Why is process monitoring necessary?

Biogas plants are biological systems involving various interacting micro-organisms that anaerobically degrade organic matter. The main product is biogas, a gas rich in methane (CH4) that can be used as a renewable fuel for vehicles or to generate heat or electricity for local use or for use via energy distribution grids.

The degradation involves four consecutive biological processes: hydrolysis, acidogenesis, acetogenesis and methanogenesis.

 

If one of these processes is negatively affected in any way there is an immediate influence on the other processes and the biogas plant can become unstable. Typical process failures include, among others, organic overload, hydraulic overload and ammonia inhibition. Process monitoring can help to understand what happens in a biogas plant and help to maintain a stable process. In many cases, a strongly inhibited micro-organism population or a total crash of the whole plant can have severe financial consequences for the biogas plant operator.

In general, process monitoring in anaerobic or other fermentation processes can help to:

• Give an overall picture of the biogas process
• Identify upcoming instabilities in anaerobic digester before a crash happens
• Accompany a successful start-up or re-start of a plant

The costs of online monitoring are often much lower than the costs and lost revenue associated with re-establishing a biologically destabilised plant.

For example, if a biogas plant has totally crashed it may have to be emptied and filled again with new inoculum. This, together with the necessary start-up period, means that several months can be lost during which the plant could have operated at full load. The financial consequences can be devastating for the plant operator.

Key parameters in Biogas monitoring

In biotechnological processes, the detailed monitoring of the fermentation product, in this case biogas, provides valuable information. Therefore, it is recommended to monitor both the volume of gas produced and the gas composition. With regard to process monitoring, a change in either gas production or gas composition can be an indicator of process imbalance.

Biogas primary constituents

• CH4 and H2S measured during the methanation process give a good overall view of the performance of the degradation process and bacterial activity. A decrease in methane content can be a first sign of organic overload, provided that the feedstock mix has not recently changed. Similarly, a sudden increase in H2S can provoke process instability.
• Simultaneous real-time monitoring of CH4, H2S and CO2 empower process automation (i.e. operation of scrubbers)
• Before entering the distribution chain, the product must be qualified, again for economic reasons (excess H2S and/or CO2 in the CH4 would make it ‘sour’ and potentially corrosive enough to damage distribution infrastructure)
• CH4, H2S, CO, CO2 and O2 Continuous Stack Emissions Monitoring required for environmental compliance

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Stationary Biogas Monitoring Systems

In general, a large variety of devices can be applied for measuring biogas production but in practice, as biogas is of variable gas composition, dirty, corrosive, wet, and produced at low pressure, measuring biogas volume and composition accurately is one of the most challenging parameters at a biogas plant. In the long run not properly designed systems can pose considerable problems due to corrosion, fouling or general deterioration when measuring raw biogas.

AquaGas Pty Ltd introduces the OFCEAS technology and Proceas BioGas analyser  designed by AP2e to address the analytical (online monitoring of biogas primary constituents) and operational (zero drift, minimum maintenance, no consumable, autonomous operation) biogas plants and methanation process monitoring requirements.

The ProCeas BioGas analyser features two patented technologies:

• The OFCEAS IR Laser technology for enhanced specificity, selectivity, accuracy and stability (no instrumental response drift.):  The OFCEAS is a self-referencing spectrometer. This means that there is no need to re-analyse a zero and/or a span gas on a regular basis. The “zero” information is contained in the direct measurement spectrum while the “span” information is intrinsic to the analyser (pre-calibrated with 4 OD linear response).

•  The Low Pressure Sampling (LPS) system enabling low-cost installation thank to non-heated lines and reduced maintenance. Lowering the pressure of a gas sample reduces the bandwidth of the absorbance bands. At atmospheric pressure, simultaneous measurement of H2S (ppm’s) in presence of CO2 (10’s %), CH4 (10’s %) can be tainted by cross response / false positive due to spectral overlapping of CO2 and CH4 bands with H2S  information.  If operating at 100 mbar, all absorption bands have narrowed down to the point where there is no spectral overlap of the absorption bands. Cross response have been eliminated. 

When sampling gas from landfill, anaerobic digestion/bacteria and other fermentation processes, the use of a dedicated sampling system is necessary to ensure a specific sample preparation and to preserve your monitoring equipment. Another advantage of the LPS System is the ability to reduce the sample dew point (vapour pressure) to prevent the risk of condensation. The sample extraction is achieved by the use of a sonic nozzle. The sample is taken at a very low pressure (50 to 200 mbar abs, 100mbar nominal). At 100 mbar abs. the ambient temperature is almost always above the dew point eliminating the risk of condensation.

• No heated line required to eliminate condensation: low power consumption, no related ongoing maintenance
• Sample does not need to be treated – True Direct Extractive CEMS and Process analyser
• No risk of absorption/desorption

The ProCeas offers sampling solutions tailored to the needs of the biogas production industry with automated sampling sequences and multipoint monitoring enabling fast, accurate and detailed analysis at various location with a single instrument: Raw biogas (1), processed biomethane (2), vented biogas (3) and stack emissions (4).

The Proceas Biogas Monitoring System is a cost effective online monitoring solution offering application specific features including:

• Field proven technologies in the Biogas production industry
• Simultaneous measurement of all primary constituents of Biogas
• Fully automated standalone system (automatic calibration and sampling sequences)
• Low Pressure Sampling (no sample condensation)
• Deep particulate filtration
• PFA and PTFE gas path (essential to avoid loss of components of interest)
• Fast, accurate and reliable online analysis
• No drift or deviation therefore minimal calibration requirements
• Low-maintenance and user friendly system
• Site specific sampling solutions

The ProCeas analysers are manufactured and pre-calibrated by AP2e in France. The systems are built, integrated and tested in Australia by AquaGas. Standard systems are delivered fully integrated in a 19 inch rack suitable for installation in industrial settings (indoor 15-35C). For installation in hazardous area the BioGas Monitoring System is available in ATEX certified version.

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Portable Biogas Analyser

 

Short term measurement during start up conditions

The start-up of a biogas plant is a very sensitive process. Due to slow multiplication of some of the micro-organisms involved in anaerobic sludge and the consequent risk of hydraulic overload, the start-up of a biogas plant can take much longer than in other biotechnological processes. A start-up time of 1-2 months is nothing exceptional in biogas plants.

Therefore, the effort in process monitoring has to be highest during start-up. If the start-up is too fast, a sub-optimal biogas process can be the consequence because the most favourable micro-organisms have not multiplied in the biogas plant. In contrast, a slow start-up can cause a possible loss of income as time is taken to reach full load capacity. The frequency of the measurements should be increased during this crucial process steps.

The  ANKERSMID Portable Biogas Analyser is especially designed so that detailed gas analysis can be carried out at any time in any place. The entire gas conditioning system is housed in a compact and robust carrying case which ensures that the components can be removed easily and gas analysis performed in an efficient manner: quickly, safely and with a  minimum maintenance. This system ensures reliable sample preparation without loss and prevents damage on the  analysis system used downstream.

The ABYSS Portable Biogas Analyser uses a dual beam Infra-red cell highly resistant to corrosive flue gases and harsh environments of operation.  Its light weight rugged enclosure fitted LCD screen, safety filter and Quick Connect fittings enables efficient short term measurement of emissions and process gas in waste recycling facilities, industrial fermentation plants and associated odour filtration systems.