Abstract
Nitrogen-rich biomass pyrolysis has been explored as a green, distributed, and simple alternative for sustainable ammonia production at atmospheric pressure. In this paper, tea waste was selected as a promising feedstock, and H2 was employed as an enhancer to increase the yield of NH3. The nitrogen distribution among three-phase pyrolytic products affected by various temperatures and different atmospheres was compared and discussed. The evolution pathways for fuel–N during tea waste pyrolysis under a H2-rich atmosphere were concluded. Results indicated that the introduction of H2 was favorable for the increase of the gas–N yield but decreased the yields of char–N and tar–N.
At lower temperatures, the bond cleavage of amide–N (N–A) in fuels was enhanced by H2, which then yielded more NH3–N through deamination. Subsequently, H2 improved the production of nitrile–N in tar, as well as NH3–N and HCN–N, by accelerating the secondary cracking of amine–N (tar–N) generated from the decomposition of amide–N. However, the formation of heterocyclic–N in tar through the polymerization of amine–N was restrained under a H2-rich atmosphere.
Pyrolysis in the presence of H2 generated a large amount of H radicals. When the temperature continued to increase, sufficient H radicals impressively advanced the ring rupture (into HCN–N) and full hydrogenation (into NH3–N) of pyridinic–N/pyrrolic–N (N-6/N-5) in char. Meanwhile, H radicals also intensified the thermal cracking of nitrile–N and ring opening of heterocyclic–N to form more HCN–N and NH3–N. Overall, more nitrogen evolved into NH3–N and HCN–N during pyrolysis in a H2-rich atmosphere, especially at high temperatures. The highest NH3 yield of 43.33 wt % was achieved at 800 °C under a 25% H2 + 75% Ar atmosphere.