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Nitrogen compounds (NO, NO2, N2O and NH3) in NOx emissions from commercial EURO VI type heavy-duty diesel engines with a urea-selective catalytic reduction system Joonho Jeon, Jong Tae Lee, and Sungwook Park Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01331 • Publication Date (Web): 29 Jul 2016 Downloaded from http://pubs.acs.org on August 5, 2016

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Energy & Fuels 1

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Nitrogen compounds (NO, NO2, N2O and NH3) in NOx emissions

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from commercial EURO VI type heavy-duty diesel engines with a

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urea-selective catalytic reduction system

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Joonho Jeon†, Jong Tae Lee‡, Sungwook Park§,*

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†

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University, Seoul 04763, Republic of Korea

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‡

National Institute of Environmental Research, Incheon 22689, Republic of Korea

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§

School of Mechanical Engineering, Hanyang University, Seoul 04763, Republic of Korea

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Submitted to “Energy & Fuels”

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Submission date: July 22, 2016

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* Corresponding author

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Sungwook Park

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Associate Professor

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School of Mechanical Engineering,

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Hanyang University,

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222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea

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Tel. : +82-2-2220-0430

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Fax : +82-2-2280-4588

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E-mail : [email protected]

Department of Mechanical Convergence Engineering, Graduate School of Hanyang

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ACS Paragon Plus Environment

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Abstract

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Selective catalytic reduction (SCR) systems have been widely used in heavy-duty (HD) diesel

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engines to meet the stringent emission standards for nitrogen oxides. Mobile SCR system has

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improved the considerable reduction of NOx emission from HD diesel engines. Many

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investigations have performed to enhance the catalytic performance and optimize the SCR

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system within the diesel engines. The purpose of the present study was to investigate the

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types of nitrogen components (NO, NO2, N2O, and NH3) in the NOx exhaust emissions prior

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to and after passing through an SCR device. A EURO VI type commercial heavy-duty diesel

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engine was equipped to an AC dynamometer. World harmonized stationary cycle and

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transient cycle were introduced to operate the test engine under road conditions. A new

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quantum–cascade laser analyzer was employed to measure the nitrogen emission species in

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real time. The engine-out NOx emissions were strongly affected by the load conditions of the

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engine, which also determined the efficiency of SCR conversion. Total conversion rates of up

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to 96% were achieved for both test modes. Various concentration of urea were used as a

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reducing agent in the SCR system. Based on the urea concentration, the conversion efficiency

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and composition of nitrogen oxides varied under the same engine conditions. The fraction of

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nitrogen compounds in NOx emissions changed during the catalytic processes within the SCR

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system. The results yielded accurate concentration values for nitrogen compounds in the

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commercial heavy-duty engine, warning of the possibility of a new greenhouse gas due to

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converted NOx emissions.

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Keywords

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EURO VI, Heavy–duty, Nitrogen oxide, Selective catalytic reduction, Nitric oxide, Nitric

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dioxide, Nitrous oxide, Ammonia, WHSC, WHTC


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Nomenclature

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Carbon monoxide

CO

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Coefficient of variation

CoV

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Constant volume sampler

CVS

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Diesel oxidation catalyst

DOC

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Engine control unit

ECU

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Greenhouse gas

GHG

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Heavy–duty

HD

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Nitrogen oxides

NOx

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Nitrogen dioxide

NO2

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Nitric oxide

NO

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Nitrous oxide

N2 O

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Particulate matter

PM

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Portable emissions measurement system PEMS

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Quantum–cascade laser

QCL

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Selective catalytic reduction

SCR

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Total hydrocarbon

THC

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World harmonized stationary cycle

WHSC

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World harmonized transient cycle

WHTC

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1. Introduction

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Selective catalytic reduction (SCR) systems have been applied to heavy–duty (HD)

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diesel engines as an after-treatment system to reduce nitrogen oxides emissions, which are

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considered to be noxious gases within the air. To meet strict emissions regulations, EURO VI,

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an SCR technology that has been used in industrial stationary applications, was introduced to

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treat the nitrogen oxide (NOx) emissions of mobile diesel engines

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SCR technology developments, urea-SCR devices were selected by a number of

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manufacturers for the light– and heavy–duty diesel engines 4. Urea, NH2–CO–NH2, is an

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SCR reductant substitute for ammonia, which is toxic and difficult to handle. An aqueous

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urea solution was injected into the exhaust gas stream at the entrance of the SCR system.

