A site trial demonstration of CO2 capture from real flue gas by novel carbon fibre composite monolith adsorbents

International Journal of Greenhouse Gas Control 42 (2015) 415–423

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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc

A site trial demonstration of CO2 capture from real flue gas by novel carbon fibre composite monolith adsorbents Ramesh Thiruvenkatachari, Shi Su ∗ , Xin Xiang Yu, Yonggang Jin Commonwealth Scientific and Industrial Research Organisation (CSIRO), 1, Technology Court, Pullenvale, Qld 4069, Australia

a r t i c l e

i n f o

Article history: Received 23 June 2015 Received in revised form 24 August 2015 Accepted 25 August 2015 Keywords: Carbon composite CO2 capture Flue gas Site trial Adsorption and regeneration

a b s t r a c t A carbon fibre composite solid sorbent CO2 capture prototype unit was developed and site trialled at a coal-fired power station using actual flue gas. Over 200 test cycles of adsorption and sorbent regeneration were performed to evaluate the performance of the adsorbents. Under the study conditions, the CO2 adsorption efficiency of the carbon fibre composite adsorbents was found to be consistently over 98% with or without flue gas pre-treatment. The adsorption performance of the solid sorbents was maintained even after more than 200 tests and it was demonstrated for the first time that the carbon fibre composite solid sorbents were very stable under real flue gas conditions without any noticeable impact of SO2 and NOx on their CO2 adsorption performance. CO2 desorption efficiency of the adsorbent material using combined heat and vacuum swing was found to be between 90 and 95%. Besides, the adsorbents exhibited consistently very high removal performance of SO2 and NOx from flue gas and can very well eliminate the need for conventional chemical based caustic scrubber pre-treatment systems, which is generally required for solvent based capture processes. The carbon composite adsorption systems also have potential to be a pre-treatment or pre-cleaning technology, particularly for Australian coal-fired power stations which lack flue gas de-sulphurisation and de-nitrification facilities. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Growing concerns for global warming and climate changes have attracted widespread efforts to develop efficient and cost-effective technologies for post-combustion capture (PCC) of CO2 from large point sources, such as coal-fired power plants. PCC is viewed as one of the key technology options to reduce greenhouse gases (GHG), because it has the potential to be retrofitted to existing coalfired power stations without requiring substantial changes to the combustion process. Among various PCC technologies for CO2 capture from flue gas, use of porous solid adsorbents is showing great promise in reducing anthropogenic CO2 emissions (Samanta et al., 2012; Sayari et al., 2011; Wang et al., 2011a,b; D’Alessandro et al., 2010). A variety of porous solids such as metal-organic frameworks (Sumida et al., 2012; Wang et al., 2008; Yang et al., 2012; Banerjee et al., 2008), covalent organic frameworks (Mohanty et al., 2011), zeolites (Chue et al., 1995), alumina (Chen and Ahn, 2011; Lee et al., 2011), amine-functionalized silicas (Chen et al., 2009), and porous carbons (Silvestre-Albero et al., 2011; Thiruvenkatachari et al., 2009, 2013; Hao et al., 2010, 2011; Sevilla and Fuertes, 2011;

∗ Corresponding author. E-mail address: [email protected] (S. Su). http://dx.doi.org/10.1016/j.ijggc.2015.08.018 1750-5836/© 2015 Elsevier Ltd. All rights reserved.

Sevilla et al., 2011; Xia et al., 2011; Zhao et al., 2010; Gonzalez et al., 2013; Plaza et al., 2010), have been extensively investigated. To be competitive with other available technologies, porous solid sorbents must be of low-cost, offer substantially greater adsorption capacities and selectivity for CO2 , be less sensitive to toxic materials like SOx , NOx and moisture in the gas stream, readily able to be regenerated without compromising on the performance during repeated cycles of capture and discharge operations and possess good thermal and mechanical properties. In an US DoE funded study (Sjostrom and Krutka, 2010; Sjostrom et al., 2011), evaluating over 100 different potential CO2 solid sorbents under similar conditions using flue gas of 10–12% CO2 , saturated moisture (90% relative humidity), 5–6% O2 , 100–120 ppm NOx , and 50–250 ppm SO2 , it was observed that even though carbon-based sorbents had a lower CO2 capacity, they exhibited superior cyclic stability and greater tolerance to impurities in flue gas, in comparison to amine supported materials and zeolites (Ruiz et al., 2013). Carbon based adsorbents with its low cost and high chemical, thermal and mechanical stability (Wang et al., 2011a,b; D’Alessandro et al., 2010; Plaza et al., 2010) are proving to be a more suitable candidate for use in real coal-fired flue gas streams, which typically contain 75–77% N2 , 9–15% CO2 , 5–8% H2 O, 4–6% O2 , trace amount of CO, NOx , and SO2 gases and fly-ash dust. Such adsorbents are potentially even more desirable for Australia’s coal-fired

