A new route for hydrogen production with simultaneous CO2 … PDF... · 2006-05-24 · WHEC 16 / 13-16 June 2006 – Lyon France 3/12 Steam cycle Air Prereformer SE-SRM Purge H 2

WHEC 16 / 13-16 June 2006 – Lyon France

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A new route for hydrogen production with simultaneous CO2 capture

Hendricus Th.J. Reijersa, Paul van Beurdena, Gerard D. Elzingaa, Steven C.A. Kluitersa, Jan-Wilco Dijkstraa

and Ruud W. van den Brinka

aHydrogen and Clean Fossil Fuels, Energy reseach Centre of the Netherlands (ECN), Westerduinweg 3, 1755 LE, Petten, Netherlands, e-mail: [email protected]

ABSTRACT:This paper presents the results of a pre-combustion decarbonisation route for electricity production. Here, the fuel (natural gas) is converted into hydrogen with simultaneous CO2 capture. The hydrogen is fed to a gas turbine for electricity production. Efficiencies have been calculated for four systems in which this concept is realized using three different CO2 sorbents. All systems show an efficiency gain with respect to the reference system with CO2 capture. Hydrotalcite is shown to be a suitable CO2 sorbent. The adsorption is enhanced by impregnating HTC with K2CO3 and by the presence of steam in the feed gas. Loading percentage of K2CO3 or temperature have a smaller effect on the adsorption. The desorption is strongly dependent on the purge gas flow and the duration of the desorption step. The principle of sorption enhancement has been shown for steam reforming of methane on lab-scale. The results of this experiment could be described by a simple Excel spreadsheet model.

KEYWORDS : sorption enhanced, steam reforming, hydrogen production, CO2 capture, model.

IntroductionEven though fossil fuels will remain the most important energy source for at least the first half of this century, there is a growing awareness that energy must be produced at lower greenhouse gas emissions. This has led to new technologies to reduce the emission of the CO2 produced from the burning of fossil fuels. One possibility is to use pre-combustion decarbonisation. Here, the CO2 produced is captured prior to combustion, while transferring the energy content of the fuel to hydrogen. Various pre-combustion routes for electricity production are being investigated including sorption enhancement of the reaction equilibrium during hydrogen production by CO2 capture. One important question to be answered is where should the sorption enhancement take place, for example during methane steam reforming (SMR), or only during a water-gas shift (WGS) and how this effects the efficiency of the chosen systems. Additionally, many different CO2 sorbents have been proposed, but they have very different properties with regards to regeneration (pressure swing or temperature swing) and operating window during adsorption. Rarely, have different sorbent/system combinations been compared. The first part of this paper attempts to address some of these issues in a comparison of four differing system concepts together with three very different sorbent materials. This is done in the context of production of electricity, though the produced hydrogen could be used for other purposes as well. The concept-material combinations are compared in turn to the relevant base cases. Not all combinations have been compared, but the trends can be clearly seen. As reference, a gas turbine combined cycle is used with a power output of 380 MWe and an efficiency of 57.1%. The goal is to find a sorbent-concept combination that has only half the efficiency penalty that is suffered when using a post-combustion technique. Post-combustion results in an efficiency loss of 9.1%, so the target value is 52.6%. One of the investigated systems (sorption-enhanced steam methane reforming with CaO sorbent) actually equals this value. The second part of the paper shows results of experiments using hydrotalcite, a promising sorbent material. Parameters affecting the adsorption properties (promoter loading, steam content of the feed gas, temperature) have been investigated. It has been shown that at WGS and SMR conditions, nearly complete conversion of respectively CO and CH4 can be obtained. The most important challenge is to reduce the amount of purge gas.

Systems studiesSystems studies have been performed to compare the performance of a SERP-based electricity production system with that of a similar one using conventional, post-combustion amine scrubber for CO2 capture. Table 1 shows the investigated systems: - ATR(Air)-SE-WGS: This system is a combination of an air-driven autothermal reformer followed by a

shift section and then a sorption-enhanced shift reactor as known from the work of Air Products. Essentially the majority of the feedstock has already been converted into H2, CO and CO2 before entering the sorption enhanced reactor.


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- ATR(O2)-SE-WGS: This system is very similar to ATR(Air)-SE-WGS except that an Air Separation Unit (ASU) is included, which can for example dramatically reduce the size of the ATR reactor.