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Under high temperature conditions, the urea solutions decomposed via the following

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hydrolysis reaction.

1,-3

. As a result of many

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(1)

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Equation (1) reveals the global urea decomposition process. Among the various urea

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decomposition products, ammonia is used as a reducing agent in the SCR system to reduce

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NOx emissions. The main SCR reactions are suggested as follows:

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(2)

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(3)

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(4)

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With regard to the NOx emissions produced by a diesel engine, nitric oxide (NO)

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comprises over 90% of the emissions. Equation (2) is thus considered to be the primary

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reaction for the reduction of NOx in the urea-SCR system. Equation (3) does not consume


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oxygen, which is a much slower reaction 5. The presence of nitrogen dioxide (NO2)

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accelerated the NOx conversion reactions seen in Eq. (4). When the quantities of NO and NO2

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were equimolar, the reaction rate was much faster than that of the main reaction (2)

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Related to the NO2 concentration in emissions, DeNOx reaction rates have been investigated

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by various research groups. Since the mobile SCR system should be concerned with costs,

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catalyst materials have been developed as the primary subject of SCR research in the last

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decade. State-of-the-art commercial SCR system have been developed with Vanadium-base 9-

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11

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systems in industry are operated under steady-state conditions, mobile SCR devices are

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driven under transient temperature conditions. DeNOx performance is also strongly

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influenced by temperature. Commercial SCR after-treatment systems are optimized to operate

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under a wide range of temperature conditions with a large number of investigations.

, Cu-zeolite

12, 13

, and Fe-zeolite

14-17

6-8

.

based SCR catalysts. Additionally, although the SCR

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However, most of the performed research was conducted with chemical aspects under

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laboratories’ conditions. There were few results to concern the SCR device coupled to the 18

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actual engine system. Misra et al.

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2010 and 2011 HD diesel engines and evaluated the emitted NOx levels during road driving

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with a portable emissions measurement system (PEMS). Four different trucks were used to

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compare the reduction in NOx and the effects of various vehicle conditions. Misra et al.

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reported their experimental results, revealing the effects of temperature and load conditions

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on the potential to control NOx emissions. Myung et al.

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HD engine performance and emission characteristics under various driving cycle modes,

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referencing the European cycle (ESC/ETC) and worldwide harmonized driving cycles

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(WHSC/ WHTC). They compared the regulated emissions (THC, CO, NOx, and PM)

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concentrations under four driving cycles. The test results reported that the newly introduced

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cycles (WHSC/WHTC) featured lower exhaust temperatures and a better reflection of

investigated the in-use NOx emissions for model year

19

conducted a study with regard to


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practical HD diesel engines than the European modes. SCR system research coupled with

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hybrid– and conventional diesel–buses was reported by Guo’s groups

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types of bus engines on the road conditions with PEMS to measure the NOx emissions. Two

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hybrid buses and two conventional diesel buses were compared in terms of their fuel

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economy and converted NOx emissions after passing through the post-treatment system. The

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results revealed that the hybrid system exhibited slightly higher NOx concentrations due to

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lower exhaust temperatures, which deteriorated SCR performance. When the hybrid buses

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were driving on city routes, NOx emissions exceeded the EURO IV standards.

20

. They tested two

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Among the previous research results, many studies have been focused on SCR catalysts

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during the early 2000’s. Recently, studies have not dealt with SCR systems related to the

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latest emissions standards. This study aims to perform a profound investigation with regard to

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nitrogen oxides in a EURO VI type HD diesel engine. The present research would be a

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milestone to DeNOx processing at EURO VI levels. The engine experiments were processed

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using the world harmonized stationary cycle (WHSC) and transient cycle (WHTC). In

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addition, a new analyzer using a quantum–cascade laser method enabled the accurate

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measurement of nitrogen emission components in exhausted nitrogen oxides.

