Simulation of an oxygen membrane

Transcription

PAPER
www.rsc.org/ees | Energy & Environmental Science
Simulation of an oxygen membrane-based combined cycle power plant:
part-load operation with operational and material constraints
Konrad Eichhorn Colombo,*a Olav Bolland,a Vladislav V. Khartonb and Christoph Stillerc
Received 22nd May 2009, Accepted 10th September 2009
First published as an Advance Article on the web 4th November 2009
DOI: 10.1039/b910124a
This paper presents the design and part-load performance of a natural gas-fired oxy-combustion
combined cycle power plant for CO2 capture. The combustion chamber of a conventional gas turbine
was replaced by a membrane reactor, making it possible to obtain a highly concentrated CO2 stream for
long-term storage. The focus was on power plant operation with a view to operational and material
constraints of individual process components to ensure their proper performance and required lifetime.
In this respect, the mixed-conducting membrane modules were of particular interest. Temperature as
well as concentration limitations for CO2 and other chemical species narrowed the operating window.
Other critical reactor components added further constraints. For part-load operation of the power
plant, two load control strategies were analysed for the gas turbine operating at constant rotational
speed. In the first load control strategy, variable guide vanes were used to manipulate the mass flow of
air to the gas turbine compressor. This degree of freedom was used to control the turbine exit
temperature. In the second control strategy, variable guide vanes were not used and the turbine exit
temperature was allowed to vary. For both load control strategies, the mean solid-wall temperature of
the membrane modules was maintained close to its design value, which led to improved stability. The
load-control strategy using variable guide vanes was superior to the strategy without variable guide
vanes due to higher combined cycle efficiencies and increased load-reduction capability. Moreover, the
performance of the catalytic combustors in the membrane reactor, operating at near stoichiometric
conditions, also improved as a result of increased oxygen concentrations at part-load operation.
Relevant process components were based on spatially distributed conservation balances for energy,
species, mass, and momentum. A stability diagram was incorporated into the membrane module model
to investigate the risk of degradation. Performance maps were used for turbomachinery components.
1. Introduction
The power and industry sectors combined contribute more than
60% of all anthropogenic CO2 emissions.1 The increasing
demand for energy will lead to further emissions increases unless
a
Department of Energy and Process Engineering, Norwegian University of
Science and Technology, Kolbjørn Hejes vei 1B, 7491 Trondheim, Norway.
E-mail: [email protected]; [email protected]; Tel: +47 735
92744
b
Department of Ceramics and Glass Engineering, CICECO, University of
Aveiro, 3810-193 Aveiro, Portugal. E-mail: [email protected]
c
Ludwig-B€
olkow-Systemtechnik GmbH, Daimlerstr.15, 85521 Ottobrunn,
Germany
environmental issues are addressed. CO2 capture and storage is
one option in the portfolio of actions for stabilizing atmospheric
greenhouse gases. Novel power plants with CO2 capture, along
with system improvements of existing power plants, offer
potential for controlling emissions. These novel power plants can
be largely classified into oxy-combustion, pre-combustion, and
post-combustion power plants.1 The principle of an oxycombustion power plant concept is given in Fig. 1, and a detailed
process flow sheet is shown in Fig. 2. In this oxy-combustion
system, the combustor in a conventional gas turbine (GT) power
plant (Fig. 3) is replaced by a membrane reactor, which is
comprised of mixed-conducting membranes for integrated air
separation.1–6 Dilution of the CO2 with nitrogen in the air coming
Broader context
At present, fossil fuels (oil, gas, and coal) represent the largest share of all primary energy sources and will most likely remain
dominant over the coming decades. The combustion of fossil fuels produces large amounts of CO2 during the energy conversion
process. With a share of over 60%, power generation represents the largest source of anthropogenic CO2. Carbon dioxide capture
and storage is one option in the portfolio of mitigation methods to stabilize emissions of CO2. Oxy-combustion power plants using
membrane technology have been proposed as one possible approach for CO2 capture. However, the use of membranes for integrated
air separation is very challenging due to several constraints that limit power plant performance and the lifetime of critical process
components.
1310 | Energy Environ. Sci., 2009, 2, 1310–1324
This journal is ª The Royal Society of Chemistry 2009
Fig. 1 Membrane-based combined cycle power plant. The temperature
distribution is indicated throughout the process.
from the GT compressor is avoided and only oxygen-depleted air
is emitted to the atmosphere after it passes the heat recovery
steam generator. However, the use of afterburners (Fig. 2) leads
to some CO2 emissions, although efficiency and power output are
both increased. Heat is generated by the combustion of natural
gas, here assumed to be pure methane, with the oxygen that is
transported through the membranes. The combustion products
(mainly water vapour and CO2) are recycled to moderate the
combustion temperature and to increase the driving force for
oxygen transport. The recycle loop is indicated in Fig. 1 and
Fig. 2. A fraction of this stream is bled off from the recycle loop
and used to produce steam in a steam cycle for additional power
generation. After heat recovery, the CO2 is conditioned,
compressed, and transported to a storage site.
Fig. 2 Membrane-based combined cycle power plant with CO2 conditioning.
Energy Environ. Sci., 2009, 2, 1310–1324 | 1311
Fig. 3 Conventional combined cycle power plant.
