a review on solar reforming systems

7/25/2019 A Review on Solar Reforming Systems

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A review on solar reforming systemsSyed A.M. Said a ,n , Mohammed Waseeuddin a , David S.A. Simakov ba Department of Mechanical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Kingdom of Saudi Arabiab Department of Chemical Engineering, University of Waterloo, Canada

a r t i c l e i n f o

Article history:Received 15 January 2015Received in revised form

31 August 2015Accepted 16 December 2015

Keywords:Methane reformingHydrogenSolar energySyngas

a b s t r a c t

Direct combustion of fossils is a low ef ciency process that leads to excessive carbon dioxide emissions.Highly ef cient and clean combustion processes can be achieved by the use of hydrogen as fuel.Hydrogen can be produced by steam reforming of methane which is a highly endothermic process. Heatrequired for such endothermic process can be achieved by the use of solar energy. The literature indicatesthat solar reforming is restricted to high temperature generating solar technologies like solar towers anddishes due to the high heat demand of the reforming process. A novel idea of using solar parabolictroughs in conjunction with a membrane reformer has been developed which allows low temperatureoperation of the process due to the equilibrium shift effect. Molten salt heated in the solar parabolictrough facility (up to 600 C) provides heat needed for the endothermic reforming reactions. In thisreview, we provide critical assessment of the latest published developments in solar reforming tech-nologies. We focus on reactor design concepts employed to couple the heat requirements of methanereforming process with concentrated solar power. The emphasis is novel, alternative routes which havepotential of commercialization.

& 2015 Elsevier Ltd. All rights reserved.

Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. H2 production via reforming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.1. Reforming methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Steam reforming of methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Dry reforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.1.3. Partial oxidation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4. Autothermal reforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2. Catalysts for reforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3. Recent numerical work in steam reforming of methane. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

3. Reforming using solar energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Indirectly heated reformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2. Tubular reformer receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Windowed/Volumetric reformer receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.3.1. ASTERIX experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.3.2. Directly irradiated annular pressurized receiver (DIAPR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1533.3.3. Thermochemical receiver/reactor system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.3.4. CAESER experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.3.5. The Weizmann Institute reactor/receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.3.6. Sodium re ux reformer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.7. Dry reforming using FeO catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4. Potential for solar parabolic troughs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Numerical and experimental works on membrane reformers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1574.2. Coupling membrane reformers to solar parabolic troughs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

Contents lists available at ScienceDirect

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Renewable and Sustainable Energy Reviews

http://dx.doi.org/10.1016/j.rser.2015.12.0721364-0321/ & 2015 Elsevier Ltd. All rights reserved.

n Corresponding author. Tel.: 966 13 8603123; fax: 966 13 8602949.E-mail address: [email protected] (S.A.M. Said).

Renewable and Sustainable Energy Reviews 59 (2016) 149 159
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5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction

The energy sector worldwide is largely dominated by theburning of fossil fuels which leads to the emissions of CO 2 into theatmosphere and is known to contribute to the undesired globalwarming. It is generally accepted that the increase in the tem-perature of the earth is linked to the increase in greenhouse gases,mostly CO 2 , in the atmosphere. CO 2 is a major greenhouse gasreleased whenever fuels containing hydrocarbon, such asmethane, are burned [1]. Most of the world's energy needs arecurrently met by combustion of hydrocarbon fuels. The use of renewable energy is an alternative which has potential to providea favorable solution to the problem of greenhouse gases emissions.But the current cost of technologies based on renewable energy iscurrently competitive enough to fossil fuels based technologiesand more advancements are still required.

Fossil fuels are heavily used in power industry as it is relativelycheap and abundant. Various current estimates place the worldpeak for fossil fuel consumption between 2009 and 2035 [2],implying that there is at least as much oil still available in theworld as has been consumed since the industrial revolution. Thediscovery of methane hydrates in vast quantities indicates thatfossil fuel will continue to be used for decades to come. Unfortu-nately combustion of these fuels produces pollutants and CO 2 , themain contributor to global warming. One of the main challengesfacing engineers now is how to reduce or prevent the negativeimpact of CO 2 emissions due to burning of fossil fuels on the cli-mate, while allowing the continued use of fossil fuels for powerproduction.

Carbon capture and sequestration (CCS) means capturing CO 2and trapping it away from the atmosphere. There are manymethods of sequestering CO 2 either by utilization in industrialprocesses or by storage underground or in deep seas. Enhanced OilRecovery (EOR) is another option which involves injecting CO 2 intopartially depleted oil wells to increase oil production. This is cur-rently common practice in many oil companies. CO 2 is also utilizedin Enhanced Coal Bed Methane Recovery (ECBM) for harvestingmethane from unmineable coal by injecting CO 2 into the coal bed.The neutralization of alkali pollutants can be done by using CO 2 inpolluted areas. Yet another method is to absorb CO 2 into coal bedsthat are inaccessible and cannot be mined. Some minerals willreact with CO 2 to form a solid product. There are two methods of doing this; the rst is mining the reactant mineral and the secondis reacting it with CO 2 to produce a product which can be used in

road pavement, or injecting CO 2 into subterranean caverns con-taining the reactant mineral, and allowing it to slowly react overtime. Other methods of CO 2 storage include injection into somegeological formations, particularly porous rock.

Many methods have been proposed for capturing CO 2 producedby combustion of fossil fuels for production of electricity. The fuelmay be burned in air, and the exhaust products separated tocapture CO 2 . This usually involves either cryogenic or chemicalprocesses, or use of membranes to separate gases. These methodsare costly and consume a large amount of energy, thus resulting inreducing the power plant ef ciency. Since it is now necessary tolook for a clean energy source, the available choices are hydrogenand green solutions. The carbon is removed from hydrocarbon fuelbefore combustion, by conversion of methane to syngas, a mixture

of carbon monoxide (CO) and hydrogen (H 2 ), followed by

separation of H 2 from the mixture. This process is called reform-ing, being the oldest, cheapest and most widely used method toproduce hydrogen commercially worldwide [3]. It is possible toproduce pure H 2 by this method, which is an energy rich fuelsource. Hydrocarbons usually used for hydrogen production arenatural gas, liquid gas, naphtha, coal and methane among whichmethane is highly used due to its high hydrogen content and lowcapital costs as compared to other hydrocarbons.

