Influence of temperature on products characteristics derived from biomass nitrogen-enriched pyrolysis in NH₃ environment

Conversion of biomass to high-value products by pyrolysis is a significant development direction for biomass utilization. As a promising thermochemical conversion technology of biomass, biomass pyrolysis can obtain the bio-char, bio-oil and higher heating value gas product. By introducing the exogenous nitrogen into the biomass pyrolysis process to form nitrogen-enriched pyrolysis, high-value products can be potentially generated. In this study, NH₃ was chosen as the exogenous nitrogen. The influence of pyrolysis temperature on the properties of sawdust and corn stalk pyrolysis product was investigated. Furthermore, the forming mechanism of nitrogen-contained compounds was analyzed. Products' characteristics were investigated with variant approaches, such as chromatography-mass spectroscopy(7890A/5975C, Agilent, America), ultimate analysis(Vario Micro cube, Germany), diffuse reflectance Fourier transform infrared spectroscopy(Vertex 70, Bruker, Germany), and X-ray photoelectron spectroscopy(Axis ultra DLD, Kratos, United Kingdom). The results indicated that, with pyrolysis temperature increasing, the yield of the bio-char derived from sawdust and corn stalk decreased significantly, while the trend of gas products′ yield turned out to be the opposite. The bio-oil yield increased firstly and then decreased, reaching the maximum(48.7% and 42.7% for sawdust and corn stalk, respectively) at 550℃. Bio-oil contained a lot of amine compounds(methylamines, propylamines and trace amides) and nitrogen-contained heterocyclic compounds(pyrroles, piperidines and trace morpholines) and a few nitriles(acetonitriles and propionitriles). The contents of amine compounds, nitrogen-contained heterocyclic compounds and nitriles were 50%-80%, 10%-20% and lower than 10%, respectively. With the temperature increasing, the content of amine compounds increased due to the fact that NH₃ reacted with biomass at high temperature leading to the formation of NH2, NH and H free radicals, which were then combined with pyrolysis volatiles to generate amine compounds more easily. However, nitrogen-contained heterocyclic compounds and nitriles began to decompose and converse, and hence their contents declined to varying degrees. With the pyrolysis temperature increasing, the content of nitrogen and carbon of the bio-chars increased obviously while the oxygen content decreased significantly. The highest nitrogen contents of the bio-chars from sawdust and corn stalk were 4.08% and 3.92%, respectively. The surface of bio-char contained a lot of nitrogen-contained functional groups, such as pyridine C=N, N-COO, C-N and N-H at 350 ℃, which could be derived from the reaction of NH₃ with the carbon skeleton. The intensities of pyridine C=N, N-COO and C-N increased obviously at 550 ℃. After 750 ℃, none of nitrogen-contained functional groups could be detected. Pyridinic-N, amide-N and pyrrolic/pyridine-N were the main existing forms of nitrogen-contained functional groups on the surface of bio-char. At lower temperature (350 ℃), amide-N and pyrrolic/pyridine-N were the main nitrogen-contained functional groups. With the temperature increasing, the content of amide-N decreased dramatically, while the content of pyridinic-N increased significantly. At higher temperature (750-850 ℃), only pyridinic-N and pyrrolic/pyridine-N existed, indicating that amide-N was not stable and easy to convert to the more stable pyridinic-N and pyrrolic/pyridine-N. In conclusion, nitrogen-contained bio-oil and nitrogen-doped bio-char are produced by biomass nitrogen-enriched pyrolysis, and the bio-oil contains numerous high-value chemicals. However, the detailed forming mechanisms of nitrogen-contained products need to be further researched.