The industrial use of such membranes for power generation is
demanding with respect to properties that determine oxygen
permeation and long-term stability of the materials used. Some
general target properties are given in Table 1. In addition to
desired oxygen transport properties, the membrane must maintain sufficient stability under all foreseeable operation conditions
to prevent bulk decomposition and membrane failure caused by
thermo-mechanical and chemical stresses. So far, there is no
membrane material that has optimal properties for all potential
operating conditions as well as for processing and cost-related
aspects. For example, any increase in stability-related properties
is usually associated with worsening of other properties, such as
the oxygen permeation rate.7 Optimization of operating conditions of the membrane reactor should thus account for the
material-specific properties of the membrane, including possible
microstructural and surface degradation. Most of the highly
permeable perovskite-related compositions containing alkalineearth elements are partly or fully unstable under the operation
conditions considered in this work because of interaction with
gas species such as CO2 and water vapour.8,9 However, perovskite-related compositions without alkaline-earth cations give
relatively low oxygen permeation fluxes.7,10,11 In this work, the
dense membrane layer was assumed to be made of the perovskite-related material La2NiO4+d 12,13 and coated onto a porous
support substrate. Using the same material for the porous
support ensures thermo-mechanical and chemical integrity.14
Although this family of materials does not have the highest
oxygen permeation rate, it has the major advantages of relatively
low thermal and chemical expansion, the absence of alkalineearth constituents, and relatively easy processing.13,15 The
membrane modules and heat exchangers (HXs) in the membrane
reactor are assumed to be counter-current two-fluid monoliths
with a high area-to-volume ratio for high heat and mass transport,9,16 as shown in Fig. 4. A drawback of this monolith technology is the complex manifold system9 and a low tolerance of
pressure differences, which lead to mechanical stress. Furthermore, Selimovic17 recently indicated that two-fluid monoliths
exhibit a large pressure drop due to the flow distributors.
Although a large number of membrane materials and configurations have been evaluated to date (see ref. 10, 11, 18–20, and
references therein), there are only a few studies in the published
literature that address incorporating membranes for air separation in power plants. Kvamsdal et al.21 analysed a membranebased power plant using steady-state simulation tools, where
process components were represented by lumped models. Such
models provide information about efficiency and power output
as well as average heat and mass transport rates. But a more
realistic picture of performance, i.e. where operational and
material constraints are incorporated into the analysis, cannot be
given. Selimovic22 developed a two-dimensional model of the
membrane module and analysed the membrane reactor using
a zero-dimensional model. Further, Selimovic et al.23 developed
a one-dimensional membrane module model based on a finite
volume method. But the very important link between the
membrane reactor and turbomachinery has yet to be evaluated.
The membrane-based power plant shares several similarities
with solid oxide fuel cell GT hybrid cycles. For instance, both
operate over similar temperature ranges.9,24 However, solid oxide
Table 1 Target properties of oxygen mixed-conducting membranes for
power generation applications
Relative oxygen
nonstoichiometry
variations with
respect to the values
under operation
conditions
Creep rate
Oxygen ionic
conductivity
Electronic conductivity
Thermal expansion
coefficient
Chemical stresses
Dd/doperation < 20%
Mechanical stresses
Oxygen permeation
<108 cm s1 75
>0.1 S cm1
Oxygen permeation
Thermo-mechanical
stresses
>10–30 S cm1
8–20 ppm K1 75
Fig. 4 Oxygen membrane module: two-fluid monolith (top), modelling
manifold (bottom).
fuel cell-based hybrid cycles have mostly been analysed for use in
small-scale power plants with less than a few MW of power
output. The variable rotational speed of the turbomachinery can
thus be applied to load control, providing an additional degree of
freedom in the system. The GT is here assumed to be connected
to the electrical grid, which means that a variable shaft speed is
not practical.
In the present work, a membrane-based combined cycle power
plant (CCPP) was analysed with respect to design as well as partload operation. Operational and material constraints were
emphasized to maintain sound power plant performance where
a reasonable lifetime of critical process components could be
achieved, such as for the membrane modules. The membranebased power plant was further compared to a conventional
CCPP without CO2 capture to demonstrate the effect of CO2
capture on efficiency and power output.
2. Combined cycle power plant
In the detailed flow sheet of the membrane-based power plant
(Fig. 2), fuel is injected into the membrane reactor by subsonic
ejectors (Fig. 5). Ejectors provide compression without moving
parts, which results in lower stress and high system reliability.
Moreover, they feature a simple design, high temperature resistance, and low-cost fabrication and operation.25–28 Steam that is
taken from the steam cycle must be added to maintain proper
ejector performance. Fuel pre-heating is required to avoid twophase flow in the ejectors. Compressed air is split after the GT
compressor, where the majority is passed through the branch
comprising low-temperature HXs, membrane modules, and
high-temperature HXs. The remainder is led to the bleed-gas
HXs. These two streams are mixed afterwards and fed to the
afterburners. In addition to oxygen separation, the membrane
modules operate as HXs. Oxygen is transported from the air to
the sweep gas stream, whereas heat transport occurs in the
opposite direction. In the catalytic combustors, the fuel reacts
with highly diluted oxygen. Excess oxygen is required for stable
and complete combustion.29,30 Afterburners increase the turbine
inlet temperature to the thermal limit of the turbomachinery,
thus increasing efficiency and power output. The CO2 produced
in the afterburners is emitted to the atmosphere. It is not practical to capture these rather small amounts of CO2 due to their
high nitrogen dilution. The exhaust gas is then fed into the heat
recovery steam generator for further power generation, and also
for producing steam that is used in the ejectors. After passing the
bleed-gas HXs, the sweep gas enters the CO2-compression system
including HXs for the cooling of the CO2-rich gas, flash
condenser, compressor, purification system, and pump. In the
purification system, impurities such as oxygen are removed to
produce a highly concentrated stream of CO2. The gas separation
can be accomplished by distillation.31–33 Carbon dioxide purities
of more than 99% can be achieved by these processes.32 However,
if the CO2 is used for enhanced oil recovery, the specifications
may be more stringent.31,32 In this work it is assumed that all
impurities, i.e. water and oxygen, are removed prior to end
compression.
3. Gas turbine process component modelling
3.1 Monolithic membrane modules and heat exchangers
Oxygen permeation of the mixed-conducting membrane is
controlled by both bulk diffusion and surface kinetics.12 Bulk
diffusion was based on an approximate of the Wagner equation20
i
c0 D0 h n
(1)
pO2 ; air pnO2 ; sweep gas
JO2 ¼
4thml n
with c0 the concentration and D0 the self-diffusion coefficient of
oxygen, thml the membrane thickness, n represents an additional
fitting parameter used for pressure dependency, and pO2 the
oxygen pressure. Please note that c0, D0, and n are temperaturedependent parameters. Fitting parameters were used to modify
eqn (1) as well as the source term in the species conservation
balances to make allowance for surface kinetic contributions.
