Methodology to Calculate the CO2 Emission Reduction at the Coal-Fired Power Plant: CO2 Capture and Utilization Applying Technology of Mineral Carbonation

This study introduces a novel methodology to calculate the carbon dioxide (CO2) emission reduction related to residual emissions, calculating the CO2 emission reduction through a 2 MW (40 tCO2/day) carbon capture and utilization (CCU) plant installed at a 500 MW coal-fired power plant in operation, to evaluate the accuracy, maintainability, and reliability of the quantified reduction. By applying the developed methodology to calculate the CO2 emission reduction, the established amount of CO2 reduction in the mineral carbonation was evaluated through recorded measurement and monitoring data of the 2 MW CCU plant at the operating coal-fired plant. To validate the reduction, the accuracy, reproducibility, consistency, and maintainability of the reduction should be secured, and based on these qualifications, it is necessary to evaluate the contribution rate of nationally determined contributions (NDCs) in each country. This fundamental study establishes the concept of CCU CO2 reduction and quantifies the reduction to obtain the validation of each country for the reduction. The established concept of the CCU in this study can also be applied to other CCU systems to calculate the reduction, thereby providing an opportunity for CCU technology to contribute to the NDCs in each country and invigorate the technology.


Introduction
Under the Paris Agreement agreed upon in 2015, all countries must submit their nationally determined contributions (NDC), a voluntary reduction goal, to the United Nations (UN), and undergo a performance evaluation every five years for the submitted NDC [1]. Thus, to reduce their GHG emissions, countries are developing greenhouse gas (GHG) reduction technologies in various fields. South Korea is developing diverse policies and technologies to achieve its set goal, which is to reduce GHG by 37%, compared to business as usual (BAU), by 2030 [2].
The development of GHG reduction technologies can be categorized into technology to reduce CO 2 emission, the main emission source, and technology to reduce non-CO 2 GHG emissions. Among the technologies, active research efforts have been made on carbon capture and utilization or storage (CCUS), as it is one of the technologies to reduce CO 2 emissions. Korea has arranged a CCUS study to reduce CO 2 by 10.3 million tons by 2030 for CCUS technology commercialization [2].

Overview of Object Technology
By reacting CO 2 with the reagent CaO, the CO 2 concentration in power generation emission decreases from 10-15% to under 1% and allows the resulting permanently captured CO 2 to be utilized as construction materials in building and civil construction. This study is based on the operating data of a 40 tCO 2 /day class (12,000 tCO 2 /y, 2 MW) CCU pilot plant installed at a coal-fired power plant in Korea. Figure 1 shows that the CCU process of mineral carbonation technology consists of a two-tiered reaction tower. In the reaction tower, the reagent, CaO, reacts with water, resulting in an aqueous solution of Ca(OH) 2 . Then, CO 2 from the flue gas is injected by the blower to the Ca(OH) 2 solution, resulting in a carbonation reaction. Carbon capture and storage (CCS) is considered the key technology for mitigating emissions from fossil-fuel power plants while these are still operational [3]. Much research has been carried out on how to store the carbon using various methods, such as geological sequestration [4,5], biological fixation [6,7], and ocean disposal [8,9]. However, CCS technologies has some barriers, such as the fact that it is an unprofitable activity that requires large capital investment [10], and in some countries CCS is not optional as their geological capacity is limited or, in some cases, only available offshore, which increases transportation and injection costs [11]. Owing to these downsides of CCS, carbon capture and utilization (CCU) has gained better evaluation for its cost-effectiveness and practical measuring and monitoring of GHG emission reduction compared to CCS [12,13].
CO2 is utilized in a variety of ways: chemicals or fuels through electro-catalytic conversion [14], plastics based on CO2-based polymers [15], urea [16], mineral carbonation, etc. The mineral carbonation technology, CCU technology, converts CO2 to solid inorganic carbonate through a chemical reaction, and uses it in construction materials and coal mine landfills, or safely disposes of the CO2 without potential CO2 leakage, which may be threatening to human and environmental safety [17,18].
Although CCU has different GHG emission reduction calculation methods for each of its various technologies, even the UNFCCC, which provides the most effective methodology for GHG emission reduction, is not able to provide an adequate methodology [19].
In this study, we establish a novel methodology to calculate the CO2 emission reduction related to residual emissions, where convention methods, e.g., UNFCCC CDM or ISO 14064-2, does not consider the residuals. The novel methodology has evaluated accuracy, maintainability, and reliability of quantified CO2 emission reduction based on the CO2 emission reduction in 2 MW class (40 tCO2/day) installed at a currently operating 500 MW coal-fired power plant. This novel method could be the opportunity to quantify the CO2 reduction in CCU and increase the reliability of CCU technology to gain more attention, and ultimately contribute to sustainable development worldwide.

