Research On The Organic Nitrogen Transfer Mechanism During Pyrolysis Of Wheat-straw

Biomass is one of the importent renewable energy sources and the use of biomass energy has received increasing attention.Since the biomass contains more nitrogen,it will generate nitrogen oxides which can be harmful to the environment during the heat utilization process.The research on the transformation mechanism of organic nitrogen during the biomass heat utilization process is beneficial to the clean utilization of biomass energy.Wheat is an important crop and wheat straw is also one of the important fuels for biomass power plants in northern China.Studying the transformation mechanism of organic nitrogen using the wheat straw rather than many biomass pyrolysis can not only be easier to discover its pyrolysis mechanism during wheat straw burning,but also provide guidance for the study of nitrogen oxide conversion mechanism in other biomass heat utilization processes.The conversion of biomass organic nitrogen is significantly affected by the biomass chemical composition(the form of nitrogen,the content of cellulose,hemicellulose,lignin and minerals in biomass)and pyrolysis conditions(pyrolysis temperature,heating rate,fuel particle diameter and pyrolysis atmosphere),making it difficult to predict the transformation mechanism of nitrogen in direct pyrolysis of biomass.In order to explore the transformation mechanism of organic nitrogen in the process of wheat straw pyrolysis,the transformation mechainism of organic nitrogen in wheat straw under different pyrolysis conditions was studied;the model compound was made according to the nitrogen form of wheat straw and pyrolysis of the model compounds and their constituents were carried out under different pyrolysis conditions.The structure of the model compound is relatively simple and easy to speculate its reaction mechanism;accordingly,the reaction mechanism of organic nitrogen in wheat straw can be inferred.To explore the microscopic reaction pathway of the model compound,the density functional theory was used to reveal the reaction mechanism of the model compound through theoretical calculation.The pathways reveal the transformation mechanism of organic nitrogen in the pyrolysis process of wheat straw more clearly.In order to explore the pyrolysis of wheat straw,the method of thermogravimetry-Fourier infrared gas analyzer(TG-FTIR)combined with tube furnace-Fourier infrared gas analyzer was used to conduct the rapid pyrolysis and programmed pyrolysis of wheat straw.The study found that as the temperature increases,organic nitrogen would enter the volatiles,tar and coke,respectively.As the temperature increases,the volatile nitrogen and coke nitrogen would be cleaved to form nitrogen oxide precursors(mainly ammonia,hydrogen cyanide and isocyanic acid);the tar would form nitrogen oxide precursors by volatile nitrogen and coke nitrogen,respectively.When pyrolysis heating rate is increased,the residue after pyrolysis remained less;the smaller the fuel particle size,the less the residue of fuel pyrolysis;increased carrier gas flow rate can also reduce the residue.These phenomena are dependent on whether the volatile nitrogen can escape from the fuel particles in time and whether it is polymerized into coke or pyrolysis to form small molecular gases.During rapid pyrolysis,when the pyrolysis temperature increases,the amount of nitrogen-containing gas increases rapidly,the amount of HCN is increased,and the amount of NH3 is reduced.As the particle size becomes smaller,the amount of gas-generated increases,possibly due to the higher particle size,the smaller the thermal resistance and the easier the release of pyrolysis volatiles,producing more gas and more NH3 while forming less coke.At the same time,in the presence of O2 in the pyrolysis atmosphere,NO is formed,when O2 is at 10%,however,the increase of O2 causes a decrease in HCN and NO.The main nitrogen gas produced is HCN during straw rapid pyrolysis.A model straw composed of amino acids and cellulose was mixed according to the proportion in the straw to study the reaction mechanism of organic nitrogen in wheat straw.In the slow pyrolysis and rapid pyrolysis experiments,it was found that the release characteristics of the model straw and the nitrogen-containing body of the pickled straw were strongly consistent.Hence,the straw model could be used to study the pyrolysis mechanism of the fuel nitrogen of the wheat straw.Temperature-programmed pyrolysis and rapid pyrolysis was conducted on nine kinds of amino acids with high content and representative structures the wheat straw(glycine,alanine,phenylalanine,leucine,glutamic acid,aspartic acid,proline,histidine,and cyst)and mixture of two from 10 amino acids were subjected to pyrolysis experiments.The experimental results show that the amino acid pyrolysis first dehydrates to form a cyclic dipeptide,decarburizes to form carbon dioxide and amine,deamination to acid and side chain cleavage,during which aspartic acid and glutamic acid mainiy undergo intramolecular dehydration reaction to generate large molecule tar.Mixing two different amino acids would cause a condensation reaction between the amino acids to form cyclic dipeptide or dipeptide.Different pyrolysis conditions can affect the rate of evolution of volatiles from solid particles,causing the reaction of tars to small molecules or to coke,and different reaction pathways would result in different distributions of nitrogen oxide precursors.The possible path in the pyrolysis process of glycine and glycine anhydride was carried out at the level of 6-31G(d)base group by B3LYP method based on the density functional theory.The transition state calculation shows that the direct activation of glycine to generate ammonia radicals and further produce ammonia has the lowest activation energy,but it is also prone to intermolecular condensation reaction to form cyclic dipeptide(glycine anhydride).The pyrolysis of glycine anhydride has the lowest activation energy for the formation of hydrogen cyanide by C-C cleavage,so the pyrolysis of glycine anhydride mainly produces HCN.At the same time,the reaction pathway of glycine anhydride to form coke nitrogen is also easy to occur.The coke nitrogen pyrolysis mainly generates ammonia gas.In the programmed pyrolysis of glycine anhydride,NH3 is more than HCN,which may be due to the high bulk density,low heating rate during the thermogravimetry,the intermolecular dehydration of the external glycine anhydride occurs in the first step,resulting in more coke nitrogen.In conclusion,the migration mechanism of organic nitrogen in wheat straw is not only related to the form of nitrogen,but also to the precipitation process of tar.Due to different pyrolysis conditions,the concentration of volatiles in localized areas of the fuel particles is different,causing the tar to be polymerize into a coke or cleav to small molecular gas,leading to the different distribution of nitrogen oxides.