基于CPFD方法的流化床生物质气化数值模拟

doi:10.11949/0438-1157.20200613

摘要/Abstract

摘要：

基于计算颗粒流体动力学（CPFD）建立了三维鼓泡流化床水蒸气-空气混合气化的数值模型，并进行了模型验证，结果表明模拟和实验具有良好的一致性。在该模型的基础上，研究了气化炉内气体分布以及温度分布；同时探究了生物质属性（颗粒粒径、含水率、种类）以及操作条件（气化温度、床料高度）对气化特性的影响。结果表明，生物质颗粒粒径对气化性能的影响存在一个最优值，平均粒径为0.6 mm是最佳的；较高的含水率会降低可燃气体产量，不利于气化反应的进行；四种生物质中，锯末气化的效率最高、可燃气体产量最大、气体热值最高，稻壳仅次于锯末但其碳转化率高于锯末；提高气化温度可以增加可燃气体的比例、提高气化效率；而初始床层高度的变化可以改变H₂/CO的比例。本实验为生物质水蒸气/空气气化提供了理论参考，有助于生物质原料的选取和处理，也有助于气化炉的放大和优化。

关键词: 生物质, 气化, 数值模拟, CPFD, 流化床

Abstract:

Based on computational particle fluid dynamics (CPFD), a three-dimensional bubbling fluidized bed steam-air mixed gasification numerical model was established, and it was verified with experiment trials. The results show that the simulation and experiment have good consistency. Based on the model, the gas distribution and temperature distribution in the gasifier were studied; meanwhile, the biomass properties (particle size, water content, types) and operating conditions (gasification temperature, bed height) were investigated. The results show that there is an optimal value for the impact of biomass particle size on gasification performance, with an average particle size of 0.6 mm being the best; a higher water content will reduce the output of combustible gas and is not conducive to the gasification reaction. Among the four types of biomass, sawdust gasification has the highest efficiency, the largest combustible gas production, and the highest gas calorific value. Rice husk is second only to sawdust but its carbon conversion rate is higher than that of sawdust; increasing the gasification temperature can increase the proportion of combustible gas and increase gasification efficiency; while the change of initial bed height can change the ratio of H₂/CO. This experiment provides a theoretical reference for biomass steam/air gasification, which is helpful for the selection and processing of biomass raw materials, and also facilitates the amplification and optimization of the gasifier.

Key words: biomass, gasification, numerical simulation, CPFD, fluidized bed

中图分类号:

TK 6

任喜熙,陈祁,杨海平,张世红,王贤华,陈汉平. 基于CPFD方法的流化床生物质气化数值模拟[J]. 化工学报, 2020, 71(12): 5763-5773.

REN Xixi,CHEN Qi,YANG Haiping,ZHANG Shihong,WANG Xianhua,CHEN Hanping. Numerical simulation of 3D fluidized bed biomass gasification based on CPFD[J]. CIESC Journal, 2020, 71(12): 5763-5773.

图/表 22

图1 鼓泡流化床气化反应器示意图

Fig.1 Schematic diagram of bubbling fluidized bed gasification reactor

表1 稻壳的工业分析和元素分析

Table 1 Proximate analysis and ultimate analysis of rice husk

元素分析/%(mass, dry)					工业分析/%(mass, dry)				HHV/ (MJ/kg)
C	H	N	S	O	FC	V	M	Ash	HHV/ (MJ/kg)
38.43	2.97	0.49	0.07	36.36	14.99	55.54	9.95	19.52	15.68

表2 控制方程[18-19,21]

Table 2 Governing equations[18-19,21]