Overview of Object Technology
By reacting CO2 with the reagent CaO, the CO2 concentration in power generation emission decreases from 10-15% to under 1% and allows the resulting permanently captured CO2 to be utilized as construction materials in building and civil construction. This study is based on the operating data of a 40 tCO2/day class (12,000 tCO2/y, 2 MW) CCU pilot plant installed at a coal-fired power plant in Korea. Figure 1 shows that the CCU process of mineral carbonation technology consists of a two-tiered reaction tower. In the reaction tower, the reagent, CaO, reacts with water, resulting in an aqueous solution of Ca(OH)2. Then, CO2 from the flue gas is injected by the blower to the Ca(OH)2 solution, resulting in a carbonation reaction.  The characteristics of this process involve three phases (gas (CO 2 ), liquid (water), and solid (CaO)) in the reaction, and the slow melting speed of solid reactant. The reaction equations are as follows [20,21] CaO(s) + H 2 O → Ca(OH) 2 (aq) (1) After carbonation, the CaCO 3 slurry produced is dehydrated and dried, to be utilized in the final powder form.

Reaction Mechanism by Mineral Carbonation
To confirm the introduced reaction mechanism from Section 2.1, chemical analysis was conducted on the materials before and after the reaction with CO 2 , CaO, and H 2 O in the 40 tCO 2 /day class (2 MW) CCU pilot plant.

Results of the Chemical Analysis of Pre-Reaction Liquid Reagent
For chemical component analysis of the pre-reaction liquid agents, produced before the reaction of CaO, H 2 O, and CO 2 in the CCU facility, a glass disc was made by fusing the agent with sodium tetraborate (Na 2 B 4 O 7 ) or lithium tetraborate (Li 2 B 4 O 7 ), after the loss of ignition (Ig.loss) was measured. Moreover, after measuring the loss of ignition of the concentration-known standard material, glass disc was made with the same method. The calibration curve of the fluorescence X-ray intensity was drawn according to the content changes in the corresponding components (silicon dioxide, aluminum oxide, calcium oxide, etc.), and the fluorescence X-ray intensity of these components was measured and quantified using wavelength dispersive X-ray fluorescence spectrometry (WD-XRF) and inductively coupled plasma-optical emission spectrometry (ICP-OES).
The pre-reaction liquid agent composition was based on three reagents, mainly CaO of 63-68%, and partially SiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, etc. Table 1 shows that the loss of ignition was confirmed to be 27-28%. The produced product from the reaction of liquid reagent and CO 2 in CCU facility was chemically analyzed as per the Section 2.2.1 analysis. After the reaction with CO 2 , the product starts to react. Ten samples were obtained from sampling every 2 h during the reaction of the product. The analysis results of the product showed that CaO of 51-53% was the principal component; SiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, etc. were confirmed to be partially included and the Ig.loss was determined to be 41-43% (Table 2). To verify the reaction mechanism according to the reaction progress of the final product, the X-ray diffraction (XRD) chart was used to check the mineral phases' changes. Figure 2 shows the analysis results, which confirm that the pre-reaction liquid reagent was mainly lime (Ca(OH) 2 ) ( Figure 2a); but through the reaction with CO 2 , lime was converted to calcite (CaCO 3 ) and aragonite (CaCO 3 ) (Figure 2b,c).  To verify the reaction mechanism according to the reaction progress of the final product, the Xray diffraction (XRD) chart was used to check the mineral phases' changes. Figure 2 shows the analysis results, which confirm that the pre-reaction liquid reagent was mainly lime (Ca(OH)2) ( Figure 2a); but through the reaction with CO2, lime was converted to calcite (CaCO3) and aragonite (CaCO3) (Figure 2b,c). A thermogravimetric analyzer (TGA) was also employed to analyze the reaction mechanism according to the progression, which enables the detection of mass changes as the temperature changes, as it increases along with the constant heating rate. As the result shown in Table 3, the first mass loss of water evaporation occurred at 40-600 °C and the desorption of CO2 was confirmed at 600-900 °C resulting in a mass loss of 37-39%, which confirmed the chemical conversion of CO2 to the final product of CaCO3 [22]. A thermogravimetric analyzer (TGA) was also employed to analyze the reaction mechanism according to the progression, which enables the detection of mass changes as the temperature changes, as it increases along with the constant heating rate. As the result shown in Table 3, the first mass loss of water evaporation occurred at 40-600 • C and the desorption of CO 2 was confirmed at 600-900 • C resulting in a mass loss of 37-39%, which confirmed the chemical conversion of CO 2 to the final product of CaCO 3 [22]. Table 3. Thermogravimetric analyzer (TGA) analysis according to the reaction time. Unit for the results are mass %.
Step 1 (40-600 • C) Step 2 (600-900 • C) TGA (Total) The TGA results show that as the reaction progresses (Figure 3a-c), the CO 2 contents increase, maintaining constant mass percent for the 2nd-6th steps (Figure 3b), and as the CaO concentration decreases, the CO 2 content gradually decreases for the 7th-10th steps (Figure 3c).  The TGA results show that as the reaction progresses (Figure 3a-c), the CO2 contents increase, maintaining constant mass percent for the 2nd-6th steps (Figure 3b), and as the CaO concentration decreases, the CO2 content gradually decreases for the 7th-10th steps (Figure 3c).

