Analysis of process steam demand reduction and electricity generation in sugar and ethanol production from sugarcane
Introduction
Sugarcane production is one of the most important economical activities in Brazil, mainly due to its high efficiency and competitiveness. In this segment are found sugar factories, alcohol distilleries and integrated sugar and alcohol plants that can produce both products from the sugarcane. In the last few years electricity is becoming a new product too, since sugarcane bagasse can be used as fuel in cogeneration systems.
Currently, there are more than 300 sugarcane plants in operation in Brazil [1]. A total of 394.4 Mt of cane were processed in the last harvest season (2005/2006) for sugar and ethanol production [2].
Nowadays almost all sugarcane plants in Brazil are self-sufficient in thermal, mechanical and electrical energy. Generally low efficiency cogeneration systems based on steam cycle with live steam at 22 bar and 300 °C are found in these plants [3]. Reduction of process steam requirements and more efficient cogeneration systems can increase the surplus of electric power generated. Currently, cogeneration systems begin to operate with live steam generation pressure higher than 60 bar [4] attending the plant energy requirements and producing surplus electricity that can be sold. The total electricity generation capacity installed in the sugarcane plants using bagasse in Brazil is around 2300 MW [5].
The purpose of this paper is to analyze the steam demand reduction on sugar and ethanol process and alternatives for the cogeneration systems in sugarcane plants, aiming at the surplus electricity generation increase. A “base case”, with high process steam demand was simulated. This case intends to represent the thermal energy consumption of typical sugar and ethanol plants in Brazil. A thermal integration procedure was implemented in an “improved case”, reducing the steam demand reduction. Different alternatives of cogeneration systems were simulated for both cases, considering steam cycles and biomass gasification based cycles.
Section snippets
Process description
In sugarcane segments in Brazil most of the plants produce sugar and ethanol in integrated plant. Some of them use sugarcane juice for sugar production, being the molasses, a by-product of sugar process, used to produce ethanol in annexes distilleries. The use of sugarcane juice for sugar and ethanol production is very common too, reducing the sugar production in order to produce more ethanol. Plants which use half of the juice for sugar production and half for ethanol production were
Process integration in sugar and ethanol process
Process integration techniques or methods can be used to improve energy recovery in sugar and ethanol process, allowing the increase of electricity generation by the cogeneration system. Some works are found in the literature indicating the best options for the process thermal integration in sugar process. Many of them use the pinch point method and apply an exergetic analysis of sugar factories. Guallar [6] made an exergetic analysis of a beet sugar process with thermal integration.
Process simulation
A sugar and ethanol plant that operates 4000 h and crushes 2,000,000 t of sugarcane during the harvest season was simulated using the EES software [17]. Sugar and ethanol are produced, being half of the juice produced at the extraction system used for sugar production and half for ethanol production. Molasses obtained as a by-product of the sugar process is used for ethanol production, being mixed with the juice for ethanol in the mash preparation step. The bagasse produced at the extraction
Cogeneration systems
Four configurations of cogeneration systems were chosen for the analysis of different alternatives that could be applied on sugarcane plants. Thermal and electrical energy requirements of the process must be supplied by this system and surplus electricity generated is considered available for sale to the grid. The simulations were performed using EES software [17] assuming electricity generation just during the harvest season, when the sugar production process is in operation.
Table 6 shows the
Configuration I
For the configuration I the following results presented in Table 10, Table 11 were obtained.
As can be seen in Table 10, surplus bagasse between 4% and 8% was calculated for the base case. Even increasing live steam pressure and temperature, steam requirements can be attended by available bagasse. New systems with higher live steam parameters can increase surplus electricity generation considerably, reaching 70 kWh/t of cane for the highest live steam level as shown in Table 11.
For improved case,
Conclusions
The analysis performed in this paper showed that the process steam demand for sugar and ethanol plants can be reduced significantly with process thermal integration, being important to promote better use of energy in these industries.
For traditional steam cycles with back-pressure turbines (configuration I), a significant amount of surplus bagasse can be obtained with process steam demand reduction, allowing its use as raw material for other by-products of the sugarcane plants, as additional
Acknowledgements
The authors would like to acknowledge Usina Cruz Alta, Guarani and Santa Isabel engineers, technicians and managers for available process data and FAPESP, CAPES and CNPq for the financial support to do this study.
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