f a methane autothermal chemicallooping reforming experiment based on exergyanalysis
Fan Zhang, Lin Zhu* and Dong Rao
Compared with a double circulation fluidized bed, an internally circulating reactor (ICR) has a wider pressure
range, thereby increasing the conversion efficiency ofmethane and reducing the cost. However, since the ICR
is a new reactor, there is a lack of detailed analysis and research on the internal processes inside the reactor.
Consequently, in this paper, a detailed exergy analysis and an advanced exergetic analysis are utilized to
evaluate a methane autothermal chemical looping reforming experiment carried out on an ICR. A detailed
analysis of the internal exergy destruction of the ICR is carried out by establishing a mathematical model
for the enthalpy and exergy of the substance. The exergy destruction inside reactors is divided into the
exergy destruction in the chemical reaction process, the destruction of thermal exergy and other flowing
exergy destruction. The influences of the CH4/NiO molar ratio and addition of steam or not on exergy
destruction are studied. With the increase of CH4/NiO molar ratio, the chemical looping reforming process
gradually supersedes the chemical looping combustion process and becomes the dominating reaction,
which exerts a strong beneficial influence on the reduction of exergy destruction of chemical reactions.
When the CH4/NiO molar ratio is 0.60, the exergy efficiency of the product is up to 52.17%. The advanced
exergetic analysis provides a direction for improving the experiment. Based on these results, the ICR is
evaluated in all aspects, including comparative evaluation, existing limitations, and possible future work.
1 Introduction
Syngas with CO and H2 as the main components is one of themost important chemical materials and can be used to producemethanol, gasoline and other chemical products. The syngasproduction unit takes up most of the equipment cost andproduction cost in the production process of the above prod-ucts.1,2 Therefore, it is of great signicance to optimize theproduction process of syngas. Syngas is oen chemicallyproduced from coal, petroleum fractions, natural gas, biomass,and other hydrocarbons. Among them, the production of syngasfrom natural gas is mature and has a good effect and a low cost,thus becoming the fastest-growing and most-studied technicaldirection.3 Traditional ways to produce syngas from natural gasinclude steammethane reforming (SMR), partial oxidation, andcarbon dioxide reforming. In general, these methods have thedefects of high energy consumption, large carbon dioxideemission and serious carbon deposition in the reactor.4,5
As a fast-growing technology, chemical looping reforming(CLR) has outstanding advantages and dispenses with an airseparation plant and a capture device which is used to treat CO2
in the tail gas. It is the most promising process in the future
ng, School of Chemistry and Chemical
y, Chengdu 610500, P. R. China. E-mail:
44
energy utilization.6,7 Chemical looping reforming technology isdivided into chemical looping steam methane reforming (CL-SMR) and autothermal chemical looping reforming (a-CLR).CL-SMR was rst presented by Ryden et al.8 The schematicdiagram is shown in Fig. 1(a). CL-SMR is a combination of twounits, that is, chemical looping combustion (CLC) and SMR. Inthe CLC unit, the fuel reactor (FR) surrounds the reformingreactor, and the CLC unit offers the heat required by reformingreaction. Special attention should be paid to the fact that the heatrequired by the reforming reaction is offered by the sensible heatthat the high-temperature oxygen carrier (OC) carries from the airreactor (AR) to FR. In chemical looping technology, the OC is notonly a supplier of oxygen but also a heat transfer medium anda catalyst (used to catalyze the reforming reaction of CLR).Through water-gas shi reaction and pressure swing adsorption,the syngas generated in the reforming reaction further produceshydrogenwith high purity. Aer treatment, the tail gas can returnto the combustion reactor, as fuel for CLC. The a-CLR is highlysimilar to the CLC. It was rst presented by Mattisson andLyngfelt.9 The schematic diagram is shown in Fig. 1(b). Differentfrom CLC which aims at obtaining efficient thermal energy, a-CLR uses a low OC circulation rate to make the fuel oxidizedincompletely under insufficient oxygen supply, therebyproducing syngas. Sometimes, in order to improve the hydrogen-carbon ratio of the product, a certain amount of steam may beadded when fuel is fed. The reduced OC is oxidized and
Fig. 1 Schematic diagramof (a) the chemical looping-steammethane reforming and (b) the auto-thermal chemical looping-steammethane reforming.
