Simulation of an Oxygen Membrane-Based Gas Turbine Power Plant

Dec 2, 2009 - Simulation of an Oxygen Membrane-Based Gas Turbine Power Plant: Dynamic Regimes with Operational and Material Constraints. Konrad Eichho...
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Energy Fuels 2010, 24, 590–608 Published on Web 12/02/2009

: DOI:10.1021/ef9004253

Simulation of an Oxygen Membrane-Based Gas Turbine Power Plant: Dynamic Regimes with Operational and Material Constraints Konrad Eichhorn Colombo,*,† Vladislav V. Kharton‡ and Olav Bolland† †

Department of Energy and Process Engineering, Norwegian University of Science and Technology, Kolbjoern Hejes vei 1B, NO-7491 Trondheim, Norway, and ‡Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal Received May 8, 2009. Revised Manuscript Received August 19, 2009

This paper investigates the transient behavior of a natural gas-fired power plant for CO2 capture that incorporates mixed-conducting membranes for integrated air separation. The membranes are part of a reactor system that replaces the combustor in a conventional gas turbine power plant. A highly concentrated CO2 stream can then be produced. The membrane modules and heat exchangers in the membrane reactor were based on spatially distributed parameter models. For the turbomachinery components, performance maps were implemented. Operational and material constraints were emphasized to avoid process conditions that could lead to instability and extensive stresses. Two load-control strategies were considered for the power plant with a gas turbine operating at constant rotational speed. In the first load-control strategy, variable guide vanes in the gas turbine compressor were used to manipulate the mass flow of air entering the gas turbine compressor. This degree of freedom was used to control the turbine exit temperature. In the second load-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 the design value. Simulation reveals that the membrane-based gas turbine power plant exhibits rather slow dynamics; fast load following was hence difficult while maintaining stable operation. Comparing the two load-control strategies, load reduction with variable air flow rate and controlled turbine exit temperature was found to be superior because of the considerably higher and faster load reduction capability, increased stability of the catalytic combustors in the membrane reactor, and higher power plant efficiencies. (Figure 2) is replaced by the membrane reactor, which is composed of ceramic high-temperature membrane modules, as well as heat exchangers (HXs), and catalytic combustors.3-6 The membrane modules and HXs in the membrane reactor were assumed to be fabricated as two-fluid monoliths7 (Figure 3). The membrane-based GT power plant shares certain similarities with solid oxide fuel cell GT hybrid cycles. For example, both are operated at a similar temperature range.8 But whereas solid oxide fuel cell systems have been widely investigated in terms of operational and material constraints (e.g., refs 9-11 and

1. Introduction The power and industry sectors combined contribute more than 60% of all anthropogenic emissions of CO2.1 The increasing demand for energy will lead to further emissions increases unless environmental issues are addressed. CO2 capture and storage (CCS) is one option in the portfolio of actions for the stabilization of atmospheric greenhouse gases.2 In addition to system improvements to existing power plants, novel power plant configurations with CO2 capture have been developed. These may be largely classified into pre-, post-, and oxy-combustion power plants. One concept in the oxy-combustion family for natural gas is shown in layman terms in Figure 1. The combustor in a conventional gas turbine (GT) power plant

(6) Griffin, T.; Sundkvist, S. G.; A˚sen, K.; Bruun, T. Advanced zero emissions gas turbine power plant. J. Eng. Gas Turbines Power 2005, 127 (1), 81–85. (7) Bruun, T., Werswick, B., Kristiansen, K., Grønstad, L. Method and equipment for feeding two gases into and out of a multi-channel monolithic structure. U.S. Patent 7285153, 2007. (8) O’Hayre, R. P.; Colella, W.; Cha, S.-W.; Prinz, F. B. Fuel Cell Fundamentals, 2nd ed.; Wiley: Hoboken, NJ, 2009; pp XXV, 546. (9) Bove, R., Ubertini, S., Modeling Solid Oxide Fuel Cells Methods, Procedures and Techniques; Springer: Netherlands, 2008. (10) Vielstich, W.; Gasteiger, H. A.; Lamm, A. Handbook of Fuel Cells: Fundamentals, Technology and Applications; Wiley: Chichester, U.K., 2009; Vol. 6. (11) Vielstich, W.; Gasteiger, H. A.; Lamm, A., Handbook of Fuel Cells: Fundamentals, Technology and Applications; Wiley: Chichester, U.K., 2009; Vol. 5. (12) Campanari, S. Full load and part-load performance prediction for integrated SOFC and microturbine systems. J. Eng. Gas Turbines Power 2000, 122 (2), 239–246. (13) Milewski, J.; Miller, A.; Saacinski, J. Off-design analysis of SOFC hybrid system. Int. J. Hydrogen Energy 2007, 32 (6), 687–698. (14) Stiller, C.; Thorud, B.; Bolland, O.; Kandepu, R.; Imsland, L. Control strategy for a solid oxide fuel cell and gas turbine hybrid system. J. Power Sources 2006, 158 (1), 303–315.

*To whom correspondence should be addressed. E-mail: (K.E.C.) [email protected]); (V.V.K.) [email protected]; (O.B.) olav. [email protected]. (1) Metz, B.; Davidson, O.; de Coninck, H.; Loos, M. Meyer, L. Summary for Policymakers. Carbon Dioxide Capture and Storage; IPCC Special Report; Cambridge University Press: Cambridge, U.K., 2005. (2) Pacala, S.; Socolow, S. R. Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies. Science 2004, 305, 968–972. (3) Bruun, T. P., Grønstad, L., Kristiansen, K., Werswick, B., Linder, U. Device for combustion of a carbon containing fuel in a nitrogen free atmosphere and a method for operating said device. WO 02/053969 A1, 2002. (4) Bruun, T., Hamrin, S. Combustion installation. Int. Patent WO 2007/060209A1, 2007. (5) Linder, U.; Eriksen, E. H.; A˚sen, K. I. A method of operating a combustion plant, and a combustion plant using separated oxygen to enrich combustion air. US 6,877,319 B2; 2005. r 2009 American Chemical Society

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the dynamic simulation of oxy-combustion and other CCS power plants is not available to date,17 with one exception,18 to the best of the authors’ knowledge. A number of questions arise when looking at the transient performance of the membrane-based GT power plant with regard to nearly stoichiometric combustion and large amounts of recycled gas. When operation conditions change in the membranebased GT power plant (e.g., changes in load), the whole system responds because of the strong interaction of process components. The GT power plant presented here was more complicated than the oxy-combustion process in ref 18 because of the use of membrane modules as an integrated air separation device. Hamrin19 stated (qualitatively) that fast transient changes are not possible for this membrane-based GT power plant. Load changes must be executed slowly, considering the operational and material constraints of the membrane reactor components. This paper is divided into 8 sections. Section 2 describes the GT power plant and its main components in more detail. Section 3 outlines operational and material constraints that must be considered to maintain a reasonable lifetime for process components. Section 4 summarizes the time scales in the system. Section 5 explains the two loadcontrol strategies for the GT power plant. Section 6 presents a possible start-up and shut-down procedure for the power plant. Section 7 provides simulation results for a transient load decrease and transient load increase for both loadcontrol strategies. Section 8 concludes with the key points of the paper. A detailed model description is given in the Appendix.

Figure 1. Principle of the membrane-based combined cycle power plant with CO2 capture. The temperature distribution is indicated throughout the process.

Figure 2. Conventional gas turbine power plant.