The energy conservation balance for the gas phase is given by34
vTg
v
vTg
vhO2 ;g
¼
þ ahf Ts Tg 4JO2
(2)
rg cp;g vg
lg
vz
vz
vz
vz
and for the solid-wall (dense membrane layer and porous
support)34
0 ¼ ls
2
X
v2 Ts
vDhO2
þa
hf;i Tg;i Ts þ JO2
2
vz
vz
i¼1
where the following Nusselt number correlation was used35
Nu ¼
hf;i wch
¼ 3:091
lg;i
(4)
Species conservation in the gas phase for oxygen reads34
vg
vCg;O2
¼ kJO2
vz
The pressure drop was based on36
1wch DP
f ¼
2 L
rv2
(5)
(6)
with the friction factor35
f Re ¼ 56.91
Fig. 5 Section of the constant-area ejector model.
(3)
(7)
where r represents the density, cp is the heat capacity, T the
temperature, v the fluid velocity, z the spatially distributed
variable in axial direction, l the thermal conductivity, a the areato-volume ratio of the monoliths, hf the heat transfer coefficient,
hO2 the enthalpy of oxygen in the gas phase, wch the channel
width, L the monolith length, DP the pressure drop, and Re the
Reynolds number. The index ‘g’ denotes the gas phase and ‘s’ the
solid-wall.
The monolithic HXs in the membrane reactor were based on
the same set of conservation balances. However, the source term
for oxygen transport was omitted. Individual species conservation balances were replaced by a total mass balance to moderate
model complexity.
3.5 Combustor
The combustor model (catalytic combustors and afterburners)
was based on the following equilibrium reaction
CH4 + 2O2
2H2O + CO2
(14)
3.2 Gas turbine
Isentropic efficiency calculations were applied for the basic GT
compressor and turbine model.37 Steady-state performance maps
were incorporated to calculate the mass flow, pressure ratio,
isentropic efficiency, and cooling flow rate during off-design
operation.38,39 Variable guide vanes (VGVs) were used to control
the mass flow of air entering the GT compressor.37,39 The surge
margin (SM) was calculated by39
Psurge Pworking line
(8)
SMc ¼ 100
Pworking line
Off-design performance of the GT turbine was represented by the
Stodola equation40
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
u
u
Pout
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffirffiffiffiffiffiffiffiffiffiffiffirffiffiffiffiffiffiffiffiffiffiffiffiu 1 Pin
MW
gin
Tin;DP u
m_
Pin
u
¼
2 (9)
Tin u
m_ DP Pin;DP MWDP gin;DP
t
Pout;DP
1
Pin;DP
3.6 Recycle loop
The pressure in the recycle loop on the sweep gas side was
calculated by the ideal gas law36
PV ¼
mRT
MW
(15)
with m as the total mass in the recycle loop, V the volume, and R
the universal gas constant.
3.7 Combined cycle power plant
The efficiency of the CCPP was calculated by37
h¼
Pgen Pcons
n_ fuel LHVfuel
(16)
where m_ is the mass flow, MW the molecular weight, and g the
isentropic exponent. The indexes ‘in’, ‘out’, and ‘DP’ indicate the
inlet, outlet, and design point conditions, respectively.
with Pgen as the generator power output, Pcons is the sum of work
related to power plant internal consumers, n_ fuel the mole flow of
fuel and LHVfuel its lower heating value.
Further modelling assumptions for the combined cycle power
plant can be found in Table 2.
3.3 Ejector
3.8 Model implementation
The one-dimensional ejector model was divided into five
sections: (i) nozzle critical section, (ii) nozzle outlet, (iii) mixing
section inlet, (iv) diffuser inlet/mixing section outlet, and (v)
diffuser outlet. The description of ejector sections has been
generalized here, where the indexes 1, 2, and 3 refer to inlet and
outlet sections, respectively.
The energy balance was defined by36
The models of the membrane-based GT power plant and
conventional GT power plant as well as the CO2-compression
system were executed in gPROMS.41 The HX for steam
production and sweep gas cooling were modelled in Aspen
HYSYS.42 For both simulation tools, a Soave–Redlich–Kwong
equation of state was used with a very good match for the
physical properties, meaning that consistency was maintained.
The steam cycle and HRSG were modelled in GTPRO/
GTMASTER.43 The steam pressures for the dual-pressure steam
cycle were selected in accordance with current industry practice
for this range of power output and this type of steam cycle.
m_ 1(h1 + ½v21) + m_ 2(h2 + ½v22) ¼ m_ 3(h3 + ½v23) + Hloss
(10)
where h is the enthalpy, and Hloss is the heat loss. The mass
balance is given by36
(11)
4. Operational and material constraints for
individual process components
(12)
where A is the area.
The operational and material constraints for the membranebased power plant are explained and summarized in Table 3.
Constraints that also need to be considered in a conventional
power plant are indicated by italic letters.
3.4 Pipe
4.1 Mixed-conducting membranes
The pipe models account for further pressure drops in the power
plant using the following friction factor35
At isothermal conditions, the membrane expands with
decreasing oxygen pressure. When decreasing the temperature at
constant oxygen pressure, the membrane contracts because of
thermal and chemical geometric changes as a result of a increased
equilibrium oxygen stoichiometry.44 These volume changes must
be considered to keep mismatches low between the porous
m_ 1 + m_ 2 ¼ r3v3A3
The momentum balance for the mixing section reads36
P3A3 P2A2 P1A1 ¼ m_ 1v1 + m_ 2v2 m_ 3v3
f Re ¼ 64
in conjunction with eqn (6).
