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A large coal-fired power plant needed to be retrofitted with a complex desulphurization facility to prevent the release of harmful sulfur dioxide and monoxide gas emissions. The power station has a generating capacity of 1,200 MW. As a result of installing the desulphurization process, the plant has reduced sulfur dioxide emissions by 95 percent. In the desulphurization process, hot exhaust gas from burning powdered coal is channeled into an absorber. In this tall tank, lime slurry is sprayed through nozzles. Purified air from the atmosphere and flue gas also are blown in. The sulfur dioxide and monoxide react with the oxygen from the air and the lime forming gypsum. The resultant gypsum suspension is desiccated using a hydrocyclone and belt filters. The non-toxic gas is then emitted to the atmosphere. All control parameters from this desulphurization process - such as pH value of the gas, removal of suspended materials (i.e., gypsum) and temperature - must be checked continuously to ensure proper and safe operation. The process must be adjusted if the equilibrium between pH value, temperature and lime infeed breaks down. A total of 2,200 digital and 350 analog values must be recorded to maintain this balance; 700 digital and 50 analog values are output, depending on measured values.
It is critical that the desulphurization process is not interrupted due to the failure of the automation system. An interruption could lead to a malfunction and the release of damaging emissions. Therefore, redundant control systems using PLC-based control and INTERBUS are used to offer high levels of availability and ensure that faults can be tolerated.
Control System The process master PLC calculates the output data based on the input data and its programming, passing it on to the process. At the same time, it transmits the same output data to a “hot standby” PLC via a high speed parallel link. This ensures it is possible to smoothly switch from one PLC to the other. To ensure that both PLCs are accessing the same historical data, only the process master PLC is allowed to calculate the output data. The PLCs also exchange control and dead man’s signals via two digital I/O cards. This guarantees there are no repercussions in the plant during a change-over if the process master PLC suffers a spontaneous failure In this redundant configuration, each PLC also has one or more INTERBUS controller boards which operate fully autonomous bus systems. Thus, there are two input or output channels for each sensor and actuator. To supply single input and output signals to the double modules, two diodes (in the case of a digital input) are used to decouple the modules. With analog signals, complicated networks of Zener diodes are used.
Results/Benefits Summary
Future Plans As a further safety option, the redundant backup module provides local intelligence which is capable of operating the sub-bus even if there is a total failure of the control mechanisms. The redundant 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. As in this application, one PLC is the process master and the other runs in hot standby mode. 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; availability of a remote bus interface makes it possible to use INTERBUS-compatible equipment.
Other Relevant Applications
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APPLICATION OVERVIEW