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2. Experimental methodology

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2.1 Engine system

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The measurement of nitrogen species (NO, NO2, N2O, and NH3) was performed on a

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commercial heavy-duty diesel engine, meeting the EURO VI regulations. The heavy–duty

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engine possessed a displacement of 12.7 liters over 6 cylinders, which can generate 323.6 kW

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maximum power at the 1800 rpm. Due to stringent emission standards, additional after-

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treatment devices were required to conform to regulations. In the present system, state-of-the-

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art technologies were employed to reduce exhaust emissions to within standard levels. First,

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diesel oxidation catalysts (DOC) were connected to the exhaust pipe to promote the oxidation

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of carbon monoxide (CO), total hydrocarbons (THC), and the organic fraction of diesel

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particulates. After the DOC device, a selective catalytic reduction (SCR) composed of a

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Cu/Zeolite catalyst was linked to convert nitrogen oxides to non-noxious species such as

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nitrogen gas and water. Harmful emitted particulate matters were burned through the diesel

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particulate filter system. Finally, ammonia oxidation catalysts removed ammonia after the

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SCR system. The test engine also employed an exhaust gas recirculation system to minimize

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NOx production during the combustion process. Detailed engine specifications are tabulated

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in Table 1. The heavy-duty engine was linked to an AC dynamometer (HD 460 LC,

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HORIBA), which is an AC induction motor that can control the engine speed and torque.

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Rated power and torque of the dynamometer were up to 462 kW and 2680 Nm, respectively.

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A commercial engine control unit (ECU) commanded entire engine electrical system based

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on the designated engine conditions set by the manufacturer. The software (STARS,

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HORIBA) simultaneously communicated cycle information to the dynamometer and ECU.

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With regard to emissions measurements, a constant volume sampler (CVS) system was

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employed after the post-treatment devices, which is an official method to measure engine


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exhaust emissions. The CVS system detected regulated emissions (CO, THC, and NOx)

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during the test cycle operation. The emissions analyzer (MEXA–7200D, HORIBA) measured

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diluted emissions using a bag sampling system, which can evaluate and compare emissions

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levels to the EURO VI limits. In this study, a quantum–cascade laser (QCL) analyzer was

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introduced to measure emissions related to nitrogen components such as nitric oxide, nitrogen

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dioxide, nitrous oxide, and ammonia gases. The QCL analyzer uses an infrared absorption

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method, which improved the precision of the emissions measurement compared to the

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Fourier Transform Infrared method. The device could measure and record four gases in real

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time over 1 second intervals. There were two measuring points for the QCL detectors; one

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location was before the after-treatment device inlet and the other was at the rear of the SCR

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system. Figure 1 illustrates a schematic of the test engine and emissions system. Analyzing

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and comparing the values measured between the two points showed the emission conversion

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efficiency of the post–system along with other relevant information.

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2.2 Experimental procedures

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The official engine dynamometer schedules of the EURO VI emissions regulations were

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used for the heavy–duty engine. The schedules are composed of transient and steady modes.

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The United Nations Economic Commission for Europe defined two representative test cycles,

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the world harmonized transient cycle (WTHC) and world harmonized stationary cycle

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(WHSC), serving to cover the typical driving conditions in various countries. Normalized

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engine speeds and torques for each cycle can be seen in Fig 2. The WHTC mode has the wide

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range of engine speeds and loads for the entire duration of 1800 seconds. The WHSC mode

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consisted of a ramped steady-state cycle composed of 13 modes, including an idling mode.

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Although WHTC officially separated cold and hot conditions, a new warm-up condition was


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introduced in the present study due to test time limitations. The pre-operation conditions for

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the WHSC were performed following formal test procedures using the 9 mode for 10 minutes.

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All tests were repeated 3 times and their results were averaged to reduce variations between

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tests. The coefficient of variation (CoV) was used to evaluate errors of the averaged NOx

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emissions values. The CoVs of averaged NO, N2O, and NO2 emissions were under 10%

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except for the near zero results, which were below measurable minimum values (< 0.2 ppm).

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Detailed test conditions are summarized in Table 2. The urea injection strategies in terms of

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injection timings and mass were designed by the manufacturer. In the present system, the urea

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injection system was used without any modification, which performed the same injection

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strategies under different urea concentrations. Various urea concentrations were used to

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investigate the effects of urea on DeNOx performance. The reference concentration was

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32.5% m/m and other tested urea solutions were 20% and 40%. The test urea solutions were

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obtained by mixing solid urea and distilled water based on mass fractions.

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3. Results and discussion

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This section presents the experimental results of the test engine under official conditions

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to validate the engine system under EURO VI regulations. With this regard, various nitrogen

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species within the exhausted emissions were investigated under a new set of testing

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conditions, using different urea concentrations.

