Siloxanes, present in small concentrations in biogas, interfere with the operation of biogas-to-energy facilities. Unlike sulphides, silicones do not react with water to form acids. However, during combustion, the siloxane molecules are broken down and release oxygen and silicon.
Silicon combines with other elements forming silicates, silica and other crystalline compounds which are deposited in the combustion chamber, in cylinder heads and on the faces of valves. These deposits abrade and wear down different internal parts of the engines. When the deposit is considerable, there are abrasion problems, ignition failure and even engine seizure. The mixture of the two effects mentioned above is the worst possible case, since the silicon paste is added to the particles of silica, forming a semirigid layer which has disastrous effects on cogeneration equipment.
Permissible silicone contents for proper functioning of the production engines should in general be < 5 mg/Nm3, although each machine manufacturer sets its own limits.