Emission Reduction of NOx, CO, HC, PM2.5, and PAHs by Using a Catalyst in a Diesel Engine

Received: April 19, 2022
Revised: July 11, 2022
Accepted: July 17, 2022

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

Cite this article:

Chou, F.C., Jhang, S.R., Lin, S.L., Chen, C.B., Chen, K.S., Lin, Y.C. (2022). Emission Reduction of NO_x, CO, HC, PM_2.5, and PAHs by Using a Catalyst in a Diesel Engine. Aerosol Air Qual. Res. 22, 220180. https://doi.org/10.4209/aaqr.220180

HIGHLIGHTS 

• The emission of NO_x, CO, and HC is reduced by 42.0%, 38.5%, and 1.73%.
• The EGP-SCR effectively filters the total p-PAHs with efficiency of 56.7%.
• The total PAHs reduction efficiency of gas-phase PAHs is 3.23%.
• Calculating the lifetime carcinogenic risk of gas-phase PAHs is worthy of attention.

Keywords: Diesel engine, EGP-SCR, PM2.5, PAHs, Lifetime carcinogenic risk

1 INTRODUCTION

Diesel exhaust gas has long been regarded as a key factor affecting global ambient air quality (Lin et al., 2020). In Taiwan, regulations on diesel vehicles focus on subsidy schemes for old vehicle replacement, and restrictions and encouragement for the use of gas control devices to reduce emissions. According to the Environmental Protection Administration in Taiwan, 23 million motor vehicles were in use in 2021 and diesel engines powered 4.07% of them.

Specifically, the number of vehicles with heavy-duty diesel engines totaled 26,627 under the Euro V standard. To comply with the Euro V standard, the existing after-treatment systems for diesel engines often come with selective catalytic reduction (SCR), whereby urea is injected as a reducing agent to convert NO_x emissions into nitrogen. Presently, SCR-based exhaust systems are considered to have the highest NO_x reduction efficiency, followed by SCR-coated diesel particulate filters (DPFs) and NO_x traps (Jung et al., 2019; Kim et al., 2022).

Recent studies have conducted WHTC and European Transient Cycle (ETC) tests on SCR systems coated with the commercially available V₂O₅-WO₃/TiO₂ catalyst. The results reveal that the SCR catalyst helps to reduce NO_x emissions when exhaust temperature ranges from 70°C–400°C for WHTC and 250°C–400°C for ETC (Table 1). This suggests that, if diesel vehicles run in cities, with the sudden acceleration, deceleration, and stopping causing drastic changes in engine torque and an average exhaust temperature of ≤ 200°C, this will ultimately cause the activity of the SCR catalyst to drop significantly.

Polycyclic aromatic hydrocarbons (PAHs), often identified in aerosols emitted by diesel engines, are a group of toxic chemical compounds comprising two or more benzene rings that exist in both particle and gas phases. A study discovered that short-term exposure to particulate matter (PM_2.5)-PAH and gaseous PAH pollutants in diesel exhaust gases is correlated with an increased rate of hospital admission related to respiratory and cardiovascular conditions (Samburova et al., 2016; Shi et al., 2021).

Strategies involving after-treatment technologies for DPFs are often used to capture and reduce particulate matter (PM) and PAHs emissions (Hu et al., 2013; Huang et al., 2015). In addition, the p-PAHs can reduce significantly if the exhaust system is equipped with a DPF and a DOC (diesel oxidation catalyst), and the elimination efficiency can reach 97.8% to 99.7% (Huang et al., 2015; Tan et al., 2017). Weber et al. (1999) conducted a vehicle driving test using V₂O₅-WO₃/TiO₂ as an after-treatment catalyst. It failed to remove gaseous PAHs (g-PAHs) if the exhaust temperature was lower than 200°C. Conversely, the catalyst could start to activate if the exhaust temperature was above 250°C, converting naphthalene to 70%. This result also demonstrates a strong correlation between catalyst temperature and naphthalene conversion rate (Shah et al., 2012).

