Coal-fired power stations are often linked to the scourges of forest decline and acid rain, not least due to the sulfur dioxide and monoxide which they emit. A complex desulfurization plant has been retrofitted to the Jaworzno III power station in Poland in order to prevent such occurrences. The coal-fired power station has a generating capacity of 1200 MW, representing about 5% of Poland's entire energy consumption.
One of the most important requirements was that the entire plant should offer high levels of availability, i.e. it should be tolerant to any faults which might occur. Redundant systems are used in order to achieve this objective. Important components are repeated several times in such systems.

Pure process engineering:
complex throughflow technology
controlled with INTERBUS

In the case of a digital input, such as a switch, two diodes are used for decoupling the modules. A similar procedure can be used for the digital outputs. The situation is different with analog signals, however, because complicated networks of Zener diodes have to be set up.
The hot exhaust gas from burning powdered coal is channeled into an absorber in which the desulfurization takes place. This is a tall tank in which a lime slurry is sprayed through nozzles. Purified air from the atmosphere and flue-gas are blown in. The sulfur dioxide and monoxide reacts with the O² from the air and the lime, forming gypsum. The resultant gypsum suspension is desiccated using a hydrocyclone and belt filters. The purified flue-gas is not just stripped of its invisible SO^x, most of the dust is also removed as well. The gas is then allowed to escape into the atmosphere. Before the flue-gas desulfurization plant came into use, Jaworzno used to emit 123 318 tonnes of sulfur dioxide every year. This value has now been reduced by 95% to about 6 200 tonnes. All control parameters such as the pH-value, removal of the gypsum suspension and the temperature, have to be checked continuously. The process has to be run in again if the equilibrium between the pH-value, temperature and lime infeed breaks down. This would cause too much sulfur to be emitted and the quality of the gypsum would be impaired.
2 200 digital and 350 analog values have to be recorded so as to maintain the balance. 700 digital and 50 analog values are output depending on the measured values. The process control system runs on Modicon A500 PLCs from AEG Schneider Automation. These control the start-up, operation and shut-down. They are designed as a hot-standby system: the process master PLC calculates the output data based on the input data and its programming and passes the output data to the process. At the same time, it transmits the output data to the hot-standby PLC via a high-speed parallel link. This ensures it is possible to switch from one PLC to the other without causing any problems. However, such a changeover can only function if both PLCs are accessing the same historical data. For this reason, only the process master PLC is allowed to calculate the output data.

New Redundancy Concept
The configuration involving two PLCs with up to three INTERBUS lines each has proven its worth in Jaworzno. However, a method is now available for implementing redundant backups in an INTERBUS system more easily. The system in Jaworzno has the advantage of redundancy right through the module. However, there is only one sensor so it is a moot point whether redundancy is really necessary right through the module. In future applications similar to Jaworzno, attempts will be made to use only one input or output module if there is only one sensor system. In order to provide bus redundancy nevertheless, PHOENIX CONTACT and Cegelec AEG Anlagen- und Automatisierungstechnik have developed a new redundancy concept. Two host controller boards and two bus lines are used in this concept, as in Jaworzno. The host controller boards can be installed in one PLC or accommodated in two separate PLCs. Both lines are routed to special IB redundant backup modules. These allow a redundant remote bus to be connected to non-redundant I/O modules. Each redundant backup module forms the entry to a so-called subbus which does not have a redundant backup and collects data from single modules. This concept does without doubled I/O modules and the complicated diode networks. As far as the control system is concerned, there is no difference between this and a conventional doubling of the I/O modules. Any faults in one or the other bus system are reported to the operator who can then make an individual decision as to whether the process management should be changed over to the non-faulty system. As a further safety option, the redundant backup module provides local intelligence which is capable of operating the subbus even if there is a total failure of the control mechanisms. The redundant backup module combines the function of two bus terminal blocks and an INTERBUS submaster. Both remote buses are connected to the redundant backup module. The intelligent submaster selects one of the remote buses and supplies the data to its subordinate ring. The same principle as at Jaworzno is followed in this arrangement: one PLC is the process master and is permitted to set outputs and read inputs. The other PLC runs in hot-standby mode and is only allowed to read inputs. The subordinate ring has the characteristics of a remote or local bus and is non-redundant. Modules from the ST range can be directly connected to the redundant backup module; the activation of a remote bus makes it possible to use INTERBUS-capable equipment.

For Process Engineering
The applicability of this concept also makes the INTERBUS with redundant backup an attractive proposition for process engineering. It is also possible to utilize the advantages, such as high speed, deterministic functions and a potent diagnostic system, wherever it is essential that a process must not be interrupted due to the failure of the automation system. Examples include pharmaceuticals production, waste water purification and airport systems.

Dipl.-Ing. Bodo Seifert