Evolvement behavior of microstructure and H2O adsorption of lignite pyrolysis☆

The effect of pyrolysis on the microstructure and moisture adsorption of lignite was investigated with low field nuclear magnetic resonance spectroscopy. Changes in oxygen-containing groups were analyzed by Fourier transform infrared spectroscopy(FTIR), and H2 O adsorption mechanism on the surface of lignite pyrolysis was inferred. Two major changes in the pore structure of lignite char were observed as temperature increased in 105–200 °C and500–700 °C. Pyrolysis temperature is a significant factor in removing carboxyl and phenolic hydroxyl from lignite.Variation of ether bond content can be divided into three stages; the content initially increased, then decreased,and finally increased. The equilibrium adsorption ratio, content of oxygen-containing groups, and variation of pore volume below 700° were closely correlated with each other. The amount of adsorbed water on char pyrolyzed at700 °C increased. Moreover, the adsorption capacity of the lignite decreased, and the adsorption state changed.     

Moisture readsorption and low temperature oxidation characteristics of upgraded low rank coal

This study investigated the behavior of upgraded low rank coal produced by a coal-oil slurry dewatering process regarding moisture readsorption, low temperature oxidation, and spontaneous combustion. The upgraded low rank coal had higher heating values than raw coal. It also showed lower moisture readsorption than raw coal and had less susceptibility to low temperature oxidation and spontaneous combustion. This seemed to result from the coating of the asphalt on the surface of the coal, which covered fine pores and suppressed the active functional groups from reacting with moisture and oxygen in the air. The increasing upgrading pressure negatively affected the moisture readsorption, low temperature oxidation, and spontaneous combustion.     

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.     

低阶煤高温高压水热处理改性及其成浆特性

褐煤、亚烟煤等低阶煤内水、氧含量大,直接制备浆体燃料,成浆浓度低、粘度大、流动性差。通过高温高压对低阶煤进行水热处理(热水反应),可以在较短时间内提升其煤阶和热值,同时改变其理化特征,进而较大程度提高其成浆浓度。结果表明,热水反应后,小龙潭褐煤最大成浆浓度可以由44.6%(原煤)提高到64.55%;印尼MIP亚烟煤最大成浆浓度则由39.71%(原煤)提高到64.61%。不同反应终温对于热水反应产物的成浆性能改善程度不同,从含氧基团化学分析以及煤水界面接触角两个方面进行了机理探讨。     

Pyrolysis of brown coals. 1. Decomposition of acid groups in coals containing carboxyl groups in the acid and cation forms

The effect of low-temperature pyrolysis (up to 300 °C) on the acid groups of two low-rank coals (a brown coal from Victoria, Australia, and a lignite from Texas, U.S.A.) has been studied for samples in both the acid and cation forms. A preliminary study at temperatures above 300 °C was made on the brown coal. The carboxyl groups of coals in the acid form decompose to give one mole of carbon dioxide for each equivalent of carboxyl content. Cation-form coals yield more carbon dioxide on pyrolysis than can be accounted for by the carboxyl groups present. Water is evolved in proportion to the carbon dioxide evolved from both acid- and cation-form coals, but the ratios differ. Findings have been interpreted as indicating that some other oxygen-containing group is associated with the carboxyl group. In the case of the acid-form coal this group decomposes to give water. When the carboxyl group is in the cation form, decomposition of the associated groups gives carbon dioxide as well as water. Phenolic groups appear to be stable, at least to 300 °C.     

加压水蒸气下年轻煤脱氧改质的研究

年轻煤是煤液化的良好原料 ,但它的氧含量高增加了煤液化过程中无用的氢耗 ,对这些煤进行脱氧改质有重要的意义 .选择了四种年轻煤——霍林河、小龙潭、义马和神华煤在高压釜内水蒸气气氛下进行了脱氧改质的研究 .结果表明 ,处理后煤样的氧含量和含氧官能团降低显著 ,氧的脱除率最高达到了 2 0 .7% .此外 ,煤质还有一些其他的变化 ,如热值和碳含量有所提高 ,最高内在水分和挥发分降低 ,表明煤阶有所提高 .对煤中的总酸性基、羧基和酚羟基的化学分析显示 ,脱氧改质后煤样的羧基、酚羟基等含氧官能团明显降低 ,羧基和酚羟基的最高脱除率分别达到了78.5 %和 31 .3% ,达到了脱氧改质的目的 .     