The use of DPFs is often limited. For vehicle scenarios, catalytic filter cakes coated with metal catalysts have become popular and promising in practice, as they can simultaneously manage PM and NO_x pollutants (Lin et al., 2018). As the Euro V standard reveals, the concepts behind an after-treatment SCR system comprising an exhaust gas processor (EGP-SCR) are similar to those behind catalytic filter cakes. Adopting cold start WHTC as the test program for diesel engines, this study explores the effects of the EGP-SCR system on NO_x, CO, HC, PM_2.5, gaseous and particulate PAH emissions. By simultaneously measuring emissions upstream and downstream of the system, the present research obtained integrated emission coefficients related to traditional gaseous pollutants. These related to gaseous PAHs and particulate PAHs and their distribution attributes prior to, and following, the EGP-SCR treatment, after which the cancer risk of human exposure to PAHs was determined.

 
2 METHODS

 
2.1 Experimental Setup

In this study, the target gas emissions from a heavy-duty diesel engine were sampled as illustrated in Fig. 1. The engine test was performed on a dynamometer at the research and development center of Chinese Petroleum Co. in Chiayi, Taiwan. Table 2 and Table 3 detail specifications of the tested engine and tested fuel, respectively (Lin et al., 2008). The exhaust after-treatment system comprises four units: a dosing control unit, a dosing unit, a urea supply unit, and a urea tank.

Fig. 1. Experimental setup.

Urea was injected into the engine exhaust and mixed with the air in the experiment. Afterwards, the urea was fully diffused through a flow equalization device before entering the ceramic-fiber filter brick for treatment, where V₂O₅-WO₃/TiO₂ was adopted as a porous ceramic-metal coating. Subsequently, the research simultaneously recorded changes in the concentrations of gaseous pollutants (i.e., NO_x, CO, and HC) in the diesel engine under the WHTC standard, upstream and downstream of the EGP-SCR system.

PM_2.5 were extracted at a constant rate to 47-mm filters with 2 µm pore size (Whatman, UK). The filters were placed in a glass jar covered with aluminium foil before being moved to a dry box and brought back to the laboratory for weight measurement and subsequent particulate PAH (p-PAH) extraction and analysis. The gaseous PAHs (g-PAHs) were collected by first using a condenser to deliver air under temperatures of ≤ 52°C, the cartridge was filled with XAD-16 resin to absorb all PAHs. After the sampling, the glass tubes were wrapped with aluminium foil. The airtight containers were analyzed in the laboratory, where the samples were stored in a fridge at a temperature below −4°C for at least seven days until the analysis ended.

 
2.2 Test Cycle and Methodology

A WHTC test comprises three steps: cold start, 10-min soak, and hot start. The test program in this study was regarded as only a part of the cold start process. The present research studied three cycles under the WHTC cold start, representing urban, rural, and highway driving (Zhang et al., 2017). The urban driving cycle (900 s), which averaged a speed of 21.3 km h^–1, comprised operations involving idling, stopping, and frequent starts. The rural driving cycle (481 s) averaged a speed of 21.3 km h^–1 with a peak of 75.9 km h^–1. For the highway driving cycle (419 s), the highest speed was 87.8 km h^–1. Fig. 2 reveals the exhaust temperature ranges of the three cycles, namely, 42°C–225°C, 162°C–288°C, and 163°C–323°C for urban, rural, and highway driving, respectively.

Fig. 2. Air filter temperatures under the WHTC driving cycle.

 
2.3 PAHs Analysis

In this study, 21 PAHs were analysed. The PAHs were subdivided into three groups:

• Low molecular weight hydrocarbons (LMW-PAHs), i.e., (Nap), (AcPy), (Acp), (Flu), (PA), and (Ant)
  
  
• Moderate molecular weight hydrocarbons (MMW-PAHs), i.e., (FL), (Pyr), (BaA), and (CHR)
  
  
• High molecular weight hydrocarbons (HMW-PAHs), i.e., (CYC), (BbF), (BkF), (BeP), (BaP), (PER), (DBA), (BbC), (IND), (Bghip), and (COR)
  
  

The total PAH data for fuel exhaust is given by the sum of 21 individual PAHs (Lin et al., 2019a). To classify and quantify the PAHs, an after treatment was performed using the Soxhlet extractor with a blended dissolvable (n-hexane and dichloromethane; vol/vol, 1:1; 500 mL each) for 24 h. Each substance was regulated by HP gas chromatograph (GC) (HP-6890; Hewlett-Packard, Wilmington, DE, USA) coupled to a mass selective detector (MSD) (HP-5973) (Chen et al., 2019; Mueller et al., 2019). The results indicate that the recovery rates of the 21 PAHs range between 83.6% and 101%, with an average of 93.2%.

