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Integration of a municipal solid waste gasification plant with solid oxide fuel cell and gas turbine
Renewable Energy 55 (2013) 490e500
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Integration of a municipal solid waste gasification plant with solid oxide fuel cell and gas turbine Filippo Bellomare a, Masoud Rokni b, * a b
University of Padua, Dept. of Industrial Engineering, Sede V - via Venezia, 1-35131 Padova (PD), Italy Technical University of Denmark, Dept. of Mechanical Engineering, Thermal Energy System, Building 403, 2800 Kgs, Lyngby, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 October 2012 Accepted 9 January 2013 Available online 11 February 2013
An interesting source of producing energy with low pollutants emission and reduced environmental impact are the biomasses; particularly using Municipal Solid Waste (MSW) as fuel, can be a competitive solution not only to produce energy with negligible costs but also to decrease the storage in landfills. A Municipal Solid Waste Gasification Plant Integrated with Solid Oxide Fuel Cell (SOFC) and Gas Turbine (GT) has been studied and the plant is called IGSG (Integrated Gasification SOFC and GT). Gasification plant is fed by MSW to produce syngas by which the anode side of an SOFC is fed wherein it reacts with air and produces electricity. The exhausted gases out of the SOFC enter a burner for further fuel combusting and finally the off-gases are sent to a gas turbine to produce additional electricity. Different plant configurations have been studied and the best one found to be a regenerative gas turbine. Under optimized condition, the thermodynamic efficiency of 52% is achieved. Variations of the most critical parameters have been studied and analyzed to evaluate plant features and find out an optimized configuration. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Biomass Municipal solid waste Gasification SOFC
1. Introduction The word “Biomass” refers to vegetables and animals substances, not from fossil origin; these can be used as fuel in a power plant for the production of electrical energy. Biomasses derive from living or recently living biological organisms and can be considered as a particular kind of renewable energy source, because the carbon dioxide placed in the atmosphere by their use derives from the carbon amount absorbed during their life. In this way, the most important pollutants linked to biomass utilizations are related to transport, manufacture and transformation processes. Municipal Solid Waste can be considered a valid biomass to use in a power plant. Some advantages can be obtained; the principal is the reduction of pollutants and greenhouse gases emissions. Another advantage is that by their use it is possible to reduce the storage in landfills and devote these spaces to other human activities. It is also important to point out that this kind of renewable energy suffers significantly less availability which characterizes
* Corresponding author. E-mail addresses: fi[email protected] (F. Bellomare), MR@ mek.dtu.dk (M. Rokni). 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.01.016
other type of renewable energy sources such as in wind and solar energy. As proposed in Morris et al. [1], with a well management of waste, the following points should be considered: -
prevention of waste generation; recycling of waste materials; reduction at minimum of landfilling disposal; incineration with energy recovery at efficiencies comparable with alternative technologies and sophisticated exhaust gas cleaning equipment; - gasification processes. In a gasification process, waste is subject to chemical treatments through air or steam utilization; the result is a synthesis gas, called “Syngas” which is principally composed of hydrogen and carbon monoxide. Traces of hydrogen sulphide could also be present which can easily be separated in a desulphurisation reactor. The gasification process is usually based on an atmospheric-pressure circulating fluidized bed gasifier coupled to a tar-cracking vessel; the gas produced is then cooled and cleaned. Syngas can be used as fuel in different kind of power plant such as gas turbine cycle, steam cycle, combined cycle, internal and external combustion engine and Solid Oxide Fuel Cell (SOFC).
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500
SOFC based power plants are known as efficient power generators not only as stands alone (about 50% thermal efficiency) but also in hybrid cycles, where SOFC is integrated with another plants, such as gas turbine or Rankine cycle (above 60% thermal efficiency). SOFC plants are also flexible in using different kind of fuels after minor fuel pre-treatment such as desulphurisation. SOFC based power plants have been studied in the past and some manufacturer are trying to realize such systems for CHP (combined heat and power) applications [2]. In literature it is possible to find studies about SOFC plant combined with CC (combined cycles) to achieve ultra-high electrical efficiencies [3,4]. SOFC and gas turbine integrated systems have been extensively studied for CHP applications [5] and with biomass gasification [6,7]. In Refs. [8,9], the characterization, quantification and optimization of hybrid SOFCeGT systems have been studied. The dynamics and control concept of a pressurized SOFCeGT hybrid system has also been studied in Ref. [10]. In Ref. [11] modeling results were compared with measured data for a 220 kW SOFCeGT integrated plant, while details of the design, dynamics and startup of such hybrid power plants are studied in Ref. [12]. Part-load characteristics of an SOFCemicro GT were also studied in Ref. [13]. In the current study a gasification plant is considered which is modeled and compared with a gasifier named Viking gasifier. This biomass gasifier is an autothermal (air blown) fixed bed gasifier situated located at Technical University of Denmark, see e.g. Refs. [14,15]. Hofmann et al. [16] had operated an SOFC on cleaned syngas from the Viking gasifier without noticing any degradation. They concluded that their gasification plant provided a biogas which could directly be fed into a solid oxide fuel cell. It shall be mentioned that the Viking gasifier has not been used with Municipal Solid Waste and its’ functionality remains unknown. However, the mathematical model developed here is general and can be used with almost any type of gasifier unless the adjusting parameter to be decided, see below. In the present study, a Municipal Solid Waste gasification plant integrated with SOFC is combined with a gas turbine to recover the energy of the off-gases from the topping SOFC cycle. In addition, a regenerative gas turbine has been introduced to recover more energy from the off-gases. The latter configuration has been compared with the one without regeneration; hybrid recuperator was shown to be very efficient and could increase significantly the plant efficiency. In addition, some relevant parameters have been studied to optimize the plant efficiency in terms of operating conditions. Compared with modern waste incinerators with heat recovery, the gasification process integrated with SOFC and gas turbine permits an increase in electricity output up of 50%, see e.g. Refs. [1]., thus solid waste gasification process can compete with incineration technology. Moreover waste incinerators require the installation of sophisticated exhaust gas cleaning equipment that can be large and expensive; these systems are not necessary in the studied plant. No investigation on Municipal Solid Waste Gasification plant integrated with SOFC and GT has been found in the open literature, and therefore, the current investigation seems to be completely novel and might bring up new ideas on designing new energy system configurations for future applications. 2. Plant model The plant studied in this investigation is represented through the following block scheme: The principal components of the plant are the Gasification plant, the SOFC plant and the Gas Turbine. Through the Gasification plant, Municipal Solid Waste is converted into syngas; a mixture of H2, N2, CO, CO2, H2O, CH4 and Ar. The produced syngas is previously cleaned to remove tracks of H2S that could poison the SOFC.
491
The cleaned syngas is then sent to the SOFC plant to produce electricity. The SOFC stacks cannot consume all the fuels and therefore the remaining fuel is sent to the burner to complete the combustion. The combusted gases after the burner are expanded in a gas turbine (acts as a bottoming cycle) for further electricity production. The heat released through the exhausted gases can be used for other applications (i.e. district heating or district cooling). In fact, from the second principle of the thermodynamic all the heat coming from the combustion process cannot be converted into electric energy and a part of it will be lost. From the block scheme one can also identify two other sources of losses: ashes and tar from the gasification plant. Apart from the fuel, the other inputs of the plant are air feeding the gasifier and air feeding the cathode side of the SOFC stacks. To introduce these, auxiliary energy is necessary for a compressor; another use of auxiliary energy is that of blowing the Syngas out from the Gasification Plant to the SOFC. The efficiency of the plant can be expressed as the ratio between the net produced electric power and the fuel power, where “net power” means the difference between the produced power and the power used in the auxiliary components (compressors, blowers, control systems, etc.):
h¼
PNET
¼
mfuel LHVfuel
PTOT PAUX
(1)
mfuel LHVfuel
2.1. Gasifier model The gasification plant used in this study is based on the model developed in Ref. [17] which is repeated here for the sake of clarification. Pyrolysis process requires a certain amount of moisture (around 10%) inside the fuel; therefore superheated steam makes gasification process applicable for fuel with high humidity content (up to 60%). Depending on the type of fuel used, moisture can be significantly variable; this makes the solution suitable for example for biomass with high moisture content. Higher process rates are achieved when a mixture of air and steam is used as gasification agent; temperature can be also lowered. Hydrogen content is also increased and this makes the produced syngas composition suitable for an SOFC plant. A simple Gibbs reactor was implemented, meaning that the total Gibbs free energy had its minimum when the chemical equilibrium was achieved. Such characteristic could be used to calculate the gas composition at a specified temperature and pressure without considering the reaction pathways [18]. The procedure is shortly described here. The Gibbs free energy of a gas (assumed to be a mixture of k perfect gases) is given by
G ¼
k i X h ni gi0 þ RT ln ðni pÞ
(2)
i¼1
where g0, R and T are the specific Gibbs free energy, universal gas constant and gas temperature respectively. Each atomic element in the inlet gas is in balance with the outlet gas composition, which shows that the flow of each atom has to be conserved. For N elements, this balance is expressed as (see, e.g. Ref. [17].) k w X X ni;in Aij ¼ nm;out Amj i¼1
for j ¼ 1; N
(3)
m¼1
The N elements correspond to H2, O2, N2, CO, NO, CO2, steam, NH3, H2S, SO2, CH4, C, NO2, HCN (hydrogen cyanide), COS (carbonyl sulfide), Ar and Ashes (SiO2) in gasifying process. Amj is the number
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of atoms of element j (H, C, O, N) in each molecule of entering compound i (H2, CH4, CO, CO2, H2O, O2, N2 and Ar), while Aij is the number of atoms of element j in each molecule of leaving compound m (H2, O2, N2, CO, NO, CO2, steam, NH3, H2S, SO2, CH4, C, NO2, HCN (hydrogen cyanide), COS, Ar and Ashes). The minimization of the Gibbs free energy can be formulated by introducing a Lagrange multiplier, m, for each of the N obtained constraints. After adding the constraints, the expression to be minimized is then
f ¼ Gtot;out þ
N X
mj
j¼1
k w X X ni;out Aij nm;in Amj
(4)
By setting the partial derivation of this equation with respect to ni;out zero then the function 4 can be minimized as
vf
vni;out
¼
0 0gi;out
vGtot;out
vni;out
þ
N X
mj Aij ¼ 0 for i ¼ 1; k
j¼1
þ RT ln ni;out pout þ
N X
ENernst ¼
Dgf0 RT þ ln ne F ne F
pffiffiffiffiffiffiffiffi pH2 ;tot pO2 ; pH2 O
(7)
!
