Chapter twelve: Technical and social constraints in integrating biogas plants into farms

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Constraints delaying the diffusion of installations
Reguirements for an optimally integrated biogas installation
New incentives to build biogas plants

 

The number of constructions of agricultural biogas plants in Europe has been steadily decreasing during the last few year. Willinger (1988) tries to point out a few of the major technical, financial and social problems which led to that development. A technical check-up list and some ideas on marketing strategies should help to overcome this bottleneck.

Starting with the first oil crisis in 1973, considerable research efforts in the field of renewable energies have been undertaken all over the world, in particular on the topic of anaerobic digestion. The phase of basic research was subsequently followed by applied and pilot plant research and is at present carried on with demonstration plants. Despite the progress of research and the support of pilot and demonstration plants by the governments and the EC, the technique has not become as widespread in European agriculture as had been hoped. The expectations may have been too high: it took 100 years to replace firewood by coal, and a further 30 years passed before oil became the predominant energy source. Finally, the development of nuclear energy has taken more than 20 years, even though the investments in research were, by the power of three, higher than for anaerobic digestion.

Currently, something over 500 biogas installations are in use on European farms (Demuynck and Nyns 1984). There are many reasons why farmers have constructed these plants. The main incentives include substitution for oil, autonomy in energy, hygiene and odour reduction of the manure, improvement of fertilizer quality, and to protect the environment. Whereas in the first years of the development, the aspect of energy was the major, if the not the only reason to build a biogas unit, in recent years the environmental impact (fertilizer quality, odour reduction) has gained considerable importance. Unfortunately, the rate of construction has slowed down remarkably during recent years. Development has also followed a slow growth rate. The phase of initiation, after the oil crisis, was followed by a fast growth phase, which around 1984, turned into disenchantment, caused by a number of factors.

 

Constraints delaying the diffusion of installations

General Constraints: Biogas production suffers the drawback of all renewable energy sources: it is a very complex technology, requiring the combination of a variety of classical fields of engineering, i.e. of professionals who by tradition are not accustomed to collaborate. In other words, when it comes to alternative energy, the traditional electric or mechanical engineer is outdated. Unfortunately, this simple remark encompasses the major constraint against a fast diffusion of renewables. At present, there are too many highly trained engineers in the field of nuclear energy.

Technical Constraints: Even under the broadest definitions, the 500 biogas plants described in the EC survey (Demuynck and Nyns 1984) can still be divided into roughly 17 different systems. Particularly during the first years of development, every constructor designed his own biogas system. Hence, the same mistakes have been repeated many times. As a result, many installations have been discarded , but only after they had contributed to a bad image of biogas technology. Nowadays, the technique has become simpler and more efficient. Construction costs could be reduced at slightly higher planning costs. Unfortunately, the market is diminishing, so the dispersion of the better new plants is very restricted. However, there are still possibilities to make designs more sophisticated, with a higher degree of standardization. Progress could be enhanced if public funds were available to support small, but flexible engineering firms, specializing in anaerobic digestion, rather than large offices, which often stick to known but outdated techniques, because of their high overhead costs. In addition, smaller firms are more willing to share knowledge with other groups, which is an obligatory precondition for the fast development of small-scale techniques. As a negative example of misunderstood application of biogas technology, the gigantic MBB-projects in Germany and Asia could be mentioned. But also the often-cited village biogas plants in Denmark still suffer from the narrow thinking typical of large engineering firms. As one of many positive approaches of technical cooperation, the initiative of the Folkecenter in Denmark could be mentioned. Apart from personnel-related problems, there is another major obstacle reducing the diffusion of biogas plants. Only in recent years have liquid manure systems been developed, whereas roughly 80% of all farms have a solid-waste manure system. However, the technology of solid-waste digestion is still based on the batch digester, essentially designed in the 50s. In many cases, another renewable energy source such as wood or straw competes with biogas.

Financial Constraints: The major financial restriction is the price of oil. Until about 1985, most of the biogas plants had pay- back periods of less than 15 years, which was compatible with its life expectation. With the decreasing oil price, however, pay- back periods became almost infinite. Unfortunately, energy costs are always base* of the price of oil, which is not quite correct because the latter does not include social costs. With renewable energies, the cost of reduction in air pollution is actually paid by the owner of the respective device, while future generations will have to pay the price for the current pollution.

An improved basis of costing could be achieved on the basis of the price of electricity, if electricity companies were to pay realistic costs. This means prices reflecting the real energy costs of a newly built power plant. As long as the world market is saturated with cheap electricity, from about 0.04 SFr./kwh), chances are small that higher prices will be reimbursed for electricity from renewable sources, despite the fact that, from the point of view of political economy, the production of electricity from biogas would be the most favourable solution. The overall process efficiency is high (more than 80%) and the electricity produced has a very high energy capability. Another financial constraint, which affected the development and ultimately the diffusion of biogas plants was the reduction of research money in several countries.

Social Constraints: Certainly, the negative image of biogas production, which was created by the early installations, has considerably reduced the interest of farmers to build biogas plants. In particular, the malfunction of the widely announced, large-scale demonstration units, such as the village biogas plants in Denmark, or the installations erected in agricultural schools in Switzerland, had a detrimental effect on the local diffusion, even though a considerable number of problems arose only from poor maintenance, because nobody felt responsible. Since biogas is a "new" technology, the social pressure to build one's own installation is lacking. Due to the low density there is no feeling of social pressure, nor a pressure from agricultural consultants, because they are not familiar with the technology either.

