A simplified model of a boiler furnace to understand effects on grate temperatures and potential overheating events

Boiler grates are a key part of a furnace, ideally providing combustion air along the depth of the furnace where the fuel (usually bagasse, wood chips or coal lumps) is being directed to, as well as removing ash. There are at least three types of grates used in the Australian sugar industry: moving grates, dumping grates, and pin-hole grates. Grate overheating and damage (in particular for travelling grates) has cost some of the industry significant maintenance, capital and operational costs. The adoption of improved fuel-feeding technologies such as swirl spreaders has been hindered by concerns on the effect on grate temperatures due to changed combustion conditions in the furnace, such as the location of the main flame region. The industry has been well served in the last 30 years by a Computational Fluid Dynamics (CFD) model called FURNACE, which has been used to model (and improve) boiler combustion, erosion and corrosion. This paper presents the development of a separate simplified model with the aim of predicting grate temperatures for different furnace and grate designs and operating conditions. The simplified model was developed in ANSYS Fluent and can combine the effects of radiation, convection and conduction on grate temperature, whilst having the capability of modelling the physical and material characteristics of a grate in detail.