Study on mixing, modelling and control of an SCR system.pdf

1、In recent years, more and more research attention has been paid to the NOx emissions caused by marine diesel engines. Selective catalytic reduction (SCR) system has been proven to be an effective technology for the remov

2、al of NOx emitted from marine diesel engines.In order to comply with stringent International Maritime Organization(IMO)Tier III NOx emission regulations,a number of engine manufacturers have developed their own SCR syste

3、ms with an option of installing SCR reactors before or after the turbines of engine turbochargers.This thesis focuses onmodelling of evaporation and decomposition of urea-water-solution(UWS) droplets, design and optimisa

4、tion of static mixers,modelling of an SCR reactor and developing model-based urea dosing control strategy.
　　The amount of ammoniaconverted from UWS has a significant effect on the NOx removal efficiency of SCR system

5、s. Due to a limited installation space for SCR systems on board,choosingthe location of urea injection nozzle appropriately has become a critical issue for SCR systemdesign. An evaporation and decomposition model of UWS

6、droplets has been developed in this researchin orderto determine the total depletion time of a UWS droplet, which is helpful to calculate the proper length between the urea nozzle and reactor of an SCR system.
　　In or

7、der to achieve a high NOx removal rate andreduce the quantity of NH3 slip, static mixers arecommonly used before SCR reactors to improve the mixing between ammonia and exhaust gases. 4 novel static mixershave beendesigne

8、d and the performance of the mixersis comparedin the study.An experiment has been conducted to validate the mixing performance and pressure loss of the static mixers developed. It shows that there is a satisfied agreemen

9、t between the simulationand experiment results.
　　A mathematical model of SCR reactorshas been established. The unknown parameters of the model are identified by minimising the error between the model predicted and me

10、asured values of both the temperature and the species concentration after the SCR reactor.The SCR reactor model is further used in a simulation for the purpose of developing model-basedurea dosing control strategies.

11、　　A state observer is used to determine the actual states in the reactor which supplies the mandatory information for developing model-based urea dosing control strategies. The NH3 cross-sensitivity of NOx sensors can be

12、 described by a lineare quation. The simulation results of the observer show that the NH3 cross-sensitivity of NOx sensors can be neglected when estimating the actual states of the reactor if NH3 is of a low concentratio