Influences Of Co Addition On Sulfidation Performance Of Lignite Semi-coke Supported Zn-based Mid-temperature Desulfurization Sorbent

Lignite is relatively abundant in China, but its utilization is strictly limited because of its drawbacks such as high moisture content, low calorific, active chemical reactivity, easy weathering in the air and difficult to transport. But it can be used as a sorbent support thanks to its porous textural structure, surface with abundant polar oxygen-containing functional groups and relatively high mechanical strength. Our research group impregnated the precursor solution of the Zn-based active component directly on the lignite at high pressure, after which the impregnated lignite was pyrolysed in the absence of oxygen to prepare the semi-coke supported sorbent. This procedure combine the preparation of the semi-coke and the thermal decomposition of the active component, which simplified the sorbent preparation process. Former research of our group has optimized the condition of the high pressure impregnation and the pyrolysis during the sorbent preparation process. The optimal condition of the high pressure impregnation process and the pyrolysis was obtained: Zn（NO₃）₂ solution as the precursor solution, impregnation concentration of 36 wt%, impregnation pressure of 20 atm, impregnation time of 5 h, pyrolysis temperature of 550 oC, pyrolysis time of 180 min and heating rate of 10 oC/min.Some problems have been found in the former experiments. COS was found in the tail gas before the concentration of H₂S reached 1 ppm. COS is a kind of organic sulfur compound and it’s more difficult to remove from the coal gas than H₂S. So, the COS formed during the desulfurization process may cause a decrease in the desulfurization performance of the sorbent. Zn-based sorbent has some limitations, what’s more, composite metallic oxide sorbents have superior desulfurization performance not only in terms of textural characteristic but also in terms of desulfurization activity. Co is an active component for H₂S removal, and it is also usually used as a catalyst for COS hydrogenolysis reaction. Thus, we modified the Zn-based sorbent with Co to improve the desulfurization performance and the COS removal capacity of the Zn-based sorbent.On the basis of our former research, we prepared the Zn Co sorbent in the optimal high-pressure impregnation conditions and pyrolysis conditions. The best Zn/Co ratio was investigated, and the reason why Co modification can promote the desulfurization performance of Zn-based sorbent was also studied by the characterization of the sorbents. The influence of Co addition on the formation and removal of COS during the desulfurization process and the influence of desulfurization temperature on the desulfurization performance of the Zn Co sorbent was also investigated. And we re-optimized the pyrolysis condition of the Zn Co sorbent preparation process. Some conclusions were drew and listed as follows:1) The Co addition can promote the desulfurization performance of Zn-based sorbent, and the desulfurization performance of the Zn Co sorbents increased first and then decreased as the Co addition content increased. The best Zn/Co ratio was 36 : 9. Sorbent Zn₃₆Co₉ had the longest breakthrough time and the biggest sulfur capacity, which was 1290 min and 5.11 g S/100 g sorbent, respectively. Synergistic effect existed between Zn and Co in the sorbent.2) COS formed during the desulfurization process was removed through the hydrolysis and hydrogenolysis reaction of COS. The hydrolysis of COS dominated in the desulfurization process of sorbent Zn₃₆Co₀. The Co addition can improve the capacity of the sorbent to remove organic sulfur because of its ability to catalyze the hydrogenolysis reaction of COS.3) The desulfurization performance of sorbent Zn₃₆Co₀ and sorbent Zn₃₆Co₉ decreased as the desulfurization temperature declined. But the Co addition can slow down the decline of desulfurization performance of the sorbent in some extent.4) The optimal pyrolysis condition of the Zn₃₆Co₉ preparation process was: pyrolysis temperature of 550 oC, holding time of 1 h and heating rate of 10 oC/min. The sorbent prepared under this optimal pyrolysis condition had the longest breakthrough time and the biggest sulfur capacity, which was 1620 min and 6.71 g S/100 g sorbent, respectively.