CORROSION IN COAL-GASIFICATION ENVIRONMENTS AT 550-DEGREES-C

Coal gasification combined cycle power plants promise lower emissions and higher efficiencies than conventional pulverized coal power plants . Process economics favor compact, high pressure, slagging gasifiers, usually of the entrained type, for most coals. The high raw gas temper ature in such plants prevent the use of uncooled metals in contact wit h the gas. Direct metal/gas contact usually occurs in downstream synga s coolers. Water or steam cooled heat exhangers are used to extract he at from the syngas to produce steam at temperatures generally not exce eding 500-degrees-C. Under the dynamic process conditions in the synga s cooler, the syngas composition composition does not shift to low tem perature equilibrium, but the high temperature(1100-130O-degrees-C) ga s composition is quenched in. Laboratory corrosion studies on model al loys, with 20Cr and 35% Ni, were carried out at 550-degrees-C using no nequilibrium gas compositions, representative of commercial oxygen blo wn entrained slagging gasifiers. Such processes can broadly be divided in two types: those in which little or no water or steam is added to the gasifier, and processes where 10-30% steam or water is added (dry coal basis). The former processes produce syngas with a very high CO c ontent, and a very low CO2 and H2O content. The latter processes produ ce a syngas which usually contains 10-20% steam and have a somewhat lo wer CO content. In the water-free syngas nonprotective oxide/sulfide s cales formed and significant corrosion rates were observed. Oxide/sulf ide scales also formed in syngas containing 15% steam, but corrosion r ates were generally low.