Comparison of reaction characteristics and kinetics between tar thermal cracking and steam reforming in a micro fluidized bed reaction analyzer

doi:10.11949/0438-1157.20211244

Abstract

Abstract:

In this study, a micro fluidized bed reaction analyzer (MFBRA) was used to examine the reaction characteristics of tar steam reforming and thermal cracking in the temperature range of 750—950℃ and steam partial pressure (SP) of 10%—30%. The production of gaseous products and the conversion of total carbon in the gaseous products and the conversion of tar were completely analyzed. The reaction kinetics was also calculated. The results show that during tar thermal cracking, with the increase of reaction temperature, the yields of H₂, CH₄, CO and CO₂ and the conversion of total carbon in the gaseous products increased, and the reaction time decreased. However, in the process of tar steam reforming, by raising the reaction temperature, the reaction time was prolonged largely, and the yields of H₂, CH₄, CO and the conversion of total carbon in the gaseous products increased significantly, while that of CO₂ reached the maximum value at 850℃. During this process, it included not only tar thermal cracking but also the reforming reaction between tar components and the reaction intermediates. At 950℃ and SP of 30%, the conversion of total carbon in the gaseous products reached 92.34%. For the tar steam reforming, with the increase of SP, although the yields of gas components and the conversion of total carbon in the gaseous products increased, the corresponding reaction rate decreased. At the same reaction temperature, with the increase of SP, the generation rates of CO, CH₄ and the rate of total carbon conversion in the gaseous products increased, the H₂ reaction rate gradually decreased with a long stable zone, while the CO₂reaction rate reached its maximum value at 850℃. The activation energy (E_a) of the gaseous products (H₂, CO, CO₂ and CH₄), the conversion of total carbon in the gaseous products and the tar conversion were 90.10, 42.01, 58.56, 64.92, 61.44 and 63.26 kJ/mol, respectively. The corresponding values were obviously less than that of tar thermal cracking. It indicated the promotion effect of steam on tar conversion. Finally, the tar thermal cracking kinetics data was compared with the literature data to verify the feasibility of the MFBRA tar steam reforming reaction test and the accuracy of the analysis results.

Key words: biomass, tar, thermal cracking, steam reforming, micro fluidized bed

摘要：

采用微型流化床反应分析仪(MFBRA)考察了不同温度(T，750~950℃)和水蒸气分压(SP，10%~30%)下生物质焦油水蒸气重整过程中的气体生成、气体产物中总碳转化和焦油转化等反应特性，求算反应动力学，并与焦油热裂解特性进行比较。在热裂解过程中，随温度增加，各气体(H₂、CH₄、CO、CO₂)产率和气体产物中的总碳转化率增加，反应时间缩短。而在焦油水蒸气重整过程中，等温下的反应时间明显延长，且H₂、CH₄、CO产率和气体产物中的总碳转化率显著提升，而CO₂产率在850℃时有最大值。在焦油水蒸气重整过程中，不仅有焦油裂解，还有裂解产物与水蒸气的反应，促进碳转化。在950℃、SP=30%条件下，气体产物中的总碳转化率达到92.34%。水蒸气作用下，气体组分的产率和气体产物中的总碳转化率增加，而等温条件下的反应速率下降。水蒸气分压对各气体组分的影响具有差异性。随分压增加，CO、CH₄的生成速率和气体产物中的总碳转化的反应速率增加；H₂生成速率逐渐下降，速率稳定段扩大；CO₂生成速率在850℃时有最大值。采用均相模型求取焦油水蒸气重整反应过程中的活化能，气体产物的生成活化能(H₂、CO、CO₂和CH₄)、气体产物中的总碳转化及焦油转化的活化能明显偏低，分别为90.10、42.01、58.56、64.92、61.44和63.26 kJ/mol，对应数值明显小于焦油热裂解，说明水蒸气对焦油重整反应的促进作用。最后，将焦油热裂解动力学数据与文献数据对比，验证了MFBRA对焦油水蒸气重整反应测试的可行性和分析结果的准确性。

关键词: 生物质, 焦油, 热裂解, 水蒸气重整, 微型流化床

CLC Number:

TQ 522.64

Peng WU, Fang WANG, Xi ZENG, Hongren ZHAN, Junrong YUE, Tingting WANG, Guangwen XU. Comparison of reaction characteristics and kinetics between tar thermal cracking and steam reforming in a micro fluidized bed reaction analyzer[J]. CIESC Journal, 2022, 73(1): 362-375.

武鹏, 王芳, 曾玺, 战洪仁, 岳君容, 王婷婷, 许光文. 微型流化床中焦油热裂解和水蒸气重整的反应特性及动力学对比[J]. 化工学报, 2022, 73(1): 362-375.