(13)
Table 2 Detailed assumptions for the membrane-based and conventional combined cycle power plant
Fuel preparation and handling of power consumption
- Fuel is assumed to be pure methane, available at 4 MPa
- Fuel temperature in the membrane reactor after preheating is 398 K (fuel heating, e.g. by the CO2-rich exhaust gas prior to the CO2-compression
system; additional heat integration was not included in the analysis)
- Fuel temperature to the afterburners 278 K
- Lower heating value ¼ 8 MJ mol1
Membrane reactor
- Number of manifolds of type 1 is 12
- Number of manifolds of type 2 is 48
- Membrane module length is 0.1625 m; high-temperature heat exchanger length is 0.355 m; low-temperature heat exchanger length is 0.9825 m; bleedgas heat exchanger length is 1.5 m; the height and width of all monoliths is 0.15 m
- Gas channel width of membrane module and monolithic heat exchangers is 1.5 mm
- Thickness of the membrane layer is 30 mm
- Length of porous support with protective additives is 0.01625 m
- Area-to-volume ratio of the monoliths is 895 m2 m3
- Number of gas channels of the monoliths is 6690
- Split fraction of air to the bleed-gas heat exchanger is 15%
- Radiative interaction between reactor components is neglected
Gas turbine
- Variations of the isentropic efficiency for compressors are calculated by performance maps38 (84.5% in design)
- Variations of the isentropic efficiency for the turbine and the amount of cooling air that is extracted from the compressor discharge are calculated by
performance maps39 (86.8% in design)
- Shaft mechanical efficiency is 99%76
- Generator mechanical efficiency is 98.5%76
- Cooling air fraction of compressed air is 11.6%39
Pipes
- Diameter of pipe 1 is 0.39 m; diameter of pipe 2 is 0.53 m; length of pipe 1 and pipe 2 is 5 m
- Diameter of pipe 3 is 0.2 m and length is 10 m
- Average velocity of pipe 1 is 53.7 m s1; average velocity of pipe 2 is 56.1 m s1; average velocity of pipe 3 is 21.1 m s1
Ejector
- Heat loss ¼ 1%
- Isentropic efficiency for the section-wise ejector model is based on data from ref. 77 and 78
- Operation in the critical mode70–72 (the mass flow of steam is therefore set)
- Diffuser area ¼ 0.058 m2; diffuser throat area ¼ 9.86 cm2; nozzle area ¼ 0.064 cm2
- Velocity in the mixing section and diffuser outlet is 121 m s1 and 2 m s1, respectively
Catalytic combustors and afterburners
- Heat loss ¼ 1%76
Heat recovery steam generator and steam cycle
- Steam temperature delivered to the GT power plant is 573 K
- Pressure levels in steam cycle are 0.5 MPa and 8 MPa, respectively
- UA for steam-producing heat exchanger is 16 kW K1
- UA for cooler is 103.4 kW K1
CO2 purification and compression process
- Variations of the isentropic efficiency for compressors are calculated by performance maps38
- Flash condenser: phase equilibrium between vapour and liquid phase operating at constant pressure79
- Purification system: total work input ¼ 0.44 MJ per kg CO2;31,32 cooling water flow rate ¼ 23.2 kg per kg CO2;32 complete removal of all impurities
- Pump isentropic efficiency ¼ 75%80
Other assumptions
- Ambient air: T ¼ 288 K; pressure ¼ 0.1 MPa; composition: 79% nitrogen, 21% oxygen
- Cooling water: T ¼ 283 K, temperature raise ¼ 10 K, pressure ¼ 0.1 MPa
support material and the dense membrane layer as well as
between all connections of the membrane-reactor components.
Membrane compositions such as La2NiO4+d, and its derivatives
offer some advantages because of their relatively low thermal and
chemical expansion.13,15 But the relatively large concentration of
La3+ cations may still reduce membrane performance. Hence,
essentially isothermal conditions and the highest possible operating temperature must be maintained to minimize stresses.45,46
In theory, the thermo-mechanical stability of the membrane may
be improved by applying additional constituents such as Ag or
Pd,47 but the long-term performance of these kinds of
membranes must still be carefully evaluated, because similar
stability problems may arise with these materials. Membrane
material decomposition may be associated with redox instability,
interaction with gaseous species and/or kinetic demixing,
resulting from a slow, but non-negligible cation transport under
the oxygen chemical potential gradient.48–50 Chemically-induced
expansion is among the most important thermo-mechanical
properties that relate to the volume changes under the oxygen
chemical potential gradients and determines the applicability of
Table 3 Operation and material constraints for the membrane-based combined cycle power plant. Constraints for the conventional power plant are
indicated by italic letters
Process component
Constraint
Effect
Limit
Catalytic combustors
Minimum oxygen mole fraction
0.5 mol%30
Catalyst in the catalytic combustors
Maximum sulfur concentration
Catalyst in the catalytic combustors
Maximum temperature gradients
Ejectors
Performance loss due to operation
outside the critical mode
Performance in critical range
025,81
GT compressor
Thermodynamic conditions with
respect to temperature, pressure,
and composition
Minimum total pressure difference
in mixing section
Pressure of actuating (fuel and
steam mixture) fluid
Maximum variable guide vane angle
limit
Minimum surge margin
Performance loss and failure due to
unstable and incomplete
combustion
sulfur poisoning
thermal shocks
Performance loss due to formation
of vapour–liquid phase
GT turbine
Cooling air fraction to GT turbine
GT turbine
Maximum turbine inlet temperature
High-temperature heat exchangers
Maximum temperature
Low-temperature heat exchangers
Minimum temperature
Low-temperature heat exchangers
High-temperature heat exchangers
Dense membrane layers and porous
support
support
support
Ejectors
Ejectors
GT compressor
support
support
support
Membrane modules and
monolithic heat exchangers
Pipes
All reactor components
Sealings and joined ceramics
Minimum temperature
Maximum temperature
respect to membrane
temperature, CO2 and O2
pressure in sweep gas
respect to membrane
respect to membrane
Maximum total pressure difference
Maximum fluid velocity
Maximum difference in thermal
expansion coefficients
Maximum leakage rate
membrane materials. These volume changes lead to oxygen
anion concentration gradients, variable-valence cation oxidation
states and average cation size across the membranes. The resultant differential strains induce mechanical stresses that can lead
to fracture. Similar effects can also arise from thermal expansion.
