Efficient drying and oxygen-containing functional groups characteristics of lignite during microwave irradiation process

To remove the high moisture of ZhaoTong lignite, the efficient drying characteristics and oxygen-containing functional groups changes in lignite during microwave irradiation process were highlighted in this study. As the microwave absorbers, lignite char and NaNO₃ were added to microwave drying of ZhaoTong lignite. The minimum chemical oxygen demand of waste water generated from microwave drying process of lignite was 99.89?mg?O₂?L^?1. The effects of microwave power, lignite mass, the weight ratio of lignite char to lignite and NaNO₃ content on the drying rate, and moisture diffusion coefficient of lignite were investigated during lignite microwave irradiation process. It was found that the drying rate and moisture diffusion coefficient of lignite increased with increasing microwave power, the weight ratio of lignite char to lignite and NaNO₃ content, but decreased with increasing lignite mass. Lignite char and NaNO₃ were mixed with lignite that can enhance the instantaneous surface temperature of lignite sample under microwave irradiation. Compared with addition of lignite char to lignite, the addition of NaNO₃ to lignite can decrease the unit electric power consumption of moisture evaporating. And the minimum unit electric power consumption of moisture evaporating was 9.44?Wh?g^?1. The FTIR technology was used to investigate the oxygen-containing functional groups changes in lignite during microwave drying process. The oxygen-containing functional groups of lignite were effectively removed with increasing microwave power.     

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.     

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.     

褐煤物化结构对水分复吸的影响

阐述了褐煤中水分的赋存形态及其在受热过程中的转化行为。分析了不同提质干燥过程中褐煤物化结构的变化,探讨了褐煤物化结构与水分脱除及复吸的关系。结果表明:褐煤表面丰富的孔结构和大量亲水性含氧官能团是其水分较高的主要原因。干燥提质过程中,孔结构坍塌和交联反应的剧烈程度对于抑制复吸非常重要。加热过程中褐煤表面活性含氧官能团的数量、形成和分解行为直接影响褐煤的持水和吸氧能力,含氧官能团的分解会使煤的疏水性增加,自燃倾向性降低。最后说明褐煤提质的关键是水分的高效安全脱除,脱水煤表面氧化和水分复吸的抑制,并提出了抑制提质褐煤复吸水分的可能途径是依据不同组成、结构褐煤的水分赋存和脱除行为,针对性地改善干燥操作条件,调节样品孔径分布、比表面积和表面含氧官能团的存在形态及数量。     

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.     

Experimental study on the effects of drying methods on the stabilities of lignite

The drying processes are always applied prior to the transportation or utilization of lignite, and result in notable changes in the stabilities of lignite. In this paper, the study on the effects of nitrogen and MTE drying process on the physico-chemical properties and stabilities of Zhaotong lignite was carried out. The briquettes produced by MTE drying in this study were 150 mm in dimension, and so had a much larger particle size than nitrogendried samples. Nitrogen adsorption, mercury intrusion porosimetry and scanning electron microscopy all suggested that drying was accompanied by the transformation of larger pores into smaller ones. Compared to nitrogen drying, the pore structures could be stabilized by the MTE process. The soluble salts were removed during MTE drying which resulted in the decrease in ash and the concentrations of some of the major metals.The removal of water enhanced the hydrophilicity of nitrogen dried samples, but did not affect the hydrophilicity of MTE dried samples. The moisture holding capacity of MTE dried samples reduced faster than nitrogen dried samples with the decrease of residual moisture content. Themoisture readsorption processes of MTE dried sampleswere strongly inhibited due to themuch larger particle size of sample produced byMTE drying than nitrogen drying. The susceptibility to spontaneous combustion, indicated by cross point temperature and self-heating tests, of nitrogen and MTE dried samples increased with the decrease of residual moisture content. The MTE dried samples are more liable to spontaneous combustion than nitrogen dried samples with the same residual moisture and particle size. However, the larger particle size of the MTE product made it more stable with respect to spontaneous combustion and also moisture readsorption.     

