Effect Of Different Drying Methods On The Physicochemical Properties And Thermal Conversion Characteristics Of Lignite

The energy resources of our country are characterized by"rich coal,less oil and poor gas",which determines that the main energy consumption structure in our country must be coal based for a long time,and the tense situation of crude oil and natural gas supply is also created objectively.In the utilization of coal resources,high quality anthracite and bituminous coal play the leading role.Under the situation of increasing global energy consumption,the supply of high quality coal is becoming increasingly intense.Low quality coal is a young coal with low metamorphism and late coal.Lignite is an important low quality coal,which accounts for about 45%of the world’s coal.However,the moisture content of lignite is usually greater than 30%,and some even up to 60%.It is easy to weathering and spontaneous combustion.It is not suitable for long-term storage.The transportation cost is high and the combustion efficiency is low.Therefore,the key to the large-scale exploitation and utilization of lignite is to improve the quality and dehydration of lignite,and it is also the main method to overcome the defects of lignite（high moisture,low calorific value and spontaneous combustion）,and to achieve partial pyrolysis,gasification,liquefaction and coal based chemical raw materials.Xilinguole Meng,Inner Mongolia is the main producing area of lignite.However,there are various kinds of lignite drying and dewatering technology at present.There are few studies on the influence of hydrothermal dehydration,microwave dehydration and hot air drying on the change of lignite structure.The thermal transformation characteristics of lignite,including pyrolysis,gasification and combustion,will change after dehydration and upgrading.Therefore,three different dehydration processes were applied to the dehydration and upgrading of lignite in Xilinguole Meng area of Inner Mongolia province.And the effect of the physicochemical properties of lignite on its subsequent thermal transformation after dehydration,has an important guiding significance for the clean and efficient transformation of the lignite.The main conclusions of this paper are as follows:（1）Hydrothermal dehydration shows that when the hydrothermal temperature is constant,and the pressure is greater than 3Mpa,the reaction process of coal sample in hydrothermal treatment will be less affected.With the increase of the reaction pressure,the lignite gradually entered the stage of deep modification,and the degree of thermal decomposition increased,which reduced the yield of the solid,and the average particle size of the hydrothermal treatment dehydrated coal samples gradually decreased.Compared with the average particle size of dehydrated coal samples,the influence of hydrothermal pressure on the average particle size of dehydrated coal samples is small.With the increase of hydrothermal dehydration pressure,the internal moisture and oxygen-containing functional groups of lignite are removed more thoroughly,and the pore structure of dehydrated lignite is also changed.The equilibrium moisture of hydrothermal treatment dehydrated coal is gradually reduced.The active chemical composition in the coal macromolecular structure is gradually transformed into a stable compound before the volatiles are formed.When the pressure is increased from 1Mpa to 4Mpa,the yield of volatile matter is reduced from 44.11%to 42.12%,and the carbon content in the coal sample increases from 74.49%to 75.99%,and the hydrogen content is reduced from 4.71%to 4.57%.The reduction of oxygen content increased from 29.19%of raw coal to 18.14%.（2）Microwave dehydration shows that the initial stage of lignite in the microwave drying is in the preheating stage as a whole.The surface of lignite absorbs microwave energy to heat the coal particles as a whole,because there is almost no removal of water.With the prolongation of the dehydration time,the water content in the microwave drying lignite decreased,and the water saturated surface of the coal sample was destroyed.The evaporation area of the surface moisture of the coal samples moved further to the coal particle center,which resulted in the slow evaporation rate of the coal particles.When the microwave drying power is greater than 700W,the pyrolysis reaction occurs in microwave drying lignite.Some aliphatic hydrocarbon side chains and oxygen containing functional groups are decomposed,and the tar like substances are dissolved in water,which leads to the trend of the high concentration of COD,TOC,BOD and NH₃-N in the waste water obtained by microwave drying lignite.（3）The lignite dehydration in the fixed bed reactor shows that the dehydration rate of the coal samples is different,and the effect of the drying medium on the dehydration rate from high to low is nitrogen>oxygen.Compared the lignite with larger particle size,the lignite with smaller the size is heated and the temperature field is more homogeneous.Therefore,the particle with particle size of 0.5⁰ mm has greater dehydration rate than the fine size lignite sample.Compared with the water quality of condensate obtained from lignite dehydration at 140℃,the condensed water from lignite dehydration at high temperature contains a large amount of organic matter,and the COD content of the condensed water from coal sample dehydration at 210℃and 280℃is very high.When the coal sample dehydration temperature rises from140℃to 280℃,the NH₃-N content in condensed water from the dried coal sample with particle size of 6³mm in oxygen increases from 47 to 338,and increases 7.19times,while the TON content also increases from 78 to 480,and increases 6.15 times.The precipitation of four kinds of heavy metal ions increased with the increase of dehydration temperature,but the content of heavy metal ions in coal sample drying condensate was much lower than that of the standard V standard.