Biogas – the Natural Source of Energy with a Balanced CO2 Offset

A biogas plant requires daily support and control, as it involves a sensitive biological process.  Key process characteristics are, for example, the tempera­ture, the dwell time and rearrangement of the substrate, as well as the quantity and frequency of the substrate feed. In order to optimise and control the plant, it is absolutely essential to analyse the gas composition on a continuous basis.

Table 1 shows the typical compositions of the main components of biogas in the fermenter prior to purification.

Essentially, the process of biogas production consists of fermentation, pro­cessing and purification.

The fermenter is the heart of the plant, as this is where the majority of the biogas is produced. The typical agricultural fermenters are tower constructions, usually with a capacity of at least 1 000 m•, part of which is sometimes below ground level. Inside the fermen­ter an agitator ensures the necessary homogeneity of the substrate and prevents the formation of sinking and floating layers.

In conventional biogas plants, conden­sate separators and pollution filters are used in the preparation and purification stage, in order to not only remove solids carried by the gas flow, but also to prevent the icing of gas lines and to avoid corrosion.

The desulphurisation (H2S removal) is also an important element of the gas purification. The H2S in the gas can be separated by means of chemical, physi­cal or biological procedures. In agricul­tural renewable resource biogas plants, the H2S components are usually removed by blowing in air.

Apart from the legal requirements for infeed into the gas network and the requirements for the protection of the motor when generating electricity, it is necessary to remove the hydrogen sulphide (H2S) for the general protection of the materials used in the overall plant, since H2S has a strongly corrosive effect in combina­tion with water.

In addition, higher concentrations of hydrogen sulphide in the biogas are problematical, as sulphur oxides are created during the combustion. These also have a greater corrosive effect on metallic plant components and also acidify the motor oil used in combustion engines. As a rule, for safe CHP opera­tion, limit values of between 100 and 500 ppm H2S in the biogas must not be exceeded. Considerably more stringent requirements apply to the feeding of gas into the network and for the protec­tion of gas-fueled engines.

In principle, there are various options for using biogas obtained from the fermenter. Apart from conversion into heat, fuel and/or electricity, it can also be fed directly into the gas network.

The biogas production is ideally moni­tored and controlled by a continuous measurement of methane (CH4), carbon dioxide (CO2), oxygen, and hydrogen sulphide.

For the various measurement points used in biogas applications, the continuous-measurement ULTRAMAT 23, ULTRAMAT 6, OXYMAT 6, the gas chro­matographs MicroSAM and SITRANS CV, and for special applications the MAXUM Ed II are used for analytical purposes.