CO2 Capture from Flue Gas in an Existing Coal-Fired Power Plant by

Article pubs.acs.org/IECR

CO2 Capture from Flue Gas in an Existing Coal-Fired Power Plant by Two Successive Pilot-Scale VPSA Units Lu Wang,† Ying Yang,† Wenlong Shen,† Xiangming Kong,† Ping Li,†,* Jianguo Yu,† and Alirio E. Rodrigues‡ †

State Key Laboratory of Chemical Engineering, College of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ Laboratory of Separation and Reaction Engineering (LSRE), Associate Laboratory LSRE/LCM, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, s/n, 4200-465, Porto, Portugal ABSTRACT: CO2 capture and storage (CCS) is an effective method for achieving CO2 mitigation while simultaneously keeping energy supplies secure. To put CCS into practice, it is important to develop energy-efficient industrial technologies for CO2 capture. In this work, a pilot-scale demonstration of carbon capture from flue gas by adsorption technology was performed in an existing coal-fired power plant in China, and the power energy consumption to capture 1 kg of CO2 was measured onsite; furthermore, the feasibility and efficiency of adsorption technology for postcombustion CO2 capture were investigated. The pilotscale carbon capture plant consisted of two successive VPSA units coupled with a dehumidifying unit. In the dehumidifying unit, water vapor in the desulfurized flue gas was removed by alumina adsorption. Then, CO2 in the dehumidified flue gas was captured by two successive VPSA units, where the three-bed eight-step VPSA process was employed in the first unit packed with zeolite 13X APG, and the second two-bed six-step VPSA unit was packed with pitched activated carbon beads. A roots blower was used to supply the desulfurized flue gas to the pilot-scale carbon capture plant at a controlled flow rate, and both a reciprocating pump and a diaphragm pump were employed to desorb adsorbents under vacuum pressure in the two-stage units and recover high-purity CO2 for subsequent storage or utilization. Some key assessment parameters were measured onsite, including the flow rate of flue gas, CO2 recovery from flue gas, CO2 purity in the product gas, and power energy consumption to capture 1 kg of CO2, and the experimental results were verified by numerical simulations using a multibed VPSA modeling framework. Based on the experimental and simulated results, CO2 capture from flue gas in an existing coal-fired power plant by two successive VPSA units was evaluated.

1. INTRODUCTION The continuous increase of atmospheric greenhouse gases as a result of fossil fuel burning has been identified as the major contribution to global warming and climate change.1 Among the greenhouse gases, CO2 contributes more than 60% of global warming because of the large amount of carbon dioxide emissions.2,3 However, fossil fuels will continue to play an important role in both heat and power generation and heavy industrial manufacturing operations for the foreseeable future.4 Therefore, this global recognition has attracted great attention to the development and improvement of technologies and strategies for mitigating CO2 emissions. Carbon capture and storage (CCS) is considered a vital approach to greenhouse gas control and the reduction of the climate impact of power generation in the near future.5,6 Although efforts are needed to develop new and cleaner technologies such as advanced combustion and gasification technologies in new power plants, postcombustion CO2 capture is still required to avoid excess emissions of CO2 from existing power plants. The carbon capture target is to obtain a concentrated stream of 90−95 vol % CO2 for further sequestration or utilization. To date, four candidate approaches are avilable for postcombustion CO2 capture, including absorption, adsorption, low-temperature distillation, and membrane separation.7 Chemical absorption using an amine-based aqueous solution is known as a promising carbon capture method. In © 2013 American Chemical Society

fact, amine scrubbing has been used to separate CO2 from natural gas and hydrogen since 1930, and it is a robust technology, ready to be tested and used on a large scale for CO2 capture from flue gas in existing coal-fired power plants.8 However, amine scrubbing technologies have some drawbacks such as a high corrosion rate for equipment and high energy consumption for regeneration.9 The typical energy consumption to capture 1 kg of CO2 by amine scrubbing is 2.75−4.6 MJ/kg of CO2.1,7 Another promising carbon capture method, membrane separation, has also gained increasing interest in recent years for making postcombustion CO2 capture more energy-efficient. The energy demand for postcombustion CO2 capture with membrane separation is 1−3.5 MJ/kg of CO2 depending on the use of different types of membrane materials such as polymeric, ceramic, and zeolite membrane materials.10 Adsorption technology with well-engineered solid adsorbents is a competitive method for carbon capture because of the reusable nature of adsorbents, low capital investment, and easy automatic operation, especially for carbon capture from smallto medium-scale CO2 emitters.11−26 Pressure swing adsorption/ vacuum pressure swing adsorption (PSA/VPSA) processes have Received: Revised: Accepted: Published: 7947

March 27, 2013 May 13, 2013 May 14, 2013 May 14, 2013 dx.doi.org/10.1021/ie4009716 | Ind. Eng. Chem. Res. 2013, 52, 7947−7955

