alumina dual catalysts under the diesel methanol dual fuel exhaust conditions

Chemical Engineering Science 211 (2020) 115320

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Selective catalytic reduction of nitrogen oxides with methanol over the (cobalt-molybdenum)/alumina dual catalysts under the diesel methanol dual fuel exhaust conditions Chao Chen, Anren Yao, Chunde Yao ⇑, Hui Wang, Mingkuan Liu, Zhuangzhuang Li State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China

h i g h l i g h t s
Methanol plays two prominent roles as the fuel and as the reductant.
The catalytic performance of methanol-SCR was studied on the DMDF engine test bench.
The SCR attains the reductant by either active or passive methanol supply modes.
The DMDF engine with methanol-SCR (cobalt + molybdenum) achieves the ultra-low NOX emissions.

a r t i c l e

i n f o

Article history: Received 3 May 2019 Received in revised form 23 October 2019 Accepted 25 October 2019 Available online 26 October 2019 Keywords: Diesel methanol dual fuel engine Methanol-SCR Nitrogen oxides (cobalt-molybdenum)/alumina

a b s t r a c t The (cobalt-molybdenum)/alumina ((Co-Mo)/Al2O3) dual catalysts have an excellent performance on the selective catalytic reduction (SCR) of nitric oxide with methanol (methanol-SCR) on the synthetic gas test bench. The catalytic performance of (Co-Mo)/Al2O3 needs to be studied on the engine test bench due to the complicated emissions of engines. Tests were carried out on the diesel methanol dual fuel (DMDF) engine. The effect of methanol supply methods, methanol dosing, injection timings of diesel fuel and exhaust gas recirculation (EGR) ratios on the performance of methanol-SCR was investigated. Results show that the performance of methanol-SCR on the engine test bench slightly deteriorates compared with that on the synthetic gas test bench. The performance of methanol-SCR on the active methanol supply mode is superior to that on the passive methanol supply mode. The low nitrogen oxides (NOX) conversion with passive methanol supply mode suggests that the ratio of carbon monoxide to hydrocarbons (more than 1) and the ratio of nitrogen dioxide to NOX are the critical factors on NOX conversion during the catalyst selection. However, the passive methanol supply mode has less effect on the fuel consumption. The catalytic efficiency of methanol-SCR is improved by the increased methanol dosing and EGR ratios and the retarded injection timings. The methanol dosing increases the particulate emissions and the fuel consumption. Increasing EGR ratios is less bad for the fuel consumption than retarding injection timings. A combination of the DMDF engine and the methanol-SCR realizes the ultra-low NOX emissions. Ó 2019 Elsevier Ltd. All rights reserved.

Abbreviations: SCR, selective catalytic reduction; (Co-Mo)/Al2O3, (cobaltmolybdenum)/alumina; DMDF, diesel methanol dual fuel; NOX, nitrogen oxides; NO, nitric oxide; HP-EGR, high-pressure exhaust gas recirculation; HC, hydrocarbons; LTC, low temperature combustion; LNT, lean NOX traps; PM, particulate matter; PN, particulate number; CO, carbon monoxide; H2, hydrogen; DOC, diesel oxidation catalyst; POC, particulate oxidation catalyst; Al2O3, alumina; Ag/Al2O3, silver/alumina; Co/Al2O3, cobalt/alumina; Mo/Al2O3, molybdenum/alumina; ECU, electronic control unit; PWM, pulse-width modulation; NO2, nitrogen dioxide; FSN, filter smoke number; CHN, China; Euro, Europe; CPSI, channels per square inch; GHSV, gas hourly space velocity; wt., weight; F.S., full scale; N2, nitrogen; O2, oxygen; CO2, carbon dioxide; MSR, methanol substitution ratio; ESC, Europe steadystate cycle; PMSM, passive methanol supply mode; AMSM, active methanol supply mode; BMEP, brake mean effective pressure; BSFC, brake specific fuel consumption; CA, crank angle; ATDC, after top dead center. ⇑ Corresponding author at: Tianjin University, No.92 Weijin Road, Nankai District, Tianjin 300072, China. E-mail address: [email protected] (C. Yao). https://doi.org/10.1016/j.ces.2019.115320 0009-2509/Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Diesel engines, for their high reliability and low operating costs, have been widely used in commercial transportations (Wu et al., 2017). However, the consumption of fossil energy will increase with the gradual increase in number of diesel engines. Nitrogen oxides (NOX), one of the main emissions of Diesel engines, can cause the formation of acid rain and ground ozone layer. NOX emissions are harmful to environment and human health (Boningari and Smirniotis, 2016). The frequently-used technologies of NOX abatement are the low temperature combustion (LTC) (Agarwal et al., 2017), the alternative fuels (Bae and Kim, 2017; Yusri et al., 2017) and the NOX after-treatment devices (Piumetti et al., 2016). The LTC mode achieves high thermal efficiency, and

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decreases the emissions of NOX and particulate matter (PM) (Agarwal et al., 2017). There are five kinds of NOX aftertreatment systems for diesel engines: direct catalytic decomposition, selective catalytic reduction (SCR), lean NOX traps (LNT), plasma-assisted abatement and the combination of NOX reduction and soot combustion (Piumetti et al., 2016). Furthermore, the researchers have paid much attention on the alternative fuels in recent two decades. Using alternative fuels, such as natural gas (Korakianitis et al., 2011), dimethyl ether (Bae and Kim, 2017) and alcohols (Yao et al., 2017; Yusri et al., 2017), can decrease not only the consumption of diesel fuel but also the NOX emissions. The methanol, which can be produced from fossil fuels, biomass and carbon dioxide (CO2), is attractive as an alternative fuel to solve the energy sustainability and environment issues (Yao et al., 2017). There are two main methods for burning methanol on compression ignition engines: methanol-diesel blends (Huang et al., 2004; Sayin et al., 2010) and diesel methanol dual fuel (DMDF) (Chen et al., 2019; Yao et al., 2017). Huang et al. (2004) studied the effect of methanol-diesel blends (10%, 14% and 18%) on the combustion of a single-cylinder diesel engine. Sayin et al. (2010) reported the effect of the injection pressure and timing on the performance and emissions of a diesel engine fueled with methanol-diesel blends (5%, 10% and 15%). The DMDF, which can overcome the problem of immiscibility between methanol and diesel fuels and achieve the high methanol substitution ratio (MSR), has been widely studied in the past years (Wang et al., 2018; Yao et al., 2017). The DMDF with the high MSR can significantly reduce NOX and PM emissions but increase the carbon monoxide (CO) and unburned hydrocarbons (HC) emissions (Chen et al., 2019; Wang et al., 2018). In recent study, the emission standard of Euro V can be achieved without urea-SCR by using DMDF combined with a diesel oxidation catalyst (DOC) and a particulate oxidation catalyst (POC) (Wei et al., 2017). Compared with the Euro V legislation, the NOX limit of Euro VI largely decreases, and the number of test points at low loads in Euro VI cyclic tests increases. However, the MSR will reduce when the DMDF engine works at low loads, which weakens the effect of the DMDF mode on the reduction of NOX emissions. Hence, a combination of in-cylinder emission reduction methods and NOX after-treatment devices needs to be applied on the DMDF engine to meet increasingly stringent emission standards. The SCR technology, which can perform the superior performance of NOX reduction in the presence of excess oxygen, has been widely used in diesel engines (Piumetti et al., 2016). Depending on the nature of reductant, the SCR systems can be classified into four groups: CO-SCR (Hamada and Haneda, 2012), hydrogen (H2)-SCR (Furfori et al., 2010; Satokawa et al., 2007), urea-SCR (Metkar et al., 2013; Zhao et al., 2017), and HC-SCR (Arve et al., 2004; Frobert et al., 2012; Mrad et al., 2015), respectively. The iridium has been regarded as an only high active catalyst for CO-SCR systems (Hamada and Haneda, 2012). Hence, the application of COSCR to diesel engines needs to be studied further. The H2, as a reductant (Furfori et al., 2010) or accelerant (Satokawa et al., 2007), is applied to improve the NOX conversion especially at low temperatures. However, the NOX conversion of H2-SCR greatly reduces at high temperatures. The urea-SCR system has a high NOX conversion in a wide temperature window. However, some issues exist in the application process, such as the low temperature crystallization and the onboard urea tank. On the contrary, the reductants of HC-SCR are from vehicle fuels. Therefore, the HC-SCR system is considered as one of the most promising technologies to compete with urea-SCR (Cheng and Bi, 2014). The extensively used reductants of HC-SCR are ethers, alkanes, and alcohols. Frobert et al. (2012) studied the effect of the nature of reductant on NOX conversion over a silver/alumina (Ag/Al2O3) catalyst. Results showed that compared with decane, diesel fuel lowered

