FREQUENTLY ASKED QUESTIONS (FAQ)
There are many reasons for using an online gas analysis system:
- The process of fermentation can be constantly monitored and overloading can be detected quickly in the gas phase.
- In connection with the recording of data gas values can be proof of adhering to the required limits (e.g. hydrogen sulphide within the CHP operation).
- The air supply for the microbiological desulphurization can be optimized and it is possible to automate it using a PI controller.
- Security relevant concentrations (upper and lower explosion limit) can and must be monitored, predominantly during batch processes such as dry fermentation.
- CHP management – biogas can only be fed into the CHP if it has a certain fixed concentration.
- Process values can be used for a superior control system and for the detection of overloads (e. g. hydrogen measurement for fuzzy logic control).
- methane (0-100 vol.-%)
- carbon dioxide (0-100 vol.-%)
- hydrogen sulphide (0 – 10 /30 / 200 / 1500 / 3000 / 5000 ppm)
- oxygen (0-25 vol.-%)
- hydrogen (0 – 2000 / 5000 / 20000 / 50000 ppm)
- other sensors; more measuring ranges available on request
- All gases are measured undiluted, since dilution of gases
represents an additional inaccuracy.
- Multi-point calibration for all sensors, compensating for
residual inaccuracies and achieving higher measuring accuracy over
the entire measuring range.
- Infrared dual beam temperature and pressure compensated for CH4 and CO2
without cross sensitivity to other gases, no influences by
water vapor, aging etc.
- Electrochemical for H2S, H2 and O2 without cross-sensitivity to other
gas constituents/filtering of other constituents.
- Optionally, paramagnetic sensors for O2.
- Caution with measurement methods such as heat tone and thermal conductivity.
These are not suitable without restriction for
multi-component mixtures due to cross-sensitivity to other gases
(Attention, biogas contains 6 and more gas components with a wide range of physical and chemical properties).
- If needed, the devices can be fitted with more redundant sensors with the same or a different measuring range.
Every gas measuring device must be calibrated and maintained. Electrochemical sensors such as hydrogen sulphide, hydrogen and oxygen always show a drift downwards due to the measuring principle (on average about 5 % per year, but up to a maximum of 2 % per month). We recommend a first check after 6 months (you will be informed by us). Depending on the result of this first check, the further intervals are between 9 and 12 months. During these calibrations, the sensors for methane and carbon dioxide are also checked, which only show a low drift (maximum 2 % per year) – although the accuracy requirement for these gases is usually higher.
Methane and carbon dioxide concentrations are measured with infrared sensors (2-beam method). These sensors have a life-span of several years. The life-span is limited predominantly by the life-span of the infrared source of light. The decrease in intensity is compensated to a large extent by means of comparison with the second beam.
Electrochemical sensors (hydrogen sulphide, oxygen, hydrogen) are used up (even without measuring, simply in the air). As we use high-quality sensors, we can guarantee a life-span of 2 years – also with frequent measurements up to more than once an hour.
Because we are dealing with a measuring device a regular maintenance is essential. The sensors should be calibrated every 6 to 12 months. The calibration, which is carried out by us, currently costs € 75 per sensor. The prices for new electrochemical exchange sensors are between € 200 – € 350. For long-term operation of the systems the yearly maintenance cost (incl. travel costs) can be calculated at 5 – 10 % of the investment.
As the systems are adapted to the needs of the customers there is no established price. Depending on the equipment prices range between € 4,000 and € 12,000.
Every measuring interval of at least 30 minutes is possible. In normal operation mode a measurement every 1 to 2 hours is recommended. If measuring intervals are longer, changes in the biological process may not be recognized in time. A continuous analysis is also possible but the technical equipment would need to be adapted accordingly.
Yes. It is possible to both integrate sensors and to attach external sensors (e. g. fill level, pressure, pH, gas flow, CHP performance).
Although industrial components are expensive, they are much more robust and available as spare parts over decades. Thereby we achieve a very high availability of the systems.
Using our systems data can be transferred optionally by electrical signal (4..20 mA), serial RS232 or RS422/485 interface (ASCII, Profibus, Modbus RTU) and Ethernet (Modbus TCP, Simatic Fetch/Write, TCP socket). More options are available on demand.
In contrast to the electrical signal a bus can transmit other data, such as the device status, to the control system. Moreover certain functions can be triggered by a control system/computer. The greater the distance and the number of the values to be transferred, the cheaper the solution with the bus compared to the analogue signal, as wiring and the analogue digital changer become redundant.
Each analysis system is fitted with a detonation arrestor, which prevents a flashback of flames. The exhaust pipe of the gas measuring device has to be led off or fed back into the main gas pipe in order to prevent the formation of an explosive mixture in the room where the device is installed.
