Conversion of carbon monoxide in supercritical water and the influence of alkaline potassium salts
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摘要: 构建了CO高压溶解的进气系统，在连续式反应系统中对超临界水条件下CO的转化规律进行了研究；针对生物质超临界水气化中钾盐的多样性，选择KHCO3、K2CO3和KOH等三种钾盐成分，研究了它们在不同工艺条件（450-600℃、23-29 MPa、停留时间3-6 s）下对超临界水中水煤气转化过程的影响。结果表明，在无催化条件下，提高反应温度、延长停留时间均提高了CO的转化率，而压力对其影响在低压下（23-25 MPa）比较大，高压下（25-29 MPa）比较小，水煤气转化的反应动力学方程为k=103.75×exp（-0.66×105/RT）（s-1）。碱性钾盐均能显著提高CO转化率，其催化促进程度从大到小依次为：KHCO3>K2CO3>KOH。添加碱性钾盐时，提高反应温度、延长停留时间均提高CO转化率，而压力的影响比较复杂。碱性盐对水煤气转化反应的催化是通过草酸盐（HC2O4-）和甲酸盐（HCOO-）作为中间产物进行的。Abstract: The conversion of carbon monoxide in supercritical water was investigated in a continuous reaction system which could dissolve carbon monoxide in water at high pressure. Meanwhile, due to the diversity of potassium salts in biomass supercritical water gasification, the influence of various alkaline potassium salts (KHCO3, K2CO3 and KOH) on the water gas shift reaction were investigated at 450-600 ℃, 23-29 MPa and with a residence time of 3-6 s. The results show that under non-catalytic conditions, the increases in reaction temperature and residence time both lead to higher CO conversion. The effect of pressure on CO conversion is distinct at low pressure (23-25 MPa), but rather minor at higher pressure (25-29 MPa). The rate expression for the non-catalytic water gas shift reaction is k=103.75×exp(-0.66×105/RT)(s-1). The potassium salts can promote the CO conversion significantly and the activity of three salts follows the order of KHCO3 > K2CO3 > KOH; the conversion of CO is enhanced at higher temperature and longer reaction time, whereas the effect of pressure on CO conversion is much complicated. The catalytic effect of alkaline potassium salts in the CO conversion may be explained by the formation of oxalate (HC2O4-) and formate (HCOO-) intermediates.
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Key words:
- alkaline potassium salts /
- water gas shift /
- supercritical water /
- CO conversion
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图 1 连续式超临界水反应系统示意图
Figure 1 Schematic diagram of continuous supercritical water gasification system
1: feedstock tank; 2: carbon monoxide saturator; 3: high pressure pump; 4: needle valve; 5: precooler; 6: cooling water; 7: reaction area; 8: high pressure ceramic tube; 9: electric stove wire; 10: K type thermocouple; 11: heat insulator; 12: post-cooler; 13: pressure gage; 14: 50 μ m filter; 15: 5 μ m filter; 16: back pressure valve; 17: gas collection; 18: gas-liquid separator
图 2 压力对CO转化率的影响
Figure 2 Conversion of CO under different pressures at 600 ℃ and CO/H2O (mol ratio) = 0.0006
图 3 温度对CO转化率的影响
Figure 3 Conversion of CO along with reaction time under different temperatures at 25 MPa and CO/H2O=0.0006 (mol ratio)
图 4 甲酸根的液相产物分析结果谱图
Figure 4 Ion chromatography spectra of formic acid at 450 ℃, 25 MPa, with a residence time of 6 s and CO/H2O (mol ratio)= 0.0006
图 5 -ln(1-x)随反应时间的变化
Figure 5 Plot of -ln(1-x) against residence time for the WGSR at 25 MPa and CO/H2O (mol ratio)=0.0006
图 6 水气变换反应速率的阿伦尼乌斯图
Figure 6 Arrhenius plot of rate constant for the noncatalytic WGSR in supercritical water
图 7 不同温度时钾盐对CO转化率的影响
Figure 7 Effect of potassium salts on the conversion of CO at 25 MPa and with a residence time of 6 s and CO/H2O (mol ratio) = 0.0006 450-600 ℃, [K+]=0.0015 mol/L
图 8 不同压力时钾盐对CO转化率的影响
Figure 8 Effect of potassium salts on the conversion of CO at 600 ℃ and with a residence time of 6 s and CO/H2O (mol ratio)= 0.0006 [K+]=0.0015 mol/L
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