Fangel Bioenergy, located in Odense, Denmark, is a producer and supplier of biogas and biomethane. The company generates around 20,000,000 m³ of biogas annually from livestock manure sourced from nearby farms and organic residual materials from local food manufacturers.

The biogas is primarily used in biogas engines to generate electricity for the power grid, and heat for consumers connected to a local district heating network. The company plans to primarily supply biomethane to the national gas grid in the future, making its current biogas engines part of the country’s backup power capacity to help balance the electricity market when solar and wind generation are insufficient.

Given its production capacities, the company transports biogas from the digester to the biogas cleaner through a large DN300 (~12") steel pipe. Immediately after the digester, the biogas typically consists of 65% methane and 35% CO₂ and is fully saturated with water. The operating pressure is low, around 100 mbar / 1.45 psi, and process temperatures range between +20…40°C / +68…104°F. Because the pipe runs outdoors, the biogas cools as it flows, resulting in significant condensation inside the pipeline.

The biogas producer required a reliable, long-term stable solution for flow measurement. The combination of high humidity, variable gas composition, high CO₂ content, low pressure, and the relatively large pipe diameter makes this a challenging application for most flowmeters, limiting the suitability of several measurement principles. Previous tests with an oscillator gas flowmeter failed to produce satisfactory results.

Having considered and tested other flow measurement methods, the biogas producer ultimately opted for an ultrasonic flowmeter, as this principle proved most suitable for the demanding application. While commercially available ultrasonic biogas flowmeters had previously been limited to line sizes up to DN200 (~8”), KROHNE was able to provide the OPTISONIC 7300 Biogas flowmeter in DN300 (~12”) for this application.

The ultrasonic biogas meter is designed to handle varying gas compositions and introduces no obstruction to the flow. As a full-bore meter, it measures without any pressure loss. Its sophisticated electronics efficiently process the ultrasonic signals that travel through the pipeline, which can be significantly dampened by CO₂ content and condensate. For this reason, the OPTISONIC 7300 Biogas was equipped with adapted transducers, optimising signal strength in challenging process conditions with high moisture and CO₂ content. To enhance robustness and resist corrosion from hydrogen sulphide (H₂S), the transducers are made of titanium without any plastic parts.

To compensate for temperature and pressure fluctuations affecting gas volume flow, the meter includes integrated pressure and temperature sensors. This enables the device to output both standard volume flow rate and totalised standard volume flow. In addition, the flowmeter can estimate methane content based on temperature and the measured velocity of sound, which reflects variations in gas composition and the differing molar masses of the constituent gases.

The customer benefits from an ultrasonic flowmeter capable of measuring in large biogas pipelines exceeding DN200, which is unique in this meter class. Despite challenging process parameters such as the large pipe diameter, high CO₂ content, and significant condensation, the OPTISONIC 7300 Biogas provides reliable, temperature- and pressure-compensated flow measurement without the need for additional measuring devices.

This performance is achieved through a combination of advanced signal processing and an adapted design of the titanium transducers, which significantly mitigate the influence of CO₂ on the ultrasonic signal. The OPTISONIC 7300 Biogas operates without pressure drop, offers a high turndown ratio, and performs independently of gas composition. In addition, the ultrasonic flowmeter is inherently drift-free and requires virtually no maintenance.

The customer also benefits from integrated methane content measurement. This provides the customer with a useful trend indication of the biomethane yield in the biogas, eliminating the need for accurate, but expensive and maintenance-intense extractive analysers at the installation.