Investigating the performance of a fluidized bed reactor for a magnesium hydroxide thermochemical energy storage system

doi:10.19799/j.cnki.2095-4239.2021.0141

Abstract

Abstract:

Thermochemical energy storage is a promising energy storage method because of its high-energy storage density, long-term storage capability, and broad temperature ranges. A fluidized bed reactor has excellent heat and mass transfer performance, suitable for a thermochemical heat-storage system. This study established a two-dimensional axisymmetric unsteady numerical model that included the multiphase flow and chemical reaction, based on the Eulerian–Eulerian model and heat-transfer and reaction kinetics equations. The effects of bed expansion ratio and gas flow rate on the heat storage and release efficiency were analyzed based on the investigation of the reaction processes and using magnesium hydroxide and magnesium oxide as the thermochemical heat storage materials. The authors validated the numerical model and investigated the energy flow and energy consumption optimization in the fluidized bed reactor by conducting experiments. The results indicated that the heat-transfer efficiency did not limit the bed temperature. The temperature differences among the different regions inside the reactor and between the gas phase and the solid phase were less than 1.0 K. The exothermic reaction kinetics limited the performance of the reactor. A high-temperature gas-solid reaction can significantly increase the bed's expansion rate; additionally, the bed expansion ratio and gas flow rate significantly influenced the heat storage efficiency but had little influence on the heat release efficiency. The gas that was preheated in the heat release process, as well as the sensible heat within particles in the heat-storage process, were key aspects of the reactor energy optimization. This research reflects guiding value for the numerical modeling and experimental optimization of a fluidized bed reactor in a thermochemical heat storage system.

Key words: thermochemical energy storage, magnesium hydroxide, fluidized bed reactor, numerical calculation

CLC Number:

TK 02

Bowen YANG, Jun YAN, Changying ZHAO. Investigating the performance of a fluidized bed reactor for a magnesium hydroxide thermochemical energy storage system[J]. Energy Storage Science and Technology, 2021, 10(5): 1735-1744.

Figures/Tables 18

Fig. 1

Table 1

Summary of operating conditions for hydration and dehydration tests"

物理量	表达式
Gidaspow模型的相间交换系数^[28]	$K g s = K s g = 150 ε s (1 - ε g) μ g ε g d p 2 + 1.75 ε s ρ g u ? s - u ? g d p, ε g ≤ 0.8 ? 18 1 + 0.15 ε g R e s 0.67 ε g - 2.65 ε s ρ g u ? s - u ? g d p R e s, ε g > 0.8$
应力应变张量^[29]	$τ ? = ε μ ? u ? + ? u ? T - 23 ? · u ? I ?$
固体压力^[29]	$? P s = 10 - 8.686 ε s + 8.577 ? ε s$
Gidaspow模型的固体剪切黏度^[30]	$μ s = 45 ε s ρ s d p g 0, s s 1 + e s s Θ s π 0.5 + 10 ρ s d p Θ s π 96 ε s 1 + e s s g 0, s s 1 + 45 ε s g 0, s s 1 + e s s 2$
Gidaspow模型的径向分布函数^[31]	$g 0, s s = 1 - ε s ε s, m a x 13 - 1$
颗粒温度	$Θ s = 13 u ? s u ? s$
颗粒碰撞恢复系数	$e s s = 0.9$
Reynolds数	$R e s = ρ g d p u ? s - u ? g μ g$

Table 1

Table 2

Table 3

Fig. 2

Fig. 3

Table 4

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Fig. 10

Fig. 11

Table 5

Fig. 12

Fig. 13

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参数	值
网格数	29×220
网格形状及面积	矩形，10^-6 m²
时间步长	0.001～0.01 s，变步长
收敛准则	最大值0.001
每个时间步长最大迭代次数	30

网格数/个	时间步长/s	床体平均温度/K
20 × 100	0.001 ～ 0.01 s，变步长	604.3
29 × 220	0.001 ～ 0.01 s，变步长	604.5
58 × 440	0.001 ～ 0.01 s，变步长	604.5
29 × 220	0.01 s，固定步长	604.5
29 × 220	0.001 s，固定步长	604.5

编号	氮气流量/(L·min^-1)	水蒸气分压/kPa	气体预加热温度/℃	反应初始温度/℃	反应物质量/g Mg(OH)₂
1	10	47.4	110	144	140
2	7	70.1	110	161	80
3	7	47.4	110	162	60
4	4	57.8	110	159	60

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