Case Study
Digester Biogas Flow Measurement
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•Case Study: Digester Biogas Flow Measurement
By Steven Craig, Sr. Member Technical Staff
Fluid Components International
The engineers at a municipal wastewater treatment utility were expanding their biogas digester system and were seeking an improvement over their existing, maintenance-intensive flow metering technology. They needed to measure the amount of gas contained within a gas bag as well as the flow rate of the gases traveling from the gas bag system into a co-generation power system.
Digester gas is a mixed composition, consisting of methane, CO2 and trace gases. The amount of biogas flowing is a critical process parameter in order to prevent an excessive build-up of gas in the digester, to minimize excess waste gas flaring into the environment, to ensure operation of the co-generation engine system and to prevent the release of the potentially hazardous gas that might threaten plant personnel or send high levels of obnoxious odors into areas outside the plant.
Such digester gas bag systems are designed as domeshaped, engineered membrane stations to hold methane biogas, provide for sludge storage and prevent odors. There are multiple types and manufacturers of gas bags. A typical bag system includes two durable membranes. The outer membrane is cable restrained and remains inflated in a fixed position. The inner membrane moves freely as it stores or releases gas generated from an anaerobic digestion process.
The utility measures the gas flow in and out of the digester gas bag system with flow meters connected to a primary programmable logic controller (PLC). The flow meter and PLC provide a real-time, running calculation of the gas quantity in the gas bag system. The PLC then communicates with and sends commands to a distributed control system (DCS) to take actions based upon the quantity of gas.
In the event of excessive biogas (methane) build-up, a system alarm notifies the operations staff and the primary PLC issues a command to the DCS telling it to take action by using the extra gas in one of several ways:
- Increase the load for an online co-gen power engine.
- Start a secondary co-gen power engine.
- Increase the load of both online co-gen power engines.
- Send it to a flaring stack and ignite the flare to burn off the excess gas.
•Case Study: Digester Biogas Flow Measurement
SOURCE: Fluid Components International