Methane reforming is highly endothermic that requires largeheat inputs. In industrial plants, reformer tubes packed with cat-alyst pellets are heated (typically to temperatures of 850 950 C)by burning a fraction of natural gas (ca. 30%). An attractive alter-native is to use concentrated solar energy to provide the heatrequired to drive the reaction, thus saving the otherwise com-busted fraction of the natural gas. The resulting solar fuel , whichcontains solar energy stored in chemical bonds, can be used foreither electricity generation in gas turbines or as a chemicalfeedstock. There are a number of recent reviews on solar ther-mochemical conversion which discuss thermodynamics andkinetics [79] , catalysis [79] , system design [80] and various ther-mochemical conversion routes [81] . Herein, we speci cally focuson natural gas reforming and provide critical assessment of thelatest developments in solar thermal reforming technologies. Theemphasis is on solar reformer design concepts and on alternativedesigns which can eventually allow widespread commercializationof solar thermochemical conversion technologies.

2. H 2 production via reforming

2.1. Reforming methods

2.1.1. Steam reforming of methaneSince steam reforming of methane produces syngas with the

highest hydrogen to CO ratio, it is considered to obtain highly purehydrogen. The overall process of steam reforming of methane canbe described by the following reactions (steam reforming, watergas shift and overall reaction):

CH4 H2 O CO 3H2 1

CO H2 O CO2 H2 2

CH4 2H2 O CO2 4H2 3

The overall process is highly endothermic which requires largeamounts of heat for the process to take place. In a conventionalplant, the required heat is typically provided by combusting afraction of natural gas (up to 30%) to re the reformer tubespacked with catalyst from outside, while another fraction is fed tothe reformer tubes. The CO produced by the SMR reaction isreacted with steam in downstream reactors to produce morehydrogen by water gas shift (WGS).

Being an endothermic process, steam reforming requires veryhigh temperatures and makes the process expensive. However,alternative processes of methane reforming are being studied togain economic viability according to the requirement of syngasproduced. The concern with the economic viability lead to thedevelopment of alternative processes of methane reforming,

which are dry reforming, partial oxidation and autothermal

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reforming which are being studied by researchers for producingsyngas [4 6].

2.1.2. Dry reforming Dry reforming is a process which produces syngas; with

hydrogen to CO ratio of 1. In this process, methane is reacted withcarbon dioxide in the presence of a catalyst, to produce syngas[4,7] . The dry reforming of methane reaction is given as:

CH4 CO2 2CO 2H2 4

This reforming process is considered when the produced syn-gas is used for the material synthesis of fuel liquids and requiressyngas as raw materials. Coke, which is deposited on the surface of the catalyst, reduces the activity of the reaction and is consideredas a disadvantage of dry reforming as it reduces its useful life. Cokeformation is due to the presence of CO 2 as a reagent in the reac-tion. Thus, dry reforming is a unique process of methane reform-ing that is affected by two reagents that contain carbon (CH 4 andCO2 ) [4,8,9]. There is likely more chances of coke formation in thisprocess due to the low H/C ratio when compared to steamreforming [8] . Low coke formation rate on the catalyst surface isone of the main challenges for dry reforming for which activecatalysts need to be developed. Use of supports which favor thedissociation reaction of CO 2 into CO and O can reduce the cokeformation to some extent [9] .

2.1.3. Partial oxidationAnother method for production of syngas is the partial oxida-

tion of methane in which methane reacts with oxygen in thepresence of a catalyst. The resultant syngas obtained by this pro-cess is of good hydrogen to CO ratio [10] . The reaction for partialoxidation of methane is expressed as:

CH4 12

O2 CO 2H2 5

Since partial oxidation of methane is an exothermic processand requires a lower amount of thermal energy, it is considered tobe economical compared to other reforming processes. Fromanother point of view, the process is considered expensive as itrequires a ow of pure oxygen for combustion of methane. Speci ccare must be taken during the process since the two reagentsinvolved (CH 4 and O 2 ) are highly explosive and may prove to bedangerous [11] .

2.1.4. Autothermal reforming Steam reforming and partial oxidation methods were com-

bined to give a new process to produce syngas namely Auto-thermal reforming. Thus, in the steam reforming there is contactwith a ow of gaseous oxygen, in the presence of a catalyst [5]. Thethree reagents for autothermal reforming of methane are CH 4 , H2 Oand O 2 . The thermal energy required for the steam methane

reforming process is generated by partial oxidation and thus savesa signi cant amount of energy. Since the produced syngas con-sumes the thermal energy it generates, it is named autothermal[12] . Hydrogen to CO ratio in the syngas depends on the gaseousreactant fractions introduced in the process. A value of H 2 /CO ratioranges between 1 and 2 [13] .

Overall, methane reforming is an important process in theenergy sector for the conversion of natural gas into highly purehydrogen. The ultimate purpose of methane reforming is to obtainhigh-purity gaseous hydrogen; however, the type of method usedin the conversion process of methane in syngas in uences on H 2 /CO ratio obtained. According to the authors of [5,6,14] main type of reforming processes of methane is the steam reforming process,because it generates syngas with the highest H 2 /CO ratio. The

product obtained from the reforming process is a fuel which is

considered ideal for the development of catalytic processes toobtain high-purity gaseous hydrogen and to be used as feedstockin the petrochemical industries to produce liquid fuels andmethanol. However, alternative methods for natural gas reformingwere developed with the aim of making savings in thermal energyconsumption; steam reforming method is found to be an ef cientone for bulk hydrogen production. Thus, the choice of the mostappropriate catalytic chemical process of methane reforming

should account for the economic viability of the process withregard to the purpose given to the syngas produced. In otherwords, it can be said in short that the choice of the chemicalprocess of reforming to be used in the conversion of methane insyngas should be made based on the nal application that will begiven to syngas obtained. Partial oxidation, dry reforming andautothermal reforming can also serve as options to produce syn-gas, nevertheless depending on the value of H 2 /CO ratio to beobtained, especially when it comes to reduce the consumption of thermal energy, one of the most important factors in the imple-mentation of an industrial project.

