Transformation of methane to synthesis gas over metal oxides without using catalyst

Availableonlineatwww.sciencedirect.comDRN群FScienceDirectN NATURAL GASELSEVIERJournal of Natural Gas Chemistry 18(2009)124-130www.elsevicr.com/locatejngeReviewTransformation of methane to synthesis gas over metaloxides without using catalystReza Alizadeh*, Esmail Jamshidi2, Guangqing Zhang1. Department of Chemical Engineering, Sahand University of Technology Tabriz 51335-1996, Iran:2. Department of Chemical Engineering, Amirkabir Universiry of Technology(Tehran Polytechnic), Tehran, 15875-4413, Iran;3. School of Material Science and Engineering. The University of New South Wales, UNSW Sydney, NSW 2052, AusraliaReceived December 10, 2008; Revised May 4, 2009; Available online June 22, 2009AbstractThis article reviews a new developing method in the field of metal oxide reduction in chemical and metallurgical processes, which usesmethane as a reducing agent. Commonly, coal is used as the reducing agent in the reduction of metal oxide and other inorganic materials; Metalproducing factories are among the most intensive and concentrated source of greenhouse gases and other pollutants such as heavy metals, sulfurdioxide and fly ash. Thermodynamically, methane has a great reducing capability and can be activated to produce synthesis gas over a metaloxide as an oxygen donor. Metal oxide reduction and methane activation, two concurrent thermochemical processes, can be combined as anefficient and energy-saving process; nowadays this kind of technologies is of great importance. This new reduction process could improveare synthesis gas that is more valuable than methane. In this paper, thermodynamic studies and advantages of this promising method werediscussed. The major aim of this article is to introduce methane as a best and environmentally friendly reducing agent at low temperaturemethane; metal oxide; reaction; reduction; greenhouse gases; synthesis gas1. Introductionin contact with carbon, as follows:M-O,+yCo rM+In metal processing industry usually three methodsThis CO2 diffuses back into carbon to generate more COused: electrometallurgical, hydrometallurgical and pyromet- according to the well-known Boudouard reactionallurgical method. Electrometallurgical and hydrometallurgyC+CO= 2C0cal methods characterized by high-energy consumption, dehus. in the solid-state reduction of metal oxide with carived mostly from the combustion of fossil fuels, for process bon, Co is the gaseous intermediateheat and electricity generation. The common method in metalThe reduction of inorganic mineral with coal(coke)is notprocessing factories is pyrometallurgical method based on us- environmentally friendly. These reactions produce poisonousing coal as a reducing agent. Reduction of metal oxide or gases such as SO, CO and large amount of Co2. It would beate ore is an important step for the production of metal favorable if an alternative technique could be developed withchemicals from their natural mineral. The various reducing no discharge of greenhouse gases to the atmosphere. Naturalagents currently employed in reduction processes are carbon, gas, whose major component is methane, is an excellent alhydrogen or mixture of hydrogen and carbon monoxide. Ini- ternative for playing this role. Commonly in chemical plantstial reduction of metal oxide with carbon takes place accord- methane is oxidized in catalytic reformer for producing syning to the following equation [1]thesis gas. The existing commercial technology and estab-lished process for synthesis gas production from natural gas iswhere both solid oxide and carbon are in contact. Te (1) based on the endothermic steam or wet reforming of methaneM-O, +yC(gr)=rM+yCoover中国煤化工 ly be represented by2lgenerated diffuses and reacts withoxide, which is notCNMHG3H2Comesponding author. Tel: +984123459156; Fax: +984123444355; E-mail: r alizadeh@sut.acirDalian Institute of Chemical Physics, Chinese Academy of Sciences. All nights reserveddoi:10.1016S10039953(08)60105XJoumal of Natural Gas Chemistry Vol. 18 No. 2 2009Global warming is one of the most important environmen-The equilibrium constant of this reaction can be calcutal problems in the recent decades and in recent years, lated from the following equation 19great attention has been paid to this problem. This