the NOX conversion but ethanol increased it. Prikhodko et al. (2015) reported that the lean gasoline engine fueled with ethanol/gasoline blends containing at least 50% ethanol could attain the high NOX conversion over Ag/Al2O3 catalyst. At present, the lean combustion engines are fueled with alcohols-blended fuels for decreasing the consumption of fossil fuels, such as ethanolblended fuels (Frobert et al., 2012; Prikhodko et al., 2015), and methanol-blended fuels (Eyidogan et al., 2010; Sayin et al., 2010; Wei et al., 2017). A vehicle with a DMDF engine has two fuel tanks, where one is used to carry diesel and the other one is used to carry methanol (Wei et al., 2017). Moreover, the reductant of SCR can be supplied by the additional methanol injection system installed on exhaust pipe or the unburned methanol from the cylinder on the DMDF mode (Wei et al., 2017; Wang et al., 2018). The methanol, as a reductant of HC-SCR, is not easy to crystallize at low temperatures because of its low condensation point (Nova and Tronconi, 2014; Zhen and Wang, 2015). Therefore, the scheme of the DMDF engine combined with methanol-SCR is feasible and attractive. The selection of catalyst is crucial to attain the excellent performance of methanol-SCR (Mrad et al., 2015). Tabata et al. (1995) and Burch et al. (1998) found that the methanol could selectively reduce NOX over an alumina (Al2O3) catalyst in lean conditions. The subsequent research indicated some metal oxides supported on Al2O3 could improve the catalytic activity. Masters and Chadwick (1998) reported that molybdenum/alumina (Mo/Al2O3) catalyst improved the low-temperature activity of catalyst. Park et al. (2007) found that cobalt/alumina (Co/Al2O3) catalyst performed the NOX conversion of 100% between 300 and 350 °C. Männikkö et al. (2012, 2013, 2016) have recently carried out some meticulous studies on enhancing the catalytic activity of methanol-SCR. Results showed that the Ag/Al2O3 catalyst with 3 wt.% Ag was active for NOX conversion between 200 and 500 °C, and had a high low-temperature activity (Männikkö et al., 2012, 2013). Results also indicated that both small and somewhat larger silver species achieved the high low-temperature activity, as well as the high conversion of NO to N2 (Männikkö et al., 2016). However, these tests were all performed under the excess oxygen (O2) atmosphere over the only one catalyst on the synthetic gas test bench. Moreover, a large number of CO and HC emissions can be produced on the DMDF mode. Therefore, we recently investigated the effect of different catalysts (selected from literature (Burch et al., 1998; Masters and Chadwick, 1998; Männikkö et al., 2013; Park et al., 2007)) on the SCR of nitric oxide (NO) by methanol in the presence of excess O2 and CO on the synthetic gas test bench (Yao et al., 2018). Results indicated that the (CoMo)/Al2O3 dual catalysts performed a high activity at low and at high temperatures. However, the gas components of synthetic gas test bench are less than those of engines (Prikhodko et al., 2015). Therefore, the catalytic performance of (Co-Mo)/Al2O3 needs to be researched on the engine test bench. Exhaust conditions that affect NOx conversion on the engine are mainly the exhaust temperature, the space velocity, the catalyst inlet concentration of NOX, and reductant (Schmieg et al., 2008). The space velocity and the exhaust temperature are mainly affected by the speeds and loads of engines. The amount of reductant used for NOX conversion can be controlled by the methanol injector at the upstream of SCR catalyst or the DMDF combustion mode. The NOX concentration can be adjusted by some NOX reduction methods for in-cylinder combustion processes. For example, the DMDF mode, the delayed injection timing of diesel fuel and the increased exhaust gas recirculation (EGR) ratio can be used to reduce the engine-out NOX emissions. Hence, the effect of the DMDF mode, injection timing and EGR ratio on the catalytic performance of (Co-Mo)/Al2O3 is worth exploring. Considering the research gap in the performance evaluation of (Co-Mo)/Al2O3 catalysts on the engine test bench, the objective of

C. Chen et al. / Chemical Engineering Science 211 (2020) 115320

this study was to understand the NOX emissions and reduction with methanol over (Co-Mo)/Al2O3 dual catalysts in the DMDF engine. Therefore, the effect of methanol supply methods, methanol dosing, injection timings and EGR ratios on the methanol-SCR performance was researched. Furthermore, a combination of the DMDF engine and the methanol-SCR system was proposed to achieve the ultra-low NOX emissions, which is the novelty of this study. On the basis of better understanding of those impacts, it was desired to provide some guidelines for the application of methanol-SCR to the DMDF engine.

