrograms show marked changes in the predominance and distribution of specific molecular classes as a function of lithotype. Hydrogen-rich triterpenoid components are predominantly associated with the lighter lithotypes, whereas hydrogen-deficient phenolic components are more abundant in pyrolysates from the darker lithotypes. Carbon preference indices (CPIs) are >1 for the alkanes but <1 for the alkenes released by pyrolysis. All CPI values generally increase with darkening lithotype. Correlations of the components released with the maceral composition of the lithotypes have also been established. This information is used to establish some details of the mechanism of pyrolysis and the probable relative liquefaction behaviour of these lithotype samples.     

In-bed char combustion of Australian coals in PFBC. 3. Secondary fragmentation

Secondary fragmentation occurs during the char combustion stage of coal combustion in fluidised beds and was investigated at pressure in a bench-scale PFBC rig for five Australian black coals. The critical diameter of char particles above which secondary fragmentation occurred was 2 mm for all the coals studied. In contrast with previous work [1 and 2] at atmospheric pressure, secondary fragmentation was found to occur during the early stage of char combustion, explaining its influence on char burnout and in-bed char loading in PFBC.

Two normal distribution functions were found to describe the secondary fragmentation behaviour in PFBC. The first determined randomly which particle broke up, and the second predicted the size distribution of fragments. Neither char properties nor oxygen concentration significantly affected the parameters in these two statistical functions. However, the rank of the parent coal correlated with the ratio of maximum number of char fragments to initial number of char particles. This secondary fragmentation behaviour should be included in mathematical modelling of char combustion in PFBC to predict the char burnout correctly.     

Liquefaction of subbituminous coals with a basic nitrogenous model process solvent

Liquefaction reactions in a tubing-bomb reactor have been carried out as a function of coal, coal sampling source, reaction time, atmosphere, temperature, coal pre-treatment, SRC post-treatment and process solvent. Pyridine as well as toluene conversions ranging from 70 to > 90 wt% involving both eastern bituminous and western subbituminous coals are obtained. 1,2,3,4-Tetrahydroquinoline (THQ) has been extensively used as a process solvent under optimized liquefaction conditions of 2:1 solvent: coal, 7.5 MPa H₂, 691 K and 30 min reaction time. Comparisons of THQ with other model process solvents such as methylnaphthalene and tetralin are described. Liquefaction product yield for conversion of subbituminous coal is markedly decreased when surface water is removed from the coal by drying in vacuo at room temperature prior to liquefaction. The effect of mixing THQ with Wilsonville hydrogenated process solvent in the liquefaction of Wyodak and Indiana V coals is described.     

Ash deposition behavior of upgraded brown coal in pulverized coal combustion boiler

Ash with a low melting point causes slagging and fouling problems in pulverized coal combustion boilers. Ash deposition on heat exchanger tubes reduces the overall heat transfer coefficient due to its low thermal conductivity. The purpose of this study is to evaluate the ash deposition for Upgraded Brown Coal (UBC) and bituminous coal in a 145 MW practical coal combustion boiler. The UBC stands for Upgraded Brown Coal. The melting temperature of UBC ash is relatively lower than that of bituminous coal ashes. Combustion tests were conducted on blended coal consisting 20 wt.% of UBC and 80 wt.% of bituminous coal. Before actual ash deposition tests, the molten slag fractions in those coal ashes were estimated by means of chemical equilibrium calculations. The calculation results showed the molten slag fraction for UBC ash reached approximately 90% at 1523 K. However, that for blended coal ash decreased to 50%. These calculation results mean that blending UBC with bituminous coal played a role in decreasing the molten slag fraction. This phenomenon occurred because the coal blending led to the formation of alumino-silicates compounds as a solid phase. Next, ash deposition tests were conducted using a practical pulverized coal combustion boiler. A water-cooled stainless-steel tube was inserted in locations at both 1523 K and 1273 K in the boiler to measure the amount of ash deposits. The results showed that the mass of ash deposition for blended coal did not greatly increase, compared with that for bituminous coal alone. Therefore, appropriately blending UBC with bituminous coal enabled the use of UBC without any ash deposition problems in practical boilers.     

