Evolution Of Physical And Chemical Structures Of Lignite During Thermal Dewatering And Its Influence On Re-adsorption Moisture Of Upgraded Coal

The high moisture content in lignite is the main reason that the resource reserves of lignite are relatively abundant, but it has not been large-scalely exploited and effectively utilized. The moisture re-adsorption of the conventionally and thermally upgraded lignite becomes the bottleneck of the effective utilization of lignite. The basic theory researches on the variations of physical and chemical structures druring drying and re-adsorption of upgraded lignite based on China typical lignite and the suppression of the moisture re-adsorption capacity of upgraded lignite by improving its surface activity will afford the theoretical support for reasonable transformation of lignite resource.In this thesis, XM lignite from Inner Mongolia in China, which was uniformed in the project of National Basic Research Program of China(973 Program), was selected for the main research object and HLH lignite was used for complementary object. The drying characteristics of lignite were systematically examined using the fixed-bed reactor. The influences of drying atmosphere, drying temperature and time, the particle size of coal sample on drying characteristics were analyzed. In addition, the drying characteristics under the microwave and conventional heat treatment were inspected using the microwave thermogravimetric analyzer, which was self-designed by our research groups. The difference between drying curves through heating effect and pure microwave treatment was analyzed. The moisture re-adsorption behavior of upgraded lignite dried at different conditions was systematically examined in the constant temperature and humidity chamber. The physical and chemical structure changes of lignite during drying were measured respectively using scanning electron microscopy, nitrogen adsorption instrument, in-situ infrared diffused reflectance instrument and chemical titration method. These changes were also associated with the moisture re-adsorption behavior of upgraded lignite. Through MB45 Halogen moisture meter, the drying kinetics of lignite was analyzed and the kinetic model and activation energy were respectively optimized and calculated. Through the DVS(dynamic vapor sorption) advantage type automatic high-pressure gas adsorption analyzer the re-adsorption kinetics was analyzed and the adsorption heat and activation energy were calculated. These theory analyses about moisture re-adsorption process were associated with the existing forms of re-adsorbed moisture.Main results obtained from this research are summarized as follows:(1) Drying efficiency of lignite increases with the increase of drying temperature and time under different heat treatment conditions and the removal maximum moisture content at a certain temperature also increases. Because of different physical and chemical properties of carrier gas, the influence of ambient atmosphere on drying efficiency is different. The higher heat conductivity coefficient of nitrogen results in the higher drying efficiency. The oxidation reaction of oxygen in the air atmosphere on the lignite surface accelerates the moisture removal but it reduces the drying efficiency of lignite at the higher temperature. The expansion and cracking phenomena on the lignite surface which caused by CO2 improve the drying efficiency of lignite. The greater particle size of coal sample, the smaller specific surface area, whic leads to the decrease of contact area between the coal sample and heat transfer medium and then the reduction of drying efficiency.(2) Under the condition of the same thermal track, the drying efficiency of microwave treatment increases obviously compared with that of conventional heat treatment and the difference of drying efficiency between two kinds of treatment is about 25%. The drying efficiency increases with the increasing of microwave power, but when the microwave power is 560 W, the promotion role of microwave irradiation reaches optimum.(3) Mesoporous accounts for the majority in the lignite and the macropore will collapse to generate mesoporous with the proceeding of drying process untill to 140 oC. When drying time reaches to 50 min at 140 oC, the collapse phenomenon of pore structure suddenly intensifies and then recedes. Under nitrogen, argon and air atmospheres, the specific surface area and pore volume of upgraded lignite all reduce. The change of pore structure is more obvious under nitrogen atmosphere compared with that under argon atmosphere. Under air atmosphere, the low temperature oxidation on the surface of lignite occurs in the drying process and releases the heat. So that improves the temperature of lignite surface and intensifies the pore structure collapse. Under CO2 atmosphere in addition to the pore structure collapse, the swelling effect of CO2 on lignite surface with colloidal properties also occurs, which lead to the increase of pore size.(4) The oxygen-containing functional groups in lignite are mainly: Ar-OH, COOH, C = O and COO –. When the temperature is higher than 160 oC, COOH firstly begins to decompose and its decomposition rate increases with the increase of drying temperature and time. When the temperature is higher than 200 oC, Ar-OH and C = O obviously decompose and the decomposition rate of C = O is greater than that of Ar –OH. The decomposition rate order of different oxygen-containing functional groups is as follows: COOH > C = O>-OH>COO –.(5) Drying process of raw coal can be divided to three periods, which are the increasing rate, constant rate and falling rate stages, respectively. Modified Henderson and Pabis model fits the lignite thin-layer drying process well and the average active energy is 25 k J/mol for lignite. The contents of different type moisture dominate the average active energy during lignite drying.(6) The equilibrium moisture content of the upgraded coal after moisture re-adsorption is significantly lower than that of raw coal. There is no correspondence between the re-adsorption moisture and that removed during drying. The relation between the equilibrium moisture content of the upgraded coal after re-adsorption Y(mmol/g) and the specific area S(m2/g) and the oxygen-containing functional groups content ACOOH, AOCH3, AC=Oå'Œ AAr-OH(mmol/g) is quantitatively related and as follows: Y=1.53[S] + 4.36[ACOOH] + 1.06[AAr-OH] + 0.78[AOCH3] + 0.82[AC=O]- 0.44.(7) The type of re-adsorption moisture is relatively simple and mainly exists in the forms of physical and chemical adsorption forms on surface of upgraded lignite. The proportion of physical adsorption moisture reduces and that of chemical adsorption increases with the rise of drying temperature, thus the adsorption heat released by the re-adsorption of same amount of moisture increases. The process of moisture re-adsorption can be divided into two sections and their activation energies are respectively 7.9 and 17.3 k J?mol-1. The activation energy of the first section is smaller, adsorption rate is greater and it occurs more easily at low temperature. That of the second section is higher and it proceeds at high temperature and more gently changes.