2. Experimental apparatus and methods 2.1. Tested engine and equipment The tests were carried out on a turbocharged and intercooled diesel engine equipped with an electronic fuel injection unit pump system. Table 1 shows the main parameters of this engine. Fig. 1 illustrates the schematic diagram of engine test system. Before tests, the original diesel engine was modified to the DMDF engine. The DMDF engine based on the original engine was required to install a set of methanol injection and supply systems. The methanol injection system installed in the intake port included three methanol injectors and a methanol supply rail. The methanol supply system consisted of a fuel tank, a filter, a pump and a pressure regulator. An exhaust methanol injection system at the upstream of SCR system was inserted into the exhaust pipe to supply adequate methanol reductant. This exhaust injection system was composed of a methanol injector, a cooling chamber and a methanol supply rail. The distance between the injection system and the SCR system was at 0.8 m to ensure the reliable evaporation of methanol. The cooling chamber was filled with liquid coolant (around 30 °C) to ensure the normal operation of injector. The methanol fuel pressure was controlled steadily at 0.4 MPa by the pump and pressure regulator. The specialized methanol electronic control unit (ECU) was applied to produce the pulse-width modulation (PWM) signal to drive the injector, where the duty ratio was devoted to regulate the injection amount of methanol. The injection timing of diesel fuel was precisely adjusted by the diesel ECU. The actuation type of the methanol injector was the solenoid-driven type. The number and diameter of methanol injector holes were 4 and 0.2 mm, respectively. A high pressure EGR circuit with an intercooler was installed on the DMDF engine, which was also controlled by the diesel ECU. Table 2 shows the controlling and measuring accuracy of facilities. The speeds and torques of the engine were controlled accurately by the dynamometer and its own measuring and controlling system. The air mass flow was measured by an air flowmeter (20 N, Toceil Inc.). The consumption of diesel and methanol fuels was measured by the fuel consumption meters. The average temperature of SCR was calculated by the inlet and exit temperature of SCR system measured by the K-type thermocouple sensors. The temperature of intake air and cooling water was measured by the PT100 type thermal resistance sensors. The

Table 1 Main technical parameters of engine. Parameter

Value

Engine type Displacement (L) Compression ratio Bore  stroke (mm) Fuel injection system Rated power/speed (kW/rpm) Max. torque/speed (Nm/rpm)

4-Cylinder, inline, DI diesel engine 4.214 17:1 108  115 Electronic unit pump 103/2800 440/(1400–1800)

3

exhaust pressure was measured by a diffusion silicon pressure sensor. The raw exhaust was sampled at the inlet and outlet of the SCR system, respectively. A gas analyzer (7100DEGR, Horiba Mexa) was used to measure regulated gases including HC and CO, and it also calculates EGR ratio by the following formula (1):

REGR ¼

V CO2 in  V 0  100% V CO2 ex  V 0

ð1Þ

where V 0 (Vol.%) is the volume fraction of CO2 in the atmosphere. V CO2 in (Vol.%) and V CO2 ex (Vol.%) are the volume fraction of CO2 at the intake and exhaust pipes, respectively. NO and nitrogen dioxide (NO2) emissions were simultaneously measured with a gas analyzer (6000FT, Horiba Mexa). NOX emissions were the sum of NO and NO2 emissions. The PM emissions were attained by a smoke meter (415S, AVL Inc.). The result of the smoke meter was displayed as filter smoke number (FSN) based on the standard ISO 10054. The particulate number (PN) was measured by a differential mobility spectrometer (DMS500SKII, Cambustion Inc.). The DMS measures particulate size in the range of 5–1000 nm. The DMS consists of the two-stage dilution systems. The first-stage dilution ratio is usually set to 5:1 according to the requirements of manufacturer. The second-stage dilution ratio is adjusted to maintain a good signalto-noise ratio and extend the cleaning cycle. The fuels used in the tests were CHN 5 diesel fuel (the mass fraction of sulfur was under 10 ppm) and methanol (purity was 99.5%). Table 3 displays the physicochemical properties of diesel and methanol fuels. 2.2. Preparation of tested catalysts The dual catalysts, formed by combining with the Co/Al2O3 catalyst in the front and the Mo/Al2O3 catalyst in the rear, performed a wider temperature operation window (Yao et al., 2018). The (CoMo)/Al2O3 dual catalysts were packaged and installed on the exhaust pipe to research the performance of the dual catalysts on the engine test bench. During the activity tests of the methanolSCR system, one block Co/Al2O3 catalyst, and one block Mo/Al2O3 catalyst were integrated in sequence as indicated in Fig. 1. The substrate of the dual catalysts was cordierite, and the structure of the dual catalysts was permeable honeycomb. The cell density of cordierite honeycomb monoliths was 400 cpsi (channels per square inch). The cordierite honeycomb monoliths had a diameter of 190 mm and a length of 252.4 mm. The volume of catalysts was 7.16 L per each block. The mass ratios of transition metals to catalysts in the Co/Al2O3 and Mo/Al2O3 catalysts were 0.5 wt.% and 5 wt.%, respectively. The mass of powder catalysts coated over cordierite honeycomb monoliths was well controlled at 100 g/L. 2.3. Experimental method and procedure The activity tests of NOX reduction using methanol as a reductant over (Co-Mo)/Al2O3 catalysts (denoted as the dual catalysts) were researched on the DMDF engine. During the tests, the intake air temperature and cooling water temperature of engine were controlled at 35 ± 2 °C and 85 ± 2 °C, respectively. The temperature of diesel and methanol fuels was maintained at 30 ± 2 °C. Table 4 exhibits the main parameters of test points. The torques and speeds of engine were fixed at the constant when the operation of engine switched from pure diesel mode to DMDF mode. In order to study the percentage of methanol used in the DMDF mode, the MSR is used and defined as the following formula (2):

MSR ¼

qd  qm;d  100% qd

ð2Þ

where qd (kg/h) and qm;d (kg/h) are the consumption of diesel fuel on the pure diesel and DMDF modes, respectively. In the tests, the

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Data acquisition system

Dynamometer controller

Methanol ECU

Methanol supply rail Methanol injectors

Diesel ECU

Electronic unit pump Thermal sensor

Intake

methanol-air mixture

Air cooler Mo/Al2O3

Co/Al2O3

Thermal sensor

Methanol supply rail Methanol injector Cooling chamber

Smoke meter AVL415S

Pressure sensor

Methanol tank

Diesel filter

Turbo charger

Methanol filter

Air flow meter

Methanol pump

Exhaust EGR valve

Pressure regulator Fuel consumption meter

Diesel injector

EGR cooler

Gas analyzer Horiba Meax-7100DEGR

Particulate analyzer DMS 500SKII

Dynamometer Gas analyzer Horiba Meax-6000FT

Thermal sensor

Fuel path Signal path Gas path

Diesel tank

Thermal sensor Methanol-SCR

Fig. 1. Schematic diagram of tested engine bench.

Table 2 Controlling and measuring accuracy of devices. Device

Variable controlled/ measured

Accuracy

Dynamometer

Engine speed Engine torque Air mass flow Fuel mass flow Exhaust temperature Intake air temperature

±0.1% ±0.2% ±1.0% ±1.0% ±1 °C ±1 °C

Cooling water temperature Exhaust pressure HC emissions

±1 °C

AVL-415S smoke meter

CO emission EGR ratio NO emission NO2 emission PM emissions

DMS 500SKII particulate analyzer

particulate size

±1.0% F.S. ±1.0% F.S. ±1.0% F.S. ±1.0% F.S. ±0.001 FSN 5– 1000 nm ±3.0% F.S.

