Available online at ScienceDirect. Energy Procedia 69 (2015 )

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1 Available nline at ScienceDirect Energy Prcedia 69 (2015 ) Internatinal Cnference n Cncentrating Slar Pwer and Chemical Energy Systems, SlarPACES 2014 Slar-t-fuel energy cnversin analysis f slarised mixed refrming f methane Y. Sun a* and JH. Edwards b a CSIRO Energy Technlgy, PO Bx 330, Newcastle, NSW 2300, Australia b CSIRO Energy Technlgy, PO Bx 136, Nrth Ryde, NSW 2113, Australia Abstract An energetic upgrade factr has been defined as the rati f the lwer heating value (LHV) f the syngas (H 2 plus CO) prduced plus unreacted feedstck t that f the feedstck prcessed. It is used t evaluate the thermal perfrmance f the steam refrming f methane, mixed refrming f methane with steam and carbn dixide, and carbn dixide refrming f methane based n the calculated equilibrium prduct cmpsitins. A nn-stichimetric equilibrium mdel was develped using FactSage 6.3 sftware t cnduct the thermdynamic calculatins fr prductin f syngas. The results shw that increasing temperature r decreasing pressure can enhance the energetic upgrade factr f all three refrming prcesses. The rati f the feedstck cmpnents has an effect n the energetic upgrade factr fr the three refrming prcesses. There is an ptimum rati exisitng that prvides the highest energetic upgrade factr fr each f the three refrming prcesses. Replacement f H 2 O by CO 2 can enhance the energetic upgrade factr, particularly at high temperatures with n carbn frmatin. The analysis als identifies perating regimes where carbn frmatin is thermdynamically pssible as well as discusses the undesirable effect that this carbn frmatin has n the energetic upgrade factr The Authrs. Published by Elsevier Ltd. This is an pen access article under the CC BY-NC-ND license 2015 The Authrs. Published by Elsevier Ltd. ( Peer Peer review review by by the the scientific scientific cnference cnference cmmittee cmmittee f SlarPACES f SlarPACES 2014 under 2014 respnsibility under respnsibility f PSE AG f PSE AG. Keywrds: energetic upgrade factr, steam refrming f methane, mixed steam-carbn dixide refrming f methane, carbn dixide refrming, slar thermal applicatin * Crrespnding authr. Tel.: ; fax: address: Yanping.Sun@csir.au The Authrs. Published by Elsevier Ltd. This is an pen access article under the CC BY-NC-ND license ( Peer review by the scientific cnference cmmittee f SlarPACES 2014 under respnsibility f PSE AG di: /j.egypr

2 Y. Sun and J.H. Edwards / Energy Prcedia 69 ( 2015 ) Intrductin One f the challenges t the widespread applicatin f slar energy is the develpment f cst-effective energy strage technlgies that enable slar energy t be utilized utside daylight hurs in the generatin f electricity and prcess heat fr the energy-intensive industries. Strage f slar energy in chemical bnds is cnsidered t be a prmising methd because f high energy strage density and the ptential fr strage at ambient temperature. Slar-driven highly endthermic reactins such as steam refrming f methane (SRM) (Reactin (1)) r CO 2 refrming f methane (CDRM) (Reactin (2)), can stre slar energy therm-chemically int the syngas [1-3]. Australia has large areas f high inslatin which, in many cases, are c-lcated with significant resurces f natural and cal seam gases, which cntain varying levels f CO 2. Australia therefre is an ideal place t cnduct these tw chemical reactins fr capturing slar energy. When methane is refrmed using bth steam and CO 2 simultaneusly, the prcess is knwn as mixed refrming f methane (MRM). CH 4 + H 2 2 CH 4 + CO 2 2 = ± 206 kj/ml (1) H 25 C = ± 247 kj/ml (2) H 25 C The water-gas shift reactin (WGS, reactin (3)) r its reverse can als ccur in the abve refrming prcesses. CO + H 2 O (g) 2 + CO 2 = /+ 41 kj/ml (3) H 25 C An imprtant cnsideratin when cnducting these reactins n a large-scale is the pssibility f slid carbn frmatin which can ptentially ccur via the fllwing reactins (4), (5) and (6): CH 4 (s) + 2H 2 2CO (s) + CO 2 CO + H 2 (s) + H 2 O (g) = +/ 75 kj/ml (4) H 25 C = /+172 kj/ml (5) H 25 C = /+131 kj/ml (6) H 25 C Carbn frmatin via any f these rutes and its build-up n reactr surfaces and/r catalyst is unacceptable and wuld render the prcess inperable. Refrming typically uses high temperatures and in the cnventinal refrming prcess this heat is generally prvided by the