Basic Theory Study On Micro-scaled Physicochemical Properties Of Compound Regeneration Of Diesel Particulate Filter

Diesel engines are used more and more widely in the world,but the PM emission is a serious environmental pollution and increasingly threaten human health.DPF（Diesel Particulate Filter）is one of the most effective post-processing technology to remove PM emission.DPF regeneration is the focus of the research.Experts are trying hard to improve the regeneration efficiency of DPF.In this paper,the microscopic physicochemical properties of the DPF composite regeneration process were studied.The research content of this paper mainly includes:Based on AVL-Fire and Boost,DPF flow field model,pressure drop model,PM prediction and regeneration chemical reaction model were constructed.The influence of DPF-related physical parameters on its internal PM trapping process,active regeneration frequency,and maximum active regeneration temperature was studied using numerical simulation.Based on CHEMKIN-PRO,the simplified mechanism of low-temperature combustion of post-spray oxidant was studied in the active regeneration of DOC-DPF system.The effects of the type of post-spray oxidant and the internal distribution ratio on its combustion and emissions were studied.In this paper,the kinetic mechanism of the chemical reaction between NO_x（including N₂O）and PM is constructed.The diesel emission test system was used to study its emission law,and the test results of the reaction mechanism were verified by the test results of the emission test.The NO_x-PM reaction process under different inlet conditions of the DPF was studied.This study found that high porosity,high inlet and outlet ratios,hexagonal channels,high wall permeability,low carbon soot permeability,high ash distribution coefficient,low initial ash content,large pore wall thickness and other physical properties are favorable realize energy saving and effective regeneration of DPF.The pressure drop curves of filter cake soot and deep soot were studied separately.The intersection of the two is defined as the intersection TR between the filter cake layer and the deep bed soot pressure drop curve.Monitoring the relationship between T_Rand DPF regenerative signals can effectively reduce the active regeneration frequency.T_R will be delayed as the DPF cell wall thickness decreases and soot penetration increases.In this paper,an 11-step simplified reaction mechanism of methane-hydrogen mixture as a DPF activator and a 32-step simplified reaction mechanism of n-heptane-toluene mixture as a diesel substitute as a DPF activator were studied.The blending ratio of the two mixtures has a great influence on their properties.With the increase of the blend ratio of H₂ in the methane-hydrogen gas mixture,the combustion rate of the fuel is accelerated,the ignition delay period is shortened,the CO emission is reduced,and the NO_x emission is increased.When the H₂ blend ratio exceeds 20%,the value of NO_x emissions increases significantly.With the increase of the mixing ratio of toluene in the n-heptane-toluene mixture fuel,the burning rate is reduced,the ignition delay period becomes longer,and the combustion is slower.CO emissions are about 50%higher than methane-hydrogen mixtures that react under the same conditions,but NO_x emissions are extremely low.In this paper,the detailed reaction mechanism of NO_x-PM containing N₂O in the DPF is constructed.By studying the chemical reaction process between NO_x and PM under different inlet conditions,it was found that the DPF inlet temperature should be between 400 and 500℃.At this point,high PM removal efficiency can be ensured without preventing NO_x removal due to excessive temperature.The ratio of NO₂ to total NO_x content in the exhaust gas at the inlet of the DPF is introduced asα.Controlling the working process of the DOC so as to control theαvalue in the range of 0.7 to 0.8 can achieve better regeneration and removal of the PM without excessively deteriorating the NO_x removal efficiency.The molar ratio of carbon in NO_x and PM introduced into the DPF inlet diesel exhaust isβ.The removal efficiency of NO_x is the highest whenβ=1.Whenβ≥8,PM can rely on the mutual chemical reaction of DPF internal reactants under this condition to complete continuous and complete passive regeneration.However,in the actual engine operation,NO_x emissions must still be taken into consideration and supplemented by corresponding active regeneration methods.N₂O is an important intermediate product in the process of NO_x-PM reaction,and its presence can further reduce the ignition temperature of PM.CO is also an important intermediate product in the NO_x-PM reaction process.It can continue to react with other oxidants and eventually produce CO₂.During the reaction of CO with NO_x,the effect of the CO ratio to NO_x is much greater than the effect of CO and different types of reactants.Properly increasing the proportion of NO_x in the reactants helps to increase the CO removal efficiency.In this paper,based on the research of the DPF microcosmic physical and chemical properties,DPF active-passive regeneration change rules and corresponding control strategy were mastered.This paper has laid a theoretical basis for future research about the DPF energy-efficient regeneration.