Thermochemical storage performance of steam methane reforming in tubular reactor with simulated solar source

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9th International Conference on Applied Energy, ICAE2017, 21-24 August 2017, Cardiff, UK

Thermochemical storage performance of steam methane reforming The 15th International Symposium on District Heating and Cooling in tubular reactor with simulated solar source Assessing the Rong feasibility using the heat bb a,b,demand-outdoor Guaa, JingofDing , Jianfeng Lua,b, * temperature function for a long-term district heat demand forecast School School of of Engineering, Engineering, Sun Sun Yat-Sen Yat-Sen University, University, Guangzhou, Guangzhou, 510006, 510006, China China a a

a,b,c

I. Andrić

b bSchool

School of of Engineering, Engineering, Sun Sun Yat-Sen Yat-Sen University, University, Guangzhou, Guangzhou, 510006, 510006, China China

*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc

a Abstract IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal Abstract b

c

Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France

Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 ruereactor Alfred Kastler, Nantes, France The storage performance of methane in heated by solar The thermochemical thermochemical storage performance of steam steam methane reforming reforming in aa tubular tubular reactor heated 44300 by simulated simulated solar source source was was investigated under different conditions. As inlet flow rate increases, the methane conversion obviously decreases, investigated under different conditions. As inlet flow rate increases, the methane conversion obviously decreases, while while the the thermochemical thermochemical energy energy storage storage efficiency efficiency first first increase increase for for more more reactants, reactants, and and then then it it decreases decreases because because the the methane methane conversion conversion decreases. decreases. 3D 3D numerical numerical model model considering considering unilateral unilateral solar solar irradiation irradiation with with Gaussian Gaussian distribution distribution was was established established to to predict predict heat heat Abstract chemical reaction inside the reactor. The simulation results very well fit with experiment, and the heat transfer of the transfer transfer and and chemical reaction inside the reactor. The simulation results very well fit with experiment, and the heat transfer of the reactor reactor was was further further investigated investigated with with the the impact impact of of energy energy flux flux density. density. As As energy energy flux flux density density increases, increases, the the methane methane conversion conversion District heating networksthermochemical are commonly addressed in the literature as one of the most effective solutions for decreasing the sharply sharply grows, grows, while while peak peak thermochemical energy energy storage storage efficiency efficiency exists. exists. greenhouse gas emissions frombytheElsevier building sector. These systems require high investments which are returned through the heat © © 2017 2017 The The Authors. Authors. Published Published by Elsevier Ltd. Ltd. to theresponsibility changed climate conditions and buildingthe renovation policies,Conference heat demandApplied in the future could decrease, ©sales. 2017 Due The Authors. Published by Elsevier Ltd. committee Peer-review under of the scientific 9th International Energy. Peer-review under responsibility of the scientific committee of of 9th International International Conference Conference on on Applied Energy. Energy. Peer-review responsibility the scientific committee of the the 9th on Applied prolonging under the investment returnofperiod. The mainsteam scopemethane of thisreforming; paper is to assess the feasibility of using solar the heat demand – outdoor Keywords: thermochemical storage; simulated simulated source; numerical model; temperature function for heat demand Keywords: steam methane reforming; thermochemical storage; solar source; numerical model; forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were 1. Introduction 1.compared Introduction with results from a dynamic heat demand model, previously developed and validated by the authors. The results showed that when only weather change is considered, the margin of error could be acceptable for some applications Thermochemical energy storage is one the most promising energy storage for high energy low Thermochemical energy was storage is than one of of thefor most promising energyconsidered). storage ways ways for its its after high introducing energy density, density, low (the error in annual demand lower 20% all weather scenarios However, renovation energy loss and easy transported products [1]. Steam methane reforming is a typical reaction system for energy loss products [1]. Steam methane is ascenarios typical combination reaction system for scenarios, the and error easy value transported increased up to 59.5% (depending on the weatherreforming and renovation considered). thermochemical energy storage and can be used to collect the concentrating solar thermal power [2]. thermochemical energy storageincreased and canon be average used to within collectthe therange concentrating solar thermal power that [2]. corresponds to the The value of slope coefficient of 3.8% up to 8% per decade, Many on steam reforming. Halabi et [3] the hydrogen decrease in the numberfocus of heating of methane 22-139h during the heating on the of weather by and Many researchers researchers focus on the thehours steam methane reforming. Halabiseason et al. al. (depending [3] enhanced enhanced the combination hydrogen production production by investigated low temperature catalytic steam methane reforming. Hafizi et al. [4] analyzed the effect of renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending the investigated low temperature catalytic steam methane reforming. Hafizi et al. [4] analyzed the effect of calcium calciumonand and coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and cerium promoters during the steam methane reforming process. Kim et al. [5] researched the kinetics of steam cerium promoters during the steam methane reforming process. Kim et al. [5] researched the kinetics of steam improve the accuracy of heat demand estimations.

© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * * Corresponding Corresponding author. author. Tel.: Tel.: +8620-3933-2320; +8620-3933-2320; fax: fax: +8620-3933-2319. +8620-3933-2319. Cooling.

E-mail E-mail address: address: [email protected] [email protected] Keywords: Heat demand; Forecast; Climate change 1876-6102 1876-6102 © © 2017 2017 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. Peer-review Peer-review under under responsibility responsibility of of the the scientific scientific committee committee of of the the 9th 9th International International Conference Conference on on Applied Applied Energy. Energy.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy. 10.1016/j.egypro.2017.12.369

Rong Gu et al. / Energy Procedia 142 (2017) 1139–1146 Rong Gu/ Energy Procedia 00 (2017) 000–000

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methane reforming in small stationary reformers. Gokon et al. [6] studied established the steam methane reforming in porous media solar thermochemical reactor model by Monte Carlo ray tracing and finite volume method. In order to carry out the reforming reaction effectively and safely, suitable catalyst is in need and nickel is used most frequently for high catalytic activity, availability and low price [7]. Horiuchi et al. [8] studied the oxidation resistance of Ni/Al2O3 catalyst adding Na, K, Mg and Ca oxides as additives. It was found that the catalyst with αAl2O3 as carrier showed the best anti-carbon deposition performance. Kolaczkowski et al. [9] studied the combined effect of diffusion and chemical reaction in the catalyst pellets with heat and mass transfer. The reactor structure is also an important factor for the thermochemical energy storage and the optimal distribution of the catalyst in different reactors remains to be studied. Klevin et al. [10] tested the effect of different reaction temperature and hydrogen/carbon monoxide ratio on the performance of the steam methane reforming reaction in the catalyst-free particle reactor was. The arrangement of the catalyst inside the reforming tube may cause local overheating, resulting in material failure [11]. So it is necessary to predict the change of the temperature along the tube and Jeongmin Lee [12] studied the numerical simulation of heat transfer and chemical reaction in steam methane reforming. Although various kinds model of reactor for steam methane reforming had been developed, there are few investigations about steam methane reforming reactor heated by simulated solar source. In this article, the thermochemical storage performance of steam methane reforming tubular reactor under a simulated solar source was tested, and a 3D reactor model based on the experimental data to investigate the effect of reactant inlet condition and the structure of reactor was proposed. 2. Experimental system and results 2.1. Experimental system The whole thermochemical storage system includes simulated solar source, tubular reactor and gas cylinders, as illustrated in Fig. 1(a). Simulated solar source consists of 7 parabolic mirrors and 7 xenon lamp with power of 6 kW of each lamp. The tubular reactor is made of quartz and is filled with Ni/Al2O3 with a porosity of 0.42 and bed length of 200 mm, while the outer and inner diameters are 30 mm and 26 mm, as shown in Fig. 1(b). Three K-type thermocouple was respectively positioned inside the reactor. 6

2 13 12

4

14

8

5

1 3

11

7

CH4

9

10

1. methane tank, 2. flow meter, 3. water reservoir, 4. metering pump, 5. evaporator, 6. simulated solar source, 7. tubular reactor, 8. thermocouples, 9. data acquisition system, 10. computer, 11. cold trap, 12. sampler, 13. gas chromatography, 14. tail gas treatment Fig. 1 (a) Experimental system (b) Schematic diagram of system

The main reaction and side reaction of steam methane reforming are: CH4+H2O ⇋ CO+3H2, ΔH1,298K= -205.8 kJ·mol-1 CO+H2O ⇋ CO2+H2, ΔH2,298K= 41 kJ·mol-1

(1) (2)



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The methane conversion is described as:  X CH4

FCH4 ,i  FCH4 ,o FCH4 ,i

(3)

100%

where FCH4,i and FCH4,o respectively represent the inlet and outlet methane mass flow rate. X H2O 