（4）The N_g value of the free radical in the hydrothermal treatment dehydrated coal sample increases with the gradual increase of the hydrothermal treatment pressure.As for the microwave drying lignite and the dehydrated lignite in the fixed bed reactor,the free radical concentration increases with the increase of microwave drying power and dehydration temperature,and there is no tendency to decrease like hydrothermal treatment.With the increasing of the water thermal pressure,the g factor gradually decreases,the content of oxygen functional groups in the dehydrated coal at 1Mpa is higher,and the paramagnetic center of the unpaired electrons is in the oxygen atom with the higher spin orbit coupling constant,so the value of g factor is higher.As for the microwave drying lignite and the dehydrated lignite in the fixed bed reactor,the value of g factor gradually decreases with the increase of microwave drying power and dehydration temperature.This shows that the lignite gradually enters the stage of deep modification with the increase of dehydration temperature and microwave power.（5）Compared with raw coal,when the hydrothermal treatment pressure reaches4Mpa,the mass fraction of light tar in pyrolysis tar increased from 47.56%of raw coal to 62.27%.In the microwave drying lignite,with the increase of microwave drying power,the tar yield of coal sample is gradually increasing,but the increase is small,only from 7.28%when the drying power is 300W to 7.39%when the drying power is1000W.The contents of C,H,S and N in pyrolysis tar continue to increase with the prolongation of drying time or increase of the hydrothermal temperature,while the oxygen content is gradually reduced.The contents of S and N in pyrolysis tar increased with the increase of dehydration process parameters.（6）When the pyrolysis temperature reaches 500℃,the yield of tar produced by individual pyrolysis of demineralized coal was slightly higher than that of raw coal,which reached 7.51%and 6.92%,respectively.In the process of catalytic upgrading,tar yield was decreased,and the light tar mass fraction and gas product yield were increased.The effect of tar upgrading by catalytic cracking is the best when the addition proportion of raw coal and demineralized coal were 40%,and the addition proportion of char is 30%.The tar yield of catalytic pyrolysis increased in the order are as follows:demineralized coal catalytic bed>raw coal catalytic bed>char catalytic bed.While the yield and mass fraction of light tar increased in the order are as follows:raw coal catalytic bed>char catalytic bed>demineralized coal catalytic bed.The content of single aromatic ring,two aromatic rings and three aromatic compounds in the demineralized coal catalytic bed was higher than that of the other two beds,and tetracyclic aromatic compounds yield slightly lower than that of raw coal catalytic bed and char catalytic bed.The content of wash oil,anthracene oil,asphaltene in catalytic pyrolysis tar over raw coal catalytic bed was significantly lower than that of the demineralized coal catalytic bed and char catalytic bed.（7）With the increase of the hydrothermal temperature,the number of large aromatic ring structures（I_G/I总）in the coal sample pyrolysis char increases,while the defects in the char and other disordered structures(I_D3/I总)and the amorphous carbon structure(I_D1/I_G)are gradually reduced.In microwave drying,the disordered graphite lattice and atomic content in the pyrolysis char and the lattice vibration amplitude of the aromatic layer increase with the increase of microwave drying power.The large aromatic ring structure（I_G/I总）in the coal sample pyrolysis char increases,while the defects in the char and other disordered structures(I_D3/I总)and amorphous carbon structure(I_D1/I_G)gradually decrease with the increase of microwave drying power.When the microwave drying power is greater than 700W,the defects and other disordered structures(I_D3/I总)and amorphous carbon structure(I_D1/I_G)in pyrolysis char decrease slowly.With the increase of gasification reaction temperature,the gasification reaction time needed to reach the same conversion rate decreased obviously,and the gasification reaction temperature rises 100℃,the reaction time is shortened by 1/4.When the gasification temperature reaches 1000℃,the carbon conversion can reach100%when the heat holding time reaches 10 min,while the gasification temperature reaches 750℃,and the carbon conversion is 30 min when the carbon conversion reaches 100%.The curve shape of gasification reaction rate of hydrothermal and microwave dehydrated lignite is very similar.It shows that although the form of drying supply energy is different,the coal samples also have different degrees of modification,but it has no effect on the change of the large molecular structure in coal.（8）The crystal structure parameters of pyrolysis char of lignite in three different dehydration methods are similar,but there are obvious differences.For the interlayer spacing,the pyrolysis char of coal samples with hydrothermal dehydration is the smallest for the interlayer spacing,reaching 0.384nm,while the pyrolysis char of coal samples with hydrothermal dehydration is the biggest for the interlayer spacing,reaching 0.463nm.The change trend of aromatic lamellae stacking height L_c and aromatic layer diameter L_a,and layer spacing d₀₀₂ are similar.The results showed that the condensed ring structure of pyrolysis char from hot air dehydrated coal sample pyrolysis appeared more condensation,and the basic lattice unit of pyrolysis char also had longitudinal binding polymerization and internal growth.It leads to the loss of the marginal active sites and reduces the activity of the semi coke,which is in accordance with the conclusion that the reaction activity of the pyrolysis of the hot air dehydrated coal sample is lower.