Available online at GHGT-9. Elsevier use only: Received date here; revised date here; accepted date here

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1 doi: /j.egypro Available online at Energy Procedia00 1 (2009) (2008) Energy Procedia GHGT-9 A odeling software linking approach for the analysis of an integrated reforing cobined cycle with hot potassiu carbonate CO 2 capture Lars Olof Nord a, *, Anusha Kothandaraan b, Howard Herzog c, Greg McRae b, Olav Bolland a a Departent of Energy and Process Engineering, the Norwegian University of Science and Technology, NO-7491 Trondhei, Norway b Departent of Cheical Engineering, Massachusetts Institute of Technology, Cabridge, MA 02139, USA c Laboratory for Energy and the Environent, Massachusetts Institute of Technology, Cabridge, MA 02139, USA Elsevier use only: Received date here; revised date here; accepted date here Abstract The focus of this study is the analysis of an integrated reforing cobined cycle (IRCC) with natural gas as fuel input. This IRCC consisted of a hydrogen-fired gas turbine (GT) with a single-pressure stea bottoing cycle for power production. The reforing process section consisted of a pre-reforer and an air-blown auto theral reforer (ATR) followed by water-gas shift reactors. The air to the ATR was discharged fro the GT copressor and boosted up to syste pressure by an air booster copressor. For the CO 2 capture sub-syste, a cheical absorption setup was odeled. The design case odel was odeled in GT PRO by Theroflow, and in Aspen Plus. The Aspen Plus siulations consisted of two separate odels, one that included the reforing process and the water-gas shift reactors. In this odel were also nuerous heat exchangers including the whole pre-heating section. Air and CO 2 copression was also incorporated into the odel. As a separate flow sheet the cheical absorption process was odeled as a hot potassiu carbonate process. The odels were linked by Microsoft Excel. For the CO 2 capture syste the odel was not directly linked to Excel but instead a siple separator odel was included in the reforing flow sheet with inputs such as split ratios, teperatures, and pressures fro the absorption odel. Outputs fro the potassiu odel also included pup work and reboiler duty. A ain focal point of the study was off-design siulations. For these steady-state off-design siulations GT MASTER by Theroflow in conjunction with Aspen Plus were used. Also, inputs such as heat exchanger areas, copressor design point, etc., were linked in fro the Aspen Plus reforing design odel. Results indicate a net plant efficiency of 43.2% with approxiately a 2%-point drop for an 80% part load case. Another off-design siulation, at 60% load, was siulated with a net plant efficiency around 39%. The CO 2 capture rate for all cases was about 86%, except for the reference case which had no CO 2 capture. c Elsevier Ltd. All rights reserved. Keywords: Carbon capture and storage (CCS); CO 2 capture; Pre cobustion capture; Integrated reforing cobined cycle (IRCC); hot potassiu carbonate; odeling; process siulation 1. Introduction Pre-cobustion CO 2 capture is one possible route to fossil fueled power generation with low CO 2 eissions. There exist any possible configurations for a pre-cobustion plant, not the least in relation to the fuel feed. Eide and Bailey [1] describe and discuss different pre-cobustion decarbonization processes. One such process is the integrated reforing cobined cycle (IRCC). An IRCC is fueled by natural gas which is refored to a synthetic gas, ainly consisting of H 2 and CO. The refored gas is water-gas-shifted, the CO 2 can be separated out, and the resulting hydrogen-rich fuel used in a gas turbine (GT). For the CO 2 separation any options exist. One alternative is to use a cheical absorption syste utilizing a hot potassiu carbonate solution. The potassiu carbonate solvent is an aqueous alkaline solvent particularly suited for processes with high total pressure and high CO 2 concentration. The process perfors on the principle of a pressure swing absorption-desorption cycle * Corresponding author. Tel.: ; fax: E-ail address: lars.nord@ntnu.no.

