Municipal Solid Waste Gasification with Solid Oxide Fuel Cells and Stirling Engine

Transcription

1 Downloaded from orbt.dtu.dk on: Apr 05, 2018 Muncpal Sold Waste Gasfcaton wth Sold Oxde Fuel Cells and Strlng Engne Rokn, Masoud Publshed n: Proceedngs of the Internatonal Conference on Clean Energy 2014 Publcaton date: 2014 Document Verson Peer revewed verson Lnk back to DTU Orbt Ctaton (APA): Rokn, M. (2014). Muncpal Sold Waste Gasfcaton wth Sold Oxde Fuel Cells and Strlng Engne. In Proceedngs of the Internatonal Conference on Clean Energy 2014 (pp ) General rghts Copyrght and moral rghts for the publcatons made accessble n the publc portal are retaned by the authors and/or other copyrght owners and t s a condton of accessng publcatons that users recognse and abde by the legal requrements assocated wth these rghts. Users may download and prnt one copy of any publcaton from the publc portal for the purpose of prvate study or research. You may not further dstrbute the materal or use t for any proft-makng actvty or commercal gan You may freely dstrbute the URL dentfyng the publcaton n the publc portal If you beleve that ths document breaches copyrght please contact us provdng detals, and we wll remove access to the work mmedately and nvestgate your clam.

2 MUNICIPAL SOLID WASTE GASIFICATION WITH SOLID OXIDE FUEL CELLS AND STIRLING ENGINE Masoud Rokn Techncal Unversty of Denmark, Dept. of Mechancal Engneerng, Thermal Energy Secton Buldng 403, 2800 Kgs, Lyngby, Denmark e-mal: ABSTRACT Muncpal Sold Waste (MSW) can be consdered a vald bomass to be used n a power plant. The major advantage s the reducton of pollutants and greenhouse gases emssons not only wthn large ctes but also globally. Another advantage s that by ther use t s possble to reduce the waste storage n landflls and devote these spaces to other human actvtes. It s also mportant to pont out that ths knd of renewable energy suffers sgnfcantly less avalablty whch characterzes other type of renewable energy sources such as n wnd and solar energy. In a gasfcaton process, waste s subject to chemcal treatments through ar or/and steam utlzaton; the result s a synthess gas, called Syngas whch s prncpally composed of hydrogen and carbon monoxde. Traces of hydrogen sulfde could also be present whch can easly be separated n a desulfurzaton reactor. The gasfcaton process s usually based on an atmospherc-pressure crculatng fludzed bed gasfer coupled to a tar-crackng vessel. Syngas can be used as fuel n dfferent knd of power plant such as gas turbne cycle, steam cycle, combned cycle, nternal and external combuston engne and Sold Oxde Fuel Cell (SOFC). In the present study, a MSW gasfcaton plant ntegrated wth SOFC s combned wth a Strlng engne to recover the energy of the off-gases from the toppng SOFC cycle. Detaled plant desgn s proposed and thermodynamc analyss s performed. Relevant parameters have been studed to optmze the plant effcency n terms of operatng condtons. Compared wth modern waste ncnerators wth heat recovery, the gasfcaton process ntegrated wth SOFC and Strlng engne permts an ncrease n electrcty output up of 50%, whch means that the sold waste gasfcaton process can compete wth ncneraton technology. Moreover waste ncnerators requre the nstallaton of sophstcated exhaust gas cleanng equpment that can be large and expensve and are not necessary n the studed plant. INTRODUCTION Due to the ever-ncreasng demand for more effcent power producton and dstrbuton, the man topcs of research and development n the feld of electrcty producton are mprovng effcency and reducng pollutant emssons. There s currently an ncreased nterest n developng a dstrbuted system of smaller-scale facltes rather than a large-scale faclty at a sngle locaton, allowng electrcty and heat to be produced and dstrbuted close to the end user and thereby mnmzng the costs assocated wth transportaton, see (Sanchez et al., 2008) and (Rokn, 2013a). The word Bomass refers to vegetables and anmals substances, not from fossl orgn; these can be used as fuel n a power plant for the producton of electrcal energy. Bomasses derve from lvng or recently lvng bologcal organsms and can be consdered as a partcular knd of renewable energy source, because the carbon doxde placed n the atmosphere by ther use derves from the carbon amount absorbed durng ther lfe. In ths way, the most mportant pollutants lnked to bomass utlzatons are related to transport, manufacture and transformaton processes. Muncpal Sold Waste can be consdered a vald bomass to use n a power plant. Some advantages can be obtaned; the prncpal s the reducton of pollutants and greenhouse gases emssons. Another advantage s that by ther use t s possble to reduce the storage n landflls and devote these spaces to other human actvtes. It s also mportant to pont out that ths knd of renewable energy suffers sgnfcantly less avalablty whch characterzes other type of renewable energy sources such as n wnd and solar energy. As proposed n (Morrs et al., 1998), wth a well management of waste, the followng ponts should be consdered: - preventon of waste generaton; - recyclng of waste materals; - reducton at mnmum of landfllng dsposal; - ncneraton wth energy recovery at effcences comparable wth alternatve technologes and sophstcated exhaust gas cleanng equpment; - gasfcaton processes. In a gasfcaton process, waste s subject to chemcal treatments through ar or steam utlzaton; the result s a synthess gas, called syngas whch s prncpally composed of hydrogen and carbon monoxde. Traces of hydrogen sulfde could also be present whch can easly be separated n a desulfurzaton reactor. The gasfcaton process s usually based on an atmospherc-pressure crculatng fludzed bed gasfer coupled to a tar-crackng vessel; the gas produced s then cooled and cleaned. Syngas can be used as fuel n dfferent knd of power plant such as gas turbne cycle, steam cycle, combned cycle, nternal and external combuston engne and Sold Oxde Fuel Cell (SOFC). Sold oxde fuel cell (SOFC) stacks wll soon enter the commercalzaton phase, and small Strlng engnes are approachng ths phase. It therefore would be nterestng to ntegrate these two technologes nto a sngle system, combnng the benefts of each system to establsh a new technology. Together wth an ntegrated gasfcaton plant that gasfes MSW n a two-step gasfcaton process, electrcty and heat could then be produced n an envronmentally frendly way.