Toceil-20 N air flow meter Fuel consumption meter K type thermocouple sensor PT100 type thermal resistance sensor

Diffuse silicon pressure sensor Horiba Mexa-7100DEGR gas analyzer

Horiba Mexa-6000FT gas analyzer

PN emissions

F.S. F.S. F.S. F.S.

±0.4% F.S. ±1.0% F.S.

Table 3 Main physicochemical properties of diesel and methanol fuels. Physicochemical property

Diesel

Methanol

Molecular formula Molecular weight Oxygen content (wt.%) Cetane number Stoichiometric air/fuel ratio Low heating value (MJ/kg) Heat of evaporation (kJ/kg) Sulfur content (ppm wt.) Density at 20 °C (kg/m3)

C10-C15 190–220 0 51 14.7 42.5 260 <10 840

CH3OH 32 50 <5 6.45 19.7 1178 0 790

increment of MSR was 10%. The maximum MSR was primarily restricted by the misfire or knock for each test point. To ensure the normal combustion of the engine, the peak pressure and pressure-rise rate in the cylinder needed to be less than 15 MPa and 1.5 MPa/°CA, respectively (Chen et al., 2019). The speeds and loads of tested engine were chosen from the Europe steady-state cycle (ESC) test of emission standards of heavy-duty diesel engine. The tests were divided into four groups. The detailed test processes are as follows. Firstly, the reductant to methanol-SCR was provided by unburned methanol from the DMDF mode (denoted as passive methanol supply mode (PMSM)) or by injecting methanol upstream of catalyst (denoted as active methanol supply mode (AMSM)). As for the AMSM, the main component of HC upstream of catalyst was methanol, and the CO emission upstream of catalyst was low in the exhaust gas. While the emissions of CO and HC upstream of catalyst, and the ratio of NO2 to NOX upstream of catalyst were high under the PMSM in the exhaust gas. These two kinds of methanol supply modes had a great impact on the components of exhaust gas upstream of catalyst. Hence, the effect of different methanol supply modes on the performance of methanol-SCR was studied at A75 test point. The A75 test point represented the speed of 1660 rpm and the brake mean effective pressure (BMEP) of 0.99 MPa. The reason for selecting speed A is that the speed was common for heavy-duty diesel vehicles. The HC concentration measured at the inlet of the SCR system was regarded as the amount of reductant due to the limitation of equipment and the complex species of HC in the exhaust gas. The HC/NOX ratio (mass based) was defined as the mass ratio of the HC emissions to NOX emissions measured at the inlet of the SCR system. The HC/NOX ratio was also referred to as the methanol dosing in AMSM. This AMSM treatment method has also been adopted by others in literature (Gough et al., 2012; Prikhodko et al., 2015; Viola, 2009). The NOX conversion is defined as the following formula (3):

NOx conv ersion ¼

NOxin  NOxout  100% NOxin

ð3Þ

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C. Chen et al. / Chemical Engineering Science 211 (2020) 115320 Table 4 Main parameters of test points. Test point

Speed (rpm)

BMEP (MPa)

Injection timing (°CA ATDC)

EGR ratio (%)

MSR (%)

HC/NOX ratio (mass based)

A100 A75 A50 A25 B75

1660 1660 1660 1660 2090

1.32 0.99 0.66 0.33 0.99

0 -10/-6/-2/0/2/6 0 0 0

0 0/5/10/15 0 0/28 0/21

0 0/10/20/30/40 0 0/10 0/35

0.01/0.94/1.72/2.26/3.00 0.04/1.17/2.05/3.06/3.71 0.06/1.70/2.82/3.82 0.13/3.07/6.46 0.10/11.5

_ D þ HM  m _M HD  m  1000 HD  P e

ð4Þ

where HD (MJ/kg) and HM (MJ/kg) are the low heating value of diesel and methanol fuels, respectively. The value of HD and HM is shown in _ M (kg/h) are the total consumption of diesel _ D (kg/h) and m Table 3. m and methanol fuels, respectively. Pe (kW) is the effective power of the engine. The fuel penalty is calculated by the following formula (5):

Fuel penalty ¼

_ MSCR HM  m  100% _ D þ HM  m _M HD  m

ð5Þ

_ MSCR (kg/h) is the mass of methanol fuel injected ahead of where m the SCR catalyst. Secondly, in addition to the properties of the catalyst, the catalytic performance of the SCR system was also influenced by the operating parameters such as gas hourly space velocity (GHSV), and exhaust temperature (Nova and Tronconi, 2014; Maunula, 2007). In the engine tests, the GHSV and exhaust temperature were mainly affected by the speeds and loads of engine. Therefore, the tests chose four different loads of 25%, 50%, 75% and 100% of engine full loads at fixed speed of 1660 rpm (speed A). The impact of four different engine loads on the performance of methanol-SCR was investigated at speed A on the AMSM. The engine operated on pure diesel fuel. The methanol dosing (HC/NOX ratio) is exhibited in Table 4 for each engine load. The GHSV is expressed by the following formula (6) (Schar et al., 2006):

GHSV ¼

qm;exh  R  T  1000 P  V cat

ð6Þ

where qm;exh (kg/h) is the exhaust flow of engine. R (J/kg K) is the specific gas constant of exhaust gas. T (K) is the inlet temperature of SCR. P (Pa) is the inlet pressure of SCR. V cat (L) is the total volume of SCR. Thirdly, the impact of injection timing of diesel fuel and EGR ratio on the performance of methanol-SCR was investigated at A75 under the AMSM. The engine operated on pure diesel fuel. The increments of diesel injection timing and EGR ratio were 4 °CA and 5%, respectively. Finally, a combination of the DMDF engine and the methanolSCR system was proposed to achieve the ultra-low NOX emissions at three different test points. Three test points were A25, A75 and B75, respectively. The NOX reduction methods for the DMDF engine included the retarded injection timing, the increased EGR ratio and the maximum MSR value. As displayed in Table 4, the optimal value of NOX reduction methods at A25 and B75 test points was determined based on the test processes of A75 test point. Furthermore, the effect of NOX reduction methods on the emissions of PM and PN and the fuel consumption was evaluated. The sampling was carried out after the inlet and outlet temperature of SCR was steadily maintained for 8 min. The sampling fre-

3. Experimental results and discussion 3.1. Effect of methanol supply modes on the catalytic performance of methanol-SCR There are two kinds of methanol supply modes to provide reductant for the methanol-SCR, i.e., AMSM and PMSM. Two kinds of methanol supply modes have a great impact on the components of exhaust gas upstream of catalysts. Therefore, the effect of AMSM and PMSM on the performance of methanol-SCR was researched at A75 test point in this section. The A75 test point represents the speed of 1660 rpm and the BMEP of 0.99 MPa. Fig. 2 shows the effect of HC/NOX ratios on the catalytic performance of methanol-SCR. In Fig. 2, the HC/NOX is changed by altering the amount of methanol injected upstream of catalyst. The NOX conversion increases from 3% to 48% with the increase of HC/NOX ratio. The NO2 emission increases, while the NO emission decreases with the increased HC/NOX ratio. The NOX conversion is 3% when the HC/NOX ratio is 0.04 with no methanol injected upstream of

5

50

40

4

NOX conversion

30

NO NO2

3

20 2 10 1

0

-10

0

0.04

1.17

2.05

3.06

Brake specific emissions of NO and NO2 (g/kWh)

BSFC ¼

quencies of the DMS500 MKII instrument, the Horiba Mexa-6000FT analyzer and the Horiba Mexa-7100DEGR analyzer were set to 1 Hz. The emissions of NO and NO2 were recorded 45 data by the Horiba Mexa-6000FT analyzer under each test point. The emissions of HC and CO were recorded 45 data with the Horiba Mexa7100DEGR analyzer for each test point. The PM emissions were recorded 5 data with the AVL 415S smoke meter at every test point. The PN emissions were recorded 60 data with the DMS 500SKII instrument at every test point. Finally, the average value of recorded data under every test point was taken to study. The data error analysis adopted the method of one standard deviation. The error bars are given in the figures.