2. Gas Turbine Power Plant In the membrane-based GT power plant (Figure 4), fuel is injected into the membrane reactor by means of subsonic ejectors. Steam is extracted from the steam cycle and added to the fuel for proper ejector performance. The fuel to the catalytic combustors must be preheated to avoid two-phase flow in the ejectors. The compressed air from the GT compressor is split, with the majority led through the lowtemperature HXs, membrane modules, and high-temperature HXs. The remainder is fed to the bleed-gas HX branch. The air in these two branches is heated and mixed afterward. In the membrane modules, oxygen is transported from the air side to the sweep gas side. The temperature of the oxygendepleted air is further increased in the afterburners before being fed to the GT turbine. It should be noted that the CO2 produced in the afterburners is emitted to the atmosphere. Capturing these small amounts of CO2 is not practical because of the low CO2 pressure. Geometric and operational conditions were adjusted to obtain required power plant performance in conjunction with satisfying all operational and material constraints. 2.1. Gas Turbine. In this paper, a steady-state approximation for the turbomachinery was assumed. The compressor maps are based on Mach number similarity, where secondorder effects are neglected, such as changes caused by

Figure 3. Oxygen mixed-conducting membrane module (top) and modeling manifold (bottom) with design specifications.

references therein), part-load operation (e.g., refs 12-15), and transient behavior (e.g., refs 14-16), published literature on (15) Magistri, L.; Traversa, A.; Cerutti, F.; Bozzolo, M.; Costamagna, P.; Massardo, A. F. Modelling of pressurised hybrid systems based on integrated planar solid oxide fuel cell (IP-SOFC) technology. Fuel Cells 2005, 5 (1), 80–96. (16) Roberts, R. A.; Brouwer, J. Dynamic simulation of a pressurized 220 kW solid oxide fuel-cell-gas-turbine hybrid system: Modeled performance compared to measured results. J. Fuel Cell Sci. Technol. 2006, 3 (1), 18–25. (17) Alie, C.; Douglas, P.; Croiset, E. Scoping Study on Operating Flexibility of Power Plants with CO2 Capture; www.ieagreen.org.uk; 2008. (18) Imsland, L.; Snarheim, D.; Foss, B. A.; Ulfsnes, R.; Bolland, O. Control issues in the design of a gas turbine cycle for CO2 capture. Int. J. Green Energy 2005, 2 (2), 303–315.

(19) Hamrin, S. Control of a gas turbine with hot-air reactor. US2006/ 0230762A1; 2006. (20) Riegler, C.; Bauer, M.; Kurzke, J. Some aspects of modelling compressor behavior in gas turbine performance calculations. J. Turbomachinery 2001, 123 (2), 372–378. (21) Kurzke, J. Compressor and Turbine Maps for Gas Turbine Performance Computer Programs;Component Map Collection; GasTurb: Dachau, Germany, 2004; http://www.gasturb.de.

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Figure 4. Membrane-based gas turbine power plant.

variations in fluid composition.20,21 The single-shaft GT is operated at a constant rotational speed. The GT compressor was based on performance maps,21 which are shown in Figure 5. Variable guide vanes (VGVs) were applied to reduce the mass flow of air into the GT compressor by as much as 30%.22,23 Variations of the turbine isentropic efficiency were based on a GT turbine chart.24 It is standard engineering practice to pass a fraction of compressed air over the GT turbine disk and blades. This cooling air fraction was also determined by charts24 and represents a fixed parameter in off-design. In general, when matching GT components, individual process components should have the highest possible efficiency. Furthermore, operation in regions of instability must be omitted, for instance where GT compressor surge is likely to occur. Maximum isentropic efficiency for the GT compressor could not be obtained because of the matching between individual process components. It should also be noted that the high-temperature HXs were operated at temperatures that are approximately 100 K less than the limit reported in Sundkvist et al.25 Furthermore, state-ofthe-art GTs allow for much higher turbine inlet temperatures than was assumed here. Hence, the combination of operational and material constraints remains the same in principle. 2.2. Membrane Reactor Process Components. 2.2.1. Membrane Modules and Heat Exchangers. The dense membrane (22) Kim, T. S.; Hwang, S. H. Part load performance analysis of recuperated gas turbines considering engine configuration and operation strategy. Energy 2006, 31 (2-3), 260–277. (23) Kim, J. H.; Kim, T. S.; Sohn, J. L.; Ro, S. T. Comparative analysis of off-design performance characteristics of single and twoshaft industrial gas turbines. J. Eng. Gas Turbines Power 2003, 125 (4), 954–960. (24) Walsh, P. P.; Fletcher, P. Gas Turbine Performance, 2nd ed.; Blackwell Science: Maden, MA, 2004. (25) Sundkvist, S. G.; Julsrud, S.; Vigeland, B.; Naas, T.; Budd, M.; Leistner, H.; Winkler, D. Development and testing of AZEP reactor components. Int. J. Greenhouse Gas Control 2007, 1 (2), 180–187. (26) Smith, J. B.; Norby, T. On the steady-state oxygen permeation through La2NiO4þδ membranes. J. Electrochem. Soc. 2006, 153 (2), 233– 238. (27) Vigeland, B.; Glenne, R.; Breivik, T.; Julsrud, S. Membrane and use thereof. U.S. 6,503,296B1, 2003.

Figure 5. Off-design performance maps for the gas turbine compressor with respect to variations in mass flow and pressure ratio (bottom) and mass flow and isentropic efficiency (top).21

layer in the membrane modules was assumed to be based on intergrowth La2NiO4þδ,26,27 which shows a suitable combination of substantially high oxygen permeability, low chemical (28) Paulsen, O. Rigid bonded glass ceramic seals for high temperature membrane reactors and solid oxide fuel cells. PhD thesis. Norwegian University of Science and Technology, Trondheim, Norway, 2009.

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and thermal expansion, and relatively good phase stability. The dense membrane layer was assumed to be coated on a porous support substrate made of the same material, to ensure thermomechanical and chemical integrity.29 The heat exchangers in the membrane reactor have the same geometric dimensions as the membrane modules, except for the length. 2.2.2. Staged-Catalytic Combustors. The membrane-based GT power plant contains two types of combustors. The afterburners were assumed to be of a standard type. The membrane reactor includes staged-catalytic combustors where catalytic partial oxidation of natural gas is followed by complete oxidation under lean conditions.30 Changes in operation conditions in terms of temperature, mass flow, and pressure have a significant effect on the catalytic partial oxidation stage (e.g., H2 and CO yields) and therefore on the overall combustor performance.31 Other hydrocarbons may also be found during partial oxidation of CH4, but these small quantities were neglected. Carbon deposition was also neglected. To maintain stable and complete combustion, excess oxygen must be available.30,32 Both the staged catalytic combustor and afterburner model were based on complete oxidation. This assumption leads to improved numerical stability. To substantiate this approach, an equilibrium-based combustor was developed and implemented in gPROMS.33 In addition, these two gPROMS models were compared to a HYSYS34 model. There was a very good match between the three models. 2.2.3. Ejector. The one-dimensional ejector model was divided into five sections as shown in Figure 6: (i) nozzle critical section, (ii) nozzle outlet, (iii) mixing section inlet, (iv) diffuser inlet/mixing section outlet, and (v) diffuser outlet. Studies have shown that in solid oxide fuel cell-GT hybrid cycles, ejectors have a considerable effect on the overall power plant performance.35,36 The ejectors were assumed to operate in the critical regime, which means at elevated pressures of the actuating fluid (fuel and steam mixture).37 2.2.4. Recycle Loop in Membrane Reactor. The pressure in the recycle loop depends on the total mass accumulated, which in turn depends on the volume of individual membrane reactor components. The outlet of the bleed-gas HXs was chosen as the pressure control location, but other locations can be used for this purpose as well.