2 742 L.O. Nord et al. / Energy Procedia 1 (2009) Nord et al. / Energy Procedia 00 (2008) with absorption taking place at high pressures. CO 2 capture by the use of potassiu carbonate is, for exaple, described by Kohl and Nielsen [2]. The topic of this study is analysis of an IRCC process, with a special focus on off-design siulations. Siilar pre-cobustion process configurations have been studied by Andersen et al. [3] and Ertesvåg et al. [4]. Consonni and Viganò [5] also analyzes a pre-cobustion setup but with co-generation of power and hydrogen. Hoffann et al. [6] investigates a pre-cobustion cycle using partial oxidation reforing. The cited studies focus on design case analysis. There is liited aount of literature in ters of off-design analysis of CO 2 capture cycles. Part load analyses of natural gas post-cobustion systes are perfored by Möller et al. [7]. Haag et al. [8] and Naqvi et al. [9] analyze the part load behavior of soe of the proposed oxy-fuel cycles. The reainder of the paper is divided into the following sections: Section 2 describes the process. Section 3 describes the details of the ethodologies used in the paper. The results are shown and analyzed in Section 4, and concluding rearks are given in Section Process description The process refors natural gas to a syngas as shown in Figure 1. The syngas is water-gas shifted converting CO to CO 2 and the CO 2 separated out before the hydrogen-rich fuel is used for the gas turbine. As the auto-theral reforer (ATR) is air-blown there will be a significant portion of nitrogen in the gas. This nitrogen is used as fuel diluent for NO x abateent in the GT cobustor. The air needed for the ATR is bled fro the GT copressor discharge plenu and boosted up to syste pressure with a booster copressor. There are a nuber of heat exchangers in the syste. The pre-heating of the reforing streas is handled in various zones in the heat recovery stea generator (HRSG). The syngas cooler, located after the ATR, acts as an evaporator for the high-pressure (HP) stea cycle. The other heat exchangers for the process streas either generate lowpressure (LP) stea for the reboiler in the capture sub-syste or pre-heat the fuel for the GT. The selected gas turbine is a GE 9FB set up for an integrated gasification cobined cycle (IGCC). The requireents for an IRCC GT and an IGCC GT are very siilar. The bottoing stea cycle, including the HRSG and a stea turbine (ST), is a one-pressure syste at approxiately 85 bar. The CO 2 capture sub-syste consists of a hot potassiu carbonate process. The CO 2 is copressed to 150 bar in the CO 2 copression and pup train. Figure 1. Process flow sheet of integrated reforing cobined cycle

3 L.O. Nord et al. / Energy Procedia 1 (2009) Nord et al. / Energy Procedia 00 (2008) Methodology The design case odel was odeled in GT PRO by Theroflow, and in Aspen Plus. The Aspen Plus siulations consisted of two separate odels, one that included the reforing process and the water-gas shift reactors. In this odel, nuerous heat exchangers were included, aong those the whole pre-heating section. Air and CO 2 copression was also incorporated into the odel. The cheical absorption process was odeled as a hot potassiu carbonate odel in a separate flow sheet. The odels were linked by Microsoft Excel utilizing Aspen Siulation Workbook and the Theroflow E-LINK. For the CO 2 capture syste the odel was not directly linked to Excel, instead a siple separator odel was included in the reforing flow sheet with inputs such as split ratios, teperatures, and pressures fro the absorption odel. Outputs fro the capture odel also included pup work and reboiler duty. In a scenario where CO 2 capture plants becoe coon-place, part load operation will be an iportant part of the operation schee. For a plant such as the one odeled in this work the goal is certainly to run it at base load operation for the ajority of the tie but as part of an overall grid strategy part load operation will coe into play. For these steady-state off-design siulations, GT MASTER by Theroflow in conjunction with Aspen Plus were used. Also, inputs such as heat exchanger areas, copressor design point, etc., were linked in fro the Aspen Plus reforing design odel. The overall siulation overview with the linking is displayed in Figure 2. Figure 2. Overall siulation overview with software linking 3.1. Design case assuptions The process was designed with a requireent of at least 85% CO 2 capture rate. The capture rate is here defined as the fraction of fored CO 2 that is captured. To achieve an overall capture rate of 85% the cheical absorption syste was odeled for a 90% capture rate. It can be argued that the 90% capture rate is too low and that a higher design capture rate for the absorption sub-syste would have been preferable and instead operate the syste at a lower stea-to-carbon ratio (S/C) to achieve the overall capture rate of 85%. A lower S/C would lead to a higher ethane slip fro the ATR and a lower CO conversion in the water-gas-shift reactors but the lower aount of stea used would increase the net plant efficiency. The selected S/C for the siulations is 1.5. During the siulation work it was noted that the low-pressure and interediate-pressure sections in the HRSG becae quite sall because of the significant pre-heating requireents (which could have been copensated with duct firing in the HRSG). Because of this and to siplify the process it was decided to have a one-pressure level in the HRSG. The pressure level was set at approxiately 85 bar for the design case. Other assuptions include a condenser pressure of 0.04 bar, a 3% pressure drop (of inlet pressure) in the heat exchangers and reactors, and ISO abient conditions Off-design analysis One of the ain focal points for the process analysis was related to off-design conditions. Two part load points were analyzed; 80% and 60% of the design case gas turbine load respectively. The off-design analyses were steady-state based. It should also be entioned that off-design considerations can affect the design of the process. For exaple, in this process, stea extracted for the reboiler in the absorption syste had to be extracted at a higher pressure than necessary to achieve a sufficient pressure level also at part load. In addition, off-design considerations for the booster copressor ay lead to a selection of a less than optiu design point to achieve the required pressure ratio at reduced ass flow rates. In the following sub-sections the theory and ethodology used for the part load scenarios will be presented.

