Thermodynamic analysis of mixed and dry reforming of methane for solar thermal applications

Transcription

1 Thermdynamic analysis f mixed and dry refrming f methane fr slar thermal applicatins Y Sun a*, T Ritchie a, S S Hla b, S. McEvy a and W. Stein a a CSIRO Energy Technlgy, PO Bx 330, Newcastle, NSW 2300, Australia b CSIRO Energy Technlgy, PO Bx 883, Pullenvale, QLD 4069, Australia Crrespnding authr. Tel.: ; fax: address: Yanping.Sun@csir.au Abstract Thermdynamic analysis f the refrming f methane with carbn dixide alne ( dry refrming ) and carbn dixide and steam tgether ( mixed refrming ) was perfrmed as part f a prject investigating the suitability f these endthermic reactins fr the strage f slar thermal energy. The methd f Gibbs Free Energy minimisatin was emplyed t identify thermdynamically ptimal perating cnditins fr dry refrming, as well as mixed refrming with a desired H 2 /CO mlar rati f 2. The nnstichimetric equilibrium mdel was develped using FactSage sftware t cnduct the thermdynamic calculatins fr carbn frmatin, H 2 /CO rati, CH 4 cnversin and H 2 yield as a functin f reactin temperature, pressure, and reactant mlar ratis. Thermdynamic calculatins demnstrate that in the mixed refrming prcess, ptimal perating cnditins are at H 2 O / = 1.0/1.0/0.5, P = 1 t 10 bar and T = 800 t 850 C fr prductin f syngas with a H 2 /CO mlar rati f 2 in a carbn-free zne. Under these cnditins, the maximum CH 4 cnversin f 99.3% is achieved at 1 bar and 850 C while the maximum value f H 2 yield is 88.0%. In the dry refrming prcess, a carbn frmatin regime is always present at a mlar rati f 1 fr T = C and P = 1 30 bar, whereas a carbn-free regime can be btained at a mlar rati greater than 1.5 and fr T 800 C. Keywrds: mixed steam refrming f CH 4 with refrming, refrming, syngas, slar thermal applicatin 1. Intrductin Slar energy is by far the mst abundant frm f clean, renewable and carbn-free energy. Hwever, the intermittent nature f slar energy is a majr impediment t its large-scale arund-the-clck utilizatin in industrial heat and pwer applicatins. One methd f string slar energy is by the use f reverse chemical reactins with large heat f reactin. Tw such reactins are steam refrming f methane (SRM, reactin 1) and carbn dixide refrming f methane (CDRM r dry refrming, reactin 2), bth f which are reversible and highly endthermic, resulting in the frmatin f syngas, a mixture f CO 1

2 and H 2. When methane is refrmed using bth steam and carbn dixide simultaneusly, the prcess is knwn as mixed refrming f methane. CH 4 + H 2 O (g) CO + 3H 2 CH 4 + 2CO + 2H 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 ccur in the mixed refrming and CDRM prcesses. CO + H 2 O (g) H 2 + = 41 kj/ml (3) H 25 C Reactins 1 and 2 take place at high temperatures. In the cnventinal refrming prcess this heat is prvided by the cmbustin f extra methane. In slar thermal refrming, hwever, the heat is btained by cncentrated sunlight. As a result, up t 30% f the energy embdied in the prduct syngas can be derived frm slar input [1]. This is useful either as a way f string slar thermal energy (which can subsequently be recvered via the reverse exthermic reactin) r as a syngas prductin methd with lwer emissins than cnventinal refrming. Steam refrming has been successfully demnstrated in several slar cncentrating facilities using Rh-based catalysts and a pwer f 200 t 400 kw [2,3]. In 2004, the Australian Cmmnwealth Scientific and Research Organizatin (CSIRO) built a singletwer helistat field f 500 kw capacity at its Newcastle site with the bjective f demnstrating a larger scale slar refrming prcess (Figure 1) [4]. Slar resurce at this site typically prvides a direct nrmal incident radiatin peak f W/m 2. The slar steam refrming is cnducted at reactin temperatures f C ver a cmmercial catalyst at 5 10 bar. Figure 1 The CSIRO slar twer facility (left) and receiver assembly (right) 2

