Importantly, there is no causal correlation between the NOx emission amount (corresponding nitrogen conversion ratio) and the nitrogen content in algae biomass. Moreover, the nitrogen conversion ratio almost reaches its maximum at 700 C. The algae En, Sa and Ch have NOx emission amounts ranging from 0.82 to 0.81 mg, 0.84e0.86 mg, and 1.30 to 1.13 mg at 700e900 C, respectively. This phenomenon is related to both the catalytic effect of active alkali metals and heterogeneous reduction by char in addition to the combustion temperature; e.g., the K content in Sa (53.43 mg/g) is significantly higher than that of En (39.27 mg/g) and Ch (2.83 mg/g), promoting the NOx formation. Meanwhile, the low carbon content weakens the reductive atmosphere during combustion and then increases the NOx emission.