Thermal gasification or direct combustion? A technical assessment of energy generation in Indonesian sugar factories

In this study, I compare different cogeneration system scenarios for efficient energy generation in an Indonesian sugar and ethanol factory. These scenarios include the use of condensing-extraction steam turbines, variable speed electric drives for sugar processing equipment, measures to reduce the low pressure steam demand for sugar and ethanol processing, and two advanced cogeneration systems. The advanced cogeneration systems considered are an 80 bar high pressure direct combustion steam Rankine cycle (advanced SRC) system and a biomass integrated gasifier combined cycle (BIGCC) system. Using steady-state thermodynamic models, I estimate a maximum net electricity generation potential of 180 kWh/tc for the BIGCC system. This is 38 percent greater than the electricity generation potential of 130 kWh/tc for the advanced SRC system. The net electricity generation potentials of the BIGCC and advanced SRC systems are approximately eight and six times the potential of the existing factory, respectively. Although the BIGCC system has a greater electricity generation potential than the advanced SRC system, it needs a 50 percent higher minimum bagasse feed rate to satisfy the factory low pressure steam demand for sugar and ethanol processing, which may affect its ability to provide steam and electricity during the off-season. For the Indonesian sugar factory, the annual revenue potential of the BIGCC system is US$15 million per year, approximately 50 percent higher than the US$10 million per year for the advanced SRC system, assuming an electricity sale rate of US$45/MWh and Certified Emissions Reduction carbon credit price of US$13.60. BIGCC technology is in the development stage, with no commercial systems in operation today. More importantly, to date no large scale BIGCC system has operated successfully in a commercial sugar factory. Given these risks, an advanced SRC system may be more suitable for efficient cogeneration at the Indonesian sugar factory in the near future.