Industrial Application of a Preventive Knock Technique

CHP power plants, supplied by natural gas, have a great interest due to more and more stringent environmental regulations. Natural gas has a low C/H ratio resulting in low CO2 emissions in spark ignition engines. For economical reasons, CHP gas engines are normally designed to operate under their optimal settings. Small variations in the composition of the supplied gas can then lead to knock occurrence. In this paper, a preventive knock device is developed for CHP power plants. It is based on the instantaneous measure of the Methane Number (MN) of the supplied gas. The measure is performed through an online MN gas sensor (it measures as well Wobbe index and the calorific value of the supplied gas). Prediction of knock, following the MN of the gas is developed in previous works. Correction of knock is performed through an engine map, established thanks to numerical simulations.

The preventive system was tested off-line on a CHP SI engine (138 l/32 cylinders/P=2 MWe) in order to compare it with the original knock curative system of the engine when knock occurs. Gas Methane Number was measured during three consecutive months (MN variation observed from 90 to 66). A major knocking problem was detected in many cylinders (MN=66). The protection device recommended a decrease of 2 degrees on SA and a 20% decrease on the engine load (P=1.6 MWe). Two main advantages are shown:

1. 1
   - The device recommends the adaptation of engine settings before knock appears preventing from engine damages.
2. 2
   - The device keeps the engine running (even if it is not in an optimal way) avoiding any power production losses induced by a shutdown of the engine.