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Biogas - what is this?

Biogas is made from biomass and/or biological degradable parts of wastes by anaerobic fermentation.

Possible sources are as follows:

  • Agricultural material; e.g. grasses, corn, beet, silage, energy plants as well as residues from the production of grain, corn and sugar
  • Biogenous residues from industry and business; e.g. residues from production of food and luxury foodstuffs, wastes from the utilisation of animal carcasses, municipal sewage
  • Municipal wastes; e.g. separate collection (green biotons), cut grass, leaves
  • Animal excrements from agriculture; e.g. pig, cattle and horse manure, poultry manure

Figure: The biogasplant in Bruck an der Leitha; left: Mashing tanks and fermenters; right: Post-digestion and gas storage tank

The production of biogas is done in special reactors (fermenters) by fermentation (digestion) in absence of oxygen. Main components of biogas are methane and carbon dioxide. Depending of the used substrate and the process parameters biogas also contains secondary components like oxygen, nitrogen and contaminations from bacterial degradation of organic sulphurous substances (hydrogen sulphide). In praxis, a wide mixture of materials is used as substrate for the biogas production. Therefore, the composition of the produced gas is also fluctuating within a, more or less, wide range. As a result, a definition of untreated raw biogas according to the gas composition can only be given within a certain range of concentrations.

Parameter Unit Raw biogas
(typical values)
Gas quality
for grid injection
according ÖVGW G31/G33
Gas quality
for fuel purposes
according BGBL 417-II 2004
Upper heating value [kWh/m³STP] 6,0 to 9,3 10,7 to 12,8 8,4 to 13,1
Wobbe-index [kWh/m³STP] 6,0 to 11,1 13,3 to 15,7 12,1 to 15,7
Relative density [-] 0,70 to 1,20 0,55 to 0,65 0,55 to 0,70
Methane content [mol%] 40 to 80 ≥ 97,0 (absence H2) * ≥ 89,5 (absence H2) *
Carbon dioxide content [mol%] 14 to 55 ≤ 2,0 unspecified
Ammonia content [mg/m³STP] ≤ 1.000 technically free unspecified
Hydrogen sulphide content [mg/m³STP] ≤ 2.000 ≤ 5,0 unspecified
Oxygen content [mol%] ≤ 2,0 ≤ 0,5 unspecified
Nitrogen content [mol%] ≤ 20,0 ≤ 5,0 unspecified
Water content (dewpoint) [°C] < 37 @ 1 bar ≤ -8 @ 40 bar unspecified

*...not directly specified, but calculated from other specified parameters (upper heating value or Wobbe-index)

This table shows that a certain degree of purification is needed in order to obtain a product gas that is suitable for the substitution of natural gas (grid injection) or suitable to be used as a vehicle fuel (Bio-CNG). Basically, this purification/upgrading contains the removal of carbon dioxide, water, hydrogen sulphide and ammonia. Furthermore, numerous other unwanted gas species might be present in the biogas depending on the type of the biogas production plant. These species comprise e.g. silicon compounds (siloxanes, silanes), chlorine compounds or CFCs, additional sulphurous compounds like mercaptans or COS. Moreover, raw biogas always contains liquids and solids (droplets and dust). All named impurities together with their relevant limits are covered by the named engineering standards and regulations as well as the effective technical process guidelines of the gas utilisation facilities. In most cases additional advanced gas upgrading process steps are necessary, if these components are contained in the biogas.

Figure: Separation steps necessary for the upgrading of biogas to biomethane