Improvement of biomethane potential of Elephant grass (Napier grass, Pennisetum purpureum Schum) by addition of biomass ash and ash-extracts as supplements

Abstract

The effect of supplementing an AD process with biomass ash and ash-extracts was investigated in a 42-days BMP test using Elephant grass silage as feedstock. The results showed that the supplements increased BMP in the range between 314.6 and 359.6 N mL CH4/g VS fed. Buswell equation showed that the theoretical methane potential (TMP) of the grass was 491 N mL CH4/g VS fed and that the various treatments resulted in biodegradation rates ranging between 64.6 and 73.2%, corresponding to a 4–19% increase in the volume of methane produced. Also, hydrolysis constants from the first-order kinetic model ranged between 0.27 and 0.28 day−1. Gompertz model correctly predicted methane productivity and gave good fits (R2 > 0.97); lag time (λ) between 1.04 and 1.52 days from days 1 to 5. These results strongly suggest that the trace elements and alkalinity content of the ash extracts enhanced the degradation efficiency of Elephant grass and improved its methane yield.