Carbonization of coals to produce anisotropic cokes.: 4. Improving co-carbonization behaviour of bituminous and subbituminous coals with pitch by pretreatment coals to remove cations

Several de-ashing pretreatments of selected coals have been examined to establish simple and effective procedures to improve, in terms of anisotropic development, the co-carbonization behaviour of the coals with pitches. Refluxing pretreatments of the coals in 1N HCI containing methanol and in boiling water containing EDTA-2Na (EDTA-2Na/coal = 7/100 byweight) improve the co-carbonization behaviour of Witbank, Miller, Taiheiyo and Wandoan coals, all of which, in their original, untreated form, are modified in terms of anisotropic development only very slightly in co-carbonizations with A240 petroleum pitch. Pretreatment of coals of very low rank appears to be ineffective in co-carbonizations with A240; however, co-carbonization with hydrogenated A240 shows that the pretreatments are effective. Analyses and FT-i.r. spectroscopy of mineral matter suggest that improved behaviour due to the pretreatment is related to the removal of divalent cations and to the modification of the oxygen functionality of the coals.     

低NOx燃气燃烧技术研究进展

气体燃料具有易于点火、燃烧迅速、燃烧完全等特点,且氮、硫、灰分低,因此燃烧后产生的污染物相对较少,属于较清洁的燃料,且国家燃气补贴政策的实施,使气体燃料燃烧近年来有很好的发展前景。但随着国家对大气污染物的控制更加严格,控制气体燃料燃烧过程中NOx的生成至关重要。笔者介绍了不同种类NOx的产生机理及影响因素,并基于NOx的产生机理提出控制措施,分析目前应用较广泛的燃气燃烧技术的低氮原理及应用现状,最后提出燃气燃烧器应用的展望。燃气燃烧过程中主要以热力型NOx及快速型NOx为主,温度和过量空气系数是影响NOx生成的主要影响因素。燃烧温度高于1 500℃时,热力型NOx呈指数型增长,温度是影响NOx生成的最重要因素。根据NOx产生机理,低NOx燃烧技术的实质是降低最高燃烧温度,控制燃烧区燃料浓度以及氧浓度,缩短烟气在高温区的停留时间,破坏NOx生成的最佳条件,最终抑制NOx的生成。低NOx燃烧技术一定程度降低了NOx的生成,但又会破坏整个燃烧进程,对燃烧和放热过程造成不利影响,降低了燃烧效率和传热效率,因此如何解决这些矛盾是亟需解决的问题。在实际应用中,应根据需求选择合适的燃烧技术,同时可将不同燃烧技术相结合起到稳燃、低氮的效果。应用较广泛的燃气燃烧技术主要是阶段型燃烧技术、烟气再循环燃烧技术、无焰燃烧技术等,其中催化燃烧技术发展前景较好,目前已应用于多个领域,其催化剂的热稳定性和寿命问题是限制其工业上广泛应用的核心问题。     

Exploring the possibilities of using Brazilian subbituminous coals for blast furnace pulverized fuel injection

The current study investigates the combustion and blast furnace injection performance of three Brazilian subbituminous coals (Mina do Recreio) and their beneficiation products using laboratory scale combustion tests. The coals have relative high ash yields (up to 40 wt%) that were reduced stepwise to levels as low as 12 wt%, dry basis. The reduction of ash yields is paralleled by a significant decrease in sulphur and inertinite contents.The combustion tests were performed in a drop tube reactor operating at 1300 °C using two different atmospheres (2.5 and 5% O₂). The chars exhibited preferentially rounded shapes with thick walls and abundant secondary porosity for the 2.5% O₂ chars, whereas the 5% O₂ chars showed very thin walls as a consequence of extensive burnout. The intrinsic reactivities of both set of chars were similar. The differences in conversion between the two working atmospheres were 24-37% and roughly tend to increase with increasing mineral matter content. Conversions as high as 76-81% were reached operating under 5% O₂ indicating that the coals are easy to burn. The small differences in burnout among the coals and their beneficiation products cannot be clearly attributed neither to mineral matter or inertinite content. A rough inverse relationship was found between the intrinsic reactivity of the chars and the inertinite content of the parent coal indicating that the char material derived from inertinite was intrinsically less reactive than that derived from vitrinite. These differences were no longer relevant at high temperature.Blast furnace injection performance was studied through thermobalance experiments using CO₂ atmosphere and 1050 °C temperature. It is apparent that the beneficiation process has no effect on the reactivity of the coals from Recreio Mine. The only exception is the low ash coal-2-LabB (11.5 wt%), for which a higher reactivity is indicated. The reactivity tests show also that the coals have adequate properties to be used together with imported coal blends in pulverized coal injection in the blast furnace (PCI).     

