Effects of a Diesel Particulate Filter on Emission Characteristics of a

Aug 25, 2017 - National Laboratory of Auto Performance & Emission Test, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100...
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Effects of a Diesel Particulate Filter on Emission Characteristics of a China II Non-road Diesel Engine Hao Zhong,† Jianwei Tan,‡ Yulong Wang,† Jinling Tian,† Naitao Hu,† Jin Cheng,† and Xuemin Zhang*,†,‡ †

College of Engineering, China Agricultural University, 17 Qinghua East Road, Haidian District, Beijing 100083, People’s Republic of China ‡ National Laboratory of Auto Performance & Emission Test, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ABSTRACT: The emission standard of China’s non-road diesel engines is experiencing the transition period from China II to China III, and the relative non-road diesel engine emission issues need to be urgently solved. As a kind of effective aftertreatment technology, a diesel particulate filter (DPF) has become increasingly important in non-road diesel engine emission research. To evaluate the emission characteristics of a non-road diesel engine retrofitted with a DPF, particulate matter (PM) and gaseous emissions emitted from a China II non-road diesel engine loaded with a DPF were investigated on the engine bench. The mass (specific emissions) and number concentrations of PM emissions were obtained by a DLS7200 mass collection system and matter CU-2 number instrument with and without a DPF, respectively. Additionally, gaseous emissions [NOx, CO, and hydrocarbon (HC)] were quantitatively analyzed with and without a DPF by a MEXA-7100EGR emission analysis chamber. Results showed that a DPF could lower the mass and number concentrations of PM emissions effectively. The mass trapping efficiency of PM was more than 90%, and the number trapping efficiency of PM was over 99% . NOx total specific emissions were not obviously influenced by a DPF. However, the emission ratio of NO2/NOx significantly increased by 10−30% compared to the situation without a DPF. Because of the oxidation catalyst of the diesel oxidation catalyst (DOC), the specific emissions of CO and HC decreased by more than 90%. Additionally, there was a substantial reduction of the free acceleration smoke value above 95% after installing a DPF. The DPF is one of the great technological routes that can readily make PM emissions of China II non-road diesel engines meet the emission demands of China IV.

1. INTRODUCTION Diesel engines are widely equipped in the field of non-road vehicles as a result of their good power performance, low fuel consumption rate, and high reliability;1−3 however, serious particulate matter (PM) emissions have resulted in great harm to both the environment and human health.4−7 To date, China has formulated the national standard GB 20891-2014 for nonroad diesel engines, Limits and Measurement Methods for Exhaust Pollutants from Diesel Engines of Non-road Mobile Machinery (CHINA III, IV).8 PM emission limits reached 0.025 g kW−1 h−1. During the implement of this standard, it is required that, from Dec 1, 2016, diesel engines that do not meet the China III emission standard are banned, from the manufacture and imports to sales of non-road machines. However, Chinese domestic non-road diesel engine emission control technologies lag behind emission standards; thus, many diesel engine manufacturers are confronted with serious technological challenges. Currently, most Chinese non-road diesel engines are at the China II level, that is, low emission requirements and serious pollution. Hence, on the basis of the China II non-road diesel engine, deepening the study of emission control technologies plays quite an important role in the current stage. In China, non-road machinery could be an important pollutant source of the deterioration in air quality in Chinese urban areas as a result of its large quantity.9 In comparison to road diesel engines, non-road diesel engines usually work under heavy-load and fast acceleration conditions; © XXXX American Chemical Society

