Dynamic Modeling of the Reactive Side in Large-Scale Fluidized Bed Boilers

Guillermo Martinez Castilla,Filip Johnsson,David Pallarès,Rubén M Montañés

doi:10.1021/acs.iecr.0c06278

Abstract

This work presents a dynamic model of the reactive side of large-scale fluidized bed (FB) boilers that describes the in-furnace transient operation of both bubbling and circulating FB boilers (BFB and CFB, respectively). The model solves the dynamic mass and energy balances accounting for the bulk solids, several gas species, and the fuel phase. The model uses semi-empirical expressions to describe the fluid dynamics, fuel conversion, and heat transfer to the furnace walls, as derived from units other than the studied ones. The model is validated against operational data from two different industrial units: an 80 MW CFB and a 130 MW BFB, both at steady-state and transient conditions. The validated model is used to analyze: (i) the performance of the reactive side of two FB boilers under off-design, steady-state conditions of operation; and (ii) the open-loop transient response when varying load or fuel moisture. The results underline the key role of heat capacity on the stabilization time. Within a given unit, the differences in heat capacity between the top and bottom of the furnace affect also the stabilization times, with the furnace top (lower heat capacity) being 1–3 times faster in the CFB unit and up to 10 times faster in the BFB unit. Due to the differences in gas velocity, the investigated boilers are found to stabilize more rapidly to input changes when running at full load than at partial load. Lastly, a variable ramping rate analysis shows that the inherent transient responses of the reactive side disappear when disturbances are introduced at (slower) rates, typical of industrial operation. Thus, the reactive side could be modeled as pseudo-static.

Highlights

Fluidized bed combustion (FBC) units are characterized by strong mixing and heat transfer capabilities, which make them the preferred option for the thermochemical conversion of low-grade and/or renewable solid fuels, such as municipal solid waste and biomass
The model presented in this paper describes the reactive side of fluidized bed (FB) boilers through a number of perfectly mixed control volumes exchanging mass and energy and for which the coupled dynamic balances of those scalars are solved
It should be noted that combustion in FB units entails inherent variations attributable to the actions taken to control the solids inventory impacting on the average solids size and the riser pressure drop or the variation in the feedstock composition moisture and volatile content due to fuel storage

Summary

Introduction

Fluidized bed combustion (FBC) units are characterized by strong mixing and heat transfer capabilities, which make them the preferred option for the thermochemical conversion of low-grade and/or renewable solid fuels, such as municipal solid waste and biomass. This means that FBC units have a high level of fuel flexibility, allowing co-firing of different fuel mixtures depending on fuel price and availability. FB boilers have gained global recognition in the past decades as a viable technology for the thermal conversion of solid fuels and play crucial roles in many energy systems across the world.[1,2] In particular when operated with biomass and renewable waste fractions, FB boilers represent a viable alternative to conventional coal boilers. In markets where combined heat and power (CHP) plants are well-established, replacing old grate or pulverized coal-fired boilers with FB boilers creates the possibility to implement polygeneration facilities that are capable of providing flexible shares of sustainable heat, power, and transportation fuels

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