3 SOFCs are one of the most promsng types of fuel cells, partcularly regardng energy producton. They are expected to produce clean electrcal energy at hgh conventon rates wth low nose and low pollutant emssons (Calse et al., 2006). To date, studes on syngas from coal and bomass gasfcaton to feed SOFC are carred out, such as (Doherty et al., 2010) and (Ghosh and De, 2006). Usng synthetc wood gas for operatng of SOFC s also expermentally studed n (Buchnger et al., 2007) whch showed that wood gas from ar gasfcaton always gave a stable performance whle wood gas from steam gasfcaton dd not gve clear results. The exhaust temperatures of SOFCs are hgh due to the hgh operatng temperature of the cells. Addtonally, because the fuel utlzaton n the fuel cell never reaches 100 percent, the unreacted fuel needs to be combusted n a burner. Ths combuston n turn produces even hotter off-gases that are perfectly suted for use n a heat engne, such as a Strlng engne, for the producton of power and heat for domestc purposes. Numerous studes have nvestgated SOFC-based power systems and suggested hgh thermal effcences n the lterature. However, the majorty of these studes use gas turbnes as the bottomng cycle, see, e.g., (US Department of energy, 2004), (Rensche et al., 2000) and (Hasel et al. 2008). A steam turbne has also been used as a bottomng cycle (Rokn 2010a and Rokn 2010b), resultng n hgh plant effcency. Only a few studes have been carred out wth a Strlng engne as a bottomng cycle when a fuel cell cycle s used as the toppng cycle, see, e.g., (Sanchez et al., 2008) and (Rokn, 2013a). At present, usng the Brayton and Rankne cycles as bottomng cycles seems to be the most practcal because of the maturty of these technologes. Gven that the development trends suggest that the operatng temperature of the SOFC wll decrease, usng gas turbne as bottomng cycle wll become less benefcal over tme. Introducng a heat engne (Strlng) as bottomng cycle for SOFC compared to gas turbne and steam cycles has several advantages. Such hybrd cycle s sgnfcantly less complex, heat producton wll be as much as electrcal power (hgh heat-power rato), small scale CHP (combned heat and power) plants sutable for hotels, hosptals, shoppng centers can be bult and the plant cost wll be much lower. Integrated gasfcaton SOFC systems have also been studed, see, for example, (Proell et al., 2004), (Bang- Møller and Rokn, 2010) and (Rokn, 2012). However, there has been an absence of research nto ntegrated MSW gasfcaton SOFC-Strlng CHP plants n the lterature, formng the bass of ths study. The present work s an analytcal study that conducts a thermodynamc nvestgaton of systems wth ntegrated gasfcaton of MSW, where the syngas s used as fuel for a SOFC plant that also functons as a toppng cycle for a Strlng engne usng the heat from the off-gasses exhausted from the toppng cycle. The system s net capacty s 120 kw, whch s sutable for decentralzed CPH plants. The gasfer type used for the analyss s based on the Vkng twostage gasfer bult at DTU-Rsø, whch s an autothermal (ar-blown) fxed bed gasfer and produces a clean syngas that can be drectly fed nto a SOFC. More nformaton on the gasfer plant can be found n (Henrksen et al., 2006), (Ahrenfeldt et al., 2006) and (Hofmann et al., 2007). The SOFC s based on a theoretcal model wth emprcal coeffcents calbrated from expermental data. The Strlng engne s parameters are chosen by fttng these parameters to a valdated feasble engne. No nvestgaton on muncpal sold waste gasfcaton plant ntegrated wth SOFC and Strlng engne has been found n the open lterature, and therefore, the current nvestgaton seems to be completely novel and mght brng up new deas on desgnng new energy system confguratons for future applcatons. It should also be noted that the system presented here was studed thermodynamcally and that the objectve of ths study was not to present or dscuss the assocated costs. The performances of the varous plants are compared n terms of effcency, fuel consumpton and other related parameters. Thus, the man dea s shortly Usng muncpal sold waste to generate electrcty through gasfcaton, SOFC and Strlng engne. PLANT MODEL The prncpal components of the plant are the gasfcaton plant, the SOFC plant and the Strlng engne. Through the gasfcaton plant, MSW s converted nto syngas whch s a mxture of H2, N2, CO, CO2, H2O, CH4 and Ar. The produced syngas s then cleaned to remove the tracks of H2S that could poson the SOFC. Ar Ar Muncpal Sold Waste Gasfcaton Plant Syngas SOFC Plant Electrc Energy Ash H 2 S Strlng Engne Heat Electrc Energy Domestc Heat and Hot water Exhaust gases Fgure 1. Block scheme of the plant.