Figure 6. Sections of the constant-area ejector model.

2.2.5. Model Implementation. The GT power plant model was executed in gPROMS,33 an equation-oriented modeling tool. Physical gas properties were obtained from the property data package Multiflash33 using a Soave-RedlichKwong equation of state. A total of 30 and 50 discretization elements in an axial direction were used for the membrane module and monolithic HXs, respectively. Five discretization elements were used in the radial direction. A higher number of discretization elements did not improve results in terms of accuracy, but significantly increased model complexity and required CPU time. Heat and mass balances for the cold-side fluid were discretized by a forward finite difference method, whereas a backward finite difference scheme was applied to the counter-current hot fluid. The solid-phase energy balance was discretized by a central finite difference method1. A central finite difference method was used for the insulation layer. Pipes were discretized using a backward finite difference method with a total number of 15 distribution elements. The approximation of partial derivatives for all spatially distributed models was second order. The whole system was solved by a variable order backward differentiation formulas (DASOLV solver).33 A general rule for the required calculation time cannot be defined a priori. The solver varies time increments depending on discontinuities and current changes in process variables. The whole of the membranebased GT power plant model consists of more than 12 000 equations. 3. Operational and Material Constraints for Power Plant Components Table 1 shows the most important operational and material constraints that need to be considered for the membranebased GT power plant. Some of these constraints are further discussed in this section. Additional information can be found in Eichhorn Colombo et al.38 3.1. Mixed-Conducting Membrane Modules. At isothermal conditions, the membrane expands with decreasing oxygen pressure. When decreasing the temperature at constant oxygen pressure, the membrane contracts due to geometric changes caused by an increased equilibrium oxygen stoichiometry.39 These volume changes must be taken into account during operation to lower the risk of mismatches between the dense membrane layer and the porous support material and also between all connections of the membrane reactor

(29) Kovalevsky, A. V.; Kharton, V. V.; Snijkers, F. M. M.; Cooymans, J. F. C.; Luyten, J. J.; Marques, F. M. B. Oxygen transport and stability of asymmetric SrFe(Al)O3-[δ]-SrAl2O4 composite membranes. J. Membr. Sci. 2007, 301 (1-2), 238–244. (30) Griffin, T.; Winkler, D.; Wolf, M.; Appel, C.; Mantzaras, J. In Staged Catalytic Combustion Method for the Advanced Zero Emissions Gas Turbine Power Plant; Vienna, Austria, 2004; American Society of Mechanical Engineers: New York, 2004; pp 705-711. (31) Zhu, J.; Zhang, D.; King, K. D. Reforming of CH4 by partial oxidation: Thermodynamic and kinetic analyses. Fuel 2001, 80 (7), 899– 905. (32) A˚sen, K. I. Gas Processing and CO2, StatoilHydro ASA: Porsgrunn, Norway, personal communication, 2008. (33) gPROMS (General Process Modelling and Simulation Tool), v 3.1.5, Process Systems Enterprise Ltd.: London, www.psenterprise.com, 2008. (34) Aspen Technology Aspen HYSYS; Aspen Technology: Cambridge, MA, 2006. (35) Marsano, F.; Magistri, L.; Massardo, A. F. Ejector performance influence on a solid oxide fuel cell anodic recirculation system. J. Power Sources 2004, 129 (2), 216–228. (36) Zhu, Y.; Cai, W.; Li, Y.; Wen, C. Anode gas recirculation behavior of a fuel ejector in hybrid solid oxide fuel cell systems: Performance evaluation in three operational modes. J. Power Sources 2008, 185 (2), 1122–1130. (37) Yinhai, Z.; Wenjian, C.; Changyun, W.; Yanzhong, L. Fuel ejector design and simulation model for anodic recirculation SOFC system. J. Power Sources 2007, 173 (1), 437–49.

1 The application of mixed finite difference methods was necessary to obtain stable reduced-order models after linearization. (38) Eichhorn Colombo, K.; ’Bolland, O.; Kharton, V. V.; Stiller, C. Simulation of an oxygen membrane-based combined cycle power plant: Part-load operation with operational and material constraints. Energy Environ. Sci. In press (DOI: 10.1039/b910124a). (39) Carolan, M. F. A.; Watson, M. J.; Minford, E.; Motika, S. A.; Taylor, D. M. Controlled heating and cooling of mixed conducting metal oxide materials. U. S. Patent7,122,072 B2, 2006.

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a

Constraints for the conventional gas turbine power plant are indicated by italic letters.

maximum fluid velocity

thermodynamic conditions with respect to membrane temperature, CO2 and O2 pressure in sweep gas thermodynamic conditions with respect to membrane temperature, CO2 and O2 pressure in sweep gas thermodynamic conditions with respect to membrane temperature, H2O and O2 pressure in sweep gas maximum pressure difference

effect

limit

60 m/s97

0.1 MPa

depending on process conditions (favored at low temperatures)43

performance loss and failure due to hydroxide formation performance loss and failure due to mechanical stresses safety; performance loss and failure due to noise and vibration problems

depending on process conditions25

depending on process conditions25,43

1323 K25

1173 K25

10 ppb25

50 ppb25

depending on process conditions (poisoning is favored at low temperatures) 673 K25

1573 K25

1530 K 96

depending on technology24

5%63,64

30%22,23

depending on process conditions37

035,95

depending on process conditions

ppb range57

0.5 mol %32

performance loss and failure due to oxidation

mass flow reduction by variable guide vanes performance loss and failure due to a sudden drop in pressure and detrimental aerodynamic pulsation performance loss and failure due to thermomechanical stresses performance loss and failure due to thermo-mechanical stresses performance loss and failure due to thermomechanical stresses performance loss and failure due to sulfur poisoning performance loss and failure due to thermomechanical stresses performance loss and failure due to sulfur poisoning performance loss and failure due to sulfur poisoning performance loss and failure due to thermomechanical stresses performance loss and failure due to thermomechanical stresses and chemical interaction performance loss and failure due to carbonate formation

performance loss and failure due to unstable and incomplete combustion performance loss and failure due to sulfur poisoning performance loss due to formation of vapor-liquid phase performance loss due to operation outside the critical mode performance in critical range

: DOI:10.1021/ef9004253

membrane modules and monolithic heat exchangers pipes

dense membrane layers and porous support

dense membrane layers and porous support

dense membrane layers and porous support

maximum temperature

minimum temperature

maximum sulfur concentration

dense membrane layers and porous support dense membrane layers and porous support dense membrane layers and porous support

maximum sulfur concentration

high-temperature heat exchangers

maximum sulfur concentration

maximum temperature

high-temperature heat exchangers

low-temperature heat exchangers

maximum turbine inlet temperature

GT turbine

minimum temperature

cooling air fraction to GT turbine

GT turbine

low-temperature heat exchangers

minimum surge margin

GT compressor

GT compressor

ejectors

ejectors

thermodynamic conditions with respect to temperature, pressure, and composition minimum pressure difference in mixing section pressure of actuating (fuel and steam mixture) fluid maximum variable guide vane angle limit

maximum sulfur concentration

catalyst in the catalytic combustors ejectors

constraint minimum oxygen mole fraction

catalytic combustors

Process component

Table 1. Operation and Material Constraints of the Membrane-Based GT Power Plant (Figure 4)a

Energy Fuels 2010, 24, 590–608 Eichhorn Colombo et al.

