(371w) Dynamic Modeling and Simulation of the Material Stress Profile in a Supercritical Pulverized Coal Boiler Under Load-Following Operation

In recent years, more renewable sources of energy – which are intermittent and dependent on variable environmental conditions – are being integrated into the power grid. This intermittency forces traditional power sources, such as supercritical pulverized coal-fired (SCPC) power plants to cycle rapidly and sustain low-load operations beyond their original design. This cycling and low-load operation causes drastic changes in the thermal and pressure profiles of the supercritical boiler as the pressure slides to follow the load, which can cause damage, especially to the tubes inside the boiler.

For capturing the tube metal stress evolution, it is important to consider the spatial and temporal profiles of the temperature at both the inner and outer tube surfaces, as well as the through the tube wall. These profiles are practically unmeasurable but can be estimated using first-principles modeling. Furthermore, the pressure profiles inside the tube must also be known for estimating tube metal stress evolution. Due to the severe operating conditions, these variables (especially at the interior of the tubes) are sparingly measured even in highly instrumented boilers and therefore must be estimated. In addition, fouling on the external surface, which is mainly due to ash and slag deposits from the flue gas, and scaling in the internal surfaces affect the tube internal and external surface temperatures and the dynamic characteristics of the through-wall temperature profile, which, in turn, affect both creep and fatigue damages as well as internal and external corrosion. Localized heating can result in creep void formation along the grain boundary leading to intergranular cracking and embrittlement along the grain boundary. This need to estimate the thermal profiles of the tube surfaces and through the tube walls as well as the pressure profiles within the tubes necessitates a first-principles-based process model for an SCPC boiler that is capable of simulating the coal/flue gas and water/steam sides simultaneously in order to evaluate these temperatures and pressures as the SCPC boiler cycles its load.

Many of the available boiler process models in the literature use a lumped-parameter approach[1], [2], which is inadequate for estimating the transient stress profiles along and through the tube walls. There are a few distributed-parameter CFD models available in the literature that have focused on the impact of ash deposition and scaling[3], [4] but mainly on their impact on the temperature. Furthermore, these CFD models focus exclusively on one component of the boiler, such as the superheater, rather than the boiler as a whole. In process models that do consider the entire boiler, there is either a lack of resolution at the tube level that is necessary to determine the effect of fouling on the dynamics of the system[5] or the requirement, again, of a CFD model to simulate the flue gas side of the boiler and therefore calculate the tube surface temperatures[6]. While CFD models exist with these capabilities, to the best of our knowledge, there is no distributed-parameter process model for the water/steam and coal/flue gas sides of all components of an SCPC boiler that has investigated the tube metal stress profiles under load-following operation in presence of time-varying ash buildup.

To fill this gap in the existing literature, a dynamic, distributed-parameter one-dimensional model of an SCPC boiler was developed in Aspen Custom Modeler. This model simulates the water/steam and coal/flue gas sides of the boiler together in a computationally-efficient model that also has enough resolution to capture the effects of fouling on the boiler dynamics and determine the changes in the thermal profile as a function of the load. The model includes distinct submodels of each component in the boiler (water wall, superheaters, reheaters, economizer), that are connected together to model and simulate the entire boiler. This modular approach allows the components to be customized in a variety of boiler configurations, so it can be used to match existing boilers quickly compared to adapting a statically configured model. Each submodel also considers the dynamics of the tubes to more accurately characterize the thermal holdup in the boiler and to allow for due consideration of the fouling caused by ash deposition and scaling. The efficacy of the full boiler model is then demonstrated by simulating cycling operation of an SCPC boiler with varying levels of fouling. Both creep and thermo-mechanical fatigue damage models were also developed and included in the boiler model. The creep and fatigue assessments are conducted following mechanistic models and international standards such as the EN 13345 Standard, and the rainflow-counting method used to determine stress ranges and cycles. Finally, the impact of various load-following and low-load operations on the creep and fatigue evolution is studied with a time-varying ash buildup profile.

References

[1] G. Go and U.-C. Moon, “A Water-Wall Model of Supercritical Once-Through Boilers Using Lumped Parameter Method,” J. Electr. Eng. Technol., vol. 9, no. 6, pp. 1900–1908, 2014.

[2] E. Oko and M. Wang, “Dynamic modelling, validation and analysis of coal-fired subcritical power plant,” Fuel, vol. 135, pp. 292–300, Nov. 2014.

[3] D. Taler, M. Trojan, P. Dzierwa, K. Kaczmarski, and J. Taler, “Numerical simulation of convective superheaters in steam boilers,” Int. J. Therm. Sci., vol. 129, pp. 320–333, Jul. 2018.

[4] M. Trojan and D. Taler, “Thermal simulation of superheaters taking into account the processes occurring on the side of the steam and flue gas,” Fuel, vol. 150, pp. 75–87, Jun. 2015.

[5] D. Rakopoulos, I. Avagianos, D. Almpanidis, N. Nikolopoulos, and P. Grammelis, “Dynamic Modeling of a Utility Once-Through Pulverized-Fuel Steam Generator,” J. Energy Eng., vol. 143, no. 4, p. 04016070, Aug. 2017.

[6] C. Schuhbauer, M. Angerer, H. Spliethoff, F. Kluger, and H. Tschaffon, “Coupled simulation of a tangentially hard coal fired 700°C boiler,” Fuel, vol. 122, pp. 149–163, Apr. 2014.