名称	控制方程
连续性方程	$? ? t α g ρ g + ? · α g ρ g u g = δ m ˙ s, ????? δ m ˙ s = - ? f d m s d t d m s d u s d T s$
动量方程	$? ? t α g ρ g u g + ? ? α g ρ g u g u g = - α g ? p + α g ρ g g + ? ? τ g + F$
组分输运方程	$? ? t α g ρ g Y g, i + ? ? α g ρ g Y g, i u g = ? ? α g ρ g D g Y g, i + δ m ˙ g, i + δ m ˙ s, i$
能量守恒方程	$? ? t α g ρ g h g + ? ? α g ρ g h g u g = α g ? p ? t + u g ? ? p + φ - ? ? α g q + Q ˙ + S h + q ˙ D$
颗粒运动方程	$A = d u s d t = D s u g - u s + g - 1 α s ρ s ? τ s - ? p ρ s + F s$
颗粒碰撞模型	$τ s = 10 P s α s β m a x α c p - α s, δ 1 - α s$
颗粒能量传递模型	$m s C v d T s d t = ∑ j = 1 N p Q i j + Q s g + Q r a d i + Q r e a c$

表2 控制方程[18-19,21]

Table 2 Governing equations[18-19,21]

名称	控制方程
连续性方程	$? ? t α g ρ g + ? · α g ρ g u g = δ m ˙ s, ????? δ m ˙ s = - ? f d m s d t d m s d u s d T s$
动量方程	$? ? t α g ρ g u g + ? ? α g ρ g u g u g = - α g ? p + α g ρ g g + ? ? τ g + F$
组分输运方程	$? ? t α g ρ g Y g, i + ? ? α g ρ g Y g, i u g = ? ? α g ρ g D g Y g, i + δ m ˙ g, i + δ m ˙ s, i$
能量守恒方程	$? ? t α g ρ g h g + ? ? α g ρ g h g u g = α g ? p ? t + u g ? ? p + φ - ? ? α g q + Q ˙ + S h + q ˙ D$
颗粒运动方程	$A = d u s d t = D s u g - u s + g - 1 α s ρ s ? τ s - ? p ρ s + F s$
颗粒碰撞模型	$τ s = 10 P s α s β m a x α c p - α s, δ 1 - α s$
颗粒能量传递模型	$m s C v d T s d t = ∑ j = 1 N p Q i j + Q s g + Q r a d i + Q r e a c$

表3 非均相动力学参数及其表达式[18-19]

Table 3 Heterogeneous kinetic parameters and expressions[18-19]

反应名称	化学方程式	化学反应速率表达式
焦炭燃烧R₁	$C + O 2 → C O 2$	$R 1 = 5.957 × 102 T e x p - 17975 T 6 d p O 2$
二氧化碳气化R₂	$C + C O 2 → 2 C O$	$R 2 = 1.272 m s T e x p - 1.88 × 108 R T C O 2 - ??????? 1.0436 × 10 - 4 m s T 2 e x p - 1.97 × 107 R T - 20.92 C O 2$
水蒸气气化R₃	$C + β H 2 O → 1 - β C O + β - 1 C O 2 + β H 2$	$R 3 = 1.272 m s T e x p - 1.88 × 108 R T H 2 O - ??????? 1.0436 × 10 - 4 m s T 2 e x p - 5.25 × 107 R T - 17.29 C O$
甲烷化反应R₄	$C + 2 H 2 → C H 4$	$R 4 = 1.368 × 10 - 3 m s T e x p - 8087 T - 7.087 H 2 - ??????? 0.151 m s T 0.5 e x p - 13578 T - 0.372 C H 4 0.5$

表3 非均相动力学参数及其表达式[18-19]

Table 3 Heterogeneous kinetic parameters and expressions[18-19]