3 The SRM prcess prduces syngas with a H 2 /CO mlar rati higher than 3, which is nt suitable fr direct pst-applicatins as such Fischer-Trpsch synthesis. In additin, excess steam is used (a H 2 O mlar rati f ) which reduces the energy efficiency f the refrming prcess due t the evapratin f excess water. These prblems can be vercme using mixed refrming r CDRM instead f SRM. Firstly, mixed refrming and CDRM generate syngas with lwer H 2 /CO mlar ratis than SRM, creating a mre desirable feedstck fr synthesis f liquid fuels. Secndly, slar energy is stred mre efficiently due t less demand fr evapratin f excess water. Thirdly, mixed refrming and CDRM allw cal seam gas, which in sme instances ccurs naturally as a mixture f and CH 4, t be used as a feedstck. In spite f these attractive efficient and envirnmental benefits, there are n cmmercial technlgies available fr mixed refrming and CDRM prcesses. The main challenge is that slid carbn is mre easily frmed in mixed refrming and CDRM than in SRM. Carbn frmatin is a cnsequence f ne r mre f the fllwing reactins: CH 4 C (s) + 2H 2 2CO C (s) + CO + H 2 C (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 in CDRM is mre severe than that f SRM due t the lw H/C atmic rati f this reactin [5]. Nble metals-based catalysts are less sensitive t carbn depsitin [6], but cnsidering the high cst and limited availability f nble metals, it is mre practical frm an industrial standpint t emply Ni-based catalysts with high perfrmance and high resistance t carbn depsitin. Ni-based catalysts shw high activity fr SRM and CDRM, but deactivate due t carbn depsitin. Recently, ur labratry has develped highly active and stable Ni-based catalysts fr mixed refrming and CDRM. In additin, perating cnditins are anther imprtant influence n carbn frmatin, and can be ptimised t achieve carbn-free r minimal carbn frmatin refrming regimes. This paper will present thermdynamic analysis n the ptimisatin f syngas prductin in a slar thermal prcess. Fr the case f mixed refrming, thermdynamically ptimal perating cnditins have been identified fr the prductin f syngas that has a H 2 /CO mlar rati f 2, and invlves carbn-free frmatin. In additin, thermdynamically ptimal perating cnditins are als calculated fr prductin f the syngas frm CDRM. A nn-stichimetric equilibrium mdel has been develped using FactSage sftware fr the thermdynamic analysis. The apprach is capable f handling multiple feed streams and multiphase prducts, which is extremely useful in the refrming prcess where ptential liquid and slid phase prducts can be frmed. 2. Simulatin methdlgy 3

4 The equilibrium cmpsitins f the mixed refrming and CDRM prcesses can be calculated using the Gibbs free energy minimisatin methd [7] where the ttal Gibbs free energy (G) is minimum and its differential is zer at the equilibrium state at given temperature and pressure as described belw [8]. (dg) T,P = 0 (7) Fr a mixture f N species, the ttal Gibbs free energy f a system can be expressed by the sum f the chemical ptential f all cmpnents as belw: N G = n iµ i (8) i= 1 where n i is the mles f species i and µ i the chemical ptential f cmpnent i can be given by Eq. (9) where G f, i fi µ i = G f, i + RTln( ) (9) f i is the standard Gibb s free energy f frmatin f cmpund i, R the mlar gas cnstant, T the system temperature, fi the system fugacity, fugacity; cmbinatin f Eqs (8) and (9) gives Eq. (10) N fi the standard-state fi G = n i G fi + n i RT ln( ) (10) f i= 1 N i= 1 The equilibrium state at specified T and P is determined by minimising the Gibbs free energy fr a given set f species withut any specificatin f the pssible reactins which might take place in the system. Minimisatin was accmplished with the use f FactSage sftware capable f perfrming analysis f single r multiple phases f several cmpnents in equilibrium. The H 2 O mlar rati and/r mlar rati, temperature and reactin pressure were varied. T evaluate the perfrmance f the tw refrming systems, CH 4 cnversin, H 2 yield and the H 2 /CO mlar rati were defined as belw: i F(CH - 4)i F n (CH ut Cnversin (CH 4 ) % = 4 ) 100 F Yield(H )(%) = (2F F (CH 4) in (H 2) ut 2 (CH ) 4) + F in (H2O)in 100 (11) (12) H 2 / CO F (H 2) ut = (13) F (CO) ut 4