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power stations, where there are no flue gas de-sulphurisation and de-nitrification facilities (Dave et al., 2011). Given the large volume of flue gas to be treated (for a typical 350 MW unit, about 425 m3 s−1 (Cottrell et al., 2010)), the adsorption system using structured monolith solid sorbents, such as the honeycomb shaped sorbents with flow through channels, is generally more suited for this application than the packed bed or fluidized bed reactors, as it allows the use of dust laden flue gas at high flow rates without large pressure drop, offers greater geometric surface area for better gas–solid contact and ease of scale-up, and can avoid the fluidisation of adsorbents. Depending on the type of adsorbent system and the adsorption operating conditions, desorption occurs in different ways such as pressure or vacuum swing adsorption (P/VSA), thermal swing (TSA) and electrical swing (ESA) that may also be combined (Thiruvenkatachari et al., 2013; Gonzalez et al., 2013; Yu et al., 2004; Gupta and Gosh, 2015; Chaffee et al., 2007; Maring and Webley, 2013). Carbon based adsorbents studied for post-combustion CO2 capture from flue gas so far have been evaluated using simulated flue gas and under laboratory conditions. Table 1 summarises the performance of carbon adsorbents evaluated for flue gas CO2 adsorption and regeneration reported in some of the earlier studies. A study by Wang et al. (2013), carried out site trial investigation at the power plant with activated carbon. However, the CO2 concentration in the flue gas sent to the activated carbon adsorbent was 70–80% and was not subjected to actual flue gas conditions. Experiences from actual site trial studies using real flue gas operated over a number of adsorption and regeneration cycles are critical for the development of carbon based solid sorbent CO2 capture technology. A number of existing and planned large-scale demonstration plants (equipped with a capture capacity of 1 MtCO2 /year) for the post-combustion capture of CO2 from coal-fired power plants are solvent based (Haszeldine, 2009). Here we present for the first time the results from a CO2 capture site trial demonstration using CSIRO developed honeycomb monolithic carbon fibre composite solid adsorbents at a coal-fired power station in New South Wales, Australia. The stability and performance of these carbon composite adsorbents were evaluated for an extended period of time under actual field conditions subjected to real flue gas containing a mixture of various gas constituents over a number of cycles of adsorption combined with thermal and vacuum swing regeneration. This study aims to provide an insight into the material performance under actual field conditions for further development of this novel CO2 capture technology. Based on the outcomes from this study, the application of these adsorbents can be expanded to gas purification and gaseous contaminant removal from other industries. Such a study also helps to bridge the

gap between fundamental lab-scale experiments and large-scale demonstrations. 2. Experimental 2.1. Flue gas and adsorbent material The composition of major components in the raw flue gas from the coal-fired power station used in this study is given in Table 2. The honeycomb carbon fibre composites were fabricated using the raw materials petroleum pitch carbon fibre (Sinocarb CO. Ltd., China) and phenolic resin (Durez 7716, USA). A series of preparation steps such as moulding, drying and curing, carbonisation and physical activation were applied for the preparation of these carbon composite adsorbents. A detailed description of the adsorbent fabrication procedure, including the temperature and duration of each of the preparation steps have been discussed previously in Thiruvenkatachari et al. (2009, 2013). The fabricated adsorbents were 123 mm in diameter and a series of 150 mm length composites were stacked inside the 2 m long adsorbent column. A series of flow-through gas channels (267 channels of each 2.5 mm diameter each) for the passage of flue gas and a series of channels (13 of each 12.7 mm diameter) to indirectly heat and cool the adsorbents, were present, giving a honeycomb structure to the adsorbent. Stainless steel tubes were used to pass hot air and cold water through the 13 channels in the adsorbents. Total weight of the carbon fibre composite adsorbent inside the adsorbent chamber was 4.891 kg. 2.2. Prototype test unit for site trial The pilot prototype test unit designed to treat up to 200 SLPM of flue gas was developed and installed near the flue stack at the coal-fired power station. A simple schematic of the prototype test unit is shown in Fig. 1. The test unit extracted the flue gas from the stack using a fan (F78, Aerotech Pty. Ltd., Australia), filtered the dust using the HEPA cartridge filter (CSL-HE850Q, Solberg, USA), and pre-treated the flue gas in the scrubber wash column using dilute caustic solution (1% sodium hydroxide solution at around pH 9, counter-current to flue gas) to remove (absorb) acid gas contaminants and provide the pre-treated flue gas to the adsorbent column (2 m length, 0.125 m diameter). During the adsorption (CO2 capture) step, the concentrations of gases at the exit of the adsorption column were monitored and when the CO2 concentration in the column outlet began to increase (indicating breakthrough point), the adsorption process was stopped. The flue gas or air was heated using the furnace (Tetlow Kilns & Furnaces Pty. Ltd., Australia) to supply the hot gas (up to 423 K) for adsorbent regeneration and CO2 desorption. The adsorption