cmbustin f additinal methane. In slar thermal refrming, hwever, the heat wuld be prvided by cncentrated slar energy. Depending n the type f refrming reactin, the methane cnversin and the extent f the reverse f its reactin, it is likely that mre than 30% f the energy embdied in the prduct syngas n a lwer heating value (LHV) basis can be derived frm slar input [4]. This is useful either as a way f string slar thermal energy at ambient temperature (which can subsequently be recvered via the reverse exthermic reactin) r as a syngas prductin methd with lwer CO 2 emissins than cnventinal refrming. SRM has been successfully demnstrated in several slar cncentrating facilities arund the wrld with slar energy inputs ranging frm 25 t 500 kw [5-10]. In 2004, the Australian Cmmnwealth Scientific and Industrial Research Organisatin (CSIRO) built a single-twer helistat field f 500 kw capacity at its Newcastle site and used part f this field t demnstrate the slar SRM in a tubular reactr with 25 kw f slar energy input [11]. A theretical thermdynamic analysis f these chemical reactins is an imprtant first step in determining the ptimum refrming mde and perating cnditins fr the therm-chemical capture f slar energy. It is als imprtant t establish thse perating cnditins where carbn frmatin is thermdynamically pssible s that these can be avided in practice.

3 1830 Y. Sun and J.H. Edwards / Energy Prcedia 69 ( 2015 ) The bjective f this paper is t use the thermdynamic analysis t identify the mst prmising refrming reactin and perating cnditins fr the prductin f slarised syngas with the maximum pssible cnversin efficiency under carbn-free cnditins. Nmenclature A SRM_1.2 (H 2 O/CH 4 =1.2); SRM_2.5 (H 2 O/CH 4 =2.5); SRM_3.5 (H 2 O/CH 4 =3.5) B CDRM_1.2 (CO 2 /CH 4 =1.2); CDRM_2.0 (CO 2 /CH 4 =2.0); CDRM_2.5 (CO 2 /CH 4 =2.5) C MRM_0.8/0.4/1.0 (H 2 O/CO 2 /CH 4 =0.8/0.4/1.0); MRM_1.0/1.0/1.0 (H 2 O/CO 2 /CH 4 =1.0/1.0/1.0); MRM_1.5/1.0/1.0 (H 2 O/CO 2 /CH 4 =1.5/1.0/1.0); MRM_1.0/1.5/1.0 (H 2 O/CO 2 /CH 4 =1.0/1.5/1.0) 2. Methdlgy The cnversin efficiency f slar energy int chemical energy by the slar SMR is represented by the energetic upgrade factr (U c ) mdified based n the literature [12]: U c m syngas LHV m syngas m LHV feedstck unreacted feedstck feedstck LHV feedstck (7) m syngas : mles f syngas prduced LHV syngas : lwer heating value f syngas, kj/ml m feedstck : mles f CH 4 fed t refrmer LHV feedstck : lwer heating value f CH 4, kj/ml m unreacted feedstck : mles f unreacted CH 4 In cmparisn t the literature, the LHV f unreacted feedstck (CH 4 in this case) is included in Equatin (7) because it is still a useful frm f energy. By cntrast, any slid carbn frmed within the reactr, either n the catalyst r reactr wall, culd nt generally be regarded as a useful frm f energy and s has nt been included. U c was evaluated using Equatin (7), where m syngas was calculated based n the equilibrium cmpsitins f the refrming prcesses as determined by minimizing the Gibbs free energy fr a given set f species with a cnsideratin f the pssible reactins which might take place in the system. This was accmplished using FactSage 6.3 sftware and its assciated data base, a package capable f determining the equilibrium cmpsitins f cmplex multi-phase and multi-cmpnent systems [2]. 3. Results and discussin 3.1 Effect f temperature and pressure n the perfrmance f three refrming mdes Fig. 1a shws that U c increases with increasing reactin temperature frm 760 t 850 C at 1 bar fr SRM_1.2, CDRM_1.2 and MRM_0.8/0.4/1.0. This indicates that increasing temperature can enhance U c fr the three refrming prcesses as a result f the increased CH 4 cnversin. At 760 C, CDRM has the lwest U c amng these refrming prcesses due t substantial carbn frmatin as shwn in Fig 1a. As mentined previusly this carbn culd nt generally be used as a fuel due t its depsitin within the refrming reactr, thus wasting CH 4 energy which has a negative impact n U c. Carbn frmatin decreases with increasing temperatures t reach zer when C, thus liminating wasted CH 4, and enhancing U c. CDRM has the highest U c amng the three refrming C, indicating that the replacement f H 2 O by CO 2 in the feedstck can increase U c at high temperatures and at 1 bar. Cnsequently, mre slar energy is captured int syngas at high temperatures in CDRM_1.2 than that in the ther refrming prcesses.

