Green production of ammonia from nitrogen-rich biomass pyrolysis: Evolution of fuel-N under H2-rich atmosphere

Ammonia (NH3) is an emerging carbon-free energy carrier. A sustainable route for NH3 production is proposed in this work via pyrolysis process of nitrogen-rich renewable resources. Microalgae, the carbon-neutral and renewable resource with a high content of nitrogen, were selected in this work. H2 was used as a reinforcing agent to enhance NH3 production. The influence of H2 on the fuel‑nitrogen transfer during the microalgae pyrolysis was experimentally investigated. The formation and conversion pathways of the N-containing species in bio-char, bio-oil and gaseous products were evaluated. Results indicate that H2 promotes the nitrogen transformation into gaseous products, while reduces the contents of the nitrogen in solid char and bio-oil. Meanwhile, H2 greatly promotes the conversion of protein-N to pyridinic-N, pyrrolic-N and quaternary-N in biochar, but also facilities the formation of N-heterocyclic compounds in bio-oil. Pyrolysis temperature is an important factor affecting the distribution and conversion of N-containing species. NH3 and HCN yields increase gradually with temperature. H2 is beneficial for the thermal cracking of stable proteins, the deamination of amides, and the ring-opening of N-heterocyclic species, thereby facilitating the formation of NH3. The highest NH3 yield of 37.34 wt% is obtained at 800 °C under 15%H2 + 85%Ar atmosphere.