- SE-SMR: This system combines a pre-reformer with a sorption enhanced methane steam reformer which can be underfired by either product (H2) or fuel (CH4) to direct the CO2-capture ratio.

- SE-ATR: This system combines a pre-reformer with an air-driven autothermal reformer. For regeneration by temperature swing, the reformer can be underfired by either product (H2) or fuel (CH4) to direct the CO2-capture ratio.

The emphasis in this study is to look at the efficiency of the complete system as opposed to the individual units. The sorption-enhanced reactors are modelled as black boxes and continuous processes although this is not necessarily the expected behaviour in a final system. This systems analysis makes use of a one-pressure steam cycle, uses isentrope efficiencies, ignores pressure drops and delivers the resulting CO2 at 110 bar. The gas turbine inlet temperatures are held constant as well as the gas inlet stream. This results in difference in electricity production and fuel usage.Three different materials which have been identified as promising high-temperature CO2 adsorbents, have been used for sorption enhancement: hydrotalcite (HTC) [1], Li4SiO4 [2] and CaO [3]. Regeneration of the latter two sorbents is performed by increasing the temperature with respect to the adsorption step, while keeping the system pressure equal (temperature swing). The HTC is regenerated at the same temperature as the adsorption step, while lowering the system pressure (pressure swing). Since the temperature ranges where CO2 adsorption is effective, differ for these sorbents (400 - 500 ºC for HTC, 600 - 700 ºC CaO and Li4SiO4), CaO and Li4SiO4 are excluded from SE-WGS, whereas HTC is excluded from SE-ATR. The SE-SMR system is shown in Figure 1. For the air-blown SE-WGS and SE-ATR systems, a side branch configuration is used, where compressed air from the gas turbine is fed to the ATR reactor. As reference, a gas turbine combined cycle based on a Siemens V94.3A turbine of nominal power output 380 MWe and efficiency 57.1% is used. The efficieny penalty due to CO2 capture using an amine scrubber, amounts to 9.1%. The carbon capture ratio is 86.2%.

Table 1 Investigated sorbent/system combinationssorbent system

air/O2 SE-WGS SE-SMR SE-ATRHTC x xLi4SiO4 x xCaO x x

Assumptions with regard to adsorption and desorption conditions are summarized in Table 2. ‘CH4conv.’and ‘CO conv.’ are the conversion of CH4 and CO respectively, ‘S/C’ is the steam-to-carbon ratio of the feed gas, ‘S/CO2’ is the steam-to-adsorbed CO2 ratio required for bed regeneration. For HTC, the value of ΔH is taken from Ding and Alpay [4], for Li4SiO4 and CaO from a thermodynamic database [5]. The operation conditions of the SE-WGS systems are taken from Air Products [6], the CH4 and CO conversions and S/C of CaO-based SE-SMR are taken from Meyer [7]. The adsorption pressure is determined by the operation pressure of the combustion chamber of the gas turbine. For the other parameters, reasonable values have been assumed.

Table 2 Operation conditions of investigated systems ('-' means not applicable)air/SE-WGS

O2/SE-WGS

SE-SMR SE-SMR SE-ATR SE-SMR SE-ATR

sorbent HTC HTC HTC Li4SiO4 Li4SiO4 CaO CaOAdsorptionCH4 conv. (%) - - 93 93 93 93 93CO conv. (%) 95 95 96.8 96.8 96.8 96.8 96.8S/C (-) 1.5 1.5 3.0 3.0 3.0 3.0 3.0T (ºC) 400 400 400 600 700 600 700p (bar) 17 17 17 17 17 17 17ΔH (kJ/mol) 17 17 17 170 170 170 170DesorptionS/CO2 (-) 1.8 1.8 1.8 1.8 1.8 1.8 1.8T (ºC) 400 400 400 750 750 1000 1000p (bar) 2.8 2.8 2.8 17 17 17 17ΔH (kJ/mol) -17 -17 -17 -170 -170 -170 -170


