Semi-Dry Carbonation Process Using Fly Ash from Solid Refused Fuel Power Plant

The increasing CO2 concentration in the Earth’s atmosphere, mainly caused by fossil fuel combustion, has led to concerns about global warming. Carbonation is a technique that can be used as a carbon capture and storage (CCS) technology for CO2 sequestration. In this study, the utilization of the fly ash from a solid refused fuel (SRF) power plant as a solid sorbent material for CO2 capture via semi-dry carbonation reaction was evaluated as a simple process to reduce CO2. The fly ash was exposed to accelerated carbonation conditions at a relative humidity of 25, 50, 75, and 100%, to investigate the effects of humidity on the carbonation kinetics of the fly ash. The reaction conditions such as moisture, concentration of CO2, and reaction time can affect CO2 capture performance of fly ash. Due to a short diffusion length of H2CO3 in water, the semi-dry process exhibits faster carbonation reaction than the wet process. Especially, the semi-dry process does not require a wastewater treatment plant because it uses a small amount of water. This study may have important implications, illustrating the possibility of replacing the wet process with the semi-dry process.


Introduction
CO 2 is a main greenhouse gas and undoubtedly a major contributor to global warming.Capturing CO 2 from the atmosphere is an essential parameter of the carbon management for sequestrating CO 2 from our environment.The concentration of CO 2 in our atmosphere is promoted by the combustion of fossil fuels for generating electricity [1,2].In addition, the amount of the fly ash produced from power plants is expected to increase continuously as the demand for electricity increases.Thus, the interest in fly ash utilization has increased [3][4][5][6].
Among many CO 2 reduction and sequestration techniques, solid-looping using calcium oxide (CaO) is a promising CO 2 capture process, known as carbonate looping [7][8][9][10][11][12][13].This process is based on the reversible reaction between CaO and CO 2 to form calcium carbonate.The carbonation reaction can be used to remove CO 2 from the atmosphere.Derevschikov et al. prepared a CaO/Y 2 O 3 sorbent to capture CO 2 at high temperatures [14].J. Shi et al. synthesized a CaO/sepiolite sorbent by the hydration reaction [15].However, these manufacturing processes are complex and uneconomical for industrial application.
Meanwhile, solid refused fuel (SRF), a highly heterogeneous mixture of high calorific fraction of non-hazardous waste materials, has been recognized as a viable alternative to fossil fuels, and is already being used as a fuel in various industrial sectors, including power plants [16].The fly ash from SRF plants contains about 20 wt% of lime (CaO) which can be used to sequester CO 2 by aqueous carbonation.Thus, studies about utilizing fly ash as a solid sorbent material for CO 2 capture have increased.In the carbonation reaction between CaO and CO 2 , water plays an essential role in hydrating the calcium-based materials to form calcium hydroxide and most studies were conducted in environments with a high amount of water.Loo et al. conducted accelerated wet carbonation experiments on circulating fluidized bed combustion (CFBC) ash (the prepared solution possessed an ash-to-water ratio of 1:10) [17].Dananjayan et al. and Ebrahimi et al. conducted aqueous carbonation with the water-to-ash ratio of 15:1 with carbonation capacities of 50.3 g and 32 g of CO 2 /kg of fly ash, respectively [18,19].
However, although a Ca(OH) 2 aqueous solution can be effectively used as an absorbent to capture CO 2 , commercialization of the CO 2 capture process is still hindered because of the high amount of energy required to evaporate water.Moreover, the process needs a wastewater treatment plant which can be a source of greenhouse gasses.
In this study, we investigated the utilization of fly ash as a solid sorbent material for CO 2 capture via semi-dry carbonation reaction.The effects of the amount of moisture, CO 2 concentration, and reaction time on the performance of fly ash was investigated.The rest of the paper is structured as follows: Section 2 describes the experimental framework used to evaluate the semi-dry carbonation.Section 3 contains the composition changes after the carbonation of SRF ash.Sections 4 and 5 present our discussions and conclusions.

Materials
Fly ash obtained from Korea District Heating Corporation in Korea was used for this research.The fly ash was dried in an oven at 100 • C for 10 h to remove moisture.

