An introduction to biogas technology
Biogas technology. . . is a modern, ecology-oriented form of appropriate technology based on the decomposition of organic materials by putrefactive bacteria at suitable, stable temperatures. A combustible mixture of methane and carbon dioxide, commonly referred to as biogas, develops under air exclusion (leaving behind digested slurry) in the digester - the heart of - any biogas plant.
To ensure continuous gas production, the biogas plant must be fed daily with an ample supply of substrate, preferably in liquid and chopped or crushed form. The slurry is fed into the digester by way of the mixing pit. If possible, the mixing pit should be directly connected to the livestock housing by a manure gutter. Suitable substrates include:
- dung from cattle, pigs, chickens, etc.,
- green plants and plant waste,
- agroindustrial waste and wastewater.
Wood and ligneous substances are unsuitable.
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| ig. 1.1: A typical biogas-system configuration (Source: OEKOTOP) |
| Biogas guideline data | |
| Suitable digesting temperature: | 20 - 35 °C |
| Retention time: | 40 - 100 days |
| Biogas energy content: | 6 kWh/m³ = 0.61 diesel fuel |
| Biogas generation: | 0.3-0.5 m³ gas/m³ digester volume x day |
| 1 cow yields: | 9-15 kg dung/day = 0.4m³ gas/day |
| 1 pig yields: | 2-3 kg dung/day = 0.15 m³ gas/day |
| Gas requirement for cooking: | 0.1-0.3 m³ /person |
| for 1 lamp: | 0.1-0.15 m³ /h |
| for engines: | 0.6 m³/kWh |
- cook 2-3 meals or
- operate one refrigerator all day and two lamps for 3 hours or
- operate a 3 kW motor generator for 1 hour.
Of the many alternative forms of agricultural biogas systems, two basic types have gained widespread acceptance by reason of their time-tested reliability and propagability:
- floating-drum plants with a floating metal gasholder,
- fixed-dome plants with gas storage according to the displacement principle.
The main difference between the two is that the biogas generated in a fixed-dome plant collects in the domed roof of the digester, while that produced in a floating-drum plant collects in a metal gasholder. The gasholder, the purpose of which is to cover peak demand, is directly hooked up to the consumers (kitchen, living quarters, refrigerator, motor generator, . . .) by way of pipes.
Plant construction is effected with as much local material as possible, i.e.:
- bricks, rocks, sand, cement for the digester,
- metal or plastic tubes for the gas pipes,
- metal for the gasholder,
- gas valves, fittings and appliances.
Target groups and applications
The prime field of application for biogas plants is family farms, particularly those engaging in animal husbandry. Also, biogas plants are a proven successful means of disposal for wastewater and organic waste. Differentiation is made between the following groups of users:
- Small and medium-sized farms equipped with family-size plants (6-25 m³ digester) use biogas for cooking and lighting. The installation of a biogas plant usually goes hand in hand with a transition to either overnight stabling or zero grazing. The modified stabling, coupled with the more intensive care given to the animals, improves the quality of animal husbandry as an inherent advantage of biogas technology.
- Specialized stock-farming operations involving the medium to large-scale production of cattle, pigs and/or poultry can use medium-to-large biogas systems with digester volumes ranging from 50 m³ upward. The resultant safe disposal of fresh manure is a real contribution toward environmental protection, particularly with regard to the prevention of water pollution. Moreover, that contribution is rewarding for the farmer, too, since the biogas constitutes an autonomous source of energy for production processes.
- For agroindustrial estates and slaughterhouses, the pro-biogas arguments are similar to those mentioned above in connection with stock farms: safe disposal of potentially hazardous solid and liquid waste materials, coupled with a private, independent source of energy for generating electricity, powering coolers, etc.
- Biogas plants in schools, hospitals and other public institutions provide a hygienic means of toilet/kitchen-waste disposal and a low-cost alternative source of energy. Schools in particular can serve as multipliers for the dissemination of information on biogas.