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dashed line in Figure 7) and left (vertical dotted line in Figure 7) when the oxygen chemical potential increases or decreases, respectively. However, both processes could promote carbonate formation, thus leading to a more complex shape of the membrane stability boundary. The dense membrane layer was assumed to be coated on the feed side of the porous support because of the high risk of carbonate formation at the sweep gas inlet. The porous support acts as protection layer by limiting diffusion of gaseous species such as CO2. Exposing the membrane to high oxygen pressures on the feed side increases the risk of oxidation. But for the operation conditions studied in this work, the risk of carbonate formation was considered to be more severe than oxidation risks. Stabilizing components were assumed to be incorporated into the pores of the membrane support layer that were at high risk of carbonate formation (at the sweep gas inlet) to provide sufficient longterm stability. 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.43 For Ni-containing materials, this degradation mechanism involves primarily nickel hydroxide volatilization.46 The resulting La-enrichment at the membrane surface may facilitate other degradation processes such as lanthanum carbonate or hydroxide formation. Pressure differences between the two fluids in the monolithic membrane modules and HXs larger than one bar should be avoided to maintain mechanical stability. Minimizing the wall thickness leads to increased oxygen and heat transport rates. On the other hand, using thinner walls comes at the expense of less operability and robustness as well as the requirement for a more advanced control strategy for the power plant. Industrial supplies of natural gas typically contain 1 ppm SO2,47 which is still too high for the mixed-conducting material assumed here. Deep desulfurization is needed. The maximum difference in thermal expansion coefficients of process components in the membrane reactor should not exceed 10-17%. 3.2. Ceramic Heat Exchanger Monoliths and Seals. The monolithic HXs in the membrane reactor are fabricated of materials that are stable in atmospheres containing high oxygen pressure as well as gaseous species such as H2O, CO2, and CO.48 However, the HXs in the membrane reactor put some additional constraints on the power plant in terms of temperature limits (Table 1). Sulfur limitations and pressure differences between the two fluids are similar to those specified for the membrane modules. The sealing of membrane reactor components poses additional challenges. The sealant must be hermetic, as well as chemically and thermomechanically stable,28,49,50 for the

Figure 7. Stability diagram for the dense membrane layer and porous support based on ref.,25 showing stable and unstable operation regions with respect to CO2 and oxygen pressure in the sweep gas as well as solid-wall temperature.

components. Essentially isothermal conditions and the highest possible operating temperature should hence be employed to minimize stresses.40,41 Decomposition of the membrane material can occur as a result of redox instability, interaction with gaseous species or kinetic demixing.42,43 The chemically induced expansion is among the most important thermomechanical properties that relates to the volume changes under the oxygen chemical potential gradients and determines the applicability of membrane materials. Similar effects can also result from thermal expansion. These volume changes can ultimately lead to fracture. When operating the membrane at a temperature range of 973-1073 K at high CO2 and water vapor pressures, the use of any membrane material containing substantial amounts of alkaline-earth elements may be generally problematic, if carbonate formation or oxidation is thermodynamically favorable under the corresponding equilibrium conditions.44 Sundkvist et al.25 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 (Figure 7). Temperatures below 1173 K in conjunction with high CO2 pressures lead to the formation of oxycarbonates, primarily La2O2CO3,44 followed by complete decomposition. Increasing the oxygen pressure leads to moderately higher CO2 stability (nonvertical dashed line in Figure 7). Reduced oxygen pressure has an opposite and stronger effect (nonvertical dotted line in Figure 7). At moderate operation temperatures, the La2NiO4þδ membrane may degrade because of oxidation.25,45 The stability limit at low temperatures is shifted to the right (vertical (40) Blond, E.; Richet, N. Thermomechanical modelling of ion-conducting membrane for oxygen separation. J. Eur. Ceram. Soc. 2008, 28 (4), 793–801. (41) Hamrin, S. Control of a gas turbine with hot-air reactor. US2006/ 0230762A1; 2006. (42) Fontaine, M. L.; Larring, Y.; Norby, T.; Grande, T.; Bredesen, R. Dense ceramic membranes based on ion conducting oxides. Ann. Chim. 2007, 32 (2), 197–212. (43) Drioli, E.; Giorno, L. Membrane Operations: Innovative Separations and Transformations; Wiley-VCH: Weinheim, 2009; p XXV, 551 s. (44) Foger, K. H., M.; Turney, T. W. Formation and thermal decomposition of rare earth carbonates. J. Mater. Sci. 1992, 27 (1), 77–82. (45) Bannikov, D. O.; Cherepanov, V. A. Thermodynamic properties of complex oxides in the La-Ni-O system. J. Solid State Chem. 2006, 179 (8), 2721–2727.

(46) Vigeland, B. E.; Bruun, T. Protection of process equipment with significant vapor pressure by adding an evaporating component to gas in contact with said equipment. WO 2008/007967 A1, 2008. (47) Carroni, R.; Schmidt, V.; Griffin, T. Catalytic combustion for power generation. Catal. Today 2002, 75 (1-4), 287–295. (48) Julsrud, S. V.; Erlend, B. A ceramic heat exchanger. Int. Patent WO 03/033986 A1, 2003. (49) Weil, K. S.; Koeppel, B. J. Comparative finite element analysis of the stress-strain states in three different bonded solid oxide fuel cell seal designs. J. Power Sources 2008, 180 (1), 343–53. (50) Budd, M. Barium lanthanum silicate glass-ceramics. U. S. Patent 7,189,668B2, 2007.

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entire operation range. The thermal expansion coefficients of the sealant and joined ceramic materials should be similar to avoid significant mismatches.25,28,50-52 The maximum leakage rate should not be higher than 0.1%. 3.3. Catalytic Combustors. Standard combustion technology cannot be applied to the combustors in the membrane reactor because of low excess oxygen concentrations in conjunction with high CO2 and water vapor dilution. A staged catalytic combustor has therefore been proposed, where catalytic partial oxidation of methane is followed by complete oxidation under lean conditions. In the catalytic partial oxidation, a mixture of CO and H2 is produced, which provides hydrogen-stabilized combustion for the second combustion stage.30,53,54 Supported Rh-based catalysts were found to be suitable for the combustors because of their high activity, selectivity, resistance to carbon deposition, and sulfur resistance.55,56 Nevertheless, deep desulfurization is required to prevent poisoning of the catalyst.57 The mechanical integrity of the catalyst and their supports should withstand the possibility of thermal shocks that might occur during rapid load changes as well as during start-up and shut-down of the power plant.47 Excess oxygen is required to maintain stable and complete combustion.30,32 But high concentrations of oxygen in the sweep gas are problematic since higher energy and cooling water requirements are needed for the purification system.58-60 As a result, power plant efficiency drops.38 Moreover, degradation of process components located downstream of the membrane reactor can also occur.61 3.4. Turbomachinery and Further Fluid Components. The GT should be operated with a sufficient surge margin, which for power applications is typically in the range of 15-20%.24 This margin was relaxed to approximately 10% to increase the part-load capability of the membrane-based power plant for the load-control strategy that uses VGVs. However, additional reduction below 5% should be avoided because the GT may then be operating under a regime where subsonic

