Question

(a) Explain why highly efficient diesel engines tend to produce more NOx than low performance diesel engines.

(b) Describe, with the aid of a sketch, a Selective Catalytic Reduction (SCR) unit for a marine propulsion diesel engine.

(c) Explain why accurate monitoring of the exhaust gas flows entering and leaving a Selective Catalytic Reduction unit are required and how these readings are used to control the reduction chemical supplied to the SCR unit.

Answer 1

Part (a)

The formation of NOx depends particularly on the temperature of the combustion. Highly efficient engines operate at a higher temperature and pressure than normal diesel engines. Higher the temperature higher the emissions of NOx (because more energy promotes the chemical reaction). These conditions favour the production of NOx gases. The quantity depends on the volume and duration of the hottest part of the flame.

Part (b)

SCR (Selective Catalytic Reduction) is a method used to control NOx emission. This method involves injection of a fine mist of urea plus water (called as Diesel Exhaust Fluid - DEF) into the engine’s exhaust system to create a chemical reaction to turn NOx into Nitrogen and Water Vapour.

DEF is a non-hazardous solution, which is 32.5% urea and 67.5% de-ionised water.

The SCR system consists of a reactor, catalyst elements, soot blower, sensors, air reservoir, mixing devices, dosing unit urea injection nozzle, urea pump and safety control system.

Part (c)

Urea is sensitive to temperature. At low temperature, urea cannot be decomposed to ammonia (NH3) and cannot be evaporated to absorb NOx from the exhaust gas. 300-350C is suitable for urea decomposition. At lower temperature, urea will deposit forming ammonium sulphate and block the exhaust passage. If temperature is above 500C, NH3 will be burnt and unable to absorb NOx. So accurate monitoring of exhaust temperature is important to monitor urea decomposition.

✓ Verified Model Answer (Text Solution)

SCR (Selective Catalytic Reduction) is a method used to control NOx emission. This method involves injection of a fine mist of urea plus water (called as Diesel Exhaust Fluid - DEF) into the engine’s exhaust system to create a chemical reaction to turn NOx into Nitrogen and Water Vapour.