排气温度和催化基质特性对柴油机排放的影响_英文_陈朝辉

第30卷第9期农业工程学报 V ol.30 No.942 2014年5月Transactions of the Chinese Society of Agricultural Engineering May 2012Influence of exhaust temperature and catalytic substrateproperties on diesel exhaustChen Zhaohui, Zhang Wei, Chen Guisheng, Shen Yinggang (Yunnan key Laboratory of Internal Combustion Engine, Kunming University of Science and Technology, Kunming 650500,China)Abstract: The influence of different temperatures and CDPF (catalyzed diesel particulate filter) substrate properties on NOx reduction and PM oxidation were studied by catalytic experiments, engine bench tests and simulation. From activity evaluation and characterization tests, it was found that owing to the higher mobility of lattice oxygen and the maximum concentrations of oxygen vacancies, La2Cu0.7Fe0.3O4 showed a relatively better catalytic performance between 300°C to 500°C. The NO conversion efficiency on SiC (Silicon carbide) substrate was better than that on cordierite substrate from 350℃ to 500℃under simulated diesel emission conditions. The engine bench test results showed that NOx conversion efficiency increased from 340℃ to 528℃. Due to higher porosity and stronger thermal diffusion characteristics, the soot oxidation rate and NOx conversion rate on SiC substrate CDPF is better than that on cordierite substrate, under condition of 75% and 90% loads of engine at 1 600 r/min. From simulation researches, CDPF with higher cell density and specific surface-area cell resulted in lower internal mass-transfer resistances, and higher mass-transfer coefficients, which yielded better soot and NOx reduction performances.Key words:diesel; catalytic; temperature; exhaust; substrate property; NOx; sootdoi：10.3969/j.issn.1002-6819.2014.09.006CLC number:TK411+.5 Document code:A Article ID:1002-6819(2014)-09-0042-08Chen Zhaohui, Zhang Wei, Chen Guisheng, et al. Influence of exhaust temperature and catalytic substrate properties on diesel exhaust[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2014, 30(9):42－49. (in English with Chinese abstract)陈朝辉，张 韦，陈贵升，等. 排气温度和催化基质特性对柴油机排放的影响[J]. 农业工程学报，2014，30(9)：42－49.0 IntroductionDue to the high thermal efficiency, good fuel economy and strong power, diesel engines have been widely used in various industries around the world[1-3]. However, NOx and PM are the main harmful emissions of diesel, they are trade-off in cylinder, and difficult to be removed simultaneously.Application of after-treatment technology like CDPF (catalyzed diesel particulate filter) can solve this problem[4-7].Yoshid, et al [8] first proposed the idea that, DPF was used to trap soot particles and then reacted with NOx, thus both particles and NOx can be simultaneously removed under the action of catalysts. There are several catalysts available to remove NOx and PM, such as precious metals, metal oxide Received date：2013-10-09 Revised date：2014-04-02Foundation item：National Natural Science Foundation of China (No.51276128), Yunnan Provincial Science and Technology Department(2013FB017), Yunnan Provincial Department of Education (KKJA201256011), Kunming University of Science and Technology (KKSY201256142).Biography：Chen Zhaohui(1980－), Ph.D. Instructor, Kunming, City College, Kunming University of Science and Technology, 650500.Email: chenzhaohuiok@ catalysts[9], perovskite catalysts[10] and spinel-type catalysts[11]. Some literatures[12-15] reported that, with the act of metal oxides, perovskite and spinel-type catalysts, the reaction temperature of NOx conversionto N2 is high, which is not suitable for commonly used conditions of diesel engine. In recent years, some authors[11,16-17] confirmed that the application of perovskite-type catalysts containing Cu to simultaneously reduce PM and NOx is practicable for engine. For instance, according to the research by Zhao, et al[11], NOx conversion rate was 22% and soot ignition temperature decreased to 376℃, while La1.5K0.5CuO4 and soot were in loose contact condition. However, in some literatures[18-21], sample experiments were taken to study the conversion ratesof removing NOx and PM under simulated conditionsof diesel exhaust. In this paper, the CDPF was prepared with catalysts for engine bench tests to verify the removal efficiency of actual diesel emissions NOx and PM, and to research the influence of temperature and substrate properties on catalytic reduction NOx and soot, which has great significance.