Development and Performance Evaluation Experiment of a Device for Simultaneous Reduction of SO_x and PM

Abstract

Mitigating air pollutants such as SO_x and PM emitted from ships is an important task for marine environmental protection and improving air quality. To address this, exhaust gas after-treatment devices have been introduced, but treating pollutants like SO_x and PM individually poses challenges due to spatial constraints on ships. Consequently, a Total Gas Cleaning System (TGCS) capable of simultaneously reducing sulfur oxides and particulate matter has been developed. The TGCS combines a cyclone dust collector and a wet scrubber system. The cyclone dust collector is designed to maintain a certain distance from the bottom of the wet scrubber, allowing exhaust gases entering from the bottom to rise as sulfur oxides are adsorbed. Additionally, the exhaust gases descending through the space between the cyclone dust collector and the wet scrubber collide with the scrubbing solution before entering the bottom of the wet scrubber, facilitating the absorption of SO_x. In this study, the efficiency of the developed TGCS was evaluated, and the reduction effects based on design parameters were investigated. Furthermore, the impact of this device on ship engines was analyzed to assess its practical applicability. Experimental results showed that increasing the volume flow rate of the cleaning solution enhanced the PM reduction effect. Particularly, when the height of the Pall ring was 1000 mm and the volume flow rate was 35 L/min, the sulfur oxide reduction effect met the standards for Sulfur Emission Control Areas (SECA). Based on these findings, suggestions for effectively controlling atmospheric pollutants from ships were made, with the expectation of contributing to the development of systems combining various after-treatment devices.

1. Introduction

Air pollution is a significant global environmental issue, impacting human health and ecosystems, apart from inducing climate change. Among pollutants emitted from ships, sulfur oxides (SO_x) and particulate matter (PM) are identified as major contributors to air pollution. SO_x primarily arise during fuel combustion, acting as a major cause of acid rain, leading to soil and water contamination. PM, comprising particles with diameters less than 10 μm, poses various health risks such as respiratory and cardiovascular diseases. Consequently, international regulations are in place to regulate the emission of SO_x and PM. Aakko-Saksa et al. emphasized the need to use sulfur-free fuels in conjunction with exhaust after-treatment technologies to achieve carbon neutrality. They particularly analyzed that the transition to a medium-term decarbonization process can be achieved only by combining carbon-neutral fuels with emission control technologies [1].

Regulation of SO_x is enforced through initiatives like the International Maritime Organization (IMO) Sulfur 2020 regulations, limiting the sulfur content in fuels used globally to 0.5% or less. Particularly, in Sulfur Emission Control Areas (SECAs), fuels with sulfur content of 0.1% or less are mandated. However, low-sulfur fuels are relatively expensive and pose issues of engine fouling in aged engines. Hence, methods like scrubbers are employed to reduce SO_x emissions. Scrubbers often discharge wash water overboard without treatment. Osipova et al. evaluated the environmental impact of untreated scrubber wash water and concluded that it causes significant pollution. They recommended that regulations should be strengthened to address not only air pollution but also sea pollution by imposing restrictions on the discharge of wash water [2]. Teuchies et al. also evaluated the impact of marine pollution caused by scrubber wash water. They argued that discharging scrubber wash water near the coast would negatively impact the ecology. While scrubbers are useful devices for reducing air pollution, they can cause sea pollution; therefore, they must be designed with considerations for purifying the wash water [3].

Black carbon, a type of PM emitted from ships, has been a subject of concern due to its potential to accelerate ice melting in Arctic routes. Measures like Diesel Particulate Filters (DPF) are used to mitigate atmospheric PM, but periodic regeneration is required as trapped particles can obstruct filters, affecting engine performance. Hence, there’s a pressing need for effective technologies to reduce SO_x and PM emissions. Cao et al. analyzed the effect of fuel injection pressure on controlling NOx and PM emissions from diesel engines. They evaluated combustion conditions that can minimize NOx and PM emissions. Although emissions could be minimized by controlling the engine’s combustion mode, this approach is limited because it affects engine performance. Therefore, it is appropriate to install after-treatment devices in marine engines to reduce emissions [4].