Semi-Dry Carbonation Reactor
CO 2 capture was conducted in a round-bottom flask with a single neck as shown in Figure 1.The mixture of fly ash (200 g) and water was delivered into the flask.The rotation speed of the flask was fixed at 40 RPM to shake the mixture.The CO 2 stream (99.99%) and N 2 stream (99.99%) were flowed into the reactor using the regulator and the flow meter to control the CO 2 concentration.The total flow rate of the mixture gas was 10 L min −1 .The temperature in the reactor was maintained at 25 • C. The flow chart of the experimental method of semi-dry carbonation is shown in Figure 2.However, although a Ca(OH)2 aqueous solution can be effectively used as an absorbent to capture CO2, commercialization of the CO2 capture process is still hindered because of the high amount of energy required to evaporate water.Moreover, the process needs a wastewater treatment plant which can be a source of greenhouse gasses.
In this study, we investigated the utilization of fly ash as a solid sorbent material for CO2 capture via semi-dry carbonation reaction.The effects of the amount of moisture, CO2 concentration, and reaction time on the performance of fly ash was investigated.The rest of the paper is structured as follows: Section 2 describes the experimental framework used to evaluate the semi-dry carbonation.
Section 3 contains the composition changes after the carbonation of SRF ash.Sections 4 and 5 present our discussions and conclusions.

Materials
Fly ash obtained from Korea District Heating Corporation in Korea was used for this research.
The fly ash was dried in an oven at 100°C for 10 h to remove moisture.

Semi-Dry Carbonation Reactor
CO2 capture was conducted in a round-bottom flask with a single neck as shown in Figure 1.
The mixture of fly ash (200g) and water was delivered into the flask.The rotation speed of the flask was fixed at 40 RPM to shake the mixture.The CO2 stream (99.99%) and N2 stream (99.99%) were flowed into the reactor using the regulator and the flow meter to control the CO2 concentration.The total flow rate of the mixture gas was 10 L min -1 .The temperature in the reactor was maintained at 25°C.The flow chart of the experimental method of semi-dry carbonation is shown in Figure 2.

90
The fly ash was composed of fine particles, while the bottom ash was coarse and granular.The 91 color of the bottom ash was dark, dull brown while the color of the fly ash was light brown.Figure 4 92 shows the morphologies of the fly ash and the bottom ash.The microstructure of the bottom ash

93
(Figure 4b) had a much larger particle size compared to fly ash, which was about the size of sand but 94

Results
When SRF is burned in a bottom boiler, most of the unburned material is caught in the flue gas and captured as fly ash.Bottom ash is an incombustible byproduct that is collected from the bottom of the furnaces that burn SRF for generating steam.Therefore, fly ash and bottom ash are quite different physically and chemically.The fly ash and the bottom ash used in the study are shown in Figure 3.

