Structure characteristics of dewatered lignite from different drying temperature and time and effects on its behaviors of re-adsorbing moisture

By changing the drying temperature and residence time, a series of different degree dewatered lignite samples were prepared in the fixed-bed reactor under inert gas and their adsorbing moisture behaviors were investigated under certain re-adsorption conditions. Using SEM, nitrogen adsorption and in-situ FTIR analysis methods, the structure changes of dewatered lignite were measured and associated with the drying and re-adsorption behaviors. The results show that the higher the drying temperature, the higher the drying efficiency and the more obvious structure change of dewatered lignite. But the moisture re-adsorption amount of dewatered lignite showed different changing trends. Under the drying condition of 140°C and 50min, the total moisture content of coal after moisture re-adsorption was the lowest. Most pores are mesoporous in lignite and the macropore structure collapsed into mesopore during drying. When the drying temperature was higher than 120°C, oxygen-containing functional groups began to decompose; with the increase of drying temperature and time, decomposition rate increased. The hydrophilic strengths of -OH and COOH groups are stronger than that of others.     

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.     

Comparison of drying methods on physical and chemical properties of Shengli lignite

The physicochemical properties of Shengli lignite dewatered by three methods in hot air, microwave, and vacuum conditions were investigated. The effects of drying method on the coal pore structure, surface morphology, surface carbon-containing groups, and moisture readsorption performance of dewatered lignite were studied by the Brunauer-Emmett-Teller (BET) method, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and dynamic water vapor sorption analysis (DVS). The results indicate that a large quantity of semi-open micropores and open-ended mesopores with a slit-like or cylinder shape exist in lignite. The specific surface area and total pore volume value of dewatered lignite decrease as follows: microwave-dried lignite (MD) > vacuum-dried lignite (VD) > hot air–dried lignite (HAD), which is consistent with MIP results. SEM images show that lignite dried by different methods is characterized by varied surface morphology. The XPS results reveal that VD has the highest content of C–C/C–H and the lowest content of C–O, and HAD has a higher content of C–O and a higher surface oxidation degree (α) than MD. In addition, the moisture readsorption capability of the dewatered lignite follows the sequence MD > VD > HAD.     

高温干燥对褐煤孔隙结构及水分复吸的影响

利用卧式固定床实验炉制得不同温度下干燥处理的煤样,采用低温氮吸附法测试煤样的比表面积、孔体积和孔径分布等孔隙特征参数,使用复吸实验装置测定不同干燥程度褐煤煤样的平衡含水量,探索了高温干燥处理后褐煤孔隙结构的演变与其复吸特性之间的关联规律。结果表明：褐煤原煤及不同干燥温度(600~800℃)下半焦的等温吸附曲线均属于第Ⅱ类吸附等温线,褐煤原煤、600℃和700℃干燥半焦的吸附回线均属于L1型,800℃干燥半焦的吸附回线有从L1型转变为L2型的趋势;随着干燥温度的增加,干燥半焦的比表面积先增大后减小,介孔峰值的孔径微分同样先增大后减小,而大孔孔径微分基本保持不变;分形维数D₁和D₂呈相反的变化趋势,且D₂>D₁;不同干燥程度半焦的复吸曲线变化趋势相同,且平衡含水量随着干燥温度的升高而减小;半焦复吸特性与孔隙结构有关,平衡含水率与其孔容积之间呈较好的线性关系。     

Drying behavior of lignite under microwave heating

Because of lignite’s high moisture content, it must be dried before most applications. Microwave radiation may be suitable for efficient drying because of its special heating properties. This study investigated the drying behavior of lignite samples from eastern Inner Mongolia by microwave thermogravimetric analysis. Three stages of microwave drying were observed: preheating, fast weight loss, and falling rate drying periods. Samples’ surface temperatures increased dramatically during preheating, dropped slightly in the second period, and rose again in the final period. The measured surface temperature was <95°C during microwave heating. The overall moisture content decreased more rapidly under higher microwave power. Fine lignite particles (diameter <0.2?mm) and lump samples (particle size 10?mm) dried better than granular lignite (particle size 1–2?mm). The samples also underwent slight natural drying (1–2% point reduction in moisture content) after microwave treatment. The critical moisture content of lignite (11–15% under experimental conditions) was redefined. Energy consumption was analyzed to evaluate the feasibility of the proposed drying process.     

