Biogas Plant Or Methane?

A biogas plant is actually methane, which is the main component of a biogas mixture. It is a colorless, tasteless and odorless gas, so it has properties that makes its detection problematic. At high indoor concentrations when atmospheric 02 is displace, it acts as a suffocate gas. Even though, this is extremely rare due to the fact that it is lighter than the air.

In a complete combustion reaction methane produces only water vapor and carbon dioxide, according to the following equation CH4 +2 02 - C02 +2 H20

Methane flammability limits very between 5 to 15%. So when the gas content is less than 5% it cannot be burned, as if the content exceeds 15%.

Additionally, methane ignition requires temperature above 600*C (which is called ignition temperature). During combustion, the theoretical exhaust combustion temperature (exhaust gas temperature with no losses) is 1950*C.

As methane is lighter than air it accumulates under roofs with the risk of creating an explosive mixture. Thus the methane can displace the whole or part of the air in a confined space. The risks posed by this phenomenon may vary greatly. A 10% replacement of the air in a methane gas mixture does not create asphyxiate problems but it poses a significant risk of explosion (since the air -methane mixture is within its flammability limits). On the other hand, by replacing 90% of the air of methane gas mixture it creates an intolerable breathing situation but without the possibility of explosion or ignition.

On this basis, naked heaters should not exist in a digester and the required electrical equipment should be "explosive" type. Also, any sources of ignition, such as iron or steel tools, electrical machinery, electrical switches, etc, should be avoided nearby the digesters.

Other gases

Carbon dioxide and hydrogen sulfide are heavier than air and tend to settle and accumulates in low height. Ammonia (as methane ) is lighter than air and concentrates under roofs. Ammonia is both irritating corrosive. Hydrogen sulfide is poisonous (toxic ) and has the characteristic odor of rotten egg.

The concentration of hydrogen sulfide in the biogas is variable, depending on the substrate used and cannot be easily predicted. Exposures to sulfide concentrations above 1.000 ppm is fatal. Concentrations between 10 and 20 ppm cause eye and lung irritation. At concentrations close to 150 ppm it can be detected by the sense of smell.

Carbon monoxide (Co) can be present in the exhaust system of an incomplete biogas combustion process. Carbon monoxide is formed due to the lack of the required stoichiometric oxygen amount during the combustion. Co is tasteless, colorless, odorless, with a density slightly less than air. It is highly flammable and toxic. Carbon monoxide poisoning symptoms begin with headache, tachycardia, dizziness, nausea, and can lead to unconsciousness and death.