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Heat transfer and thermochem i cal energy storage process of methane dry reforming in a disk reactor with focused solar simulator was modeled and analyzed. The results showed that thermochemical energy storage efficiency of disk reactor can reach 28.4%, and that is higher than that of tubular reactor. The maximum reaction rate occurs at catalyst bed corner near the baffle, because the corner has high temperature and high reactant molar fraction. As reactant flow increases, methane conversion and thermochemical energy storage efficiency decrease as catalyst bed temperature and heat loss decrease. The thermochemical energy storage efficiency increased first and then decreased with methane molar ratio increasing, while methane conversion and the thermochemical energy storage efficiency increased with reactant temperature increasing. As catalyst bed porosity rises, methane conversion and thermochemical energy storage efficiency increased first and then decreased, and optimum porosity is 0.31.

Solar energy is a kind of abundant clean energy, but the cost of energy development and utilization is high and the efficiency is low due to its dispersion and instability. Thermochemical energy storage technology is the most promising high-temperature energy storage method. Research in the past two decades has shown that solar thermal energy can effectively drive chemical conversion reactions [_{4} and CO_{2}.

In recent years, solar reforming reactors are mainly developed into three types: indirectly heated reformer, tubular reformer-receiver and the windowed or volumetric reformer-receiver [

In addition to experimental research, the researchers conducted a large number of numerical simulation studies. Rubin et al. [_{2}-ZrO_{2} catalyst through experiments and simulations. Akbanri et al. [

At present, the research on methane dry reforming thermochemical energy storage process in enhanced reactor still need to be further investigated. In this paper, the thermochemical storage performance of methane dry reforming disk reactor was studied with focused solar simulator. The reactant flow, reactant methane molar ratio, reactant temperature and catalyst bed porosity were analyzed for mechanism of methane dry reforming process in the disk reactor, to find a new method to improve the methane conversion and energy storage efficiency of the system.

focused solar simulator. The reformer is a disk with radius of R_{3} and height of H, and catalyst is also a disk with radius of R_{1}. The inlet region and outlet region are separated by catalyst bed and two baffles. The outer surface with quartz glass at y = 0 is heated by concentrated heat flow from solar simulator.

The main reaction of methane dry reforming reaction is:

CH 4 + CO 2 ↔ 2 CO + 2 H 2 , Δ H m = + 247.3 kJ / mol (1)

The main side reaction is:

CO 2 + H 2 ↔ CO + H 2 O , Δ H s = + 41.1 kJ / mol (2)

Inlet mole fraction of methane is:

y = F CH 4 , i F CH 4 , i + F CO 2 , i (3)

where F CH 4 , i and F CO 2 , i denote inlet flow rates of methane and carbon dioxide under standard condition (1 atm, 20˚C), respectively.

Methane conversion is:

X CH 4 = F CH 4 , i − F CH 4 , o F CH 4 , i (4)

where F CH 4 , o is methane outlet flow under standard condition.

Carbon dioxide conversion rate is:

X CO 2 = F CO 2 , i − F CO 2 , o F CO 2 , i (5)

where F CO 2 , o is carbon dioxide outlet flow under standard condition.

The energy flow received by reactor from solar simulator is:

E i n = ∬ S w q r d S (6)

where q r is concentrated radiant heat flux on reactor surface, S w is surface area of reactor irradiate by solar simulator.

q r = q c exp ( − 1464.3 r 2 ) (7)

where q c denotes central heat flux, and r denotes radius from focus.

Thermochemical energy storage is:

Q c h = F CH 4 ⋅ X CH 4 ⋅ Δ H m v CH 4 + F CO 2 ⋅ ( X CO 2 − X CH 4 ) ⋅ Δ H s v CO 2 (8)

where v denote mole volume under standard condition. Δ H m and Δ H s are reaction heat of main reaction and side reaction.

Sensible heat increment is [

Q s e = ∑ i ∫ T s T 0 F o , i ρ o , i c p , o , i d T (9)

where ρ o , i , c p , o , i are density and specific heat of species i in product, T s and T 0 are surrounding and outlet temperatures.

Thermochemical energy storage efficiency and total energy storage efficiency are:

η c h = Q c h E i n (10)

η t o t a l = Q t o t a l E i n = Q c h + Q s e E i n (11)

Based on existing experimental data, a three-dimensional model of disk reactor with solar simulator is established. The entire process is assumed to be steady, and fluid is assumed to be ideal gas. The model contains solid domain of reactor wall, fluid domain with porous media inside reactor. Catalyst bed is assumed as porous media.

The fluid zone includes inlet and outlet regions, and porous zone of catalyst bed. Continuity equation can be expressed as:

∂ ( ρ f u i ) ∂ x i = 0 (12)

where ρ f is density of mixed fluid, u i is superficial velocity vector based on the total cross-sectional area of fluid and porous medium.

Momentum conservation equation is expressed as:

∂ ∂ x j ( ρ f u i u j ) = ∂ ∂ x j ( μ ∂ u i ∂ x j ) − ∂ p ∂ x i + ρ f g i + S m , i (13)

where μ is dynamic viscosity, p is fluid pressure, g i is gravitational acceleration and S m , i is momentum source caused by flow in porous media. In the inlet and outlet regions, S m , i = 0 .

The momentum source for homogeneous porous media consists of viscosity loss term and inertia term as [

S m , i = ( μ α u i + C 2 1 2 ρ f | u | u i ) (14)

The permeability and internal resistance factor [

α = D p 2 150 γ 3 ( 1 − γ ) 2 (15)

C 2 = 3.5 D p 1 − γ γ 3 (16)

where D p is the diameter of catalyst particles.

