Usability of catalytic gas cleaning in a simplified IGCC power system: Deactivation of Ni/AI2O3 catalysts: Literature review

Nickel-based catalysts have proved to be efficient for tar and ammonia decomposition in laboratory-scale gas purification experiments, in which biomass, peat and coal gasification was applied. A potential location for a separate catalyst reactor for an IGCC process using biomass gas derived from a fluidized-bed gasifier is downstream of cyclones before the ceramic filter unit. Complex nature of biomass and peat gas cannot be simulated completely in the laboratory. Long-term tests using a gas stream from an operating gasifier are likely the best way to test catalyst deactivation. Catalyst deactivation may be chemical, mechanical or thermal. Poisoning, fouling, thermal degradation and vaporization are the four intrinsic mechanisms. Poisoning and thermal degradation are generally slow and irreversible; fouling with coke and carbon is rapid but easily reversed by gasification. Loss of metals by vaporization is completely irreversible. Deactivation is more easily prevented than cured. Poisoning by impurities may be prevented by purifying the reactants. Carbon deposition and coking may be prevented by minimizing formation of precursors and by mani?pulating mass-transfer regimes to minimize the effect of carbon or coke on activity. Sintering is avoided by operating at a low temperature. The catalyst should also have a sufficient mechanical strength so that it does not dust or crack while in operation. Thermodynamic calculations showed that in the process conditions likely to be used in the catalytic cleaning unit, nickel oxide is reduced to metallic nickel, carbon (graphite) and nickel sulphide is formed depending on the temperature, pressure and the gas composition of the process. The higher the pressure the more probable is the formation of carbon and nickel sulphide. The probability for carbon formation decreases when the moisture content of the gas increases.