Mechanical/thermal dewatering of lignite. Part 3: Physical properties and pore structure of MTE product coals

The mechanical thermal dewatering (MTE) process has been shown to effectively dewater high moisture content low rank coals via the application of mechanical force at elevated temperatures.Using mercury intrusion porosimetry (MIP) as an investigative tool, this study examines how MTE processing conditions, such as temperature and pressure, affect the compressibility, pore size distribution, apparent (skeletal) density and shrinkage behaviour of three low rank coals sourced from Australia, Greece and Germany. As both pore filling and sample compression occurred at high mercury intrusion pressures, all MIP data were corrected for compression effects by using compressibility values derived from mercury extrusion data.The MTE process is shown to produce a low porosity coal, which, depending upon the processing conditions used, undergoes further shrinkage upon oven drying at 105 °C. An increase in MTE temperature (above about 85 °C) led to an increase in mesopore volume, which is caused by a hardening of the coal structure, leading to pore volume retention and a consequent reduction in percent shrinkage on oven drying. The increase in measured mesopore volume is also associated with an increase in measured surface area.The reverse trend is seen with increasing MTE pressure, where both the macro and mesopore volume decrease with pressure, causing the percent shrinkage to increase accordingly. This effect may be due to an increase in capillary forces caused by a decrease in the average pore diameter. The percent shrinkage increased up to a pore volume of about 0.1 cm³/g, beyond which no further reduction in pore volume was achieved. The decrease in mesopore volume is also associated with a decrease in measured surface area.Compressibility values derived from mercury extrusion data show that the MTE process has little impact on the network strength of the skeletal network structure of all three coals investigated. Likewise, the skeletal density remained relatively unchanged.The reduction in water content, pore volume and the changes in shrinkage behaviour under increasingly severe MTE conditions are suggestive of the physical changes that accompany increased coalification (rank) within the lignitic range.     

Effect of industrial microwave irradiation on the physicochemical properties and pyrolysis characteristics of lignite

The surface functional groups and pyrolysis characteristics of lignite irradiated by microwave were comparatively studied to evaluate the feasibility of using industrial 915 MHz for lignite drying. The drying kinetics, micro structure, chemical functional groups, re-adsorption properties, and pyrolysis characteristics of the dried coal were respectively analyzed. Results indicated that for typical Chinese lignite studied in this paper, 915 MHz microwave drying was 7.8 times faster than that of the hot air drying. After industrial microwave drying, the sample possessed much higher total specific surface area and specific pore volume than that of air dried sample. The oxygen functional groups and re-adsorption ratio of microwave irradiated coal decreased, showing weakened hydrophilicity. Moreover, during the pyrolysis of the coal dried by hot air and microwave, the yield of tar largely increased from 1.3% to 8.5% and the gas production increased correspondingly. The composition of the tar was also furtherly analyzed, results indicated that Miscellaneous hydrocarbons (HCs) were the main component of the tar, and microwave irradiation can reduce the fraction of polycyclic aromatic hydrocarbons (PAHs) from 26.4% to 22.7%.     

Efficient <Emphasis Type="Italic">in situ</Emphasis> drying of low rank coal in a pressurized down-flow flash dryer

Flash drying of low rank coal with synthesis gas was addressed by using a pressurized down-flow dryer. The proposed method is a potential approach to secure gaseous water that is required in coal processing by utilizing moisture in the low rank coal. The drying process was promoted by increasing the initial temperature of the synthesis gas as a drying medium and decreasing the particle size of the coal. The moisture removal rate of the coal using synthesis gas at 9 bars and 500 °C reached up to 97% within ten seconds. Although it is a higher temperature than that of fixed bed or moving bed dryer, outlet moisture laden synthesis gas had the low level of tar enough to be a feedstock of downstream catalytic process due to the short residence time in the dryer. The chemical composition changes of the coal during the drying resulted in reducing oxygen content to the atomic ratio of oxygen to carbon as 0.1 and enhancing its calorific value. Disappearance of hydroxyl functional group from the surface and physical reduction of the surface area of the coal decreased the moisture re-adsorption capacity, which could prevent the spontaneous combustion of the low rank coal.     

