Effects Of Gas Residence Time On The Products Of Coal Pyrolysis In Fluidized Bed

Coal is the main energy source in China.The high efficiency and clean utilization of coal has been paid more and more attention.Coal pyrolysis technology causes coal to decompose at high temperature under anoxic conditions,and induce a series of changes in physical and chemical properties.It can classify products at the same time reduce environmental pollution,having the advantages of low investment and simple system.In the coal pyrolysis conversion system,with the increasing complexity of the reactor,volatile products would be affected by a certain heat in the reactor and gas transport pipeline after leaving the reaction zone.The gas phase secondary reaction takes place such as cracking,dehydrogenation,condensation,and then eventually changes the composition of pyrolysis products.The vapor transport time of pyrolysis gas in effective heating section has a great influence on the yield and composition of the final products.There are many problems in the past research of the influence of gas residence time on the pyrolysis products,such as the residence time is short,the heating rate is not high,and the temperature research is not comprehensive.It is based on the small fluidized bed rapid pyrolysis reactor in this paper,changing the gas residence time by adjusting the flow rate of carrier gas in the furnace and building the two reaction zone of the different size in the outside of the furnace,researching the changes of pyrolysis products under the short gas residence time?1.8?7s?and longer gas residence time?5.5?18.7s?.So as to reveal the secondary reaction mechanism on gas phase area during pyrolysis,and provide basis for the design and operation of pyrolysis reactor.?1?In the study of the short gas residence time influence in the furnace,the Xiaolongtan lignite was used as raw material,and the reaction parameters such as fluidizing gas velocity and pyrolysis time were determined.It adjusted the carrier gas flow to control of pyrolysis gas residence time within 1.8?7s.Pyrolysis temperature was 500?800?.The yield and content of char,tar,gas and water were collected and calculated.The experimental results show that the pyrolysis tar increases first and then decreases with the increase of gas residence time,and there is a maximum.The tar yield was influenced by both gas phase secondary reactions and heating rate changes.It was affected by internal reaction consumption and lack of particle residence time under gas residence time of 4s,so the tar yield was insufficient.The gas phase secondary reaction began to dominate after 4 and 5.5s,and then make tar crack into small molecular products.There is a certain increase in gas and water rate at longer residence times.The four component chromatography experiment of pyrolytic tar on 700 ?illustrated that the aromatic hydrocarbon content of 5.5s and 7s increased at the longer residence time compared with other components.The degree of tar fragrance increased.It should be related to aromatization and polycondensation.The tar collection rate of 7s was decreased by 5.32%compared with 1.8s.It should be related to the volatility of lightweight components.The decrease of asphalt content also showed the increase of the degree of tar.The CO and CO₂ yield increased slightly after 5.5s,which should be related to the cleavage of oxygen-containing functional groups.CH₄ yield change was not obvious.The H₂ component had a reverse growth trend in the shorter residence time,which may be related to the crosslinking reactions during early pyrolysis.The highest yield of C₂H₆ was at 700 ?.And it began to decline later than others.The changes of C₃H₈ and C₃H₆ were more obvious at high temperature above 600 ?.The yield decreased and the gas began to break down into smaller molecules after 5.5s.?2?In the study of the longer gas residence time influence outside the furnace.By comparing the response of different size reaction section and no secondary reaction section,studying the law of gas phase secondary reaction in 5.5?18.7s residence time,it found that the tar yield decreased with the increase of residence time,and the Gas production rate increased.With the extension and expansion of the gas transport pipeline,the tar component was gradually broken down into small molecular gas and water.The tar secondary reaction was on enhancement.The sight component segmentation and GC-MS analysis of tar showed the content of tar in low boiling point increased.The content of aliphatic hydrocarbons in light tar decreased obviously.And it was gradually cracking or going on aromatization reaction.The main aromatics components increased significantly.It should be related to the aromatization,condensation and condensation of aromatic molecules.The phenolic components in the non-hydrocarbon components decreased obviously,which was one of the main forms of secondary tar cracking.Other non-hydrocarbon components such as oxygen heterocycles were also reduced.Tar structure became simple under the condition of 18.7s.The main gas components and water increased from 5.5s to 18.7s.The increase of CH₄ component was mainly due to the decomposition of fat structure and the degradation of aromatic methyl and aromatization reaction.The H₂ component increased more obviously at 700 and 800?,which should be related to condensation and dehydrogenation reactions.CO and CO₂ were mainly derived from the cleavage of various oxygen-containing functional groups and oxygen heterocycles.The yield of H₂O was increased obviously,which was the main products of phenol hydroxyl cleavage.The yield of C₂-C₃ increased first and then decreased with the residence time,which were from decomposition of aliphatic hydrocarbons and aliphatic side chains.It began to break down into smaller molecules after 8.6s.Olefin components increased in 18.7s,which is the main product of secondary cracking pyrolysis.?3?The experiment also selected Shenhua and Yulin coal for pyrolysis experiments under the condition of 5.5?18.7s.The experimental results were analyzed with Xiaolongtan lignite,which showed that with the increase of residence time Xiaolongtan lignite tar decline and gas,aquatic rise was more obvious than other two bituminous coals.Lignite pyrolysis tar component is unstable,which is more likely to crack under the same conditions.And in the research of raising the temperature of the secondary pyrolysis section,The initial temperature of the fluidized bed was maintained at 600?,The temperature of the secondary reaction zone was increased to 700 ? and 800 ?.The secondary reactions of tar and main gas products were obviously strengthened.The increase of secondry temperature made the secondary pyrolysis reaction more deeply with the increase of residence time.