Analysis Results of the Reaction Mechanism
Chemical analysis was performed according to the reaction progress of the three input reagents, CaO, H 2 O, and CO 2 , in the CCU facility. The XRD and TGA analysis results showed that through the reaction, the input reagents were converted into CaCO 3 , which confirmed that this technique is able to permanently capture CO 2 by converting CO 2 into CaCO 3 which has a lower energy level ( Figure 4) [23].

Analysis Results of the Reaction Mechanism
Chemical analysis was performed according to the reaction progress of the three input reagents, CaO, H2O, and CO2, in the CCU facility. The XRD and TGA analysis results showed that through the reaction, the input reagents were converted into CaCO3, which confirmed that this technique is able to permanently capture CO2 by converting CO2 into CaCO3 which has a lower energy level (Figure 4) [23].

Concept of CO2 Emission Reduction
The UNFCCC's Clean Development Mechanism (CDM) and ISO 14064-2 concepts were introduced to evaluate the CO2 reductions. In CDM, the reduced amount of net CO2 is calculated based on continuous measurement and sampling, by considering the amount of CO2 emission and CO2 residual emissions before and after the reduction facility was introduced. Therefore, in this study, the CO2 emissions from a coal-fired power plant, which is the baseline scenario before the installation of a reduction facility, are treated with desulfurization and a dust collection facility. The baseline CO2 emission was calculated in terms of emission into the atmosphere without CO2 capture. After the facility installation, CO2 emission was continuously measured to calculate the project CO2 emissions, while CO2 residual emissions were calculated by analyzing the captured CO2 amount in the product, to calculate the net CO2 reductions.

Method to Calculate the Baseline CO2 Emission
The baseline CO2 emissions calculation method excludes the calculated CO2 in outlet gas from the calculated CO2 in inlet gas. The amount of inlet CO2 gas was calculated by multiplying the inflow rate of flue gas, infused into the CO2 capture and reaction facility, and the CO2 concentration. The CO2 in outlet gas was calculated by multiplying the CO2 concentration value and flow rate of the flue gas emitted from the facility. Flow rates and concentrations were measured continuously during the monitoring period (Equation (4) and Table 4).

Concept of CO 2 Emission Reduction
The UNFCCC's Clean Development Mechanism (CDM) and ISO 14064-2 concepts were introduced to evaluate the CO 2 reductions. In CDM, the reduced amount of net CO 2 is calculated based on continuous measurement and sampling, by considering the amount of CO 2 emission and CO 2 residual emissions before and after the reduction facility was introduced. Therefore, in this study, the CO 2 emissions from a coal-fired power plant, which is the baseline scenario before the installation of a reduction facility, are treated with desulfurization and a dust collection facility. The baseline CO 2 emission was calculated in terms of emission into the atmosphere without CO 2 capture. After the facility installation, CO 2 emission was continuously measured to calculate the project CO 2 emissions, while CO 2 residual emissions were calculated by analyzing the captured CO 2 amount in the product, to calculate the net CO 2 reductions.

