Biogas Technology in Nepal
History
The beginning of biogas activities in Nepal was a programme carried out by the United Missions to Nepal within the context of the Agricultural Year 1974/1975 whose objective was to introduce biogas plants similar to in India. The Agricultural Development Bank of Nepal (ADB/N) assisted in financing the plants by providing a special credit framework. The Gobar Gas Tatha Krishi Yantra Vikas (P.) Ltd. (Biogas and Agricultural Equipment Development (Pvt) Ltd.), normally called Gobar Gas Company (GGC), was founded in 1977 by the ADB/N, the United Missions to Nepal (UMN) and the Nepal Fuel Corporation (today: Timber Corporation Nepal - TCN).
During the history of the company various foreign sources have been involved in promoting biogas dissemination through the GGC. Since 1988 the Netherlands Development Organisation (SNV Nepal), has been working with the GGC with the involvement of two Dutch experts in the "Research Unit" and "Workshop" divisions. This cooperation was expanded into a financially more extensive Biogas Support Programme at the beginning of 1992.
Gobar Gas Company (GGC) Biogas Programme
The GGC biogas programme is the only supra-regional dissemination programme in Nepal. The building of plants is organised by two regional offices in western (Butwal) and eastern Terai (Biratnagar) and by 11 district offices belonging to these as well the dissemination office in Kathmandu belonging to the main office. Each of the dissemination offices of the company sells, installs, repairs and services biogas plants. The research department is responsible for the development of technical solutions and dissemination structures where possible cost reduction, improvement of customer service and the solution of specific problems in daily use are concerned.
In 1992,there were approximately 6,000 biogas plants in Nepal. Of these, about 4,500 were installed by the Gobar Gas Company in various parts of the country. About 70% of biogas plants are located in Terai. Not counting the biogas plants in upland valleys, only approximately 13% are in mountainous regions. This is explained by difficult access conditions to farms in the mountains which makes the transport of building material very expensive so that farmers in mountainous regions can hardly afford biogas plants.
Types of Plant
So far, mainly two types of plant have been built and disseminated in Nepal. These are the floating-drum plant based on the Indian type (with an overflow at the top rim of the cylindrical digester instead of an outlet pipe) and fixed-dome plants with a flat floor, cylindrical digester and a dome made of concrete. The market-oriented procedure soon led (and earlier than in India) to fixed-dome plants becoming the standard model: since 1980 fixed-dome plants have primarily been disseminated mainly for reasons of cost. These are offered in digester sizes of between 4 m3 and 50 m3.
The Nepali fixed-dome plant is a development peculiar to Nepal which has been modified in various ways over the years.
The construction reflects the particular dissemination conditions of the country:
- The form of the digester, the compensation chamber and inlet are simple geometrical figures which, if bricks are not available on site or are difficult to transport there, can be built from natural stones.
- The dome storing gas is plastered or moulded out of concrete on a clay mould. In this case, the building material necessary (normally only concrete since gravel and sand are often available locally) can be brought to the site packed in sacks.
Construction of a Fixed-dome Plant in Nepal
Building a fixed-dome plant - with regard to the work invested by the farmer - involves a great amount of work. After a cylindrical pit has been excavated to the prescribed depth, the floor slab is laid and the cylindrical wall of the digester is erected on it. In cases where the ground is very firm or rocky, a masoned digester wall is dispensed with; the wall is then only plastered from the inside. The digester is subsequently filled with the excavated earth. At the top the inside form of the dome is shaped in clay using a template. Finally, the concrete is applied on a layer of sand with a trowel and smoothing board. After the dome has hardened, the earth is shovelled out through the large opening in the compensation chamber.
The majority of newly installed plants by far have a nominal digester volume of 10 m3 which includes the total volume of the digester and gas storage tank but not the compensation chamber. According to the definition of average digester volume of fixed-dome plants, this corresponds to a digester volume of 7.8 m3. (With fixed-dome plants the level of liquid varies according to how much gas is in the dome. To determine an average value for digester volume half of the maximum gas storage volume plus permanently present digester volume is assumed - i.e. at the highest volume of gas and lowest level of liquid in the digester - according to the BORDA definition). The other sizes offered show no clear tendency as not only smaller 8 m3 plants but also larger 15 m3 plants were sold in almost the same numbers. The smallest plants with 4 m3 digester volume were not ordered in 1991/92.
The plants are designed for a hydraulic retention time of 65 - 70 days. With a daily filling of 60 kg dung (from around 4 - 6 cattle) - in the climatic conditions of Terai - a gas yield of 2.4 m3/d is to be expected. This quantity provides sufficient energy for cooking for a household of 7 - 9 people. If a toilet is connected the expected gas yield rises by about 15%.
In Nepal the construction material is also the largest cost factor in a biogas plant. It amounts to between 71% (for digester volume = 20 m3) and 83% (digester volume = 4 m3) of the total costs according to the size of plant. The most expensive individual item is cement, followed by the gas pipes.
The GGC plants fulfil the same requirements on reliability of operation as fixed-dome plants in other countries which have been constructed in a different way. Their type of construction however, seems somewhat exotic (especially in Terai where bricks are the normal building material) in the face of existing experience throughout the world with masoned gas domes. Savings could be made in constructing the dome as, with masoned domes, the extra work involved in re-filling the excavated pit and re-digging is dispensed with.
Despite an unfavourably positioned gas outlet which is too low, the plants mostly operate without any trouble. The gas outlet in the dome is located - depending on the size of the plant - between 11 and 17 cm lower than the overflow at the compensation chamber and, for this reason, can become blocked with sludge if the gas is let out at the same time as the plant is being filled with dung. Problems are reported only during the colder months of the year when sludge is occasionally pressed into the gas pipes with the gas and blocks these.