But it should be noted stall and blade flutter can occur. that instability phenomena, apart from surging, were not included in the model because of a lack of available data.38 The GT compressor maps for off-design performance are presented in Figure 5. 4. System Time Scales The response of the oxygen transport in the membrane layer was assumed to be instantaneous for the sake of simplicity. This assumption is valid when all kinetic parameters are constant, no microstructural changes in the membrane bulk and on the surface occur, and surface activation/ passivation processes are negligible. While the latter assumptions can be used for most mixed-conducting materials that are stable under the membrane-based power plant conditions,25,65 the applicability of the former simplification is very limited. For instance, the bulk oxygen diffusivity and the interfacial exchange rate in La2NiO4þδ are both dependent on the oxygen chemical potential.66 At the same time, the transient times observed for dense single-layer nickelate membranes operating under oxidizing conditions at temperatures above 1200 K66 were indeed short, suggesting that the response of membrane modules should be primarily determined by mass transfer phenomena in the membrane modules. As an example, during isothermal cycling of the permeate-side oxygen partial pressure in the range 102-103 Pa, steady-state oxygen fluxes through 0.6 mm thick La2NiO4þδ membranes at 1223 K and feed side oxygen partial pressure of 21 kPa were reached after 30-400s; increasing temperature to 1273 K makes the relaxation processes faster. Steady-state can then be reached within 100 s. The relatively fast equilibration kinetics is in agreement with data in the literature on La2NiO4-based materials (refs 67-69 and references therein). In fact, the rate of these transient processes at elevated temperatures becomes similar to that of the mass propagation in the gaseous phase, making the two virtually indistinguishable. The time scales of the monolithic membrane modules and HXs were analyzed previously with respect to energy, mass, and species conservation, respectively. The temperature of the membrane modules reaches steady-state within minutes.70

(51) Sunarso, J.; Baumann, S.; Serra, J. M.; Meulenberg, W. A.; Liu, S.; Lin, Y. S.; Diniz da Costa, J. C. Mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen separation. J. Membr. Sci. 2008, 320 (1-2), 13–41. (52) Budd, M. Method of forming a glass ceramic material. U. S. Patent 6,475,938B1, 2002. (53) Eriksson, S.; Boutonnet, M.; Jaras, S. Catalytic combustion of methane in steam and carbon dioxide-diluted reaction mixtures. Appl. Catal., A 2006, 312 (1-2), 95–101. (54) Eriksson, S.; Wolf, M.; Schneider, A.; Mantzaras, J.; Raimondi, F.; Boutonnet, M.; Jaras, S. Fuel-rich catalytic combustion of methane in zero emissions power generation processes. Catal. Today 2006, 117 (4), 447–453. (55) Eriksson, S.; Nilsson, M.; Boutonnet, M.; Jaras, S. Partial oxidation of methane over rhodium catalysts for power generation applications. Catal. Today 2005, 100 (3-4), 447–451. (56) Shamsi, A. Partial oxidation of methane and the effect of sulfur on catalytic activity and selectivity. Catal. Today 2009, 139 (4), 268–273. (57) Cimino, S.; Torbati, R.; Lisi, L.; Russo, G. Sulphur inhibition on the catalytic partial oxidation of methane over Rh-based monolith catalysts. Appl. Catal., A 2009, 360 (1), 43–49. (58) Pipitone, G.; Bolland, O., Power generation with CO2 capture: Technology for CO2 purification. Int. J. Greenhouse Gas Control In Press. (59) Li, H.; Yan, J.; Anheden, M. Impurity impacts on the purification process in oxy-fuel combustion based CO2 capture and storage system. Appl. Energy 2009, 86 (2), 202–213. (60) Aspelund, A.; Jordal, K. Gas conditioning;The interface between CO2 capture and transport. Int. J. Greenhouse Gas Control 2007, 1 (3), 343–354. (61) Plasynski, S. I.; Litynski, J. T.; McIlvried, H. G.; Srivastava, R. D. Progress and new developments in carbon capture and storage. Crit. Rev. Plant Sci. 2009, 123–138. (62) Saravanamuttoo, H. I. H., Rogers, G. F. C., Cohen, H., Straznicky, P. V. Gas Turbine Theory; 6th ed.; Prentice Hall: 2009.

(63) Roumeliotis, I.; Mathioudakis, K., Evaluation of water injection effect on compressor and engine performance and operability. Appl. Energy (doi:10.1016/j.apenergy.2009.04.039). (64) Brun, K.; Kurz, R.; Simmons, H. R. Aerodynamic instability and life-limiting effects of inlet and interstage water injection into gas turbines. J. Eng. Gas Turbines Power 2006, 128 (3), 617–625. (65) Vigeland, B., Glenne, R.; Breivik, T; Julsrud, S. Membrane and use thereof. U.S. 6,503,296B1, 2003. (66) Shaula, A. L.; Naumovich, E. N.; Viskup, A. P.; Pankov, V. V.; Kovalevsky, A. V.; Kharton, V. V., Oxygen transport in La2NiO4 þ δ: Assessment of surface limitations and multilayer membrane architectures. Solid State Ionics 2009, 180 (11-13), 812-816. (67) Vashook, V. V.; Yushkevich, I. I.; Kokhanovsky, L. V.; Makhnach, L. V.; Tolochko, S. P.; Kononyuk, I. F.; Ullmann, H.; Altenburg, H. Composition and conductivity of some nickelates. Solid State Ionics 1999, 119 (1), 23–30. (68) Mauvy, F.; Bassat, J. M.; Boehm, E.; Dordor, P.; Grenier, J. C.; Loup, J. P. Chemical oxygen diffusion coefficient measurement by conductivity relaxation-correlation between tracer diffusion coefficient and chemical diffusion coefficient. J. Eur. Ceram. Soc. 2004, 24 (6), 1265–1269. (69) Kim, G.; Wang, S.; Jacobson, A. J.; Chen, C. L. Measurement of oxygen transport kinetics in epitaxial La2NiO4þ thin films by electrical conductivity relaxation. Solid State Ionics 2006, 177 (17-18), 1461– 1467. (70) Eichhorn Colombo, K.; Imsland, L.; Bolland, O.; Hovland, S. Dynamic modelling of an oxygen mixed conducting membrane and model reduction for control. J. Mem. Sci. 2009, 336 (1-2), 50–60.

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Table 2. Range of Time Scales of Individual Process Components of the Membrane-Based GT Power Plant response in process components

time scale [s]

oxygen permeation in membrane layer pressure and mass propagation in fluid machinery (GT, ejector) thermal in combustion mass propagation in membrane modules and HXs thermal in membrane modules and HXs thermal in insulation material (membrane reactor, pipes)

essentially instantaneous71 essentially instantaneous9 milliseconds30 seconds minutes hours

The membrane reactor and pipes were assumed to be coated with a high-performance insulation material that prevents large heat losses. Consequently, steady-state is reached after several hours. Shorter start-up times, for example, after a short stop, are also possible because the membrane reactor can be maintained at a fairly high temperature. Griffin et al. 30 state that the required time for complete methane combustion in a highly diluted atmosphere is in the range of milliseconds. A time range that is smaller than one second was not considered in the power plant model to reduce numerical stiffness. Pressure and mass propagation in fluid machinery (GT, ejector) are assumed to occur instantaneously.9 The range of time scales of individual GT power plant components is shown in Table 2.

Figure 8. Steady-state part-load performance maps for the loadcontrol strategy with variable guide vanes and controlled turbine exit temperature.