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3.1 Emission results of official test modes

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The test engine was evaluated to determine acceptability within EURO VI regulations

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under the official WHTC and WHSC cycle modes. Emissions in terms of CO, THC, NOx,

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NH3, and PM were measured using certified instruments. In this test, the emission results of

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the WHTC cycle were obtained by integrating the cold and hot mode results based on a

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weighting factor. Table 3 presents the final emissions concentrations for the two test modes,

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compared to the EURO VI emissions standards. The emission values from the test engine

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were within regulation limitations for each tested condition. With the employed after-

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treatment system, engine emissions could be significantly reduced to below 10% of the limit

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values with the exception of the nitrogen oxides. Although the SCR system was used to

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reduce NOx emissions with high conversion efficiencies over 90%, the exhausted NOx

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concentrations were close to the standard values in both modes that were tested. This

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outcome accounted for the challenge of the DeNOx process with regard to engine-out

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emissions.

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3.2 NOx emissions for reference urea

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For a profound investigation of nitrogen oxides, Figure 3 shows the conversion

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efficiency of the reduction of NOx and its component species (NO, NO2 and N2O), while

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comparing the upstream and downstream levels of the SCR DeNOx device. Upstream refers


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Energy & Fuels 11

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to the location prior to the after-treatment system. Downstream indicates the emissions

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converted by the additional post-treatment devices. In general, the cold WHTC mode

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produced a higher quantity of nitrogen oxides due to low exhaust temperatures, an important

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factor in activating the SCR system. The new WHTC warm-up condition increased exhaust

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temperatures up to 310oC, regarded as hot operation conditions. Thus, the total NOx

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emissions for the WHTC decreased compared to that of the official WHTC mode. For raw

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NOx emissions, the total quantity during steady-state driving mode was larger than during the

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transient condition and was strongly related to the driving schedule (Fig. 2), as shown in Figs.

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4 and 5. Although there was a difference in the quantity of NOx exhausted between the two

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test modes, the conversion rates were around 96% for both conditions using the reference

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urea. Interestingly, the fraction of nitrogen species were changed after the catalytic process

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occurred in the DeNOx apparatus. When NOx emissions were released directly from the

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engine, nitric oxide occupied nearly the entirety of the emitted NOx. Nitrogen dioxide and

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nitrous oxide comprised only around 5–7% although they were summed. However, catalytic

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reactions reduced the NO component by up to 9.8% and increased the fraction of N2O by

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87.5% in the WHSC mode.

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Figures 4 and 5 present the variation of each nitrogen component emission as a function

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of time. The QCL analyzer simultaneously measured NO, NO2, N2O, and NH3 emissions in

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real time. In Figs. 4(a) and 5(a), nitric oxide was predominantly exhausted at the exit of the

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engine. In addition, the variation in concentration followed the torque movement for each test

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mode as shown in Fig. 2. These results showed that engine torque was a significant factor in

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generating NOx emissions. According to the extended Zeldovich mechanism, nitric oxide

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formation depended highly on the temperature of the engine

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increased, combustion temperatures and pressures were also elevated within the engine

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cylinder. Moreover, higher quantities of NO were produced during WHSC driving because


21

. When the engine torque

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the engine sustained higher load conditions for a while. This evidence was also observed in

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the exhaust temperature graph in Figs. 4 and 5. As the engine load reached high states, the

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engine-out temperature achieved high values. With regard to nitrogen dioxide, small fractions

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were occupied in the NOx emissions for both modes. The mechanism for NO2 production was

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the following:

243

(5)

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Since HO2 radicals formed in relatively low-temperature conditions, the formation of

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NO2 would be accelerated during low load conditions 22. Thus, the NO2 concentration under

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the WHTC mode increased slightly compared to that of the WHSC mode. Although NO2

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emissions play a negative role in the environment due to acid rain and photochemical smog,

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NO2 promotes the catalytic reaction to reduce NO in the SCR system 5, 23. The toxic gas, NO2,

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was consumed completely during post-processing in the DeNOx system.

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In Figs. 4(b) and 5(b), it is remarkable that nitrous oxides, secondary emissions, were

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increased, removing engine-out NO emissions via the after-treatment system. Although the

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absolute value of N2O formation was low, the fraction increased considerably up to 87.5% in

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the WHSC cycle. These results announced that mobile SCR system could induce GHG

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emissions as opposed to significantly reducing NO. There are potential pathways to form

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nitrous oxide in the urea-SCR system:

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(6)

257

(7)

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Equations (6) and (7) were reported by Colombo et al.