4 The cleaned syngas s then sent to the SOFC plant to produce electrcty. The SOFC stacks cannot consume all the fuels and therefore the remanng fuel s sent to the burner to complete the combuston. The combusted gases after the burner are then sent to a Strlng engne (acts as a bottomng cycle) for further electrcty producton. Both engne coolng crcut and the heat released through the exhausted gases can be used for space heatng and domestc hot water (DHW) producton, see the block scheme shown n Fg. 1. Apart from the fuel, the other nputs of the plant are ar feedng the gasfer and ar feedng the cathode sde of the SOFC stacks. To ntroduce these, auxlary energy s necessary such as compressors. Another use of auxlary energy s to blow the syngas out from the gasfcaton plant. The effcency of the plant can be expressed as the rato between the net electrc power and the fuel power, where net power refers to the dfference between the total produced power and the power used n the auxlary components such as compressors, blowers, control systems, etc.: Net Tot Aux (1) m fuel P LHV fuel P m fuel P LHV fuel Gasfer and Methanator Modelng The gasfcaton plant used n ths study s based on the model developed n (Rokn, 2012) whch s repeated here for the sake of clarfcaton. A smple Gbbs reactor s mplemented, meanng that the total Gbbs free energy has ts mnmum when the chemcal equlbrum s acheved. Such characterstc can be used to calculate the gas composton at a specfed temperature and pressure wthout consderng the reacton pathways (Smth et al., 2005). The Gbbs free energy of a gas (assumed to be a mxture of k perfect gases) s gven by G k 1 n g 0 RT ln n p (2) where g 0, R and T are the specfc Gbbs free energy, unversal gas constant and gas temperature respectvely. Each atomc element n the nlet gas s n balance wth the outlet gas composton, whch shows that the flow of each atom has to be conserved. For N elements, ths balance s expressed as, see, e.g. (Rokn, 2012) k 1 w n, naj n m, outamj for j 1,N (3) m1 The N elements correspond to H 2, O 2, N 2, CO, NO, CO 2, steam, NH 3, H 2 S, SO 2, CH 4, C, NO 2, HCN (hydrogen cyande), COS (carbonyl sulfde), Ar and Ashes (SO 2 ) n gasfyng process. A m s the number of atoms of element j (H, C, O, N) n each molecule of enterng compound (H 2, CH 4, CO, CO 2, H 2 O, O 2, N 2 and Ar), whle A j s the number of atoms of element j n each molecule of leavng compound m (H 2, O 2, N 2, CO, NO, CO 2, steam, NH 3, H 2 S, SO 2, CH 4, C, NO 2, HCN (hydrogen cyande), COS, Ar and Ashes). The mnmzaton of the Gbbs free energy can be formulated by ntroducng a Lagrange multpler,, for each of the N obtaned constrants. After addng the constrants, the expresson to be mnmzed s then N k w G tot, out j n outaj n, m, n Amj (4) j1 1 m1 By settng the partal dervaton of ths equaton wth respect to n, out g G n 0, out tot, out, out RT ln N j1 A j j 0 for 1, k n, out N n, outpout ja j 0 for 1, k j1 to zero then the functon can be mnmzed as Thus a set of k equatons were defned for each chemcal compound leavng the system. Fnally, t was found that by assumng chemcal equlbrum at the gasfer the methane content n the product gas was underestmated. Therefore, a parameter called METHANE was appled to allow some of the methane bypasses the gasfer wthout undergong any chemcal reactons. By adjustng ths parameter the product gas could be calbrated aganst the expermental data of the Vkng gasfcaton plant and found to be 0.01 meanng that 1% of the methane was bypassed n the gasfer, although very small amount, see (Rokn, 2012). Changng the fuel from bomass to muncpal waste shall change ths parameter slghtly but due to lack of expermental data ths value was assumed to be the same. (5)

5 The basc MSW composton and propertes used n ths study s shown n Table 1, whch s based on the study of (Channwala and Parkh, 2002). Ths composton should be consdered vald unless other values are provded. Note that the compostons are referred to a dry bass, meanng that percentages are expressed n terms of weght fracton wthout moster content. The MSW composton s then changed as dscussed below. Table 1. Muncpal sold waste compostons and propertes used n ths study. MSW Dry-based percentage C [%] 47.6 H [%] 6 O [%] 32.9 S [%] 0.3 N [%] 1.2 Ash [%] 12 LHV [kw], (dry bass) c p [kj/kg] 1.71 Mosture SOFC Modelng The SOFC model developed n (Rokn, 2012) s used n ths nvestgaton, whch were calbrated aganst expermental data for planar SOFC type. For the sake of clarty, t s shortly descrbed here. In such modelng one must dstngush between electrochemcal modelng, calculaton of cell rreversblty (cell voltage effcency) and the speces compostons at outlet. For electrochemcal modelng, the operatonal voltage (E cell ) was found to be E cell E E E E (6) Nernst act ohm conc where E Nernst, E act, E ohm and E conc are the Nernst deal reversble voltage, actvaton polarzaton, ohmc polarzaton and concentraton polarzaton. Assumng that only hydrogen s electrochemcally converted, then the Nernst equaton can be wrtten as 0 Δg f RT ph,tot p 2 O2 E Nernst ln (7) nef nef ph2o p p p (8) H2, tot H2 CO 4pCH4 where g f 0 s the Gbbs free energy (for H 2 reacton) at standard pressure. Its value s J/mol at K and 1 bar. The water-gas shft reacton s very fast and therefore the assumpton of hydrogen as only speces to be