Landfill gas (biogas) released from landfills contains small quantities of siloxane. It can also be found in biogas from sewage treatment plants.
A siloxane is any chemical compound composed of elements of the form R2SiO, where R is a hydrogen atom or a hydrocarbon group. They belong to a wide class of compounds known as organo silicon compounds -or “silicon” to most people.
Siloxane was the cause of many landfill gas engine failures when in the late 1970s landfill gas engines were first introduced, and until the problem was identified. Nowadays it should not be a problem. The manufacturers and plant maintainers have developed strategies for the avoidance of build-up in engines and turbines by treating the biogas before it enters the gas engine, or turbine, and this prevents damage to the plant.
In internal combustion engines, siloxanes are oxidised to silicon dioxide which then forms deposits on pistons and cylinder heads which are extremely abrasive and cause damage to the internal components of the engine.
In bad examples of siloxane contamination the landfill gas power generation engines can require a complete overhaul at 5,000 h or less of operation.
Silicon dioxide accumulates on the heated surfaces in combustion equipment, especially in the cylinders, on the valves and on the heads of IC generator engines.
Other web sites quote an example of an IC generator engine running on 400 SCFM of biogas containing just 1 ppmv of siloxane D5, for example, which is said to have generated approximately 59 kg or 130 lb. of silicon dioxide per year.
Due to the fact that this is a solid, some of this silicon dioxide will remain in the engine and if not removed can cause considerable damage that increases the cost of operating the generation equipment.
If there are also elements such as sodium, aluminum, magnesium, iron, or similar elements present then glass-like materials called silicates can form. It is these silicates which are extremely abrasive to generator engine moving parts.
There are at least five different removal methods for siloxane which have been applied to varying extents to the removal of siloxane before a biogas like landfill gas is combusted.
Siloxane contaminants are in theory removed by passing the biogas through a bed containing activated alumina, which absorbs the siloxanes. The alumina eventually reaches saturation.
When the activated alumina becomes saturated with siloxanes, the absorption capability of the activated alumina can be recovered by passing a regeneration gas through the bed of activated alumina, however, it would be necessary to obtain specialist advice before doing this.
Further sources of information on siloxane removal from landfill gas are the specialist landfill gas engine and turbine equipment manufacturers.
We have further information on trace components in landfill gas on here.