 
2.4 Health Risk Assessment of P-PAHs

The health risk of g-PAHs was estimated using a human intake model, as defined in Eq. (1) and Eq. (2). The incremental lifetime carcinogenic risk (ILCR) for humans can be determined by calculating the lifetime average daily dose (LADD) of PAHs (Kong et al., 2015):

where Ci is the emission factor of BaP_eq in g-PAHs (µg kWh⁻¹); IR is the inhalation rate (m³ day⁻¹), 7.6 for children and 20 for adults; ED denotes the lifetime exposure duration (6 years for children and 24 years for adults); EF is the exposure frequency (350 days year⁻¹, used to calculate the lifetime exposure of human receptors); BW refers to the body weight (15 kg for children and 70 kg for adults); ALT is considered the average lifetime for carcinogens (70 years × 365 day year⁻¹ = 25,550 days); and CSF is the cancer slope factor, which for BaP from inhalation is defined as 3.14 (mg kg⁻¹ day⁻¹)⁻¹ (Jamhari et al., 2014).

 
2.5 Health Risk Assessment of G-PAHs

The lifetime lung carcinogenic risk (LLCR) of particulate PAH (p-PAH) exposure through inhalation was calculated using Eq. (3) and Eq. (4). BaP_eq is calculated by multiplying the individual PAH emission factor (Ci) by the relative toxic equivalency factor (TEF_i). ∑BaP_eq denotes the sum of individual BaP_eq values:

where ET, EF, ED, and AT denote the exposure time (6 hr day⁻¹), frequency (350 day year⁻¹), exposure duration (6 yers for children and 24 years for adults), and average lifetime (70 years × 365 day year⁻¹ = 25,550 d), respectively. ADAF is the age-dependent adjustment factor (3 for children and 1 for adults) (Song et al., 2021; Nam et al., 2021), while IUR represents the inhalation cancer unit risk, 8.7 × 10^–5 (ng m⁻³)⁻¹.

3 RESULTS AND DISCUSSION

 
3.1 Regulated Emissions

Table 4 specifies the regulated emissions measured simultaneously upstream and downstream of the SCR system during the cold start of the WHTC. The hydrocarbon (HC) emission level was the lowest among other pollutants, ranging from 0.0278 to 0.349 g (kWh)^–1 for the upstream and from 0.0267 to 0.347 g (kWh)^–1 for the downstream. CO emissions at the upstream and downstream lay between 0.967–1.56 and 0.136–1.17 g (kWh)^–1, respectively. CO₂ emissions exhibited the highest values among other pollutants, reaching 2.05–2.38 g (kWh)^–1 for the upstream and 2.41–3.03 g (kWh)^–1 for the downstream. Upstream NO_x emissions reached 1.20–1.53 g (kWh)^–1, compared with downstream of 0.643–1.01 g (kWh)^–1. In terms of PM_2.5, its emission at the upstream was 0.0435 g (kWh)^–1 and decreased to 0.00948 g (kWh)^–1 following the SCR treatment.

The use of SCR (commercially available V₂O₅-WO₃/TiO₂) catalysts usually activate high conversion efficiencies at temperatures between 250°C and 400°C (Ma et al., 2015;Tan et al., 2019). In terms of our results, under urban driving conditions, low exhaust temperature (156 ± 47.8°C), causing low CO and NO_x removal efficiency which are accounting for 22.3% and 25.0%, respectively. Under the highway driving cycle (228 ± 56.1°C), the higher temperature increased the activity of the catalyst and improved CO and NO_x removal efficiency, reaching 83.8% and 86.1%, respectively. It is suspected that the oxygen content in the combustion chamber may regulate HC emissions. Lack of sufficient oxygen for combustion under urban driving conditions would cause higher HC emissions. However, cold driving cycle with low temperature also leads to the low activity of the SCR, resulting in unsatisfactory emission reduction rates of HC (Mrad et al., 2015). Similar studies have suggested that equipped with an EPG-SCR control system could improve to reduce NO_x, CO, and other regulated pollutants in the engine exhaust, while the exhaust temperature is an important factor influencing tailpipe emissions (Bai et al., 2018; Zhang et al., 2017; Borillo et al., 2018; Yusuf and Inambao, 2019).