electrochemcally converted s justfed, see (Holtappels et al., 1999) and (Matsuzak and Yasuda, 2000). In the above equatons p H2 and p H2O are the partal pressures for H 2 and H 2 O respectvely. The actvaton polarzaton can be evaluated from the Butler Volmer equaton (Keegan et al., 2002), whch s solated from other polarzatons to determne the charge transfer coeffcents and exchange current densty from the experment by the curve fttng technque. The ohmc polarzaton (Zhu and Kee, 2003) depends on the electrcal conductvty of the electrodes as well as the onc conductvty of the electrolyte. Ths was also calbrated aganst expermental data for a cell wth anode thckness, electrolyte thckness and cathode thckness of 600 m, 50 m and 10 m respectvely. The concentraton polarzaton s domnant at hgh current denstes for anode-supported SOFCs, wheren nsuffcent amounts of reactants are transported to the electrodes and the voltage s then reduced sgnfcantly. Agan the concentraton polarzaton was calbrated aganst expermental data by ntroducng the anode lmtng current, (Costamagna et al., 2004), n whch the anode porosty and tortuosty were also ncluded among other parameters. The fuel composton at anode outlet was calculated usng the Gbbs mnmzaton method as descrbed n (Smth et al., 2005). Equlbrum at the anode outlet temperature and pressure was assumed for the followng speces: H 2, CO, CO 2, H 2 O, CH 4 and N 2. Thus the Gbbs mnmzaton method calculates the compostons of these speces at outlet by mnmzng ther Gbbs energy. The equlbrum assumpton s far because the methane content n ths study s very low. To calculate the voltage effcency of the SOFC cells, the power producton from the SOFC (P SOFC ) depends on the amount of chemcal energy fed to the anode, the reversble effcency ( rev ), the voltage effcency ( v ) and the fuel utlzaton factor (U F ). It s defned n mathematcal form as P SOFC LHV H n H n LHV CO n CO, n LHV CH n CH, n rev v U F (9) 2 2, 4 4 where U F was a set value and v was defned as E cell (10) v E Nernst

6 The reversble effcency s the maxmum possble effcency defned as the relatonshp between the maxmum electrcal energy avalable (change n Gbbs free energy) and the fuels LHV (lower heatng value) as follows, (see e.g. Wnnck, 1997) g f fuel rev (11) LHV fuel Addtonally, equatons for conservaton of mass (wth molar flows), conservaton of energy and conservaton of momentum were also ncluded nto the model. Table 2 dsplays the man parameters for the SOFC stacks used n ths study. Table 2. The man SOFC parameters used n ths study. Parameter Value Fuel utlzaton factor Number of cells n stack 74 Number of stacks 160 Cathode pressure drop rato (bar) 0.05 Anode pressure drop rato (bar) 0.01 Cathode nlet temperature ( C) 600 Anode nlet temperature ( C) 650 Outlet temperatures ( C) 780 Generator effcency 0.97 Strlng Modelng The model for Strlng engne used n ths study s adopted from the model developed n (Rokn, 2013a), whch s a pseudo Strlng cycle, and more closely approxmates actual engne performance, developed n (Reader, 1979). A bref explanaton of how the model s mplemented n the n-house program s heren provded. The man dfference between the pseudo Strlng cycle and the deal Strlng cycle s the assumpton of sentropc compresson and expanson n the former rather than sothermal compresson and expanson n the latter. It s assumed that sentropc compresson and expanson provde more realstc cycle performance because, by ncorporatng these processes, the losses encountered n the Strlng engne are accounted for. In modelng, the engne s dvded nto three parts: the heater, engne and a cooler. The most mportant parameters of a Strlng engne are the temperature rato, the compresson rato, the regenerator effectveness and the heater effectveness. Engne power can be determned from engne effcency and the dfference between the heat source and heat snk. Heat added to and removed from the engne can be done by two dfferent heat exchangers and ther effectveness. The losses from the Strlng engne are the result of varous loss mechansms, ncludng mechancal and thermal processes. Therefore, a loss factor s ncorporated that accounts for all mechansms of losses n the engne, ncludng both mechancal and thermal. The hghest temperature of the workng flud (helum) s lower than the heater wall temperature, and the lowest temperature of the workng flud s a weghted temperature and s the average between the nlet and outlet temperatures. Therefore, T heater and T cooler are ntroduced whch refer to the temperature dfference over the heater and cooler, respectvely. The man parameters for the Strlng engne used n ths study are shown n Table 3. Table 3. The man Strlng engne parameters used n ths study. Parameter Value Heater and cooler p (bar) 0.01 Heater wall temperature ( C) 600 Heater T ( C) 125 Heater effectveness 0.95 Cooler T ( C) 60 Compressor rato ( ) 1.44 Regenerator effectveness 0.98 Mechancal loss factor 0.8 Modelng of Other Components Modelng of other components such as heat exchangers, pumps, desulfurzaton reactor, etc. are adopted from the study of (Rokn, 2013a), n whch the relablty of the components modelng was justfed by buldng a benchmark system consstng SOFC, methanator, heat exchanger, etc. and fed wth dfferent fuels such as natural gas, ethanol, methanol and d-methyl ether (DME). The obtaned results agreed well wth the correspondng data obtaned by other researchers n the open lterature for all cases studed. PLANT CONFIGURATION The system nvestgated here s presented n Fg. 2, whch s a small-scale CHP consstng of an ntegrated MSW gasfcaton plant wth an SOFC system functonng as a toppng cycle, whle a Strlng engne wth domestc hot water

