Thermodynamic modeling and evaluation of high efficiency heat pipe integrated biomass GasifiereSolid Oxide Fuel CellseGas Turbine systems

Energy 109 (2016) 751e764 Contents lists available at ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy Thermodynamic modeling and evaluation of high efficiency heat pipe integrated biomass GasifiereSolid Oxide Fuel CellseGas Turbine systems S Santhanam 1, C Schilt, B Turker, T Woudstra, P.V Aravind* Delft University of Technology, Energy Technology Section, Leeghwaterstraat, 2628 CB, Delft, The Netherlands a r t i c l e i n f o a b s t r a c t Article history: Received 25 November 2015 Received in revised form 25 April 2016 Accepted 28 April 2016 This study deals with the thermodynamic modeling of biomass GasifiereSOFC (Solid Oxide Fuel Cell) eGT (Gas Turbine) systems on a small scale (100 kWe) Evaluation of an existing biomass GasifiereSOFC eGT system shows highest exergy losses in the gasifier, gas turbine and as waste heat In order to reduce the exergy losses and increase the system's efficiency, improvements are suggested and the effects are analyzed Changing the gasifying agent for air to anode gas gave the largest increase in the electrical efficiency However, heat is required for an allothermal gasification to take place A new and simple strategy for heat pipe integration is proposed, with heat pipes placed in between stacks in series, rather than the widely considered approach of integrating the heat pipes within the SOFC stacks The developed system based on a GasifiereSOFCeGT combination improved with heat pipes and anode gas recirculation, increases the electrical efficiency from approximately 55%e72%, mainly due to reduced exergy losses in the gasifier Analysis of the improved system shows that operating the system at possibly higher operating pressures, yield higher efficiencies within the range of the operating pressures studied Further the system was scaled up with an additional bottoming cycle achieved electrical efficiency of 73.61% © 2016 Elsevier Ltd All rights reserved Keywords: Solid Oxide Fuel Cell Gas Turbine Biomass gasification Heat pipes Exergy Introduction Solid Oxide Fuel Cells are highly efficient devices Their second law efficiencies are usually above 90%, as they produce electricity and heat at very high temperatures By utilizing this heat to produce mechanical work, subsequently, electricity is expected to result in very high efficiencies in electricity production [1e8] Efficient heat management, helps to achieve high efficiencies in such systems especially with a direct or indirect internal reforming in the fuel cell when carbonaceous fuels are used SOFCeGT (Solid * Corresponding author E-mail addresses: Santhanam.Srikanth@dlr.de (S Santhanam), A PurushothamanVellayani@tudelft.nl (P.V Aravind) Permanent address: German Aerospace Center (DLR), Institute of Engineering Thermodynamics, 38-40 Pfaffenwaldring, 70569, Stuttgart, Germany Tel.: þ49 7116862755 http://dx.doi.org/10.1016/j.energy.2016.04.117 0360-5442/© 2016 Elsevier Ltd All rights reserved Oxide Fuel CelleGas Turbine) systems using natural gas as fuel are expected to attain thermal efficiencies of 60%e80% [9] Operating such systems loaded with biosyngas and produced in a biomass gasifier, are expected to result in highly efficient and sustainable electricity production Biosyngas can be produced by endothermic reactions, when gasification agents, such as steam and CO2 are used The heat produced in SOFCs being partly used for gasification, is expected to help minimize exergy losses in such integrated systems The use of heat pipes for exchanging heat between solid oxide fuel cells and gasifiers, has been studied in the past [10e12] The EU (European Union)-funded project ‘BioHPR’ was successfully completed, using heat pipes in gasifiers while producing biosyngas with a high LHV (Lower Heating Value) Mol fractions of 40% H2, 20% CO and 5% CH4 in the biosyngas are reported [13] Another EU funded project, Biocellus, has gained significant progress in integrating heat pipes with the gasifiers System studies at Delft, Imperial College and other places have shown that electrical efficiencies above 60% from solid fuels are achievable for SOFC based systems [14,15] Recently it has been shown a better thermal 752 S Santhanam et al / Energy 109 (2016) 751e764 integration between the SOFC and biomass gasifier can further assist in reducing the exergy losses in the gasifier and thereby increase the efficiency [16] In this work a new and simple strategy for heat pipe integration is proposed, with heat pipes placed in between stacks in series, rather than the widely considered approach of integrating the heat pipes within the SOFC stacks Further the use of SOFC anode off gas as gasification agent is an additional new concept proposed in this work To the best of authors knowledge, solutions presented in this work are new This paper presents a detailed description of the second law of evaluation and optimization of very high efficiency, small power level, gasifiereSOFCeGT systems, having novel integration concepts A scaled up version of the proposed system model with a steam rankine bottoming cycle is later presented in this