Method to Calculate the Baseline CO 2 Emission
The baseline CO 2 emissions calculation method excludes the calculated CO 2 in outlet gas from the calculated CO 2 in inlet gas. The amount of inlet CO 2 gas was calculated by multiplying the inflow rate of flue gas, infused into the CO 2 capture and reaction facility, and the CO 2 concentration. The CO 2 in outlet gas was calculated by multiplying the CO 2 concentration value and flow rate of the flue gas emitted from the facility. Flow rates and concentrations were measured continuously during the monitoring period (Equation (4) and Table 4).
Q in,m,gas × C in,m,CO2 × γ − Q out,m,gas × C out,m,CO2 × γ (4) Table 4. Explanation of symbols used in the Equation (4). Project CO 2 emission is the emission of greenhouse gases caused by the additional energy use, energy penalty, in project activity. Furthermore, when power generation is 100% renewable, the CO 2 eq emissions of electricity use is zero. Therefore, this value should reflect the proper emission factor according to the contribution of renewables to electricity production during the monitoring and project period, e.g., national electric power emission factor of Korea.

BE
In this study, the calculated greenhouse gas emission from additional electricity consumption was applied to calculate the project CO 2 emission (Equation (5) and Table 5).

Method to Calculate the Residual Emissions
The residual emission amount is the amount of residual emissions of CO 2 by the production of the product, which implies that the captured CO 2 from the flue gas is not captured by the product, but dissolved, and remains in the business boundary by water or other substance. The residual emissions factor, RE ef, was developed to calculate the residual emission amount during the sampling period. The RE ef was developed by applying the difference between analyzed amount during sampling period of the utilized CO 2 at the CCU facility and the fixed CO 2 at the actual product, by the CO 2 content analysis of samples (Equation (6) and Table 6).
S m=1 Q in,m,gas × C in,m,CO2 × γ − Q out,m,gas × C out,m,CO2 × γ (6) Table 6. Explanation of symbols used in the Equation (6). The residual emissions amount (RE M ) was calculated by applying the developed RE ef (Equation (7) and Table 7).

Method to Calculate the CO 2 Emission Reduction
The CO 2 reduction amount was calculated by excluding the sum of project CO 2 emissions and residual amount from the baseline CO 2 emissions amount (Equation (8) and Table 8).

Results and Discussion
To evaluate the CO 2 reduction in applying technology, the monitoring measurement points were defined (yellow points on Figure 5), and the normal operating data collected during 22 h of monitoring period.

The Baseline CO2 Emission
The measured CO2 from the measuring instrument was used to determine the baseline CO2 emissions, the results of which are shown in Table 9.

The Baseline CO 2 Emission
The measured CO 2 from the measuring instrument was used to determine the baseline CO 2 emissions, the results of which are shown in Table 9.

The Project CO 2 Emission
The electricity was the energy source during the monitoring period; therefore, the additional electric power consumption was measured through the watt-hour meter to determine the project CO 2 emissions. The electric power emission factor of Korea, which also considers the renewable energy, was applied to the measured electricity consumption [24]. The measured electricity consumption is 8.91 MWh, and the CO 2 emission is 4.155 tCO 2 during the monitoring period, 22 h.

Residual Emissions
The residual emissions factor (RE ef ) during the sampling period has to be calculated to estimate the residual emissions amount (RE M ). The TGA results and the sampling data of product mass were utilized to calculate the residual emissions factor (RE ef ). The CO 2 content of the sampled product are shown in column Step 2 of Table 3 in Table 10, and the product mass from the sampling data were gained from a measuring instrument (Product Mass in Table 10). Based on the Equation (6), using the analyzed data, the residual factor (RE ef ) during the sampling period was 0.1297, hence the residual emission amount is 3.085 tCO 2 during 22 h of the monitoring period (Equation (7)). Table 10. Data for residual emission calculation.

Measured Time
Step 2 of

CO 2 Emission Reduction
Following the emission results from Sections 4.1-4.3, the CO 2 reduction was confirmed to be 16.54 tCO 2 during 22 h of the monitoring period. Assuming that this facility operates for 24 h/day for 300 days, it was analyzed to reduce by 5413 tCO 2 annually.

