Simulation Study On The Nitrogen Oxides Formation And Reduction Process Of The Ammonia-Diesel Dual Fuel Engine

It is of great practical significance to study the efficient and clean combustion theory and technology of ammonia fuel engine.The development of ammonia as a single engine fuel is limited by its bad combustion characteristics;Such as high spontaneous combustion temperature,low flame propagation speed,narrow flammability limit and high latent heat of vaporization.The mixed combustion of ammonia and hydrocarbon can effectively improve the combustion of ammonia fuel,and has shown good performance in the application of internal combustion engine,which has certain feasibility.However,from the aspects of economy and safety,the diesel-ammonia mixed fuel system is also a suitable choice.However,the combustion of ammonia and its mixed fuel will produce a large amount of nitrogen oxide emissions,and the research on ammonia-diesel dual-fuel engine is still in the basic stage,so the research on the dynamic mechanism of ammonia-diesel dual-fuel chemical reaction is helpful to promote the development of its engine in essence.In addition,a good chemical reaction kinetics model of ammonia mixed fuel including NO_x generation and reduction is the key to elucidate the NO_x generation mechanism of ammonia mixed fuel and reduce NO_x emissions.Therefore,the study on the chemical reaction kinetics mechanism of NO_x generation and reduction in ammonia-diesel dual-fuel combustion process can theoretically provide theoretical guidance for reducing NO_x emissions of ammonia and its hybrid fuel engines.At first,with the Chemkin software developed a containing NO_xformation and reduction of ammonia and diesel hybrid combustion mechanism of chemical reaction kinetics of simplified,and the mechanism of the dual fuel was zero peacekeeping three-dimensional validation and the corresponding correction and optimization,with the analysis of isotope labeling method,the dual fuel combustion NO_x in the process of production means and proportions.The results show that when the combustion temperature and equivalent ratio of the system increase,the proportion of nitrogen oxides generated from N₂ combustion in air increases.And then,the development of ammonia-n-heptane dual-fuel combustion mechanism coupled to the Converge three-dimensional engine model and with the help of three-dimensional CFD software CONVERGE simulation results,combined with the processing of Matlab simulation results,explore the NO_x generation and reduction boundary conditions under different conditions and the corresponding chemical reaction path.To provide theoretical guidance for improving NO_x emission.The results show that there are four main NO generation paths in the combustion process.N₂→NO,HNO→NO,HONO→NO,NO₂→NO;There are four main consumption paths of NO in the cylinder.From NO→NO₂,NO→HNO,NO→NNH,NO→N₂,respectively.The main consumption paths are NO→NNH and NO→N₂.Because the other two routes actually reflect the dynamic conversion balance of NO and NO₂,they have NO substantial impact on NO consumption.At the early stage of combustion,the reduction of NO in the cylinder is mainly concentrated in the whole combustion region and more in the high temperature region,but the reduction amount is small.With the continuous combustion,the reduction of NO mainly occurs in the middle and low temperature region and the temperature range is 960 K to 1500 K.As the combustion continues,the amount of NO reduced in the cylinder increases gradually.The temperature range is close to the suitable temperature range for selective non-reduction denitrification process.Finally,the simulation results of 3D CFD software CONVERGE were coupled to the calibrated SCR post-processing model to explore the NO_x reduction efficiency under different working conditions.Because the conventional SCR model could not reduce N₂O emission,the measures to reduce N₂O generated in the in-cylinder combustion process of ammonia and its mixed fuel engine were analyzed and verified.It provides theoretical guidance for the design of engine aftertreatment.The results show that NO_xcan be effectively reduced to N₂ when the exhaust gas in the cylinder of ammonia-diesel dual-fuel engine is directly isothermal into the SCR model without additional reducing agents such as ammonia.In order to reduce the N₂O emission of ammonia and its mixed fuel engines,the effective measures are mainly by means of post-treatment devices.The three-effect catalytic conversion device,oxidation catalytic conversion device and SCR post-treatment device are available.In order to ensure the conversion efficiency of the post-treatment unit,it is important to pump excess air or oxygen into the unit.