Figures/Tables 15

Fig.1 Simplified reaction diagram of tar steam reforming

Table 1 Reactions involved in the tar steam reforming process

序号	反应方程式	反应类型
R1	tar $→$ char + gas + tar	热裂解反应
R2	C_nH_m + n H₂O $? ????????????$ n CO + (n + m/2) H₂	重整反应
R3	C_nH_x + CO₂ $→$ H₂ + CO	干重整反应
R4	CO + H₂O $?$ CO₂ + H₂	水煤气变换反应
R5	C_nH_x $? ????????????$ n C + (x/2) H₂	积炭反应
R6	C + H₂O $?$ CO + H₂	炭气化反应
R7	C + CO₂ $?$ 2CO	二氧化碳还原反应
R8	CH₄ + H₂O $?$ CO + 3H₂	甲烷蒸气重整

Table 1 Reactions involved in the tar steam reforming process

序号	反应方程式	反应类型
R1	tar $→$ char + gas + tar	热裂解反应
R2	C_nH_m + n H₂O $? ????????????$ n CO + (n + m/2) H₂	重整反应
R3	C_nH_x + CO₂ $→$ H₂ + CO	干重整反应
R4	CO + H₂O $?$ CO₂ + H₂	水煤气变换反应
R5	C_nH_x $? ????????????$ n C + (x/2) H₂	积炭反应
R6	C + H₂O $?$ CO + H₂	炭气化反应
R7	C + CO₂ $?$ 2CO	二氧化碳还原反应
R8	CH₄ + H₂O $?$ CO + 3H₂	甲烷蒸气重整

Fig.2 Schematic diagram of experimental device (a) and structure of micro-fluidized bed reactor (b)

Fig.3 Data treating approch of tar reforming in steam

Fig.4 Variation of H2 yield in tar thermal cracking (a) and steam reforming [(b)—(d)]

Fig.5 Variation of CO yield in tar thermal cracking (a) and steam reforming [(b)—(d)]

Fig.6 Variation of CO2 yield in tar thermal cracking (a) and steam reforming [(b)—(d)]

Fig.7 Variation of CH4 yield in tar thermal cracking (a) and steam reforming [(b)—(d)]

Fig.8 Variation of carbon conversion in the gaseous products during tar thermal cracking (a) and steam reforming [(b)—(d)]

Fig.9 Relationship of reaction rate vs. gas generation yield and carbon conversion at different conditions

Fig.10 Variation of reaction rate vs. gas yield or carbon conversion at different SP

Fig.11 Fitting of homogeneous model data of gas generation and total carbon conversion in gaseous products at different temperatures

Fig.12 Linear fitting of rate constant vs. 1/T at different SP

Table 2 Reaction kinetics data of tar thermal cracking and steam reforming

气体	水蒸气分压	E_a/(kJ/mol)	A/s^-1	R²
H₂	Ar	117.05	427.32	0.993
	SP=10%	86.40	13.14	0.997
	SP=20%	89.40	17.44	0.996
	SP=30%	94.49	26.87	0.999
	average (10%—30%)	90.10	19.15	0.997
CO	Ar	45.28	23.60	0.987
	SP=10%	44.87	12.39	0.999
	SP=20%	39.67	6.25	0.989
	SP=30%	41.49	7.11	0.995
	average (10%—30%)	42.01	8.58	0.994
CO₂	Ar	62.24	94.66	0.982
	SP=10%	56.33	10.79	0.991
	SP=20%	59.45	17.79	0.999
	SP=30%	59.91	11.92	0.990
	average (10%—30%)	58.56	13.50	0.993
CH₄	Ar	62.55	46.89	0.995
	SP=10%	62.90	24.12	0987
	SP=20%	67.51	37.77	0.993
	SP=30%	64.36	26.36	0.999
	average (10%—30%)	64.92	29.42	0.993
C	Ar	69.64	725.83	0.998
	SP=10%	62.51	101.79	0.998
	SP=20%	60.55	125.24	0.989
	SP=30%	61.26	78.63	0.991
	average (10%—30%)	61.44	101.87	0.996
Total	Ar	71.14	706.32	0.992
	SP=10%	63.55	105.14	0.993
	SP=20%	62.81	163.92	0.988
	SP=30%	63.42	99.67	0.999
	average (10%—30%)	63.26	122.91	0.993

Table 3 Comparison of Ea between tar thermal cracking in this study and literatures

产物	仪器	原料	T/K	E_a/(kJ/mol)	文献
total	MFB	biomass tar	1023—1223	69.64	this study
	MFB	biomass tar	1023—1223	78.2	[38]
	fixed bed	biomass tar	773—1073	70.5	[39]
	TGA	biomass tar	1223	79.6	[40]
	fixed bed	biomass tar	773—1373	76.6	[41]
H₂	MFB	biomass tar	1023—1223	117.05	this study
	fixed bed	biomass tar	773—1073	129	[42]
CO	MFB	biomass tar	1023—1223	45.28	this study
	MFB	model compound	1023—1223	53.35	[43]
CH₄	MFB	biomass tar	1023—1223	62.55	this study
	MFB	biomass tar	1023—1223	63.19	[38]

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