第9期陈朝辉等：排气温度和催化基质特性对柴油机排放的影响431 Catalytic tests1.1 Catalyst preparation and characterizationAll required chemical reagents and citric acid were mixed and prepared into solution with appropriate stoichiometry. The solution was prepared into sol-gel using ultrasonic vibration and infrared light irradiation.After being pre-calcined for 2 h and further calcined at 800℃for 4 h, the perovskite-type oxide catalysts were prepared. The X-ray photoelectron spectroscopy spectra were obtained with a PHI-1600 ESCA spectrometer, and the binding energy was calibrated with respect to the C1s peak (284.6 eV) of contaminated carbon. NO desorption measurements were performed in a 6 mm quartz tube reactor, with a flow of N2 and O2 passing through the reactor at the flow rate of 20 mL/min, from 100°C to 960 °C and with heating rate of 10°C/min.1.2 Catalytic activity testThe catalyst and soot particles were mixed and placed in a fixed-bed quartz reactor, and were evaluated with a temperature programmed reaction process. Reactant gases included 10% O2 and 5% NO balanced with He, at the flow rate of 50 mL/min. The catalytic performance was evaluated in terms of soot ignition temperature (T ig), the maximum productivity of N2 (P N2) and its corresponding temperature (T max). T ig can be calculated by extrapolating the steeply ascending portion of the carbon dioxide formation curve to zero carbon dioxide concentration[22-23]. P N2 can be calculated by 2[N2]out/[NO]in, where [N2]out and [NO]in are the concentrations of N2 in outlet gas and of NO in inlet gas respectively[22].1.3 X-ray Photoelectron Spectroscopy resultsFig.1 shows the O1s peak separation and binding energy of La2Cu1-x Fe x O4(x=0.1, 0.2, 0.3), where x represents the amount of Fe substituted for Cu. We inferred that there are two kinds of oxygen on catalyst surface, which is lattice oxygen and adsorbed oxygen. The lower binding energy (located at 529.2-529.78 eV) is lattice oxygen, and the higher binding energy (531.77-531.92 eV) is adsorbed oxygen. With increase of Cu partial substitution with Fe, the mobility of lattice oxygen increased gradually.1.4 NO desorption tests under different temperaturesThe NO desorption tests results of La2Cu1-x Fe x O4 (x=0.1-0.3) catalysts are shown in Fig. 2. There are three desorption peaks in NO desorption, γpeak (150℃≤T≤300℃)，ηpeak (300℃≤T≤500℃) andδ peak (T ≥500℃). The desorption species after NO reacts on catalyst surface are N2, N2O, NO2 and NO. According to IR-study on adsorption of NO over different absorbents by Zhao[24-25], we concluded the three desorption peaks are N2O, NO and N2 species. The difference in adsorbed amount of NO on perovskite catalysts would be attributed to the different concentrations of oxygen vacancies. In literature[25] La2CuO4 only has a small NO desorption peak because of its small amount of nonstoichiometric oxygen, and the intensity of desorption peak increases with B-site partial substitution, which is due to a large amount of oxygen vacancy formed in the La2Cu1-x Fe x O4samples. Among all of the samples, the area of NO desorption peak is the largest over La2Cu0.7Fe0.3O4, indicating that it has the maximumconcentration of oxygen vacancies.Note: x represents the amount of Fe substituted for Cu, the same below.Fig.1 O1s X-rayphotoelectron spectroscopy spectraFig.2 NO desorption curves under different temperatures 1.5 Thermogravimetry and differential thermal analysis of sootThermogravimetry and differential thermal analysis tests were carried out in a fixed bed reactor, printex-U soot were placed in it in the absence of catalysts, reactant gases included 20% O2 and 80% He. The results are shown in Fig.3.From thermogravimetry curve, the weight of soot particles decreased significantly at 448℃, and the weightlessness finished at 673℃. From the differential thermal curve, soot combustion exothermic peak temperature is 580°C. From temperature-programmed农业工程学报 2014年44reaction (TPR) curve of soot, the ignition temperature is 500°C, and the generation of CO 2 peak concentration temperature is 580°C, the weightlessness slope of soot in thermogravimetry curve showed that the maximum oxidation rate of soot particles is 580℃. The burnout temperature is 680°C, and the differential thermal analysis curve tends to 0 corresponding to temperatures, and the weight of soot on the thermogravimetry curve also tends to 0. The above analysis showed that the tested data by catalyticevaluation apparatus are reliable.Fig.3 Thermogravimetry and differential thermal analysis ofsoot1.6 NOx and soot reduction characteristics under different temperaturesFig.4 shows the results of NOx and soot reduction characteristics