15

and Madia et al.

24

,

259

respectively. This result could happen if the NO2 content exceeds the NO levels in the

260

reactant gases. Another possible reaction is the partial oxidation of ammonia:


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(8)

262

In the copper zeolite catalyst, the oxidation of ammonia started between 250 and 300oC

263

and ammonia conversion was accelerated at higher temperatures of over 400oC 15. During the

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ammonia oxidation process, N2O gas could be produced through partial oxidation. In the

265

present study, it was believed that the partial oxidation of ammonia was predominant in

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forming nitrous oxide due to the high nitric oxide concentrations at the inlet. This conclusion

267

was clearly observed in the next section. As the urea concentration increased, the exhausted

268

N2O also increased. A relationship between the inlet gas temperature and N2O concentration

269

was discovered in Figs. 4(b) and 5(b). Nitrous oxide increased with a decrease in exhaust

270

temperatures. Moreover, increased temperatures suppressed the formation of nitrous oxide. It

271

seemed that the temperature decrease led to the partial oxidation of ammonia, resulting in a

272

larger quantity of N2O emissions. With regard to ammonia slip, the concentrations of NH3

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was strongly related to the injection quantity of urea solution over the driving modes. The

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largest amount of ammonia slip was observed under the WHSC mode between 1200s and

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1300s, which was under the highest speed and torque conditions. The larger amount of urea

276

solution was injected to reduce the high NOx emissions with a short time. The instantly

277

injected urea which was not participated to the NOx conversion process was emitted to the

278

exhaust pipe. This operation period led the increase in the ammonia slip of WHSC mode

279

compared the WHTC mode as shown in Table 4.

280

3.3 Nitrogen oxide emissions for various urea concentration

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The effects of urea concentration on SCR performance were investigated using two

282

tested urea agents with concentrations of 20% (m/m) and 40% (m/m). The engine test

283

conditions were the same as those of the previous reference test. Table 4 shows the nitrogen

284

oxides and ammonia slip at the final tail pipe, after being processed through the after-


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treatment devices. The standard values refer to the official EURO VI limitations. With regard

286

to NOx emissions, the conversion efficiency increased with an increase in the concentration

287

of urea. With the 20% urea agent, the NOx emissions exceeded the standard values by more

288

than three times. Since the lower urea concentration supplied a smaller quantity of ammonia

289

reactant to the catalyst, the conversion rates deteriorated. Conversely, the 40% agent induced

290

the lowest NOx emissions values, which were half that of the reference values. The ammonia

291

slip, however, surpassed the standard values due to an excess of supply. Figure 6 illustrated

292

the composition fraction of nitrogen oxides after the DeNOx process. Engine-out meant that

293

raw emissions were measured prior to the SCR system. The total values were normalized to

294

the raw emission concentration.

295

Comparing the 20% and 40% agents, there were remarkable results with regard to the

296

fraction of nitrogen species. The portion of nitrogen dioxide increased in the low

297

concentration case, whereas the high concentration agent converted NO2 to near zero levels.

298

The increased in nitrogen dioxide originated from the NO oxidation reaction in the copper

299

zeolite system (9).

300

(9)

301

In general, the produced NO2 was used to convert nitric oxide with NH3. Due to

302

deficiencies in the ammonia component, a larger quantity of NO2 gas was exhausted with

303

unconverted NO emissions. For nitrous oxide emissions, they were also influenced by the

304

NH3 concentration. As mentioned above, the formation of N2O was primarily due to the

305

partial oxidation of ammonia. Thus, the 40% urea case exhausted the greatest fraction of N2O

306

gas although the total quantity of gas was significantly reduced by the high conversion

307

efficiency of the SCR. Figure 7 shows the nitrogen species concentrations as a function of

308

test times at the downstream of the SCR. For 20% urea solution in Fig. 7(a), the nitrogen


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dioxide concentration was high during the entire cycle period with low conversion rate of NO.