5 where F (x) is the mlar flw rate f species x in kml/h, and additinal subscripts dente inlet r utlet flw. 3. Results and discussin 3.1. Mixed refrming T analyse the thermdynamic equilibrium f the mixed refrming prcess, the verall reactin equatin is expressed as: CH 4 + αh 2 O + β prducts (14) The prducts cnsist f H 2, CO, CH 4,, C (graphite), C 2 H 6, H 2 CO, C 2 H 4, H, OH, HCO et al, 38 species in ttal. Hence, there are many elementary reactins ging n within the prcess. In this simulatin, t ptimise perating cnditins t btain carbnfree znes with the H 2 /CO mlar rati f 2, the stichimetric cefficient α was varied frm 0.5 t 1.5, and β was varied frm 0.5 t 1.5 fr reactin temperatures f C and pressures f 1 30 bar. Amng all the prducts, H 2, CO and C (graphite) are f mst interest in the mixed refrming prcess. The frmatin f slid carbn is a majr cncern in mixed refrming because it can deactivate the catalyst and lwer the perfrmance f the refrming system. It is imprtant t predict carbn-frming cnditins s that they can be avided. It has been reprted [9] that carbn is frmed as graphite in the refrming prcess and s slid carbn is regarded as graphite phase in these thermdynamic calculatins. Carbn frmatin is significantly affected by perating temperature and the mlar ratis f H 2 O and. Figure 2a shws that carbn frmatin decreases with increasing temperature. In principle, reactins 4 6 can all cntribute t carbn frmatin, but in the SRM and CDRM prcesses, carbn frmatin mainly cmes frm reactin 5 [9]. Because the reactin is exthermic and hence favured by lw temperature, increasing temperatures result in the reductin in carbn frmatin. Furthermre, carbn frmatin decreases with increasing ratis f H 2 O and because excess and water can prmte the reverse reactins 5 and 6 respectively, inhibiting carbn frmatin. Hence, adding H 2 O r t the mixed refrming prcess can als effectively prevent slid carbn frmatin at temperatures greater than 700 C. The cnclusins agree with the wrk f Assabumrung et al. [10]. It can be cncluded that carbn-free znes can be achieved at T > 700 C in the range f the ratis f bth and H 2 O frm 1.0 t 1.5 at 1 bar. CH 4 cnversin strngly depends n the mlar ratis f bth and H 2 O as well as perating temperature. Figure 2b illustrates that CH 4 cnversin increases with perating temperatures frm 700 t 1000 C at 1 bar fr ratis f and H 2 O f Furthermre, CH 4 cnversin rises with increasing mlar ratis f as T < 850 C. Hwever, the psitive effect is insignificant fr T 850 C because CH 4 cnversin reaches almst 100%. In additin, CH 4 cnversin increases with increasing mlar ratis f H 2 O frm 0.5 t 1.5 at T 750 C because excess and/r H 2 O can react with unreacted CH 4 at high temperatures, resulting in enhanced CH 4 cnversin. Hwever, CH 4 cnversin decreases with increasing mlar ratis f H 2 O frm 0.5 t 5

