Multiobjective Dynamic Optimization for Optimal Load-Following of Natural Gas Combined Cycle Power Plants under Stress Constraints

Abstract

With the increasing penetration of intermittent renewable energy sources into the electric grid, there is an associated need for conventional thermal power plants that are designed to operate at base-load conditions to cycle their load rapidly and frequently and operate under low-load conditions. Fast load-following operations lead to plant efficiency loss and the reduction of equipment life. A high-fidelity dynamic model of a natural gas combined cycle power plant, with rigorous equipment level submodels, is developed to capture the plant transient performance and the thermomechanical stress evolution at the high-pressure drum is computed to assess the drum life consumption. Stress evolution is modeled for the location of the edge at the drum/downcomer junction that experiences high circumferential stress amplitude during the fast load-following operation, leading to higher fatigue damage than locations considered under existing design standards. A dynamic optimization problem is solved for optimal load-following operation. Depending on the value of the stress constraint and the desired ramp rate, satisfying the desired stress constraint may be infeasible without relaxing the ramp rate. A multiobjective dynamic optimization problem is solved using a lexicographic approach that minimizes ramp rate relaxation and maximizes efficiency while satisfying stress constraints, avoiding spraying to saturation, and maintaining main steam and reheat steam temperature within bounds. It was observed that the optimal ramp rate can be highly nonlinear as opposed to the industry-standard linear ramp rate. Nonlinear optimal ramp-rate profiles not only help to avoid stress constraints that may be unavoidable by using the linear profile but also result in higher efficiency than the linear profile. The study shows that there is a strong tradeoff between the relaxation in the plant ramp rate and the time-average thermal efficiency.

Design and Multiobjective Dynamic Optimization of Superheaters for Load-Following Operation in Pulverized Coal Power Plants

“…Identification and quantification of the localized damage are challenging without inspection during the plant shutdown. Even with inspections, it can be cost-prohibitive to inspect all possible locations for damage . One way of reducing the probability of failure occurrences is to optimally design the superheater components for flexible operations.…”

“…However, studies have reported that these undesired operations lead to creep and fatigue damages, as well as damages caused by the synergistic effects of creep and fatigue, significantly shortening the life of the boiler components. 5 Among the critical components, superheaters operating at the highest temperatures are particularly affected by cycling operations. Superheaters not only suffer from fatigue damage but also face the risk of exceeding their design temperature during low-load and cycling operations, leading to creep damages.…”

“…Even with inspections, it can be cost-prohibitive to inspect all possible locations for damage. 5 One way of reducing the probability of failure occurrences is to optimally design the superheater components for flexible operations. Traditionally superheaters have been designed neither for rapid ramp-up/down operation nor for low-load operations.…”

Hot Corrosion Monitoring in Boilers Using a Nonlinear Estimator and Electrochemical Noise-Based Corrosion Sensors

“…9 While several approaches have been recommended to minimize damage, 10,11 it is difficult to avoid excursion in temperature under aggressive cycling. 12 Monitoring of rotary equipment items such as the steam turbine has been well established over decades, 13−15 but it is difficult to monitor the health of static equipment such as high-temperature boiler components. One such key boiler component that undergoes damage is the waterwall (WW).…”

ACS Editors’ Choice Virtual Collection from I&EC Research

“…There are several notable contributions within the realm of thermodynamics that examined CO 2 solubility in polymers, thermodynamics tools used in industry, and equilibria for ternary mixtures of formaldehyde, water, and methanol . Systems engineering articles examined dynamic optimization of combined cycle power plants and systematic design of molecules and materials, whereas transport-related articles reported new advances in using data driven models to describe flow in porous media and fluidized beds and methane hydrate dissociation in porous sands . Several of the contributions report advances on the use of bioinspiration or biopolymers , or smaller biomolecules to create new materials or improve processes.…”