Effect of Temperature and Mineral Matter on the Formation of NOx Precursors during Fast Pyrolysis of 2,5-Diketopiperazine

2,5-diketopiperazine (DKP) was used as a N-containing model compound to investigate the formation pathway of NOx precursors (HCN, NH3, and HNCO) during biomass pyrolysis. The experiment was carried out using a tube furnace coupled with a Fourier Transform Infrared Spectrometer in an argon atmosphere. The results showed that NH3, HCN, and HNCO were the major N-containing species formed during DKP fast pyrolysis. The largest yield was HCN, followed by NH3 and lastly HNCO. When the pyrolysis temperature was increased, the yield of NH3 increased slowly, but the yield of HCN decreased slightly at 800~950 °C and the change accelerate rapidly above 950 °C. Then NH3 became the main product above 1020 °C. The temperature influence was negligible on the selectivity between HCN and NH3 from pyrolysis of DKP. H radicals played an important role in competitive reactions. It was also noted that the presence of Na+, K+, Ca2+, and Mg2+ exhibited a catalytic effect on nitrogen conversion during the DKP fast pyrolysis process. K+ and Na+ were beneficial to the yield of NH3, but not to the yield of HCN. Ca2+ and Mg2+ could promote the formation of HCN, but prevent the formation of NH3.