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ScienceDirect Energy Procedia 00 (2017) 000–000

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1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.

The 15th International Symposium on District Heating and Cooling

Assessing the feasibility of using the heat demand-outdoor temperature function for a long-term district heat demand forecast

I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc

aIN+ Center for Innovation, Technology and Policy Research – Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal bVeolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France

cDépartement Systèmes Énergétiques et Environnement – IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France

Abstract

District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease, prolonging the investment return period. The main scope of this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were compared with results from a dynamic heat demand model, previously developed and validated by the authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations.

© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.

Keywords: Heat demand; Forecast; Climate change

Energy Procedia 138 (2017) 518–523

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 2017 International Conference on Alternative Energy in Developing Countries and Emerging Economies. 10.1016/j.egypro.2017.10.238

10.1016/j.egypro.2017.10.238 1876-6102

© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 2017 International Conference on Alternative Energy in Developing Countries and Emerging Economies.

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 00 (2017) 000–000

www.elsevier.com/locate/procedia

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Organizing Committee of 2017 AEDCEE.

2017 International Conference on Alternative Energy in Developing Countries and Emerging Economies 2017 AEDCEE, 25‐26 May 2017, Bangkok, Thailand

Computational Fluid Dynamics Model of CO2 Capture in Fluidized Bed Reactors: Operating Parameter Optimization

Chattan Sakaunnapaporna, Pornpote Piumsomboona,b, Benjapon Chalermsinsuwana,b,* aFuels Research Center, Department of Chemical Technology, Faculty of Science, Chulalongkorn University,

254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand bCenter of Excellence on Petrochemical and Materials Technology, Chulalongkorn University,

254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand

Abstract

Alternative energy is one of the methods for decreasing fossil fuel consumption. However, conventional fossil fuel process improvement is also considerably interesting issue due to the fact that adjusting existing process is easier and cheaper comparing to the development of the process compatible with the alternative energy. At present, the global warming and climate change phenomenon cause the increasing of average earth temperature. The CO2 emission to the atmosphere is mainly produced by fossil fuel combustion from power industry. This is because the CO2 has high heat capacity. Therefore, in order to use the conventional fossil fuel process efficiently, CO2 should be eliminated from the flue gas before releasing it to the environment. Currently, there are many methods that use to capture CO2 such as using circulating fluidized bed riser with solid sorbent. The advantages of circulating fluidized bed riser are uniform solid particle and temperature distributions, high contacting area between gas-solid particle and suitable for continuous operation. In this study, the effect of operating parameters on CO2 capture in circulating fluidized bed riser with solid sorbent is investigated using 2D computational fluid dynamics model. The basic simulation step has to find the suitable computational mesh cells or grid independency test (5,000, 10,000, 15,000 and 20,000 cells) and compare the simulation result with the real experimental result. According to the simulation results, the suitable mesh cell is 10,000 cells and the obtained result is matched with the experimental results. Then, the effect of operating parameters on the CO2 capture conversion is optimized. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Organizing Committee of 2017 AEDCEE.

Keywords: Circulating fluidized bed reactor, CO2 capture; Computational fluid dynamics,Optimization; 2k factorial design.

* Corresponding author. Tel.: +662-218-7682; fax: +662-255-5831.

E-mail address: benjapon.c@chula.ac.th

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 00 (2017) 000–000

www.elsevier.com/locate/procedia

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Organizing Committee of 2017 AEDCEE.

2017 International Conference on Alternative Energy in Developing Countries and Emerging Economies 2017 AEDCEE, 25‐26 May 2017, Bangkok, Thailand

Computational Fluid Dynamics Model of CO2 Capture in Fluidized Bed Reactors: Operating Parameter Optimization

Chattan Sakaunnapaporna, Pornpote Piumsomboona,b, Benjapon Chalermsinsuwana,b,* aFuels Research Center, Department of Chemical Technology, Faculty of Science, Chulalongkorn University,

254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand bCenter of Excellence on Petrochemical and Materials Technology, Chulalongkorn University,

254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand

Abstract

Alternative energy is one of the methods for decreasing fossil fuel consumption. However, conventional fossil fuel process improvement is also considerably interesting issue due to the fact that adjusting existing process is easier and cheaper comparing to the development of the process compatible with the alternative energy. At present, the global warming and climate change phenomenon cause the increasing of average earth temperature. The CO2 emission to the atmosphere is mainly produced by fossil fuel combustion from power industry. This is because the CO2 has high heat capacity. Therefore, in order to use the conventional fossil fuel process efficiently, CO2 should be eliminated from the flue gas before releasing it to the environment. Currently, there are many methods that use to capture CO2 such as using circulating fluidized bed riser with solid sorbent. The advantages of circulating fluidized bed riser are uniform solid particle and temperature distributions, high contacting area between gas-solid particle and suitable for continuous operation. In this study, the effect of operating parameters on CO2 capture in circulating fluidized bed riser with solid sorbent is investigated using 2D computational fluid dynamics model. The basic simulation step has to find the suitable computational mesh cells or grid independency test (5,000, 10,000, 15,000 and 20,000 cells) and compare the simulation result with the real experimental result. According to the simulation results, the suitable mesh cell is 10,000 cells and the obtained result is matched with the experimental results. Then, the effect of operating parameters on the CO2 capture conversion is optimized. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Organizing Committee of 2017 AEDCEE.

