Biogas plants

A biogas plant is the name often given to an anaerobic digester that treats farm wastes or energy crops. It can be produced using anaerobic digesters (air-tight tanks with different configurations). These plants can be fed with energy crops such as maize silage or biodegradable wastes including sewage sludge and food waste. During the process, the micro-organisms transform biomass waste into biogas (mainly methane and carbon dioxide) а также digestate. The composition of biogas varies depending upon the substrate composition, as well as the conditions within the anaerobic reactor (temperature, pH, and substrate concentration).

Dangers

The air pollution produced by biogas is similar to that of natural gas. The content of toxic hydrogen sulfide presents additional risks and has been responsible for serious accidents. Leaks of unburned methane are an additional risk, because methane is a potent greenhouse gas.

Biogas can be explosive when mixed in the ratio of one part biogas to 8–20 parts air. Special safety precautions have to be taken for entering an empty biogas digester for maintenance work. It is important that a biogas system never has negative pressure as this could cause an explosion. Negative gas pressure can occur if too much gas is removed or leaked; Because of this biogas should not be used at pressures below one column inch of water, measured by a pressure gauge.

Frequent smell checks must be performed on a biogas system. If biogas is smelled anywhere windows and doors should be opened immediately. If there is a fire the gas should be shut off at the gate valve of the biogas system

Contaminants

Sulfur compounds
Toxic and foul smelling Hydrogen sulfide is the most common contaminant in biogas, but other sulfur-containing compounds, such as thiols may be present. Left in the biogas stream, hydrogen sulfide is corrosive and when combusted yields sulfur dioxide а также sulfuric acid, also corrosive and environmentally hazardous compounds.

Ammonia
Ammonia is produced from organic compounds containing nitrogen, such as the amino acids in proteins. If not separated from the biogas, combustion results in nitrous oxide emissions.

Siloxanes
In some cases, biogas contains siloxanes. They are formed from the anaerobic decomposition of materials commonly found in soaps and detergents. During combustion of biogas containing siloxanes, silicon is released and can combine with free oxygen or other elements in the combustion gas. Deposits are formed containing mostly silica or silicates and can contain calcium, sulfur, zinc, phosphorus. Such white mineral deposits accumulate to a surface thickness of several millimeters and must be removed by chemical or mechanical means.

Practical and cost-effective technologies to remove siloxanes and other biogas contaminants are available.

Sources: Various Internet Sites, Wikipedia