5. Load-Control Strategies Two load-control strategies were analyzed in this paper for the GT operating at constant rotational speed: (i) load-control using VGVs in the GT compressor, while the turbine exit temperature was controlled2, and (ii) load-control, where VGVs are not used and the turbine exit temperature remained uncontrolled. For both load-control strategies, process conditions were adjusted such that the membrane modules were kept close to their design temperature when power output was changed continuously. The steady-state part-load analyses of the membrane-based combined cycle power plant operated with these two loadcontrol strategies can be found elsewhere.38 Steady-state partload operation maps for the membrane-based GT power plant are shown in Figures 8 and 9, respectively. The steadystate operating line on the GT compressor map for the loadcontrol strategy using VGVs is shown in Figure 5. It should be emphasized that these operating lines represent the case of perfect control, that is, where the output of the controller is identical to the reference value. In reality, measured variables may vary due to measurement errors. Furthermore, the turbomachinery and the membrane reactor components are exposed to degradation, and changes in ambient conditions also have a strong effect on the power plant performance.72

Figure 9. Part-load operation maps for the load-control strategy without variable guide vanes and uncontrolled turbine exit temperature. Table 3. Set of Controlled and Manipulated Variables of the Membrane-Based GT Power Plant controlled variable set power output pressure in recycle loop mean solid-wall temperature of membrane modules ejector operational mode (critical) turbine exit temperature

manipulated variable set fuel flow rate to the afterburners recycle loop valve opening fuel flow rate to the membrane reactor steam valve opening air mass flow to GT compressor by VGVs

After a degree-of-freedom analysis of the power plant model, a set of controlled and manipulated variables was found, shown in Table 3. Pairs of corresponding controlled and manipulated variables were obtained by relative gain array analyses at different load points and frequencies.73

2 In modern GTs, however, the turbine exit temperature may still vary when basically applying VGVs as a result of an optimization of efficiency for the combined cycle power plant. (71) Zhang, W.; Smit, J.; van Sint Annaland, M.; Kuipers, J. A. M. Feasibility study of a novel membrane reactor for syngas production. Part 1: Experimental study of O2 permeation through perovskite membranes under reducing and non-reducing atmospheres. J. Membr. Sci. 2007, 291 (1-2), 19–32.

(72) Eichhorn Colombo, K.; Kharton, V. V.; Viskup, A. P.; Kovalevsky, A. V.; Shaula, A. L.; Bolland, O. Simulation of an Oxygen Membrane-based Gas Turbine Power Plant: System Level Analysis of Operation Stability and Individual Process Unit Degradation. In preparation. (73) Skogestad, I. S. P. Multivariable Feedback Control: Analysis and Design. 2nd ed.; Wiley: Chichester, U. K., 2005.

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Figure 10. Membrane-based gas turbine power plant with auxiliary process components for safe start-up and shutdown.

Manipulated variables were varied such that all operational and material constraints were satisfied (Table 1). In the membrane module model, the mean solid-wall temperature represents a spatially distributed variable and cannot directly be measured. However, related gas properties of the air and sweep gases, at somewhat lower temperatures, can be used to calculate this value. Manipulated variables were represented by nonlinear regression functions with respect to generator power output. For the purposes of simplification, the temperature of steam and preheated fuel that is supplied to the membrane reactor was assumed to be constant and also high enough to avoid the formation of a two-phase fluid.

remaining combustible gases from previous operation or failed start attempts. The GT is started and operated at low rotational speed3 to purge the membrane reactor and to build up resistance to the fluid that is introduced into the membrane reactor. Simultaneous with the GT starting procedure, the “split valve” and “N2 valve” are opened to allow nitrogen into the membrane reactor, which is heated by an auxiliary combustion system. At the same time, the “air blow-off valve” is controlled to keep the macroscopic stoichiometric composition in the membranes constant4. This method is known as iso-compositional heating.39,75,76 In the chemical equilibrium between gas and solid phase, oxygen will not pass into or out of the membranes. That means there is no gradient in the anion concentration and variable-valence cations oxidation state, so that the strains caused by differential chemical expansion are therefore reduced. During the start-up process, the membrane modules are not homogeneously heated but there will be a temperature gradient resulting in internal oxygenstoichiometry differences.76 The maximum allowable differential strain must be determined experimentally or by numerical methods.40,77 Carolan75 reported typical heating and cooling rates in the range of 0.25-10 K/min, but these may also be as low as 1 K/h at low temperatures. Sundkvist et al.25 reported heating rates of approximately 3 K/min and cooling rates of approximately 5 K/min25 for the ceramic HX monoliths, which may also apply to the membrane modules.

6. Start-Up and Shut-Down Procedures A special class of transients for (power) processes are the start-up and shut-down procedures. In what follows, one possible strategy is presented on a qualitative basis for the membrane-based power plant. Simulations were not performed because several other material-specific properties as well as operational effects need to be considered that were not included in the GT power plant model. Changes in operation conditions during start up and shut down must be performed at a rate that is slow enough to prevent excessive stresses. But the required time should also be as short as possible to operate the power plant economically. At any rate, unfavorable operational conditions must be avoided. The number of auxiliary systems should also be minimal. This includes hot valves as well as valves in main streams, additional combustors, and the need for technical gases.74 The extended flowsheet for the membrane-based GT power plant with its proposed auxiliary components for startup and shut-down is shown in Figure 10 (see Figure 4 for comparison). Auxiliary process equipment, which is not necessary for normal operation, is indicated by red bold capital letters. 6.1. Start-Up Procedure. Initially, all valves in the GT power plant as well as the VGVs in the GT compressor must be fully closed. Before any fuel or oxygen is fed to the membrane reactor, it must be purged to avoid ignition of

3

It is engineering practice to use the generator as starting motor.62 Gases other than nitrogen may also be used, but CO2 and H2O should be avoided to prevent degradation of the membranes. (74) Ferrari, M. L.; Traverso, A.; Pascenti, M.; Massardo, A. F. Early start-up of solid oxide fuel cell hybrid systems with ejector cathodic recirculation: Experimental results and model verification. Proc. Inst. Mechanical Eng., Part A: J. Power Energy 2007, 221 (5), 627–635. (75) Carolan, M. F. Control of differential strain during heating and cooling of mixed conducting metal oxide membranes. U. S. 2006/ 0060080, A1, 2006. (76) Carolan, M. F. Operation of mixed conducting metal oxide membrane systems under transient conditions. U. S. Patent 7,468,092 B2, 2008. (77) Hendriksen, P. V.; Larsen, P. H.; Mogensen, M.; Poulsen, F. W.; Wiik, K. Prospects and problems of dense oxygen permeable membranes. Catal. Today 2000, 56 (1-3), 283–295. 4