Fig. 1. Schematic of the solid sorbent CO2 Capture and regeneration prototype unit with flue gas pre-treatment facility [red represents adsorbent regeneration line and green represents gas capture line with gas bypass represented by green broken line. 1) flue stack, 2) blower, 3) dust filter, 4) furnace, 5) scrubber wash column, 6) heat exchanger, 7) volume meter, 8) carbon composite adsorbent column, 9) Vacuum pump].

Adsorbent type

Scale/Adsorber type

Pitch based activated carbon

Lab Scale/fixed bed

Activated carbon

Lab Scale/fixed bed

Norit activated carbon

Lab Scale/fixed bed

Pitch based activated carbon

Field/fixed bed

Commercial activated carbon pellets

Lab Scale/fixed bed

Biomass activated carbon

Lab Scale/fixed bed

Norit activated carbon

Lab Scale/sound assisted fluidised bed Lab scale/honeycomb monolith fixed bed Large scale/honeycomb monolith fixed bed

CSIRO carbon nanotube composite

CSIRO carbon fibre composite

Gas composition

CO2 adsorption capacity

Regeneration

Ref.

Analysis conditiona

mmol g−1

Method

Simulated flue gas, 15% CO2 in N2 Simulated flue gas, 17% CO2 , 79% N2 , 4% O2 Simulated flue gas, 17% CO2 in N2

Pure CO2 @15 kPa, 303 K Pure CO2 @15 kPa, 298 K

1.1

TSA VSA ESA

75–80

76–100

0.75

PSA

99.8

34

17 vol% CO2 , bal N2 , 303 K

0.77

TSAVSAVTSA

43

Plaza et al. (2010)

Enriched flue gas 74.5% CO2 Simulated flue gas, 15% CO2 in N2 Simulated flue gas, 14% CO2 in N2 Simulated flue gas, 15% CO2 in N2

Pure CO2 @15 kPa, 303 K 15 v% CO2 , bal N2 , 320 K Pure CO2 @15 kPa, 298 K 15 vol% CO2 , bal N2 , 298 K

0.7

VPSA

95.6

40 87 97 90.2

0.09

–

–

–

Ruiz et al. (2013)

1.02–1.08

–

–

–

0.37–0.55

–

–

–

Gonzalez et al. (2013) Raganati et al. (2014)

Simulated flue gas, 12% CO2 , 5% O2 , bal N2 Simulated flue gas, 13–15% CO2 , 5–6% O2 , bal N2

Pure CO2 @15 kPa, 298 K

1.18

–

–

–

Jin et al. (2013)

Pure CO2 @15 kPa, 298 K

0.92

TSAVSAVTSA

96

Thiruvenkatachari et al. (2013)

Max CO2 conc. (%)

∼100

Capture efficiency (%) Wang et al. (2011a,b) Na et al. (2001)

Wang et al. (2013)

a Reported pure CO2 at 15 kPa values represents the equilibrium CO2 adsorption capacity for single component CO2 at a partial pressure of 15 kPa measured at 298–303 K, which corresponds to a typical CO2 concentration in flue gas in the range of 10–15%.

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Table 1 Analysis of carbon adsorbents used for CO2 capture from flue gas from reported literatures.

