In Japan, biogas generated from sewage treatment plants are mostly utilized as auxiliary fuel for sewage sludge combustion and Combined Heat and Power (CHP) fuel, however, some excess biogas is usually just burned without energy recovery. Since Japanese major gas companies are legally required to utilize 80 % or more of excess biogas by 2015, they are expected to promote on-site and off-site utilizations of biogas. In general, biogas has a composition of around 60 vol % methane, 40 vol % carbon dioxide and trace impurities such as hydrogen sulfide, water vapor and siloxane. Even if these trace components are removed before fuel supply, available gas appliances are limited to boilers and CHPs with special specification because its calorific value is lower than city gas. In order to expand application of biogas, it is required to be upgraded to natural gas quality. Today, there are several types of technologies for biogas upgrading and the most popular technologies are pressurized water scrubbing, pressure swing adsorption (PSA), organic physical scrubbing and chemical scrubbing. In Japan, pressurized water scrubbing and PSA are adopted for biogas upgrading before gas grid injection. However pressurized water scrubbing is suitable for only large-scale plants and PSA method is low in methane recovery rate. Currently membrane separation technology has gained attention because of its high performance in methane separation from biogas and its compactness suitable for smaller scale plants. In addition, capital and operational costs of membrane separation system are expected lower than other systems although there are few cases of biogas upgrading with membrane technology in Japan. Since membrane separators are not developed for biogas upgrading in usual, there is not enough data of upgrading digestion gas from sewage treatment plants with membrane separators to estimate long-term performance and membrane replacement cycle. A long-term test of upgrading digestion gas with organic membrane separators was carried out on a pilot scale. Raw digestion gas generated from a sewage treatment plant in Yokohama city was provided for the test. Targeted values of the methane concentration and the recovery rate of the upgraded biogas were over 98 vol % and over 90 %, respectively.
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