CRFD modeling of high-temperature reciprocating grate degradation in a 15 t/h eucalyptus wood chip boiler

Grate firing presents a favorable solution for biomass combustion, accommodating a range of fuels with different bulk sizes and moisture contents, thus reducing the need for extensive fuel preparation and handling. However, it often suffers from considerable degradation, which leads to unscheduled maintenance shutdowns attributed to temperature overload and corrosion. This study aims to investigate the high-temperature degradation processes affecting reciprocating grates in boilers using eucalyptus wood chips as fuel and thus identify critical operating points due to temperature gradients and corrosion conditions in the grate. The degradation of the grate bars results from both the high temperatures inside the grate and the chemical composition of its surface. These conditions can promote thermomechanical fatigue and facilitate high-temperature corrosion of the grate metal. The combustion reaction, temperature, and gaseous species can be modeled using ANSYS Fluent Computational Reactive Fluid Dynamics (CRFD) code. Thermo-Calc software can identify regions prone to metal dusting on the grate metal. The study analyzed the degradation of the grate at high temperatures, particularly emphasizing three zones with high surface temperatures associated with the release of volatiles, char gasification, and oxidation while examining parameters such as maximum, average, uniformity temperature, and the presence of metal dust corrosion. The CFRD simulation results demonstrated the temperature distributions in the furnace grate bars, with maximum temperatures of 1250 K and an average of 700 K, which can cause thermomechanical fatigue. The Calphad calculations identified zones prone to graphite formation and consequent metal dust formation, zones of volatile release up to 1000 K, zones of intense char gasification up to 1250 K, and zones of significant char oxidation across the entire temperature spectrum. The study mapped the high-temperature zones throughout the grate and identified the regions prone to metal dust corrosion, providing knowledge and methodologies that are to be used to improve steam generator designs.