Environmental Frame Conditions of Biogas Technology
Climatic Conditions for Biogas Dissemination
Temperatures
Biogas technology is feasible in principle under almost all climatic conditions. As a rule, however, it can be stated that costs increase for biogas production with sinking temperatures. Either a heating system has to be installed, or a larger digester has to be built to increase the retention time. Unheated and un-insulated plants do not work satisfactory when the mean temperature is below 15 °C. Heating systems and insulation can provide optimal digestion temperatures even in cold climates and during winter, but the investment costs and the gas consumption for heating may render the biogas system not viable economically.
 Global 15oC isotherms for January and July, indicating the biogas-conducive temperature zoneSource: OEKOTOP |
Not only the mean temperature is important, also temperature changes affect the performance of a biogas plant adversely. This refers to day/night changes and seasonal variations. For household plants in rural areas, the planner should ensure that the gas production is sufficient even during the most unfavorable season of the year. Within limits, low temperatures can be compensated with a longer retention time, i.e. a larger digester. Changes of temperature during the course of the day are rarely a problem as most simple biogas digesters are built underground.
Precipitation
The amount of seasonal and annual rainfall has mainly an indirect impact on anaerobic fermentation:
- Low rainfall or seasonal water scarcity may lead to insufficient mixture of the substrate with water. The negative flow characteristics of substrate can hamper digestion.
- Low precipitation generally leads to less intensive systems of animal husbandry. Less dung is available in central locations.
- High precipitation can lead to high groundwater levels, causing problems in construction and operation of biogas plants.
Suitability of Climatic Zones
Tropical Rain Forest
annual rainfall above 1.500 mm, mean temperatures between 24 and 28°C with little seasonal variation. Climatically very suitable for biogas production. Often animal husbandry is hampered by diseases like trypanosomiasis, leading to the virtual absence of substrate.
Tropical Highlands
rainfall between 1.000 and 2.000 mm, mean temperatures between 18 and 25°C (according to elevation). Climatically suitable, often agricultural systems highly suitable for biogas production (mixed farming, zero-grazing).
Wet Savanna
rainfall between 800 and 1.500 mm, moderate seasonal changes in temperature. Mixed farming with night stables and day grazing favor biogas dissemination.
Dry Savanna
Seasonal water scarcity, seasonal changes in temperatures. Pastoral systems of animal husbandry, therefore little availability of dung. Use of biogas possible near permanent water sources or on irrigated, integrated farms.
Thornbush Steppe and Desert
Permanent scarcity of water. Considerable seasonal variations in temperature. Extremely mobile forms of animal keeping (nomadism). Unsuitable for biogas dissemination.
Firewood Consumption
A unique feature of biogas technology is that it simultaneously reduces the need for firewood and improves soil fertilization, thus substantially reducing the threat of soil erosion. Firewood consumption in rural households is one of the major factors contributing to deforestation in developing countries. Most firewood is not acquired by actually cutting down trees, but rather by cutting off individual branches, so that the tree need not necessarily suffers permanent damage. Nonetheless, large amounts of firewood are also obtained by way of illegal felling.
In years past, the consumption of firewood has steadily increased and will continue to do so as the population expands - unless adequate alternative sources of energy are developed. In many developing countries such as India, the gathering of firewood is, strictly speaking, a form of wasteful exploitation. Rapid deforestation due to increasing wood consumption contributes heavily to the acceleration of soil erosion. This goes hand in hand with overgrazing which can cause irreparable damage to soils. In the future, investments aimed at soil preservation must be afforded a much higher priority than in the past. It will be particularly necessary to enforce extensive reforestation.
Soil Protection and Reforestation
The widespread production and utilization of biogas is expected to make a substantial contribution to soil protection and amelioration. First, biogas could increasingly replace firewood as a source of energy. Second, biogas systems yield more and better fertilizer. As a result, more fodder becomes available for domestic animals. This, in turn, can lessen the danger of soil erosion attributable to overgrazing. According to the ICAR paper (report issued by the Indian Council of Agricultural Research, New Delhi), a single biogas system with a volume of 100 cft (2,8 m3) can save as much as 0.3 acres (0,12 ha) woodland each year.
Taking India as an example, and assuming a biogas production rate of 0.36 m3/day per livestock unit, some 300 million head of cattle would be required to produce enough biogas to cover the present consumption of firewood. This figure is somewhat in excess of the present cattle stock. If, however, only the amount of firewood normally obtained by way of deforestation (25.2 million trees per year) were to be replaced by biogas, the dung requirement could be satisfied by 55 million cattle. Firewood consumption could be reduced to such an extent that - at least under the prevailing conditions - a gradual regeneration of India's forests would be possible.
According to empirical data gathered in India, the consumption of firewood in rural households equipped with a biogas system is much lower than before, but has not been fully eradicated. This is chiefly attributable to a number of technical and operational short-comings. At present,
- many biogas systems are too small to handle the available supply of substrate;
- many biogas units are operated inefficiently;
- many of the existing biogas systems are not used due to minor mistakes;
- biogas users tend to increase energy consumption to the point of wastage, then requiring additional energy in the form of firewood.