Mass flow reduction by variable
guide vanes
a sudden drop in pressure and
detrimental aerodynamic
pulsation
thermo-mechanical stresses
sulfur poisoning
sulfur poisoning
sulfur poisoning
thermo-mechanical stresses and
chemical interaction
carbonate formation
ppb range64
Depending on process conditions
Depending on process conditions70
30%73,74
5%66,67
Depending on technology39
1530 K82
1573 K16
673 K16
50 ppb16
(poisoning is favoured at low
temperatures)
10 ppb16
1173 K16
1323 K16
Depending on process
conditions9,16
oxidation
Depending on process conditions16
hydroxide formation
(favoured at low temperatures)9
mechanical stresses
Safety; performance loss and failure
due to noise and vibrational
problems
leakage
0.1 MPa
60 m s1 83
10–17%
0.1%
In general, when operating the membrane in a temperature
range of 973–1073 K at high CO2 and water vapour pressures,
the use of any membrane materials containing substantial
amounts of alkaline-earth elements may be problematic, if
carbonate formation or oxidation is thermodynamically
Fig. 6 Stability diagram for the dense membrane layer and porous
support based on ref. 16. Stable and unstable operation regions are
shown with respect to CO2 and oxygen pressure in the sweep gas as well
as solid wall temperature.
favourable under the corresponding equilibrium conditions.51
Sundkvist et al.16 provided a tentative stability diagram for
a membrane that shows regions where carbonate formation and
oxidation are likely to occur. This stability diagram was extended
by incorporating the oxygen pressure as an additional parameter,
as shown in Fig. 6. Temperatures below 1173 K in conjunction
with high CO2 pressures lead to the formation of oxycarbonates,
primarily La2O2CO3,51 followed by the complete decomposition
according to the general reactions
La2 NiO4þd þ 3CO2
La2 ðCO3 Þ3 þ NiO þ
d
O2
2
(17)
or, more likely,
La2 NiO4þd þ CO2
La2 O2 CO3 þ NiO þ
d
O2
2
(18)
Increasing the oxygen pressure leads to a moderately higher
stability in CO2 (non-vertical dashed line in Fig. 6). A reduced
oxygen pressure has an opposite and stronger effect (non-vertical
dotted line in Fig. 6).
At moderate operation temperatures, the La2NiO4+d
membrane may degrade due to oxidation.16,52 For undoped
La2NiO4+d, this process occurs via several reactions, leading to
the separation of layered Ruddlesden–Popper compounds and
lanthanum oxide.52 The latter may then react with CO2 or water
vapour. At the final stage, the oxidative decomposition can be
expressed as
La2NiO4+d + (½ d g)O2
LaNiO3g + ½La2O3 (19)
The stability limit at low temperatures is shifted to the right
(vertical dashed line in Fig. 6) and left (vertical dotted line in
Fig. 6) when the oxygen chemical potential increases or
decreases, respectively. However, both processes could promote
carbonate formation. A more complex shape of the membrane
stability boundary could therefore emerge.
The dense membrane layer is assumed to be on the air side
because of the high risk of carbonate formation at the sweep gas
inlet. The porous support acts as a protection layer by limiting
diffusion of gaseous species such as CO2. Caro53 suggested adding
small amounts of oxygen to improve stability. But this would
reduce oxygen transport because of a lower driving force in addition to a more complex reactor design. Exposing the membrane on
the feed side to high oxygen pressures increases the risk of oxidation. In this work the risk of carbonate formation was considered
to be more serious than oxidation. Stabilizing components were
assumed to be incorporated into the pores of the membrane
support layer at a high risk of carbonate formation. These additives should be catalytically active to increase oxygen permeation.
All known membrane materials contain chemical elements
that may evaporate under operational conditions, for
instance during start-up and shut-down of the power plant.9 For
Ni-containing materials, this degradation mechanism
involves primarily nickel hydroxide volatilization.54 The
resulting La-enrichment at the membrane surface may facilitate
other degradation processes such as lanthanum carbonate and/or
hydroxide formation. Under equilibrium conditions, the presence of metal hydroxides on the membrane surface is thermodynamically favourable only at low temperatures, far from the
operation conditions considered in this work. Hydroxide
formation was consequently not incorporated into the
membrane module model.
Pressure differences of the two fluids in the membrane modules
and HXs larger than one bar must be avoided to maintain
mechanical stability. Minimizing the wall thickness leads to
increased oxygen and heat transport rates. But improved mass
and heat transport by using thinner walls comes at the expense of
less operability and robustness as well as the requirement of more
advanced control equipment for the membrane reactor. The total
pressure in the recycle loop is measured at the outlet of the bleedgas HXs, but other locations are also possible.
Industrial supplies of natural gas typically contain 1 ppm
SO2,55 which is still too high for the mixed-conducting material
assumed here. Deep desulfurization is required.
4.2 Ceramic heat exchanger monoliths and sealing
The monolithic HXs in the membrane reactor are fabricated of
materials that are stable in atmospheres with high oxygen pressure and gaseous species such as H2O, CO2, and CO.56 However,
the HXs in the membrane reactor add some further constraints
with respect to temperature limits (Table 3). Sulfur limitations
and pressure differences between the two fluids are similar to
those specified for the membrane module. Additional issues
relate to the sealing of membrane reactor components. The
sealant should be hermetical and chemically and thermomechanically stable15,57,58 for the entire operating range. In
addition, the sealant and joined ceramic materials, such as flow
distributors, should have similar thermal expansion coefficients
to avoid significant mismatches.11,15,16,58,59 The number of sealants that satisfy these requirements is limited and includes
primarily SiO2-containing glass ceramics, phosphates and
various composites. Sealant materials based on BaO–La2O3–
SiO2–SrO have been proven to show a close match to La2NiO4+d
ceramics.15,58
4.3 Catalytic combustors
Due to the low excess oxygen concentrations in conjunction with
the high dilution of CO2 and water vapour, standard combustion
technology cannot be used in the combustors in the membrane
reactor. Instead, a staged catalytic combustor has been developed, where catalytic partial oxidation of methane is followed by
complete oxidation under lean conditions. A mixture of CO and
H2 is produced by catalytic partial oxidation, which provides
a hydrogen-stabilized combustion process in the second
combustion stage.29,60,61 Supported Rh-based catalysts are suitable for the combustor due to their high activity, selectivity,
resistance to carbon deposition, and sulfur resistance.62,63
Nevertheless, deep desulfurization is required to prevent
poisoning of the catalyst.64 The mechanical integrity of the
catalyst and its supports in the combustors should withstand
possible thermal shocks that can occur during rapid load changes
as well as during start-up and shut-down of the power plant.55
Excess oxygen is required to maintain stable and complete
combustion.29,30 But high concentrations of oxygen in the sweep
gas give rise to higher energy and cooling water requirements for
the purification system.31–33 Moreover, degradation of process
components located downstream of the membrane reactor may
also occur.65 If thermodynamic conditions are such that the
generation of carbon monoxide cannot be avoided, the hightemperature HXs and bleed-gas HXs could be coated with
a catalytic layer, where the carbon monoxide and all other
combustible compounds are converted before being fed to the
membrane modules and HX, respectively.
