Experimental and modeling approaches to simulate temperature-gradient induced intradeposit chemical processes with implications for biomass boiler corrosion

A4 Conference proceedings


Internal Authors/Editors


Publication Details

List of Authors: Daniel Lindberg, Jonne Niemi, Markus Engblom, Tor Laurén, Patrik Yrjas, Mikko Hupa
Publication year: 2018
Publisher: The Korean Institute of Chemical Engineers (KIChE)
Book title: 23rd International Conference on Fluidized Bed Conversion - Innovative Fluidized Bed Conversion Technology for a Sustainable Development
ISBN: 978-89-950005-7-1


Abstract

The heterogeneous nature of the ash chemistry of biomass fuels gives rise to challenges in predicting the intradeposit chemical processes relevant for deposit melting, sintering and enrichment of corrosive ash species.

An experimental method has been developed to study the evolution of ash deposit chemistry and morphology in temperature gradients simulating conditions similar to real superheater deposits. The method is based on applying synthetic ash mixtures on an air-cooled corrosion probe, which is inserted into a tube furnace in air or synthetic flue gas. Gas temperatures vary between 700 and 900 °C and probe temperatures vary between 300 and 600 °C. Focus has been on how melting behavior of alkali salt-rich deposits, i.e. KCl-K2SO4-NaCl-Na2SO4 mixtures, affects the sintering of the deposits, as well as on studying vaporization-condensation of KCl and NaCl within the deposits. The interaction of reactive gas components, such as SO2 and gaseous KCl, with the deposits was also studied.

The vaporization-condensation mechanism was shown to lead to enrichment of alkali chlorides towards colder surfaces within the porous parts of the deposit. It leads to condensation and build-up of chlorides on the steel surface, which causes accelerated corrosion, due to the formation of low-melting FeCl2 mixtures. Liquid-phase sintering and temperature gradient zone melting were shown to be the main mechanisms for the supersolidus sintering of the deposits.

The vaporization and condensation of alkali chlorides within the deposits was modelled to explain the time-dependent build-up of the chlorides using both CFD and thermodynamic modeling. Temperature gradient induced Fickian concentration diffusion was shown to be the main mechanism and accurately predicted the alkali chloride build up as a function of deposit composition, local temperature and time. A CFD-model for predicting the alkali chloride enrichment in superheater deposits in full-scale boilers has also been developed.


Keywords

Alkali chloride, high-temperature corrosion, Temperature gradient

Last updated on 2019-18-07 at 05:58