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Steam cycleAir

Prereformer SE-SRM

PurgeH2O

CO2H2O To cooler, CO2

compressor andwater knock-out

Natural gas

H2, H2OSteam

Q

Natural gas/Hydrogen

product gasunderfiring

Adsorption mode

Desorption mode

Figure 1 SE-SMR system

A summary of the results is shown in Table 2. The efficiency is defined as the net export electricity divided by the lower heating value fuel input. Apart from the efficiency of the complete system, efficiencies of the SE-SMR and SE-ATR systems are given for the variants without desorption gas turbine and without pressurized underfiring of the reformer. The desorption gas turbine uses the gas products released during the regeneration step of the sorbent. To obtain the required CO2 capture ratio, the reformer must be underfired partly with product gas from the reformer. At pressurized underfiring, the underfiring is performed at the pressure of the product gas so that it can still be used as feed gas for the gas turbine. Without pressurized underfiring, product gases only pass through the steam cycle. The CCR (carbon capture ratio) is the fraction of recovered CO2. For SE-SMR and SE-ATR systems, part of the H2 rich product gas from the SERP reactor has been used for underfiring, as a supplement of natural gas, to increase the CCR to 85%, though with associated efficiency penalty.

For all cases, the obtained efficiencies exceed that of the reference with CO2 capture (48%). The highest efficiency is obtained for CaO-based SE-SMR, though the CCR is 5% points lower than that of the SE-WGS systems. If a desorption gas turbine is not used, the efficiency drops by a few percent points for the Li4SiO4 and CaO-based systems. If pressurized underfiring is not applied, then the efficiency drops ca. 4% for Li4SiO4, and 7% points for CaO. For the HTC based system, the differences are smaller because the desorption gas is at a much lower pressure (2.8 bar) and because the desorption temperature is much lower (400 ºC). Further, the efficiency proves to be very sensitive to the amount of steam needed for the process, thus it depends on the S/C and S/CO2 values.

Table 2 Results of systems studies ('-' means not applicable)air/SE-WGS

O2/SE-WGS

SE-SMR SE-SMR SE-ATR SE-SMR SE-ATR

sorbent HTC HTC HTC Li4SiO4 Li4SiO4 CaO CaOEfficiency (%) 50.4 48.5 51.6 52.2 52.0 52.6 52.5No desor. turb. - - 51.4 51.0 50.6 50.8 50.4No pressurized underfiring

- - 49.3 47.7 48.1 45.7 45.4

CCR (%) 90.0 90.0 85.0 85.0 85.0 85.0 85.0

For the above calculations, the reformer or water-gas shift reactor is considered as a 'black box'. The actual layout can be quite complex as shown by e.g. the configuration developed by Air Products for the SE-WGS system [6]. For the systems requiring a temperature swing for desorption (Li4SiO4 and CaO-based systems), a fluidized-bed tandem configuration is anticipated [7]. For SE-ATR, a high-temperature reaction (partial oxidation of CH4) and low-temperature reaction (CO2 adsorption) must be integrated in a single reactor, for which a new reactor concept must be developed. The workability of these reactor concepts is a


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point of further research, as well as the technology of pressurized underfiring. Experiments must show if the low S/CO2 values allow long operation times of the catalyst/adsorbent bed. Finally, a cost analysis of the investigated systems must be performed to allow a selection on the basis of lowest CO2 removal cost per ton of avoided CO2.

CO32-H2O

Mg(OH)6 - octahedron

Al(OH)6 - octahedron

CO32-H2O

Mg(OH)6 - octahedron

Al(OH)6 - octahedron

Figure 2 Structure of hydrotalcite

ExperimentalThe research has focused so far on HTC because the HTC based system performs best. HTCs (=

Hydrotalcites), also called layered double hydroxides or Feitknecht compounds, belong to the family of anionic clays. Their general formula is

OmHAOHMM nxn

xx 2/232

1 .)()( −++− ,

where M2+ and M3+ are divalent and trivalent metal ions respectively, and An- is an anion. The value of x should be in the range 0.20 - 0.33. The metal ions and anions appear in different layers (Figure 2). The metal ion host layer has the brucite structure of Mg(OH)2, in which the metal ions are octahedrally coordinated by OH- ions. Part of the divalent metal ions is replaced by trivalent ions, leaving the brucite structure intact. Consequently, this layer has a net positive charge which is compensated by the charge of the anion layer. The empty sites of the anion layer are filled with water molecules. More details can be found in a review paper [8]. For our work, four commercially available HTCs (PURAL MG70, PURAL MG61 HT, PURAL MG50 and PURAL MG30) were obtained from SASOL. They are aluminum magnesium hydroxide carbonates of general formula (Mg)1-x(Al)x(OH)2(CO3)x/2.mH2O. Their specifications are given in Table 3. We also prepared several HTC samples in-house, called ECN-HTC. To a well-stirred NaHCO3 solution of 65 ºC, a solution containing the metal nitrates in the ratio Mg/Al = 3 was added dropwise resulting in precipitation of the HTC precursor. The pH was kept at a constant value of 8.00 by adding a NaOH solution when needed. After that, the solution was allowed to cool down to room temperature overnight under vigorous stirring. Next day, the precipitate was separated from the suspension by filtration and dried for 24 hours at 120 ºC.