m¼1
i¼1
where ENernst, DEact, DEohm and DEconc are the Nernst ideal reversible voltage, activation polarization, ohmic polarization and concentration polarization Assuming that only hydrogen is electrochemically converted, then the Nernst equation can be written as
(5)
mj Aij ¼ 0 for i ¼ 1; k
j¼1
Thus a set of k equations were defined for each chemical compound leaving the system. Finally, it was found that by assuming chemical equilibrium at the gasifier the methane content in the product gas was underestimated. Therefore, a parameter called METHANE was applied to allow some of the methane bypasses the gasifier without undergoing any chemical reactions. By adjusting this parameter the product gas could be calibrated against the experimental data of the Viking gasification plant and found to be 0.01 meaning that 1% of the methane was bypassed in the gasifier, although very small amount, see Rokni [17]. Changing the fuel from biomass to municipal waste shall change this parameter slightly but due to lack of experimental data this value was assumed to be the same. Table 1 reports the considered MSW composition referred to a dry basis; percentages are expressed in terms of weight fraction. 2.2. SOFC model The SOFC model developed in Ref. [19] is used in this investigation, which were calibrated against experimental data for planar SOFC type. For the sake of clarity, it is shortly described here. The model is assumed to be a zero-dimensional, thus enabling calculation of complicated energy systems. In such modeling one must distinguish between electrochemical modeling, calculation of cell irreversibility (cell voltage efficiency) and the species compositions at outlet. For electrochemical modeling, the operational voltage (Ecell) was found to be
Ecell ¼ ENernst DEact DEohm DEconc ;
(6)
pH2;tot ¼ pH2 þ pCO þ 4pCH4
where Dgf0 is the Gibbs free energy (for H2 reaction) at standard pressure. The water-gas shift reaction is very fast and therefore the assumption of hydrogen as only species to be electrochemically converted is justified, see Refs. [20] and [21]. In the above equations pH and pH2 O are the partial pressures for H2 and H2O respectively, The activation polarization can be evaluated from the Butlere Volmer equation [22], which is isolated from other polarizations to determine the charge transfer coefficients and exchange current density from the experiment by the curve fitting technique. The ohmic polarization depends on the electrical conductivity of the electrodes as well as the ionic conductivity of the electrolyte. This was also calibrated against experimental data for a cell with anode thickness, electrolyte thickness and cathode thickness of 600 mm, 50 mm and 10 mm respectively. The concentration polarization is dominant at high current densities for anode-supported SOFCs, wherein insufficient amounts of reactants are transported to the electrodes and the voltage is then reduced significantly. Again the concentration polarization was calibrated against experimental data by introducing the anode limiting current [23], in which the anode porosity and tortuosity were also included among other parameters. The fuel composition at anode outlet was calculated using the Gibbs minimization method as described in Ref. [18]. Equilibrium at the anode outlet temperature and pressure was assumed for the following species: H2, CO, CO2, H2O, CH4 and N2. Thus the Gibbs minimization method calculates the compositions of these species at outlet by minimizing their Gibbs energy. The equilibrium assumption is fair because the methane content in this study is very low. To calculate the voltage efficiency of the SOFC cells, the power production from the SOFC (PSOFC) depends on the amount of chemical energy fed to the anode, the reversible efficiency (hrev), the voltage efficiency (hv) and the fuel utilization factor (UF). It is defined in mathematical form as
PSOFC ¼ LHVH2 n_ H2 ;in þ LHVCO n_ CO;in þ LHVCH4 n_ CH4 ;in hrev hv UF ; (9) where UF was a set value and hv was defined as
hv ¼ Table 1 Municipal solid waste compositions used in this study. Component
Percentage [%]
C H O S N ASH
47.6 6 32.9 0.3 1.2 12
(8)
DEcell ENernst
(10)
The reversible efficiency is the maximum possible efficiency defined as the relationship between the maximum electrical energy available (change in Gibbs free energy) and the fuels LHV (lower heating value) as follows (see e.g. Ref. [24].)
hrev ¼
Dgf
fuel
LHVfuel
(11)
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500
1 yH2 ;in þ g f gf H2 O H2 O2 CO2 2
h 1 y gf g þ gf CO CO2 2 f O2 CO;in i yCH4 ;in þ 2 gf gf 2 gf
gf
gf
493
In addition, equations for conservation of mass (with molar flows), conservation of energy and conservation of momentum were also included into the model.
community, the mass flow of fuel produced in the gasification plant and the efficiency of the entire power plant, seems to be the best way to establish the size of such plants. Therefore, in this study such couplings have been investigated as first step. Further, different plant configurations have been studied to find out the most optimized plant configuration with respect to efficiency. This in turn implies that the placement of the heat exchangers plays an important role for the optimized configuration. It is better to have a realistic values of pressure drops in the heat exchangers rather than too low. The real pressure drops of the heat exchangers remain unknown until separate study is carried out to estimate their pressure drops at each side. This can be done after initial study of the complete plant configuration. Therefore, the initial values for the heat exchangers have firstly been studied depend on their mass flow estimation from a configuration and afterward it was found that the pressure drops vary between 0.005 bar and 0.04 bar depending on fluids type and mass flow. The lower value corresponds to the fuel side and the higher value corresponds to the gas side of the heat exchangers. Other input data are shown in Table 2.
3. Plant configurations
4. Results and discussions
Waste enters the gasification plant to obtain a synthesis gas known as “syngas”, which after a cleaning process to remove H2S tracks, is used for a SOFC plant. The off-fuel and the off-air which come from the anode and cathode side respectively are then sent to a burner for complete combustion. To recover more energy, the flue gases from the burner are expanded in a gas turbine to obtain additional electric power. Two different configurations have been studied; the second one includes a regenerative/recuperated gas turbine, as shown in Fig. 1 Fig. 2.
Simulations, calculations and analysis have been carried out using DNA (Dynamic Network Analysis), an in-house componentbased code for energy systems analysis developed at the DTU Thermal Energy Systems department. The solution is provided solving a system of nonlinear equations through the Newtone Raphson modified algorithm, see e.g. Rokni [19]. Ahrenfeldt et al. [15] report that the “Viking gasifier” offers some interesting features such as low tar content in produced syngas (<5 [mg/Nm3]), stable unmanned operation, high cold gas efficiency (>95%), low environmental impact (clean condensate, high carbon conversion ratio), and gasification at ambient pressure. Higher process rates are achieved when a mixture of air and steam is used as gasification agent to lower the operating temperature and increase the hydrogen content. This makes the produced syngas composition suitable for feeding a SOFC plant. It was tested experimentally without any remark. Similar results have also been obtained through the DNA model using MSW as fuel in the Viking Gasifier; however it is not possible to confirm this result with experimental data because the considered gasifier has not been used with such kind of biomass. The resulted output data are shown in Table 3 wherein case A refers to the configuration without hybrid recuperator and case B refers to the configuration including a hybrid recuperator, as was shown in Fig. 2. As can be seen, plant performances enhance significantly by introduction of the hybrid recuperator that allows preheating the air in additional step before entering the cathode pre-heater. The principle consequences of such hybrid recuperator are:
Dgf
fuel
¼
H2 O
CH4
O2
(12) The partial pressures were assumed to be the average between the inlet and outlet as
yj;out þ yj;in p j ¼ fH2 ; CO; CH4 ; CO2 ; H2 O; N2 g 2 yO2 ;out þ yO2 ;in ¼ pc 2
pj ¼ pO2
(13)
3.1. Input parameters It could be interesting to study a power plant running on solid waste and to reveal some interesting features such as efficiency and power generated. For such plants having high efficiency is important but this is not the main feature, since municipal waste is used as fuel. Thus, the environmental impact would be the most important advantage, while power production would be another important issue to show how much of the power needed in the community can be produced environmental friendly. The compromise between the amount of waste production in the Air
Waste
Air
Gasification
Syngas
Electric SOFC
Energy
Plant H2S
-
Heat
Ash Electric
Gas Turbine
Energy
larger available heat for the bottoming cycle (gas turbine here); higher temperature of flue gas entering the burner; higher thermodynamic mean temperature; recovering more energy of the smoke gases and consequently higher plant efficiency; - better integration of heat flows inside the plant. - lower losses caused by the temperature level of smoke outlet from turbine.
Heat
4.1. Sensitivity analysis Exhaust gases Fig. 1. Block scheme of the plant.
A sensitivity analysis is conducted by varying some crucial parameters while all other values are kept constant. The analyzed
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F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500
Fig. 2. Plant scheme without recuperator (a) and with hybrid recuperator (b).
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500 Table 2 Simulations data input.
LHVw ¼ LHV MOIðLHV þ rÞ
Component
Data input
Value
Dryer
MSW mass flow Tin fuel side pin fuel side Tout fuel side pout fuel side Heat losses Dp fuel side Dp steam side Tout syngas side Tin water pout H2S Heat losses Operating p Operating T Dp Water to fuel ratio C conversion factor Non-equilibrium CH4 Tin air side Heat losses Dp syngas side Dp air side Tout steam side Heat losses Dp syngas side Dp steam side
2.4 [kg/s] 25 [ C] 1.01325 [bar] 150 [ C] 1.008 [bar] 0 [kW] 0.05 [bar] 0.005 [bar] 800 [ C] 150 [ C] 1.01325 [bar] 0 [kW] 0.998 [bar] 800 [ C] 0.005 [bar] 0 1 0.01 25 [ C] 0 [kW] 0.005 [bar] 0.005 [bar] 200 [ C] 0 [kW] 0.005 [bar] 0.005 [bar] 80% 98% 0 [kW] 0.0049 [bar] 0 [kW] 650 [ C] 650 [ C] 0.8 780 0.01 [bar] 0.04 [bar] 75 50,500 0 [kW] 0.01 [bar] 0.01 [bar] 0 0.04 [bar] 0.04 [bar] 1.01325 [bar] 25 [ C] 2.5 [bar] 0.8 0.98 0.8 0.98 0 [kW] 0.95 1.01325 [bar] 0.86 0.98 0.04 [bar] 0.04 [bar] 0 [kW] 1.01325 [bar]
Gasifier
Air p_h
Steam heater
Steam blower
his hm
Desulphuriser
Heat losses Dp Heat losses Tin cathode side Tin anode side Utilization factor Operating T Dp anode side Dp cathode side Cells per stack Number of stacks Heat losses Dp used fuel side Dp syngas side Heat losses Dp flue gas side Dp air side pin Tin pout
SOFC stack
Anode p_h
Cathode p_h
Compressor
Syngas blower Burner Turbine Generator Recuperator
495
his hm his hm Heat losses Dp pout
his hel Dp smoke side Dp air side Heat losses pout smoke side
(14)
where LHVw is the lower heating value on wet basis, LHV is the corresponding value for dry basis fuel, MOI is the moisture content in the fuel and r is the vaporization heat of water (2500 kJ/kg). By reviewing a large amount of articles in the literature one can conclude that the values of moisture content in MSW ranges between 8% and 20% as shown e.g. in Bébar et al. [26] and Hernandez et al. [27]; the lower the moisture is, the better the fuel would be. In fact MSW composition could change day by day and it could be beneficial to control this parameter before introducing the fuel into the gasifier. Fig. 3 shows the LHV of MSW as function of moisture content. Power production and efficiency are influenced significantly with fuel moisture content. The power production by SOFC and GT decreases with increasing moisture content, see Fig. 4 This is also true for auxiliary components in which power consumption decreases; particularly the syngas blower and the compressor which are due to reduction of syngas mass flow and consequently the required air. However, the decrements of the auxiliary consumptions are lower than the reduction of total power production which explains why the net power decreases. Variations of plant efficiency are negligible which can be explained by the fact that the reduction of net power corresponds to the reduction of fuel inlet power and the ratio between these two values remains practically unchanged. SOFC plant efficiency is not affected significantly by moisture variation while the GT efficiency decreases which is due to circumstances that the reduction of power production is higher than the reduction of inlet fuel power, see Fig. 5. The most peculiar result is that the gasification plant efficiency increases slightly when moisture content is increased. Of course, syngas output energy decreases but its reduction is slower than the fuel input energy. In any case it is better to have low moisture content, approximately 10%, mainly because the output net power production will be reduced otherwise. 4.3. Gasification temperature Gasification temperature is another important parameter to be studied because it affects the fuel composition which will be fed to SOFC. Increasing gasification temperature results in a higher air mass flow required for oxidation (more nitrogen is diluted in the syngas) and hence the produced amount of syngas will be increased. However, the heat value of the fuel is lowered because of chemical equilibrium leading to higher amount of CO and lower amounts of CO2, CH4 and H2 content. Such fuel compositions influence the power production by SOFC plant negatively which in turn influences the produced electric power and efficiency as shown in Fig. 6. SOFC produced power decreases due to a decrement of syngas LHV during the gasification process which is because of higher air mass flow is required for oxidation and for this reason auxiliary consumption in the air compressor increases. However, the generated electric power in the GT increases as Table 3 Simulations data output.
parameters in this study are identified as fuel moisture content, gasification temperature and SOFC operating temperature. The results are provided in the following subsections.
