Effects of sodium on nitrogen evolution characteristics during the fast co-pyrolysis of coal and biomass

The co-combustion of biomass and coal is a crucial initiative for advancing carbon neutrality. However, the specific influence of alkali metals on nitrogen evolution during fast co-pyrolysis remains unclear. This study investigates the regulatory effects of sodium (Na) on the migration and transformation of volatile-N and char-N during the rapid pyrolysis and co-pyrolysis of sawdust (SD) and lignite coal (LC). Acid-washed and Na-loaded samples, including individual SD/LC and blended samples (blending ratios of 5% and 15%), were pyrolyzed in a vertical fixed-bed reactor at 500 °C and 800 °C. The evolution of volatile-N (measured as NH₃ and HCN) and char-N functional groups was analyzed using an FTIR gas analyzer and employing X-ray photoelectron and Raman spectroscopies. Results show that at low temperatures of 500 °C, labile biomass N-A decomposes into NH₃, while coal N-5 and N-6 remain relatively stable. At 800 °C, heterocyclic nitrogen undergoes ring-opening to form HCN, while coal preferentially retains nitrogen as N-Q. Crucially, Na addition alters these pathways, promoting the apparent NH₃ and HCN release from biomass at 500 °C while inhibiting heterocyclic ring-opening and N-Q hydrogenation at 800 °C. During co-pyrolysis, Na consistently suppresses volatile-N precursors by inhibiting the decomposition of heterocyclic nitrogen and promoting char-N stabilization mainly as N-5 and N-Q. Raman analysis further reveals that Na regulated the evolution of aromatic carbon structures in blended chars, thereby affecting char-N retention within the carbon matrix. These findings provide valuable insights into controlling NO _x precursor emissions during the fast co-pyrolysis of coal and biomass through alkali metal management.