Comparative analysis of electricity cogeneration scenarios in sugarcane production by LCA

Abstract

Purpose The decentralization of the Brazilian electricity sector in association with the internal electricity supply crisis has encouraged companies in the sugarcane industry to produce electricity by burning sugarcane bagasse in cogeneration plants. This approach reduces the environmental impact of the sugarcane production and has opened up opportunities for distilleries and annex plants to increase their product portfolios. Potential scenarios for technically and environmentally improving the cogeneration performance were analyzed by using thermodynamic analysis and Life Cycle Assessment (LCA). Methods The method used in this study aimed to provide an understanding and a model of the electrical and thermal energy production and the environmental impacts of conventional vapor power systems which operate with a Rankine cycle that are commonly used by Brazilian distilleries. Vapor power system experts have suggested focusing on the following technical improvement areas: increasing the properties of the steam from 67 bar and 480°C to 100 bar and 520°C, regeneration, and reheating. Eight case scenarios were projected based on different combinations of these conditions. A functional unit of "To the delivery of 1.0 MWh of electricity to the power grid from a cogeneration system" was defined. The product system covers the environmental burdens of the industrial stage and the agricultural production of sugarcane. Results and discussion Technical evaluation indicated that the energy efficiency improves as the pressure at which the vapor leaves the boiler increases. Simultaneously, the net power exported to the grid increases and the makeup water consumption in the cooling tower and the makeup water supplied to the boiler reduce. From the LCA, it was noted that the improved energy performance of the system is accompanied by reduced environmental impacts for all evaluated categories. In addition, vapor production at 100 bar and 520°C results in greater environmental gains, both in absolute and relative terms. Conclusions Reheating and regeneration concepts were found to be considerably effective in improving the energy and environmental performance of cogeneration systems by burning sugarcane bagasse. For the evaluated categories, the results indicate that the proposed modifications are favorable for increasing the efficiency of the thermodynamic cycle and for decreasing the environmental impacts of the product system.

Thermodynamic and Environmental Analysis of Scaling up Cogeneration Units Driven by Sugarcane Biomass to Enhance Power Exports

“…Dry saturated steam is extracted from the turbine at about 2.5 bar in order to supply the thermal demand of the distillery and for reaching the deaeration set point (110 • C) [37]. The boiler feedwater pump pressure is designed to overcome the maximum possible fluid flow at the maximum design operating condition.…”

“…This correction factor is usually applied on boiler systems and a feedwater control valve adjusts the flow. (a) Straw consumption is only related to scenarios VI-X; (b) Dias et al [45,46]; (c) Efficiencies of steam generator, electrical generator, pumps and steam turbine are based on Guerra et al [37], Dias et al [19,46].…”

“…The liquid fraction also passes through the deaerator prior to being reintroduced into the boiler, closing the cycle [37,[45][46][47].…”

“…The operational scale was defined by the properties of superheated steam at boiler exit, which ranges from 20 bar up to 100 bar [19]. According to Guerra et al [37] most cogeneration units located in São Paulo are running the Rankine cycle in this range of pressure without reheating.…”

“…In this study, EES was employed to develop an analysis that a certain parameter is varying within a suitable range in order to find the optimum result. The algebraic equations used for modeling the thermodynamic behavior at each step of the Rankine cycle follow mass and energy conservation principles [37].…”