Plant
4.2. Fuel moisture content Fuel moisture is an important parameter to be studied since its variation influences the fuel properties through the LHV (Lower Heating Value) as discussed in Ref. [25]:
SOFC GT
Data output
A
B
Ptot [MW] Pcons [MW] Pnet [MW] Cfuel [MW] h [%] P [MW] h [%] P [MW]
29.5 12.22 17.3 42.62 40.7 18.5 43 11
34.32 12.19 22.13 42.62 51.93 18.53 43.5 15.79
20 19 18 17 16 15 14
η [%]
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500
LHV [MJ/kg]
496
9 10 12 14 16 18 20 22 24 26 28 30 MOI [%]
100 90 80 70 60 50 40 30 20
Fig. 3. LHV as function of fuel moisture.
GASIFICATION
PLANT SOFC GAS TURBINE
9 10 12 14 16 18 20 22 24 26 28 30 MOI [%]
Fig. 5. Plant efficiency as function of moisture content.
a consequent to reduction of elaborated mass flow. For this reason total net power production remains more or less unchanged. Generated electric power for each plant affects the corresponding plant efficiency, meaning that all efficiency values decrease as result; except for the GT plant efficiency which is increased which is explained above. These are revealed in Fig. 7.
- Lost exergetic efficiency; εL
εL ¼
EL
(16)
EFuel - Total exergetic efficiency; εTOT
εTOT ¼ εD þ εL
4.4. SOFC operating temperature SOFC operating temperature has been changed from 780 C to 950 C while anode and cathode inlet temperatures are adjusted accordingly and maintaining a constant temperature difference over the cells of the SOFC stacks (130 C) as discussed in Rokni [19]. The reason is to avoid thermal stresses over the cells which otherwise may damage the cells. SOFC power production and SOFC efficiency decrease when operating temperature increases which are due to reduction of fuel cell equilibrium potential [28] and a thermal expansion among materials [29]. However, auxiliary consumption increases consequently and the plant efficiency decreases. Gas turbine power production and efficiency increase due to a higher energy input through the used fuel coming from SOFC, see Fig. 8 and Fig. 9.
5. Exergy analysis Lost and Destroyed exergy for each component have been calculated through a system of equation for which the solution has been provided through EES (Engineering Equation Solver). The following parameters have been evaluated for each component of the plant:
ED
(15)
(17)
where ED , EL and EFuel are respectively the destroyed, the lost and the fuel exergy of the i-th component. The calculated destroyed, lost and total exergy for each component are shown in Table 4. Principle losses can be identified in the following components: - gasifier, because of the chemical reactions that take place; - burner, due to the oxidation of fuel that requires the conversion of chemical in thermal energy; - SOFC, due to the chemical reactions that have place; - cathode Pre-Heater, because the high difference between flue gas and air temperature profiles; flue gas and SOFC air mass flow is also high, so exergy destruction affects more on global losses; - compressor and Turbine, due to the high mass flow of fluid; - Recuperator, the principle source of loss, due to the high mass flow and temperature of the off gases. Total exergetic efficiency for the whole plant can be evaluated as:
hEx ¼
EProduct
¼ 1
EFuel
- Destroyed exergetic efficiency defined as εD
E D þ EL
(18)
EFuel
Where ED and EL are the total Destroyed and Lost Exergy for the complete plant; EFuel is the inlet exergy through the introduced waste. For the plant analyzed here, an exergetic efficiency of 48.62% is obtained.
EFuel
35
35 TOTAL
25 SOFC NET
20 15
GAS TURBINE
10 5
AUXILIARY
9
10 12 14 16 18 20 22 24 26 28 30 MOI [%]
Fig. 4. Electric power as function of moisture content.
TOTAL
30 P [MW]
30 P [MW]
εD ¼
25
NET
20
SOFC
15
GAS TURBINE AUXILIARY
10
700
750
800
850
900
TGAS [°C] Fig. 6. Electric power as function of gasification temperature.
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500
90
70
60
PLANT
50
60
η [%]
η [%]
70
SOFC
30
30
GAS TURBINE
700
750
800 TGAS [°C]
850
800
850
900
950
TSOFC [°C] Fig. 9. Plant efficiency as function of SOFC operating temperature.
½V=h
(19)
For the thermoeconomic analysis of the plant, appropriate cost functions must firstly be formulated to include the purchase cost for each component; the capital cost per annum, the operating cost per annum, and the total cost per annum. The expressions for each component purchase costs I are summarized in detail in Table 5. The cost of dryer, burner and SOFC have been obtained through a market review and expressions found in literature have been used for the other components [30e33]. For the kth component, Capital Investment IKTOT is amortized in n years, as shown by the following formula:
IK
780
900
Thermoeconomic analysis aims to build a system of equations through the costs balances for each component; the unknown variables are the Unitary Costs c [V/kWh] for each node. Once the exergy value of each node is known, then it would possible to calculate the thermoeconomic cost of the kth node in terms of cost rate (cost per hour), as follows:
TOT
GAS TURBINE
20
6. Thermoeconomic analysis
SOFC
40
Fig. 7. Plant efficiency as function of gasification temperature.
Ck ¼ ck Ek
PLANT
50
40 20
GASIFICATION
80
GASIFICATION
80
497
¼ f IKTOT
(20)
The values for equipment life-span n, construction period CP, interest rate int, inflation rate ri and operating hours Hr are shown in Table 6, which are based on the report in Ref. [33]: With 7000 operating hours per year, an 80% of availability factor is obtained which is the ratio between operating hour and the maximum amount of hours in one year (8760 h). CI
Capital Investment Cost Z K is calculated as: CI
TOT
ZK ¼
iK Hr
(23) OM
Maintenance and operating cost Z K is considered by a “maintenance factor” M, assumed to be 1.1 [35] and the relation with the TOT
is:
total cost Z K OM
ZK
TOT
¼ MZ K
(24) CI
OM
Table 7 shows the calculated values for Z K , Z K each component in the plant
TOT
and Z K
for
In the cost balance the “cost rate” associated to the product (C P [V/h]) is set equal to the total rate of expenditures made for the
product, which in turn is sum of the fuel cost rate (C F V/h]) and the
cost rates associated to the capital investment (Z CL [V/h]) and
where f is the “annuity factor”, calculated as:
" f ¼
qnþCP i ðqi
1ÞqnþCP i
#1
qCP i 1
(21)
ðqi 1ÞqCP i
operating and maintenance (Z OM [V/h]). For the kth component, operating at steady state, it is formulated as:
C P;k ¼ C F;k þ Z k ¼ C F;k þ Z CI;k þ Z OM;k
(25)
where qi is the “interest factor”, defined as:
int r 1þ i 1þ 100 100
P [MW]
qi ¼
44 39 34 29 24 19 14 9
(22)
TOTAL
NET SOFC GAS TURBINE AUXILIARY
780
800
850 900 TSOFC [°C]
950
Fig. 8. Electric power as function of SOFC operating temperature.
Table 4 Destroyed, lost and total exergetic efficiency for each component. Component
εD [%]
εL [%]
εTOT [%]
Dryer Gasifier Desulphuriser Air Pre-Heater Mixer Splitter Steam Generator Steam Blower Syngas Blower Anode Pre-Heater SOFC Cathode Pre-Heater Burner Turbine Electric generator Hybrid Recuperator Compressor
0.2026 8.569 0.001325 0.727 0.1395 0 0.4913 0.007344 0.2953 0.01458 5.529 1.951 6.881 2.002 0.7002 6.593 3.633
0 0.4389 0.361 0 0 0 0 0 0 0 0 0 0 0 0 13.25 0
0.2026 9.008 0.362325 0.727 0.1395 0 0.4913 0.007344 0.2953 0.01458 5.529 1.951 6.881 2.002 0.7002 19.85 3.633
498
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500
Table 5 The cost of other components.
Table 7 Calculated components cost in this study.
Component
Cost [$]
Dryer Gasifier Desulphuriser
130,000 2:9 106 ð3:6mWASTE Þ0:7 [32] 800,000 AEX 0:78 [30] 130 0:093
Heat Exchangers
75ðm=0:9 hm hIS Þrc ln rc [32] 3000 Pel,SOFC 192,000 (98.328 ln WTURBINE þ 1318.5) WTURBINE [30] 0:5 [33] 60 Pel
Compressor SOFC Burner Gas Turbine Generator
At this stage, product and fuel costs for each component of the system must be defined, therefore, a system of equations are built with a cost-balance equation for each unit. Unit cost equations for external flows into the system for which costs are externally defined and losses for which the unit cost is set equal to zero. It would thus be possible to solve the system equations when auxiliary equations, according to the two following principles, are added (or assumed): - if definition of “fuel” of a component includes a stream that goes through another component and is used in it, then the unit cost of the stream flowing into and out of the component is the same; - if the product of a component is composed of two or more streams then the unit cost of those streams are equal. TEC (thermoeconomic) method requires to know the exergy for each node of the system; exergetic analysis of the plant is carried out by DNA by fixing a thermodynamic state for the ambient (Tamb ¼ 25 C and pamb ¼ 1 bar). To solve the linear system of equations, other auxiliary equations are also necessary. For this, unit cost of absorbed power in the auxiliary systems is set equal to the weighted average unit cost of electric power produced by the SOFC and the gas turbine:
cAuxiliary ¼
cSOFC ESOFC þ cGT EGT ESOFC þ EGT
(26)
Now the system of equations is completed and it can be solved; no other auxiliary equations are necessary to obtain the results. These set of equations has been solved by the equation solver software, EES. Afterward, it would be possible to define and to calculate for each component a Unitary Cost for the “Fuel” cF [V/kWh] and a Unitary Cost for the “Product” cP [V/kWh]. The following two main thermoeconomic parameters have been calculated to evaluate and to optimize the performance of each component: - relative Cost Difference Factor DrK; - exergoeconomic Factor fK. Table 6 Economic parameters used in this study. Parameter
Value
Equipment Life-Span “n” Construction Period “CP” Interest Rate “int” Rate of Inflation “ri” Operating Hours “Hr”
15 [years] 1 [year] 6 [%] 2 [%] 7000 [h/year]
Component
Capital investment cost [V/h]
Operating and maintenance cost V/h]
Total cost [V/h]
Dryer Gasifier Desulphuriser Air Pre-Heater Mixer Splitter Steam Generator Steam Blower Syngas Blower Anode Pre-Heater SOFC Cathode Pre-Heater Burner Turbine Electric generator Hybrid Recuperator Compressor Total
4.59 313.33 28.24 0.75 0.00 0.00 1.48 0.01 0.15 0.65 1365.44 2.96 6.78 144.33 0.19 2.75 3.42 1875.05
0.51 34.81 3.14 0.08 0.00 0.00 0.16 0.00 0.02 0.07 151.72 0.33 0.75 16.04 0.02 0.31 0.38 208.34
5.10 348.14 31.38 0.83 0.00 0.00 1.65 0.02 0.17 0.72 1517.15 3.29 7.53 160.36 0.21 3.05 3.80 2083.39
These two variables have been calculated for the kth component through the following equations:
Drk ¼
cP;k cF;k cF;k
(27)
fk ¼
Z k Z k þ cF;k ED;k þ EL;k
(28)
where cp,k and cF,k are the unit costs of product and fuel respectively. For the k component andZ k [V/h ] is the investment cost. The relative cost difference factor DrK suggests that for which component it is possible to enhance a major efficiency, however, this evaluation is not enough to have a complete analysis. The exergoeconomic factor fK in fact suggests how it would better to set in; this parameter expresses the ratio between the lost exergoeconomic cost and the total lost exergoeconomic cost, sum of the economic investment and the inefficiency due to irreversibility. A low value of this parameter (close to zero) suggests that exergetic losses are decreased at the expense of a higher total cost of investment for the component. On the other hand, a high value (close to 1) suggests decrease the purchase equipment cost by enhancing the exergetic losses of the component. Optimization of one component by means of exergoeconomic factor might not lead to a more optimized system, when analyzing a complex energy system. Modifications may reflect negatively on the other components and therefore major attention should be paid to the components that have high exergetic losses and investment costs. In Table 8, exergoeconomic parameter for each component is provided. As shown in Table 8, the highest values of DrK have been obtained for the following components: - gasifier and SOFC, where irreversibility due to chemical reactions are present - anode preheater and steam generator, because of the inefficiency in the thermal exchange which in turn is due to small difference between the inlet and outlet temperatures of fluids. The major values of fK are in those components where both exergy losses and total investment cost are high, particularly in
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500 Table 8 Exergoeconomic parameter for each component. Component
DrK [%]
fK [%]
Dryer Gasifier Desulphuriser Air Preheater Mixer Splitter Steam Generator Steam Blower Syngas Blower Anode Preheater SOFC Cathode Pre-Heater Burner Turbine Electric generator Hybrid Recuperator Compressor
2.262 336.7 6.624 42.25 6.599 0 142.8 20.2 9.442 189.2 202.4 10.51 7.255 14.83 2.051 16.52 19.2
10.42 97.13 6.624 18.59 0 0 40.18 5.691 1.26 87.73 97.55 2.681 2.282 61.44 0.5169 0.2377 2.265
gasifier and SOFC. Gasifier investment cost is dependent on the biomass input meaning that decreasing the waste mass flow may allow to obtain a lower exergoeconomic factor, and therefore, obtain a more optimized system. The moisture content in the fuel, and consequently the LHV, could also be another source of inefficiency. The exergoeconomic factor for gasifier is around 97.13%. In SOFC system, enhancement could be obtained by decreasing SOFC temperature [34]; for this component exergoeconomic factor is around 97.55%. Decreasing the operating temperature will lower the purchase cost as well as the exergetic losses. In this way exergoeconomic factor should strongly be reduced. Thermoeconomic analysis also confirms that the anode preheater is a source of significant loss in the plant which is due to the inefficiency in the thermal exchange. Further investigation is needed to reveal how to decrease the loss of this component. Finally, through such kind of analysis it is possible to obtain the unit costs of the produced electric power by the SOFC and the gas turbine. Therefore, price of electricity can be calculated by
Pel ¼
ccost ESOFC þ cGT EGT
(29)
ESOFC þ EGT
Inserting the values calculated above, electricity price is obtained as:
Pel ¼ 0:098 V=kWh which is in fact comparably low in the European market. Purchase cost of SOFC affects strongly on the electricity price. As mentioned above, the SOFC purchase cost is estimated to be around 3000 V/kW, which is in fact very expensive. If SOFC cost decreases to one-third in the future after market entry, then the electricity price would be significantly lower. In other words, it would be beneficial to build the system even with today’s cost of SOFC and much more beneficial in the future. 7. Conclusions Gasification technology allows using biomass energy with high efficiency; in particular through Municipal Solid Waste gasification in which energy is obtained from a “low-cost” fuel instead of incinerating and/or throwing it in the landfills. Integration of such gasification plant with a hybrid SOFC e GT plant permits to achieve an efficiency of 52% in an optimized configuration which is very high plant efficiency without presence of any dangerous pollutants in the off-gases.