Keywords: Circulating fluidized bed reactor, CO2 capture; Computational fluid dynamics,Optimization; 2k factorial design.

* Corresponding author. Tel.: +662-218-7682; fax: +662-255-5831.

E-mail address: benjapon.c@chula.ac.th

2 Author name / Energy Procedia 00 (2017) 000–000

Nomenclature

k number of considered parameter

1. Introduction

Nowadays, the emission of carbon dioxide (CO2) from chemical industry is a major cause of the global warming because CO2 can absorb and maintain the heat which then has an impact on climate change. Currently, alternative energy is one of the methods for decreasing fossil fuel consumption such as solar energy and wind energy. It can decrease air pollution which is primary cause of the global warming, but the investment of equipment in building alternative energy plant is very expensive. Thus, conventional fossil fuel process improvement is also considerably interesting issue due to the fact that adjusting existing process is easier and cheaper comparing to the development of the process compatible with the alternative energy. There are many methods that use to capture CO2 such as using circulating fluidized bed riser with alkali-based solid sorbent. Alkali metal carbonates such as Na2CO3 and K2CO3 react with CO2 and H2O and transform to alkali metal hydrogen carbonates after CO2 adsorption [1]. In fluidized bed reactor, the solid flow pattern is important quantitatively due to difference solid flow pattern will affect the rate heat and mass transfers.

There are many researches that study the effect of operating parameter on CO2 adsorption in circulating fluidized bed riser. Wang et al. [2] researched about CO2 capture using potassium-based sorbents in circulating fluidized bed reactor at different inlet gas velocities using simulation method by considering effect of particle clusters. According to their results, the simulation with particle cluster effect predicted the system hydrodynamics similar to the experimental result more than the simulation without particle cluster effect. Yi et al. [3] studied the effect of operating parameters, gas inlet velocity, solid circulation rate and water content in feed gas, on CO2 removal percentage in circulating fluidized bed reactor by using K2CO3 solid sorbent. As a result, the increase of the overall CO2 removal is owing to the increasing solid circulation rate and water vapor content and the decreasing gas velocity. Zhao et al. [4] studied the effect of amount of K2CO3 on CO2 sorption capacity. The CO2 sorption capacity increased when increasing the amount of K2CO3. Yafei et al. [5] investigated the CO2 capture performance of some wood materials by using fluidized bed reactor. The component of employed wood materials was investigated by XRD which showed high K2CO3 component. According the results, the CO2 capture capacity increased when the reaction temperature decreased (60 to 100oC) and mole ratio between water and CO2 increased. Apart from the experimental method, the simulation method was used to study the CO2 capture processes. Emadoddin et al. [6] simulated CO2 sorption in circulating fluidized bed using deactivation kinetic model and compared the results with experimental information and other chemical reaction models. According the results, differential pressure from simulation result was similar to experimental result [3]. In addition, the deactivation kinetic reaction model predicted the CO2 removal percentage accurately more than the other chemical reaction model. However, the systematically study of the effect of operating parameters on the CO2 removal percentage is still lacking in the literature. Most of the studies were considered the experiment using one factor at a time methodology. With this methodology, the interaction effect between operating parameters cannot be obtained.

The main objective in this study is therefore to investigate the effect of the different inlet gas velocities and the solid circulation rate on the CO2 conversion using two-dimensional computational fluid dynamics model. The numerical model is comparing its correctness with the literature experimental data by Yi et al. [3]. In this study, the response surface via 2k factorial statistical experimental design (with literature base case condition) was found for determining the operating parameter optimization on the CO2 conversion in circulating fluidized bed reactor.

2. Methodology

2.1 Computational model

In this study, the circulating fluidized bed riser was constructed by using computer–aided design program, DESIGN MODULER and was simulated by using computational fluid dynamics simulation program, ANSYS FLUENT. The model in two-dimensional Cartesian coordinate system which consisting of 5,000, 10,000, 15,000, 20,000 mesh cells and 80 s flow time was used. The gas and solid particles entered to the circulating fluidized bed

Chattan Sakaunnapaporn et al. / Energy Procedia 138 (2017) 518–523 519

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 00 (2017) 000–000

www.elsevier.com/locate/procedia

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Organizing Committee of 2017 AEDCEE.