褐煤干燥特性及产品特性研究

通过对内蒙古锡林浩特褐煤的热重分析可知,褐煤干燥过程约30 min,干馏过程约1.5 h。对不同粒径褐煤进行了干燥特性研究,说明+100,100～50,-50 mm原煤完全干燥分别需要4.0,1.0,0.5 h,粒径越小,煤样失重率越大,干燥速度越大,达到相同干燥效果所需的干燥时间也越短。最后分析了干燥温度对产品特性的影响,同时测试了干燥、干馏提质产品的自燃特性和复吸水特性,结果表明:与原煤相比,150,200℃产品挥发分分别提高了33.72%和31.13%,更易发生自燃,而550℃干馏过程中挥发分降低了30.89%,热稳定性大幅增加;150,200,550℃产品吸氧量分别提高了0.43,0.65,0.72 cm3/g;干燥产品燃点要低于原煤,而干馏产品燃点则高于原煤;干馏煤因改变了孔隙结构,最高内在水分降低,即复吸水的能力降低。因此,干燥提质产品与褐煤性质基本一致,而干馏提质产品性能则获得较大改善。     

Evolution of microstructure and combustion reactivity of lignite during high-temperature drying process

The evolutions of the microstructure characteristics and the combustion reactivity of lignite during high temperature drying process at 600–800°C were addressed in the paper. The information about the oxygen functional groups, carbon skeleton, pore structure, and combustion reactivity of the lignite before and after the drying treatment was obtained by Fourier transform infrared spectroscopy, Brunauer–Emmett–Teller technique, Raman spectroscopy and thermogravimetric analyzer, respectively. The results show that a different evolutionary trend of the chemical structure of the lignite appears at the high drying temperature of 700°C at which the carbonyl, carboxyl, and carboxylate significantly decomposed and the treatment gave a slight elimination of structural imperfections and the formation of new cross-linking structures. Then, at the drying temperature of 800°C, a slight increase of the oxygen functional groups was found. The break of the aromatic ring or heterocycle contributed to the rapid increase of amorphous carbon to some extent. During the drying process, the surface area of the resulted lignite gradually increased until 700°C and then decreased. The evolution may be attributed to the decomposition of organic compounds, shrinkage forces, and thermal relaxation effect. The investigation indicates that the change in chemical nature of the coal had a greater impact on the combustion reactivity than the internal pore structure did. In addition, the combustion intensity and performance of the treated lignite was enhanced by the drying process.     

褐煤深度脱水的实验研究

针对褐煤低温烟气干燥技术脱水幅度难以提高,发热量增加不多等问题,提出了低温干燥与深度脱水的联合工艺。研究了不同干燥温度、干燥时间和煤样粒度对褐煤深度脱水效果的影响,结果表明:褐煤深度脱水适宜的干燥温度为500～800℃,较为适宜的干燥时间为80 s左右,当粒度小于50 mm时,干燥产物发热量随粒度变化不大。最后提出了褐煤深度脱水最佳工艺条件和参数,即当预干燥煤的Mar约为18%,煤样粒度为-50 mm,干燥温度为700℃,干燥时间为80 s时,深度脱水产物的Mar为8%,Vdaf在46%左右,折算Qnet,ar约为21 kJ/g。褐煤深度脱水促进了褐煤发热量的进一步提高,实现了褐煤资源的增值,研究结果为褐煤低温干燥与深度脱水联合工艺技术方案的确定奠定了基础。     

Effect of mechanical thermal expression drying technology on lignite structure

Mechanical thermal expression (MTE) is a developing nonevaporative lignite dewatering technology. It has been proved to be effective to dewater high moisture content in low-rank coals via the application of mechanical force and thermal energy at elevated temperatures. In this paper, the dewatering behavior of the Xiaolongtang lignite in Yunnan province, China during the MTE process was studied with three process parameters: time, temperature, and pressure. Meanwhile, the mechanism was also explored of how variations in temperature and pressure during the MTE process affect the surface oxygen functional groups and pore structure, which was mainly conducted by means of Fourier transform-infrared spectrometer (FTIR) and mercury intrusion porosimetry (MIP). Increases in MTE temperature and pressure resulted in significant reductions in residual moisture content and moisture-holding capacity, along with the increase in fixed carbon content and content reductions of other elements, especially oxygen content, this could be largely attributed to the destruction of the surface oxygen functional groups and porosity in the lignite. Technologically, the optimal conditions for temperature and pressure are 150–220°C and 6–10?MPa. The residual moisture content of the lignite treated by MTE at 200°C, 10?MPa is lower than 8%; the dewatering rate reaches over 76% with the calorific value being approximately 22?MJ/kg. Carboxyl and hydroxyl groups break down at drying temperatures above 120°C and constant applied pressure 10?MPa; with the pore volume significantly reduced, only few pores (diameter?相似文献   

