Digester Flow Meter for Landfill & Municipal Gas Flow Measurement

• Thermal mass flow meters are critical for monitoring gas process efficiencies and to report emissions in landfills and wastewater processes.
• Upon the collection of landfill gas (biomethane), operators can send it to flare or upgrade into higher value fuels. Otherwise, it can be of use directly in industrial processes to replace fossil fuels.
• Thermal gas mass flow meters are ideal for measuring the quantity of methane that emits from landfills and wastewater digesters.

The production of landfill gas (LFG) occurs from the decomposition of organic material in landfills. These gas mixtures from landfills require monitoring and reporting. Additionally, methane emissions from landfills represent an opportunity to capture and reuse it as a significant source of energy. Accurately measuring these flows is critical to meeting process efficiencies, cost reduction goals and reporting requirements. Furthermore, in wastewater treatment plants, a digester flow meter is necessary for the same purpose.

Municipal Solid Waste

The EPA’s Rule 40 CFR Part 98, since October 2009, requires the reporting of greenhouse gas emissions from large sources and suppliers in the United States. This requirement enables the collection of accurate emissions data to help make important policy decisions. According to the EPA, by 2012, municipal solid waste (MSW) landfills trail only industry and agriculture as significant sources of human methane emissions in the United States. As such, landfills account for almost 20% of all methane emissions in the USA.

When municipal solid waste first deposits in a landfill, it undergoes an aerobic (with oxygen) decomposition stage and very little methane generates. However, this usually changes in less than 1 year of operation. Anaerobic conditions (without oxygen) develop and methane-producing bacteria begin to decompose the waste. Thus, landfills generate increasing amounts of methane as they age. Methane (60–70%) and carbon dioxide (CO2) (30–40%) comprise the majority of most landfill gas with additional trace amounts of nitrogen, oxygen, and other gases. Although methane doesn’t last as long in the atmosphere as CO2, it is a dangerous greenhouse gas with a global yearly warming. For example, methane is 25 times greater of a greenhouse gas than that of CO2.

Methane Biogas for Energy Production

Instead of allowing the landfill gas to escape into the environment, operators can collect it and use it as an important and convenient energy source. By using the LFG, municipalities are eliminating odors and reducing other hazards in associates with the LFG emissions. At the same time, they are preventing methane from entering the atmosphere. Thus they limit its contribution to local smog and global climate change.

Operators typically use a series of wells and a blower / flare (or vacuum) system to extract and collet landfill gas. The LFG then directs to a central point for processing and treatment for its particular use. Then the gas either goes to flare, powers turbines that generate electricity, or replaces fossil fuels in industrial and manufacturing operations. Furthermore, the LFG can potentially receive upgrading to higher quality gas that is suitable for direct combustion. Otherwise, it can undergo further processing into an alternative vehicle fuel. The accurate measurement of the LFG flow by a digester flow meter is a critical aspect of any of these efforts.

Potential points of measurement between the landfill and the cogeneration plant are:

1. individual collection lines
2. common header lines
3. process lines through sumps, knock-out pots, and blowers / compressors
4. oxidizer/flare stacks;
5. supplemental natural gas supply

Digester Flow Meter for Measuring Biomethane Production

Constant temperature thermal mass flow meters, such as those produced by Eldridge Products Inc. (EPI), operate on the principle of thermal dispersion or heat loss from a heated Resistance Temperature Detector (RTD) to the flowing gas. Essentially, two active RTD sensors operate in a balanced state. One acts as a temperature sensor reference, meanwhile, the other is the active heated sensor. Heat loss to the flowing fluid tends to unbalance the heated flow sensor and it is forced back into balance by the electronics.

With this method of operating the constant temperature sensor, only the fluid flow heat loss affects the skin temperature. Thus, it allows the sensor core temperature to be maintained and produces a very fast response to fluid velocity and temperature changes. Additionally, with power only applying as necessary, the technology has a wide operating range of flow and temperature. The heated sensor maintains an index of overheat above the environmental temperature sensed by the unheated element. This virtually eliminates the effects of variations in density by molecular heat transfer and sensor temperature corrections.

Specifying the Requirements for a Landfill and Digester Flow Meter

Operators must consider a number of factors when selecting and specifying any instrumentation. This is true for thermal mass flow meters for use in LFG systems. To specify the best configuration, operators must determine:

What are the flow measurement conditions, such as the minimum and maximum flow rates, the process pipe size, and the gas temperature and line pressure?

Every landfill and digester flow meter has a minimum and maximum flow limit for any given pipe size. In addition, it will require specific temperature and pressure ranges for the physical construction. Assuring that the flow meter meets these basic requirements is the first step in specifying the proper thermal mass flow meter. These parameters will determine the calibration scale and accuracy, as well as helping to identify potential issues with the overall installation.

Where will the flow meter be installed and what is the piping configuration upstream and downstream of that location?

The flow readings will be their most accurate where the gas flow profile in the pipe is uniform and consistent so that the sensor output at the point of measurement is truly representative of the overall flow through the pipe. All instrument manufacturers recommend straight run requirements for the installation of flow meters. These recommendations intend to help end-users determine suitable locations for the flow meters. However, it is important to recognize that these are only guidelines and not guarantees of optimal positioning.

Is there moisture present at the point of measurement?

By its nature, most landfill gas is “wet” meaning it contains water. Simply stated, thermal mass flow meters will not read accurately* if water droplets come into contact with the sensor RTDs. Although there are successful strategies for minimizing the potential for problems, manufactures advise installing the flow meter at a location where the gas is dry.

*The heat loss to a liquid such as water droplets is much greater than the heat loss to a dry gas. So, the digester flow meter’s signal will typically rise to a higher value producing an error that will remain until the heated RTD is dry again. Although these droplets will affect our flow meters, they will not cause damage to the flow sensor. Non-condensing water vapor in the gas is acceptable.

Master-Touch thermal flow meters comply with the requirements of 40 CRF 98.34 (c)(1). As such, we calibrate each digester flow meter in our own NIST-traceable facility. We use CH4 / CO2 gas mixtures for the best accuracy. Our flow meters provide real-time mass flow measurement and totalization for continuous LFG flow monitoring. In addition, Master-Touch flow meters can store as many as four independent flow ranges for multiple gas mixtures. Furthermore, they have two fully-programmable relays for a variety of notification functions and/or pulse totalization of flow. A variety of sizes and configurations are available to meet virtually any installation requirement.