基于动力学分析的核桃壳最佳炭化工艺

doi:10.16085/j.issn.1000-6613.2023-0809

摘要/Abstract

摘要：

核桃壳产量多，固定碳含量极高且灰分少。选取核桃壳作为研究对象，对其炭化过程的热动力学参数进行分析，探究其炭化进程及原理，最终通过实验和响应面模拟分析得到核桃壳炭化的最佳工艺。研究发现，综合炭化特性指数随着升温速率的增大呈先增大后减小的趋势，且在10℃/min左右时指数达到峰值，此升温速率下炭化反应更剧烈。核桃壳的炭化过程是一个多阶段的复杂反应过程，其分阶段进行半纤维素、纤维素和木质素的分解，该过程活化能总体逐渐升高。最后通过模拟和实验分析得出最佳工况为：升温时间为14.8min，最终温度为324.7℃，保温时间为60min，材料粒径为5mm左右，最佳炭保留率为69.4%。

关键词: 炭化, 动力学分析, 响应面分析, 最佳炭化工况, 农林废弃物

Abstract:

Walnut shell has high yield, high fixed carbon content and low ash content. In this paper, the walnut shell was selected as the research object, the comprehensive pyrolysis characteristic index was proposed, the thermal dynamics analysis was carried out on it, and the carbonization process and principle were studied. Finally, the best carbonization process was obtained through experiments and response surface simulation analysis. It was found that the comprehensive characteristic index of pyrolysis first increased and then decreased with the increase of heating rate, and reached the peak at about 10℃/min^{since the pyrolysis reaction was more intense at this heating rate. The carbonization process of walnut shell was a multi-stage complex reaction process, which decomposed hemicellulose, cellulose and lignin in stages, and the activation energy of this process increased gradually. Finally, through simulation and experimental analysis, the optimal process was obtained as follows: heating time was 14.8min, final temperature was 324.7℃, soaking time was 60min, material particle size was about 5 mm and the optimal carbonization rate was 69.4%.}

Key words: carbonization, dynamic analysis, response surface analysis, optimal carbonization condition, agricultural and forestry waste

中图分类号:

X705

刘阳, 王云刚, 修浩然, 邹立, 白彦渊. 基于动力学分析的核桃壳最佳炭化工艺[J]. 化工进展, 2023, 42(S1): 94-103.

LIU Yang, WANG Yungang, XIU Haoran, ZOU Li, BAI Yanyuan. Optimal carbonization process of walnut shell based on dynamic analysis[J]. Chemical Industry and Engineering Progress, 2023, 42(S1): 94-103.

图/表 14

表1 核桃壳工业/元素分析

原料	工业分析（质量分数）/%				元素分析（质量分数）/%					Q_net·ar/MJ·kg ^-¹
原料	A_ad	V_ad	M_ad	FC_ad	C	H	O	N	S	Q_net·ar/MJ·kg ^-¹
核桃壳	0.28	70.73	7.77	21.22	48.4	6.18	44.29	0.33	0.01	17.44

图1 不同升温速率炭化特性曲线

表2 不同升温速率下核桃壳热解特性参数

参数	数值
参数	5℃·min ^-¹	10℃·min ^-¹	20℃·min ^-¹	30℃·min ^-¹
T_d/℃	228.5	247.8	259.2	267.3
T_e/℃	389.9	395.1	405.3	416.6
T_P/℃	346.3	361.7	365.2	367.7
ΔW_max/%	51.1	52.7	52.3	52.9
η_∞/%	76.6	77.0	76.9	77.4
(dW/dt)_max/%·min ^-¹	0.71	0.69	0.69	0.70
(dW/dt)_mean/%·min ^-¹	0.1	0.1	0.1	0.1
P/10 ^-⁵%³·min ^-²·℃ ^-¹	9.8	10.1	9.5	9.3

图2 所有升温速率条件下的核桃壳炭化改良等转化率方法曲线图

表3 不同升温速率下等转化率法得到的活化能

转化率 $α$	拟合曲线	活化能E/kJ·mol ^-¹	线性相关系数R²
0.05	y=10.47x+21.07	40.92942	0.96724
0.1	y=11.32x+18.37	74.23701	0.96137
0.15	y=14.93x+18.52	123.99989	0.96492
0.2	y=16.99x+21.47	141.1927	0.97774
0.25	y=23.23x+31.85	192.93921	0.99983
0.3	y=19.52x+24.87	168.19976	0.97851
0.35	y=20.73x+26.42	172.21079	0.96724
0.4	y=22.85x+29.43	175.81415	0.96137
0.45	y=25.21x+32.78	176.44426	0.9587
0.5	y=25.37x+32.41	210.77128	0.95368
0.55	y=26.29x+33.33	218.43556	0.95423
0.6	y=24.82x+30.51	206.15721	0.95912
0.65	y=25.81x+30.69	214.45144	0.96901
0.7	y=25.86x+31.39	214.83034	0.9575
0.75	y=26.27x+31.62	218.23444	0.95142
0.8	y=10.47x+21.07	230.85888	0.91083
0.85	y=27.79x+33.49	383.15627	0.94912
0.9	y=40.92x+47.70	339.92115	0.95142
0.95	y=53.78x+57.11	280.62832	0.99983

