High Temperature Corrosion in Gasification of Biomass and Waste The effect of H2 and H2O
Licentiate thesis, 2014
Gasification of biomass and waste is a key issue in reducing the usage of fossil fuels especially in the transportation section (i.e. fuels for vehicles) and thus enabling a more sustainable society. The chemical composition of the produced gas dictates the boundaries within which e.g. superheaters and product gas coolers can operate. However, scientific investigations of corrosion problems in biomass gasification plants are scarce.
To investigate corrosion in a biomass gasfier environment, a stainless steel (304L) and a low alloy steel (T22) were exposed for 4 hours at 600 °C in the 2 MW Chalmers gasifier. The results showed that a thick deposit layer containing high amounts of carbon in addition to K2SO4, ZnS and KCl formed on the sample surface. As expected, the corrosion of stainless steel was less severe than for low alloy steel. On the stainless steel, most of the sample surface was covered with a thin protective oxide layer. However, signs of breakaway oxidation were visible on parts of the surface. A continuous thick oxide layer (about 10 μm) was formed on the low alloyed steel. The oxide consisted of two distinct layers. The outer layer consisted of iron oxide and the inner layer consisted of (Fe, Cr) spinel type oxide.
To provide a more detailed understanding of the corrosion processes in low oxygen environment, significant for the gasifier environment, well-controlled and simplified laboratory exposures were performed. This work concerns environments containing H2 and H2O. Stainless steel (304L) was exposed at 600 °C for 24 and 72 hours in environments containing different H2 and H2O contents. Firstly, a protocol was developed for performing the H2/H2O exposures in order to provide well-defined low a(O2) exposure conditions. Subsequently, the effect of H2 and H2O on corrosion behaviour was investigated. The results showed that an increase of H2 content does not significantly change the oxidation rate. In contrast, by increasing the H2O content, the mass gain and oxide thicknesses were increased considerably. The oxide layers consisted of an outward growing iron oxide (Fe3O4) and inward growing (Fe Cr Ni Mn) spinel type oxide. Cr rich bands were present in the inner oxide layer. Cr was also enriched close to the steel grain boundaries.