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From waste to electricity through integrated plasma gasification/fuel cell (IPGFC) system G. Galeno a, M. Minutillo b, A. Perna a,* a b
Department of Industrial Engineering, University of Cassino, Via G. Di Biasio 43, Cassino, Italy Department of Technology, University of Naples “Parthenope”, Centro Direzionale, Naples, Italy
article info
abstract
Article history:
The waste management is become a very crucial issue in many countries, due to the ever-
Received 6 August 2010
increasing amount of waste material, both domiciliary and industrial, generated.
Received in revised form
The main strategies for the waste management are the increase of material recovery (MR),
29 October 2010
which can reduce the landfill disposal, the improvement of energy recovery (ER) from
Accepted 1 November 2010
waste and the minimization of the environmental impact.
Available online 4 December 2010
These two last objectives can be achieved by introducing a novel technology for waste treatment based on a plasma torch gasification system integrated with a high efficiency energy
Keywords:
conversion system, such as combined cycle power plant or high-temperature fuel cells.
Waste-to-energy
This work aims to evaluate the performance of an Integrated Plasma Gasification/Fuel Cell
RDF
system (IPGFC) in order to establish its energy suitability and environmental feature.
Plasma gasification
The performance analysis of this system has been carried out by using a numerical model
Solid oxide fuel cell
properly defined and implemented in Aspen Plus code environment. The model is based
Integrated energy system
on the combination of a thermochemical model of the plasma gasification unit, previously developed by the authors (the so-called EquiPlasmaJet model), and an electrochemical model for the SOFC fuel cell stack simulation. The EPJ model has been employed to predict the syngas composition and the energy balance of an RDF (Refuse Derived Fuel) plasma arc gasifier (that uses air as plasma gas), whereas the SOFC electrochemical model, that is a system-level model, has allowed to forecast the stack performance in terms of electrical power and efficiency. Results point out that the IPGFC system is able to produce a net power of 4.2 MW per kg of RDF with an electric efficiency of about 33%. This efficiency is high in comparison with those reached by conventional technologies based on RDF incineration (20%). ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The waste management is become a very crucial issue in many countries, due to the ever- increasing amount of waste material, both domiciliary and industrial, generated. The main strategies for the waste management are the increase of material recovery (MR), which can reduce the
landfill disposal and the improvement of energy recovery (ER) from waste. Conventional technologies for the energy recovery from waste are based on the incineration process and pyrolysis or gasification processes [1e9]. While both pyrolysis and gasification are feasible technologies to handle municipal waste, commercial applications of these technologies have been limited.
* Corresponding author. Tel.: þ39 07762993634; fax: þ39 07762993886. E-mail addresses:
[email protected] (G. Galeno),
[email protected] (M. Minutillo),
[email protected] (A. Perna). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.11.008
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Nomenclature EPJ IPGCC IPGFC LHV RDF Pe hPG hIPGFC S/C Uf UAir E0 Vcell Icell IL,cat Qcell F
EquiPlasmaJet Integrated Plasma Gasification/Combined Cycle Integrated Plasma Gasification/Fuel Cell Lower Heating Value, kJ/kg Refuse Derived Fuel Net Power, kW Plasma gasification efficiency Electric efficiency of IPGFC system Steam to carbon ratio at the pre-reforming reactor Fuel utilization factor Air utilization factor Ideal voltage for hydrogen oxidation at ambient pressure, V Cell voltage, V Cell current, A Cathode limiting current, A Thermal power, kW Faraday constant, 96485 C/mol
Recently, an innovative technology, based on the plasma torch gasification, seems to be the most effective and environmentally friendly method for biomass/solid waste treatment and energy utilization [10e19]. The plasma gasification process works at very high temperatures in an oxygen-starved environment and decomposes completely the input waste material into very simple molecules. The organic compounds are thermally decomposed into their constituent elements and converted into a synthesis gas, which mainly consists of hydrogen and carbon monoxide, while the inorganic materials are melted and converted into a dense, inert, non-leachable vitrified slag [10,19]. Therefore, the syngas generated by the plasma gasification is cleaner than that produced by conventional gasification processes [10], due to the high temperatures involved, which allow to broken down all the tars, char and dioxins. However, the production of a very high temperature plasma gas requires an external energy source and thus a high electric consumption of the plasma torches [11,13,19,20]. This drawback can be overcome if the syngas produced by gasification is used as fuel in high efficiency power generation systems, such as combined cycle power plants or hightemperature fuel cells. In a previous paper [20], the authors focused on conventional technologies for energy generation and thus, they studied the behaviour and the performance of an integrated plasma gasification combined cycle (IPGCC) power plant. In this paper the integration between the plasma gasification unit and a solid oxide fuel cell (SOFC) system is proposed and the energy suitability of the Integrated Plasma Gasification/Fuel Cell system (IPGFC) is analyzed by means of thermochemical and electrochemical models properly developed.
