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.