paper to evaluate the efficiency at higher capacities Description of the employed base-case model and subsystems The model presented earlier by our team [17] is considered as the basis for the present study In this model (Fig 1), biosyngas is formed in the gasifier and it is cleaned using a set of high temperature gas cleaning devices Clean biosyngas is fed to the SOFC which operates at an average temperature of 950  C, and part of the anode and cathode gas is recycled to maintain the SOFC inlet temperature at 900  C Since not all the fuel is utilized in the fuel cell (85%), anode gas is combusted with cathode gas before the turbine Turbine exhaust is used to preheat the cathode air flow, the gasification agent and for generating steam required to prevent carbon deposition Detailed process system scheme is shown in the Fig in Appendix B The model presented in this study which is a modified system involving heat pipes integration and anode off gas recirculation, is similar to the model presented in the earlier publication by the team of present authors [14] 2.1 Biomass gasification Biomass gasification is a thermochemical process of converting solid biomass fuel to high calorific gas product when the biomass reacts with a suitable gasifying agent at high temperatures The product of the gas composition is primarily a mixture of H2 (hydrogen), CO (carbon monoxide), CO2 (carbon dioxide), CH4 (methane), as well as other hydrocarbons such as ethane and so on Some amounts of H2O (water) and N2 (nitrogen), are also present depending on the gasification agent used Air, oxygen, steam or carbon dioxide is majorly used as the gasification agents Autothermal gasification take place when air or oxygen is used, Fig Process flow diagram of the base case, biomass GasifiereSOFCeGasTurbine, system and where the heat required for endothermic gasification reactions are supplied by the oxidation reaction occurring in the gasifier The advantage of autothermal gasification is that no external heat is required, but it produces a significant amount of nitrogen in the gas product Allothermal gasification occurs when steam or carbon dioxide is used as the gasifying agent In such process, an external heat source is required to support the endothermic gasification reactions, more so, a higher energy content of gas product can be obtained The main advantages and technical challenges of using different gasifying agents are summarized in Table given below Gasifier design, operating parameters and bed catalysts majorly determine the gas composition and the formed contaminants The gasification process is modeled in the study, while assuming chemical equilibrium However, simplified assumptions are taken, based on literature, from the percentage of char/carbon and methane produced in the high temperature gasification process and the carbon and hydrogen required for the production of char/carbon and methane are bypassed in the gasifier model 2.2 Carbon deposition As the syngas is heated or cooled down, solid carbon may get deposited When carbon deposition occurs, it can lead to blockage in the pipes and apparatuses This deposition is dependent on the thermodynamic CeHeO equilibrium composition of the biosyngas at different temperatures and pressures In general, a higher carbon content in the gas tends to increase carbon deposition, while hydrogen and oxygen helps to reduce it Addition of steam thus, helps to reduce carbon deposition A discussion on carbon deposition in gasifiereSOFC systems is shown in Refs [33,34] 2.3 Heat pipes Steam or carbon dioxide gasification is an allothermal process, where high LHV biosyngas can be obtained, but additional heat is required for the process Meanwhile, SOFCs produce heat as a result of exothermal electrochemical oxidation and internal losses The heat produced by SOFC is usually removed by providing excess air on the cathode side, which affects the system's performance By using heat pipes between the SOFC (acting as a heat source) and the allothermal gasifier (acting as a heat sink), the excess heat produced at SOFC can be transferred to the gasifier, where heat is required for gasification Such an integration using heat pipes result in the cooling of SOFC stack, leading to a reduction in the required cathode air flow Furthermore, integrating SOFC with gasifier using heat pipes, also reduces the exergy losses in the gasifier Therefore, a higher system performance can be achieved, due to the two effects 2.3.1 Principle Heat pipes are simple and effective heat transfer equipment, without moving parts A heat pipe is a hollow tube with layers of wire screen along the inner wall-the so-called wick The wick is filled with the liquid, having properties similar to the evaporation and condensation temperature of the application Heat pipes utilize the vaporizing liquid in order to create high heat fluxes from any heat source, in our case, it is utilized from the SOFC to the gasifier, where the endothermic gasification reactions take place High temperature heat pipes are usually metallic pipes containing an alkali metal (Na, K, and so on) Heat is transferred into the heat pipe at the evaporation zone This heat is released at the condensation zone, from