Numerical Modeling on Catalytic Tri-reforming Reaction of Methane for Syngas Production

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 105 (2017) 4198 – 4203

The 8th International Conference on Applied Energy – ICAE2016

Numerical Modeling on Catalytic Tri-reforming Reaction of Methane for Syngas Production KT Wua, CT Yub, RY Cheinc* a Dept. of Forestry, National Chung Hsing University, Taichung, Taiwan, 40227 Division of Chemistry, Institute of Nuclear Energy Research, Taoyuan, Taiwan, 32546 c Dept. of Mechanical Engineering, National Chung Hsing University, Taichung, Taiwan, 40227 b

Abstract In this study, a two-dimensional numerical model was built to study the tri-reforming of methane (TRM) in a tubular fixed-bed reactor under various operation pressures, inlet temperatures, and reactant compositions. It was found that TRM cannot be activated if the reactant inlet temperature were too low. With lower O2/CH4, higher H2O/CH4 and lower CO2/CH4 ratios in the reactant composition, higher H2/CO ratio can be obtained. It was also found that watergas shift (WGS) reaction played an important role for controlling the resulted H2/CO ratio in syngas. © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.

Keywords: Tri-reforming of methane (TRM); Syngas; Reactant composition; Water-gas shift reaction; H2/CO ratio.

1. Introduction CH4-CO2 reforming combines two of the most problematic greenhouse gases to generate syngas for the synthesis of clean liquid fuels and valuable chemicals, and therefore is a promising reaction for the control of global warming. However, this process requires high energy input and there is a risk of carbon formation that would deactivate the catalyst. To overcome these problems, a novel tri-reforming of methane (TRM) process that combines three generally used methane reforming for syngas production were proposed by Song and Pan [1]. In TRM, the following reactions are coupled and carried out in a single reactor: Steam reforming of methane (SRM): 0 (1) 206 kJ / mole CH 4  H 2O l CO  3H 2 , 'H 298 K Dry reforming of methane with CO2 (DRM): 0 CH 4  CO2 l 2CO  2 H 2 , 'H 298 247 kJ / mole K

* Corresponding author. Tel.: +886-4-22840433 ext 307; fax: +886-4-22877170. E-mail address: [email protected]

1876-6102 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.895

ġ (2)

4199

KT Wu et al. / Energy Procedia 105 (2017) 4198 – 4203

Partial oxidation of methane (POM): 0 CH 4  0.5O2 l CO  2 H 2 , 'H 298 36 kJ / mole K

(3)

As shown in Eqs. (1)~(3), the TRM combines the endothermic reactions of SRM and DRM with exothermic reaction of POM. The heat released from POM is used as the heat supply for the SRM and DRM and makes the TRM more energy efficient [2]. Although catalytic TRM have been studied extensively [3-4], further understanding and modeling on TRM are still needed. Because numerical simulation has the advantage of allowing easily studying the influence of the operating conditions for a chemical reactor, a two-dimensional numerical model is established in this study to investigate the TRM performance under various operating conditions and reactant compositions. Particular attention is focused on the high pressure reaction because of the practical syngas application requirements. 2. Physical and mathematical models The tubular fixed-bed reactor used in this study is shown in Figure 1. The reactor has a length of L and a radius of Rb as shown in Fig. 1. A reactant consisting of CH4, O2, H2O, CO2, and N2 is introduced into the reactor from the reactor bottom. Note that N2 serves as the balanced inert gas in the reaction. The pressure and temperature of reactant at reactor inlet are denoted as pin and Tin, respectively. Transport phenomena in the reactor can be described using the conservation equations for mass, momentum, energy and species leading to a set of non-linear partial differential equations. Detail governing equations and boundary conditions for the mass conservation, fluid flow, energy transport, and species transport can be found in the study of Chein et al. [5].

