Rural Electricity Resource Council

The recovered biogas can be utilized in a variety of ways. The gas consists of 60-80 percent methane, with a heating value of approximately 620 - 800 Btu/ft3. If the economic evaluation shows that it is not cost effective to invest in equipment to generate electricity, or if the maintenance and labor involved in operating the generator hampers other farm activities, it's best to flare off the gas or use it as a heat source. This approach might also be the best for any new installation. Once the methane recovery process is up and running for at least a year, the decision to add a generator can be made.

Flaring Off The Gas - In many cases, the objective in using a methane digester is odor control. If so, simply discharging the gas in a safe manner is all that's needed. There are three reasons why a flame is used, rather than simply letting the gas escape into the atmosphere. The first is safety. Although methane is lighter than air, there is the chance that this combustible gas could travel or be confined in an area where it creates a fire or asphyxiation hazard.

Another reason to ignite the gas is to visibly monitor the process. If a flame is visible at all times, it means that the digestion process is working.

The third reason is environmental protection. Methane is classified as a greenhouse gas, which means it can degrade the earth's layer of ozone in the atmosphere. Burning the biogas creates carbon dioxide. Although CO2 is also known as a greenhouse gas, it is significantly less harmful to the environment than methane.

Biogas As A Heat Source - As an alternative to flaring the gas, it can also be contained and used as fuel for a boiler, space heat, or to power refrigeration equipment. It may also be directly combusted as a cooking or lighting fuel. Most equipment that uses natural gas, propane, or butane as fuel can be fueled by biogas.

Biogas can also be used directly on-site as a fuel for facility operations. Equipment that normally uses propane or natural gas can be modified to use biogas. Such equipment includes boilers, heaters, and chillers.

Boilers and Space Heaters. Boilers and space heaters fired with biogas produce heat for use in the facility operations. Although this may not be the most efficient use of the gas, in some situations it may be a farm's best option.

Chilling/Refrigeration. Dairy farms use considerable amounts of energy for refrigeration. Approximately 15 to 30% of a dairy's electricity load is used to cool milk. Gas-fired chillers are commercially available and can be used for this purpose. For some dairies, this may be the most cost effective option for biogas utilization.

Other energy use options may exist. For example, a nearby greenhouse could be heated with the biogas, and carbon dioxide from the heater exhaust could be used to enhance plant growth. These options need to be evaluated on a case-by-case basis.

To make another selection from the Components of a Biogas System, click here.

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