the heat pipe to its environment For application in S Santhanam et al / Energy 109 (2016) 751e764 753 Table Summary of the main advantages and challenges of the different gasifying principles Gaisfying agent Air Steam Carbon dioxide Main advantages Main technical challenges 2 2 Partial combustion for heat supply of gasification Moderate char and tar content High heating value (10e15 MJ N mÀ3) H2 rich biosyngas (>50% by vol) High heating value of the biosyngas High H2 and CO, low CO2 in the biosyngas gasifiereSOFC sodium systems is a suitable working fluid with a boiling point at 883  C [35] 2.4 Gas cleaning Cleaning biosyngas either at ambient temperatures or at higher temperatures to meet the requirements of SOFCs is a challenging task Further research on the gas cleaning requirements of SOFCs and gas cleaning technologies is required for developing a suitable biosyngas cleaning systems for gasifiereSOFC systems A detailed analysis of the gas cleaning options is beyond the scope of this paper, but it is available (from the team of present authors) in literature [36] 2.5 SOFCeGT systems SOFCs can be connected to gas turbines with the SOFC anode off gas directly combusted in the Gas Turbine combustor or the heat from the SOFC off gas transferred to an externally heated, closed gas turbine cycle The latter case will decrease the Turbine Inlet Temperatures, which often leads to reduced system efficiencies The first option is widely considered in the study of systems available in literature [37e51] System configuration & assumptions 3.1 Biomass gasification The biomass input is selected from the Phyllis database provided by ECN (Energy research Centre of Netherlands) [52] The selected biomass is casuarina, which has a composition of; C 49.3%, H 5.9%, O 44%, N 0.6%, S 0.02%, Cl 0.162%; by weight The biomass composition is shown in Table 2, with a lower heating value of 15.500 kJ/kg, and a water content of 15 wt% The biomass is fed to the gasifier at a temperature of 25  C and at an operating pressure of gasifier The biomass feed rate to the gasifier is kept constant at 0.011 kg/s The biomass input has an energy source of 170.50 kW, which has an exergetic value of 192.87 kW Table Biomass composition in mass fractions Component Mass concentration (%) C H O N S H2O CH4 SiO2 39.41 4.19 36.12 0.47 0.05 15.00 4.08 0.68 Low heating value (3e6 MJ N mÀ3) Large amount of N2 in biosyngas Require indirect or external heat supply for gasification High tar content in biosyngas Require catalytic tar reforming Require indirect or external heat supply Require catalytic tar reforming References [18,19] [20e26] [27e32] The gasifier operates at a temperature of 800  C and the chemical equilibrium is assumed at the outlet To obtain a more realistic situation, mol% of the carbon is bypassed in the gasifier as an unconverted carbon Additionally, mol% of methane is bypassed in the gasifier, as biosyngas from air gasifiers, which is often reported as containing methane around this concentration In the base case situation, no heat is added to or released from the gasifier 3.2 Gas cleaning After the gasifier, biosyngas is cleaned close to a gasification temperature of 800  C The first step is a cyclone in which the large particles are removed (the bypassed carbon from the gasifier), and a barrier filter, for removing the small particles Tar cleaning is also done at high temperatures, around 800  C, the suggested catalysts are based on dolomite and nickel After the dust and tar cleaning, the syngas is allowed to cool down to a temperature of 600  C, for HCl and H2S removal When steam gasification or recirculation of anode gas is applied, significantly less steam is required to prevent carbon deposition The biosyngas is cooled down in two steps In the first step, the heat exchanger decreases the temperature, by increasing the temperature of the cleaned biosyngas, while in the second step, the heat exchanger reduces the temperature to 600  C by cooling with an external air flow In the model HCl and H2S, cleaning is modeled as a pressure drop Just before the SOFC, an alkali getter is modeled as a pressure drop, operating at a temperature of 900  C No change in composition of the main gas components due to gas cleaning devices is assumed Only pressure drop due to gas cleaning is assumed and heat loss to the environment is not considered Please refer to for a detailed discussion on the development of such, and similar gas cleaning concepts for gasifiereSOFC integration in the reference 3.3 SOFCeGas Turbine The SOFC system is implemented with a recycling of anode and cathode gases, in which a fraction of output gas is recycled in order to increase the SOFC inlet temperatures to 900  C The SOFC is assumed to operate at a temperature of 950  C with an outlet temperature of 1000  C Reasonable assumptions were made for various input parameters for the SOFC, based on the measurements of Ni/GDC anodes, as described in literature For the present calculations, cell resistance is taken as  10À5 Ohm m2 at an average SOFC temperature of 950  C The mean current density is taken as 2500 A/m2 The fuel cell is elaborated as a direct-internal-reforming fuel cell, which means that methane is