Modelling of SO3 Formation in the Flame of a Heavy-oil Fired Furnace

D. R. Schneider and Ž. Bogdan

Abstract
The accurate description of sulphur trioxide formation in the flame is a prerequisite for developing a realistic SO3 model that could be used in an analysis of reduction of low-temperature corrosion in steam generators. The scope of this paper is limited only to SO3 produced in the furnace by homogenous gas-phase reactions during heavy-oil fuel combustion and does not include heterogeneous catalytic reactions on heat exchanger's surfaces and ash particles. As a first attempt, SO3 was modelled as a part of globally applied combustion model – the probability density function/equilibrium chemistry model. By comparison with known data, it was concluded that this model does not give realistic results. An extensive literature review revealed dominant reactions in which SO3 participates. Further analysis showed that the SO3 reactions could be considered as slow ones comparing to the main reactions during combustion, which means that the chemical equilibrium assumption could not be employed. Based on these reactions, a new SO3 model, which takes into account finite kinetic character of reactions, was defined. It was implemented into the FluentTM CFD code as a user subroutine. Such mathematical model was used for simulation of a full-scale furnace of the oil-fired steam generator. The accuracy of the model was assessed by comparison with measured data and available data from the literature. Satisfying conformance was established.

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Keywords
SO3, numerical modelling, furnace, heavy-oil fuel combustion