4 744 L.O. Nord et al. / Energy Procedia 1 (2009) Nord et al. / Energy Procedia 00 (2008) Heat exchanger analysis In the off-design scenarios the overall heat transfer coefficient U in the heat exchangers will vary. With inclusion of surface fouling and fin effects (extended surface) U can be expressed as: 1 1 R f, c R f, h 1 R (1) UA ha A A ha w o c o c o h o h In the off-design analysis presented in this paper an as-new plant is assued with no aging or fouling. The fouling factors R f, cand R f, h are therefore set to 0. The wall conduction ter Rw is also neglected. o is the overall surface efficiency of a finned surface and A is the heat transfer area. Subscripts c and h refer to the cold and hot side of the heat exchanger, respectively. In the pre-heating heat exchangers in the HRSG the cold side has a high stea content. In the syngas cooler, as well as in HE2 and HE4 (refer to Figure 1) the cold side has water and stea only. Copared to the hot side, which contains gas with a lower level of stea, the cold side heat transfer coefficient is assued uch larger, that is hc hh. Equation (1) can then be siplified to: 1 1 UA ha (2) o The area A and fin efficiency o are constant when coparing design to off-design conditions. Using the Nusselt nuber and an epirical correlation including the Reynolds and Prandtl nubers (Incropera and DeWitt [10]): h Nu D hd C ReD Pr k n (3) The constants C,, and n, are assued independent of the nature of the fluid. The Prandtl nuber Pr and theral conductivity k are assued constant fro design to off-design conditions. The diaeter D is constant. For the siulations, it is of interest relating the off-design UA to the design (UA) d. Equation (2) can then be written as: UA UA d Re (4) Re D D,d D By using Re D, where the dynaic viscosity μ is assued constant, a siple expression for correction of the UA-value A when going fro design to off-design siulations can then be derived: UA UA, d h hd (5) The -constant is dependent on the geoetry of the shell and tube heat exchanger. is the fluid ass flow. Subscript d refers to design conditions Pressure drop analysis Assuing fully developed turbulent flow, eaning the pressure gradient dp/dx is a constant, the pressure drop fro axial position x 1 to x 2 can be expressed as: p2 2 x2 2 u u p dp f dx f x2 x1 2D 2D p1 x1 (6) where u is the ean fluid velocity and the density of the fluid. The Darcy friction factor f is defined as: f 2 dp dx D (7) u 2

5 L.O. Nord et al. / Energy Procedia 1 (2009) Nord et al. / Energy Procedia 00 (2008) By using u A, and coparing to design conditions the following expression can be derived for off-design considerations: p p d d 2 d (8) Copressor ap For the air booster copressor in Figure 1, a copressor ap has been used for calculating the outlet pressure and isentropic efficiency in off-design operating points. The ap has been adopted fro the original, presented in a ap collection by Kurzke [11], to fit the process in the analysis. The following non-diensional ratios have been used: 02 The pressure ratio p p, the corrected ass flow 01 T N corr, and the corrected rotational speed Ncorr 01 p01 T. p 01 is the 01 stagnation pressure at copressor inlet and p 02 at copressor discharge. T 01 is the stagnation teperature at copressor inlet. In Figure 3, corr is plotted versus and the isentropic efficiency is for different N corr. The corrected ass flow and the corrected speed are relative to design value. The surge line is also visible in the figure. The chosen design point for the booster air copressor is indicated in the graph. For off-design operating conditions it is assued that the copressor can be speed controlled. For the GT copressor, GT MASTER used aps built-in to the progra surge line Pressure ratio design point 1.4 N T 01 (relative to design value) T01 p01 (relative to design value) 1.0 design point 0.8 Isentropic efficiency is N T 01 (relative to design value) T01 p 01 (relative to design value) Figure 3. Copressor ap for air booster copressor When choosing the design and off-design operating points it is iportant to consider the surge argin to ensure stable copressor operation and avoid surge conditions. The surge argin is here defined as: SM corr,rel corr,rel corr,rel,surge (9)

6 746 L.O. Nord et al. / Energy Procedia 1 (2009) Nord et al. / Energy Procedia 00 (2008) Here corr,rel is the corrected ass flow relative to the design corrected ass flow. corr,rel,surge is the corrected ass flow at surge conditions on the operating line (constant speed line) relative to the design corrected ass flow. 4. Results and discussion The ain results for the design and off-design cases are presented in Table 1. The net plant efficiency is 43.2% for the design case with about two percentage-points drop to each subsequent part load case. The net plant efficiency is defined as: net,plant W W W W W W LHV t c gen s gen cop p drive aux NG NG (10) Here W t is the GT turbine power, W c the GT copressor power, W s the ST power, W cop the total power consuption by the air and CO 2 copression. W p is the pup power for feed water pups, pups