Importance Of The Measured Variables - PRONOVA SSM 6000 User Manual

SSM 6000 INTRODUCTION
CONT
Gas mixtures with critical concentration relationships between methane and oxygen are flammable. Although critical gas mixtures of
this kind are very rare in biogas plants, the utmost must be done in order to avoid the risk of ignition. In the SSM 6000, a detonation
protection unit separates the analyzer from the biogas plant. Furthermore, the interior of the device is flushed with ambient air, so
that no flammable gas mixture can occur in the device even in the case of pipe leaks.
Advantage: Increased safety.
1.2

Importance of the measured variables

Methane CH
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Methane is the only energy carrier in biogas worth mentioning. A high yield is hence vital for the profitability of the plant.
Furthermore, the methane concentration provides important information concerning the fermentation process and hence the
condition of the biogas plant. A decreasing concentration is the first sign of a disturbed fermentation process. Continuous monitoring
of the methane content hence makes it possible to search for and identify the causes of a problem at an early stage. Depending on
the purpose for which the biogas is used, the methane content must be within certain limits, for example, in order to permit its safe
use as a fuel in pilot injection units or in gas-fuelled spark ignition engines. Too high or low a methane content can cause damage to
the motor. Furthermore, too low a methane content strongly affects economic efficiency.
Hydrogen sulphide H S
2
The block cogeneration plant burns the hydrogen sulphide contained in biogas to form SO from which sulphurous or sulphuric acid
x
are produced in the presence of water. These acids lead to corrosion of those parts of your plant (for example, pilot injection unit or
gas-fuelled spark ignition engine) which contain nonferrous heavy metals. This is why hydrogen sulphide must be eliminated to the
largest extent possible.
Regular measurement of the hydrogen sulphide content provides a good indication of how good the biogas "desulphurization"
process is working.
Oxygen O
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In most plants, desulphurization of the biogas is accomplished by injecting additional ambient air oxygen into the process.
Experience has shown that an ambient air oxygen content of more than 5Vol.%ume does not lead to any further reduction of the
hydrogen sulphide content by biological degradation. Furthermore, higher concentration can reduce the reaction capability of
bacteria and a flammable gas mixture can occur in the fermenter. Monitoring the oxygen content in the biogas hence makes sense.
If too high a hydrogen sulphide level is measured even though the oxygen content is within the optimum range, insufficient
desulphurization must then be due to other causes. Possible explanations then include too low an ambient temperature for the
bacteria or too short a contact time during which the bacteria is exposed to the biogas.
Carbon dioxide CO
2
Besides methane, carbon dioxide is the second largest quantitative component of biogas. Both components account for around 98%
of the volume. This is, however, only an approximate value because other gases, such as ammonia (NH ) or hydrogen (H ), can be
3 2
produced in more than insignificant concentrations during the fermentation process and hence occur in biogas. The total
concentration of methane, carbon dioxide, oxygen and nitrogen contained in the injected air should amount to around 100Vol.%ume.
The nitrogen concentration corresponds to around 3.8 times the O concentration.
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Hydrogen H
2
The determination of the hydrogen content is becoming increasingly important for the assessment of the conversion processes in the
fermenter. Hydrogen is a pre-product and/or intermediate product of methane during the process of anaerobic decomposition, so that
monitoring of the hydrogen concentration enables an even faster detection of disturbances during the fermentation process.
800C-07.07.EN SSM 6000C User Manual 12