PREDICTING THE EFFECTS OF FUEL PROPERTIES ON COMBUSTION PERFORMANCE

AS THE SUGAR industry generates and exports more renewable electricity it will be more common for sugar mill boilers to be fired with bagasse that has been stored for extended periods. In many cases, year round cogeneration requires the use of supplementary, preferably renewable, fuels. Consequently, sugar factory boilers will have to overcome the challenge of reliably producing steam with a wider range of fuels. If a boiler cannot maintain stable combustion of the fuel in suspension then its steam output and efficiency will be adversely affected. Determining how well a fuel or fuel combination will burn is often done by trial and error but this can be time consuming and expensive. Computer modelling is an attractive alternative to the trial and error approach though experimental verification will remain an essential adjunct. In this work the boiler heat transfer modelling program BOILER and the Computational Fluid Dynamics (CFD) code FURNACE were used to predict how changing fuel properties will affect the operation of a typical sugar factory boiler. The calculations indicate that if all the heat liberated during slow oxidation of the combustible fibre in stored bagasse is used to evaporate moisture from the stockpile then the amount of combustible bagasse fibre saved due to improved boiler efficiency will more than offset the combustible fibre lost during storage. The calculations assume that the heat generation per unit mass of combustible fibre during storage is the same as the gross calorific value of the combustible fibre. More detailed study of the slow oxidation reactions is required to determine whether or not this is the case. The FURNACE model simulations predict that bagasse with reduced volatile matter content, a possible consequence of long term storage, will give higher unburnt fuel losses and therefore will significantly reduce boiler efficiency.