Research On Nitrogen Transformation And N-Containing Target Products Control During Biomass Nitrogen-Enriched Pyrolysis

Biomass pyrolysis for high-valued products is an important development direction for biomass utilization.Biomass contains large amounts of O-containing groups.If using nitrogen to substitute oxygen,we may obtain abundant N-containing species,such as pyrrole,pyridine,indole,which can be used to produce pharmaceuticals,dye or other chemicals.N-doped carbon materials contain lots of active N-containing groups,such as pyridinic-N,pyrrolic-N,quaternary-N and pyridone-N-oxide,which can be applied in catalysis,adsorption,energy storage.This work focused on investigating nitrogen transformation mechanism during high-nitrogen biomass pyrolysis,and formation mechanisms of N-containing species and N-doped carbon during introducing exogenous nitrogen,and mechanism of biomass N-enriched pyrolysis for high-valued N-containing chemicals and N-doped carbon materials,which is in favor of realizing efficient and valuable utilization of biomass wastes.The main works are shown as follows.Firstly,we studied the pyrolytic poly-generation?body N-enriched pyrolysis?characteristics of three typical algae?EP,SP and NS?with different components and nitrogen content,and discussed the influence of components difference on pyrolytic products.At 400-600°C,the major compositions of EP bio-oil were furans resulted from the dyhydration of carbohydrates.SP yields large amounts of N-containing chemicals and phenols due to the cracking of proteins,and dehydration,cyclization and Maillard reaction of N-containing reactive intermediates.While long-chain carboxylic acids,hydrocarbons and esters,originated from the cracking and steam reforming of lipids,are the dominated components for NS.Pyrolytic char contained a certain amount of C-O/C-O-C/C=N,C=O/C-N and COO-groups.At 700-800°C,aromatics gradually became the major compounds for all bio-oil due to the cyclization,aromatization and polycondensation of char,deoxygenation and aromatization of reactive intermediates derived from EP,denitrogenation and aromatization of that from SP,chain scission,and cyclization and aromatization of that from NS.Meanwhile,large amounts of CO,H₂ and CH₄ released out through decarbonylation,dehydrogenation,demethylation and chain-end scission.Besides,N-/O-containing groups in char decreased further and polyaromatics were produced due to further ring polymerization.On-line and off-line testing technologies were combined to comprehensively analyze the distribution properties,existing forms and regulation evolution of nitrogen in char,bio-oil and gas products druing algae pyrolysis.A mechanism of nitrogen transformation during algae pyrolysis was proposed based on amino acids decomposition.Nitrogen in algae is mainly in the form of protein-N?⁹0%?with some inorganic-N.At lower temperature,inorganic-N decomposed to NH₃ and NO.Pyridinic-N and pyrrolic-N in char stemed from cyclization reactions of amino acids?leucine,arginine,proline,valine,glutamic acid,and aspartic acid?.Some pyridinic-N converted to quaternary-N through ring condensation.Phenylalanine,tyrosine,and tryptophan decomposition could form indoles in bio-oil.The cracking of other amino acids could release large quantities of NH₃/NH₂*,which reacted with fatty acids to form amides in bio-oil.Nitriles could then be generated through dehydration reactions of amides.At higher temperature,the dehydration?cyclization?of glutamic acid and aspartic acid could form pyridinic-N and pyrrolic-N.Pyrrolic-N and quaternary-N decomposition could form pyridines,pyrroles,and indoles in bio-oil,respectively.HCN and NH₃ release could be attributed to the cracking of pyridinic-N and pyrrolic-N in char.Microalgae with high nitrogen content was used as exogenous nitrogen to realize exogenous N-enriched pyrolysis of biomass?bamboo wastes?with low nitrogen content through co-pyrolysis.Nitrogen transformation mechanism during co-pyrolysis was revealed.And a novel method was proposed that introducing biochar addition to control nitrogen emission.N-enriched pyrolysis promoted the Calkoxy-O bond scission,and inhibited the secondary cracking of long-chain fatty acids and NH3 reacting with long-chain fatty acids,thus generated large quantities of long-chain fatty acids.NH3 and N-containing intermediates reacted with N-containing intermediates,thus decreased the content of acetic acid,aldehydes,ketones in bio-oil.Amide groups?-NH2?in NS may react with carbonyl groups?-C=O?in NS through Maillard reaction to form N-heterocyclic ring,then through dehydration or decarbonylation reactions to generate pyridinic-N or pyrrolic-N in char product.But it also promoted NH3 and HCN release.Considering the limited nitrogen in nature,NH₃ was used as exogenous nitrogen to investigate the regulation evolution of N-containing species from bamboo wastes N-enriched pyrolysis at different temperature,and compare them with the products characteristics from pyrolysis under Ar.The formation mechanism of N-containing bio-oil and N-doped carbon was revealed.NH3 reacted with biomass to generate NH2*and NH*,which further reacted with O-containing intermediates from biomass pyrolysis to form N-containing species?such as amines?through Maillard reactions.Some amines transformed into N-heterocyclics?pyridines,pyrroles?through dehydration,condensation and cyclization reactions.And some O-heterocyclics?furans and furfurals?could also convert to N-heterocyclics?pyrroles?through substitution reactions.While phenols might directly react with NH3,NH2*and NH*to form anilines through dehydration reactions.O-containing groups in char reacted with NH3,NH2*and NH*,and formed large quantities of N-containing groups?pyridinic-N,pyrrolic-N,quaternary-N and pyridone-N-oxide?through Maillard reactions.Firstly,O-containing groups reacted with-NH2 to generate pyridinic-N and pyrrolic-N,then some pyridininc-N transformed into quaternary-N through polymerization reaction.To improve the quality N-containing bio-oil and N-doped carbon materials,KOH activator was introduced in N-enriched pyrolysis to investigate the synergistic effect of NH₃and KOH on N-containing bio-oil and N-doped carbon materials.The co-generation mechanism of high-valued N-containing chemicals and N-doped carbon materials was revealed.KOH reacted with active functional gourps in biomass,and removed large amounts of O-containing groups to generate abundant active vacancies.NH₃ reacted with biomass and formed lots of NH₂*and NH*,which occupied active vacanicies quickly,thus enriched large amouts of nitrogen in char?10.4 wt.%?product and bio-oil?>50%N-containing species?.NH3 and KOH also etched carbon fragments to form large quantities of pores,thus obtained N-doped carbon materials with high specific surface area?1873 m²/g?and developed mesoporous structure.The specific capacitance of the N-doped carbon electrodes of supercapacitor reached 187 F/g with excellent rate characteristics and cycle stability.Finally,it realized the objective of biomass N-enriched pyrolysis for high-valued N-containing chemicals and N-doped carbon materials.