Studies on exhaust gas treatment systems for reducing SO_x, PM, and NOx emissions from ships have been conducted. Among various SO_x reduction technologies, Zannis et al. compared and analyzed wet scrubbers utilizing seawater or freshwater solutions with sodium hydroxide (NaOH). Among these, a closed-loop wet scrubber using NaOH solution was found to be the most effective system for reducing SO_x emissions [5]. Numerical analytical methods are used to simulate SO₂ adsorption during dynamic interactions between combustion gases and water droplets. Amoresano et al. performed conditions analysis to optimize spray volume distribution and conditions to enhance capture efficiency [6]. Kunche et al. analyzed SO_x and NOx emissions using simulation-based methods from industrial boilers. While useful for analyzing emissions from existing factories, there are limitations in predicting emissions from newly constructed facilities [7]. Yang et al. evaluated the performance of wet scrubbers installed on ships in actual operation. The results showed that wet scrubbers were effective in removing more than 95% of SO₂ emissions. However, they only removed about 10% of particulate matter (PM). It was determined that this was because small sulfuric acid particles formed in the scrubber fluid, and PM was not efficiently removed by impact [8].

As such, there is a limit to reducing PM with just one scrubber. Hu et al. conducted an experimental study on the dust removal performance of a wet scrubber. The efficiency of dust removal was improved by installing an additional 24 blades on a wet scrubber installed on land, but this resulted in a pressure drop. Since scrubbers installed on ships affect engine performance, it is not appropriate to use a blade installation method that causes such a pressure drop [9] of DPF catalyst related to PM reduction performance. Yang et al. verified the relationship between the capture efficiency, air pollutant removal rate, regeneration effect, carbon load, and pressure loss of DPF catalyst to comply with Tier III emission regulations. In addition, by utilizing simulation, the system velocity field and flow field distribution uniformity were evaluated, and the DPF structure was optimized [10]. A study by Syrek-Gerstenkorn et al. demonstrated that clean energy sources have proven effective in reducing emissions of SO_x and PM, while coal energy usage has significantly increased pollution levels, especially PM concentrations [11]. Research on reducing PM emissions using biodiesel is also underway, and a study by Lu et al. showed reductions in harmful emissions like polycyclic aromatic hydrocarbons and soot, thereby minimizing adverse effects on engines [12].

Methods for reducing SO_x emissions include using low-sulfur fuels. Qi et al. conducted a comparative analysis of the effects of fleet scrubbers and fuel change on emissions, revealing cost-effective reductions when scrubbers are employed [13]. Research is continuously being conducted to reduce environmental pollutants by changing fuel. Chivu et al. studied the effects of turpentine addition on diesel engine emissions and engine performance. Lower emissions were observed at high engine loads and low speeds with blends of up to 30% turpentine [14]. In this way, technologies to control ship emissions are continuously being researched. Zhao et al. argued that clean energy and high-efficiency emission control technologies should be adopted to cope with increasingly stringent emissions regulations. They emphasized the need to comprehensively review regulations, changes in fuel, and post-treatment technology to find a solution to the emission problem [15].

Challenges exist in installing multiple after-treatment devices on small ships due to spatial constraints, prompting research into developing integrated devices capable of reducing various pollutants. Choi et al. performed an optimized design to increase the nitric oxide (NO) reduction rate while reducing the space occupied by the after-treatment device by redesigning the filter shape of the DPF and Selective Catalytic Reduction (SCR) systems [16]. Wilailak et al. performed a parametric analysis and design optimization of wet scrubbers in the marine industry. They identified that the main parameters affecting SO_x removal efficiency are the cleaning fluid temperature, circulation flow rate, and chemical input amount [17]. Research on air pollutants emitted from boilers has been utilizing Computational Fluid Dynamics (CFD). Delcourt et al. conducted a numerical analysis of a residential biomass boiler to compare PM emissions. When comparing the experimental results with a CFD model considering particle transport, they obtained valid results [18]. Additionally, research is underway on the reduction effect of air pollutants when scrubbers are installed on ships. Karatug et al. analyzed the feasibility of scrubbers on crude oil tankers, during which they confirmed that scrubbers increased the emission of pollutants other than SO_x [19]. For these reasons, installing only a scrubber does not align with IMO’s environmental pollution reduction goals. Therefore, there is a need to develop devices capable of simultaneously reducing PM and NOx emissions, among others. Research on reducing PM emissions is analyzing the performance of wet electrostatic precipitators applied in the maritime sector. Javinen et al. studied the particle filtration characteristics of an emission reduction system consisting of scrubbers and wet electrostatic precipitators, evaluating the emission reduction capabilities of each scrubber and wet electrostatic precipitator [20]. There was a need to install two or more emission reduction devices, but there was a limitation that they took up a lot of installation space and were difficult to apply to ships.