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82

The use of the heating rates provided by Sundkvist et al. would lead to start-up times of five hours or longer. In comparison, a mid merit gas turbine as assumed in this work, requires approximately 10-15 min for start-up.24 Blond and Richet40 suggested an oxygen pressure rate not exceeding 0.14 per hour. The oxygen pressure may be changed slowly enough to allow (partial) creep relaxation of the membrane. The “N2 valve”, “air blow-off valve”, and the GT speed must hence be carefully controlled. After the membrane reactor components have adapted to the new conditions, “fuel valve 1” opens, and fuel is introduced. Steam may also be required for the start-up of the membrane reactor, controlled by the “steam valve”, to allow for a smooth transition of the membrane to high water vapor concentrations and proper ejector performance. But this is considered to be very critical because of the formation of hydroxides at low temperatures.43 The amount of fuel must be carefully controlled by “fuel valve 1” to keep the pressure of the combustion products H2O and CO2 low enough to avoid hydroxide and carbonate formation. Next, the “anti-evaporation valve” opens, and small amounts of the evaporating components of the membrane and porous support layer are injected into the recycle loop. However, after passing the membrane modules, the added components need to be recollected to avoid clogging membrane reactor components downstream of the membrane modules.46 The catalyst used in the catalytic combustors (e.g., Rh78-80) must be able to withstand large thermal shocks during transient operation.47 Propagating hot spots may occur78,81 that can lead to high surface temperatures and ultimately to sintering and volatilization of the catalyst. Tavazzi et al.78 showed that the catalyst aging phenomenon was enhanced when using high preheating temperatures which could be useful in removing carbon traces. On the other hand, loss of catalyst activity could be avoided when applying moderate preheating temperatures. Hence, careful control of process conditions is also essential for the catalytic combustors. During start-up, all gases are vented to the atmosphere. When the membrane reactor components have equalized to the new process conditions, the rotational speed of the GT is increased. The “air blow-off valve” is further controlled to maintain optimal conditions for both the membrane reactor and the turbomachinery. The switch from auxiliary operation to self-sustaining operation of the process must be carefully scheduled. In conventional GTs, this switch corresponds to a rotational speed of the GT of about 15-20% of normal.62 The ignition system (not shown in Figure 10) is turned off when the engine has reached self-sustaining conditions.62 The GT is then accelerated to idle by a steady opening of “fuel valve 2” while continuing starter assistance. The starter is cut off when the speed reaches around 60% of the rated speed. The starter continues to assist the GT in reaching a higher rotational speed even after the torque of

the starter and GT are equalized. The time to bring the GT to a speed of either 2% lower than that for idle or of fullpower is 10-15 min for aero-derivatives whereas it is 30 min to 4 h for heavy-duty engines.24 The fuel scheduling for engine acceleration during start-up must be such that undesired performance phenomena are avoided.24,82 Further, the use of “fuel valve 2” and the starter cutoff time need to be carefully scheduled to prevent extensive temperature gradients. VGVs can assist in controlling instabilities,82 but can also be the cause of instabilities in addition to reduced efficiency.83 After the GT power plant has reached selfsustaining conditions, the auxiliary system is cut off and the fuel mass flow to the membrane reactor and afterburners is increased. The GT power plant is then synchronized with the electrical grid. The auxiliary valves are closed. During start-up, steam is produced by an auxiliary steam generator until the steam cycle can generate steam with the required properties by the GT exhaust or sweep gas. All auxiliary components are in operation until the GT and membrane reactor have reached self-sustaining conditions. The load is then further increased by opening the angle of the VGVs until the desired operation point is reached. Purging of the heat recovery steam generator (HRSG) (Figure 1) is needed before the exhaust gas from the gas turbine can be introduced.82 Appropriate steam properties for a steam turbine start-up in conventional combined cycle power plants are reached at approximately 50-60% GT load. Before starting the steam turbine, the gland steam system5 must be in operation and the condenser evacuated. Until the steam turbine takes over the available steam flow, the excess steam flows across the steam turbine bypass.84 Modern combined cycle power plants in the range of 50-400MW can be started within 40-50 min after 8 h of standstill (hot start), within 75-110 min after 60 h standstill (warm start), and within 75-150 min after 120 h standstill (cold start).84,85 The CO2 purification and compression stage (Figure 1) can be considered as rather decoupled from the combined cycle because of the low operation temperature with a time scale lower than that of the GT power plant and steam cycle and HRSG.86,87 The gas flow to the CO2 compression system is gradually increased by closing the “sweep gas blow-off valve”. During the start-up procedure, CO2 capture is regarded as not feasible. 6.2. Shut-Down Procedure. Hamrin41 claimed that a rapid shut-down of the GT power plant can be achieved by using a GT blow-off valve which bypasses the membrane reactor (not shown in Figure 10). Even faster cool-down of the GT turbine can be achieved by fully opening the “air blow-off valve”. 5 The gland steam system prevents both out-leakage of high-pressure steam and in-leakage of air into the turbine, which would degrade the efficiency of the turbine by raising the condenser pressure. (82) Kim, J. H.; Song, T. W.; Kim, T. S.; Ro, S. T. Dynamic simulation of full startup procedure of heavy-duty gas turbines. J. Eng. Gas Turbines Power 2002, 124 (3), 510–16. (83) Tsalavoutas, A.; Mathioudakis, K.; Stamatis, A.; Smith, M. Identifying faults in the variable geometry system of a gas turbine compressor. J. Turbomachinery 2001, 123 (1), 33–39. (84) Kehlhofer, R. H., Warner, J., Nielsen, H., Bachmann, R. Combined-Cycle Gas & Steam Turbine Power Plants, 2nd ed.; 1999. (85) Sanaye, S.; Rezazadeh, M. Transient thermal modelling of heat recovery steam generators in combined cycle power plants. Int. J. Energy Res. 2007, 31 (11), 1047–1063. (86) Soerensen, E.; Skogestad, S. Optimal startup procedures for batch distillation. Comput. Chem. Eng. 1996, 20 (Suppl 2), S1257–S1262. (87) Han, M.; Park, S. Startup of distillation columns using profile position control based on a nonlinear wave model. Ind. Eng. Chem. Res. 1999, 38 (4), 1565–1574.

(78) Tavazzi, I.; Beretta, A.; Groppi, G.; Maestri, M.; Tronconi, E.; Forzatti, P. Experimental and modeling analysis of the effect of catalyst aging on the performance of a short contact time adiabatic CH4-CPO reactor. Catal. Today 2007, 129 (3-4), 372–379. (79) Williams, K. A.; Leclerc, C. A.; Schmidt, L. D. Rapid lightoff of syngas production from methane: A transient product analysis. AIChE J. 2005, 51 (1), 247–260. (80) Pena, M. A.; Fierro, J. L. G. Chemical structures and performance of perovskite oxides. Chem. Rev. 2001, 101 (7), 1981–2017. (81) Maestri, M.; Beretta, A.; Groppi, G.; Tronconi, E.; Forzatti, P. Comparison among structured and packed-bed reactors for the catalytic partial oxidation of CH4 at short contact times. Catal. Today 2005, 105 (3-4), 709–717.

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Table 4. Design Case Results for the Membrane-Based GT Power Plant (Figure 4) stream number

T [K]

P [MPa]

m_ [kg/s]

xN2 [mol %]

xO2 [mol %]

xCH4 [mol %]

xH2O [mol %]

xCO2 [mol %]

1 2 3 4 5 6 7 8 9 10 11 12 13

288 707 489 802 1473 1330 1258 1189 1258 278 1531 875 710

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

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

79 79 0 0 0 0 0 79 83.7 0 81.7 81.7 0

21 21 0 8.1 1.1 1.1 8.7 21 16.3 0 15.3 15.3 1.1

0 0 55.8 3.5 0 0 0 0 0 100 0 0 0

0 0 44.2 65.8 72.8 72.8 67.2 0 0 0 2 2 72.8

0 0 0 22.6 26.1 26.1 24.1 0 0 0 1 1 26.1

But these two shut-down strategies are not regarded to be safe shut-down methods if target lifetime of critical power plant components is to be maintained. Instead, the reverse start-up procedure should be applied to the shut-down of the GT power plant. At zero load the GT and generator are disconnected. In the event of an emergency, the breakers as well as the fuel flow rate will be tripped immediately without continuous reduction of the load to a minimum.88 In such incidents, the “air blow-off valve” will be opened. The CO2 compression stage is not strongly affected by transient phenomena in the power plant during shut-down. The load in the HRSG and steam cycle can be shut down by reducing the mass flow of the exhaust and sweep gas from the GT by opening the “sweep gas blow-off valve”. When the exhaust gas temperature from the GT has reached a defined minimum level, the steam turbine is shut down. The HRSG is then further unloaded and also shut down.84