NOX conversion (%)

where NOxin (ppm) and NOxout (ppm) are the NOX volume concentration at the inlet and outlet of the SCR system, respectively. The brake specific fuel consumption (BSFC) and the fuel penalty were introduced to analyze the fuel loss of two kinds of methanol supply modes. The BSFC is calculated by the following formula (4):

3.71

HC/NOX ratio (mass based) Fig. 2. Effect of HC/NOX ratios on the NOX conversion and the brake specific emissions of NO and NO2 under the active methanol supply mode at A75 test point (A75 represents the speed of 1660 rpm and the BMEP of 0.99 MPa).

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catalyst. This is because the conversion reaction in the catalyst between the unburned HC (about 82 ppm, not shown) from the cylinder and NOX occurs. This conversion indicates that the (CoMo)/Al2O3 dual catalysts can utilize HC to reduce NOX, not just methanol. The maximum NOX conversion is 48% at A75 test point on the engine test bench with the inlet temperature of SCR at 355 °C. However, the previous study indicated that the temperature window of NO conversion (NO conversion beyond 50%) over (Co-Mo)/Al2O3 catalysts was in the range of 182–454 °C at the GHSV of 20,000 1/h on the synthetic gas test bench (Yao et al., 2018). The likely three reasons for the decrease of NOX conversion on the engine test bench are as follows. Firstly, unlike the calculation of NOX conversion, the calculation of NO conversion does not eliminate the effect of the oxidation reaction between NO and O2 to produce NO2. Secondly, the GHSV of the engine test bench is 2.25 times greater of that of the syngas platform. The residence time of reactants over catalysts decreases inversely with the increased GHSV. Hence, the NOX conversion decreases. Finally, the engine exhaust gases contain water vapor but the feed used on the synthetic gas test bench does not have. The research done by Tabata et al. indicated that water vapor inhibited the NOX conversion, and the extent of inhibition enhanced as the content of water vapor increased (Tabata et al., 1995). In fact, water vapor competes with reductant for active sites over catalysts (Cheng and Bi, 2014). Therefore, a part of active sites will briefly deactivate and thus the NOX conversion decreases. Fig. 3 indicates the effect of MSR on the catalytic performance of methanol-SCR. In Fig. 3, the HC/NOX ratio is changed by altering MSR on the DMDF mode. The maximum MSR was controlled at 40% to ensure the combustion stability and reduce rough of combustion under A75 test point. It can be seen that the NOX conversion and the HC/NOX ratio increase with the increase of MSR. The NOX conversion is 8% at MSR40. Hence, the catalytic performance on the AMSM is better than that on PMSM. Fig. 4 indicates the effect of MSR on the NO, NO2, CO and HC emissions before and after the methanol-SCR system. Before the SCR system, the brake specific emission of NO reduces, but the HC, CO and NO2 emissions increase with the increased MSR. There is nearly no NO2 emission after the SCR. The HC and CO emissions after the SCR are lower than those before the SCR at any given MSR. However, the NO emission after the SCR is higher than that before the SCR. The possible explanation for these observations is that NO2 inhibits the oxidation of NO, CO and HC in the exhaust gases (Al-Harbi et al., 2012). The NO2, as a strong oxidant, preferentially reacts with HC

4

8

NOX conversion 6

3

4

2

1

2

HC/NOX ratio (mass based)

NOX conversion (%)

HC/NOX ratio

0

0 0

10

20

30

40

Methanol substitution ratio (%) Fig. 3. Effect of methanol substitution ratios on the NOX conversion and HC/NOX ratios under the passive methanol supply mode at A75 test point.

and CO to produce NO under the action of dual catalysts. The formation process of NO can be described as follows (Al-Harbi et al., 2012; Wei et al., 2015):

fHCg þ NO2 ! NO þ COX þ H2 O

ð7Þ

CO þ NO2 ! NO þ CO2

ð8Þ

where COX represents CO and CO2. ThefHCg represents the absorbed HC. The HC and CO can be oxidized by O2 when the HC and CO exist and NO2 is consumed totally. The oxidation of HC and CO can be described by the following reactions (Maunula, 2007; Zou et al., 2015):

fHCg þ O2 ! COX þ H2 O CO þ O2 ! CO2

ð9Þ ð10Þ

Except to the above oxidation reactions, the main surface reactions over catalysts also includes the selective reduction reaction between HC and NOX. The NOX reduction reaction can be described by the following reactions (Maunula, 2007; Tabata et al., 1995):

NO þ O2 ! NO2

ð11Þ

fHCg þ NO2 ! N2 þ COX þ H2 O

ð12Þ

The active sites over catalysts are limited. Meantime, the HC competes with CO for active sites over catalysts (Frobert et al., 2012). The absorbed HCfHCg decreases. Furthermore, one part of thefHCg is oxidized by NO2 or O2 (reactions (7) and (9)), while the other part of the fHCg reduces NOX (reactions (11) and (12)). The mass of fHCgused to reduce NOX decreases, and then the NOX conversion reduces. Therefore, a mass of CO and NO2 in the PMSM inhibits the reduction between NOX and HC over (Co-Mo)/ Al2O3 catalysts. Although CO was added to the feed, the CO/HC ratio was only set to 0.2 and 0.4 in the synthesis gas tests (Yao et al., 2018). The condition where the CO/HC ratio is more than 1 was not done. Meanwhile, the NO2 was not introduced into the feed on synthesis gas tests (Yao et al., 2018). The high CO/HC (more than 1) and NO2/NOX ratios usual occur at DMDF mode with the high MSR (Wang et al., 2018). Hence, results from PMSM suggests that the effect of CO/HC (more than 1) and NO2/NOX ratios on NOX conversion needs be studied deeply during the catalyst selection. Fig. 5 exhibits the effect of methanol supply modes on the BSFC and the fuel penalty. As shown in Fig. 5a, the BSFC and the fuel penalty increase as HC/NOX ratio increases. It can be seen from Fig. 5b that the BSFC slightly increases as MSR increases. A slight increase of BSFC is because that the low heating value of methanol is lower than that of diesel (Yusri et al., 2017). However, the fuel penalty does not exist due to the PMSM without injecting methanol into the exhaust pipe. Although the NOX conversion on the PMSM is lower than that on AMSM, the NOX concentration from exhaust gas in the cylinder effectively reduces on the PMSM (Fig. 4a). 3.2. Effect of methanol dosing on the methanol-SCR at different engine loads under AMSM The exhaust temperature influences the evaporation of methanol in exhaust pipe and the rate and selectivity of NOX reduction. The engine loads have a great effect on the exhaust temperature. Therefore, the effect of engine loads on the performance of methanol-SCR under the AMSM was studied in this section. Fig. 6 shows the effect of HC/NOX ratios on NOX emissions at the exit of SCR with different engine loads. In Fig. 6, the HC/NOX is changed by altering the amount of methanol injected upstream of catalyst. It can be seen that the NOX emissions at the exit of SCR reduce with the increased HC/NOX ratio at any given load. But the decrease amplitude of NOX emissions reduces with the

