Experimental Investigation of Precombustion CO2 Capture Using a

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Experimental Investigation of Precombustion CO2 Capture Using a Fixed Bed of Coal Particles in the Presence of Tetrahydrofuran Jin Yan, Yi-Yu Lu, Jia-Le Wang, Sheng-Lan Qing, and Yu-Rui Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00998 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 25, 2016

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Experimental Investigation of Precombustion CO2 Capture Using a Fixed Bed of Coal Particles in the Presence of Tetrahydrofuran Jin Yan a, Yi-Yu Lu a, *, Jia-Le Wang b, Sheng-Lan Qing a, Yu-Rui Wang b

a

State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China

b

College of Power Engineering, Chongqing University, Chongqing 400044, China

* Corresponding author. Tel: +86-23-65105640 E-mail: [email protected] (Y.Y. Lu)

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Abstract This work presents a hybrid gas adsorption and hydrate formation process for CO2 capture from fuel gas (40 mol% CO2/H2) using a fixed bed reactor (FBR) of coal particles. To better understand the CO2 capture performance of this hybrid process, the effects of particle size and tetrahydrofuran (THF) concentration on gas consumption and CO2 selectivity were experimentally investigated at 100% liquid saturation of the fixed bed. The experiments were conducted at 274.2 K with the initial pressure fixed at 3.0 MPa. The results indicated that medium-sized coal particles (0.5-1.0 mm) are more favorable than small (0.1-0.5 mm) and large (1.0-3.0 mm) coal particles as higher gas uptake, CO2 recovery, and separation factor were obtained in the FBR filled with medium-sized coal particles. Gas uptake was observed to increase while increasing THF concentration from 1.0 to 5.6 mol%, but the optimum CO2 selectivity was obtained at 1.0 mol% THF. The gas uptake obtained at 1.0 mol% THF in the FBR of coal particles was comparable with that obtained at 5.53 mol% THF in the FBR of silica sand at the same temperature and pressure conditions. Therefore, the FBR of coal particles saturated by 1.0 mol% THF solution can be used as a viable format for CO2 capture from fuel gas when considering gas uptake and CO2 selectivity at the same time.

Keywords: Gas hydrates; Carbon dioxide capture; Precombustion; Porous coal particles; Adsorption; Gas separation


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1. Introduction Gas hydrates are ice-like inclusion compounds formed at low temperature and high pressure conditions with small gas molecules such as CH4, C2H6, and CO2 trapped in the hydrogen-bonded water cavities. So far, three different structures (sI, sII, and sH) of gas hydrates have been identified depending on the size of hydrate formers as well as the formation conditions.1-3 Generally, one volume of gas hydrates is able to store approximately 180 volumes of gas at standard temperature and pressure condition.3 This feature of gas hydrates has expanded the hydrate-based technologies to natural gas storage and transport,4 gas separation,5-9 and CO2 sequestration,10-14 etc. The basis of separating gas mixtures (CH4/N2, CO2/N2, SF6/N2, etc.) through hydrate formation is that one component of the gas mixture will preferentially incorporate in the hydrate phase at moderate temperature and pressure conditions while other components remaining in the gas phase, and thus a higher concentration of the gas component enriched in the hydrates will be obtained after hydrate dissociation. Most recently, the hydrate based gas separation (HBGS) process has been proposed for CO2 capture from fuel gas (CO2/H2) which is used in the integrated gasification combined cycle (IGCC) power plants for electricity generation.15-20 The concept of HBGS process for CO2 capture from CO2/H2 is that CO2 is preferred to enter the hydrate phase at mild temperature and pressure conditions while most H2 molecules are remaining in the gas phase. This is referred to as “precombustion CO2 capture”, which can be used to raise the efficiency of power generation and reduce CO2 emission to the atmosphere. However, the pressure required for hydrate formation from CO2/H2 at a given temperature is considerably high when using liquid water without additives.21 Thus, thermodynamic promoters like tetrahydrofuran (THF), cyclopentane (CP), and tetra-n-butyl ammonium bromide (TBAB) have been employed to shift the hydrate phase equilibrium conditions to high temperatures and low pressures.22-26 In this way, during the hydrate formation process the amount of energy consumed for gas compression and gas/liquid cooling will