 
3.2 Total PAH Emissions

The characteristics of total PAHs (g-PAHs and p-PAHs) both upstream and downstream of EGP-SCR over the cold start WHTC are presented in Table 5, while Fig. 3 shows the distribution of p-PAHs and g-PAHs. The g-PAHs dominated in comparison with the p-PAHs, the distribution of g-PAHs accounted for more than 90% of total PAHs, regardless of the EGP-SCR use. LMW and MMW-PAHs, such as Nap, FL, Pyr, BaA and CHR, are compounds that have two to three rings. Those regarded as HMW-PAHs, with five to seven rings, were CYC, BbF, BkF, BeP, BaP, PER, DBA, BbC, IND, Bghip and COR (Dhital et al., 2021; Wang et al., 2021a).

Fig. 3. The distribution of particulate PAHs (p-PAHs) and gaseous PAHs (g-PAHs) (a) upstream and (b) downstream of EGP-SCR over WHTC driving cycle.

PAHs comprise carbon and hydrogen with chemical structures of at least two combined benzene rings in straight. The combustion of fossil fuels predominantly produces PAHs during the pyrolysis and pyro-synthesis processes, in which the gas phase of the hydrocarbons can rapidly restructure and participate on PAH formation. PAHs come from the incomplete combustion of lubricants and hydrocarbon wastes. During the combustion, either the oxygen concentration or the temperature is low, which can easily lead to the formation of PAHs. The formation of LMW-PAHs is correlated of diesel fuel adopted (Tsai et al., 2020; Lin et al., 2020b).

In this study, LMW-PAHs—with low steam concentration and high volatility—mostly existed in gas-phase wastes. Conversely, MMW- and HMW-PAHs tended to adsorb on the surface of PM_2.5 particles, due to their higher steam pressure. These results were consistent with Lin et al. (2019c), who examined the distribution of PAHs adsorbed on PM_2.5 particle surfaces among 15 diesel vehicles.

3.3 G-PAHs Emissions

Fig. 4 shows the emission factors and removal efficiencies for g-PAHs at the cold start phase of the WHTC. G-PAHs are mainly generated by gaseous hydrocarbons (HC). Considering the sum of g-PAHs, the emission factors were 55.5 µg (kWh)^–1 for the upstream and 53.7 µg (kWh)^–1 for the downstream, indicating a 3.23% reduction when SCR was applied. The finding was consistent with Shah et al. (2012), who reported a significant reduction of g-PAHs following the use of an SCR catalyst.

Fig. 4. Emission factor and removal efficiency of (a) total g-PAHs, (b) the difference between LMW/MMW and HMW g-PAHs, (c) EF BaP_eq, W/O and W/EGP-SCR.

LMW g-PAHs (two- and three-ring PAHs) provided the largest contribution to exhaust emissions, accounting for more than 90% of total PAHs, followed by their MMW and HMW counterparts. After the SCR treatment was applied, 3.19%, 0.987%, and 1.53% reductions for LMW-PAHs, MMW-PAHs, and HMW-PAHs were identified. Concerning individual PAHs, Nap (55.0 µg (kWh)^–1), PA (0.352 µg (kWh)^–1), and Acp (0.0828 µg (kWh)^–1) had the highest concentration. Nap contributed the highest in comparison to the other g-PAHs.

According to Shah et al. (2012), the conversion rate of Nap and Ant are highly correlated with catalyst temperature, suggesting that high temperatures could promote the effectiveness of SCR catalysts and thus reduce concentrations of the two substances downstream. Shah et al. (2012) reported that the Nap conversion rate reached ≥ 50% after the SCR catalyst was adopted. Compared to this study, conversion rates were low for both Nap (2.73%) and Ant (19.6%), probably because of the difference in test cycle design and fuels. Additionally, the g-PAHs, caused by the incomplete combustion of the diesel engine, were mainly 2-ring Nap, which were mostly adsorbed on the V₂O₅-WO₃/TiO₂ catalyst surface. However, the oxidation temperature of V₂O₅-WO₃ was insufficiently high; consequently, the catalyst could not convert the PAHs adsorbed on its surface effectively (Tseng et al., 2011).

 
3.4 P-PAHs Emissions

The emission factors and removal efficiencies of p-PAHs under the cold start phase of WHTC are illustrated in Fig. 5, with emissions factor of 0.0487 µg (kWh)⁻¹ for the upstream and 0.0211 µg (kWh)⁻¹ for the downstream, achieving a 56.7% removal rate.