7

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Brake specific emissions of CO and HC (g/kWh)

Brake specific emissions of NO and NO2 (g/kWh)

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NO2 before the SCR

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Fuel penalty (%)

(a) active methanol supply mode

Brake specific fuel comsuption (BSFC) (g/kWh)

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Fuel penalty (%)

Brake specific fuel comsuption (BSFC) (g/kWh)

Fig. 4. Effect of methanol substitution ratios on the brake specific emissions of gaseous pollutants under the passive methanol supply mode at A75 test point: (a) NO and NO2 emissions and (b) CO and HC emissions.

0.0 0

10

20

30

40

Methanol substitution ratio (%)

Fig. 5. Effect of methanol supply modes on the brake specific fuel consumption (BSFC) and fuel penalty at A75 test point: (a) active methanol supply mode and (b) passive methanol supply mode.

increase of HC/NOX, which is mainly due to the limitation of active sites. The NOX conversion increases with the increased HC/NOX at low and medium loads. However, there is a limitation for the increase of NOX conversion with a high methanol dosing. There are three likely reasons for this limitation. Firstly, the catalytic activity of catalysts reduces due to the low exhaust temperature at low and medium loads. Secondly, the low exhaust temperature results in the HC deactivation of catalysts. The HC deactivation of catalysts means that carbon-rich surface species transformed from HC upstream of catalyst are deposited on the active sites of catalyst, and the mismatch of the adsorption and desorption processes of HC may occur (Cheng and Mulawa, 2009; Theinnoi et al., 2008). Thirdly, a high methanol dosing further increases the rate of the HC deactivation site coverage at low temperatures. Therefore, the methanol dosing must be controlled at a low level under low and medium loads to reduce the fuel consumption. At the BMEP of

1.32, 0.99, 0.66 and 0.33 MPa, the maximum NOX conversion is 41%, 48%, 26% and 25%, respectively. The exhaust temperature increases with the increased engine load. The catalytic activity improves and the rate of HC-deactivation decreases with the increased exhaust temperature. Hence, the maximum NOX conversion increases. But the maximum NOX conversion of full load is less than that of 75% of full load. There are two main reasons for the difference of NOX conversion. Firstly, the inlet temperature of SCR is about 460 °C at full load (Fig. 7a). Results of syngas tests (Yao et al., 2018) done by our group showed that the non-selective oxidation of reductant and O2 over (Co-Mo)/Al2O3 catalysts was significantly strengthened when the inlet temperature of catalysts was more than 400 °C. Meantime, the selective reduction between NOX and reductant over dual catalysts was weakened. Hence, the total oxidation reaction, as shown in reaction (9), becomes the main reaction on the surface of catalysts as the HC/NOX ratio

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(a) BMEP= 1.32 MPa

NOX emissions (g/kWh)

(b) BMEP= 0.99 MPa NOX conver.

NOX emissions w/o SCR: 6.17 g/kWh

NOX conver. 6

7

6.9%

6

NOX conver. 20.6%

5

NOX conver. 29.8% NOX conver.

4

40.5% NOX conver. 39.7%

3

NOX conver.

5

26.2% NOX conver. 37.5%

4

NOX conver. 48.2%

2

0.01

0.94

1.72

2.26

0.04

3.00

1.17

(c) BMEP= 0.66 MPa

8

3.71

NOx emissions w/o SCR: 6.76 g/kWh

NOX conver.

6 NOX conver.

NOX conver.

NOX conver.

25.6%

24.2%

24.9%

4

NOX emissions (g/kWh)

7

7.6%

5

3.06

(d) BMEP= 0.33 MPa

NOX emissions w/o SCR: 7.36 g/kWh 7

2.05

HC/NOX ratio (mass based)

HC/NOX ratio (mass based)

NOX emissions (g/kWh)

NOX conver. 43.5%

3

2

8

NOX emissions w/o SCR: 6.45 g/kWh

3.3%

NOX emissions (g/kWh)

7

NOX conver. 6

13.7% NOX conver. 21.5%

5

NOX conver. 24.6%

4

3

3 0.06

1.70

2.82

3.82

HC/NOX ratio (mass based)

0.13

3.07

6.46

HC/NOX ratio (mass based)

Fig. 6. Effect of HC/NOX ratios on the NOX emissions and NOX conversion under different loads at a fixed speed of 1660 rpm: (a) BMEP = 1.32 MPa, (b) BMEP = 0.99 MPa, (c) BMEP = 0.66 MPa and (d) BMEP = 0.33 MPa.

increases under A100 test point. After the SCR system, as shown in Fig. 8a, the CO emission significantly increases and the HC emissions reduce to around zero as the HC/NOX ratio increases. The temperature at the exit of SCR dramatically increases (Fig. 7a) due to the exothermic character of the total oxidation reaction. The total oxidation reaction between HC and O2 decreases the amount of reductant for the SCR system (Prikhodko et al., 2015). Therefore, the NOX conversion decreases. The research done by Zou et al. (2015) also showed that the methanol-SCR reaction over dual catalysts (W8/Ag/Al2O3 + Ag/Al2O3) was mainly the total oxidation as the exhaust temperature was over 340 °C. Secondly, the GHSV at A100 is higher than that at A75. The residence time of reactants on the surface of catalysts decreases with the increased GHSV. Hence, the NOX conversion further reduces. Figs. 7 and 8 exhibit the effect of HC/NOX ratio on the temperature and the emissions of CO and HC before and after the SCR at different loads, respectively. In Fig. 7a, the exit temperature of SCR dramatically increases with the increased HC/NOX ratio. The difference between the inlet and exit temperature of SCR also increases as the HC/NOX ratio increases. It can be seen from

Fig. 8a that the HC emissions after the SCR are close to zero at any given HC/NOX ratio. While the CO emission after the SCR significantly increases with the increased HC/NOX ratio. The decreased HC and the increased CO suggest that a drastic total oxidation reaction occurs inside the SCR system at A100 (reactions (9) and (10)). As shown in Fig. 7b, the inlet temperature of SCR reduces from 380 °C to 357 °C with the increased HC/NOX ratio, which is due to the evaporation of methanol upstream of catalyst. The exit temperature of SCR is higher than the inlet temperature of SCR when the HC/NOX is more than or equal to 1.17. In Fig. 8b, HC emissions decrease and CO emission increases with the increased HC/ NOX ratio. The selective reduction reactions between NOX and HC (reactions (11) and (12)) are the main surface reaction over catalysts at A75. This selective reduction is an exothermic reaction (Cheng and Mulawa, 2009). Therefore, the exit temperature of SCR is higher than the inlet temperature of SCR. But the heat released from the selective reduction is lower than that released from the total oxidation. Hence, the difference between the upstream and downstream temperature of SCR at A75 is less than that at A100.