2.
Technology review
This section provides a brief description of plasma torch gasification processes in waste management and introduces
R Tcell p pi n N J kA,an kA,cat EA,an EA,cat Ai i i0,an i0,cat DO2 b s 3
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Universal gas constant, 8.314 J/molK Cell operating temperature, K Ambient pressure, Pa Partial pressure of species “i” Number of electrons Number of elements in which the cell is discretized along the tube axis Single element in the scheme of the tubular cell Pre-exponential coefficient of the Anode Pre-exponential coefficient of the Cathode Anodic activation energy, J/mol Cathodic activation energy, J/mol Area of each section of the cell, m2 Current density, A/m2 Anodic exchange current density, A/m2 Cathodic exchange current density, A/m2 Oxygen ordinary diffusion coefficient, m2/s Electronic transfer coefficient Tortuosity Porosity
the high-temperature fuel cells as a promising power technology for the integration with waste gasification systems.
2.1.
Plasma gasification
The plasma is created by applying energy to a gas in order to reorganize the electronic structure of the species (atoms, molecules) and to produce excited species and ions. This energy can be thermal, or carried by either an electric current or electromagnetic radiations [21]. Depending on the type of energy supply and the amounts of energy transferred to the plasma, the properties of the plasma change, in terms of electronic density or temperature. Huang et al. [19] distinguish two main groups of laboratory plasmas, the high temperature or fusion plasmas, in which all species (electrons, ions and neutral species) are in a thermodynamic equilibrium state, and the low-temperature plasmas (a further distinction can be made between the thermal plasmas, in which a quasi-equilibrium state occurs, and the cold plasmas where a non-equilibrium state takes place). Among all the plasmas processes, the thermal plasmas is the most suitable for waste materials treatment, because the organic compounds, under high temperature conditions, are decomposed into their constituent elements and the inorganic materials (glass, metals, silicates, heavy metals) are melted and converted into a dense, inert, non-leachable vitrified slag [10,16,21]. With respect to the plasma sources for this application field, they can be the arc plasma torches fed by a DC power supply or the metallic torches, in which the plasma is generated by a microwave discharge [21,22]. In an arc plasma torch a DC-ARC discharge provides high energy density and high temperature region between two electrodes and, in the presence of a sufficiently high gas flow, the plasma extends beyond one of the electrodes in the form of a plasma jet [21]. The temperature in the core of the plasma plume can be greater than 3$104 C, whereas in the marginal zones, it
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decreases rapidly and the average operating temperature can be as high as 5000 C. The arc plasma generators can be divided into non-transferred arc torch and transferred arc torch. In a non-transferred arc torch, the two electrodes do not participate in the processing and have the function of plasma generation. In a transferred arc reactor, the substance to be processed is placed in an electrically grounded metallic vessel and acts as the anode, hence the reacting material should be an electrically conductive material [19]. The syngas produced by the plasma gasification process contains the plasma gas components, usually oxygen and/or nitrogen, if air or nitrogen is used as plasma gases respectively. The use of other inert gases, such as argon, can allow to improve the performance of the gasification process, even if the costs of the whole system increase. There are several manufactures of plasma torches, such as Westinghouse Plasma Corporation, PEAT (Plasma Energy Applied Technology, Inc.), Phoenix Solution. The commercially available models are in the range of 75 kW to 10 MW of power with a thermal efficiency (the percentage of arc power that exits the torch and enters the process) of about 90% [16].
