Biogas production from Napier grass at various cutting intervals

Abstract

This work focused on a study of methane (CH4) production from Napier grass at various cutting intervals. Digestion of Napier grass at cutting intervals of 30, 45, 60, 90 days was investigated in two-stage (acidogenic and methanogenic) anaerobic reactors. Four sets of reactors were constructed with plastic bottles. The reactor working volume was 4 and 5 L for acidogenic and methanogenic reactor, respectively. Acidogenic reactors were fed once daily at a feed rate of 200 mL/day with a slurry of 1:5 Napier grass:water. Hydraulic retention times (HRT) were set at 20 days for acidogenic and 25 days for methanogenic reactor stage. Mixed ruminal microorganisms from cow of approximately 10 g mixed liquor volatile suspended solid/L were used as inoculum. The reactors were operated at ambient temperature of 30 ± 1 °C. pH was adjusted to be 7.5 for all methanogenic reactors at the start-up period. The reactors functioned without pH control. The CH4 yield obtained from Napier grass at cutting intervals of 30, 45, 60 and 90 days was 160, 127, 104 and 74 L at STP/kg of dry Napier grass added to the reactor, respectively, which indicated that 6.25, 7.87, 9.62 and 13.51 kg of dry Napier grass, respectively, is needed to produce 1 m3 of pure CH4. However, the grass yield at the cutting intervals of 30, 45, 60 and 90 days was 16.88, 37.50, 62.50 and 81.25 tons dry weight/hectare/year, respectively. When considering the grass yield of cultivation, the annual CH4 yield per area obtained from Napier grass at the cutting intervals of 30, 40, 60 and 90 days was 2700, 4763, 6500 and 6013 m3/hectare/year, respectively. Napier grass at the cutting interval of 60 days gave the highest CH4 yield of all various cutting intervals.