Development of water gas shift/membrane hybrid ... - ScienceDirect

Energy Procedia 4 (2011) 1139–1146 Energy Procedia 00 (2010) 000–000

Energy Procedia www.elsevier.com/locate/procedia www.elsevier.com/locate/XXX

GHGT-10

Development of water gas shift/membrane hybrid system for precombustion CO2 capture in a coal gasification process See Hoon Leea, Jeong Nam Kima, Won Hyun Eoma, Young Deok Koa, Seong Uk Hongb, Il Hyun Backa* a

Climate change Tchnology Research Division, Korea Institute of Energy Research 71-2 Jang-Dong, Yuseong-Gu, Daejeon 305-340, Republic of Korea b Dept. of Chem. Eng., Hanbat National University 16-1 Duckmyoung-Dong, Yuseong-Gu, Daejeon 305-719 Republic of Korea Elsevier use only: Received date here; revised date here; accepted date here

Abstract In this study, a 2l/min water gas shift/membrane hybrid system for pre-combustion CO 2 capture has been developed. To control the concentration of major components such as H 2, CO, and CO 2, MFCs were used in experimental apparatus. The gas concentration in these experiments was equal with syngas concentration from dry coal gasifiers (H 2: 25-35, CO: 60-65, CO 2: 515 vol%). The operation conditions of WGS/membrane hybrid system were 200-400 oC, 1-15bar. Steam/Carbon ratios were between 2.0 and 5.0. To separate hydrogen from mi xed gas stream, the palladium membrane will be adopted. As steam/carbon ratio increased, the conversion in the HTS reactor increased from 85% to 91% at the condition of 350 oC, 1,000ml/min, CO: 65, H 2: 30, CO 2: 5%. However the conversion decreased with increasing of gas flow. In WGS experi ments, the conversion reached 99.5% at the condition of 1,000ml/min and CO: 65, H 2: 30, CO 2: 5%. In the experiments of WGS with membrane reactor, the gas concentration before membrane reactor was H 2: 56.28, CO 2: 43.48, CO: 0.24%. The gas concentration of retentate flow was H 2: 35.74, CO 2: 63.27, CO: 0.99%. © All rightsLtd. reserved c 2010 ⃝ 2011 Elsevier PublishedLtd. by Elsevier Open access under CC BY-NC-ND license. Keywords: Water gas shift; Membrane; CO2 capture; Gasification

1. Introduction Concerns about climate change and the environmental consequences of increased levels of atmospheric CO2 will require the power generation industry to reduce CO2 emissions from their current levels. Especially, Integrated Gasi fication Combined Cycle (IGCC) with CCS (Carbon Capture and Storage) technologies has been under development as a remedy for environmental pollution and as a means for maximizing energy efficiency [1]. The problem of continuously increasing the CO2 content of the Earth’s atmosphere requires a solution, which will allow the generation of sustainable power at increased levels. Pre-combustion, a technology for reducing CO2 is a highly economical and efficient process that involves the exhausted heat and the use of micro equipment in combination with several technologies.

doi:10.1016/j.egypro.2011.01.166

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Author / Energy Procedia 00 (2010) 000–000 S.H. Leename et al. / Energy Procedia 4 (2011) 1139–1146

A gasification process with pre-combustion CO2 capture process, which converts coal into environment-friendly synthetic gas, might be promising option for sustainable power generation. In the coal gasification for power generation, coal is converted into H2 , CO and CO2 . To reduce the cost of CO2 capture and to maximize hydrogen production, the removal of CO and the additional production of hydrogen might be needed. Especially the concentration of CO in a entrained flow coal gasification is about 50-65%. Therefore a novel process with water gas shift reactor and membrane has been developed. When a water gas shift reaction (WGSR) is carried out in two adiabatic reactors, a low temperature shift (LTS: 200-220o C) reactor and a high temperature shift (HTS: 300-380o C) reactor in series, H2 purity increases [1]. Water gas shift reaction is well known catalytic process in the chemistry industry [2]. In the reforming of fossil fuel, WGSR is adopted for the removal of CO and the additional production of hydrogen. Currently fixed-bed reactors are used in commercial system with two different types of catalysts, which are named as HTS and LHS in order to produce high yields of hydrogen by steam reforming of natural gases [3]. Gasification of coal will be another promising option for produce hydrogen due the large amount of coal deposits. However, the concentration of CO is about 30~50 % much larger than that of CO from reforming of natural gas, which is about 10 %. The separation of hydrogen with hydrogen membrane might make the high concentration of CO2 stream and reduce the CO2 capture cost in the coal gasification process. To increase CO2 concentration from coal gasification, hydrogen separating membrane might be used. Generally palladium alloys have been studied as hydrogen-separating membranes in applications that include hydrogenation and dehydrogenation reactions, H2 recovery from plant streams, and coal gasification [4]. It is well known that the presence of other gases along with hydrogen influences permeation of hydrogen not only by means of the dilute effect [5]. To separate hydrogen from syngas and maximize CO2 concentration, a water gas shift/membrane hybrid system for pre-combustion CO2 capture in a coal gasification process has been developed. In this study, WGSR was carried out using two catalytic reactors and Pd-Cu membrane reactor in order to investigate the CO conversion performance with simulated syngas which had a large amount of CO in reformate gas. Also the change of CO2 concentration after the Pd-Cu membrane reactor was measured. 2. Ex perimental apparatus A water gas shift/membrane hybrid system for pre-combustion CO2 capture consisted of simulated syngas injection module, water injection system, gas and steam preheater high temperature shift reactor, low temperature shift reactor, back pressure regulator, mass flow meter, Pd-Cu membrane, ND-IR analyzer and gas chromatograph (Fig. 1).