499
The best performance is obtained with a regenerative gas turbine in which the plant efficiency increases to about 11% due to the recovery of the energy content in the exhausted off-gases. As discussed the most suitable plant size is of approximately 25 MWe, according to the actual SOFC market which in turn permits to obtain high plant efficiency with a fixed input fuel mass flow. However with such size it is still possible to remove quantities of MSW comparable with the mean production of some small cities which concludes that in the immediate future the analyzed system is going to be feasible and the advantage of integrating such hybrid system with an MSW Gasifier provides a synthesis gas which will be fed to a competitive energy system from a fuel with negligible costs and high plant efficiency. References [1] Morris M, Waldheim L. Energy recovery from solid waste fuels using advanced gasification technology. J Waste Manage 1998;18:557e64. [2] Fontell E, Kivisaari T, Christiansen N, Hansen JB, Pålsson J. Conceptional study of a 250 kW planar SOFC system for CHP application. J Power Sources 2004; 131:49e56. [3] Rokni M. Introduction of a fuel cell into combined cycle: a competitive choice for future cogeneration. ASME Cogen e Turbo, IGTI 1993;8:255e61. [4] Riensche E, Achenbach E, Froning D, Haines MR, Heidug WK, Lokurlu A, et al. Clean combined cycle SOFC power plant e cell modeling and process analysis. J Power Sources 2000;86:404e10. [5] Pålsson J, Selimovic A, Sjunnesson L. Combined solid oxide fuel cell and gas turbine systems for efficient power and heat generation. J Power Sources 2000;86:442e8. [6] Proell T, Aichering C, Rauch R, Hofbauer H. Coupling of biomass steam gasification and an SOFCegas turbine hybrid system for highly efficient electricity generation. ASME Turbo Expo Proceeding, GT2004-53900 2004:103e12. [7] Bang-Møller C, Rokni M. Thermodynamic performance study on biomass gasification, solid oxide fuel cell and micro gas turbine hybrid systems. J Energy Conversion Manage 2010;51:2330e9. [8] Subramanyan K, Diwekar UM. Characterization and quantification of uncertainty in solid oxide fuel cell hybrid power plants. J Power Sources 2005;142:103e16. [9] Calise F, Dentice d’ Accadia M, Vanoli L, von Spakovsky MR. Single-level optimization of hybrid SOFC e GT power plant. J Power Sources 2006;159: 1169e85. [10] Wächter C, Lunderstädt R, Joos F. Dynamic model of a pressurized SOFC/gas turbine hybrid power plant for the development of control concept. J Fuel Cell Sci Technol 2006;3:271e9. [11] Roberts RA, Brouwer J. Dynamic simulation of a pressurized 220 kW solid oxide fuel-cell e gas-turbine hybrid system: modeled performance compared to measured results. J Fuel Cell Sci Technol 2006;3:18e25. [12] Rokni M, Fontell E, Ylijoki Y, Tiihonen O, Hänninen M. Dynamic modeling of Wärtsilä 5kW SOFC system. In: Singhal SC, Mizusaki J, editors. Ninth International Symposium on solid oxide fuel cell (SOFC IX), Quebec; 2005. p. 865e75. [13] Costamagna P, Magistri L, Massardo AF. Design and part-load performance of a hybrid system based on a solid oxide fuel cell reactor and a micro gas turbine. J Power Sources 2001;96:352e68. [14] Henriksen U, Ahrenfeldt J, Jensen TK, Gøbel B, Bentzen JD, Hindsgaul C, et al. The design, construction and operation of a 75 kW two-stage gasifier. J Energy 2006;31:1542e53. [15] Ahrenfeldt J, Henriksen U, Jensen TK, Gøbel B, Wiese L, Kather A, et al. Validation of a continuous combined heat and power (CHP) operation of a twostage biomass gasifier. J Energy Fuels 2006;20:2672e80. [16] Hofmann PH, Schweiger A, Fryda L, Panopoulos KD, Hohenwarter U, Bentzen JD, et al. High temperature electrolyte supported Ni-GDC/YSZ/LSM SOFC operation on two-stage Viking gasifier product gas. J Power Sources 2007;173:357e66. [17] Rokni M. Thermodynamic analysis of an integrated gasification plant with solid oxide fuel cell and steam cycle. J GREEN 2012;2:71e86. [18] Smith JM, Van Ness HC, Abbott MM. Introduction to chemical engineering thermodynamics. 7th ed. Boston: McGraw-Hill; 2005. [19] Rokni M. Plant characteristics of an integrated solid oxide fuel cell and a steam cycle. J Energy 2010;35:4691e9. [20] Holtappels P, DeHaart LGJ, Stimming U, Vinke IC, Mogensen M. Reaction of CO/CO2 gas mixtures on Ni-YSZ cermet electrode. J Appl Electrochem 1999; 29:561e8. [21] Matsuzaki Y, Yasuda I. Electrochemical oxidation of H2 and CO in a H2-H2-COCO2 system at the interface of a Ni-YSZ cermet electrode and YSZ electrolyte. J Electrochemical Soc 2000;147:1630e5. [22] Keegan KM, Khaleel M, Chick LA, Recknagle K, Simner SP, Diebler J, Analysis of a planar solid oxide fuel cell based automotive auxiliary power unit, SAE Technical Paper Series No. 2002-01-0413; 2002. [23] Kim JW, Virkar AV. The effect of anode thickness on the performance of anode e supported solid oxide fuel cell. Proc. of the Sixth Int. Symp. on SOFCs (SOFC e VI), PV99 e 19. J. Electrochemical Society 1999:830e9.
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[24] Winnick J. Chemical engineering thermodynamics. New York: John Wiley & Sons; 1997. [25] Cocco D, Palomba C, Puddu P. Tecnologie delle energie rinnovabili. Padova: SGE-EDITORIALE; 2008. [26] Bébar L, Stehlik P, Havlen L, Oral J. Analysis of using gasification and incineration for thermal processing of wastes. Appl Thermal Eng 2005;25:1045e55. [27] Hernandez-Atonal FD, Ryu C, Sharifi VN, Swithenbank J. Combustion of refusederived fuel in a fluidised bed. Chem Eng Sci 2007;62:627e35. [28] Ni M, Leung DYC, Leung MKH. Modeling of methane fed solid oxide fuel cells: comparison between proton conducting electrolyte and oxygen ion conducting electrolyte. J Power Sources 2008;183:133e42. [29] Hirschenhofer JH. It’s time to take fuel cell seriously part II. Power Eng 1995; 99:30. [30] Arsalis A. Thermoeconomic modeling and parametric study of hybrid solid oxide fuel cell-gas turbine-steam turbine power plants ranging from 1.5 MWe to 15 Mwe. J Power Sources 2008;181:313e26. [31] Lozano MA, Valero A. Theory of exergetic costs. J Energy 1993;18:939e60. [32] Bridgwater AV. The technical and economic of biomass gasification for power generation. J Fuel 1995;74:631e53. [33] Lian ZT, Chua KJ, Chou SK. A thermoeconomic analysis of biomass energy for trigeneration. J Appl Energy 2010;87:84e95. [34] Rokni M. Thermodynamic analysis of an integrated SOFC cycle with a Rankine cycle. J Energy Conversion Manage 2010;51:2724e32.