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decrease and the energy costs will be cut. However, it was found that gas consumption (gas uptake) during the HBGS process was compromised in the presence of promoters as the hydrate cavities that are expected to trap CO2 molecules would be filled by the promoter molecules. Besides, the CO2 selectivity obtained in the presence of thermodynamic promoters is reduced as H2 molecules may compete to enter the hydrate cages. Therefore, one solution for this problem is to increase the liquid conversion to hydrates and incorporate more CO2 molecules in the hydrate phase. The present work intends to use a coal filled fixed bed reactor (FBR) to promote hydrate formation kinetics and raise the CO2 selectivity because the coal particles employed are able to provide large gas/liquid interface for hydrate nucleation and have strong CO2 adsorption capacity.27 To improve the kinetics of hydrate formation and increase gas consumption, the FBRs packed with porous media such as silica sand, silica gels, glass beads, and coal particles have been employed instead of the stirred tank reactor (STR).28-32 A prominent advantage of FBR is that the gas/liquid contact surface is enlarged significantly as compared to the STR. One recent study shows that the FBR filled with coal particles increased the gas consumption dramatically as compared to the FBR of silica sand, and higher CO2 selectivity was obtained.32 It was also found that CO2 capture in the liquid-saturated FBR of coal particles is not a single hydrate formation process but a hybrid adsorption-hydrate formation (HAHF) process, which is different from those occurred in other FBRs (silica sand, silica gel, and glass beads). This is probably related to the high CO2 adsorption capacity of coal particles,27 and one possible interpretation for the mechanism of this HAHF process was presented in the literature.32 However, the performance of this HAHF process occurring in the FBR of coal particles has not yet been well understood. For example, the change in particle size and additive concentration will influence the adsorption kinetics, hydrate formation kinetics, and CO2 selectivity from the CO2/H2 gas mixture. The data of these influences are not available in the literature. The purpose of this work is to experimentally investigate the kinetics of gas adsorption and hydrate


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formation as well as CO2 selectivity from the CO2/H2 gas mixture using different-sized coal particles in the presence of different THF concentrations, so as to report underlying data to optimize CO2 capture efficiency of the CO2/H2 gas mixture in the coal filled FBR.

2. Experimental Section 2.1. Materials. The CO2/H2 gas mixture containing 40 mol% CO2 and 60 mol% H2 was supplied by Chongqing Rising Gas with a reported composition uncertainty of ± 0.05 mol%. It was used to model a typical fuel gas that was produced from fossil fuels for power generation in IGCC power plants. THF with a mass purity higher than 99.9% was used as a thermodynamic promoter to reduce the hydrate phase equilibrium pressure at a given temperature. The coal particles employed in the experiments were prepared following the steps of grinding, screening, and drying. The properties such as pore diameter, particle size, and BET surface area were measured using a surface area and porosity analyzer (Micromeritics ASAP 2020, Norcross, GA) and were presented in Table 1. The volume of liquid water required to fill the inner pores of coal particles and the interstitial spaces among coal particles (100% water saturation) was determined using the method given in the literature7 and was presented in Table 1 as well. Deionized water was used to prepare the THF solution for all experiments. 2.2. Apparatus. Detailed description of the experimental apparatus can be seen in our previous work.33 Briefly, it consists of a high pressure stainless steel reactor with an internal volume of 600 cm3 and can be operated at a maximum pressure of 10 MPa. The reactor was immersed in a water bath, and the temperature was controlled by an external refrigerator. The temperatures of gas phase and the fixed bed were measured by two platinum resistance probes with an uncertainty of 0.1 K. The reactor pressure was measured by a pressure transducer (Yokogawa Electric Corporation, Tokyo, Japan) with an uncertainty of 0.06%


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in the range of 0-10 MPa. A data acquisition unit (Agilent 34970A, USA) was used to collect the temperature and pressure data every 10 seconds and transmit these data to a computer at same time. A gas chromatograph (GC-2014, Shimadzu Corporation, Kyoto, Japan) with an uncertainty of 0.1 mol% was used to determine the compositions of the gas mixture remaining in the reactor and the gas mixture collected from hydrate dissociation at the end of the experiments. 2.3. Procedures. The experiments were conducted in batch mode with the temperature fixed at 274.2 K. The experimental procedure was given as follows. Prior to the experiments, the reactor was cleaned with deionized water and dried. To carry out the experiments at a constant gas volume of 310 cm3, the coal particles with a volume of 290 cm3 were prepared to fill the reactor. Thus, the mass of coal particles in different sizes was determined based on the densities given in Table 1. Subsequently, the fixed bed of coal particles was completely saturated by the THF solution. The volumes of THF solution were calculated according to the coal particle saturations (Table 1). The THF solution with a volume of 59, 61, and 58 cm3 was used to saturate the small, medium, and large coal particles, respectively. The reactor and tubing were purged three times using the fuel gas (40 mol% CO2/H2) to remove any air remaining in the system. Then the reactor was charged with the fuel gas to a desired pressure. When the temperature and pressure in the reactor reached desired values, the inlet valve was closed to prevent gas supply to the reactor during the experiments. This was time zero for the experiments of separating CO2 from fuel gas using the fixed bed of coal particles. Meanwhile, the temperature and pressure data were acquired by the data acquisition unit and logged in the computer every 10 seconds. The experiment was stopped when no pressure drop in the reactor was observed for at least 2 h and the temperature returned to approximately 274.2 K. At the end of the experiments, the gas mixture remaining in the reactor was sampled for composition analysis using GC as well as the gas mixture collected from hydrate dissociation. Each experiment was