in the absorption sub-syste, etc. W aux is the auxiliary power requireent. is the echanical efficiency and gen is the generator efficiency. drive is the efficiency of the drives for the different copressors and pups. NG is the natural gas ass flow entering the syste and LHV NG the lower heating value of the natural gas. Note that all the power ters are defined as their absolute values eaning all power ters are considered positive and the sign is handled in the equation itself. The capture rate is just above 86% for all cases, except for the reference case which has no CO 2 capture. The reference case is based on a natural gas cobined cycle plant with a GE 9FB gas turbine and a triple-pressure stea cycle. Table 1. Result suary for design case (100%), off-design cases (80% and 60%) and reference case (100% ref.) Gas turbine relative load [%] (ref.) Natural gas LHV input [MW] Gross power output GT [MW] Gross power output ST [MW] Gross power output [MW] Gross power output [% of LHV input] Air copression [MW] Air copression [% of LHV input] CO 2 copression [MW] CO 2 copression [% of LHV input] CO 2 capture pups [MW] CO 2 capture pups [% of LHV input] BFW pups in pre-cob process [MW] BFW pups in pre-cob process [% of LHV input] Auxiliaries [MW] Auxiliaries [% of LHV input] Net power output [MW] Net plant efficiency [% of LHV input] Efficiency capture penalty [%-point loss to ref. case] CO 2 eissions [g CO 2 / net kwh el.] CO 2 capture rate [%]

7 The results indicate a rather low net plant efficiency copared to the reference case. This is for one, strongly influenced by the conservative selection of turbine inlet teperature. An increase of the teperature would significantly reduce the capture penalty. Secondly, the reboiler duty could be lowered, again leading to a higher plant efficiency. Other ites to consider is L.O. Nord et al. / Energy Procedia 1 (2009) Nord et al. / Energy Procedia 00 (2008) The turbine inlet teperature for the gas turbine set is a critical paraeter for the overall plant perforance. For the base case of this analysis, a conservative teperature of 1297C has been selected. The reason for this assuption is two-fold. For one, the IGCC setup of GE s 9FB GT includes replacing the hot gas path of the FB with FA parts. The 9FA design turbine inlet teperature is 1327C. Secondly, because of the hydrogen fuel which leads to an increase in stea content in the turbine copared to when firing natural gas, the heat transfer rate to the turbine blades increase, leading to a higher blade etal teperature. Because of this, another 30C decrease in firing rate has been ipleented in the odel leading to the 1297C turbine inlet teperature. If the GT could be fired at the full 9FB firing rate of 1427C the net plant efficiency would increase fro 43.2% to 44.7%. Chiesa et al. [12], and Todd and Battista [13] addresses issues related to firing hydrogen in gas turbines. The CO 2 capture reboiler duty is another paraeter affecting the plant efficiency. With the setup as shown in Figure 1, a part of the stea for the reboiler in the capture sub-syste is extracted fro the ST. If the reboiler duty would decrease fro the current 1980 kj/kg CO 2 captured down to approxiately 1250 kj/kg CO 2 captured, no stea extraction fro the ST to the capture sub-syste would be necessary and the ST output would increase. This would increase the net plant efficiency fro the base case efficiency of 43.2% to 43.9%. Figure 4 shows a T-Q diagra of the HRSG. Notable is that the boiler (HPB1 in Figure 4) is sall copared to a ore standard HRSG design. The reason for this is the large aount of stea generated in the syngas cooler. This eans the econoizer and the superheater in the HRSG are rather large but with a saller boiler. The vertical jups of the gas teperature in the figure are due to the pre-heating of the process streas. It should be noted that the ajority of the pre-heating is upstrea of the superheater eaning a significant portion of the available heat of the gas strea is reoved before any stea is generated. The unconventional design is because of the integration with the reforing process. One can argue that a triple-pressure stea cycle would have a higher efficiency copared to the single-pressure syste applied here. However, because of all the pre-heat streas and the syngas cooler acting as an evaporator, the low-pressure and interediate-pressure sub-systes would have been very sall adding very liited value at an increased coplexity. Duct firing could have changed this picture, however for this work it was decided not to utilize suppleentary firing. Figure 4. GT PRO T-Q diagra for single-pressure heat recovery stea generator 5. Conclusions By cobining siulation tools for cheical engineering and power plant engineering analyses respectively, a helpful representation of the overall syste can be accoplished for an IRCC process. The IRCC process ay involve a significant integration between the power cycle and the reforing process as is the case for the cycle studied. It can therefore be advantageous to cobine the tools as is shown in this paper.