Fig. 2 Schematic diagram of the circulating fluidized bed. (1) Airreactor, (2) fuel reactor, (3) cyclone for particle separation, (4) and (5)
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regenerated in AR and prepared for the next circulation. Throughthe regulation of the feed ratio of fuel, steam, and air, the wholeunit can stay in an autothermal process without the need for anexternal heat source.10 At the same time, it has the advantages ofcapturing CO2 and generating syngas with a H2/COmolar ratio of2. This ratio is suitable for Fischer–Tropsch synthesis or meth-anol synthesis.11,12 At present, the research interests of CLRmainly include the selection of an efficient and economic wear-resistant OC, the design of a unit with high energy efficiencyand the development of a reactor adapting to high temperatureand high pressure, with excellent mass and heat transfer capacityand airtightness.13–15
The circulating uidized bed reactor (CFBR), applicableto CLC and CLR processes with coal and natural gas as rawmaterials, is the most-studied reactor type.16–18 It was rstpresented by Lyngfelt et al. and composed of a FR, an AR,a cyclone separator, a riser and looping seals, as shown inFig. 2.9 According to relevant studies, pressurization hasa lot of benets to the CLR process, such as improving theconversion rate of carbon, increasing the combustion effi-ciency, improving the purity of CO2 captured and reducingthe total capital cost of hydrogen production.19–21 However,the CFBR has obvious disadvantages in pressurization. Thisis because with the increase of pressure, the efficiency ofcyclone separator would decrease, while and the destructionof OC particles would increase.22,23 Under pressurization,the circulation effect of solids in the looping seals is alsoa big problem.24
Therefore, the proposal of an internally circulating reactor(ICR) offers a new approach to achieve the pressurization of CLR,as shown in Fig. 3. The difference between this reactor and CFBRwas that it replaces the looping seal in the traditional structurewith simple ports and replaces the cyclone separator at the topwith a freeboard. And the ICR was divided into AR and FR witha vertical partition. An L-type connecting port is designed at thetop and bottom respectively, to act as a circulation channel of theOC. The enlarged freeboard zone at the top of AR acted asa “speed bump” of the OC, avoided the elution of particles andmade the OC fall into the upper port at a reduced speed, so thatthey can be circulated to FR. The circulation of solids was stillachieved by feeding them into the reactor using gases at differentspeeds. The gas ow rate in AR was faster than that in FR.
A bubbling uidized system and a fast uidized system inAR and FR were set up respectively, by regulating the gasow rate in each reactor. The solid particles in FR gatheredat the bottom port, thus creating a pressure difference onboth sides of the port and offering a drive for solids tocirculate back to AR. The particles at the port formeda physical plug and thus minimized the gas leakage. Tosimplify the design of the inlet of ICR, the gas inow intoeach reactor was realized through a cylindrical tube at thebottom. An electric heater was set at the bottom of thereactor, to preheat the reactor and regulate the autothermal
process of the reaction. The reactor was put into a cylin-drical shell. At the same time, to avoid large heat losses,insulating material (glass wool) needed to be placed aroundthe reactor.
With a simple design, the ICR integrated the functions ofAR, FR, cyclone separator and looping seal and simpliedthe operating process of these devices. The cyclone sepa-rator, the looping seal and the riser for solids transport werereplaced with short and simple ports, so that the ICR caneffectively control the high solids circulation rate even ata high pressure. Moreover, since the internal pressure wasalmost constant, the ICR didn't require a separate pressureshell for each component as the CFBR did, thereby reducingthe cost of pressure equipment. Currently, the research forICR is mainly focused on hydrodynamics and the estab-lishment of reactive multiphase ow models.25,26 However,the detailed thermodynamic evaluation of the CLR processin ICR has not been involved.
Exergy refers to the maximum useful work possibleduring a process that brings a system or substance intoequilibrium with the environment. The exergy analysisprovides a measure for assessing the “quality” of energy. Inprevious studies, the way researchers used exergy analysiswas to treat a reactor or unit as a black box and ignored thedetailed process of internal exergy destruction, that is, theexergy of the input minus the exergy of the output.27,28
In the present study, the ICR concept designed by Osmanet al. and their experiment data are used.29 By establishinga mathematical model for the process inside ICR and the
22034 | RSC Adv., 2019, 9, 22032–22044
thermodynamic data of substance, a thermodynamic eval-uation is carried out on the experiment. The exergy analysisis gradually applied from the system to each reactor, thevarious processes inside the reactor, and the variouschemical reactions to explore the specic factors of exergydestruction. Using an advanced exergetic model, the mainsource of exergy destruction is analyzed and a correspond-ing optimization scheme is proposed. The inuences ofCH4/NiO molar ratio and the addition of steam or not onvarious exergy destructions in the ICR system, AR and FR areanalyzed. The feasibility of future development of ICR issummarized and evaluated.
2 Experiments and methods2.1 Brief description of the operations and results of ICRexperiment29
In the CLR experiment, methane was used as the fuel and NiOwas used as the OC and a total of 7 cases were tested. Supposethat the methane input was entirely converted, that is, the gasow rate aer the conversion was 3 times that of the methaneinput. All of the experimental processes were performed at anoperating pressure of 1.7 bar. The reactor exhaust was cooledusing a water cooler. For ease of analysis, the cooler wasincorporated into a corresponding reactor unit. Tables 1 and 2summarize the operations of all cases.
The original experiment dened the reactivity between fueland OC as the carbon conversion rate of methane at the outletof FR, as follows:
gCH4¼ xCO2
þ xCO
xCH4þ xCO2
þ xCO
(1)
xi is the volume fraction of gas species i leaving the dry gases inFR in the total output.
Due to the inherent defects of experimental equipment,there would be inevitably gas leakage in the system. From thegas leakage experiment, it can be learned that about 9 vol% ofthe gases in FR leaked into AR. There was also the same amountof gases in AR leaking into FR. But in the actual CLR process,the reaction process inuenced gas ow, so re-measuring theleakage amount of gas in process was of great signicant forprocess exergy analysis. Through the denition of the recoveryrate and purity of syngas, the amount of syngas leaking from FR
Oxygen carrier NiO/Al2O3 (mass ratio was 65/35)Particle size �161.7 mmParticle loading 2 kg (10 cm static bed-height)Temperature 650 �C (initial temperature)Pressure 1.7 barFlow rate in AR 80 N l min�1
Flow rate in FR 15 N l min�1
Steam At a temperature of 200 �C
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to AR and the amount of nitrogen leaking from AR to FR can beobtained respectively, as follows:
Syngas recovery
¼�1� syngas leakage flowrate from the FR to the AR
syngas produced flowrate in the FR
�
� 100% (2)
Syngas purity
¼�1� N2 leakage flowrate from the AR to the FR
syngas produced flowrate in the FR
�
�100% (3)
The results of CLR experiment designed by Osman et al. ineach case are shown in Table 3.