2017 International Conference on Alternative Energy in Developing Countries and Emerging Economies 2017 AEDCEE, 25‐26 May 2017, Bangkok, Thailand

Computational Fluid Dynamics Model of CO2 Capture in Fluidized Bed Reactors: Operating Parameter Optimization

Chattan Sakaunnapaporna, Pornpote Piumsomboona,b, Benjapon Chalermsinsuwana,b,* aFuels Research Center, Department of Chemical Technology, Faculty of Science, Chulalongkorn University,

254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand bCenter of Excellence on Petrochemical and Materials Technology, Chulalongkorn University,

254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand

Abstract

Alternative energy is one of the methods for decreasing fossil fuel consumption. However, conventional fossil fuel process improvement is also considerably interesting issue due to the fact that adjusting existing process is easier and cheaper comparing to the development of the process compatible with the alternative energy. At present, the global warming and climate change phenomenon cause the increasing of average earth temperature. The CO2 emission to the atmosphere is mainly produced by fossil fuel combustion from power industry. This is because the CO2 has high heat capacity. Therefore, in order to use the conventional fossil fuel process efficiently, CO2 should be eliminated from the flue gas before releasing it to the environment. Currently, there are many methods that use to capture CO2 such as using circulating fluidized bed riser with solid sorbent. The advantages of circulating fluidized bed riser are uniform solid particle and temperature distributions, high contacting area between gas-solid particle and suitable for continuous operation. In this study, the effect of operating parameters on CO2 capture in circulating fluidized bed riser with solid sorbent is investigated using 2D computational fluid dynamics model. The basic simulation step has to find the suitable computational mesh cells or grid independency test (5,000, 10,000, 15,000 and 20,000 cells) and compare the simulation result with the real experimental result. According to the simulation results, the suitable mesh cell is 10,000 cells and the obtained result is matched with the experimental results. Then, the effect of operating parameters on the CO2 capture conversion is optimized. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Organizing Committee of 2017 AEDCEE.

Keywords: Circulating fluidized bed reactor, CO2 capture; Computational fluid dynamics,Optimization; 2k factorial design.

* Corresponding author. Tel.: +662-218-7682; fax: +662-255-5831.

E-mail address: benjapon.c@chula.ac.th

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 00 (2017) 000–000

www.elsevier.com/locate/procedia

254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand

Abstract

* Corresponding author. Tel.: +662-218-7682; fax: +662-255-5831.

E-mail address: benjapon.c@chula.ac.th

2 Author name / Energy Procedia 00 (2017) 000–000

Nomenclature

k number of considered parameter

1. Introduction

Nowadays, the emission of carbon dioxide (CO2) from chemical industry is a major cause of the global warming because CO2 can absorb and maintain the heat which then has an impact on climate change. Currently, alternative energy is one of the methods for decreasing fossil fuel consumption such as solar energy and wind energy. It can decrease air pollution which is primary cause of the global warming, but the investment of equipment in building alternative energy plant is very expensive. Thus, conventional fossil fuel process improvement is also considerably interesting issue due to the fact that adjusting existing process is easier and cheaper comparing to the development of the process compatible with the alternative energy. There are many methods that use to capture CO2 such as using circulating fluidized bed riser with alkali-based solid sorbent. Alkali metal carbonates such as Na2CO3 and K2CO3 react with CO2 and H2O and transform to alkali metal hydrogen carbonates after CO2 adsorption [1]. In fluidized bed reactor, the solid flow pattern is important quantitatively due to difference solid flow pattern will affect the rate heat and mass transfers.

There are many researches that study the effect of operating parameter on CO2 adsorption in circulating fluidized bed riser. Wang et al. [2] researched about CO2 capture using potassium-based sorbents in circulating fluidized bed reactor at different inlet gas velocities using simulation method by considering effect of particle clusters. According to their results, the simulation with particle cluster effect predicted the system hydrodynamics similar to the experimental result more than the simulation without particle cluster effect. Yi et al. [3] studied the effect of operating parameters, gas inlet velocity, solid circulation rate and water content in feed gas, on CO2 removal percentage in circulating fluidized bed reactor by using K2CO3 solid sorbent. As a result, the increase of the overall CO2 removal is owing to the increasing solid circulation rate and water vapor content and the decreasing gas velocity. Zhao et al. [4] studied the effect of amount of K2CO3 on CO2 sorption capacity. The CO2 sorption capacity increased when increasing the amount of K2CO3. Yafei et al. [5] investigated the CO2 capture performance of some wood materials by using fluidized bed reactor. The component of employed wood materials was investigated by XRD which showed high K2CO3 component. According the results, the CO2 capture capacity increased when the reaction temperature decreased (60 to 100oC) and mole ratio between water and CO2 increased. Apart from the experimental method, the simulation method was used to study the CO2 capture processes. Emadoddin et al. [6] simulated CO2 sorption in circulating fluidized bed using deactivation kinetic model and compared the results with experimental information and other chemical reaction models. According the results, differential pressure from simulation result was similar to experimental result [3]. In addition, the deactivation kinetic reaction model predicted the CO2 removal percentage accurately more than the other chemical reaction model. However, the systematically study of the effect of operating parameters on the CO2 removal percentage is still lacking in the literature. Most of the studies were considered the experiment using one factor at a time methodology. With this methodology, the interaction effect between operating parameters cannot be obtained.