The cost of syngas produced by means of the steam reformingof methane process is directly dependent on natural gas prices andis found to be one of the least expensive methods among all bulkhydrogen production technologies. It is a common fact that naturalgas is readily available around the world and thus hydrogen gen-eration by means of steam reforming becomes very attractive.Steam reforming of methane is widely used in industry todaywherein hydrogen is produced in large centralized plants for usein numerous applications, including chemical manufacturing andpetroleum re ning. Research and development is currently con-cerned with the development of small-scale technologies forsteam reforming of methane to enable distribution of hydrogenand improve delivery infrastructure [15] . Regardless of the type of reforming process employed in syngas production, it is importantto emphasize that the success of the conversion of methane isstrongly dependent on the catalytic system used in the process, aswell as on operational conditions such as reaction temperature. Inthis case one should always have in mind that the catalytic systemis the material that activates the conversion reaction of methane.One of the appropriate reforming process which produces hydro-gen on a large scale using a nickel based catalyst is steamreforming. Nickel being easily available and relatively cheaper thanother metal catalysts, attracts researchers towards the study anddevelopment of steam reforming.

2.2. Catalysts for reforming

Steam reforming uses nickel based catalysts as the activematerial. However, cobalt and other noble metals also catalyze thesteam reforming reaction but are generally too expensive to ndwidespread use. Ruthenium and Rhodium have higher activity permetal area than nickel but are usually not considered due to theirhigh costs being rare earth metals. A number of different carriersincluding alumina, magnesium aluminum spinel, and zirconia areemployed. Some of the problems faced by SMR process are lowmethane conversion due to reversibility, coke formation, catalystdeactivation, heat transfer issues and diffusion limitations [16 18].Being catalytically promoted, the process is sensitive to the cata-lyst used not only chemically but also due to the physical phe-nomena taking place. Thus, complete insight of the phenomena isnecessary to run the process ef ciently and economically.

As mentioned, the most commonly used catalyst for steammethane reforming (SMR) is nickel. Ni is used unsupported as wellas supported on certain supports such as SiO 2 , Al2 O3 , Al2 O4, ZrO2 ,Ce ZrO2 [19 23] . Low conversion, deactivation and coke formationare issues that makes certain catalysts unsuitable for the SMR

process since in real life, reformers operate far away from ideal

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conditions. Usually operating pressure are set very high asrequired in synthesis loop but at the same time, reduce conver-sion. The presence of higher hydrocarbons and sulfur contentalong with high pressure make the environment severe for thecatalyst. Ni supported on certain supports such as Al 2 O4 , Al2 O3 andSiO2 were reported to be deactivated severely due to oxidation of Ni [24] . It was found that NiAl 2 O3 [25] catalyst deactivatedseverely even when operated under different reaction systems to

check the dependence of heat transfer mechanism [26] . Also,pretreatment of the catalyst with other metals like Rh was foundto give higher activity also at elevated temperatures after under-going certain oxidation reduction reactions for Al 2 O3 [23] .

A comparative study of Ni supported over Ni/Ce ZrO2 , Ni/ZrO2 ,Ni/CeO2 , Ni/MgAl2 O4 and Ni/Al 2 O3 [27] indicated that Ni over Ce ZrO2 catalyst have higher activity due to high interaction betweenNi and Ce ZrO2 supports and also due to high oxygen carryingcapacity. In another study, Ni/Ce ZrO2 Al2 O3 catalyst was testedagainst the steam treatment effects and it was found that catalystdeactivated in steam due to formation of inactive Ni Al2 O4reduced to Ni and Al 2 O3 in presence of H 2 . Same effect wasreported to be the reason for the deactivation of Ni Al2 O3 catalystsbut this deactivation was low due to the coverage provided byZrO

2. Thus, the catalyst was found to be highly active by using low

steam concentrations in the reactor feed initially so that the pro-cess proceeds in the forward direction and some hydrogen isproduced [24] .

Since steam methane reforming occurs on the surface of thecatalyst (surface reactions), increasing the amount of active metalwill increase the surface area for species to absorb resulting inhigher reaction rates. However, the loading of active metal islimited due heat and mass transfer effects. Ni loading on reducedNiAl2 O3 catalyst was optimized to 12% within a range of 6 15%loading and also it was reported that below 3% loading of Ni,inactive Ni Al 2 O4 is formed. Also, reduction of the catalyst makes itliable for higher conversion and thus conversion of 80% wasreported at atmospheric pressure, temperature of 1023 K and S/Cof 1 [28] .

Sintering is another problem associated with the steamreforming catalyst. It is the loss of surface area of the active speciesof the catalyst due to migration and coalescence of nickel particleson the carrier surface and is a complex process in uenced byseveral parameters including chemical environment, catalyststructure and composition [29 33] . Factors like high temperatureand high steam partial pressure enhance sintering. Also, steamreforming catalysts are susceptible to sulfur poisoning. At steamreforming conditions, all sulfur compounds are converted intohydrogen sul de, which is chemically absorbed on the nickelsurface, enhancing the phenomena. It is reported that for Nisupported over MgAl 2 O4 catalyst, sintering occurs in the initial200 h of the reaction, after which, change in the size of Ni particlesis small. The size of the nickel particles is limited after sinteringand is related to the support surface area and Ni loading yieldingto sintering being proportional to nickel loading [34] . The sinteringprocess is more affected by temperature than time and at highertemperatures, particle migration is dominant whereas atomicmigration dominates at low temperatures [35] . The operatingconditions in a reformer depend on the type of feedstock and theapplication. The typical inlet temperature is between 350 C and550 C. The selection of the operating conditions is in many casesdictated by the limits of carbon formation on the catalyst. For agiven feedstock and pressure, the reformer must be operatedwithin a certain temperature window. The formation of a whiskertype of carbon will occur above the upper temperature limitwhereas operation below the lower temperature limit may resulteither in a polymeric type of carbon formation (gum) or lack of

suf cient catalyst activity.

