Kinetics and Mechanism of Copyrolysis of Walnut Shell and Shenfu Coal Using Attapulgite as Catalyst Based on TG-MS Method
Author:Yang Fu Sheng
Supervisor:zhou an ning
Copyrolysis is an important way to achieve efficient and sustainable utilization of coal and biomass.The int.eraction between coal and biomass during copyrolysis,results in very complex reaction process,which leads to much controversy over theoretical issues about copyrolysis interaction and regulation mechanism of copyrolysis products for a long period.In view of the above problems,copyrolysis and catalytic copyrolysis characteristics and kinetics of the walnut shell(WS)and the Shenfu coal(SFC)with attapulgite as catalyst,were studied by thermogravimetric and differential thermogravimetric analysis.Effects of tcemperature and catalyst on volati.le products in the copyrolysis were investigated,by thermogravimetric-mass spec.trometry,combined with pyrolysis of cellulose and lignin as model compounds;reaction characteristics and synergy mechanism between the WS and the SFC was also revealed.The results have certain theoretical significance for development of new high-efficiency co-pyrolysis technology of coal and biomass.(1)Research on the synergistic effect occurred in the WS and the SFC copyrolysis.Copyrolysis characteristics and kinetics of the WS and the SFC,were studied by thermogravimetric and diff-erential therm.ogravimetric method.The results showed that devolatilization temperataure range of the walnut shell and the Shenfu coal overlapped partially.Synergistic effect could be observed in 300-500 ℃ copyrolysis of the walnut shell and the Shenfu coal,leading to increase in volatile matter production.Mass fraction of the walnut shell was an important factor determining the synergistic effect.The synergistic effect was more obvious when WS:SFC mass ratio were 3:1 and 2:1,and heating rate was not lower than20℃·min-1,(2)Kinetics of copyrolysis of the walnut shell and the Shenfu coal.Apparent activation energy of copyrolysis of the waln.ut shell and the Shenfu coal,was calculated according to four iso-conversional methods,namely Friedman method,FWO method,KAS method and Starink method.Copyrolysis mechanism model of the walnut shell and the Shenfu coal,was calculated and analyzed by Achar method and Coats-Redfern method.The results showed that apparent activation energy of the walnut shell/Shenfu coal(3:1)copyrolysis in 260-360 ℃was 278.20 kJ·mol-1,copyrolysis process during 260-340 ℃ accorded with three-dimensional diffusion(D3)mechanism.Apparent activation energy of the walnut shell/Shenfu coal(3:1)pyrolysis calculated by model-fitting method,was lower than that obtained by iso-conversional method,due to the fact that some chemical reaction processes in the walnut shell and the Shenfu coal pyrolysis was neglected,in calculation using model-fitting method,where single heating rate and preset reaction mechanism were adopted.(3)Investigation into copyrolysis of the walnut shell and the Shenfu coal by TG-MS method.TG-MS was used to investigate variation with temperature in volatile products,such as water,carbon dioxide,acetic acid,furan,methylfuran and furfural,during individual pyrolysis of the walnut shell and the Shenfu coal,together with copyrolysis of the walnut shell and the Shenfu coal.It was showed that when mass ratio of the walnut shell to the Shenfu coal was 2:1,production of water,acetic acid,furan,methylfuran and furfural,reached the highest value compared with other mass ratios,among which maximal relative strength of acetic acid was 3.9 times that for individual pyrolysis of the walnut shell,7.5 times that for the Shenfu coal pyrolysis,and acetic acid yield was 2.6 times theoretical one in 300-500 ℃,attributed to the synergistic effect which promoted formation of water,acetic acid,furan,methyl furan and furfural.(4)Study on kinetics of catalytic copyrolysis of the walnut shell and the Shenfu coal.Effects of the attapulgite on copyrolysis activation energy and mechanism model of the walnut shell and the Shenfu coal were focused,by means of Friedman,FWO,KAS and Starink methods,as well as Achar and Coats-Redfern methods,respectively.It was suggested that the attapulgite catalyzed copyrolysis of the walnut shell/Shenfu coal(3:1)at 380 ℃significantly;averaged apparent activation energy of the walnut shell/Shenfu coal(3:1)in 260-380 ℃,decreased by 22 kJ-mol-1,when attapulgite was added at mass ratio 1:9.Three-dimensional diffusion(D3)mechanism remained for the walnut shell/Shenfu coal(3:1)catalytic copyrolysis in 260-340 ℃,but second-order(F2)reaction mechanism appeared in 380-500 ℃.(5)Research on catalytic mechanism of the attapulgite on copyrolysis of the walnut shell and the Shenfu coal.Relationship between theoretical weight losses and experimental ones,for catalytic copyrolysis of the walnut shell and the Shenfu in presence of the attapulgite with different mass ratios was studied by thermogravimetric analysis;formations of volatile products were compared,before and after the attapulgite addtion at different mass ratios,by thermogravimetric-mass spectrometry.It was shown that the catalytic effect of the attapulgite was influenced by its pore structure and surface structure.In presence of the catalyst,amount of acetic acid,furan,methyl furan and furfural from copyrolysis before 380℃,multiplied compared to those formed in absence of the attapulgite;production of acetic acid,furan and furfural decreased,but benzene production increased after 380℃.When mass ratio of the attapulgite to the walnut shell/Shenfu coal(3:1)was 1:3,yield of benzene reached the highest level.(6)Research on copyrolysis of the walnut shell and the Shenfu coal.Firstly,hydrogen radicals and hydroxyl radicals,produced by pyrolysis of the walnut shell before 300 ℃induced and promoted pyrolysis of the Shenfu coal,including breakage of bridge bond and alkyl side-chains,mobile phase devolatilization,aliphatic group pyrolysis,resulting in formation of free radical fragments;meanwhile,active cellulose melted from the walnut shell at 300 ℃,underwent chain breakage and depolymerization,lignin underwent phenolic hydroxyl decomposition.Finally,great variety of free radical fragments,including methyl and carboxyl groups,produced by pyrolysis of the walnut shell and the Shenfu coal,promote formation of acetic acid through free radical stabilization.(7)Research on catalytic copyrolysis of the walnut shell/Shenfu coal.The attapulgite catalysis promoted synergistic effect during copyrolysis.Above all,silanol active sites and ionic active centers in the attapulgite,catalyzed chain breaking and depolymerization of active cellulose,along with decomposition of lignin in the walnut shell,induced and promoted pyrolysis of the Shenfu coal,with aid of generated hydrogen and hydroxyl radicals;secondly,a large quantity of free radical fragments,produced by the pyrolysis of the walnut shell and the Shenfu coal,were converted with the attapulgite catalysis into furan and furfural,through dehydration,fragmentation,condensation,decarbonylation and decarboxylation;part of furan and furfural,were catalyzed into light aromatic hydrocarbons by continued dehydration,decarboxylation and decarbonylation.Finally,toluene and xylene in light aromatic hydrocarbons,reacted with acetic acid,furan,methylfuran and furfural,in presence of the attapulgite catalysis and high temperature,forming polycyclic aromatic hydrocarbons,which condensed with free radicals containing aromatic ring from coal pyrolysis,giving semicoke.Based on the above researches,a new catalytic co-pyrolysis process,named continuous gradient pyrolysis and step-wise recovery,was established,which was expected to be used in step-wise recovery of tar components.Using this gradient pyrolysis and step-wise recovery process,condensable small molecular chemicals,such as acetic acid and furan compounds,can be recovered in 200-380 ℃;light aromatic hydrocarbons can be recovered in 380-500℃;heavy tar can be recovered after 500℃.