Danish Biogas plant preparing for dynamic FeCl₂ dosing

Background
A Danish biogas plant operator with an annual production of more than 5 million m³ of biomethane wants to optimise his running costs for desulphurisation.

The plant’s primary feedstock is chicken manure, hay and crops from nearby operations, and the sludge is used as fertilizer for the fields. Although only a couple of years old, the plant is already highly successful and therefore in the process of being upgraded to increase the production capacity even further.

Desulphurisation setup
The biogas is purified via a 3-stage desulphurisation process. In the first stage, the operator doses FeCl₂ directly into each of the plant’s five anaerobic digesters. Following this, the gas enters activated carbon filters for further gas desulphurisation before a membrane system upgrades the gas to biomethane for the natural gas grid. The operator currently monitors the H₂S levels in the process gas using a traditional multi gas analyser installed just before the activated carbon filters. The multi gas analyser provides spot measurements of conditioned gas at fixed intervals.

Purpose
Despite being a critical part of biogas operations, desulphurisation is an often-overlooked optimisation parameter due to the difficulties involved with acquiring accurate and reliable H₂S measurements in biogas processing.

With this project, the operator wants to dynamically adjust the FeCl₂ injection rate to optimise his cost of chemicals while maintaining the optimal lifespan on costly activated carbon filters and protecting the delicate membrane system from corrosion induced by hydrogen sulphide spikes.

Solution
An inline SulfiLogger^TM H₂S sensor was installed outdoors in a side-stream right after one of the five anaerobic digesters. The sensor measured directly in the wet and unconditioned biogas with temperature and pressure ratings well within the specifications of the sensor. The ATEX certified SulfiLogger^TM sensor provided continuous, online H₂S data via a cloud solution. The sensor could also have been integrated into the company’s existing PLC/SCADA system using the sensor’s 4-20 mA output.

Results
The SulfiLogger^TM H₂S sensor provided the plant operator with a complete overview of the real-time presence of hydrogen sulphide after the digester. Disregarding the fluctuations, the H₂S levels in the gas coming out of the digester were generally quite low, suggesting the operator is dosing excessive quantities of FeCl₂ for long periods of time.

Perspectives
Sample-based H₂S analyzers do not provide an ideal input for dynamic chemical dosing in biogas, as they deliver a static snapshot rather than a full overview over time. By relying solely on sample-based measurements for adjusting chemical injection rates, biogas operators risk dosing either excessive quantities to match peak loads of H₂S or insufficient amounts minimising the lifespan of assets. Using the novel SulfiLogger^TM H₂S sensor, biogas operators can optimise H₂S mitigative actions for lower running costs and improved asset management.

Stay tuned for more updates in the next stage of the project, as the operator starts adjusting the injection rate to match actual H₂S levels.

An overview of the plant’s 3-stage desulphurisation process and the sensor’s location in the setup

The sensor is installed in a sidestream right after one of the anaerobic digesters

H₂S readings from the SulfiLogger^TM H₂S sensor measuring right after an anaerobic digester. The top graph shows the full 5 day data set, while the bottom graph highlights a period of 2½ hours at the end of the second day (marked in red).
Both graphs show frequent process-induced fluctuations in the H₂S levels in the gas coming out of the digester. The half-hourly fluctuations shown in the bottom graph are caused by the mixer turning on and off at that rate.