Technoeconomic and Environmental Evaluation of Sodium

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Process Systems Engineering

Techno-economic and environmental evaluation of sodium bicarbonate production using CO2 from flue gas of a coal-fired power plant Ji Hyun Lee, Dong Woog Lee, Choonyong Kwak, Kijun Kang, and Jay Lee Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02253 • Publication Date (Web): 04 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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Industrial & Engineering Chemistry Research

Techno-economic and environmental evaluation of sodium bicarbonate production using CO2 from flue gas of a coal-fired power plant Ji Hyun Lee †, ‡, Dong Woog Lee †, Choonyong Kwak §, KiJun Kang §, Jay H. Lee †,*

† Department

of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology

(KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ‡ Climate

and Environmental Laboratory, KEPCO Research Institute, 105 Munji-ro, Yuseong-gu, Daejeon 34056,

Republic of Korea § benit

M, Ulsasn General Business Center 38, Heohak 3- gil, Onsan-eup, Ulju-gun, Ulsan, Republic of Korea

* Corresponding author: Tel.: +82 42 350 2114; fax: +82 42 350 2210 E-mail address: [email protected] (Jay H. Lee).

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ABSTRACT This study analyzes the technical, economic and environmental feasibility of a CO2 mineralization process, which has great potential in terms of CO2 utilization capacity. The chosen process is the sodium bicarbonate manufacturing process that uses sodium carbonate solution with CO2 that is obtained from the flue gas produced from a coal-fired power plant. The technical feasibility analysis involves a performance evaluation, which is conducted using a bench-scale apparatus capable of producing sodium bicarbonate with a purity of 99% and greater. According to the analysis, the CO2 reduction potential of the proposed CO2 utilization process is approximately 0.33 ton of CO2 per ton of sodium bicarbonate produced. When comparing CO2 emissions for the production of 1 ton of sodium bicarbonate, the CO2 utilization process of this study produces approximately 0.10 ton of CO2 emissions excluding the CO2 footprint of sodium carbonate, which is the key raw material. If the footprint of sodium carbonate from the Solvay process is used to account for it, this number goes up to 1.96 tons compared to the 1.69 tons produced by the conventional process. In addition, this study evaluates the economic feasibility of a commercial-scale plant based on the proposed technology with the capacity of 30,000 tons of sodium bicarbonate by utilizing approximately 10,000 tons of CO2 per year. Most of the analyzed cases indicate strong economic potential with a benefit-cost ratio and internal rate of return value of 1.45 and 80.5%, respectively.

KEYWORDS Carbon dioxide; Techno-economic assessment; Solvay process; sodium bicarbonate


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1. INTRODUCTION CO2 utilization can be defined as the application of captured and concentrated CO2 that adds value and partially offsets the cost of CO2 capture.1 As CO2 utilization technologies can potentially reduce greenhouse gas emissions and generate revenue at the same time, a multitude of such technological methods are being proposed for the production of various compounds such as polymers, liquid fuel, and algae.2-5 Despite such efforts, there have been few cases of commercialization. In order to take currently proposed CO2 utilization technologies to commercially meaningful scales, studies should be conducted to establish the technical and economic feasibilities of the technologies as well as to confirm the potential CO2 reduction relative to conventional technologies that produces the same product. However, it is difficult to develop technological, economic, and life-cycle models of commercial-scale plants that satisfy all three requirements. As CO2 is chemically stable 6, additional energy is required from external sources to convert it to substances of higher energy levels (e.g., fuels), and this results in many cases in which the amount of CO2 emitted to produce the energy for CO2 conversion exceeds the amount of CO2 directly utilized in the process.1 In addition, from the perspective of commercialization, it is difficult to confirm the economic competitiveness of proposed CO2 utilizing processes compared to existing processes that produce the same products. This is because costs for CO2 utilization processes are typically higher compared to conventional processes due to the need for building and operating the units for CO2 supply and conversion. 6-8 Among the various CO2 utilization technologies being examined, CO2 mineralization is considered to have great potential in terms of the CO2 utilization capacity and economics. With regard to this matter, the Global CCS Institute (GCCSI) conducted life cycle assessment for various CO2 utilization technologies and suggested that CO2 mineralization technology demonstrated greater CO2 reduction compared to the other technologies that were examined.1



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The GCCSI results are supported by the fact that plants based on CO2 mineralization processes are already being commercially operated. For example, the Capitol SkyMine® plant of Skyonic Corporation began operations March 2015 in San Antonio, Texas at Capitol Aggregates cement factory. The plant manufactures marketable products such as sodium bicarbonate, caustic soda, and bleach by utilizing 75,000 tons of CO2 obtained from the flue gas from a cement factory without CO2 capture processes. 9 However, this plant requires a total capital cost of over 200 million USD, and involves a series of complicated processes including carbonation and brine electrolysis. 10 Twence, in the Netherlands, has also developed a CO2 mineralization process that utilizes CO2 from the flue gas of waste incineration to produce sodium bicarbonate.

11

The CO2

mineralization technology developed by Twence captures low-concentration CO2 generated from a waste to energy (WTE) plant to produce high-purity CO2, which is then reacted with a sodium carbonate solution to produce a sodium bicarbonate slurry. The resulting sodium bicarbonate slurry is directly applied for the cleaning of acid components in flue gas. The plant in Hengelo, the Netherlands, produces 8,000 tons of sodium bicarbonate annually by utilizing 2,000 tons of CO2 from flue gas. 11 Analysis of this business model shows that, despite the increase in capital costs for the installation of the CO2 capture process, the overall economics of the project is expected to improve due to the adoption of the semi-dry process that directly uses the sodium bicarbonate slurry instead of a drying process that uses sodium bicarbonate in powder form 12 (the direct use of the slurry for acid component cleaning eliminates the need for the dewatering and drying processes of the sodium bicarbonate slurry after the carbonation process). However, this technology may affect post-treatment processes such as fluid bed dryer filter bags as significant amounts of water vapor are added to the process. For this reason, the implementation of this


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technology may induce significant risks in terms of safe and stable plant operation. Motivated by the current status of CO2 utilization technologies, this study suggests a new CO2 mineralization technology. In contrast to the aforementioned technologies of Skyonic Corporation & Twence, this study proposes a sodium bicarbonate production process that involves directly injecting low-concentration CO2 (12 - 15 vol%) contained in the flue gas of a coal-fired power plant into a carbonation process without the need for separate processes for CO2 capture. The overall feasibility of the proposed technology is analyzed by calculating the economic feasibility of a sodium bicarbonate manufacturing plant with a capacity of 30,000 tons per year based on design data from the results of bench-scale performance evaluations, and the amount of CO2 reduction is evaluated through comparisons with a conventional sodium bicarbonate manufacturing process.

2.

PROCESS DEVELOPMENT & EVALUATION

2.1. Methodological framework. For the evaluation of environmental and economic feasibilities of the proposed CO2 mineralization technology, a framework (refer to Figure 1) used by the authors of this study in a previous research serves as a basis.13 The bench-scale CO2 mineralization process is configured according to the aforementioned framework, and by conducting performance tests, key performance data of the CO2 mineralization technology regarding technical feasibility are obtained. This is followed by a survey on the market of the target product, with the results serving as a basis for determining the manufacturing capacity of a commercial-scale CO2 mineralization plant. The overall feasibility is evaluated based on the metrics described in the literature.13



Figure 1. Flow diagram of the methodological framework14 of this study, D and E denote decision and execution steps, respectively.

2.2. Chemical reaction and boundary conditions. In the U.S., the main method of obtaining sodium bicarbonate is through trona (trisodium hydrogendicarbonate dihydrate, also sodium sesquicarbonate) mining, whereas in other areas, the ammonia-soda process or the Solvay process is primarily utilized.15 Trona mining-based processes include the monohydrate process and the sesquicarbonate process, both of which are operation-wise quite similar. For example, the monohydrate process involves crushing the trona ore, removing water and CO2 through a calcination process to produce sodium carbonate, and subsequently reacting the sodium carbonate with CO2 to produce the final product, sodium bicarbonate.15 The Solvay process produces sodium carbonate (Na2CO3) from concentrated brine (NaCl solution) and limestone (CaCO3) with ammonia acting as a catalyst. The produced sodium


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carbonate solution is transported to a bicarbonate reaction step where sodium bicarbonate (NaHCO3) is precipitated by utilizing CO2 from lime kilns. In the Solvay process, CaCl2 is also produced as a by-product. However, CaCl2 as a solution requires a high-energy-consuming distillation process to be recovered as a powder. In addition, as the market for CaCl2 is limited, many Solvay plants currently dispose the produced CaCl2 as wastewater rather than recovering it.16 The CO2 mineralization process of this study for sodium bicarbonate manufacturing comprises the steps of bicarbonation and post-treatment including dewatering & drying processes (Figure 2(b)-(c)). The CO2 contained in the flue gas and the sodium carbonate solution are the reactants of the bicarbonation reaction. The CO2 source of the proposed CO2 mineralization technology is flue gas that is emitted from coal-fired power plant. It is possible to avoid the capital & operating cost of CO2 capture processes, as the technology uses lowconcentration CO2 without CO2 capture processes. The sodium carbonate solution and the CO2 in the flue gas take part in the bicarbonation reaction (equation 1) shown below to form sodium bicarbonate. Bicarbonation reaction:

Na2CO3 + H2O + CO2 → 2NaHCO3

(1)

As shown in equation 1, one mole of sodium carbonate is used to produce two moles of sodium bicarbonate through the bicarbonation reaction. In terms of the weight of the chemicals, this indicates that it is possible to produce 1.6 tons of sodium bicarbonate using 1 ton of sodium carbonate and 0.4 tons of CO2. In addition, this study analyzes a case in which imported Na2CO3 is supplied to the process against a case that considers the CO2 emissions during Na2CO3 production through the Solvay process to compare the CO2 footprints.



(a)

(b)

(c)


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Figure 2. System boundary: (a) conventional sodium bicarbonate manufacturing process 10; (b) CO2 mineralization process in which imported Na2CO3 is supplied; (c) CO2 mineralization process considering the CO2 emission during the Na2CO3 production via the Solvay process. 2.3. Product demand. For the techno-economic evaluation of the CO2 mineralization process of this study, the market analysis is confined to South Korea. This is because it is possible for us to obtain relatively trustworthy data from domestic companies and experts of the field. According to the market research analysis, the size of the South Korean sodium bicarbonate market was 173,162 tons in 2018 17, as shown in Figure 3, of which approximately 80,000 tons were used for acid gas removal applications in steelworks, biomass power plants and waste-to-energy plants.18-20 The price of sodium bicarbonate as of 2018 is 300 - 350 USD per ton and sodium carbonate is 220 - 230 USD per ton19, 21. In addition, we can see from Figure 3 that, in 2003 and prior, a large portion of sodium bicarbonate was produced in South Korea and even exported to other countries. However, after 2003, all such plants were decommissioned. In 2019, all sodium bicarbonate is imported from other countries such as China or the U.S.

Figure 3. Sodium bicarbonate market growth in South Korea.



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In this study, the target application of the sodium bicarbonate produced is set as the treatment of acid gas component emitted from steelworks and biomass power plants. Regarding the selection of product applications, as this study directly uses flue gas from coal-fired power plant, personal care & animal feed-based applications, which require high purity of product (> 99.5%), are avoided. Considering these points, this study conducted technical and economic evaluation of a CO2 mineralization plant capable of annually manufacturing approximately 30,000 tons of sodium bicarbonate, which is approximately one-third of the total market size for the acid gas treatment application in South Korea.

2.4. System boundary identification. The CO2 mineralization process that is the subject of this research comprises of sodium bicarbonate slurry production, dewatering and drying processes. For the target CO2 emission source, this study designated supercritical coal-fired power plants as the emission source. As high-purity CO2 is not required for the CO2 mineralization process, the flue gas can be directly utilized without CO2 capture process. When compared to existing CO2 utilization processes that require highly concentrated CO2, the direct use of flue gas results in significant cost reductions.2,

3, 22

The system boundaries of the techno-economic assessment of this study

include CO2 bicarbonation and sodium carbonate solution preparation and exclude the stages related to the transportation and the consumption of the product. To analyze the change in CO2 footprint with respect to the supply method of Na2CO3, the CO2 utilization process of the two cases presented in section 2.2 are evaluated. In addition, the Solvay process is considered as a reference to compare the CO2 footprint of the proposed process. The process schemes for each case is already shown in Figure 2.


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2.5.1. Key reaction of the process. Sodium bicarbonate of this study is produced by the bicarbonation reaction between CO2 contained in the flue gas and the sodium carbonate solution. The formula below describes the chemical reaction.23 For the bicarbonation reaction, Bjerrum plot for the carbonate system indicates that the carbonation column should be operated at pH values between 8.5 to 9.5 to maximize sodium bicarbonate production. 10 Bicarbonation reaction Na2CO3(aq) + H2O(l) + CO2(g) → 2NaHCO3(aq)

ΔHo= -108.48 KJ/mol,

ΔG= -42.992 KJ/mol

(2)

2.5.2. Bench-scale unit experiment. Prior to the performance test, a CO2 carbonation column design model was developed in conjunction with the bench-scale performance test to obtain reliable CO2 carbonation column design data. Detailed information on the CO2 carbonation column design model is provided in Supporting Information. And using the design data from the carbonation column design model, a bench-scale CO2 carbonation column was constructed. As described above, the bubble column is configured for the bicarbonation reaction that directly uses low concentration CO2. In the bicarbonation reaction, feed gas that contains CO2 enters the bubble column inlet at the bottom and flows upwards. A down-comer pipeline is configured outside the bubble column for the purpose of transporting the sodium bicarbonate slurry, a product of the bicarbonation reaction, to the bottom where a discharge line is connected. An overview of the bench scale CO2 mineralization process is depicted in Figure 4.



Figure 4. Schematic representation of the bench-scale CO2 mineralization test unit with a bubble column.

Based on the developed process scheme, a bench-scale CO2 mineralization process is developed. Performance tests are conducted using the bench-scale test unit, which is capable of producing approximately 16 kg of sodium bicarbonate per day using 10 kg of CO2. Considering the saturation concentration of sodium carbonate in water, the concentration of the sodium carbonate solution is set at 30 wt% for the bicarbonation reaction. The sodium carbonate solution is stored in a jacket-type stirred tank reactor which is operated at 60 ℃. A peristatic pump is used to feed the sodium carbonate solution to the upper section of the carbonation column. A simulation gas that possessed a similar CO2 concentration conditions to flue gas that is obtained from supercritical coal-fired power plants (CO2 14 vol%, N2 86 vol%) is used.24 and the CO2 is consumed in the bicarbonate reaction as the gas travels through the carbonation column. The CO2 is treated to below 7 vol% in the bicarbonation process before


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being emitted to the atmosphere. The carbonation column is constructed with transparent acrylic material to ensure flooding and bubble conditions are visible during operation. In the column, a pH probe (Orion Star A211) and a non-dispersive infrared CO2 analyzer (Horiba VIA-510) are installed to measure pH changes as well as the CO2 concentration of the exit gas during operation. The following are specific details of the continuous operation performance tests conducted using the bench-scale unit. The carbonate solution and the simulation gas containing CO2 are injected at rates of 45 g/min and 30 l/min, respectively. After the continuous operation for approximately six hours, the data show stable results after the initial injection of the gas (Figure 5(a)). According to the pH sensor, the carbonation column is maintained at pH levels of 8.5-9.0. The internal temperatures of the column are maintained according to the specifications of the bottom section (40 ℃ ), the middle section (38-40 ℃ ), and the upper section (4045 ℃ ). In addition, the CO2 conversion rate calculated from the experimental data is maintained at approximately 45 vol%. The sodium bicarbonate is finally produced in a powder form through subsequent dewatering & drying processes. After the dewatering & drying of samples, acid-base titration and X-ray diffraction analysis (RIGAKU model: Smart Lab) show that the peaks mostly corresponded to reference sodium bicarbonate (purity: ≥ 99%, CAS 144-55-8, Sigma-Aldrich) (Figure 5(b)).



