Biogas System

As countries, cultures and economies aim to move away from fossil fuels in favor of energy sources more benign in atmospheric impact, they have an array of options from which to choose. Solar panels, wind turbines, hydropower dams and geothermal units dot the various landscapes, reflecting a shift in thinking about how power is produced. Yet another naturally occurring phenomenon is taking center stage among renewable energy actors — biogas. Emerging from the oxygen-free decay of organic substances, biogas is proving to be a reliable stand-in for fossil fuels. Acknowledging this development, scientists continue to work on systematizing its production.

Where does biogas originate?

Biogas can come forth from wastewater and sewage; cow and pig manure; food waste and garbage; dead tree limbs and dead forest creatures; and a host of other organic materials. Essential is that this matter decomposes cut off from any and all oxygen supplies. Thus biogas is created consistently under natural circumstances: under a pile of fallen leaves or within a heap of grass clippings, for instance, or in a swamp or the corner of a barn. Whether made spontaneously or by design, biogas consists mostly, though not exclusively, of methane (CH4) and carbon dioxide (CO2). The methane is key to its energy capacity. Additionally, biogas contains small percentages of water, nitrogen, hydrogen sulfide and other volatile organic compounds. In fact, raw biogas has a very similar chemical composition to the natural gas extracted from mineral deposits — without the greenhouse gas emissions. Though miniscule among the overall content, these lesser components can do damage to public health, tanks, pipelines and the general efficiency of the biogas. Nature can not remove them but people can.

From manure to renewable energy

Using swine manure as a model organic material — sometimes called substrate or feedstock — the first order of business is to supply an anaerobic digester, or biogas reactor, with an appropriate amount of manure to yield the needed quantity of biogas. Manure from pigs is ideal since it is stored as slurry, keeping oxygen from infiltrating it. Adding water also helps to dilute the ammonia content; ammonia can suppress methane production. Pre-heating the manure (to 95 degrees F or more) can likewise expedite the digestion process. The feedstock enters the digester through an inlet. Within the digestion tank itself, an agitator begins rotating to more evenly distribute the microorganisms throughout the substrate. Extension researchers at the Pennsylvania State University conclude that pig manure substrate requires seven to 10 days retention in the digester to allow for maximum biogas production. Once digestion runs its course, biogas is collected for further treatment. The substrate — now called digestate or effluent — can serve as a highly nutritious fertilizer, animal bedding or manufactured product components.

Gas Cleaning

Removing toxic and corrosive chemical compounds from biogas has many variants. On a farm biogas system, the gas stream is run through iron shards or steel wool. More sophisticated membranes may be employed at other facilities, e.g. water treatment plants. Alternately, the gas stream can flow through an amine solution that absorbs the extraneous compounds or be adsorbed by a chemically treated solid like activated carbon. The type and extent of cleansing depends on the purpose of the biogas. Biomethane, for vehicle fuel, calls for the most exhaustive refinement.