Study On Emission Characteristics And Removal Of PM And NO_X With After-Treatment For In-Use Diesel Vehicles In The Guanzhong Plain,Shaanxi,China

In-use diesel vehicular emission has become one of the major sources of ambient particulate matter（PM）and nitrogen oxides（NO_x）in the Guanzhong Plain（GZP）.Apart from phasing out in-use diesel vehicle（DV）with high emissions,installing after-treatment devices can also effectively reduce the PM and NO_x emissions of DV,so as to improve the ambient air quality.Because of the low emission standard,high emissions and large differences in emitted pollutants of in-use DV,the parameters of DV emission are prerequisites for designing after-treatment device.In addition,the limited space in in-use DV suggests that the conditions of after-treament device in DV needs to be adjusted before use.Therefore,it deserves insightful investigation to match the after-treatment technology with corresponding DV,so as to obtain the stable performance of reducing PM and NO_x emissions.In this study,the concentration,size distribution,morphology,chemical composition of PM,as well as concentration of NO_x（NO+NO₂）in the DV emission were investigated.This thesis studied the effects of key parameters on PM emission reduction efficiencies of Diesel Particulate Filter（DPF）and NO_x emission reduction efficiencies of Urea-Selective Catalytic Reduction（Urea-SCR）through model simulation,in which PM and NO_x emission parameters were model inputs.The simulation helped determine the specific after-treatment technologies that can simultaneously reduce the PM and NO_x emissions（de PM+de NO_x）.Two typical DVs were selected,which were intentionally equipped with de PM+de NO_x after-treatment system（DOC+DPF+Urea-SCR）.The PM and NO_x reduction efficiencies and durability of de PM+de NO_x system were studied by a 3×10⁴km running.As a scenario,this thesis evaluated the environmental and economic benefits for GZR that was brought by this system.The results were as follows.The number concentration of fine particle with D_p≤2.5μm(PM_2.5)was 3.01×10⁸#/cm³.The PM_0.1 accounted for>69%of PM_2.5 number concentration,suggesting that the particles in DV emission were mainly ultrafine particles.The mass fraction of carbonaceous component in PM was～65%.The scanning electron microscopy analysis showed that the soot in DV emission was agglomerated by spherical or ellipsoidal particles with～10 nm diameter and grape-like,flocculent or short-chain aggregates.The soot fractal dimension（D_f）was～1.8 with loose structure and large surface area.In addition,the concentration of NO_x in the tested DV emission was 1658±793 ppm,and the concentration of NO accounted for 82%of NO_x.Lattice-Boltzmann Method was applied to investigate the effect of micro-structure of porous walls on capture capability of DPF and its influence on the pressure drop（p_r）.The results showed that:smaller porosity of DPF filter led to better filtration efficiency and greater p_r.The suitable porosity of DPF installed on DV was 0.55-0.60.The growth probability（D_i）of micropore can be used to characterize the distribution of porous media skeleton.The quantitative relationship between p_r of the filter and transverse growth probability（D_i）of porous media was p_r=-0.548D_i+0.697（R²=0.959）.The major macro-structure parameters,which posed remarkable impacts to DPF filtration efficiency and pressure drop,included filter diameter,length and pore density.The diameter of DPF installed on DV should be larger than 250 mm.DPF regeneration and Urea-SCR performance were simulated by GT-Power.The scheme of DPF regeneration was as follows:3g/L initial deposition density of PM,120 g/s exhaust gas mass flow rate,12%O₂ concentration,and 350℃exhaust temperature.The optimum set-up scheme of Urea-SCR was:exhaust temperature in the range of 225-250℃,NO₂/NO_x=0.18 and NH₃/NO_x=1.0 in DV exhaust.The study evaluated the removal efficiency and durability of de PM+de NO_x system installed on tested large diesel passenger bus and heavy-duty diesel truck.The results showed that:the smoke value,i.e.extinction coefficient,of tested vehicles was reduced by～63%and the removal efficiency of PM_2.5 number concentration decreased to 58%and 97%at the initial stage of operation（20 km）.After 3×10⁴ km operation,the smoke removal efficiency and PM_2.5number concentration decreased to 31%-46%and 50%-97%,respectively.The NO_x conversion efficiency when using Urea-SCR was in the range of 46-60%.Assuming that 4.02×10⁴ DVs in the GZR are installed with the system of DOC+DPF+Urea-SCR in 2017,a scenario can be expected in this region that 1207 tons of PM2.5 and 13289 tons of NO_x are reduced in vehicular emissions.Total cost of after-treatment system installation is 1.407billion yuan in this scenario,corresponding to an increase of 90.5 million yuan in annual cost.