6 C_graphite (ml/ml f CH 4 )(%) 1.0 at the rati f < 1.0 as T < 750 C. A pssible reasn is that carbn is frmed under the peratin cnditins (shwn in Figure 1a) and thus additin f a small amunt f H 2 O favurs the reverse reactin (6), inhibiting the SRM t sme degree. H 2 yield is als affected by the mlar ratis f bth and H 2 O as well as perating temperature, but their influences n H 2 yield are mre cmplicated than they are n CH 4 cnversin. Figure 2c illustrates that H 2 yield is prprtinal t reactin temperatures at P=1 bar under bth and H 2 O mlar ratis f 0.5, which are the stichimetric ratis fr mixed refrming. H 2 yield reaches the maximum value f 98.4% at 1000 C. When the mlar ratis f and H 2 O are greater than 0.5 a maximum H 2 yield appears ver the temperature range C. Its actual value is different under different perating cnditins. Fr example, the maximum H 2 yield is 79.9% at 850 C with H 2 O = 1.0 and CH 4 / = 1.0 while it is 73.0% at 800 C with H 2 O = 1.5 and CH 4 / 1.0 respectively. The higher the tw ratis, the lwer the temperature required fr achieving a maximum H 2 yield. Hwever, it shuld be remembered that the influence f the rati f H 2 O n H 2 yield depends n the definitin f H 2 yield. Based n the definitin f Eq.12, ne mlecule f H 2 O culd prduce ne mlecule f H 2. When the rati f H 2 O is greater than 0.5, the stichimetric rati fr the mixed refrming, excess H 2 O culd make little cntributin t prductin f H 2, but it is still taken int accunt in the denminatr. Hence, amunts f H 2 O greater than the stichimetric requirement culd lead t a decrease in H 2 yield. H 2 O = 0.5 (tp), 1.0 (middle), 1.5 (bttm) H 2 O = 0.5 (bttm), 1.0 (middle), 1.5 (tp) CH 4 Cnversin (%) Temperature ( C ) (a) (b) 6

7 YH 2 (%) H 2 /CO H 2 O = 0.5 (tp), 1.0 (middle), 1.5 (bttm) H 2 O = 0.5 (bttm), 1.0 (middle), 1.5 (tp) (c) (d) Figure 2 (a) Carbn frmatin, (b) CH 4 cnversin, (c) H 2 yield and (d) H 2 /CO rati, all as functins f and H 2 O ratis at P = 1 bar. Syngas with a H 2 /CO mlar rati f 2 is a desired feedstck fr the Fischer-Trpsch and methanl syntheses. The rati f H 2 /CO heavily depends n the mlar ratis f and H 2 O as well as perating temperature. Figure 2d shws that the H 2 /CO rati decreases significantly with increasing temperature fr H 2 O and ratis f because the WGS reactin is exthermic, favuring lw temperatures. Increasing temperatures inhibits the WGS reactin and simultaneusly prmtes the reverse WGS reactin. Hence, a decreased H 2 /CO rati with increasing temperature is bserved which agrees with experimental results reprted previusly [11]. Figure 2d als shws that the H 2 /CO rati decreases with increasing rati. The pssible reasns are that additin f culd prmte three reactins namely: CDRM (reactin 2), the reverse WGS reactin (reactin 3) and reactin 5. In the CDRM prcess, cnsumptin f ne mle f culd prduce tw mles f CO and tw mles f H 2, which des nt affect the H 2 /CO rati; in the reverse reactin 3, the reactin f ne mle with ne mle H 2, prduces ne mle f CO, leading t the decrease in the H 2 /CO rati; in the reverse reactin 5, cnsumptin f ne mle f prduces tw mles f CO, thus resulting in a decreased rati. Hence, regardless f any kinetic effects, additin f leads t a decrease in the mlar rati f H 2 /CO. Varying the H 2 O rati als has an impact n the H 2 /CO rati, and the degree f its influence depends n bth temperature and rati. At T 750 C and P = 1 bar, the H 2 /CO rati increases with increasing H 2 O rati (in the range f the ratis f frm ) because high temperature favurs the SRM, resulting in the increase in the H 2 /CO rati. In cntrast, at T < 750 C, the H 2 /CO rati decreases with increasing H 2 O mlar rati at < 1.0) as the H 2 O mlar rati is less than 1.0. A pssible explanatin is that carbn is frmed under these peratin cnditins (shwn in Figure 1a) and thus additin f a small amunt f H 2 O favurs reactin (6) which inhibits the SRM reactin and reduces the H 2 /CO rati. 7

