Sulfur transformations in catalytic hot-gas cleaning of gasification gas: Dissertation

The aim of the present work was to obtain new knowledge on the poisoning effects of sulfur on nickel catalysts used for tar and ammonia decomposition in gasification gas. Catalyst performance tests and sulfur poisoning tests were carried out in atmospheric and pressurized fixed-bed tube reactors and in a pressurized honeycomb reactor. The desorption behavior of chemisorbed sulfur from the bed materials was monitored using temperature-programmed hydrogenation. A closed-loop gas-recirculation system was used to measure the isosteric heat of sulfur chemisorption on supported nickel catalysts under hot-gas cleaning conditions. Under the same conditions, sulfur affected the hydrocarbon (tar, methane)-decomposing activity less than the ammonia decomposing activity. When the temperature was increased or the total pressure decreased, the effect of sulfur poisoning likewise decreased. To prevent sulfur poisoning of nickel catalysts in tar and ammonia decomposition, the catalytic process should operate at temperatures above 1173 K. It turned out that bulk nickel sulfide was active in decomposing ammonia under high-temperature gasification gas-cleaning conditions. The methane decomposing activity of the catalyst, however, was not affected by bulk nickel sulfide formation under pressurized conditions, but that of toluene clearly decreased. The activity of the catalyst in ammonia decomposing already increased before the H2S concentration in the gas phase reached the bulk nickel sulfide formation limit. This activity change caused by adsorbed sulfur species, therefore, was not related to the phase change only but was explained by the decrease in enthalpy resulting from sulfur chemisorption on nickel. The dissociative adsorption of ammonia is probably facilitated on the nickel surfaces when the binding energy of sulfur on nickel decreases. Sulfur was adsorbed on nickel catalysts in different chemical states, depending on the process conditions applied. In high-temperature gasification gas (T > 1 173 K) the sulfur adsorbed on the catalyst formed an irreversible monolayer on the catalyst surfaces, while at lower temperatures (T < 1 173 K) the adsorbed sulfur, probably composed of multilayer sulfur, was desorbed from the catalyst in a sulfur-free hydrogen-containing atmosphere. A monolayer of sulfur, however, still remained on the catalyst after desorption. The enhanced effect of high total pressure on sulfur-poisoning of nickel catalysts could be accounted for by the increased amount of sulfur adsorbed on the catalyst. During sulfur adsorption in an H2S/H2 atmosphere, reconstruction (sintering) of the catalysts occurred and probably under some conditions, melt formation on the catalyst surfaces. High surface area and high basicity of support materials favored H2S adsorption on these materials. Under steady-state conditions, the strong sulfur adsorption on a catalyst could be facilitated due to smaller crystallites of nickel.