2.2. High-temperature fuel cells application in the gasification field Waste gasification processes are more commonly integrated with combined cycle power plants for large scale applications or with internal combustion engines for medium/small scale applications. The overall performance of integrated gasification/power systems can be enhanced by the use of more advanced power generation technology such as high-temperature fuel cells (MCFC, SOFC). High-temperature fuel cells have more fuel flexibility than low-temperature fuel cells (PEMFC) because the carbon monoxide does not poison the anode electrocatalyst, whereas it works as a fuel [23e25]. As a result, various fuels can be processed to produce a reformate (containing primarily hydrogen and carbon monoxide) for direct use in high-temperature fuel cells. The main fuel sources, used to produce this reformate, include fossil fuels (natural gas, oil and coal) and renewable fuels (i.e. biomass and waste) [24,25]. Nagel et al. [26] studied biomass integrated gasification fuel cell systems with an electrical power output of about 1 MW. In this work they assessed the technical and economical feasibility of integrated systems, based on three existing or soon available biomass gasification processes, various gas processing technologies and four SOFC designs for stationary applications. Lobachyov et al. [27], studied the integration of a biomass gasification system with a Molten Carbonate Fuel Cell (MCFC) instead of a gas turbine. Their analysis showed that feeding the gasification product gas into an MCFC, instead of a gas turbine, the efficiency and the environmental emissions could be improved. Panopoulos et al. [28] studied the integration between an allothermal biomass gasifier and a SOFC power unit, both operating at atmospheric pressure, for small scale CHP generation. The biomass gasifier, modelled by using the AspenPlus simulation software, consisted of two fluidised bed reactors, in
which the secondary one, that supplied heat to the primary, was fed with SOFC depleted off-gases, un-reacted gasification char and additional biomass, if required. The electrical efficiency obtained, 36%, was comparable with that of a biomass gasification combined cycle. Thus, the high-temperature fuel cells can be considered as the most promising fuel cell type for the application in integrated gasification systems due to their low requirements in the fuel gas quality, their capability of oxidizing carbonous fuels [25e29] and their high performance.
3.
Plasma gasification modelling (EPJ model)
The plasma gasification process has been modelled by using the thermochemical code AspenPlus. Fig. 1 shows the flowsheet of the plasma gasification reactor model, called EquiPlasmaJet (EPJ), and Table 1 reports a short description of the main blocks used to model the system [20]. According to the expected temperature profile inside the plasma gasifier, the reactor is divided into two reaction zones for the purpose of modelling. As a consequence, the gasification of the organic fraction of the solid waste is carried out in two reactors, HTR (High Temperature Reactor) and LTR (Low Temperature Reactor), in which the chemical equilibrium is solved by a non-stoichiometric formulation. In this approach the equilibrium composition is found by the direct minimization of the Gibbs free energy for a given set of species without any specification of the possible reactions which might take place in the system. The HTR reactor, which operates at an average temperature of about 2500 C, simulates the main zone of the plasma reactor where the plasma jet directly impacts the refuse. In the LTR reactor, which operates at temperatures of about 800e1200 C, the gasification process is completed and the organic fraction is converted into a synthesis gas. Moreover, in order to simulate the broken down phase of the organic fraction an RYIELD reactor (DECOMP) is placed before the HTR reactor. In this block, where the waste yield distribution is specified according to the proximate and ultimate analysis, the organic fraction of the solid waste is decomposed into its constituent elements. The heat of reaction associated with the waste decomposition (broken down phase) is considered in the gasification energy balance as a “heat stream” (HEAT1) that connects the DECOMP reactor with the HTR reactor. In order to model the plasma jet, a heat exchanger (DC-ARC), which supplies the heat needed to generate the plasma gas, has been introduced. The plasma gas (PLASMA) is produced at 4000 C, and the power consumption of the plasma torch is calculated by the thermal power transferred to the stream GAS in the DC-ARC heat exchanger, considering a plasma torch thermal efficiency (the ratio between the energy transferred to the gas and the discharge energy) of 90%. Because the waste is forced gravitationally downward of the plasma reactor, it is preheated by the hot syngas that travels upward. Therefore, the water into the waste evaporates and leaves the reactor together with the syngas. In order to model this behaviour, two heat exchangers (HEX1 for the solid waste and HEX2 for the gas phase), a separation unit (SEP) and a mixer (MIX) have been used in the plasma gasification reactor model.