Carbonization of coals to produce anisotropic cokes: 3. Evaluation of low-rank bituminous and subbituminous coals by single and co-carbonizations with pitch

Single carbonizations and co-carbonizations of 17 low-rank bituminous and subbituminous coals have been studied to evaluate their suitability as sources of blast furnace coke in terms of pore-wall profile and anisotropic development within the cokes. Co-carbonizations suggest the possible use of low-rank coals which from single carbonizations would not have been considered suitable. To evaluate semi-quantitatively the coke quality, two structural characteristics of the cokes produced by single and co-carbonizations are graded on a scale of 1 to 5. Overall assessments for each coal are plotted against the atomic H/C and 0/C ratios of the original coals. Although there are a few exceptions, coals with similar assessments are located in the same region of the plot, indicating that, to a first approximation, the H/C and 0/C ratios are suitable indicators of the single and co-carbonization properties of a coal. The presence of cations in the coal appears to be an additional factor influencing the carbonization properties and may explain the exceptional behaviour of some coals. Removal of these cations by pretreatment of the coals improves the carbonization properties.     

Kinetic parameters of oxidation of bituminous coals from heat-release rate measurements

Arrhenius parameters of oxidation of two Scottish bituminous coals were determined from heat-release rate measurements at laboratory oven temperatures. When the heat release rate expression so obtained was applied at stockpiling temperatures (∼300 K), one coal was shown to have a significantly greater propensity to spontaneous combustion than the other.     

Supercritical gas extraction of Polish brown coals

Properties of nine selected samples of Polish brown coals from both strip mines and perspective deposits varying in lithotype have been determined. This involved technical analysis of the elementary, group and petrographie compositions. The coals were extracted with supercritical toluene at 410 °C and 13 M Pa. The humic acid content and humodetrinite content were found to be well correlated with the yield of coal extract. The optimal conditions of gas extraction for a selected coal sample were determined.     

用于混煤燃烧控制砷排放模型研究

混煤掺烧是控制燃煤砷排放的有效方式,但由于缺乏相关的配煤模型,限制了该技术的应用。根据燃煤过程中砷的挥发释放机制,提出一种采用砷的释放指数P表征煤燃烧过程砷的释放特性的配煤模型。该模型综合考虑煤的灰分、灰中主要矿物元素含量、各矿物元素对砷的固定系数以及煤中砷含量等因素。研究结果表明,随着煤灰固定系数由23.12增至50.90,煤灰的气相砷吸附量由3.39 mg/g增至6.14 mg/g;随着释放指数P增大,砷的固定率减小,且随着温度升高,两者相关性由900℃的0.67增至1 300℃的0.86。根据P值筛选煤种进行掺烧,当掺混煤种P值差异较大时,掺烧低P值煤种不仅可降低混煤中的砷含量,还能促进高P值煤的砷在灰中富集,促进率达77.14%;掺混煤种P值差异较小时,掺烧低P值煤种会促进砷的释放。本模型可较好地筛选煤种,为混煤掺烧控制砷等痕量元素的排放提供了新的思路。     

Genesis of micrinite in some Australian coals

Seven Australian coals have been examined petrographically and two types of micrinite, disseminated micrinite and cavity-filling micrinite, are described. The disseminated micrinite appears to be very fine fragments of fusinite, semifusinite and other low reflectance inertinite. Cavity-filling micrinite occurs only in inertinite-rich coal seams and was formed under aerobic conditions. It is considered that porigelinite and other granular material in peat are the major progenitors of the cavity-filling micrinite. Such progenitors would have entered cell cavities and other voids, possibly as a water slurry, before or after they had been partially oxidized.     

Integrated instrumental analysis of soft brown coals

A method for the simultaneous determination of calorific value Q^r, ash yieldA^r and total water content W^r_t of soft brown coals is described. The method involves instrumental analysis of brown coal samples using neutron thermalization and gamma back-scattering techniques. Over 80 coal samples (1.1 kg) were analysed, having Q^rvalues up to 15.5 MJ kg^?1,A^r ranging from 3.6 to 76.1 wt% and W^r_t ranging from 11.4 to 61.7 wt%. A comparison of the results from the instrumental method with those of standard laboratory analyses is given, followed by a discussion of the accuracy of the method and of possible ways for improving it.     

