TECHNOLOGY

from 2022 Biomass Magazine Issue 2

ASSET MANAGEMENT

CRITICAL MEASUREMENT TECHNOLOGIES FOR LIVE BIOGAS MONITORING

BY NARGE SPARAGES

Biogas has been produced, cleaned up and used for several decades. Also for several decades, millions of tons of biogas has been lost

every year.

However, the focus on optimizing and recovering all the renewable gases produced from biomass, livestock and other waste production has significantly increased over the past few years, driven by climate change and the road to net zero. Still, the growth potential for biogas use is significant.

Today, there are critical measurement solutions that support biogas collection and use, regardless of its source. For example, Panametrics deploys process analyzers to monitor the oxygen content in the anaerobic digester and the carbon dioxide content in the biogas separation process, as well as measure the moisture content of biomethane prior to transportation and end use. Ultrasonic flow meters monitor the natural gas flow rate and determine its methane content to provide a good indication of the gas mixture calorific value.

This article will explain how these technologies work, where they are employed and how they enable producers to optimize operations by accurately measuring biogas and biomethane quality and flow, primarily focusing on the upgrading of the biogas to biomethane.

Separating Carbon Dioxide from Methane

The most useful part of the biogas is the biomethane. Most commonly, membrane filters separate the carbon dioxide and methane. Operators will want to know the ratio of the carbon dioxide and methane going into the separation process, and then the amount of methane on either side of the separation process, as an indicator of the efficiency. Given that carbon dioxide and methane are the two predominant components, this can be treated as a binary gas mixture, and a thermal conductivity analyzer like the Panametrics XMTC is used at all three points to measure percent levels of carbon dioxide in methane.

Carbon dioxide and methane have different thermal conductivities. A thermal conductivity analyzer will have one sensor exposed to air and the other exposed to the sample gas. The transmitter is calibrated with a zero and span that represents pure carbon dioxide or methane, as the zero and the span will be a mixture of the two. The transmitter measures the loss of heat from the sensor exposed to the sample gas compared to the loss of heat to air, and can then easily calculate the concentration of methane in carbon dioxide or vice versa. With no moving parts, this methodology requires little maintenance and is easy to implement.

Water Vapor Doesn’t Burn

Compression of the biogas will drop out the bulk of the water vapor that comes off the biogas collection process. The resultant gas is saturated with water vapor at that pressure. After separating the methane from the carbon dioxide, the biomethane will either be used as a fuel locally or sold into the natural gas grid for pipeline transportation

to its point of use. Moisture in this natural gas can cause pipeline and infrastructure corrosion, and for this reason, there are tariff limits on the amount of moisture that can be present in this gas.

To monitor the dehydration process and ensure that the tariff limits are met, the Panametrics pro.IQ using aluminum oxide sensor technology, or the Aurora tunable diode laser moisture analyzer, provide the versatility and the accuracy needed to meet the industry standards. Aluminum oxide is the lower cost method but requires annual sensor calibration for highest levels of accuracy. Tunable diode laser moisture analyzers do not require this maintenance and are faster to come back online after a moisture upset.

When a dedicated analyzer is not required, spot checks on the drying process can be made using the portable analyzer versions of the pro.IQ and Aurora.

Safety Matters

During the entire collection and upgrading process, air is present and can enter the biogas or biomethane. The oxygen content must be kept below the lower explosive level. An analyzer can alarm when the oxygen level exceeds 2%, giving the operator time to investigate and mitigate any issues. A thermal paramagnetic oxygen analyzer has no moving parts that can be impacted by light mists of water or corrosive constituents that can be found in the biogas. These analyzers use the paramagnetic properties of oxygen to measure percent levels of oxygen in the biogas or biomethane.

In addition to knowing the moisture content in the upgraded biomethane, more customers are insisting on knowing the trace levels of oxygen. Here, galvanic fuel cell technology is best suited for the measurement.

Biogas upgrading skids are manufactured and maintained by skilled artisans, experts in the collection and cleanup of biogas to biomethane. They rely on the expertise of analyzer manufacturers to provide an integrated solution that prepares the sample gas for the measurement of methane, carbon dioxide, moisture and oxygen. Each customer has their individual requirements, and each application has its own nuances.

More Than Just a Flow Meter

Accurate flow measurement of biogas in the upgrading process—from collection through to the clean biomethane—is critical. There are challenges in making flow measurements in the raw biogas. This is low-pressure gas saturated with water and containing impurities such as high carbon dioxide contents (an attenuative gas). Ultrasonic transit-time flow meter technology is the perfect solution. The flow meter has no obstructions in the flow path of the gas, which results in no pressure drop to cause energy loss, nor moving parts that can erode to cause calibration drift. It simply sends and receives ultrasonic signals across the flow path, measuring the time to go against the flow and with the flow, to calculate the velocity of the gas. This smart technology can then be used to calculate volumetric flow. The Panametrics advantage is the transducer design and the measurement algorithms that are used to ensure both accuracy and reliability in the measurement system.

The not-so-hidden benefit of using this technology is that the Panametrics PanaFlow Z1G and Z2G meters employ smart ultrasonic signal calculations to calculate the percentage of carbon dioxide in methane at the same time they are calculating the flow measurement. Just as these two gases have different thermal conductivities that the XMTC can differentiate, they also have different speed of sound characteristics that the meters can differentiate to output the concentration ratio.

Author: Narge J. Sparages Global Commercial Development Leader Process Analyzers Division, Panametrics www.panametrics.com

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