The studies introduced above did not test the devices installed on actual ships because it would cost a lot to conduct a performance evaluation on reducing exhaust emissions using the devices on actual ships. This study covered the analysis of the factory performance test, which was the stage before installation on an actual ship, and then the device was installed on an actual ship and a test run was conducted. The results of the analysis performed on an actual ship were not covered in this study because the validity of the results was limited due to the influence of the surrounding environment, such as bad weather.

In addition, since the after-treatment devices to reduce exhaust emissions are installed separately, economic costs are incurred for each device. The biggest problem is the space constraints for installing the after-treatment devices on ships because they are large in volume. Therefore, there is a need for the development of integrated devices capable of simultaneously reducing SO_x and PM.

This study aims to develop such an innovative device capable of simultaneously reducing SO_x and PM and evaluate its performance. To achieve this goal, a new device combining a composite filtering system, cyclone dust collector, and wet scrubber was designed and developed. The reduction efficiency of this device was verified through experiments, and its impact on ship engines was evaluated for practical application.

2. Development and Configuration of Total Gas Cleaning System

The simultaneous SO_x and PM reduction device developed in this study was called the Total Gas Cleaning System (TGCS). Figure 1 illustrates the concept of TGCS. The cyclone collector was installed inside the scrubber to clean the exhaust gas, and the device was designed to have the same volume as the scrubber.

The cyclone dust collector functions as a cylindrical unit designed to absorb PM in exhaust gases, where gases spiral downward, allowing PM to be captured. Sludge precipitates along the conical surface and is collected at the bottom.

Figure 2 depicts a photograph of the developed TGCS apparatus. Centrifugal cyclones are integrated into the outer shell of the wet scrubber in a circular tube format. This design enables external PM capture while simultaneously removing SO_x in the wet scrubber.

The TGCS’s cyclone dust collector is strategically positioned to maintain a certain distance from the bottom of the wet scrubber. This arrangement allows exhaust gases entering from the bottom to rise, facilitating SO_x absorption. Additionally, the space between the cyclone dust collector and the wet scrubber allows descending exhaust gases to collide with the cleaning solution before entering the wet scrubber’s bottom, enhancing SO_x absorption.

In the design process of TGCS, numerical analysis was performed on three design models using CFD to predict performance. Model 1 applies only to the scrubber and serves as a benchmark for performance evaluation. Model 2 combines the scrubber with a cyclone dust collector, while Model 3 combines the scrubber with a cyclone dust collector and additionally installs guide vanes. The subjects of analysis are the effect of fine dust reduction and the trend of pressure drop at the inlet and outlet.

Figure 3a shows the change in flow velocity when passing through the TGCS. The particle mass flow rate at the outlet decreased by 48% in Model 2 and 67% in Model 3 compared to Model 1. This indicates that the performance of the cyclone dust collector can be increased by installing guide vanes to direct the exhaust gas to flow in a helical path. Figure 3b shows the pressure inside the TGCS, and in Model 1, a pressure drop occurred inside the scrubber due to the crossflow. In Models 2 and 3, the pressure drop was 34% less compared to Model 1. This is attributed to the structural shape of the cyclone and guide vanes, which facilitate a smoother flow stream.

3. Experimental Procedure

To evaluate the reduction effects on PM and SO_x of the developed TGCS, experiments were conducted using a 1.5 ton/h boiler (DM-150, Daelim Royal EnP, Seoul, Republic of Korea), as depicted in Figure 4.

For assessing SO_x reduction performance, a Heavy Fuel Oil (HFO) with a sulfur content of 3.0% was utilized in the 1.5 ton/h boiler. Figure 5 illustrates the positions where exhaust gas components were measured to evaluate TGCS’s PM and SO_x reduction performance.

