Study on mixing, modelling and control of an SCR system

• Main Author: Tian, Xinna
• Published: University of Strathclyde 2016
• Subjects: 623.8
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.694568

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 removal 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 systems with an option of installing SCR reactors before or after the turbines of engine turbochargers. This thesis focuses on modelling of evaporation and decomposition of urea-water-solution (UWS) droplets, design and optimisation of static mixers, modelling of an SCR reactor and developing model-based urea dosing control strategy. The amount of ammonia converted from UWS has a significant effect on the NOx removal efficiency of SCR systems. Due to a limited installation space for SCR systems on board, choosing the location of urea injection nozzle appropriately has become a critical issue for SCR system design. An evaporation and decomposition model of UWS droplets has been developed in this research in order to 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 order to achieve a high NOx removal rate and reduce the quantity of NH₃ slip, static mixers are commonly used before SCR reactors to improve the mixing between ammonia and exhaust gases. 4 novel static mixers have been designed and the performance of the mixers is compared in 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 agreement between the simulation and experiment results. A mathematical model of SCR reactors has been established. The unknown parameters of the model are identified by minimising the error between the model predicted and measured 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-based urea dosing control strategies. 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 described by a linear equation. 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 concentration in the exhaust.