2.3. Recent numerical work in steam reforming of methane

There has been an extensive work on the numerical analysis of the steam reforming in order to study and improve the process.Seo et al. [36] numerically investigated a reforming system con-sisting of a combustion zone and Steam Methane Reforming (SMR)and Water Gas Shift (WGS) bed. The numerical results from theanalysis were found to be in good agreement with the experi-

mental values. Concentration of the species at the exit, tempera-ture in the WGS bed, heat transfer to the catalyst bed and thesteam methane reforming reactions were studied depending onthe operating parameters. Also, analysis was made on the outerwall of the system by providing a cooling ux, effect of whichreduced the methane conversion and carbon monoxide yield inthe SMR bed. Increasing steam to methane ratio showed increasein the methane conversion along with a decrease in the carbondioxide concentration. Zhai et al. [37] developed a simulationmodel for the steam methane reforming reaction in a microreactor consisting of a combustion channel and a reaction channel.The SMR reaction was Rh-catalyzed whereas the combustion wasPt-catalyzed. The CFD model built included the elementary reac-tion kinetics for the SMR reactions and global reaction kinetics wasapplied for combustion. Heat conduction ability of the wall wasgiven importance in the study which affects the interplay betweenthe endothermic and exothermic reactions occurring simulta-neously in the reactor. A characteristic value of 0.5 mm was sug-gested by the authors for the thickness of the walls. A simulationstudy on the Zone Flow Reactor was done by Wilde and Froment[38] . Zone Flow is a tubular reactor having two types of internals: acore type and a casing type which is adjacent to the wall, having athin layer of catalyst over them. A very complex reactor withcorrugations, blades and cones was studied in order to improvethe heat transfer. Higher catalyst ef ciency per unit reactorvolume was achieved due to the higher geometric surface area.Also, a judicious design for the relative position of the internalsand their geometrical characteristics and higher catalyst effec-tiveness, higher energy ef ciency and lower steam to carbon ratioswas aimed in the study. Irani et al. [39] did numerical analysis of the steam methane reforming reactions in a monolith reactorusing surface based and volume based reaction models. In thesurface based approach, the reaction takes place near the wallwhere the catalyst is coated whereas reaction of chemical speciesthrough penetration inside a thin catalyst layer occurs in a volu-metric approach. The numerical models developed utilized theLangmuire Hinshelwood type of kinetic reaction rates for the SMR and Water Gas Shift reactions. The modeling results of the surfacebased reactions were found to be more accurate with the experi-mental data. However volumetric reaction models are of goodchoice as long as they predict the experimental data closely.Pantoleontos et al. [40] studied the steam reforming of methane ina packed bed reactor considering the dynamic behavior of theprocess. A set of partial differential equations were developed andvalidated against experimental results for the physico-chemicalprocess. The physico-chemical process taking place in the solidand gaseous phase were set to the operating conditions of anactual reformer with high temperatures and pressures. The heat atthe reactor wall was optimized for hydrogen yield. The dynamicconversion, temperature and partial pressures pro le at the bedand particle level were well demonstrated by the use of 2-phasereactor concept and optimized wall temperature pro le.

Butcher et al. [41] presented a study on a bench-scale annularmicrochannel reactor (AMR) prototype for the endothermic steamreforming of methane. The experimental results at a steam tomethane ratio of 3, reactor temperature of 1023 K, and pressure of 11 bar showed catalyst power densities of 1380 W per cm 3 and

hydrogen yields greater than 98% of thermodynamic equilibrium.



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Also, a two-dimensional steady-state computational uid dynamicmodel was developed and validated using experimental data toidentify suitable operating conditions for mass production AMR.Thermal ef ciencies, de ned based upon methane and producthydrogen higher heating values (HHV's), of 72.7% and 57.7% wereobtained from simulations for methane capacities of 0.5-2S LPM athydrogen yields corresponding to 99 75% of equilibrium values.Further, the analysis of local composition, temperature and pres-

sure indicated that catalyst deactivation via coke formation ornickel oxidation was not thermodynamically favorable. Mondalet al. [42] worked on the economic and environmental analysis of thermocatalytic decomposition of methane (TCDM) a processthat decomposes methane thermally to produce hydrogen fromnatural gas. Although, TCDM is not yet commercial, it may prove tobe competitive with the more popular SMR process due to itslower carbon footprint. The low temperature (600 C) operation of the process (TCDM) could lead to a signi cant energy and methanegas saving when compared to the conventional steam reforming.Angeli et al. [43] studied the effect of presence of higher alkanes inthe natural gas feedstock on carbon accumulation during lowtemperature steam reforming of methane. The feedstock wasinvestigated over Ni and Rh catalysts supported on lanthanumdoped ceria zirconia mixed oxide. Both catalysts showed highresistance to coke formation and especially in the case of Rh/La/CeO2 ZrO2 , the carbon accumulation detected was low even after10 h on stream in steam reforming of all mixtures of hydrocarbonstested. The presence of higher alkanes in methane increased theamount of carbon on Ni compared to pure methane as well as thenature of the carbonaceous species. Increase in the C-number of the additive alkane had almost no in uence on the total amount of carbon formed but favored the formation of lamentous carbon.

