Distinct Roles of Dilute Acid and Ammonia in Steam Explosion of Corn Stover: Linking Biomass Structure to Bioconversion.

Dilute acid- and ammonia-assisted steam explosion (SE) are promising pretreatments for lignocellulosic biorefineries and induce distinct structural changes in biomass. Here, sulfuric acid-, phosphoric acid-, and ammonia-assisted SE were comparatively investigated within a wash-free framework to elucidate additive-dependent mechanisms linking biomass structural reconstruction to downstream bioconversion. Mechanistic analyses revealed that sulfuric acid-assisted SE intensified lignin-related inhibition, resulting in the highest surface lignin coverage (SLC) and the strongest non-productive cellulase adsorption. By contrast, phosphoric acid-assisted SE preferentially promoted hemicellulose removal while alleviating lignin condensation, yielding the lowest SLC. Ammonia-assisted SE suppressed lignin condensation despite limited lignin removal; however, its higher water retention capacity impaired enzymatic hydrolysis under high-solids conditions. Notably, SLC showed the strongest correlation with glucose yield (R2 = 0.999), highlighting it as an integrated indicator of enzymatic hydrolysis performance. Consistent with these structural differences, the sulfuric acid-, phosphoric acid-, and ammonia-assisted corn stover achieved glucose yields of 85.72%, 93.28%, and 92.16%, respectively. Moreover, retained soluble components in the wash-free system appeared to benefit downstream fermentation, as all hydrolysates remained fermentable without detoxification, with the ammonia-assisted SE hydrolysate showing the best co-production of yeast protein and ethanol. These findings provide mechanistic guidance for additive selection in water-efficient lignocellulosic biorefineries.