Analysis of D2 law in case of Shengli lignite drying under inert and uninert environment

Shengli lignite coal, originated from inner Mongolia China, contains significantly high amount of moisture (more than 30%) which can cause spontaneous combustion or other application problems. Thus, it is of interest to understand the heat and mass transfer mechanism of the low-rank lignite drying under different drying environments such as N₂, CO₂, air, argon, and helium. In this study, fundamental drying experiments with different drying agents were performed on coal samples using thermogravimetric analysis (TGA) method. Lignites with size of 0.045–0.075?mm were heated up from ambient temperature to a target temperature of 175°C under different environments at heating rates of 5, 10, 20, 40, and 80 °C/min, respectively. It was found that thermal conductivity of drying media, heating rate, and initial moisture content are three most significant factors affecting lignite drying process. The highest moisture release rate and the lowest T_peak (when maximum moisture release rate occurred) were observed when drying with helium due to its highest evaporation constant (i.e., highest thermal conductivity). Moreover, higher heating rate and moisture content resulted in higher evaporation rate and T_peak. In the meantime, the classical D² law, which is used to simulate the liquid fuel droplet combustion, was further developed to describe the “group effect” of moisture evaporation process of solid fuel during drying. The D² law well explains the experiment results. Finally, the structures of the dried lignite samples under different drying mediums were investigated through scanning electron microscopy studies. It was found that lignite coals shrank and became more compact when dried out, especially with drying agent CO₂.     

Comparison of some physico–chemical properties of Victorian lignite dewatered under non-evaporative conditions

A Victorian lignite, designated Loy Yang low ash, run of mine (LYLA (R)) has been dewatered using mechanical thermal expression (MTE) at 150–200 °C and 6–25 MPa and by hydrothermal dewatering (HTD) at 200–300 °C and the products compared. Total acidity values for all samples as measured by a pyrolysis thermogravimetric Fourier transform infrared (TG–FTIR) method were similar to those measured by barium ion exchange. Stronger (carboxylic) acid values determined by pyrolysis TG–FTIR tended to be lower than ion exchange values, except for the 300 °C HTD sample, for which both methods gave similar values although these were much lower than at all of the other treatment temperatures. Equilibrium moisture contents (EMC) for the MTE products and the 200 °C HTD product were similar to those of the original lignite at relative vapour pressure (RVP) ⩽ 52%, but lower at RVP 92%. EMC values for 300 °C HTD products were all lower than for the original lignite, indicating that processing temperature was the most important factor in determining these properties. CO₂ adsorption surface area was also mainly a function of processing temperature, decreasing with increasing temperature. However, the pore volume as determined by mercury porosimetry was influenced by whether dewatering was effected by MTE or HTD, the mechanical pressure applied in the MTE process resulting in a lower porosity.     

不同干燥程度胜利褐煤燃烧与自燃特性

为了探究高水分褐煤干燥后的燃烧与自燃特性变化,采用一维火焰炉、煤粉着火炉以及自燃试验台对不同干燥程度的胜利褐煤进行了试验研究。试验结果表明,试验样品的着火温度随着干燥程度的增加而降低,随着风煤比的增加而增加,煤粉细度同样会对着火温度产生影响。在燃用干燥褐煤过程中宜采用较高一次风率以提高制粉系统的安全性。胜利褐煤及其干燥褐煤均属于极易燃尽煤种,燃尽率均在99.4%以上,水分的变化对燃尽率影响不大。建议胜利褐煤与20%水分干燥褐煤的运行氧含量控制在3.5%左右。随着干燥程度的加深和粒径的减小,褐煤越容易自燃。胜利褐煤干燥到20%以下可能有自燃的风险。     