Changes of oxygen-containing groups during thermal treatment and their influences on moisture readsorption of lignite

The changes in oxygen-containing groups in the thermal treatment process of lignite were analyzed and the moisture readsorption amounts of upgraded coals were related in this study. A chemical titration method was applied to determine the types and contents of oxygen-containing groups in raw lignite and upgraded coals obtained in the temperature range of 353–633 K under atmospheres of argon, air, and carbon dioxide. Experimental results showed that the oxygen-containing groups mainly involved carbonyl, phenolic hydroxyl, and carboxylic groups (including carboxyl and carboxylate), among which phenolic groups were dominant. These different oxygen-containing groups decomposed discriminatingly with temperature according to thermal stability. The removal of oxygen-containing groups proceeded more easily in a CO₂ atmosphere compared to in Ar. The oxidation reaction occurring on the surface of lignite exposed to air led to an increase in oxygen-containing groups. The different types of oxygen-containing groups possessed various combining abilities with water, among which carboxyl was considered the most important group for the moisture readsorption. Decomposition of oxygen-containing groups in the thermal treatment process of lignite mainly caused a reduction in monolayer water adsorption and further decreased the multilayer water.     

Investigation of combustion reactivity and NO emission characteristics of chars obtained from the devolatilization of raw and partially dried lignite

In an attempt to achieve the clean and efficient utilization of lignite, drying pre-treatment was performed in this study before lignite combustion. The combustion reactivity and NO emission characteristics of the partially dried lignite samples in the char combustion stage were investigated by means of TG and isothermal combustion tests, and the reactivity could be summarized as the following order: L1>L0.5>raw>L3>LT>L5 (chars obtained from the devolatilization of the raw and partially dried coals at 378 K for 0.5, 1, 3, 5, and 120 minutes) and the NO conversion ratio of L1 was the lowest. When the moisture content in the coal particles was relatively high (19.68%-35.84%), the drying treatment could increase the combustion reactivity and inhibit NO emission in the char combustion stage; When the moisture content was within a relatively low range (0.17%-19.68%), the moisture removal had negative effects on the reactivity and NO emission in the char combustion stage. The surface behaviour and microstructure of the raw coal char and chars derived from the partially dried coals were clarified by temperature programmed desorption/reduction (TPD/TPR) and Raman spectroscopy. The results illustrated that L1 derived from L_c1 (19.68%) was the most reactive sample with the largest amount of C(O) on the particle surface. There were also more reactive aromatic structures in L1 than other samples. Compared with direct combustion or excessive drying treatment, lignite should be dried to a certain degree (19.68%) for optimized lignite combustion.     

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   

Effect of Drying Conditions on Moisture Re-Adsorption Performance of Dewatered Lignite

In order to improve the drying efficiency of lignite and restrain the moisture re-adsorption of dewatered coal, the drying characteristics of typical Chinese lignite, the re-adsorption performances of dewatered samples and the change in pore structure throughout the entire processes were investigated in this study. Lignite samples with four different particle size fractions were dried in a fixed-bed reactor in the temperature range 60–160°C. The re-adsorbing moisture behaviors of dewatered coal samples containing different water contents were investigated at temperatures of 20–40°C and humidities of 55–95%. The changes in the pore structure of raw coal and different dried samples were measured by mercury intrusion porosimetry (MIP) and the relations between their re-adsorption performance and change in pore structure were explored. The moisture removal yields of lignite increased with an increase in drying time and temperature and was close to 100% above 120°C and over 100% after holding 40 and 15 min at temperatures of 140 and 160°C due to the release of CO₂ from the decomposing carboxyl group in the coal matrix. The re-adsorbed moisture content in dewatered coal was influenced by drying temperature and coal particle size through varying pore structure. The temperature and relative humidity in the re-adsorbing process were the main factors that influenced the moisture re-adsorption capacity of dewatered lignite, in which the re-adsorbing temperature mainly operated by varying the bonding ability of water on the surface of dewatered coal, and the relative humidity was connected with the pore structure as well. The mesopore was the main factor that influenced the re-adsorption of dewatered coal and the re-adsorption of moisture in dewatered coal at 100°C was highest due to the narrow range of the pore radius and because the relative volume ratio of 5 to 50 nm mesopore (above 91%) was high. The water loss yield of lignite with smaller particle size was higher due to its larger pore volume and surface area, but its re-adsorption capacity was lower because of lower volume ratio of 5 to 50 nm mesopore volume in dewatered coal obtained from the smaller size lignite.     

