Power. Continuous

14/03/1998
Down To Earth

Power. Continuous most active participants engaged in the field of energy recovery from wastes agree that a rapid expansion of the acti-vity can result only if this area attains the status of an industry, fully self-sufficient and standing on its own legs. Recently a team of scientists led by P Rajabapiah from the centre for Application of Science and Technology to Rural Areas (astra), successfully tested new biogas plants in a bid to make them more efficient and thus assist the Indian farmers to attain self sufficiency as far as energy production is concerned.

Indian biogas plants are usually designed to use cowdung for biogas production. When water is mixed with cowdung in equal proportion, the concentration of the resultant mixture becomes a homogenous input for the plant which then utilises this mixture to produce energy.Due to the nature of this feedstock, the biogas plant discharges the digested effluent slurry correspondingly whenever it is fed. Therefore, there is neither settling nor floating of the matter inside the digestor if the plant is operated properly. These plants can be operated almost conti-nuously. Both gas production and slurry flow are quite constant like in ideal plug-flow digestors. Since both gas production and slurry flow are conti-nuous, these plants are categorised as continuous plants (Bio Energy News , Vol 1, No 4).

However, plants which are essen-tially designed for cowdung digestion cannot be operated continuously when fed with biomass feedstocks other than dung. Fermentable cellulosic biomass materials other than dung often create flow problem in liquid-phase biogas digestors either by floating to the top, thus forming a thick scum or stratifying to the bottom of the digestor. Though biomass can be fed into the digestor by force, as in Chinese fixed-dome plants, it cannot come out after digestion by gravity, thus limiting the plant to batch mode. Therefore, these plants are categorised as semi-continuous plants.

This is the reason that in these biogas plants, there is no constant detention time. The detention time is directly proportional to the operation time. In fact, the maximum detention time is about six months and in some cases it is even upto a year. This implies that the digestor volume should be large enough to accommodate the daily feed until it is emptied once or twice every year. As the volume is large, the cost of the plant often offsets the cost of its construction. And this is the reason why volumetric gas yield, or the volume of the gas-per-unit-volume of the digestor, is low in Chinese plants compared to the other designs of biogas plants.

To enable the non-dung based biogas plants to work continuously and make them low-cost, P Rajabapiah and his team at astra recently experimented with plug-flow fermentors for non-homogenous feed. They have successfully developed biomass plug-flow fermentors which, in long-term field operation, can work continuously. Green-leafy biomass, dry-leaf litter and kitchen wastes have been successfully fed into this new plant daily and an equivalent amount of the digested material has been taken out. These new fermentors combine the advantages of both fixed-dome and floating-drum types of biogas plants.

These new biomass plug-flow fermentors are shallow, horizontal structures that are constructed partially underground. But the astra team claims that these can also be constructed fully underground by creating differential ground levels for better aesthetics, cleanliness and performance.

The digestors are connected to separate galvanised iron gas-holders. Except for a little space for these gas holders, the inlet and outlet chambers of the plant can remain fully underground. In fact, the astra team suggests that lawns with ornamental plants be grown on the soil above the digestor. The team claims that these biomass plug-flow fermentors will pave the way for better, energy-efficient biogas plants of the future.

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