8 Figure 3a illustrates that at a fixed rati f CH 4 /H 2 O/ = 1.0/1.0/0.5, carbn frmatin increases with increasing pressure at T 700 C because increasing pressure prmtes the frward reactin 5 which is cnsidered t be a main cntributr t carbn frmatin in bth SRM and CDRM reactins. Fr example, carbn cncentratin increases frm 0 t 0.19 mles per CH 4 mle as the pressure increases frm 1 t 30 bar at 700 C. Hwever, the effect f pressure n carbn frmatin culd be negligible at temperatures greater than 850 C because reactin 5 is exthermic and thus favured by lw temperature. High temperature inhibits the reactin, leading t the decrease in carbn frmatin. Based n the thermdynamic calculatin, it is cncluded that the carbn-free regime can be btained at the rati f H 2 O 1 and the rati f 0.5 at T 850 C and P 30 bar in the mixed refrming prcess. Nte that slid carbn that wuld actually depsit n the catalyst may have a different frm with lw Gibbs free energy f frmatin rather than graphite n which these thermdynamic calculatins were based. In the present study, slid carbn is represented by graphite, as in mst equilibrium studies, but any nn-ideality may significantly influence the carbn frmatin bundary. Figure 3b shws the effect f pressure and temperature n CH 4 cnversin at the CH 4 /H 2 O/ mlar rati f 1.0/1.0/0.5. It can be seen that CH 4 cnversin declines with increasing pressure because bth SRM and CDRM reactins cause an increase in vlume. Fr example, CH 4 cnversin decreases frm 99.3% t 54.1% at 850 C when the pressure increases frm 1 t 30 bar. The effect f temperature n CH 4 cnversin is related t pressure. At the lw pressure f 1 bar, CH 4 cnversin increases with temperature frm 700 t 850 C. At T > 850 C, almst 100% CH 4 cnversin is achieved and thus the temperature effect is insignificant. As P 5 bar, CH 4 cnversin increases with increasing temperature in the temperature range 700 t 1000 C. Figure 3c shws that H 2 yield decreases with increasing pressure, as des CH 4 cnversin. Fr instance, H 2 yield decreases frm 88.0% t 47.7% at 850 C when the pressure increases frm 1 t 30 bar. The effect f temperature n H 2 yield als depends n pressure. At 1 bar, H 2 yield attains the maximum value f 88.0% at 850 C, but at P 5 bar, H 2 yield increases with increasing temperature in the range 700 t 1000 C. Figure 3d illustrates that at the fixed CH 4 /H 2 O/ mlar rati f 1.0/1.0/0.5, the H 2 /CO rati increases with increasing pressure frm 1 t 30 bar at T 800 C whereas the rati appraches 2.0 at T = 850 C. Beynd this pint, the effect f pressure n the H 2 /CO rati becmes insignificant despite the fact that the rati slightly decreases with increasing temperature. 8

9 0.20 P = 1 bar C_graphite (ml/ml f CH 4 ) P = 5 bar P = 10 bar P = 30 bar 0.00 (a) 100 CH 4 Cnversin (%) P = 1 bar P = 5 bar 20 P = 10 bar P = 30 bar 0 (b) 9

10 H 2 Yield (%) P = 1 bar 20 P = 5 bar P = 10 bar P = 30 bar 0 (c) 4 P = 1 bar P = 5 bar Mlar rati f H 2 /CO 3 2 P = 10 bar P = 30 bar 1 (d) Figure 3 (a) Carbn frmatin, (b) CH 4 cnversin, (c) H 2 yield and (d) H 2 /CO rati, all as a functin f temperature and pressure at the CH 4 /H 2 O/ rati f 1.0/1.0/0.5 In summary, carbn frmatin is a majr cncern in the mixed refrming prcess. Hence, ptimum perating cnditins wuld be in a carbn-free zne fr slar refrming applicatins. Furthermre, a H 2 /CO rati f 2 can be achieved by adjusting the 10