9

Temperature ( C)

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C. Chen et al. / Chemical Engineering Science 211 (2020) 115320

80

(a) BMEP=1.32 MPa

40 0 -40

T=Tafter the SCR-Tbefore the SCR

600

Tbefore the SCR

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Tafter the SCR

500 450 400 350 0.01

0.94

1.72

2.26

3.00

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HC/NOX ratio (mass based) 10

(b) BMEP=0.99 MPa

0

-10

-20

T=Tafter the SCR-Tbefore the SCR

400

Tbefore the SCR

390

Tafter the SCR

380 370 360 350 0.04

1.17

2.05

3.06

3.71

HC/NOX ratio (mass based) Fig. 7. Effect of HC/NOX ratios on the temperature difference and the temperature before and after the SCR under two kinds of loads at a fixed speed of 1660 rpm: (a) BMEP = 1.32 MPa and (b) BMEP = 0.99 MPa.

3.3. Effect of injection timing of diesel fuel and the EGR ratio on the performance of methanol-SCR The impact of the injection timing and EGR ratio on the NOX emissions from the cylinder has been known to everyone. But the effect of the injection timing and EGR ratio on the catalytic performance of methanol-SCR is unknown and will be investigated in the following section: Fig. 9 shows the effect of the injection timing of diesel fuel (Fig. 9a) and EGR ratio (Fig. 9b) on the NOX conversion, the HC/

NOX ratio, the GHSV, and the average temperature of SCR. The reductant of methanol-SCR was supplied by injecting methanol with the fixed mass into exhaust pipe during tests. Meantime, the engine operated on the pure diesel fuel. It can be seen from Fig. 9a that the NOX conversion increases from 28% to 55% when the injection timing retards from 10 °CA ATDC to 6 °CA ATDC. It can also be seen that the average temperature of SCR, the HC/ NOX ratio and the GHSV increase with the retarded injection timing. As exhibited in Fig. 9b, the NOX conversion, the HC/NOX ratio and the average temperature of SCR increase with the increase of

C. Chen et al. / Chemical Engineering Science 211 (2020) 115320

(a) BMEP=1.32 MPa 20

CObefore the SCR

HCbefore the SCR

COafter the SCR

HCafter the SCR

Brake specific emissions of CO and HC (g/kWh)

Brake specific emissions of CO and HC (g/kWh)

10

15

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5

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0.01

0.94

1.72

2.26

3.00

HC/NOX ratio (mass based)

(b) BMEP=0.99 MPa 25

CObefore the SCR

HCbefore the SCR

COafter the SCR

HCafter the SCR

20

15

10

5

0 0.04

1.17

2.05

3.06

3.71

HC/NOX ratio (mass based)

Fig. 8. Effect of HC/NOX ratios on the brake specific emissions of CO and HC before and after the SCR under two kinds of loads at a fixed speed of 1660 rpm: (a) BMEP = 1.32 MPa and (b) BMEP = 0.99 MPa.

EGR ratio. The NOX conversion increases from 44% to 64% as the EGR ratio increases from 0 to 15%. The GHSV reduces with the increased EGR ratio, which is due to a part of exhaust gas recycled into cylinder. There are three main reasons for the increase of NOX conversion with the retarded injection timing and the increased EGR ratio. Firstly, the average temperature of methanol-SCR increases with the retarded injection timing and the increased EGR ratio. Hence, the catalytic activity of catalysts improves with the increased average temperature of SCR. Secondly, as shown in Fig. 10, the NOX emissions from the cylinder reduce with the retarded diesel injection timing and the increased EGR ratio. The study of HC-SCR done by Schmieg et al. (2008) indicated that the lower NOX level in the reactant feed could improve NOX conversion and make the temperature window extend toward low temperatures. Furthermore, compared with the amount of HC (methanol) injected upstream of catalyst, the amount of engine-out HC is negligible. Therefore, the HC/NOX ratio increases. Thirdly, the GHSV reduces with the increase of EGR ratio. The residence time of reactants over catalysts increases. More NOX can be reduced by reductants to be converted to N2. Hence, increasing EGR ratio is more beneficial to the improvement of methanol-SCR performance than retarding injection timing. Fig. 10 exhibits the effect of injection timing (Fig. 10a) and EGR ratio (Fig. 10b) on the engine-out emissions of NOX and PM. Fig. 11 shows the effect of injection timing (Fig. 11a) and EGR ratio (Fig. 11b) on the BSFC and fuel penalty. In Fig. 10a, the NOX emissions reduce and the PM emissions increase as the injection timing retards. The PM emissions are below 0.02 g/kWh when the injection timing is before the top dead center of 0 °CA. In Fig. 11a, the BSFC dramatically increases and the fuel penalty slightly decreases as the injection timing retards. A slight decrease in the fuel penalty is due to an increase in the consumption of diesel fuel. Previous results (Wang et al., 2018) showed that the MSR could attain the maximum limit with the injection timing at 0 °CA ATDC under A75 test point on the DMDF engine. A high MSR can reduce the consumption of diesel fuel and the NOX emissions. Hence, considering the NOX conversion, the fuel consumption and the PM emissions, the injection timing of the A75 test point is chosen at 0 °CA ATDC. In Fig. 10b, the engine-out NOX emissions reduce and the engine-out PM emissions increase as the EGR ratio increases. In

Fig. 11b, the BSFC increases and the fuel penalty slightly decreases as the EGR ratio increases. Increasing EGR ratios is less bad for the fuel consumption than retarding injection timings. Hence, the EGR ratio of the A75 test point is set at 15% to attain the maximum NOX conversion. In conclusion, the catalytic performance of methanolSCR can be improved by retarding injection timing and increasing EGR ratio. 3.4. Realization of the lowest NOX emissions on the DMDF engine at different test points According to the above sections, a combination of DMDF engine and methanol-SCR was proposed to achieve the ultra-low NOX emissions. Fig. 12 displays the effect of NOX reduction methods on the NOX (Fig. 12a) and PM emissions (Fig. 12b). In Fig. 12, the relative reduction is defined as the ratio of the emissions difference of NOX or PM between the current and the previous test conditions to the NOX or PM emissions of the previous condition. Three test points, i.e., A25, A75 and B75, are chosen from the ESC test procedure. Fig. 12 also shows the test conditions, which are set by combining with five different NOX reduction methods. Table 4 exhibits the optimal value of NOX reduction methods at A25, A75 and B75 test points. In Fig. 12, the engine-out unburned HC can selectively reduce NOX over the dual catalysts without methanol injected into exhaust pipe at the second test condition. At the second test condition, the NOX conversion of A25, A75 and B75 is 14%, 5% and 13%, respectively. The likely explanation for the difference in NOX conversion is as follows. The HC/NOX ratios of A25, A75 and B75 are 0.13, 0.04 and 0.10, respectively. According to the results of Section 3.2, the catalytic performance of methanolSCR increases with the increase of HC/NOX ratio. Hence, the NOX conversion of A75 is lower than that of A25 and B75. Fig. 13 displays the effect of test conditions on the particulate size distribution. As shown in Figs. 12 and 13, the presence of dual catalysts in the exhaust stream has less effect on the increase of PM and PN emissions, which is due to a slight increase in the exhaust backpressure. A high EGR ratio can significantly decrease NOX emissions from the engine at the third test condition. However, the PM and PN emissions dramatically increase