The HTC samples, both commercial and in-house prepared, were activated by heating them in air to 400 ºC and by keeping them at this temperature for 4 hours. After that, the obtained powders were loaded with 22 wt% K2CO3 using dry impregnation and dried overnight [9]. The impregnated powders were compacted at a pressure of 275 atm and at room temperature. A sieve fraction of these particles wasobtained with sizes between 0.212 and 0.425 mm.

The experiments were performed using a reactor tube of 16 mm inner diameter and 150 mm length. The reactor was filled with 3 g of particles. The sample was heated under N2 to the temperature of the experiment, usually 400 ºC. Humidified N2 (29% H2O) was passed along the sample for 75 minutes to remove any calcination products, the so-called pre-desorption step. After that, a series of adsorption/desorption cycles was applied. The conditions of the adsorption and desorption steps of a


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standard experiment are given in Table 4. The duration of both steps is 75 minutes and 60 gas samples per step from the dry effluent are analysed by a MicroGC (Hewlett Packard M200H), using a sample interval of 75 s. A permapure is used for drying the effluent. At the end of the experiment, the sample is cooled down to room temperature under dry N2. The water is evaporated and mixed with the feed gas flow in a CEM (Controlled Evaporation Mixer) unit. The mass flow controllers, CEM unit and valves are computer-controlled. A schematic diagram is shown in Figure 3.

Table 3 Specifications of HTCs according to the supplierspec. surface

(m²/g)pore volume

(g/cm³)bulk density

(g/cm³)MgO:Al2O3(wt%:wt%)

PURAL MG70 > 1801) > 0.21) 0.35 - 0.55 70:30PURAL MG61 HT 16 not specified 0.15 61:39PURAL MG50 > 2001) > 0.21) 0.45 - 0.65 50:50PURAL MG302) > 2501) > 0.51) 0.35 - 0.55 30:701) After 3 hours of activation at 550 ºC2) PURAL MG30 contains a significant amount of boehmite

Table 4 Standard experimental conditionsadsorption desorption

flow (ml/min) 30 100composition 5% CO2/29% H2O/66% N2 29% H2O/71% N2T (ºC) 400 400duration (min) 75 75

N2

CO2

7 vent

permapure

ventbypass

throughreactor

reactorvalve

CEM

liquidwater

water flow controller

CH4

Mass flow controller

He

flow meter

MicroGC

N2

CO2

7 vent

permapure

ventbypass

throughreactor

reactorvalve

CEM

liquidwater

water flow controller

CH4

Mass flow controller

He

flow meter

MicroGC

Figure 3 Schematic diagram of the lab-scale experimental apparatus

To determine the amount of CO2 adsorbed and desorbed by the HTC samples only during respectively the adsorption and desorption steps, a so-called blank experiment was performed to correct for instrumental effects. For the blank experiment, a non-adsorbing material (SiC) of the same particle size range and bed volume as the adsorbing samples was used. The experimental conditions were identical to those applied in the adsorption and desorption steps of the experiments using an adsorbing sample. The following parameters were varied: HTC composition, K2CO3 loading, H2O fraction of the feed gas, promoter,


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operation temperature, desorption flow and desorption time. Finally, the HTC sorbent was mixed with a low-temperature steam reforming catalyst and a CH4/H2O/N2 gas mixture was fed to the reactor to investigate sorption-enhanced reforming of methane.