11 CH 4 /H 2 O/ mlar rati, temperature and pressure f the prcess which is cnsistent with the results f Hegarty et al.[12]. Optimal perating cnditins are at H 2 O / = 1/1/0.5, P = 1 t 10 bar and T = 800 t 850 C fr prductin f syngas with the H 2 /CO rati f 2 in a carbn-free zne. Under these cnditins, a maximum CH 4 cnversin f 99.3% and a maximum H 2 yield f 88.0% are achieved at 1 bar and 850 C CDRM The general reactin mechanism fr the thermdynamic equilibrium f the CDRM prcess can be expressed as: CH 4 + α prducts (15) The same prduct species and elementary reactins assumed fr mixed refrming als applies t CDRM. In this simulatin, the stichimetric cefficient f α fr was varied frm 1.0 t 2.0 while the reactin temperatures were varied in the range frm 700 t 1000 C and pressure in the range f 1 t 30 bar. Amng all the prducts, H 2, CO and C (graphite) are f mst interest in the CDRM prcess. Carbn frmatin As mentined earlier, carbn depsitin is a majr barrier fr cmmercial applicatin f the CDRM in chemical industry because slid carbn depsits n the surface f catalyst, leading t catalyst deactivatin. Hence, it is imprtant t ptimise perating cnditins t avid r minimise carbn frmatin within the CDRM prcess. Carbn frmatin is affected by many parameters such as temperature, pressure, catalyst, and the mlar rati. Figure 4a shws that carbn frmatin at 1 bar decreases with bth increasing temperature frm 700 t 1000 C and increasing mlar rati frm 1.0 t 2.0. It has been reprted that carbn frmatin mainly riginates frm CO disprprtinatin (reactin 5) in the CDRM prcess [9]. Increasing temperature can decrease carbn frmatin as this reactin is exthermic. Furthermre, increasing the mlar rati can prmte the reverse reactin 5, thus inhibiting carbn frmatin. Figure 4a als illustrates that a carbn frmatin zne is always present at a rati f 1.0 in the temperature range 700 t 1000 C, whereas carbn-free cnditins can be btained at a rati greater than 1.5, at 1 bar and T 800 C. 11

12 0.6 C_graphite (ml/ml f CH 4 ) = 1.0 = 1.5 = (a) 1.0 P = 1 bar C_graphite (ml/ml f CH 4 ) P = 5 bar P = 10 bar P = 30 bar 0.0 (b) Figure 4 Carbn frmatin as a functin f (a) mlar rati and temperature at 1 bar, and (b) temperature and pressure at a rati f 1.0 Figure 4b shws that carbn cncentratin increases with increasing pressure frm 1 t 30 bar and decreases as temperature increases frm 700 t 1000 C at the mlar rati f = 1.0. N carbn-free regin appears at the rati f = 1.0 and at P 1 bar in the temperature range 700 t 1000 C. Hwever, carbn-free znes are btained at the rati 1.5, high temperature and lw pressure as described abve. It is unlikely, hwever, that the additin f a large amunt f will be a practical methd f preventing carbn frmatin as the separatin and recycling f is an energyintensive prcess. Hence, in rder t realise cmmercial applicatin f the CDRM 12

13 prcess, there is a need t develp nvel CDRM catalysts which can incrprate a kinetic inhibitin f carbn frmatin under cnditins where carbn frmatin is thermdynamically favurable. CH 4 cnversin CH 4 cnversin is a functin f the mlar rati, reactin temperature and pressure. Figure 5a illustrates that CH 4 cnversin at 1 bar increases with the mlar rati in the temperature range 700 t 1000 C because excess can react with unreacted CH 4, resulting in enhanced CH 4 cnversin. In additin, CH 4 cnversin increases with increasing temperature. The psitive effects are bvius at lw ratis and lw temperatures, but they becme insignificant at temperatures ver 850 C fr ratis higher than 1.5 because CH 4 cnversin almst attains 100% CH 4 Cnversin (%) = = 1.5 = (a) 13