Glossary Aij: number of atoms of element j in each molecule of leaving compound m Amj: number of atoms of element j in each molecule of entering compound i cF: Fuel Unitary cost, V/kWh ck: Unitary cost for the kth node, V/kWh cp: Product Unitary cost, V/kWh Ck: Thermoeconomic Cost for the kth node, V/h E: Nernst ideal reversible voltage, V
ED : Destroyed Exergy
E k : Exergy for the kth node, kW EL : Lost Exergy
EFuel : Fuel Exergy f: annuity factor fk: Exergoeconomic factor for the kth component F: Faradays constant, C/mol G: Gibbs free energy, J g0: specific Gibbs free energy, J/kmol Hr: Operating hours int: interest rate IkTOT : Capital Investment
Uf: Utilization factor V: Potential W: electrical work Zk: Cost for the kth component, V/h Greek symbols d: partial derivation DVact: Activation polarization, V DVconc: Concentration polarization, V DVohm: Ohmic polarization, V DG: Gibbs free energy, W Dp: Pressure drops, bar Drk: Relative Cost Difference Factor for the kth component εD: Destroyed Exergetic Efficiency εL: Lost Exergetic Efficiency εTOT: Total Exergetic Efficiency h: Efficiency hEx: Exergetic Efficiency m: Lagrange multiplier n: stoichiometric coefficient Subscript aux: auxiliary cons: consumed d: destroyed el: electrical ex: exergy F: fuel fuel: fuel property gas: gasification i: i-th element in: in value is: isentropic j: j-th element l: lost m: mechanic max: maximum net: net amount out: out value P: product prod: product property r: water vaporization latent heat value reag: reagent real: real value rev: reversible tot: total w: wet
TOT
I k : Capital Investment cost n: equipment life-span ne: electrons number n: molecular weight P: Power, W Pel: Price of electricity, V/kWh p: Pressure, bar pH2 : Partial pressures for H2, bar pH2 O : Partial pressures for H2O, bar pO2 : Partial pressures for O2, bar q: interest factor r: rate of inflation R: Universal gas constant, J/kmol K T: Operating temperature, K
Abbreviations CC: Combined Cycle CHP: Combined Heat and Power CI: Capital Investment CP: Construction Period DNA: Dynamic Network Analysis EES: Engineering Equation Solver GT: Gas Turbine IGSG: Integrated Gasification SOFC and GT LHV: Lower Heat Value MOI: Moisture MSW: Municipal Solid Waste OM: Operating and Maintenance factor SOFC: Solid Oxide Fuel Cell
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Integration of a municipal solid waste gasification plant with solid oxide fuel cell and gas turbine Filippo Bellomare a, Masoud Rokni b, * a b
University of Padua, Dept. of Industrial Engineering, Sede V - via Venezia, 1-35131 Padova (PD), Italy Technical University of Denmark, Dept. of Mechanical Engineering, Thermal Energy System, Building 403, 2800 Kgs, Lyngby, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 October 2012 Accepted 9 January 2013 Available online 11 February 2013
An interesting source of producing energy with low pollutants emission and reduced environmental impact are the biomasses; particularly using Municipal Solid Waste (MSW) as fuel, can be a competitive solution not only to produce energy with negligible costs but also to decrease the storage in landfills. A Municipal Solid Waste Gasification Plant Integrated with Solid Oxide Fuel Cell (SOFC) and Gas Turbine (GT) has been studied and the plant is called IGSG (Integrated Gasification SOFC and GT). Gasification plant is fed by MSW to produce syngas by which the anode side of an SOFC is fed wherein it reacts with air and produces electricity. The exhausted gases out of the SOFC enter a burner for further fuel combusting and finally the off-gases are sent to a gas turbine to produce additional electricity. Different plant configurations have been studied and the best one found to be a regenerative gas turbine. Under optimized condition, the thermodynamic efficiency of 52% is achieved. Variations of the most critical parameters have been studied and analyzed to evaluate plant features and find out an optimized configuration. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Biomass Municipal solid waste Gasification SOFC
1. Introduction The word “Biomass” refers to vegetables and animals substances, not from fossil origin; these can be used as fuel in a power plant for the production of electrical energy. Biomasses derive from living or recently living biological organisms and can be considered as a particular kind of renewable energy source, because the carbon dioxide placed in the atmosphere by their use derives from the carbon amount absorbed during their life. In this way, the most important pollutants linked to biomass utilizations are related to transport, manufacture and transformation processes. Municipal Solid Waste can be considered a valid biomass to use in a power plant. Some advantages can be obtained; the principal is the reduction of pollutants and greenhouse gases emissions. Another advantage is that by their use it is possible to reduce the storage in landfills and devote these spaces to other human activities. It is also important to point out that this kind of renewable energy suffers significantly less availability which characterizes
* Corresponding author. E-mail addresses: fi[email protected] (F. Bellomare), MR@ mek.dtu.dk (M. Rokni). 0960-1481/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2013.01.016
other type of renewable energy sources such as in wind and solar energy. As proposed in Morris et al. [1], with a well management of waste, the following points should be considered: -
prevention of waste generation; recycling of waste materials; reduction at minimum of landfilling disposal; incineration with energy recovery at efficiencies comparable with alternative technologies and sophisticated exhaust gas cleaning equipment; - gasification processes. In a gasification process, waste is subject to chemical treatments through air or steam utilization; the result is a synthesis gas, called “Syngas” which is principally composed of hydrogen and carbon monoxide. Traces of hydrogen sulphide could also be present which can easily be separated in a desulphurisation reactor. The gasification process is usually based on an atmospheric-pressure circulating fluidized bed gasifier coupled to a tar-cracking vessel; the gas produced is then cooled and cleaned. Syngas can be used as fuel in different kind of power plant such as gas turbine cycle, steam cycle, combined cycle, internal and external combustion engine and Solid Oxide Fuel Cell (SOFC).
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500
SOFC based power plants are known as efficient power generators not only as stands alone (about 50% thermal efficiency) but also in hybrid cycles, where SOFC is integrated with another plants, such as gas turbine or Rankine cycle (above 60% thermal efficiency). SOFC plants are also flexible in using different kind of fuels after minor fuel pre-treatment such as desulphurisation. SOFC based power plants have been studied in the past and some manufacturer are trying to realize such systems for CHP (combined heat and power) applications [2]. In literature it is possible to find studies about SOFC plant combined with CC (combined cycles) to achieve ultra-high electrical efficiencies [3,4]. SOFC and gas turbine integrated systems have been extensively studied for CHP applications [5] and with biomass gasification [6,7]. In Refs. [8,9], the characterization, quantification and optimization of hybrid SOFCeGT systems have been studied. The dynamics and control concept of a pressurized SOFCeGT hybrid system has also been studied in Ref. [10]. In Ref. [11] modeling results were compared with measured data for a 220 kW SOFCeGT integrated plant, while details of the design, dynamics and startup of such hybrid power plants are studied in Ref. [12]. Part-load characteristics of an SOFCemicro GT were also studied in Ref. [13]. In the current study a gasification plant is considered which is modeled and compared with a gasifier named Viking gasifier. This biomass gasifier is an autothermal (air blown) fixed bed gasifier situated located at Technical University of Denmark, see e.g. Refs. [14,15]. Hofmann et al. [16] had operated an SOFC on cleaned syngas from the Viking gasifier without noticing any degradation. They concluded that their gasification plant provided a biogas which could directly be fed into a solid oxide fuel cell. It shall be mentioned that the Viking gasifier has not been used with Municipal Solid Waste and its’ functionality remains unknown. However, the mathematical model developed here is general and can be used with almost any type of gasifier unless the adjusting parameter to be decided, see below. In the present study, a Municipal Solid Waste gasification plant integrated with SOFC is combined with a gas turbine to recover the energy of the off-gases from the topping SOFC cycle. In addition, a regenerative gas turbine has been introduced to recover more energy from the off-gases. The latter configuration has been compared with the one without regeneration; hybrid recuperator was shown to be very efficient and could increase significantly the plant efficiency. In addition, some relevant parameters have been studied to optimize the plant efficiency in terms of operating conditions. Compared with modern waste incinerators with heat recovery, the gasification process integrated with SOFC and gas turbine permits an increase in electricity output up of 50%, see e.g. Refs. [1]., thus solid waste gasification process can compete with incineration technology. Moreover waste incinerators require the installation of sophisticated exhaust gas cleaning equipment that can be large and expensive; these systems are not necessary in the studied plant. No investigation on Municipal Solid Waste Gasification plant integrated with SOFC and GT has been found in the open literature, and therefore, the current investigation seems to be completely novel and might bring up new ideas on designing new energy system configurations for future applications. 2. Plant model The plant studied in this investigation is represented through the following block scheme: The principal components of the plant are the Gasification plant, the SOFC plant and the Gas Turbine. Through the Gasification plant, Municipal Solid Waste is converted into syngas; a mixture of H2, N2, CO, CO2, H2O, CH4 and Ar. The produced syngas is previously cleaned to remove tracks of H2S that could poison the SOFC.
491
The cleaned syngas is then sent to the SOFC plant to produce electricity. The SOFC stacks cannot consume all the fuels and therefore the remaining fuel is sent to the burner to complete the combustion. The combusted gases after the burner are expanded in a gas turbine (acts as a bottoming cycle) for further electricity production. The heat released through the exhausted gases can be used for other applications (i.e. district heating or district cooling). In fact, from the second principle of the thermodynamic all the heat coming from the combustion process cannot be converted into electric energy and a part of it will be lost. From the block scheme one can also identify two other sources of losses: ashes and tar from the gasification plant. Apart from the fuel, the other inputs of the plant are air feeding the gasifier and air feeding the cathode side of the SOFC stacks. To introduce these, auxiliary energy is necessary for a compressor; another use of auxiliary energy is that of blowing the Syngas out from the Gasification Plant to the SOFC. The efficiency of the plant can be expressed as the ratio between the net produced electric power and the fuel power, where “net power” means the difference between the produced power and the power used in the auxiliary components (compressors, blowers, control systems, etc.):
h¼
PNET
¼
mfuel LHVfuel
PTOT PAUX
(1)
mfuel LHVfuel
2.1. Gasifier model The gasification plant used in this study is based on the model developed in Ref. [17] which is repeated here for the sake of clarification. Pyrolysis process requires a certain amount of moisture (around 10%) inside the fuel; therefore superheated steam makes gasification process applicable for fuel with high humidity content (up to 60%). Depending on the type of fuel used, moisture can be significantly variable; this makes the solution suitable for example for biomass with high moisture content. Higher process rates are achieved when a mixture of air and steam is used as gasification agent; temperature can be also lowered. Hydrogen content is also increased and this makes the produced syngas composition suitable for an SOFC plant. A simple Gibbs reactor was implemented, meaning that the total Gibbs free energy had its minimum when the chemical equilibrium was achieved. Such characteristic could be used to calculate the gas composition at a specified temperature and pressure without considering the reaction pathways [18]. The procedure is shortly described here. The Gibbs free energy of a gas (assumed to be a mixture of k perfect gases) is given by
G ¼
k i X h ni gi0 þ RT ln ðni pÞ
(2)
i¼1
where g0, R and T are the specific Gibbs free energy, universal gas constant and gas temperature respectively. Each atomic element in the inlet gas is in balance with the outlet gas composition, which shows that the flow of each atom has to be conserved. For N elements, this balance is expressed as (see, e.g. Ref. [17].) k w X X ni;in Aij ¼ nm;out Amj i¼1
for j ¼ 1; N
(3)
m¼1
The N elements correspond to H2, O2, N2, CO, NO, CO2, steam, NH3, H2S, SO2, CH4, C, NO2, HCN (hydrogen cyanide), COS (carbonyl sulfide), Ar and Ashes (SiO2) in gasifying process. Amj is the number
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F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500
of atoms of element j (H, C, O, N) in each molecule of entering compound i (H2, CH4, CO, CO2, H2O, O2, N2 and Ar), while Aij is the number of atoms of element j in each molecule of leaving compound m (H2, O2, N2, CO, NO, CO2, steam, NH3, H2S, SO2, CH4, C, NO2, HCN (hydrogen cyanide), COS, Ar and Ashes). The minimization of the Gibbs free energy can be formulated by introducing a Lagrange multiplier, m, for each of the N obtained constraints. After adding the constraints, the expression to be minimized is then
f ¼ Gtot;out þ
N X
mj
j¼1
k w X X ni;out Aij nm;in Amj
(4)
By setting the partial derivation of this equation with respect to ni;out zero then the function 4 can be minimized as
vf
vni;out
¼
0 0gi;out
vGtot;out
vni;out
þ
N X
mj Aij ¼ 0 for i ¼ 1; k
j¼1
þ RT ln ni;out pout þ
N X
ENernst ¼
Dgf0 RT þ ln ne F ne F
pffiffiffiffiffiffiffiffi pH2 ;tot pO2 ; pH2 O
(7)
!
