To investigate the formation of nitrogenous pollutants (NPs) during biomass thermo-chemical conversion (pyrolysis and gasification) is significant for the control of air pollution as these NPs are an important factor for the formation of PM_2.5. Research progress on the formation mechanism and influence factors of NPs during two processes are reviewed. Consistent conclusions from the literature can be summarized as follows:1) NPs formed from two processes resemble in their formation paths but differ in their types & components. Either NH₃ or HCN is confirmed to be main NPs for pyrolysis while NH₃ is dominant for gasification. 2) Comparing the influence factors, it is demonstrated that the increase of any factor such as the heating rate, the content of fuel nitrogen and the concentration of steam involved will enhance the formation of NPs for two processes. Meanwhile, the effect of temperature on the selectivity of NPs towards two processes are similar as well as higher temperature is inclined to decrease the amount of NPs. 3) Comparing the results of nitrogen distribution, it is found that the percentage of NPs in gaseous phase are approximately 50% for pyrolysis and as much as 90% for gasification. Therefore, to control the formation of NPs in gaseous phase is effective to reduce the pollutants during biomass thermo-chemical conversion. Meanwhile, based on the current conclusions obtained, the deficiencies of formation mechanism are summarized as well as the prospective developments are proposed for further research.

Contents
1 Introduction
2 Formation paths of NPs during biomass thermo-chemical conversion
2.1 Formation paths of NPs during pyrolysis process
2.2 Formation paths of NPs during gasification process
3 Effect of nitrogen occurrence characteristics in fuels
3.1 Effect of nitrogen structure
3.2 Effect of nitrogen content
4 Effect of thermal conditions
4.1 Effect of heating rate
4.2 Effect of temperature
5 Effect of reaction atmosphere
6 Effect of other conditions
6.1 Effect of the physicochemical properties of fuels
6.2 Effect of the catalytic performance of additives
7 Nitrogen distribution during two processes
8 Conclusion