Industrial & Engineering Chemistry Research

Article

been widely applied to recover CO2 from various gas mixtures, such as off-gas of steam reformers, natural gas, and flue gas from coal-fired power plants.27 The configuration of the PSA process changes depending on the CO2 concentration in the gas mixture treated. When the concentration of CO2 in the feed is higher than 20%, a single-stage PSA process is sufficient to obtain 99% CO2 gas with high recovery.28 However, it is difficult to capture CO2 with both high purity and high recovery in a single-stage PSA/VPSA process when the CO2 concentration in the flue gas is lower, such as 3−20 vol % from power plants. Two successive VPSA units are recommended to capture and concentrate CO2 from flue gas to obtain product purity higher than 90% with a relatively high CO2 recovery ratio.29 Adsorbent materials with superior properties are an essential requirement for carbon capture by adsorption processes to obtain a high-purity CO2 stream.1,28,30−32 Carbon materials and zeolite molecular sieves have been widely used for CO2 capture from flue gas.28,33−35 In our previous works,36−39 two types of new adsorbent materials were recommended for capturing CO2: zeolite 13X APG developed by UOP Company and pitchbased activated carbon beads (ACBs) prepared in Prof. Ling’s laboratory.35 The adsorption equilibrium isotherms, kinetics, and cyclic processes for the separation of CO2 and N2 were discussed in our previous research studies. One objective of this article was to evaluate the performance of zeolite 13X APG and activated carbon beads for CO2 capture from a real flue gas in an existing coal-fired power plant. The development of energy-efficient CO2 capture technologies is very important for CCS industrialization. To date, reports about onsite pilot-scale CO2 capture from real flue gases by adsorption technology have been rare, even though pilotscale experiments are important for evaluating the actual separation performance and power consumption. In this study, an experimental evaluation was performed for CO2 capture from flue gas in an existing coal-fired power plant using a twostage VPSA unit composed of a three-bed eight-step cycle for the first unit packed with zeolite 13X APG and a two-bed sixstep cycle for the second unit packed with pitch-based ACBs. In addition, the CO2 recovery from flue gas, CO2 purity in the product gas, and power energy consumption to capture 1 kg of CO2 from flue gas were measured onsite.

Table 1. Physical Parameters of Zeolite 13X APG and PitchBased Activated Carbon Beads zeolite 13X APG pellet radius (average) (m) pellet density (kg/m3) pellet porosity crystal diameter (μm) adsorbent specific heat [J/(kg K)] specific surface area (m2/g) total pore volume (cm3/g)

ACBs 0.00135 1099.5 0.31 1.5 920 454.54 0.267

pellet radius (average) (m) pellet density (kg/m3) pellet porosity micropore average size (nm) adsorbent specific heat [J/(kg K)] specific surface area (m2/g) total pore volume (cm3/g)

0.001 805.85 0.506 0.6 880 1457.02 0.350

Figure 1. Three-dimensional configuration diagram of two successive pilot-scale VPSA units.

in the dehumidified flue gas was controlled at less than 0.5% for CO2 capture. Two successive VPSA units were used to capture and concentrate CO2 from the dehumidified flue gas, where a three-bed eight-step VPSA process was employed in the first unit packed with zeolite 13X APG, and the second unit was packed with pitched activated carbon beads and operated as a two-bed six-step VPSA process. A roots blower from Changsha Blower Co. with a variable-frequency drive (VFD) was used to supply the desulfurized flue gas for the pilot-scale carbon capture plant at a controlled flow rate, and a reciprocating pump and a diaphragm pump were used to desorb adsorbents under vacuum pressure and recover CO2 with higher purity for subsequent storage or use. A reciprocating pump (WLW) with a rated power of 4 kW from Shanghai Vacuum Pump Co. was used in the first VPSA unit, and a diaphragm pump with a rated power of 1.1 kW from ILMVAC was used in the second VPSA unit. Part of the effluent gas from the adsorber in the first VPSA unit was heated (about 473 K) and passed through the dehumidifying unit to regenerate alumina. The effluent gas from the adsorber in the second VPSA unit was recycled to the inlet of the first VPSA unit to recover CO2. There were two tanks in the first and second units to store the desorbed CO2 product gases at a given pressure for recirculation to the packed column at the rinse steps. The first VPSA unit with three beds (columns C−E) was packed with 261 kg of zeolite 13X APG, where each stainless steel bed had a 400-mm diameter and a 1000-mm packed

2. EXPERIMENTAL SECTION 2.1. Adsorbent Materials. Based on our previous research studies, two types of adsorbents were used to capture CO2 from the real flue gas in an existing coal-fired power plant: zeolite 13X APG and pitch-based activated carbon beads. Zeolite 13X APG was purchased from UOP Company (Shanghai, China). Pitch-based activated carbon (AC) beads were prepared in Prof. Ling’s laboratory with coal tar pitch by the emulsion, air stabilization, and carbonization method, and the synthesized pitch spheres were activated with CO2 or H2O.35,36 The characteristic properties of zeolite 13X APG and pitch-based AC beads can be found elsewhere36,38 and are summarized in Table 1. Adsorption equilibrium isotherms, kinetics, and cyclic processes for the separation of CO2 and N2 by zeolite 13X APG and pitch-based AC beads can be found in our published articles.36−39 2.2. Pilot-Scale Carbon Capture Plant. The pilot-scale carbon capture plant consisted of two successive VPSA units coupled with a dehumidifying unit, as shown in Figure 1. In the dehumidifying unit, two beds were packed with alumina, and temperature swing adsorption (TSA) was employed to remove water vapor from the desulfurized flue gas; the relative humidity 7948