3. Reforming using solar energy

Research in the last two decades has demonstrated the ef cientuse of solar thermal energy for driving chemical reforming reac-tions [44 53] . In these highly endothermic reactions, hydro-carbons are reacted with steam or carbon dioxide (CO 2 ) over acatalyst to form a synthesis gas (syngas) composed primarily of hydrogen (H 2 ) and carbon monoxide (CO). The solar heat isapplied to the reactor either indirectly through a working uid(such as air heated in a solar receiver) or directly via reactor tubesor a porous catalytic reactor exposed to concentrated solar radia-tion. In open-loop systems, a hydrocarbon feedstock (e.g., naturalgas, pyrolized or gasi ed coal or oil shale, or low-quality hydro-carbon gases or waste) is upgraded in energy content with solarenergy. In closed-loop systems, a high-quality hydrocarbon feed-stock such as methane (CH

4) is converted to syngas via solar

reforming; the syngas is then stored or transported off-site prior toconversion back to CH 4 in a methanation reactor that recovers thesolar energy as heat or industrial processes for power generation.

The high temperatures required for solar reforming effectivelylimit the concentrator choices to solar dishes and central receivers.The dish technology is modular and is well suited to distributedapplications such as the destruction of toxic wastes. On the otherhand, bulk energy production, whether in closed-loop or open-loop con guration, probably must be carried out on a large scale tocompete with fossil fuels and probably requires the tower (centralreceiver) technology. In recent years, several basic solar reformerconcepts have been investigated. These reformers can be classi edas the indirectly heated reformer, the tubular reformer receiver,

and the windowed or volumetric reformer

receiver [54] .

3.1. Indirectly heated reformer

The indirectly heated reformer consists of a tube bundle con-taining catalyst within the tubes through which the process gas iscirculated, and heated by a secondary uid that gets its thermalenergy from a solar receiver [45] . Heating agents that have beenconsidered are air, helium and condensing sodium vapor. Theindirectly heated steam reformer has potential advantages of uti-

lizing commercially proven tubes and catalyst, and can be equip-ped with thermal storage or auxiliary fossil ring to give extendedor 24-h operation. This mode of operation is desirable to reducecapital costs and provide a uniform product. The process pressurecan be optimized independent of the solar receiver pressure. Onthe other hand, the indirectly heated system has more equipment,and the secondary uid introduces additional pumping and tem-perature losses.

3.2. Tubular reformer receiver

The tubular reformer/receiver incorporates the catalyst-bearingtubes directly into the solar furnace where they are heated by solarradiation [51] . While this concept eliminates the costs and energylosses associated with the secondary heat transport loop, a largerand more costly solar receiver is required. A limited amount of heat storage is associated with the receiver, suf cient to damp theeffect of solar transients. Auxiliary fossil-fuel or electrical heatingcan be used to extend the heating time, and there is freedom inselecting the optimal process pressure.

3.3. Windowed/Volumetric reformer receiver

The windowed or volumetric reformer/receiver places thereforming catalyst in a position where it is heated directly by thesolar beam, making very high volumetric reaction rates possible[46] . As a result, the receiver is quite compact and potentiallyinexpensive. However, this technology requires good matching of

ow rate with solar ux and also the development of reliablewindows (which may limit operating pressure) and does not lenditself to energy storage or non-solar operation. Nevertheless,prospects for low capital costs and a good match to dish con-centrators make this concept attractive. The individual receivercells are limited in capacity by the window area, but large modulararrays are feasible for solar towers.

3.3.1. ASTERIX experiment ASTERIX [44 ,45] experiment for solar steam reforming of

methane was conducted in the early 90's for investigating thedetails and problems associated with the process heat demand foran industrial chemical process with solar-generated high tem-perature heat, using an indirectly heated reformer. The speci cobjectives of the ASTERIX experiment were to collect and store anoptimum amount of solar energy, to obtain maximum conversionof methane and to produce consistently high-quality synthesis gas.A Gas-Cooled Solar Tower (GAST) system [45] was used to producehot air and drive a separate steam reformer which was again fedinto the GAST cycle. Results of the test were found to be moretemperature dependent in comparison with pressure achieving93% methane conversion at 803 C and 6 bar pressure. Fig. 1 showsthe schematic of the reformer reactor used in ASTERIX study.

3.3.2. Directly irradiated annular pressurized receiver (DIAPR)A DIAPR system which uses steam reforming of methane with a

Ru/Al 2 O3 catalyst promoted by Mn oxides was studied by Bermanet al. [46] . A directly irradiated annular pressurized receiverreactor ( Fig. 2) was used as the receiver reactor which was used in

conjunction with a eld of heliostats. The absorber of the receiver



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consisted of pins arranged in the shape of a porcupine made of sintered alumina (99.7% Al 2 O3 ). The catalyst activity and stabilitywas tested for methane conversion at different reforming tem-peratures ranging from 500 1100 C using fresh catalyst and cat-

alyst that has been used for 506 h at 1100

C. It was found that the

activity of the catalyst does not change signi cantly even afterprolonged use which suggested that the catalyst was thermallystable. The same tests when conducted with the same catalyst butwithout Mn oxides showed that the activity of the catalystdecreases drastically at higher temperatures.

3.3.3. Thermochemical receiver/reactor systemA receiver reactor paired with a solar furnace with heliostats to

collect the solar radiation and re ect it onto the parabolic re ectorwas studied by De Maria et al. [47,48] . The catalyst used in thisstudy was 10% Ni deposited on Al 2 O3 pellets. The reformer wastested for different steam to methane ratios (1 3) and tempera-tures (680 750 C) at atmospheric pressure. It was found that atthe highest temperature studied (750 C), methane conversionwas within 20% of the equilibrium and the heating value of theproduct gases was approximately 20% higher than that of the inputmethane. The experimental setup of this study is shown in Fig. 3.

