The Korean Society of Marine Engineering

[ Original Paper ]

Journal of Advanced Marine Engineering and Technology - Vol. 47, No. 4, pp.187-194

ISSN: 2234-7925 (Print) 2765-4796 (Online)

Print publication date 31 Aug 2023

Received 06 Jul 2023 Revised 07 Aug 2023 Accepted 22 Aug 2023

DOI: https://doi.org/10.5916/jamet.2023.47.4.187

A study on carbon dioxide emissions reduction in marine diesel engines using a chemical carbon absorber

Kyung-Seok Ko¹ ; Yeong-Gyun An² ; Yung-Deug Son^†

1M. S., Mechanical Facility Control Engineering, Korea University of Technology and Education, Tel: 041-640-8613 k2shs@koreatech.ac.kr
2M. S., Mechanical Facility Control Engineering, Korea University of Technology and Education, Tel: 041-640-8613 tui7@koreatech.ac.kr,

Correspondence to: ^†Professor, Mechanical Facility Control Engineering, Korea University of Technology and Education, 1600, Chungjeol-ro, Byeongcheon-myeon, Dongnam-gu, Cheonan 31253, Korea, E-mail: ydson@koreatech.ac.kr, Tel: 041-640-8613

Copyright © The Korean Society of Marine Engineering
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The shipping industry is responsible for a considerable portion of global carbon dioxide (CO₂) emissions, and reducing these emissions is crucial to mitigate the impacts of climate change. In this study, a chemical carbon absorber (CCA) was used to reduce CO₂ emissions from marine diesel engines, and an experiment was performed to determine the efficiency of the method and quantify the reduction achieved. The experiment consisted of injecting 30% and 100% of aqueous CCA into the exhaust gas pipeline via the nozzle of a selective catalytic reduction system. The CO₂ concentration in the exhaust gas before and after the CCA injection point was measured to monitor the CO₂ emission reduction. The results showed that using CCA can reduce CO₂ emissions from the exhaust gas of marine diesel engines. However, further research is needed to optimize the process and improve its efficiency. Three potential areas for future research include installing reductions within the exhaust pipe to increase reaction contact time, recycling waste heat energy from the engine to increase reaction temperature, and improving mixing units to enhance reaction efficiency at higher engine loads. Overall, the results of this study confirm that using CCA technology can reduce CO₂ emissions from marine diesel engine.

Keywords:

CO₂ capture, Marine diesel engines, KOH based metal, Selective catalytic reduction, Carbon capture absorber

1. Introduction

Global warming and climate change have become a global issue, leading to various problems such as rising of sea levels, more frequent and severe extreme weather events, and risks to biodiversity and human health. The international community has recognized the gravity of the situation and taken efforts to improve it. Despite the growing concerns, annual emissions from the shipping industry are close to 1 billion tons of carbon dioxide (CO₂), representing approximately 3% of global emissions [1], and continue to increase owing to the rising demand for maritime trade, with projections indicating that emissions will reach 2–3 billion tons by 2050 [2]. To address this issue, the international maritime organization (IMO) has established targets to reduce 40% of CO₂ emissions by 2030 and reach net zero greenhouse gas emissions by 2050, in comparison to 2008 levels, in which international shipping was responsible for emitting 794 million tons of CO₂ [3]-[5]. As the engines of ships are the primary source of CO₂ emissions in shipping industries, the application of CO₂ capture technology to marine diesel engines has emerged as a viable solution.

CO₂ capture technology has already been effectively employed in various industries, such as power plants, and has been found to considerably reduce CO₂ emissions from marine diesel engines. The integration of CO₂ capture technology into marine diesel engines is critical for the long-term sustainability and profitability of the industry, particularly under the pressure of IMO regulations like the energy efficiency existing ship index [6] and carbon intensity indicator [7]. Furthermore, considering the increasing public demand for a sustainable shipping industry, companies that do not achieve CO₂ emission reductions are expected to eventually lose space in the market. To address these challenges, this study aimed to perform an applied test employing a chemical carbon absorber (CCA) for the exhaust gas of ship internal combustion engines. The three primary CO₂ capture systems for commercial use encompasspre-combustion, oxy-fuel combustion, and post-combustion methods [8]-[10]. Among these, post-combustion CO₂ capture is the preferred industrial system, as it can treat substantial flue gas volumes and serves as an appealing end-of-pipe solution [9]-[11]. In this experiment the post-combustion method was implemented using the selective catalytic reduction (SCR) equipment, which is already installed on existing commercial ships, to confirm whether SCR components including injection nozzle and dosing unit can be utilized.