dx.doi.org/10.1021/ie4009716 | Ind. Eng. Chem. Res. 2013, 52, 7947−7955


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and CO2 in the flue gas was captured by zeolite 13X APG in the packed column. Step 3 was depressurization (D). After adsorption, the pressure in the column was reduced to atmospheric pressure (0.1017−0.1019 MPa) by emitting gas into the atmosphere. Step 4 was rinse (RINSE). Part of the CO2-rich gas in the product tank of the first VPSA unit was recycled into the adsorbed column to displace the light component (N2) at a rinse flow rate of 20 standard liters per minute (SLPM). Step 5 was provided pressure equalization (EQ). Two columns at different pressure levels from the top were interconnected to conserve the mechanical energy contained in the high-pressure gas. The pressure in the column could be equalized to 0.05 MPa in this step. Step 6 was blowdown (B). CO2 adsorbed on zeolite 13X APG was desorbed by the reciprocating pump under 7−8 kPa vacuum pressure, and CO2-rich gas was stored in the product tank of the first VPSA unit. Step 7 was purge (PUR). Part of the effluent gas from another adsorber was used to purge the desorbed column in counter-current operation under vacuum pressure at a flow rate of 20 or 50 SLPM and to further recover CO2. The CO2-rich gas was collected and stored in the product tank of the first VPSA unit. Step 8 was received pressure equalization (EQ). The column under vacuum pressure received the gases from the column with high pressure to reduce the power consumption for pressurization. The second VPSA unit was packed with pitch-based ACBs,and a two-bed six-step cycle was performed for further concentration, as follows: Step 1 was feed (FEED). The CO2-rich gas from the first VPSA unit was passed through the adsorption column in the second VPSA unit, and CO2 was adsorbed by pitch-based ACBs in the packed column. The adsorption pressure was about 0.122−0.126 MPa. Step 2 was depressurization (D). After the feed step, the pressure in the column was reduced to atmospheric pressure by releasing gas into the atmosphere. Step 3 was rinse (RINSE). Part of the product gas in the product tank obtained in the second VPSA unit was recycled to the adsorbed column from the bottom to displace the light component (N2) with 20 SLPM flow rate. Step 4 was provided pressure equalization (EQ). Two columns at different pressure levels from the top are interconnected to conserve the mechanical energy contained in the high pressure gas. Step 5 was blowdown (B). CO2 adsorbed on ACBs was desorbed by the diaphragm pump under 20 kPa vacuum pressure, and CO2-rich gas was stored in the product tank of the second VPSA unit. Step 6 was received pressure equalization (EQ). The column under low pressure received the gases from the column with a high pressure to save the power consumption for pressurization. Before the experiments, the adsorbents packed in the columns were regenerated using the vacuum pump for several hours. According to the results simulated using a multibed VPSA modeling framework, the cyclic sequences and time schedules in the first and second VPSA units were selected, as shown in Figure 2. It should be mentioned that the desorption pressure and time were designed through a comprehensive consideration of adsorbent regeneration and the mechanical properties of the vacuum pump. In this work, the desorption pressure and time were determined from the data measured onsite for the reciprocating and diaphragm pumps.

height. The second VPSA unit with two beds (columns F and G) was packed with 68 kg of pitch-based ACBs, where the column diameter was 200 mm and the packed height was 1000 mm. The details are listed in Table 2. The pilot-scale carbon capture Table 2. Column Properties of First and Second VPSA Units columns C−E packed with zeolite 13X APG column radius (m) packed height (m) column porosity column density (kg/m3)

0.2 1 0.37 693

density of column wall (kg/m3) column wall thickness (m)

8238 5 × 10−3

wall heat capacity [J/(kg K)] 500 weight of adsorbent in each column (kg)

87.0

columns F and G packed with zeolite ACBs column radius (m) packed height (m) column porosity column density (kg/m3) density of column wall (kg/m3) column wall thickness (m) wall heat capacity [J/(kg K)] weight of adsorbent in each column (kg)

0.1 1 0.32 984 8238 5 × 10−3 500 34.3

plant was operated automatically by programmable logic software (Siemens) controlling the pneumatic solenoid valves. The power consumption of the blower and vacuum pumps was monitored by watt meters. The flow rate of flue gas was adjusted with the blower’s variable-frequency drive, and the gas flow rates at the purge and rinse steps were adjusted by mass flow controllers (MFCs). The CO2 concentrations in the inlet and outlet of the columns were analyzed online by an infrared analyzer. A backpressure regulator was used to control the adsorption pressure in the packed columns. Pressure transmitters were installed in the inlet and outlet of the columns to display the changes in pressure online. Temperature transmitters were located at positions of 0.25, 0.55, and 0.90 m along the column from the bottom to display the changes in temperature online. 2.3. Process Description. In the existing coal-fired power plant, the desulfurized flue gas was emitted to the atmosphere through a chimney, so it would be more energy-efficient to design the adsorption pressure to be near atmospheric pressure (0.117 MPa) in the first VPSA unit for CO2 capture. After dehumidification, the relative humidity in the dried flue gas was controlled at less than 0.5%, and the temperature of the dried flue gas was about 303−323 K. The desorption pressure in the first VPSA unit was set at 7−8 kPa to consider good regeneration of zeolite 13X APG and the mechanical limits of the reciprocating pump. The CO2-rich stream in the product tank of the first VPSA unit could directly pass through the adsorption column in the second VPSA unit at 0.122−0.126 MPa (the allowable pressure of the reciprocating pump). The desorption pressure in the second VPSA unit was set at 20 kPa for ACB regeneration using the diaphragm pump. The dried flue gas was fed to the first VPSA unit, where carbon dioxide was captured and concentrated to 70−80% purity, and then the CO2-rich stream was further concentrated above 95% in the second VPSA unit. A three-bed eight-step VPSA process was employed in the first carbon capture unit, and the detailed steps are described as follows: Step 1 was pressurization. The packed column was pressurized with the dried flue gas from the bottom of column to the given adsorption pressure (0.117 MPa). Step 2 was feed (FEED). The dried flue gas passed through the packed column from the bottom at a controlled flow rate, 7949