Table 5. Modelling Assumptions for the Membrane-Based GT Power Plant (Figure 4) fuel preparation, handling of power consumption, and ambient conditions • fuel is assumed to be pure methane, available at 4 MPa • fuel temperature in the membrane-reactor after preheat = 398 K (fuel heating, e.g., by the CO2-rich exhaust gas prior to the CO2 compression system, but the additional heat integration was not included in the analysis.) • fuel temperature at the afterburners 278 K • lower heating value = 0.8 MJ/mol • ambient air: temperature = 288 K; pressure = 0.1 MPa; composition: 79% nitrogen, 21% oxygen membrane reactor • number of manifolds of type 1 is 12 • number of manifolds of type 2 is 48 • membrane modules length is 0.1625 m; high-temperature heat exchanger length is 0.355 m; low-temperature heat exchanger length is 0.9825 m; bleed-gas 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 μm • 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

7. Results and Discussion 7.1. Design Case. Table 4 shows results for selected flow streams of the GT power plant. The outlet temperature of the catalytic combustors was approximately 1470 K. The excess oxygen mole fraction was approximately 1%. A turbine inlet temperature of approximately 1530 K was assumed. Further model assumptions can be found in Table 5. At design point operation, the GT power plant reaches an efficiency of 33.4% with a power output of 19.9 MW. 7.2. Transient Load Reduction with Variable Inlet Guide Vanes in the Gas Turbine Compressor and Controlled Turbine Exit Temperature. Figure 11 shows the continuous load reduction profiles for the membrane-based GT power plant using VGVs in the GT compressor for mass flow modulation. Fast load reduction rates (4.2% load/min) led to a smaller part-load operation window due to limitations of the ejectors (see Figure 12). There was a rapid decrease in the pressure difference between the actuating fluid (fuel and steam mixture) and the induced fluid (sweep gas in recycle loop). Values below zero must be avoided to maintain physically correct performance of the ejector model. Also shown in Figure 11 is the load profile (1.2% load/min) with the maximum load reduction rate at which part-load can be fully obtained (58%), that is, until the limit of VGVs was reached (see Figure 8). Note that the pressure difference in the ejectors is also close to the limit. Approximately 36 min is required to reach this minimum. A combination of alternating high load reduction rates of 3% load/min for the first reduction and 1.2% load/min for the second with periods of relaxation (500 s)

gas turbine • variations in the isentropic efficiency for compressors are calculated by performance maps21 (84.5% in design) • variations in the isentropic efficiency for the turbine and the amount of cooling air that is extracted from the compressor discharge are calculated by performance maps24 (86.8% in design) • shaft mechanical efficiency is 99%98 • generator mechanical efficiency is 98.5%98 • cooling air fraction of compressed air is 11.6%24 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/s; average velocity of pipe 2 is 56.1 m/s; average velocity of pipe 3 is 21.1 m/s ejector • heat loss = 1% • isentropic efficiency for the section-wise ejector model is based on data from refs 99 and 100 • operation in the critical mode36,37,101 (the mass flow of steam is therefore set) • diffuser area = 0.058 m2; diffuser throat area = 9.86 cm2; nozzle area = 0.064 cm2 • velocities in the mixing section and diffuser outlet are 121 and 2 m/s, respectively. catalytic combustors and afterburners • heat loss = 1%98

(88) Boyce, M. P. Gas Turbine Engineering Handbook, 3rd ed.; Gulf Professional Pub.: Boston, MA, 2006.

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Figure 14. Excess oxygen fraction in the catalytic combustors for load reduction rates shown in Figure 11.

Figure 11. Load reduction profiles of the membrane-based gas turbine power plant for the load-control strategy with variable guide vanes and controlled turbine exit temperature.

Figure 12. Pressure difference in ejector mixing section for load reduction rates shown in Figure 11.

Figure 15. Gas turbine compressor surge margin for load reduction rates shown in Figure 11.

catalytic combustors decreased during load reduction (Figure 13) while the excess oxygen mole fraction in the catalytic combustors increased (Figure 14). The surge margin of the GT compressor was reduced at part-load but remained within the specified limit (Figure 15). The pressure difference between the two fluids in the membrane modules increased during load reduction as a result of matching between the turbomachinery and membrane reactor (Figure 16). At maximum part-load operation, a pressure gradient of 77 kPa was obtained, which was still below the assumed limit of 0.1 MPa. Figure 17 shows the thermal efficiency of the GT power plant and specific emissions of CO2 produced by the afterburners. Less fuel was required during load reduction to keep the turbine exit temperature at its design value, which resulted in reduced emissions of CO2 in the GT exhaust gas. Figure 18 shows the variations in turbine inlet temperature. Additional limiting variables, given in Table 1, were not exceeded. 7.3. Transient Load Reduction without Variable Inlet Guide Vanes in the Gas Turbine Compressor and Uncontrolled Turbine Exit Temperature. The power plant behavior for three continuous load reduction rates was analyzed during which VGVs in the GT compressor were not used (Figure 19). Compared to the former load-control strategy, part-load capability was considerably reduced, that is, approximately to 81% (Figure 9). But faster load-reduction rates can be applied without large penalties in load-reduction capability. Since the mass flow of air was basically

Figure 13. Outlet temperature of the catalytic combustors and mean solid-wall temperature of the membrane modules for load reduction rates shown in Figure 11.

can be applied to reduce the required time to reach maximum part-load in conjunction with the VGV limit. One possible scenario for stepped load reduction is shown in Figure 11, under which the required time could be reduced by 5 min. Further reduction could be achieved by using even higher load reduction rates in conjunction with more frequent and shorter relaxation periods. The mean solid-wall temperature of the membrane modules for all three rates remains close to the design value (Figure 13). The outlet temperature of the 601

Energy Fuels 2010, 24, 590–608

: DOI:10.1021/ef9004253

Eichhorn Colombo et al.

Figure 16. Mean pressure gradient of the membrane modules for load reduction rates shown in Figure 11. Figure 19. Load reduction profiles of the membrane-based gas turbine power plant for the load-control strategy without variable guide vanes and uncontrolled turbine exit temperature.

Figure 17. Efficiency of the GT power plant and specific emissions of CO2 for load reduction rates shown in Figure 11. Figure 20. Excess oxygen in the catalytic combustors for continuous load reduction rates shown in Figure 19.

Figure 18. Turbine inlet temperature for load reduction rates shown in Figure 11.

Figure 21. Outlet temperature of the catalytic combustors and mean solid-wall temperature of the membrane modules for continuous load reduction rates shown in Figure 19.

unchanged, there was a strong cooling effect on the membrane reactor. More fuel needed to be fed to the catalytic combustors (Figure 9) to keep the mean solid-wall temperature of the membrane modules close to their design value. Load reduction leads to a rapid decrease in excess oxygen in the catalytic combustors (Figure 20). It was assumed that stable operation of the catalytic combustors can be maintained

with excess oxygen fractions of 0.5 mol % (Table 1). Fast load reduction rates (6% load/min) result in an overshoot (oxygen mole fraction >0.5%), followed by an undershoot (oxygen mole fraction