Fig. 1 Physical domain of the numerical model. As noted in the studies of Cho et al. [6] and Arab Aboosadi et al. [7], the alternative representation of chemical reactions involved in TRM can be considered as the combination of steam-methane reforming (SMR) and complete oxidation of methane (COM). That is, the chemical reactions in TRM are Eq. (1) along with the following reactions, Reverse CO2 methanation (RCM): 0 (4) CH 4  2 H 2 O l CO2  4 H 2 'H 298 165 kJ / mole K Water-gas shift (WGS): 0 CO  H 2O l CO2  H 2 , 'H 298 K

Complete methane oxidation (COM):

41 kJ / mole

(5)

4200

KT Wu et al. / Energy Procedia 105 (2017) 4198 – 4203 0 CH 4  2O2 l CO2  2 H 2O , 'H 298 K

803 kJ / mole

(6)

The kinetic models for these reactions were also described in the studies of Cho et al. [6] and Arab Aboosadi et al. [7] and given as, p H3 2 pCO k1 (7) SRM: r p p [  ] / DEN 2 CH 4 H 2O 1 p H2.25 K eq ,1 WGS: r2

pH pCO2 k2 [ pCO p H 2O  2 ] / DEN 2 pH2 K eq, 2

(8)

RCM: r3

p H4 2 pCO2 k3 2 [ pCH4 p H 2O  ] / DEN 2 3.5 pH2 K eq,3

(9)

DEN

1  K CH 4 pCH 4  K CO pCO  K H 2 p H 2 

COM: r4

k p

(1  K

4 a CH 4 C CH 4 CH 4

p

pO2

 K pO2 ) C O2

2



K H 2O p H 2O

(10)

pH2

k p

(1  K

4b CH 4 C CH 4 CH 4

p

pO2

(11)

 KOC2 pO2 )

In Eqs. (7)~(11), K eq ,i and ki are the chemical equilibrium constant and rate constant, respectively, for reaction i (i=1,2,3,4), p j is the partial pressure of species j, and K j and K Cj are the adsorption constants of species j (j=CH4, CO2, H2O, H2, CO). All of these kinetic parameters are given in the Arrhenius function type and are functions of temperature and can be found in the studies of Cho et al. [6] and Arab Aboosadi et al. [7]. 3. Numerical methods All of the governing equations along with the boundary conditions were solved simultaneously using COMSOL multi-physics (Comsol Inc., version 4.4). Multi-physics modules for weakly compressible Navier-Stokes, energy transport and Maxwell-Stefan were applied to solve the velocity, temperature and species concentration distributions inside the reactor. TRM performance is characterized by the following dimensionless groups, FCH 4 ,in  FCH 4 ,out (12) CH4 conversion: X u 100% CH 4 FCH 4 ,in H4 yield: Y H2

FH 2 ,out  FH 2 ,in , CO yield: YCO FCH 4 ,in

FCO ,out  FCO,in FCH 4 ,in

, H2/CO ratio: H / CO 2

FH 2 ,out

(13)

FCO ,out

Where Fi,in and Fi ,out are the molar flow rates at the reactor inlet and outlet of the ith species. Based on these definitions, CH4 conversion is the ratio of CH4 consumption rate to the fed CH4 flow rate at the reactor inlet, H2 and CO yields are defined as the net increased amounts of H2 and CO from reaction per fed CH4 flow rate. 4. Results and discussion The experimentally verified numerical results reported by Arab Aboosadi et al. [7] was used to validate the numerical model built in this study. In their study, optimization of TRM performance in a tubular fixed-bed reactor with length of 2 m was reported by a 1-D model. With the feed temperature, reactant composition, and operation pressure are 1100 K, CH4/CO2/H2O/O2=1/1.3/2.46/0.47, and 20 atm, respectively, the optimized results of methane conversion, hydrogen yield and H2/CO ratio are 97.9%,

4201

KT Wu et al. / Energy Procedia 105 (2017) 4198 – 4203

1.84 and 1.7, respectively. Using the same operation conditions and reactant compositions, Fig. 2 shows the results predicted from the present model. By comparing with the results reported in the study of Arab Aboosadi et al. [7], good agreement is obtained. Based on this comparison, the built numerical model in this study is adequate to be extended to further study on the TRM characteristics subject to variations in operating conditions and syngas compositions. 1800

0.6

(a)

(b)

1700 0.5 1600

mole fraction yi

T (K)