its fuel-injection quantity increases, and mixture gas becomes heavier, causing PM emission growth and emission issues to be more serious.10 Therefore, PM is the key point for non-road diesel engine emission control. A diesel particulate filter (DPF) is currently recognized as an after-treatment technology that can effectively meet the requirements of non-road diesel engine PM emission limits.11−14 At present, foreign countries enjoy many cases about DPF and other advanced after-treatment technologies applied in non-road diesel engines. Switzerland established its non-road vehicle emission lists in 2003.15 To meet the European emission standards, Switzerland specially made the new standard SN 2777206 aiming to evaluate the DPF system. The diesel engines, larger than 37 kW, adopt diesel oxidation catalyst (DOC), DPF, exhaust gas recirculation (EGR), turbo charge, and turbo-charging intercooling technologies.16 The Energy and Environment Research Institution of Germany Federation Environment Department17 applied a TREMOD MM Mobile Machinery model to analyze the non-road emission situation. It is found that NOx and PM emissions are larger than and equal to road vehicle emissions, respectively; thus, most German companies vigorously developed selective catalytic reduction (SCR) and DPF Received: February 27, 2017 Revised: August 25, 2017 Published: August 25, 2017 A

DOI: 10.1021/acs.energyfuels.7b00590 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels technologies to reduce non-road diesel engine emissions. In 2008, the U.S.A. built major technological parameter lists, emission lists, and a research model of non-road diesel engines, which were authenticated by the United States Environmental Protection Agency (U.S. EPA) and sold in the North American market. Currently, there are two major technology roadmaps to reach the Tier 4 standard: EGR + DPF technology and combustion optimization + SCR technology.18 In addition, researchers have conducted a number of studies on the problem of DPF trapping efficiency in PM. Kati et al.,19 who used ELPI equipment to analyze the emissions of non-road high-pressure common rail diesel engines with a DPF, showed that the mass and number concentrations of PM in different particle diameter ranges can be reduced by approximately 90%. Additionally, the results showed that DPFs had a high PM trapping efficiency. Subsequently, Theodoros et al.20 installed a DPF on a diesel engine that can meet the standards of Euro 1 and Euro 2; they analyzed and compared the PM emissions of the diesel engine before and after the installation of a DPF. Their results showed that, under real road conditions, a DPF can still make the diesel engine meet Europe four emission standards and that it is also a reasonable technical route to reduce PM emissions for non-road diesel engines. Li and Jing21 studied the PM trapping efficiency of a DPF under lug-down working conditions; results showed that the filtration efficiency in the number of DPF to PM was more than 95%, decreasing the PM mass by approximately 80−90%. Lou et al.22 used GTPower software to establish the DPF model; results showed that the structural parameters of a DPF have a significant influence on the capture efficiency of PM. These studies have verified the excellent PM trapping performance of DPFs, and the emission characteristics of diesel engine gaseous pollutants [NOx, hydrocarbon (HC), and CO] have been systematically studied. However, in China, the research about the effect of a DPF on emission characteristics of non-road diesel engines is rare. Under the Chinese current situation, it is of great realistic significance to strengthen research about the effect of a DPF on emission characteristics of existing non-road diesel engines. It is equally important for reducing existing diesel engine emissions, providing the technological reference to Chinese domestic nonroad diesel engine factories, and making after-treatment technologies, such as a DPF, more suitable for Chinese domestic non-road diesel engines, etc. Therefore, the effect of DPFs on the emission characteristics of non-road diesel engines needs further research. In this study, we focused on the effects of a DPF on the emission characteristics of a China II non-road diesel engine under different operating conditions, and with reference to GB 20891-20148 and the VERT evaluation system23,24 of the Swiss Environmental Protection Agency, the PM trapping characteristics of the DPF and the emission characteristics of the gaseous pollutants were analyzed.

Table 1. Main Parameters of Fuel item

index

test method

sulfur content (mg kg−1) cetane number density at 20 °C (kg m−3) viscosity at 20 °C (mm2 s−1) 50% distillation temperature (°C) 90% distillation temperature (°C) 95% distillation temperature (°C) aromatic content (%) oxidation stability (mg 100 mL−1) flash point (°C)

7.2 52 832.4 3.904 268.2 347.1 333.6 4.6 0.3 68.5

SH/T 0689-2004 GB/T 386-2010 GB/T 1884-2004 GB/T 265-2004 GB/T 6536-2010 GB/T 6536-2010 GB/T 6536-2010 SH/T 0606-2005 SH/T 0175-2004 GB/T 261-2008

