Study On NH₃ Dosing Strategy For Solid Ammonium SCR System

"Optimizing combustion + SCR" technical route can effectively reduce diesel emissions for NO_x and PM, which is one of the technology road suitable for C hina to reduce diesel pollution. Traditional SC R uses Urea-Aqueous to generate NH₃ by the hydrolysis,to reduce the NO_x emission. However, the Urea-SCR has some inherent shortcomings that is difficult to avoid, such as crystallization and poor decomposition when the temperature is lower. That makes Urea-SCR system can not effectively NO x emissions for diesel engines in some small load work conditions. Compared with the conventional Urea-SCR, SCR solid ammonium uses NH₃ direct injection to guarantee the supply of NH₃, which can broaden the temperature operating range of the system, with NH₃ produced by heating solid ammonium salts.With increasingly stringent emission for diesel, the emissions requirements of small and medium load conditions are more and more harsh. Solid ammonium SC R can effectively reduce diesel emissions for NO x, that makes it become the new hot research for the SCR technology. NH₃ dosing strategy is an important part of solid ammonium SCR systems. This paper designed a model-based NH₃ closed- loop control strategy, main content as follows:Complete the establishment o f the catalyst control model, based on the Elay-Rideal reaction mechanism. This model consists of four parts: the model of gaseous NH₃, gaseous NO_x model, ammonia storage model and temperature model. The inputs for the model are the upstream catalytic NO_x and NH₃, the exhaust gas flow rate, inlet air temperature and ambient temperature with the output are downstream NO_x and N H₃. And the states of the model are the downstream outlet gas temperature, the NO_x concentration, NH₃ concentration, ammonia storage conditions and temperature. Based on the nonlinear least squares method to identify the parameters, by using the lsqnonlin of Matlab function to complete the identification process of realization.Complete the design of the model based on NH₃ injection control strategy. The Open-loop portion uses "state observer + feed- forward algorithm" mood for the structure: state observer can get the information of ammonia storage and temperature inside the catalytic; feed- forward algorithm makes the NH₃ slip amount for the control target to ensure the maximize control for NO_x emissions. In order to ensure the stability of the system and improve the system anti- jamming capability for environmental and other factors, using conventional PI control algorithm we design closed-loop feed-back section that using the downstream NO x sensor signal feed-back to adjust the control strategy. Completed the simulation research of the strategy.Design the Systems Management software to achieve coordination of control system software and hardware. The state machine design is based on management software theory and it has three parts:, the System State Module, Status Monitoring Module and the Underlying Hardware Modules. Underlying Hardware Module sends information of the engine operation to State Monitoring Module and drives the underlying hardware according to the instruction from EC U; The state Monitoring Module is responsible for the transmission of information between software and the underlying hardware and the real-time monitoring for the engine condition; System Status Module is the part of the brain control software which ensure the working of the after-treatment under the normal conditions and shut down the entire system when a fault occurs.Complete the building of engine test be nch in order to verify the availability of NH₃ injection control strategy.Use Mototron rapid development platform, we completed the experiments on N H₃ injection closed- loop control strategy.