SIEMENS

Thanks to years of experience, Josef Götz can follow his gut instinct. Götz, a farmer from Markt Indersdorf in Bavaria, also needs to call on this experience for his biogas power plant, as the bacteria that transform forage crops and slurry into methane in the unit’s underground fermenter literally work in the dark.

This guessing game is required because until recently no affordable measuring technique existed for continually monitoring the complex operation of the fermenter. Creating optimal conditions for measurements is also difficult. “At the moment, we can only send samples to an external lab, so there’s a time lag until we can make adjustments,” Götz explains.

Biogas is converted into electricity and heat in a cogeneration plant. Ideally, the plant should generate 860 kilowatts of electricity, whereby each kilowatt hour translates into 15 cents of income for Götz. But Götz can also lose hundreds of thousands of euros if the plant fails to operate optimally. “Operation near maximum capacity gives you the best methane yield,” he says.

The type, amount, and composition of the nutrients fed to the bacteria primarily determine whether they develop to their most advanced stage, weaken, or even die. If the latter occurs, the entire facility will wither like a garden pond with too much fertilizer. This can lead to complete plant failure — a risk that virtually no operator is willing to take. If a plant fails, the fermenter has to be emptied, cleaned, and refilled. The entire fermentation process then has to be restarted, and this can take months.

With this in mind, researchers at Siemens Corporate Technology (CT) have developed a solution in the form of measurement technology that ensures the full automation of biogas plants. The system might soon be tested with Götz’s unit. The heart of the system is a spectrometer the size of a briefcase. The device operates with light in the near-infrared spectrum, which contains slightly less energy than that emitted by a heat lamp. “The spectrometer can measure the acidity of the mixture in the fermenter around the clock,” says Prof. Maximilian Fleischer, whose team at CT developed the device.

Acid concentration is a key indicator of the conditions inside a fermenter. If it exceeds a critical level, the fermenting process will shut down. “Continual process monitoring therefore allows biogas plant operators to react very quickly to problems by, for instance, changing the bacteria feed composition as soon as acid concentrations rise,” Fleischer explains. If countermeasures are to function automatically, the measurement data must be interpreted and converted into clear commands such as “add corn” or “decrease slurry content.”

Achieving this kind of transparency is the job of Volker Hirsch from the Siemens Industry Sector. “We use Simatic process controls that have already proved their value in the chemical industry,” he explains. Siemens technology is now used in Götz’s unit to collect data on temperature and gas composition. But Götz nevertheless has to take a sample to a lab every week for acid analysis, a process that involves substantial costs and runs the risk that information will be late. That’s why Götz has high hopes for real-time measurements.

Siemens researchers first had to understand the details of the complex processes in biogas facilities before they could tailor nearinfrared spectroscopy to meet the requirements of these facilities. “Methane is produced in four phases,” Fleischer explains. A different type of bacteria handles each phase. In the first two phases, bacteria break down nutrients into interim products such as butyric and acetic acids, which can be digested by their “coworkers” in phases three and four. These then convert acid into methane. “You can run into problems if too large a volume of easily digestible substances like sugar beets is on the menu,” Fleischer says. In this case, too much acid will form in too short a time, causing the mixture to turn sour and the methane-producing bacteria to become less healthy and thus less productive. Less acid is then broken down, and that damages the bacteria even further. The whole process may then come to a complete halt, at which point the bacteria in the fermenter will die.

CT’s new measuring unit transmits infrared rays into the bacterial soup via glass fiber cables. The radiation that returns is measured by detectors the size of a fist.

“Fatty acids alter infrared light in a unique way,” Fleischer explains. The more acid there is in the mixture, the greater the changes. The data can also be used to determine how many of which types of bacteria are working in the reactor, and how large the ratio is between solids and liquids. “This method is relatively inexpensive and robust as compared to other chemical analysis techniques,” says Fleischer.

Farmers, Sewage Plants, and Landfills. The results of CT’s pilot study will generate interest around the world. Biogas can be easily stored, fed into the natural gas grid or, as is the case with Götz, used on site to produce electricity and heat. A study by the Trend Research Institute estimates that German exports of biogas power plants alone will more than double over the next ten years.

Germany now has over 5,000 biogas units — more than any other country. “Collectively, these produce a lot of bio-methane, which, when converted into electricity, generates the same output as two large power plants,” says Hirsch. The biggest users of the technology are farmers looking to make extra money. However, biogas facilities can also be found at sewage plants and landfills, where they make bio-methane from wastewater and garbage.

To accelerate this trend, Siemens researchers plan to closely study bacteria feed with the help of infrared lamps in order to improve feed quality. “Cost considerations have made feed mixtures more and more heterogeneous,” says Fleischer. Many biogas plant operators now use food industry waste, for example. Götz has tried out robust wild plants, and even weeds from public and private gardens could be used to make biogas in the future, says Fleischer. Siemens experts will have to refine their method if the methane-producing bacteria are to be kept happy under such trying nutritional conditions. And they’ll need a lot of specialized knowledge here — not to mention good gut instincts.