Results
When SRF is burned in a bottom boiler, most of the unburned material is caught in the flue gas and captured as fly ash.Bottom ash is an incombustible byproduct that is collected from the bottom of the furnaces that burn SRF for generating steam.Therefore, fly ash and bottom ash are quite different physically and chemically.The fly ash and the bottom ash used in the study are shown in Figure 3.The fly ash was composed of fine particles, while the bottom ash was coarse and granular.The color of the bottom ash was dark, dull brown while the color of the fly ash was light brown.Figure 4 shows the morphologies of the fly ash and the bottom ash.The microstructure of the bottom ash (Figure 4b) had a much larger particle size compared to fly ash, which was about the size of sand but The fly ash was composed of fine particles, while the bottom ash was coarse and granular.The color of the bottom ash was dark, dull brown while the color of the fly ash was light brown.Figure 4 shows the morphologies of the fly ash and the bottom ash.The microstructure of the bottom ash (Figure 4b) had a much larger particle size compared to fly ash, which was about the size of sand but was more porous.As shown in Figure 5, the particle size of the fly ash was between 2 and 130 µm, with D50 (medium diameter) of 25 µm, D10 of 7 µm and D90 of 160 µm.was more porous.As shown in Figure 5, the particle size of the fly ash was between 2 and 130 µ m, with D50 (medium diameter) of 25 µ m, D10 of 7 µ m and D90 of 160 µ m.Table 1 shows the results of X-ray fluorescence analysis of the fly ash and bottom ash.
Concentration of the major elements in the fly ash was given in the form of oxides by XRF.The chemical compositions of the fly ash showed that SiO2, Al2O3, Fe2O3, CaO, and Na2O were the main oxides.On the other hand, the major chemical composition of the bottom ash was SiO2.Lead and copper were the most common heavy metals in both ashes (Table 2).Table 1 shows the results of X-ray fluorescence analysis of the fly ash and bottom ash.
Concentration of the major elements in the fly ash was given in the form of oxides by XRF.The chemical compositions of the fly ash showed that SiO2, Al2O3, Fe2O3, CaO, and Na2O were the main oxides.On the other hand, the major chemical composition of the bottom ash was SiO2.Lead and copper were the most common heavy metals in both ashes (Table 2).Table 1 shows the results of X-ray fluorescence analysis of the fly ash and bottom ash.Concentration of the major elements in the fly ash was given in the form of oxides by XRF.The chemical compositions of the fly ash showed that SiO 2 , Al 2 O 3 , Fe 2 O 3 , CaO, and Na 2 O were the main oxides.On the other hand, the major chemical composition of the bottom ash was SiO 2 .Lead and copper were the most common heavy metals in both ashes (Table 2).The content of CaO (17.1%) in the fly ash was much higher than that in the bottom ash (7.27%).Therefore, the fly ash, which has high calcium-based contents, was selected for CO 2 capture.
Therefore, the fly ash, which has high calcium-based contents, was selected for CO2 capture.3).
To investigate the effect of the amount of moisture on the performance of fly ash for CO2 capture, reaction with CO2 was conducted with the following amounts of water: 25 (25-W/A), 50 (50-W/A), 75 To investigate the effect of the amount of moisture on the performance of fly ash for CO 2 capture, reaction with CO 2 was conducted with the following amounts of water: 25 (25-W/A), 50 (50-W/A), 75 (75-W/A), and 100% (100-W/A).The CO 2 concentration and reaction time were fixed at 100% and 60 min, respectively.
Figure 7 and Table 4 show the crystal structure of the fly ash after CO 2 capture using different amounts of moisture and materials by using the relative intensity ratio (RIR) technique from the XRD.The CaCO 3 components of the 25-W/A, 50-W/A, 75-W/A, and 100-W/A were 14.13, 21.31, 26.16, and 21.98%, respectively.The amount of CaCO 3 increased with increasing the amount of water.This suggests that the presence of water played an essential role in hydrating the calcium-based materials to form calcium hydroxide, which then sequestrated carbon by forming calcium carbonate.Notably, the amount of CaCO 3 increased from 3.02 to 26.16% when the ratio of the ash-to-water was 1:0.75.The results showed that fly ash of 100 g from a SRF fired power plant captured CO 2 of 10.17 g.However, when the ratio of the ash-to-water was 1:1, the amount of CaCO 3 decreased.CO 2 in gaseous phase does not react with calcium-based materials; it has to dissolve in the water first to form carbonic acid (H 2 CO 3 ) [22].As shown in Figure 8, the diffusion length of the H 2 CO 3 from the outside of the water to the calcium-based materials increased with the increasing amount of water.Therefore, the fly ash with too much water reacted slowly with CO 2 .
Figure 7 and Table 4 show the crystal structure of the fly ash after CO2 capture using different amounts of moisture and materials by using the relative intensity ratio (RIR) technique from the XRD.
The CaCO3 components of the 25-W/A, 50-W/A, 75-W/A, and 100-W/A were 14.13, 21.31, 26.16, and 21.98%, respectively.The amount of CaCO3 increased with increasing the amount of water.This suggests that the presence of water played an essential role in hydrating the calcium-based materials to form calcium hydroxide, which then sequestrated carbon by forming calcium carbonate.Notably, the amount of CaCO3 increased from 3.02 to 26.16% when the ratio of the ash-to-water was 1:0.75.
The results showed that fly ash of 100 g from a SRF fired power plant captured CO2 of 10.17 g.
However, when the ratio of the ash-to-water was 1:1, the amount of CaCO3 decreased.CO2 in gaseous phase does not react with calcium-based materials; it has to dissolve in the water first to form carbonic acid (H2CO3) [22].As shown in Figure 8, the diffusion length of the H2CO3 from the outside of the water to the calcium-based materials increased with the increasing amount of water.Therefore, the fly ash with too much water reacted slowly with CO2.To investigate the properties of the fly ash with a small amount of water in the environments with low CO 2 concentrations, CO 2 captures were conducted at CO 2 concentrations of 10 (10-C/A), 20 (20-C/A), 50 (50-C/A), and 100% (100-C/A).The moisture content and reaction time were fixed at 20% and 10 min, respectively.Figure 9 and Table 5 show the crystal structure of the fly ash after CO 2 capture using different concentrations of CO 2 and the amount of materials by using the relative intensity ratio (RIR) technique from the XRD.The CaCO 3 components of the 10-C/A, 20-C/A, 50-C/A, and 100-C/A were 15.21, 19.46, 19.86, and 21.98%, respectively.As shown in Table 5, the CaCO 3 component stabilized at CO 2 concentration of 20%.This result indicates that the semi-dry process can be applied to the power plant without a CO 2 concentration process.6 show the crystal structure of the fly ash after CO 2 capture in a reaction time of 1 (1-T/A), 5 (5-T/A), 10 (10-T/A), and 30 min (30-T/A), and the amount of materials.The CaCO 3 component of the 1-T/A, 5-T/A, 10-T/A, and 30-T/A was 16.26, 15.16, 15.21, and 16.72%, respectively.As shown in Table 6, the carbonation performed well despite the short reaction time of 1 minute.This result indicates that the semi-dry process can be designed as a continuous process to capture large scales of CO 2 .6, the carbonation performed well despite the short reaction time of 1 minute.This result indicates that the semi-dry process can be designed as a continuous process to capture large scales of CO2.