2.2 Supplementary explanation on the experiment andprocessing of results
The chemical reactions in the ICR mainly include ve hetero-geneous catalytic reactions ((R1)–(R4) and (R7)) and two homo-geneous catalytic reactions ((R5) and (R6)), as shown below.Among them, (R1) and (R2) serve as the main reactions in thereduction step of the OC, while (R7) serve as the main reactionin the oxidation step of the OC.
CH4 + NiO / CO + 2H2 + Ni (R1)
CH4 + 4NiO / CO2 + 2H2O(g) + 4Ni (R2)
H2 + NiO / H2O(g) + Ni (R3)
Table 3 Summary of the main results of the CLR experimental campaig
Product compositions (vol%)of fuel reactor Fuel reactor Air reactor
When the oxidation rate of OC in AR is too low, the solidscirculating to FR contained Ni. The methane has a carbondeposition reaction on the catalyst Ni. This not only leads tothe deactivation of the OC, but also affects the circulationefficiency of solids.30,31 According to relevant research, theaddition of steam can effectively inhibit carbon deposition,especially when the steam/carbon molar rate is greater than1,32 the reactions are as follows.
CH4 / C + 2H2 (R8)
CO2 + C / 2CO (R9)
H2O + C / CO + H2 (R10)
Since the internal ow of reactor is substantially stable, it is nowassumed that the mass of the OC cycle is a stable value as follows.
m ¼ mNiO + mNi + mAl2O3+ mC (4)
When calculating the solids circulation rate from FR to AR,the cycle rate of C is zero. The amount of NiO recycled from theFR to the AR is calculated from the CH4/NiO molar ratio andfeed quantity of methane. In Cases 3–8, the CH4/NiO molarratios are 0.27, 0.31, 0.36, 0.45, 0.40 and 0.60, respectively. NiOand Ni are always attached to the inert OC (Al2O3) in a certainmolar fraction. In particular, the OC stream circulating to theFR in Case 8 carries a certain amount of Ni, too. Whencalculating the solids circulation rate from AR to FR, the cyclerate of Ni is obtained from the amount of oxygen-containinggas at the FR outlet. The amount of C is calculated from CO2
at the AR outlet minus the carbon-containing gas leaking intothe AR.
2.3 Establishing a thermodynamic model
As a relative quantity different from enthalpy, the value ofexergy requires us to select a particular environment as the
benchmark. The environmental benchmark is composed ofphysical benchmark and chemical benchmark. The physicalbenchmark includes ambient temperature and ambientpressure. So far, the physical benchmark is basicallyuniform, that is, the benchmark temperature is 298.15 Kand the benchmark pressure is 1 atm or 1 bar. The chemicalbenchmark includes the composition of environmentalreferences and their concentrations in the environment.This is the major difference between different environ-mental benchmark models. In this study, an advancedDaning Zheng model, dened as follows is referred to:33
(1) The setting of physical benchmark: the benchmarkambient temperature (T0) is 298.15 K and the benchmarkambient pressure (P0) is 1 bar;
(2) The setting of chemical benchmark: the air in the U.S.Standard Atmosphere, 1976 is selected as the referencesubstance for atmospheric elements. Pure water is selectedas the reference substance for hydrogen. While the referencesubstances of other elements are pure compounds in theearth layer.
Suppose that the molecular formula of pure substance i isAxBy, then the mathematical models of standard enthalpy andstandard exergy of the substance in the reference state areshown in eqn (5) and (6).
HqAxBy
¼ xHqA þ yHq
B þ DfHqAxBy
(5)
3qAxBy¼ x3qA þ y3qB þ DfG
qAxBy
(6)
where HqAxBy
and 3qAxByare the standard enthalpy and standard
exergy of the substance AxBy in kJ mol�1. Hiq and 3qi are the
standard enthalpy and standard exergy of the element i,in kJ mol�1. DfHq
AxByand DfGq
AxByare the standard enthalpy of
formation and standard Gibbs free energy of formation of thesubstance AxBy, in kJ mol�1. The element standard data used inthis study are shown in Table 4. The standard enthalpy offormation and standard Gibbs free energy of formation areconsulted from relevant literature.34
When a material ow composed of a variety of substanceschange from the reference state to a new state (the systemtemperature is T and the pressure is P), the mathematicalmodels of material ow enthalpy (Hm) and material owexergy (Em) in the new state are shown in eqn (7) and (8).
Hm ¼ NH�T ;P; y
�
¼ NX
yiHqi þN
Xyi
�ðTT0
CpdT
þðPP0
½V � TðvV=vTÞ�pdp� RT2�v ln
�f if qi
vT�p;x
�
(7)
Table 4 Standard enthalpies and exergies for the elements involved33
where Hi and Si are the enthalpy and entropy of the species i inthe new state respectively. Hi
q and Siq are the standard enthalpy
and standard entropy in the reference state, in kJ mol�1. f andfiq are the fugacity and standard fugacity of the species i, in kPa.
