Corrosion in coal gasification environments at 550 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 temperature in such plants prevent the use of uncooled metals in contact with the gas. Direct metal/gas contact usually occurs in downstream syngas coolers. Water or steam cooled heat exhangers are used to extract heat from the syngas to produce steam at temperatures generally not exceeding 500 OC. Under the dynamic process conditions in the syngas cooler, the syngas composition does not shift to low temperature equilibrium, but the high temperature (1 100-1300 "C) gas composition is quenched in. Laboratory corrosion studies on model alloys, with 20Cr and 35% Ni, were carried out at 550 OC using nonequilibrium gas compositions, representative of commercial oxygen blown 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 content, and a very low Con and H 2 0 content. The latter processes produce a syngas which usually contains 10-20% steam and have a somewhat lower CO content. In the water-free syngas nonprotective oxide/sulfide scales formed and significant corrosion rates were observed. Oxide/sulfide scaIes also formed in syngas containing 15% steam, but corrosion rates were generally low.