Biogas still has the "pioneer" image in Europe, instead of carrying prestige. Despite high investment costs, it is considered as low technology, only good for Developing Countries. A professional marketing approach, including advertisement, courses for consultants and engineers, as well as introductory seminars with site visits, for politicians, would probably renew the diffusion of biogas plants. A few approaches are currently under way, but international coordination would certainly speed up the process.

 

Requirements for an optimally integrated biogas installation

Before any concept of propagation is initiated, one has to make sure that a number of biogas systems are available which can satisfy not only from a purely technical point of view (which has to be proven in practice) but also from the view of an optimal integration of the system into an existing or a new farm.

Figure 12.1 is compiled for the construction of a biogas installation.

Stable Within the barn, the manure removal system is the most crucial point which often leads to operational problems. The design of the manure channels is very important. Their size and the type of the removal device should be coordinated, as several authors have pointed out (e.g. Robertson 1977; Nosal and Steiner 1986). For more diluted materials, gravity systems without mechanical device are to be preferred. Best results have been obtained from sluice-gate slurry channels and overflow slurry channels. Excessive use of water for flushing lead. to a high process-energy demand, and requires higher digester volumes. Whenever slurry channels have to be flushed, fresh or digested manure should be used. The addition of untreated solid waste should be avoided, since it tends to form scum. Chopping of long straw, in the liquid phase, requires more energy than shredding before bedding. Reception pits should be placed within the barn, if possible, or should be insulated, to minimize heat loss from the manure. Their volume should be as small as possible. One should be aware that for every degree Celsius lost per m of manure, on the way to the digester, about 260 1 of biogas have to be invested, to recover the temperature.

Figure 12.1: Compiled for the construction of a biogas installation.

Process Parameters With mesophilic digestion, HRT should be as short as possible. Best yields were achieved at 10 - 15 days with piggery waste (Van Velsen 1981), at 10 days with waste from fattening cattle (Baserga 1984) and at 18 days with straw containing cow manure (Wellinger 1985). However, recent developments have shown that in cold climates, the best net energy yields were achieved at HRTs of 40 - 50 days and a fermentation temperature around 22C (Sutter and Wellinger 1987). For countries which require, by law, long storage capacities of manure, such as Germany or Switzerland, a combined system for storage and digestion (ACF-System, Wellinger 1988) brought excellent results.

Design Criteria. Whenever possible, sunken digesters should be built. They do not need pumping, either from the barn into the digester, or from the digester into the storage tank. The investment cost for pumps is low, but their lifetime is very short, and replacement costs are high. With a direct flow from the barn into the digester, an important amount of heat in the fresh manure can be conserved. In addition, sunken digesters have considerably lower heat losses in winter. However, in the majority of cases the digester has to be erected above ground. Good insulation, preferably on the outside of the digester, with a thickness of 12 cm or more, yielding a U-value of 0.4 W/m3.K or less, is strongly recommended.

Heating systems are preferably built of plastic materials, in order to prevent corrosion by differences in galvanic potential. Optimal performances vary from 150 W/m3 to 270 W/m3 of digester volume in function of TS-content and HRT (Walther 1985). To avoid crusting of the heat exchangers, the water temperature should not exceed 55C. For mixing, the best results were obtained from slow rotation stirrers (5 - 25 rpm), which provide enough motion to keep a possible scum moistened, and prevent the formation of a heavy sediments. Good stirring can be achieved with mixing powers of 25 40 W/m3 digester volume, and an energy input of more than 40 W/m3 /d. If straw or other bulky material is fed, stirring alone can not prevent scum formation, as long as the scum- forming material is not removed (Baserga et al. 1985). Another way of stirring the mass inside the digester is by circulating the biogas through the slurry. This has been done for many years in the anaerobic digestion of sewage, and is therefore a proven technology.

Gas Utilization. The possibilities of biogas utilization are the same as for natural gas. In an optimally-designed installation, the gas produced is utilized close to 100% all year round. This is the only way to achieve economical operation. The gas produced should therefore cover the base-line energy demand, while the peaks are covered with a different energy source, such as wood or straw. If the peak energy has to be combined with biogas, in many cases hot water storage is the energy-store to be compared with other types of gas storage. For most electrical applications, desulphurization of the gas is recommended (Egger 1984). The removal of carbon dioxide however, is not necessary, even when the gas is compressed (Wellinger et a]. 1984).

If all factors for optimal integration are carefully evaluated, the running costs of an installation will automatically be as low as possible.

 

New incentives to build biogas plants

During recent years, the construction of new biogas plants has continuously decreased, but there are signs that this trend has bottomed out. Increasing environmental problems and air pollution catastrophes in recent years, leading to increased diseases of the respiratory organs, and to dying forests, have led to considerable changes in public opinion. In several countries, virtually no new community project is accepted (energy concepts, waste disposal, constructions of public buildings, etc.) where not at least one possibility of alternative energy application has been studied. If this new social pressure continues, it will bring with it new motivation for the farmer. But even more important, a number of new laws on energy consumption and environmental control have been introduced and accepted, which, before long, will also bring better financial support for the application of renewable energies, in the form of subsidies or tax reductions.

If we now seize the opportunity of the positive social and political atmosphere, and succeed in pushing through the construction of a significant number of efficient biogas plants, either in industry or agriculture , we may well see a break- through of anaerobic digestion, independent of the present price.


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