A more serious problem, however, is the fact that a household biogas system program can only reach the small percentage of farmers who have the investment capital required. The majority of rural households will continue to use firewood, dried cow dung and harvest residues as fuel.
Reduction of the Greenhouse Effect
Last but not least, biogas technology takes part in the global struggle against the greenhouse effect. It reduces the release of CO2 from burning fossil fuels in two ways. First, biogas is a direct substitute for gas or coal for cooking, heating, electricity generation and lighting. Additionally, the reduction in the consumption of artificial fertilizer avoids carbon dioxide emissions that would otherwise come from the fertilizer producing industries. By helping to counter deforestation and degradation caused by overusing ecosystems as sources of firewood and by melioration of soil conditions biogas technology reduces CO2 releases from these processes and sustains the capability of forests and woodlands to act as a carbon sink.
Methane, the main component of biogas is itself a greenhouse gas with a much higher "greenhouse potential" than CO2. Converting methane to carbon dioxide through combustion is another contribution of biogas technology to the mitigation of global warming. However, this holds true only for the case, that the material used for biogas generation would otherwise undergo anaerobic decomposition releasing methane to the atmosphere. Methane leaking from biogas plants without being burned contributes to the greenhouse effect! Of course, burning biogas also releases CO2. But this, similar to the sustainable use of firewood, does only return carbon dioxide which has been assimilated from the atmosphere by growing plants maybe one year before. There is no net intake of carbon dioxide in the atmosphere from biogas burning as it is the case when burning fossil fuels.
Biogas and the Global Carbon Cycle
Each year some 590-880 million tons of methane are released worldwide into the atmosphere through microbial activity. About 90% of the emitted methane derives from biogenic sources, i.e. from the decomposition of biomass. The remainder is of fossil origin (e.g. petrochemical processes). In the northern hemisphere, the present tropospheric methane concentration amounts to about 1.65 ppm.
Further Information
With anaerobic digestion, a renewable source of energy is captured, which has an important climatic twin effect.
- The use of renewable energy reduces the CO2-emissions through a reduction of the demand for fossil fuels.
- At the same time, by capturing uncontrolled methane emissions, the second most important greenhouse gas is reduced:
1m3 cattle manure = 22,5 m3 biogas = 146 kWh gross = 36 kg CO2- Emissions
Smaller agricultural units can additionally reduce the use of forest resources for household energy purposes and thus slow down deforestation (about 1 ha of forest per rural biogas plant), soil degradation and resulting natural catastrophes like flooding or desertification.
1 m3 biogas (up to 65% CH4) = 0,5 l fuel oil = 1,6 kg CO2
1 m3 biogas = 5,5 kg fire wood = 11 kg CO2
When applied for industrial or municipal wastewater treatment, surface waters and other water resources (rivers, sea, ground and drinking water resources) are being protected. Often the purified wastewater can be reused, e.g. as process water in industry or as irrigation water in agriculture. Costs saved for providing additional water can be directly translated into benefits.
The introduction, promotion and broad-scale dissemination of anaerobic technology into agro-industrial, domestic and agricultural sector combined with efficient power and heat generation or household energy appliances allows by now an efficient and viable reduction of environmental pollutants.
The Impact on the Greenhouse Effect
The greenhouse effect is caused by gases in the atmosphere (mainly carbon dioxide CO2) which allow the sun's short wave radiation to reach the earth surface while they absorb, to a large degree, the long wave heat radiation from the earth's surface and from the atmosphere. Due to the "natural greenhouse effect" of the earth's atmosphere the average temperature on earth is 15°C and not minus 18°C.
The increase of the so called greenhouse gases which also include methane, ozone, nitrous oxide, etc. cause a rise of the earth's temperature. The World Bank Group expects a rise in sea levels until the year 2050 of up to 50 cm. Flooding, erosion of the coasts, salinization of ground water and loss of land are but a few of the consequences mentioned.
Until now, instruments to reduce the greenhouse effect considered primarily the reduction of CO2-emissions, due to their high proportion in the atmosphere. Though other greenhouse gases appear to a smaller extend in the atmosphere, they cause much more harm to the climate. Methane is not only the second most important greenhouse gas (it contributes with 20% to the effect while carbon dioxide causes 62%), it has also a 25 times higher global warming potential compared with carbon dioxide in a time horizon of 100 years (Table 1).
| Gas | Relative global warming potential 20 years after emission |
Relative global warming potential 100 years after emission |
|---|---|---|
| CH4 | 63 | 24,5 |
| N2O | 270 | 320 |
| FCKW12 | n. | 8.500 |
| CF3Br (Halon 1301) | 5.600 | |
| C2F6 (Perflourethan) | 12.500 |
The reduction of 1 kg methane is equivalent to the reduction of 25 kg CO2. The reduction of greenhouse gases with a high global warming potential can be more efficient compared with the reduction of CO2.