3.3.4. CAESER experiment The CAtalytically Enhanced Solar Absorption Receiver (CAESAR)[49 ,50] test was conducted to determine the thermal, chemical,and mechanical performance of a commercial-scale, dish moun-ted, direct catalytic absorption receiver (DCAR) reactor over arange of steady state and transient (cloud) operating conditions.The focus of the test was to demonstrate proof-of-concept anddetermine global performance such as reactor ef ciencies andoverall methane conversion. The objectives of the project were todemonstrate the solar DCAR concept using a commercial-scalereceiver/reactor on a parabolic dish, and to develop numericalsimulation models capable of predicting the global performance of the receiver/reactor unit and the thermal, chemical and mechan-ical performance of the absorber. The focus, therefore, was onobtaining global and absorber performance data over a range of steady-state and transient operating conditions (e.g., cloud tran-sients) and comparing these results with model predictions. Thesystem was operated during both steady-state and solar transient(cloud passage) conditions with total solar power absorbed valueof 97 kW and maximum CH 4 conversion of 70%. Receiver thermalef ciencies ranged up to 85% and chemical ef ciencies peaked at54%. Global model predictions such as reactor ef ciencies and CH 4conversion compared well with test data. For example, modelpredictions of 71.9%, 48.2%, and 46.5% for thermal ef ciency, che-mical ef ciency, and CH 4 conversion, respectively, for one of theCAESAR tests, compared favorably with the corresponding testvalues of 79.3%, 50.7%, and 45.9%. Fig. 4 shows a pictorial of the

CAESER unit.

Fig. 1. Cross-section of ASTERIX [45] heat exchange reformer.

Fig. 2. Schematic of DIAPR solar facility [46] .

Fig. 3. Experimental set up of DeMaria et al. [47] .



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3.3.5. The Weizmann Institute reactor/receiver A 480 kW testing facility of reformers was developed at

Weizmann Institute of Science [51] . A solar receiver with storageand transportation, for development of high temperature tech-nology was designed for both carbon dioxide (CO 2 ) and steamreforming. The test facility (shown in Fig. 5) operated between 1 to18 bar pressure, coupled with a methanator system that recoversthe heat energy from the reverse reaction. The cavity receiver

contained eight vertical reformer tubes (2-in. schedule 80), 4.5 mlong (active length). The overall dimensions of the device wereabout 5 m high, 4.5 m wide and 3 m deep. The reactor wasdesigned to produce syngas at 800 C which resembled commer-cial reformers except that a solar cavity receiver had replaced theconventional gas-fueled radiant furnace.

3.3.6. Sodium re ux reformer Another chemical reactor was integrated into a sodium re ux

heat pipe receiver for carbon dioxide reforming of methane andtested in the solar furnace of the Weizmann Institute of Science[52] . The receiver/reactor was a heat pipe containing seven tubesinside an evacuated metal box containing sodium as shown inFig. 6. Two of the tubes were lled with catalyst, 0.5 wt% Rh on

alumina and the front surface of the box served as the solar

absorber. In operation, concentrated sunlight heated the frontplate and vaporized sodium from a wire mesh wick attached to theother side of the plate. Sodium vapor condensed on the reactortubes, releasing latent heat and returning to the wick by gravity.Adequate performance of the receiver system was observed duringmany tests under varying ow conditions. The maximum powerabsorbed was 7.5 kW at temperatures above 800 C. The feasibilityof operating a heat pipe receiver/reactor under solar conditionswas proven, and the advantages of re ux devices were con rmedin this study.

3.3.7. Dry reforming using FeO catalyst To convert solar energy into chemical energy, an infrared fur-

nace was designed and tested with FeO powder as catalyst for thedry reforming of methane [53] . The FeO powder was suspended ina molten salt solution wherein the reactants were own andobserved at different ow rates. It was observed that the productconsisted of CO, H 2 and H 2 O with a ratio of 3:1:1 respectively at a

ow rate of 200 ml/min and that of CO and H 2 with a 2:1 ratio at aowrate of 50 ml/min. Theoretical dry reforming of methane was

attained at a owrate of 50 ml/min due to the contact ef ciencybetween the catalyst (FeO powder) and CH 4 /CO2 mixed gases. Thisexperiment of methane reforming reaction with CO 2 alsodemonstrated the use of FeO as catalyst in the molten salt mediumfor converting solar into chemical energy. The experimental setup

of this study is shown in Fig. 7.

Fig. 4. The CAESER Unit [50] .

Fig. 5. The Weizmann Institute 480 kW reformer/receiver [51] .

Fig. 6. Sodium heat pipe reformer [52] .

Fig. 7. Dry reforming using FeO as catalyst setup [53] .



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Table 1 summarizes the solar reforming experiments con-ducted by various researchers in the past few years and showsparticularly the type of solar facility and catalysts used intheir study.

4. Potential for solar parabolic troughs

Economic viability of the process equipment should also beconsidered while applying the solar facilities like solar towers anddishes for performing solar reforming. Also there is a higherprobability of heat losses to the environment when working withhigher temperatures, thus making the process inef cient. Inaddition, capital cost investments should be made on thermalinsulations which restrict the heat losses from the solar facility(towers and dishes) to the reactor. Another mature and relativelyinexpensive solar technology is the solar parabolic trough, but thetemperatures obtained (max. 600 C) are not suf cient for thereforming process and thus solar reforming using solar parabolictroughs technology is not investigated in the past. Although solarparabolic troughs do not generate the temperatures required forreforming (850 950 C), this technology can be applied for solarreforming processes by developing a reformer which operates atlower temperatures than those required for traditional, indust-rially practiced reforming ( Fig. 8).

Among various technologies related to production, separationand puri cation of H 2 , membrane reactor technology seems to bepromising and membrane separation assisted reforming is nowa-days increasingly considered as a good candidate for substitutingconventional systems [58] . The speci c thermodynamic constrainslimiting traditional reactors can be circumvented by using inno-vative integrated systems, such as the membrane reactors (MRs),systems in which both reaction and separation are carried out inthe same device. Basically, membranes are barriers which allowthe ow of a speci c component in the gas stream to the otherside. The permeated component is known as permeate while theretained mixture is the retentate. High selectivity, high ux, lowcost and high mechanical and chemical stability are some of thecharacteristics which a separation membrane should possess.

