Figures 13.4 and 13.5 show the refining process of the RCF facility. The facility produces less than 100 Nm3/day, making it a Class 2 producer according to the High Pressure Gas Safety Act [13]. It is also qualified by law to be a “mobile production facility”. As this mobile production facility can be moved with a crane, the fixed asset tax does not apply. Biogas purification is carried out by membrane separation inside the facility. In order to create a standardized caloric quantity required for town gas 12A, a device that adjusts the caloric content by adding propane was installed. The gas at the completion of the refining process is temporarily stored in storage units below the RCF facility. High-pressure filling of transportable cylinders using high-pressure boosters makes it possible for the biogas to be utilized outside the farm production system as fuel for general households and compressed natural gas (CNG) vehicles.

13.2 On-site Field Testing at Japanese Rural Area

13.3.1 Biogas Production from Biogas Plant

The farm where the biogas purification facility was established is located in central Hokkaido (Hokkaido: northern part of Japan). A free-stall

The total volume of the anaerobic fermentation tank is 396 m3 and the effec­tive volume is 330 m3. Operating conditions include a fermentation temperature at 55 °C and a hydraulic retention time (HRT) of 15 days. The daily material input is 22 m3/day of dairy cow slurry (density assumption: 1 kg/L dairy cow slurry=1 kg/L water). Material is added to the fermenter twice after manure removal. This is done at 9:30 in the morning and 4:30 in the evening. In order to prevent hydrogen sulfide from affecting the biogas purification membrane film, the hydrogen sulfide con­centration must be virtually 0. Desulfurization equipment that combines microbial desulfurization and dry desulfurization was used. After desulfurization, the biogas hydrogen sulfide concentration was virtually 0 ppm.

Figure 13.7 shows biogas production from an on-farm biogas plant. Table 13.1 shows the operation conditions in the anaerobic fermentation tank. In winter, the biogas produced was reduced to a low raw material supply volume, because slurry on the free-stall floor was frozen by the cold outside air.

The loading rate was 3.87 kg VS/m3/day, the average amount of biogas produced was 450 ± 23 m3/day, and the average methane concentration was 58 ± 1.8 %. The input materials and digestive slurry had a high ammonia nitrogen level compared to

Table 13.3 Average composition of raw gas, off-gas, and refined gas

the typical value, but the concentration of methane was the same as in other reported cases [14, 15].

Furthermore, lack of accumulation of propionic acid meant that the fermentation state was favorable. In Japan, the merits of methane fermentation include not only energy production, but also its role in odor mitigation countermeasures. Proprionic acid, one of the main malodorous organic acids produced in the dairy cow slurry due to the anaerobic fermentation process, was 95 % decomposed by methane fermentation processing. The odor intensity of the post-treatment digestive slurry was reduced to 1/50 of the input material. Thus, the refinement process provided highly effective odor control in neighboring villages when the digestive slurry was sprayed over grasslands.