Pyrolysis characteristics and kinetics analysis of Qinghua coal, Ningxia based on chemical bonding characteristics of macerals

doi:10.11949/0438-1157.20191156

Abstract

Abstract:

X-Ray photoelectron spectroscopy and Fourier transform infrared spectroscopy were used to characterize the functional group types, surface structure element valence distribution and chemical bond occurrence characteristics of Ningxia Ningdong Qinghua coal. Then the mass loss behavior of vitrinite and inertinite of Qinghua coal at different pyrolysis temperatures and the changes of key gas components were investigated by means of the thermogravimetry coupled with mass spectrometry (TG-MS). Furthermore, the differences of pyrolysis behavior between vitrinite and inertinite were discussed from the point of view of chemical bonding breaking and pyrolysis kinetics based on Coats-Redfern model. The results show that the pyrolysis weightlessness peaks of Qinghua coal macerals at different temperatures are in good agreement with the information of corresponding chemical bond breaking. The TG curves of different macerals of Qinghua coal are similar, but the mass loss rate of vitrinite is always higher than that of inertinite at the same pyrolysis temperature. In the fast pyrolysis stage, vitrinite exhibits greater mass loss rate and also maximum mass loss rate than inertinite. The main reason is that the relative content of aliphatic functional groups in vitrinite is higher, and more C_al—C_al bonds would break down during the fast pyrolysis stage. The order of activation energy and frequency factor of three main pyrolysis stages of Qinghua coal macerals at different pyrolysis temperatures is: fast pyrolysis stage > fast polycondensation stage > slow pyrolysis stage. In the fast pyrolysis stage, the average activation energies of vitrinite and inertinite are both about 75 kJ/mol, but the frequency factor of vitrinite is higher.

Key words: fuel, pyrolysis, kinetic theory, coal macerals, chemical bond characteristics

摘要：

通过X射线光电子能谱和傅里叶红外光谱表征宁夏宁东庆华煤不同显微组分的官能团种类、表面结构元素价态分布及化学键赋存特征。采用热重-质谱联用考察庆华煤镜质组和惰质组在不同热解温度下的失重行为和关键气体组分变化。进一步基于Coats-Redfern模型从化学键断裂特征和反应动力学角度探讨煤镜质组和惰质组的热解行为差异。结果表明，庆华煤显微组分的热解失重峰与相应化学键断裂信息能够很好地吻合。不同显微组分的热重曲线变化趋同，但相同热解温度下镜质组的失重率始终高于惰质组。快速热解阶段镜质组较惰质组表现出更大的失重率和最大失重速率。其主要原因在于镜质组的脂肪族官能团相对含量更高，快速热解阶段会发生更多的C_al—C_al断裂。不同热解温度下庆华煤显微组分三个主要热解阶段的活化能和频率因子大小次序为：快速热解阶段>快速缩合阶段>缓慢热解阶段。在快速热解阶段，镜质组和惰质组的平均活化能均约为75 kJ/mol，但镜质组的频率因子更高。

关键词: 燃料, 热解, 动力学理论, 煤样显微组分, 化学键特征

CLC Number:

TQ 530.2

Ning MAO, Qiang WANG, Yan YANG, Dunxin XU, Wei FENG, Jinpeng ZHANG, Hongcun BAI, Qingjie GUO. Pyrolysis characteristics and kinetics analysis of Qinghua coal, Ningxia based on chemical bonding characteristics of macerals[J]. CIESC Journal, 2020, 71(2): 811-820.

毛宁, 王强, 杨妍, 徐敦信, 冯炜, 张金鹏, 白红存, 郭庆杰. 基于显微组分化学键特征的宁夏庆华煤热解特性及动力学分析[J]. 化工学报, 2020, 71(2): 811-820.

