Simulation of a CFB Boiler Integrated With a Thermal Energy Storage System During Transient Operation

Dionisis Stefanitsis,Aristeidis Nikolopoulos,Bernd Epple,Jens Peters,Jochen Ströhle,Konstantina Koutita,Nikolaos Nikolopoulos,Athanasios Nesiadis

doi:10.3389/fenrg.2020.00169

Abstract

In the current work, a transient/dynamic 1-dimensional model has been developed in the commercial software APROS for the pilot 1 MWth CFB boiler of the Technical University of Darmstadt. Experiments have been performed with the same unit as well, the data of which are utilized for the model validation. The examined conditions correspond to the steady-state operation of the boiler at 100%, 80% and 60% heat loads, as well as for transient conditions for the load changes from 80% to 60% and back to 80%. Fair agreement is observed between the simulations and the experiments regarding the temperature profiles in the riser, the heat extracted by the cooling lances as well as the concentration of the main species in the flue gases; a small deviation is observed for the pressure drop, which, however, is close to the results of a CFD simulation run. The validated model is extended with the use of a thermal energy storage (TES) system, which utilizes a bubbling fluidized bed to store/return the particles during ramp up/down operation. Simulations are performed both with and without the use of TES for the load path 100%-80%-60%-80%-100% and the results showed that the TES concept proved to be superior in terms of changing load flexibility, since the ramp up and down times proved to be much faster and lower temperature drops between the loads are observed in this case.

Highlights

The 2030 and 2050 EU frameworks for climate and energy(1,2) aim at the decrease of greenhouse gas emissions with improved energy efficiency as well as with larger share than nowadays of Renewable Energy Sources (RES) in the energy supply
In order to assess the validity of the model, the results of the steady-state and transient simulations are compared against the experimental data of Technical University of Darmstadt (TUDA) performed in the pilot circulating fluidized bed (CFB) boiler, as well as against data from CFD simulations
The purpose of the current work was to investigate the operation of a CFB boiler equipped with a thermal energy storage (TES), which uses a bubbling fluidized bed (BFB) to store/return the hot particles during ramp up/down operation

Summary

Introduction

The 2030 and 2050 EU frameworks for climate and energy(1,2) aim at the decrease of greenhouse gas emissions with improved energy efficiency as well as with larger share than nowadays of Renewable Energy Sources (RES) in the energy supply. The replacement of the conventional power plants with RES with stochastic nature, such as wind or solar, creates a flexibility gap between power supply and demand. During a windy day with low power demand, the conventional power plants, such as those operating with lignite or gas, need to operate at partial load for a certain period of time. This creates new challenges for such plants, which should operate efficiently at partial loads and have high ramp up/down rates, including fast start up and shut down operation. The costs depend on the specific technology and plant, to give an idea about its magnitude, for a coal fired power plant the retrofitting costs range from 100 up to 500 e/kW (Energiewende, 2017)

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