Experimental study of thermal fragmentation of lignite in drying process

Abstract

This paper investigated the thermal fragmentation of three lignite samples with temperature of 100?°C–200?°C and drying time of 0–90?min using the fixed bed reactor. The effects of a variety of factors such as temperature and drying time on fragmentation ratio were studied. The results showed that fragmentation ratio was positively related to temperature and drying time, and the degree of fragmentation was different for different lignite samples. Additionally, it was demonstrated that the point load strength remarkably decreased with the increase of temperature and drying time. The fragmentation ratio as a function of point load strength represented that lignite with higher strength had a lower fragmentation ratio, and the possibility of secondary fragmentation of dried lignite decreased with the increase of point load strength. Fragmentation ratio decreased with the increase of lignite density. The mineral matters of lignite significantly had influence on fragmentation. The mass inhomogeneity degree was given to depict the mineral element compositions. A fragmentation prediction equation was established based on mass inhomogeneity degree, which was associated with moisture content and ash content.     

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。     

Experimental study and energy analysis on microwave-assisted lignite drying

For value-added utilization of high-humidity lignite, an investigation on its drying behavior was carried out, using a method of Microwave Thermogravimetric Analysis (MTGA). The characteristic of heat and mass transfer during drying process was studied. The effects of initial lignite mass on its drying process and energy consumption were also addressed. In addition, changes of lignite pore structure, sulfur content, and calorific value were tested after drying experiment. The results indicated that lignite microwave drying was divided into three stages: incubation stage, high-efficient dehydration stage, and deep dehydration stage. It was in agreement with the variation of temperature inside the sample. High-efficient dehydration stage and deep dehydration stage were separated at a critical moisture content. Microwave power and initial mass could be properly matched, which was effective to increase dehydration rate and decrease energy consumption. It was further noted that the best power for achieving the least energy consumption was in advance to that for achieving the greatest dehydration rate, especially for initial mass of 20?g and 25?g. The optimum power for initial mass of 20?g and 25?g was found to be 385?W and 539?W, respectively. Pore structure of dried lignite was basically deteriorated. Meanwhile, lignite microwave drying at 231?W for 6?min could lead to an increase of calorific value by 8.1% and a decrease of sulfur content by 68.6%.     

Influence of vacuum impregnation pretreatment combined with IR drying on quince quality with shrinkage modeling by ANN

The effects of infrared power on drying behavior of quince slice were investigated. The samples were pretreated under vacuum impregnation (VI) and atmospheric pressure with sucrose sirup. The quality attributes measured included moisture content, bulk density rehydration, water loss, solid gain, texture, porosity, color, non-enzymatic browning, and effective moisture diffusivity. In addition, the modeling of shrinkage by ANN. VI increased the effective moisture diffusivity, bulk density, and softening of the dried fruit tissues while decreasing the time of drying (p?<?0.05). The highest porosity was observed for the control samples treated under VI and dried at 1200?W. The desired color was achieved in the osmotic samples treated under atmospheric conditions and dried at 800?W. The rate of rehydration was reduced in the osmotic samples under vacuum. MLP neural network was used to model the shrinkage of the best topology 3-3-1 by LM learning algorithm and threshold function of Tangent sigmoid function, with a correlation coefficient of 0.9963 and the error MSE of 0.000340.     

Wettability modification of lignite by adsorption of alkyltrimethylammonium bromides with different alkyl chain length

To overcome the problem of moisture re-adsorption of dried lignite with common evaporation drying methods, a set of linear alkyl quaternary ammonium surfactants (C₁₀TAB, C₁₂TAB, and C₁₆TAB) were used to modify lignite surface, and the effects of alkyl chain length on the adsorption characteristics of surfactants and wettability of lignite surface were evaluated. Pseudo-first and pseudo-second-order kinetic models and Langmuir model were, respectively, used to simulate adsorption kinetics and adsorption isotherms. The results showed that three surfactants gradually formed double-layer adsorption on lignite surface and the loading of surfactant increased with the length of alkyl chain. X-ray photoelectron spectroscopy analysis showed that C₃H₉N⁺ moiety of the surfactants would preferentially interact with O?C?O groups of lignite. Results of wetting heating and moisture re-adsorption showed that three surfactants obviously decreased hydrophilicity and restrained moisture re-adsorption of lignite, but with the formation of double-layer adsorption, hydrophilic headgroups of surfactant faced outward, which caused increase in hydrophilicity of lignite. As a result of two opposite effects of surfactant chain length on lignite wettability, the effect of C₁₂TAB on decreasing hydrophilicity was the best among the three chosen surfactants.     