11

(a) injection timing

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NOX conversion 50

HC/NOX

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Injection timing (°CA ATDC)

EGR (%) Fig. 9. Effect of different parameters on the methanol-SCR system at A75 test point: (a) injection timing of diesel fuel and (b) EGR ratio.

due to the introduction of EGR. Moreover, the introduction of EGR makes the peak particle diameter move toward the large particles as the speed or load increases (Fig. 13). The optimized methanol dosing can further reduce the NOX emissions under any given test point at the fourth test condition. In Fig. 13, the methanol dosing upstream of catalyst has some effect on the particulate size distribution. The concentration of peak particle increases greatly at high speeds and high loads. In Fig. 12a, the minimum NOX emissions are obtained at the fifth test condition after changing the combustion mode from pure diesel to DMDF. At the fifth test condition, the NOX emissions of A25, A75 and B75 are 1.03 g/kWh, 0.59 g/kWh and 0.44 g/kWh, respectively. As exhibited in Fig. 12b, the DMDF mode effectively reduces the formation of PM. In Fig. 13, the DMDF mode makes the concentration of peak particle reduce observably at high speeds and high loads. In the DMDF mode, the decrease in diesel consumption and the addition

of methanol containing oxygen result in a significant reduction in PM and PN emissions. Fig. 14 shows the effect of test conditions on the fuel penalty and BSFC at three test points. The BSFC slightly increases at any given test point when the test condition changes from the first to the third. The BSFC greatly increases at the fourth test condition due to the methanol dosing. Moreover, injecting methanol at the upstream of the methanol-SCR results in the fuel penalty. The BSFC basically remains unchanged except the A25 test point when the test condition changes from the fourth to the fifth. The peak temperature in the cylinder is lower at low loads. Therefore, substituting for the diesel fuel of the same calorific value requires more methanol (Wang et al., 2018), and then the BSFC increases. The DMDF mode has less effect on the fuel penalty. In conclusion, a combination of the DMDF engine and the methanol-SCR system is feasible to achieve the ultra-low NOX emissions.

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(a) injection timing

0.00

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0.02 NOX

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EGR (%) Fig. 10. Effect of different parameters on the engine-out emissions of NOX and PM at A75 test point: (a) injection timing of diesel fuel and (b) EGR ratio.

4. Conclusions This research was carried out on the DMDF engine which was modified from an electronic unit-pump heavy-duty diesel engine. The catalysts of methanol-SCR system were the (Co-Mo)/Al2O3 dual catalysts. The fuel consumption was primarily characterized by the BSFC and the fuel penalty. The effect of methanol supply modes, methanol dosing, injection timings of diesel fuel and EGR ratios on the catalytic performance of methanol-SCR was studied. The route to achieve ultra-low NOX emissions was proposed and discussed. The conclusions are drawn as follows. 1. The NOX reduction performance of (Co-Mo)/Al2O3 dual catalysts on the engine test bench slightly deteriorates compared with that on the synthetic gas test bench. Furthermore, the catalytic

performance of the methanol-SCR on the AMSM is superior to that on the PMSM. A mass of CO and NO2 emissions in the PMSM inhibits the reduction between NOX and HC emissions. Hence, the low NOX conversion with PMSM suggests that the CO/HC (more than 1) and NO2/NOX ratios are the critical factors on NOX conversion during the catalyst selection. 2. There is an applicable scope of methanol dosing that can keep a balance between the fuel consumption and the NOX conversion. Less amounts of methanol are required at low temperatures due to the HC-deactivation of catalysts. The methanol cannot be injected too much at high temperatures due to the total oxidation reaction between HC and O2. 3. Retarding injection timings of diesel fuel and increasing EGR ratios can enhance the catalytic performance of methanolSCR. Increasing EGR ratios is more beneficial to the improve-

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(a) injection timing

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210 0

5

Injection timing ( °CA ATDC)

10

15

EGR (%)

Fig. 11. Effect of different parameters on the brake specific fuel consumption (BSFC) and fuel penalty at A75 test point: (a) injection timing of diesel fuel and (b) EGR ratio.

Condition1: injection timing

Condition2: injection timing+methanol catalysts

Condition3: injection timing+methanol catalysts+EGR Condition4: injection timing+methanol catalysts+EGR+methanol dosing

A25 A75 B75

6

4

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PM emissions (g/kWh)

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Condition5: injection timing+methanol catalysts+EGR+methanol dosing+DMDF (a) NOX (b) PM 0.20 0.15

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1

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Condition

Fig. 12. Effect of test conditions on the brake specific emissions and reduction of NOX (a) and PM (b) at different speeds and loads (where the test points of A25 and A75 represent the BMEP of 0.33 and 0.99 MPa at the speed of 1660 rpm, respectively. The B75 point represents the BMEP of 0.99 MPa at the speed of 2090 rpm).

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Particulate number (dN/dlog/Dp/cc)

2.0x109

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(b) A75

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Appendix A. Supplementary material

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.ces.2019.115320.

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Fuel penalty (%)

Brake specific fuel consumption (BSFC) (g/kWh)

Fig. 13. Effect of test conditions on the particulate size distribution at different speeds and loads: (a) A25, (b) A75 and (c) B75 (Conditions 1–5 are described in Fig. 12.).

0

150 1

2

3

4

5

Condition Fig. 14. Effect of test conditions on the brake specific fuel consumption (BSFC) and fuel penalty at different speeds and loads (Conditions 1–5 are described in Fig. 12.).

ment of methanol-SCR performance than retarding injection timings. Moreover, increasing EGR ratios is less bad for the fuel consumption than retarding injection timings. 4. A combination of the DMDF engine and the methanol-SCR system is feasible to achieve the ultra-low NOX emissions. The presence of dual catalysts in the exhaust stream slightly increases the PM and PN emissions. The increased EGR ratio, the retarded injection timing and the methanol dosing all increase the PM and PN emissions. But the DMDF mode effectively reduces the PM and PN emissions. In addition, the DMDF mode has less effect on the BSFC and fuel penalty except at low loads.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors wish to thank the Natural Science Foundation of China (No. 51676134) for the financial support.

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