Effect of moisture distribution in pore structure on fragmentation characteristics of lignite

ABSTRACT

The experiments were performed to explore the effect of moisture on fragmentation properties of Mengdong (MD) lignite, Xiao Longtan (XLT) lignite, and Zhaotong (ZT) lignite with temperature of 200°C and particle size of 13–10?mm using the fixed bed reactor. It showed fragmentation ratio and pulverization ratio increased, and particle size variation ratio decreased with an increase in initial moisture of lignite. The distribution of moisture in pores under different initial moisture contents was analyzed by nuclear magnetic resonance. The distribution of moisture in pores between raw and dried lignite was different. The water content in macropores rapidly decreased, but water content in mesopores remained constant in prior period and then slightly decreased with the residence time increasing. There was a good connectivity between macropores and mesopores. The water in micropores, which hardly influenced fragmentation, was generally closed and not favored for the mobility of water. The water in macropores had a significant effect on fragmentation of lignite. However, when the moisture content in macropores was less than 0.10?g/g coal, the degree of fragmentation became little.     

褐煤低温干燥特性的实验研究

中国褐煤储量丰富,但较高水分极大地限制了其开采和利用,对其进行脱水提质是解决褐煤高效利用的关键。通过热重分析仪（TGA）对HL和YN的不同粒径褐煤,分别在50,80和110℃等温干燥2h。结果表明：在每个干燥温度下煤样都能达到恒重,随着温度的升高煤样总失重增加,说明煤中水分与煤表面之间具有不同的结合强度;随着水分的降低,水分蒸发所需能量增加,煤水之间的相互作用加强,其中包括氢键和微孔对水的束缚力。粒径0．250～0．150mm、水分26．61％的HL褐煤在50c（二干燥后,水分降至6．96％,此时水分以分子层水的形式存在;干燥温度升至110℃时,煤中水分并未明显降低,说明煤中官能团与水分子间形成的氢键对水分有强烈的吸附作用。HL褐煤50℃干燥后,0．150～0．074mm和0．074～0．038mm煤样残留水分分别为6．52％和3．93％,均低于0．250～0．150mm煤样的6．96％,说明0．250～0．150mm煤样中不能脱除的残留水是孔隙水,被禁锢在狭小空间内。     

干燥后褐煤水分复吸性的试验研究

在实验室条件下考察了深度干燥和轻度干燥后的褐煤在高湿度环境和自然环境中的水分复吸性。结果表明：干燥后褐煤由于自身特点,具有较大的吸水能力,但受环境条件的影响比较显著,深度干燥后的褐煤水分复吸性较强,当干燥后褐煤处于不同的湿度环境下,或复吸水分或复失水分,但最终将会趋于某一稳定水分值。建议工业化干燥过程中,干燥后褐煤的含水量略大于内含水量,当处于正常自然环境中,干燥后褐煤不会发生显著的水分复吸现象,过度干燥既不经济也无必要。     

Numerical Study on Moisture Transfer in Ultrasound-Assisted Convective Drying Process of Sludge

A coupled heat and moisture transfer model for ultrasound-assisted convective drying process of sludge was established. In this model, the permeable flow caused by acoustic pressure gradient in sludge was considered. The pore structure variety in sludge with ultrasonic irradiation was microscopically studied, and the pore size distribution of sludge was described by fractal geometry. Based on the fractal characterization, the physical properties of sludge including permeability, porosity, and tortuosity factor were determined, and the effective moisture diffusion coefficient of sludge under ultrasonic irradiation was also derived considering the effects of ultrasonic excitation energy and thermal effect on migration rate of water molecule. The effects of ultrasonic energy density and convective air temperature on convective drying process of sludge were numerically analyzed. The results showed that the ultrasonic irradiation changes the pore size distribution in sludge, the sludge flocs are dispersed, and the connectivity of pore structure is improved. Ultrasonic treatment is favorable to accelerating the moisture transport in the convective drying process of sludge, and the ultrasonic influence on moisture transport in sludge intensifies gradually with the increase of acoustic energy density from 0.2 to 0.6 W/ml. Furthermore, it can be also found that the enhancement effect of ultrasound on the average drying rate of sludge is more obvious at the connective air temperature of 65°C than that at 40°C under the uniform acoustic energy density and air velocity of 1.5 m/s.