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3. RESULTS AND DISCUSSION 3.1. Arrangement for Two Types of Adsorbents in Two Successive VPSA Units. Single-component adsorption isotherms for CO2 and N2 on zeolite 13X APG and pitch-based ACBs measured at 303, 333, 363, 393 K in the pressure range of 3−100 kPa were reported in previous works36,38 and are shown in Figure 3. The points represent the experimental data, and the solid lines represent the results predicted by the multisite Langmuir model. According to the multisite Langmuir model equations obtained for single-component adsorption isotherms, the competitive adsorption equilibria of CO2 and N2 on zeolite 13X APG and pitch-based ACBs can be predicted by the equations

The working capacity for a given adsorbent is defined as the difference between the adsorption capacities under adsorption and desorption conditions Δqi = qi(Pads) − qi(Pdes),

2

2

aCO ,13X ⎛ qCO ,13X qN ,13X ⎞ 2 2 2 ⎟ × ⎜⎜1 − − qm,CO ,13X qm,N ,13X ⎟⎠ ⎝ 2 2

⎛ 31904 ⎞ ⎟ = 5.249 × 10−7 × 6.929PyCO exp⎜ 2 ⎝ 8.315T ⎠ qCO ,13X qN ,13X ⎞3.362 ⎛ 2 × ⎜1 − − 2 ⎟ ⎝ 6.929 6.270 ⎠

(1)

qN ,13X = qm,N ,13X K N2,13X PyN 2

2

2

a N ,13X ⎛ qCO ,13X qN ,13X ⎞ 2 2 2 ⎜ ⎟ × ⎜1 − − qm,CO ,13X qm,N ,13X ⎟⎠ ⎝ 2 2

⎛ 17805 ⎞ ⎟ = 4.390 × 10−7 × 6.270PyN exp⎜ 2 ⎝ 8.315T ⎠ qCO ,13X qN ,13X ⎞3.715 ⎛ 2 × ⎜1 − − 2 ⎟ ⎝ 6.929 6.270 ⎠

(2)

qCO ,ACB = qm,CO ,ACBK CO2,ACBPyCO 2

2

2

aCO ,ACB ⎛ qCO ,ACB qN ,ACB ⎞ 2 2 2 ⎜ ⎟ × ⎜1 − − qm,CO ,ACB qm,N ,ACB ⎟⎠ ⎝ 2 2

⎛ 24556 ⎞ ⎟ = 2.409 × 10−7 × 16.493PyCO exp⎜ 2 ⎝ 8.315T ⎠ qCO ,ACB qN ,ACB ⎞6.614 ⎛ 2 × ⎜1 − − 2 ⎟ ⎝ 16.493 14.346 ⎠

(3)

qN ,ACB = qm,N ,ACBK N2,ACBPyN 2

2

2

a N ,ACB ⎛ qCO ,ACB qN ,ACB ⎞ 2 2 2 ⎟ × ⎜⎜1 − − qm,CO ,ACB qm,N ,ACB ⎟⎠ ⎝ 2 2

⎛ 17934 ⎞ ⎟ = 2.171 × 10−7 × 14.346PyN exp⎜ 2 ⎝ 8.315T ⎠ qCO ,ACB qN ,ACB ⎞7.604 ⎛ 2 × ⎜1 − − 2 ⎟ ⎝ 16.493 14.436 ⎠

(5)

where Δq is the working capacity; q(Pads) is the adsorption capacity under adsorption pressure; q(Pdes) is the adsorption capacity under desorption pressure. In the existing coal-fired power plant, after desulfurization and dehumidification of the flue gas, the CO2 concentration in the dried flue gas is 15−17%, and CO2 is captured in the first VPSA unit at near-atmospheric pressure (0.117 MPa), so the CO2 partial pressure in the feed is estimated as 20 kPa for adsorption. The desorption pressure is designed as 8 kPa for the regeneration of zeolite 13X APG, and the temperature of flue gas is about 303 K. According to the cyclic adsorption/desorption conditions designed for the first VPSA unit, the working capacities for CO2 and N2 on zeolite 13X APG and pitch-based ACBs were estimated by eqs 1−5, as reported in Table 3. It was found that the CO2 working capacity on zeolite 13X APG is higher than that on ACBs for carbon capture from the flue gas of the investigated coal-fired power plant. In the second VPSA unit, the CO2-rich gas obtained in the first unit with 70−80% purity is fed to the ACB-packed column, and CO2 is adsorbed in the second VPSA unit at a given pressure of 120 kPa, so the CO2 partial pressure in the feed is estimated as 90 kPa for adsorption. The desorption pressure can be set at a relatively high value (about 20 kPa) to reduce the power consumed by the vacuum pump. The CO2 working capacities between 90 kPa for adsorption and 20 kPa for desorption in the second VPSA unit demonstrate that pitch-based ACBs have a larger CO2 working capacity than that of zeolite 13X APG. Therefore, ACBs are a better adsorbent material for the second VPSA unit. Based on a comparison of the CO2 working capacities of zeolite 13X APG and pitch-based ACBs, it is recommended that zeolite 13X APG be packed in the first VPSA unit to capture CO2 from flue gas with a high efficiency, whereas ACBs are a better adsorbent material for the second VPSA unit to concentrate CO2 from the CO2-rich stream. A schematic flow sheet of the two successive VPSA units is illustrated in Figure 4, where the effluent gas from the adsorber in the second VPSA unit is recycled to the inlet of the first VPSA unit to increase the CO2 recovery from the flue gas. Dried flue gas with 15−17% CO2, fed to the first VPSA unit, is concentrated to 70−80%, and the CO2-rich stream obtained from the first VPSA unit is further concentrated in the second VPSA unit to >95%, giving a product stream that can be directly applied for the compression and storage step. 3.2. Experiment and Simulation for CO2 Capture from Flue Gas by Two Successive VPSA Units. As mentioned previously, the pilot-scale carbon capture plant consists of two successive VPSA units coupled with a dehumidifying unit. When desulfurized flue gas with a relative humidity of 70−90% at 303−323 K passes through the dehumidifying unit at a flow rate of 35.5−37.0 N m3/h, water vapor and impurities (SO2, NOx) are removed by alumina adsorption, the relative humidity in the dried flue gas is controlled at less than 0.5%, and the CO2 concentration in the dried flue gas is measured as 16−16.5% in the existing coal-fired power plant. According to our previous works,38,39 the process parameters are defined as

