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High End Gas Analysis in Wastewater Plantsusing INFRA .sens®

Nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) are greenhouse gases that can be produced in large quantities during wastewater treatment. Wi.Tec-Sensorik GmbH has developed a highly accurate and long-term stable multi-channel NDIR gas analyser (INFRA.sens®) for this area of application.

 N2O emissions from wastewater treatment plants generally exhibit large daily and seasonal fluctuations. Due to the considerable spatial and temporal fluctuations in emissions, long-term online monitoring approaches are therefore required. Exhaust gas monitoring systems based on NDIR technology are best suited to measuring emissions due to their low maintenance costs and high temporal resolution. In addition, the monitoring of process emissions makes it possible to quantify the oxygen transfer in the biological treatment process, which leads to process optimization and thus considerable energy savings. 

N2O concentrations are normally between 0 and 300 ppm. However, during the high emission phases of certain plants, concentrations of up to 3000 ppm N2O can be detected. The N2O gas analyser must therefore have a high dynamic range. 

HXXO20230311121740 3000

NDIR INFRA.sens® 

The highly selective gas analysis is carried out using NDIR technology (INFRA.sens®). The INFRA.sens® utilises the known absorption bands for methane, carbon dioxide and nitrous oxide in the 3μm – 5μm range. The radiation source is a blackbody radiator that can be modulated in the frequency range of 1-10Hz. To achieve the highest possible resolution, a 250 mm long analyser cuvette is used, which is coated with a special gold layer. The gold coating has a high reflectivity and leads to an optimum signal level on the detector side. The gold coating also protects against corrosion. The IR detector is located on the opposite side and consists of 4 elements. Interference filters are used to filter out the gas-specific spectral components for the respective detectors. The interference filters have a very narrow bandwidth and an efficient blocking factor. This leads to a very high selectivity and negligible cross-sensitivity to other gas components. 

An additional reference measurement ensures drift-free measurement results. The detection limit of N2O in this arrangement is <0.5ppm, so that even the smallest changes in concentration are reliably detected. 

The entire signal processing takes place in an electronic evaluation unit (base board), which is located below the optical unit. Data is transmitted via an RS232 interface. CAN interface, MODBUS (option) and analogue output 0-10V (option) are also available. 

HXXO20220807162559 3000

Thermobox 

The sample cell is thermostatically controlled to 50°C to prevent condensation of water vapour inside the analysis cell. The entire assembly is integrated into a galvanised sheet steel housing, which is insulated on the inside to shield it from external temperature influences. The gas analyser is specified for an ambient temperature of 5-45°C. The warm-up time is less than 45 minutes. 

O2.sens (Oxygen sensor)

Oxygen-Measurement 

This set-up also includes an electrochemical (EC) oxygen sensor (galvanic cell) for measuring the oxygen concentration in the gas mixture. The O2.sens D has a measuring range of 0-25 vol.% O2 and is very selective for oxygen, even in the presence of other gases in high concentrations. Compared to physical gas sensors, the service life of electrochemical gas sensors is limited due to the chemical reactions in the sensor. The service life is therefore specified in vol.%-h. The typical sensor lifetime is >500,000 vol.%-h. In the presence of 10-20 vol.% oxygen, the calculated service life is approx. 3-6 years. The O2.sens D can communicate with the INFRA.sens® electronics via an I2C interface. Compared to analogue transmission, the I2C interface ensures loss-free and high-resolution transmission of measured values. 

Specificationen

General
Ranges 0-100ppm, 0-500ppm, 0-2000ppm, 0-5000 N2O 
0-1000ppm, 0-2000ppm CH4 
0-1 vol.%, 0-5vol.%, 0-20 vol.% CO2 
0-100 Vol.% O2 
Gas flow 0.1-1.5 l/min. 
Dimensions 345mmx153mmx103mm (Thermobox) 
Weight app. 720g 
Lifetime >40 000h (NDIR) 
>500.000Vol.%·h (EC) 
Warm-up time <45min. (initial) 
Response time t90 1.5s-15s (NDIR) 
<5s (EC) 
Detection limit <0.5ppm N2O 
<5ppm CH4 
< 0.01 Vol.% CO2 
<0.1 Vol.% O2 
Linearity error <1% F.S. 
Supply voltage 15-30 VDC 
Power consumption <35 W@24VDC (Warm-up phase) 
Output signal RS232, Modbus or CANbus 
Operating temperature 5-45°C 
Air pressure 300-1200 hPa (mbar)