4 Y. Sun and J.H. Edwards / Energy Prcedia 69 ( 2015 ) The reasn fr this is that, fr a given ttal mles f prduct, CDRM results in a greater increase in LHV than SRM due t the increased amunt f CO relative t that f H 2 in prduct gases as illustrated in Table 1, since the LHV f CO (283 kj/mle) is higher than that f H 2 (242 kj/mle). Fig. 1. Energetic upgrade factr and carbn frmatin as a functin f reactin temperature at 1 bar (a), 5 bar (b) and 10 bar (c) fr three refrming prcesses

5 1832 Y. Sun and J.H. Edwards / Energy Prcedia 69 ( 2015 ) Table 1 Amunts f each prduct gas per mle f CH 4 input at 850 C and 1 bar fr the three refrming mdes Feed mlar rati H 2 CO CO 2 CH 4 H 2 O Ttal (mle) (mle) (mle) (mle) (mle) (mle) H 2 O/CH 4 = H 2 O/CO 2 /CH 4 =0.8/0.4/ CO 2 /CH 4 = When pressure increases frm 1 t 5 and t 10 bar, Fig.1b and 1c illustrate that U c increases with increasing temperature and decreases with increasing pressure fr all three mdes f refrming. There is n carbn frmatin fr SRM, but carbn frmatin ccurs fr CDRM under the perating cnditins investigated. Carbn is frmed fr MRM at 5 and 10 bar when T<850 C, but there is n carbn frmatin at 850 C. Carbn frmatin increases with increasing pressure, but decreases with increasing temperature. Fig.1b shws that SRM_1.2 has the highest U c when T<800 C at 5 bar while the value f U c fr MRM_0.8/0.4/1.0 is similar t that f SRM_1.2 at T>800 C and this trend is repeated at 10 bar (Fig.1c). This indicates that partial replacement f H 2 O by CO 2 in the SRM prcess can enhance U c at high temperatures. The CDRM_1.2 has the lwest U c under the cnditins investigated due t severe carbn frmatin, which results in U c being less than 1.0 under certain cnditins. In the latter case, there is n energy benefit by using slar energy t drive the refrming reactin. Hence, the peratiing cnditins fr the slar refrming mde shuld be designed and cntrlled t avid carbn frmatin in the practical prductin f slar syngas. It is desirable, simply frm the pint f view f maximizing U c, t perfrm slar refrming at as high a temperature and lw a pressure as pssible as well as t use CO 2 as a feedstck. Hwever, the temperature is limited t that which can be reliably attained within the receiver with an acceptable perating life f the reactr materials f cnstructin. Als, peratin at 1 bar is nt practical because this wuld unacceptably increase bth the size f the reactr and the syngas cmpressin requirements which increase bth capital and perating csts. On the ther hand, replacement f H 2 O by CO 2 clearly increases the likelihd f carbn frmatin. Hence, there is an ptimum perating cnditin existing fr cnducting slar refrming in any practical applicatin which is discussed in detail in Sectin Effect f the H2O/CH4 rati fr SRM Fig. 2 illustrates the effect f the H 2 O/CH 4 rati n U c in the temperature range f 760 t 850 C and at pressures frm 1 t 10 bar fr the SRM reactin. There is n carbn frmatin under the cnditins investigated. Fig. 