Modelling the effects of bagasse pre-drying in sugar mill boiler systems.

Cogeneration in the Australian sugar industry currently represents only a small portion of the electricity generation in Australia. There is significant potential to expand cogeneration as an alternative to traditional fossil fuels, delivering environmental and economic benefits. Central to increasing cogenerated energy output is the use of bagasse drying. This paper uses process systems modelling, verified against a case study mill, to investigate operational constraints and opportunities arising from the integration of bagasse dryers into raw sugar mills. International case studies show that there are operational advantages in using pre-dried bagasse as fuel in boiler systems. These include increased boiler efficiency, lower emissions and an overall decrease in bagasse usage. In this work a steady state model which included bagasse furnace, steam boiler, air pre-heater, economiser and flue gas heated bagasse dryer was developed and verified against Pioneer Mill process information (PI) data. The model was used to quantify the effect of bagasse drying on boiler efficiency, specific steam production and acid dew point (ADP) corrosion potential. The results show that increases in boiler efficiency could be achieved by drying bagasse from 50wt% moisture to 37.5wt% moisture. Pre-drying of bagasse can deliver up to 16% reduction in bagasse usage during normal boiler operation and specific steam production per unit mass of bagasse can be increased by up to 24%. Bagasse drying showed negligible effect in reducing ADP temperature. However pre-dried bagasse facilitates system operating conditions that avoid flue gas temperatures less than the ADP temperature. This paper also provides a brief review of current literature and practice in bagasse drying. The accuracy and reliability of experimental and theoretical drying rate models used to design bagasse dryers are discussed and future research necessary to advance bagasse dryer design is identified.