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conducted three times at the same experimental conditions following the above mentioned procedure so as to obtain reproducible results. The experimental conditions for the experiments carried out in the fixed bed of coal particles were given in Table 2 and Table 3. As seen, the experiments were carried out at 274.2 K with the fixed bed of coal particles saturated by THF solutions. To study the effect of particle size on CO2 capture from fuel gas, the coal particles with three different sizes were used (Table 2). Three THF concentrations (1.0, 3.0, and 5.6 mol%) were employed to saturate the fixed bed of coal particles to study the effect of THF concentration on CO2 capture from fuel gas (Table 3). 2.4. Calculation of the amount of gas mixture consumed. Based on gas compositions measured at the beginning and end of the experiments, the number of moles of the CO2/H2 gas mixture consumed during the experiments (gas consumption, △nH) was calculated as follows

∆nH = ng,0 − ng,t = (

PV PV )0 − ( )t ZRT ZRT

(1)

where ng,0 and ng,t are the numbers of moles of the CO2/H2 gas mixture in the reactor at time 0 and time t, P is the pressure in the reactor, T is the temperature of gas phase, V is the volume of gas phase, R is the universal gas constant, and Z is the compressibility factor which is calculated by the Pitzer correlation for the second Virial coefficient 34

Z = 1 + B0

Pr P + ω B1 r Tr Tr

(2)

where Tr is the reduced temperature, Pr is the reduced pressure, ω is the acentric factor, B0 and B1 are functions of the reduced temperature only and determined by the equations of Abbott as follows

B0 = 0.083 −

0.422 1 0.172 , B = 0.139 − 4.2 1.6 Tr Tr

(3)

Since the amount of water used to saturate the FBR of coal particles varied with the


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particles size, gas consumption (gas uptake) was normalized using the following equation for comparison. ∆nH, normalized =

∆nH n H 2O

(4)

where ∆nH is moles of the gas mixture consumed at the end of the experiment and n H 2 O is the moles of water used in each experiment. Note that gas consumption in this work included gas dissolution in liquid phase, gas adsorption on coal particles, and gas incorporation in hydrate phase. The rate of gas consumption was calculated using the forward difference method as

 d ∆nH  ∆nH, t+∆t − ∆nH, t , ∆t = 10s   = ∆t  dt  t

(5)

The average value of these rates was calculated every 30 min and reported as   d ∆nH   d ∆nH   d ∆nH     dt  +  dt  + L +  dt   1  2   m  , m = 180 Rav =   m    

(6)

2.5. Calculation of CO2 recovery and separation factor. CO2 recovery ( RCO2 ) and separation factor (S.F.) were two metrics proposed by Linga et al.35 to evaluate the efficiency of CO2 separation from fuel gas through hydrate formation. The values of CO2 recovery ( RCO2 ) reported in this study was calculated as follows

RCO2 =

H nCO 2 feed nCO 2

× 100%

(7)

feed where nCO is the number of moles of CO2 supplied to the vessel at the beginning of the 2

H is the total number of moles of CO2 consumed during the experiment, and nCO 2

H experiments. nCO includes the moles of CO2 dissolved in liquid, adsorbed on coal 2


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particles, and incorporated in the hydrate crystals at the end of the experiments. The separation factor (S.F.) was determined as follows

S.F . =

H nCO × nHgas2 2 gas nCO × nHH2 2

(8)

gas where nCO and nHgas are the numbers of moles of CO2 and H2 remaining in the gas phase 2

2

at the end of the experiments. The symbol nHH is the number of moles of H2 trapped in 2

hydrates at the end of the experiments.

3. Results and discussion 3.1. Gas adsorption in the fixed bed of coal particles without liquid. Figure 1 shows a comparison of gas consumption obtained in the FBR using different sized coal particles without liquid. The experimental data were presented in Table S1 in the supporting information. Since no liquid was added in the fixed bed of coal particles, gas consumption was only caused by gas adsorption on coal particles. The experiments were conducted at 274.2 K with the initial pressure fixed at 3.0 MPa, and gas volume in the reactor was kept at 310 cm3. The number of moles of the gas adsorbed was calculated using eqs.1-3. It can be seen from Figure 1 that gas consumption is increased with the increase of particle size. This result can be attributed to the BET surface area of coal particles that is enlarged with the increase of particle size (Table 1), so the number of moles of the gas mixture adsorbed on the surface of coal particles is increased in the FBR using large coal particles (1.0-3.0 mm). 3.2. The combined gas adsorption and hydrate formation process for CO2 capture. It was found that a hybrid gas adsorption-hydrate formation process occurs in the fixed bed of coal particles while the coal particles were saturated by THF solution, and the process is dominated by hydrate formation when the liquid saturation is increased from 40% to 100%.32 Therefore, in order to better understand the properties of hydrate formation for CO2 capture during the hybrid adsorption-hydrate formation process, the 9 ACS Paragon Plus Environment