表3 不同升温速率下等转化率法得到的活化能

转化率 $α$	拟合曲线	活化能E/kJ·mol ^-¹	线性相关系数R²
0.05	y=10.47x+21.07	40.92942	0.96724
0.1	y=11.32x+18.37	74.23701	0.96137
0.15	y=14.93x+18.52	123.99989	0.96492
0.2	y=16.99x+21.47	141.1927	0.97774
0.25	y=23.23x+31.85	192.93921	0.99983
0.3	y=19.52x+24.87	168.19976	0.97851
0.35	y=20.73x+26.42	172.21079	0.96724
0.4	y=22.85x+29.43	175.81415	0.96137
0.45	y=25.21x+32.78	176.44426	0.9587
0.5	y=25.37x+32.41	210.77128	0.95368
0.55	y=26.29x+33.33	218.43556	0.95423
0.6	y=24.82x+30.51	206.15721	0.95912
0.65	y=25.81x+30.69	214.45144	0.96901
0.7	y=25.86x+31.39	214.83034	0.9575
0.75	y=26.27x+31.62	218.23444	0.95142
0.8	y=10.47x+21.07	230.85888	0.91083
0.85	y=27.79x+33.49	383.15627	0.94912
0.9	y=40.92x+47.70	339.92115	0.95142
0.95	y=53.78x+57.11	280.62832	0.99983

图3 等转化率方法得到的活化能曲线图

图4 核桃壳炭化热解的活化能、转化率和温度的关系

表4 核桃壳炭化热解的活化能与组分反应过程

转化率范围	温度范围	反应	有效活化能E_a
α<0.11	T₀~T₁	水分蒸发，低温易分解组分进行分解	从40.9kJ/mol上升至 86.2kJ/mol
0.11≤α<0.44	T₁~T₃	半纤维素分解出挥发分	从86.2kJ/mol上升至181.7kJ/mol再下降至173.6kJ/mol后续基本没有变化
0.44≤α<0.85	T₃~T₅	纤维素分解出挥发分、木质素分解成炭	从173.6kJ/mol迅速上升至352.2kJ/mol
α≥0.85	T₅~T_e	木质素分解成炭	从352.2kJ/mol下降至280.4kJ/mol

图5 不同尺寸核桃壳炭化原料

表5 响应面分析因素及水平

因素	水平
因素	低水平 (-1)	中间水平 (0)	高水平 (+1)
X₁	0	15	30
X₂	300	400	500
X₃	20	40	60
X₄	0.25	2.625	5

表6 回归方程方差分析

方差来源	平方和	自由度	均方差	F值	P值	显著性
R²=0.9913, Adj R²=0.9792, 信噪比=27.2898
模型	5387.31	14	384.81	81.80	< 0.0001	显著
X₁	39.79	1	39.79	8.46	0.0156
X₂	4338.08	1	4338.08	922.14	< 0.0001
X₃	119.20	1	119.20	25.34	0.0005
X₄	110.84	1	110.84	23.56	0.0007
X₁X₂	4.62	1	4.62	0.9826	0.3449
X₁X₃	0.54	1	0.54	0.1148	0.7417
X₁X₄	2.04	1	2.04	0.4347	0.5246
X₂X₃	0.73	1	0.73	0.1554	0.7017
X₂X₄	23.38	1	23.38	4.97	0.0499
X₃X₄	0.02	1	0.02	0.0042	0.9498
X₁²	6.09	1	6.09	1.29	0.2818
X₂²	406.73	1	406.73	86.46	< 0.0001
X₃²	5.57	1	5.57	1.18	0.3021
X₄²	3.09	1	3.09	0.6565	0.4367
误差	47.04	10	4.70	—	—
失拟项	28.93	3	6.58	7.43	0.0771	不显著
纯误差	1.32	4	0.97	—	—
校正总和	5434.35	24	—	—	—

图6 炭保留率实际值和预测值之间的关系图

图7 炭化变量的交互作用曲面图

表7 模型验证

X₁	X₂	X₃	X₄	q
X₁	X₂	X₃	X₄	实验值	预测值
14.8min	324.7℃	60min	5mm	68.8%	72.3%
14.8min	324.7℃	60min	5mm	70.9%	72.3%
14.8min	324.7℃	60min	5mm	68.6%	72.3%

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