310

This results were caused by the lack of ammonia species in the SCR system as the

311

aforementioned it. In the WHTC mode, a larger amount of NO2 was measured with extremely

312

low N2O emissions. Under the high percentage of urea solution (40%), the conversion rate of

313

NO was dramatically increased compared to the standard urea agent (32.5%). The nitrous

314

oxide was formed with around 24% more concentration during the test time than that of the

315

32.5% urea solution.


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4. Conclusion

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Among the exhaust emissions of a heavy-duty engine, nitrogen oxide was profoundly

318

investigated based on the recent EURO VI emission regulations. Theoretically, it is well-

319

known that the nitric oxide and nitrogen dioxide that constitute NOx emissions are due to

320

transportation. However, this study experimentally showed the effects of a selective catalytic

321

reduction system on the fraction of nitrogen components in exhaust emissions. A quantum–

322

cascade laser (QCL) analyzer allowed each nitrogen–species (NO, NO2, N2O and NH3) to be

323

measured with high accuracy. Using three urea concentrations, nitrogen oxides and ammonia

324

slip were studied and the following conclusions were drwan:

325

1. For a EURO VI type engine, the mobile SCR system in a heavy–duty engine induced

326

a high NOx reduction rate of up to 96% under official steady-state and transient

327

driving cycles. The copper–zeolite catalyst converted the fraction of nitrogen oxides,

328

decreasing the total quantity of NOx emissions. During the catalytic reactions,

329

nitrous oxides were formed that occupied more than half the percentage of the NOx

330

exhaust emissions.

331

2. Although the absolute quantities of nitrous oxide are low, nitrous oxide can induce

332

greenhouse effects and smog in the city air. The heavy–duty engine primarily

333

operated under highway conditions for long-distance public transportations or cargo

334

trucks. Under steady–state conditions, the formation of N2O gas increased compared

335

to the transient driving case.

336

3. N2O production was proportional to an increase in the urea agent concentration. The

337

partial oxidation of ammonia was a main reaction in the formation of nitrous oxides.

338

When 40% urea was used in the SCR device, the fraction of N2O emissions


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Energy & Fuels 17

339

increased by up to 93%. Moreover, an oversupply of ammonia induced a larger

340

amount of ammonia slip over the regulated values.

341 342


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Page 18 of 34 18

343

References

344

1. Hirata, K.; Masaki, N.; Ueno, H.; Akagawa, H., Development of Urea-SCR System for

345

Heavy-Duty Commercial Vehicles. SAE 2005, 2005-01-1860.

346

2. Girard, J. W.; Montreuil, C.; Kim, J.; Cavataio, G.; Lambert, C., Technical Advantages of

347

Vanadium SCR Systems for Diesel NOx Control in Emerging Markets. SAE Int. J. Fuels Lubr.

348

2008, 1, (1), 488-494.

349

3. Ku, K. W.; Hong, J. G.; Park, C. W.; Chung, K. Y.; Sohn, S. H., Effects of Various Factors

350

on the Conversion Efficiency of Urea Solution in a Urea Selective Catalytic Reduction

351

System. Energy Fuels 2014, 28, (9), 5959-5967.

352

4. Johnson, T. V., Diesel Emission Control in Review. SAE Int. J. Fuels Lubr. 2009, 2, (1), 1-

353

12.

354

5. Koebel, M.; Elsener, M.; Kleemann, M., Urea-SCR: a promising technique to reduce NOx

355

emissions from automotive diesel engines. Catal. Today 2000, 59, (3-4), 335-345.

356

6. Scott Sluder, C.; Storey, J. M. E.; Lewis, S. A.; Lewis, L. A., Low Temperature Urea

357

Decomposition and SCR Performance. SAE 2005, 2005-01-1858.

358

7. Grossale, A.; Nova, I.; Tronconi, E.; Chatterjee, D.; Weibel, M., NH3-NO/NO2 SCR for

359

Diesel Exhausts Aftertreatment: Reactivity, Mechanism and Kinetic Modelling of

360

Commercial Fe- and Cu-Promoted Zeolite Catalysts. Top. Catal. 2009, 52, (13-20), 1837-

361

1841.

362

8. Ruggeri, M. P.; Nova, I.; Tronconi, E., Experimental Study of the NO Oxidation to NO2

363

Over Metal Promoted Zeolites Aimed at the Identification of the Standard SCR Rate

364

Determining Step. Top. Catal. 2013, 56, (1-8), 109-113.