14 100 CH 4 Cnversin (%) P = 1 bar P = 5 bar P = 10 bar P = 30 bar 60 (b) Figure 5. CH 4 cnversin as a functin f (a) the / CH 4 mlar rati and temperature (P = 1 bar), and (b) temperature and pressure ( = 1.0). As T 800 C, it is difficult t btain CH 4 cnversin greater than 99% except at ratis greater than 2. Hwever, this circumstance is nt practical in reality because excess can reduce H 2 yield and increase energy cnsumptin required fr separatin and recycling f. Figure 5b demnstrates that CH 4 cnversin is a functin f pressure and temperature at the mlar rati f 1.0. It can be seen that CH 4 cnversin decreases with increasing pressure in the temperature range 700 t 1000 C. Fr instance, CH 4 cnversin decreases frm 90.7% t 68.7% at 800 C when the pressure increases frm 1 t 30 bar, while CH 4 cnversin increases frm 90.9% t 99.1% with increasing temperature frm 700 t 1000 C at 1 bar. It is bvius that high CH 4 cnversin can be achieved at high temperature and lw pressure and at the rati f 1.0. The calculatin results are cnsistent with the experimental findings [13], where Sng et al. studied the effect f pressure (frm 1 t 53 atm at 750 C) and f temperature (frm 200 t 800 C at 27 atm) fr the CDRM reactin ver an 8wt.% Ni/Na-Y catalyst. Hence, increasing reactin temperature can increase catalyst activity whereas increasing pressure has an adverse effect. H 2 Yield Thermdynamic calculatin has indicated that H 2 yield is a functin f mlar rati, perating temperature and pressure. Figure 6a shws that H 2 yield decreases as the 14

15 rati is increased at 1 bar and T 750 C, whereas this influence is insignificant at 700 C. A pssible reasn is that the CDRM and reverse WGS reactins are endthermic, favuring high temperature. Hence, at lw temperatures, increasing the cncentratin des nt have a significant impact n H 2 yield. Hwever, at high temperatures, these reactin rates and CH 4 cnversins are enhanced and s additin f has a greater influence n H 2 yield than that at lw temperatures. Temperature als affects H 2 yield, but its influence is related t the mlar rati. At the rati f 1.0 and 1 bar, H 2 yield increases frm 75.9% t 98.5% when temperature increases frm 700 C t 1000 C. At this rati 1.5, H 2 yield increases with temperature at lw temperatures whereas it decreases with increasing temperature at high temperatures. The maximum H 2 yield appears at 800 C with 88% at the rati f 1.5 and with 80% at the rati f 2.0 respectively. The pssible reasn is that extra prmtes the reverse reactin 3, leading t the decrease in H 2 yield. Figure 6b shws that H 2 yield decreases with increasing pressures at the mlar rati f = 1 in the temperature range C. Fr example, it reduces frm 91% t 36% at 800 C when pressure increases frm 1 bar t at 30 bar. The effect f reactin temperature n H 2 yield varies with pressure. At 1 bar, H 2 yield increases rapidly with temperature frm 700 t 900 C. At temperatures greater than 900 C, the increasing trend slws dwn. It is pssible that H 2 yield attains almst 100% under the high temperature cnditins. Hence, further increasing temperatures have little effect n H 2 yield. Hwever, H 2 yield increases with increasing temperature almst linearly when pressure is greater than 5 bar, which is similar t the trend f CH 4 cnversin = 1.0 = 1.5 H 2 Yield (%) = (a) 15

16 H 2 Yield (%) P = 1 bar 20 P = 5 bar P = 10 bar 0 (b) P = 30 bar Figure 6: H 2 yield as a functin f (a) mlar rati and temperature (P = 1 bar), and (b) temperature and pressure ( = 1.0) H 2 /CO mlar rati Figure 7a shws that the H 2 /CO rati significantly decreases bth as temperature increases and as the rati f increases frm 1.0 t 2.0. A pssible explanatin is that the reverse WGS reactin is endthermic and thus favured by high temperatures, which leads t the lw mlar rati f H 2 /CO at high temperatures. Increasing the rati als prmtes the reverse WGS reactin, leading t the increased CO cncentratin and the decreased H 2 cncentratin. Hence, increasing bth temperature and the rati f, results in the decrease in a H 2 /CO rati. Figure 7b illustrates that at the fixed mlar rati f 1.0, the H 2 /CO rati increases with increasing pressure frm 1 t 30 bar at T 850 C whereas the effect f pressure n the rati becmes insignificant at T 900 C. 16

