生物质热解过程中氮迁移转化机理研究进展

生物质热解产物中热解气和热解油具有较高能源利用价值，可作为替代燃料或化工原料，但伴随热解过程迁移至热解气/油中的氮元素不仅会影响其品质，热解气/油进一步利用后也会污染大气环境。该研究围绕生物质资源制备清洁能源的总目标，系统分析生物质热解过程中氮迁移转化机理，重点论述气相氮、液相氮和焦炭氮的生成与转化机理。通过总结前人研究，得出生物质热解气中的含氮物质主要为HCN、NH3等，其中NH3主要来源于氨基酸热解释放的氨基以及HCN在焦炭表面的水解转化；HCN主要来源于腈、含氮杂环等一次热解产物的二次裂解；热解油中的含氮物质主要为含氮杂环、腈与酰胺，其中含氮杂环主要由部分氨基酸片段或氨基酸间的脱水缩合反应产生；腈主要来源于氨基酸分子脱H2反应以及酰胺脱H2O反应；酰胺主要来源于NH3与羧基的置换反应。不同生物质种类与热解工况下氮的迁移转化特性复杂多样，生物质种类以及热解过程中的压力、停留时间、升温速率、温度、热解气氛、粒径、催化剂等因素均会影响热解过程中氮的迁移转化路径，最终影响生物质热解气/油中含氮物质的组成及分布。进一步提出生物质热解过程中氮排放控制未来研究方向，以期为实现农村生物质资源高效清洁利用提供参考。

Review of nitrogen migration and transformation during biomass pyrolysis

Biomass pyrolysis can generate tar and gas products with high industrial value. But the nitrogen (N) element in the biomass can inevitably migrate to the products along with the pyrolysis process, thus possibly polluting the environment. Focusing on the overall goal of preparing clean energy from biomass resources, this study systematically analyzes nitrogen migration and conversion mechanism during biomass pyrolysis, focusing on the research progress of the generation and conversion mechanism of gas nitrogen, tar nitrogen and char nitrogen. The NOx precursors can be the HCN and NH3 in the biomass pyrolysis gas. Specifically, the NH3 comes from the amino acids that are released from the amino acid pyrolysis and hydrolysis of HCN on the surface of char, while the HCN is from the secondary cracking of primary pyrolysis products, such as nitrile and N-containing heterocycle. The N-containing substances in the pyrolysis oil include the N-containing heterocycles, nitrile, and amide. Furthermore, the N-containing heterocycles can be produced by the fragmentation of some amino acids and by dehydration condensation between the amino acids. The nitrile is derived from the de-H2 reaction of amino acid molecules and the de-H2O reaction of amides. The substitution reactions can also be used to form amides from NH3 and carboxyl groups. More importantly, the biomass varies greatly in the different pyrolysis characteristics and products, due to the composition during the reaction. The higher heating rates can promote tar cracking for higher NOx precursor production during biomass pyrolysis, while the lower heating rates can contribute to tar production for better quality. The pyrolysis temperature and atmospheres of biomass can pose a large effect on the yield and composition of the pyrolysis products. The pyrolysis in the O2 and H2O atmosphere can enhance the conversion of HCN to NH3, while the pyrolysis in the CO2 atmosphere can reduce the production of NOx precursors. In terms of the pyrolysis pressure, the gas-N residence time can facilitate the reaction path of the secondary pyrolysis for the migration path of nitrogen. The larger particle sizes of the biomass can increase the NOx precursors but less the tar production, whereas, the smaller particle sizes can promote the N fixation in the char. The catalysts can reduce the pyrolysis time and the temperature for the N migration and conversion during biomass pyrolysis. The mineral elements (such as K, Ca, and Fe) in the biomass can promote the conversion of nitrogenous substances in the coke into the HCN. By contrast, the metal oxides (such as Fe2O3, Co3O4, and NiO) can be used to enhance the production of Tar-N, where Co3O4 has the best performance. The KOH can reduce the types of hydrocarbon compounds in the pyrolysis oil, but for less NH3 and HCN production. The current NOx treatments are the catalytic, plasma, microbial, absorption, and adsorption methods. All tail-end treatments cannot reduce the emission of pollutants with low efficiency and high energy consumption. Anyway, the N migration and transformation mechanism in the pyrolysis of biomass can reduce the emission of N-containing pollutants at the source during the pyrolysis process.