Discussions
The utilization of SRF ash has emerged as one of the most important interventions in SRF power plants.Due to the calcium-based materials in the SRF ash, the SRF ash can be reused as a material for CO 2 capture.According to Loo et al., who studied the utilization of circulating fluidized bed combustion (CFBC) ash, high humidity conditions during the carbonation process were a critical factor that significantly enhanced the CO 2 uptake in the CFBC ash [17].However, in this study, the results show that the high amount of water led to a low efficiency of carbonation reaction.This may be due to the fact that the diffusion length of the H 2 CO 3 increased correlatively with the amount of water.These results demonstrate that the semi-dry carbonation process is more effective in the aspect of cost and time compared to the wet process.The carbonated SRF ash can be used in various construction industries such as a mineral admixture for concrete [23].For the utilization of the carbonated SRF ash prepared via semi-dry carbonation process, further research should be conducted on controlling the conditions of the process such as temperature and pH value.

Conclusions
In this study, the utilization of the fly ash from SRF power plant as a solid sorbent material for CO 2 capture via semi-dry carbonation reaction was evaluated as a simple CO 2 reduction technique which does not require a high cost wastewater treatment plant and an evaporating process.Quantitative analysis was conducted by using the relative intensity ratio technique from the XRD to study the potential for fly ash carbonation under semi-dry carbonation process.The amount of CO 2 capture increased as the amount of water increased.However, when the ratio of the ash-to-water was 1:1, the amount of CO 2 capture decreased because the diffusion length of the H 2 CO 3 , from the outside of the water to the calcium-based materials, increased with the increasing amount of water.The results indicated that H 2 O plays an important role in the carbonation of calcium-based materials.The fly ash of 100 g can capture 10.17 g of CO 2 by semi-dry carbonation reaction without any treatment.The moisture content can affect the CO 2 capture capacity but CO 2 concentration and reaction time do not significantly affect the carbonation reaction.In the semi-dry carbonation process, the carbonation was well performed in the short reaction time and with low CO 2 concentration.These results show that the semi-dry process can be designed as a continuous process and applied to a power plant directly without CO 2 concentration processes.

Figure 1 .
Figure 1.Schematic diagram of the semi-dry carbonation system for the CO2 capture.

Figure 1 .
Figure 1.Schematic diagram of the semi-dry carbonation system for the CO 2 capture.

Figure 3 .
Figure 3. Photographs of (a) fly ash and (b) bottom ash.

Figure 3 .
Figure 3. Photographs of (a) fly ash and (b) bottom ash.

Figure 4 .
Figure 4. Morphologies of the (a) fly ash and (b) bottom ash.

Figure 5 .
Figure 5. Particle size distribution of fly ash.

Figure 4 .
Figure 4. Morphologies of the (a) fly ash and (b) bottom ash.

Figure 4 .
Figure 4. Morphologies of the (a) fly ash and (b) bottom ash.

Figure 5 .
Figure 5. Particle size distribution of fly ash.

Figure 5 .
Figure 5. Particle size distribution of fly ash.

