(614b) Health Monitoring of an Industrial Supercritical Pulverized Coal Boiler

Due to increasing penetration of renewables in the electric grid, fossil-fired power plants are being forced to rapidly change their load, tremendously impacting the pressure and thermal profiles throughout these plants, which in turn affect health of the power plant equipment. This is of particular concern for supercritical pulverized coal-fired (SCPC) power plants, which operate at extreme conditions, and especially the boilers of SCPC plants, which experience the highest temperatures and pressures in the plant. Thus, to ensure that SCPC power plants are able to operate reliably under load-following conditions, it is necessary to monitor the health of the boiler to determine when such operation will cause enough damage to render the boiler inoperable and lead to costly unplanned shut downs for maintenance / replacement.

To monitor boiler health, it is necessary to know not only the conditions of the water/steam and flue gas in the boiler but also the temperatures of the steam tubes and header which are the primary components susceptible to damage. While the temperatures of these components are vital to monitoring the stresses and damage to the boiler, they are not typically measured, even in highly instrumented boilers, due to the harsh operating conditions in SCPC boilers. Thus, they must be calculated based on available measurements and those calculated temperatures used to determine the stresses on the components [1]–[4]. However, these methods still rely on measurements being available at the exact points at which stresses need to be calculated, and these locations are not usually known especially for load-following operation. Thus, a distributed-parameter dynamic model of the boiler can instead be used to find the full thermal and pressure profiles of the boiler to calculate the thermomechanical stresses at all points along the boiler. However, current boiler models tend to either neglect to consider key variables necessary for health monitoring, like tube wall temperatures,[5] or focus on only one component of the boiler,[6], [7] which cannot adequately capture the dynamics of the entire boiler system in response to large load changes. Typically, these reductions in the fidelity and scope of the boiler model are done to avoid the computational expenses, allowing the model to run in real- or near real-time. Those model details are instead often considered in more complex computational fluid dynamics (CFD) models that can take weeks to simulate seconds of boiler operation.[8] Thus, there is a need for an SCPC boiler model that is detailed enough to calculate the stresses at all points along the boiler during load-following in real-time and assess the damage that may be caused by such operation.

The model developed here is a first-principles, dynamic, and distributed-parameter model of a boiler capable of simulating load-following scenarios. Additionally, the model considers not only the tube wall temperatures, but the entire through-wall profile of the tubes and headers, which are necessary to characterize the stresses on these components. The model was developed in Aspen Custom Modeler (ACM) in order to take advantage of ACM’s detailed property models, which in the distributed model can account for steep property changes as the boiler transitions from supercritical to subcritical operation. Furthermore, the model is computationally efficient enough to run in real- or near real-time, depending on the magnitude of the load change considered.

The boiler model is validated against load following transient industrial data from an SCPC plant. Such validation tends to be lacking in the open literature where first-principles SCPC models are either not validated[9] or have only a few variables validated – primarily the load and main steam temperature, pressure, and flow.[10] The boiler model presented herein is validated against industrial operational data for multiple points along the boiler at locations where the measurements for temperature, pressure, or flow were available. To perform this validation, the industrial data was reconciled using the comprehensive, dynamic boiler model itself with full consideration of the thermohydraulics within the boiler rather than the simplistic mass and energy balances that are common in the literature[11], [12]. This ensures the dynamic behavior of the boiler is adequately characterized during the reconciliation.

Implemented alongside the boiler dynamic model, the tri-axial stresses are calculated throughout the boiler for monitoring during transient operation. From the tri-axial stresses, damage indicators are calculated for both creep and fatigue damage endured by the boiler. Tube and header damage due to cycling operation is based on the European standard EN 12952-3 for the design and calculation of water-tube boilers,[13] while the tube rupture time is estimated based on the material properties[14]. Using the integrated boiler-stress model, the impact of load-following operation on an industrial SCPC boiler was studied for a variety of scenarios, including various ramp rates and final loads in order to determine the cumulative effect of such operation on the health of the boiler. The transient results of these studies will be presented in the context of number of allowable load cycles and time until tube / header failure. This approach can also provide a framework for the on-line monitoring of SCPC boilers to prevent or reduce the damage incurred when they are forced to follow the load.

References

[1] J. Taler, P. Dzierwa, M. Jaremkiewicz, D. Taler, K. Kaczmarski, and M. Trojan, “Thermal stress monitoring in thick-walled pressure components based on the solutions of the inverse heat conduction problems,” Journal of Thermal Stresses, vol. 41, no. 10–12, pp. 1501–1524, Dec. 2018, doi: 10.1080/01495739.2018.1520621.

[2] J. Taler et al., “Thermal stress monitoring in thick walled pressure components of steam boilers,” Energy, vol. 175, pp. 645–666, May 2019, doi: 10.1016/j.energy.2019.03.087.

[3] J. Taler et al., “Monitoring of transient 3D temperature distribution and thermal stress in pressure elements based on the wall temperature measurement,” Journal of Thermal Stresses, pp. 1–27, Apr. 2019, doi: 10.1080/01495739.2019.1587328.

[4] M. Jaremkiewicz and J. Taler, “Online determining heat transfer coefficient for monitoring transient thermal stresses,” Energies, vol. 13, no. 3, 2020, doi: 10.3390/en13030704.

[5] M. Trojan, “Modeling of a steam boiler operation using the boiler nonlinear mathematical model,” Energy, vol. 175, pp. 1194–1208, May 2019, doi: 10.1016/j.energy.2019.03.160.

[6] S. Grądziel and K. Majewski, “Simulation of heat transfer in combustion chamber waterwall tubes of supercritical steam boilers,” Chemical and Process Engineering - Inzynieria Chemiczna i Procesowa, vol. 37, no. 2, pp. 199–213, Jun. 2016, doi: 10.1515/cpe-2016-0017.

[7] W. Zima and M. Nowak-Ocłoń, “A New One/Two-Dimensional Model of the Conjugate Heat Transfer in Waterwall Tubes of the Supercritical Steam Boiler Combustion Chamber,” Heat Transfer Engineering, vol. 39, no. 13–14, pp. 1272–1282, Aug. 2018, doi: 10.1080/01457632.2017.1364066.

[8] M. Granda, M. Trojan, and D. Taler, “CFD analysis of steam superheater operation in steady and transient state,” Energy, vol. 199, p. 117423, Mar. 2020, doi: 10.1016/j.energy.2020.117423.

[9] J. Taler et al., “Mathematical model of a supercritical power boiler for simulating rapid changes in boiler thermal loading,” Energy, vol. 175, pp. 580–592, May 2019, doi: 10.1016/j.energy.2019.03.085.

[10] A. K. Olaleye, “Modelling and Operational Analysis of Coal Fired Supercritical Power Plant Integrated with Post Combustion Carbon Capture,” 2015.

[11] X. Jiang, P. Liu, and Z. Li, “A data reconciliation based framework for integrated sensor and equipment performance monitoring in power plants,” Applied Energy, vol. 134, pp. 270–282, Dec. 2014, doi: 10.1016/j.apenergy.2014.08.040.

[12] S. Guo, P. Liu, and Z. Li, “Data processing of thermal power plants based on dynamic data reconciliation,” Chemical Engineering Transactions, vol. 61, pp. 1327–1332, 2017, doi: 10.3303/CET1761219.

[13] “Water-tube boilers and auxiliary installations - Part 3: Design and calculation of pressure parts,” London, UK, May 2002.

[14] ECCC, “ECCC Data Sheets 2005.” D G Robertson & S R Holdsworth ETD Ltd., Sep. 2005.