FH2O,i  FH2O,o FH2O,i

(4)

100%

The incident energy flux on the reactor surface from simulated solar source is calculated as: Ein   qr，i dS

(5)

Sw

where qr,i is the local distribution of the energy flux and Sw is the heating surface of reactor facing to the simulated solar source. During steam methane reforming, the thermochemical energy storage power can be calculated as:  Qchem

FCH4 X CH4 H1 vCH4



( FH2O X H2O  FCH4 X CH4 )H 2

(6)

vH2O

where vCH4 and vH2O are respectively molar volumes of methane and steam under standard condition, and ΔH1 represents the reaction heat for main reaction, while ΔH2 represents the reaction heat for side reaction. The chemical energy storage efficiency is described as: chem 

Qchem Ein

(7)

2.2 Experimental results Fig. 2 shows the methane conversion under different reactants flow rate and energy flux density. The methane conversions both decrease obviously with reactants increase under different energy flux density. Besides, the methane conversion is much higher when the energy flux density is bigger under the same reactants flow rate. During the reactants flow rate increase from 4 L/min to 16 L/min, the methane conversion are dropped from 53.2% to 23.8% and 29.5% to 12.9% when the energy flux densities are qc=288.3 kW/m2 and qc=565.8 kW/m2, respectively. 80 2

qc=288.3 kW/m

XCH4 (%)

60

2

qc=565.8 kW/m

40 20 0

4

6

8

10

12

14

16

F (L/min) Fig. 2 Methane conversion vary with reactants flow rate and energy flux density

Fig. 3 presents the thermochemical energy storage performance under different reactants flow rates, where the energy flux densities qc=288.3 kW/m2 and qc=565.8 kW/m2, respectively. Generally, thermochemical energy storage performance has similar tendency under different energy flux densities. When reactants flow rate rises, the thermochemical energy storage efficiency first increase for more reactants, and then it decreases because the

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methane conversion decreases. The maximum thermochemical energy storage efficiency of present system reaches the maximum of 16.1% at 10 L/min when the energy flux density is 228.3 kW/m2. 20

chem (%)

15 10 2

qc=288.3 kW/m

5 0

2

qc=565.8 kW/m

4

6

8

10 12 14 16 F (L/min) Fig. 3 Thermochemical energy storage efficiency vary with reactants flow rate and energy flux density

3. Numerical model of tubular reactor According to the simulated solar source, a 3D symmetry reactor model is developed to study the inner heat and mass transfer and the energy storage performance. The reactor surface is separated into heating wall and back wall. The catalyst filled inside the tubular reactor, and a uniform reactant is imposed in the inlet. The mass conservation equation of porous zone is: (8) (  f ui )  0 where ρf is the fluid density, and ui is the vector of fluid velocity. The momentum conservation equation can be described as:    uk  p   g  Si uiuk     xi xi  xi  xk where μ is the the dynamic viscosity, and Si is the source term. The energy equation of solid zone is:    T   c pT     kw  t xi  xi  where kw is the thermal conductivity of steel wall. The energy equation of porous zone is:     T   c pT    c puiT      keff   Sh t xi xi  xi  where Sh is the source term caused by chemical reaction, and keff is the effective thermal conductivity. The species mass balance can be calculated as: ( Ym,i )  ( J i )  Ri

(9)

(10)

(11)

(12)

where Ym,i, Ri and Ji mean the mass fraction, reaction rate and mass flow of the species. Only forward reaction is to be considered, the reaction rate is determined by Arrhenius expression:

k  Ae

 Ea / RT

(13) where k is the forward rate constant, A is pre-exponential factor, and Ea is activation energy. In the porous media domain, the energy equation of the solid phase and the fluid phase is defined in order to reveal the heat exchange process between the internal fluid and the catalyst solid better. The energy equation is as follows: Fluid phase energy equation:



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T f       f c p , f T f     f c p , f uiT f   Sh  Ssf     keff , f  t xi xi  xi 

(14) where Tf is the temperature of the mixed fluid, cp,f is the specific heat capacity of the mixed fluid, Sh is the energy source term added by the methane steam reforming reaction, Ssf is the volume heat source caused by the heat transfer between the solid and the fluid, keff is the effective thermal conductivity of the mixed fluid calculated by:

0.5PrRe p k f , Re p  0.8 keff ,f   Re p  0.8  0.7 k f ,

(15)

where Rep is the Reynolds number of the mixed fluid and Pr is the Prandtl number. The energy equation for the solid phase is:

Ts     (1   )  s c p , sTs  +Ssf   keff ,s   t xi  xi 

(16) where ρs is the density of the catalyst particles, cp,s is the specific heat capacity of the catalyst, and Ts is the temperature of the catalyst bed. keff,s is the effective thermal conductivity of the solid, can be expressed as:

keff , s  k f (

ks 0.280.757log  0.057log( ks / k f ) )  0.5k f Re p Pr kf

(17)

The heat source Ssf caused by the heat exchange between the solid and the fluid can be expressed as:

Ssf  hsf (Ts -T f )

where hsf is the convective heat transfer coefficient between solid and fluid, and its expression is: 6(1   )k f (2  1.8 Re1/p 2 Pr1/3 ) hsf  Dp2

(18)

(19) Fig. 4 is the physical model of methane steam reforming tubular reactor under radiant heating of solar simulator. The outer wall of the reactor is divided into two parts: the heating surface and the heat dissipation surface along the axis. In Fig. 4, the outer surface above the x-axis is the reactor heating surface heated by the converging heat flow, and the outer surface below the x-axis is the heat dissipation surface where the heat is exchanged with the environment.

Fig. 4 The physical model of the tubular reactor

The boundary condition of heat dissipation surface can be expressed as: T h Tw  Ts 

(20)

The boundary condition of heating surface can be expressed as: T qrad  h Tw  Ts    (Tw4  Ts4 ) where h is the heat transfer coefficient of natural convection, Tw and Ts are the wall temperature and surrounding temperature, σ is the blackbody radiation constant. qrad is the concentrated solar radiation.

(21)

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Concentrated heat flux distribution of focused solar simulator was measured by indirect method. The brightness distribution of the target surface obtained from any angle using the Lambertian target is independent of the incident angle of the light and the target surface brightness is linearly related to the heat flux at that point. And the image of the gray value captured by the CCD camera and the brightness of the point is also subject to a linear relationship, so the gray value and heat flux density have the following correspondence:

E  Kc  GV

(22) where E is the heat flux value at that point, Kc is the scale factor and GV is the gray value at that point. Fig. 5 shows the grayscale and heat flux distributions of the plate heatsink. According to the experimental results, the heat flux density of the convergent heat flux is Gaussian distribution. The expression is:

 qrad Fc exp(570r 2 )

(23) where Fc is the maximum value of energy density at the center of the spot, r is the distance from the center of the spot. R2

R2

R1

R1

(b) Fc=221.0KW/m2

(a) Fc=288.3 KW/m2

Fig. 5 The gray scale of the water - cooled Lambertian target

In present simulation, the pre-exponential factor and activation energy of Arrhenius expression is respectively 1.33×109 and 1.34×105 kJ·mol-1. Table 1 present the comparison of methane conversion between experiment and calculation, where the reactants flow rate varies from 8 L/min to 14 L/min and the energy flux density is 228.3 kW/m2. The calculation results fit the experiment well, as the maximum relative error is 15.3% and the minimum is 1.3%. Table 1. Comparison of methane conversion between experiment and calculation

Experimental conditions No.

Fc (kW/m2)

F (L/min)

1 2 3 4

288.3 288.3 288.3 288.3

8 10 12 14

4. Simulation results

XCH4,e (%) 23.2 19.3 15.4 13.3

Results XCH4,s (%) 26.7 20.3 15.6 12.0

Deviation (%) -15.3 -5.1 -1.3 9.7



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Fig. 6 presents the temperature distribution of the reactor symmetry, where the reactants flow rate is 4 L/min and the energy flux density is 288.3 kW/m2. In catalyst bed section, along the flow direction, the catalyst bed temperature first increases with the energy flux increase, and then decreases as the thermochemical storage and the decrease of energy flux, while the heating face temperature is higher than the coating face. At the zone near the inlet, the temperature decreases because of the heat transfer from the environment and the reactor wall. o

C

1137

998 859 (a) 加热面

720 581

(b) 散热面

442 303

164 (c) 对称面

25

Fig. 6 Temperature distribution of the Heating surface, heat dissipation surface and symmetrical surface of the reactor (qc =288.3 kW/m2，F=4 L/min)