In this experiment, CCA was injected in concentrations of 30% and 100% into the exhaust gas pipeline through SCR nozzles to confirm the efficiency of CO₂ absorption by the reaction of CCA with exhaust gases. To monitor CO₂ emissions, the emission rate of CO₂ in the exhaust gas before and after the CCA injection point was measured. The CO₂ emission reduction rate was determined and the amount of CO₂ emission was quantified.

This experiment enabled to verify the potential of using CO₂ capture technology in shipping industries [12].

2. Overall system description

2.1 Marine diesel engine

The experiment was conducted under the following conditions. The experiment was aimed to confirm the actual reduction rate of CO₂ by applying CCA to commercially used engines for maritime shipping. CCA, selected as the experiment subject, is an aqueous solution capable of reducing CO₂ by trapping and converting it into sodium carbonate or potassium carbonate, as described in the following chemical reactions [13].

The marine diesel engine used in the experiment was an STX-MAN engine (6 cylinder, in-line type), as shown Figure 1, which is mainly used in cargo ships, tankers, Ro-ro vessels, and tug-boats. This engine can use multiple types of fuel including marine gas oil, marine diesel oil, and heavy fuel oil, and it emits approximately 100 tons of exhaust gas per hour. The specifications of the diesel engine and fuel are listed in Table 1 [14].

Figure 1:

STX-MAN marine diesel engine

Table 1:

Specifications for marine diesel engine and fuel

2.2 Measurement of CO₂ capture ratio

Horiba MEXA (Figure 2), a widely used measurement tool in the maritime field, was selected for the analysis of exhaust gas emissions. The equipment was used to measure the carbon ratio in the generated exhaust gas. The substances measured from the exhaust gas are NOx, CO₂, O₂, and total hydrocarbons (THC) [15]. Based on the measurement results, the influence of other environmental pollutants in addition to CO₂ can be confirmed.

Figure 2:

Exhaust gas analyzer (Horiba MEXA)

2.3 Measuring and injection equipment

Many ships have SCR systems installed to reduce NOx emissions considering environmental factors. The principle of SCR systems to reduce NOx is the chemical reaction between urea/catalyst and NOx in the exhaust gas to change from NOx to Nx and H₂O. The STX-SCR system consists of SCR controller, pump unit, dosing unit, mixer, nozzle, catalyst, and reactor, as shown in Figure 3. The SCR operation sequence is as follows:

Figure 3:

SCR system configuration

①Exhaust gas emission from engine → ② SCR controller→ ③ Pump unit → ④ Dosing unit → ⑤ Nozzle & mixer → ⑥ Catalyst → ⑦ Elimination of NOx from the emission exhaust gas

⑤step in Figure 3 is the chemical reaction to produce ammonia from N, N₂ in the exhaust gas as follows [16]:

(NH₂)2CO → (NH₂)2CO (s) + H₂O

(NH₂)₂CO → NH₃ (g) + HNCO

HNCO (g) + H₂O → NH₃ (g) + CO₂

⑥step in Figure 3 is the chemical reaction to produce Nx,H₂O from ammonia as follows [16]:

4NO + 4HN₃ + O₂ → 4N₂ + 6H₂O

2NO + 2NO₂ + 4NH₃ → 4N₂ + 6H₂O

2NO₂ + 4NH₃ + O₂ → 3N₂ + 6H₂O

Through steps ⑤ and ⑥, NOx becomes Nx and H₂O. Exhaust gas analysis (utilizing MEXA equipment in Figure 2 is conducted in step ⑦ to confirm NOx elimination from the exhaust gas.

For this experiment, the STX-SCR system in Figure 3 was installed with STX-MAN diesel engine and urea was replaced by CCA to confirm the CO₂ capture rate. The reactor was also replaced by a straight pipe 500 A, as a catalyst was not required, and utilized for NOx reduction.