7 heaters comprses the bottomng cycle. Such small scale consstng of a MSW gasfcaton ntegrated wth SOFC- Strlng whch has a hgh heat-power rato s has not been nvestgated prevously. MSW are fed nto a gasfer for the producton of syngas va a two-step process. The frst step s a pyrolyss of the feedstock, and the second step utlzes a fxed bed gasfer, where the pyrolysed feedstock s gasfed by steam and ar as gasfcaton agents. A gas cleaner s ntroduced to remove the small contamnants present n the syngas, manly sulfur. The operatng temperature of the gas cleaner s assumed to be about 250 C. For the toppng SOFC cycle, the ambent ar at 15 C s compressed to the workng pressure of the SOFC (normal pressure) and then heated n the cathode ar preheater (CP) to 600 C before enterng the cathode sde of the SOFC stacks. The cathode preheater uses some of the SOFC off-ar to heat the ncomng ar. The off-ar s splt nto two streams: one enterng the CP and the other enterng the catalytc burner. For the anode sde, the cleaned syngas s frst pumped to compensate for the pressure drop along ts way. The syngas s then preheated to about 650 C before enterng the anode sde of the SOFC usng the off-fuel out of the fuel cell. The operatng temperature of the fuel cell s assumed to be 780 C whch s enough to preheat the ncomng syngas. The burner s mplemented because all of the fuel wll not be reacted n the fuel cell stacks due to fuel utlzaton. The enterng temperatures mentoned above are the mnmum enterng temperatures and are essental requrements for the proper functonng of SOFC stacks, not only to ntate the chemcal reactons but also to avod cell thermal fractures. SOFC Gas Cleaner Gasfer CP AP SG GAP MSW Pyrolyser Ar Burner Sulfur Ar Ash Steam Strlng DHW Off gases Domestc Hot Water SH Space heater Fgure 2. Basc plant confguraton. Confguraton 1 Table 4. System operatng nput parameters. MSW temperature ( C) 15 Pyrloss temperature ( C) 150 Gasfer temperature ( C) 800 Gasfer pressure (bar) 1 Gasfer pressure drop (bar) Gasfer carbon converson factor 1 Gasfer non-equlbrum methane 0.01 Steam blower sentropc effcency 0.8 Steam blower mechancal effcency 0.98 Ar temperature nto gasfer ( C) 15 Syngas blower sentropc effcency 0.7 Syngas blower mechancal effcency 0.95 Syngas cleaner pressure drop Cathode compressor ar ntake temperature ( C) 15 Compressor sentropc effcency 0.7 Compressor mechancal effcency 0.95 Gas heat exchangers pressure drop 0.01 Pnch temperature for cathode preheater ( C) 20 Burner nlet outlet pressure rato 0.97 Water pump effcency 0.95 Inlet water temperature for hot water ( C) 20 Outlet water temperature for hot water ( C) 60 Off gas temperature after water heater ( C) 95

8 For the bottomng cycle, a Strlng engne s mplemented. The Strlng engne utlzes the combuston products, leavng the burner as a heat source. The water used as the heat snk enters at 20 C and exts at 60 C, makng t approprate for, both domestc hot water and space heatng. In partcular, ts temperature s suffcent to address problems related to bactera, e.g., Legonella. The heat remanng after the Strlng engne s used for domestc hot water producton. Water s constraned n the same manner as the heat snk, and the combuston products leave the system nto the envronment at approxmately 95 C, whch s hot enough to avod corroson problems. Other system operatng parameters are mentoned n Table 4. In another confguraton t s proposed to nclude a methanator after the fuel pump and thereby ncrease the methane contan of the syngas, see Fg. 3. Thus the syngas s reformed exothermcally n a methanator, wheren the CH 4 content n the gas s ncreased from a molar fracton of approxmately 0.01 to nearly 0.05, whch s the result of the reacton between H 2 and CO. Introducng methanator wll decrease electrcal producton from the SOFC stacks slghtly, but on the other hand, because the reformaton s hghly exothermc, less heat needs to be extracted from the SOFC off-fuel to heat the ncomng fuel to the fuel cell. Ths wll eventually provde the Strlng engne wth a larger amount of heat to be used because the fuel wll be at a hgher temperature when enterng the burner, and the combuston processes wll therefore occur at a hgher temperature. Secondly, the use of a larger molar fracton of CH 4 n the SOFC causes endothermc nternal reformng, reducng the amount of ar used for coolng purposes and mantanng the SOFC operatng temperature at 780 C. Thus, the workload of the cathode compressor wll decrease. SOFC Gas Cleaner Gasfer CP AP Methanator SG GAP MSW Pyrolyser Ar Burner Sulfur Ar Ash Steam Strlng DHW Off gases SH Space heater Domestc Hot Water Confguraton 2 Fgure 3. Improved plant confguraton by ntroducng a methanator nto the basc confguraton. SOFC Gas Cleaner Gasfer CP Methanator SG GAP MSW Pyrolyser Ar Burner Sulfur Ar Ash Steam Strlng DHW Off gases SH Space heater Domestc Hot Water Confguraton 3 Fgure 4. Fnal plant confguraton when methanator s ncluded and anode preheater s removed.

