The sugar industry

Boilers

Old style fire-tube boilers, used widely by industry until well into the 1920s, were succeeded by water tube boilers. The sugar industry engineers and boiler manufacturers developed various grate designs, the stepped grate being popular until the introduction, in 1962, of the first bagasse suspension-fired boiler. This design of American origin, with a capacity of 27 tonne/hour, demonstrated the advantages of suspension firing together with the associated dump grate, in minimising the effects of dirt in the bagasse. Formerly, dirt had caused build-up of clinker on brickwork, caused cleaning problems in chambers and rear passes as well as general wear, and created difficulties with the effective burning of bagasse heaped on the grate.

Australian engineers became active in improving the designs to handle local conditions and to improve the performance. Travelling grates were included in the installations, and within 16 years of the first installation of a bagasse suspension fired boiler, a unit of 227 tonne/hour capacity was operating successfully. Some good work was done in computer modelling of boilers by the combined efforts of the Sugar Research Institute (SRI), CSR Ltd., IBM, and the Bureau of Sugar Experiment Stations.

Until this time, the policy of mills in progressively expanding boiler stations by purchasing additional small units, had led to a multiplicity of boilers, with their attendant high cost of maintenance and manning. The present trend in Australia is towards a single large installation.

Bagasse is a difficult material to handle. It chokes readily, does not flow freely in convergent or even parallel conduits and needs divergent chutes for easy flow. It will clog in screw conveyors and its handling behaviour varies with moisture level, cane variety, and growing conditions. Australian engineers have shown great ingenuity in devising methods of conveying, plowing-off belts and reclaiming this difficult material from storage.

Suspension fired boilers' furnace designs have resulted in much taller furnaces, with faced boiler tubes lining the walls. The use of baffles in the path of the flue gases in some designs, to improve the heat transfer from flue gases to the tubes, also introduced tube wear by erosion. Partial re-design and protection of the tubes have been successful in minimising the problem in existing units.

Before the oil crisis of the 1970s, auxiliary oil firing was practised, but the industry was quick to find methods of minimising and almost completely eliminating oil usage. The solutions included: better bagasse firing control and the limited use of extraneous fuels such as coal, woodchips, sawdust, and nutshells.

At the beginning of the crushing season, bagasse may be in short supply due to plant commissioning, wet weather, cane shortage and low levels of fibre in cane at that time. Later in the season it is common for mills to have excess bagasse and some method of disposal is required. A solution adopted by some mills is to incinerate bagasse in their boilers and to use air cooled condensers to condense that quantity of steam which is in excess of the factory's requirements. This gives an efficiency variation as required and saves the cartage and disposal problems of excess bagasse (Levy, 1981). It is disappointing that, in Australia, the electricity generating authorities are not disposed to offer payment which would be adequate to enable the mills to purchase generating plant in excess of their needs in order to supply electricity to the grid.

Australian Academy of Technological Sciences and Engineering