Gas appliances
A number of Third World manufacturers offer specially designed cooking burners and lamps that operate on biogas. Standard commercial cookers and lamps can also be converted to run on biogas.
Diesels and spark-ignition engines can be fueled with biogas following proper modification; diesel engines prefer a mixture of biogas and diesel fuel. Biogas-fueled refrigerators, though not very efficient, are attractive alternatives for hospitals, schools and restaurants without electrification.
Slurry utilization
The digested slurry from biogas plants is a valuable organic fertilizer, since most of the main nutrients (N, P, K) are preserved. In areas where regular fertilizing is uncommon, the use of digested slurry for that purpose requires intensive counseling of the farmer. Biogas technology can play an important role in self-sustaining ecofarming.
The advantages of biogas technology
. . . for the user consist chiefly of direct monetary returns, less work and various qualitative benefits.
The monetary returns consist mainly of:
- savings on kerosene, diesel fuel, bottled gas and, possibly, wood or charcoal,
- an additional energy supply for commercial activities,
- savings on chemical fertilizers and/or additional income from higher agricultural yields.
The qualitative benefits are:
- easier, cleaner cooking and better hygiene,
- better lighting during the evening hours,
- energy independence,
- improved stock-farming practice,
- good soil structure thanks to fertilization with digested sludge.
The regional and overall domestic significance derives from the following merits and aspects:
- development of a reliable, decentralized source of energy operated and monitored by the users themselves,
- less local deforestation,
- improved conditions of agricultural production,
- more work and income for local craftsmen,
- infrastructural development,
- expanded indigenous technological know-how.
While the absolute figures corresponding to the above effects may often be marginal as compared to the overall economy; they nonetheless have a noticeable impact within the project region.
Cost of construction, amortization
As a rule, it costs DM 1000 or more to install a masonry biogas plant, including all peripheral equipment, i.e. improved stabling, gas appliances, piping, etc. A favorable payback period of less than 5 years can be anticipated for such an investment, if the biogas is used in place of a commercial energy source like kerosene or firewood, but not if it is used as a substitute for "free" firewood.
Dissemination of biogas technology
Thanks to the broad scale of potential uses for biogas, in conjunction with an increasingly advanced state of technical development' numerous developing countries are intensively promoting the dissemination of biogas plants. The undisputed leaders are the PR China (4.5 million plants), India (200 000 plants) and Brazil (10 000 plants). Other countries also have launched biogas dissemination programs with some or all of the following components:
- development of appropriate appliances and plants,
- establishment of technology and advisory-service centers,
- continuous support for the users,
- training of biogas practicians,
- advertising and promotional activities,
- assistance for private craftsmen,
- provision of financing assistance.
Criteria for the utilization of biogas technology
Building a biogas plant is not the kind of project that can be taken care of "on the side" by anyone, least of all by a future user with no experience in biogas technology. The finished plant would probably turn out to be poorly planned, too expensive and, at best, marginally functionable - all of which would disappoint the user and spoil the prospects for the construction of additional plants. Consequently, the following rules of thumb should be observed:
- There are workable alternatives to biogas technology:
Regarding energy: energy-saving cookstoves, afforestation, wind/solar energy, small-scale hydropower, etc.; better access to commercial energy supplies
Regarding fertilization: spreading or composting of fresh dung
Regarding animal husbandry: pasturing instead of stabling in combination with a biogas plant.
Any decision in favor of or against the installation of a biogas plant should be based on due consideration of how it compares to other alternatives according to technical, economic, ecological and socioeconomic criteria.
- Both the available supply of substrate and the energy requirements must be accurately calculated, because the biogas plant would not be worth the effort if its energy yield did not cover a substantial share of the energy requirements.
- The system must be properly built in order to minimize the maintenance & repair effort.
- Siting alternatives must be painstakingly compared, and only a really suitable location should be selected for the biogas plant.
The financial means of the plant's user must not be overextended (risk of excessive indebtedness).