m¼1
i¼1
where ENernst, DEact, DEohm and DEconc are the Nernst ideal reversible voltage, activation polarization, ohmic polarization and concentration polarization Assuming that only hydrogen is electrochemically converted, then the Nernst equation can be written as
(5)
mj Aij ¼ 0 for i ¼ 1; k
j¼1
Thus a set of k equations were defined for each chemical compound leaving the system. Finally, it was found that by assuming chemical equilibrium at the gasifier the methane content in the product gas was underestimated. Therefore, a parameter called METHANE was applied to allow some of the methane bypasses the gasifier without undergoing any chemical reactions. By adjusting this parameter the product gas could be calibrated against the experimental data of the Viking gasification plant and found to be 0.01 meaning that 1% of the methane was bypassed in the gasifier, although very small amount, see Rokni [17]. Changing the fuel from biomass to municipal waste shall change this parameter slightly but due to lack of experimental data this value was assumed to be the same. Table 1 reports the considered MSW composition referred to a dry basis; percentages are expressed in terms of weight fraction. 2.2. SOFC model The SOFC model developed in Ref. [19] is used in this investigation, which were calibrated against experimental data for planar SOFC type. For the sake of clarity, it is shortly described here. The model is assumed to be a zero-dimensional, thus enabling calculation of complicated energy systems. In such modeling one must distinguish between electrochemical modeling, calculation of cell irreversibility (cell voltage efficiency) and the species compositions at outlet. For electrochemical modeling, the operational voltage (Ecell) was found to be
Ecell ¼ ENernst DEact DEohm DEconc ;
(6)
pH2;tot ¼ pH2 þ pCO þ 4pCH4
where Dgf0 is the Gibbs free energy (for H2 reaction) at standard pressure. The water-gas shift reaction is very fast and therefore the assumption of hydrogen as only species to be electrochemically converted is justified, see Refs. [20] and [21]. In the above equations pH and pH2 O are the partial pressures for H2 and H2O respectively, The activation polarization can be evaluated from the Butlere Volmer equation [22], which is isolated from other polarizations to determine the charge transfer coefficients and exchange current density from the experiment by the curve fitting technique. The ohmic polarization depends on the electrical conductivity of the electrodes as well as the ionic conductivity of the electrolyte. This was also calibrated against experimental data for a cell with anode thickness, electrolyte thickness and cathode thickness of 600 mm, 50 mm and 10 mm respectively. The concentration polarization is dominant at high current densities for anode-supported SOFCs, wherein insufficient amounts of reactants are transported to the electrodes and the voltage is then reduced significantly. Again the concentration polarization was calibrated against experimental data by introducing the anode limiting current [23], in which the anode porosity and tortuosity were also included among other parameters. The fuel composition at anode outlet was calculated using the Gibbs minimization method as described in Ref. [18]. Equilibrium at the anode outlet temperature and pressure was assumed for the following species: H2, CO, CO2, H2O, CH4 and N2. Thus the Gibbs minimization method calculates the compositions of these species at outlet by minimizing their Gibbs energy. The equilibrium assumption is fair because the methane content in this study is very low. To calculate the voltage efficiency of the SOFC cells, the power production from the SOFC (PSOFC) depends on the amount of chemical energy fed to the anode, the reversible efficiency (hrev), the voltage efficiency (hv) and the fuel utilization factor (UF). It is defined in mathematical form as
PSOFC ¼ LHVH2 n_ H2 ;in þ LHVCO n_ CO;in þ LHVCH4 n_ CH4 ;in hrev hv UF ; (9) where UF was a set value and hv was defined as
hv ¼ Table 1 Municipal solid waste compositions used in this study. Component
Percentage [%]
C H O S N ASH
47.6 6 32.9 0.3 1.2 12
(8)
DEcell ENernst
(10)
The reversible efficiency is the maximum possible efficiency defined as the relationship between the maximum electrical energy available (change in Gibbs free energy) and the fuels LHV (lower heating value) as follows (see e.g. Ref. [24].)
hrev ¼
Dgf
fuel
LHVfuel
(11)
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500
1 yH2 ;in þ g f gf H2 O H2 O2 CO2 2
h 1 y gf g þ gf CO CO2 2 f O2 CO;in i yCH4 ;in þ 2 gf gf 2 gf
gf
gf
493
In addition, equations for conservation of mass (with molar flows), conservation of energy and conservation of momentum were also included into the model.
community, the mass flow of fuel produced in the gasification plant and the efficiency of the entire power plant, seems to be the best way to establish the size of such plants. Therefore, in this study such couplings have been investigated as first step. Further, different plant configurations have been studied to find out the most optimized plant configuration with respect to efficiency. This in turn implies that the placement of the heat exchangers plays an important role for the optimized configuration. It is better to have a realistic values of pressure drops in the heat exchangers rather than too low. The real pressure drops of the heat exchangers remain unknown until separate study is carried out to estimate their pressure drops at each side. This can be done after initial study of the complete plant configuration. Therefore, the initial values for the heat exchangers have firstly been studied depend on their mass flow estimation from a configuration and afterward it was found that the pressure drops vary between 0.005 bar and 0.04 bar depending on fluids type and mass flow. The lower value corresponds to the fuel side and the higher value corresponds to the gas side of the heat exchangers. Other input data are shown in Table 2.
3. Plant configurations
4. Results and discussions
Waste enters the gasification plant to obtain a synthesis gas known as “syngas”, which after a cleaning process to remove H2S tracks, is used for a SOFC plant. The off-fuel and the off-air which come from the anode and cathode side respectively are then sent to a burner for complete combustion. To recover more energy, the flue gases from the burner are expanded in a gas turbine to obtain additional electric power. Two different configurations have been studied; the second one includes a regenerative/recuperated gas turbine, as shown in Fig. 1 Fig. 2.
Simulations, calculations and analysis have been carried out using DNA (Dynamic Network Analysis), an in-house componentbased code for energy systems analysis developed at the DTU Thermal Energy Systems department. The solution is provided solving a system of nonlinear equations through the Newtone Raphson modified algorithm, see e.g. Rokni [19]. Ahrenfeldt et al. [15] report that the “Viking gasifier” offers some interesting features such as low tar content in produced syngas (<5 [mg/Nm3]), stable unmanned operation, high cold gas efficiency (>95%), low environmental impact (clean condensate, high carbon conversion ratio), and gasification at ambient pressure. Higher process rates are achieved when a mixture of air and steam is used as gasification agent to lower the operating temperature and increase the hydrogen content. This makes the produced syngas composition suitable for feeding a SOFC plant. It was tested experimentally without any remark. Similar results have also been obtained through the DNA model using MSW as fuel in the Viking Gasifier; however it is not possible to confirm this result with experimental data because the considered gasifier has not been used with such kind of biomass. The resulted output data are shown in Table 3 wherein case A refers to the configuration without hybrid recuperator and case B refers to the configuration including a hybrid recuperator, as was shown in Fig. 2. As can be seen, plant performances enhance significantly by introduction of the hybrid recuperator that allows preheating the air in additional step before entering the cathode pre-heater. The principle consequences of such hybrid recuperator are:
Dgf
fuel
¼
H2 O
CH4
O2
(12) The partial pressures were assumed to be the average between the inlet and outlet as
yj;out þ yj;in p j ¼ fH2 ; CO; CH4 ; CO2 ; H2 O; N2 g 2 yO2 ;out þ yO2 ;in ¼ pc 2
pj ¼ pO2
(13)
3.1. Input parameters It could be interesting to study a power plant running on solid waste and to reveal some interesting features such as efficiency and power generated. For such plants having high efficiency is important but this is not the main feature, since municipal waste is used as fuel. Thus, the environmental impact would be the most important advantage, while power production would be another important issue to show how much of the power needed in the community can be produced environmental friendly. The compromise between the amount of waste production in the Air
Waste
Air
Gasification
Syngas
Electric SOFC
Energy
Plant H2S
-
Heat
Ash Electric
Gas Turbine
Energy
larger available heat for the bottoming cycle (gas turbine here); higher temperature of flue gas entering the burner; higher thermodynamic mean temperature; recovering more energy of the smoke gases and consequently higher plant efficiency; - better integration of heat flows inside the plant. - lower losses caused by the temperature level of smoke outlet from turbine.
Heat
4.1. Sensitivity analysis Exhaust gases Fig. 1. Block scheme of the plant.
A sensitivity analysis is conducted by varying some crucial parameters while all other values are kept constant. The analyzed
494
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500
Fig. 2. Plant scheme without recuperator (a) and with hybrid recuperator (b).
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500 Table 2 Simulations data input.
LHVw ¼ LHV MOIðLHV þ rÞ
Component
Data input
Value
Dryer
MSW mass flow Tin fuel side pin fuel side Tout fuel side pout fuel side Heat losses Dp fuel side Dp steam side Tout syngas side Tin water pout H2S Heat losses Operating p Operating T Dp Water to fuel ratio C conversion factor Non-equilibrium CH4 Tin air side Heat losses Dp syngas side Dp air side Tout steam side Heat losses Dp syngas side Dp steam side
2.4 [kg/s] 25 [ C] 1.01325 [bar] 150 [ C] 1.008 [bar] 0 [kW] 0.05 [bar] 0.005 [bar] 800 [ C] 150 [ C] 1.01325 [bar] 0 [kW] 0.998 [bar] 800 [ C] 0.005 [bar] 0 1 0.01 25 [ C] 0 [kW] 0.005 [bar] 0.005 [bar] 200 [ C] 0 [kW] 0.005 [bar] 0.005 [bar] 80% 98% 0 [kW] 0.0049 [bar] 0 [kW] 650 [ C] 650 [ C] 0.8 780 0.01 [bar] 0.04 [bar] 75 50,500 0 [kW] 0.01 [bar] 0.01 [bar] 0 0.04 [bar] 0.04 [bar] 1.01325 [bar] 25 [ C] 2.5 [bar] 0.8 0.98 0.8 0.98 0 [kW] 0.95 1.01325 [bar] 0.86 0.98 0.04 [bar] 0.04 [bar] 0 [kW] 1.01325 [bar]
Gasifier
Air p_h
Steam heater
Steam blower
his hm
Desulphuriser
Heat losses Dp Heat losses Tin cathode side Tin anode side Utilization factor Operating T Dp anode side Dp cathode side Cells per stack Number of stacks Heat losses Dp used fuel side Dp syngas side Heat losses Dp flue gas side Dp air side pin Tin pout
SOFC stack
Anode p_h
Cathode p_h
Compressor
Syngas blower Burner Turbine Generator Recuperator
495
his hm his hm Heat losses Dp pout
his hel Dp smoke side Dp air side Heat losses pout smoke side
(14)
where LHVw is the lower heating value on wet basis, LHV is the corresponding value for dry basis fuel, MOI is the moisture content in the fuel and r is the vaporization heat of water (2500 kJ/kg). By reviewing a large amount of articles in the literature one can conclude that the values of moisture content in MSW ranges between 8% and 20% as shown e.g. in Bébar et al. [26] and Hernandez et al. [27]; the lower the moisture is, the better the fuel would be. In fact MSW composition could change day by day and it could be beneficial to control this parameter before introducing the fuel into the gasifier. Fig. 3 shows the LHV of MSW as function of moisture content. Power production and efficiency are influenced significantly with fuel moisture content. The power production by SOFC and GT decreases with increasing moisture content, see Fig. 4 This is also true for auxiliary components in which power consumption decreases; particularly the syngas blower and the compressor which are due to reduction of syngas mass flow and consequently the required air. However, the decrements of the auxiliary consumptions are lower than the reduction of total power production which explains why the net power decreases. Variations of plant efficiency are negligible which can be explained by the fact that the reduction of net power corresponds to the reduction of fuel inlet power and the ratio between these two values remains practically unchanged. SOFC plant efficiency is not affected significantly by moisture variation while the GT efficiency decreases which is due to circumstances that the reduction of power production is higher than the reduction of inlet fuel power, see Fig. 5. The most peculiar result is that the gasification plant efficiency increases slightly when moisture content is increased. Of course, syngas output energy decreases but its reduction is slower than the fuel input energy. In any case it is better to have low moisture content, approximately 10%, mainly because the output net power production will be reduced otherwise. 4.3. Gasification temperature Gasification temperature is another important parameter to be studied because it affects the fuel composition which will be fed to SOFC. Increasing gasification temperature results in a higher air mass flow required for oxidation (more nitrogen is diluted in the syngas) and hence the produced amount of syngas will be increased. However, the heat value of the fuel is lowered because of chemical equilibrium leading to higher amount of CO and lower amounts of CO2, CH4 and H2 content. Such fuel compositions influence the power production by SOFC plant negatively which in turn influences the produced electric power and efficiency as shown in Fig. 6. SOFC produced power decreases due to a decrement of syngas LHV during the gasification process which is because of higher air mass flow is required for oxidation and for this reason auxiliary consumption in the air compressor increases. However, the generated electric power in the GT increases as Table 3 Simulations data output.
parameters in this study are identified as fuel moisture content, gasification temperature and SOFC operating temperature. The results are provided in the following subsections.
