同步去除柴油机排气微粒和NOx的试验研究

An experimental investigation on removing PM and NOx simultaneously from diesel exhaustWu Qianli1，MA Chaochen1，Zhong lei1，Han jing21 School of Mechanical and Vehicular Engineering，Beijing Institute of Technology，Beijing（100081）2 Beijing Command College of CAPF，Beijing （100012）E-mail：mcc1900@AbstractIn order to achieve simultaneous removal of particulate matters (PM) and NOx in diesel exhaust, a new kind of aftertreatment proto type has been developed. The proto type combined effects of static, cyclone, non-thermal plasma and hydrocarbon selective catalytic reduction. Experiments have been carried out with standard gases simulating diesel exhaust. Physical and chemical effects that took place in the proto type are as follows: the collection of PM by electrostatic-cyclone system, the oxidative combustion of PM, the selective catalytic reduction of NOx, and the reaction between PM and NOx. The effect of non-thermal plasma makes the density of NO decrease and that of NO2 increase, whereas, the amount of NOx remains the same. Employing catalyst coupled with non-thermal plasma debase the temperature by about 50℃, there the peak value of transform rate appears. All of catalyst, non-thermal plasma, and deoxidizing HC are active to decrease combustion temperature of PM, moreover, the effect of all of them together decrease the temperature to about 280℃, which is informing. It is realized that PM and NOx were removed by Physics and chemical effects mentioned above.Keywords：plasma；hydrocarbon selective reduction；diesel；particulate matters；NOx1. IntroductionAt present, it is an urgent task to take all kinds of methods to remove PM and NO X in diesel exhaust simultaneously.The selective reduction of NOx by hydrocarbons (HC-SCR) in the presence of excess oxygen is a potential method to remove NO X from exhaust. Researchers are attracted by HC-SCR for there is a small amount of HC in diesel exhaust and can be easily obtained. There are two main problems when HC-SCR is used to engine exhaust: narrow active temperature window, especially in low temperature region; catalyst is sensitive to SO X and water vapor.Since 1990’s researcher have combined non-thermal plasma with HC-SCR to study the NO X reduction in oxygen rich exhaust[1-3]. It also reported that reaction can take place between soot and NO X, thus PM and NO X can be removed simultaneously[4-5].Plasma has the characteristic of having effect on both PM and NO X. There are two main aspects on decreasing diesel emission of plasma: The physical collection of diesel PM in ESP; The chemical effect of plasma on NO X and PM removal in exhaust.A new kind of attertreatment proto type has been developed. The proto type combined electrostatic-cyclone technology together with non-thermal plasma selective catalytic reduction technology to remove PM and NO X simultaneously. Experiments have been carried out with simulated gases and with real diesel exhaust .This paper is mainly focused on test with simulated gases and test with real diesel exhaust will describe in other papers.2. Experiment2.1 Experimental setupFigure 1 shows the experimental setup for simulated gas reaction test. The simulated gas consisted of NO/N2, O2; C3H6.The composition was controlled by MFCs (mass flow controller, Brooks, 0154).The plasma reactor was a pulsed corona type. The reactor was power by an AC+DC power supply. The voltage and current wave-forms were recorded by a digital oscilloscope (Tektronix-220)using PV1 high-voltage probe and PA1 current probe. Gases were analyzed in catalyst estimateFig.1. Schematic diagram of experimental setup1~6.gas bottles 7.MFC 8.blending room 9.reactor 10.oscilloscope11.power supply 12.gas analyzers puter 14.water bubblertest bench. The input energy density was calculated by multiplying the voltage, current wave-forms and the pulse repetition rate .A bubbler was used to control the humidity of gas flow and a cold trap was installed at the sample inlet of the NO X analyzer to prevent the water condensation in the analyzer. Catalyst used in the test is Ag/Al2O3. The crystallographic feature of catalyst was observed with X-Ray Diffraction Method (Shimadzu XRD-6000).Fig2 is the sketch of the proto type. The proto type is a “two stage” reactor, the first stage is a plasma generator, and gases are pretreated here and then goes to the chemical reaction stage by virtue of catalyst. The first stage is also an electrostatic-cyclone collector. When exhaust from diesel flows through the collector tangentially, PM is charged in electric field and attracted to the outer electrode forced by the electric field .Thus most PM is collected. When aggradation layer is thick enough, it will be blowed off under enough high inlet wind speed and fall into the collection groove .Uncollected PM goes through the second stage of the system where PM will oxidize further.Fig.2. “two-stage” reactor proto type1. Steel sleeve2.seal layer3.inner electrode4.outer electrode5. Insulated cirque6.collecting groove2.2 RESULT2.2.1 Plasma effect on NO conversionIn NO/O2/C3H6/N2 mixture, we examined the effects of hydrocarbon and energy density on NO, NO2 concentration changes. Fig3 shows the concentration changes as a function of energy density. In the existence of plasma, the concentration of NO decreased while NO2 concentration increased with total NO X concentration unchanged. This is because O radical dissociated from O2 induced by plasma makes NO be oxidated to NO2. When HC is added to the NO/O2/N2 mixture, the oxidation of NO to NO2 was greatly enhanced as the concentration of HC increased. The oxidation processof NO to NO2 was increased as the input energy density increased .But the increase rate decreased as the increase ofinput energy density.Fig .3. Effect of HC concentration and energy density on plasma oxidation of NO to NO2.O28%，T=250℃.