H2O

0.4

1500 1400

H2

0.3

1300

CO2

0.2

CO

1200 0.1

1100 1000

0

0.5

1

reactor length (m)

1.5

2

0

O2 CH4 0

0.5

1

reactor length (m)

1.5

2

Fig. 1 (a) temperature and (b) species mole fraction distributions along reactor length using the optimized operation conditions reported by Arab Aboosadi et al. [7]. Table 1 Reactor geometry and base operation conditions of TRM Reactor length, L Reactor diameter, d Inlet pressure, pin Inlet temperature, Tin Reactant flow rate, F Mole fraction ratio, CH4/O2/H2O/CO2/N2 Catalyst weight, W Catalyst size, dp W/F ratio Heat transfer condition

2m 5 mm 20 atm 300~1000°C 3 mLN min-1 0.1/0.05/0.1/0.1/0.65 0.25 g Ni/Al2O3 0.42~0.5 mm 0.014 ghL-1 Adiabatic

To carry out the parametric study on TRM performance, the base operation conditions and reactant compositions listed in Table 1 are used. It is noted that stoichiometric ratio of reactant species for TRM based on Eqs. (1)~(3) is chosen as the base reactant composition in this study. With the energy expected to be provided from COM, the reactor is operated in adiabatic condition. Fig. 2 shows the effect of inlet temperature on the temperature and species mole fraction distributions in the reactor using the base operation conditions listed in Table 1. From Fig. 2(a), it is seen that TRM cannot be activated as the inlet temperature is low. This is because COM cannot be activated for providing the energy for SRM and DRM reactions as Tin is too low. Based on Fig. 2(a), a peak temperature can be found if the COM were activated. With the energy provided from COM, H2 and CO can be yielded from SRM and DRM reactions. Fig. 2(b) shows the species mole fraction distributions along reactor centerline for Tin=500°C. At the reactor length of 0.8 m, all the O2 was consumed completely. At this reactor length, maximum values of temperature, H2O and CO2 contents were resulted due to COM reaction. After this length, mole fractions of CH4, CO2 and H2O decrease while the mole fractions of CO and H2 increase due to the reactions of SRM and DRM.

4202

KT Wu et al. / Energy Procedia 105 (2017) 4198 – 4203 1400

0.15

(a)

(b)

Tin=500oC

1200

H2 H2O

1000oC o

1000

0.1

mole fraction

900 C

T (oC)

o

800 C o 700 C

800

600oC o

500 C

600

CO2 CO O2

0.05

o

400 C

400

CH4

Tin=300oC 0

200

0.5

1

reactor length (m)

1.5

2

0

0.5

1

reactor length (m)

1.5

2

Fig. 2 (a) Effect of inlet temperature on temperature distribution in the reactor. (b) Species mole fraction distributions in the reactor for Tin=500°C. Fig. 3 shows the effect of operation pressure and W/F ratio on H2/CO ratio under base operation conditions. From Fig. 3(a), the pressure produces insignificant effect on H2/CO ratio. Note that highpressure syngas is usually required for practical applications. From Fig. 3(b), it is seen that effect of W/F ratio on H2/CO ratio is also insignificant except that higher H2/CO ratio can be obtained at lower inlet temperatures with high W/F ratio. With high W/F ratio, the volumetric flow rate is low. Less energy requirement and longer reactant residential time lead to lower inlet temperature for high H2/CO result shown in Fig. 3(b). Also note that H2/CO ratio decreases with the increased Tin. 5

5

(a)

-1

W/F=0.014 ghL

4

(b) p= 20 atm

p=10 atm p=20 atm p=30 atm

W/F=0.14 ghL-1 -1 W/F=0.014 ghL -1 W/F=0.0014 ghL

4

H2/CO

3

H2/CO

3

2

2

1

1

0 500

600

700

800

Tin (oC)

900

1000

0 300

400

500

600

700

Tin (oC)

