Graphical Analysis of Plant-Wide Heat Cascade for Increasing Energy Efficiency in the Production of Ethanol and Sugar from Sugarcane

Increased energy efficiency in the production of renewable energies can contribute to sustainable economic growth, with less consumption of fossil resources, less greenhouse gas emissions, and more energy supply certainty. The most abundant bio-based fuel for automobile transportation is presently ethanol. Brazil is an important producer of ethanol and uses sucrose from sugarcane, which is currently the most efficient feedstock for bioethanol production. The improvement of ethanol production in existing plants by heat integration can lead to a significant increase in energy efficiency. This paper presents diagrams in which data necessary for heat integration are organized in a new way. For the first time, the entire heat cascade through the individual components of ethanol and sugar production is analyzed, including the boiler, steam turbine, heat exchangers, and process operations. In autonomous plants, the produced ethanol and electricity correspond to about 35 and 8% of the inlet energy, respectively; in combined ethanol-sugar plants, the produced ethanol, sugar, and electricity correspond to about 16, 22, and 9% of the inlet energy, respectively. The remaining energy (53–57%) leaves the plant as residues or is rejected to the ambient as heat. Opportunities for increasing the plant energy efficiency by modifying the process operations, heat exchangers, turbine system, and boiler are identified and discussed. The analysis of the plant-wide heat cascade makes it possible to understand the relation between the combustion energy in a boiler, the exergy of combustion gases and high-pressure steam, the production of electricity through a turbine, and the thermal energy consumption in heat exchangers and process operations. This holistic perspective helps improve the energy performance in the production of ethanol and sugar.