Fig. 1 Diesel Generating Plant Configuration with SCR
Exhaust gases leaving the engine exhaust manifold pass through the exhaust-gas duct to the SCR system. Exhaust gases in this instance measure 300 to 400 deg. C in the half-/high-load operation, to the satisfaction of temperature requirement necessary for the activation of denitrification catalyzer.
Exhaust gases treated for NOx removal in the SCR system enters the exhaust turbocharger and, after driving the turbocharger turbine for the associated compressor to charge air into the engine cylinders, is discharged out of the turbocharger as used exhaust gases relatively low in temperature at about 250 deg. C.
2-2. Change in Plant Dynamic Characteristics from Use of SCR System
Fig. 2 shows how the diesel generating plant configured as in Fig. 1 would vary in turbocharger speed, exhaust-gas temperature, etc. which constitute its dynamic characteristics, when the plant output power is brought down from 2800 kW to 2400 kW.
It can be seen that though the plant output power remains at 2400 kW following the reduction in engine output, the hunting occurs in the scavenging and exhaust circuits affecting the turbocharger speed, scavenging pressure, exhaust pressure, and engine exhaust-gas temperature etc., the hunting increasing in amplitude with time and ending up in the state of divergence. All this would naturally result in abnormal rise in engine exhaust-gas temperature to the point of rendering the plant operation impossible as a matter of particular concern.
Such a hunting phenomenon is triggered by the mechanism explained in the engine exhaust outlet and turbocharger gas inlet and is a characteristic feature intrinsic to the diesel generating plant of the configuration referred to. Overcoming this problem, therefore, is of vital importance to the successful plant operation.
2-3. Mechanism Triggering Hunting
With reference to the hunting which occurs in the scavenging and exhaust circuits of diesel generating plant fitted with the SCR system in the process of bringing down the plant output power, how such a hunting phenomenon is triggered is explained as follows.
Fig. 3 delineates the mechanism of the hunting occurring in the scavenging and exhaust circuits when bringing down the plant output power in the form of diagram, from which it can be seen that without the SCR system, the turbocharger inlet gas temperature and its speed go down to proper levels without delay in response to the load change (as indicated with the dotted lines in the diagram).
When fitted with the SCR system between the dynamo engine and its turbocharger, however, the hunting is triggered in the scavenging and exhaust circuits by the following mechanism.
1] When the engine output is reduced, the SCR system, being large in heat capacity, lags in responding to it with consequent delay in decline of the SCR system and its discharge exhaust-gas temperature.
2] The delay in decline of the SCR system discharge exhaust-gas temperature causes the turbocharger to keep running without going down in speed immediately; the turbocharger running at higher speed than what is suitable for the reduced engine output results in charging an excessively larger quantity of air into the engine cylinders than is warranted.
3] Consequently the exhaust gases discharged from the engine cylinders (into the exhaust manifold) go down in temperature below what is normal with the engine output.
