Study On The Forms Of Water Readsorption Of Different Kinds Of Coal Based On Low Field ~1H-NMR Technology

China is a country with coal as its main source of energy,but with the continuous consumption of high quality coal,the exploitation and utilization of lignite is gradually paid attention.Indonesian lignite is one of the main types of coal used in thermal power generation in eastern coastal provinces of China,but high water content seriously restricts the utilization of Indonesia lignite.The thermal upgrading technology can effectively reduce the water content and improve the maturity of coal,but the coal after upgrading is easy to adsorb water again.This study investigated the coal characterstics and the differences in physicochemical structure of Indonesian lignite,Wucaiwan coal and Shenhua bituminous coal.The Indonesia lignite was thermally upgraded at 200⁵00°C under the atmosphere of N₂ and CO₂respectively,and the effects of temperature and atmosphere on the physical and chemical structure were analyzed.Three raw coals and upgraded Indonesian lignite were subjected to adsorb water to study the effect of physicochemical structure,ambient temperature and humidity on the readsorption characteristic,and then the readsorption law between coal rank,thermal upgrading temperature and atmosphere was obtained.The water isotherm adsorption process was described using different models.The water of raw coal and thermally upgraded Indonesian lignite was determined by ¹H-NMR technology and the water type was divided.The main conclusions are as follows:Wucaiwan coal belongs to bituminous coal but its moisture as received is higher than Indonesian lignite and Shenhua bituminous coal.The content of C gradually increased while the content of O gradually decreased.The CO₂ atmosphere is more advantageous to remove O than N₂,but has little influence on H.The specific surface area and the total pore volume of Wucaiwan coal are the smallest while the contents of OH and COOH are the highest.The specific surface area and the total pore volume of Indonesian lignite is less than Shenhua bituminous coal,while he content of OH and COOH is more than Shenhua bituminous coal.In the process of thermal upgrading,the specific surface area and pore volume of the upgraded lignite decreases contiously,and they are larger under the N₂ atmosphere.The content of OH and COOH decreases continuously,of which the thermal stability of OH is higher and CO₂ is more conducive to the decomposition and removal of OH and COOH than N2.The relationship of water readsorption rate relationship between three raw coals is Wucaiwan coal>Indonesian lignite>Shenhua bituminous coal,and all less than the moisture as received.Wucaiwan coal’s high OH and COOH content makes it have a high water readsorption rate.While the Shenhua bituminous coal has a large pore volume,but the oxygen functional group is very few,which leads to the lowest readsorption rate.OH and COOH have stronger influence on water adsorption than specific surface area and pore volume.The water readsorption rate decreases contiously with the increase of upgrading temperature,but because of less specific surface area and pore volume,OH and COOH content under CO₂ atmosphere,its water readsorption rate is lower.The three raw coals and upgraded lignite experience the process of rapid water adsorption,slow water adsorption and the balance of water adsorption.The water readsorption rate is lower at higher ambient temperature and lower relative humidity.With the increasing upgrading temperature,the water readsorption rate decreases continuously under CO₂ atmosphere.The Henderson model is suitable for describing the water isothermal adsorption process of Shenhua bituminous coal,while the Mod-BET model is more suitable for the Wucaiwan coal and Indonesia lignite,and the three raw coals water isothermal adsorptions are allⅡadsorption.With the increase of upgrading temperature and decrease of relative humidity,the primary adsorption and secondary adsorption water continuously decrease.And the primary adsorption water content undedr N₂ atmosphere is significantly greater than that in CO₂ atmosphere,while the difference of secondary adsorption water content is small.The water in coal can be devided into four types including capillary water,strongly bound water,weakly bound water and free water according to the different location in coal.Strongly bound water and free water are the main water in Indonesian lignite and Shenhua bituminous coal,and because there are more oxygen functional groups between 10¹00nm,the content of Strongly bound water is higher than Shenhua bituminous coal.A large number of pores above 50nm and higher content of OH and COOH leads Wucaiwan coal more strongly bound water and weakly bound water and higher water readsorption rate.Upgraded lignite has same water forms and its content is strongly bound water>free water>capillary water>weakly bound water capmared with dry lignite.The difference is that the strongly bound water and free water are significantly less than dry brown coal,while the difference of weakly bound water and capillary water is small.With the increasing upgrading temperature,the content of strongly bound water and free water decreased rapidly,but the reduction of the weakly bound water and the capillary water is smaller.The study on the readsorption process and water forms can provide theoretical support for the drying of different kinds of coal and the storage and efficient utilization of the upgraded lignite.