Rule of heat and mass transfer during the pyrolysis process of pine sawdust particles

doi:10.16085/j.issn.1000-6613.2022-0337

Abstract

Abstract:

To explore the mechanism of heat and mass transfer during the pyrolysis process of organic solid waste particles in a fixed-bed reactor, the pyrolysis of organic solid waste, pine sawdust chosen as raw materials, was simulated and analyzed at the particle scale. The secondary cracking reaction of tar, the mass and momentum transfer of volatile in the pores of particles were considered. Additionally, the flow phenomenon of volatile in the particles was simulated by Darcy’s law. The effects of endothermic reaction and convective heat transfer on particle temperature during particle pyrolysis were also considered. Based on the model of two-step reaction kinetic, the effects of different particle sizes and pyrolysis temperature on the pyrolysis of organic solid waste particles were discussed. The results showed that the endothermic reaction and the convective heat transfer of volatile hindered heat transfer to particle center and prolonged the time for the particles to reach the average temperature. During pine sawdust particles pyrolysis, there was a significant temperature gradient inside the particles. The rate-control step for the pyrolysis of the particles surface was the chemical reaction kinetics while that inside the particles was heat transfer. Additionally, the results showed that the lower pyrolysis temperature was and the larger the particle size was, the more significant the heat transfer resistance was inside the particle. The time required for complete pyrolysis of pine wood chips particles would rise with the increase of particle size, but when particle size was above 10mm, the increment of time required for complete pyrolysis of pine sawdust was greater than that of particles below 10mm.

Key words: pyrolysis of organic solid waste, heat transfer, mass transfer, numerical simulation

摘要：

为探索在固定床反应器中有机固废颗粒热解过程中的热量、质量传递机理，本研究从颗粒尺度上对有机固废松木屑颗粒热解过程建模分析，模型中考虑了焦油的二次裂解反应及挥发分在颗粒孔隙中的质量、动量传递过程，并采用达西定律模拟了挥发分在颗粒孔隙内的流动现象，对颗粒热解过程的吸热反应以及挥发分逸出时的对流换热对颗粒温度的影响进行考察。基于两步反应动力学模型，探讨了不同颗粒尺寸、热解温度对有机固废松木屑颗粒热解过程的影响。结果表明，热解吸热反应和挥发分的对流换热阻碍了热量向颗粒中心的传递，延长了颗粒达到均温的时间；松木屑颗粒热解时，颗粒内会存在明显的温度梯度，在颗粒表面主要受化学反应动力学限制，在颗粒内部则主要受热量传递过程限制。此外，热解温度越低，粒径越大，颗粒内部的传热阻力越大。松木屑颗粒完全热解所需时间会随着颗粒粒径的增大而增加，但当颗粒粒径在10mm以上时，随着颗粒粒径的增大，颗粒完全热解所需时间的增量要大于10mm以下颗粒。

关键词: 有机固废热解, 传热, 传质, 数值模拟

CLC Number:

TQ021

XUE Xiaohui, YUAN Mengli, SONG Yuncai, FENG Jie. Rule of heat and mass transfer during the pyrolysis process of pine sawdust particles[J]. Chemical Industry and Engineering Progress, 2022, 41(12): 6245-6254.

薛小慧, 袁梦丽, 宋云彩, 冯杰. 松木屑颗粒热解过程中的热/质传递规律[J]. 化工进展, 2022, 41(12): 6245-6254.

Figures/Tables 15

工业分析/%					元素分析（daf）/%
M_ad	A_ad	V_ad	FC_ad		C	H	N	S	O^*
5.76	1.03	65.72	27.49		52.41	6.58	0.09	0.14	40.78

组分	w/%	ξ/%	组分	w/%	ξ/%
T^*	55	0	H₂O	16.3	0
C	18.3	0	T₂	0	5.4
CH₄	0.4	5.43	C₂H₄	0	5.17
CO	3.2	31.13	C₂H₆	0	0.38
CO₂	6.8	6.4	H₂	0	1.09

反应	指前因子A/s^-1	活化能E/kJ·mol^-1	反应热∆H/kJ·kg^-1
1	2.66×10¹⁰	106.5	64
2	10^4.98	93.3	-42

物性参数	值或表达式
比热容/J·kg^-1·K^-1	$c p, O = 1112.0 + 4.85 (T - 273)$ $c p, C = 1003.2 + 2.09 (T - 273)$ $c p, T = - 100 + 4.4 (T - 273) - 1.57 × 10 - 3 (T - 273) 2$ $(c p) g, m i x = 1390$
热导率/W·m^-1·K^-1	$λ O = 0.13 + 0.0003 (T - 273)$ $λ C = 0.08 - 0.0001 (T - 273)$ $λ g, m i x = 0.0742$
摩尔质量/g·mol^-1	$M T = 144.05$ ， $M T 2 = 78$
颗粒的初始密度 /kg·m^-3	$ρ 0 = 650$
黏度/Pa·s	$μ T = - 3.73 × 10 - 7 + 2.62 × 10 - 8 T$ ， $μ g, m i x = 0.0742$
环境压力/Pa	$p a m b = 101325$
特征孔径/m	$10 - 7$
黑度	$0.9$

物性参数	值或表达式
比热容/J·kg^-1·K^-1	$c p, O = 1112.0 + 4.85 (T - 273)$ $c p, C = 1003.2 + 2.09 (T - 273)$ $c p, T = - 100 + 4.4 (T - 273) - 1.57 × 10 - 3 (T - 273) 2$ $(c p) g, m i x = 1390$
热导率/W·m^-1·K^-1	$λ O = 0.13 + 0.0003 (T - 273)$ $λ C = 0.08 - 0.0001 (T - 273)$ $λ g, m i x = 0.0742$
摩尔质量/g·mol^-1	$M T = 144.05$ ， $M T 2 = 78$
颗粒的初始密度 /kg·m^-3	$ρ 0 = 650$
黏度/Pa·s	$μ T = - 3.73 × 10 - 7 + 2.62 × 10 - 8 T$ ， $μ g, m i x = 0.0742$
环境压力/Pa	$p a m b = 101325$
特征孔径/m	$10 - 7$
黑度	$0.9$

位置	最大热通量/W·m^-2
r=0	772.26
r=1/4R	5189.2
r=1/2R	8575.9
r=3/4R	11648
r=R	22114

指标参数	2mm	4mm	6mm	8mm	10mm	12mm	14mm	16mm
颗粒表面最大温升速率/K·s^-1	246	232	220	186	172	154	136	115
最大温差/K	130	170	257	292	330	338	353	366
均温/K	868	871	870	870	870	871	871	871
达均温时间/s	17	38	62	92	127	153	184	215
完全热解时间/s	9	20	32	46	61	77	94	110

指标参数	773.15K	823.15K	873.15K
颗粒表面最大温升速率/K·s^-1	124	153	172
最大温差/K	244	275	330
均温/K	771	820	870
达均温时间/s	135	132	127
完全热解时间/s	77	69	61

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