|Blending of New and Traditional Technologies - Case Studies (ILO - WEP, 1984, 312 p.)|
|PART 2: CASE STUDIES|
*Prepared by Umberto Colombo and Danielle Mazzonis. Italian Commission on Nuclear and Alternative Energy Sources (ENEA).
THE PRODUCTION OF biogas (mixture of carbon dioxide and methane) from the anaerobic digestion of waste products is a technology traditionally used in certain developing countries such as India and China. In the industrialised countries, anaerobic digestion has only begun to spread in recent years as a system to reduce pollution and as a source of energy.
The technologies employed in the Western countries, designed mainly to reduce the pollutant content of animal and agro-industrial wastes as much as possible, have led to the development of better systems. The organic matter conversion indexes have been extremely high, in certain cases reaching 80 per cent in energy terms.
In Italy, more than 150 large-scale (from 100 to 12,000 m3 of working volume) anaerobic digestion plants have been established. In 1982 the Italian Commission on Nuclear and Alternative Energy Sources (ENEA) took a decision to test the application of emerging technologies in this sector of energy. Towards this end, it has begun to install data-gathering systems in these plants using a personal computer.
I. THE USE OF PERSONAL COMPUTERS
The use of personal computers has the following three objectives:
- to determine the optimal operating parameters, optimising the digesters and improving the conversion indexes to analyse the energy and material balances;
- to reduce investment and management costs;
- to achieve perfection in automated control systems.
Some of the results of this activity can be transferred to developing countries. For this reason, it might be worthwhile to examine in depth certain aspects of ENEAs experience.
Plug-Flow Plant for the Digestion of Cow Dung
The system is composed of a cement digestor covered by a curtain made of synthetic material (rubber). It has a heating system that reaches a temperature of 35 degrees Centigrade through the circulation of hot water in polyethylene pipes resting on the bottom. There is no shaking system. The gas produced is used to feed an eight-cycle engine with a capacity of 5 kW; the engine is equipped with a system to recover heat from the discharge gas and from the cooling circuit.
Compared to traditional systems already known and used in developing countries, the novel aspect of this plant is the absence of shaking and the presence of a heating system that uses a portion of the hot water obtained from the co-generator. The gathering of data with a personal computer has permitted a study of the optimal geometry of the plant, into which several septa have been inserted to force the sewage to take certain routes which prevent the sedimentation and flotation of the matter. The efficiency of this plant, built in Emilia-Romagna, is approximately twice as high as the figures for simplified systems reported in the United Nations bulletins. Furthermore, present production is approximately 2 m3 of biogas per 1 m3 of working volume. Thus plant efficiency is almost five times greater than that of traditional systems. In Italy, the cost of construction is five to ten times lower than that of a completely mixed type of plant. Since a major cost item is construction of the cement tank, construction costs might be even lower in countries in which labour costs are lower than in Italy.
In certain areas large-scale plants are being built to collect effluents of different provenance (swine, cattle and poultry-breeding, and agro-industrial plants).
These plants require advanced technology to prevent pollution, to maximise energy recovery and to improve fertiliser efficiency. Management of the different plant operations must, therefore, be very carefully controlled. The best instrument to achieve this is a personal computer of the kind utilised to monitor the plants equipped with systems to control the machinery.
In particular, the personal computer must manage the flows of sewage from the different sources, control the sludge heating and recirculation system and intervene in waste treatment. It is of the utmost importance that it manages the systems for the utilisation of biogas.
In the plant now being built at Marsciano (Perugia) the biogas will be used to produce electric power. The generators will also produce hot water to heat the digestor; in the cold months an auxiliary boiler will have to be used. The biogas will also be used to heat greenhouses for the cultivation of flowers and vegetables; there will be two plants for the dessication of tobacco. Biogas will be used to heat the delivery rooms of three swine herds in the zone. A plant will be installed to compress the excess biogas for use in motor vehicles. The management of all these functions, in an industrial plant that will produce over 16,000m3 of biogas per day must be entrusted to a personal computer. The operations involved are extremely complex, also because the different uses are not contemporary, and certain uses must be given preference over others (for example, heating the digestion plant).
Certain developing countries have similar requirements. ENEA contacts with Argentinian officials have revealed the need to construct large-scale plants to utilise all human (sewage sludge, solid wastes) and animal wastes, to which aquatic plants (water hyacinths) have been added. These aquatic plants grow naturally in the Parana River and are a serious obstacle to navigation. Industrial plants will be constructed in different towns along the river to supply enormous quantities of organic fertiliser and considerable quantities of biogas which can be converted into electric power or used for direct combustion. In this case as well, the use of automated systems to obtain optimal efficiency and the optimal use of energy are recommended.
II. SOLAR POWER AND BIOGAS: ANOTHER TYPE OF INTEGRATION OF DIFFERENT TECHNOLOGIES
Solar power and biogas require very simple systems for communities like those in rural Kenya whose energy requirements are still rather limited. A project on solar power and biogas is now being started in Nairobi by an Italian firm.
The objective of the project is to supply, as economically as possible, a system to cook food without firewood, which destroys forest resources and involves great efforts in wood-gathering. This system will also be used to provide gas-lighting (replacing the present system of oil-lights), to preserve vaccines, refrigerate food (through absorption refrigerators) and feed combustion engines for water pumping or for other uses. It will also make available a considerable quantity of organic fertiliser, which is particularly useful for soils subject to erosion.
Electric power conversion is not considered necessary in this initial phase given the small size of the industrial plants and the particular cultural situation of the villages in which the plants would operate. The lack of electric power also means that the heating system of the digester and the controls on the refrigerators must be modified. For this reason, consideration has been given to integrating the biomass source with solar energy and heating the anaerobic digestion plant with a small solar panel equipped with a mechanical thermostatic valve. At night the digestor will have to be insulated with straw or other plant wastes.
The biogas plant will be fed with cow dung, plant wastes and other biomass that can be gathered locally.