Study On The Effect Of Mechanical Vibration On The Thermal Fragmentation Characteristics Of Lignite During Superheated Steam Drying

Lignite is a low-rank coal with high water content,which can be used as energy or a raw material for chemical industrial products.Drying and upgrading is the first and an indispensable step in using lignite.The existing mature lignite drying and upgrading processes at home and abroad are mainly based on evaporative dehydration technology.Most of them need to crush the lignite particles into smaller particles in advance.The water content of Zhaotong lignite is close to 60%,it difficult for the crushing equipment to crush it to the target particle size.Small particles are easy to bond and difficult to sieve,resulting in failure to meet the drying process requirements.In view of the abovementioned problems in the drying and upgrading process of high-moisture lignite,this paper proposes a lignite vibrating mixed flow screening and drying system and conducts experiments on an independently built test device to explore the lignite drying characteristics and crushing rules of different operating conditions during the vibration drying process and reveals the promotion of the vibration field on the crushing behavior and drying effect of lignite,and provides useful thinking for the design of a new lignite drying device.In this paper,Zhaotong Sanshantang lignite was selected as the research object to explore the changing rule of discharge particle size and water content in vibration field(vibration rate 300 cpm)under different experimental conditions(drying temperature 190 ?-250 ?,feeding particle size 15-30 mm,drying time 0-20 min,feeding volume 200-400g).The results show that the feeding material are continuously broken into smaller particle size.The content of the intermediate particle size is changing by the influence of its own generation rate and fragmentation rate.In the latter part of the drying process,the feeding material are completely broken,and the particle size composition of the discharge particles is mainly < 1,1-3 and 3-6mm,which gradually stabilizes;the longer the vibration drying time,the smaller the feeding particle size and the less the feeding material volume and the higher the drying temperature,the more obvious the increasing trend of the smaller particle size content in the discharge material,among the various factors,the temperature has a more profound influence on the drying process,at 250 ?,the material quickly enters the constant speed drying stage,after only 10 minutes,the water content of the material drops from 59.08% to 4.59%,< 1,1-3,3-6mm accounted for 77.53% of the output particle size,1-3 mm accounted for the most,reaching 37.45% of the total output;under different experimental conditions,except for the abnormal increase in the particle size water content caused by the condensation of water vapor,the water content of the discharge particles increases with the particle size and showing an obvious gradient.Using the fragmentation strength based on the concept of fragmentation potential energy can better describe the overall fragmentation degree of the particles of lignite after vibration drying process.The study found that the fragmentation strength is more sensitive to changes in the particle size distribution of the particle group under the influence of multi-factors(feeding particle size,drying temperature,drying time,feeding volume).As the vibration drying time passes,the cumulative curve of the discharge particle size shifts to the left,the fragmentation strength continues to increase,and eventually it tends to be stable.The particle size-residual moisture gradient based on the particle size content can directly express the unevenness of dryness of particles during the vibration drying process.Under different operating conditions,the discharge particle size-residual moisture gradient increases first and then decreases with the increase of drying time,indicating that there is a moment when the particle drying effect is most uneven during the drying process.The faster the drying rate,the earlier this moment appears.The Weibull model was used to fitting the moisture ratio change during the lignite vibration drying process.It has a good fit under different experimental conditions,and the corresponding mathematical model was established.In order to further explore the promoting effect of the vibration field on the drying and fragmenting of lignite,by adjusting the vibration frequency and changing the loading time of the vibration field,the effects of different loading behaviors of vibrational energy on the drying characteristics and fragmenting characteristics of lignite particles were studied.The study found that there is a critical vibration frequency for the impact of the vibration frequency on the drying and crushing of Zhaotong lignite.Only when the vibration frequency is higher than the excitation frequency,the vibration form energy can promote the crushing and drying effect of the particles.Under the test conditions of this paper,when the vibration frequency is 300 cpm,compared with 200 cpm,the difference in the particle size composition and the degree of drying reaches the maximum after 15 minutes,the average water content is reduced by 16.75% and the crushing strength is increased by 50.63%,The difference between the total moisture content and the crushing strength of the output below 200 cpm are within 10%;The drying degree of Zhaotong lignite increases linearly with the increase of the vibration loading time,and the drying effect of the vibration loading time 5,10,15 min and the vibration loading are improved by 17.07%,46.08%,and 81.85% respectively.After the vibration drying reaches a certain level,If the vibration frequency is not changed,the particle size distribution will gradually stabilize.The gray correlation model was used to analyze the influence weight of the five factors of drying time,drying temperature,feeding particle size,vibration frequency and feeding volume on fragmentation strength and pulverization rate.The study found that the weight of the impact on the fragmentation strength is: drying time > vibration frequency > drying temperature > feeding particle size > feeding volume;the weighting on the pulverization rate is: vibration frequency > drying time > feeding particle size > feeding volume > drying temperature.A neural network-based model for predicting the fragmentating characteristics of lignite during vibration drying process is established,with the drying time,drying temperature,vibration frequency,feeding volume,and feeding particle size as input neurons,average water content of the discharge particles,fragmentation strength and pulverization rate as output neurons,the results show that the neural network model has good adaptability in the drying field.The paper has 39 pictures,8 tables,and 112 references.