(a)

(b)

Figure 5. Bench scale test results: (a) CO2 conversion rate from bench scale CO2 carbonation test (b) X-ray diffraction pattern of powdered sample.


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In addition to the aforementioned performance tests, the bicarbonation reaction performance inside the carbonation column is analyzed using the developed in-house column design model. Figure 6 compares the outlet CO2 concentration measurement with the prediction of the column design model. Exp 1~3 are bench-scale tests according to changes in the bubble column height (Exp 1: 600 mm, Exp 2: 900 mm, Exp 3: 1200 mm, all the other test conditions are the same). Through these tests, we can observe that the CO2 conversion rate is linearly proportional to the bubble column height, and that the column design model developed in this study is capable of effectively simulating the bench-scale performance tests, as shown in Table 1.

100 Column ID: 150 mm without tower internals 1/4" pipe gas nozzle

80

CO2 exit conversion, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Exp 1 Exp 2 Exp 3 Model

Temp.:40-43C (column inside) Feed Gas: CO2 14 vol% Feed Gas Rate: 25-30 l/min Feed Liquid: Na2CO3 30 wt%

60

40

20

0 0

200

400

600

800

1000

1200

1400

Column height, mm

Figure 6. Comparison of the outlet CO2 concentrations: The bench scale test vs. the column design model.

As previously mentioned, this study aims to directly use flue gases produced from power plants. Flue gases from coal-fired power plants contain various impurities on top of CO2, such as SOx, NOx, and particulates. However, this study designated the flue gas extraction point



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after the particulate control device and the flue gas desulfurization process of the coal-fired power plant. At this point, the amount of detected pollutants in the flue gas is substantially low. In Korea, the SOx and NOx emissions from the power plants are managed below 30 ppm and 50 ppm respectively

24.

Therefore, even if the entire pollutants are added to the sodium

bicarbonate production process, the amount is too low even to be detected through the XRD analysis and will not affect the purity of the final sodium bicarbonate.

Table 1. Results of the Column Design Model and the Bench Scale Test Parts

Units

Column Design Model

Column diameter

mm

150

Column height

mm

1,200

Rate

l/min

25

CO2 concentration

vol%

14

Rate

ml/min 44.3

Na2CO3 concentration

wt%

30

CO2 removal rate

kg/d

3.36

CO2 concentration

vol%

8.3

8.3-9.3

CO2 conversion rate

%

44.5

36.8-44.8

NaHCO3(s) rate

kg/d

10.93

Gas feed Liquid feed Gas output

Bottom product

Bench Scale Test

25-30

30

2.5.3. Commercial scale CO2 carbonation plant. Using the developed column performance prediction model, a commercial-scale CO2 carbonation column is designed, and the results are shown in Supporting information Table S3. As described in the results, a bicarbonation bubble column should be approximately 2.9 m in inner diameter and 27 m in total height to enable the annual manufacturing of approximately 30,000 tons of sodium bicarbonate. The utility consumption (electricity, steam, and water) of the CO2 carbonation plant is calculated using the proposed carbonation column design model and the pilot-scale dewatering & drying test using a cake grinding dryer of Jangwoo Machinery


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Inc. The results are described in Supporting information Table S4. 2.5.4. Material balance of the plant Table 2 describes the overall material balance of the commercial-scale CO2 bicarbonation process. In the bicarbonation reaction of the CO2 mineralization process, approximately 0.33 tons of CO2 is utilized to manufacture 1 ton of sodium bicarbonate. In addition, based on the material balance results obtained using the in-house column design model, the column size of the bicarbonation process is determined using the equations described in Supporting information Table S1.