under different temperatures over La 2Cu 1-x Fe x O 4. With the B-site substitution increases from 0.1 to 0.3, NO conversion increased significantly. When the substitution is 0.3, the maximum conversion efficiency of NO is up to 45% at 450℃. When the substitution is 0.1, the soot ignition temperature is only 302℃, but the NO conversion efficiency is also the lowest, only 17%. When the substitution is 0.3, the ignition temperature is 350℃, although this temperature is higher than substitution 0.1 and 0.2, it is still 150℃ lower than that with absence of catalysts. Because exhaust temperatures can be increased by post-injection [26] or add DOC(diesel oxidation catalyst) before CDPF [27,28], we selected La 2Cu 0.7Fe 0.3O 4 with the highest NO conversion efficiency for engine bench tests, to research the NOx and PM reduction efficiency of actual engine.1.7 Influent of different DPF substrates on NOx and soot reduction under simulation conditionThe effects of cordierite and SiC substrates on catalytic activities were tested under simulation condition. Cordierite substrate was treated by acid, SiC substrate was activated at 500℃, catalysts wereFig.4 NO conversion to N 2 and soot oxidation to CO 2coated onto surface of substrate by volume impregnation method. The results of two substrates influence on NO conversion efficiency and CO 2 concentration are shown in Fig.5. We can see that conversion efficiency of NOx and oxidation efficiency of soot is almost equal between 200℃ and 350℃ when catalysts on the two substrates. NO conversion rate on SiC substrate was better than that on cordierite substrate from 350℃ to 500℃, which indicated that SiC substrate was better to reduce NOx in diesel exhaust temperature. NO conversion rate of cordierite substrate is slightly higher than SiC substrate from 550℃ to 700℃. Compared to cordierite substrate, SiC substrate with higher porosity and stronger thermal diffusion characteristics, the temperature distribution on SiC substrate surface in TPR test was more uniform, and the catalytic reaction temperature is lower than that on cordierite substrate internal surface [29]. So from CO 2 concentration curve, the soot oxidation rate on SiC substrate is lower than that oncordierite substrate.Fig.5 Effects of different substrates on NOx and sootreduction第9期 陈朝辉等：排气温度和催化基质特性对柴油机排放的影响452 Engine bench test results2.1 Test engine and experiment setupLa 2Cu 0.7Fe 0.3O 4 was coated onto the wall-flow cordierite and SiC DPF substrates, and was packaged into CDPF. The DPF specification is 110 mm× 160 mm (1.6 L), with 200 CPSI (cells per square inch) cell densities. The test engine is WP12 with six-cylinders and common rail fuel injection system, and the displacement is 11.596 L. Because the displacement of engine is larger than the volume of CDPF, the sixth cylinder was modified as working cylinder and exhausted alone, the other five didn’t work. The CDPF was put on the exhaust pipe of test cylinder, and was matched with exhaust gas flow. Fig.6 shows the schematic diagram of engine test bench. By measuring NOx concentrations at front and back ends of CDPF, catalytic conversion of NOx was achieved, and by measuring pressure difference of the front and back ends of CDPF, PM capturing and oxidizationinformation was acquired.Fig.6 Schematic diagram of engine test bench2.2 Effects of different engine exhaust temperatures on reduction of NOx and PMFuel injection pulse width of engine at 1 600 r/min is adjusted to change engine loads. Electric heater was installed in front of CDPF in exhaust pipe, which can change temperatures of exhaust gas. Exhaust flow rate was kept at 0.046 kg/s. Fig.7 shows the influence of different temperatures of exhaust gas on NOx removal efficiency and on pressure drop of CDPF. From Fig.7a, the NOx conversion rate increased from 340℃ to 528℃, it was only 7.4% when temperature is 340℃, while it increased to 9.9% when temperature reaches 528℃. This is because since the activity of surface oxygen increases as growing temperature when catalyst works at its active window, which is beneficial for NO adsorption and desorption on oxygen vacancy, so NOx conversion efficiency increases. However, whentemperature reaches 567℃, NOx conversion rate decreases, the reason is that the very high temperature is not conducive for NO desorption on catalyst surface.Fig.7b is the impact of different temperatures of exhaust gas on pressure drop. Engine ran for 2 h for capturing soot. When exhaust gas temperature is 340℃, the pressure drop increases to about 15 kPa for