With the use of perm-selective membranes, it is possible toperform steam reforming of methane reactions at lower tem-peratures than the conventional ones. This is done by removingthe hydrogen formed from the products stream, shifting theequilibrium towards the product side and promoting the reactionkinetics according to Le Chatelier's principle. The removal of hydrogen is done by use of membranes which are highly selective

to hydrogen within the vicinity of the reactor. Dense metalmembranes like Pd and its alloys are currently the most suitabledue to remarkably high hydrogen selectivity. Recent developmentsin Pd and Pd alloy membranes [59 ,60 ] allow the use of thesemembranes to a temperature range of 300 700 C. The Pd-basedmembranes which characterize the high hydrogen selectivity fol-low a solution diffusion mechanism. The hydrogen ux across themembrane is governed by Sivert's Law [61] according to which the

ux across the membrane is proportional to the difference of thesquare root of hydrogen partial pressures at the two sides of themembrane. Sivert's law is given by:

J H 2 AH 2 exp E H 2R g T p0 : 5H 2 p0 : 5H 2 ; M 12

where, J H 2 is the hydrogen ux, AH 2 is the pre-exponential factor,P H 2 and P H,M are the partial pressures of hydrogen at the feed andpermeate side.

Table 1Comparison of different solar reforming systems.

Author Solar facility Catalyst Temperature ( o C) Conversion

ASTERIX[44 ,45]

Solar tower N/A 702 803 91

CAESER [49 ,50] Solar dish Rh Al2 O3 750 1100 66TCRR [47 ,48] Solar furnace Ni-Cr 650 850 60Kodama et al.

[55]Xe Arc lamp Ru/ Al2 O3 730 1030 73

Levy et al. [56] Solar furnace Rh Al2 O3 700 900 67Gokon et al.

[53]Infraredfurnace

FeO 900 950 70

Dahl et al. [57] Solar furnace No catalyst 1770 1860 77

Fig. 8. Simpli ed scheme of the reformer heated by solar parabolic troughs.



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4.1. Numerical and experimental works on membrane reformers

Uemiya et al. [62] compared the SMR performance of a sup-ported Pd membrane and a porous Vycor glass membrane in amembrane reactor in a temperature range of 623 773 K. While thePd membrane ef ciently exceeded the equilibrium conversion,only a small change in equilibrium was observed for porous Vycorglass. Simakov and Sheintuch [63] provided experimental

demonstration of the autothermal Pd membrane reformer andstudied the effects of various parameters on the reformer perfor-mance. It was shown that the reactor performance is stronglyaffected by the ratio of the feed rate to the maximal H 2 separationrate (membrane area and permeability). Chibane and Brahim [64]studied a one dimensional model of a Pd membrane reactor forSMR, evaluating the reactor performance in terms of CH 4 conver-sion, H 2 /CO ratio and H 2 recovery. The reactor performance wasfound to be optimal (nearly complete conversion and high H 2recovery) for the temperature range of 580 600 C, steam-to-methane ratio of 3, pressure range of 300 600 kPa and a sweepgas ratio of 3. In another simulation study conducted by Fernandesand Soares [65] , it was shown that the membrane thickness is animportant parameter in the membrane reactor performance andthat the use of ultrathin membranes ( m scale) can signi cantlyenhance methane conversions. Simakov and Sheintuch [66]de ned a one dimensional non-isothermal model of the auto-thermal Pd membrane reformer and studied systematically theeffects of various design and operation parameters on the reformerperformance. The simulations mapped the acceptable domain of operation and the optimal set of operating parameters. A novelhybrid plant for the production of a mixture of methane andhydrogen (17 vol%) from a steam-reforming reactor whose heatduty is supplied by a concentrating solar power (CSP) plant bymeans of a molten salt stream was demonstrated by De Falco et al.[67] . The results showed that a CSP plant with an active area of about 16,000 m 2 coupled with a tube-and-shell reactor, with fourreformers is able to supply the electricity and enriched methane toabout 2930 domestic users.

Numerical simulations of membrane using nite-volumeapproach are very few. In the numerical work done by Marinet al. [68] , a hydrogen perm-selective reactor was modeled andstudied in 2D. The catalyst considered for the study was Ru/SiO 2which is favorable for steam methane reforming at low tempera-tures and steam to methane ratios whereas the membrane forhydrogen permeation was that of palladium. Analysis was made onthe inlet temperature, space velocities, pressures and sweep gasrate. The inlet temperature and space velocity showed signi canteffect on methane conversion among the operating variablesconsidered. At a space velocity of 8900 h 1 , hydrogen permeationrate was found to be maximum, however to attain a completeconversion of methane, space velocity of 6000 h 1 was suggestedby the authors.

Coroneo et al. [61] carried out numerical and experimentalinvestigation of mass transfer performance of Pd Ag membranemodules for hydrogen separation. In the analysis, impact of con-centration polarization and non-ideal ow on design of membraneseparation units was evaluated by using computational uiddynamic. In the analysis, the source and sink terms are introducedon either side of the membrane boundary cell in order to accountfor the permeation of hydrogen in the numerical study developed.The study covered different module con guration for H 2 and N 2mixture and compared with the experimental results which werefound to be in good agreement. The analysis extended to the use of baf es which permits the production of compartments inside themodule, avoiding mixing and increase permeation driving force.The results show the separation performances of the module

increases with the number of baf es. In all modules, studied the

module with baf e resulted in maximum permeates of H 2 . ShigeoGoto [70] studied the effect of hydrogen permeation rate throughcomposite palladium membrane for tubular con guration. A pal-ladium lm was coated on composite membrane tube. The ana-lysis was done for increasing temperatures for two differentmodes. In the rst mode, CP mode, H 2 was passed through thetube and pressure was reduced in shell side. Therefore, H 2permeates rst ceramic tube and then through the palladium lm.

