Classification Of Water Form In Lignite And Energy Consumption Analyses Of Microwave And Fixed Bed Drying Technologies

With the large consumption of high quality coal in China,the application of low-rank coal,such as lignite,has attracted increasing attention.China is abundant in lignite resources,but high water content of lignite results in lower calorific value and difficult transportation.Therefore,lignite should be dewatered before its utilization.Dewatering technologies of lignite includes fixed bed drying,fluidized bed drying,microwave drying and hydrothermal drying,among which microwave drying has amount of advantages,such as rapid drying velocity and higher selectivity and fixed bed technology is mature.However,the existed forms of water in lignite are different,and require different energies during dewatering in operating conditions.Based on this,the main results are shown as follows:?1?Inner Mongolia lignite and Yunnan lignite with different water contents are used as the research object.Differential scanning calorimetry?DSC?was conducted to classify the water forms in lignite.The result shows that the water in lignite contains freezable water and non-freezable water.The freezable water can be divided into free water and freezable bound water.More than 40%db of water is free water;27%⁴0%db is freezable bound water;and less than 27%db is non-freezable water.The theoretical dewatering energy consumption of Yunnan lignite and Inner Mongolia lignite obtained through the DSC under N₂are 2080±19 J/g H₂O and 2084±16 J/g H₂O,respectively.The result shows that drying atmosphere can interact with oxygen containing groups on coal surface and water molecules,thereby affecting dewatering energy;the dewatering energy consumption of lignite under CO₂ are 2652±8 J/g H₂O and 2448±27 J/g H₂O,respectively,which are larger than the theoretical dewatering energy consumption under N₂.?2?The microwave drying device was employed to dry lignite.The effect of operating conditions on the lignite drying characteristics and dewatering energy consumption was investigated.The result shows that it is difficult to completely drying the lignite in 450 W microwave field.When the microwave power is larger than 700 W,the lignite is prone to slight pyrolysis.The atmosphere has little effect on drying characteristics of lignite when use amount of coal sample.The Inner Mongolia lignite will release volatiles in high power microwave field and needs to be humidified.Drying 50 g lignite in microwave field under 500⁶00 W,the dewatering energy of total moisture is 8.6¹5.3 kJ/g H₂O,and the dewatering energy of freezable water is 8.7¹2.5 k J/g H₂O.The dewatering energy of non-freezable water is 8.5-21.8 k J/g H₂O.The dewatering energy consumption of total water in Inner Mongolian lignite with particle sizes of 1.00-3.00 mm and 3.00-6.00 mm is 6.5¹6.7 kJ/g H₂O and 6.1¹6.6 kJ/g H₂O,respectively.?3?The self-made fixed bed device were employed to dry lignite.The effect of operating conditions on the lignite drying characteristics and dewatering energy consumption was investigated.The best drying temperatures for Yunnan lignite and Inner Mongolia lignite in fixed bed are 140¹60 ^oC and 120¹40 ^oC,respectively.The slow drying of lignite can also completely remove the water in lignite,and the drying process of lignite is slow in the early stage of slow drying.With the increase of temperature,the drying process is accelerated.The dewatering energy consumption of fixed bed drying 50 g lignite at 120-160 ^oC was 1.3-3.6 kJ/g H₂O.The dewatering energy consumption increases with the drying temperature increases.The atmosphere has little effect on dewatering energy consumption.The dewatering energy consumption of Inner Mongolia lignite is higher than that of Yunnan lignite.?4?The fixed bed drying and the microwave drying were coupled and explored the effect of the methods on drying behaviors and the moisture re-absorption behavior of the dried lignite was explored.It is recommended to use microwave drying to remove free water first and then remove the remaining water by fixed bed drying.The energy consumption is lower at this time.The water re-absorption of the upgraded coal is about 1/3 than that of raw coal.