Table 1. Emission characteristics of 1.5 ton/h boilers.

Item	Value	Unit
Exhaust gas velocity	8	m/s
Exhaust gas standard flow rate	1750	Sm3/h
Exhaust gas temperature	280	°C

presents the emission characteristics of the 1.5 ton/h boilers used in the experiments, with multi function measuring instrument (Testo 480, Baden-Württemberg, Germany) equipment employed for measuring exhaust gas flow rate, temperature, and pressure.

The Isokinetic Stack Sampler (M5, CleamAir, Chicago, IL, USA), equipped with an internal PM trapping filter, was utilized for PM measurement. The trapping filter was dried completely and compared using the gravimetric method for accurate reduction rate analysis. Vario Plus equipment (MRU, Neckarsulm, Germany) was utilized to measure SO₂ in the exhaust gas, capable of measuring from 0 to 5000 ppm with an accuracy of ±5%.

Table 2. Design factors of TGCS core components.

Component	Design Factor	Unit
Pall ring height	500, 1000	mm
Volumetric flow rate	15, 25, 35	L/min (liters/min)

outlines the design factors of TGCS’s core components, such as Pall ring height and volumetric flow rate of cleaning solutions.

The shape of the liquid film formed varies with the height of the Pall ring, with appropriate liquid film formation crucial to avoid boiler system damage due to excessive pressure. To select the height of the Pall ring, it was manufactured at 100 mm intervals from 300 mm to 1300 mm, and the allowable pressure drop results were analyzed. The findings showed that at heights of 400 mm, 500 mm, 1000 mm, and 1100 mm, the pressure drop values were within acceptable limits. The TGCS in this study consists of a total of four spray assemblies. To apply Pall ring heights of 400 mm and 1100 mm, additional components for the close fitting of the spray assemblies were required, which increased the manufacturing cost. For this reason, Pall ring heights of 500 mm and 1000 mm were selected in this study. Additionally, the volumetric flow rate of the cleaning solution per minute was set as a design factor to analyze the optimal flow rate for PM and SO_x reduction. The reduction performance of exhaust gases was evaluated by comparing the Pall ring’s height and the cleaning solution’s volumetric flow rate.

To minimize experimental result errors, experiments were conducted over a total of 30 days, and data excluding the five highest and five lowest results were analyzed. Excluded results were deemed to have significant deviations due to external factors such as weather. Among the 40 experimental results conducted under similar conditions of ambient temperature and humidity, deviations were found to be less than 1.0%. Based on these results, a performance evaluation was conducted.

4. Results and Discussions

4.1. Pall Ring Height 500 mm

Table 3. Exhaust gas measurement results according to volumetric flow rate at Pall ring height 500 mm.

Measuring Item	Measuring Position	Volumetric Flow Rate (L/min)			Unit
		15	25	35
Particle Matter (PM)	Upstream	185.6	185.6	185.6	mg/Sm3
	Downstream	70.5	42.9	28.5
PM reduction rate		62.0	76.9	84.6	%
SO2	Upstream	1690	1690	1690	ppm
	Downstream	352	213	112
SO2 reduction rate		79.2	87.4	93.4	%
CO2	Upstream	10.9	10.9	10.9	%
	Downstream	10.8	10.8	10.8
SO2 (ppm)/CO2 (%)		32.6	19.7	10.4	ppm/%
TGCS Pressure	Upstream	98	100	101	mmH2O

displays the results of exhaust gas analysis concerning the cleaning solution’s volumetric flow rate in the design with a Pall ring height of 500 mm, while Figure 6 compares the reduction effects of PM and SO₂ based on the volumetric flow rate.

Increasing the cleaning solution’s volumetric flow rate enabled PM and SO₂ reduction, with minimal impact on TGCS’s upstream pressure. It was feasible to increase the volumetric flow rate without considering the pressure difference of the boiler exhaust gas at a Pall ring height of 500 mm. According to SECA regulations, the fuel’s sulfur content must be below 0.1%. This corresponds to a SO₂ (ppm)/CO₂ (%) ratio of 4.3, which was not met at 500 mm. However, the global sulfur content of 0.5%, corresponding to a SO₂ (ppm)/CO₂ (%) ratio of 21.7, was achieved at volumetric flow rates of 25 and 35 L/min.