反应名称	化学方程式	化学反应速率表达式
焦炭燃烧R₁	$C + O 2 → C O 2$	$R 1 = 5.957 × 102 T e x p - 17975 T 6 d p O 2$
二氧化碳气化R₂	$C + C O 2 → 2 C O$	$R 2 = 1.272 m s T e x p - 1.88 × 108 R T C O 2 - ??????? 1.0436 × 10 - 4 m s T 2 e x p - 1.97 × 107 R T - 20.92 C O 2$
水蒸气气化R₃	$C + β H 2 O → 1 - β C O + β - 1 C O 2 + β H 2$	$R 3 = 1.272 m s T e x p - 1.88 × 108 R T H 2 O - ??????? 1.0436 × 10 - 4 m s T 2 e x p - 5.25 × 107 R T - 17.29 C O$
甲烷化反应R₄	$C + 2 H 2 → C H 4$	$R 4 = 1.368 × 10 - 3 m s T e x p - 8087 T - 7.087 H 2 - ??????? 0.151 m s T 0.5 e x p - 13578 T - 0.372 C H 4 0.5$

表4 均相动力学参数及其表达式[6,18-19,25]

Table 4 Homogeneous kinetic parameters and their expressions[6,18-19,25]

反应名称	化学方程式	化学反应速率表达式
一氧化碳燃烧R₅	$C O + 0.5 O 2 → C O 2$	$R 5 = 5.62 × 1012 e x p - 133031 R T C O O 2 0.5$
氢气燃烧R₆	$H 2 + 0.5 O 2 → H 2 O$	$R 6 = 5.159 × 1015 T - 1.5 e x p - 28517 R T O 2 H 2 1.5$
甲烷燃烧R₇	$C H 4 + 2 O 2 → C O 2 + 2 H 2 O$	$R 7 = 3.552 × 1011 T - 1 e x p - 130530 R T O 2 C H 4$
甲烷与蒸汽反应R₈	$C H 4 + H 2 O → 3 H 2 + C O$	$R 8 = 3.0 × 108 e x p - 125000 R T H 2 O C H 4$
水煤气转化反应R₉	$C O + H 2 O ? C O 2 + H 2$	$R 9 = 7.68 × 1010 T e x p - 304641 R T C O 0.5 H 2 O - ???????? 6.4 × 109 T e x p - 326425.7 R T H 2 0.5 C O 2$

表4 均相动力学参数及其表达式[6,18-19,25]

Table 4 Homogeneous kinetic parameters and their expressions[6,18-19,25]

反应名称	化学方程式	化学反应速率表达式
一氧化碳燃烧R₅	$C O + 0.5 O 2 → C O 2$	$R 5 = 5.62 × 1012 e x p - 133031 R T C O O 2 0.5$
氢气燃烧R₆	$H 2 + 0.5 O 2 → H 2 O$	$R 6 = 5.159 × 1015 T - 1.5 e x p - 28517 R T O 2 H 2 1.5$
甲烷燃烧R₇	$C H 4 + 2 O 2 → C O 2 + 2 H 2 O$	$R 7 = 3.552 × 1011 T - 1 e x p - 130530 R T O 2 C H 4$
甲烷与蒸汽反应R₈	$C H 4 + H 2 O → 3 H 2 + C O$	$R 8 = 3.0 × 108 e x p - 125000 R T H 2 O C H 4$
水煤气转化反应R₉	$C O + H 2 O ? C O 2 + H 2$	$R 9 = 7.68 × 1010 T e x p - 304641 R T C O 0.5 H 2 O - ???????? 6.4 × 109 T e x p - 326425.7 R T H 2 0.5 C O 2$

表5 模拟的边界条件和参数[7,15,18,27-29]

Table 5 Simulated boundary conditions and parameters[7,15,18,27-29]

参数	数值	参数	数值
生物质入口质量流率/(kg/h)	0.3	时间步长（Δt）/s^-1	0.0001
生物质颗粒直径/mm	0.25~0.35	粒子与壁面的法线动量保留系数，e_n	0.3
生物质颗粒密度/(kg/m³)	750	粒子与壁面的切线动量保留系数，e_t	0.99
焦炭密度/(kg/m³)	1300	固相应力模型的无量纲常数，β	3
沙粒密度/(kg/m³)	2300	固相应力模型的无量纲常数，σ	10^-8
气化温度/℃	800	固相最大堆积密度下的体积分数，α_cp	0.6
压力/Pa	101325	碰撞最大动量重定向，ξ	40%

