Formation Characteristics And Mechanism Of Low-Temperature Pyrolysis Products From Low Rank Coals

The internal structure of coal,the peripheral functional groups of structure unit,the side chain,surface and spatial structure properties are changes constantly with the degree of coal metamorphism;the content of carbon,hydrogen,oxygen,nitrogen,sulfur and other elements are also different,and these lead to the different pyrolysis characteristics coals.Low rank coals are characteristics of its developed pore high content of water,volute,and oxygen with low calorific value.so the low rank coals are easy to be weathering and spontaneous combustion,and not conducive to long distance transportation and keep in storage.As it have great limitations for the large-scale gasification.At present,low rank coals are mainly used as a local power fuel with low efficiency and large emissions of contaminants and carbon.And this will cause to large waste of energy and serious environmental pollution.At present,the research on the low rank coal upgrading utilization are mainly in hydrogenation liquefaction,volute remove,CH4 releasing in pyrolysis gas and research on low rank coals using TG and DTG.There is rare study on low rank coal pyrolysis upgrading,especially about formation mechanism and properties of low rank coal pyrolysis products.Therefore,systematic and in-depth study is very necessary to realize the characteristics and formation mechanism of coal pyrolysis products including water,tar,pyrolysis gas and char,the systematic study of the formation and this is very important to provide some data to support for the goal pollutant control and new technology development;understanding of low rank coal pyrolysis mechanism would help to understand the pyrolysis behavior of low rank coal,has certain instruction function to the low rank coal upgrading transformation.The report were focused to the factors which influence pyrolysis of low rank coals such as coal species,heating rate,temperature and maintain time;and using BET,FT-IR,GC/MS and TG/DTG to analysis the characteristics of the pyrolysis products;calculating the generation rate of the pyrolysis products,preliminary study was done on the pyrolysis kinetics and reaction mechanism of low rank coal in the different reaction series.The main results and conclusions were as follows:1)Established an calculation method on low rank coal pyrolysis products generate characteristic parameters during drying,pyrolysis and char formation process,which include the pyrolysis products began to generate temperature(Ti)and fast pyrolysis product finish temperature(Tf),and these two parameters are the temperatures of ?=5%and ?=85%respectively;maximum pyrolysis product formation rate point(Rmax)and its corresponding temperature(Tmax).2)Drying process of low rank coal was divided into three stages.The drying rate was the fastest in first stage(0-20min),the lignite and non caking coal in the dehydration rate of the segment linear equations with y=0.62 x+1.64 and y=0.64 x+2.32;and the water yield was increased slowly with the drying time in the second stage(the stage of lignite as 20-40 min interval,non caking coal for the 20-50 min interval),drying and dehydration rules were fit for linear equations y=0.13x+7.37 and y=0.14 x+5.22;the drying and dehydration reaction activation energy of low rank coals increases with the increase of coal rank,and the activation energy E are 18.21 KJ/mol and 21.03 KJ/mol respectively with the heating rate of 10?,activation energy of non stick coal was increased by about 18%than the lignite in the period of drying;drying reaction activation energy decreases as the heating rate increases,when the heating rate of 5?/min,lignite and non caking coal drying activation energy E=21.91 KJ/mol and 22.36 KJ/mol,and when the heating rate increased to 20?/min,lignite and non caking coal drying activation energy E were 16.63 KJ/mol and 20.16 KJ/mol,the activation energy decreased by 24%and 10%respectively So,therefore,reduce the coal rank and increasing the heating rate,was conducive to the low rank coal moisture faster removal;drying rates of total and rapid drying time of 200 C are similar to that of the temperature of 250 C,therefore,the drying temperature of low rank coal should be considered in selection of 200?.3)Established a liquid pyrolysis product collecting device to study the generation characteristics of tar from low rank coal pyrolysis.And it used a big curve shaped channel of pyrolysis products which garantee high velocity and low residence time of volatile within the channel to avoid the poor airflow of high temperature gas occurred two times cracking in the pyrolysis apparatus.Tar yield were 5.10%and 7.97%with traditional aluminum retort and the big curve shaped channel pyrolysis apparatus respectively,tar yield increased rapidly with the increasing of heating rate,tar yield of lignite and non caking coal followed the linear equation y=0.26 x+8.38 and y?0.07x+3.40 with the heating rate of 10?/min,respectively,then the yield of tar as the heating rate increased the growth rate slowed down,when the heating rate increased to 15?/min and 20?/min,lignite and non sticky coal tar yield were consistent with linear equation y?0.04x+10.66 and y=0.02x+3.84;the tar formation reaction activation energy increases with the increase of coal rank,lignite and non caking coal tar formation activation energy of E were 40.62 KJ/mol and 49.16 KJ/mol such as heating rate of 10?/min,the tar yield activation energy of non caking coal is about 17%higher than the lignite,tar yield activation energy decreases with increasing heating rate,lignite and non caking coal tar generated activation energy were decreased by 13%and 5%,when heating rate was dropped from 20/min to 5?