Located in Duisburg, the Schwelgern coke plant, which is operated by KBS Kokereibetriebsgesellschaft Schwelgern GmbH, runs continuously 24 hours a day, seven days a week, 365 days a year. If any of the 140 coke ovens were to cool down, they would be irreparably damaged. Optimized production and process monitoring is therefore essential. Accordingly, field devices have been connected to the distributed control system at the coke plant via the Profibus PA fieldbus system for many years, an approach which has proven to be successful (lead image).
In the first step of the furnace process, coke is combined with iron ore to create pig iron, which is subsequently processed to make steel. The Schwelgern coke plant produces a great deal of the coke required by thyssenkrupp Steel Europe AG (tkSE AG) to recover pig iron. The production of coke requires mixtures of various types of coal fines, which are defined as the raw material. Today, 100% of this coal is imported. The supplied coal types are prepared and stacked onto two mixing beds. From here, the mixed feed coal is transported to the intermediate storage area of the central coal tower, where a coal charging car transports one batch to one coke oven in each case and fills one of the 70 oven chambers of batteries 1 and 2. Each chamber has a working volume of 93 m3 and receives around 79 tons of feed coal.
During normal operation, the heating walls in the oven chambers are heated with a gas mixture primarily consisting of the coke oven gas from the furnaces and converter gas from the steel plants and are set at a temperature of around 1350°C. In order to fully change into coke, the coal must be hermetically sealed in the oven chamber and heated for 25 hours. During this dry distillation process, all of the volatile components of the coal are driven out of the coal in the form of raw gas. At the end of the coking time, the coke oven doors on both sides of the oven chamber open and the ram pushes roughly 56 tons of red-hot coke into the quenching car. The red-hot coke is cooled in one of the wet quenching towers (Fig. 1). Covered belt conveyors are used to transport the cooled coke to the coke screening plant, where it is classified before being transported directly to the tkSE AG blast furnaces, either by belt conveyor or in cars. After it is cooled and cleaned in the gas treatment system, the raw gas is fed into the tkSE AG gas network as coke gas, where it is used to fuel the plants and power stations (Fig. 2).
The Schwelgern coke plant is one of the most technologically advanced coke plants in the world and also contains one of the largest coke ovens. It was commissioned in 2003 and been continuously in operation for more than 16 years. Every year, more than 2.6 million tons of coal are processed at the plant, which is located in Duisburg, directly next to the river Rhine.
Early detection of communication errors
Bernd Wemhöner, Head of Process Technology at KBS: “We were already impressed with the performance of the Profibus system back in 2003. After we tested it in a smaller application, we knew before the start of plant construction that we would use the communication standard in the new coke plant.” This decision marked a crucial step in ensuring reliable operation of the plant in the future. Around 2,000 field devices of all types are currently connected via Profibus to the distributed control system at the coke plant. The majority of the measuring transducers used, for example for measuring pressure and temperature, and the positioners for pneumatic drives have a Profibus PA connection (Fig. 3). These devices are coupled with the distributed control system by means of transparent DP/PA converters. Other sensors and actuators that are not equipped with Profibus DP or Profibus PA interfaces are integrated into the system via Profibus DP-I/Os. Due to the long distances on the plant premises and in order to protect against interference, many kilometers of fiber optic cables have been installed as the transmission medium. This was the first time Profibus was used in a coke plant, and one of the largest applications of a fieldbus system ever.
As mentioned previously, Profibus DP connects the control systems and the field devices in shared networks. This required much less cabling work compared to parallel wiring, where each sensor and actuator needed to be connected to the network using a separate cable. As a result, the plant was able to cut costs and save time for documentation during the planning phase, and also subsequently during assembly and installation. Naturally, remote query and remote configuration via the FDT/DTM technology in place at the coke plant is possible with Profibus. The diagnostic capability of the standard makes it possible to identify communication errors early on, which in turn increases system availability. Another reason that Profibus was chosen is the openness of the protocol, which is not dependent on the devices of any particular manufacturer. The coke plant must run continuously 24 hours a day, seven days a week. In order to safeguard operations around the clock, the Profibus DP systems and the process controllers are redundantly configured, which means two of each are installed (Fig. 4).
Transparent data conversion
Thanks to the web-based monitoring of Profibus networks, the Profibus DP/PA coupler from Phoenix Contact helps to ensure that there is zero downtime. The device transparently converts the Profibus DP communication into Profibus PA data and also offers detailed network diagnostics functions. Each configuration requires a head station for administration purposes, a Profibus DP repeater, and a Profibus PA module to interface with the Profibus PA network. This modular concept makes it possible to configure specific applications (Fig. 5).
The preconfigured FB-HSB-DP/PA unit provides a basic package which is suitable for most applications. A bus inside the base of each module allows the connection of ten Profibus DP or PA modules that can establish a connection with up to four separate Profibus DP/PA networks. The Profibus DP repeater modules have a capacity of up to ten redundant segments per head station, and can therefore also support network redundancy. By interfacing with the communication interface module, the user can access the primary element of the coupler/link as well as configuration and diagnostic data via the integrated web server using an Ethernet connection. The integrated network diagnostics can then be accessed via the communication interface module. This includes physical layer measurements of Profibus DP/PA segments, a list of active devices, the network configuration and oscilloscope functionalities. Moreover, the connected field devices can be configured and maintained via FDT/DTM as a remote function (Fig. 6).
Fast implementation during continuous operation
Maik Kirchhoff, an employee in the maintenance division who is responsible for maintenance of the plant: “The simple, redundant design and the comprehensive diagnostics with integrated web and DTM technology makes it considerably easier to monitor the field devices installed in the system.” When asked how long the coke plant will remain in operation, Bernd Wemhöner estimates at least another 20 years during which time no major malfunctions are expected. This effectively means that the coke plant in Duisburg would run continuously for around 40 years at maximum availability. This requires an open, compatible communication system. If suppliers update or discontinue their devices, a replacement device with a full scope of functions must be available. “This is an area in which Phoenix Contact offered us comprehensive support and worked out a solution for replacing a discontinued product that we could quickly implement during continuous operation,” concluded Wemhöner.
Direct integration into the Profibus PA system
Only devices that are equipped with their own fieldbus interface can be connected to a Profibus PA segment. With the new Profibus PA-I/O gateway from Phoenix Contact, I/Os can now be directly integrated into the Profibus PA system. Analog and digital I/Os are integrated into a Profibus PA network via a PA head station and the corresponding I/O modules. Since all signals are transferred to the control room from the field via Profibus PA, the entire installation can be performed using standard Profibus PA cables. Should the system be expanded, the existing infrastructure can still be used. The head station comes pre-configured so that the user requires no additional software. Integration into the higher-level host system is performed by means of an EDD or GSD file (Fig. 7).
Fig. 1: Every 10 minutes, after the press-out and quenching process, white steam rises from the quenching tower
Fig. 2: The coke gas obtained during this process is processed on the KBS premises in Duisburg
Fig. 3: A PT100 sensor monitors the temperature at the cooling tower
Fig. 4: To ensure optimum availability, a redundant configuration is used based on two Profibus DP/PA coupler stations each with two Profibus PA segments
Fig. 5: The redundant Profibus DP/PA coupler takes up very little space in the control cabinet
Fig. 6: Illustration of the diagnostic options with the Profibus DP/PA coupler
Fig. 7:The Profibus PA mixer integrates digital and analog signals into the Profibus PA network
More information: www.phoenixcontact.de/prozess
Dipl.-Ing. Mehmet Akcit, Business Development Manager Asia/Pacific, Phoenix Contact Electronics GmbH, Bad Pyrmont