4.4 Turbomachinery and further fluid components
The GT compressor must have a sufficient SM (Fig. 7). Surging is
a complex, transient flow reversal that can cause severe damage
to the compressor airfoils.37,39 Walsh and Fletcher suggested
a SM for GT compressors in the power sector of 15–20%.39 This
value has been relaxed to increase the part-load capability of the
membrane-based power plant. But further reduction below 5% is
not recommended because the GT may then be operated in
a regime of subsonic stall and blade flutter.37,66,67 Material
constraints for the GT arise as a result of creep, oxidation, and
low-cycle fatigue.37,68 It should be noted that techniques for inlet
air cooling and NOx reduction can also significantly affect the
operating point of the GT.66 Due to the lack of available data,
such phenomena were not incorporated into the GT model.
The split ratio of air coming from the GT compressor is not
controlled for reasons of control simplicity. Valves in the main
streams should in general be avoided. After passing the two
branches comprising (i) bleed-gas HXs and (ii) low-temperature
HXs, membrane modules, and high-temperature HXs, both
streams are mixed. The length of both paths is nearly equal. In
practice, the total pressure drop could be different since the fluid
that is passed through the bleed-gas HX branch meets fewer flow
distributors. This could lead to oscillations in the membrane
reactor. The split ratio of the combustor exhaust gas is indirectly
controlled by means of the valve controlling the pressure in the
recycle loop.
5. Part-load operation
In a conventional CCPP (Fig. 3), external power demand is
controlled by varying the fuel flow rate to the combustors. This
type of load-control method is not practical for the membrane1318 | Energy Environ. Sci., 2009, 2, 1310–1324
Fig. 7 Performance maps for the gas turbine compressor with respect to
pressure ratio and isentropic efficiency.38
based power plant. Frequent and rapid changes of temperature
in the membrane modules and other critical reactor components
must be avoided to reduce stresses. For this reason, the mean
solid-wall temperature of the membrane modules was chosen as
a controlled variable. This variable is spatially distributed and
cannot be directly measured. Inferential control69 can be used in
order to establish a characteristic relationship between related
variables, which are more easily accessible, and the mean-solid
wall temperature. The fuel flow rate to the catalytic combustors
is manipulated to keep the mean solid-wall temperature of the
membrane modules close to the design value. Mechanical stresses
in the membrane modules and monolithic HXs in the membrane
reactor can be minimized by controlling the pressure difference
between the two fluids. The pressure on the air side throughout
the GT power plant is a result of thermodynamic property
matching of the turbomachinery components and does not
represent an available degree of freedom in the system. But the
pressure in the recycle loop can be used to control the pressure
difference. Furthermore, for proper ejector performance in the
critical mode,70–72 the mass flow of steam must be adjusted. The
power output is controlled by manipulating the mass flow of fuel
to the afterburners. The set of controlled and manipulated
variables is given in Table 4. The mass flow rate of air entering
the GT compressor can be controlled by VGVs.39,73,74 This
additional degree of freedom can be used to maintain a nearly
constant turbine exit temperature, hence improving steam cycle
performance. In modern GTs, however, the turbine exit
temperature may still vary when VGVs are applied to optimize
Table 4 Set of controlled and manipulated variables
Controlled variable set
Manipulated variable set
Power output
Total pressure in recycle loop
Mean temperature of the
membrane modules
Ejector operational mode (critical)
Turbine exit temperature
Fuel flow rate to the afterburners
Recycle loop valve opening
Fuel flow rate to the membranereactor
Steam valve opening
Air mass flow to GT compressor by
variable guide vanes
efficiency for the CCPP. In this paper, two load-control strategies
are investigated: (i) where the turbine exit temperature is
controlled using VGVs, and (ii) where VGVs are not used, which
allows the turbine exit temperature to vary. In the following
analysis, the load is reduced until one of the operational and
material constraints in Table 3 is close to the limit (a safety
margin was considered). It is worth mentioning that other loadcontrol strategies proved insufficient. For instance, when seeking
to keep the oxygen mole fraction in the catalytic combustors
constant, no numerical solution could be obtained for the loadcontrol strategy (i), thus indicating unfavourable thermodynamic
conditions. For the load-control strategy (ii), rapid cooling of the
membrane modules was observed so that degradation becomes
more likely. Moreover, maintaining the combustion temperature
at its design (maximum) value leads to part-load behaviour that
is similar in terms of constraints to the two load-control strategies analysed in this paper, but calculation of the relative gain
arrays69 showed that the assignment of the design case cannot be
sustained, i.e. large variations at different frequencies and load
points occurred. This load-control strategy would therefore be
difficult to realize with standard control techniques. It should be
noted that the use of afterburners gives substantial improvements for external power demands while operational and material constraints are not exceeded. Without afterburners,
theoretically all the CO2 that is produced in the catalytic
combustors could be captured. On the other hand, power plant
performance and lifetime of critical reactor components would
generally worsen.
6. Results and discussion
Fig. 8 Carbon dioxide pressure difference between the limit (zero) and
that of the sweep gas, indicating the risk of carbonate formation for the
dense membrane layer and porous support. The oxygen flux, oxygen
pressure difference between the two fluids, and solid wall temperature in
axial direction are also shown.