Cp and V are the heat capacity and volume respectively. N is thetotal molar ow rate of the material ow, in mol min�1. Amongthem, the second terms of eqn (7) and (8) can be calculated fromthe Peng–Robinson equation of state in Aspen Plus.
2.4 Establishing an exergy destruction process model
Exergy destruction refers to the system's loss of ability toperform work because of the occurrence of an irreversibleprocess inside the system.
The exergy balance model shown in eqn (9) is used, toanalyze the exergy destruction in a steady ow system.
XN1
i¼1
EF;i ¼XN2
j¼1
EP;j þXN3
k¼1
ED;k (9)
where EF,i is the ith exergy ow input by the system and EP,j is the j
th
exergy ow output by the system. ED,k is the kth exergy destruction
ow caused by the irreversible process inside the system.Apart from the material ow exergy mentioned in the
previous section, in a common thermodynamic system, there isalso workow exergy and heat ow exergy. The workow exergy(EW) is equal to the workow (W) itself, while the heat owexergy needs to be determined by the initial temperature T1,nal temperature T2 and he heat transferred Q in the heattransfer process, as shown in eqn (10) below.
EQ ¼ðT2
T1
�1� T0
T
�dQ (10)
The irreversible processes within a system include chemicalreaction processes and physical processes (including owingprocess, heat transfer process, mixing & separation process andother processes). All of the Gibbs free energy of chemical reac-tions involve in the present study under reaction conditions isless than zero. �DG stands for the maximum useful work thatcan be converted in the reversible process of chemical reaction.But in actual cases, part of �DG is converted to heat. Supposethat g stands for the proportion of�DG converted to heat in theirreversible process of chemical reaction. Then the exergydestruction during the reaction can be calculated by eqn (11)below. In a special combustion reaction where �DG is entirelyconverted to heat. At this point, g ¼ 1.
Since all of the other irreversible processes belong to phys-ical processes. The exergy destruction can be calculated fromenthalpy change DH and entropy change DS during the process,as shown in eqn (12) below.
ED,k ¼ DH � T0DS (12)
During the operation of system, since the material owand waste heat discharged are not effectively utilized, someexergy is lost in the irreversible process. This part of exergydestruction is called unused exergy destruction (Eunout). Theremaining exergy ow is called beneted exergy owEbenout . From here, it can be seen that the total exergy destruc-tion of the system includes exergy destruction caused by thepotential difference inside the system and exergy destructioncaused by the dissipation effect outside the system, as shownin eqn (13) below.
ED ¼XN2
j¼1
Eout;jun;j þ
XN3
k¼1
ED;k (13)
The exergy efficiency of the equipment k or the process k isdened as follows.
xk ¼EP;k
EF;k
¼ 1� ED;k
EF;k
(14)
2.5 Establishing an advanced exergetic analysis model
In the chemical looping reaction system, the exergy destructionsof AR and FR are not only related to the irreversible processduring their own operation, but also related to the energyconsumption of the other reactor. For a circulation process, theexergy destruction of each reactor inside the system needs to beanalyzed in detail. In an advanced exergetic analysis model, theexergy destruction of reactors can be divided into unavoidableexergy destruction, avoidable exergy destruction, exogenousexergy destruction and endogenous exergy destruction, depend-ing on whether the destruction is avoidable and whether thedestruction is caused by internal reasons.
As to the exergy destruction of a given reactor x in the system,the exergy destruction caused by the limitations of technology,economy and other realistic factors are called unavoidable exergydestruction (EUND,x). While the remaining exergy destruction thatcan be reduced by improving process and equipment are calledavoidable exergy destruction (EAVD,x), as shown as follows.
ED,x ¼ EUND,x + EAV
D,x (15)
The calculation of unavoidable exergy destruction relies onthe decision-maker's judgement about the optimal runningstate of system, aer taking technology, economy and otherconstraints into account, so it is somewhat subjective. In thepresent study, the unavoidable exergy destruction is expressedas the ratio of the exergy destruction and output exergy underthe optimal running state of the reactor x and the output exergy(EP,x) in the actual process, as shown below.
The optimal running state in the CLR experimental processis calculated by simulating using Aspen Plus soware from eqn(15) and (16), the calculation model of avoidable exergydestruction can be obtained as below.
EAVD,x ¼ ED,x � EUN
D,x (17)
Since the CLR system is a closed looping system, the irrevers-ibility of a certain reactor x in the system is not simply caused bythe low efficiency of the reactor itself, but also related to the lowefficiency of the other reactor. Considering the interaction betweentwo reactors in the system, the exergy destruction of a given reactorx can be divided into endogenous exergy destruction (EEND,x) andexogenous exergy destruction (EEXD,x), as follows.
ED,x ¼ EEND,x + EEX
D,x (18)
The main ways to calculate endogenous exergy destructionare thermodynamic cycle and engineering methods. Since theideal operation of equipment and the ideal parameters of cyclecannot be obtained accurately, thermodynamic cycle methodhad poor calculation accuracy. The engineering method, whichis based on the exergy sensitivity analysis of the whole system, isa method that solves using drawings. Although this method hasa heavy calculation burden, it is capable of obtaining moreaccurate results. The calculation model of the engineeringdrawing method is as follows.