(filled:NO,blank:NO2)2.2.2 Plasma effect on catalytic reduction of NOXIn this experiment the gas temperature in the plasma region was fixed at 150℃.The temperature of the catalyst region was controlled 100-500℃.The space velocity was keep at 10000h -1. Fig.4 shows the NO X conversion as a function of temperature under the effect of plasma/catalyst, or catalyst only. When the temperature is lower than 330℃, plasma/catalyst has higher conversion rate than catalyst only, and the peak conversion rate is about 70%.However the catalyst only has higher conversion rate than plasma/catalyst above 330℃.It can be also seen from Fig.4 that the peak conversion rate temperature shift about 50℃ to lower temperature zone with plasma/catalyst than catalyst only. This indicates that plasma can create active species at low temperature which are reductive and increase the activity of catalyst. This is particularly promising for engine use.2.2.3 PM oxidationFig5 shows the change of CO 2 density as a function of catalyst temperature .In order to find the direct influence of catalyst on PM removal, no010020030040050060001020304050energy density/(J.L -1)N O x c o n c e n t r a t i o n /(p p m )500ppm 1000ppm 500ppm 1000ppm 0ppm 0ppmC HFig.4. comparison of catalyst reduction of NO with and without plasmaNO 500ppm,O28%,energy density=30J/LFig5. catalyst effect on PM oxidationNO 、HC was added to the mixture. Soot from real exhaust gas was dipped to the surface of catalyst to simulate the real soot accumulation. CO 2 produced at 380℃.This indicates the start of PM oxidation. Generally ,soot react with O 2 at 500～600℃.When catalyst was used to the oxidation of PM, the temperature was greatly dropped.Fig 6 shows the change of PM oxidation temperature with plasma/catalyst. When no HC was added to the mixture, the PM oxidation temperature is 350℃.The temperature is lower than that of catalyst only. These results revealed that plasma strengthen the activity of catalyst.0.20.40.60.80200400600temperature/℃C O 2 d e n s i t y /%Fig.6 HC effect on PM oxidation NO 500ppm 、O2 8%、C1:NOX=6When HC was added to the mixture, the PM oxidation temperature further decreased to 280℃.Plasma has a better influence on PM oxidation when HC was added to the mixture than no HC. When HC went through the reactor, it turned into many highly active radicals, which were absorbed to catalyst and soot. They react on the surface of catalyst .Thus the oxidation of PM was enhanced.2.2.4 Plasma/catalyst effect on NOX and PM removalWhen NO was added to the mixture ,the oxidation temperature of PM is still 280℃ This indicates that NO has no influence on PM oxidation.Fig7.shows the NO X conversion rate with and without the presence of PM.Fig.7. PM effect on NOX conversion3 ConclusionsSimultaneous removal of NO X and PM by plasma/catalyst is evaluated in a “two-stage” plasma system. This paper focuses on the effect of plasma and catalyst with simulated gas. Based on the0.10.20.30.40.50.60.70.80100200300400500temperature/℃C O 2 d e n s i t y /%2040 6080 0 100200300400500temperature/℃N O x c o n v e r s i o n /%experimental results, the following is concluded:z In the existence of plasma, the concentration of NO decreased while NO2 concentration increased with total NO X concentration unchanged.z Peak conversion rate temperature shift about 50℃ to lower temperature zone with plasma/catalyst than catalyst only.z All of catalyst, non-thermal plasma, and deoxidizing HC are active to decrease combustion temperature of PM, moreover, the effect of all of them together decrease the temperature to about 280℃, which is informing.z It is realized that PM and NOx were removed by physics and chemical effects mentioned above.References[1] Held W, Koening A ,Richter T et al. Catalytic NO X reduction in net oxidizing exhaust gas.SAE,1990,900469.[2] Iwamoto M, Yahiro H Yu-u Y et al. Selective reduction of NO by lowerHydrocarbons in the presence of O2 and SO2 over copper ion-exchange zeolites.Shokubai.1990,32(6):430-433.[3] Li Y, Armor J N. Catalytic reduction of nitrogen oxides with methanein the presence of excess oxygen. Appl. Catal. B, 1992, 1(4):L31-L40.[4] Fuming Xiao et al. Plasma aftertreatment of diesel particulate for a minibus.SAE,2000-01-3459.[5] Thomas ,S.E.,A.R. Martin et al. Plasma aftertreatment of particulates: Theoretical limits and impact on reactordesign. SAE, 2000-01-1926.。