Biogas typical components

The typical biogas components which are contained in its gaseous mixture are those that make it capable of producing renewable energy.

Biogas typical components

During anaerobic digestion of organic materials which contain certain groups of anaerobic bacteria, the organic substrate is converted into biogas, a gaseous combustible mixture, which has the ability to be used in various applications for energy production. The main compound consisted in biogas mixture is methane (CH4) which is actually the compound that gives biogas combustible properties. Methane is easily burned according to the following well-known exothermic combustion equation:

CH4 + 2O2 CO2 + 2H2O + 192 Kcal/mol

The complete combustion of 1m3 of CH4 provides about 8.570 kcal of heat. Thereby, biogas can be used in the following applications:

  • Direct combustion in a boiler for heat production
  • Utilization, after the proper pretreatment, in internal combustion engines for electric energy production (or both heat and electricity if a cogeneration engine is used).
  • Upgrading into biomethane so as to be used as a transportation fuel. Alternatively, upgraded biomethane can substitute natural gas (a non-renewable fuel), at its various applications in chemical industry.

Apart from methane, biogas contains significant amounts of carbon dioxide (CO2), which is non-combustible, along with smaller quantities and traces of other compounds. A typical composition of biogas can be seen on the following table:

Table 1: Biogas typical composition

Component

Formula

Concentration (% by vol.)

Methane

CH4

55-70

Carbon dioxide

CO2

30-45

Nitrogen

N2

0-5

Oxygen

O2

<1

Hydrocarbons

CnH2n+2

<1

Hydrogen sulfide

H2S

0-0.5

Ammonia

NH3

0-0.05

Water (vapour)

H2O

1-5

Siloxanes

CnH2n+1SiO

0-50 mg/m3

 

Carbon dioxide is removed from biogas only when the target is to upgrade it into biomethane. In conventional biogas CHP plants the equipment for capturing CO2 is not required.

Nitrogen and oxygen, the two main components of air, are found usually in biogas in a ratio 4:1. Their presence is mainly due to the air introduced in biogas mixture after the anaerobic digestion so as to remove the excess hydrogen sulfide amounts.

Ammonia concentration in biogas is most of the times very low, not exceeding 0.1 mg/m3. The presence of ammonia in higher concentration is attributed to the increased nitrogen content of the substrate used (e.g. poultry manure).

biogas plant 2Hydrogen sulfide quantity in biogas is a decisive factor for its quality. Without the integration of a biogas desulfurizing step, hydrogen sulfide concentration may exceed 0.2 % by volume which is a significant amount, capable of causing damages due to corrosion effects to the downstream piping or to the cogeneration engine. Actually, many engine manufacturers and providers suggest an upper limit in hydrogen sulfide concentration of 0.05 % by volume, for a long-life lasting effective operation of the engine.

The presence of water, in the gaseous form of vapour, is inevitable in biogas mixture due to the type of biochemical reactions which take place during anaerobic digestion. Similarly to sulfide, water is undesirable in biogas stream to the engines since it is responsible for the appearance of corrosion effects. High vapour concentration may turn the non-corrosive carbon dioxide into a corrosive compound due to formation of carbon acid traces. As a result, water removal from biogas is another necessary pretreatment step so as to eventually introduce biogas into the cogeneration engines.

Finally siloxanes are silica compounds which derive from the anaerobic digestion of specific type organic substrates. Such materials are household waste and municipal wastewater. That means that siloxanes are present mainly in landfill gas or sewage sludge biogas, while they are not a burden for agricultural feedstock biogas plants. Siloxanes presence in biogas is extremely undesirable, since they have the ability to react with oxygen at high temperatures and form silica dioxide (SiO2), which is a substance that remains in various parts of the equipment thus causing serious damages on them.

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