Fig. 7 shows the catalyst bed temperature and methane conversion varies when the energy flux density rises from 200 kW/m2 to 700 kW/m2. The catalyst bed temperature and methane conversion both increases with the energy flux density rising. During the energy flux density increases from 200 kW/m2 to 700 kW/m2, the catalyst bed temperature rises from 843oC to 1251oC and the methane conversion sharply grows from 26.2% to 95.9%. 1400

100 75

o

Tca ( C)

50 1000

Tca

XCH4

800 200

300

400

500

600

XCH4 (%)

1200

25 0 700

2

qc (kW/m ) Fig. 7 Catalyst bed temperature and methane conversion under different peak energy flux density

Fig. 8 shows the thermochemical energy storage efficiency under different energy flux density. When the energy flux density rises, thermochemical energy storage efficiency first increases with the methane conversion increasing, and then it decreases for high heat loss at high reactor surface temperature. There exists peak thermochemical energy storage efficiency 18.3% when the energy flux density is 400 kW/m2.

Rong Gu et al. / Energy Procedia 142 (2017) 1139–1146 Rong Gu/ Energy Procedia 00 (2017) 000–000

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18

ch (%)

ch

16

14 200

300

400 500 2 qc (kW/m )

600

700

Fig. 8 Thermochemical energy storage efficiency under different peak energy flux density

5. Conclusion In this paper, thermochemical storage performance of steam methane reforming reactor heated by a simulated solar source was investigated. A 3D symmetry numerical model was developed and fit well with the experimental data. The simulation result indicates that the catalyst bed temperature first increases and then decreases along the flow direction. The catalyst bed temperature and the methane conversion both increase with the energy flux density increase. And there exists optimal thermochemical energy storage efficiency when the energy flux density rises. Acknowledgements This work was supported by funding of National Natural Science Foundation of China (U1601215, 51476190), Fundamental Research Funds for the Central Universities and Natural Science Foundation of Guangdong Province (2017B030308004). References [1] Steinfeld APR. Solar thermochemical process technology [J]. Encyclopedia of physical science and technology 2001.15(1): 237. [2] Romero M, Steinfeld A. Concentrating solar thermal power and thermochemical fuels[J]. Energy & Environmental Science 2012.5(11): 92349245. [3] Halabi MH, De Croon M, Schaaf J, et al. Low temperature catalytic methane steam reforming over ceria–zirconia supported rhodium. Applied Catalysis A: General 2010.389: 68-79. [4] Hafizi A, Rahimpour MR, Hassanajili S. Hydrogen production via chemical looping steam methane reforming process: effect of cerium and calcium promoters on the performance of Fe2O3/Al2O3 oxygen carrier. Applied Energy 2016.165: 685-694. [5] Kim TW, Park JC, Lim T, et al. The kinetics of steam methane reforming over a Ni/γ-Al2O3 catalyst for the development of small stationary reformers. International Journal of Hydrogen Energy 2015.40: 4512-4518. [6] Gokon N, Nakamura S, Hatamachi T, et al. Steam reforming of methane using double-walled reformer tubes containing high-temperature thermal storage Na2CO3/MgO composites for solar fuel production. Energy 2014.68: 773-782. [7] Bradford MCJ, Vannice MA. CO2 reforming of CH4. Catal Rev 1999. 41:1-42. [8] Horiuchi T, Sakuma K, Fukui T, et al. Suppression of carbon deposition in the CO2-reforming of CH4 by adding basic metal oxides to a Ni/Al2O3 catalyst [J]. Applied Catalysis A: General 144:111-120. [9] Kolaczkowski S, Chao R, Awdry S, et al. Application of a CFD code (FLUENT) to formulate models of catalytic gas phase reactions in porous catalyst pellets. Chemical Engineering Research and Design 2007.85:1539-1552. [10] Klein HH, Karni J, Rubin R. Dry methane reforming without a metal catalyst in a directly irradiated solar particle reactor[J]. Journal of solar energy engineering 2009.131:021001. [11] Swaminathan J, Guguloth K, Gunjan M, et al. Failure analysis and remaining life assessment of service exposed primary reformer heater tubes. Engineering Failure Analysis 2008.15:311-331. [12] Jeongmin L. Characteristics of heat transfer and chemical reaction of methane-steam reforming in a porous catalytic medium. Journal of Mechanical Science and Technology 2016.30:473-481