Vattenfall, a leading European energy company, operates with Vattenfall Europe Mining its own lignite mines in the east of Germany and is a well-known supplier of pulverized lignite. Pulverized lignite is a fine-grained fuel with a high heating value and a liquid-like character. It is used as an alternative fuel to heating oil, natural gas and hard coal in many industries.
Delivery is made by trucks or railway using special tanks which are filled from silos
(fig. 1) by the supplier.
In general, when flammable, fine-grained bulk materials are stored in silos or transported in tanks, undetected smoldering fires or hot spots cause the danger of spontaneous combustion or explosions. During smoldering, methane and carbon monoxide are generated; they are good indicators for a beginning smoldering. Hot spots are able to ignite lignite powder when the powder is mixed with air in parts of a silo – and may cause fire or dust explosion.
Early detection of smoldering fires and the resulting gases respectively by suitable gas analysis equipmet is considered as best means to prevent such hazardous incidents.
The solution (Fig. 3) is based on the continuous analysis of methane content in sample gases which are extracted from the loading plant at different spots. Four gas sampling probes are installed, one each at the three outlet pipes and one at the top of the silo. Separate sample gas lines carry the gas samples to two dual channel NDIR-based gas analyzers (ULTRAMAT 23, fig. 2) where they are analyzed continuously for their methane content. The concentration signals are recorded and limit switches will immediately respond when a critical level of methane is reached.
All gas analyzers are equipped with interfaces to Profibus DP and communicate measuring values and status that way with the plant controller (Simatic S7). As soon as limit signals are recorded, the controller will immediately activate an alarm and, if required, the entire loading system will be purged with nitrogen. Furthermore, to achieve highest possible system reliability, the analyzers are recalibrated periodically through their internal AUTOCAL function using ambient air. However, recalibration is innitiated by the controller via Profibus only during breaks in the loading process to ensure an efficient, uninterrupted and safe loading.
Smoldering fire detection is a well-known and proved application of continuous NDIR gas analyzers. Connecting the gas analyzers directly to Profibus, however, makes this safety system an integrated part of the entire plant control system. Vattenfall Europe Mining uses Profibus communication generally for years in their plants but it is the first time to integrate gas analyzers into the network. Stefan Weichert, responsible for the löading plant, states: The Profibus communication allows the controller to regularly adjust the automatic calibration cycles of the gas analyzers into time slots where the loading process is interrupted and to block them during loading. So we can be sure that the safety control function of the analyzers is definitely available during all loading cycles.