Pyrolysis Characteristics And Nitrogen Transformation Patterns Of Biomass Pretreated With Ammonia Torrefaction And Urea Solution Impregnation

Nitrogenous compounds are an important class of organic compounds that have important applications in the synthesis of pharmaceuticals and fine chemicals.However,the current industrial production of such nitrogen-containing chemicals almost entirely relies on non-renewable coal and petroleum resources.Moreover,nitrogenous heterocyclic compounds have complex structures and rich heteroatoms,which still have many drawbacks in the synthesis process,leading to high production costs.Compared with petroleum-based hydrocarbons,using biomass as raw materials to form nitrogen-rich chemicals through nitrogen-rich pyrolysis is more conducive to ammonia formation.At the same time,nitrogen-rich pyrolysis of biomass can co-produce nitrogen-doped carbon materials,which have important application prospects in catalysis,electrode materials,supercapacitors,and organic pollutant removal.Therefore,using renewable and eco-friendly biomass resources to prepare high-value nitrogen-containing chemicals and nitrogen-rich biochar has important research significance.In this paper,agricultural and forestry biomass(cotton stalks,pine powder)and the main component of biomass(cellulose)were used as experimental materials,and ammonia torrefaction and urea impregnation pretreatment techniques were used to prepare nitrogenous compounds and nitrogen-rich biochar.By controlling the synergistic pyrolysis conditions of raw material pretreatment,the physicochemical properties and structural evolution of pretreated biomass were revealed,and the characteristics of pyrolysis products and the evolution of activation energy of pretreated biomass were clarified.The migration and transformation of nitrogen during the pretreatment-pyrolysis process were explored,and the possible pathways for the generation of main nitrogenous compounds were proposed.The main contents and results are as follows:The effect of torrefaction temperature and duration on torrefaction and pyrolysis product characteristics was studied using cotton stalks as raw materials.The results showed that ammonia torrefaction promoted the formation of nitrogenous heterocyclic compounds,especially pyridine compounds,while reducing the content of oxygen-containing substances.With increasing torrefaction temperature and duration,the formation of amide-N,pyrrole-N,and pyridine-N were all promoted.When the torrefaction temperature increased to 290℃,the nitrogen content increased significantly from 0.98%to 6.85%,and the nitrogen doping ratio was the largest at this temperature,which was 3.22%.Additionally,the duration of torrefaction had a positive effect on the relative content of nitrogenous chemicals.To further investigate the conversion pathway of nitrogen in the torrefaction-pyrolysis system,we employed cellulose,a common biomass model substance,as the raw material to explore the effect of torrefaction temperature on cellulose’s chemical structure evolution and the nitrogen doping and migration conversion mechanism during ammonia torrefaction.The results indicate that ammonia torrefaction pretreatment effectively optimized the distribution of nitrogen and oxygen elements in cellulose and increased the relative content of nitrogenous compounds.Specifically,increasing the torrefaction temperature led to a marked rise in nitrogen content,from 0 to 11.18%.The carbon skeleton first captured active nitrogen-containing free radicals to form–NH_n-N,with pyridine-N and pyrrole-N arising from the conversion of–NH_n-N.Analysis of biomass oil components demonstrated that ammonia torrefaction pretreatment boosted the relative content of nitrogen-containing compounds from 0 to 68.34%(TCA-320).The presence of C=O played a central role in nitrogen fixation,and nitrogen in biomass oil mainly existed as five-membered and six-membered heterocycles.Correlation analysis indicated that the main precursors for the formation of nitrogen-containing heterocyclic compounds are firstly derived from the transformation of nitrogenous species in the torrefied solid phase products,secondly,the reaction of oxygen-containing straight chains and five-membered oxygen-containing heterocycles with nitrogen-containing groups,rather than six-membered oxygen-containing heterocycles.Finally,we summarized the product distribution characteristics in the torrefaction-pyrolysis system and proposed the nitrogen doping and conversion mechanism.Pine powder was used as a raw material to investigate the impact of urea impregnation concentration and pyrolysis temperature on the composition of various nitrogen-and oxygen-containing compounds in the bio-oil produced by pine powder pyrolysis.The results showed that the total phenol content in bio-oil increased significantly after nitrogen-rich pyrolysis,and four types of phenols were identified.Furthermore,compared to conventional biomass pyrolysis,the bio-oil after nitrogen-rich pyrolysis had a reduced content of aldehydes and straight-chain ketones.The content of nitrogenous compounds in bio-oil was significantly and positively correlated with urea concentration.When the urea concentration increases,the amount of pyridines was the largest.Finally,the characteristics and similarities and differences between ammonia torrefaction and urea impregnation pretreatment technologies for the production of nitrogen-containing compounds were analyzed.