Table 2. Primary Parameters of the Diesel Engine item type emission level bore (mm) stroke (mm) displacement (L) rated power (kW) maximum torque (N m) air intake method fuel supply method injection timing (deg crank angle) after-treatment, EGR

index in-line six-cylinder water-cooled direct-injection engine China II 102 120 5.9 132 (2200 revolutions/min) 790 (1500 revolutions/min) turbo-charged mechanical 6±1 nothing

2.2. DPF Parameters for Test. The overall dimension of the DPF carrier is determined by parameters of the diesel engine. The result of the exhaust flow is measured by a hot-film air flow meter when the engine is operating at a rated power of 26.51 m3· min−1. The engine speed is 2200 revolutions min−1, and the estimation of the specific velocity of the DPF carrier under normal conditions is 36.7 s−1.25 With the exhaust flow and specific velocity at the rated speed, the volume of the DPF carrier is determined to be 1.25 × 107 mm3; additionally, considering the installation of the DPF on the engine bench, the overall dimension of the DPF carrier is Φ of 228 × 305 mm. Parameters of the DPF carrier are shown in Table 3.

Table 3. Parameters of the DPF item

DPF

DOC

material diameter, Φ (mm) length (mm) porosity (%) micropore diameter (μm) thickness (mm) density of pore (25.4 mm−2) regeneration mode catalyst amount (g L−1)

SiC 228 305 50−59 10−25 0.3 150 DOC-assisted passive regeneration 30

metal 220 230

0.05 300 5

2. TEST APPARATUS AND METHOD 2.1. Fuel and Diesel Engine for Test. Fuel for the engine bench test was diesel on sale in Beijing and was chosen according to the Beijing DB11/239-2012 vehicle diesel standard, to ensure that the test could be carried out normally; its basic parameters are shown in Table 1. At present, the vast majority of China’s non-road diesel engines only meet the China II emission standards. A diesel engine that can meet the China II emission standards for a bench test is produced by a domestic engine plant; this engine is widely used in construction machinery. Its basic parameters are shown in Table 2.

2.3. Test Conditions. The test referred to eight conditions of ISO 8178-426,27 for diesel engines. The method contains four kinds of applied working conditions, just as shown in Table 4. This method makes reference to the Switzerland VERT evaluation system. From the table, we could see that differences of torques and speed between these four conditions are obvious; therefore, these conditions can be used as typical operating conditions of non-road diesel engines. In addition, a free acceleration test was also added to measure the exhaust smoke of diesel. All steady-state condition test methods were in accordance with GB 20891-2014, Limits and Measurement Methods for Exhaust B

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Energy & Fuels Table 4. Engine Testing Conditions working condition number

engine speed (revolutions min−1)

load percentage (%)

5 7 3 1

1500 1500 2200 2200

100 50 50 100

Pollutants from Diesel Engines of Non-road Mobile Machinery (CHINA III, IV)8 and HJ 451-2008, Technical Requirement for Environmental Protection ProductAftertreatment Devices for Diesel Vehicle Exhaust.25 2.4. Test Process. The emission test system is shown in Figure 1. Analytical instruments used in the emission testing mainly included an

Figure 2. PM-specific emissions under different conditions.

conditions. After installation of the DPF, the specific emissions of each working condition were significantly lower than before. Under different working conditions, there is little difference in the specific emissions between a new DPF and a regenerative DPF, all lower than 0.025 g kW−1 h−1. Before the installation, because of the high engine speed, low-load condition, poor fuel atomization quality, and low combustion temperature, condition 3 had a peak value of PM-specific emissions reaching 0.28 g kW−1 h−1. Showing the deterioration of PM emissions under this condition but after being filtered by the DPF, PMspecific emissions quickly decreased to 0.0082 g kW−1 h−1. The DPF can maintain specific emissions to a similar level after regeneration, reaching 0.0084 g kW−1 h−1. Figure 3 shows the situation that mass trapping efficiency changed with working conditions. Mass trapping efficiency of