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LTR SYNGAS ORG1
HEX1 HEAT2
HEX2
MIX ORG2 SYNGAS
HEAT1
GAS
PLASMA
DC-ARC
DECOMP HTR ORG3
SEP
ORG4 WATER
Fig. 1 e Plasma gasification reactor model (EPJ model).
The working conditions and the performance of the plasma torch gasifier calculated by developed model, deeply described in ref. [20], are summarized in the Table 2. The data refers to the optimal values found by applying a sensitivity analysis (air as plasma gas and additional oxygen in the plasma gasifier).
4.
SOFC modelling
Solid oxide fuel cells are based on the concept of an oxygen ion conducting electrolyte through which the oxide ions migrate from the air electrode (cathode) side to the fuel electrode (anode) where they react with the fuel (H2, CO, etc.) to generate an electrical voltage. During the 1990s, SOFC stacks have been developed in an industrial scale and, since 1999, SiemenseWestinghouse operated a trial SOFC, with a power output of 100 kW and 46% efficiency, at 1000 C [30]. Smaller apparatuses already operate from the Japanese Chubu Electric Power, the Ceramic Fuel Cells in Australia and the Sulzer Hexis in Switzerland. In a pilot scale a significant number of cells have been tested successfully with an efficiency that exceeds 45%. SOFC of medium production
Table 1 e Main blocks description. Block Name
Block type
Description
DECOMP
RYIELD
HTR LTR
RGIBBS
HEX1 HEX2 SEP DC-ARC MIX
HEATER
Non-stoichiometric reactor based on known yield distribution Rigorous reactor and multiphase equilibrium based on Gibbs free energy minimization Heat exchanger
SEPARATOR HEATER MIXER
Separator unit Heat exchanger Stream mixer
(50 kW–1 MW) are an attractive solution for a distributed energy generation. Power generation systems in which a SOFC stack is used in combination with other generating equipment can provide higher electrical efficiency than the simple SOFC system [23]. This is because the high-temperature SOFC exhaust heat also contributes to power generation in the other generating devices. In order to analyse the SOFC system integrated with the novel gasification process based on the plasma torch technology, a one-dimensional model has been developed.
4.1.
Model description
The issue of modelling in fuel cell systems can be dealt following two approaches: at cell-level, to improve understanding of complex physical and chemical phenomena, and at fuel cell system-level to investigate the impact of the operating conditions on the overall system. In this study the second approach has been followed. In order to evaluate the SOFC performance, in terms of electric and thermal power generated, a one-dimensional model, which considers the thermochemical (i.e. reforming and shifting reactions) and electrochemical (i.e. electrooxidation of hydrogen) reactions, has been developed by using the AspenPlus code. This model is valid for either tubular or planar SOFC configuration. For the present analysis, the tubular fuel cell configuration, based on the SiemenseWestinghouse concept with cathode-supported cell, has been assumed [31]. Geometry parameters and operating data of the SOFC module are summarized in Table 3 [32e34]. The basic assumptions in fuel cell modelling are:
Steady state; Isothermal fuel cell; The fuel composition varies only in the outlet direction; All chemical species of working fluids are treated as ideal gas; All gas reactions are in chemical equilibrium; The operating cell voltage is the same for every tubular cell.
