(186m) On the Temporal Evolution of the Material Stress Profile in a Supercritical Pulverized Coal Boiler Under Load-Following Operation

As more variable renewable generators of energy are penetrating the power grid, coal-fired power plants are increasingly forced to follow the load. Under load-following conditions, supercritical pulverized coal (SCPC) boilers experience considerable changes in their thermal and pressure profiles. Operation at higher temperatures than the design condition causes creep while thermal cycling leads to fatigue. Combined material creep-fatigue damage under cycling operation can eventually lead to the failure of the boiler system. For understanding the stress evolution in an SCPC boiler, a dynamic model of the boiler that can accurately estimate the temperature and pressure profiles in the boiler during transient operation is highly desired. Under sliding pressure operation, not only can the region where the water transitions to supercritical steam change, the boiler can also eventually slide into the subcritical phase under low-load conditions.

Current dynamic models for SCPC boilers mostly use a lumped-parameter approach. This approach fails to accurately capture the local transients of variables since there are highly nonlinear changes in physical properties and heat transfer characteristics near the critical point for water [1]. Effects of the burner configuration [2] and tube layout and design [3] on the boiler transients are also mostly neglected. Furthermore, there is hardly any work that has studied the stress profile in SCPC boilers under load-following operation.

In this work, a rigorous and modular dynamic process model for a once-through SCPC boiler is developed in Aspen Custom Modeler. Separate one-dimensional, distributed-parameter sub-models are considered for the different sections of the boiler (e.g. economizer, final superheater, platen superheater, water wall). It should be noted that due to the complicated flow paths of the water/steam and flue gas through the boiler, it is important to model the entire boiler with all its components to capture the transients accurately. Within each sub-model, rigorous property calculations are carried out. The heat transfer model considers various heat transfer mechanisms, and also accounts for the change in the heat transfer characteristics due to the change in the hydrodynamics and thermal properties of the steam under supercritical and subcritical conditions. This high-fidelity dynamic boiler model provides spatial and temporal changes in the thermal and pressure profiles in the boiler.

A material damage model is developed and integrated with the boiler dynamic model. The damage model considers the combined effect of creep and fatigue degradation phenomena. The creep model is developed with due consideration of the material properties, working load, plastic collapse load, and the yield stress. The fatigue damage model is developed by considering the principal strains at a given fatigue cycle with due consideration of the wall temperature variation along the tube, the temperature gradient along the tube wall and the variation of internal pressure. The integrated creep-fatigue damage model is used to study the dynamic evolution of the spatial stress profile under different load-following scenarios. The model suggests optimal load-following strategies that can significantly reduce material damage due to load-following.

References

[1]	O. Palmqvist, Dynamic Modelling of Heat Transfer Processes in a Supercritical Steam Power Plant, Göteborg, Sweden: Chalmers University of Technology, 2012.
[2]	D. Rakopoulos, I. Avagianos, D. Almpanidis, N. Nikolopoulos and P. Grammelis, "Dynamic Modeling of a Utility Once-Through Pulverized-Fuel Steam Generator," Journal of Energy Engineering, vol. 143, no. 4, 2017.
[3]	K. Yamamoto, H. Suganuma, K. Domoto, Y. Yamasaki, Y. Kanemaki and H. Nakaharai, "Design Technology for Supercritical Sliding Pressure Operation Vertical Water Wall Boilers - First report: History of Practical Application and Introduction of Enhanced Rifled Tube," Mitsubishi Heavy Industries Technicla Review, vol. 50, no. 3, pp. 59-68, 2013.