(412d) Methane Fermentation of Seaweed Biomass | AIChE

(412d) Methane Fermentation of Seaweed Biomass

Authors 

Matsui, Jr., T. - Presenter, Tokyo Gas Co., Ltd.
Amano, T. - Presenter, Tokyo Gas Co., Ltd.
Koike, Y. - Presenter, Tokyo Gas Co., Ltd.
Saiganji, A. - Presenter, Tokyo Gas Co., Ltd.
Saito, H. - Presenter, Tokyo Gas Co., Ltd.


Recently seaweeds cause social problems in Japan, which is surrounded by the sea. Seaweeds glow in large quantities and pile up on the seashore (green tight). They spoil view and release the stench because they rot quickly. Local governments have been collecting and incinerating huge amount of such seaweeds. Ulva sp. is the most typical among those seaweeds. In contrast seaweeds have been planted recently for ocean remediation, not for foodstuff. Cultivation of Laminaria sp. has been tested for such purpose. Though it is effective, it has caused new problems of treatment for harvested seaweeds. In Japan, such troublesome seaweeds exist numerously. It is strongly desired to utilize them. Producing energy from them is the one of the effective way to use. It contributes remediation of the ocean and the problem of green house effect. Though there were reports about the methane fermentation of seaweeds, they only show results of laboratory scale experiments. So a field test plant was built for practical use and it has been tested to produce biogas from seaweeds in large scale. Maximum treating capacity of this plant is one ton-seaweed/day. This plant has a gas engine generator. The effective way to exchange biogas to energy has been researched at same time. A project of this field test is cooperated with new energy and industrial technology development organization (NEDO) in Japan. Seaweeds of Ulva sp. and Laminaria sp. were used as materials in this field test. Methane fermentation process is proper to exchange seaweeds to fuel of gas because of the high concentration of water (about 90%). This field test plant consists of four parts (pre-treatment, fermentation, biogas storage and generation). In pre-treatment part, seaweeds are smashed and diluted with water to suppress the effect of salt and make appropriate slurry. In fermentation part, there are two processes (pre-fermentation and methane fermentation) for a higher efficiency of fermentation. The seaweed slurry is treated by pre-fermentation (acid production) and use of methane fermentation as substrate. Capacity of a pre-fermentation tank is 5 kl. In the methane fermentation process biogas is produced. Capacity of a methane fermentation tank is 30 kl. The methane fermentation tank contains porous matrix inside for inmobilizing bacterial cells. Biogas is refined (de-sulfur) and stored in a gasholder (30 kl). Residue of methane fermentation is dehydrated and used as fertilizer. Biogas is mixed with city gas (natural gas) and fed to the gas engine generator (a co-generation system). A gas mixer of the engine was improved to mix with biogas and city gas. Electricity (10 kW) generated by the gas engine is used for electric equipments of the plant. Heat (23 kW) from the engine is used for heating-up energy for fermentation tanks. An equipment of deodorization using microorganisms is set. Seaweeds of Laminaria sp. as test materials were continuously supplied (0.2 to finally 1 ton/day). Total solid concentration (TS) after adding dilution water was 1 to 5%. Retention time of the pre-fermentation was 2 to 3days. The temperature was controlled at 25-35 °C. Total concentration of produced organic acid (mainly acetic, lactic, and butylic) was 1000 to 5000 ppm. In the case of the methane fermentation, retention time was 15 to 25 days and temperature was controlled at 55 °C. Consumption of organic acid in the methane fermentation tank was confirmed. Concentration of ammonium ion was low (under 150 ppm). Level of ammonium ion was lower than that to prevent methane fermentation. Biogas was produced continuously. Composition of the biogas was about 60 % methane and 40 % carbon dioxide. Several thousands ppm hydrogen sulfide was also contained. Hydrogen sulfide was removed by iron oxide in this plant. Results of pH variation tests showed that it is better to control pH over 7.5 to gain much methane gas. One ton-seaweed yields 22 kl methane gas. In this test biogas had been produced continuously for over 150 days. Seaweeds of Ulva sp. collected on seashore were also tested. Collected seaweeds contained foreign bodies such as sand. Such foreign bodies do not affect fermentation directly but decrease available volume of tanks. Thus they are washed by water and foreign bodies were removed before they were used for fermentation tests. Amount of 0.6 ton/day (TS=3%) of Ulva sp. was supplied. Conditions of the fermentation were same as the test of seaweeds of Laminaria sp.. Organic acid was also produced in the pre-fermentation tank. Concentration of the organic acid was 1000 to 3000 ppm. Organic acid was consumed in methane fermentation tank. Concentration of ammonium ion was about 500 ppm. Level of it was also low. It does not affect methane fermentation. Composition of the biogas was about 60 % methane and 40 % carbon dioxide. Yield of methane gas was 17 kl/ton-seaweed. The yield was lower than the case of the seaweeds of Laminaria sp.. It is expected that Ulva sp. has more components which are not decomposed easily by bacteria. The biogas had been continuously produced for over 70 days. Yield of methane gas was stable. Results of these tests show it is possible to produce biogas from seaweeds (Lminaria, Ulva sp.) in condition of practical use. Biogas after de-sulfur is used as fuel of gas engine for generating electricity in this field test plant. Quantity and composition of collected biogas is often changed because of source of materials, condition of the sea or weather, and fermentation condition. Thus it is effective to use biogas as fuel mixing with other fuel, such as city gas which is supplied stably. By controlling the ratio of city gas in mixed gas, it is possible to keep engine operation stabilize easily against fluctuation of quantity of biogas. Adding city gas to biogas is conducted so as to control heat value of fuel gas. This contributes to reduce concentration of carbon dioxide contained in fuel gas, which is obstruction of combustion, and leads to increase thermal efficiency of gas engine. From our experimental results, thermal efficiency of gas engine supplied mixed gas (biogas and city gas) is over 10% higher than that of one using only biogas as fuel. It has been succeeded to gain biogas from seaweeds continuously for a long time. Biogas has been used for fuel of the gas engine generator effectively. It is desired that utilization of seaweed biomass will extent according to this result.

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