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Table 2 e Plasma torch gasifier performance [20].
Table 3 e Geometry parameters of 100 kW SOFC [32e34].
RDF mass flow RDF Low Heating Value (LHV) Plasma gas mass flow (Air) Oxygen mass flow (additional flow) Syngas mass flow Syngas outlet temperature
kg/s MJ/kg kg/s kg/s kg/s C
1 12.9 0.505 0.207 1.60 1245
Syngas composition (vol %) H2 CO CH4 CO2 H2O N2 HCl H2S COS Syngas Low Heating Value (LHV) Torch power ASU power hPG
% % % % % % % % % MJ/kg MW MW %
28.65 37.37 e 1.41 14.19 17.12 0.31 0.22 0.01 9.20 2.75 0.195 69.1
Fig. 2 shows the flowsheet of the SOFC system model based on SiemenseWestinghouse design. It consists of a fuel prereforming reactor (PRE-REF), a catalytic burner (CB), in which part of the anodic exhaust (A-OUT) is burnt with the cathodic one (C-OUT) to supply the thermal energy needed for the air reactant preheating, and a heat exchanger (HEX). In the pre-reforming reactor, where the remaining of the anodic exhaust is also recirculated to improve the system efficiency, the feeding fuel is converted into a synthesis gas containing mainly H2, CO, CO2 and H2O. The pre-reforming reactor is a Gibbs reactor in which the reforming reaction is simulated considering the chemical equilibrium solved by the direct minimization of the Gibbs free energy for a given set of species. The SOFC stack is simulated as a hierarchical block that provides containers for simulation objects. The HIERARCHY block allows to forecast the SOFC power unit performance by assigning the number of the cells and the stacks and some of the operating parameters reported in Table 3. Each tubular cell is discretized in N-elements along the tube axis (that is the direction of both the fuel and oxidant flow). Fig. 3 shows a schematic diagram of the SOFC cell geometry, in which a single J element is highlighted with a dashed line. According to this schematic diagram, a model for predicting material, heat and electrical outputs of the J element has been performed by using the AspenPlus code, as depicted in Fig. 4 (it is worth noting that the numerical results of the J element are the input data for the next element). The J element comprises mainly a cathode block (CAT,J), whose function is to separate out the O2 required for the electrochemical reaction and an anode block (AN,J), modelled as a stoichiometric reactor, in which the hydrogen oxidation occurs. During the SOFC operation, the anodic gas composition varies along the channel, due to both the electrochemical oxidation of hydrogen and the water gas shift reaction (WGSR). The electrochemical oxidation of CO has been neglected because the WGSR is very fast (the nickel-based catalyst that covers the anode speeds up this reaction), so that it is possible
Geometry parameters Stack number Cell number Cell active area Cell length Cell outer diameter Anode thickness Cathode thickness Electrolyte thickness Interconnection thickness
3 384 834$104 1.5 2.2$102 100$106 2.2$103 40$106 85$106
m2 m m m m m m
Nominal operating data Cell temperature Cell pressure UF/UAir S/C in the pre-reformer Input air/fuel temperatures DC Power
C bar
910 1.08 0.85/0.19 1.8 630/200 C 120
C kW
to assume that the whole CO is converted into hydrogen (the electricity is entirely produced from the electrochemical oxidation of hydrogen) [35]. Thus, the reactions occurring in the cell are: H2 þ 0:5 O2 /H2 O
(1)
CO þ H2 O/H2 þ CO2
(2)
In order to take into account the reaction (2), a Gibbs reactor (SHIFT,J) has to be placed down to the AN,J block. The energy (QBAL,J block allows to estimate the thermal fluxes generated in each J element during cell operation) and mass balances are solved by an iterative procedure: assigned the fuel utilization factor (UF), the air utilization factor (UAir)
EXAUST-2 HEX AIR
EXAUST-1 PRE-REF CB FUEL
A-OUT
A-IN
C-OUT
SOFC
HIERARCHY
Fig. 2 e Flowsheet of the SOFC system based on SiemenseWestinghouse configuration.