8 748 L.O. Nord et al. / Energy Procedia 1 (2009) Nord et al. / Energy Procedia 00 (2008) suppleentary firing in the HRSG to allow for a ore standard design of the stea generator and the use of low-teperature process heat instead of low-pressure stea for the reboiler in the cheical absorption syste. Also, a higher design CO 2 capture rate for the absorption syste could be advantageous. However, it should be entioned that the IRCC process is coplex and any options and configuration possibilities are present. In the end, a line had to be drawn how far to extend the analysis work. The off-design siulation results show the possibility to run a plant like this at part load conditions down to approxiately 60% gas turbine load. Reducing the load further down ay not be practical for several reasons. For exaple, the CO eissions fro the GT would increase and potentially also NO x. Further, the efficiency drop would at soe point be too large to justify. The air booster copressor pressure ratio would continue to decrease (if using a one-copressor train and not ultiple copressors) eaning the overall syste pressure would keep decreasing until the level is too low for realistic plant operation. The possibility would then be to switch to natural gas fuel for the gas turbine. Indeed, the plant is designed for having natural gas as back-up fuel for the GT. In fact, to start up a plant like the one in the study, natural gas is required. At a load around 30% or above the switch-over to the hydrogen-rich fuel would take place during a start-up. 6. Acknowledgents We would like to thank the Norwegian Research Council and StatoilHydro for providing the funding for the project. 7. References 1. Eide, L.I. and D.W. Bailey, Precobustion decarbonisation processes. Oil and Gas Science and Technology, (3): p Kohl, A. and R. Nielsen, Gas purification. 5th ed. 1997, Houston, Texas, USA: Gulf Publishing Copany. 3. Andersen, T., H.M. Kvasdal, and O. Bolland. Gas turbine cobined cycle with CO 2 capture using auto-theral reforing of natural gas. in ASME Turbo Expo Munich, Gerany. 4. Ertesvåg, I.S., H.M. Kvasdal, and O. Bolland, Exergy analysis of a gas-turbine cobined-cycle power plant with precobustion CO 2 capture. Energy, (1): p Consonni, S. and F. Vigano, Decarbonized hydrogen and electricity fro natural gas. International Journal of Hydrogen Energy, (7): p Hoffann, S., et al., Perforance and cost analysis of advanced gas turbine cycles with pre-cobustion CO 2 capture, in ASME Turbo Expo. 2008: Berlin, Gerany. 7. Möller, B.F., M. Genrup, and M. Assadi, On the off-design of a natural gas-fired cobined cycle with CO 2 capture. Energy, (4): p Haag, J.C., et al. Turboachinery siulation in design point and part-load operation for advanced CO 2 capture power plant cycles Montreal, Que., Canada: Aerican Society of Mechanical Engineers, New York, NY, USA. 9. Naqvi, R., J. Wolf, and O. Bolland, Part-load analysis of a cheical looping cobustion (CLC) cobined cycle with CO 2 capture. Energy, (4): p Incropera, F.P. and D.P. DeWitt, Fundaentals of heat and ass transfer. 3rd ed. 1990: John Wiley & Sons, Inc. 11. Kurzke, J., Copressor and turbine aps for gas turbine perforance coputer progras - Coponent ap collection : Gerany. 12. Chiesa, P., G. Lozza, and L. Mazzocchi, Using hydrogen as gas turbine fuel. Journal of Engineering for Gas Turbines and Power, (1): p Todd, D.M. and R.A. Battista, Deonstrated applicability of hydrogen fuel for gas turbines, in Gasification 4 the Future. 2000: Noordwijk, Netherlands.