qCO ,13X = qm,CO ,13X K CO2,13X PyCO 2

i = CO2 , N2

(4)

where q is the adsorption capacity of CO2 or N2 on 13X APG or ACBs; qm is the maximum amount adsorbed of pure gas CO2 or N2 on 13X or ACBs; K is the adsorption constant; a is the number of neighboring sites occupied by a molecule; P is the gas pressure; y is the percentage composition of CO2 or N2 in gas phase.

t

CO2 purity =

7950

t

∫0 blow CCO2u|Z = 0 dt + ∫0 purge CCO2u|Z = 0 dt t

t

∫0 blow Ctu|Z = 0 dt + ∫0 purge Ctu|Z = 0 dt

(6)



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Figure 2. Cyclic time sequences in the (a) first and (b) second VPSA units. t

CO2 recovery =

t

∫0 blow CCO2u|Z = 0 dt + ∫0 purge CCO2u|Z = 0 dt t

∫0 feed

CO2 productivity =

tblow

(∫

0

+ t press

(1 − r )

Wvacuum =

CCO2u|Z = 0 dt

CCO2u|Z = 0 dt + ∫ 0

t purge

(7)

(11)

CCO2u|Z = 0 dt )A

The notation for the symbols in eqs 6−11 can be found in our previous works.38,39 Dried flue gas with a relative humidity of less than 0.5% passes through the first VPSA unit for CO2 capture, where three beds are packed with 261 kg of zeolite 13X APG and three-bed eight-step VPSA process is employed (adsorption/ desorption pressures: 0.117 MPa/8 kPa). In the first VPSA unit, 85−95% CO2 can be recovered from flue gas with 73%∼82% CO2 purity in product gas. But the CO2 purity in the product gas is lower than the required specification (>95%) for further CO2 compression, liquefaction and storage (>95%). So, the CO2-rich stream obtained in the first VPSA unit enters the second VPSA unit for further concentration, where two beds are packed with 68 kg of pitch-based ACBs operated as a twobed six-step VPSA process (adsorption and desorption pressures = 122 and 20 kPa, respectively). The experimental results for CO2 capture from flue gas by two successive VPSA units are summarized in Table 4. It was found that, when flue gas of 35.5 N m3/h passes through the pilot-scale carbon capture plant, 90.2% of the CO2 can be recovered from the flue gas with 95.6% CO2 purity in the product gas, and the power

t totalwads (8)

The theoretical definition of power consumption for VPSA processes is t

specific power ave =

∫0 press (∫

0

+ t feed + t rinse

t blow

Wblower dt + ∫ 0

CCO2u|Z = 0 dt + ∫ 0

t blow + t purge

t purge

Wvacuum dt

CCO2u|Z = 0 dt )A

(9)

where Wblower represents the power consumption of the blower during the pressurization and high pressure feed step and Wvacuum represents the power consumption of the vacuum pump in the blowdown and purge steps, which can be described by the equations Wblower

γ − 1/ γ ⎡⎛ ⎤ P ⎞ P γ − 1⎥u(0)πR c 2 blow RTblow ⎢⎜ atm ⎟ ⎢⎝ Pblow ⎠ ⎥ γ−1 R gTblow ⎣ ⎦

γ − 1/ γ ⎡⎛ ⎤ Pfeed ⎞ P γ ⎢ = − 1⎥u(0)πR c 2 feed RTfeed ⎜ ⎟ ⎢⎣⎝ Patm ⎠ ⎥⎦ γ−1 R gTfeed

(10) 7951



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Figure 3. Adsorption equilibrium isotherms of CO2 and N2 on zeolite 13X APG and pitch-based ACBs.

Table 3. Working Capacities for the Adsorption of CO2 and N2 on Zeolite 13X APG and Pitch-Based ACBs first VPSA unit

second VPSA unit

adsorbent

gas

adsorption capacity (20 kPa) (mmol/g)

desorption capacity (8 kPa) (mmol/g)

Δqi (303 K) (mmol/g)

adsorption capacity (90 kPa) (mmol/g)

desorption capacity (20 kPa) (mmol/g)

Δqi (303 K) (mmol/g)

13X APG

CO2 N2 CO2 N2

3.05 0.030 0.83 0.23

2.30 0.000068 0.35 0.0065

0.75 0.030 0.48 0.23

4.25 0.0028 2.26 0.037

3.11 0.00037 0.89 0.0025

1.14 0.0024 1.37 0.034

ACBs

Figure 4. Arrangement for two types of adsorbents in two successive VPSA units.