phelogK=∑ UplooK/P-∑vgK/R(house gases(mainly CO2)are accumulating in the atmosphere where k is the equilibrium constant of the reaction and kjas a result of human activity(anthropogenic)and various tants and products respectively. The change of reaction Gibbsindustries [3-5]Carbon dioxide contributes significant greenhouse gaseffect, for instance, 82 percent of United States greenhouse△G0=∑vG-∑m=-R2TLnK(8)gas emission is carbon dioxide [6]. There are a huge amount where GD and GD are the Gibbs free energies of productspapers about the researches for COz concentration from the and reactants, respectively, at temperature T and atmosphericflue gas and further CO2 sequestration or conversion in theliteratures[7-16]. The largest part of carbon dioxide(about temperaturepressure. Rg is the gas constant and T is the absolute10%)is produced by metal oxide reduction processes as a re-sult of carbon consumption during the processes as a reducingFrom these equations the approximate commencing tem-ratures(AG=0 of several metal oxides reaction with car-agent For that reason, the reduction of greenhouse gases es- bon and methane according to Reactions(1)and(5)are com-pecially carbon dioxide in extractive metallurgy is becoming puted and tabulated in Table 1. In which AG and AGs are themore important[I7Using methane instead of carbon as a reducing agent has Gibbs free energy change for Reactions()and(5),respec-recently received an increasing interest in metal processing and methanothermal (Reaction(5) reductionCO2 and not any SOr. But Coal based method produceTable 1. Approximate temperatures(K)for spontaneouslarge amount of fly ash and a lot of heavy metal such as mer-proceeding of metal oxide reduction with carbon,cury that is emitted in atmosphere due to its existence in coal.Reaction (1), and methane Reaction(5)oal is not abundant but large sources of natural gas are avail- Metal oxide( ste temperature (Kk)There are some countries in the world(such as Iran)where△TKable, therefore using methane or natural gas for the ore reduc-Fe,0890tion process will be economically beneficial and favorable inthese countries [18]1570Metal oxide can be reduced by hydrogen as well. Buthydrogen is very expensive for practical use compared to1337methane: moreover hydrogen is frequently produced bymethane reforming process.97019504302. Thermodynamic considerationsCommon method for the metal oxide reduction is basedTable 1 shows that methanothermal reduction temperaon high temperature reaction with coal which is briefly called tures under standard conditions are much lower than in car-Carbothermal reduction. Coal or coke is a preferred reducing bothermal reduction with solid carbondition to thermo-agent because of its price. Carbothermal reactions are com- dynamic advantage, the gas-solid reaction(methanothermalplex, but the corresponding overall chemical reaction may be reduction)has a higher kinetic rate in comparison with therepresented as Equation(1)solid-solid reaction(carbothermal reduction). This shows thatM-O +yC(gr)=rM+yCO(1) methane has greater reducing capability than carbon, whichUsing methane or natural gas as a reducing agent com- creates a potential to decrease the operating temperature ofthe reduction process(30 K for Fe2O3 to 550 K for Al2O3)methane for the co-production of metals and synthesis gas It should be emphasized that Table I is just illustrating the(mixture of hydrogen and carbon monoxide). The general reaction can be writtenreduction temperature decreases more than these values.M-Ov+yCH4= M+y(Co +2H2Variation of Gibbs free energy versus temperature for theIn the above reaction metal oxide is playing a role of oxy- general Reactions(1)and (5)also are shown in Figures I and2 for some metal oxidesThermodynamic calculation in this kind of reactions is中国煤化工very important. Consider a general equation, as follows3.M∑tR=∑vPCNMHGwhere R and P are the reactants and products, and uR and upThe new method for reforming methane is using methaneare the stoichimetric coefficients of reactants and products, as a reducing agent of metal oxide. this process not onlyReza Alizadeh et al/ Joumal of Natural Gas Chemistry Vol. 18 No. 2 2009M, O, +)CH,"xM+yCo+2H2yHyo-M,O,+yH210Figure 3. Representation of hydrogen and synthesis gas producing scheme3. Iron oxide reduction- Pbohe reduction of porous