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experiments shown in Tables 2 and 3 were carried out at 100% liquid saturation. Figure 2 shows a typical temperature and gas uptake profile for the experiment performed in the FBR packed with small-sized coal particles at 1.0 mol% THF (experiment 2 in Table 2). As seen, temperature in the fixed bed was observed to increase sharply at the beginning of the experiment and reached a climax (temperature peak) at 9.6 min, indicating the onset of CO2 adsorption on coal particles and hydrate nucleation in the fixed bed due to the exothermic nature of gas adsorption and hydrate formation. However, it is difficult to distinguish gas adsorption from hydrate formation on the basis of temperature rise as there is only one apparent temperature peak observed during the experiment. This indicates that hydrate formation in the FBR of coal particles was immediately induced by gas adsorption when the experiment was started. Meanwhile, gas uptake was observed to increase dramatically along with the rapid temperature increase between 0 and 9.6 min. This means that a large amount of gas mixture was adsorbed on coal particles and incorporated in the hydrate phase during this period, which was designated as stage I in Figure 2. Then, temperature in the fixed bed was seen to return to the set value (274.2 K) as the heats of gas adsorption and hydrate formation were gradually removed by water bath. Gas uptake was observed to increase slowly and reached 90% of the total gas consumption at about 238 min (stage II). This is probably because hydrate formation in the FBR of coal particles was restricted due to the slow transport of gas molecules through the hydrates that were formed in stage I. Finally, the gas uptake curve was observed to reach a plateau after 238 min (stage III in Figure 2). This indicates the completion of the hybrid adsorption-hydrate formation process in the FBR of coal particles. The gas uptake caused by gas adsorption and dissolution was determined in the fixed bed of coal particles that were fully saturated by pure water (without THF). In this case gas hydrates cannot form and the number of moles of gas consumption was calculated using eqs.1-3. The results were presented in Table S2 in the supporting information. It


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was found the total gas consumption caused by gas adsorption and gas dissolution was 0.0616 mol, which is approximately 75% of the gas consumption obtained in the hybrid gas adsorption-hydrate formation process (experiments 1-3 in Table 2), and CO2 recovery and separation factor (S.F.) were much lower than that obtained in the presence of 1.0 mol% THF (Table S2). This indicates that CO2 capture from CO2/H2 was enhanced when hydrate formation occurring in the fixed bed of coal particles. 3.3. Effect of particle size on CO2 capture in the liquid-saturated fixed bed of coal particles. Table 2 shows a summary of the experimental conditions and the results of CO2 separation from fuel gas using the fixed bed of coal particles in the presence of 1.0 mol% THF. The experiments were conducted at 274.2 K with the initial pressure fixed at 3.0 MPa. Coal particles in three size distributions (0.1-0.5 mm, 0.5-1.0 mm, and 1.0-3.0 mm in diameter) were used to explore the effect of particle size on the performance of CO2 capture in the fully saturated FBR of coal particles. The results shown in Table 2 include normalized gas uptake (∆nH,

normalized),

H CO2 concentration in hydrates ( xCO ), CO2 2

recovery ( RCO2 ), and separation factor (S.F.). Figure 3 shows a comparison of gas uptake for the experiments performed in the FBR using different-sized coal particles. It was observed in the figure that gas uptake for the experiments using medium-sized coal particles (0.5-1.0 mm in diameter) was higher than that using small (0.1-0.5 mm) and large (1.0-3.0 mm) coal particles. At the end of the experiments, gas uptake obtained from the experiments using medium-sized coal particles (0.5-1.0 mm) was 0.03 mol of gas/mol of water, which was 1.5 times higher than that using small and large coal particles (0.02 mol of gas/mol of water). As seen in Table 1, the pore volume of coal particles increases with the increase of particle size. However, it is noted that the measured pore volumes of different sized coal particles are very small. For example, the pore volume of large coal particles (1.0-3.0 mm in diameter) was 0.0028 cm3/g, so the total pore volume for 279 g of coal particles was 11 ACS Paragon Plus Environment

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only 0.8 cm3, which was much smaller than the amount of liquid (58 cm3) used to saturate the fixed bed of coal particles. This indicates that hydrate formation in the FBR of coal particles would primarily occur in the interstitial spaces of coal particles rather than in the tiny particle pores. On the other hand, as illustrated in Figure 4, the interstitial space among coal particles was observed to increase with the increase of particle size. In this case, the continued growth of gas hydrates in the confined interstitial space will become difficult with the increase of particle size. This is because the mass transfer for the further growth of hydrates in the large interstitial space might be prevented by the hydrate films formed at the beginning of the adsorption-hydrate formation process, as a result causing a reduction of water conversion to gas hydrates in the interstitial spaces of coal particles. Therefore, gas consumption obtained from the experiments using large coal particles was lower than that obtained from the experiments using medium-sized coal particles (Figure 3). Also, it can be seen in Table 1 that the specific surface area of small particles was lower than that of the medium-sized particles, so gas adsorption on the surfaces of small particles was reduced, causing smaller amounts of hydrates formed in the confined spaces of coal particles. This might be the reason why gas consumption obtained in the FBR of small coal particles was lower than that obtained in the FBR of medium-sized coal particles (Figure 3). Figure 5 shows a comparison of temperature profiles for the experiments using liquid-saturated coal particles with different size distribution. As seen in Figure 5, although different-sized coal particles were used in the FBR the temperature peaks were observed to appear approximately at the same time and were encountered shortly after the initiation of the experiments. This implies that gas adsorption and hydrate formation in the FBR of coal particles occurred immediately as the experiments were started. Interestingly, the intensity of temperature peak for the medium-sized coal particles was observed to be higher than that for the small and large coal particles, which can be attributed to the larger amounts of gas hydrates formed at the beginning of the experiment.