reformed in the fuel cell Methane reforming is an endothermic process and thus, a fraction heat will be consumed within the fuel cell The fuel utilization is fixed at 85% of the inlet flow, and the pressure drop in the fuel cell is 0.05 bars on both sides The 754 S Santhanam et al / Energy 109 (2016) 751e764 anode product gas consisting of the unutilized combustible gases of H2, CO and CH4, is combusted with the remaining oxygen in the cathode output gas, found in the combustor The pressure drop in the combustor is assumed to be 0.02 bars The amount of cathode air flowing through the SOFC unit, determines the mass of flue gas entering the gas turbine Secondly, the amount of cathode air flowing through the system is determined by the energy balance of the SOFC unit The flue gas from the turbine exit is fed to the heat recovery unit to preheat the cathode air, steam and gasification air (for the base case system) The exhaust stack inlet temperature and pressure are measured at 100  C and 1.013 bar Due to the high level of cleanliness expected for the syngas, such low stack temperatures are considered reasonable 3.4 Heat pipes and anode off gas recirculation for gasifier For the modified system with heat pipe and anode gas recirculation for gasifier (gasification), the base case system is altered accordingly Normally, the heat pipes are integrated within SOFC stack, which makes the stack design complex and difficult to fabricate [53e56] Here, we propose to place the heat pipes in between the two stacks, rather than integrating the heat pipes within the SOFC stack The proposed method is intended to make the fabrication significantly simpler, as no new stack design is required The SOFC is now separated into two parts, each operating within a temperature range of 900  Ce1000  C Heat is removed from the first and the second fuel cell by cooling hot product gases (1000  C) to 900  C, before entering the second fuel cell The heat pipes usually operate with sodium, however, for modeling purpose, steam is used The exergy loss is not affected by the modeling approach, as long as heat pipe temperatures are equal to the existing temperatures in practice The heat pipe model only transfers an amount of heat and is independent of the heat pipe fluid medium Apart from the heat extracted from the fuel cell system, a small fraction of the total heat required for the gasification is extracted from the flue gas after combustion, using heat pipes A fraction of anode off gas is extracted, before entering into the combustor and is fed to the gasifier as gasification agent Detailed process system scheme is shown in the Fig 10 in Appendix C 3.5 Cycle tempo component models and exergy efficiency The Cycle Tempo is a thermodynamic modeling software, developed by the Department of Process and Energy at TU Delft The program calculates mass and energy balances of all the individual apparatus in the system model Based on this approach, a system matrix is developed, which is later solved to obtain the individual values In the present work, the biomass input feed determines the total power of the process system Cycle Tempo utilizes the Gibbs energy minimization routine to calculate the outlet gas compositions of the Gasifier, Combustor and internal reforming Fuel cell components The mass flow of air to the cathode side is determined by the energy balances of the fuel cell based on the cooling required to maintain the specific outlet temperatures of the exhaust streams The outlet temperature of the fuel cell is specified as input and it is assumed that both anode streams and cathode streams exit at same temperature The fuel cell component is based on the following set of equations Firstly, the gas inlet is taken to equilibrium state Two different methods can be used for fuel cell calculations In the first method, operating voltage and current density are provided as input to determine the area-specific resistance and active area of the fuel cell whereas in the second method, the area-specific resistance and active are input to determine the operating voltage and current density Additionally, in both cases the power output is calculated by specifying the fuel utilization This is performed based on the assumption that temperature, pressure and compositions are constant in cross section perpendicular to the fuel flow The total cell current is calculated based on the anode inlet flow rate and gas composition I¼  4m;a in  2F yH ỵ y0CO ỵ 4y0CH U f Ma (1) In the above equation 4m;a in represents the mass flow rate of anode gas at the inlet, y0i denotes the mass fractions of the gas components at the inlet F represents the faradays constant and final U f is the fuel utilization M a is the molar mass of the anode gas mixture The transfer of oxygen from the cathode side to the anode side is related to the cell current via the faraday relation Based on the specified outlet temperature of the gas, the mass flow rate of cathode is determined using the energy balance The local reversible or Nernst voltage, current density and gas compositions are calculated based on the following set of equations The local Nernst equations across a cross section area x is given by V rev;x ¼ V 0rev 1=2 RT