Biogas Energy

Farms all over America have the opportunity to produce their own electricity from animal feedlot operations and reduce environmental problems that stem from the animal waste: by installing biogas digesters on site. Biogas is produced when manure decomposes anaerobically (without the presence of oxygen). Methane makes up about 60-70% of that biogas, and it can be used as a clean burning fuel. Biodigester systems collect manure, capture the gas and burn it to produce electricity. The electricity can be used to run farm operations and the collected waste is prevents water contaminations, odor pollution and greenhouse gases from reaching the atmosphere.

How It Works

Biogas recovery systems have a few main parts: a manure collection system, an anaerobic digester which optimizes the manure for methane production, and a biogas collection system which then pipes the biogas to the gas use device (e.g. a generator). Biogas is most commonly used to produce electricity for on-farm use, and any excess electricity can often be sold back to the electricity grid, or biogas can be used as boiler fuel for space or water heating.

Covered anaerobic lagoons are the most common units used. They have a cover installed over the lagoon, methane is recovered and sent though a pipe to the combustion device. They have the simplest design but are only viable below the 40th parallel because warmer weather is needed to maximize methane production using these systems.

Plug-flow digesters are long and narrow heated tanks, often installed partially underground to retain heat. These units work only with dairy manure that is collected by scraping.

Complete mix digesters are heated tanks with a mechanical, hydraulic or gas mixing system, and generally require a diluted manure mix (e.g. process water can be used).

Where It’s Used
Large dairy and swine operations have the greatest potential to produce significant amounts of methane, and therefore electricity, at least cost because their manure is can be collected and stored as a liquid, slurry or semi-solid, unlike other livestock industries.

AgSTAR, a program administered jointly by the U.S. Environmental Protection Agency, the U.S. Department of Agriculture and the U.S. Department of Energy, promotes methane recovery systems at confined animal feeding operations (CAFOs) all across the country.

How Much It Costs
Profitability of biogas digesters largely depends on the size of the operation, the method of manure management and local energy costs. According to AgSTAR, biogas recovery becomes profitable for operations of at least 500 cows or 2,000 swine. Manure should be collected in a liquid, slurry or semi-solid state and be collected frequently (at least weekly). Generally the electricity produced is used on-site and any excess can be sold back to the local electric utility.

Farmers who reduce greenhouse gas emissions from their farms can be paid money by carbon offset companies for using biodigester systems that capture greenhouse gas emission. Carbon offset companies have emerged in recent years in response to the growing demand from private companies and individuals who want to reduce their environmental impact by “offsetting” the emissions they produce from their every day activities. Carbon offset companies sell “carbon credits” to companies/individuals and with that money they pay people who reduce greenhouse gas emissions, such as farmers who use biodigesters to capture methane emissions.

Advantages
Odor control is significantly enhanced by biodigesters, because they reduce the volatile organic compounds in manure to odorless methane and carbon dioxide. Hydrogen sulfide (“rotten egg”odor) is captured in the biogas and destroyed during combustion.

Water protection is provided by biodigesters because they prevent 90% of disease-causing bacteria from entering surface waters. Digesters also reduce chemical oxygen demand (COD) which is a measure of the ability of organic wastes to remove oxygen from waters. Aquatic species depend on dissolved oxygen in water for survival, so farm practices that reduce COD help protect aquatic ecosystems.

Greenhouse gas reduction is critical in mitigating climate change, and methane is one of the most potent greenhouse gases: 21 times as powerful as carbon dioxide in trapping heat. Seven percent of methane emissions in the US come from livestock and poultry manure, the majority of which is from swine and dairy operations. Biodigesters eliminate nearly all methane emissions, and as a renewable source of energy they reduce our reliance on fossil fuels.

In the EPA’s Biodigester Market report (Feb. 06), Ag Star found that about 7,000 farms could utilize biodigesters cost effectively, providing about 700 MW of energy and reducing greenhouse gas emissions by about 1.3 million metric tons of methane – the equivalent of taking 4.7 million cars off highways.