2a shws that SRM_2.5 mdes have the highest U c at 1 bar when T<80 00 C, SRM_1.2 has the highest U c fllwed by SRM_2.5 and SRM_3.5 respectively. Althugh the stichimetric rati f H 2 O/CH 4 is 1.0 fr SRM as shwn in Equatin (1), traditinal SRM is perfrmed with H 2 O/CH 4 ratis f at abut 800 C and at abut 30 bar. The high temperature is required t achieve high methane cnversin in this prcess. Excess H 2 O nt nly prmtes higher CH 4 cnversin but als suppresses carbn frmatin that wuld therwise deactivate the catalyst and ultimately blck the reactr. Hwever, the psitive effect f enhancing CH 4 00 C and at 1 bar due t the fact that CH 4 cnversin is almst 100% under these cnditins [13]. On the ther hand, the excess H 2 O can facilitate the water-gas shift reactin which cnverts CO int H 2 and CO 2, resulting in a greater decrease in U c fr the SRM due t the lwer amunt f CO prduced relative t H 2. Hence, the SRM with the lwer H 2 O/CH 4 rati has the higher U c at high temperature and 1 bar. There is als a significant prcess efficiency penalty fr excess water due t the enthalpy required fr bth evapratin and sensible heating f the steam.

6 Y. Sun and J.H. Edwards / Energy Prcedia 69 ( 2015 ) Fig. 2. Energetic upgrade factr as a functin f H 2O/CH 4 rati at (a) 1 bar (b) 5 bar (c) 10 bar and at temperatures frm 760 t 850 C fr SRM When the pressure increases t 5 bar, SRM_3.5 has the highest U c at T 800 C whereas SRM_3.5 has a similar value f U c t that f SRM_2.5 Fig. 2b. At 10 bar, SRM_3.5 has the highest U c in the temperature range investigated as illustrated in Fig. 2c. SRM_1.2 has the lwest U c at these elevated pressures due t lw CH 4 cnversin [13]. It is clear that U c fr the SRM reactin is affected by the H 2 O/CH 4 rati, pressure and temperature.

7 1834 Y. Sun and J.H. Edwards / Energy Prcedia 69 ( 2015 ) Effect f the H2O/CO2/CH4 rati fr MRM Fig. 3 shws U c fr MRM increases with increasing temperature and decreases with increasing pressure in the temperature range 760 t 850 C and at pressures frm 1 t 10 bar. Hwever, the effect f temperature and pressure n carbn frmatin is ppsite t that f U c. Mrever, U c varies as a functin f the H 2 O/CO 2 /CH 4 rati, depending n temperature and pressure At 1 bar (Fig. 3a), U c decreases in the rati f H 2 O/CO 2 /CH 4 in the fllwing rder: 1.0/1.5/1.0>1.0/1.0/1.0>1.5/1.0/1.0 >0.8/0.4/1.0 in the temperature range 800 t 850 C, where there is n carbn frmatin. At 5 bar (Fig. 3b) and 10 bar (Fig. 3c), MRM_0.8/0.4/1.0 has the lwest U c amng the fur MRM mdes due t severe carbn frmatin whereas MRM_1.0/1.5/1.0 has the highest U c in the temperature range 780 t 850 C The carbn frmatin decreases with increase in the rati f H 2 O/CO 2 /CH 4 in the fllwing rder: 0.8/0.4/1.0>1.0/1.0/1.0>1.0/1.5/1.0 under the cnditins investigated. N carbn is frmed in MRM_1.5/1.0/1.0 under the cnditins investigated, which implies that adding extra water is mre efficient than adding extra CO 2 t suppress carbn frmatin in MRM.

