The research progress of formation and control on the N-containing compound of biomass pyrolysis gas

Biomass energy plays an important role in combating global warming and the depletion of fossil energy sources. Although different recovery technologies of biomass energy were utilized industrially, the development level of different recovery technologies varies. The application of biomass energy includes technologies such as combustion, pyrolysis, gasification, and fermentation. The pyrolysis technology is an efficient and economical method to utilize biomass energy, which combines the advantage of energy recovery and product diversification. However, the N-containing compounds in the biomass pyrolysis gas make the pyrolysis gas of low quality, which combustion leads to secondary pollution of air. This review summaries the research status of N-containing compounds in the biomass pyrolysis gas, mainly reviewing the differences in the thermos-gravimetric behavior of typical biomass and the four compositions in biomass (cellulose, hemicellulose, lignin, and proteins). There were significant differences in the thermos-gravimetric behavior of biomass with different material compositions, but the whole TG curve can be divided into three stages: in the first stage, the pyrolysis of easily decomposable components in biomass releases small molecule gases and steam; in the second stage, the pyrolysis of cellulose, hemicellulose, and lignin in biomass released a large amount of O-containing bio-oil; in the third stage, the volatile components attached to the surface of the bio-char were cracked again and condensation reaction occurs. The nitrogen content in biomass was high, and during the pyrolysis process, nitrogen migrated into the solid-liquid-gas three-phase, and the migration transformation process was extremely complex. This review also discussed the generation mechanism of N-containing compounds in biomass pyrolysis gas and analyzed the distribution and control research of N-containing compounds. The NH₃ in the low-temperature pyrolysis gas was mainly derived from the direct pyrolysis of protein in biomass. With the increase of pyrolysis temperature, the biomass pyrolysis volatiles were cracked secondly to generate N-containing heterocyclic substances, nitriles, and cyclic amides, and further cracked to produce HCN. Under the high-temperature atmosphere, partial HCN reacts with ·H and generates NH₃ with the biomass char catalysis, leading to a decrease in the concentration of HCN. The N-containing heterocyclic substances from the second cracking of volatiles were the main resource of HCNO, and HCNO has a relatively lower concentration and is easily reduced to HCN and NO. Thus, with the pyrolysis temperature increase, the main components of N-containing compounds in the pyrolysis gas were gradually converted from NH₃ and HCNO to NO and HCN. When the temperature was 800 ℃, the concentration of NO accounted for 40% of the N-containing compounds in pyrolysis gas. While, at 900 ℃, NH₃ and HNCO were barely detectable. At the same time, it pointed out the difficulties and challenges faced in the practical application of N-containing compound removal. It is necessary to establish a generalized mechanism for nitrogen conversion during the thermal conversion of biomass. The nitrogen transport and control mechanisms during biomass pyrolysis need to be further improved. And, the key research directions in the process optimization and economic analysis of N-containing control are further anticipated. This review aims to provide a theoretical basis and technology support for biomass pyrolysis gas purification.