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Measurement and Testing
BIOGAS BATCH FERMENTATION SYSTEM FOR LABORATORY USE WITH AUTOMATIC GAS ANALYSIS IN REAL TIME
Biogas is a renewable energy source that is obtained through the fermentation of organic materials. A common system for optimizing the biogas process is the so-called batch fermentation system. In this report, we will take a closer look at the RITTER biogas batch fermentation system and explain how it works and its advantages.
Introduction
The biogas batch fermentation system is a type of anaerobic reactor used to produce biogas. It consists of a closed container with a stirrer called a digester or fermenter.
The fermentation process begins with the addition of organic materials to the digester. This can be various types of biomasses, such as animal excrement, plant residues, food waste or energy crops such as maize or grass. The organic material is added to the digester together with water to create optimal conditions for the anaerobic bacterial culture that carries out the fermentation process.
After the organic material has been placed in the fermenter, the container is sealed airtight to ensure that no oxygen is added. This is important as the fermentation process takes place under anaerobic conditions, where the bacteria break down the organic matter and produce biogas. During the fermentation process, the fermenter is kept at a constant temperature that is suitable for the activity of the bacteria, usually between 35 and 40 degrees Celsius.
The fermentation process itself consists of several successive stages. First comes hydrolysis, in which complex organic molecules are broken down into simpler compounds. This is followed by acidogenesis, in which the products of hydrolysis are converted into organic acids. In the third stage, acetogenesis, the organic acids are converted into acetic acid. Finally, in methanogenesis, acetic acid is converted into methane, which forms the main component of the biogas.
The fermentation process in the biogas batch fermentation system usually takes several days to weeks, depending on the type of biomass used and the specifi c conditions in the fermenter.
The entire biogas batch fermentation system consists of 3 parts: 1. Fermentation vessels (biogas fermenter) 2. Gas analysis (CH4
and CO2 3. Flow measurement Fig. 1 shows the schematic structure of the entire device. ) Fig. 1: Principle structure of the biogas batch fermentation system Fermentation vessels
The fermentation vessel is made of PMMA (polymethyl methacrylate) and is equipped with a gas-tight stirrer. A permanently gas-tight shaft feed-through is always a challenge for gas measurements. Even O-rings or lip seals fail in the long term with aggressive gas components or the smallest particles in the gas fl ow. In stirrers for fermentation bottles, unsatisfactory compromises are therefore made due to ineffi cient stirrer rods and/or the smallest gaps on the stirrer shaft. For this reason, RITTER uses a technically sophisticated magnetic coupling between the stirrer motor and stirrer blade. This not only ensures a permanently gas-tight transmission, but also enables the use of individually manufactured stirrer blades adapted to the fermentation substrate.
Special features
• Absolutely gas-tight thanks to magnetic coupling between stirrer motor and stirrer blade
• Various sizes: 0.5 / 1 / 2 liters
• High break resistance due to PMMA construction (without fragile glass gas outlet nozzle)
• Application-specifi c stirrer blades available • Stirring speed adjustable via the software from 1 to 30 rpm • Interval stirring adjustable via the software
PIN OCTOBER / NOVEMBER 2024
• Suitable for media with low and high viscosity (≤ 450 mm²/sec)
• Gas outlet connection: PVDF tube fi tting for tube Ø 4/6 mm Gas analysis
) are determined using the NDIR method. This method is based on the selective absorption of infrared radiation by the different gas molecules. The CO2 place at 4.3µm and the CH4
(CH4 measurement at 3.4µm. Spectral
overlaps are excluded using narrow-band interference fi lters, so that this type of gas measurement guarantees high selectivity (Wiegleb 2022). The
INFRA.sens®
uses a multi-channel detector
with which both gases can be detected simultaneously in just one sample cell. This has the advantage that both gas concentrations are measured at the same time. If gas sensors were connected in series, there would inevitably be a time offset, which would cause a considerable error, especially at low gas fl ow rates.
The
INFRA.sens® has a very high measuring accuracy of 1%. To achieve this, all measuring errors are compensated electronically.
Methane and carbon dioxide gas concentrations are measured using the tried and tested
INFRA.sens modules. As the concentrations can reach values of over 50 % by volume, the respective measuring ranges have been designed for the range from 0 to 100 % by volume. Both gas concentrations cx cx
(CO2
) and measurement therefore takes
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