capturing soot particles. And then the engine ran for 40 min with exhaust gas temperatures respectively are 340℃, 528℃ and 567℃. The pressure drop increases to approximately 19 kPa at 160 minutes when engine ran at 340℃, because it does not reach ignition temperature of PM. The back pressure drops to about 10 kPa when engine ran at 528℃. This is due to the oxidation of soot particles, while the back pressure drops to about 8 kPa when engine ran at 567℃ due tothe faster oxidation rate of soot particles.Fig.7 NOx conversion rate and back pressure of PM trap and oxidation in CDPF under different exhaust temperatures2.3 Effects of different DPF substrates on NOx and soot reduction in diesel exhaust pipeCatalyst was coated on internal surface of Cordierite and SiC DPF substrate, to test effects of different DPF substrates on NOx and soot reduction in diesel exhaust pipe. The test conditions were 50%, 75% and 90% loads of engine at 1 600 r/min. The results are shown in Fig.8. From Fig. 8a, we can see that when the load is 50%, NOx conversion rate on SiC substrate is lower than that on cordierite substrate. However, in case of 75% and 90% loads, the conversion efficiencies are on the contrary.As Fig. 8b shows, rate of pressure rise on SiC substrate is higher than that on cordierite substrate. When engine was running at 120 min, the back农业工程学报 2014年46pressure on SiC substrate was 14 kPa, which was 1 kPa higher than that on cordierite substrate. As the thermal conductivity and thermal diffusion of SiC is better than cordierite, the oxidation rate of soot on SiC substrate is faster. When the engine was running at 160 min, the back pressure on SiC substrate CDPF dropped to 10 kPa, which was 0.7 kPa lower than thaton cordierite substrate.Fig.8 Different substrates on CDPF removal of NOx and PM3 Cell characteristics on NOx and soot reduction by simulationSimulation method was used to study SiC CDPF with different cell densities, cell shapes and catalytic wall heights on soot oxidation and NOx reduction at 450℃. From Fig.9a, soot oxidation rate increased with rising cell density. The soot load of 100 CPSI was 4.25 g /L after 500 s, however, it reduced to 3.59 g/L with 400 CPSI. Cell density has greater impact on CDPF overall backpressure, the maximum back pressure of 100 CPSI CDPF was 6.29 kPa at 320 s, while it was only 2.01 kPa with 400 CPSI. Fig.9b showed that the rate of NOx absorbed on catalyst surface also increased with rising cell density, and the generated coverage of nitrate enhanced. Therefore, a general positive trend in NO conversions was observed, going from 100 CPSI monolith catalysts to 400 CPSI monolith catalysts. This is because in external of CDPF with higher cell density, there are smaller hydraulic diameters and higher mass-transfer coefficients. In internal of CDPF, higher CPSI will relate to thinner wall thickness andwashcoat layer, which resulted in internal lower mass-transfer resistances [30].Fig.9 Different cell densities on CDPF removal NOx and PMFig.10 Different cell shapes on CDPF removal NOx and PMFig.10 is characteristics of three cell shapes (square 400 cpsi, hexahex 400 cpsi, octagon GSE 400 cpsi) on soot oxidation and NO reduction. As showed in Fig.10a, because the square cell, hexagons and第9期陈朝辉等：排气温度和催化基质特性对柴油机排放的影响47octagons cell had larger specific surface-area and thinner washcoat layer, which resulted in greater mass transfer coefficient. The soot load of hexahex cell was 3.51 g/L at 500 s, it reduced to 3.46 g/L when CDPF with octagon GSE cell, which resulted lower overall pressure drop. As can be seen from Fig.10b, GSE CDPF also shows a faster NOx absorption and higher NO reduction rate.Fig.11 shows two different catalytic wall heights effect, namely, a 400 CPSI monolith sample with 100% and 60% catalytic wall height. We can see that because there is less catalyst participates in catalytic reaction, the 0.6 catalytic wall height shows a lower soot oxidation rate and higher pressure drop, also it displays a less NOx absorption and lower NO conversion rate.Fig.11 Different catalytic wall heights on removal NOx andPM4 Conclusions1) La2Cu x Fe1-x O4 has been prepared by citric acid complexation method. The maximum oxidation rate of soot without catalysts is 580℃. 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Res. 2011, 50(1):299－309.第9期陈朝辉等：排气温度和催化基质特性对柴油机排放的影响49排气温度和催化基质特性对柴油机排放的影响陈朝辉，张韦，陈贵升，沈颖刚（昆明理工大学云南省内燃机重点实验室，昆明 650500）摘要：为了提高催化器CDPF（catalyzed diesel particulate filter）去除NOx和PM的性能，该文运用催化试验、发动机台架试验及模拟计算，研究了排气温度和催化器的基底材料特性对NOx还原和PM氧化的影响特性。