The main objective in this study is therefore to investigate the effect of the different inlet gas velocities and the solid circulation rate on the CO2 conversion using two-dimensional computational fluid dynamics model. The numerical model is comparing its correctness with the literature experimental data by Yi et al. [3]. In this study, the response surface via 2k factorial statistical experimental design (with literature base case condition) was found for determining the operating parameter optimization on the CO2 conversion in circulating fluidized bed reactor.

2. Methodology

2.1 Computational model

In this study, the circulating fluidized bed riser was constructed by using computer–aided design program, DESIGN MODULER and was simulated by using computational fluid dynamics simulation program, ANSYS FLUENT. The model in two-dimensional Cartesian coordinate system which consisting of 5,000, 10,000, 15,000, 20,000 mesh cells and 80 s flow time was used. The gas and solid particles entered to the circulating fluidized bed

520 Chattan Sakaunnapaporn et al. / Energy Procedia 138 (2017) 518–523 Author name / Energy Procedia 00 (2017) 000–000 3

riser and entrained out the circulating fluidized bed riser at the bottom and top sections, respectively. The simplified schematic drawing of the circulating fluidized bed riser is shown in Fig. 1. The mixing zone was set about 0.6 m height and the fast fluidization zone was set about 5.6 m height.

Fig. 1. The simplified schematic drawing of the circulating fluidized bed riser.

2.2 Mathematical model

The mathematical model that used in this study consisted of four conservation equations, which were mass, momentum, energy and fluctuating kinetic energy (granular temperature) conservation equations, and other related constitutive equations similar to the ones formulated by Chalermsinsuwan et al. [7]. For the constitutive equations, the kinetic theory of granular flow concept was used to explain the solid particle flow behaviour. In this study, three reaction kinetic models for simulation the CO2 adsorption that were the Homogenous model [8], the Deactivation model [9] and the Equilibrium model [10] were simulated and compared the result with experimental information by Yi et al. [3].

2.3 Boundary and initial conditions

In this study, the gas phase consisted of CO2, H2O and N2 that had mass fraction of 0.10, 0.15 and 0.75, respectively. The solid particles were potassium carbonate (K2CO3) particles, with average diameter of 98 microns and bulk density of 1,100 kg/m3. For the boundary condition, no slip condition was used for gas phase at the wall and partial slip condition was used for solid particle phase. For the initial conditions, there were no gas and solid phases in the circulating fluidized bed riser. The operating gravitational force was –9.81 m/s2 in y direction and the operating pressure was set equal to 101,325 Pa. To analyse the system hydrodynamics and the CO2 conversion, the 2k factorial statistical experimental design (with literature base case condition) was used to determine the effect of the inlet gas velocity and the solid circulation rate on the CO2 conversion as summarized in Table 1.

Table1. The statistical experimental design cases.

Case Inlet gas velocity (m/s) Solid circulation rate (kg/m2s) CO2 removal percentage (-)

0 (base case) 1 21 58.46

1 1 10 32.35

2 1 30 70.38

3 3 10 0.65

4 3 30 4.23

4 Author name / Energy Procedia 00 (2017) 000–000

3. Results and discussion

3.1 Grid independency test and experimental validation The grid independency test and the comparison of the simulation result with literature experimental results are

important steps for performing computational fluid dynamics simulation. This study results were averaged after the system reached quasi steady state condition (simulation time of 60-80 s). Fig.2 (a) shows the differential pressure at four elevation heights (at elevation heights of 0.52 m, 2.27 m, 4.07 m, 5.87 m, respectively) comparing between simulation result and the experimental result. It was found that simulated differential pressure results were consistent with the experimental result. The selected suitable mesh cell should be the lowest mesh cells for saving time but still could predict the obtained result accurately. Fig.2 (b) illustrates the averaged CO2 mass fraction with different mesh cells and chemical reaction kinetic models. As a result, the suitable mesh cells was 10,000 cells because the predicted average CO2 mass fraction was similar to the ones with 15,000 and 20,000 cells and similar to experimental result of Yi et al. [3] with the outlet CO2 mass fraction of 0.042. All the reaction kinetic model, homogenous model, deactivation model and equilibrium model, was well predicted the CO2 capture process in circulating fluidized bed riser. However, due to the experimental data comparison and the easier of the methodology, the homogeneous model was then used in the subsequence simulation. From the validation of the results, this confirms the correctness of the employed computational fluid dynamics model.