EF,tot � EP,tot ¼ ED,other + EEND,x (19)
where EF,tot, EP,tot and ED,other are the total input exergy of thesystem, the total output exergy of the system, and the totalexergy destruction of other devices than the reactor x. Bydrawing of a curve of EF,tot � EP,tot with respect to ED,other, theresearchers found that when ED,other ¼ 0, the intercept of thecurve on the y-axis is equal to the endogenous exergy destruc-tion of the reactor x. From the study of Kelly et al.,35 it can belearned that EF,tot � EP,tot is linearly correlated with ED,other. It isnoteworthy that when the reactor x is calculated, its runningstate should remain unchanged, that is, its exergy efficiencymust remain unchanged (xx ¼ const).
Aer the division of exergy destruction in reactors, thecalculation formulas for unavoidable endogenous exergydestruction (EUN,END,x ), unavoidable exogenous exergy destruction(EUN,EXD,x ), avoidable endogenous exergy destruction (EAV,END,x ) andavoidable exogenous exergy destruction (EAV,EXD,x ) are obtained asfollows:
From eqn (5) and (6), the standard enthalpy and standard exergyof the substance under study can be obtained, as the basic datafor exergy analysis, as shown in Table 5.
3.1 Analyzing the system exergy
The CH4/NiO molar ratio is the main factor controlling whethera reaction process is CLR or CLC. Theoretically, when the CH4/NiO molar ratio is less than 0.25, the whole system ran a CLCprocess. First of all, in Cases 3–6, a sufficient and the sameamount of air is fed into AR. The CH4/NiO molar ratio is regu-lated by changing the feed quantity of methane. Secondly, bycomparing Cases 7 and 8 with Cases 5 and 6 respectively,through the regulation of the feed quantity of oxygen, theinuence of incomplete oxidation of nickel-based OC on thereforming reaction can be studied. Finally, in Case 9, theaddition of steam changed the thermodynamic equilibriumrestriction in the reaction system and was a possible path toimprove the CLR process.
Fig. 4 shows the exergy destruction of the system andproduct exergy in each case. Initially, with the increase ofmethane input, the exergy destruction of the system didn'tincrease signicantly. Moreover, through the reduction of airinput as appropriate, up to 18% of exergy can be saved.Compared with the addition of steam, the reduction of oxygeninput is more conducive to the reduction of the total exergydestruction of the system. In Case 8, the exergy efficiency wasthe highest, at 52%. From the above, it can be seen that throughthe increase of the CH4/NiO molar ratio as appropriate, that is,
Fig. 4 System exergy destruction and product exergy for Cases 3–9.
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to control CLR process as the main reaction process, the use ofexergy can be improved and exergy destruction in the processcan be reduced, thereby achieving the efficient use andconversion of energy.
The main product of the CLC process is thermal exergy,while the main product of the CLR process is chemical exergy.Therefore, in the CLR study, the output product exergy referredto the chemical exergy of syngas which is discharged from FR.Since the chemical exergies of nitrogen and carbon dioxide inthe syngas are very small, the product exergies are dominated bythe chemical exergies of hydrogen, carbon monoxide andmethane. In Cases 3–6, the target product of experiment (i.e.syngas) gradually grew and the output product exergy also grewaccordingly. Case 3 had an almost complete CLC process andthe output exergy of target product is only 0.05 kW. As the mostobvious CLR process, the output exergy of target product inCase 6 is about 1.01 kW. In Cases 7 and 8, as the oxidation rateof the OC in AR decreased, the NiO circulating to FR decreased,while the Ni increased. In these two cases, the CLR process isfurther enhanced. The exergy which hadn't been used originallyis converted to product exergy and then output. On the otherhand, according to the study of Moayeri et al., Ni can directlycatalyze the decomposition of methane into hydrogen andcarbon monoxide and also promote the output of productexergy.36 In the comparison between Cases 5 and 9, the additionof steam can signicantly increase the product exergy and raisethe hydrogen output by about two times.
Fig. 5 shows the exergy destruction of AR and the exergydestruction of FR in each case. The exergy destruction of ARaccounted for more than 82% of the total exergy destruction ofthe system. The reaction process in AR is the main exergonicprocess of the system. The main factors affecting the exergy
Fig. 5 AR exergy destruction and FR exergy destruction for Cases 3–9.
destruction of AR included the conversion rate of OC in FR andthe oxygen input in AR. In Cases 3–6, when the oxygen input inAR was sufficient, a constant amount of NiO reacted withgrowing methane and produced more Ni. The oxidization of Niin AR led to more exergy destruction. From the results of Cases7–8, it can be learned that the circulation of OC at a lowoxidation rate can help reduce exergy destruction in the process.But on the other hand, it would also increase deposited carbon(methane was decomposed under the catalysis of nickel). Whenthe deposited carbon was leaked to AR, oxygen would beconsumed and further reduce the circulation rate of oxygen inthe system, making the system unstable and diminishing thereaction efficiency. Considering the long-term stable operationof the system, there existed a minimum for oxygen input. Theaddition of a certain amount of steam can effectively inhibit theproblem of carbon deposition and reduce the exergy destruc-tion of AR.37
As for the exergy destruction of FR, in Cases 3–6, with theincrease of the CH4/NiO molar ratio, the combustion process inFR was gradually weakened. In contrast, the reforming processwas gradually enhanced. The enhancement of reformingprocess can reduce the exergy destruction inside the reactor. InCase 4, the exergy destruction of FR reached a maximum of 0.33kW. In Cases 7 and 8, the exergy destruction of FR furtherdecreased. On the one hand, the decrease of the CH4/NiO molarratio in the system can further enhance the reforming reaction.On the other hand, the decrease of the conversion rate ofmethane can also diminish the exergy destruction of FR.