Impact of Swell-Drying Process on Water Activity and Drying Kinetics of Moroccan Pepper (Capsicum annum)

The impacts of total hot air drying (THD) and the swell-drying process (SD), which is hot air drying coupled with an instant controlled pressure drop DIC process, on the drying kinetics and adsorption isotherm behavior, were determined in the case of Moroccan peppers. Drying kinetics were studied through a model that takes into account the starting accessibility and the internal diffusion model with Fick's law. Adsorption experimental data were obtained by using the gravimetric method at 25, 40, and 50°C. Data were fitted to the GAB, Halsey, and Oswin models. Studied responses were: effective diffusivity (D_eff), starting accessibility (δW_s), monolayer moisture content (X_m), specific surface area (σ), internal porosity (?), and microstructure. Mean relative percentage deviation (E) and root mean square (RMS) were used to evaluate the fitting of models. Results showed that: pepper adsorption isotherms followed the type II isotherm curve;the equilibrium moisture content (X_eq) depended on the temperature and the drying method; the GAB and Halsey models fitted well the experimental data; at selected conditions of the DIC treatment (0.35 MPa and 5 s), the monolayer moisture content (X_m), the specific surface area (σ), and the internal porosity values were higher than those of THD samples; the DIC process intensified the drying operation, by increasing the effective diffusivity and the starting accessibility. Although the DIC process improves the drying kinetics, this behavior could be ignored related to a higher a_w. This study shows that, thanks to the new expanded structure obtained by the DIC process (higher porosity), the adsorption capacities of products are improved (a_w is reduced). Thus, the SD process improves both the drying operation and the stability of dried pepper products.     

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。     

Pulsed vacuum drying of wolfberry: Effects of infrared radiation heating and electronic panel contact heating methods on drying kinetics,color profile,and volatile compounds

ABSTRACT

The drying kinetics and quality attributes of wolfberry were investigated under pulsed vacuum drying based on two different heating ways of far-infrared radiation (PVD-FIR) and electronic panel contact (PVD-EPC) heating. They were operated at different drying values of heating panel temperatures (60, 65, and 70°C) with 15 and 2?min as the constant vacuum pressure and atmospheric pressure duration, respectively. Drying time for wolfberry dried by PVD-FIR was lower by 17–19% compared with that by PVD-EPC at the same drying temperature. The effective moisture diffusivity (D_eff) determined by Weibull distribution model ranged from 3.72?×?10^?10 to 6.59?×?10^?10?m²/s and 3.34?×?10^?10 to 6.88?×?10^?10?m²/s for PVD-FIR and PVD-EPC, respectively. The drying activation energy was 54.30 and 68.59?kJ/mol for the samples dried by PVD-FIR and PVD-EPC, respectively. The color parameters L*, a*, and b* of wolfberry dried by PVD-FIR were higher than those dried by PVD-EPC. The product dried by PVD-FIR contained more vivid luster compared to that dried by PVD-EPC. The contents of aldehydes, esters, phenols, and the heterocyclic compound in PVD-FIR sample were higher than those in PVD-EPC samples. Additionally, the alcohols, ketones, and acid contents in PVD-FIR sample were lower than those in PVD-EPC sample. In summary, PVD-FIR is more suitable for wolfberry drying as it enhances drying rate and product’s quality compared with PVD-EPC.     

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。     

Energy,exergy and economic analyses on heat pump drying of lignite

Abstract

Evaporative drying of lignite is an energy intensive process. In this study, the heat pump is integrated with a lignite drying system to decrease the energy consumption rate of lignite drying. The performance of heat pump drying is energetically and exergetically evaluated with developed models. Results show that the power consumption rates to dehydrate 1?kg of water from raw lignite in the heat pump drying system without and with lignite preheater are 660.82 and 585.62?kJ (kg H₂O)^?1, respectively. Exergetic analysis indicates that most exergy is destructed in the condenser and the evaporator in the heat pump drying. The case of lignite-to-electricity process (i.e., a lignite-fired power plant integrated with heat pump drying) is studied to examine additional benefits of heat pump drying to the downstream industrial processes that consume dried lignite. Thermodynamic and economic models are developed. Net efficiency of the lignite-to-electricity process can be increased by 1.4 and 1.57 percentage points for heat pump drying without and with lignite preheater, respectively. Preliminary economic analysis shows that the integration of heat pump drying without and with lignite preheater can earn additional 1.42 and 1.73 million USD, respectively. The influences of drying system and heat pump parameters are also analyzed.