9 As mentoned above, ncludng a methanator after the fuel pump causes exothermc reformaton of the syngas and thereby ncreasng ts temperature. The calculatons show that the syngas temperature after the methanator wll be hgher than the requred 650 C, resultng n elmnaton of anode preheater. Fgure 4 shows the fnal confguraton when the anode preheater s replaced by a methanator. RESULTS AND CONCLUSIONS The performances of the plants proposed above are shown n Table 5. In the table confguraton 1 denotes the basc confguraton wthout methanator. Confguraton 2 s the basc plant when a methanator s ntroduced and anode preheater s kept. Confguraton 3 s the fnal one when a methanator s ntroduced and the anode preheater s removed. As shown n the table, the electrcal effcency of the fnal plant s more than 1 percent pont hgher than the basc one. Electrcty producton by the SOFC s decreased whle power from Strlng engne s ncreased and auxlary power consumpton s decreased, as the drect result of methanator ncluson. On the other hand both space heatng and hot water producton wll also decrease when a methanator s ncluded. Removng or keepng the anode preheater does not have a sgnfcant effect on plant effcency, power and heat producton. As shown n the table, the methanator has two major benefts for the plant, frst heat need to preheat the fuel wll be decreased resultng n hgher heat avalable for the Strlng engne (through hgher burner temperature). Second, compressor load decreases due to lower coolng effect for fuel cells to mantan ts operatng temperature at the desred level. Table 5. Plant performance for dfferent confguratons. Confguraton Confguraton 1 Confguraton 2 Confguraton 3 Electrcal effcency, (%) Plant electrcal power, (kw SOFC power Strlng power, (kw) Auxlary power, (kw Space heatng, (kj/s) DHW, (kj/s) Total heat, (kw) CHP effcency, (%) MSW mass flow, (kg/h) Methane content, (molar %) Burner temperature, ( C) Confguraton 1: basc plant wthout methanator Confguraton 2: basc plant wth methanator and anode preheater Confguraton 3: basc plant wth methanator but wthout anode preheater Effect of MSW composton The MSW composton can be changed dependng on the garbage type and natonal recyclng polcy. In fact MSW composton could change day by day and t could be nterestng to nvestgate dfferent MSW composton feedng to the gasfer and study ther effect on plant performance. Therefore, a varous MSW compostons presented n the lterature s studed here and the results are shown n Table 6. Table 6. Plant performance wth dfferent waste compostons; basc (Channwala and Parkh, 2002), waste 1 (Buah et al. 2007), waste 2 to 5 (Cozzan et al., 1995), waste mean Compound Basc Waste 1 Waste 2 Waste 3 Waste 4 Waste 5 Waste Mean C H O S N Ash Cl LHV, (kw), (dry bass) c p, (kj/kg) Mosture In the table waste mean represents the mean composton of 19 dfferent waste compostons provded by 12 dfferent references as, (Buah et al., 2007), (Cozzan et al., 1995), (Guan et al., 2009), (Bebar et al., 2005), (Dala K. et al., 2009), (Galvagno et al., 2009), (Gordllo and Annamala, 2010), (Baggo et al., 2009), (Hernandez-Atonal et al., 2007), (Pao et al., 1998), (Pao et al., 2000), (Channwala and Parkh, 2002). Thus, a huge number of data are analyzed and the results are presented as mean composton values for MSW. The expermental values for LHV of the MSWs are gven n some references but such value s mssng n the other references. The LHV (dry bass) value of the mssng MSW s calculated from, LHV HHV (H2500) (Mosture 2500) (12)

10 Plant electrcal effcency [%] SOFC current densty [A/cm2] ICCE 2014 Conference, Istanbul, Turkey, 8 12 June 2014, pp: where HHV s the hgher heatng value. The mosture of MSW s provded n the correspondng reference and H s the weght percentage of hydrogen. HHV of the MSW s calculated from Dulong expresson (Channwala and Parkh, 2002) as HHV C 1.443(H O / 8) S MJ / kg. (13) In ths expresson C, H, O and S are the weght percentage of carbon, hydrogen, oxygen and sulfur, respectvely. The heat capacty s calculated from the weghted average of the values of all the elementary components n the fuel; x c p, c p, fuel (14) x where x are the mass fractons of every component and c p, s the specfc heat capacty of each elementary component. The values for specfc heat capacty of each component are adopted from (Incropera et al., 2006) at a temperature of 300K. The calculatons results are shown n Table 7. As shown n the table the electrcal effcency of the plant changes from about 43.6% to about 48.1% dependng on the MSW composton. The lowest plant effcency s obtaned wth waste 5 whle the hghest plant effcency attaned wth the waste 1. The mean waste composton gves a plant effcency of about 45.1%. Generally, plant effcency s ncreased when SOFC cell effcency s hgher and ts current densty s lower. Ths can also be vsualzed n Fg. 5. Table 7. Plant performance wth dfferent MSW compostons. Waste Type Electrcal Effcency (%) SOFC E cell (V) SOFC Current densty (A/cm 2 ) Basc Waste Waste Waste Waste Waste Waste mean SOFC cell voltage [V] Fgure 5. Plant effcency versus cell voltage and current densty wth dfferent muncpal sold wastes. Effect of Number of Stacks and Utlzaton Factor Both number of stacks and utlzaton factor have consderable effect on plant performance n term of electrcal effcency. As dscussed n (Rokn, 2013b), ncreasng number of stacks s n favor for plant effcency. The plant cost s also drectly depended on number of stacks and therefore for choosng number of stacks one should also study the economy of the plant, ether n terms of thermoeconomy or technoeconomy. Fgure 6 demonstrates ths effect. For SOFC utlzaton factor of the electrcal effcency ncreases from about 45.1% to about 47.5% for 100 and 4000 number of stacks respectvely. Smlarly, cell voltage ncreases from to Of course, t s not economcal to desgn the plant wth 4000 stacks when power output s only 120 kw, see Fg. 6a. As can be seen n the fgure, plant effcency as well as cell voltage does ncrease sgnfcantly when number of stacks s more than Increasng utlzaton factor to 0.8 show also smlar trend. Plant effcency sharply ncreases from about 47.5% to about 48% when number of stacks s ncreased to about Further ncreasng the number of stacks has small nfluence on plant effcency. It s possble to reach an electrcal effcency of about 48.2% when number of stacks s

11 Thermal Effcency [%] Cell Voltage [V] Thermal Effcency [%] Cell Voltage [V] ICCE 2014 Conference, Istanbul, Turkey, 8 12 June 2014, pp: about Agan, ths would not be economcal. Choosng number of stacks to about 150 or 160 wll be more vable and ths why n the above calculatons 160 stacks was chosen. U F = Effcency Cell voltage Number of Stacks U F = Effcency Cell voltage Number of Stacks a) b) Fgure 6. Effect of number of stacks on plant effcency and cell voltage, for utlzaton factor, U F, of (a) and 0.8 (b). CONCLUSIONS An ntegrated muncpal sold waste gasfcaton combned wth SOFC and Strlng engne for decentralzed CHP plant of 120 kw electrcty power s presented and thermodynamcally studed. Plant electrcal effcency up to 48% and CPH effcency up to 95% s possble to attan, dependng on the plant desgn and MSW composton. It s shown that applyng a methanator after the syngas offers two man advantages, ar compressor workload decreases due to less coolng requrement for SOFC stacks. Secondly, less heat wll be needed to preheat the ncomng fuel to the SOFC offerng hgher heat avalable for bottomng cycle and ncreasng ts electrcty power producton. Despte less fuel cell voltage, plant electrcal effcency ncreases wth such methanator. Dfferent MSW compostons provde dfferent plant effcency whch ranges from 43 to 48% when 7 dfferent MSW compostons are used n the smulatons.19 dfferent MSW compostons are then screened to fnd out a mean composton and study ts effect on plant performance whch shown to be about 45% electrcal effcency. NOMENCLATURE c p specfc heat, J/kgC E Voltage, V F Faradays constant, C/mol g 0 Standard Gbbs free energy, J/mol g f Gbbs free energy, J/mol m Mass flow, kg/s n Molar reacton rate, mol/s n e number of electron P Power, W p Pressure, bar T Operatng temperature, K R Unversal gas constant, J/mol K U F Fuel utlzaton factor x mass fracton Greek Letters dfference effcency Subscrpts act actvaton conc concentraton ohm ohmc rev reversble v voltage Abbrevatons AP Anode pre-heater

12 CHP CP GAP HHV LHV MSW SG SOFC SH DHW Combned heat and power Cathode ar pre-heater Gasfer ar pre-heater Hgher heatng value Lower heatng value Muncpal sold waste Steam generator Sold oxde fuel cell Space heater Domestc hot water REFERENCES [1] Ahrenfeldt J., Henrksen U., Jensen T.K., Gøbel B., Wese L., Kather A. and Egsgaard H Valdaton of a contnuous combned heat and power (CHP) operaton of a two-stage bomass gasfer. Energy and Fuels 20(6): [2] Baggo P., Barater M., For L., Grgante M., Av D. and Tos P Expermental and modelng analyss of a batch gasfcaton/pyrolyss reactor. Energy Converson and Management 50(6); [3] Bang-Møller C. and Rokn M Thermodynamc performance study on bomass gasfcaton, sold oxde fuel cell and mcro gas turbne hybrd systems. Energy Converson and Management 51(12): [4] Bebar L., Stehlık P., Havlen L. and Oral J Analyss of usng gasfcaton and ncneraton for thermal processng of wastes. Appled Thermal Engneerng 25(7); [5] Buah W.K., Cunlffe A.M. and Wllams PT Characterzaton of products from the pyrolyss of muncpal sold waste. Process Safety and Envronmental Protecton 85(B5); [6] Buchnger G., Hnterreter P., Raab T., Gresser S., Lawlor V,Klen K., Kuehn S., Stte W. and Messner D Stablty of mcro tubular SOFCs operated wth synthetc wood gases and wood gas components. IEEE /07: [7] Calse F., Dentce d Accada M., Palombo A. and Vanol L Smulaton and exergy analyss of a hybrd sold oxde fuel cell (SOFC) Gas turbne system. Energy 31: [8] Channwala S.A. and Parkh P.P A