By using a filter (“Fritte”) the gas measuring device sucks in the ambient air and feeds the interior with fresh air. In case of possible leaks in the interior the gas would be led off via the exhaust pipe. In addition, most devices (according to the kind of sensor they are fitted with) are checked for the gas composition of the air for flushing or in the interior. Thus, a possible methane atmosphere can be detected in the interior or in the room where the device is installed.
When carrying out comparison measurements please check the specified accuracy values and the calibration status of the devices. Deviations questioned by a user are often within the specified accuracy values. When calculating the total please consider that there is still at least 4 times the amount of oxygen in form of nitrogen available. Using up oxygen can mean the percentage is even higher. It also needs to be taken into account whether the device used for the comparison is fitted with a temperature and pressure compensation. Without this compensation errors in measurement can account for more than 10 %. Moreover it needs to be considered whether the devices measure the gas as is or whether the humidity is removed with a gas cooler.
When comparing values against lab analyses it has to be taken into account that the latter are often normalized to 100 %, i. e. the total is exactly 100 %. If various components are considered, this does not constitute a problem, but otherwise values for the other components may be too high. If working with lab results it should be checked how accurate the analyses are. Based on our experience the accuracy is 1 to 2 %. When taking samples please make sure to use suitable containers or to carry out the analysis quickly in order to avoid falsification of the results due to gas diffusion. Bags out of plastic are often not suitable for all components (diffusion, absorption).
It depends on how accurate the efficiency needs to be determined. For a general assessment it might be enough, but for a more accurate determination appropriate standards have to be applied. For determining the efficiency the methane concentration and the gas volume flow are required. The gas analysis system as well as the volume measurement should be calibrated immediately before and afterwards. We specify an accuracy of 2 % of the final value of the measuring range for the methane concentration. However, even by calibrating the system the maximum accuracy is only between 0.5 and 1 %, because the certified calibration gases already show an inaccuracy of this scale. Calibration gases with a similar concentration to that of biogas should be used as all sensors are (despite compensation) to a certain extend non-linear. When carrying out flow measurements it is important to know for which gas composition or gas density the specified accuracy is applicable. In practice the accuracy of gas flow measurements are rarely better than 5 %. Often the results of the mass flow are converted to the volume by assuming a notional gas composition. Therefore, when confronted with deviating gas compositions many systems calculate the volume inaccurately.
Also make sure that humidity is not calculated twice by mistake.
According to the principle of measurement many gas flow measuring devices are dependent on the gas composition when rendering the measured value as a volume or a mass flow. The devices for thermal mass flow measurement are calibrated at 65 5 CH4 and would show an error in measurement of 10 % at a methane concentration of 50 % depending on the flow, just because of the altered gas composition. If these flow sensors are connected to AWITE’s process analysis systems, these errors are recalculated and corrected, because the 3-dimensional optional field and the current gas composition are known.
As we do not adjust the zero points of the sensors automatically, the measured value in the air can be a non-zero value. For example, for a measuring range of 5000 ppm it can happen that 20 ppm are shown at the air measuring point. But it always has to be considered that the ambient air might not completely pure. Therefore the zero point of the sensors with a very small measuring range (e. g at 0..10 ppm) is not adjusted with ambient air but with calibration gas.
Yes, this can happen. Hydrogen is generated mainly during the acidification of fresh substratum. If highly acidic substratum infiltrates into the system, little or no hydrogen is generated as the hydrogen has already been discharged from the system. For example, if the substratum was pure vinegar, no hydrogen would be generated. Therefore biogas plants cannot be operated purely based on the hydrogen concentration. However, a high hydrogen concentration prevents the degradation of propionic acid, butyric acid etc. and therefore should be avoided.
The explosion limits of biogas are between 5 and 15 vol.-% for a methane concentration. Thus the gas is diluted in the CHP. For the inflammability it makes a big difference whether a low methane concentration of say 40 % was formed by a bad gas (i. e. residual of carbon dioxide) or by the air supply for the desulphurization process. In the latter case the gas certainly is more inflammable as the gas has only been mixed with air. Remark: The answers presented here refer to the most modern devices. As our devices are constantly being developed, diverting statements concerning older devices and purpose-built systems may apply.
Though air is measured in regular intervals, we do not adjust the oxygen value automatically to 20.95 % and the other sensors to 0. If we did, the operator would have less control over the drift behaviour of the sensors. Another disadvantage of the automatic adjustment is that leakages or lack of oxygen are not recognized and sensors are adjusted. For example, the oxygen concentration decreases considerably and measurably by 1 % in a room with people during a working day and increases again after working hours. With AWITE devices the operator can use the display to perform an air adjustment for all sensors.