A simple Excel spreadsheet reactor model has been developed in which the reactor is considered to be a series connection of many small reactors. In each small reactor, the outlet gas composition and flow, and the CO2 loading are calculated from the inlet gas by using reaction rate expressions of the WGS and SMR reactions and a model for the CO2 adsorption. The outlet gas of the one reactor is the inlet gas for the next reactor in the series. For the next time step, the calculated CO2 loading becomes the actual loading. The composition and flow of the outlet gas of the last element are those of the effluent gas of the total reactor. The model is isothermal and isobaric, and does not include molecular diffusion. For steam reforming, the kinetics of Wei and Iglesia has been used, who investigated the kinetics of a Rh/alumina catalyst for CH4partial pressures in the range 0.05 - 4.5 bar and for temperatures in the range 600 - 850 ºC [10]. They found that the kinetics is independent of the H2O and CO2 partial pressures in the same pressure range. For the water-gas shift reaction, the equation of Xu and Froment was applied who investigated the steam reforming kinetics of a Ni/alumina catalyst including the kinetics of water gas shift [11]. For CO2 adsorption, the linear driving force equation is used:

( )eqLD nnk

dtdn

−−= ,

where n is the CO2 loading at time t, and neq is the equilibrium loading as determined by the adsorption isotherm i.e. by the CO2 partial pressure. The proportionality constant kLD is determined experimentally.

Table 5 Values of Freundlich and Langmuir parametersparameter value parameter valuek (mol/kg) 0.518 m (mol/kg) 0.38n (-) 4.205 b (Pa-1) 5.29e-4

Table 6 CO2 adsorption capacities at breakthrough and during the whole adsorption step at various conditions for the 20th cycle

tbt(min)

qads(tbt) (mmol/g)

qads(t=75 min) (mmol/g)

HTC samplePURAL MG70 7.5 0.14 0.33PURAL MG61 HT 8.8 0.18 0.28PURAL MG50 7.9 0.17 0.33PURAL MG30 12.5 0.29 0.44Mg-Al HTC 9.0 0.18 0.35K2CO3 loading (wt%)0 2.5 0.02 0.0611 6.4 0.13 0.1822 6.9 0.14 0.1833 6.4 0.12 0.1644 5.2 0.09 0.13H2O content of feed gas (%)0 5.0 0.10 0.217.5 6.1 0.13 0.2915 8.8 0.20 0.3329 8.8 0.21 0.33temperature (ºC)400 8.8 0.18 0.33450 8.7 0.18 0.32500 7.5 0.14 0.27

To describe the adsorption by K-promoted hydrotalcite (kHTC), we used the Freundlich isotherm given by:

nCOeq

ppkn

/1

0

2

= ,


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as well as the Langmuir isotherm:

2

2

1 CO

COeq

bpbp

mn+

= ,

where pCO2 is the partial CO2 pressure, p0 = 1 bar, and k and n, and m and b are pairs of parameters which characterize the strength of the adsorption. Their values, denoted in Table 5, have been found by fitting the above expressions to the measured CO2 adsorption at various pressures in the range 0,01 – 0,3 bar and at 400 ºC. The Freundlich.expression for the adsorption isotherm was preferred to the Langmuir expression since the CO2 adsorption continues to increase even at high CO2 pressure (up to 16 bar).

0.0

0.4

0.8

1.2

1.6

PURAL MG70/22 wt% K2CO3



PURAL MG61 HT/22 wt% K2CO3

ECN-HTC/22 wt% K2CO3

blank

adsorption

0

1

2

3

4

0 10 20 30 40

Elapsed time (min)

CO

2flo

w (m

l/min

)





blank

desorption

0.0

0.4

0.8

1.2

1.6





ECN-HTC/22 wt% K2CO3

blank

adsorption

0

1

2

3

4

0 10 20 30 40

Elapsed time (min)

CO

2flo

w (m

l/min

)





blank

desorption

Figure 4 Adsorption and desorption profiles of various potassium- promoted HTC samples (the desorption profiles of ECN-HTC/22 wt% K2CO3 coincides with that of PURAL MG70/22 wt% K2CO3)

Results and discussionFigure 4 shows the adsorption and desorption profiles corresponding with the 20th cycle of the

experiments up to t = 40 min using the various PURAL samples and ECN-HTC, all loaded with 22 wt% K2CO3. The CO2 adsorption decreases during the first 20 cycles for all samples. After the 20th cycle, it becomes approximately constant. The results of the blank experiments are indicated by dashed lines.

