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

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)

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.

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.

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

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.

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%.

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.

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.

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%.

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.

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.

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.