|Boiling Point No. 22 - August 1990 (ITDG Boiling Point, 1990)|
Reproduced from the GTZ/SEP booklet "Dissemination of Biogas Plants in the Rural Areas of Kenya - 1987"
In principle, biogas can be used in the same way as any other combustible gas. It has a calorific value of approximately 21.5 MJ/m³ . It must be borne in mind, however, that the composition of biogas - and, as a result, both its calorific value and its combustion behaviour - vary within a certain range.
The biogas produced in the Meru plants (N. E. Kenya) is used for cooking and lighting. It could also be burnt to operate a gas-powered refrigerator or a chicken incubator or warmer. A gas pressure of between 5 and 20 cm water column is required for cooking. Lamps require a pressure of about 10 cm water column, which can be achieved with the larger gasholders now being installed on the Meru plants. Like other family plants, the Meru units do not produce enough gas to operate engines, e.g. generators.
The members of the Special Energy Programme (SEP) biogas team are well aware of the fact that the availability of reliable biogas appliances for cooking and lighting is essential for the success of the entire programme. Not only do the biogas appliances have to be reliable and safe; they must also be cheap, easy to operate and capable of meeting the users' cooking and lighting needs in a way that is generally compatible with the prevailing local customs and practices. Moreover, the design of the devices should be such that the members of the target group find them basically attractive and aesthetically pleasing, as it is very important for the success of the dissemination activities that biogas be seen by the users as having a certain prestige value that enhances their status in the local community.
A family that has acquired aMeru plant will usually require at least one biogas cooker and one biogas lamp. Frequently, a user will initially install one cooker and one lamp and, after a short time, request a second cooker and a second lamp - and in some cases, even two additional lamps.
Both propane and butane (bottle gases) have a higher calorific value and a higher combustion speed than biogas. As a result, standard commercial propane or butane burners cannot be used with biogas unless they have been modified in certain ways. The gas injector jet and the mixing chamber must be enlarged, and the number of burner jets as well as their cross-section - must be increased. The dimensions of the air inlet should be such that a biogas-air ratio of approx. 1:4.5 is maintained.
Taking into account not only the different calorific values of biogas, fuelwood and charcoal, but also the varying end-use efficiencies of biogas cookers' and open fires and jikos, it can be assumed that 1 m³ of biogas will substitute for up to 5.6 kg of wood or as much as 1.7 kg of charcoal.
The KIE biogas cooker consists of a round burner mounted in a rectangular frame constructed of angle iron that has a pot support in each corner.
Two versions can be supplied: a single and a double-burner model. The retail price (19&7 prices) of the KIE biogas cooker is about KSh 450. for the single - burner model and KSh 750. for the double-burner version. The unit works fairly well and has a specific biogas consumption per burner of not more than 450 litres/hour.
In 1986, the biogas field workers of the SEP/Kenya developed a new biogas cooker design. As the prototype was produced in a metal workshop in Meru District, the unit is called the "Mew biogas cooker". The Meru cooker consists of a metal liner, a ceramic insert and the burner. The metal liner is either round or square and has either three or four pot supports. Alternatively, it may be equipped with a separate metal ring which serves as a potholder. The appearance of the round model is quite similar to that of a jiko.
The ceramic insert can be purchased from Kenyan potteries. It was originally developed as a component of improved fuelwood-saving cookstoves in the framework of the SEP/Kenya's stove dissemination programme, which is being implemented in co-operation with the women's organisation Maendeleo ya Wanawake (MyW).
The burner consists of four 1" metal pipe elbows that have been welded together; following welding, a total of about 50 jets are drilled in the burner in two rows. The air inflow can be regulated by means of a simple air control sleeve. The gas pipe is opened and closed with a 0.5" gate valve.
The Meru biogas cooker has several advantages:
It reminds potential users of a jiko, a quite common and well-known stove design that is already in widespread use in rural Kenya, and is thus a "familiar" appliance that is not rejected by the target group on the basis of its outward appearance.
It is heavy and is therefore not likely to tip over when cooks are preparing dishes that require considerable stirring, e.g. ugali or githeri.
Use of the ceramic insert, which serves to insulate the pot and conserve heat, results in higher energy efficiency, and thus shorter cooking times.
All components can be produced in local workshops using locally available materials. The retail price of the Meru biogas cooker is not more than about KSh 500., and it has a specific biogas consumption of 600 litres/hour. The SEP intends to disseminate the technical know-how required to produce this cooker among artisans in other districts as well, thus facilitating its adoption in all the biogas programme's target regions.
The lighting efficiency of biogas lamps is generally quite low, averaging between 3% and 5%. Nonetheless, a good biogas lamp can illuminate a room far better than a wick kerosene lamp, and produces a light intensity comparable to that which can be obtained with a pressure kerosene lamp or an electric light bulb in the power range of 25-75 W.
Many parts of rural Kenya still have no access to electric power. In such areas, biogas makes it possible for farm households to improve their work areas for longer periods every day. This in turn enhances the overall quality of farm families' lives, for example by enabling women to do their housework - and children to do their school work - during the evening hours (after 6.30 pm) under good lighting conditions.
High-quality, "modern" household lighting is also a status symbol among rural people and, as a result, biogas lamps contribute significantly to the attractiveness of the biogas technology among the relevant target groups.
KIE began development work on biogas lamps in 1984, and the first model produced by the firm was based on a standard commercial LPG lamp that had been modified for biogas operation. Unfortunately, however, this prototype did not perform satisfactorily because of the nature and scope of the modifications that had been undertaken. Consequently, another lamp was developed.
To date, more than 120 lamps of this second type have been produced and distributed, and a number have also been supplied to users in Tanzania. The lamps have performed satisfactorily. Some users have complained about flickering, or noted that the light was yellow rather than white, or that black spots had developed on the mantle. However, such problems are not attributable to design flaws in the lamp, but rather to insufficient gas pressure in the biogas plant or to the presence of water in the gas piping.
Recently it was decided to modify this design in order to eliminate certain manufacturing problems which had had an adverse effect on the overall quality of the output:
The mixing chamber was assembled from three separate parts. The bores were not of a uniform diameter, which meant that in some cases the gas did not flow properly and extensive threading was required to join the parts.
In some lamps the injector jet was not in line with the axis of the mixing chamber. Unfortunately, the new KIE lamp is not yet on the market. However, with demand for biogas lamps increasing rapidly in Kenya, and in view of the crucial role of improved domestic lighting in promoting the dissemination of the technology, it is important to ensure that adequate supplies of these suitable appliances are available to consumers. With this in mind, the SEP/Kenya initiated the importation of Brazilian "Jackwal" brand biogas lamps. The importation and distribution of the units is now being handled by private merchants.
Like other gas or pressure lamps, the "Jackwal" lamp employs a gas mantle. The lampshade reflects the light downwards and the lamp-glass helps maintain the operating temperature at the required high level. Both the gas and the air inlet can be regulated. The specific biogas consumption of the "Jackwal" lamp works out at about 100 litres/hour. Its retail price in Nairobi is KSh 820. The new KIE design should be every bit as good as the "Jackwal" lamp - if not better - and it is not expected to be any more expensive than the Brazilian import.