Drying Characteristics of East Inner Mongolia Lignite and Effect on the Readsorption Moisture
|School||Taiyuan University of Technology|
|Course||Chemical Engineering and Technology|
|Keywords||lignite moisture dry re-adsorption pore structure|
Coal has been playing a significant role in China for the past few years. In recent years, though the proportion of coal in first energy consumption has been decreased, but its amounts of production and consumption continue increasing. What was worse, the ratio of coal reserve and production in China is far more less than that of the world average level. It is also a prominent problem to block the sustainable development of energy industry that the high quality coal resources seem gradually inadequate. Therefore, the exploiting of low rank coals in a large scale, especially the lignite, is of a great significance to satisfy the rapid growing energy demands and to realize the goal of energy conservation and emission reduction. Lignite resources is relative rich in China, but its utilization efficiency is low due to the characters of high water content, low calorific value, being easy to weathering and spontaneous combustion, and high re-adsorption capacity after dried. It is necessary to upgrade lignite before large-scale utilization. There exist less fundamental researches about the lignite dehydration mechanism and its main influencing factors, the relation between removing process of moisture and readsorption capacity of dried coal, which limit the using range of lignite in a large extent. So, we chose the east Inner Mongolia lignites as the experimental samples and study the effects of various factors (such as temperature, particle size, drying time and atmosphere) on the processes of lignite drying and re-adsorption moisture. The main experimental conclusions include the following aspects:The drying efficiency of lignite increases with the increase of temperature in the drying process and achieves almost100%at the120℃. The deepening of pore structure damage extent is to the benefit of water removal from lignite. As the temperature rises, the carboxyl groups on the surface of lignite start to decompose at160℃and result to the releasing of carbon dioxide.The re-adsorption capacity of dried coal increases firstly and then deceases with the rise of drying temperature, which should be related to the shareof mesopores volume. The total moisture content of dried lignite is the lowest at160℃in all samples after readsorption of dried coal.The surface structure of lignite is destroyed and the expansion and cracking phenomenon appear in the initial stage of drying because of the low surface energy. During this process, the macropores firstly collapse and fragment into smaller porous structure leading to specific surface area and porosity increase. With the drying temperature rising, the pore structure is destroyed to a deeper level and parts of pore channels are clogged owing to cross-linking among pores. Comparing the changes of pore structure with the corresponding content of moisture re-adsorption, a significant conclusion is given that mesopores structure greatly influences the re-adsorption behavior of upgraded lignite.The samples with smaller particle sizes present the high drying efficiency because of absorbing more heat through the bigger specific surface area. By comparing the results of MIP characterization, it is found that the pore structure of samples with smaller particle sizes is less developed than that of samples with larger particle sizes, and the degree of pore structure damage is also deeper under the similar drying conditions. For this reason, the re-adsorption amount of drying sample decline with the decreasing particle sizes of lignite. From these results, it is revealed that the changes of mesopores structure contribute greatly to the moisture re-adsorption process of dried lignite.The drying efficiency of lignite emerges the regular variation with the change of drying time. When the drying time is extended, the moisture removal amount firstly increases and then tends to be constant. The balance time becomes shorter with the drying temperature rising. The re-adsorbed moisture amount is diverse at different drying time and it increases to the maximum when the drying time is30min and then decreases. The total moisture contents of different drying samples represent that it is minimum when the drying time is20mm.The unique characteristic of CO2linear molecule is beneficial to replacing water molecules of coal and making the ability of occupying the active sites on the surface of lignite stronger than CH4molecule. Compared with Ar atmosphere, CO2atmosphere has a higher drying efficiency, but their difference decreases with increasing drying temperature. CO2gas can make the surface of the lignite swell and crack early, and its pore enlargement is more obvious. Compared with the samples dried under Ar atmosphere, the amount of mesopores of the coal samples dried under CO2atmosphere is larger and the re-adsorption capacity is greater. The changes of pore structure characteristics of lignite caused by drying under CO2gas is a step-by-step process. The drying efficiency of lignite is greater under CO2atmosphere than that under Ar atmosphere and the difference is gradually increased with the drying time extending.