Table 2. Results of the Commercial-scale Column Design Model Parts

Units

Column Design Model

Column diameter

mm

2,900

Column height

mm

27,000

Rate

Nm3/hr

11,500

CO2 concentration

vol%

14.0

Rate

m3/hr

16.6

Na2CO3 concentration

wt%

30.0

Rate

Nm3/hr

10,703

CO2 removal rate

ton/hr

1.1

CO2 concentration

vol%

7.6

CO2 conversion rate

%

49.5

NaHCO3(s) rate

ton/hr

3.4

H2O

ton/hr

13.4

Gas feed Liquid feed Gas output

Bottom product

2.6. Evaluation of the CO2 reduction potential. Using the data on the direct CO2 emissions, indirect CO2 emissions, and CO2 consumption of the proposed CO2 mineralization process and conventional processes13, 16, the overall net CO2 emission index is evaluated. Separate calculations are conducted to obtain data for the



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direct CO2 emissions, indirect CO2 emissions, and CO2 consumption involved with the manufacturing of 1 ton of sodium bicarbonate, as shown in Supporting information Table S5. For the estimation of indirect CO2 emissions of the process, which arises from electricity consumption, electrical energy is assumed to be sourced from coal-fired power plant. The corresponding life-cycle greenhouse gas emissions per unit of electricity is considered (CO2 emissions produced from a coal-fired power plant = 820 kgCO2/MWh).25 In addition, CO2 emission data for the production of 1 ton of sodium carbonate, which is the key raw material of the proposed process, are needed to account for the indirect CO2 emission. The footprint for the raw material would vary according to how it is made (Solvay process, trona & nahcolite based process, nepheline syenite process, carbonation of caustic soda16 etc.). If we assume that the Solvay process is used to produce the sodium carbonate, according to the calculation results in Supporting information, indirect CO2 emissions charged to account for the footprint of sodium carbonate would be 1.86 ton CO2/ton sodium bicarbonate produced. With this, direct and indirect CO2 emissions produced for the manufacturing of 1 ton of sodium bicarbonate are determined as approximately 0.35 tons and 1.94 tons, respectively. In the process, approximately 0.33 tons of CO2 is utilized, which results in a net CO2 emission of 1.96 tons. Conversely, for the conventional Solvay process, direct and indirect CO2 emissions are analyzed as 0.13 – 0.25 tons and 1.48 – 2.04 tons, respectively, and the overall CO2 emissions per ton of product manufactured is 1.35 – 2.03 tons, as shown in Figure 7(a). As a result, under the baseline conditions, the CO2 mineralization process of this study results in CO2 emission levels that are approximately 0.27 ton higher than the average value of the conventional Solvay process for the production of 1 ton of sodium bicarbonate. This number is subject to changes and re-interpretations, depending on how the raw material is obtained and how it is accounted for. For example, if the raw material is imported, the footprint may not be charged to the country that consumes it to manufacture another product. Note that the net emission number


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drops to 0.10 ton per ton of product if the footprint of sodium carbonate is excluded. To investigate the changes according to the carbon emission factors of various electricity sources, changes in the CO2 reduction potential are analyzed for the following five cases, as shown in Table 3. Table 3. Lifecycle CO2 Equivalent of Various Electricity Generation Sources Generation

Lifecycle CO2 equivalent (kgCO2/MWh)

ref.

Coal (baseline case)

820

25

Natural gas

480

25

Korean grid mix

500

26

Solar photovoltaics

48

25

Wind offshore

12

25

The evaluation results are shown in Fig. 7(b). As shown in the figure, net CO2 emission is linearly dependent on the carbon emission factors of the various electricity sources. In addition, whereas the case of imported Na2CO3 feed shows significantly lower net CO2 emission compared to the conventional process, the case that obtains Na2CO3 from the Solvay process shows similar net CO2 emission levels to the conventional process. (a)



(b)

Figure 7. CO2 emissions of the proposed process vs the conventional process with various electricity sources.

