Biogas Separation

The idea of biogas separation is understood in two different ways. Following the process of its creation, biogas must be segregated from the organic matter that gives rise to it. Subsequently, the unnecessary — even toxic — compounds in biogas must be set apart from the methane (CH4) in order to maximize the safety and efficiency of the biogas as an energy source. As biogas becomes a more popular recourse for those avoiding the further expenditure of fossil fuels, the prospect of making it as pure as possible is evolving from a highly desirable goal to an absolute necessity.

Biogas origination

The cycle of life leaves no living matter untouched. Seeds germinate, are given birth, grow to maturity, deteriorate, die and decompose. While decomposition may appear to be the end of the line, it actually starts something new. With oxygen present, the organic materials break down to release certain gases. Yet when oxygen is removed from this chain of events, there is a difference in the gaseous harvest — methane is an end-product that is highly valuable as an energy source. This multi-step disintegration process is referred to as anaerobic digestion. A natural occurrence that has been going on for aeons, anaerobic digestion is replicated now by human technology. The rotting tissue that activates the digestive activity can be nearly any biodegradeable substance, though some are better CH4 producers than others. Egg shells, coffee grounds, watermelon rind, dead rabbits, combined corn stalks, rotten apples, goose droppings, fallen leaves and horse manure — and numerous other examples — count as digestible organic matter. Finally, biogas, i.e. a mix of methane, carbon dioxide, water vapor and a few other minor gases, is released from the substrate heap.

Biogas collection

Although digesters are built to different scales and purposes, each operates on the knowledge that the biogas will rise to the top of the digester tank while the liquid and solid substrate — soon to become digestate — occupy the lower regions due to their higher densities. The gas, then, is free to exit the digester component of the system via a gas pipeline that leads to further treatment. The leftover digestate can consist of solid matter, liquids or a combination of both. Digestate can become a variety of useful applications, not the least of which is a beneficial fertilizer for agricultural crops. Other uses for digestate include animal bedding and base material for bioplastics and other manufactured goods.

Separating biogas compounds

If biogas is to be exploited in a manner that neither threatens the public health nor the vessels that hold and transport it, then certain compounds are best eliminated from the content. Sulphur compounds, siloxanes, water vapor and carbon dioxide (CO2) can each be problematic to the transport, storage and combustion of the methane in biogas. Some, like hydrogen sulfide, can induce serious respiratory problems — even death — when humans are exposed to it. Siloxanes can disrupt the operation of combustion engines while CO2 inhibits efficient storage and transport of biogas. Pressing a stream of gas through liquid solutions, the ammonia-derived contents which absorbs the harmful gas molecules, is but one effective biogas separator. Another removal procedure involves cooling and pressurizing the biogas in a way that condenses and frees the contaminant compounds from the CH4. Membranes of diverse materials allow smaller pollutant gas molecules to pass through while trapping the larger CH4 molecules. Finally, treated solids like activated carbon can attract contaminant molecules to their surfaces thereby improving biogas quality.