The adsorption profiles of PURAL MG70, MG61 HT and ECN-HTC are similar (breakthrough times respectively 7.5 min, 8.8 min, 9.0 min), whereas that of PURAL MG50 starts to break through at the same


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time as the before-mentioned ones (7.9 min) though it rises less strongly, whereas breakthrough of PURAL MG30 clearly occurs at a later time (12.5 min). For all samples we find that after breakthrough, the profile does not show a sharp rise towards the inlet CO2 flow (1.5 ml/min), but rather slowly creeps to this value. Table 6 shows the calculated amount of CO2 adsorbed up to the breakthrough time tbt and during the whole adsorption step (here 75 min), respectively qads(tbt) and qads(t=75min). About two thirds of all CO2 is adsorbed before breakthrough. The desorption profiles show a rapid initial CO2 release, which considerably slows down during deeper desorption and never becomes zero. At the end of the desorption step, the CO2 flow is in the range 0.05 - 0.1 ml/min. Apart from the desorption peak, which is highest for PURAL MG30, theprofiles of the various samples are quite similar. The desorbed amount exceeds the adsorbed amount of CO2. During impregnation of calcined HTC with K2CO3, the original HTC structure is partly restored due to the memory effect [8]. When the impregnated HTC is heated again, calcination of the restored structure occurs which is apparently not complete after the pre-desorption step.

0.0

0.4

0.8

1.2

0 2 4 6 8

44%

33%

11%

22%

non-impregnated

adsorption

0.0

0.4

0.8

1.2

1.6

0 10 20 30 40Elapsed time (min)

CO

2flo

w (m

l/min

)

44%

33%

11%

22%

non-impregnated

desorption

0.0

0.4

0.8

1.2

0 2 4 6 8

44%

33%

11%

22%

non-impregnated

adsorption

0.0

0.4

0.8

1.2

1.6

0 10 20 30 40Elapsed time (min)

CO

2flo

w (m

l/min

)

44%

33%

11%

22%

non-impregnated

desorption

Figure 5 Adsorption and desorption profiles of PURAL MG70 with different K2CO3 loadings

Figure 5 shows the adsorption and desorption profiles corresponding with the 20th cycle of experiments using PURAL MG70 with different K2CO3 loadings. For comparison, also the unloaded sample is included. It is seen that the CO2 adsorption is enhanced by a factor 3 when loading the HTC with K2CO3,implying that the original HTC structure is not simply restored upon contacting the calcined product with K2CO3. The CO2 adsorption is not very sensitive to the loading percentage (Table 6). The desorption profiles of the various samples are quite similar. The shift of the peak position is due to the extent of sorbent loading, not to differences in desorption properties of the various samples.

The presence of water during adsorption is important. Without water, the CO2 adsorption capacity is smaller than in the presence of water. At H2O fractions in the feed gas of 15% or higher, the CO2adsorption does not increase (Table 6). The temperature of the experiment, which refers to both the


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20

60

100

140

0 50 100 150 200 250 300Purge gas (mol gas/mol adsorbed CO2)

Des

orpt

ion

perc

enta

ge (%

)

`

75 min50 min25 min10 min

Figure 6 Desorption characteristics determined for different desorption times indicated in the figure (tads = 10 min, Fdes = 30 ml/min)

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

0 5 10 15 20 25 30 35 40

Time (min)

CH

4,CO

2,CO

con

cent

ratio

n (%

)

0

2

4

6

8

10

12

14

16

H2c

once

ntra

tion

(%)

reaction regeneration

solid line: experiment

dashed line: calculated from modelH2

CH4

CO2

Figure 7 Effluent gas composition during reaction and regeneration using a mixture of steam reforming catalyst and HTC impregnated with 22 wt% K2CO3

3

4

5

6

7

8

0 20 40 60 80 100

Cycle number

CO

2upt

ake/

rele

ase

(ml)