2.7. Process economics.


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The evaluation of the process economics is performed based on the economic metrics described in the literature.13 As previously mentioned, the aforementioned in-house column design model and the pilot-scale dewatering & dryer performance test served as a basis for the calculation of the mass and energy flows. The results in turn served as a basis for cash flow analysis to evaluate revenue and costs of production. The baseline condition for the analysis is shown in Table 4. As previously mentioned, the annual manufacturing capacity of the plant is assumed as approximately 30,000 tons of sodium bicarbonate. A Korean contractor provided the process equipment and operating costs based on preliminary design data, as shown in Tables 5 & 6. Table 4. Economic Parameters for the Evaluation of the Process Economic Specifications

Value

Year

2018

Project life (years)

20

Decommissioning cost (USD)

0

Capital distribution (%)

70/30

Construction period (years)

2

Discount rate (%)

5.5

27

Carbon credit (USD/tCO2)

21

28

Annual plant utilization rate (h/yr)

0.8

Electricity prices (KRW/kWh)

100

Korea won-US dollar exchange rate (KRW/USD)

1,200

29

Table 5. Process Equipment Cost Estimation in CO2 mineralization plant Process Equipment

Cost (kUSD)

Soda ash unloading & storage

1,441

Product Storage & Packing system

1,113

Dryer system

990

Centrifuge system

767


ref.


Carbonation column

417

Soda ash dissolution system

371

Utility & post-treatment system

71

Sum

1,441

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Table 6. Operating Costs of the Plant Specifications

Basic unit(/tNaHCO3) or percentage of cost

Unit price or Value

Variable O&M

　

　

· Raw material

　

　

Na2CO3

0.65 ton

230 USD/tNa2CO3

CO2

0.3 ton

· Utility

ref.

16

　

　

Water

0.25 ton

1.31 USD/tWater

30

Electricity

410 kW

8.3 Cents/kWh

29

Steam

0.3 ton

17 USD/tSteam

30

Fixed O&M

　

　

· Labor cost

8 persons

58,333 USD/operator

· Maintenance cost

3% of BEC

191 kUSD/year

· Administration

2% of BEC

128 kUSD/year

· Consumables

1% of BEC

64 kUSD/year

vendor data

As illustrated in Figure 8(a), the capital costs of the sodium carbonate unloading & storage


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as well as the storage and packaging systems of the product (sodium bicarbonate) are high. These categories are followed by the capital costs of the dryer and the centrifuge process. The capital costs of the carbonation process are relatively low compared to other processes due to the low manufacturing cost of the bubble column, which does not have specific internals in the column. The annual trend of operating costs shown in Figure 8(b) indicates that the cost of sodium carbonate is the most significant cost, with operating labor costs for plant operation being the second most significant. On the other hand, the consumption of steam and electricity are not high. The cash flow analysis results indicate that the manufacturing costs of sodium bicarbonate for the baseline case is 0.8 USD/tNaHCO3, and the benefit-cost ratio (BCR), net present value (NPV), and the value of internal rate of return (IRR) are 1.45, 2,373 kUSD, and 80.5%, respectively. Figure 9(c) shows a cost breakdown analysis of the baseline case. The largest contributor to the overall cost is the variable operating cost, which includes the raw material cost, with 73.3%. The fixed operating cost and the capital cost are the second and third largest contributors, respectively.

(a)



(b)

(c)

Figure 8. Economic evaluation for the CO2 mineralization plant: (a) CAPEX distribution (b) OPEX distribution (c) Cost breakdown analysis.

2.8. Sensitivity analysis. Sensitivity analysis is conducted to evaluate the degree in which the economic feasibility is influenced by changes in various variables. For this analysis, annual plant utilization rate (h/year), electricity price, product price, discount rate, and carbon credit are considered as variables, and net present value changes are analyzed with respect to ±30 % changes from the


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baseline conditions. For the sensitivity analysis, all parameters are analyzed within the same ± 30 % range. This approach allows for an analysis of the relative impacts of the parameters. An example of such approach includes the IEA report on costs of generating electricity.31 In addition, the ±30 % range for every parameter was set taking into consideration the rate of change of discount rate in the U.S. in the past year, which was approximately 33%.32 As a result of the sensitivity analysis, the prices of sodium carbonate, the main raw material, and sodium bicarbonate, the main product, are identified as the factors that have the greatest impact on the economic feasibility of the plant. In particular, fluctuations in the price of sodium bicarbonate (±30 %) can result in changes of up to ±90% in NPV. This is followed by the discount rate, and then the annual plant utilization rate in terms of having the greatest effect on NPV. Conversely, carbon credit has a smaller effect on the economic feasibility of the project, as the price of carbon credit is relatively low at approximately 21 USD/t CO2 as of 2019 28, in addition to the fact that the amount of CO2 that is used in this plant is limited to approximately 10,000 tons per year.

