Protein-rich spent coffee grounds (SCG) are promising precursors for N-doped carbon materials, yet their pyrolysis releases abundant N-containing volatiles (such as NH₃ and HNCO), which are potential NOₓ precursors. This study aims to mitigate such emissions and achieve waste valorization by regulating nitrogen fixation and N-configuration through pretreatments and co-pyrolysis with lignite. TG-IR, in situ DRIFTS, XPS, NH₃-TPD, and DFT calculations were jointly employed to investigate SCG, lignite, and their blends under torrefaction, KOH impregnation, and co-pyrolysis.
The key findings could be summarized in three aspects. (1) Pretreatments reduced N-containing volatiles release. Torrefaction at 250 °C promoted protein–polysaccharide crosslinking and deoxygenation, thereby altering N evolution. KOH impregnation increased the atomic charges on H in N–H bonds and strengthened C–N bonds in anionic diketopiperazines, suppressing NH₃ generation and enhancing N retention. (2) Gas–solid reactions during co-pyrolysis enhanced inherent nitrogen fixation. Maillard reactions formed Amadori/Heyns intermediates while lignite and its derived semicoke provided physical and chemical adsorption sites for NH₃, respectively. Synergistic interactions in SCG–hemicellulose–lignite blends further improved NH₃ capture and fixation. (3) Pretreatments regulated N configurations. Torrefaction and KOH modification facilitated the enrichment of pyridinic N, pyrrolic N, and pyridinolate configurations. The possible N transformation mechanism was speculated by DFT calculations, including side-group removal and further aromatization.
In conclusion, integrating pretreatments with enhanced gas–solid interactions could effectively regulate N-configuration and enhances inherent nitrogen fixation. This strategy reduces nitrogen-containing volatile emissions for pollution control while converting SCG into value-added N-doped carbon precursors, offering a sustainable route for valorization of industrial biomass residue.
 

Spent coffee grounds; Protein-rich biomass residue; Thermal treatment; N configuration regulations; DFT calculations