Coke evolution during the air- and oxy-firing regeneration of a spent Ni/ZnO sulfur adsorbent

Reactive adsorption desulfurization (RADS) is an effective desulfurization technology in refineries to reduce the sulfur contents in oil products. To ameliorate the desulfurization technology, the evolution of coke was first studied in the air-firing regeneration of a typical spent Ni/ZnO sulfur adsorbent used in RADS. Efforts were made to elaborate the conversion of carbon and sulfur during the oxy-firing regeneration of the spent adsorbent to test the possibility of applying that carbon capture technology – oxy-fuel combustion to the sulfur adsorbent regeneration. A lab-scale drop-tube furnace was employed to conduct the combustion experiments; and the speciation of carbon and sulfur in the regenerated adsorbents was characterized by advanced techniques including near-edge X-ray absorption fine structure (NEXAFS), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The catalytic role of metals (Ni/Zn) in the coke combustion was also verified by acid leaching and temperature-programmed oxidation (TPO) experiments. It has been founded that the carbonaceous materials deposited on the adsorbent comprise five groups, including polyaromatic and graphitic carbon with a fraction of around 27 mol% of the entire carbon, and the other groups which are mostly light aliphatic compounds and oxygen-containing functional groups. During the air-firing regeneration process, most of the light coke species were oxidized into oxygenated intermediates and then desorbed as CO and CO₂, with a small portion being polymerized into heavy molecules. Oxy-firing regeneration was verified to improve the efficiency of coke combustion due to the metal-catalyzed CO₂ gasification reaction and improved the sulfur conversion under the oxygen-lean condition such as 3 vol% O₂ in CO₂. The carbon conversion increased from 23% in 3%O₂/N₂ to 31% in 3%O₂/CO₂ at 510 ℃. In the meantime, the sulfur conversion was also increased from 50% to 57% at 750 ℃, which is probably due to that the CO₂ gasification of coke alleviated the competition of oxygen between carbon and sulfur.