Figure 9. Impact of various parameters on NPV.



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Additionally, as previously shown in Figure 8(c), the variable operating costs account for a significant portion of the total cost, with the largest portion being the raw material costs (sodium carbonate, water, steam etc.). Considering this cost structure, several strategies could be considered to improve the economic competitiveness of the technology. One such strategy involves using low-grade sodium carbonate which is produced from the Solvay process or lowcost sodium carbonate which is produced as a by-product in processes such as caprolactam manufacturing as the raw material for the CO2 mineralization process. This strategy is supported by the fact that the prices of high-purity and low-purity sodium carbonate differ by at least 10% in 201819, and that the target application of sodium bicarbonate in the proposed technology of this study is acid gas treatment, which does not require high-purity sodium carbonate. A different strategy involves producing sodium bicarbonate by applying the proposed CO2 utilization process at sites that produce large amounts of CO2 and using the produced sodium bicarbonate directly at the site. Key steel companies such as POSCO of Korea purchase sodium bicarbonate from external suppliers to remove large amounts of acidic gas (SOx etc.) emitted from the steel production processes. The substantial amounts of CO2 produced from such plants can be used to produce sodium bicarbonate to remove acid gas onsite, which is expected to significantly reduce O&M and CAPEX.

2.9. Process improvement potential. The proposed CO2 mineralization process could be further improved in terms of CO2 reduction and economic feasibility with the following approaches. The CO2 source used in this study is selected as a coal-fired power plant that emits CO2 with low concentrations of 12 - 15 vol%. According to the in-house column design model, the height of the carbonation column of the commercial plant for the annual manufacturing of approximately 30,000 tons of sodium


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bicarbonate should be at least 25m, which results in high capital costs. However, if the proposed CO2 mineralization technology is applied to different target CO2 sources instead of coal-fired power plants, such as steelworks and cement factory, the size of the carbonation column could be reduced due to the mineralization of CO2 of relatively higher concentrations (22 - 25 vol%).

3.

CONCLUSIONS This study proposed a CO2 mineralization technology that directly utilizes low-concentration

CO2 (12 - 15 vol%) obtained from emissions of coal-fired power plant to produce sodium bicarbonate. A bench-scale performance test is performed to verify the technical-economic feasibility and competitiveness of the CO2 mineralization process. Analysis of the CO2 reduction potential of the proposed CO2 utilization process shows that, for the production of 1 ton of sodium bicarbonate, 0.33 ton of CO2 utilization is possible. The result also shows that the proposed process of this study produces approximately 1.96 tons of CO2 emissions, mainly owing to the large footprint of sodium carbonate, whereas the conventional process produces 1.69 tons of CO2 emissions. Not counting the footprint of sodium carbonate, the number drops to 0.10 ton. The technical-economic evaluation results of the aforementioned CO2 mineralization plant show a BCR of 1.45 and the value of IRR of 80.5%, substantiating the economic feasibility of the process. Therefore, the main motivation for adopting the proposed technology would be on the economic side.

AUTHOR INFORMATION



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* Corresponding author Prof. Jay H. Lee: E-mail address: [email protected]; Tel.: 82-42-350-3926; Fax: 82-42-350-3910

ACKNOWLEDGMENTS This work was supported by the KEPCO Research Institute and by the Korea Carbon Capture & Sequestration R&D Center (KCRC) under the project # N01170680.

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For Table of Contents Only

Vent

Sodium carbonate

Flue gas (CO2) Coal-fired power plant

CO2 Carbonation

Baking Soda


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Technoeconomic and Environmental Evaluation of Sodium

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