C-IN
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and the number of elements, the model calculates the chemical composition of each stream and its enthalpy variation, for a given cell operating temperature. The electrical output is calculated by means of a FORTRAN block, in which the estimation of the cell voltage Vcell (V) is based on the following expressions: PN Vcell ¼
J¼1
VJ
N
VJ ¼ ENernst hact hohmic hconc
(3) (4)
where ENernst is the thermodynamic potential, hact is the activation overpotential due to slow kinetics of the electrochemical reactions taking place on the electrodes, hohmic is the ohmic overpotential due to the resistance to electron flow through both electrodes and interconnection and to the resistance to oxygen ion flow through the electrolyte, and hconc is the concentration overpotential due to mass transfer limitations. The Nernst potential is calculated as: ENernst ¼ E0 þ
RT pH2 p0:5 O2 ln pH2 O 2F
(5)
Fig. 4 e Conceptual scheme of the SOFC cell modelling.
The activation losses are estimated as the difference between the anodic and cathodic losses: hact ¼ hact;an hact;cat
(6)
in which each term can be calculated considering the Tafel’s law, derived, as known, by the ButlereVolmer relationship. Thus, the activation losses are estimated (either for anode or cathode) by the equation: hact
RT i ¼ ln nFb i0
(7)
where io is the exchange current density that is calculated as follows: pH2 pH2 O EA;an i0;an ¼ kA;an exp (8) p p RT
i0;cat ¼ kA;cat
0:25 pO2 EA;cat exp p RT
(9)
The ohmic overpotential is calculated according to: hohm ¼ Rohm Icell
(10)
where Rohm is the ohmic resistance that is calculated taking into account the four resistances (anode, cathode, electrolyte
Fig. 3 e Schematic of the tubular cell.
and interconnections resistances) to the flow of ions and electrons inside the cell: Rohm ¼
4 X ri li i¼1
(11)
Ai
In this relation ri (the specific material resistivity), li (the thickness) and Ai (the current flow area) are referred to each cell component (i.e. anode, cathode, electrolyte and interconnections). Their values, according to refs. [34,36], are reported in Tables 3 and 4.
Table 4 e Coefficients in polarization losses. Ohmic losses [34] Anode resistivity Cathode resistivity Electrolyte resistivity Interconnection resistivity Activation losses [26] Electronic transfer coefficient Number of electrons Anode activation energy Cathode activation energy Anode pre-exponential coefficient Cathode pre-exponential coefficient Coefficient m Concentration losses [34,36] Porosity 3 Tortuosity s Oxygen ordinary diffusivity
Um Um Um Um
2.98$105 exp(1392/T) 8.114$105 exp(600/T) 2.94$105 exp(10350/T) 0.025
0.5
kJ/mol kJ/mol A/m2
2 110 120 7$109
A/m2
5.5$109 1
m2/s
0.5 5.9 7.3$106
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Table 5 e Model validation results. Literature data [34] Fuel composition at cell inlet H2 CO CH4 CO2 H2O N2 Cell inlet fuel temperature Cell inlet air temperature Stack exhaust temperature Cell Voltage Cell Current density Gross Electric efficiency (LHV) DC Electric Power
Model results
small thickness, so that the concentration overpotential is only due to the slow diffusion of oxygen through the thick porous cathode. This loss is expressed as: hconc ¼
vol % vol % vol % vol % vol % vol % C C C V A/cm2 % kW
27.0 5.6 10.1 23.1 27.9 6.2 536 821 834 0.7 1780 56 120
26.6 5.4 10.7 23.2 27.9 6.2 532 820 825 0.68 1821 55 118
The concentration polarization is the result of diffusion of reactants to the interface between the electrolyte and the anode/cathode catalysts. The diffusion is associated with a resistance, resulting in a voltage drop (concentration polarization), that is dominant at high current densities where the diffusion is greatest. When the current density reaches either the anode or the cathode limiting currents, that is usually taken as a measure of the maximum rate at which a reactant can be supplied to an electrode [37], an insufficient amount of reactants is transported to the electrodes and the operating voltage is reduced to zero [38]. According to Suwanwarangkul et al. [31], the mass transfer resistance through the porous anode is ignored because of its
RTcell Icell ln 1 2F IL;cat
(12)
The cathode limiting current is evaluated as: IL;cat ¼
2FpO2 DO2;eff RTcell
(13)
where the DO2,eff is the effective ordinary diffusion coefficient: DO2;eff ¼
DO2 3 s
(14)
The material properties and all coefficients used in the polarization losses equations are summarized in Table 4.