3.2 Analyzing the reactor exergy
Fig. 6 shows the distribution of exergy destruction of AR in eachcase. In AR, the exergy destruction occurred in the exhaustcooling process, chemical reaction process heat transferprocess and other processes (including owing process, mixing& separation process and dissipation process, etc.). First of all,the unused thermal exergy referred to the thermal exergy ofreactor exhaust which is lost and dissipated in the cooler, whichwas mainly related to the outlet temperature and exhaustvolume. Since no heat recovery unit was set up in the reactor, an
Fig. 6 Exergy destruction distribution in the AR for Cases 3–9.
abundance of thermal exergy cannot be effectively used, makingthe thermal exergy in the exhaust become the greatest part ofthe exergy destruction of AR. Meanwhile, since there is nosignicant difference in terms of outlet temperature andexhaust volume, the unused thermal exergy uctuated around0.78 kW. Secondly, the exergy destruction in the chemicalreaction process had a direct bearing with the conversion rate ofoxygen. With sufficient oxygen, the conversion rate of oxygen inCase 5 reached the maximum, 34.3%, which corresponded tothe maximum exergy destruction of 0.34 kW in the chemicalreaction process. Although the conversion rate in Case 8 was thehighest, the oxygen input was still relatively low, so the exergydestruction in the chemical reaction process in Case 8 was stillrelatively small. The thermal exergy transferred to FR wasmainly used to support the endothermic reaction in FR andassociated with the conversion of methane in FR and the reac-tion temperature in AR. This part of thermal exergy was mainlytransferred through the OC and reactor wall. In Cases 3–6 andCases 7–8, with the increase of conversion rate of methane,more and more thermal exergy was transferred to FR. Moreover,in the comparison between Cases 5 and 7, Cases 6 and 8, thethermal exergy transferred to FR was mainly related to thereaction temperature and conversion rate of methane in AR. Inthe comparison between Cases 5 and 9, as the addition of steamintroduced thermal exergy to the reactor, the thermal exergytransferred from AR to FR in Case 9 decreased. Finally, theexergy destruction of other processes was mainly related to theow inside the reactor. Since the ow somewhat uctuatedduring the experiment, the value of exergy destruction wouldslightly change within a certain range.
Fig. 7 shows the distribution of exergy destruction of FR ineach case. First of all, the unused thermal exergy in FR wasconsistent with the above denition. Since the exhaust of FR isabout 0.2 times that of AR, the unused thermal exergy would alsodecrease accordingly. In Case 8, the reaction temperature in FRwas low, while the leakage amount of syngas was large, so itsunused thermal exergy was the smallest, at 0.14 kW. Secondly,the exergy destruction in the chemical reaction process wasmainly inuenced by the reacting dose of methane and the CH4/NiO molar ratio. In Case 4, when the maximum exergy
Fig. 7 Exergy destruction distribution in the FR for Cases 3–9.
destruction of 0.13 kWwas reached, with the increase of the CH4/NiO molar ratio, the exergy destruction in the chemical reactionprocess would gradually decrease. The main reaction in FR wasan endothermic reaction. And then, when being transferred toFR, this high-temperature heat from AR had a temperaturedifference, thus leading to exergy destruction. Such destructiondecreased with the decrease of temperature difference. In Cases 7and 8, the temperature differences between two reactors wererelatively small and the exergy destructions in the process were0.001 kW and 0.002 kW respectively. Finally, the exergy destruc-tions of other processes and unused thermal exergy exhibited thesame trend in FR and AR.
Fig. 9 Exergy destruction distribution of chemical reaction in the FRfor Cases 3–9.
3.3 Analyzing process exergy in chemical reactions
Fig. 8 shows the distribution of exergy destruction of chemicalreactions in AR. The chemical reactions in AR included theoxidation of nickel and the oxidation of other substances (e.g.coke, hydrogen and carbon monoxide) leaked from FR. Theoxidation of nickel served as a combustion reaction. From eqn(7), it is can be seen that the exergy destruction per mole wasrelated to the reaction temperature and Gibbs free energy. Theequilibrium temperature for the oxidation of nickel is 2762 �C(from HSC Chemistry soware). Therefore, the increase ofreaction temperature was benecial to the reduction of exergydestruction in the reaction process. Since the system ran ata constant pressure, the exergy destruction per mole was merelya function of temperature.
In all cases, the exergy destruction in the oxidation process ofnickel presented an unstable variation trend. When themethane input increased, on the one hand, the amount ofnickel oxide reacting with methane increased. On the otherhand, due to the enhancement of the CLR process, the amountof nickel oxide consumed per mole methane decreased. In Case5, the exergy destruction in the oxidation process of nickelreached a maximum of 0.33 kW. In Case 8, due to insufficientoxygen input, nickel was not completely oxidized and the exergydestruction in the process was diminished. On the other hand,the increase of exergy destruction of other reaction processes
Fig. 8 Exergy destruction distribution of chemical reaction in the ARfor Cases 3–9.