Devolatilization and combustion of large coal particles in a fluidized bed

Devolatilization and combustion of large particles of Eastern Canadian coals (Evans and Minto), 5-50 mm dia., were studied in a bench-scale atmospheric fluidized bed reactor at 1023-1173 K with 0.5 mm sand particles as the bed material. The devolatilization time, mass loss history, changes in proximate volatiles content and C/H mass ratio, and temperature history at the centre of the particle during devolatilization were determined. The mass loss during devolatilization is correlated with the proximate volatiles content of the parent coal. The devolatilization time is correlated with the initial particle diameter by a power-law relation with an exponent of 1.54-1.64. The results show insignificant effect of superficial velocity on devolatilization.     

Constitution of tars from the flash pyrolysis of Australian coals: 1. Separation

Separation of the maltene fraction of Millmerran flash-pyrolysis coal tar by ion-exchange and adsorption chromatography produced coal-tar resins, aromatic hydrocarbons (AH) and an alkane/alkene fraction. The coal-tar resins comprise acid, base, neutral and polyfunctional fractions. Derivatives of benzene and naphthalene are the main volatile constituents of the AH fraction, while the alkane/alkene fraction consists mainly of straight-chain hydrocarbons from C₁₀ to C₃₄ and small amounts of isoprenoid hydrocarbons, steranes and triterpanes. Fuller's Earth, used in the separation of the maltenes, suffers from the disadvantage of irreversibly adsorbing organic material.     

半焦空气分级燃烧NOx排放试验研究

半焦是低阶煤经低温热解后的产物,其中半焦粉与煤粉工业锅炉常用煤种烟煤相比价格低廉。若能将半焦粉用作煤粉工业锅炉的燃料,既可拓宽煤粉工业锅炉的适用燃料范围,又可增强煤粉工业锅炉的市场竞争力。由于半焦挥发分低、固定碳高,实现其着火和稳定燃烧需要更高的温度,同时,降低NOx初始排放也是一个技术难题。为了实现半焦在煤粉工业锅炉中的稳定燃烧及NOx排放的降低,采用两段式滴管炉开展半焦空气分级燃烧NOx排放规律研究。笔者对半焦空气不分级燃烧NOx排放规律进行了研究,主要探究了主燃区温度(1 000~1 400℃)及过量空气系数的影响,为后续空气分级燃烧降低NOx的效果提供对比依据。半焦空气分级燃烧试验主要研究了主燃区温度(1 000~1 400℃)及二次风比例(0.4~0.8)的影响,并从燃尽率、NOx减少比例、灰样微观孔隙和形貌等方面进行论证,试验结果表明,在空气不分级燃烧条件下,即燃尽风配风比例为0时,随着主燃区温度升高,NOx排放浓度随之迅速升高;随着过量空气系数增加,NOx浓度先迅速增加,过量空气系数大于1.15时,NOx浓度增速变缓;在空气分级燃烧中,相同主燃区温度条件下,二次风比例由高到低变化时,NOx排放呈先迅速下降后缓慢回升的变化趋势,燃尽率先快速升高而后趋于平缓。二次风比例为0.56时(即燃尽风率为0.39),燃尽率达90%,NOx排放浓度降至最低,为120 mg/m^3以下,此时是试验条件下的最佳二次风比例。     

Carbonization reactions of inertinite macerals in Australian coals

Widespread disagreement about the degree of reactivity of the inertinite group of macerais is related to variations in experimental conditions of assessment and failure to appreciate technological modifications imposed on similar macerals by dissimilar source materials and depositional conditions. This has resulted in the constant under-estimation of the coking potential of post-Carboniferous inertinite-rich coals by predictive methods developed for vitrinite-rich Carboniferous coals. Coking tests up to 1000 °C have been carried out on 20 coals of different rank in such a manner that coked portions of the samples could be correlated with their uncoked equivalents. It has been found that an inverse relationship exists between the level of precarbonization reflectance (PCR) of inertinite and the reflectance and bireflectance of its coke. The increase in the latter parameter is non-linear and involves a sudden jump which is taken as the boundary between reactive (high bireflectance) and non-reactive (low bireflectance) inertinite. In relation to coal rank a reactivity field for inertinite has been delineated which can be subdivided into two areas of high and moderate reactivity, respectively. On the whole, the proportion of reactive inertinite is larger than allowed for in most petrography-based coke stability calculations.