Use of FTIR combined with forms of water to study the changes in hydrogen bonds during low-temperature heating of lignite

The changes in the hydrogen bonds (HBs) of three types of Indonesian lignite during low-temperature heating were investigated. The amount of water loss was determined by weighing the samples before and after heating in an oven. The changes in the number of the different types of HBs were determined using Fourier transform infrared spectroscopy coupled with types of water in lignite. The number of peak positions and absorption bands in each spectrum was determined by curve-fitting analysis with a Gaussian function. The quantified integrated area of aromatic hydrogen atoms was used to accurately investigate the changes in the HBs. The results show that at low temperatures (T ≤ 50°C), free water is mainly removed, and the HBs broken are those between free water molecules. However, at medium temperatures (50 <T ≤ 100°C), bound water is mainly removed. The number of HBs significantly changes because of the breaking of bound water molecule HBs and bound water cluster–carboxyl group HBs, and the formation of nonfreezable moisture HBs. At high temperatures (100 <T < 125°C), nonfreezable moisture can be released. The number of HBs changes as a result of competition between the removal of nonfreezable moisture and the increase in the number of carboxyl groups. At higher temperatures (T ≥ 125°C), the moisture remaining in lignite is thermal decomposition moisture. In addition, the rate of decomposition of carboxyl groups is higher than the rate of generation, which means that the number of HBs markedly decreases at higher temperatures.     

Preparation and characterization of carbon adsorbents from furfural

Carbon adsorbents with different properties were obtained from furfural with variations in the activation reagents and conditions of treatment. They possess insignificant ash and sulfur contents. Pore volume, pore size distribution and the chemical character of the surface of the obtained carbon adsorbent depend on the activation reagent and temperature of treatment. Various oxygen-containing groups of acidic character (carboxyl groups, carboxyl groups in lactone-like binding, phenolic hydroxyl and carbonyl groups) and different chemical properties are present on the surface of carbon oxidized with air. The sample activated with water vapour contains predominantly oxygen-containing groups with basic character.     

Transformation of the reabsorption characteristics of lignite treated by low and high temperature drying process

Abstract

The reabsorption characteristics of the lignite treated by low and high temperature drying process were addressed in the paper. The information about the moisture form, functional groups, effective water-filled porosities and equilibrium moisture content of the lignite before and after the drying process was investigated using Differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and a self-made reabsorption device, respectively. The results show that the low drying temperature (140, 190, 230?°C, 10?min, N₂) has little impact on the effective water-filled porosities of the resulted samples, whereas it has a great influence on the main oxygen-containing functional groups, which amount firstly decreases and then increases with the drying temperature increasing. In the case of the lignite samples dried under high-temperature (600, 700, 800?°C, 30?s, N₂), the amount of the effective water-filled porosity of the sample decreases and the amount of oxygen-containing functional groups increases as the temperature increasing. The reabsorption capability of the high temperature dried sample is much lower than that of the sample treated under low drying temperature. The reabsorption characteristics of the low-temperature dried samples are affected by the amount of the oxygen-containing functional groups, while the effective water-filled porosity is main factor for the lignite samples derived from high temperature drying process. Moreover, the work gives a good evidence that the high-temperature drying process is an effective choose for lignite upgrading.     

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.     

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.     

