Thermodynamic Modelling and Performance of Combined Solid Oxide Fuel Cell and Gas Turbine Systems

Abstract

Solid oxide fuel cells (SOFCs) are a ceramic type of fuel cell operating at elevated temperatures. By utilising the thermal energy from the SOFC in a heat engine, a hybrid cycle with high performance can be achieved. One promising concept is a pressurised SOFC in combination with a recuperated gas turbine cycle. The fuel cell, being the more efficient power generator of the two, represents the major output of such a system. Most likely intended for small-scale power and heat generation these systems can employ a microturbine. This work was concerned with thermodynamic modelling of combined solid oxide fuel cell and gas turbine (SOFC/GT) systems, and the evaluation of their performance. A detailed fuel cell model was developed by Azra Selimovic, and adapted and implemented in a process simulation program by the author. The fuel cell model considered a planar SOFC design with internal reforming and heat conduction in the solid part. Electrolyte-supported cells were assumed, which are typical for a high-temperature SOFC. The modelling approach was unique, integrating a two-dimensional cell model into a flowsheet model. Operational analysis was flexible as changes in the system parameters, as well as cell parameters, could be assessed on the overall behaviour of the systems. Although no validation of the system model was possible due to a lack of data, the fuel cell model was compared with other models in the literature with good results. Both parametric and to a lesser extent, conceptual variations of a reference SOFC/GT system were made, resulting in, generally, good performance under different operating conditions. The investigation showed greater effect on the system performance of the cell parameters (e.g., maximum solid temperature, internal cell resistance, cell geometry) than of the system parameters (e.g., pressure, turbine inlet temperature, recuperator size). By introducing conceptual changes into the SOFC/GT system, such as networking of stacks and cathode gas recycling, significant improvement of the system performance was achieved. In the first case, air and fuel were ducted in series between stacks, which led to higher utilisation of the fuel in the stacks and allowed for lower air flow rates. In the second concept, the air flow required by the cell could be decoupled from the flow to the gas turbine, thereby increasing the fuel utilisation and power fraction of the fuel cell part of the system. In addition, environmental aspects of SOFC/GT systems were covered, such as SOFC systems with CO2 abatement, biomass-fuelled SOFC/GT systems and design analysis of a combustor applied to burn SOFC off-gases. In the first study the concept of multistage oxidation (networking of stacks) was elaborated to capture CO2 in the anode (reacted fuel) stream. In the second study a gasifier and gas treatment system for biomass were modelled and implemented in a SOFC/GT flowsheet model. The combustor design analysis resulted in a non-premixed burner type adapted to the SOFC/GT application. Promising results were obtained in all studies.

Abstract (Swedish)

Popular Abstract in Swedish

Fasoxidbränsleceller (eng. SOFC) är en keramisk bränslecellstyp som arbetar vid höga temperaturer. Genom att utnyttja energin hos avgaserna från en SOFC i en annan kraftvärmeprocess kan man erhålla ett s.k. hybridsystem som når höga verkningsgrader. Ett koncept som studerats i denna avhandling är en SOFC i kombination med en gasturbincykel (SOFC/GT). Eftersom bränslecellen är den mest effektiva av de två är också effektandelen därifrån störst. Dessa hybridsystem kommer troligtvis utnyttjas för småskalig el och värmeproduktion. Denna avhandling behandlar modellering av SOFC/GT system för att utröna deras uppförande och möjligheter att nå god prestanda. En ny modelleringsteknik har använts genom att införa en detaljerad bränslecellsmodell (utvecklad av Azra Selimovic) i ett processimuleringsprogram. Det framkom bl.a. att de parametrar som berör bränslecellen har störst inverkan på systemet. Olika systemkoncept har även studerats t.ex seriekoppling av flera mindre bränsleceller som leder till lägre kylluftbehov och bättre prestanda. Även miljömässiga aspekter har studerats som t.ex SOFC/GT system med biomassa som bränsle och SOFC system med fullständig eller partiell avskiljning av koldioxid. Generellt kan mycket goda miljödata från dessa system erhållas tack vare den elektrokemiska omvandlingen av bränslet. En studie av brännkammare anpassat för dessa system gav mycket låga NOx emissioner. Verkningsgraden för dessa system beror på hur stor bränslecell man kan tillåta för ett givet bränsleflöde. En relativt stor bränslecell leder till höga verkningsgrader men förmodligen höga kostnader. P.g.a viss osäkerhet i beräkningarna och det faktum att ingen fullständig optimering gjorts leder till att ingen exakt bestämning kan göras. Men en slutsats är att verkningsgrader upp emot 70 % kan erhållas men förmodligen är 60 % mer realistiskt med tanke på ekonomi för system och storlek och kostnad av komponenter.

• author: Pålsson, Jens
• supervisor: [unknown] [unknown]
• opponent: Campanari, Stefano, Dr.Ing., Politecnico di Milano, Italy
• organization: Thermal Power Engineering
• publishing date: 2002
• type: Thesis
• publication status: published
• subject: Energy Engineering
• keywords: vakuumteknik, vibrationer, akustik, Motors and propulsion systems, Motorer, framdrivningssystem, Gases, fluid dynamics, plasmas, plasma, hydraulik, Maskinteknik, vibration and acoustic engineering, hydraulics, vacuum technology, Modelling, Mechanical engineering, Solid oxide fuel cell, Hybrid system, Gaser, fluiddynamik
• pages: 148 pages
• defense location: Room M:B, M-building, Ole Römers v. 1, Lund Institute of Technology, Lund
• defense date: 2002-05-29 10:15:00
• external identifiers: other:ISRN:LUTMDN/TMHP--02/1003--SE
• ISBN: 91-628-5247-7
• language: English
• LU publication?: yes
• additional info: Article: 1. Pålsson, J., Selimovic, A. and Sjunnesson, L.,"Combined Solid Oxide Fuel Cell and Gas TurbineSystems for Efficient Power and Heat Generation",Journal of Power Sources, 6th Grove Fuel CellSymposium, Volume 86/1-2, pp. 442-448, March 2000. Article: 2. Barchewitz, L. and Palsson, J., "Design of anSOFC System Combined to the Gasification ofBiomass", 4th European SOFC Forum. Lucerne,Switzerland, July 2000. Article: 3. Palsson, J., Selimovic, A. and Hendriksen, P.,"Intermediate Temperature SOFC in Gas TurbineCycles", ASME TurboExpo 2001, Paper No. 2001-GT-0091,New Orleans, USA, June 2001. Article: 4. Pålsson, J. and Selimovic, A., "Design andOff-Design Predictions of a Combined SOFC and GasTurbine System", ASME TURBOEXPO 2001, Paper No2001-GT-0379, New Orleans, USA, June, 2001. Article: 5. Selimovic, A. and Pålsson, J., "Networked SolidOxide Fuel Cell Stacks combined with a Gas TurbineCycle", accepted for publication in the Journal ofPower Sources, in conjunction with the 7th GroveFuel Cell Symposium, London, September, 2001. Article: 6. Hermann, F., Pålsson, J.and Mauss, F., "CombustorDesign Analysis for SOFC Offgases", submitted tothe 5th European SOFC Forum, to be held in Lucerne,Switzerland, July, 2002. Article: 7. Pålsson, J., Arriagada, J. and Fredriksson, B.,"SOFC Systems with CO2 Capture", submitted to the5th European SOFC Forum, to be held in Lucerne,Switzerland, July, 2002.
• id: 84a45f3d-cf75-41ed-a21c-0c2d75b30c19 (old id 464641)
• date added to LUP: 2016-04-01 15:27:51
• date last changed: 2018-11-21 20:34:35