Biogas Plants in Agriculture: Why Mobile Spot Measurements Are Crucial for Efficiency and Safety
Agricultural biogas plants are complex biological reactors. Slurry, silage, and residues like sugar beet pulp are anaerobically fermented into biogas in fermenters – a process that is sensitive to temperature fluctuations, substrate changes, and oxygen ingress.
Especially in two-stage plants with mesophilic (approx. 35–40 °C) and thermophilic (approx. 50–55 °C) fermentation, close monitoring of the gas composition is indispensable. A current project at an agricultural operation exemplifies how mobile spot measurements with the Gas Analyzer Portable GAP100 increase process reliability – even under demanding climatic conditions.
The Starting Point: 11 Measuring Points, Two Fermentation Stages, One Mobile Device
The operator ferments agricultural residues (slurry, corn silage, sugar beet residues) in a two-stage plant with parallel mesophilic and thermophilic process control. For quality assurance, spot measurements are required twice a week at eleven defined measuring points:
- Main fermenter (mesophilic and thermophilic)
- Post-digester
- Gas storage (before and after desulfurization)
- Consumer side (CHP inlet after activated carbon filter)
The required measuring components: CH₄ (0–100 vol.%), CO₂ (0–65 vol.%), and O₂ (0–25 vol.%). A stationary installation at each point would be uneconomical – this is where a portable handheld measuring device fully demonstrates its strengths.
H₂S as a “Hidden Added Value”: Why Hydrogen Sulfide Measurement Should Be Mandatory
In addition to CH₄ and CO₂, hydrogen sulfide is a central parameter for plant protection. Raw biogas contains 100 to over 2,000 ppm H₂S, depending on the substrate. Inadequate desulfurization can lead to the following effects, among others:
- Corrosion of gas-carrying pipes, heat exchangers, and CHP components
- Catalyst poisoning in the exhaust gas aftertreatment system
- SO₂ emissions after combustion, which can exceed official limits
The GAP100 measures H₂S as standard up to 2,000 ppm via an electrochemical sensor. This allows the efficiency of the activated carbon filter to be directly verified: if the H₂S content increases after the filter, a filter change is due before downstream components are damaged. The GAP100 is a process measuring device for spot measurement of gas quality and not a personal protection or gas warning device.
The GAP100 in Detail: Stationary Measurement Accuracy in a Mobile Housing
We consistently developed the GAP100 as a mobile version of our stationary analyzers. The most important technical specifications:
| Parameter | Specification |
| measurement principle CH₄ / CO₂ | NDIR single-beam method with referencing |
| measurement principle O₂ / H₂S | Electrochemical sensors |
| measuring ranges | CH₄ 0–100 vol.% · CO₂ 0–65 vol.% · O₂ 0–25 vol.% · H₂S 0–2,000 ppm |
| Pressure compensation | 0.7–1.1 bar absolute |
| Gas flow | approx. 2.5 l/min (integrated pump) |
| Weight | 4.2 kg |
| Battery life | approx. 8 hours |
| Operational readiness | as soon as the target temperature of the measuring cell of 45 °C ± 2 K is reached |
| Interface | RS232 |
| protection class / housing | IP 54, ASA+PC-FR plastic |
| Ambient temperature | +5 to +45 °C |
Crucially: The integrated pump and the condensate separator with filter eliminate the need for external additional devices. The included accessory kit – carrying case, PTFE sample gas line, sampling tube, and the GMM evaluation software – enables immediate field use without further purchases.
Pre-conditioning: Why the Customer-Side Gas Cooler Perfectly Matches the GAP100
Biogas is saturated with water vapor and leaves the fermenter at about 35 to 55 °C, depending on the stage. Without drying, condensate would precipitate in the measuring device and impair the NDIR measuring cell as well as the electrochemical cells. In the described project, a customer-side gas cooler is upstream of the GAP100, drying the sample gas to a dew point of approximately 5 °C.
Since the measuring cell of the GAP100 is heated to 45 °C ± 2 K, it is significantly above this dew point, so no re-condensation occurs in the device. The usual low overpressure at the measuring point is within the pressure compensation range of the device. Upstream gas conditioning also extends sensor life and keeps calibration efforts within the intended annual interval.
Extreme Climate Zone: Mobile Measurement in Winter Conditions
In the project, operability at ambient temperatures down to −20 °C was requested. The GAP100 is specified by the manufacturer for +5 to +45 °C; temperatures below this are outside the device specification. In frost, the sensitivity of electrochemical sensors decreases, the usable capacity of lithium-ion batteries drops significantly – charging below 0 °C should be avoided as it damages the cells – and condensate can freeze in supply lines. The device can be operated within the specification if the following deployment concept is adhered to:
- Storage exclusively in a heated room or vehicle (e.g., technical container, operating building)
- Acclimatization to at least +5 °C before commissioning
- Only short-term outdoor use for the actual sampling at the measuring point
- Transport in an insulated carrying case, keep sample gas line warm
- Visual inspection for condensate/ice formation after each use
Practical Benefits: What the Data from 11 Measuring Points Provides the Operator
The twice-weekly collection of the four main components provides a complete picture of the plant:
- CH₄ trend in the fermenter: Indicator for biological activity and substrate quality
- CO₂/CH₄ ratio: Early indicator for acidification or process disturbances
- O₂ content: Control of biological desulfurization (targeted air dosing) and leakage detection
- H₂S before and after activated carbon filter: Residual capacity of the filter and protection status of the CHP
All measured values, including timestamps and measuring point assignments, are documented via the RS232 interface and the GMM software – a solid basis for operational documentation and maintenance management.
Mobile Precision for Growing Biogas Markets
Agricultural biogas plants are the backbone of decentralized energy generation – especially in regions with high agricultural residue volumes. Mobile spot measurements with the GAP100 offer operators an economically attractive entry into systematic process monitoring: four components, stationary measurement accuracy, eight hours of battery life, and a complete case for field use.
The standard H₂S measurement is more than a bonus – it is a crucial component for plant and occupational safety. For demanding climate zones, we define a deployment concept together with the customer that respects the device specification while still providing reliable measured values. This transforms a standard device into a customized solution for biogas production. Contact us for your individual solution!