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Therefore, more gas mixture was incorporated into the hydrate phase and hence more heat was released. This is consistent with the result of gas uptake shown in Figure 3. Figure 6 shows a comparison of CO2 recovery ( RCO2 ) and separation factor (S.F.) obtained from the experiments using three different-sized coal particles that were fully saturated by 1.0 mol% THF solution. As seen in Figure 6, CO2 recovery obtained in the FBR of medium-sized coal particles was higher than that obtained in the FBR of small and large coal particles. Similarly, the separation factor (S.F.) obtained in the FBR of medium-sized coal particles was higher than that obtained in the FBR of small and large coal particles. This result indicates that selecting proper particle size is one effective approach to increase the CO2 selectivity from fuel gas when the experiments were performed in the FBR of coal particles, and in this study the medium-sized coal particles (0.5-1.0 mm) are more favorable for CO2 capture from fuel gas than the small and large coal particles. 3.4. Effect of THF concentration on CO2 capture from fuel gas in the liquid-saturated fixed bed of coal particles The effect of THF concentration on CO2 capture from fuel gas using the FBR of coal particles was investigated at three different THF concentrations (1.0 mol%, 3.0 mol%, and 5.6 mol%). In this section, only the coal particles with optimum size (0.5-1.0 mm) were used according to the effect of particle size presented above. The experiments were carried out at the same temperature and pressure conditions as that used in section 3.3 (Table 3). Figure 7 shows a comparison of gas consumption obtained from the experiments that were conducted in the fixed bed of coal particles at three THF concentrations. As seen in Figure 7, the gas uptake obtained at 5.6 mol% THF was observed to be larger than those obtained at 1.0 and 3.0 mol% THF. The average value of gas uptake obtained at the end of the experiments was 0.030, 0.032, and 0.037 mol of gas/mol of water corresponding to 1.0, 3.0, and 5.6 mol% THF respectively (Table 3). This indicates that a larger amount of 13 ACS Paragon Plus Environment

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gas mixture was consumed when increasing the THF concentration from 1.0 to 5.6 mol%, and a higher water conversion to gas hydrates was achieved with the increase of THF concentration. This result was in good agreement with that reported by Babu et al

30

.

They studied the hydrate formation kinetics using a fixed bed of silica sand for CO2 separation from CO2/H2, and found the total gas uptake obtained at 5.53 mol% THF was 1.8 times higher than that obtained at 1.0 mol% THF. Note that the phase equilibrium pressure for hydrate formation from CO2/H2 at a given temperature decreases with the increase of THF concentration

22, 36

, so the increasing trend of gas uptake observed with

the increase of THF concentration can be attributed to the elevated driving force (△P = Pexp - Peq) for hydrate formation at a fixed temperature. As a result, more gas hydrates were formed at 5.6 mol% THF due to the increased driving force and the number of moles of the gas mixture incorporated in the hydrate phase was increased. Gas uptake obtained in the FBR of coal particles was also compared with that obtained in a stirred reactor. Lee et al. reported the hydrate based CO2 capture in a stirred reactor using the same CO2/H2 gas mixture.36 It was found that the final gas uptake obtained in the FBR of coal particles (0.0295 mol of gas/mol of water) was 3.7 times larger than that obtained in the stirred reactor (0.008 mol of gas/mol of water) at 1.0 mol% THF. This indicates that hydrate formation in the FBR of coal particles was promoted due to the increased gas/liquid contact areas. On the other hand, Lee et al. found gas uptake obtained at 1.0 mol% THF was higher than those obtained at 0.5 mol% and 3.0 mol% THF. They explained that the amount of hydrate cavities available for CO2 and H2 gases decreased with the increase of THF concentration, so gas uptake decreased at higher THF concentration. It should be noted that one major limitation of hydrate formation in stirred reactors is that gas hydrates would agglomerate at the gas/liquid interface and prevent gas diffusion for the further growth of gas hydrates, so one reason for the higher gas uptake obtained at 1.0 mol% THF might be the mass transfer that was enhanced at 1.0 mol% THF somehow as compared to 0.5 and 3.0 mol% THF. As seen in Figure 7, gas uptake