Figure 5. Temperature profiles at packed heights of 0.25, 0.55, and 0.9 m in two successive VPSA units during a single cycle in cyclic steady state.

consumption (for a blower and two vacuum pumps) was measured onsite by watt meters as 2.44 MJ/kg of CO2. Furthermore, the experimental data were compared with the simulation results calculated by the mathematical model described before.39,40 In the two successive VPSA units, the effluent gas from the adsorber in the second VPSA unit is recycled to the inlet of the first VPSA unit to increase the CO2 recovery ratio from the flue gas. The recirculation results in a fluctuation of the CO2 concentration in the feed to the first VPSA unit from 16% to 18−20%. During the simulations, the CO2 concentration in the feed was set at an average value. As shown in Table 4, the CO2 purity in the product gas and the CO2 recovery ratio from the flue gas predicted by the model are in good agreement with the experimental data. The CO2 recovery from the flue gas was found to decrease with increasing feed flow rate, and the the CO2 purity in the product gas

increased because of the greater amount of CO2 adsorbed in the packed column during the same feed time, resulting in a decrease of the power consumption. When the purge flow rate was increased from 20 to 50 SLPM (cycles 1 and 3), the CO2 recovery from flue gas was increased, but the product CO2 purity decreased by about 3%. Therefore, a tradeoff should be considered among CO2 purity, recovery, and power consumption when using the light product (N2) gas to purge. The operating conditions for CO2 capture by the pilot-scale plant were designed previously by simulations using a mathematical model that was described before.39,40 Because the CO2 recovery from flue gas is set above 90%, the CO2 purity in product gas is expected to be more than 95% to reduce the 7952



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Table 4. Experimental and Numerical Results for CO2 Capture from Flue Gas in an Existing Coal-Fired Power Plant Using the Two Successive Pilot-Scale VPSA Units first-stage unit

feed

second-stage unit

two-stage VPSA

yCO2,feed Q Qfeed Pfeed purity recovery power consumption Pfeed purity power consumption purity recovery power consumption Poutlet purity (%) [MJ/(kg of CO2)] (kPa) (%) [MJ/(kg of CO2)] (%) (%) [MJ/(kg of CO2)] cycle (kPa) (N m3/h) (%) (SLPM) (kPa) (%)

a

1

126

37.0

16.0

2

122

35.5

16.5

3

126

37.0

16.0

20

118 116

50

118

79.4 79.6 74.5 74.9 72.4 71.8

83.7 85.2 90.2 91.3 88.2 89.0

2.22 0.587 2.04 0.584 2.10 0.586

125 123 125

98.8 96.8 95.6 95.2 96.0 95.8

0.43 0.174 0.40 0.172 0.41 0.173

98.8 96.8 95.6 95.2 96.0 95.8

83.7 85.2 90.2 91.3 88.2 89.0

2.65 0.761 2.44 0.756 2.51 0.758

expt sima expt sima expt sima

Simulations were performed with the mathematical model described previously.39,40

Table 5. Comparison of Performances among Several Experimental Processes for CO2 Capture from Flue Gasa process

adsorbent

two-stage VPSA PTSA PSA VPSA two-stage PSA TSA MEA absorption

first stage, 13X APG; second stage, ACBs CaX-type zeolite 13X 5A 13 X 5A −

a

yCO2,feed (%)

QF

desorption conditions

16.0

35.5−37.0 N m3/h

11.5 12.6 15.0 10.5

1000 N m3/h 120 L/min 32.1−45.9 N m3/h 110 N m3/h

10 13

20 N dm3/min −

first stage, 7−8 kPa; second stage, 20 kPa 0.05−0.15atm, 323−373 K 5−6 kPa 5.5 kPa first stage, 6.67 kPa; second stage, 13.34 kPa 423 K −

CO2 purity CO2 recovery (%) (%)

power consumption

ref

result type

95.6

90.2

2.44 MJ/kg of CO2

this work

expt

99 90−95 71−81 99

90 60−70 79−91 80

2.02 kJ/kg of CO2 6−10 kW/TPDc 2.64−3.12 MJ/kg of CO2 2.3−2.8MJ/kg of CO2

29 42 40 16

expt expt expt expt

≥94 >99

75−85 90

6.12−6.46MJ/kg of CO2 3−6MJ/kg of CO2

43 14, 44

expt −

All experiments for CO2 capture were carried out at near-atmospheric pressure.

