ODS alloys are candidate materials for the critical section in the high temperature heater for indirect fired coal conversion systems, although little has been published on their corrosion characteristics in this situation. Salient features of indirect fired systems are described. Under a European Union COST 501 programme, a natural gas fned ODS alloy heater has been built and operated for a short period as part of a Closed Cycle Gas Turbine Demonstrator. Although the planned environment was clean, a number of critical oxidation related problems emerged during development work associated with the programme. These included scale spallation, breakaway corrosion due to aluminium exhaustion, and nitridation. With respect to the material, oxidation led to porosity induced growth, due to the Kirkendall effect, and low temperature embrittlement. The effect of these can be controlled by establishing criteria for the levels of oxide growth and aluminium exhaustion during service, Tentative limits are suggested. Providing that direct attack from coal ashes and slags is eliminated by good furnace design, as seems possible, current alloys should permit the construction of an indirect coal fired system with an efficiency of around 50%.
Keywords; Indirect Fired Exchangers, ODS Alloys, Breakaway Corrosion, SpaRation, Nitridation, Explosive Welding