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obtained at 1.0 mol% THF was higher than that obtained at 3.0 and 5.6 mol% THF between 0 and 250 min. This result is consistent with that reported by Lee et al.36 Then, gas uptake obtained at 5.6 mol% THF became higher than that obtained at 1.0 and 3.0 mol% THF. Note that when hydrate formation with the CO2/H2 mixture was carried out in batch operation using a STR or FBR, gas composition in the reactor would vary with time due to the preferred incorporation of CO2 in the hydrates, so the phase equilibrium pressure of hydrates formed with CO2/H2 at a given temperature will change with time. In this case, the driving force for hydrate formation at a fixed THF concentration will change with time and influence hydrate growth. The driving force at 1.0 mol% THF might be larger than that at 3.0 and 5.6 mol% THF before 250 min and became smaller after 250 min, so gas consumption at 1.0 mol% THF was higher before 250 min and became lower after 250 min. This analysis indicates that in a bath reactor (STR, FBR) the change of driving force with time might be another reason for the higher gas uptake at 1.0 mol% THF before 250 min as well as for the higher gas uptake at 5.6 mol% after 250 min. It is also noted in Figure 7 that determination of the optimum THF concentration for hydrate formation will depend on when the experiments were stopped. For example, 1.0 mol% THF would be the optimum concentration if the experiments were terminated before 250 min, whereas 5.6 mol% would be the optimum concentration if the experiments were stopped after 250 min. Figure 8 shows a comparison of CO2 recovery ( RCO2 ) and separation factor (S.F.) obtained in the fixed bed of coal particles at different THF concentrations. As seen in the figure, CO2 recovery was observed to decrease when increasing THF concentration from 1.0 to 5.6 mol%. For example, the average value of CO2 recovery was 57%, 51%, and 47% corresponding to 1.0, 3.0, and 5.6 mol% THF, respectively. A decreasing trend of the separation factor (S.F.) was also observed with the increase of THF concentration. This result indicates that although gas uptake was enhanced by increasing THF concentration from 1.0 to 5.6 mol% in the FBR of coal particles (Figure 7), CO2 selectivity from the 15 ACS Paragon Plus Environment

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fuel gas was compromised with the increase of THF concentration. In addition, it can be H seen in Table 3 that the average value of CO2 concentration of the gas mixture ( xCO ) 2

sampled from hydrate dissociation decreased from 93.4 mol% to 86.5 mol% with THF concentration increasing from 1.0 to 5.6 mol%. This implies that more H2 molecules were enclathrated in the hydrate cavities at a higher THF concentration. 3.5. Comparison of gas consumption in different fixed bed reactors. It was found in the literature that the highest gas consumption for CO2 capture from fuel gas using the HBGS process was obtained in the FBR of silica sand at 5.53 mol% THF (0.0519 mol of gas/mol of water), and the experiments were carried out at 279.2 K with the initial pressure fixed at 6.0 MPa.30 However, the parameters of CO2 selectivity such as CO2 recovery and separation factor (S.F.) in that system were not reported. In this study, as seen from the results presented above, the highest CO2 selectivity in the FBR of coal particles was obtained at 1.0 mol% THF in the FBR of medium-sized coal particles (0.5-1.0 mm in diameter). To compare the performance of HBGS process for CO2 separation from fuel gas in the FBR of coal particles with that in the FBR of silica sand, we carried out experiments in the FBR of coal particles at the same temperature and pressure conditions as that used in the literature.30 The results were summarized in Table 4. As seen, gas uptake obtained in the FBR of coal particles was 0.0492 ± 0.0004 mol of gas/mol of water, which is comparable with that obtained in the FBR of silica sand. Besides, CO2 recovery and separation factor (S.F.) obtained in the FBR of coal particles were presented in Table 4 as well. This result indicates that the FBR filled with coal particles can be used as a comparative format for CO2 capture from fuel gas while taking CO2 selectivity and gas uptake into account at the same time, and the optimum THF concentration determined in this work (1.0 mol%) was greatly reduced as compared to that used in the FBR of silica sand (5.53 mol%). According to the results reported in this work, CO2 selectivity in the fixed bed of coal particles can be improved when using coal particles with large pore volumes or selecting proper particle size. 16 ACS Paragon Plus Environment

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4. Conclusions In this work, a hybrid gas adsorption and hydrate formation process for CO2 capture from fuel gas (40 mol% CO2/H2) was accomplished using a FBR of coal particles. To better understand the performance of this hybrid process for CO2 capture from CO2/H2, the effects of coal particle size and THF concentration on gas consumption and CO2 selectivity were studied at 100% liquid saturation of the fixed bed. The results indicated that the medium-sized coal particles (0.5-1.0 mm) are more favorable than the small (0.1-0.5 mm) and large (1.0-3.0 mm) coal particles as higher gas uptake, CO2 recovery, and separation factor were obtained comparing to that obtained using the small and large coal particles. Gas uptake was increased with increasing the THF concentration from 1.0 to 5.6 mol%, but the optimum CO2 selectivity was obtained at 1.0 mol% THF. The gas uptake obtained at 1.0 mol% THF in FBR of coal particles was comparable with that obtained at 5.53 mol% THF in the FBR of silica sand. Therefore, the FBR of coal particles saturated with 1.0 mol% THF solution can be used as a comparative format for CO2 capture from fuel gas when considering gas uptake and CO2 selectivity at the same time.

