Materials for Gasifier Heat Exchangers

Abstract:

<p>A wide variety of gasification systems are continuing to be developed around the world, including Integrated Gasification Combined Cycle (IGCC) and the UK developed Air Blown Gasification Cycle (ABGC) systems. Originally, these systems were developed to be fired on various grades of coal, but there is now interest in using a more diverse range of solid fuels (e.g. co-firing coal with waste or biomass, using low grade coals and heavy fuel oils) in order to reduce environmental impact and fuel costs.</p> <p>All gasification technologies require a heat exchanger (often called either a syngas cooler or fuel gas cooler) between the gasifier and the gas cleaning system. The duty required from this heat exchanger varies depending on the type of gasifier, gas-cleaning requirements (e.g. hot dry cleaning or wet scrubbing) and steam cycle needs. However, gasifier hot gas path environments are potentially very aggressive for materials both during plant operation and off-line periods. This has the effect of imposing a temperature window for the safe operation of these heat exchangers (with current materials restricting their use to modest steam conditions and preventing their use as superheaters with commercially viable lives) and dictates that downtime corrosion control precautions are required during off-line periods. There are significant differences in the hot gas path environments between the various gasification systems and with different fuels, but unfortunately these just have the effect of changing the balance between different potential degradation modes arising from the gasification environments.</p> <p>The project has assessed the potential corrosive effects of deposits formed on coal-fired and coal/waste co-fired gasifier fuel-gas/syngas heat exchangers in ABGC and IGCC systems. This has included determining the ranges of deposit compositions formed on heat exchangers with different fuels and quantitatively assessing the effects of such deposits on downtime corrosion (including the effects of potential preventative measures) and synergistic interactions. These activities have lead to the identification of combinations of fuels, operating conditions and materials that could produce rapid heat exchanger failures due to interactions with the deposits formed during the heat exchanger operation.</p> <p>The following candidate gasifier heat exchanger alloys were investigated; AISI 316L, AISI 310, AISI 347H, Alloy 800, Sanicro 28, Haynes 160, Esshete 1250, Haynes 556, IN625 and T23. In terms of cost and performance Sanicro 28 appears to be the best choice for evaporative heat exchangers in the range of test conditions investigated.</p> This report is divided into the following sections: <ol> <li>Introduction</li> <li>Aims and Objectives</li> <li>Background</li> <li>Deposit Formation on Gasifier Hot Gas Path Components</li> <li>Review of Trace Element Partitioning in Gasifier Hot Gas Paths</li> <li>Downtime Corrosion Testing</li> <li>Downtime Corrosion and Preventative Measures</li> <li>Synergistic Testing</li> <li>Identification of Safe Operating Window</li> <li>Conclusions</li> <li>Further Work</li> <li>References</li> </ol>

Author(s):

Kilgallon, P., Simms, N.J. and Oakey, J.E.