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Automating Biogas Plants: Diagnostics and Monitoring Increase Yields

Germany has 7,100 biogas plants, according to Fachverband Biogas e.V., the country's biogas association. In 2011 they had a combined capacity of about 2780 MW and contributed about 3.1 per cent of the nation's energy mix. But the sector must adopt new production methods to meet its baseload potential, claims Biogas Pirates, a firm active in Schleswig-Holstein in northern Germany which specialises in the construction and servicing of biogas plants.

Automation systems can both raise plant availability and help meet the transparency requirements of Germany's EEF energy laws, says Biogas Pirates' manager Ralf Breckling. "We used to only take care of service and replacement needs," he said. "During that time we quickly learned that integrated, robust complete systems based on durable, flexible industrial solutions are the best investment for plant operators."

The need for information

Operational efficiency and biogas yields are potentially raised through gathering various types of data, which must also be available for further processing. Information can include flow rates, pressures or fill levels as well as motor performance data or methane content.

Some parameters must be monitored cyclically. It is important, for instance, to ensure that the fermentation process runs smoothly. If the bacteria in the fermenter are not "fed" for six hours, methane production drops significantly. After only a few more hours, fermenter contents can no longer be regenerated and must be removed in an elaborate, time-consuming and costly process.

Detailed diagnostics and high-availability systems are vital. Before purified biogas — or biomethane — can be fed into the natural gas network, its composition and quality parameters such as calorific value and density must be known and checked against public guidelines. Gas and energy production must also be documented along with the quantities of all feed materials. By constantly checking ingredients, operators can monitor and control the fermentation process to maintain the quality of the generated biogas.

To achieve this, all subsystems must communicate simply and securely with each other. Ethernet-based data cables — such as those used in offices to connect printers with PCs — are an effective way to exchange data. But industrial ethernet has become the standard for meeting the industrial sector's need for robust communication systems that can reliably transfer data over long distances.

Automation in practice

Automation featuring such communication systems can be seen in practice at a biogas plant built in 2010 at Leckeng, near Germany's border with Denmark.

The plant consists of a wet fermenter, a secondary fermentation tank and fermented substrate storage. New material is brought in every hour via a feed screw to keep methane production at a maximum. A continuous automation solution from the field level to the control room features synchronised components. The plant supplies gas to two combined heat and power (CHP) stations, each with a capacity of 400 kW.

The plant's operators opted for an open industrial ethernet standard for automation that offers real-time capability and supports remote diagnostics as well as implementation of internet protocols. Industrial ethernet was also considered appropriate for enabling the secure and reliable transmission of data over the 2 km between the digester plant and the outlying CHP station. In addition, components and systems linked to the system can be expanded and modified later. A flexible mini-controller monitors the entire biogas plant. Three ethernet connections on the device ensure networking and flexibility. A modem can also be easily connected for secure remote maintenance.

Rather than being fed into the gas network, methane from Leckeng fires the two CHP units to generate electricity and heat. The second "satellite" CHP station, in the engineering room of a high-rise building, supplies about 160 homes with heat. Internet and Profinet communication enable distributed generation of heat and electricity with methane piped over from the biogas plant to be run as if it were a centralised plant.

Sustainable production

Sustainability determines how biogas plants operate in several ways. The switch to industrial automation solutions, for instance, enables preventative or condition-oriented maintenance.

These functions benefit from standardised switching technology. The modular safety system, for example, includes all the components needed for switching, protecting or connecting consumers as well as for monitoring, controlling, detecting, commanding, signalling and supplying power. If any of these components need to be replaced, they can be quickly removed from the control cabinet and new components installed.

Easy connectivity was also the main reason why the Leckeng plant installed a communication interface. This interface was originally developed to provide information from the final meters — the sensors and actuators — to an automation system. While microprocessor technology has made sensors increasingly powerful, many users had been unable to apply their diagnostic and parameter information as it was unavailable in the automation system. Sensors also usually had to be wired elaborately with multi-pin cables. Using the communication interface, sensors from the meters can simply be plugged in.

Biogas plants have also benefited from standard connection blocks. They can connect individual components to form complete feeders, saving time and ensuring units are always wired correctly. At Leckeng, the motor or load feeders — consisting of contactors and circuit breakers for motor protection — were equipped with communication interfaces. As a result, only the initial starter of a series is wired to the master. Other starters are linked via plug-in connectors using pre-fabricated flat cables.

"Plug & play" and standardised connection blocks greatly cut the costs of wiring modules and of assembling individual devices into complete feeders. The user also receives diagnostic information that is unavailable with conventional wiring. Up to four feeders — direct, reversing or star-delta starter — can be connected to each other.

The system enables information to be received from even the lowest field level. The display can easily visualize information such as whether a motor contactor has triggered, or whether voltage is supplied to all phases. 

As each individual component's availability determines the biogas plant's efficiency, it is important to be able to diagnose each individual motor feeder using remote access. Operators get instant information on how to solve simple problems. Service and maintenance staff also know immediately what caused a fault or triggered an alarm. Important diagnostic information is usually programmed by the user for a company's specific requirements, but ready-made solutions are also available. 

Integrated system solutions

While the Leckeng biogas plant selected a flexible mini-controller, larger plants could implement control systems. From an automation viewpoint, a control system makes sense especially when several plants operate within an integrated system. Wastewater treatment plants or waste recovery systems can, for instance, operate in combination with biogas plants. Local authorities can also operate several biogas plants.

The need for an integrated system solution is very clear in these cases. The system solution should include graphic configuration tools, a uniform engineering platform, integrated safety technology and additional functions such as alarm and asset management. The automation system must also be scalable for easily integrating additions to the system.

Fast and straightforward engineering is another requirement, as biogas plants are often built under extreme cost and time pressure. Finally, the solution must deliver all the necessary functions including instrumentation and drive technology, safety technology and power management in a uniform environment. The scope and functionality of the automation can be adapted to the plant size and the process engineering. 

Optimizing operating costs

As well as the efficiency of processes, plant operating costs also need to be monitored. The Leckeng plant is equipped with a multifunction meter that can collect more than 50 measurement values such as voltage, currents, power and frequency.

The meter monitors network quality and can measure the difference between reference current and supply current. But the system's effectiveness hinges on power monitoring. Supply contracts set how much power is available at any moment. Exceeding this limit is expensive, so the controller is programmed to prevent energy intensive work processes from starting or running simultaneously.

For a biogas plant such as Leckeng, the overriding objective is to achieve an energy supply that is as environment-friendly as possible. Biogas Pirates considers that this is achieved through applying industrially proven state-of-the-art technology to accomplish this goal. "In the end, a biogas plant is not much different from an industrial plant," says Ralf Breckling.

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