Figure 1. Emission testing system. APA C-P-11-063 alternating current (AC) power dynamometer for control of working conditions, an FMT700-P hot-film air flow meter for measurement of exhaust gas flow, a MEXA-7100DEGR emission analysis chamber for analysis of emission pollutants, a matter CU-2 number instrument for determination of the PM number concentration, a CVS7400T dilution system for dilution of tail pipe samples, and a DLS7200 mass collection system for determination of the PM mass concentration after the sample was diluted. In Figure 1, 2 and 4 show the back-pressure sensors located 180 mm in front of the DPF entrance and 280 mm behind the entrance, respectively, and 1 and 3 are temperature sensors located 220 mm in front of the DPF entrance and 240 mm behind the entrance, respectively. The cycle order of the method that contains four kinds of working conditions is 5−7−3−1−5; the purpose of the repeated condition 5 test is to inspect the reliability and cyclicity of the data. Before and after the installation of the DPF, experiments are required to carry out the method that contains four kinds of working conditions and a free acceleration smoke test. Specifically, the free acceleration test should be repeated 6 times, taking into account the effects of regeneration on the performance of the DPF after completion of a DPF regeneration 60 min repeated test of a method that contains four kinds of working conditions. The regeneration test was under the condition when the engine was at the rated speed of 2200 revolutions min−1, taking 10% as the loading gradient to complete the regeneration from 10% to 100%.

Figure 3. PM mass trapping efficiency under different conditions.

the newly installed DPF is more than 90%; e.g., condition 1 reached a maximum of 98.6%. This is because, as the engine power increases, the exhaust velocity increases and the particle motion is enhanced; this results in an increased collection quantity.28,29 After regeneration, the inner ash content of the DPF carrier was deposited at the end of the intake channel and could not be discharged. When the amount of ash content deposits reached a certain degree, with an increased proportion of the ash blocking the occupied section, the effective trapping length of the air intake channel became shorter. At the same time, the effective filtration area of the filter was greatly reduced, the seepage velocity increased gradually, and the flow resistance of the ash layer decreased. Thus, the total filtering efficiency of PM decreased significantly.30−34 The decline of the ability of DPF to trap led to a decrease in PM trapping capacity; thus, the mass trapping efficiency is lower than that of the newly installed DPF. However, it is a minimal decline; the mass

3. RESULTS AND DISCUSSION 3.1. Impacts of the DPF on PM Emissions. Figure 2 shows the change of PM-specific emissions with the working C

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the ash deposition rate is accelerated; and the ash deposition accumulated in a large number; thus, the effective trapping length and effective filtration area of the DPF are decreased,30,31 causing a reduction of the PM number trapping quantity of DPF and leading to a decline in the number trapping efficiency. Figures 2−5 show that, before the installation of the DPF, the peak value of PM-specific emissions and number concentration appeared in working condition 3, indicating that condition 3 is the most serious condition for PM emissions. Because there is a different PM trapping mechanism in the DPF with regard to both quality and quantity, the peak value of mass trapping efficiency and number trapping efficiency appeared in different conditions and regeneration had little influence on the two kinds of trapping efficiencies. 3.2. Impacts of DPF on NOx Emissions. Figure 6a shows that the peak value of NOx-specific emissions appeared in

trapping efficiency of the regenerated DPF can still reach greater than 90%. Figure 4 shows the PM number concentration under different conditions. Before installation of the DPF, the diesel

Figure 4. PM number concentration under different conditions.

engine exhaust contains a large number of PM with a high concentration that can reach above 107 cm−3. The peak value of the concentration appeared in working condition 3 because of the high engine speed, low-load condition, low combustion temperature, and serious incomplete combustion; all of these reasons led to the increase in both the PM number and concentration. The PM number concentration dropped below 105 cm−3 with a new DPF; little difference was observed in the number concentration of PM under each condition. Condition 3 decreased most significantly. Regeneration did not affect the PM number concentration on the order of magnitude. Figure 5 shows the PM number trapping efficiency under different conditions. The PM number trapping efficiency of a

Figure 6. NOx emissions under different conditions. Figure 5. PM number trapping efficiency under different conditions.