as shown in Table 4. The theoretical power consumption is predicted as 0.756−0.761 MJ to capture 1 kg of CO2 from flue gas, so the real power consumption by the blower and two vacuum pumps is greater than the theoretically predicted value. Moreover, it was found that the power consumption of the reciprocating pump in the first VPSA unit accounts for one-half of the total measured power consumption, so it is very important to improve the mechanical properties of the vacuum pump operating under high vacuum to reduce the power consumption. The experimental results obtained in this pilot demonstration were also compared with those reported in literature for postcombustion CO2 capture by various adsorption processes and configurations, as listed in Table 5. Based on our experimental results and published works, zeolite 13X APG, 5A molecular sieve, and carbon materials are candidate adsorbent materials for CO2 capture from flue gas, because these materials have high CO2 adsorption capacities, good selectivities, and low costs. The power consumption in this study is comparable to that of others, and the power consumption for the VPSA process is relatively lower than that for the amine scrubbing process (3−4.6 MJ/kg of CO2)41 and membrane-based processes,10 suggesting that adsorption processes are effective and economical for the industrial application of CO2 capture from flue gas. The captured CO2 can be directly employed for pressurization, liquefaction, transportation, and storage steps in potential reservoirs. In this work, the choice of adsorbent materials, zeolite 13X APG packed in the first unit and ACBs packed in the second unit, was based on a comparison of the working capacities of the two adsorbents for each capture unit. For precise choice of adsorbents, both the competitive adsorption equilibrium and the adsorption/desorption kinetics should be taken into consideration. For simplification, the possible packed configurations in a two-stage VPSA unit (13X APG−ACB, 13X APG− 13X APG, ACB−ACB, ACB−13X APG) are compared based

costs of CO2 compression, liquefaction, and storage. Figure 5 shows the temperature profiles at packed heights of 0.25, 0.55 and 0.9 m in two successive VPSA units during a single cycle (in cyclic steady state). In the first VPSA unit, the temperature peak is very small at a packed height of 0.55 m, which means that the CO2 adsorption front is not allowed to advance to the packed height to ensure a high CO2 recovery from flue gas. in contrast, the temperature peaks in the ACB-packed column are higher at packed heights of 0.55 and 0.9 m, which means that the CO2 adsorption front can advance along the packed height, because the effluent gas with a small amount of CO2 from the adsorber in the second VPSA unit is recycled to the inlet of the first VPSA unit to ensure a high CO2 recovery. However, the recycled amount must be limited to reduce the power consumption. The power equipment used in the pilot-scale carbon capture plant includes a blower, a reciprocating pump, and a diaphragm pump, which were selected at the small industrial scale. As already mentioned, part of the effluent gas from the adsorber in the first VPSA unit is heated (about 473 K) and passes through the dehumidifying unit to regenerate alumina. The thermal energy consumption to heat the effluent gas is not included in the power consumption because of the use of the waste heat source for heating in the coal-fired power plant. In this experiment, the blower is coupled with a variable-frequency drive to adjust the flow rate of flue gas to save the power consumption. A reciprocating pump with a rated power of 4 kW is used for the regeneration of zeolite 13X APG under 8 kPa vacuum pressure in the first VPSA unit and simultaneously supplies the CO2-rich stream to the second VPSA unit at 0.122−0.126 kPa. A diaphragm pump with a rated power of 1.1 kW is employed for the regeneration of pitch-based ACBs under 20 kPa vacuum pressure in the second VPSA unit. The total power consumption (blower and two vacuum pumps) is measured onsite by watt meters as 2.44−2.65 MJ to capture 1 kg of CO2 from flue gas, 7953



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(3) World Energy Outlook 2006; International Energy Agency: Paris, France, 2006. (4) Meeting the Energy Challenge: A White Paper on Energy; TSO: Norwich, U.K., 2007; available at http://webarchive.nationalarchives. gov.uk/+/http://www.berr.gov.uk/energy/whitepaper/page39534. html, accessed July 2009. (5) Haszeldine, R. Carbon Capture and Storage: How Green Can Black Be? Science 2009, 325, 1647. (6) MacDowell, N.; Florin, N.; Buchard, A.; Hallett, J.; Galindo, A.; Jackson, G.; Adjiman, C.; Williams, C.; Shah, N.; Fennell, P. An Overview of CO2 Capture Technologies. Energy Environ. Sci. 2010, 3, 1645. (7) Aaron, D.; Tsouris, C. Separation of CO2 from Flue Gas: A Review. Sep. Sci. Technol. 2005, 40, 321. (8) Rochelle, G. Amine Scrubbing for CO2 Capture. Science 2009, 325, 1652. (9) Yu, C.-H.; Huang, C.-H.; Tan, C.-S. A Review of CO2 Capture by Absorption and Adsorption. Aerosol. Air Qual. Res. 2012, 12, 745. (10) Sublet, J.; Pera-Titus, M.; Guilhaume, N.; Farrusseng, D.; Schrive, L.; Chanaud, P.; Siret, B.; Durécu, S. Technico-Economical Assessment of MFI-Type Zeolite Membranes for CO2 Capture from Post Combustion Flue Gases. AIChE J. 2012, 58, 3183. (11) Na, B.; Ko, K.; Eum, H.; Lee, H.; Song, H. CO2 Recovery from Flue Gas by PSA Process Using Activated Carbon. Korean J. Chem. Eng. 2001, 18, 220. (12) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; VCH: New York, 1994. (13) Kikkinides, E.; Yang, R. T.; Cho, S. Concentration and Recovery of Carbon Dioxide from Flue Gas by Pressure Swing Adsorption. Ind. Eng. Chem. Res. 1993, 32, 2714. (14) Ho, M. T.; Allinson, G. W.; Wiley, D. E. Reducing the Cost of CO2 Capture from Flue Gases Using Pressure Swing Adsorption. Ind. Eng. Chem. Res. 2008, 47, 4883. (15) Kim, J.-N.; Park, J.-H.; Beum, H.-T.; Han, S.-S.; Cho, S.-H. PSA Processes for Recovery of Carbon Dioxide. In Greenhouse Gas Control Technologies-6th International Conference; Gale, J., Kaya, Y., Eds.; Pergamon Press: Oxford, U.K., 2003; p 1563. (16) Cho, S.; Park, J.; Beum, H.; Han, S.; Kim, J. A 2-Stage PSA Process for the Recovery of CO2 from Flue Gas and Its Power Consumption. Stud. Surf. Sci. Catal. 2004, 153, 405. (17) Chou, C.; Chen, C. Carbon Dioxide Recovery by Vacuum Swing Adsorption. Sep. Purif. Technol. 2004, 39, 51. (18) Chang, D.; Min, J.; Moon, K.; Park, Y. K.; Jeon, J. K.; Ihm, S. K. Robust Numerical Simulation of Pressure Swing Adsorption Process with Strong Adsorbate CO2. Chem. Eng. Sci. 2004, 59, 2715. (19) Yang, R. T. Gas Separation by Adsorption Processes; Butterworths: Boston, MA, 1987. (20) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley & Sons: New York, 1984. (21) Agarwal, A.; Biegler, L. T.; Zitney, S. A Superstructure-Based Optimal Synthesis of PSA Cycles for Post-Combustion CO2 Capture. AIChE J. 2009, 56, 1813. (22) Jiang, L.; Fox, V.; Biegler, L. T. Simulation and Optimal Design of Multiple-Bed Pressure Swing Adsorption Systems. AIChE J. 2004, 50, 2904. (23) Dowling, A. W.; Vetukuri, S. R. R.; Biegler, L. T. Large-Scale Optimization Strategies for Pressure Swing Adsorption Cycle Synthesis. AIChE J. 2012, 58, 3777. (24) Youssef, B.; Rodrigo, S.; Abdelhamid, S. Adsorption of CO2 from Dry Gases on MCM-41 Silica at Ambient Temperature and High Pressure. 1: Pure CO2 Adsorption. Chem. Eng. Sci. 2009, 64, 3721. (25) Shao, X.; Feng, Z.; Xue, R.; Ma, C.; Wang, W.; Peng, X.; Cao, D. Adsorption of CO2, CH4, CO2/N2 and CO2/CH4 in Novel Activated Carbon Beads: Preparation, Measurements and Simulation. AIChE J. 2011, 57, 3042. (26) Zhang, Z.; Xian, S.; Xia, Q.; Wang, H.; Li, Z.; Li, J. Enhancement of CO2 Adsorption and CO2/N2 Selectivity on ZIF-8 via Postsynthetic Modification. AIChe J. 2013, 59, 2195.

