Bagasse

In 1984-85 their total contribution to the Australian energy economy was bagasse 73, and wood 83 PJ per annum. There is no record of the total contribution due to other forms of biomass but it is probably negligible, although there are some interesting developments which will be discussed later.

Bagasse is the fuel used almost exclusively to fire the boilers in the sugar industry. It is the residue after juice is extracted from sugar cane in the sugar cane milling process; when it is discharged from the final mill of a train of mills, it is called 'final bagasse' or simply 'bagasse'. The ready availability of bagasse, as a by-product of sugar production, has always made it an attractive fuel for the sugar industry.

Although the first commercial sugar was manufactured at Port Macquarie in New South Wales in 1823 it was not until the mid 1860s that some hundreds of tonnes of sugar were produced from six mills in Queensland and nine mills in northern New South Wales. Logs of timber were used as fuel to prepare for the crushing and to supplement bagasse as a fuel, from these times until one hundred years later. In the 1950s and 1960s, oil, which was inexpensive, was used increasingly at start-up and to maintain steam, as required, during minor stoppages. When the oil crisis occurred in the 1970s, Australian sugar technologists were quick to devise methods of minimising the use of oil, almost to the point of eliminating it as a fuel for boilers in the industry. (Cullen et al., 1980; Sugar Research Institute, 1980).

Bagasse may contain various amounts of sand or soil, especially in wet harvesting conditions, and may also vary in moisture content, depending upon the harvesting conditions and the conditions of the milling train. It consists of lignocellulose, insoluble inorganic matter (ash), water soluble material (brix) and water.

The following values are typical:

Gross Calorific Value

19,400 kJ/kg dry ash free

Moisture as fired

45 to 53 per cent

Brix as fired

2 to 4 per cent

The Gross Calorific Value of dry ash free bagasse does not vary significantly between cane varieties or growing districts. A convenient indication of the value of bagasse as a fuel is given by the relationships:

tonnes equivalent bagasse =2.04 X tonnes wood
tonnes equivalent bagasse =5.88 X tonnes fuel oil
tonnes equivalent bagasse =3.45 X tonnes coal

The availability of bagasse for fuel and other purposes can be increased, when required, by the application of recent findings and by modification of mills to improve the thermal efficiencies. Such changes have not yet been made because they have not been economically attractive. In many mills there is excess bagasse for much of the season, resulting in incineration and/or carting and dumping.

The crushing season lasts from June to December and a limited amount of bagasse is stored, mostly in circular storage sheds which house Australian-designed reclamation equipment. At the mills, up to 1000 tonnes of additional bagasse are stored in the open under plastic sheeting. Sufficient bagasse is stored to fire the boilers to get the plant ready for crushing, and for use later in supplementing the bagasse coming from the crushing process. This is necessary at the beginning of the season due to wet weather, cane shortage, plant commissioning problems and lower levels of fibre in cane at that time.

Australian Academy of Technological Sciences and Engineering