Sources of Methane Emissions in the Agricultural Field
The amount of worldwide methane emissions from agricultural production comprises about 33 % of the global anthropogenic methane release. Animal husbandry alone comprises 16 %, followed by rice fields with 12 % and animal manure with 5 % . While methane released through digestion of ruminants (about 80 Mil t CH4 per year) can rarely be reduced, methane emissions from animal waste can be captured and energetically used through anaerobic treatment. The amount of methane emission mainly depends on fodder, animal type and animal waste systems. For example: the methane emission potential from dairy cattle in industrialized countries is about 0,24 m3 CH4/kg volatile solids (influence of fodder), in developing countries it is only about 0,13 m3 CH4/kg volatile solids. But taking into account the aerobic condition of solid dung systems (only 5 % of the methane emission potential is released) it is mainly the liquid waste management systems which contribute through anaerobic conditions with a high methane release to the climate change (up to 90 % of the methane emission potential is released).
From the worldwide 30 Mil t of methane emissions per year generated from the different animal waste management systems like solid storage, anaerobic lagoon, liquid/slurry storage, pasture etc. half of the emissions could be reduced through anaerobic treatment.
Eastern Europe, Asia and Far East contribute with the highest amount of 6,2 Mil t methane emissions/year each. While in Eastern Europe the emissions are caused by anaerobic animal waste management system, in the Far East they are caused by the high numbers of livestock (Figure 1).
 Fig. 1[2]: Methane-emissions from different animal waste management systems
|
Methane Reduction Potential Through the Application of Biogas Technology
Through anaerobic treatment of animal waste, respectively through controlled capture of methane and its energetic use, about 13,24 Mil t CH4/year can be eliminated worldwide. This figure includes methane emissions resulting from incomplete burning of dung for cooking purposes. By replacing dung through biogas, these emissions are avoided. In total about 4 % of the global anthropogenic methane emissions could be reduced by biogas technology.
If fossil fuels and firewood is replaced by biogas additional CO2-emissions can be avoided including a saving of forest resources which are a natural CO2 sink. Including all these effects about 420 Mil t of CO2-equivalents are avoidable (Table 2).
| CO2 Reduction [Mil t CO2/year] | ||
|---|---|---|
| CH4 | 13,24 Mil t/year CO2-equivalent: methane x 25 |
330,9 |
| Biogas | 33.321 m3/year | |
| Substitution of fossil fuels | 44,7-52,7 | |
| Fire wood savings | 4,17 - 73,8 | |
| Total | 388 - 449 = 418,5 |
Reduction Potential of Nitrous Oxide Emissions from Agriculture
The relative climatic change potential of nitrous oxide is up to 320 times higher as that of CO2 (Benefits of Biogas Technology#The impact on the greenhouse effect Table 1). Nitrous oxide generation is a natural microbial process. It is produced during nitrification and de-nitrification processes in soils, stables and animal waste management systems. In general, nitrous oxides emissions appear in soils without anthropogenic influence. Fertilizing as well as special conditions during storage can immensely increase the emissions.
Little detailed information is available about the reduction potential of nitrous oxides through anaerobic digestion of animal waste. There is still a big need for further research. Nevertheless, ongoing research results indicate that anaerobic digestion of animal waste significantly reduces nitrous oxide emissions by:
- avoiding of emissions during storage of animal waste,
- avoiding of anaerobic conditions in soils,
- reducing N2O-emissions through increased nitrogen availability for plants and a faster nitrogen absorption through crop plants,
- reducing application of inorganic nitrogen fertilizer by which N2O-emissions are reduced during production of nitrogen fertilizer.
Considering all these effects a N2O-reduction potential through anaerobic treatment of about 10 % can be assumed. This means that 49.000 t N2O/year or 15,7 Mil t CO2-equivalents could be reduced on average.
So far, the environmental costs of greenhouse gas emissions have not been calculated. One means, proposed by the US administration on the climate conference in Kyoto, is the introduction of emission rights which can be traded. In doing so, national economies could attribute a monetary benefit to the avoidance of greenhouse gas emissions.
References
Environmental Aspects
Biogas technology is feasible in principle in most climatic zones under all climatic conditions, where temperature or precipitation are not too low. (see: "Environmental Frame Conditions")
Using biogas technology is, besides direct thermal or photovoltaic use and hydropower, a form of using solar energy, mediated through the processes of photosynthesis (for build-up of organic material) and anaerobic decomposition. As such it is a renewable energy source. In many regions of the world, the consumption of firewood exceeds natural regrowth. This leads to deforestation and degradation of forests and woodlands with adverse effects on climate, water budget, soil fertility and natural products supply. Biogas is one of the solutions to this problem, because it substitutes firewood as a fuel and helps sustaining favourable soil conditions. It is also an important contribution to the mitigation of the global greenhouse effect.
The potential contribution of biogas technology to combat deforestation, soil erosion, water pollution and climate change is undisputed. But how much support biogas dissemination will receive from government institutions will depend largely on the role of environmental considerations in government decision making.
The success of biogas technology also depends on the influence of potential allies in the environmental NGO scene. Biogas programs can, if environmental policies are favorable, be perceived as "status projects" for environmental authorities.