The applicability of the nozzle for injecting urea and the dosing unit in SCR systems for controlling the injection amount were also examined. Two types of flow meter of the dosing unit were evaluated: electromagnetic type and gear type, whose specifications are listed in Table 2. These two types of flow meter were sequentially installed in the dosing unit to check the conductivity of the fluid and confirm the flow detection range. The experiment results indicated that the gear type had a measurement range of 0.1–50.0 L/h, whereas the electromagnetic type had a measurement range of 30.0–120 L/h. Therefore, the electromagnetic type flow meter was selected as the experiment equipment to be installed in the existing dosing unit.

Table 2:

Type of flowmeter to apply in the dosing unit.

The expected CCA (Concentration: 100% and 30%) consumption at each engine load, as described in Table 3, was calculated assuming a CO₂ capture amount of 30%. The spray condition was not an issue.

Table 3:

Estimated CCA consumption rate considering 30% CO2 capture condition.

Equations (1) and (2) represent the calculation formulas for consuming 100% and 30% CCA, respectively.

100 % C C A c o n s u m p t i o n r a t e = F u e l c o n s u m p t i o n C f × 15 % × 1.1

(1)

Equation (1) was used to calculate the expected CCA consumption rate considering a CO₂ emission reduction from exhaust gas of 30% when the CCA was diluted to 100%. The non-dimensional conversion factor, C_f [17], is the ratio of the fuel consumption [g] and CO₂ emission [g] based on the carbon content of the fuel. In this case, the C_f had a value of 3.151040. Based on the result of the experiment, the CCA consumption rate was estimated to be 15% when CO₂ emissions in the exhaust gas were reduced by 30%. Thus, the expected CCA consumption rate to CO₂ emission reduction was calculated by multiplying the fuel consumption by 15%. The final CCA consumption rate was estimated by applying the CCA flow uniformity coefficient of 1.1, which is a factor that accounts for the uneven distribution of CCA in the exhaust gas.

30 % C C A A c o n s u m p t i o n r a t e = F u e l c o n s u m p t i o n C f × 43.79 % × 1.1

(2)

Equation (2) was used to calculate the expected CCA consumption rate to a CO₂ emission reduction from the exhaust gas of 30% when the CCA was diluted to 30%. The non-dimensional conversion factor, C_f, was the ratio of the fuel consumption [g] and CO₂ emission [g], based on the carbon content of the fuel. In this case, C_f had a value of 3.151040. Previous Equation (1) indicated that the CCA consumption rate was estimated to be 15% when CO₂ emissions in the exhaust gas were reduced by 30%, based on the result of the experiment. Equation (2) is different from Equation (1) in that it considers the dynamic viscosity of CCA (9.4 mPa.s). This means that to achieve the same effect of reducing CO₂ by 15%, 2.9256 times more diluted CCA must be injected. The expected CCA consumption rate to CO₂ emission reduction was calculated by multiplying the CCA consumption rate (15%). The final CCA consumption rate was estimated by applying the CCA flow uniformity coefficient of 1.1.

The selection of the flow meter was based on a thorough assessment of the available range of flow supply, calculated CCA consumption rate in Table 3, CCA viscosity, and visual inspection of the spray state.

Furthermore, the experiment was conducted considering the flow allowance range of the existing SCR equipment utilized for NOx reduction. The maximum detection range of the electro-magnetic flowmeter considering the error tolerance (±2%) confirmed that it can be applied to an engine load of up to 10%. Consequently, the CO₂ capture experiment was conducted with a maximum engine load set at 10%.

2.4 Experimental diesel engine configuration

The configuration of the experiment system was same as that of the pre-existing SCR system installed in ships. This system configuration consists of engine, pipe lines, mixer, nozzle, and exhaust gas analyzer, as shown in Figure 4.

Figure 4:

Experiment system configuration.

The actual experiment facility, assembled engine, and SCR unit excluding the reactor are shown in Figure 5. Figure 6 shows the simplified flow for the CO₂ capture system in this experiment, indicating the exhaust gas flow and measuring locations before and after CCA injection. The exhaust gas coming from the engine first traverses an exhaust duct, wherein the CO₂ content under-goes initial analysis (referred to as 'Exhaust Gas #1' in Figure 4).

