Carbon Markets for Biogas Digesters

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Carbon markets constitute a new financing option for biogas dissemination projects. The number of biogas projects that are under validation, requesting registration or registered is 516, or 11.6% of the CDM projects (UNEP Risoe, March 2009). However, the highest number of biogas projects is concentrated in 5 countries, namely: Thailand, India, China, Malaysia, and the Philippines. Most of the registered projects are situated on the commercial livestock farms and the main emission reduction takes place due to switch in those cases where biogas is used for energy generation. By installing the biogas unit the animal manure that was previously deposited in an open lagoon in the baseline scenario is fermented in the biogas digester and the methane emission is avoided. The generated biogas can either be flared or used for energy generation. Biogas can be used to replace fossil fuels for heating purposes, or for producing heat and electricity by introducing a CHP unit. Apart from the benefits of replacing fossil fuels and improving the manure management system, the by-product after the fermentation of the manure is a digestate (bio-slurry) which could be used as high nutrient organic fertilizer.


Every industrial process results not only in the products wanted, but also in by-products, wastes and emissions. Amongst the emissions, normally also greenhouse gases occur. Hence, every industrial process is climate relevant, for greenhouse gases are emitted as a result of the technological processes taking place. Which substance is emitted and in which amount it is emitted depends on the specific process and the conditions of its application. Thus, the choice of the technology, the apparatus, the cleaning devise, etc. may influence the climate effects of the production process. An optimisation of the process with reference to a decision criteria reflecting the climate effects is thus a necessary precondition for the choice of the best technologies (see chapter 8.1). Some examples of industrial processes are given in the next chapter. The data used for description of the processes are either derived from assumptions of chemical reactions or upon published empirical data[1]. The emissions considered here are by-products of the industrial process itself. Typically, in such processes, raw materials are transformed from one state to another in the end product. This transformation is accompanied by the release of emissions, such as carbon dioxide (CO2), methane (CH4), or nitrous oxide (N2O). They are the reason of the greenhouse effect of the process.

The combustion of biogas results in greenhouse gases emissions. But these emissions are not addressed to climate change. As those emissions are considered to close the loop of the natural carbon cycle. The reason is, that they origin from atmospheric carbon dioxide from which they were taken by photosynthesis. CO2 produced from biogenic sources under natural conditions will return to the atmosphere at the same amount. If there are metabolites or results from processes other than CO2, such as methane, N2O, NOx, etc., than these substances are considered climate relevant and their greenhouse gas effects must be balanced [2].

Climate Relevant Emissions of a Biogas Plant

In comparison with untreated manure, methane formation from digested manure is considerably reduced by the anaerobic process, because some of the organic matter contained in the substrate has already been metabolised in the digester, which means that there is significantly less easily degradable carbon in the storage tank.

The extent to which emissions of methane are reduced will depend decisively on the degree to which the organic matter has been degraded and consequently also on the retention time of the substrate in the digester. For example, it has been demonstrated in various studies that digestates with a short digestion phase, i.e. a short retention time in the digester, will emit more CH4 than digestates with a longer retention time in the digester (see graph[3]).

If the retention time is very short, there can be increased emissions of methane in comparison with untreated manure if substrate that has just been inoculated with methane-forming bacteria is removed from the digester after a short time and transferred to digestate storage [4]. Short-circuit streams should therefore be avoided. To estimate the methane emissions from digestate,
it is possible to use the results from batch digestion experiments with digestates at 20-22 °C [5] since this more or less corresponds to the temperature in a digestate storage tank under real-world conditions. On the other hand, the values for residual
gas potential obtained under mesophilic conditions (37 °C) cannot be relied on with regard to actual emissions. Nevertheless, they can still give an indication of the efficiency of the digestion process, because they reflect the biomass potential still present in the digestate, i.e. the biomass potential that was not converted in the digester. Both parameters depend, however, on process control and also on the substrates used at the particular plant. Consequently, the values given in Table 10.6 should be regarded
merely as a guide.
Multi-stage plants tend to exhibit a lower residual gas potential both at 20-22 °C and also at 37 °C. This is due above all to the fact that a multi-stage plant has a higher retention time, which has the effect of reducing the residual gas potential. Owing to the high greenhouse potential of CH4 (1 g CH4 is equivalent to 23 g CO2), it is desirable to reduce or prevent CH4 emissions from digestate storage tanks.

Plants without gas-tight end storage should, in addition to multi-stage operation (digester cascade), meet at least one of the following requirements:

  • average hydraulic retention time of the total substrate
  • volume of at least 100 days at a continuous
  • digestion temperature throughout the year of at least 30 °C or
  • digester organic loading rate < 2.5 kg VS/Nm3/d

Calculation of the substrate volume must take account of all inputs into the digestion tank(s) (including, for example, water and/or recirculate). If the above-mentioned requirements are not met, methane emissions must be expected to exceed the average values. In such cases, it is advisable to retrofit the digestate storage tank(s) with a gas-tight seal2 for at least the first 60 days of required digestate storage.

For a complete observation on emissions related to a biogas plant constructional and logistical aspects of the volume flow have to be considered.

Emission Reduction of Environmental Impacts

The minimisation of environmental impacts aims at reducing the effects of the plant on the environment. The release of pollutants to the air, water and soil needs to be considered.

  • Seepage water (collection and utilisation of silage seepage water, runoff from storage areas)
  • Methane emissions from the biogas plant (provide digestate storage tank with gas-tight cover, identify leaks, slip from gas utilisation, engine settings, maintenance work)
  • Formaldehyde, NOx, oxides of sulphur, carbon monoxide (CHP unit only, engine settings, exhaust gas treatment)
  • Odour emissions (covered loading facility, storage areas and digestate storage tank, separated fermentation residues)
  • Noise emissions
  • After the application of fermentation residues: ammonia emissions, nitrous oxide emissions (application techniques and incorporation of the residues).

Not only do uncontrolled emissions of silage seepage water, methane and ammonia have a detrimental impact on the environment, they also signify losses in terms of the efficiency of the plant as a whole. In this respect, structural or operational measures to reduce emissions can certainly pay off financially (for example a gas-tight cover for a digestate storage tank). As a general rule the plant should be regularly checked for possible emissions. In addition to environmental and economic considerations, it is often also necessary to take safety matters into account as well.

Further Information


  1. Integrated pollution and prevention control. BREF, Madrid, 2001.
  2. FECC, GTZ, KNOTEN Weimar, 2009: VII Climate Change. Workshop „Biogas-Plant-technology planning, Beijing,
  3. FNR (2009): Ergebnisse des Biogasmessprogramm II,fckLRGülzow
  4. Clemens, J., Wolter, M., Wulf, S., Ahlgrimm, H.-J. (2002): Methan- und Lachgas-Emissionen bei der Lagerung und Ausbringung von Wirtschaftsdüngern, in: KTBL-Schrift 406, Emissionen der Tierhaltung,fckLRpp. 203-214
  5. FNR (2009): Ergebnisse des Biogasmessprogramm II, Gülzow