Figures/Tables 13

Table 1 Proximate and petrographic analyses of coal sample

Sample	Proximate analysis/%(mass)				Petrographic analysis/%(vol)
Sample	M_ad	A_ad	V_daf	FC_ad	Vitrinite	Inertinite	Exinite	Minerals
QH	0.86	10.81	19.70	71.62	88.4	7.8	0	3.8

Table 2 Ultimate analysis of coal samples

Sample	Ultimate analysis/%(mass,daf)
Sample	C	H	O	N	S
vitrinite	86.62	5.25	5.47	1.59	1.07
inertinite	86.24	5.02	6.49	1.37	0.88

Fig.1 FTIR spectra of vitrinite and inertinite

Fig.2 TG/DTG curves of vitrinite (a) and inertinite(b)

Fig.3 TG/DTG curves of vitrinite and inertinite at 900℃

Fig.4 DTG profiles of inertinite at 900℃ and fitting by 6 sub-curves through multiple Gaussian functions

Table 3 Chemical bond assignment of peaks from DTG profile

Peak	Possible origin	Peak temperature range/℃	Bond energy range/(kJ/mol)
1,2	release of bonded water and decomposition of carboxylic acid	<300	<150
3	breakage of bonds between C_al and O, S and N, and S—S	300—420	150—230
4	breakage of bonds between C_al and C_al, H, O and C_ar—N	420—550	210—320
5	breakage of bonds between C_ar and C_al, O and S decomposition of carbonates in coals to generate CO₂	550—715	300—430
6	condensation of aromatic rings to release H₂	715—900	>400

Fig.5 Evolution curves of H2, CH4, H2O and CO2 during pyrolysis of vitrinite and inertinite

Table 4 Structural parameters derived from FTIR of coal macerals

Sample	I₁	I₂	I₃	CH₂/CH₃
vitrinite	1.19	2.16	0.34	3.81
inertinite	1.12	1.68	0.71	2.79

Table 5 Fraction of C on vitrinite and inertinite

Sample	Relative content of carbon species/%
Sample	C—C/C—H	C—O	C $? ????$ O	O $? ????$ C—O
vitrinite	76.58	13.85	5.67	3.90
inertinite	75.87	12.10	6.18	5.85

Table 5 Fraction of C on vitrinite and inertinite

Sample	Relative content of carbon species/%
Sample	C—C/C—H	C—O	C $? ????$ O	O $? ????$ C—O
vitrinite	76.58	13.85	5.67	3.90
inertinite	75.87	12.10	6.18	5.85

Table 6 Fraction of O on vitrinite and inertinite

Sample	Relative content of oxygen species/%
Sample	Inorganic oxygen	C $? ????$ O	C—O	O $? ????$ C—O	Adsorbed oxygen
vitrinite	1.25	13.35	74.07	8.64	2.69
inertinite	1.35	17.66	61.92	15.12	3.95

Table 6 Fraction of O on vitrinite and inertinite

Sample	Relative content of oxygen species/%
Sample	Inorganic oxygen	C $? ????$ O	C—O	O $? ????$ C—O	Adsorbed oxygen
vitrinite	1.25	13.35	74.07	8.64	2.69
inertinite	1.35	17.66	61.92	15.12	3.95

Fig.6 Relationship between ln[-ln(1-α)]/T2]and 1/T(taking vitrinite -750℃ as an example)

Table 7 Pyrolysis kinetic parameters of coal samples under different final temperatures

T_d/℃	T/℃	E/(kJ/mol)		A/min^-1		R²
T_d/℃	T/℃	Vitrinite	Inertinite	Vitrinite	Inertinite	Vitrinite	Inertinite
750	180—420	17.26	10.97	3.37	2.88	0.9949	0.9886
	420—550	79.78	83.91	16618.86	7761.03	0.9930	0.9893
	550—715	34.84	35.68	12.76	14.68	0.9868	0.9747
800	180—420	11.89	15.26	4.17	3.86	0.9866	0.9808
	420—550	72.10	77.60	17277.45	8800.29	0.9933	0.9932
	550—715	37.93	33.35	11.64	10.63	0.9979	0.9967
850	180—420	18.25	15.70	3.74	4.64	0.9871	0.9911
	420—550	73.43	74.07	18579.39	7999.22	0.9928	0.9857
	550—715	33.84	38.26	11.23	18.26	0.9969	0.9972
900	180—420	13.63	13.82	6.01	3.86	0.9893	0.9970
	420—550	76.01	76.98	16154.16	8386.05	0.9918	0.9843
	550—715	33.82	37.55	12.63	10.25	0.9936	0.9970

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