Conservative Approach Method to CO 2 Emission Reduction
The technology in this paper utilizes the produced CaCO3 through reaction with CaO and CO 2 with coal-fired power plant flue gas as construction materials. To calculate the actual amount of captured CO 2 , the CO 2 was quantified in the reactor by the measuring instrument and the unreacted CO 2 residual emissions were quantified by analyzing the CO 2 amount in the final product. Our calculation method applied the amount of CO 2 reduction to the calculation method, which is a continuous measurement of CCU applied inlet/outlet flowrate of CO 2 . Table 11 shows that our calculation method calculated 30% less reduction than the conventional CCU reduction, where residual emissions are not considered, thereby avoiding the risk of overestimating the emission reduction amount. As is mentioned in Section 3.2.3, the residual emission factor in this study signifies the ratio of consumed unreacted CO 2 , which was determined by the difference between the amount of the actually captured CO 2 and the amount of CO 2 in the final product. Therefore, the developed residual emissions factor in this paper provides a more accurate calculation method that avoids overestimation for mineral carbonation CCU technology in various intensive greenhouse gas-emitting plants.

Maximization of the Final Product Application
The final product in this study, CaCO 3 , is expected to be used as a construction material by mixing with cement to strengthen the ground, increase the strength of structures, and produce precast pavers. Accordingly, in the case of Korea, when this technology is commercially distributed, it is suggested that it is possible to achieve a total CO 2 reduction of 6.1 million tons by using 3.8 million tons of civil engineering materials, and by using 2.3 million tons of construction material (Table 12). The Table 12 evaluation basis is as follows: (1) the CO 2 content of the product is 38.12% using Table 10; (2) Korea's soft ground surface development, 20 million m 3 /y, solidifying agent 350 kg/m 3 , 30% replacement by CO 2 capture cement; (3) Korea's soft ground surface in-depth development, 6.6 million m 3 /y, development depth 30 m, solidifying agent 300 kg/m 3 , 30% replacement by CO 2 captured solidifying agent; (4) Korea's general/special cement 450 million tons/y (the internal data in Korea cement association) 35%, 10% replacement by CO 2 captured cement; (5) Korea's blast furnace slag 96 million tons/y, 5% replacement by CO 2 captured blast-furnace slag; and (6) Korea's remicon 100 million m 3 /y, 24 MPa (350 kg/m 3 binder) 70%, approximately 10% replacement by CO 2 captured material binder (35 kg/m 3 ).

Conclusions
This study has established a method to evaluate the CO 2 reduction by mineral carbonation CCU facility, and an evaluation was made on the CO 2 reductions. The evaluation result was achieved by adopting the monitoring data and recorded measurements from a 2 MW class (40 tCO 2 /day) continuous capture process installed at a running 500 MW coal-fired power plant. The evaluation results show that the facility can annually reduce 5413 tCO 2 .
Korea has set the goal of reducing 10.3 million tons of CO 2 by 2030 through CCUS technology, and policies, such as the Green New Deal and Korea GHG emission trading system, are being implemented to achieve this goal. However, in order to validate the amount of GHG reduction and meet the national reduction target, a standard that can quantify the GHG reduction is needed. Particularly when converting to another material through a reaction with CO 2 , such as mineral carbonation technology among CCU technologies, residual emission occurs according to the conversion rate, and correction for the reduction amount is required through the measurement. Therefore, in this study, a methodology for calculating the reduction amount is presented by developing and applying a residual emission factor through continuous measurement data acquired from the operating mineral carbonization CCU facility, and analysis data through sampling.
Currently, the proposed CCU facility is a small scale that captures and utilizes 3.43% of CO 2 , which is a small part of a 500 MW thermal power plant. When the CCU facility is applied with the scaled-up technology, further studies can help to improve this study: securing storage space for raw materials and products; studies on monitoring factors such as pH; operational life time analysis of the facility through track record for continuous operation; a method of calculating the reduction amount from the LCA viewpoint according to the use of the ingredient CaO, etc.
Nevertheless, countries must achieve their reduction targets by 2030, and CCU technology is regarded as an excellent method for achieving the reduction targets. In order to validate the CO 2 emission reduction, the accuracy, reproducibility, consistency and maintainability of the reduction should be secured, and it is necessary to evaluate the contribution rate of NDCs in each country. Therefore, through this research, the concept of CO 2 reduction amount of CCU was established and quantified to conduct the elemental study, to obtain validation of the reduction in each country. We hope that the established concept in this study could be applied to the calculation of other CCU technologies, providing an opportunity for CCU technology to contribute to NDCs and the activation of CCU technology.