417

418

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Fig. 2. Overview of process procedure for the solid sorbent CO2 capture and regeneration testing.

column was closed and adsorbents were heated by passing hot air through the regeneration tubes (13 × 12.7 mm tubes through the sorbents). Once the adsorbent temperature reached 383 K, the column was opened to extract the gas that has desorbed (regeneration step). A diaphragm vacuum pump (KNF pumps, Model N 035.1.2 AT.18) then expunged the desorbed gas from the adsorbent column (applied vacuum pressure 20–25 kPa). Cool water (288–298 K) was supplied to cool the adsorbents (to 298 K) after thermal regeneration using a chiller (Aquacooler, Model R540A3). The flue gas for adsorption was also able to by-pass the caustic scrubber pretreatment system to evaluate the CO2 capture performance of the adsorbents (at 298 K) without flue gas pre-treatment, with most of the site tests carried out under such conditions. After CO2 adsorption from flue gas, the adsorbents were flushed with pure CO2 (Coregas, Grade 4.5, 99.995% purity) to replace other co-adsorbed gases (like N2 and O2 ) and to enrich the column with CO2 . It was then followed by thermal and vacuum regeneration of the adsorbents, where the adsorbed gas was desorbed from the composite material and extracted out of the column and returned to the flue stack after analysing its composition. The composite adsorbents were then flushed with air and cooled to room temperature (by passing cold water (288–298 K) through the regeneration tubes) making the column ready for the next capture and regeneration cycle. A volume meter was used at both inlet (Ritter, Model BG10) and outlet (Landis + Gyr, Model DTM 750) of the adsorbent column to measure the gas volumes. The performance of the carbon fibre composite adsorbent CO2 capture system was evaluated with and without flue gas pre-treatment. The overall gas capture and regeneration strategy or the testing sequence involved in this study using the carbon fibre composite solid sorbents is given as a flow diagram in Fig. 2. Various operating conditions adopted during this site trial testing for the adsorption, CO2 flushing and desorption steps are shown in Table 3. Time for each step during a typical capture regeneration cycle is also shown in Table 3(b).

2.3. Adsorbent characterisation and control and monitoring system N2 and CO2 adsorption measurements were carried out on Tristar 3000 and ASAP 2020 (Micrometritics Instrument Corporation, USA) volumetric analyzers after the adsorbent samples were degassed overnight under vacuum (at 473 K). The Brunauer–Emmett–Teller (BET) surface areas (1061–1312 m2 g−1 ) and the Dubinin–Astakhov (DA) pore volumes (0.31–0.78 cm3 g−1 ) for the composites were obtained from N2 adsorption isotherms at 77 K (Thiruvenkatachari et al., 2013). Adsorption isotherms for CO2 were obtained at 273 K and 298 K. Pore size distributions of the adsorbents were calculated from CO2 adsorption isotherms at 273 K by the density function theory (DFT). Surface morphologies of the adsorbents were examined using scanning electron microscopy (SEM, Nava Nano 430).

In the solid sorbent prototype test unit, parameters such as temperatures, pressures, pH, volume, flow rates of gas and liquid, and gas compositions at inlet and outlet of the wash column and adsorption column, were monitored during the site trial operations. A single analyser to measure various gas compositions present at different concentrations during testing proved early on in the study to not be an accurate method of analysis. Therefore, individual sensors to measure CO2 (Anri Instruments Pty Ltd, Madur MDIR-D Gascard), SO2 (Thermo 43i-HL), NO and NO2 (Thermo 42i-HL), CO (Thermo 48i) and O2 (PMA 10, ANRI Instruments Pty Ltd.) were used to obtain accurate, continuous and consistent data. The CO2 capture and regeneration test unit coupled with pre-treatment system was able to be remotely operated from the computer. The operation of equipment such as blower, valves, flow meters etc., and continuous data logging of the readings from the thermocouple, pressure transducers and gas sensors were carried out through the control and monitoring system we developed using LabVIEW interface. 3. Results and discussion 3.1. CO2 adsorption capacities of CSIRO developed carbon composites Fig. 3 shows the single component CO2 equilibrium adsorption capacities (at different CO2 partial pressures up to 760 mm Hg) of the fabricated carbon fibre composites obtained from CO2 adsorption isotherms at different temperatures (273–343 K). The CO2 uptake was greater at lower temperatures, as expected. The equilibrium capacity at ambient temperature (298 K) for the carbon fibre composite was 2.713 mmol g−1 (mass uptake: 12 wt%), which was about twice that of a typical commercial bituminous coal-derived activated carbon (Thiruvenkatachari et al., 2013). At a lower partial pressure of 114 mm Hg (corresponding to CO2 concentration of 15%), which is the typical flue gas concentration, the CO2 adsorption capacity was 0.92 mmol g−1 (mass uptake: 4.05 wt%). SEM (Fig. 4) image of the surface morphology of carbon fibre composites shows an interconnected carbon fibres covered with resin-derived carbon.

Table 2 Characteristics of a typical coal-fired power station flue gas. Parameter Flow rate Pressure Temperature Gas composition N2 CO2 O2 H2 O CO SO2 NOx Dust

Unit

Values (range) 3

−1

Dry gas, 1 atm, m s bar K

160–172 0.97–0.99 388–425

vol% vol% vol% vol% ppmv ppmv ppmv mg m−3 (dry)

75.9 9.9–13.0 5.5–8 6.0 9–19 100–300 200–300 2.5

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419

Table 3 Operating parameters for the site trial CO2 capture testing. (a) test conditions adopted during various stages of the capture and regeneration cycle; and (b) typical cycle times adopted (flue gas flow rate 50 L/min, CO2 flushing flow rate 15 L/min, adsorbent heated to 383 K, Vacuum column to 25 kPa, adsorbent cooled to 298 K). (a) Adsorption

CO2 flushing

Desorption

Temperature, K

Pressure, kPa

Flow rate, l/min

Temperature, K

Pressure, kPa

Flow rate, l/min

Temperature, K

Vacuum, kPa

298

101

25–75

298

101

15–50

383

20–25

(b) Adsorption, s

CO2 flushing, s

Adsorbent heating, s

Regeneration (heat and vacuum), s

Adsorbent cooling, s

300

120

2400

600

1200

4

298 K

323 K

343 K

4

313 K

3.5

3 CO2 adsorbed, mmol g-1

Amount adsorbed, mmol g-1

3.5

273 K

2.5 2 1.5 1 0.5

3 2.5 2 1.5 1

CNT composites MNS composites

0.5

0 0

100

200

300 400 500 600 Absolute pressure, mm Hg

700

800

Carbon fibre composites 0 0

100

200

Fig. 3. CO2 adsorption isotherm profiles of carbon fibre composite adsorbents at different temperatures.

400

500

600

700

800

Pressure, mmHg Fig. 5. Advancement in CSIRO carbon based composites performance with improved CO2 adsorption capacities (data for MNS composite from Bae and Su, 2013).

operational cost (Ho et al., 2008). Pore size distribution profile of the carbon adsorbents shown in Fig. 6, reveals the presence of small ˚ which are mostly responsible for the CO2 micropores less than 6 A, capture capacity at 298 K and at atmospheric pressure (Lu et al., 2014; Wang et al., 2014; Marco-Lozar et al., 2014). To the best of our knowledge, the CO2 adsorption capacities achieved by CSIRO composites are among the highest, compared to other reported porous carbon based solid adsorbents prepared by physically activation.

0.08

Microporesize Distribution, cm-3 g-1 Å-1

Further research in the carbon composite development carried out in parallel with the current study, has produced new generation adsorbents using carbon nanotubes (CNT) modified carbon fibre composites (Jin et al., 2013) and macadamia nutshell (MNS) derived biomass carbon composites (Bae and Su, 2013). Both the CNT and MNS based carbon composites have shown enhanced CO2 adsorption capacities (at 298 K and up to 1 atm) compared to originally developed carbon fibre composite adsorbents (Fig. 5). Although CNT and MNS carbon composites achieved higher CO2 adsorption capacities, in the present power plant prototype site trial studies, only the carbon fibre composite adsorbents have been trialled in the current site trial studies. The development of carbon composites with improvements in CO2 adsorption capacities can reduce the process footprint (smaller adsorber volume) and capital and

300

CNT composites

0.07

MNS composites

0.06

Carbon fibre composites

0.05 0.04 0.03 0.02 0.01 0 0

1

2

3

4

5

6

7

8

9

10

Pore Width, Å

Fig. 4. SEM images of carbon fibre composite adsorbents.

Fig. 6. Pore size distribution comparison of CSIRO developed carbon based adsorbents (data for MNS composite from Bae and Su, 2013).

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100

(a)

Gas Concentrations

300

90

SO SO2 ppmv 2 NO2 ppmv NO 2 Flowrate Col Top Temp

CO2 vol% NO ppmv CO ppmv Bottom Col Temp

350

80 70

250

60

200

50

150

40 30

100

20

50

Temperature, °C and Flowrate L/min

400

10 0

0 0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Time, min

14

(b)

CO2 Concentration, v%

12 10 8 6 4 2 0 0

5

10

15

20

25

30

35

40

45

50

Time, min Fig. 7. Capture performance of carbon fibre composites with real flue gas without pre-treatment. (a) typical adsorption of various gases and the flue gas flowrate and adsorbent temperature profiles, (b) enhanced view of the CO2 concentration profile in (a) during the first 30 min of adsorption breakthrough curve.

3.2. CO2 capture site trial with actual flue gas The capture performance of the carbon fibre composite solid sorbent using raw coal fired power station flue gas without caustic scrubber pre-treatment is shown in Fig. 7. The flue gas was only passed through the cartridge dust filter to remove dust particulates in the flue gas down to less than 1 ␮m, however no SOx and NOx removal was carried out before entering the solid sorbent adsorption column. The CO2 concentration of the raw flue gas sent into the adsorption column for CO2 capture was about 11.5%. The CO2 adsorption efficiency of the carbon fibre composite adsorbents was found to be over 98% (Fig. 7) even in the presence of SO2 and NOx without flue gas pre-treatment. The adsorption efficiency was

evaluated as the amount of CO2 retained in the bed out of all the CO2 fed during adsorption step. As the gases were adsorbed, due to the exothermic nature of the process (Masel, 1996), the heat of adsorption (mainly from CO2 ) resulted in an increase in the adsorbent temperature, as observed from Fig. 7. As the flue gas entered the column from the bottom, the rise in adsorbent temperature was observed first at the bottom of the column prior to CO2 breakthrough (Fig. 7). The heat generated from adsorption progressed from the bottom to the top of the adsorbent column and the adsorbent temperature at the top of the column peaked when the CO2 concentration in the gas stream exited the column and breakthrough occurred. The adsorbents eventually reached the point where the column was fully

Table 4 Comparison of gas capture performance from flue gas with and without caustic scrubber pre-treatment for carbon fibre composite solid adsorbents. Raw Flue Gas

Composition

Units

Values

CO2 SO2 NO NO2

vol% ppmv ppmv ppmv

11.5 132.6 315 18

Composition after pre-treatment

Composition after adsorption with pre-treatment

Adsorption efficiency with pre-treatment, %

Composition after adsorption without pre-treatment

Adsorption efficiency without pre-treatment, %

10.95 3.3 290 15.24

0.01 0.05 0.2 0.4

99.91 98.48 99.93 97.38

0.01 0.15 0.25 0.5

99.91 99.89 99.92 97.22

CO2 CO2 vol% NO ppmv NO flowrate

100

CO2 Concentration, %

90

SO2 SO2 ppmv NO2 NO2 ppmv

350 300

80

250

70 200

60 50

150

40 100

30 20

50

10 0 00:00

03:57

07:53

11:50 15:47 Time, m:s

19:44

0 23:40

Fig. 8. Gas concentration profiles for the solid sorbent CO2 capture and regeneration process for actual raw flue gas.

saturated and the outlet CO2 concentration reached that of inlet value. It can be seen from Fig. 7 that the CO2 as well as SO2 , NO and NO2 in the flue gas stream were adsorbed by the solid sorbents. It should be noted that the response of the NOx sensor was slower compared to CO2 and SO2 sensors, thus causing a delay in the recorded drop in NOx concentration at the point of switching the flue gas into the adsorption column. The breakthrough of CO was noticed soon after the flue gas was passed through the adsorbents from the bottom of the adsorption column, whereas CO2 and the trace gases (SO2 and NOx ) continued to be adsorbed. Gradually, after CO2 breakthrough, the breakthrough of NO was noticed with an increase in its concentration at the outlet of column. However, no breakthrough for SO2 and NO2 were observed during the adsorption period under the study conditions. CO2 capture was also studied using pre-treated flue gas that had passed through both the dust filter and the caustic scrubber system. The dilute sodium hydroxide scrubber solution (operated at pH 9) mainly removed SO2 (97.5%), some NO (7.9%) and NO2 (15.3%), with a negligible change in CO2 and CO concentrations. The acid gases like SOx contained in the flue gas reacts with sodium hydroxide to form neutral salts like Na2 SO4 (USEPA, 2003; Vallero, 2014). The caustic solution required periodic replacement or replenishment in order to maintain the performance of the scrubber. Nevertheless, the adsorbents exhibited consistently very high removal performance (based on mole fraction) of CO2 (over 98%), SO2 (over 99%), NO (over 99%) and NO2 (over 97%) from flue gas with or without pre-treatment, as summarised in Table 4. In this respect, the solid adsorbents proved to be a much better and efficient pre-treatment system to remove both SO2 and NOx compared to the caustic scrubber that mainly removes SO2 . The adsorption efficiency values with pre-treatment in Table 4, is the ratio between the composition after adsorption and the composition after pre-treatment. 3.3. CO2 desorption and adsorbent regeneration The process of enriched CO2 recovery and adsorbent regeneration followed the flue gas adsorption step. Almost 100% CO2 was captured from the flue gas during the adsorption step and during pure CO2 flushing for CO2 enrichment. Upon application of combined thermal and vacuum swing regeneration, the concentration of desorbed CO2 was found to be over 97%, as shown in Fig. 8. The desorption efficiency (defined as the ratio of the amount of CO2 obtained during desorption stage to the total amount of CO2 into the adsorption column from the flue gas during adsorption step and pure CO2 flushing) was found to be between 90 and 95%.

421

100 CO2 Capture Efficiency, %

110

Gas Concentration, ppmv and Flowrate, L/min

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95 90 85 80 0

25

50

75 100 125 Run Number

150

175

200

Fig. 9. Performance and stability of carbon fibre composite solid adsorbents.

During regeneration, NO was observed in the desorbed gas (Fig. 8). The flowrate profile during the operation (adsorption, CO2 flushing and regeneration processes) is also shown in Fig. 8. The effect of small quantities of flue gas impurities like NO on the properties and behaviour of recovered CO2 in compression and pipeline transportation system is uncertain, and needs to be investigated. However, this is a similar issue faced by oxy-fuel CO2 capture technology (Wall et al., 2013). Interestingly, no SO2 was noted in the desorbed gas. SO2 could possibly be retained in the adsorbents or transformed, which requires further investigation. Similar to SO2 , desorption of NO2 was also not evident. Moreover, the Thermo 42i-HL analysis instrument used for NO and NO2 analyses, required at least 1% O2 in the gas stream for the analysis of NO2 concentration. However, this was not required for NO detection. With the application of vacuum regeneration, the column was quickly removed of O2 in the desorbed gas and measurement of NO2 concentration was not feasible. A systematic evaluation of the interactions of NO, NO2 , SO2 , O2 and H2 O with carbon and their influences between each other both in the adsorption and desorption processes is required. This is especially critical for further development of this material for flue gas cleaning purpose. Although pure CO2 flushing improved the purity of captured CO2 , the amount of the product CO2 used for flushing was about four times the amount of CO2 from the flue gas adsorbed by the adsorbent. Increase in purge quantity decreases the CO2 recovery whilst also resulting in energy loss associated with adsorption heat release and an increase the desorption energy required (Xiao et al., 2008). Hence this approach of CO2 flushing and the adsorption and regeneration method adopted needs further examination and optimisation for this solid sorbent technology development.

3.4. Adsorbent stability During the course of the site trial more than 200 adsorption and regeneration cycles were conducted to demonstrate the stability of the adsorbents to real flue gas. The CO2 adsorption efficiency from real flue gas was consistently over 98% for the solid sorbents (Fig. 9) throughout the site trial study period. This demonstrated the fact that even after 200 cycles the performance of solid sorbents in terms of CO2 adsorption was able to be maintained indicating the excellent stability of the carbon fibre composite adsorbents towards real flue gas without the pre-cleaning for SOx and NOx . The adsorbent material was able to withstand the cyclic heating and cooling cycles and performed repeated capture and regeneration process without any evidence of degradation. The CO2 desorption efficiency was also over 90%, further demonstrating the stable performance of the material.