/min,increasing heating rate is conducive to the rapid generation of tar.4)Established a gas analysis apparatus for analysis the composition and content of the low rank coal in low temperature pyrolysis.The device was characterized of liquid sealing tightness on the export of pyrolysis gas and gas cylinders,so it can overcome the leakage.Pyrolysis gas yield of low rank coal was increased gradually with the increase of pyrolysis temperature;pyrolysis gas generation temperature Ti is about 400?;pyrolysis gas generation rate of Rmax and the corresponding maximum peak temperature of Tmax increased with the increasing of the coal rank,the lignite pyrolysis Tmax was at 550-600?,the corresponding Rmax was 100ml/min,but Tmax of no sticky coal pyrolysis appeared at 600?,and the corresponding Rmax 83ml/min.5)The temperature of extreme points of char formation rate increased with the increasing of coal rank.The Tmax of lignite semi-coke was 550? and the corresponding Rmax was of-0.075%/?,the Tmax of non sticky coal was increased to 600?,and the corresponding Rmax was of-0.080%/?;H/C and O/C of low rank coal value was decreased gradually with the increase of pyrolysis temperature,the H/C of lignite and non caking coal were dropped from 0.98 and 0.59 to 0.58 and 0.84 of 600?Crespectively,the O/C values were dropped from 0.42 and 0.21 to 0.08 and 0.07 of 600? respectively,that showed the fatty group content in char increased gradually with the deepen of pyrolysis degree gradually;aromatic carbon ratio of fa increased gradually,indicated that with the increase of semi coke pyrolysis temperature,the basic structural unit of aromatic ring condensation degree increases,the average condensation of semi coke ring number R value increases with increasing pyrolysis temperature increase;coal structure of unsaturated bond content and carbon,hydrogen,oxygen molar ratio values decreased with the pyrolysis process,lignite and non caking coal carbon,hydrogen,oxygen mole ratio was 0.98:0.42 and 0.59:1:1:0.21 while it dropped to 0.58:1:0.68 and 1:0.48:0.07 in 600 C char;the activation energy were increased with the increasing of coal rank,semi coke formation activation energy of non caking coal is 13.90KJ/mol,while semicoke formation activation energy of the lignite is only 7.30 of KJ/mol with heating rate of 10?/min,show that in low rank coal pyrolysis process,volatile in lignite was more easier to remove.6)With the falling off of hydroxyl(O-H),methyl(-CH3-),methylene(-CH2-)and the cracking of aromatic ring structure(-C=C-),low rank coal pyrolysis process was began;microstructure and macro morphology of coals were changed with volatiles escaping during the pyrolysis process.The surface of coal particles melted and aggregated,a cellular structure with different aperture was formed,and the whole particle size become smaller,lamellar particles with surface aggregation became oval gradually,this was due to moisture and volatile matter evolution with the increasing of temperature,due to the temperature gradient within the particles with the increasing of heat stress,the semi coke crushing,and volatile matter released.7)The reaction order was of n=2 in low temperature pyrolysis of low rank coal,the pyrolysis process can be divided into four stages.With the Coats-Redfern integral method for dynamic analysis,the kinetic parameters was obtained from ln[(-ln(1-?)/(T2)](n=1)and ln[(1-(1-?)/(T2(1-n))](n?1),then the kinetic curves of low rank coal pyrolysis at different heating rates were obtained;according to the fitting degree of dynamic curveis,reaction order n=2 and four stages are selected.The activation energy of first stage of was the lowest,second stages of activation energy was increased rapidly,indicating that the phase was the active pyrolysis stage,the activation energy decreased slightly at the third stage,while the activation energy was the largest at the fourth stage;this was due to different degree of coalification,coal pyrolysis reaction mechanism and the molecular structure were different;with the extent of poly-condensation reaction was intense enhancement of 700 C,the physical properties of the aromatic nucleus size had a significant jump,and because of poor structure and thermal stability of low ran coal,even at 700?,there are still a lot of volatile which required higher energy to release,resulting in lower activation energy at third stages between 600-700?,while the activation energy increased rapidly during the temperature of 700?-850?.8)Development of an industrial test of carbonization furnace with special structure was developed,including the pyrolysis gas collector and combustion chamber.The structure was characterized in a combustion chamber arranged on the upper and lower ends of the burner nozzle in the carbonization furnace each combustion chamber,each combustion nozzle was arranged near regenerator,so that the hot flue gas produced by combustion in the combustion chamber before leaving heat regenerator,the air entering the combustion chamber in the prior combustion nozzle and fuel for combustion is storagebody preheating;arranged at least one gaseous pyrolysis products collector in each pyrolysis chamber,gaseous pyrolysis products pass through the through channels from the outside into the pyrolysis gas collector,a baffle plate to prevent solid coal into gas collector,guide plate and the barrel along the vertical wall formed on the inner surface of the angle of 35o-55o,prevent the liquid products back to the collector outside,without changing the flow direction of coal in the pyrolysis zone at the same time,prevent the solid coal into the collector;Large low temperature carbonization bed resistance affected by filling coal particle size,particle size of the bed layer resistance is small,the coke size was 25?35mm,the carbonization furnace resistance ? P is 1722.7 Pa.