4.2. Pall Ring Height 1000 mm

Table 4. Exhaust gas measurement results according to volumetric flow rate at Pall ring height 1000 mm.

Measuring Item	Measuring Position	Volumetric Flow Rate (L/min)			Unit
		15	25	35
Particle Matter (PM)	Upstream	187.2	187.2	187.2	mg/Sm3
	Downstream	58.2	40.3	21.5
PM reduction rate		68.9	78.5	88.5	%
SO2	Upstream	1690	1690	1690	ppm
	Downstream	98	62	40
SO2 reduction rate		94.2	96.3	97.6	%
CO2	Upstream	10.9	10.9	10.9	%
	Downstream	10.8	10.8	10.8
SO2 (ppm)/CO2 (%)		9.1	5.7	3.7	ppm/%
TGCS Pressure	Upstream	140	145	150	mmH2O

presents the results of exhaust gas analysis concerning the cleaning solution’s volumetric flow rate in the design with a Pall ring height of 1000 mm, while Figure 7 compares the reduction effects of PM and SO₂ based on the volumetric flow rate.

Similar to the design with a Pall ring height of 500 mm, an increase in volumetric flow rate resulted in PM and SO₂ reduction, showing superior performance compared to 500 mm. Furthermore, considering SO_x regulations, a volumetric flow rate of 35 L/min met SECA criteria with a SO_x reduction performance of 3.7. However, TGCS’s upstream pressure exhibited a higher value, reaching 150 mmH₂O at a volumetric flow rate of 35 L/min. When installing TGCS on ship engines, analyzing interactions with turbochargers and minimizing negative impacts on engine performance due to back pressure is necessary.

Comparing the performance of TGCS designs with Pall ring heights of 500 mm and 1000 mm, several observations can be made. Firstly, in the case of a Pall ring height of 500 mm, increasing the cleaning solution’s volumetric flow rate enabled PM and SO₂ reduction, providing valuable insights into TGCS operation. However, limitations were found in meeting SECA regulations. Secondly, in the case of a Pall ring height of 1000 mm, increasing the volumetric flow rate resulted in PM and SO₂ reduction, meeting SECA criteria. However, TGCS’s upstream pressure was relatively higher, indicating potential negative impacts on ship engine performance.

In conclusion, the TGCS design with a Pall ring height of 1000 mm demonstrated more effective PM and SO₂ reduction, satisfying SECA regulations. However, additional considerations regarding back pressure and interaction with ship engines are necessary to minimize negative impacts on engine performance during TGCS design.

5. Conclusions

A Total Gas Cleaning System named TGCS capable of simultaneously reducing PM and SO_x was developed, with experimental studies conducted to evaluate its performance. The analysis focused on results concerning the Pall ring’s height, a core component of TGCS. To compare PM and SO_x reduction performance, various factors such as pressure difference, PM reduction rate, SO_x reduction rate, and SO₂ (ppm)/CO₂ (%) were analyzed based on the cleaning solution’s volumetric flow rate.

(1)

Increasing the cleaning solution’s volumetric flow rate enhanced PM reduction effects.

(2)

Compared to a Pall ring height of 500 mm, a height of 1000 mm exhibited superior SO_x reduction effects, particularly meeting SECA standards at a volumetric flow rate of 35 L/min.

(3)

As the Pall ring height increased, back pressure increased, necessitating analysis of interactions with ship engines during TGCS installation.

The development of TGCS suggests effective ways to combine after-treatment devices for reducing ship emissions, playing a crucial role in compliance with environmental regulations. Future research will focus on other TGCS design elements besides Pall ring height and volumetric flow rate, aiming to develop systems that integrate multiple after-treatment devices based on this study.

6. Patents

KR102051060B1, scrubber embedding type centrifugal cyclone which is able to reduce sulfur oxides and particulate matter together.

KR102043205B1, integrated gas purifying apparatus, which is able to reduce sulfur oxides and particulate matter together.

KR101881834B1, KR101864756B1, simultaneous removal apparatus of PM and SO_x in the exhaust gas.