Fig. 1 2l/min water gas shift/membrane hybrid system The height and internal diameter o f HTS and LTS reactors are 100 mm and 25 mm, respectively. To control gas

Author name / Energy Procedia 00 (2010) 000–000 S.H. Lee et al. / Energy Procedia 4 (2011) 1139–1146

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fl ow and gas concentration, H2 , CO, CO2 , N2 mass flow controllers (Brooks Co., 5850) were used. To inject high temperature steam, a pressurized water tank (10l), liquid flow controller (Bronkhorst co., L3O) and steam generator were used. A back pressure regulator was used to maintain the system pressure of whole hybrid system. The gas composition after reactor was analyzed with ND-IR analyzer and GC. The Al/Cr/Cu catalyst (Aluminum oxide: 87.2%, Chromium oxide: 2.01%, Chromium Troxide: 6.71%, Copper oxide: 1.9%) was used in the high temperature shift reactor and Cu/Zn catalyst (Copper oxide: 43.1%, Zinc oxide: 47.2%, Aluminum oxide: 9.7%) was used in the low temperature shift reactor (Table 1). The composition of reactant was CO : 60-65 vol%, H2 : 25-35 vol%, CO2 : 5-15 vol%. The temperature of HTS reactor was 350 -380o C and that of LTS reactor was 180-220o C. The operation o f system pressure was 1 -15 bar. The flow rate of reactant gas was 500-1,500 ml/min and steam/carbon ratio was 2.5-5. Table 1 Commercial catalyst for WGS/Membrane hybrid system HTS(SHT-4) LTS(MDC-7 ) Form

Tablet

Form

Tablet

Shape

3.2mm×3.2mm

Shape

3.2mm×3.2mm

Aluminium Oxide Chromium Oxide Chromium Troxide

87.2% 2.01% 6.71%

Cupper Oxide Zinc Oxide Aluminium Oxide

43.1t% 47.2% 9.7%

Copper Oxide

1.9%

Membrane reactor for pre-combustion CO2 capture consisted of pressure transmitter, Pd-Cu tubular membrane reactor (36mm I.D. x 100mm length), furnace, mass flow meter, mass flow controller, and GC (Fig 2). The surface and thickness of membrane was 70 cm2 and 20 !m. To maintain the pressure of m embrane reactor, the pressure of input gas was measured by using pressure transmitter and mass flow controller was changed. To calculate permeate fl ow, mass flow meter and GC-TCD were used.

Fig. 2 Apparatus of membrane reactor

3. Results Taking into account the exothermic nature of the WGS reaction, higher CO conversions are favoured at lower temperatures. But kinetics are almost instantaneous at very high temperatures. Because of the inherent temperature increase during the reaction, synthesis gas generation is typically conducted in adiabatic CO shift reactors to avoid catalyst overheating and improve the reaction conversion [6]. The bed pressure and gas flow rate affected CO conversion in the water gas shift reaction. Fig. 3 showed the conversion of CO in two stage WGS reactors at various

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gas flow rate and bed pressure. As increasing the reactant flow rate from 1,000 ml/min to 2,000 ml/min, the conversion of CO was decreased from 97% to 93% at atmosphere condition due to a decrease of contact time. But the conversion of CO was almost same at 10bar condition.

Fig. 3 CO conc. and conversion with gas flow and reactor pressure The effect of HTS and LTS temperature on CO conversion and CO concentration at 1,000 ml/min, 2.5 S/C ratio were shown in Fig. 4. As shown in Fig. 4, the CO conversion decreased with increasing the temperature of LTS and HTS reactor because of exothermic nature of WGS reaction. The operation temperature conditions of HTS and LTS o o reactors were generally 320-360 C and 190-250 C, respectively.

