ANAEROBIC DIGESTIONPAK RENEWABLE ENERGY

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A digester is a sealed vessel or container in which anaerobic digestion of organic matter occurs. The bacteria "feed" off the manure and, in the process, release biogas as a by-product. This process is referred to as anaerobic digestion, and the sealed vessel or container is thus usually referred to as an anaerobic digester. Anaerobic digestion also occurs in the anaerobic zones of open or unsealed swamps, bogs, and wastewater lagoons.

Today, farmers in developed countries are using digesters primarily to improve the quality of their manure and to reduce manure odors, the energy content of the methane being simply a by-product. However, as the price of energy increases, more farmers are looking at using anaerobic digestion as a way to generate supplemental heat and electricity for their farms. Digesters are a popular technology in rural areas of the developing world, where electricity and petroleum fuels are often unavailable or unaffordable.

Definition

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What Is a Biogas?

Biogas is a by-product of the anaerobic (without oxygen) breakdown of organic matter. The organic matter could be any of a number of materials, but on the farm, it most often comprises animal manure or other agricultural waste.

The most important component in biogas is methane, a flammable gas that can be used in furnaces, for cooking, or even as an engine fuel. However, biogas also contains carbon dioxide and small amounts of hydrogen, hydrogen sulfide, nitrogen, and water vapor.

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How Does a Digester Work?

Fresh biomass entering a digester is supplied with anaerobic bacteria by the existing digested biomass, which is tremendously rich in these microbes. The digester tank provides a conducive environment for anaerobic microbes to "digest" the biomass, resulting in digested solids, liquids, and biogas. In general, the anaerobic digestion is a living process, requiring favorable conditions (temperature, moisture content, oxygen exclusion, and pH) and a steady food supply in order to flourish.

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Manure from this dairy barn is automatically collected and delivered to a nearby anaerobic digester

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What Goes into a Digester?

Livestock manure is the most popular material, or feedstock, for anaerobic digestion on the farm, but almost any type of organic matter can be digested, including food waste, forestry residue, animal processing waste, and field crops.

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How Is Biogas Used?

Biogas generated from anaerobic digestion processes is a clean and environmentally friendly renewable fuel. There are many uses for this fuel, including use in engines, generation of electricity, heat and hot water systems, and even refrigeration.

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Biogas recovery system components

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Manure Collection Systems

In general, biogas recovery systems are technically feasible where manure is handled in a liquid, slurry, or semi-solid state:• Manure Management Systems that are Compatible With Biogas Recovery• Dairy Flushed or scraped free stall barns and dry lots.

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Chain drag scrape alley.

Receiving pit for flush dairy dry lot.

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Types of Anaerobic Digesters

Basic Types:

While many different types of biogas recovery systems are available, the three designs most commonly used are described below.

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Covered anaerobic lagoon

An anaerobic lagoon is sealed with a flexible cover, and the methane is recovered and piped to the combustion device. Some systems use a single cell for combined digestion and storage.

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Plug flow digester

A plug flow digester has a long, narrow concrete tank with a rigid or flexible cover. The tank is built partially or fully below grade to limit the demand for supplemental heat. Plug flow digesters are used only at dairy operations that collect manure by scraping.

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Complete mix digester

A complete mix digester is an enclosed, heated tank with a mechanical, hydraulic, or gas mixing system. Complete mix digesters work best when there is some dilution of the excreted manure with water. The photo above shows an externally mounted mixer.

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Additional digester typesSeveral other digester types have also been constructed in recent years, such as induced blanket reactors, fixed film digesters and batch digesters.

• Induced Blanket Reactors are digesters in which a blanket of sludge develops that retains anaerobic bacteria, providing a bacteria-rich environment through which influent must pass.• Fixed film digesters contain plastic media (e.g., pellets) on which bacteria attach and grow, instead of relying solely on suspended bacteria to break down the digester influent.• A batch digester is the simplest form of digestion, where manure is added to the reactor at the beginning of the process in a batch and the reactor remains closed for the duration of the process.

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Biogas Handling Systems

Captured biogas is transported via pipe from the digester, either directly to a gas use device or to a gas treatment system (e.g. for moisture or hydrogen sulfide removal). If the manure contains high concentrations of sulfur, then hydrogen sulfide is removed from the gas to prevent corrosion of the combustion device. In most cases, however, the only treatment needed is to remove excess moisture prior to combustion.

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Gas Use Devices

While a variety of gas use options are available, the collected biogas is most often used to generate electricity for on-farm use or sale to the local electric utility. Thermal energy in the form of waste heat, produced during electricity generation, can be recovered to heat digesters or adjacent buildings. Other uses include:• Heat generation by burning biogas in boilers• Upgrading biogas to pipeline quality• Converting biogas to compressed natural gas (CNG) for a variety of fuel applications

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A 120 kW engine generator set combusts recovered biogas, generating electricity for on-farm use.

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A heat exchanger works in conjunction with an engine-generator set to recover heat for space and water heating, thus reducing fuel oil or propane costs.

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Biogas can be fired directly in boilers or heaters as a replacement for propane

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Flares are used to combust excess biogas or to combust biogas during periods when the primary gas use device is undergoing maintenance or repair. In cases where the primary purpose of the digester is odor control or the generation of carbon credits, all of the biogas may be flared.

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Biogas can be cleaned to pipeline quality and sold to the local gas utility

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Biogas can be converted to compressed natural gas (CNG) which can be used in applications such as vehicle fuel.