Fig. 5. Emission factor and removal efficiency of (a) total p-PAHs, (b) the difference between LMW/MMW and HMW p-PAHs, (c) EF BaP_eq, W/O and W/EGP-SCR.

Specifically, LMW-PAHs occupied the largest share, accounting for 64.9% of total emissions, followed by MMW-PAHs and HMW-PAHs. After treatment, concentrations slumped by 44.1%, 78.5%, and 33.0% for LMW-PAHs, MMW-PAHs, and HMW-PAHs, respectively. The p-PAH reduction was associated with decreases in PM_2.5 after the SCR device began operating.

Euro V-certified EGP-SCR system works by modifying SCR catalysts on ceramic filter bricks. Studies have discovered that the resultant ammonium sulfate is likely to increase the size of small particles, causing them to clog small catalyst pores, thereby blocking and filtering the enlarged and agglomerated PMs. The SCR catalyst could do so mainly because of its small porous structure that can block the enlarged particles (Apicella et al., 2020). These results were consistent with Zerboni et al. (2021), who examined the decreased filtering of PAHs adsorbed on PM_2.5 particle surfaces of after-treatment systems, thereby reducing the PAH concentration and toxic BaP_eq factor. However, sampling data for this study were insufficient to verify this assumption, upon which future studies should focus, as shown in Table 1.

 
3.5 PM_2.5 and G-PAHs Health Risk Assessments

The increasingly heavy traffic in urban areas could enhance atmospheric PAH concentration. The formation of toxic PAHs is closely related to vehicular exhaust from incomplete combustion. Substantial contributions from traffic emissions lead to an increased probability of developing respiratory allergy and lung cancer (Amesho et al., 2021; Xu et al., 2021; Wang et al., 2021b). The index for health risk assessments such as ILCRs and BaP_eq are used to evaluate the cancer risk of PAHs. The ILCR value greater than 10⁻⁴, 10⁻⁶, or between 10⁻⁵ and 10⁻⁶ are considered high risk, a potential risk, or no significant risk, respectively (Cui et al., 2021). Due to the use of PM_2.5 filtration with EGP-SCR and reduce the concentration of PAHs, the resultant ILCR values for PM_2.5 were 2.80 × 10⁻⁷ and 1.42 × 10⁻⁷ in adults and children, respectively, indicating no significant cancer risk. However, the low exhaust temperature of the EGP-SCR system caused the activity of the SCR catalyst to decrease, and it was thus not capable of decomposing PAHs effectively. The ILCR values for g-PAHs were 1.93 × 10⁻³ and 1.45 × 10⁻³ in adults and children, respectively, denoting potential health risk, as shown in Table 6.

The data reveal that the EGP-SCR system can effectively filter the MMW- and HMW-PAHs adsorbed on the PM_2.5 surface and help avoid inhalation of carcinogenic substances. However, g-PAHs, proven to create serious carcinogenic hazards, cannot be suppressed by the EGP-SCR after-treatment system under cold start conditions. Therefore, catalyst modification is required to lower the active temperature range, thereby reducing the carcinogenic risk from g-PAHs.

 
4 CONCLUSIONS

By comparing the pollutant concentrations under a cold-start WHTC, before the EGP-SCR treatment, to those following the treatment, this study established the following four findings:

1. Due to the low exhaust temperature of the diesel engine (average temperature = 198°C), the activity of the SCR catalyst reduced, leading to unsatisfactory emission reduction rates of 42.0%, 38.5%, and 1.73% for NO_x, CO, and HC, respectively.
   
   
2. The EGP-SCR system can reduce PM₅ emissions, possibly because of the small-particle agglomeration caused by the catalyst. This phenomenon enabled increased EGP-SCR filtering, as PAHs were adsorbed in the form of PM_2.5 particles, thereby reducing the PAH concentration and toxic BaPeq factor. This reduced the carcinogenicity and mutagenicity of the PAHs to the human respiratory system.
   
   
3. Low diesel engine exhaust temperatures caused the activity of the SCR catalyst to decrease. Consequently, oxidation of the g-PAHs was insufficient, resulting in a high health risk.
   
   
4. Under the urban cycle, the temperature of the diesel engine was ≤ 200°C on average. The present study recommends using low-temperature catalysts to achieve greater waste processing efficiency in this scenario.
   
   

 
NOMENCLATURE

Abbreviations and Acronyms