on simulation results, where the operating conditions are the same as for cycle 2 in Table 4 used for the 13X APG−ACB configuration. The simulated results are summarized as follows: (1) For the 13X APG−13X APG configuration, a CO2 purity of 98% (higher than that for 13X APG−ACB) is obtained with a CO2 recovery of 90%. (2) For the ACB−13X APG configuration, a CO2 purity of 93% is obtained with a CO2 recovery of 96%. (3) For the ACB−13X APG configuration, a CO2 purity of 81% is obtained with a CO2 recovery of 96%. When the first unit is packed with ACBs, the CO2 recovery can be increased to 96%, and the predicted power consumption is decreased by 19% for the first stage. Further experimental validation is needed in the future.

4. CONCLUSIONS A pilot demonstration with two successive VPSA units for CO2 capture from flue gas was performed successfully in an existing coal-fired power plant. According to the competitive adsorption equilibria of CO2 and N2 on zeolite 13X APG and pitch-based ACBs, zeolite 13X APG was packed in the first VPSA unit, and pitch-based ACBs were packed in the second VPSA unit. When desulfurized flue gas passes through the dehumidifying unit at a flow rate of 35.5−37.0 N m3/h, water vapor and impurities can be removed by alumina adsorption, and the relative humidity in the dried flue gas can be controlled at less than 0.5%. When dried flue gas with less than 0.5% humidity and 15.5−16.5% CO2 passes through the pilot-scale carbon capture plant, CO2 can be concentrated up to 70−80% in the first VPSA unit and then further enriched to over 95% in the second VPSA unit. With the designed three-bed eight-step VPSA process for the first unit and two-bed six-step VPSA process for the second unit, 90.2% of the CO2 can be recovered with a purity of 95.6% in the product gas at a feed flow rate of 35 N m3/h with a total specific power consumption of 2.44 MJ/kg of CO2. Numerical simulations with a multibed VPSA modeling framework are in good agreement with the experimental results for the predictions of CO2 recovery and purity. It was found that the power consumption of the vacuum pump in the first VPSA unit accounts for one-half of the total power consumption in the carbon capture experiment, so it is very important to reduce the power consumption of the vacuum pump under a high vacuum to improve this process in the future.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support from the China 863 Program (Grant 2008AA062302). The authors are grateful to Shenergy Xinhuo Thermal Power Co. Ltd. for providing the experimental place.

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REFERENCES

(1) IPCC Special Report on Carbon Dioxide Capture and Storage; Cambridge University Press: Cambridge, U.K., 2005. (2) Albo, J.; Luis, P.; Irabien, A. Carbon Dioxide Capture from Flue Gases Using a Cross-Flow Membrane Contactor and the Ionic Liquid 1-Ethyl-3-methylimidazolium Ethylsulfate. Ind. Eng. Chem. Res. 2010, 49, 11045. 7954



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CO2 Capture from Flue Gas in an Existing Coal-Fired Power Plant by

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