4.2.
Model validation
In order to validate the SOFC model, the results obtained from numerical simulations have been compared with nominal data available from scientific literature [34], as shown in Table 5. It is worth noting that the model results are close to the nominal data available from ref. [34]. Thus, even if some changes have to be made (the pre-reformer is not needed, as it will be discussed later) the model is able to predict the performance of the SOFC power unit fed with the syngas produced by the waste gasification unit.
Fig. 5 e Flowsheet diagram of the integrated plant IPGFC.
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5. Integrated plasma gasification/fuel cell system (IPGFC) analysis In this section the energy suitability of a Plasma Gasification unit fed by RDF, integrated with a SOFC system, is investigated and its performance are analyzed by means of the thermochemical and electrochemical models described and discussed in the previous paragraphs. The EPJ model has been employed to predict the syngas composition (Table 1) and the energy balance of an RDF (Refuse Derived Fuel) plasma arc gasification reactor, whereas the SOFC model, that is a system-level model, has allowed to forecast the stack performance.
5.1.
System description
In Fig. 5 the schematic layout of the integrated plant IPGFC is depicted. The integrated system consists of two main sub-units fully integrated: the Plasma Gasification unit and the Fuel Cell unit. Moreover, in order to increase the overall efficiency, an intermediate pressure steam is generated by cooling the syngas from the plasma torch gasifier and the combustor exhausts leaving the heat exchanger HEX3. This intermediate pressure steam is then expanded in a steam turbine unit producing additional power. Because the syngas exiting the plasma gasification unit contains particulates and sulphide, a clean-up section, which remove these contaminants by means of ceramic filters and sorbent beds respectively, has to be placed before the SOFC power unit. With respect to the SiemenseWestinghouse SOFC design, the internal pre-reformer is not employed [29], because the methane and hydrocarbons concentrations in the clean product gas are very low. In addition, contrary to natural gas fuelled SOFC configurations (i.e. the SiemenseWestinghouse 100 kW SOFC module), no recirculation of the anode off-gas is considered, due to its high nitrogen content that would significantly dilute the syngas coming to the fuel cell (the nitrogen concentration in the product syngas is about 17%). In order to optimize the integration between the two main units, a part of the cathode off-gas is used as plasma gas in the plasma gasification unit, while the remaining is sent to the combustor that provides the thermal energy needed to preheat the SOFC feeding air.
5.2.
Performance analysis
The capacity of the integrated system, analyzed in this paper, is based on a SOFC module with 100 m2 of active surface resulting in gross electrical outputs of around 100 kW, so that the net electrical power estimated considering both the torch and ASU power consumption and the steam turbine power production is less than 100 kW. Nevertheless, the integrated system is envisaged for up to 1 MW, so that the constrains on size due to the costs (for gasification conversion systems, based on biomass, the most cost-effective size is expected to be plants from 1 to 30 MWe) and components availability (torch and steam turbine size) can be overcome.
