Biogas is produced at many industrial and municipal facilities and there are many opportunities to measure and improve the processes. Biogas in this post can refer to biogas plants, landfill gas and digester gas, all of which typically consist of 60-70% methane and 30-40% carbon dioxide. These gases are used to fuel boilers that produce heat or steam and to fuel generators or gas turbines to produce electricity.
Responsible energy management through level and flow instrumentation reduces environmental impact, maintains safe operations and reduces energy costs. Here are a few applications where biogas plant technology can help you achieve these goals.
Biogas is primarily produced by the biological breakdown of organic matter in the absence of oxygen – a process known as anaerobic digestion. Feed materials include biomass, manure, sewage, plants and plant waste, grease trap contents and the organic fraction of municipal solid waste.
In all forms of biogas production, safe and reliable gas flow measurement is essential in the collection, disposal or re-use of biogas. Biogas flow measurement can be especially challenging, because the instrumentation selected must provide low flow sensitivity, low pressure drop and tolerate temperature and pressure changes.
A few example applications are measuring digester gas flow and landfill gas. Measuring digester gas at wastewater treatment plants has allowed users to size generators needed to produce electricity or provide fuel flow to boilers to create heat or steam. Excess digester gas must be measured before it is flared. This gas reuse greatly reduces energy costs for the plant and reduces greenhouse gas emissions to the environment. Similarly, landfills need flow meters to measure flare gas or assist in creating their own electricity to feed back to the grid. These facilities may also participate in carbon credit programs that are verified by the flow measurement.
The most frequently used flow meter for these applications is the Thermatel® TA2 thermal mass flow meter. Thermal mass flow technology is based on heat transfer and not differential pressure. This allows a strong signal at low flows and low pressures. It also has no moving parts in the pipe and outputs flow rate that is independent of process temperature and pressure variation. If a simple relay or alarm output is needed, rather than continuous measurement, then a Thermatel TD1 or TD2 switch can be utilized.
Scrubbers are essential in gasification processes, because they remove odors, pollutants, acid gases and chemical wastes from biogas. Accurate level monitoring of the scrubbing water calls for a control that can automatically feed the correct amount of make-up water to the recycle reservoir either continuously or on a periodic basis. The level monitoring device for water-out control should be equipped with a level alarm.
Guided wave radar (GWR), such as the Eclipse® Model 706, or an ultrasonic transmitter can provide a reliable continuous level control solution. A float level switch, such as the Tuffy® II, or an ultrasonic level switch is recommended when point level control is required.
In the U.S., hundreds of landfills recover methane, which forms when waste decomposes in low-oxygen conditions. A landfill gas-to-energy system consists of a series of wells drilled into the landfill. A piping system connects the wells and collects the biogas.
As biogas emerges from a digester or a landfill, it is saturated with water that causes corrosion problems upon condensation. Dehydration systems using air, vacuum and desiccant processes to remove water typically include a holding tank for water drawn off the gas with a level control actuating a valve to vacate the tank at high level. Biogas is dehydrated according to the user’s specifications for maximum water content. Some uses, such as boiler fuel, require an extremely dry gas.
A displacer transmitter, such as the E3 Modulevel®, or GWR technology is the most suitable continuous level control for this application, while a displacer switch, such as the A15 Top Mounted Displacer Switch, can provide the point level control that this application requires.
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