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

CH

4con

vers

ion

(%)adsorbed CO2

CH4 conversion

desorbed CO2

Figure 8 Adsorbed and desorbed amount of CO2 during all cycles


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temperature of activation and the temperature at which the series of adsorption and desorption cycles is performed, does not have a strong effect on the CO2 adsorption capacity. From 400 to 450 ºC, it hardly varies, while it has slightly decreased at 500 ºC (Table 6).As pointed out in the introduction, S/CO2, the purge gas to adsorbed CO2 ratio, is an important parameter. The desorption characteristic yields the variation of the fraction of desorbed CO2 with S/CO2. The desorption flow and period have been decreased stepwise to reduce the value of S/CO2 for complete desorption, (S/CO2)c. Figure 6 shows the desorption characteristics of PURAL MG70 loaded with 22 wt% K2CO3, determined at different desorption periods (tads = 10 min, tdes = 10, 25, 50, 75 min, Fads = Fdes = 30 ml/min). Decreasing the desorption flow results in a smaller (S/CO2)c, though the CO2 adsorption capacity is also reduced such that (S/CO2)c shows a net decrease. When the desorption flow is decreased from 100 to 10 ml/min, (S/CO2)c drops by approximately a factor 5. (S/CO2)c decreases by a about factor 3 when tdes is reduced from 75 to 10 minutes. This indicates that the smaller part of the bed which is used, the more easily the adsorbed CO2 can be removed from the bed.

Combined with a SRM catalyst, the HTC sorbent enhances the CH4 conversion by taking away the produced CO2 in the reaction zone. This was done for a mixture of 1.5 g low-temperature steam reforming catalyst and 3.0 g PURAL MG70 impregnated with 22 wt% K2CO3. As before, the sample was subjected to a series of reaction/regeneration cycles at 400 ºC. Feed gas containing 2.9 % CH4, 17.4% H2O and balance N2at a flow rate of 25 ml/min was fed to the sample during the reaction step, while the standard feed gas was used for regeneration (Table 4). The results of the last (100th) cycle are shown in Figure 7. More H2 is produced during reaction than predicted by thermodynamic equilibrium without sorbent. An average CH4conversion of 95% is obtained (thermodynamic equilibrium conversion of CH4 without sorbent: 53%), demonstrating the suitability of HTC for sorption-enhanced reforming of methane. The adsorbed and desorbed amounts of CO2 during all cycles are shown in Figure 8 indicating a stable performance.

0%

1%

2%

3%

4%

5%

6%

0 10 20 30 40 50

Elapsed time (min)

CO

2con

cent

ratio

n (%

)

Freundlich

Langmuir

5th cycle

20th cycle

0%

1%

2%

3%

4%

5%

6%

0 10 20 30 40 50

Elapsed time (min)

CO

2con

cent

ratio

n (%

)

Freundlich

Langmuir

5th cycle20th cycle

Figure 9 Calculated adsorption (left) and desorption (right) profiles using the Freundlich and Langmuir fits

0

0,5

1

1,5

2

0 10 20 30 40 50Time (min)

CO

2, C

O, C

H4c

once

ntra

tion

(%)

0

2

4

6

8

10

H2c

once

ntra

tion

(%)

CO2

CH4

H2

CO0%

10%

20%

30%

40%

50%

60%

0,0 0,2 0,4 0,6 0,8 1,0Bed position (-)

CH

4con

vers

ion

Figure 10 Right : calculated effluent dry gas composition using the Freundlich (solid line) and Langmuir (dashed line) fits ; left : CH4 conversion over bed length without CO2 adsorption taking place

In addition to the experiments, calculations were performed using the Excel spreadsheet model. The reactor was divided into 2000 elements and a time step of 30 s was used. Calculations of 100 time steps were performed. For kLD, a value of 0.01 s-1 was used, determined by the slope of the adsorption profile after breakthrough. Adsorption and desorption profiles are shown in Figure 9, using both the Freundlich and Langmuir fits to the experimental adsorption isotherm, and compared with those of PURAL MG70 with 22 wt% K2CO3. It is seen that the breakthrough time is well predicted, but the rise and tail of the adsorption


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profile are not well described. This may be due to chemical interaction of HTC with CO2 next to physical adsorption [6]. The desorption profile is not well described by both isotherm fits.

The experiment with a mixture of adsorbent and catalyst was also modelled. Figure 7 shows the calculated H2 and CH4 concentration during the reaction step, shown by dashed lines. They differ from the experimental results. The calculated CH4 concentration is higher, and the calculated H2 concentration is thus lower than the experimental values, amounting to a CH4 conversion of 62%, whereas experimentally a conversion of 95% was found. Even for a six times higher catalyst amount, the calculated CH4 conversion is lower (89%). The catalyst is more active than predicted by the Wei and Iglesia kinetics. A possible explanation is that they derived their kinetic expressions in a different temperature range than used here. In Figure 10, left, the calculated effluent compositions are compared for the Freundlich and Langmuir fits. For Freundlich, a sharper breakthrough of CO2 is observed than for Langmuir. The calculated profile of CH4conversion over the bed length is shown in the left part of Figure 10, indicating that equilibrium conversion is hardly obtained once the adsorbent has become saturated with CO2, i.e. when no adsorbent is present.