Plant
4.2. Fuel moisture content Fuel moisture is an important parameter to be studied since its variation influences the fuel properties through the LHV (Lower Heating Value) as discussed in Ref. [25]:
SOFC GT
Data output
A
B
Ptot [MW] Pcons [MW] Pnet [MW] Cfuel [MW] h [%] P [MW] h [%] P [MW]
29.5 12.22 17.3 42.62 40.7 18.5 43 11
34.32 12.19 22.13 42.62 51.93 18.53 43.5 15.79
20 19 18 17 16 15 14
η [%]
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500
LHV [MJ/kg]
496
9 10 12 14 16 18 20 22 24 26 28 30 MOI [%]
100 90 80 70 60 50 40 30 20
Fig. 3. LHV as function of fuel moisture.
GASIFICATION
PLANT SOFC GAS TURBINE
9 10 12 14 16 18 20 22 24 26 28 30 MOI [%]
Fig. 5. Plant efficiency as function of moisture content.
a consequent to reduction of elaborated mass flow. For this reason total net power production remains more or less unchanged. Generated electric power for each plant affects the corresponding plant efficiency, meaning that all efficiency values decrease as result; except for the GT plant efficiency which is increased which is explained above. These are revealed in Fig. 7.
- Lost exergetic efficiency; εL
εL ¼
EL
(16)
EFuel - Total exergetic efficiency; εTOT
εTOT ¼ εD þ εL
4.4. SOFC operating temperature SOFC operating temperature has been changed from 780 C to 950 C while anode and cathode inlet temperatures are adjusted accordingly and maintaining a constant temperature difference over the cells of the SOFC stacks (130 C) as discussed in Rokni [19]. The reason is to avoid thermal stresses over the cells which otherwise may damage the cells. SOFC power production and SOFC efficiency decrease when operating temperature increases which are due to reduction of fuel cell equilibrium potential [28] and a thermal expansion among materials [29]. However, auxiliary consumption increases consequently and the plant efficiency decreases. Gas turbine power production and efficiency increase due to a higher energy input through the used fuel coming from SOFC, see Fig. 8 and Fig. 9.
5. Exergy analysis Lost and Destroyed exergy for each component have been calculated through a system of equation for which the solution has been provided through EES (Engineering Equation Solver). The following parameters have been evaluated for each component of the plant:
ED
(15)
(17)
where ED , EL and EFuel are respectively the destroyed, the lost and the fuel exergy of the i-th component. The calculated destroyed, lost and total exergy for each component are shown in Table 4. Principle losses can be identified in the following components: - gasifier, because of the chemical reactions that take place; - burner, due to the oxidation of fuel that requires the conversion of chemical in thermal energy; - SOFC, due to the chemical reactions that have place; - cathode Pre-Heater, because the high difference between flue gas and air temperature profiles; flue gas and SOFC air mass flow is also high, so exergy destruction affects more on global losses; - compressor and Turbine, due to the high mass flow of fluid; - Recuperator, the principle source of loss, due to the high mass flow and temperature of the off gases. Total exergetic efficiency for the whole plant can be evaluated as:
hEx ¼
EProduct
¼ 1
EFuel
- Destroyed exergetic efficiency defined as εD
E D þ EL
(18)
EFuel
Where ED and EL are the total Destroyed and Lost Exergy for the complete plant; EFuel is the inlet exergy through the introduced waste. For the plant analyzed here, an exergetic efficiency of 48.62% is obtained.
EFuel
35
35 TOTAL
25 SOFC NET
20 15
GAS TURBINE
10 5
AUXILIARY
9
10 12 14 16 18 20 22 24 26 28 30 MOI [%]
Fig. 4. Electric power as function of moisture content.
TOTAL
30 P [MW]
30 P [MW]
εD ¼
25
NET
20
SOFC
15
GAS TURBINE AUXILIARY
10
700
750
800
850
900
TGAS [°C] Fig. 6. Electric power as function of gasification temperature.
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500
90
70
60
PLANT
50
60
η [%]
η [%]
70
SOFC
30
30
GAS TURBINE
700
750
800 TGAS [°C]
850
800
850
900
950
TSOFC [°C] Fig. 9. Plant efficiency as function of SOFC operating temperature.
½V=h
(19)
For the thermoeconomic analysis of the plant, appropriate cost functions must firstly be formulated to include the purchase cost for each component; the capital cost per annum, the operating cost per annum, and the total cost per annum. The expressions for each component purchase costs I are summarized in detail in Table 5. The cost of dryer, burner and SOFC have been obtained through a market review and expressions found in literature have been used for the other components [30e33]. For the kth component, Capital Investment IKTOT is amortized in n years, as shown by the following formula:
IK
780
900
Thermoeconomic analysis aims to build a system of equations through the costs balances for each component; the unknown variables are the Unitary Costs c [V/kWh] for each node. Once the exergy value of each node is known, then it would possible to calculate the thermoeconomic cost of the kth node in terms of cost rate (cost per hour), as follows:
TOT
GAS TURBINE
20
6. Thermoeconomic analysis
SOFC
40
Fig. 7. Plant efficiency as function of gasification temperature.
Ck ¼ ck Ek
PLANT
50
40 20
GASIFICATION
80
GASIFICATION
80
497
¼ f IKTOT
(20)
The values for equipment life-span n, construction period CP, interest rate int, inflation rate ri and operating hours Hr are shown in Table 6, which are based on the report in Ref. [33]: With 7000 operating hours per year, an 80% of availability factor is obtained which is the ratio between operating hour and the maximum amount of hours in one year (8760 h). CI
Capital Investment Cost Z K is calculated as: CI
TOT
ZK ¼
iK Hr
(23) OM
Maintenance and operating cost Z K is considered by a “maintenance factor” M, assumed to be 1.1 [35] and the relation with the TOT
is:
total cost Z K OM
ZK
TOT
¼ MZ K
(24) CI
OM
Table 7 shows the calculated values for Z K , Z K each component in the plant
TOT
and Z K
for
In the cost balance the “cost rate” associated to the product (C P [V/h]) is set equal to the total rate of expenditures made for the
product, which in turn is sum of the fuel cost rate (C F V/h]) and the
cost rates associated to the capital investment (Z CL [V/h]) and
where f is the “annuity factor”, calculated as:
" f ¼
qnþCP i ðqi
1ÞqnþCP i
#1
qCP i 1
(21)
ðqi 1ÞqCP i
operating and maintenance (Z OM [V/h]). For the kth component, operating at steady state, it is formulated as:
C P;k ¼ C F;k þ Z k ¼ C F;k þ Z CI;k þ Z OM;k
(25)
where qi is the “interest factor”, defined as:
int r 1þ i 1þ 100 100
P [MW]
qi ¼
44 39 34 29 24 19 14 9
(22)
TOTAL
NET SOFC GAS TURBINE AUXILIARY
780
800
850 900 TSOFC [°C]
950
Fig. 8. Electric power as function of SOFC operating temperature.
Table 4 Destroyed, lost and total exergetic efficiency for each component. Component
εD [%]
εL [%]
εTOT [%]
Dryer Gasifier Desulphuriser Air Pre-Heater Mixer Splitter Steam Generator Steam Blower Syngas Blower Anode Pre-Heater SOFC Cathode Pre-Heater Burner Turbine Electric generator Hybrid Recuperator Compressor
0.2026 8.569 0.001325 0.727 0.1395 0 0.4913 0.007344 0.2953 0.01458 5.529 1.951 6.881 2.002 0.7002 6.593 3.633
0 0.4389 0.361 0 0 0 0 0 0 0 0 0 0 0 0 13.25 0
0.2026 9.008 0.362325 0.727 0.1395 0 0.4913 0.007344 0.2953 0.01458 5.529 1.951 6.881 2.002 0.7002 19.85 3.633
498
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500
Table 5 The cost of other components.
Table 7 Calculated components cost in this study.
