Kinetics of Diauxic Biogas Production from Energy Crops: Sunflower (Helianthus annus) and Napier Grass (Pennisetum purpureum) with Animal Manure

This study evaluated the kinetics of diauxic-like pattern of biogas production from energy crops, Sunflower (SF) and Napier grass (NG) with cow dung (CD). The tests were performed in a batch reactor (R) operation for 60 days in R1 - R4 and 53 days in R5 - R8 under mesophilic conditions (24 - 36OC). The characteristics of the tested energy crops suggest that they hold prospects for bioenergy production. The cumulative biogas yield/gVS showed that the best performance was R1 with a biogas yield of 15.17 dm3 (0.046 dm3/gVS) followed by R3, 13.90 dm3 (0.041 dm3/gVS) and R2, 11.01dm3 (0.032 dm3/gVS). A significant difference (P ≤ 0.05) in biogas yield was found in the reactors charged with SF/CD as against SF only. In the reactors that exhibited biphasic biogas production profile, two (2) kinetic parameters, K1 and K2 were determined by the bi-logistic function model. It was observed that the predicted values in the second phase (K2) of biogas production were considerably higher than the first phase (K1) in R2 - R5 as opposed to R6 - R8, which implies more biogas yield in phase 2 than phase 1. The results indicate that anaerobic digestion of SF and NG had a strong positive influence on biogas yield, BP, PR and λ1 but not for λ2. The bi-logistic function model suitably fitted the experimental data with a high correlation coefficient (R2) in the range of 0.986 - 0.997. Based on the kinetic parameters, the bi-logistic function model is well suited for the simulation of diauxic-like biogas production process.