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How efficient and climate-friendly is biogas production?

Biogas plants have become a familiar sight in Baden-Württemberg's rural areas. It might therefore be expected that broad experience exists in the comprehensive evaluation of this type of energy generation from renewable resources or organic materials. However, scientists draw a very differentiated picture. It is difficult to make any generalisations, although the analysis of individual facets can provide further help.

Prof. Dr. Enno Bahrs has been researching and teaching in the field of “Farm Management” at the University of Hohenheim since the beginning of the winter semester 2008/09. © University of Hohenheim

There are no simple truths when it comes to which kind of plants should be used for biogas production, where they should be planted, and the extent to which this might be useful or not. Prof. Dr. Enno Bahrs, head of the Department of Farm Management at the University of Hohenheim, believes that the perspective from which an issue is seen is crucial: are we looking at resource efficiency, climate friendliness or purely economic issues? Climate friendliness, for example, essentially depends on the quantity of greenhouse gases emitted. In the case of biogas plants, this information can be derived from the amounts of CO2, methane and nitrous oxide released. “We can compare different biogas plants in terms of the emission of these three gases and analyse their effect on the climate relatively well. However, it is rather difficult to accurately assess the global warming potential of biogas substrate production,” says Bahrs.

With biogas substrate production, the issue quickly becomes very complex where equipment and building components, e.g. the proportion of concrete parts used for building fermenters, also need to be included in the assessment. Calculation of the global warming potential (GWP) also has to take into account the amount of CO2 released during production and transportation of biogas plant components, as well as the amount of CO2 consumed during seed production, sowing, harvesting and substrate transport. Any value can theoretically be assumed. “We need to accurately determine the amount of CO2 that is released or consumed at the various stages. Nevertheless, it is still difficult to achieve absolute comparability,” says Bahrs. However, scientists tend to use functional units as benchmarks to assess the global warming potential of a biogas plant. “For example, when only electricity, and nothing else, is produced by combusting biogas in a block heat and power plant close to the biogas plant, kWh electricity is used as functional unit. It is then possible to examine how much greenhouse gas is generated per kWh electricity. Carbon dioxide is used as the reference for determining the global warming potential of different gases. Depending on the substrate used, methane has 20 times the global warming potential of CO2, and nitrous oxide is over 300 times worse than CO2,” says Bahrs.

Technological comparisons work best with model plants

Biogas production and purification leads to the release of different amounts of methane depending on the technology used. This so-called methane slip affects the climate friendliness of bioenergy. “Small amounts of methane are usually safe, but they do have an effect on the climate,” says Bahrs. Bahrs and his team have compared different biogas preparation processes in model plants with one another as part of a research project. “Amine scrubbing turned out to be the most efficient process economically, while pressurised water scrubbing proved to be more climate friendly. The results can also be quite different depending on how a biogas plant operator manages the technology used,” says Bahrs.

It always comes down to what you are looking at – this also applies to the location of biogas plants. The fact that biogas plants are usually located in the immediate vicinity of crop cultivation areas initially seems fairly reasonable. Bahrs explains why: “Generally speaking, the substrates used in Baden-Württemberg come from here or surrounding regions. As far as renewable resources are concerned, it is not very practical to transport the substrates over distances of more than 50 to 70 km. Moreover, the effect on the climate increases with length of transport.” However, Bahrs also identifies scenarios that need to be evaluated quite differently: “The situation might be quite different when drivers pick up agricultural fertiliser from a farm, drive it 100 km away and then return to the same farm with a grain delivery.” In his scientific investigations, Bahrs takes into account a number of cases like these to obtain a coherent overall picture.

Sustainability is difficult to assess, but the scientific basis is improving

It is even more difficult to assess the sustainability of biogas production. Bahrs is aware that it involves very diverse ecological factors such as minimising the ozone load, avoiding the eutrophication of the environment and maintaining biodiversity. In terms of ecological and social sustainability, the goal is to keep resource consumption as low as possible. Bahrs believes that intergenerational disproportionality is another factor that needs to be taken into account. This refers to the fact that fossil fuels such as petroleum will continue to be required in the long term for some purposes, and care therefore needs to be taken to preserve these resources for future generations.

It is difficult to include all these factors in scientific investigations due to lack of consensus or because their ecological effect is not clear. “We are trying to collaborate on the development of models that take into account these kinds of factors. Science has its limits and we are trying to push them back in an effort to create better decision alternatives,” says Bahrs. This includes simulating different plant cultivation options, while taking into account decisions on whether crops are cultivated for food or energy production purposes. “For example, we are trying to work out where to find the best economic viability under good agricultural practice. Then we will be able to come up with facts from a scientific perspective that can be used to decide which substrate to use,” says Bahrs.

Bahrs is very cautious about predicting how biogas production will develop in the future as the factors that can influence this development are far too imponderable. “I imagine that the crop composition of our agricultural land areas will not change much up to 2020; but there might be larger changes on the small-scale regional level. What will happen after 2020 depends to a large degree on national and European funding policies, which are difficult to predict.” How the cost of other types of energy develops also plays a key role. “Many biogas plants will stop operating once their guaranteed funding period ends if they are unable to produce biogas more efficiently than at present. The operation of biogas plants with renewable resources might become unprofitable when funding conditions change. However, when the cost of producing other regenerative energies increases sharply, many biogas plants will most likely continue running long into the future. However, this also very much depends on the development of petroleum and gas prices,” says Bahrs, pointing out that technical progress also plays a role in this development. He gives an example: “Corn cultivation areas in Germany have increased due to the large number of biogas plants established. Consequently, focus has also been put on improving the corn breeds used in the biogas plants.”

Website address: https://www.biooekonomie-bw.de/en/articles/news/how-efficient-and-climate-friendly-is-biogas-production