800

900

1000

Fig. 3 Effect of (a) pressure and (b) W/F ratio on H2/CO ratio under base operation conditions. In Fig. 4, effect of reactant composition on TRM performance is presented. It is found that with increased O2/CH4 ratio, higher peak temperature can be obtained and the location where peak temperature occurs is in more upstream of the reactor. The higher temperature implies that more H2O and CO2 will be produced. For the cases of increasing H2O/CH4 and CO2/CH4, variation in temperature distribution is insignificant. In Fig. 4(a), it is seen that higher H2/CO ratio can be obtained by lowering the O2/CH4 ratio. With O2/CH4=0.75, H2/CO ratio close to unity can be obtained. Note that the reverse water-gas shift reaction becomes more important as temperature is high. In Fig. 4(b), H2/CO ratio can be enhanced by increasing the H2O amount in the reactant. It is also noted that H2/CO with value of unity can be obtained for the H2O/CH4=0 case indicating DRM is the dominant reaction. In Fig. 4(c), decreased CO2 content in the reactant results in increased H2/CO ratio due to TRM dominated by SRM.

4203

KT Wu et al. / Energy Procedia 105 (2017) 4198 – 4203 5

(a)

4

5

(b) H2O/CH4=0 H2O/CH4=1 H2O/CH4=2

4

H2/CO

3

3

2

2

2

1

1

1

0 500

600

700

800

Tin (oC)

900

1000

CO2/CH4=0 CO2/CH4=1 CO2/CH4=2

4

H2/CO

O2/CH4=0.25 O2/CH4=0.5 O2/CH4=0.75

3

(c)

H2/CO

5

0 500

600

700

800

Tin (oC)

900

1000

0 500

600

700

800

Tin (oC)

900

1000

Fig. 4 Effect of ratios of (a) O2/CH4, (b) H2O/CH4, and (c) CO2/CH4 on H2/CO ratio from TRM. 5. Conclusion A two-dimensional numerical model for TRM was presented. The results predicted from the present model indicated that TRM cannot be activated if the inlet temperature of reactant were too low. Although the pressure and reactant volumetric flow rate affect the temperature distributions in the reactor, the resulted variation of H2/CO ratio was insignificant. By varying the reactant species, it was found that H2/CO ratio can be enhanced by using lower O2/CH4 ratio, higher H2O/CH4 ratio, and lower CO2/CH4 ratio in the reactant composition. Acknowledgements This work was supported by the Ministry of Science and Technology, Taiwan, under contracts MOST 104-3113-E-042A-001 and 105-3113-E-033 -001. References [1] Song C, Pan W. Tri-reforming of methane: a novel concept for catalytic production of industrially useful synthesis gas with desired H2/CO Ratios. Catalysis Today 2004;98:463-84. [2] Qian Y, Man Y, Peng L, Zhou H. Integrated process of coke-oven gas tri-reforming and coal gasification to methanol with high carbon utilization and energy efficiency. Ind. Eng. Chem. Res. 2015;54:2519-25. [3] Majewski AJ, Wood J., Tri-reforming of methane over Ni@SiO2 catalyst. Int. J. Hydrogen Energy 2014;39:12578-85. [4] Zhang Y, Zhang S, Gossage JL, Lou HH, Benson TJ. Thermodynamic analyses of tri-reforming reactions to produce syngas. Energy and Fuels 2014;28:2717-26. [5] Chein RY, Chen YC, Chung JN. Numerical study of methanol–steam reforming and methanol–air catalytic combustion in annulus reactors for hydrogen production. Applied Energy 2013;102:1022–34. [6] Cho W, Song T, Mitsos A, McKinnon JT, Ko GH, Tolsma JE, Denholm D, Park T. Optimal design and operation of a natural gas tri-reforming reactor for DME synthesis. Catal. Today 2009;139:261-7. [7] Aboosadi ZA, Jahanmiri AH, Rahimpour MR. Optimization of tri-reformer reactor to produce synthesis gas for methanol production using differential evolution (DE) method. Applied Energy 2011;88:2691–2701.

Biography RY Chein received Ph.D. degree from Washington State University in 1987. His currentġ research interest is in the transport phenomena in energy system. KT Wu received Ph.D. degree from University College London (UCL) in Chemical Engineering in 1997. His current research is in biomass gasification technology.