One use for the methane gas is to generate electricity for on-farm use, or for sale to the electric utility. Generating electricity requires a significant investment in equipment, controls and the labor to maintain the system. Before pursuing this option, it's vital that an evaluation be done to determine the expected return on investment and management time needed to keep the system running. Key factors to study are listed below:

• What type of energy does the farm use (propane, electricity, diesel, etc.)?
• How much of each energy type does the farm use and when?
• Does on-farm electricity production provide a greater advantage than using the biogas to replace propane or other heating fuel?
• What price will the local electric utility pay for farm-generated power?

If the evaluation verifies that generating electricity is the best use of the methane gas, a well-planned generation system must be assembled. Typical electricity generation systems consist of: (1) an internal combustion (IC) engine; (2) a generator; (3) a control system, and (4) an optional heat recovery system. Each component is discussed below:

1. Internal Combustion (IC) Engine. Natural gas or propane engines are easily converted to burn biogas by modifying carburetion and ignition systems. The most successful engines are industrial natural gas engines that can burn wellhead natural gas. A biogas-fueled engine generator will normally convert 18-25% of the biogas energy into electricity.

2. Generator. There are two types of generators that are used on farms: induction generators and synchronous generators.

• Induction Generator--which must operate in parallel with power provided by the utility, and cannot stand alone. An induction generator must derive its phase, frequency, and voltage from the utility. Negotiations with a utility for intertie of a small induction generator are generally much easier.
• Synchronous Generator--which was operate either isolated or in parallel with the utility. If operated as a stand alone system, the synchronous generator can be the sole supply of electricity to the farm to offset purchased power. This on-farm use will yield greater savings than sales back to the utility, because the per-kilowatt hour value is higher. Synchronous parallel generation with the utility requires sophisticated intertie equipment to match generator output to utility phase, frequency, and voltage. This is typically more expensive than controls for an induction generation.

3. Control System. Controls are required to protect the engine and to protect the utility. Control packages are available that shut the engine down due to mechanical problems such as high water temperature or low oil level. The control system will also shut off the engine if the utility power is off, or if utility electricity is out of its specified voltage and frequency range. It is important to note that the control system must operate in the moist and often dusty farm environment, and excess electronic automation often fails where simple manual and mechanical controls usually succeed.

4. Waste Heat Recovery. Approximately 75% of the input energy to an engine is rejected as waste heat. Therefore, it is common practice to recover engine heat for warming the digester and providing water and space heat for the farm. Commercially available heat exchangers can recover heat from the engine water cooling system and the engine exhaust. Properly sized heat exchangers will recover up to 7,000 BTUs of heat per hour for each kW of generator load, increasing energy efficiency 40-50%.

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

To return to the main Agricultural Methane Recovery menu, click here.