condition 5. Because the engine operated in a high-load situation, the combustion temperature increased and gas residence time became longer; additionally, the NOx emissions increased. There is little influence on NOx-specific emissions after the installation of a new DPF; only condition 7 slightly decreased. Installation of the DPF did not reduce NOx emissions. After regeneration, specific emissions of NOx were lower than emissions before the DPF was installed. The oxidation of NO is one of the main reasons for the decrease in the specific emissions of NOx.35 There is a high oxygen concentration in diesel exhaust and plenty of absorbed oxygen in precious metal active sites of DOC. Part of NO was first oxidized to NO2, and some NO2 was finally captured by the DPF in PM composed of nitrates and nitrites. The reduction of NO caused a decrease in total NOx emissions. Figure 6b shows percentages that NO2-specific emissions occupied in total NOx-specific emissions under different

newly installed DPF can reach more than 99%; the peak value appeared in working condition 3. Because of the increase of the engine speed and exhaust velocity, in addition to the enhancement in gas flow ability, the amount of PM trapped by the DPF increased. No significant change in the number trapping efficiency was observed; the number trapping efficiency remained higher than 99%. Only condition 5 had an obvious decrease. Because the DPF is made of porous media, the transport properties of porous media resulted in an increased diameter of the DPF after regeneration and the permeability increased as well;32 the exhaust flow field also changed. Many small particles pass through the DPF and then float with the exhaust gas to the exhaust outlet. In addition, the engine works under a high-load and low-speed state under condition 5. Part of the carbonaceous layer is burned into ash; D

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Before the DPF was installed, the amount of CO-specific emissions under condition 1 was at a maximum because of the high operation speed, high load, and local non-uniformity of the mixture concentration, leading to incomplete fuel combustion and increased CO emissions. HC-specific emissions appeared as a peak under condition 3; because the engine load was reduced, the oxidation effect was decreased at low temperatures, resulting in an increase in HC emissions. After regeneration, specific emissions of CO and HC appeared to have no greater volatility, indicating that performance of oxidation and catalysis was stable in DOC after regeneration. 3.4. Impacts of DPF on Smoke. Figure 8 shows the free acceleration smoke test for the diesel engine that was repeated

conditions. When the DPF was installed, the proportion of DPF emissions significantly increased; this was the result of the DPF regeneration mode. The DPF adopted the DOC-assisted regeneration mode; oxygen in the diesel engine exhaust was adequate; and DOC oxidation catalysis makes NO oxidation by O2, generating plentiful NO2, such that the proportion of NO2 emissions was significantly increased.36,37 After regeneration, the proportion of NO2 in each working condition was not significantly affected. After installation of the new DPF, the NO2 proportion of condition 7 increased the most and reached a maximum of 33.52%. Primarily because of the low engine load and low operation speed, the exhaust gas temperature did not reach the reaction-required temperature; additionally, the catalyst activity was not in the best state, carbon PM in the DPF and NO2 barely resulted in redox reactions, and NO2 produced by DOC discharged almost completely. In comparison to condition 7, the total generation amounts of NOx and NO2 in condition 3 were the lowest. The total production of NO increased; thus, there was a minimum proportion of NO2. Some NO2 was reduced to the original gas NO by carbon PM when passing through the DPF. In comparison to condition 7, the proportion of NO2 emissions declined. 3.3. Impacts of DPF on CO and HC Emissions. Figure 7 shows that the amount of CO- and HC-specific emissions

Figure 8. Free acceleration smoke test.

6 times. After the DPF was installed, the engine smoke decreased by more than 90%, mainly because the DPF captured a large number of PM, so that the soot emissions decreased. As the experiment was carried out, the proportion of the DPF to decrease smoke gradually increased and the sixth test reached the maximum value. Installation of the DPF can reduce the diesel exhaust smoke. By comparing the data from condition 5 as shown in Figures 2−7 to the data from repeating condition 5, we found that minor changes in PM, NOx, CO, and HC appeared when installing a new DPF, regenerated DPF, or no DPF, indicating results that remained within the allowable error range; thus, the test data present good accuracy, and a test program is feasible.