In the second mode, PC mode, H 2 was passed from shell side andpressure inside the membrane tube was reduced and H 2 perme-ates rst through palladium lm then composite tube. Thenumerical models were based on the combined resistances of bothpalladium lm and composite support. From the results it showedthat with increase of temperature, permeation rate increasesunder CP mode. A CFD modeling of inorganic membrane modulesfor gas mixture separation was carried out by Coroneo et al. [69] .First, they derived the permeation coef cient, based on thetransport mechanisms for the gas molecules across the membrane.The basics of molecular diffusion, Knudsen diffusion and viscous(Poiseulle) ow, and used in the source term formulation in thecontinuity and species transport equations. Of all the mechanisms,the molecular diffusion was dominant on H 2 permeation at highpressures. The mole fraction and permeation rate of hydrogenincreases with increase of working pressure and a good agreementwith the experimental results was found. Staudacher et al. [71]conducted simulative analysis of mass transfer effects in gas andvapor permeation modules. The study relates the effect of con-centration polarization and ow distribution with spacersbetween two membrane surfaces. Calculations of permeation as afunction of temperature, pressure and composition were doneusing the nite-volume method. The use of baf es in the modelwhich separates membrane into a number of compartmentshelped in velocity control of feed. The ow through the membranewas implemented as a sink in the basic transport equation for each

nite cell. A user-de ned function (UDF) was implemented for thetransport from the membrane using a function for ux propor-tional to partial pressures. Similar to the work Staudacher et al.[71] , it was found from the results that the driving force obtainedwas due to concentration polarization and the use of spacerswhich affects the performance of membrane. Iullianelli et al. [72]studied a dense Pd Ag membrane reactor packed with a Ni-basedcatalyst for the methane steam reforming reaction to be carriedout between reforming temperatures of 400 and 500 C andrelatively low pressure (1.0 3.0 bar) with the aim of obtaininghigher methane conversion and hydrogen yield than a xed bednon-membrane reactor, operating at the same conditions. A 50%methane conversion was experimentally achieved in the mem-brane reactor at 450 C and 3.0 bar whereas, at the same condi-tions, the xed bed non-membrane reactor reached a 6% methaneconversion. Moreover, 70% of high-purity hydrogen on totalhydrogen produced was collected with the sweep-gas in thepermeate stream of the membrane reactor. A membrane reactorconsisting of a dense self-supported Pd Ag tube lled with a Pt-based catalyst and used for producing pure hydrogen via oxidativesteam reforming of bio-ethanol was studied by Tosti et al. [73] .Capability of the membrane to promote the reaction conversiondue to equilibrium shift effect was demonstrated.

4.2. Coupling membrane reformers to solar parabolic troughs

As we discussed in the previous section, it is possible to over-come the thermodynamic limitations on CH 4 conversion imposedby low operating temperatures using membrane reformers. Sincein a membrane reformer complete CH 4 conversions are achievablewell below 600 C, such approach is of particular interest for solar

reforming applications since it can be used in conjunction with



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solar parabolic troughs to produce hydrogen. It was recently sug-gested to use parabolic troughs to provide the heat for methanesteam reforming using indirect approach, with molten salts as heattransfer and storage medium [74 77] . The indirect solar thermalreforming approach allows for easy incorporation of solar refor-mers into the existing parabolic troughs concentrated solar powerplants and the reformer design does not have any specialrequirements. In fact, tubular reformer geometry very similar to

industrial reformers can be used.However, the use of parabolic troughs and molten salts impliestemperature limitations which will be realized in low methaneconversions. Another issue is low activity of Ni-based catalysts atlow temperatures and their tendency to fast oxidation deactiva-tion. Carbon formation that at low temperatures occurs mainly byexothermic Boudouard coking and reverse gasi cation is also aconcern. The use of hydrogen selective membranes to shift themethane reforming equilibrium towards hydrogen generation is apromising approach. Selective separation of hydrogen by Pd-basedmembranes can allow obtaining nearly complete conversions evenat low temperatures. The price of Pd and the membrane durabilityare the main obstacles.

One such scheme of combining solar energy with a membrane

reformer was investigated by Said et al. [78] , wherein molten saltis heated via solar parabolic troughs and sent to the steammethane membrane reformer. The heat supplied by the moltensalt drives the endothermic reforming reactions for production of pure hydrogen at the exit of the reactor. In the study performed bySaid et al., low temperature methane reforming in a membranereactor heated by a molten salt was analyzed numerically bycomputational uid dynamics over a wide range of operatingparameters. Effects of temperature, steam-to-carbon ratio andspace velocity on conversion, hydrogen recovery and carbonmonoxide selectivity were investigated. The results of numericalsimulations showed that concentration polarization can be sig-ni cant and that the reformer performance can be limited by thekinetics of the reforming reaction or by hydrogen separation

ability. Nevertheless, the simulations showed that high hydrogenrecovery is achievable even at high, industrially relevant spacevelocities.

5. Conclusion

This paper reviews the recent articles that have been publishedon solar steam reforming. Steam reforming of methane is one of the processes for production of hydrogen or synthetic gas, knownfor clean combustion and high energy content. Other reformingprocesses such as dry reforming, partial oxidation and auto ther-mal reforming are reported in the literature. These technologiesare currently not suitable for bulk production of hydrogen. Steam

reforming of methane (SMR) which uses Ni based catalyst is anef cient and cost effective process for the production of hydrogenand syngas compared with other processes.

The review indicates that the heat requirement for the processcan be ful lled by solar energy using solar facilities such as solarparabolic dishes and solar towers. Different steam reformingreactors con gurations are proposed by researchers to ef cientlyconvert methane into product hydrogen or syngas. These reactors,mostly tubular, are heated directly or indirectly by solar energy.Although the solar parabolic troughs are a mature and muchcheaper technology compared to solar towers and dishes, it cannotbe used for steam reforming processes as they do not generate therequired temperature (above 800 C) for the process. Hence, thereview indicates the development of a reactor suitable for solar

steam reforming using solar parabolic trough technology. These

reactors do perform ef ciently with high methane conversions atlow temperatures generated by solar parabolic troughs.

Acknowledgments

The authors highly appreciate and acknowledge the support of King Fahd University of Petroleum and Minerals through the

research grant # R12-CE-10 offered by KFUPM-MIT Clean Waterand Clean Energy Research Collaboration Center.

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