Infrared absorption characteristics of hydroxyl groups in coal tars

Tar evolution was observed over a temperature range of 150–600 °C for four coals: Pittsburgh bituminous, Illinois No. 6, Rawhide subbituminous, and Texas lignite. Isolation of the evolved tars in a nitrogen matrix at 15 °K produced better resolved infrared spectra than those in a coal matrix, thus enhancing structural characterization of the tar molecules. Two distinct hydroxyl functional groups in the tar molecules free of hydrogen bonding were identified for the first time without interference from H₂O absorptions. These absorptions at 3626.5 and 3580.9 cm⁻¹ have been assigned to phenolic hydroxyls. It is suggested that carboxylic and aliphatic hydroxyl groups do not survive the vaporization process. Tars from Illinois No. 6 were found to contain the largest amount of phenolic hydroxyl ; Pittsburgh No. 8 tar contains approximately half of that for Illinois No. 6 while Rawhide and Texas lignite contain much less phenolic than either of the other coals.     

Evolution of organic oxygen bonds during pyrolysis of coal

Five Czech coals of different rank (70.2, 71.5, 74.5, 75.0 and 83.3 wt% carbon) were heated at different temperatures in the range 250–850 °C and the contents of oxygen-containing functional groups (OH, COOH, CO, OCH₃) were determined in the coals and carbonization residues. The coals and residues were subjected to pyrolysis chromatography at 740 °C and the yields of phenol produced were measured. The dependence of the oxygen functions and the phenol yield on temperature and coal rank is discussed. In the coals studied, the most resistant oxygen groups appear to be hydroxyl and methoxyl; carboxyl and carbonyl oxygen are eliminated below 550 °C.     

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。     

Minimization of Moisture Readsorption in Dried Coal Samples

Low-rank coals csonstitute a major energy source for the future as reserves of such high-moisture coals around the world are vast. Currently they are considered undesirable since high moisture content entails high transportation costs, potential safety hazards in transportation and storage, and the low thermal efficiency obtained in combustion of such coals. Furthermore, low-moisture-content coal is needed for the various coal pyrolysis, gasification developed. Hence, various upgrading processes have been developed to reduce the moisture content. Moisture readsorption and spontaneous combustion are important issues in coal upgrading processes. This article discusses results of laboratory experiments conducted to study the options for minimization of readsorption of moisture after drying of selected coal samples. Results suggest that there is little benefit in drying low-rank coal at high temperatures. It was found that the higher the amount of bitumen used for coating, the lower is the readsorption of moisture by dried coal. Also, mixing high-temperature-dried coal with wet coal in appropriate proportion can yield reduced moisture content as the sensible heat in the hot coal is utilized for evaporation.     

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.     

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.     

Water absorption by coals: effects of pore structure and surface oxygen

The water-holding capacity of various coals from lignite to anthracite was measured and its relation to their oxygen content and pore structure was investigated. Both factors were found to play important roles in determining the water-holding capacity. Pyrolysis of a lignite at 400 °C decreased its water-holding capacity by more than two-thirds, because of the decrease in the number of hydrophilic sites. This was caused by the decomposition of oxygen-containing groups and the decrease in surface area which resulted from the plugging of small pores by volatile matter condensation, making their surfaces inaccessible to water.     

Fuel characteristics of molasses-impregnated low-rank coal produced in a top-spray fluidized-bed reactor

Upgrading low-rank coal (LRC) through various strategies is always an important issue. Here, we report the production of hybrid coals and an evaluation of their characteristics for use as a fuel in power plants. The hybrid coals (HCKs) were prepared by a combination of drying and biomass impregnation into an Indonesian LRC followed by a precarbonization process. We used a top-spray fluidized-bed reactor for drying, biomass impregnation, and precarbonization to produce hybrid coals that have improved fuel characteristics in terms of heating value, moisture readsorption, and combustion patterns. A systematic study reveals that experimental parameters, such as a bed temperature, bioliquid spraying procedure, and precarbonization temperature strongly influence the characteristics of the resulting hybrid coal, meaning that they have an important role in upgrading LRC. In particular, the hybrid coal prepared by a process of simultaneous drying and bioliquid spraying followed by precarbonization at 200–300°C showed high contents of fixed carbon, an improved heating value, lower moisture adsorption, and single combustion patterns in which the characteristics were dramatically upgraded for practical use as a fuel in power plants. In addition, the simultaneous process using a fluidized-bed reactor has great potential because it can achieve process simplification, reduce manufacturing costs, and handle coal particles easily.