Biogas always under control!

OPTISONIC 7300 Biogas is a dedicated flowmeter for biogas applications from Krohne. The device is a specialised version of the OPTISONIC 7300 ultrasonic gas flowmeter for process gas applications.

To flow measurement, biogas presents a number of challenges such as low pressure and low flow rates and changes in gas composition: it contains 50-70 vol% of methane depending on the bio feedstock and process. The remaining part is CO2 (30-50 vol%) and small amounts of H2S, O2 and N2. While further downstream the gas may be compressed to 100 mbarg, the pressure is typically only 50-60 mbarg when measuring raw biogas e.g. directly after the fermentation. Here, biogas is also saturated with water and when the gas cools down, 5-6 vol% of free liquid water may appear in the pipeline. Temperature typically ranges from little over ambient temperature up to 50°C / 122°F. Backflow may appear in this applications, also scaling of bacteria can occur in the flowmeter. Also, process conditions are variable, e.g. when measurement is done directly after the fermentation, sudden temperature drops can occur when new sludge is fed into the fermentation tank.

Various flow measurement principles have been tested in these applications: while DP flowmeters (orifice plates) are not used due to the high pressure drop provided, Vortex flowmeters struggle with very low pressures and flow velocities, and thermal mass flowmeters are limited when water is present in the gas. Although ultrasonic is the only principle that is able to measure backflow, ultrasonic flowmeters are challenged by high damping of the acoustic signals by CO2 and high water content. In combination with a high price in relation to the other principles this has limited their use for biogas so far.

This unit has been specially designed to overcome the limitations of the ultrasonic principle here: while its signal transducers provide a stronger ultrasonic signal into the gas, its specialised signal processing allows for a better detection of small, strongly dampened acoustic signals. This and the mechanical design of transducers and transducer pockets makes the measurement insensitive to liquid water and biological scaling.

It features a maintenance-free full bore flow sensor without any moving parts. As H2S provides a corrosive solution when dissolved in water, the measuring tube is made of stainless steel 1.4404/316L and transducers are made of NACE-approved grade 29 Titanium for maximum corrosion resistance. OPTISONIC 7300 Biogas is equipped with lap joint flanges to lower its overall weight as biogas does not present high pressures and sometimes only plastic piping is used. The measurement accuracy is specified at 1% of actual measured flow rate, when calibrated with air, starting from 1 m/s / 3.28 ft/s flow velocity. When 2 % accuracy is sufficient, dry calibration can be selected as alternative at a significantly lower price.

Since the methane content of biogas may vary, it may be required for the operation of the biogas facility to know the exact methane content, e.g. when used as fuel for gas engines in a CHP unit. It features a built-in temperature sensor that in combination with the measured velocity of sound, enables direct measurement of methane content via calculation of the molar mass. OPTISONIC 7300 Biogas can also provide calculation of gas flow volume to standard conditions by using an additional pressure sensor. The electronics also include diagnostics to validate the functions of the flowmeter and the process. For output of the measured values, 4…20mA and pulse outputs are available. HART and Modbus (optional) are included for field communication.

The instrument is certified for use in explosive areas (zone 1). As in general no pressure drop is allowed in biogas pipelines, the device is available in common line sizes DN50, 80, 100, 150, 200 / 2, 3, 4, 6 and 8".