Compact Steam Bottoming Cycles: Minimum Weight Design Optimization and Transient Response of Once-Through Steam Generators

Abstract

Compactness and weight minimization are key aspects for successful and widespread implementation of waste heat recovery steam cycles in off-shore oil and gas platforms due to the site weight and volume footprint constraints. The power plant off-shore must be designed for flexibility in its operations to provide varying power demands across multiple time scales. Reliability of the heat and power production units is crucial. Within a case study in an off-shore platform in the Norwegian Continental Shelf, this article conducts design optimization of compact and low-weight steam cycles for power production from gas turbine exhaust and transient analysis of the core of heat recovery steam generators (HRSGs) via dynamic modeling and simulation, considering once-through steam generators (OTSGs) for the HRSGs. A method for simultaneous thermodynamic and heat exchanger geometry optimization design for bottoming cycles is applied, with the main objective being weight minimization and compactness of the cycle heat exchangers. Ten different optimal minimum weight bottoming cycle designs are provided by selecting ten different manufacturable tubes. The resulting bottoming cycle designs are compared in terms of weight, OTSG core weight distribution, heat transfer area, and footprint. The resulting bottoming cycle weight varies from 48.4 to ca. 87.10 ton for designs sensible for off-shore applications, and from 95.8 to 178.9 ton when selecting outer tube diameters typical of onshore applications. Smaller outer tube diameter selection in OTSG bundles is a key driver for low-weight and compact steam cycle designs. Three different designs representing light, normal, and heavy OTSG designs are compared by dynamic trajectory and response time analysis under transient scenarios by means of dynamic modeling and simulation. More compact and lighter designs respond faster to changes in the gas turbine (GT) operation upstream the OTSG. The results in this analysis indicate the need for feedforward control. Feedback control alone is probably not a good option due to the high OTSG open loop stabilization time and large sensitivity to GT exhaust gas variations. More compact and low-weight designs based on the OTSG can reduce potential challenges in controlling and stabilizing bottoming cycles for power production.

Hybrid Dynamic Surrogate Modelling for a Once-Through Steam Generator

“…Development of surrogate models generally requires data from which the main characteristics of the considered system can be extracted. In the present work, this data will stem from a previously published high-fidelity (hi-fi), dynamic OTSG model for offshore combined cycle applications [Montañés et al (2021)]. The model is developed in the Modelica language and utilizes dynamic energy and mass balances to produce a set of differential algebraic equations describing the OTSG's transient behavior.…”

“…The model is developed in the Modelica language and utilizes dynamic energy and mass balances to produce a set of differential algebraic equations describing the OTSG's transient behavior. Figure 1 (adapted from [Montañés et al (2021)]) shows the OTSG model with discretization and main states. The model is based on 1-D lumped pressure flow pipes (for gas and water/steam sides) separated from each other by a wall model (representing the metal wall for heat transfer in a bundle of tubes).…”

“…A total of n volumes and n+1 nodes are used to discretize the system in the direction of the flow. Overall, the model includes physical phenomena related to heat transfer, hydraulics, wall model thermal inertia, dynamic energy and mass balances on both the gas side and the water side [Montañés et al (2021)]. We consider the OTSG operated under valve-throttling pressure control mode, i.e., with fixed pressure set point of the produced superheated steam [Nord and Montañés (2018), Zotică et al (2022)].…”

“…The once-through steam generator (OTSG) has been identified as a particularly important component because its response to transients in the gas turbine load largely governs the GTCC's overall behavior. SINTEF Energy Research has previously developed highly accurate Modelica models for OTSGs [Montañés et al (2021)]. In this work, we explore four data-driven techniques for creating high-speed, DT-suitable surrogate models based on the Modelica model: Linear regression (LReg), polynomial regression (PReg), Gaussian process regression (GPR), and neural networks (NNs).…”

Numerical modelling of fin side heat transfer and pressure loss for compact heat recovery steam generators

“…The limitations necessitate compact designs, and the once through steam generator (OTSG) has been found to be the most suitable HRSG type for offshore steam bottoming cycles [2]. One of the key factors when optimizing OTSGs for weight and size is small diameter heat exchanger tubes, compared with their onshore equivalents [3,4]. The optimization procedures used in the design process rely on correlations for finned tube banks in order to predict the heat transfer and pressure drop of the OTSG.…”