The process conditions for design operation have been adjusted
to obtain a combustion temperature of 1473 K (stream 5), an
excess oxygen mole fraction of approximately one percent in the
catalytic combustors (stream 5), and a turbine inlet temperature
of approximately 1530 K (stream 11). The GT turbine then
requires 7.3 kg s1 (approximately 12%) of the compressed air for
cooling purposes (stream 2). Large quantities of cooling water
are needed to further cool the CO2-rich gas from the membrane
reactor before it enters the flash condenser (stream 14), and in the
distillation column to further purify the CO2 stream (stream 15).
In design operation, the membrane-based CCPP reaches an
efficiency of 47.1% with a power output of 30.4 MW. In
comparison, the efficiency of a conventional CCPP without CO2
capture (Fig. 3) is 53.3%, using the same GT specifications for
mass flow rate of air into the GT compressor and turbine inlet
temperature. The power output is 36.2 MW.
6.2 Part-load performance
6.2.1 Load-control strategy with variable guide vanes in the
gas turbine compressor. Fig. 9 shows the part-load performance
6.1 Design point performance
Table 5 Design case results for the membrane-based CCPP (Fig. 2)
6.1.1 Membrane performance. Fig. 8 shows the difference in
CO2 pressure between the limit where carbonate formation can
be avoided (the CO2 pressure difference is zero) and the sweep
gas. At the sweep gas inlet of the membrane, the oxygen pressure
reaches its minimum, whereas the pressure of CO2 is maximal, so
that decomposition is favoured. The catalytically active
components that are assumed to be incorporated into the parts of
the porous support at risk result in considerable improvement
with respect to stability. But these components also represent
resistance to oxygen permeation. Both the oxygen pressure
difference between the two fluids and the solid-wall temperature
have a large impact on the oxygen flux, as can be seen from
eqn (1).
_
xO2
xCH4
xH2O
xCO2
xN2
Stream
P/
m/
No.
T/K MPa kg s1 [mol%] [mol%] [mol%] [mol%] [mol%]
6.1.2 Combined cycle power plant. Table 5 presents selected
flow streams for the membrane-based GT power plant (Fig. 2).
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
288
707
489
802
1473
1330
1258
1189
1258
278
1531
875
710
283
283
263
0.1
1.79
4
1.78
1.76
1.76
1.76
1.77
1.76
1.75
1.7
0.1
1.75
0.1
0.1
11
63.5
7.3
1.6
37.1
37.1
32.1
35.5
47.7
44.7
0.3
60.8
60.8
5
181.1
53.6
2.3
79
79
0
0
0
0
0
79
83.7
0
81.7
81.7
0
0
0
0
21
21
0
8.1
1.1
1.1
8.7
21
16.3
0
15.3
15.3
1.1
0
0
0
0
0
55.8
3.5
0
0
0
0
0
100
0
0
0
0
0
0
0
0
44.2
65.8
72.8
72.8
67.2
0
0
0
2
2
72.8
100
100
0
0
0
0
22.6
26.1
26.1
24.1
0
0
0
1
1
26.1
0
0
100
Fig. 9 Part-load performance maps with respect to mass flow of air to
the gas turbine compressor, fuel to the catalytic combustors and afterburners, steam to the ejectors, and total pressure in the recycle loop for
the load-control strategy with VGVs and controlled turbine exit
temperature.
maps for the membrane-based power plant with VGVs in the GT
compressor for load control. Load reduction can be achieved
until the limit of the VGVs (30% from design point) is reached,
i.e. down to approximately 63%. In part-load operation, less fuel
is required for both the catalytic combustors and afterburners to
maintain a nearly constant solid wall temperature of the
membrane modules and turbine exit temperature. This is because
of the reduced mass flow of air entering the membrane reactor.
The mass flow of fuel to the afterburners is linked to the power
output and consequently decreased when the load is reduced.
The amount of steam that is required to maintain ejector
performance during the critical regime is decreased when the
power output is reduced. The decreasing SM of the GT
compressor, which is caused by matching the thermodynamic
properties of the membrane reactor and the turbomachinery, is
shown in Fig. 7. Matching between the membrane reactor and
turbomachinery makes it necessary to reduce the pressure in the
recycle loop so that the average pressure differences between the
two fluids in the membrane modules and monolithic HXs are
Fig. 10 Percentage of variable guide vane closure in the gas turbine
compressor, excess oxygen in the catalytic combustors, pressure difference in the mixing section of the ejectors, and the average pressure
difference across the membrane modules for the load-control strategy
with VGVs and controlled turbine exit temperature.
Fig. 11 Power output and efficiency of the membrane-based and
conventional combined cycle power plant for the load-control strategy
with VGVs and controlled turbine exit temperature.
minimized. The amount of steam needed to obtain proper ejector
performance decreases for lower power outputs. It should be
noted that the ejector operates not far from the specified limit
(Table 3) for low power outputs (Fig. 10). The pressure in the
recycle loop depends on the mass in all process units comprising
the recycle loop. A large decrease in the recycle loop pressure is
therefore accompanied by higher average pressure differences
between the two fluids for the membrane modules as well as
monolithic HXs (Fig. 10). Higher oxygen concentrations in the
catalytic combustors were obtained because of the lower mass in
the recycle loop (Fig. 10). The membrane-based power plant
demonstrates good performance in terms of efficiency. However,
rather heavy losses in power output arise when compared to
a conventional CCPP (Fig. 11).