Figure 6 .
Figure 6.X-ray diffraction pattern of fly ash.

Figure 6 .
Figure 6.X-ray diffraction pattern of fly ash.

Figure 7 .
Figure 7. X-ray diffraction patterns of fly ashes after carbonation with various amount of water.

Figure 7 .
Figure 7. X-ray diffraction patterns of fly ashes after carbonation with various amount of water.

Figure 8 .
Figure 8. Schematic diagram of the semi-dry process and wet process for the CO2 capture.

Figure 9 and
Figure 9 and Table 5 show the crystal structure of the fly ash after CO2 capture using different concentrations of CO2 and the amount of materials by using the relative intensity ratio (RIR) technique from the XRD.The CaCO3 components of the 10-C/A, 20-C/A, 50-C/A, and 100-C/A were 15.21, 19.46, 19.86, and 21.98%, respectively.As shown in Table5, the CaCO3 component stabilized at CO2 concentration of 20%.This result indicates that the semi-dry process can be applied to the power plant without a CO2 concentration process.

Figure 9 .
Figure 9. X-ray diffraction patterns of fly ashes after carbonation with various CO2 concentrations.

Figure 9 .
Figure 9. X-ray diffraction patterns of fly ashes after carbonation with various CO 2 concentrations.

Figure 10 and Table
Figure 10 and Table6show the crystal structure of the fly ash after CO 2 capture in a reaction time of 1 (1-T/A), 5 (5-T/A), 10 (10-T/A), and 30 min (30-T/A), and the amount of materials.The CaCO 3 component of the 1-T/A, 5-T/A, 10-T/A, and 30-T/A was 16.26, 15.16, 15.21, and 16.72%, respectively.As shown in Table6, the carbonation performed well despite the short reaction time of 1 minute.This result indicates that the semi-dry process can be designed as a continuous process to capture large scales of CO 2 .

Figure 10 and
Figure 10 and Table6show the crystal structure of the fly ash after CO2 capture in a reaction time of 1 (1-T/A), 5 (5-T/A), 10 (10-T/A), and 30 minutes (30-T/A), and the amount of materials.The CaCO3 component of the 1-T/A, 5-T/A, 10-T/A, and 30-T/A was 16.26, 15.16, 15.21, and 16.72%, respectively.As shown in Table6, the carbonation performed well despite the short reaction time of

Figure 10 .
Figure 10.X-ray diffraction patterns of fly ashes after carbonation with various reaction times.

Figure 10 .
Figure 10.X-ray diffraction patterns of fly ashes after carbonation with various reaction times.
Sustainability 2019, 11, x FOR PEER REVIEW 2 of 11 in environments with a high amount of water.Loo et al. conducted accelerated wet carbonation experiments on circulating fluidized bed combustion (CFBC) ash (the prepared solution possessed an ash-to-water ratio of 1:10) [17].Dananjayan et al. and Ebrahimi et al. conducted aqueous

Table 1 .
Oxide compositions in fly ash and bottom ash (wt%).

Table 2 .
Heavy metals in fly ash and bottom ash (mg kg -1 ).

Table 1 .
Oxide compositions in fly ash and bottom ash (wt%).

Table 2 .
Heavy metals in fly ash and bottom ash (mg kg -1 ).

Table 1 .
Oxide compositions in fly ash and bottom ash (wt%).

Table 2 .
Heavy metals in fly ash and bottom ash (mg kg −1 ).

Table 3 .
Quantitative analytical results obtained by relative intensity ratio (RIR) method (wt %) of the fly ash.

Table 4 .
Quantitative analytical results obtained by RIR method (wt%) of the fly ash after carbonation with various amounts of water.

Table 4 .
Quantitative analytical results obtained by RIR method (wt%) of the fly ash after carbonation with various amounts of water.

Table 5 .
Quantitative analytical results obtained by RIR method (wt%) of the fly ash after carbonation with various CO 2 concentrations.

Table 6 .
QuantitativeThe utilization of SRF ash has emerged as one of the most important interventions in SRF power plants.Due to the calcium-based materials in the SRF ash, the SRF ash can be reused as a material for CO2 capture.According to Loo et al., who studied the utilization of circulating fluidized bed analytical results obtained by RIR method (wt %) of the fly ash after carbonation with various reaction times.

Table 6 .
Quantitative analytical results obtained by RIR method (wt%) of the fly ash after carbonation with various reaction times.