365

9. Maunula, T.; Viitanen, A.; Kinnunen, T.; Kanniainen, K., Design of Durable Vanadium -

366

SCR Catalyst Systems for Heavy - Duty Diesel Applications. SAE 2013, 2013-01-09.

367

10. Shan, W.; Liu, F.; He, H.; Shi, X.; Zhang, C., An environmentally-benign CeO2-TiO2


Page 19 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels 19

368

catalyst for the selective catalytic reduction of NOx with NH3 in simulated diesel exhaust.

369

Catal. Today 2012, 184, (1), 160-165.

370

11. Fang, H. L.; DaCosta, H. F. M., Urea thermolsis and NOx reduction with and without

371

SCR catalysts. Appl. Catal., B 2003, 46, (1), 17-34.

372

12. Xu, L. F.; McCabe, R. W.; Hammerle, R. H., NOx self-inhibition in selective catalytic

373

reduction with urea (ammonia) over a Cu-zeolite catalyst in diesel exhaust. Appl. Catal., B

374

2002, 39, (1), 51-63.

375

13. De-La-Torre, U.; Pereda-Ayo, B.; Moliner, M.; Gonzalez-Velasco, J. R.; Corma, A., Cu-

376

zeolite catalysts for NOx removal by selective catalytic reduction with NH3 and coupled to

377

NO storage/reduction monolith in diesel engine exhaust aftertreatment systems. Appl. Catal.,

378

B 2016, 187, 419-427.

379

14. Grossale, A.; Nova, I.; Tronconi, E., Study of a Fe-zeolite-based system as NH3-SCR

380

catalyst for diesel exhaust aftertreatment. Catal. Today 2008, 136, (1-2), 18-27.

381

15. Colombo, M.; Nova, I.; Tronconi, E., A comparative study of the NH3-SCR reactions

382

over a Cu-zeolite and a Fe-zeolite catalyst. Catal. Today 2010, 151, (3-4), 223-230.

383

16. Kamasamudram, K.; Currier, N.; Szailer, T.; Yezerets, A., Why Cu- and Fe-Zeolite SCR

384

Catalysts Behave Differently At Low Temperatures. SAE Int. J. Fuels Lubr. 2010, 3, (1), 664-

385

672.

386

17. Luo, J.-Y.; Hou, X.; Wijayakoon, P.; Schmieg, S. J.; Li, W.; Epling, W. S., Spatially

387

resolving SCR reactions over a Fe/zeolite catalyst. Appl. Catal., B 2011, 102, (1-2), 110-119.

388

18. Misra, C.; Collins, J. F.; Herner, J. D.; Sax, T.; Krishnamurthy, M.; Sobieralski, W.;

389

Burntizki, M.; Chernich, D., In-Use NOx Emissions from Model Year 2010 and 2011 Heavy-

390

Duty Diesel Engines Equipped with Aftertreatment Devices. Environ. Sci. Technol. 2013, 47,

391

(14), 7892-7898.

392

19. Myung, C.-L.; Ko, A.; Kim, J.; Choi, K.; Kwon, S.; Park, S., Specific engine performance


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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393

and gaseous emissions characteristics of European test cycle and worldwide harmonized

394

driving cycle for a heavy-duty diesel engine. J. Mech. Sci. Technol. 2013, 27, (12), 3893-3902.

395

20. Guo, J.; Ge, Y.; Hao, L.; Tan, J.; Peng, Z.; Zhang, C., Comparison of real-world fuel

396

economy and emissions from parallel hybrid and conventional diesel buses fitted with

397

selective catalytic reduction systems. Appl. Energy 2015, 159, 433-441.

398

21. Heywood, J. B., Internal Combustion Engine Fundamentals. McGraw-Hill: New York,

399

1988.

400

22. Turns, S. R., An Introduction to Combustion: Concepts and Applications. McGraw-Hill:

401

New York, 2012.

402

23. Koebel, M.; Elsener, M.; Krocher, O.; Schar, C.; Rothlisberger, R.; Jaussi, F.; Mangold,

403

M., NOx reduction in the exhaust of mobile heavy-duty diesel engines by urea-SCR. Top.

404

Catal. 2004, 30-1, (1-4), 43-48.

405

24. Madia, G.; Koebel, M.; Elsener, M.; Wokaun, A., Side reactions in the selective catalytic

406

reduction of NOx with various NO2 fractions. Ind. Eng. Chem. Res. 2002, 41, (16), 4008-

407

4015.