图2 出口气体质量流率随时间的变化

Fig.2 Change of outlet gas mass flow rate with time

表6 沙粒粒径分布

Table 6 Sand particle size distribution

Size/μm	Mass fraction/%
<250	2.5
250—400	8.3
400—500	18.7
500—600	56.5
600—700	9.4
>700	4.6

图3 出口气体摩尔分数随网格数量的变化

Fig.3 Variation of outlet gas mole fraction with the number of grids

图4 实验数据与模拟数据比较

Fig.4 Comparison of experimental data and simulated data

图5 气体沿气化炉轴向变化的平均分布（ER=0.3,S/B=0.5）

Fig.5 The average distribution of gas changes along the axis of the gasifier (ER=0.3, S/B=0.5)

图6 流体温度沿轴向的变化（T=25 s,ER=0.3,S/B=0.5）

Fig.6 Fluid temperature changes along the axis

图7 流体温度沿轴向变化的云图

Fig.7 Contour of fluid temperature changing along the axis

图8 颗粒粒径对合成气摩尔分数的影响

Fig.8 Effect of particle size on the mole fraction of syngas （800℃，ER=0.35，S/B=0.5）

图9 颗粒粒径对CGE、LHV和碳转化率的影响

Fig.9 Effect of particle size on CGE, LHV and carbon conversion rate（800℃，ER=0.35，S/B=0.5）

图10 生物质含水率对合成气摩尔分数的影响（800℃，ER=0.35，S/B=0.5）

Fig.10 Effect of biomass moisture content on the mole fraction of syngas (800℃, ER=0.35, S/B=0.5)

表7 热解实验的气体组成

Table 7 Gas composition of pyrolysis experiment

原料	气体成分/%（体积）
原料	H₂	CO₂	H₂O	CH₄	CO
稻壳	1.72	23.27	8.04	13.58	53.39
锯末	1.71	25.86	11.60	12.24	48.59
树皮	1.80	30.80	9.31	11.48	46.61
玉米秸秆	1.86	28.27	11.29	12.58	45.99

表8 四种生物质的工业分析和元素分析

Table 8 Industrial analysis and elemental analysis of the four biomass

原料	元素分析/%(mass)						工业分析/%(mass)				LHV/ (MJ/kg)
原料	C_ad	H_ad	O_ad	N_ad	S_ad	M_ad	FC_ad	V_ad	A_ad	M_ad	LHV/ (MJ/kg)
稻壳	40.11	5.28	35.59	0.32	0.07	6.2	15.65	65.7	12.45	6.2	15.55
锯末	45.82	5.63	38.69	0.32	0.12	8.91	16	74.59	0.5	8.91	16.91
树皮	38.39	5.03	31.41	0.53	0.11	11.6	14.64	60.83	12.93	11.6	14.5
玉米秸秆	30.64	4.07	29.9	0.87	0.1	8.6	16.71	48.87	25.82	8.6	11.12

图11 不同生物质气化的可燃气体产量

Fig.11 Combustible gas production from different biomass gasification（800℃,ER=0.35,S/B=0.5）

图12 不同生物质气化的CGE、LHV和碳转化率

Fig.12 CGE, LHV and carbon conversion rate of different biomass gasification（800℃,ER=0.35,S/B=0.5）

图13 气化温度对气体产量的影响（ER=0.35,S/B=0.5）

Fig.13 Effect of gasification temperature on gas production (ER=0.35, S/B=0.5)

图14 初始床层高度对出口气体摩尔分数的影响（ER=0.35,S/B=0.5）

Fig.14 Effect of initial bed height on outlet gas mole fraction (ER=0.35, S/B=0.5)

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