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Part 2: Biogas Mass Flow Measurement–The Challenge

Sep 05, 2013

In my previous posts, I shared the focus on the growing need for renewable energy sources in the United States, with biogas becoming a hot commodity. Common sources include harnessing biogas from landfills, as well as digester gas produced at wastewater treatment plants.  One of the major flow measurement challenges in biogas applications is the fact that the composition of biogas varies depending on the source.  Many thermal flow companies have entered this renewable energy market, but few have been able to solve this challenge of adjusting for changing gas composition. This is the achilles heal for most thermal companies. How can the flow meter automatically adjust for changing gas compositions? The answer is, it can’t. It needs to be sent back to the factory for recalibration.

Varying Composition of Biogas

Biogas typically contains about 55-65 percent methane, 30-35 percent carbon dioxide and some hydrogen, nitrogen and other impurities. However, a representative compositional analysis (in volumetric percentage), shown in Table 1, shows the wide ranges in methane composition between 50-75 percent and carbon dioxide between 25-50 percent. This represents how the biogas composition can change with time and changing conditions in the landfill or digester tank. Such variable composition makes biogas very difficult to accurately measure. Most flow meters are calibrated for one specific gas mix composition, which means they cannot provide accurate mass flow meter readings if the composition changes without sending the meter back to the factory for recalibration.

Table 1 : Typical Compositional Changes in Biogas

The Problem: How to measure biogas and manage variable composition

Since the biogas composition is critical to its energy producing value, facilities need to assess the best flow meter measurement technology to manage the compositional changes in biogas. Many companies with varying technologies are interested in measuring the biogas as it leaves the landfill or digester tank, but this is a challenging application for many reasons. These include:

  • Varying gas compositions (see Table 1) make accurate measurements difficult because most meters are calibrated for one gas or mixture; when the composition changes, the flow measurements are no longer accurate and the meter must be recalibrated.

  • Low pressure makes differential pressure  devices like orifice plates unsuitable since they require a fairly large differential pressure to operate.

  • Biogas is often very dirty, with a high moisture and particulate content that can clog up devices like annubars and orifice plates and gum up turbine meters and similar instruments that have moving parts.

In Part 3 of this biogas measurement series, I’ll share how advancements in thermal flow meter technology have actually not only made composition management possible, but highly accurate as well. While I go into more detail in my white paper, Precise Biogas Flow Measurement, I can tell you that few flow meter systems are available on the market today that can account for variable biogas composition.

How have you solved the problem a measureing varying biogas compositions at your facility?

Scott Rouse, Product Line Director
Written By:
Scott Rouse, Product Line Director
Sierra Instruments

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