and dense pellets of ferricoxide(Fe2O)with methane in the temperature range of800 1000 1200 1400 1600 1800 2000 2200 2400 1073-1298 K was studied by Ghosh et al. [20]. Also theTemperature(K)effect of temperature, methane flow rate, pellet size and poros-Figure 1. Gibbs free energy changes for some oxide reduction with carbon ity on the rate of reduction of iron oxide pellet with methaneaccording to Equation(1)has been investigated [20]. Fe2O3 reduction is a three step reaction process and may be represented as Equation(9), and theunderstanding of the mechanism of the reaction is so difficult.Fe2O3→Fe3O4→FeO→FeSteinfeld et al. [21] studied the reduction of magnetite(Fe3 O4)with methane. The chemical reaction is as follows:Fe304+4CH4= 3Fe +8H2+4C0(10)They also proposed to combine two useful thermochem-icalprocesses as a green cycle. It consists of two steps:(1)Iron oxide reduction with methane to form iron and synthesis gas. (2) Iron oxidation with water to form hydrogen andiron oxide3. 2. Zinc oxide and lead oxide reductionThe complete kinetic study of the reduction of zinc600-Mnle by methane was made by Ale Ebrahim et al. [22, 231The equilibrium constants for Zno reduction by coke andmethane are presented in Table 2, in which KCHa and Kc arethe equilibrium constants for methane and carbon reductionFigure 2 Gibbs free energy changes for some oxide reduction with methane ity of methane with Zno is considerably greater than that ofaccording to Equation(5)creased from about 1473 K (industrial furmaces with coke)toemission, but also produces a valuable outlet gas( mixture of about 1173 K with methane. Moreover, the gaseous productsCO and H2). The produced metal could be reacted with wa- of methane reduction(C0+2H2)can be used as synthesis gaster in water splitting reaction for reproducing metal oxide and for petrochemical industries. The environmental preference ofalso hydrogen as a gaseous product. In this study a new cyclic such combined plants is omitting the greenhouse gas emissionprocess has been proposed: on the one hand metal oxide re- from the metallurgical unit to the atmosphere. Furthermoreduction with methane, on the other hand water splitting re- this technique can omit the troublesome splash zinc condenseraction. Schematic representation of thermochemical cycle of of thhydrogen and synthesis gas production using metal as an er中国煤化工 he main reaction ofZIncergy carrier is depicted in Figure 3CNMHGO+2H2Very few studies have been reported in the literature inequaion or ue temperature dependency ofwhich methane was employed directly for the reduction of this reaction is simply shownmetal oxides. This section gives a brief review of usingk=495×102exp(-67090/RT)(12)methane in metal oxide reduction processwhere k(cm/min) is the surface rate constant of reactionJoumal of Natural Gas Chemistry Vol 18 No. 2 2009Table 2 Equilbrium constants for zinc oxde reductionby other conventional methods [32]. Also the temperature de-by methane and cokependency of reaction in the range of 1143-1248 K was foundas follows [28]:1000020002l1.04k=1.6×10cexp(-47019/R2T)(21)13005339538.183.5. Manganese oxide reduction605497323946Reduction of manganese oxide by methane to manganesSu et al. theoretically proved that the gaseous product of carbide occurs in accordance with the following reaction [34]nc oxide reduction with methane is synthesis gas [24]. Syn-thesis gas production via this method by using solar energy is Mno +10/7CH4=/7Mn C3+CO+20/7H2 (22)under investigation [251Also carbothermal reduction of manganese oxide is asAlso, Ale Ebrahim et al. studied the kinetic and mathe. follows[34]matical modeling of the reduction of lead and zinc oxide mix-MnO+10/7C=1/7Mn7C3+COtures by methane[26]. Their study showed that the ArrheManganese oxide reduction reaction by methane wasnius equation for Pbo reduction by methane between 973 and studied by Anacleto et al. [35]. They could decrease reduc1123 K is as follows:tion temperature about 300 K in comparison with carbotherk=182×1012exp(-51620/R2T)13) mal reduction. Also this new process, solid-state reduction ofoxide, has been registered as a patent [36]3. 