During chemical reaction process, the fractions of reactants and products change, and mass transport equation is [

∂ ∂ x j ( ρ f Y i u j ) + ∂ J i , j ∂ x j = r i (17)

where Y i , J i , j and r i are respectively mass fraction, diffusion flux and reaction rate for species.

Chemical reaction is assumed as volumetric reaction, and the reaction rate is calculated by standard Arrhenius equation as:

k = A e − E a / R T (18)

where A and E a mean pre-reaction factor and activation energy.

In solid zone of reactor wall, heat transfer is controlled by heat conduction, and its governing equation is:

∂ ∂ x i ( k w ∂ T w ∂ x i ) = 0 (19)

where T w is temperature of reactor wall, and k w is the thermal conductivity.

In fluid zone, energy conservation equation can be expressed as:

∂ ∂ x i ( ρ f c p u i T ) = ∂ ∂ x i ( k e f f ∂ T ∂ x i ) + S h (20)

where S h is energy source caused by chemical reaction, and k e f f is effective conductivity. S h is directly calculated by enthalpy difference of reactants and products.

The effective thermal conductivity is calculated as volume average of thermal conductivities of fluid and solid as [

k e f f = γ k f + ( 1 − γ ) k s (21)

where k f and k s are conductivity of fluid and porous medium, respectively.

The heat loss from the reactor wall is primarily determined by natural convection and radiation. The boundary condition for heating surface is:

− k w ∇ T w = h n ( T w − T s ) + ε σ ( T w 4 − T s 4 ) − q r (22)

where h n is heat transfer coefficient of natural convection, σ is black body radiation constant, ε is emissivity, and T s is surrounding temperature.

The boundary condition of back surface is:

− k w ∇ T w = h n ( T w − T s ) + ε σ ( T w 4 − T s 4 ) (23)

According to the experimental results, the pre-exponential factor and activation energy of the main reaction are A 1 = 1.2 × 10 7 and E a 1 = 5.8 × 10 7 J / kmol , and those of side reaction are A 2 = 31900 and E a 2 = 1.69 × 10 7 J / kmol . The heating surface of the reactor is affected by the air-cooling system, and h h = 12.6 W / m 2 ⋅ K , while convective heat transfer coefficient on back side is h h = 4 W / m 2 ⋅ K . The emissivity ε of the outer wall surface of the reactor is 0.93, the ambient temperature was 25˚C, and the flow of the reactant inlet was uniform.

Yu et al. [

Condition (y = 0.5) | Methane conversion (%) | Thermochemical energy storage efficiency (%) | ||||
---|---|---|---|---|---|---|

Experiment | Simulation | Relative error | Experiment | Simulation | Relative error | |

q_{c} = 702 kW/m^{2}, F = 3 L/min | 30.9 | 30.0 | -2.90 | 4.02 | 3.69 | 8.15 |

q_{c} = 714 kW/m^{2}, F = 4 L/min | 36.2 | 38.3 | 5.81 | 5.74 | 6.10 | -6.34 |

q_{c} = 678 kW/m^{2}, F = 6 L/min | 40.5 | 43.5 | 7.29 | 10.1 | 10.9 | -7.66 |

mixed gas. The main reaction rate reaches the maximum value at the corner of the catalyst bed near the baffle and decreases toward the center, because reactant molar fraction as methane increase correspondingly from the center to the boundary, while product molar fraction as hydrogen decrease from the center to the boundary.

methane molar ratio increases, reaching a maximum value at y = 0.5, which is due to the best chemical reaction rate under the ideal molar ratio.

water vapor shift reaction going forward, thus the amount of by-product H_{2}O produced will decrease. On the other hand, the production of H_{2} gets the maximum at y = 0.5 under the ideal molar ratio.

reactor with different reactant temperature. As reactant temperature rises, methane conversion increases, because inside catalyst bed temperature increases, and then the reaction rate and methane conversion increase accordingly. Since the catalyst bed temperature in reactor does not increase significantly with the increase of reactant temperature, the sensible energy storage decreases significantly. Therefore, the thermochemical energy storage efficiency increases, and the total energy storage efficiency increases slightly and then gradually decreases, as shown in

The porosity of catalyst bed has important effect on heat and mass transfer process inside the reactor.

In this paper, heat transfer and energy storage performance of methane carbon dioxide disk reactor with concentrated heat flux were numerically studied, and conclusions are as follows:

1) The thermochemical energy storage efficiency of disk reactor can reach 28.4%, and that is remarkably higher than that of traditional tube reactor.

2) The maximum reaction rate occurs at catalyst bed corner near the baffle, because catalyst bed corner has high temperature and high reactant molar fraction.

3) The trends of methane conversion and thermochemical energy storage efficiency are similar. As the reactant flow increases, methane conversion and thermochemical energy storage efficiency decrease as catalyst bed temperature

and heat loss decrease.

4) Increasing reactant temperature is conducive to improving methane conversion and thermochemical energy storage efficiency.

5) As catalyst bed porosity increases, methane conversion and thermochemical storage efficiency increase first and then decrease, and optimum porosity is 0.31.

This paper is supported by Natural Science Foundation of Guangdong Province (2017B030308004) and National Natural Science Foundation of China (U1601215, 51961165101).

The authors declare no conflicts of interest regarding the publication of this paper.

Wang, Y.R., Ding, J. and Lu, J.F. (2020) Numerical Study of Methane Dry Reforming Reaction in a Disk Reactor with Focused Solar Simulator. Energy and Power Engineering, 12, 59-72. https://doi.org/10.4236/epe.2020.122005