Figure 5:

Experiment facility, STX-MAN diesel engine.

Figure 6:

Simplified flow for the CO2 capture system

The value derived from this analysis serves as the reference for emissions. Subsequently, the exhaust gas is directed through a mixer. Despite inducing an increase in back pressure, the mixer ensures uniform distribution within the exhaust pipe, facilitating the required chemical reactions. Upon the atomization of the CCA in a vaporized state via a nozzle within the mixer, the CO₂ capture process is initiated. In sequence, the exhaust gas passes through a 500A pipe before being expelled externally. Prior to this discharge, the CO₂ content undergoes a secondary analysis (referred to as 'Exhaust Gas #2' in Figure 4) within the mixer. The CO₂ capture rate is determined by comparing the primary and secondary analyses values.

3. Experimental conditions and results

3.1 Experiment conditions to measure CO₂ capture rate

The first experiment, as detailed in Table 4, was designed to measure the amount of CO₂ captured by the system under different injection amounts of 100% CCA and 30% CCA. During this experiment, the engine was maintained at a load of 0% and the amount of CCA injected into the system was gradually increased. The conditions were set to confirm the CO₂ capture rate according to the change in the amount of CCA.

Table 4:

Experiment condition at variable CCA injection amount and engine load 0% for 10 min.

The second experiment, as detailed in Table 5, was conducted to measure the amount of CO₂ captured when a fixed amount of CCA was injected under gradually increasing engine load conditions up to 10%. The conditions were set to confirm the CO₂ capture rate according to the change in the load condition from 0% to 5% and to 10%.

Table 5:

Experiment condition at variable engine load with fixed CCA injection amount for 10 min.

3.2 Experiment result of CO₂ reduction

The first experiment shown in Figure 7 was conducted to measure the CO₂ capture rate at no load. Case 1 refers to the application of CCA in a concentration of 100% and measurement of the CO₂ emission before and after the mixing pipe.

Figure 7:

Experiment results at varying CCA (100%) injection amounts with engine load 0%

The CO₂ capture rate was gradually increased with the increase of the CCA injection amount from 31.0 L/h to 61.2 L/h. As shown in Figure 7, the CO₂ emission decreased as the injection volume increased. Case 2 refers to the application of CCA in a concentration of 30% and measurement of the CO₂ emission before and after the mixing pipe. As observed in Figure 8, the CO₂ capture rate did not considerably change even when the injection quantity increased from 30.1 L/h to 61.2 L/h.

Figure 8:

Experiment results at varying CCA (30%) injection amounts with engine load 0%

Table 6 lists the CO₂ capture rates when the injection amount for each CCA concentration tested (case 1 – 100% and case2 – 30%) was changed at a fixed load. The CO₂ capture rate in case 1 gradually increased up to 9.7%, whereas that in case 2 did not considerably improve. Figure 9 shows that the carbon capture rate increased as the CCA injection rate increased under 0% engine load with relatively low exhaust gas volume. In addition, considering CCA concentrations of 100% and 30% (CCA diluted in water), the CO₂ capture rate relatively increased as the CCA injection rate increased for a concentration of 100%, whereas it slightly changed for a concentration of 30%. This means that the CCA concentration and injection amount are important factors to obtain better CO₂ capture efficiency. Based on these results, the CCA injection amount and concentration are key factors for achieving an optimized CO₂ capture rate in future investigations or experiments. To further enhance the CO₂ capture efficiency, a pumping control system in the dosing unit could be implemented. This system could utilize a proportional–integral–derivative control with a feedback mechanism to optimize the injection process.

Table 6:

CO2 capture rate for varying CCA injection amounts with engine load 0%.

Figure 9:

CO2 capture efficiency results according to CCA concentration at 0% engine load

Cases 3 and 4 in Table 7 were expanded from Cases 1 and 2, respectively. Experiments were conducted on CO₂ reduction by sequentially increasing the load to 0%, 5%, and 10%. Figure 10 is a schematic of the CO₂ reduction amount, confirming that the reduction amount depends on the load.