408


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Energy & Fuels 21

409

Acknowledgement

410

This work supported by the National Research Foundation of Korea (NRF), funded by the

411

Korea government (MSIP) (NRF-2013R1A1A2074615).

412


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413

List of Tables

414

Table 1 Test engine specifications

415

Table 2 Test mode descriptions

416

Table 3 Formal mode results of test engine for EURO VI regulation

417

Table 4 Nitrogen oxides and ammonia slip concentrations under various urea

418

concentration

419


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Energy & Fuels 23

420

421

Table 1 Test engine specifications Description

Value

Displacement (Liter)

12.7

Rated power (kW/rpm)

323.6/1800

Max. net torque (Nm/rpm)

2099/1200

Idle speed (rpm)

500

Intake system

Turbocharger

Exhaust system

EGR

After-treatment devices

DOC, DPF, SCR, AOC

422


Energy & Fuels

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423 424

Table 2 Test mode descriptions Test mode

WHSC

WHTC

Test time

1895s

1800s

Warm-up

1210rpm – 50% load

1000 rpm – 40% load

condition

10min.

Up to 309oC at SCR-out

Repeat

3 times

Urea 20%

32.5%

40%

concentration 425


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Energy & Fuels 25

426 427

Table 3 Formal mode results of test engine for EURO VI regulation CO

HC

NOx

NH3

PM

(g/kWh)

(g/kWh)

(g/kWh)

(ppm)

(g/kWh)

0.664

0.026

0.394

0.6

0.005

4.000

0.160

0.460

10.0

0.01

0.006

0.004

0.323

0.2

0.003

1.500

0.130

0.400

10.0

0.01

Emissions

WHTC Measurement WHTC Standard WHSC Measurement WHSC Standard 428


Energy & Fuels

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429 430

Table 4 Nitrogen oxides and ammonia slip concentrations under various urea concentration Urea

NOx (g/kWh)

NH3 (ppm)

concentration

WHSC

WHTC

WHSC

WHTC

20%

1.224

1.512

0.05

0.02

32.5%

0.053

0.112

9.4

5.9

40%

0.025

0.039

24.1

15.4

Standard

0.400

0.460

10.0

10.0

431


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Energy & Fuels 27

432

List of Figures

433

Figure 1 Schematic of heavy-duty diesel engine and emission measurement system

434

Figure 2 Speed and torque data for WHSC and WHTC driving schedules

435

Figure 3 Comparing nitrogen oxides concentrations by converting SCR system under

436

two test cycles

437

Figure 4 Nitrogen compounds concentration and exhaust temperature under WHSC

438

mode at (a) upstream and (b) downstream of SCR device

439

Figure 5 Nitrogen compounds concentration and exhaust temperature under WHTC

440

mode at (a) upstream and (b) downstream of SCR device

441

Figure 6 Normalized NOx concentrations for 20% and 40% test urea under (a) WHSC

442

and (b) WHTC modes


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443

444 445

Figure 1 Schematic of heavy-duty diesel engine and emission measurement system


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446

447 448

Figure 2 Speed and torque data for WHSC and WHTC driving schedules


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449

450 451 452

Figure 3 Comparing nitrogen oxides concentrations by converting SCR system under two test cycles


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453

454 455

(a) Upstream of WHSC

456 457

(b) Downstream of WHSC

458 459

Figure 4 Nitrogen compounds concentration and exhaust temperature under WHSC mode at (a) upstream and (b) downstream of SCR device


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Page 32 of 34 32

460

461 462

(a) Upstream of WHTC

463 464

(b) Downstream of WHTC

465 466

Figure 5 Nitrogen compounds concentration and exhaust temperature under WHTC mode at (a) upstream and (b) downstream of SCR device


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Energy & Fuels 33

467

468 469

(a) WHSC mode

470 471

(b) WHTC mode

472 473

Figure 6 Normalized NOx concentrations for 20% and 40% test urea under (a) WHSC and (b) WHTC modes

474


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475 476

(a) WHSC mode

477 478

(b) WHTC mode

479 480

Figure 7 Nitrogen compounds concentration for 20% and 40% urea solutions at downstream of SCR under (a) WHSC and (b) WHTC modes


Nitrogen Compounds (NO, NO2, N ... - ACS Publications

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