In Table 6 the main plant operating data and performance of the IPGFC system are listed. The SOFC power unit fed with syngas produces 111 kW and the steam turbine produces 25 kW while the torch gasification unit and the air separation unit consume about 45 kW and 4 kW respectively, thus the net power of the IPGFC is 87 kW. The electric efficiency of IPGFC has been defined as follows: hIPGFC ¼
Pe _ RDF LHVRDF m
(15)
In Fig. 6 the electric efficiencies of different conversion systems, based on waste utilization, are compared. It is worth noting that the IPGFC efficiency (32.7%) is high in comparison with the efficiency of conventional technologies based on RDF incineration (20%). Furthermore, the efficiency of the IPGFC is comparable with that of the IPGCC (the integrated plasma gasification combined cycle) analyzed in ref. [20]. This result can be explained considering that, even if the performance of a combined cycle power plant is usually higher than that of a fuel cell, in this study a better integration between the plasma torch gasification unit and the SOFC power unit has been carried out. In fact, the direct use of the cathodic off-gas (its temperature is about 900 C) as plasma gas allows to
Table 6 e IPGFC system operating parameters and performance. Operating data Plasma gasification unit RDF mass flow Plasma gas mass flow Oxygen mass flow Syngas mass flow Syngas temperature Torch power ASU power SOFC unit Fuel mass flow Air mass flow Air temperature Fuel temperature Fuel utilization factor Air utilization factor Cell Current Cell Voltage Power (3 stacks) Clean up unit Syngas outlet temperature
kg/s kg/s kg/s kg/s C kW kW
0.0206 0.0104 0.00427 0.033 1245 45 4
kg/s kg/s C C
0.033 0.28 820 400 0.82 0.23 159 0.608 111
A V kW
C
400
Steam Turbine unit Steam mass flow Steam inlet temperature Steam outlet temperature Steam inlet pressure Steam outlet pressure Power
kg/s C C bar bar kW
0.046 260 81 30 0.5 25
Performance Net Power Net electric efficiency
kW %
87 32.7
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Finally, even if the power output is less than 100 kW, the integrated system is envisaged for up to 1 MW, so that the constrains on size due to the costs (for gasification conversion systems, based on biomass, the most cost-effective size is expected to be plants from 1 to 30 MWe) and components availability (torch and steam turbine size) can be overcome.
references
Fig. 6 e Comparison of conventional and advanced technologies for waste-to-energy purpose (in this comparison the efficiency refers to plants that generally are characterized by different size).
reduce the plasma torch power consumption, increasing the overall system efficiency (using air at ambient temperature as plasma gas, the system efficiency is about 28%).
6.
Conclusions
Recently, an innovative technology, based on the plasma torch gasification, seems to be the most effective and environmentally friendly method for solid waste treatment and energy utilization. The syngas generated by the plasma gasification is cleaner than that produced by conventional gasification processes because of the high temperatures involved, which allow to break down all the tars, char and dioxins. However, the high electric consumption of the plasma torches requires its integration with high efficiency energy conversion systems. In this paper the integration between a plasma torch gasification unit and a solid oxide fuel cell (SOFC) system is proposed and the energy suitability of the Integrated Plasma Gasification/Fuel Cell (IPGFC) system is investigated and analyzed by means of thermochemical and electrochemical models properly developed. The Plasma Gasification unit fed by the RDF has been performed by applying the EPJ (EquiPlasmaJet) model previously developed by the authors, whereas, in order to evaluate the SOFC performance, in terms of electric and thermal power generated, a one-dimensional model, which considers the thermochemical (i.e. reforming and shifting reactions) and electrochemical (i.e. electro-oxidation of hydrogen) reactions inside the fuel cell, has been developed by using the AspenPlus code. Results point out that the IPGFC system produces a net power of 87 kW with an electric efficiency of about 33%. This efficiency is high in comparison with those reached by conventional technologies based on RDF incineration (20%) and comparable with the efficiency achieved by the integration of a Plasma torch Gasification unit integrated with a combined cycle (the IPGCC system).
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