Table 7:

CO2 capture rate at varying CCA injection amounts and varying engine loads.

Figure 10:

Experiment results for varying engine loads and CCA concentrations of 100% and 30%

As shown in Figure 11, the CO₂ capture rate was higher for 100% CCA compared to the case with 30% CCA for all engine loads tested. Based on the experiment results, as the engine load increased, the exhaust gas volume increased proportionately.

Figure 11:

CO2 capture efficiency according to engine load (0%, 5%, and 10%) and CCA concentration (100% and 30%)

Therefore, a larger injection volume is required to achieve an optimal CO₂ capture efficiency. Additionally, a higher CO₂ capture rate can be expected as the CCA concentration increases.

4. Conclusion

In this study, CCA was investigated as a CO₂ capture agent for the exhaust gas of a marine diesel engine. The experiment was conducted under conditions that closely resemble those of a currently operating engine of a ship, without the use of additional devices or equipment to increase CO₂ capture.

The results indicated that the CO₂ capture rate increased as the CCA concentration and injection flow rate increased. The maximum capture rate achieved using 100% concentration of CCA was 9.7% at 0% load, 7.8% at 5% engine load, and 6.3% at 10% engine load. However, the results also indicated that the reaction time between CCA and the exhaust gas decreased as the engine load increased, leading to a decrease in CO₂ capture efficiency.

By addressing these study areas, the CO₂ capture efficiency of marine diesel engines could be improved, contributing to reducing greenhouse gas emissions from shipping.

Future research can include improving the mixing unit to extend the reaction time for more efficient reactions between exhaust gas and CCA at higher engine loads. It could also include research on capturing the chemical reaction products of CCA, such as potassium carbonate and H₂O, and using them as ballast water or desiccants on ships.

Acknowledgments

Author Contributions

Conceptualization, K. S. Ko; Methodology, Y. G. An; Formal Analysis, Y. G. An; Investigation, K. S. Ko; Resources, Y. G. An; Data Curation Y. G. An; Writing-Original Draft Preparation, K. S. Ko; Writing-Review & Editing, Y. D. Son; Visualization, Y. G. An; Supervision, Y. D. Son.

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R1	2KOH+CO₂ ↔ K₂CO₃+H₂O	Overall reaction
R2	K₂CO₃+CO₂+ H₂O ↔ 2KHCO₃	Overall reaction
R3	CO₂+OH^- ↔ HCO₃^-	Bicarbonate formation
R4	HCO₃^-+OH^- ↔ H₂O	Carbonate formation
R5	CO₂+H₂O ↔ H₂CO₃
R6	H₂CO₃+OH^- ↔ HCO₃^-+H₂O
R7	CO₂(aq)+H₂O ↔ HCO₃^-+H⁺
R8	K₂CO₃ ↔ 2K⁺+CO₃^2-	Dissolution of K₂CO₃ in water
R9	CO₃^2-+H₂O ↔ HCO₃^-+OH^-	Hydrolysis of CO₃^2-
R10	2H₂O ↔ H₃O⁺+OH^-	Ionization of water

Engine model	STX-MAN 6L21/31-MK1
Cylinder bore × Stroke	210 × 310 mm
Rated power at speed	1320 kw at 900 rpm
Exhaust gas flowrate	Average 310℃
Kind fuel	ISO-F-DMC (ISO 8217 Specifications of marine fuels-2005ed)

Flowmeter Type	Electromagnetic type	Gear type
Detection of fluid conductivity	Applicable	Applicable
Detecting range	3~120 liter/h	0.1~50 liter/h

Engine load (%)	10	25	50	75	100
Fuel consumption (liter/h)	40.5	76.2	132.0	193.8	263.4
100% CCA consumption (liter/h)	21.1	39.6	68.6	100.8	136.9
30% CCA consumption (liter/h)	61.6	115.8	200.6	294.6	400.3

Case No.	Engine load	CCA consumption
Case 1 (100% CCA)	0%	31.0 kg/h
		40.6 kg/h
		50.5 kg/h
		61.2 kg/h
Case 2 (30% CCA)		31.0 kg/h
		40.3 kg/h
		50.0 kg/h
		61.3 kg/h