4. EVALUATION OF THE DPF REGENERATION POINT Figure 9 shows a DPF regeneration test. With an engine working under the condition of 2200 revolutions min−1 and 10−90% loading, DPF exhaust back pressure reached a peak

Figure 7. CO and HC emissions under different conditions.

produced by a diesel engine is very small. After the DPF was installed, the decreasing rate of CO- and HC-specific emissions was approximately 90% under each condition; this was because of the oxidative catalysis of the DPF35,36 transforming CO and HC into CO2. CO and HC emissions that were originally an emission problem can be further reduced.

Figure 9. Regeneration procedure of the DPF. E

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value of 12.8 kPa at approximately 10 000 s; with continued loading, the exhaust back pressure dropped. The exhaust back pressure of the regeneration starting point was 12.8 kPa, and the exhaust temperature was 350 °C. When the exhaust back pressure reached 12.8 kPa, the exhaust temperature increased sharply. The PM started to burn and released heat, and the exhaust temperature continued to rise until the exhaust back pressure decreased to the lowest point, when the regeneration process was complete. According to Figures 2−8, the completion of the regeneration process directly affects the emission characteristics of the diesel engine after installing the DPF. In the experiment, the generation way of the DPF is the DOC auxiliary regeneration method. Adopting DOC auxiliary regeneration for the non-road diesel engine has many advantages,38 including the following three points: (1) The regeneration frequency is low. DOC has the ability of catalytically oxidizing the particulates in the diesel engine exhaust, and the higher the exhaust temperature, the stronger the oxidation ability. NO and CO are oxidized in DOC, releasing heat. NO2 can react with the particulates in the DPF at the normal exhaust temperature, which leads to continuous regeneration. Then, the particulates that accumulated in the DPF will decrease, and the regeneration frequency of the DPF will also reduce. (2) It can reduce the release of CO and HC. (3) With a simple structure and low cost, DOC + DPF is applied to the non-road diesel engine. It can be concluded on the basis of the non-road diesel engine experiment results that, under the actual working condition, the passive regeneration method of DOC auxiliary regeneration is still suitable for the DPF, that is, loading the diesel engine to the maximum load to improve the exhaust gas temperature and lasting for a certain period, thus, realizing regeneration.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xuemin Zhang: 0000-0002-4305-6990 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Fundamental Research Funds for the Central Universities (2015GX003). This study was also supported by the China−Switzerland Cooperation Project of the Beijing Clean Air Action Plan (7F-07623.01). In addition, this work has been financially supported by the “DaBeiNong Education Fund” of the China Education Foundation Project (1071-2413003).

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5. CONCLUSION (1) PM emissions of a diesel engine are the most serious under condition 3. The DPF could lower the mass concentration and number concentration of PM emissions effectively. After installation of the DPF, the mass trapping efficiency of PM was more than 90% and the number trapping efficiency of PM was over 99%. The DPF is one of the great technological routes that can readily make PM emissions of China II non-road diesel engines meet the emission demands of China IV. (2) NOx total specific emissions were not obviously influenced by the DPF. However, the emission ratios of NO2 and NOx increased 10− 30% compared to no DPF. After installation of the DPF, because of the oxidation catalyst of the DOC, the specific emissions of CO and HC decreased by more than 90%. (3) There is a substantial reduction of free acceleration smoke value above 95% when an engine has a DPF installed. (4) When the engine operated at the rated speed condition, the exhaust back pressure of the regeneration starting point was 12.8 kPa and the exhaust temperature was 350 °C. The completion of the regeneration process directly affected the emission characteristics of the diesel engine DPF. (5) DPF is one of the great technological routes that can availably make PM emissions of China II non-road diesel engines meet the emission demands of China IV. It could be used to reduce PM emissions of existing non-road diesel engines. F

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Energy & Fuels

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DOI: 10.1021/acs.energyfuels.7b00590 Energy Fuels XXXX, XXX, XXX−XXX