6.2.2 Load-control strategy without variable guide vanes in the
gas turbine compressor. Fig. 12 shows the part-load performance
maps for the membrane-based power plant for load control
Fig. 12 Part-load performance maps with respect to mass flow of fuel to
the catalytic combustors and afterburners, steam to the ejectors, and total
pressure in the recycle loop for the load-control strategy without VGVs
and uncontrolled turbine exit temperature.
where VGVs in the GT compressor were not used. The load can
be reduced until the limit of the oxygen concentration in the
catalytic combustors (0.5 mol%) is reached, i.e. to approximately
82% of design. The mass flow of fuel to the afterburners decreases
when the power output is reduced. The mass flow of fuel to the
catalytic combustors must increase to maintain a nearly constant
mean solid wall temperature of the membrane modules. The need
for steam to maintain ejector operation in the critical mode is
decreased during part-load operation (Fig. 12). There is only
a slight decrease in the pressure ratio in the GT so that the
pressure in the recycle loop can also remain close to its design
value. Less compression work is hence required in the CO2compression system and the SM of the GT compressor increases
(Fig. 13). Maintaining a high pressure and mass in the recycle
loop comes at the expense of substantially lower excess oxygen
concentration in the catalytic combustors. The efficiency of
CCPP is generally lower when compared to that of load-control
strategy (i) since the steam cycle efficiency is sensible for changes
in the turbine exit temperature (Fig. 14). As for the load-control
strategy (i), heavy losses in power output arise compared to
Fig. 15 Carbon dioxide capture rate of the membrane-based combined
cycle power plant for the two load-control strategies.
a conventional CCPP with equal design specifications for the
turbomachinery.
6.3 CO2 emissions
For both load-control strategies, the CO2 capture rate increases
when power output is reduced because of the reduced mass flow
of fuel to the afterburners (Fig. 15).
7. Conclusions
Fig. 13 Excess oxygen in the catalytic combustors, surge margin of the
gas turbine compressor, pressure difference in the mixing section of the
ejectors, and the average pressure difference across the membrane
modules for the load-control strategy without VGVs and uncontrolled
turbine exit temperature.
Fig. 14 Power output and efficiency of the membrane-based and
conventional combined cycle power plant for the load-control strategy
without VGVs and uncontrolled turbine exit temperature.
Based on detailed modelling of individual process components,
a membrane-based combined cycle power plant (CCPP) was
analysed in design as well as part-load operation. The mixed
conducting membrane has a narrow operation window in terms
of thermodynamic properties, impurities and mechanical load.
Careful control of process conditions and use of additional
technological measures can improve stability of the membrane
modules and other critical process components. Operation of the
membrane modules at elevated and essentially isothermal
conditions results in reduced thermo-mechanical and chemical
stresses. The mean solid wall temperature of the membrane
modules was therefore chosen as a controlled variable in the
power plant, which can be manipulated by the mass flow of fuel
to the catalytic combustors in the membrane reactor. At
a constant rotational speed of the gas turbine (GT), basically two
load-control strategies can be applied, i.e. first with and second
without the use of variable guide vanes (VGVs) in the GT
compressor to manipulate the mass flow of air. For the loadcontrol strategy with VGVs, this additional degree of freedom
can be used to control the turbine exit temperature. In the second
load-control strategy without VGVs, the turbine exit temperature is allowed to vary. The former load-control strategy is
superior due to improved combined cycle efficiencies and a larger
part-load operation window. For this load-control strategy the
angle of VGVs is limiting with a relatively wide range for partload operation, i.e. the load can be reduced to approximately
62%. The performance of the catalytic combustors in
a membrane reactor operating at nearly stoichiometric conditions improves during part-load operation because of higher
oxygen mole fractions. For the load-control strategy without
VGVs, the catalytic combustors are the constraining process
components. The limit of the excess oxygen mole fraction
(0.5 mol%) is reached at approximately 82% power output.
The use of mixed-conducting membranes for integrated air
separation in this oxy-combustion power plant is very challenging due to several operational and material constraints that
need to be considered, the reduced part-load operation capability
of the GT power plant, and the need for further highly critical
process components, such as catalytic combustors. But if technical challenges can be solved that are mainly associated with the
mixed-conducting membrane itself, this membrane-based power
plant has the potential to contribute to reduce CO2 emissions in
the power generation sector.
Symbols and abbreviations
Notation
A/m2
c0/mol m3
cp/J mol1 K1
or J kg1 K1
D0/m2 s1
f
h/J mol1
hf/J s1 m2 K1
Hloss/J s1
JO2/mol m2 s1
k
L/m
LHVfuel/J mol1
m/kg
_
m/kg
s1
MW/kg mol1
n
Nu
P/Pa, J s1
pO2
R/J mol1 K1
Re
SMc
T/K
th/m
U/W m2 K1
v/m s1
wch/m
z/m
Area
Oxygen concentration
Heat capacity at constant pressure
Self-diffusion coefficient for oxygen
Friction factor
Specific enthalpy
Heat transfer coefficient
Heat loss
Oxygen flux
Constant
Monolith length
Lower heating value
Mass
Mass flow rate
Molecular weight
Fitting parameter
Nusselt number
Pressure, power output
Oxygen pressure
Universal gas constant
Reynolds number
Surge margin
Temperature
Thickness
Overall heat transfer coefficient
Fluid velocity
Gas channel width of monolith
Spatially distributed variable in axial direction
Greek letters
a/m2 m3
g
l/J s1 m1 K1
h
P
r/kg m3
Area-to-volume ratio of the monoliths
Heat capacity ratio
Thermal conductivity
Efficiency
Pressure ratio
Density
Subscripts
i
cons
DP
g
gen
ml
s
Sweep gas,air
Consumer
Design point
Gas phase
Generator
Membrane layer
Solid phase
Abbreviations
CCPP
GT
HX
SM
VGV
Combined cycle power plant
Gas turbine
Heat exchanger
Surge margin
Variable guide vane
Acknowledgements
This publication forms a part of the BIGCO2 project, which is
being conducted under the strategic Norwegian research
program called Climit. The authors acknowledge their partners:
StatoilHydro, GE Global Research, Statkraft, Aker Clean
Carbon, Shell, TOTAL, ConocoPhillips, ALSTOM, the
Research Council of Norway (178004/I30 and 176059/I30),
Gassnova (182070) and FCT, Portugal (PTDC/CTM/64357/
2006) for their support.
Knut Ingvar Asen,
Jens Bragdø Smith and John Arild
Svendsen (StatoilHydro, Norway) are gratefully acknowledged
for discussions on several aspects of the gas turbine power plant.
The reviewers are acknowledged for their valuable comments
on the paper.
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Simulation of an oxygen membrane

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