ConclusionsThe selected high temperature CO2 sorbent, Mg-Al HTC, is able to adsorb CO2 at temperatures

between 400 and 500 ºC. The adsorption is considerably enhanced by impregnating this HTC with K2CO3. Loadings between 11 and 44 wt% have been tested, but the adsorption proved not to be very sensitive to the loading percentage. The adsorption, at least the part after breakthrough, and desorption profiles of the potassium-promoted HTC consist roughly of two parts: a part characterized by fast adsorption (desorption) and a part characterized by slow adsorption (desorption). In the performed experiments, desorption was never complete. The end of the desorption profiles did not go to zero and the desorbed amounts of CO2exceeded the adsorbed amounts of CO2. This is most probably due to CO2 released from the HTC structure. Water assists the CO2 adsorption and desorption process. In the investigated temperature range, 400 - 500 ºC, the CO2 loading capacity hardly varies. Lowering the desorption flow or the duration of the desorption step, results in smaller values of (S/CO2)c. A proof-of-the-principle test has shown the workability of SERP for the steam reforming of methane.

A simple Excel spreadsheet model has been developed which describes the experimental results satisfactorily, though not sufficiently accurately. Possible reasons for that are the chemical interaction of HTC with CO2, which is not taken into account by the model, and the use of kinetic expressions for the reaction rate of the steam reforming and water-gas shift reactions which do not apply to the conditions of the experiments here.

All investigated systems show an efficiency gain with respect to the reference system (GTCC with amine scrubber). The highest efficiency is obtained for the CaO-based SE-SMR system. The efficiency is very sensitive to the inclusion of pressurized underfiring (only the CaO based systems), and to the S/CO2value. Critical issues are the reactor concept, the low S/CO2 value, and technology of pressurized underfiring.

AcknowledgementsThis research has been carried out by ECN in the CATO-project (Carbon Capture, Transport and

Storage), which is financed by the BSIK subsidy program of the Dutch Government agency SenterNovem. It is also part of ECN's DECAFF-program (Decarbonisation of Fossil Fuels), which is sponsored by the Dutch ministry of Economic Affairs.

References[1] W.F. Waldron, J.R. Hufton and S. Sircar, AIChE Journal vol. 47, p1477 (2001).[2] M. Kato M., S. Yoshikawa and K. Nakagawa, Journal of Materials Science Letters, vol. 21, 485

(2002).[3] B. Balasubramanian, A. L. Ortiz, S. Kaytakoglu and D. P. Harrison, Chemical Engineering Science,

vol. 54, p3543 (1999).[4] Y. Ding, E. Alpay, Chemical Engineering Science, vol. 55, p3461 (2000).[5] A. Roine, HSC Chemistry, Outokumpu (version 5.0), 2002.[6] R.J. Allam, R. Chiang, J.R. Hufton, P. Middleton, E.L. Weist and V. White. Chapter 13 Development

of the sorption enhanced water gas shift process. In Carbon Dioxide Capture for Storage in Deep Geologic Formations - Results from the CO2 Capture Project, Vol. 1, D.C. Thomas and S.M. Benson (Eds.), Elsevier: Oxford (2005).

[7] J. Meyer, R.J. Aaberg and B. Andresen. Chapter 12 Generation of hydrogen fuels for the thermal power plant with integrated CO2 capture using a CaO-CaCO3 cycle. In Carbon Dioxide Capture for Storage in Deep Geologic Formations - Results from the CO2 Capture Project, Vol. 1, D.C. Thomas and S.M. Benson (Eds.), Elsevier: Oxford (2005).

[8] F. Cavani, F. Trifirò, A. Vaccari, Catalysis Today, vol. 11, p173 (1991).


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[9] S. Nataraj, B.T. Carvill, J.R. Hufton, S.G. Mayorga, T.R. Gaffney, J.R. Brzozowski, Air Products and Chemicals Inc., EP Patent no. 1006079A1 (2000).

[10] J. Wei and E. Iglesia, Journal of Catalysis, vol. 225 no. 1, p116, 2004.[11] J. Xu and G.F. Froment, AIChE Journal, vol. 35 no. 1, p88 (1989).

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