Utilization of waste pine needles for the production of cellulolytic enzymes in a solid state fermentation bioreactor and high calorific value fuel pellets from fermented residue: Towards a biorefinery approach

The present study addresses valorisation of abundantly available, low-cost pine needle (PN) waste into cellulolytic enzymes and high calorific value (CV) fuel pellets (FP). Initial studies in flasks involving hot water steam pre-treatment (HWSP) using Aspergillus niger MTCC 9652 for 5 days resulted in an increase in Cellulase, Endoglucanase and Xylanase activities compared to the untreated pine needles (UPN). Fermentation experiments were scaled-up in a 6-L Solid State fermentation (SSF) packed bed reactor (PBR) at different bed heights (BH) using UPN. The maximum cellulase and xylanase yields were achieved as 9.97 ± 0.27 FPU g-ds−1 and 8276 ± 254 IU g-ds−1, respectively, at 72 h using UPN in the PBR. The fermented residue showed 53.5% consumption of holocellulose, a decrease in crystallinity index to 29.5% from 46%, with an increase in total lignin content of 45.9%. The fermented residue was processed for pellet production, resulting in a higher CV (19.13 ± 0.69 MJ kg-1) than the pellet made from unfermented PN (17.63 ± 0.66 MJ kg−1), along with enhanced compressive strength. Sulphur content (0.14%) was below the permissible limit of SOx emission for fuels with ash content less than 5%. Therefore, the present study represents a novel and sustainable biorefinery approach for PN valorisation.