NH_3气氛下温度对生物质富氮热解产物特性的影响

Influence of temperature on products characteristics derived from biomass nitrogen-enriched pyrolysis in NH_3 environment

生物质热解转化为高值产品是生物质利用的重要发展方向,在热解过程中引入外源氮素形成富氮热解,具有制备高值含氮产品的潜力。该文以NH_3为外源氮素,研究了不同热解温度对木屑和玉米秆富氮热解过程和产物特性的影响,探讨了含氮物质的形成机理,为生物质转化为高值产品提供理论依据。采用多种方式(气相色谱-质谱联用、元素分析、漫反射傅立叶变换红外光谱、X射线光电子能谱等)对热解产物特性进行了分析。结果表明,随着热解温度的升高,焦炭产率逐渐减小,热解油产率先增加后减小,550℃达到最大值。热解油中含有大量的胺态氮有机物(甲胺类和丙胺类等)和含氮杂环类物质(吡咯类和哌啶类等),以及少量的腈类。热解焦炭的含氮量随着温度升高而增加,焦炭表面存在大量含氮官能团C=N、N-COO、C-N和N-H等。含氮官能团的存在形式主要有吡啶-N、吡咯-N和酰胺-N,随着热解温度的升高,酰胺-N含量逐渐降低,而吡啶-N含量迅速增加,高温下主要为吡啶-N和吡咯-N。

Conversion of biomass to high-value products by pyrolysis is a significant development direction for biomass utilization. As a promising therrnochemical 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 D LD, 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.