Component
Cost [$]
Dryer Gasifier Desulphuriser
130,000 2:9 106 ð3:6mWASTE Þ0:7 [32] 800,000 AEX 0:78 [30] 130 0:093
Heat Exchangers
75ðm=0:9 hm hIS Þrc ln rc [32] 3000 Pel,SOFC 192,000 (98.328 ln WTURBINE þ 1318.5) WTURBINE [30] 0:5 [33] 60 Pel
Compressor SOFC Burner Gas Turbine Generator
At this stage, product and fuel costs for each component of the system must be defined, therefore, a system of equations are built with a cost-balance equation for each unit. Unit cost equations for external flows into the system for which costs are externally defined and losses for which the unit cost is set equal to zero. It would thus be possible to solve the system equations when auxiliary equations, according to the two following principles, are added (or assumed): - if definition of “fuel” of a component includes a stream that goes through another component and is used in it, then the unit cost of the stream flowing into and out of the component is the same; - if the product of a component is composed of two or more streams then the unit cost of those streams are equal. TEC (thermoeconomic) method requires to know the exergy for each node of the system; exergetic analysis of the plant is carried out by DNA by fixing a thermodynamic state for the ambient (Tamb ¼ 25 C and pamb ¼ 1 bar). To solve the linear system of equations, other auxiliary equations are also necessary. For this, unit cost of absorbed power in the auxiliary systems is set equal to the weighted average unit cost of electric power produced by the SOFC and the gas turbine:
cAuxiliary ¼
cSOFC ESOFC þ cGT EGT ESOFC þ EGT
(26)
Now the system of equations is completed and it can be solved; no other auxiliary equations are necessary to obtain the results. These set of equations has been solved by the equation solver software, EES. Afterward, it would be possible to define and to calculate for each component a Unitary Cost for the “Fuel” cF [V/kWh] and a Unitary Cost for the “Product” cP [V/kWh]. The following two main thermoeconomic parameters have been calculated to evaluate and to optimize the performance of each component: - relative Cost Difference Factor DrK; - exergoeconomic Factor fK. Table 6 Economic parameters used in this study. Parameter
Value
Equipment Life-Span “n” Construction Period “CP” Interest Rate “int” Rate of Inflation “ri” Operating Hours “Hr”
15 [years] 1 [year] 6 [%] 2 [%] 7000 [h/year]
Component
Capital investment cost [V/h]
Operating and maintenance cost V/h]
Total cost [V/h]
Dryer Gasifier Desulphuriser Air Pre-Heater Mixer Splitter Steam Generator Steam Blower Syngas Blower Anode Pre-Heater SOFC Cathode Pre-Heater Burner Turbine Electric generator Hybrid Recuperator Compressor Total
4.59 313.33 28.24 0.75 0.00 0.00 1.48 0.01 0.15 0.65 1365.44 2.96 6.78 144.33 0.19 2.75 3.42 1875.05
0.51 34.81 3.14 0.08 0.00 0.00 0.16 0.00 0.02 0.07 151.72 0.33 0.75 16.04 0.02 0.31 0.38 208.34
5.10 348.14 31.38 0.83 0.00 0.00 1.65 0.02 0.17 0.72 1517.15 3.29 7.53 160.36 0.21 3.05 3.80 2083.39
These two variables have been calculated for the kth component through the following equations:
Drk ¼
cP;k cF;k cF;k
(27)
fk ¼
Z k Z k þ cF;k ED;k þ EL;k
(28)
where cp,k and cF,k are the unit costs of product and fuel respectively. For the k component andZ k [V/h ] is the investment cost. The relative cost difference factor DrK suggests that for which component it is possible to enhance a major efficiency, however, this evaluation is not enough to have a complete analysis. The exergoeconomic factor fK in fact suggests how it would better to set in; this parameter expresses the ratio between the lost exergoeconomic cost and the total lost exergoeconomic cost, sum of the economic investment and the inefficiency due to irreversibility. A low value of this parameter (close to zero) suggests that exergetic losses are decreased at the expense of a higher total cost of investment for the component. On the other hand, a high value (close to 1) suggests decrease the purchase equipment cost by enhancing the exergetic losses of the component. Optimization of one component by means of exergoeconomic factor might not lead to a more optimized system, when analyzing a complex energy system. Modifications may reflect negatively on the other components and therefore major attention should be paid to the components that have high exergetic losses and investment costs. In Table 8, exergoeconomic parameter for each component is provided. As shown in Table 8, the highest values of DrK have been obtained for the following components: - gasifier and SOFC, where irreversibility due to chemical reactions are present - anode preheater and steam generator, because of the inefficiency in the thermal exchange which in turn is due to small difference between the inlet and outlet temperatures of fluids. The major values of fK are in those components where both exergy losses and total investment cost are high, particularly in
F. Bellomare, M. Rokni / Renewable Energy 55 (2013) 490e500 Table 8 Exergoeconomic parameter for each component. Component
DrK [%]
fK [%]
Dryer Gasifier Desulphuriser Air Preheater Mixer Splitter Steam Generator Steam Blower Syngas Blower Anode Preheater SOFC Cathode Pre-Heater Burner Turbine Electric generator Hybrid Recuperator Compressor
2.262 336.7 6.624 42.25 6.599 0 142.8 20.2 9.442 189.2 202.4 10.51 7.255 14.83 2.051 16.52 19.2
10.42 97.13 6.624 18.59 0 0 40.18 5.691 1.26 87.73 97.55 2.681 2.282 61.44 0.5169 0.2377 2.265
gasifier and SOFC. Gasifier investment cost is dependent on the biomass input meaning that decreasing the waste mass flow may allow to obtain a lower exergoeconomic factor, and therefore, obtain a more optimized system. The moisture content in the fuel, and consequently the LHV, could also be another source of inefficiency. The exergoeconomic factor for gasifier is around 97.13%. In SOFC system, enhancement could be obtained by decreasing SOFC temperature [34]; for this component exergoeconomic factor is around 97.55%. Decreasing the operating temperature will lower the purchase cost as well as the exergetic losses. In this way exergoeconomic factor should strongly be reduced. Thermoeconomic analysis also confirms that the anode preheater is a source of significant loss in the plant which is due to the inefficiency in the thermal exchange. Further investigation is needed to reveal how to decrease the loss of this component. Finally, through such kind of analysis it is possible to obtain the unit costs of the produced electric power by the SOFC and the gas turbine. Therefore, price of electricity can be calculated by
Pel ¼
ccost ESOFC þ cGT EGT
(29)
ESOFC þ EGT
Inserting the values calculated above, electricity price is obtained as:
Pel ¼ 0:098 V=kWh which is in fact comparably low in the European market. Purchase cost of SOFC affects strongly on the electricity price. As mentioned above, the SOFC purchase cost is estimated to be around 3000 V/kW, which is in fact very expensive. If SOFC cost decreases to one-third in the future after market entry, then the electricity price would be significantly lower. In other words, it would be beneficial to build the system even with today’s cost of SOFC and much more beneficial in the future. 7. Conclusions Gasification technology allows using biomass energy with high efficiency; in particular through Municipal Solid Waste gasification in which energy is obtained from a “low-cost” fuel instead of incinerating and/or throwing it in the landfills. Integration of such gasification plant with a hybrid SOFC e GT plant permits to achieve an efficiency of 52% in an optimized configuration which is very high plant efficiency without presence of any dangerous pollutants in the off-gases.
499
The best performance is obtained with a regenerative gas turbine in which the plant efficiency increases to about 11% due to the recovery of the energy content in the exhausted off-gases. As discussed the most suitable plant size is of approximately 25 MWe, according to the actual SOFC market which in turn permits to obtain high plant efficiency with a fixed input fuel mass flow. However with such size it is still possible to remove quantities of MSW comparable with the mean production of some small cities which concludes that in the immediate future the analyzed system is going to be feasible and the advantage of integrating such hybrid system with an MSW Gasifier provides a synthesis gas which will be fed to a competitive energy system from a fuel with negligible costs and high plant efficiency. References [1] Morris M, Waldheim L. Energy recovery from solid waste fuels using advanced gasification technology. J Waste Manage 1998;18:557e64. [2] Fontell E, Kivisaari T, Christiansen N, Hansen JB, Pålsson J. Conceptional study of a 250 kW planar SOFC system for CHP application. J Power Sources 2004; 131:49e56. [3] Rokni M. Introduction of a fuel cell into combined cycle: a competitive choice for future cogeneration. ASME Cogen e Turbo, IGTI 1993;8:255e61. [4] Riensche E, Achenbach E, Froning D, Haines MR, Heidug WK, Lokurlu A, et al. Clean combined cycle SOFC power plant e cell modeling and process analysis. J Power Sources 2000;86:404e10. [5] Pålsson J, Selimovic A, Sjunnesson L. Combined solid oxide fuel cell and gas turbine systems for efficient power and heat generation. J Power Sources 2000;86:442e8. [6] Proell T, Aichering C, Rauch R, Hofbauer H. Coupling of biomass steam gasification and an SOFCegas turbine hybrid system for highly efficient electricity generation. ASME Turbo Expo Proceeding, GT2004-53900 2004:103e12. [7] Bang-Møller C, Rokni M. Thermodynamic performance study on biomass gasification, solid oxide fuel cell and micro gas turbine hybrid systems. J Energy Conversion Manage 2010;51:2330e9. [8] Subramanyan K, Diwekar UM. Characterization and quantification of uncertainty in solid oxide fuel cell hybrid power plants. J Power Sources 2005;142:103e16. [9] Calise F, Dentice d’ Accadia M, Vanoli L, von Spakovsky MR. Single-level optimization of hybrid SOFC e GT power plant. J Power Sources 2006;159: 1169e85. [10] Wächter C, Lunderstädt R, Joos F. Dynamic model of a pressurized SOFC/gas turbine hybrid power plant for the development of control concept. J Fuel Cell Sci Technol 2006;3:271e9. [11] Roberts RA, Brouwer J. Dynamic simulation of a pressurized 220 kW solid oxide fuel-cell e gas-turbine hybrid system: modeled performance compared to measured results. J Fuel Cell Sci Technol 2006;3:18e25. [12] Rokni M, Fontell E, Ylijoki Y, Tiihonen O, Hänninen M. Dynamic modeling of Wärtsilä 5kW SOFC system. In: Singhal SC, Mizusaki J, editors. Ninth International Symposium on solid oxide fuel cell (SOFC IX), Quebec; 2005. p. 865e75. [13] Costamagna P, Magistri L, Massardo AF. Design and part-load performance of a hybrid system based on a solid oxide fuel cell reactor and a micro gas turbine. J Power Sources 2001;96:352e68. [14] Henriksen U, Ahrenfeldt J, Jensen TK, Gøbel B, Bentzen JD, Hindsgaul C, et al. The design, construction and operation of a 75 kW two-stage gasifier. J Energy 2006;31:1542e53. [15] Ahrenfeldt J, Henriksen U, Jensen TK, Gøbel B, Wiese L, Kather A, et al. Validation of a continuous combined heat and power (CHP) operation of a twostage biomass gasifier. J Energy Fuels 2006;20:2672e80. [16] Hofmann PH, Schweiger A, Fryda L, Panopoulos KD, Hohenwarter U, Bentzen JD, et al. High temperature electrolyte supported Ni-GDC/YSZ/LSM SOFC operation on two-stage Viking gasifier product gas. J Power Sources 2007;173:357e66. [17] Rokni M. Thermodynamic analysis of an integrated gasification plant with solid oxide fuel cell and steam cycle. J GREEN 2012;2:71e86. [18] Smith JM, Van Ness HC, Abbott MM. Introduction to chemical engineering thermodynamics. 7th ed. Boston: McGraw-Hill; 2005. [19] Rokni M. Plant characteristics of an integrated solid oxide fuel cell and a steam cycle. J Energy 2010;35:4691e9. [20] Holtappels P, DeHaart LGJ, Stimming U, Vinke IC, Mogensen M. Reaction of CO/CO2 gas mixtures on Ni-YSZ cermet electrode. J Appl Electrochem 1999; 29:561e8. [21] Matsuzaki Y, Yasuda I. Electrochemical oxidation of H2 and CO in a H2-H2-COCO2 system at the interface of a Ni-YSZ cermet electrode and YSZ electrolyte. J Electrochemical Soc 2000;147:1630e5. [22] Keegan KM, Khaleel M, Chick LA, Recknagle K, Simner SP, Diebler J, Analysis of a planar solid oxide fuel cell based automotive auxiliary power unit, SAE Technical Paper Series No. 2002-01-0413; 2002. [23] Kim JW, Virkar AV. The effect of anode thickness on the performance of anode e supported solid oxide fuel cell. Proc. of the Sixth Int. Symp. on SOFCs (SOFC e VI), PV99 e 19. J. Electrochemical Society 1999:830e9.
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Glossary Aij: number of atoms of element j in each molecule of leaving compound m Amj: number of atoms of element j in each molecule of entering compound i cF: Fuel Unitary cost, V/kWh ck: Unitary cost for the kth node, V/kWh cp: Product Unitary cost, V/kWh Ck: Thermoeconomic Cost for the kth node, V/h E: Nernst ideal reversible voltage, V
ED : Destroyed Exergy
E k : Exergy for the kth node, kW EL : Lost Exergy
EFuel : Fuel Exergy f: annuity factor fk: Exergoeconomic factor for the kth component F: Faradays constant, C/mol G: Gibbs free energy, J g0: specific Gibbs free energy, J/kmol Hr: Operating hours int: interest rate IkTOT : Capital Investment
Uf: Utilization factor V: Potential W: electrical work Zk: Cost for the kth component, V/h Greek symbols d: partial derivation DVact: Activation polarization, V DVconc: Concentration polarization, V DVohm: Ohmic polarization, V DG: Gibbs free energy, W Dp: Pressure drops, bar Drk: Relative Cost Difference Factor for the kth component εD: Destroyed Exergetic Efficiency εL: Lost Exergetic Efficiency εTOT: Total Exergetic Efficiency h: Efficiency hEx: Exergetic Efficiency m: Lagrange multiplier n: stoichiometric coefficient Subscript aux: auxiliary cons: consumed d: destroyed el: electrical ex: exergy F: fuel fuel: fuel property gas: gasification i: i-th element in: in value is: isentropic j: j-th element l: lost m: mechanic max: maximum net: net amount out: out value P: product prod: product property r: water vaporization latent heat value reag: reagent real: real value rev: reversible tot: total w: wet
TOT
I k : Capital Investment cost n: equipment life-span ne: electrons number n: molecular weight P: Power, W Pel: Price of electricity, V/kWh p: Pressure, bar pH2 : Partial pressures for H2, bar pH2 O : Partial pressures for H2O, bar pO2 : Partial pressures for O2, bar q: interest factor r: rate of inflation R: Universal gas constant, J/kmol K T: Operating temperature, K
Abbreviations CC: Combined Cycle CHP: Combined Heat and Power CI: Capital Investment CP: Construction Period DNA: Dynamic Network Analysis EES: Engineering Equation Solver GT: Gas Turbine IGSG: Integrated Gasification SOFC and GT LHV: Lower Heat Value MOI: Moisture MSW: Municipal Solid Waste OM: Operating and Maintenance factor SOFC: Solid Oxide Fuel Cell