(a) (b)

Fig. 2. (a) Differential pressure at differential elevation heights and (b) averaged CO2 mass fraction at different riser heights.

3.2 Solid volume fraction and CO2 mass fraction

The interaction between solid particle phase and gas phase in circulating fluidized bed riser has an important effect on the CO2 removal percentage inside the system. This is because the K2CO3 captures CO2 with the chemical reaction: K2CO3(s) + CO2(g) + H2O(g) 2KHCO3(s). Therefore, the contacting between solid particle phase and gas phase is crucial for occurring the adsorption. The contour of solid volume fraction for base case operating condition, inlet gas velocity of 1 m/s and solid circulation rate of 21 kg/m2s, is shown in Fig. 3 (a). The color scale bar represents the quantity of the solid volume fraction which red color is highest value and blue color is lowest value. According to the results, the mixing zone had higher solid volume fraction than the fast fluidization zone because the mixing zone had larger system diameter. The large area will have an effect on the reduction of gas and solid particle velocities. In addition, the solid particle moved down to the mixing zone by the energy loss from wall effect. These obtained phenomena are consistent with the results in Fig. 3(b). Fig. 3(b) depicts the contour of CO2 mass fraction for base case operating condition. The meaning of color scale bar is similar to the ones for solid volume fraction. The CO2 mass fraction was high and low at the bottom zone and top zone, respectively, because the CO2 reacted with K2CO3.

Chattan Sakaunnapaporn et al. / Energy Procedia 138 (2017) 518–523 521 Author name / Energy Procedia 00 (2017) 000–000 3

riser and entrained out the circulating fluidized bed riser at the bottom and top sections, respectively. The simplified schematic drawing of the circulating fluidized bed riser is shown in Fig. 1. The mixing zone was set about 0.6 m height and the fast fluidization zone was set about 5.6 m height.

Fig. 1. The simplified schematic drawing of the circulating fluidized bed riser.

2.2 Mathematical model

The mathematical model that used in this study consisted of four conservation equations, which were mass, momentum, energy and fluctuating kinetic energy (granular temperature) conservation equations, and other related constitutive equations similar to the ones formulated by Chalermsinsuwan et al. [7]. For the constitutive equations, the kinetic theory of granular flow concept was used to explain the solid particle flow behaviour. In this study, three reaction kinetic models for simulation the CO2 adsorption that were the Homogenous model [8], the Deactivation model [9] and the Equilibrium model [10] were simulated and compared the result with experimental information by Yi et al. [3].

2.3 Boundary and initial conditions

In this study, the gas phase consisted of CO2, H2O and N2 that had mass fraction of 0.10, 0.15 and 0.75, respectively. The solid particles were potassium carbonate (K2CO3) particles, with average diameter of 98 microns and bulk density of 1,100 kg/m3. For the boundary condition, no slip condition was used for gas phase at the wall and partial slip condition was used for solid particle phase. For the initial conditions, there were no gas and solid phases in the circulating fluidized bed riser. The operating gravitational force was –9.81 m/s2 in y direction and the operating pressure was set equal to 101,325 Pa. To analyse the system hydrodynamics and the CO2 conversion, the 2k factorial statistical experimental design (with literature base case condition) was used to determine the effect of the inlet gas velocity and the solid circulation rate on the CO2 conversion as summarized in Table 1.

Table1. The statistical experimental design cases.

Case Inlet gas velocity (m/s) Solid circulation rate (kg/m2s) CO2 removal percentage (-)