8 Y. Sun and J.H. Edwards / Energy Prcedia 69 ( 2015 ) Fig. 3. Energetic upgrade factr and carbn frmatin fr MRM as a functin f temperature and feed gas H 2O/CO 2/CH 4 rati at (a) 1 bar (b) 5 bar (c) 10 bar 3.4 Effect f the CO2/CH4 rati fr CDRM Fig. 4 illustrates that carbn frmatin decreases with increasing bth the CO 2 /CH 4 rati and temperature and with decreasing pressure in the temperature range 760 t 850 C at 1 t 10 bar. At 1 bar (Fig. 4a), CDRM_1.2 has the lwest U c due t carbn frmatin whereas CDRM_2.5 has the highest U c At 5 bar (Fig. 4b), CDRM_2.5 has the highest U c _2.0 has a similar value f U c t that f CDRM_2.5 2 /CH 4 =1.2) has the lwest U c due t carbn frmatin under the cnditins investigated. At 10 bar (Fig. 4c), CDRM_2.5 has the highest U c, fllwed by CDRM_2.0 whereas it is ppsite t that f U c fr carbn frmatin. U c increases with increasing CO 2 /CH 4 ratis fr CDRM frm 1.2 t 2.5. A pssible explanatin is that excess CO 2 culd react with slid carbn and then suppress carbn depsitin thrugh the reverse f the CO disprprtinatin reactin (Reactin (5)). It is unlikely, hwever, that the additin f a large amunt f CO 2 will be a practical methd f enhancing U c and preventing carbn frmatin as the separatin and recycling f CO 2 is an energy-intensive prcess.

9 1836 Y. Sun and J.H. Edwards / Energy Prcedia 69 ( 2015 ) Fig. 4. Energetic upgrade factr and carbn frmatin fr CDRM as a functin f temperature and CO 2/CH 4 rati at (a) 1 bar (b) 5 bar (c) 10 bar 3.5 Cmparisn f the energetic upgrade factrs at 850 C In practice, the maximum perating temperature is likely t be arund 850 C due t heat lss, materials restrictin and engineering issues [4]. Table 2 summarizes the values f U c fr each refrming mde at this temperature fr varius feed gas ratis and pressures. Table 2 Cmparisin f the energetic upgrade factrs fr the three refrming mdes at 850 C CO 2 /CH 4 rati (CDRM) H 2 O/CO 2 /CH 4 rati (MRM) H 2 O/CH 4 rati (SRM) P /0.4/ /1.0/ /1.5/ /1.0/ (bar) Table 2 shws that CDRM_2.5 has the highest U c at 1 bar whereas MRM_1.0/1.5/1.0 has the highest U c at 5 and 10 bar. The pssible reasn is that excess CO 2 culd faciliate CH 4 cnversins and the reverse water gas shift reactin which increases CO frmatin at the expense f H 2. As discussed abve, additin f a large amunt f CO 2 r H 2 O will nt be a practical methd fr enhancing U c because there is an energy penalty assciated with separatin and recycling f CO 2 r heating/cling excess H 2 O. With the need t minimize excess H 2 O and CO 2 in this system, it is likely that the best cmprmise is btained with CDRM_2.0 at 1 bar and with MRM_1.0/1.0/1.0 at 5 and 10 bar, where the values f U c are nly marginally lwer than the highest nes under the perating cnditins investigated here, but where the amunt f excess CO 2 r H 2 O used is reduced.