Supporting Information. Gas uptake and CO2 selectivity obtained at 0% liquid saturation (dry coal particles), and at 100% liquid saturation (with and without THF) in the FBR of coal particles is given.

Acknowledgement The financial support from the Ministry of Education Innovation Research Team (IRT13043), the National Key Basic Research Program of China (No. 2014CB239206), and the Chongqing Graduate Student Research Innovation Project (CYS15009) is greatly appreciated.


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Figure Captions:

Figure 1. Comparison of gas consumption obtained in the fixed bed reactor using different-sized coal particles without liquid. The experiments were conducted at 274.2 K with the initial pressure fixed at 3.0 MPa. Figure 2. Temperature and gas uptake profile for the experiment conducted in the fixed bed reactor using small coal particles at 274.2 K. The initial pressure was fixed at 3.0 MPa. Figure 3. Comparison of gas uptake obtained in the fixed bed reactor using coal particles in different size distributions. The experiments were conducted at 274.2 K and 1.0 mol% THF with the initial pressure fixed at 3.0 MPa. Figure 4. Schematic illustration of the interstitial space among coal particles. Figure 5. Temperature profiles obtained at 100% liquid saturation of the fixed bed reactor using different-sized coal particles. The experiments were carried out at 274.2 K with the initial pressure fixed at 3.0 MPa. Figure.6. Effect of particle size on CO2 recovery and separation factor obtained at 100% liquid saturation of the coal filled fixed bed. The experiments were carried out at 274.2 K and 3.0 MPa. Figure 7. Comparison of gas uptake obtained at different THF concentrations in the fixed bed reactor using coal particles. The particle size was in the range of (0.5 - 1.0 mm). The experiments were conducted at 274.2 K with the initial pressure fixed at 3.0 MPa. Figure 8. Effect of THF concentration on CO2 recovery and separation factor obtained in the fixed bed of coal particles. The experiments were carried out at 274.2 K and 3.0 MPa.


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Table 1. Properties of Coal Particles Used in the Fixed Bed Reactor. Property

Value

Value

Value

particle size (mm) bulk particle density (g/cm3) BET surface area (m2/g) pore volume (cm3/g) average pore diameter (nm) water saturation a (cm3/g)

0.1 - 0.5 0.825 0.366 0.0008 5.4 0.247

0.5 - 1.0 0.88 0.478 0.0016 6.9 0.238

1.0 - 3.0 0.962 0.844 0.0028 7.4 0.207

a

The amount of water measured to fill the interstitial space and inner pores of coal

particles.


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Table 2. Summary of the Experimental Results for CO2 Capture from Fuel Gas in the Fixed Bed Reactor Using Different Sizes of Coal Particles. a

Exp. no. 1 2 3 4 5 6 7 8 9 a

Pexp (MPa)

Particle size (mm)

3.0

0.1-0.5

3.0

0.5-1.0

3.0

1.0-3.0

End of experiments b

T90 (h)

2.3 2.4 2.5 2.3 2.2 2.4 2.9 3.4 3.2

T (h)

∆nH, normalized (mol of gas/mol of water)

9.5 10.2 9.6 6.0 6.0 6.0 10.9 10.9 10.7

0.0228 0.0213 0.0211 0.0301 0.0294 0.0290 0.0225 0.0211 0.0223

H xCO 2

RCO 2

(mol%)

(%)

93.2 91.5 94.4 90.4 94.6 95.1 92.3 93.5

47.7 46.0 45.0 57.9 57.0 56.5 34.3 31.9

31.3 46.5 40.6 41.5 40.5 45.4 42.9 35.6

90.1

33.1

38.1

The fuel gas was a gas mixture containing 40 mol% CO2 and 60 mol% H2. The

experiments were conducted at 274.2 K in the presence of 1.0 mol% THF. b

The time for gas consumption reaching 90% of the total gas consumption.


S.F.

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Table 3. Summary of the Experimental Results for CO2 Capture from Fuel Gas in the Fixed Bed of Coal Particles at Different THF Concentrations. a End of experiments Exp. no. 1 2 3 4 5 6 7 8 9 a

Pexp (MPa)

THF (mol%)

3.0

1.0

3.0

3.0

3.0

5.6

b

T90 (h)

2.3 2.2 2.4 3.7 3.6 3.2 4.5 3.9 4.1

T (h)

∆nH, normalized (mol of gas/mol of water)

6.0 6.0 6.0 10.9 11.0 10.7 11.1 11.0 11.1

0.0301 0.0294 0.0290 0.0323 0.0322 0.0330 0.0372 0.0366 0.0366

H xCO 2

RCO 2

(mol%)

(%)

90.4 94.6 95.1 89.6 88.7 92.1 86.4 87.3

57.9 57.0 56.5 50.7 50.4 51.0 48.1 47.0

41.5 40.5 45.4 28.5 29.8 21.1 12.4 11.3

85.7

45.9

10.3

S.F.

The fuel gas was a gas mixture containing 40 mol% CO2 and 60 mol% H2. The

experiments were conducted at 274.2 K, and the coal particles with a size distribution of (0.5 - 1.0 mm) were used. b

The time for gas consumption reaching 90% of the total gas consumption.


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Table 4. Comparison of the Results for CO2 Capture from Fuel Gas Using the Fixed Bed Reactors of Coal Particles and Silica Sand.

System coal particles silica sand b

a

T (K)

Pexp (MPa)

279.2 279.2

6.0 6.0

THF (mol%) 1.0 5.53

gas uptake (mol of gas/mol of water)

RCO 2

0.0492 ± 0.0004 0.0519 ± 0.0038

45.7

34.2

--

--

a

Coal particles with the size ranging from 0.5 to 1.0 mm were used.

b

Data reported by Babu et al.30.


S.F.

(%)

Energy & Fuels

0.10

small particles (0.1-0.5 mm) medium-sized particles (0.5-1.0 mm) large particles (1.0-3.0 mm)

0.08

Gas uptake (mol )

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

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0.06

0.04

0.02

0.00 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Time (h)

Figure 1. Comparison of gas consumption obtained in the fixed bed reactor using different-sized coal particles without liquid. The experiments were conducted at 274.2 K with the initial pressure fixed at 3.0 MPa.


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0.05

Temperature in the fixed bed Gas uptake

277

0.04

276

0.03

275

0.02

Stage I 274

0.01

Stage II

Stage III

273 0

100

200

300

400

500

Gas uptake (mol of gas/mol of water)

278

Temperature (K)

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

Energy & Fuels

0.00 600

Time (min)

Figure 2. Temperature and gas uptake profile for the experiment conducted in the fixed bed reactor using small coal particles at 274.2 K. The initial pressure was fixed at 3.0 MPa.


Energy & Fuels

0.04


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

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small particles (0.1 - 0.5 mm) medium-sized particles (0.5 -1.0 mm) large particles (1.0 - 3.0 mm) 0.03

0.02

0.01

0.00 0

2

4

6

8

10

12

Time (h)

Figure 3. Comparison of gas uptake obtained in the fixed bed reactor using coal particles in different size distributions. The experiments were conducted at 274.2 K and 1.0 mol% THF with the initial pressure fixed at 3.0 MPa.


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Coal particles

Size distribution 1.0-3.0 mm

Interstitial Space

0.5-1.0 mm 0.1-0.5 mm

Figure 4. Schematic illustration of the interstitial space among coal particles.


Energy & Fuels

280

279

Temperature (K)

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

Page 30 of 33

small particles (0.1 - 0.5 mm) medium-sized particles (0.5-1.0 mm) large particles (1.0 - 3.0 mm)

278

277

276

275

274 0

100

200

300

400

500

600

Time (min)

Figure 5. Temperature profiles obtained at 100% liquid saturation of the fixed bed reactor using different-sized coal particles. The experiments were carried out at 274.2 K with the initial pressure fixed at 3.0 MPa.


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80 70

80

CO2 recovery

70

60

60

50

50

40

40

30

30

20

20

10

10

0

Separation factor (S.F.)

S.F. CO2 recovery (%)

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

Energy & Fuels

0 0.1-0.5

0.5-1.0

1.0-3.0

Size of coal particles (mm)

Figure 6. Effect of particle size on CO2 recovery and separation factor obtained at 100% liquid saturation of the coal filled fixed bed. The experiments were carried out at 274.2 K and 3.0 MPa.


Energy & Fuels

0.05


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

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1.0 mol% THF 3.0 mol% THF 5.6 mol% THF

0.04

0.03

0.02

0.01

0.00 0

100

200

300

400

500

600

700

Time (min)

Figure 7. Comparison of gas uptake obtained at different THF concentrations in the fixed bed reactor using coal particles. The particle size was in the range of (0.5 - 1.0 mm). The experiments were conducted at 274.2 K with the initial pressure fixed at 3.0 MPa.


Page 33 of 33

80

80

CO2 recovery

70

70

60

60

50

50

40

40

30

30

20

20

10

10

0

1.0

3.0

5.6

Separation factor (S.F.)

S.F. CO2 recovery (%)

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

Energy & Fuels

0

THF concentration (mol%)

Figure 8. Effect of THF concentration on CO2 recovery and separation factor obtained in the fixed bed of coal particles. The experiments were carried out at 274.2 K and 3.0 MPa.


Experimental Investigation of Precombustion CO2 Capture Using a

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