17 = 1.0 = 1.5 H 2 /CO Rati = (a) 2.5 P = 1 bar 2.0 P = 5 bar P = 10 bar H 2 /CO Rati 1.5 P = 30 bar 1.0 (b) Figure 7. The H 2 /CO rati as a functin f (a) mlar rati and temperature (P = 1 bar), and (b) temperature and pressure ( = 1.0) 4. Cnclusins Thermdynamic analysis f the mixed refrming (SRM plus CDRM) and CDRM alne was perfrmed by Gibbs Free Energy minimizatin using FactSage sftware t identify 17

18 ptimal perating cnditins fr slar thermal applicatin. Carbn frmatin, the H 2 /CO rati, H 2 yield and CH 4 cnversin under equilibrium cnditins were calculated as a functin f the mlar ratis f H 2 O and, perating temperature as well as pressure. Thermdynamic calculatins demnstrate: Fr mixed refrming, ptimal perating cnditins fr prductin f syngas with an H 2 /CO rati f 2 in a carbn-free zne are H 2 O / = 1/1/0.5, P = 1 10 bar and T = C. Under these cnditins, the maximum CH 4 cnversin and f 99.3% and maximum H 2 yield f 88.0% are achieved at 1 bar and 850 C respectively. In the CDRM prcess, carbn-frmatin regimes are present at a rati f 1 in the temperature range f C and the pressure range f 1 30 bar. Carbn-free regimes can be btained at ratis ver 1.5, a pressure f 1 bar and T 800 C. Hwever, using excess t inhibit carbn frmatin is unlikely in reality as separatin and recycling f is an energy-intensive prcess. In additin, excess prmtes the reverse WGS reactin, reducing H 2 yield. It is imperative t develp nvel CDRM catalysts which can incrprate a kinetic inhibitin f carbn frmatin fr slar thermal applicatins. 5. References [1] Wrner, A.; Tamme, R. Catal. Tday 1998, 46, 165. [2] Abele, M.; Bauer, H.; Buck, R.; Tamme, R.; Wrner, A. J. Slar Energy Eng. 1996, 118, 339. [3] Anikeev, V. I.; Bbrin, A. S.; Ortner, J.; Schmidt, S.; Funken, K.-H.; Kuzin, N. A. Slar Energy 1998, 63, 97. [4] Stein, W.; Imenes, A.; Hinkley, J.; Benit, R.; McEvy, S.; Hart, G.; McGregr, J.; Chensee, M.; Wng, K.; Wng, J. B., R. 13th Internatinal Sympsium n Cncentrating Slar Pwer and Chemical Energy Technlgies (SlarPACES), Seville, Spain, June 20-23, [5] Edwards, J. H.; Maitra, A. M. Fuel Prc. Tech. 1995, 42, 269. [6] Rstrup-Nielsen, J. R.; Bak-Hansen, J.-H. J. Catal. 1993, 144, 38. [7] Perry, R. H.; Green, D. W.; Malney, J. O. Perry's Chemical Engineers' Hand bk; McGraw-Hill: New Yrk, [8] Smith, J. M.; Van Ness, H. C.; Abbtt, M. M. Intrductin t chemcial engineering thermdynamics McGraw-Hill Cmpanies: Nee Yrk, [9] Bradfrd, M. C. J.; Vannice, M. A. Catal. Rev.-Sci. Eng. 1999, 41, 1. [10] Assabumrungrat, S.; Lasiripjana, N.; Pirnlerkgul, P. J. Pwer Surces 2006, 159, [11] Jiang, H.; Li, H.; Xu, H.; Zhang, Y. Fuel Pr. Tech. 2007, 88. [12] Hegarty, M. E. S.; O'Cnnr, A. M.; Rss, J. R. H. Catal. Tday 1998, 42, 225. [13] Sng, C. Catal. Tday 2006, 115, 2. 18