0 (base case) 1 21 58.46

1 1 10 32.35

2 1 30 70.38

3 3 10 0.65

4 3 30 4.23

4 Author name / Energy Procedia 00 (2017) 000–000

3. Results and discussion

3.1 Grid independency test and experimental validation The grid independency test and the comparison of the simulation result with literature experimental results are

important steps for performing computational fluid dynamics simulation. This study results were averaged after the system reached quasi steady state condition (simulation time of 60-80 s). Fig.2 (a) shows the differential pressure at four elevation heights (at elevation heights of 0.52 m, 2.27 m, 4.07 m, 5.87 m, respectively) comparing between simulation result and the experimental result. It was found that simulated differential pressure results were consistent with the experimental result. The selected suitable mesh cell should be the lowest mesh cells for saving time but still could predict the obtained result accurately. Fig.2 (b) illustrates the averaged CO2 mass fraction with different mesh cells and chemical reaction kinetic models. As a result, the suitable mesh cells was 10,000 cells because the predicted average CO2 mass fraction was similar to the ones with 15,000 and 20,000 cells and similar to experimental result of Yi et al. [3] with the outlet CO2 mass fraction of 0.042. All the reaction kinetic model, homogenous model, deactivation model and equilibrium model, was well predicted the CO2 capture process in circulating fluidized bed riser. However, due to the experimental data comparison and the easier of the methodology, the homogeneous model was then used in the subsequence simulation. From the validation of the results, this confirms the correctness of the employed computational fluid dynamics model.

(a) (b)

Fig. 2. (a) Differential pressure at differential elevation heights and (b) averaged CO2 mass fraction at different riser heights.

3.2 Solid volume fraction and CO2 mass fraction

The interaction between solid particle phase and gas phase in circulating fluidized bed riser has an important effect on the CO2 removal percentage inside the system. This is because the K2CO3 captures CO2 with the chemical reaction: K2CO3(s) + CO2(g) + H2O(g) 2KHCO3(s). Therefore, the contacting between solid particle phase and gas phase is crucial for occurring the adsorption. The contour of solid volume fraction for base case operating condition, inlet gas velocity of 1 m/s and solid circulation rate of 21 kg/m2s, is shown in Fig. 3 (a). The color scale bar represents the quantity of the solid volume fraction which red color is highest value and blue color is lowest value. According to the results, the mixing zone had higher solid volume fraction than the fast fluidization zone because the mixing zone had larger system diameter. The large area will have an effect on the reduction of gas and solid particle velocities. In addition, the solid particle moved down to the mixing zone by the energy loss from wall effect. These obtained phenomena are consistent with the results in Fig. 3(b). Fig. 3(b) depicts the contour of CO2 mass fraction for base case operating condition. The meaning of color scale bar is similar to the ones for solid volume fraction. The CO2 mass fraction was high and low at the bottom zone and top zone, respectively, because the CO2 reacted with K2CO3.

522 Chattan Sakaunnapaporn et al. / Energy Procedia 138 (2017) 518–523 Author name / Energy Procedia 00 (2017) 000–000 5

(a) (b)

Fig. 3. (a) Contour of solid volume fraction and (b) contour of CO2 mass fraction (at three different quasi-steady state simulation times).

3.3 Analysis of variance for the statistical experimental design

The 2k factorial statistical experimental design methodology (with literature base case condition) is useful in performing the experiment due to its many advantages. It gives the smallest number of runs for k factors. However, the full parameter analysis can still be obtained. With this methodology, the two levels of each factor represent the low and high values. In this study, the statistical experimental design had the inlet gas velocity as a first factor and solid circulation rate as a second factor. The response variable was CO2 removal percentage at the outlet of circulating fluidized bed riser. The analysis of variance result for the statistical experimental design is summarized in Table 2. The p–value was used for the statistical testing. If the p–value is lower than 0.05, the factor significantly affects the interested response. From the results, it can be summarized that inlet gas velocity had significantly affected on the CO2 removal percentage. In addition, it was found that effect of the solid sorbent loading and the interaction between inlet gas velocity and solid sorbent loading did not have an effect on the CO2 conversion significantly. Fig. 4 illustrates the main effect plot of the inlet gas velocity and the solid circulation rate on the CO2 removal percentage. When increasing the inlet gas velocity and the solid circulation rate, the CO2 removal percentage was lower and higher, respectively. The high gas velocity will decrease the system residence time and reduce the contacting between gas and solid particles. The high solid circulation rate will increase the quantity of reactant material of the adsorption inside the circulating fluidized bed riser.

Fig. 5 shows the response surface of CO2 removal percentage with the changing of inlet gas velocity and the solid circulation rate. The response surface can be used to choose the operating condition with desired outcome or response. For this circulating fluidized bed riser, the highest CO2 removal percentage is preferred. Therefore, the low inlet gas velocity and high solid circulation rate is needed to operate the system to obtain the high CO2 removal percentage.

Table 2. The analysis of variance result for the statistical experimental design.

Source Sum of DF Mean F Prob > F Squares Square Value

A 3076.67 1 3076.67 171.75 0.05 B 440.82 1 440.82 24.61 0.13

AB 303.33 1 303.33 16.93 0.15 Residual 17.91 1 17.91 Cor Total 3920.26 4

6 Author name / Energy Procedia 00 (2017) 000–000

Fig. 4. This study main effect plot. Fig. 5. This study response surface contour.

4. Conclusion

In this study, the computational fluid dynamics model which had 10,000 mesh cells and three reaction kinetic models was accurately used to predict CO2 removal percentage and system hydrodynamics of circulating fluidized bed riser comparing with the experimental results of Yi et al. [3]. From the 2k factorial statistical experimental design (with literature base case condition), the increasing of inlet gas velocity and solid sorbent circulation rate gave lower and higher CO2 removal percentage, respectively. In addition, the analysis concluded the significant effect of inlet gas velocity on the CO2 removal. The low inlet gas velocity and high solid circulation rate is needed to operate the system to obtain the high CO2 removal percentage.

Acknowledgements

This study was financially supported by the Scholarship from the Graduate School, Chulalongkorn University to commemorate the 72nd anniversary of his Majesty King Bhumibol Aduladej, the 90th Anniversary of Chulalongkorn University Fund, the Graduate School Thesis Grant, the National Research Council of Thailand, the Thailand Research Fund for fiscal year 2016–2019 (RSA5980052), and Ratchadapisek Sompoch Endowment Fund (2016), Chulalongkorn University (CU-59-003-IC).

References

[1] Abanades JC, Antrony EJ, Wang, J, Oakey JE. Fluidized-bed combustion system integration CO2 capture with CaO. Environ Sci Technol 2005; 39:2861–2866. [2] Wang S, Wang Q, Chen J, Liu G, Lu H, Sun L. Assessment of CO2 capture using potassium-based sorbents in circulating fluidized bed reactor by multiscale modeling. Fuel 2016; 164:66–72. [3] Yi CK, Jo SJ, Seo Y, Lee JB, Ryu CK. Continuous operation of the potassium-based dry sorbent CO2 capture process with two fluidized-bed reactors. Int J Greenhouse Gas Control 2007; 1:31-36. [4] Zhao C, Chen X, Zhao C, Wu Y, Dong W. K2CO3/Al2O3 for capturing CO2 in flue gas from power plants. Part 3: CO2 capture behaviors of K2CO3/Al2O3 in a bubbling fluidized-bed reactor. Energy Fuels 2012; 26:3062–3068. [5] Yafei G, Chuanwen Z, Xiaoping C, Changhai L. CO2 capture and sorbent regeneration performances of some wood ash materials. Appl Energ 2015; 137:26–36. [6] Emadoddin A, Hamid A. CFD simulation of CO2 sorption in a circulating fluidized bed using deactivation kinetic model. 10th International Conference on Circulating Fluidized Beds and Fluidization Technology – CFB-10, US, May 1 – May 5, 2011. [7] Chalermsinsuwan B, Piumsomboon P, Gidaspow D. Kinetic theory based computation of PSRI riser: Part I–Estimate of mass transfer coefficient. Chem Eng Sci 2009; 64:1195–1211. [8] Garg R, Shahnam M, Huckaby ED. Continuum simulations of CO2 capture by dry regenerable potassium based sorbents. 7th International Conference on Multiphase Flow, ICMF 2010, US, May 30 – June 4, 2010. [9] Park SW, Sung DH, Choi BS, Lee JW, Kumazawa H. Carbonation kinetics of potassium carbonate by carbon dioxide. J Ind Eng Chem 2006; 4:522-530. [10] Kongkitisupchai S, Gidaspow D. Carbon dioxide capture using solid sorbents in a fluidized bed with reduced pressure regeneration in a downer. AIChE J 2013; 12:4519-4537.

Chattan Sakaunnapaporn et al. / Energy Procedia 138 (2017) 518–523 523 Author name / Energy Procedia 00 (2017) 000–000 5

(a) (b)

Fig. 3. (a) Contour of solid volume fraction and (b) contour of CO2 mass fraction (at three different quasi-steady state simulation times).

3.3 Analysis of variance for the statistical experimental design

The 2k factorial statistical experimental design methodology (with literature base case condition) is useful in performing the experiment due to its many advantages. It gives the smallest number of runs for k factors. However, the full parameter analysis can still be obtained. With this methodology, the two levels of each factor represent the low and high values. In this study, the statistical experimental design had the inlet gas velocity as a first factor and solid circulation rate as a second factor. The response variable was CO2 removal percentage at the outlet of circulating fluidized bed riser. The analysis of variance result for the statistical experimental design is summarized in Table 2. The p–value was used for the statistical testing. If the p–value is lower than 0.05, the factor significantly affects the interested response. From the results, it can be summarized that inlet gas velocity had significantly affected on the CO2 removal percentage. In addition, it was found that effect of the solid sorbent loading and the interaction between inlet gas velocity and solid sorbent loading did not have an effect on the CO2 conversion significantly. Fig. 4 illustrates the main effect plot of the inlet gas velocity and the solid circulation rate on the CO2 removal percentage. When increasing the inlet gas velocity and the solid circulation rate, the CO2 removal percentage was lower and higher, respectively. The high gas velocity will decrease the system residence time and reduce the contacting between gas and solid particles. The high solid circulation rate will increase the quantity of reactant material of the adsorption inside the circulating fluidized bed riser.

Fig. 5 shows the response surface of CO2 removal percentage with the changing of inlet gas velocity and the solid circulation rate. The response surface can be used to choose the operating condition with desired outcome or response. For this circulating fluidized bed riser, the highest CO2 removal percentage is preferred. Therefore, the low inlet gas velocity and high solid circulation rate is needed to operate the system to obtain the high CO2 removal percentage.

Table 2. The analysis of variance result for the statistical experimental design.

Source Sum of DF Mean F Prob > F Squares Square Value

A 3076.67 1 3076.67 171.75 0.05 B 440.82 1 440.82 24.61 0.13

AB 303.33 1 303.33 16.93 0.15 Residual 17.91 1 17.91 Cor Total 3920.26 4

6 Author name / Energy Procedia 00 (2017) 000–000

Fig. 4. This study main effect plot. Fig. 5. This study response surface contour.

4. Conclusion

In this study, the computational fluid dynamics model which had 10,000 mesh cells and three reaction kinetic models was accurately used to predict CO2 removal percentage and system hydrodynamics of circulating fluidized bed riser comparing with the experimental results of Yi et al. [3]. From the 2k factorial statistical experimental design (with literature base case condition), the increasing of inlet gas velocity and solid sorbent circulation rate gave lower and higher CO2 removal percentage, respectively. In addition, the analysis concluded the significant effect of inlet gas velocity on the CO2 removal. The low inlet gas velocity and high solid circulation rate is needed to operate the system to obtain the high CO2 removal percentage.

Acknowledgements

This study was financially supported by the Scholarship from the Graduate School, Chulalongkorn University to commemorate the 72nd anniversary of his Majesty King Bhumibol Aduladej, the 90th Anniversary of Chulalongkorn University Fund, the Graduate School Thesis Grant, the National Research Council of Thailand, the Thailand Research Fund for fiscal year 2016–2019 (RSA5980052), and Ratchadapisek Sompoch Endowment Fund (2016), Chulalongkorn University (CU-59-003-IC).

References

[1] Abanades JC, Antrony EJ, Wang, J, Oakey JE. Fluidized-bed combustion system integration CO2 capture with CaO. Environ Sci Technol 2005; 39:2861–2866. [2] Wang S, Wang Q, Chen J, Liu G, Lu H, Sun L. Assessment of CO2 capture using potassium-based sorbents in circulating fluidized bed reactor by multiscale modeling. Fuel 2016; 164:66–72. [3] Yi CK, Jo SJ, Seo Y, Lee JB, Ryu CK. Continuous operation of the potassium-based dry sorbent CO2 capture process with two fluidized-bed reactors. Int J Greenhouse Gas Control 2007; 1:31-36. [4] Zhao C, Chen X, Zhao C, Wu Y, Dong W. K2CO3/Al2O3 for capturing CO2 in flue gas from power plants. Part 3: CO2 capture behaviors of K2CO3/Al2O3 in a bubbling fluidized-bed reactor. Energy Fuels 2012; 26:3062–3068. [5] Yafei G, Chuanwen Z, Xiaoping C, Changhai L. CO2 capture and sorbent regeneration performances of some wood ash materials. Appl Energ 2015; 137:26–36. [6] Emadoddin A, Hamid A. CFD simulation of CO2 sorption in a circulating fluidized bed using deactivation kinetic model. 10th International Conference on Circulating Fluidized Beds and Fluidization Technology – CFB-10, US, May 1 – May 5, 2011. [7] Chalermsinsuwan B, Piumsomboon P, Gidaspow D. Kinetic theory based computation of PSRI riser: Part I–Estimate of mass transfer coefficient. Chem Eng Sci 2009; 64:1195–1211. [8] Garg R, Shahnam M, Huckaby ED. Continuum simulations of CO2 capture by dry regenerable potassium based sorbents. 7th International Conference on Multiphase Flow, ICMF 2010, US, May 30 – June 4, 2010. [9] Park SW, Sung DH, Choi BS, Lee JW, Kumazawa H. Carbonation kinetics of potassium carbonate by carbon dioxide. J Ind Eng Chem 2006; 4:522-530. [10] Kongkitisupchai S, Gidaspow D. Carbon dioxide capture using solid sorbents in a fluidized bed with reduced pressure regeneration in a downer. AIChE J 2013; 12:4519-4537.