Kinetics of Soot Oxidation by NO 2 MANISH SHRIVASTAVA, † ANH NGUYEN, ‡,§ ZHONGQING ZHENG, ‡,§ HAO-WEI WU, ‡,§ AND HEEJUNG S. JUNG* ,‡,§ Pacific Northwest National Laboratory, Richland, Washington 99352, Bourns College of Engineering, Center for Environmental Research and Technology (CE-CERT), University of California, Riverside, California 92521, and Department of Mechanical Engineering, University of California, Riverside, California 92521 Received December 4, 2009. Revised manuscript received April 22, 2010. Accepted April 23, 2010. Modern technologies use NO 2 to promote low-temperature soot oxidation for diesel particulate filter regeneration. In this study, the online aerosol technique of high-temperature oxidation tandem differential mobility analysis is used to study kinetics of soot oxidation by NO 2 . Soot particles are exposed to varying temperature and NO 2 mixing ratio inside the furnace resulting from thermal decomposition of NO 2 to NO. This causes soot oxidation rates to vary throughout the furnace. Variations in temperatures and NO 2 mixing ratio are thoroughly accounted for the first time. Soot oxidation rates are calculated as a function of frequency factor A soot , activation energy E soot , and concentration of NO 2 within the furnace at temperatures ranging from 500 to 950 °C. Results suggest A soot and E soot values for soot oxidation of 2.4 × 10 -14 (nm K -0.5 s -1 cm 3 molecule -1 ) and 47.1 kJ mol -1 , respectively, when reaction order to NO 2 is assumed as unity. The activation energy for soot oxidation with NO 2 is significantly lower than oxidation with air. However, parts per million levels of NO 2 cause soot oxidation at low temperatures suggesting NO 2 is a stronger oxidant than O 2 . 1. Introduction Soot has a controlling influence on earth’s temperature and climate. In addition, soot particles pose significant human health risk. Elimination of soot particle emissions from diesel engines has attracted lots of attention because of potential health risk around urban areas. Soot particles could be reduced either by modifying the combustion process inside the engine or by removing particles from the exhaust stream. Particles are removed from the exhaust stream using a diesel particulate filter (DPF), followed by catalytic soot oxidation to regenerate the filter (1). For these reasons, it is of immense interest to understand the kinetics of soot oxidation in the presence of oxidants like NO 2 and O 2 at temperatures relevant to both in-cylinder and exhaust after-treatment conditions. The high temperature (>800 °C) soot oxidation studies are relevant to modeling soot oxidation inside an engine cylinder or at flame condi- tions, while the low temperature (300-700 °C) soot oxidation studies are useful in understanding soot oxidation in the diesel exhaust and after-treatment systems such as diesel particulate filters (DPF) (2). Regeneration of DPF is performed at low temperatures (300-450 °C) to avoid thermal fatigue of the devices and to minimize energy used for regeneration. Although there are other oxidants present during diesel soot combustion, a majority of soot oxidation studies were done under oxygen environment or air (2-5). Stanmore et al. (6) reviewed soot oxidation by NO x (NO 2 , NO, and N 2 O). They reported the magnitude of soot oxidation reaction rates are in the order of NO 2 > N 2 O ≈ NO ≈ O 2 , showing NO 2 oxidation most prominent. NO 2 is a stronger oxidant than O 2 promoting low temperature oxidation of soot in the range of 200-500 °C(7). NO 2 has also been postulated to have a synergistic role with O 2 in the combustion of diesel soot (7, 8). Typically, NO 2 is 5-15% of the total NO x (less than 50 ppm) in the diesel exhaust. However, oxidation catalysts like Pt could oxidize NO to NO 2 increasing NO 2 mixing ratio to 50% of the total NO x , in the temperature range of 300-350 °C (7, 9). The continuously regenerating trap (CRT) (10) in diesel exhaust after-treatment system combines two chemical processes: platinum catalyzed oxidation of NO to NO 2 and oxidation of trapped soot in the presence of NO 2 . Thus, it is of interest to better understand kinetics of soot oxidation in the presence of NO 2 . Most of the previous research studied soot oxidation using offline methods such as thermo-gravimetric analysis (TGA) (8, 11, 12). However, this method is subject to mass and heat transfer effects that make the quantification of kinetics of soot oxidation difficult. Bulk sample of soot particles is packed by unknown packing ratio. A reasonable decoupling for mass transfer effect is achieved by measuring diffusion coefficient through the bulk sample (13). However authors are not aware of good methods to decouple heat transfer effect from bulk sample. The best experimental approach was to put quartz chips in the bulk sample and assume the temperature is uniform (14). The advantage of current TDMA (Tandem Differential Mobility Analyzer) method is to determine reaction rate by looking at size change of primary particles decoupling mass and heat transfer effects to begin with. Higgins et al. (3, 4) studied kinetics of soot oxidation from ethylene diffusion flame and from exhaust stream of medium- duty diesel engine over the temperature range of 800-1000 °C in the presence of air as an oxidant. They used the online aerosol technique of high-temperature oxidation tandem differential mobility analysis (HTO-TDMA). This technique offers several advantages over previous offline methods, such as thermogravimetric analysis (TGA) (8, 11, 12). The HTO- TDMA method minimizes effects of transport and soot aging (5). Choo et al. (15) derived oxidation characteristics of spark- discharge generated soot in the presence of NO 2 using the HTO-TDMA method. However, they made significant errors in their analysis, which will be discussed in detail at the discussion section. In this work, kinetics of soot oxidation using NO 2 as the primary oxidant are investigated using the HTO-TDMA method (3-5), at furnace temperatures ranging 500-950 °C. NO 2 inlet mixing ratios are varied from 0-600 ppm using N 2 as a carrier gas. A complication in using HTO-TDMA method to study soot oxidation with NO 2 using N 2 as a carrier gas is that NO 2 mixing ratios are not uniform throughout the heated length of the furnace. NO 2 decomposes to NO under N 2 environment inside the tube furnace at temperatures ex- ceeding 500 °C under experimental conditions of this study. NO 2 decomposition in the tube at higher temperatures is modeled using a second order kinetic rate equation. This allows calculation of NO 2 concentration at each axial location throughout the heated length of the furnace. It is noteworthy * Corresponding author e-mail: heejung@engr.ucr.edu. † Pacific Northwest National Laboratory. ‡ Bourns College of Engineering, Center for Environmental Research and Technology (CE-CERT). § Department of Mechanical Engineering, University of California. Environ. Sci. Technol. 2010, 44, 4796–4801 4796 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 12, 2010 10.1021/es903672y  2010 American Chemical Society Published on Web 05/21/2010 Downloaded by UNIV OF CALIFORNIA RIVERSIDE on September 7, 2015 | http://pubs.acs.org Publication Date (Web): May 21, 2010 | doi: 10.1021/es903672y