unfed correlaton for estmatng HHV of sold, lqud and gaseous fuels. Fuel 81(8); [9] Costamagna P., Selmovc A., Del Borgh M. and Agnew G Electrochemcal model of the ntegrated planar sold oxde fuel cell (IP-SOFC). Chemcal Engneerng 102(1):61 69 [10] Cozzan V., Petarca L. and Tognott L Devolatlzaton and pyrolyss of refuse derved fuels: characterzaton and knetc modellng by a thermogravmetrc and calormetrc approach. Fuel 74(6); [11] Dala K., Batta N., Eswaramoorth I. and Schoenau G.J Gasfcaton of refuse derved fuel n a fxed bed reactor for syngas producton. Waste Management 29(1); [12] Doherty W., Reynolds A. and Kennedy D Computer smulaton of a bomass gasfcaton-sold oxde fuel cell power system usng Aspen Plus. Energy 35(12): [13] EG&G and G Techncal Servces Inc Fuel Cell Handbook, 7th edton, U.S. Departement of Energy, Offce of Fossl Energy, Natonal Energy Technology Laboratory. [14] Galvagno S., Cascaro G., Casu S., Martno M., Mngazzn C., Russo A. and Portofno S Steam gasfcaton of tyre waste, poplar, and refuse-derved fuel: A comparatve analyss. Waste Management 29(2); [15] Ghosh S. and De S Energy Analyss of a cogeneraton plant usng coal gasfcaton and sold oxde fuel cell. Energy 31(2 3), [16] Gordllo G. and Annamala K Adabatc fxed bed gasfcaton of dary bomass wth ar and steam. Fuel (89(2); [17] Guan Y., Luo S., Lu S., Xao B. and Ca L Steam catalytc gasfcaton of muncpal sold waste for producng tar-free fuel gas. Hydrogen Energy 34(23): [18] Hasel Y., Dncer I. and Natere G.F Thermodynamc modelng of a gas turbne cycle combned wth a sold oxde fuel cell. Hydrogen Energy 33(20): [19] Henrksen U., Ahrenfeldt J., Jensen T.K., Gøbel B., Bentzen J.D., Hndsgaul C. and Sørensen L.H The desgn, constructon and operaton of a 75 kw two-stage gasfer. Energy 31(10 11): [20] Hernandez-Atonal F.D., Ryu C., Sharf V.N. and Swthenbank J Combuston of refuse-derved fuel n a fludsed bed. Chemcal Engneerng Scence 62(1-2); [21] Hofmann P.H., Schweger A., Fryda L., Panopoulos K., Hohenwarter U., Bentzen J., Ouweltjes J.P., Ahrenfeldt J., Henrksen U. and Kakaras E Hgh temperature electrolyte supported N-GDC/YSZ/LSM SOFC operaton on two stage Vkng gasfer product gas. Power Sources 173(1): [22] Holtappels P., DeHaart L.G.J., Stmmng, U., Vnke I.C. and Mogensen M Reacton of CO/CO2 gas mxtures on N-YSZ cermet electrode. Appl. Electrochem. 29: [23] Incropera F.P., DeWtt D.P., Bergman TL and Lavne A.S Introducton to Heat Transfer. 5th ed, Wley, ISBN [24] Keegan K.M., Khaleel M., Chck L.A., Recknagle K., Smner S.P. and Debler J Analyss of a planar sold oxde fuel cell based automotve auxlary power unt, SAE Techncal Paper Seres No [25] Matsuzak Y. and Yasuda I Electrochemcal oxdaton of H2 and CO n a H2-H2O-CO-CO2 system at the nterface of a N-YSZ cermet electrode and YSZ electrolyte. Electrochemstry Socety 147(5):

13 [26] Morrs M. and Waldhem L Energy recovery from sold waste fuels usng advanced gasfcaton technology. Waste Management 18(6-8): [27] Pao G., Aono S., Mor S., Deguch S., Fujma Y., Kondoh M. and Yamaguch M Combuston of refuse derved fuel n a fludzed bed. Waste Management 18(6-8); [28] Pao G., Aono S., Kondoh M., Yamazak R. and Mor S Combuston test of refuse derved fuel n a fludzed bed. Waste Management 20(5-6); [29] Proell T., Acherng C., Rauch R. and Hofbauer H Couplng of bomass steam gasfcaton and SOFCgas turbne hybrd system for hghly effcent electrcty generaton. ASME turbo Expo Proceedng;GT : [30] Reader G.T The pseudo Strlng cycle a sutable performance crteron. Proceedng of the 13th ntersocety energy converson engneerng conference. Vol. 3: , San Dego, Calforna, August [31] Rensche E., Achenbach E., Fronng D., Hanes M.R., Hedug W.K., Lokurlu A. and Adran S Clean combned-cycle SOFC power plant cell modelng and process analyss. Power Sources 86(1 2): [32] Rokn M. 2010a. Thermodynamc analyss of an ntegrated sold oxde fuel cell cycle wth a Rankne cycle. Energy Converson and Management 51(12): [33] Rokn M. 2010b. Plant characterstcs of an ntegrated sold oxde fuel cell and a steam cycle. Energy 35: [34] Rokn M Thermodynamc nvestgaton of an ntegrated gasfcaton plant wth sold oxde fuel cell and steam cycles. Green 2: [35] Rokn M. 2013a. Thermodynamc analyss of SOFC (sold oxde fuel cell) Strlng hybrd plants usng alternatve fuels. Energy 61: [36] Rokn M. 2013b. Thermodynamc and thermoeconomc nvestgaton of an ntegrated gasfcaton SOFC and Strlng engne, SDEWES2013 Conference, Dubrovnk, Croata, pp: to [37] Smth J.M., Van Ness H.C. and Abbott M.M Introducton to Chemcal Engneerng Thermodynamcs, 7th edton, Boston:McGraw-Hll. [38] Sanchez D., Chacartegu R., Torres M. and Sanchez T Strlng based fuel cell hybrd systems: an alternatve for molten carbonate fuel cells. Power Sources 192: [39] Zhu H. and Kee R.J A general mathematcal model for analyzng the performance of fuel-cell membrane-electrode assembles. Power Sources 117: [40] Wnnck J Chemcal engneerng thermodynamcs. John Wley &Sons, New Yourk.