Relationship between pore structure and moisture reabsorption of lignite dewatered by high temperature drying process

doi:10.11949/j.issn.0438-1157.20180822

Abstract

Abstract:

The coal samples dried at different temperatures were prepared by horizontal fixed bed experimental furnace. The specific surface area, pore volume and pore diameter distribution of the coal samples were determined using nitrogen adsorption isotherm (-196℃). The relationship between the pore structure and moisture reabsorption behavior of the lignite samples was investigated using a self-designed reabsorption device. The experimental results show that the N₂ adsorption behavior of the semi-coke dried under different temperatures followed the type II adsorption isotherm, and the adsorption loops of the (raw coal and the semi-coke prepared under 600 and 700℃) belonged to type L1, while the adsorption loop of the semi-coke dried at 800℃ showed a transformation tendency from type L1 to type L2. With the increasing of drying temperature, the specific surface area of the dried semi-coke first increased slowly and then decreased and the pore diameter differential corresponded to the mesopore peak showed the same trend. Meanwhile, the pore diameter differential of the macropores remained unchanged. As the drying temperature increased, the fractal dimensions D₁ and D₂ changed in the opposite trends, and the value of D₂ was higher than D₁. The moisture reabsorption behaviors of the semi-coke followed the same trend, and the equilibrium moisture content decreased with the increasing drying temperature. The moisture reabsorption characteristics of the semi-coke were closely related to the pore structure, and the equilibrium moisture content had a good linear relationship with the pore volume.

Key words: lignite, drying, adsorption, pore structure, fractals, reabsorption

摘要：

利用卧式固定床实验炉制得不同温度下干燥处理的煤样，采用低温氮吸附法测试煤样的比表面积、孔体积和孔径分布等孔隙特征参数，使用复吸实验装置测定不同干燥程度褐煤煤样的平衡含水量，探索了高温干燥处理后褐煤孔隙结构的演变与其复吸特性之间的关联规律。结果表明：褐煤原煤及不同干燥温度(600~800℃)下半焦的等温吸附曲线均属于第Ⅱ类吸附等温线，褐煤原煤、600℃和700℃干燥半焦的吸附回线均属于L1型，800℃干燥半焦的吸附回线有从L1型转变为L2型的趋势；随着干燥温度的增加，干燥半焦的比表面积先增大后减小，介孔峰值的孔径微分同样先增大后减小，而大孔孔径微分基本保持不变；分形维数D₁和D₂呈相反的变化趋势，且D₂>D₁；不同干燥程度半焦的复吸曲线变化趋势相同，且平衡含水量随着干燥温度的升高而减小；半焦复吸特性与孔隙结构有关，平衡含水率与其孔容积之间呈较好的线性关系。

关键词: 褐煤, 干燥, 吸附, 孔隙结构, 分形, 复吸

CLC Number:

TQ 536.1

Yuanmo WU, Shouyu ZHANG, Hua ZHANG, Chen MU, Hao LI, Xiaobing SONG, Junfu LYU. Relationship between pore structure and moisture reabsorption of lignite dewatered by high temperature drying process[J]. CIESC Journal, 2019, 70(1): 199-206.

吴渊默, 张守玉, 张华, 慕晨, 李昊, 宋晓冰, 吕俊复. 高温干燥对褐煤孔隙结构及水分复吸的影响[J]. 化工学报, 2019, 70(1): 199-206.

Figures/Tables 8

Table 1 Proximate and ultimate analyses of raw material

Sample	Proximate analysis/%(mass)					Ultimate analysis/%(mass)
Sample	M_ar	M_ad	A_ad	V_ad	FC_ad	C_ad	H_ad	N_ad	S_t,ad	O_ad
YM	36.70	7.08	13.50	36.19	43.23	56.96	3.61	0.95	0.59	17.31

Fig.1 Low temperature nitrogen sorption isotherms of coal samples before and after drying process

Table 2 Pore structure tests of raw coal and dried semi-coke

Samples	Surface area/ (m²/g)	Pore volume/ (cm³/g)	Average pore diameter/nm
YM	2.13173	0.006769	3.40078
YM-600℃	2.20542	0.006695	3.40177
YM-700℃	2.20848	0.006561	3.39702
YM-800℃	1.71702	0.006455	4.29893

Fig.2 Change of pore size distribution of samples before and after drying process

Fig.3 FHH function diagram of samples before and after drying process

Table 3 Fractal dimension calculation result of samples before and after drying process

Samples	Fractal dimension D₁	Fractal dimension D₂
YM	2.3935	2.5258
YM-600℃	2.3464	2.5413
YM-700℃	2.3833	2.5481
YM-800℃	2.3974	2.4951

Fig.4 Moisture reabsorption curves of samples dried under different temperatures

Fig.5 Correlation test between pore volume and equilibrium moisture content of coal samples

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