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4. Conclusions For the first time, CO2 capture performance of a prototype unit with honeycomb monolithic carbon fibre composite adsorbents was evaluated with real flue gas at a coal-fired power station. Through over 200 cycles of adsorption and regeneration with untreated flue gas containing SOx and NOx , the carbon composite adsorbent has demonstrated a very stable performance, exhibiting high CO2 adsorption efficiency consistently over 98% and desorption efficiency of 90–95%. Similar performance was obtained with pre-treated flue gas as well. These results clearly indicate that these adsorbent material are very stable against impurities in the flue gas and effective in CO2 capture from the raw flue gas and subsequent CO2 discharge. The introduction of CO2 flushing prior to thermal and vacuum swing desorption was helpful in obtaining high-purity CO2 product (97%). Further process optimisation is required and an alternative regeneration process without the need of CO2 flushing must also be investigated. The carbon adsorbents were also found to effectively remove SO2 and NOx from flue gas and could serve as a more efficient pre-treatment or pre-cleaning system compared to the caustic scrubber that mainly removes SO2 and requires frequent replenishment. The carbon fibre composite adsorbents may be particularly more suited to Australia’s coal-fired power stations which lack flue gas de-sulphurisation and de-nitrification facilities. Acknowledgements This project is part of the CSIRO Energy and received funding from Coal Innovation New South Wales (CINSW). We gratefully acknowledge the in-kind contribution from Delta Electricity. The authors express special thanks to Anthony Callan in facilitating the on-site operations for this site trial. The contributions of Paul Feron, Aaron Cottrell, Ashleigh Cousins, Jun-Seok Bae and Sanger Huang from CSIRO and former CSIRO staff members Xianchun Li and Andrew Castleden, are kindly acknowledged. We are also grateful to Clement Yoong and James Knight for their coordination and support of this project. References Bae, J.-S., Su, S., 2013. Macademia nut shell-derived carbon capture for post combustion CO2 capture. Int. J. Greenh. Gas Con. 19, 174–182. Banerjee, R., Phan, A., Wang, B., Knobler, C., Furukawa, H., O’Keeffe, M., Yaghi, O.M., 2008. High temperature synthesis of zeolite imidazolate frameworks and application to CO2 capture. Science 319, 939–943. Chaffee, A.L., Knowles, G.P., Liang, Z., Zhang, J., Xiao, P., Webley, P.A., 2007. CO2 capture by adsorption: materials and process development. Int. J. Greenh. Gas Con. 1, 11–18. Chen, C., Yang, S.T., Ahn, W.S., Ryoo, R., 2009. Amine-impregnated silica monolith with a hierarchial pore structure: enhancement of CO2 capture capacity. Chem. Commun. 24, 3627–3629. Chen, C., Ahn, W.-S., 2011. CO2 capture using mesoporous alumina prepared by a sol-gel process. Chem. Eng. J. 166, 646–651. Chue, K.T., Kim, J.N., Yoo, Y.J., Cho, S.H., Yang, R.T., 1995. Comparison of activated carbon and zeolite 13x for CO2 recovery from flue gas by pressure swing adsorption. Ind. Eng. Chem. Res. 34, 591–598. Cottrell, A., Cousins, A., Chow, T., 2010. Tarong post combustion capture pilot plant design. CSIRO Report, Australia. D’Alessandro, D.M., Smit, B., Long, J.R., 2010. Carbon dioxide capture: prospects for new materials. Angew. Chem. Int. Ed. 49, 6058–6082. Dave, N., Do, T., Palfreyman, D., Feron, P.H.M., 2011. Impact of post combustion capture of CO2 on existing and new Australian coal-fired power plants. Energy Procedia 4, 2005–2019. Gonzalez, A.S., Plaza, M.G., Rubiera, F., Pevida, C., 2013. Sustainable biomass-based carbon adsorbents for post-combustion CO2 capture. Chem. Eng. J. 230, 456–605. Gupta, T., Gosh, R., 2015. Rotating bed adsorber system for carbon dioxide capture from flue gas. Int. J. Greenh. Gas Con. 32, 172–188. Haszeldine, R.S., 2009. Carbon capture and storage: how green can black be? Science 325, 1647–1652. Hao, G.P., Li, W.C., Qian, D., Lu, A.H., 2010. Rapid synthesis of nitrogen-doped porous carbon monolith for CO2 capture. Adv. Mater. 22, 853–857. Hao, G.P., Li, W.C., Qian, D., Wang, G.H., Zhang, W.P., Zhang, T., Wang, A.Q., Schuth, F., Bongard, H.J., Lu, A.H., 2011. Structurally designed synthesis of mechanically

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