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can be interpreted as that the deposited carbon that leaked intoAR increased and led to the intensied oxidation of depositedcarbon. This conclusion can be proved by the concentration ofCO2 in the exhaust of AR. In Case 9, the addition of steam caneliminate deposited carbon to a certain extent, but due to theincrease of hydrogen yield in FR, the hydrogen concentration inthe leaked gas grew. The oxidation process of hydrogen in ARenlarged the exergy destruction of other reaction processes.
Fig. 9 shows the distribution of exergy destruction of chemicalreactions in FR in each case. In order to study the inuence ofvarious operations on major reactions in FR, the author dividedthe exergy destructions in various reaction processes in FR intoexergy destruction in reforming process, exergy destruction incombustion process and exergy destruction in other reactionprocesses. Through a comparison of exergy destructions invarious reaction processes, the reaction statuses of two majorreactions under various operating conditions can be distin-guished. A lowCH4/NiOmolar ratio can enhance the CLR processand diminish the exergy destruction of reactors. Themain reasonwas that when a unit quantity of methane had a reformingreaction on NiO, the exergy destruction accounted for 33.9%–
37.2% of the exergy destruction in combustion reaction. But onthe other hand, a low CH4/NiO molar ratio would also facilitatethe exergy destruction in other reaction processes. This wasmainly attributed to the intensied decomposition of methaneunder the catalysis of nickel. Due the limitation of thermalbalance, the CLR process had the maximum extent of reaction.When the CLR process was the most intense, the exergydestruction was 0.04 kW. Through a comparison of Cases 5 and9, the addition of steam can facilitate the CLR process, as well asthe reforming of methane steam.
3.4 Advanced exergetic analysis
For the division of unavoidable exergy destruction and avoid-able exergy destruction, the results are shown in Fig. 10. Allcases deviated from the assumed working conditions in AR atalmost the same degree, so their unavoidable exergy destruc-tions were almost equal. On the other hand, the avoidable
Fig. 10 Avoidable exergy destruction and unavoidable exergydestruction for Cases 3–9.
Fig. 12 Splitting of component exergy destruction to endogenous/exogenous and avoidable/unavoidable parts: AR.
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exergy destruction of AR was associated with the utilization ofheat. In Cases 7 and 8, due to the enhancement of the CLRprocess in FR, the heat in AR was converted to the chemicalexergy of the product, rather than being wasted by the cooler. Sothe avoidable exergy destruction obviously decreased. Theunavoidable exergy destruction of FR reached the maximum,0.26 kW in Case 4 and was mainly related to the situation ofchemical reactions. The unavoidable exergy destruction of FRwas relatively small, so it will be not be elaborated on in theoptimization process.
When the endogenous exergy destruction was calculatedusing engineering method, the running state of the reactor xmust remain unchanged. When the endogenous exergydestruction of AR was calculated, Cases 4 and 9 were selected forcalculation (i.e., to regulate the feed of FR). When the endoge-nous exergy destruction of FR was calculated, Cases 5 and 7were selected for calculation (i.e., to regulate the feed of AR).The relationship curves between EF,tot � EP,tot and ED,other fortwo reactors are shown in Fig. 11. Through the intercepts of
Fig. 11 Diagram of EF,tot � EP,tot vs. ED,other for CLR processcomponents.
various straight lines on the y-axis, the endogenous exergydestructions of various reactors can be obtained.
Fig. 12 and 13 shows the proportions of all kinds of exergydestructions of AR and FR. In AR, most of the exergy destruc-tions were endogenous, indicating that the reactions in FR hadlittle inuence on the exergy destructions of AR, while thereactions in AR had great inuence on the exergy destructionsof FR. This was because oxygen circulating to FR was one of themajor variables that directly controlled the reaction processesin FR. Through the adjustment of operating parameters in AR,the exergy destructions of FR can be effectively reduced.
In AR, the avoidable endogenous exergy destruction andunavoidable endogenous exergy destruction accounted fora vast majority of exergy destruction. In the calculation ofavoidable endogenous exergy destruction, the cooler thattreated exhaust was incorporated in the calculation of reactor.The heat discharge in AR was large, so the avoidable endoge-nous exergy destruction accounted for a large proportion. This
Fig. 13 Splitting of component exergy destruction to endogenous/exogenous and avoidable/unavoidable parts: FR.
Fig. 14 The efficiency of the CFBR and the ICR at different carrier-to-fuel radios.
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part of exergy destruction can be reduced by adding a heatexchanger to recover heat. The unavoidable endogenous exergydestruction was mainly induced by the low efficiency of chem-ical reactions. This part of exergy destruction can be reduced byreplacing the OC or changing the operating conditions, butrestricted by technology and economy. If the reaction pressureand the reaction temperature were increased blindly, theequipment investment would be increased. This deviated fromrealistic conditions. The avoidable exogenous exergy destruc-tion of AR was caused by the avoidable low efficiency of FR andaccounted for up to 16.7%. The unavoidable exogenous exergydestruction of AR can be controlled by regulating the reactionprocess in FR. The more intense the CLR process in FR, thesmaller value the unavoidable exogenous exergy destructionhad.
In FR, the unavoidable endogenous exergy destructionand unavoidable exogenous exergy destruction accountedfor a vast majority of exergy destruction. Both the reformingreaction and OC reduction reaction in the reactor wereendothermic reactions, this part of exergy destructionduring reactions was a necessary price for the procession ofreactions, the reaction processes in FR were directly inu-enced by the OC transferred from AR, and the inuencefactors included the oxygen content and heat carried by theOC. So the unavoidable exogenous exergy destructionaccounted for a relatively large proportion. Similarly, theavoidable endogenous exergy destruction can also bereduced by replacing the OC or changing the operatingconditions, although the reducible range was small. Theavoidable exogenous exergy destruction can be reduced byregulating the reaction temperature and circulation rate ofsolids in AR.
