Abstract
Nowadays, the biogas plant with anaerobic digestion is receiving much attention as a facility for livestock manure treatment and electric power generation. Generally speaking, the biogas plant reduces odor from raw manure and produces the digested slurry with rich manure components similar to the fertilizer. The special interest is in the electric generation. However, the evaluations have not been made sufficiently whether the biogas plant is practically feasible or not at any points.
The objective of this study is to evaluate the biogas system totally from the energetic points of view. The basic object of investigation is a biogas plant at the Rakuno Gakuen University (RGU). A biogas plant on farm was also investigated by the evaluation from the biogas plant at the RGU.
The biogas plant was evaluated from energetic point of view by input-output analyses. The energetic evaluation was made by comparing fossil energy inputs with available energy outputs from operating biogas plant in a year. The fossil energy inputs were those used for a biogas plant at construction, operation and maintenance. The available energy outputs were the generated electricity and heat from a biogas plant and the energy value of digested slurry. The pay back period in energy base was calculated from energy inputs and outputs analyses.
The pay back period in energy base was 59 years at the RGU biogas plant and 7 years at a biogas plant on farm when digested slurry was not utilized as fertilizer. When the digested slurry was utilized the pay back period in energy base became much shorter to 15 years and 3 years, respectively.
The total energetic evaluation of biogas plant indicated that the utilization of digested slurry as fertilizer is the indispensable element for practical feasibility at the biogas plant.