Fig. 4 CO conversion and conc. with reactor temperature

In water gas shift reaction, water is very important factor. Generally S/C ratio (H2 O/CO) used in commercial plants was 2.5 and higher. In this study, S/C ratios increased from 2.5 to 5 to lower CO concentration in H2 rich gas o o stream which left LTS reactor. At the condition of HTS : 350 C, LTS : 220 C, 1,000 ml/min, 10 bar, the effect of


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S/C ratio on CO concentrations after HTS and LTS reactor was shown in Fig. 5. As can be seen CO concentration decreased with increasing S/C ratio. At S/C ratio : 5 CO concentration in H2 rich gas stream was almost 0.3.

HTS : 350 ¡É LTS : 220 ¡É Gas flow : 1000 ml/min

CO (%) - HTS

9

1.0 0.8

8

0.6

7

0.4

6 5

0.2

HTS LTS

2.5:1

3.75:1

4:1

5:1

S/C Fig. 5 CO conc. with S/C ratio at 10bar

0.0

CO (%) - LTS

10

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For pre-combustion CO2 capture Pd-Cu(40wt%) membrane reactor was used in WGS/Membrane hybrid system. Recently a catalyst-membrane system combining the reaction and H2 separation by means of a selective membrane has been intensively investigated for the WGS reaction with the significant developments in membranes, especially Pd and Pd alloy membranes [7]. To investigate H2 permeate characteristics of tubular membrane reactor, hydrogen permeate experiments were done at various temperature and pressure (Fig. 6). As shown in Fig. 6, phase transition o 1/2 o f membrane happened at 200 C. H2 permeate flux linearly increased with an increase of SQRT(P)(bar ) .

Fig. 6 H2 flux in Pd-Cu membrane reactor

The gas concentration profile in WGS/membrane hybrid system was shown in Fig. 7. The simulated syngas (H2 : 30, CO2 : 5, CO: 65 vol%) was injected 2 stage WGS reactor. In the experiments of WGS with membrane reactor, the gas concentration before membrane react or was H2 : 56.28, CO2 : 43.48, CO: 0.24 vol%. In the permeate side, only hydrogen was detected and the gas concentration of retentate flow was H2 : 35.74, CO2 : 63.27, CO: 0.99 vol%.


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Fig. 7 Gas concentration in WGS/membrane hybrid system

4. Conclusion To develop pre-combustion CO2 capture for coal gasification, a WGS/membrane hybrid system has been developed in this study. Syngas from coal gasification has high CO concentration and low CO2 concentration. In WGS section, CO in syngas was converted into H2 and CO2 . As a result, WGS reactor in coal gasification might be helped to increase CO2 concentration and reduce the cost of CO2 capture. To separate hydrogen from hydrogen rich gas after LTS reactor, Pd-Cu membrane reactor was used in this study. At the gas concentration of CO: 65, H2 : 30, CO2 : 5% and 1,000 ml/min gas flow rate, total CO conversion in two stage WGS reactors reached 99.5%. The CO2 concentration of input gas was 43.48 vol% and the CO2 concentration of retentate flow was 63.27 vol%. WGS/membrane hybrid system for coal gasification might be applied to maximize hydrogen production and reduce pre-combustion CO2 capture. 5. References [1] Lee YJ, Lee SD, Hong SI, Moon DJ. Conceptual design and simulation study for the production of hydrogen in coal gasification system. Inter. J. Hydrogen Energy 2010; doi:10.1016/j.ijhydene.2010.04.064. [2] Navarro RM, Pea MA and Fierro JLG. Chem. Rev. 2007; 107:3953 -3956 . [3] Hydrogen from Coal Program: RD & D Plan, U.S. Dept. of Energy, 2007. p. 21. [4] Kulprathipanja A, Alptekin GO, Falconer JL, Way JD. Effects of w ater gas shift gases on Pd-Cu alloy membrane surface morphology and separation properties. Ind. Eng. Chem. Res. 2004;43:4188-4198. [5] Gallucci F, Chiaravalloti F, Tosti S, Drioli E, Basile A. The effect of mixture gas on hydrogen permeation through a palladium membrane: Experimental study and theoretical approach. Inter. J. of Hydrogen Energy 2007; 32:1837 -1845. [6] Mendes D, Mendes A, Madeira LM, Iulianelli A, Sousa JM, Basile A. The water-g as shift reaction: from conventional catalytic systems to Pd-based membrane reactors – a review. Asia-Pac. J. Chem. Eng. 2010; 5: 111-

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137. [7] Hwang KR, IHM SK, Park JS. A catalytic membrane reactor fo r water-gas shift reaction. Korean J. Chem. Eng. 2010;27:816-821.