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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 deomposes 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. However, biogas can also arise from human arrangements, like landfills and compost piles. We, then, can effectuate the generation of biogas.
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, that is. 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.
So, where do these gases, desirable or detrimental, come from? As organic matter disintegrates without oxygen, bacteria form that break the sunstances down over the course of multiple chemical reactions. As chemical compounds morph from one to another, the final reaction is the emission of biogas. As engineers and technicians develop technology to expedite this anaerobic digestion, as the process is known, they also discover more applications for biogas as well as the means to remove the tiny but pernicious trace compounds. All of this is an effort to build a comprehensive and optimal biogas system.
From Manure to Renewable Energy
Using swine manure as a model organic material — sometimes alled 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. Remember though that the digester needs space for the gas to separate from the substrate, space often afforded by an arced top. 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 supress 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 — think of a clothes washing machine — to more evenly distribute the microorganisms throughout the substrate. This also serves to diminish the size of any solid particles and thereby minimize digestion time. The slurry can be pumped in or drawn in from beneath the slats of the swine housing; from a lagoon; or from a intermediary pre-treatment vessel.
On a mid-size to large hog farm, continuous biogas reactor types are likely employed due to the substantial amount of manure produced. This means the digester is supplied with new feedstock at least two to three times each day. Using pig manure, extension researchers at the Pennsylvania State University conclude that this type of 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 sunstrate — now called digestate or effluent — can serve as a highly nutritious fertilizer, animal bedding or manufactured product components. The biogas, meanwhile, may need cleaning.
Removing toxic and corrosive chemical compunds from biogas has many variants. On a farm biogas system, the gas stream is run through iron shards or steel woll. 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. At any rate, purification is a common component in most biogas installation apparatus.
A biogas generator set can be small and fitted for single household use. On the other hand, a large biogas plant can supply renewable natural gas for hundreds of transportation vehicles or afford a power source for a public electricity utility. While sale and use may vary, each system requires a way to receive the substrate; an oxygen free digestion tank; separate collection areas for gas and digestate; and an element to scrub the biogas of harmful contaminants.