3. Cobalt oxide reductionThe calculated equilibrium partial pressure of Co forMnO reduction to Mny C3 by methane and graphite at differenttemperatures is shown in Figure 4. It shows that, the CO equiThe kinetic study of the reduction of cobalt oxide, Coo, librium partial pressure in Reaction(23)is much lower thanby methane was made by Khoshandam et al. [27]. The chenthat in reduction by methane-containing gas by Reaction(22)ical reaction is as followsCoo+CH4= Co+C0+2H2(14)Kinetic parameters of the reaction were determined inKhoshandam's work. Their study showed that the Arrheniusequation for cobalt oxide reduction by methane between 1073nd 1223 K is as follows [28:k=1.04x10exp(-37176.8/R3T)(15)3. 4. Chromium oxide reduction8°g2a一 Methane(PcwPH=I atm)H Methane(PcH-0 I atm, PH, 0.2 atm)Chromium oxide reduction by methane was studied ex-sively by scientists. Qayyum and Reeve [29], Read etal. [30] and Anacleto et al. [31] studied the reduction of00lchromium oxide by methane containing gas. Khoshandam etal. studied the reduction of chromium oxide by using onlymethane [32]. All studies are unanimous that reduction of1000105011001150120012501300chromium oxide with methane takes place according to thefollowing equationFigure 4. Calculated equilibrium partial pressure of Co in the MnO reduc-13CH4+3Cr2O3=2Cr3C2+9c0+26H216)tion by methane and graphite as a function of temperature [35]Read et al. [30]. Qayyum and Reeve [29]concluded thatthe role of methane was to supply carbon, which diffuses intothe oxide In accordance with their work the overall reduction 3.6. Titanium oxide reductionReaction(16)is a sum of Reactions(17)through(20).13CH4=13C+26H2(17)Titanium element occurs within a number of mineral de-3cn2O3+9H2=6Cr+9H20(18)d almon which are widely dis6Cr+4C= 2Cr3 C2中国煤化工 cess for the reduction9H2O+9C=9H2+9c0(20) of titCNMH GS that based on threeThe kinetic of the reduction reaction was studied by stagesn uie II accu, reduction of IcLa toKhoshandam et al. [32] using methane in the temperature titanium sponge then purification of the sponge metal [371range of 1143-1248K. Their study showed that the reduc- Using methane containing gas for the reduction of titaniumtion and carburization temperature is about 140 K lower than oxide was extensively studied by scientists [38, 39)Reza Alizadeh et al/ Joumal of Natural Gas Chemistry VoL. 18 No. 2 2009The reduction of rutile is a complex process. Titania could 3.8. Barium sulfatebe reduced to titanium oxycarbide by methane(Reaction(24)through(28)):Reduction of barium sulfate (BaSO4) to barium sulfide5TiO2+CH4= TisOg +CO+2H2(24)(BaS)is an important step for the producing of barium chemi-4TisOg+CH4= 5Ti407+CO+2H2(25) cals from barite(natural barium sulfate mineral). Barite ore3Ti407 +CH4= 4Ti305+C0+2H2 (26) is a basic starting material for manufacturing of a variety2Ti305+CH= 3Ti203+CO+2H2 (27) of barium chemicals such as BaCO3, Ba(OH)2, Ba(NO3)2.0.5Ti2O3+(0.5+2x)CHa= rTiCss+Ba(HS)2, Bao, etc In the so-called"black-ash process(1-x)nos+(0.5+x)CO+(+4x)Hreduction of barium sulfate is carried out in a rotary kiln or inSubscript"ss"in the above expression indicates that TiC and a fluidized bed at a high temperature such as 1373-1473KTiO are in a solid solutionin the presence of reducing agents, mostly coal [42]Zhang et al. also studied the effect of hydrogen in ti- tion of barium sulfate with carbon takes place accordingtanium oxide reduction. They found that hydrogen can only following equationreduce titanium dioxide(TiO2)to(Ti2O3)and further reduc-2BaSO4+4C 2Bas+4cO(32)tion is not possible by hydrogen. Beyond this point, the greatThe kinetic study of the reduction of barium sulfate byreducing capability of methane can be seen.methane was made by Alizadeh et aL. [43]. They proposed anew method for the reduction of barium sulfate by methane at3. 7. Nickel oxide reductiontemperature 1223K, about 200 K lower than the conventionalmethod. Also they improved the reaction rate by doping zincoxide as a catalyst. Their study showed the reduction reactionNickel can be extracted from concentrates and occasion- takes place according the following reactionally directly from the ore itself by hydrometallurgical, pyrometallurgical, and electrometallurgical processes. The re-BaSO4+CH4= Bas+CO2+2H20(33)Reactions(32)and (33)show the reduction of barium sul-ion of nickel oxide with methane by pyrometallurgical fate by methane produces half amount of the COz than reducwas proposed as an economical process for producing tion of barium sulfate by coal. CO2 is the main componentnickel.Reduction of the nickel oxide, NiO, by methane was of greenhouse gases, i. e, ca. 82.3%[(6]. So this diminutionstudied by Alizadeh et al. [40]. The reaction of nickel oxide in greenhouse gas emission is valuable for environment. Thiswith methane was carried out in the temperature range 873 to998K at atmospheric pressure. Complete conversion can be w process of barite reduction with methane has been regis-tered as a patent [44], also newly Jamshidi et al. have pro-achieved in 1l min at 998, which is shorter than the time posed producing of barium carbonate by this method as a newrequired by carbon, i. e, ca 120 min at 1273 K Faster reducclean process [45].tion in comparison with carbothermal reduction is attributedto high carbon activity in methane. The reduction of nickel 4, Discussionoxide with carbon takes place in the temperature range 1173to 1273 K[41]. The overall reaction is as followsIn nearly all works on metal oxide reduction reactionsNio+C= Ni+CO(29) with methane, the following mechanism has been proposedMethane is a strong reducing agent, and can react at lower [31,34-36, 38-40,43, 46)temperature easilyThe reduction process starts with adsorption of methaneNio+CH4 Ni+CO+2H2(30) on the active sites of the oxide surface and its decompositionThe equilibrium constants of Reactions(29)and(30)are described by the following reactionscomputed and tabulated in Table 3 in which Kc and KCHs areCH(gas)=CHa(ad)the equilibrium constants for the Reactions(29)and(30),re-spectively. Table 3 shows that methane has greater reducingCHa(ad)= CH3(ad)+H(ad)apability than carbon, which creates a potential to decreaseCH3(ad)=CH2(ad)+ H(ad)(36)the operating temperature of reduction of nickel oxide.CH2(ad)=CH(ad)+Had)(37)Table 3. Equilibrium constants of Reactions(29)and (30)CH(ad)= C(ad)+H(ad)2H(ad)= H2(gas)283.7988.92The overall reaction of methane adsorption and dissociation4875.28463.44may be presented as4988844176603Cad+2H21200V凵中国煤化工 -arbon species adsorbedThe temperature dependency of this reaction in the range on solCNMHGifferent from depositedof 873-998 K was found as follows (40I caLuya serves as a sink for thisk= 4. 16x 10exp(-63900/Rgr) (31) adsorbed carbon. Active adsorbed carbon provides higherThis work showed that nickel oxide reduction temperature can ducibility and reaction rate in comparison with carbothermalbe decreased about 350 K by using methane instead of carbon. reductionJoumal of Natural Gas Chemistry Vol. 18 No. 2 2009Another reaction mechanism that was proposed by process has been proposed: on the one hand zinc oxide reducQayyum and Reave[ 29]and Read et al. [30] is quite different tion with methane, on the other hand water splitting reactionfrom the above mentioned mechanism. In accordance with Schematic representation of thermochemical cycle of hydro-their work, in the reduction reaction by methane, the role of gen and synthesis gas production using zinc as energy carriermethane was to supply carbon, which diffuses into the oxide. is depicted in Figure 5 [53].This mechanism could be shown by the methane cracking re-action as follows,Synthesis gas(CO+2H2)CH4→Cd+2H2where Cd is the deposited carbon. Contrary to this mechanism that deposited carbon acts as a reductant, other findingsZno+ CH4=ZnO+CO+ 2H2have clearly indicated that carbon deposition has a detrimen-AHu23K-320 u/moltal effect on the reduction reactions. It decreases the carbonactivity in the system and blocks the access of the reducinggas to the oxide interior surface and consequently decreasesZnoZn+H 0=ZnO+Hthe overall rate of reaction [31, 34-36, 38-40,43, 461Water splitting reactorAt high temperature, the reduction process can be totallysuppressed and hindered by carbon deposition that was so- Figure 5. Schematic flowsheet of cyclic process of zinc as an energy carriercalled soot formation or sooting53One of the most important parameters of the reduction reaction is the apparent activation energy as it defines the reactorLiterature survey on the solid state reduction of metal oxdimensions and the energy consumption. Making comparison ide by methane shows the potential for industrial applicationfor activation energy of methanothermal reduction reaction is of this technology. This process provides the following ad-more interesting. The activation energy of studied reactions is vantages over the conventional technology for metal oxide reshown in Table 4. As seen from Table 4, the activation energyvalues reported for reduction reactions by methane are within(a) Lower operating temperaturesthe same order of magnitude. Therefore, a similar mechanism(b) ability to process fine materialsfor this type of reaction should exist [40, 43](c) Elimination of the coke in the overall ores processing,Reduction of metal oxide with methane sometimes is and therefore, promotion of environmentally friendly tech-more interesting when the produced gas(outlet gas) is syn-nologiethesis gas. As mentioned above, in some cases like as zincHowever, further work is needed prior to a conclusion re-oxide, nickel oxide and iron oxide(Fe3O4)reduction, the prgarding feasibility of this technology on the industrial scaleduced gases are the mixture of hydrogen and carbon monoxide This includes the following(2H2+CO) Synthesis gas is widely utilized in chemical indus-(1)Detailed heat balance and overall energy consumptiontry as a feedstock for the direct production of methanol, hy- in the reduction of ores or metal oxidesdrogen, carbon monoxide, gasoline, diesel fuel, acetaldehyde,(2)Processing of metal oxide (or ores)in the continuousacetic acid, ammonia, ethylene glycol and vinyl acetate di- reactor, which may be a fluidized bed or moving packed-bedrectly and other important fuels and chemicals(47, 48). More- reactorver, heavy hydrocarbons can be produced from synthesis gas() Processing of produced metal by step 2 in the continby Fischer-Tropsch reactions [49]uous industrial large scale water splitting reactor.(4)The use of natural gas.Table 4. Comparison of activation energy of methaneFurther study of the mechanisms and kinetics of reduction of metal oxide reaction is also recommended This canletal oxideE/kcal/mFe2O31148-122352.70include surface analysis of metal oxide in the course of reducI13-1203tion and the application of high temperature XRD; study ofPbO73-112351.62the effect of the addition of other oxides as a catalyst on metal073-1223oxide in order to improve methane reducibility1143-124847873-99863895 ConclusionsBasoal173-1248he following important points can be highlighted fromAle Ebrahim et al. combined two useful thermochemi- this中国煤化工cal processes, the methane reforming to produce synthesis gasbe decreased consider.and zinc oxide reduction to produce metallic zinc, into a sin- ably tCNMHGin chemicalgle reaction by reducing zinc oxide with methane [49, 50]. If allurgical processes. The introduced new process can be re-produced zinc reacts with water [51,5 2] in water splitting re- ceived great attention as a basis of the green chemistry foraction,the cycle will be completed. In this study, a new cyclic developing the environmentally friendly future industriesReza Alizadeh et al/ Joumal of Natural Gas Chemistry Vol. 18 No. 2 2009(2)Thermodynamic calculations showed that the reactiv- [19] Brain I. Thermochemical Data of Pure Substances. Weinheimty of methane with metal oxide is higher than coke. And thehigh carbon activity of methane is the key to achieve high rates [20] Ghosh D, Roy a k, Ghosh A. Transactions of The lron and steeland extents of reductionInstitute of Japan, 1986. 26: 186(3)By using this method, methane will be reformed to [21] Steinfeld A, Kuhn P Energy, 1993,18:239sufficiently valuable synthesis gas in the absence of catalysts. (22) Ale Ebrahim H, Jamshidi J Trans Inst Chem Eng, 2001, 79A.In this kind of reactions the role of metal oxide is donation of[231 Ale Ebrahim H, Jamshidi E Ind Eng Chem Res, 2002. 41: 2630oxygen[24] Su Z, Qin S, Tang D, Yang H, Hu C J Mol Struct: Teochem(4)By using methane as a reducing agent the impurities2006,778:41in the solid product can be decreased, which are introduced by [25]Solar Technology Laboratory at PSI(Paul Schemer Institut)ash and sulfur of the coal in the usual reduction method [54]Switzerland(5)Literature survey showed that activation energy of [26] Ale Ebrahim H, Jamshidi E Ind Eng Chem Res, 2005, 44: 495methane-metal oxide reactions are within the same order of [27] Khoshandam B. Kumar R V, Jamshidi E Metall Materials TransB,2004,35:825(6)In the reaction by methane, oxide of iron, zinc lead, [28] Khoshandam B. IPh. D. Thesis]. 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