According to the results of the advanced exergetic analysis ofAR and FR, the optimization directions for the improvement ofthe exergy efficiency of ICR can be determined.
(1) For the avoidable endogenous exergy destruction causedby the underutilization of thermal exergy in exergy destructionof AR, it is considered to carry out heat exchange treatmentbetween the high-temperature exhaust in AR and materialinow in FR. The optimization by adding a heat exchanger canalso reduce the avoidable exogenous exergy destruction of FRcaused by the transfer of thermal exergy.
(2) For the unavoidable endogenous exergy destructioncaused by the limited utilization efficiency of chemical reac-tions as a result of the redox nature of the OC, the optimizationcan be achieved by replacing the OC.
3.5 Evaluation
The research done by Luis F. et al. on hydrogen productionby chemical-looping reforming in a CFBR was comparedwith ICR experiments in Cases 3–8. The results are shown inthe gure.38 With the decrease of the NiO/CH4 molar ratio,the ICR exhibited higher utilization efficiency of exergy.There were three main reasons for this: (1) compared withthe single heat transfer mode of the CFBR, the ICR not onlytransferred heat through the OC, but also through the direct
22042 | RSC Adv., 2019, 9, 22032–22044
contact of reactor wall, thereby realizing the efficient use ofheat; (2) compared with the complex and long deliverypipeline of the CFBR, the ow mode of ICR was simple andfast; (3) the ICR can work at an operating condition higherthan the normal pressure (Fig. 14).
The advantages of ICR's pressurization operation areobvious, but its main disadvantages cannot be ignored. Gasleakage not only affects the purity and recovery of the syngasproduced, but also reduces the capture rate of CO2 andincreases the system's exergy destruction. Too high gasuidization velocity will result in increased gas leakage,while too low gas uidization velocity will result in theinability to achieve the necessary solid recycle rate.
With increasing demand for hydrogen in fuel cells,petrochemicals, aerospace and other elds, the productionof high-purity hydrogen has become more and moreimportant. However, it is clear that in the application of ICRin hydrogen production, like the traditional hydrogenproduction process, the purication device required for thesubsequent steps still cannot be got rid of. This is related tothe ICR being designed to be suitable for the CLR process atthe beginning of the design. Chemical looping hydrogengeneration (CLH) is a hydrogen production technology thatdoes not require hydrogen purication and carbon captureprocesses, which can signicantly simplify subsequentoperations and reduce costs.39,40 Therefore, in the future,ICR should be improved to a three-cavity reactor suitable forthe CLH process.
The study of the thermodynamic properties of ICRprovides a specic approach and achievable goals for theimprovement of ICR reactors. According to research results,it can be concluded that the system operating at high CH4/NiO molar rate has higher exergy efficiency. The addition ofsteam has a slight benet in the improvement of the system.Enthusiastic effective recovery plays an important role in thereduction of exergy destruction. The development of moreefficient OCs is also a way to reduce exergy destruction.
As a novel reactor suitable for chemical looping processes, ICRhas not been studied in terms of thermodynamic properties. Inthis paper, the exergy efficiency of a methane autothermalchemical looping reforming experiment carried out on an ICR isevaluated by establishing a mathematical analysis model andthe exergy destruction in all processes of each reactor during ofthe experiment are analyzed in detail. Through the model, thespecic processes and main source of exergy destructions canbe obtained. The established thermodynamic exergy destruc-tion evaluation model is also adapted to other reactors. Thepresent study provides optimization guidance for the design ofreactors and the operation of reactors.
A high CH4/NiOmolar ratio can increase the output exergy ofproduct and reduce the exergy destruction of the whole system,which suggests that the CLR process has higher energy effi-ciency than the CLC process. AR contributes to most of theexergy destruction of the system and has much room forimprovement. Since all reaction processes in the AR areexothermic, it is particularly important to use thermal exergyeffectively. Some of the thermal exergy generated is transferredto the FR to support the procession of reactions, while some aredischarged from the AR, along with the unreacted air. Thethermal exergy transferred to the FR is related to the reactionpath of methane. The unused thermal exergy accounts for thelargest part of the exergy destruction inside the reactor. The FRreceives thermal exergy from the AR. In the process of chemicalreactions, with the decrease of the CH4/NiO molar ratio, theexergy destruction gradually decreases. This is because whena unit quantity of methane has a reforming reaction on NiO, theexergy destruction accounts for 33.9%–37.2% of the exergydestruction in combustion reaction.
An advanced exergetic analysis is used to study the mainsource of exergy destructions inside the reactor. In the AR, theavoidable endogenous exergy destruction and unavoidableendogenous exergy destruction account for a large proportion,so to improve the design of the reactor becomes its main opti-mization direction. In the FR, the unavoidable endogenousexergy destruction and unavoidable exogenous exergy destruc-tion account for a large proportion, so to optimize the design ofchemical reaction becomes its main optimization direction.Based on the above two directions, two solutions have beenproposed, i.e., to add a heat recovery unit and to develop a moreefficient OC.
Conflicts of interest
There are no conicts to declare.
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