10 Y. Sun and J.H. Edwards / Energy Prcedia 69 ( 2015 ) Cnclusins U c as determined frm the thermdynamic equilibrium prduct cmpsitins, was investigated fr SRM, MRM and CDRM with different feed ratis in the temperature range 760 t 850 C and at pressures frm 1 t 10 bar. The results are summarized as the fllwing: CDRM_1.2 has the highest U c rati f H 2 O, CO 2 r (H 2 O+CO 2 )/CH 4 fr all three refrming mdes SRM_1.2 has a similar value f U c t that f MRM (H 2 O/CO 2 /CH 4 = 0.8/0.4/1.0) at T>800 C. CDRM_1.2 has the lwest U c due t severe carbn frmatin under these increased pressures. Increasing temperature r decreasing pressure can enhance U c fr the three refrming prcesses, due essentially t the increased equilibrium CH 4 cnversin and decreased carbn frmatin The rati f H 2 O, CO 2 r (H 2 O+CO 2 )/CH 4 has an effect n U c fr the three refrming prcesses, which depends n bth reactin temperature and pressure. Fr example, fr SRM at 850 C, SRM_1.2 has the highest U c at 1 bar, hwever, SRM_3.5 has the highest U c at 10 bar. Replacement f H 2 O by CO 2 can enhance the energetic upgrade factr, particularly at high temperatures and lw pressures where there is n carbn frmatin. At 850 C which is a likely perating temperature in practice, CDRM_2.5 has the highest U c (1.34) amng these refrming prcesses investigated at 1 bar. Hwever, the separatin and recycling f a large amunt f CO 2 is an energy-intensive prcess. Hence, t avid the cst and cmplexity f recvering and recycling surplus CO 2, CDRM must be perfrmed at the lwest pssible feed gas CO 2 /CH 4 rati, preferably near the stichimetric 1.0/1.0 rati. At 5 and 10 bar, MRM_1.0/1.5/1.0 has the highest U c at 1.30 and 1.28 respectivley. Hwever, in rder t minimize the surplus CO 2 in the prduct gas, it is likely that perating in the mde MRM 1.0/1.0/1.0 wuld be preferred ver MRM 1.0/1.5/1.0 despite the small reductin in U c. Acknwledgment This wrk was supprted by funds frm the CSIRO Energy Flagship. References [1] Kdama T. High-temperature slar chemistry fr cnverting slar heat t chemcial fuels. Prg. Energy Cmbust. Sci. 2003; 29: [2] Steinfield A. Slar thermchemical prductin f hydrgen-a review. Slar Energy 2005; 78: [3] Agrafitis C, Strch H, Reb M, and Sattler C. Slar thermal refrming f emthane feedstcks fr hydrgen and syngas prductin-a review. Renew. & Sust. Energy Reviews 2014; 29: [4] Wrner A and Tamme R. CO2 refrming f methane in a slar driven vlumetric receiver-reactr Catal. Tday 1998; 46: [5] Abele M, Bauer H, Buck R, Tamme R, and Wrner A. Design and test results f a receiver-reactr fr slar metahen refrming. ASME. J. Slar Energy Eng. 1996; 118: [6] Anikeev VI, Bbrin AS, Ortner J, Schmidt S, Funken K-H, and Kuzin NA. Catalytic thermchemical reactr/receiver fr slar refrming f natural gas: design and perfrmance. Slar Energy 1998; 63(2): [7] Benit R, Duffy G, D K, McNaughtn R, Edwards J, Dave H, Chensee M, and Walters C. CSIRO s Advance Pwer Generatin Technlgy Using Slar Thermal_Fssil Energy Hybrid System. in the 6th Internatinal Cnference n Greenhuse Gas Cntrl Technlgies, Kyt, Japan [8] Berman A, Karn R, and Epstein M. A New catalyst system fr high-temperature slar refrming f methane. Energy & Fuels 2006; 20: [9] Epstein M, Spiewak I, Segal A, Levy I, Lieberman D, Meri M, and Lerner V. Slar Experiments with a Tubular Refrmer. in 8th Internatinal Sympsium n Slar Thermal Cncentrating Technlgies Kln, Germany. [10] Epstein M and Spiewak I. Design and Operatin f the Weizmann Institute 480 kw Slar Refrmer in an Energy Strage Cycle. in 7th Internatinal Sympsium n Slar Thermal Cncentrating Technlgies Mscw, Russia. [11] Stein W, Imenes A, Hinkley J, Benit R, McEvy S, Hart G, McGregr J, Chensee M, Wng K, and Wng J. A New Slar Thermal Facility in Australia. in 13th Internatinal Sympsium n Cncentrating Slar Pwer and Chemical Energy Technlgies (SlarPACES) Seville, Spain. [12] Piatkwski N, Wieckert C, Weimer A, and Steinfeld A. Slar-driven gasificatin f carbnaceus feedstck-a review. Energy & Envir. Sci. 2011; 4: [13] Sun Y, Ritchie T, Hla S, McEvy S, Stein W, and Edwards J. Thermdynamic analysis f mixed and dry refrming f methane fr slar thermal applicatins. J Natural Gas Chem. 2011; 20: