Análise exergética de processos e da cogeração de bioenergia no setor sucroenergético
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Data
2020-09-25
Autores
Vieira, Marcos Alexandre
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Universidade Estadual Paulista (Unesp)
Resumo
As usinas brasileiras produzem açúcar, etanol e energia, sendo que a fonte de energia utilizada para gerar esses produtos é fornecida pela própria matéria-prima, o bagaço da cana. A etapa de cogeração na usina utiliza o bagaço como combustível para gerar o vapor, que é utilizado para produzir energia elétrica e fornecer energia térmica para o processo de produção de açúcar e etanol. Quanto maior for a eficiência térmica do processo, maior será o potencial de exportação de energia elétrica da usina. Neste estudo, realizou-se uma análise exergética do processo de transformação da cana-de-açúcar nos produtos finais, açúcar e etanol hidratado, com o objetivo de identificar uma metodologia para o cálculo da eficiência térmica do processo, podendo servir como base para qualificar o nível de eficiência térmica de usinas do setor sucroenergético. Para isso, foi elaborada a modelagem integrada de todas as etapas do processo de produção, desde a entrada da matéria-prima na usina até a produção de açúcar e etanol, calculando-se a energia térmica requerida para essa transformação. As simulações foram realizadas considerando-se a faixa de variação do mix produto (porcentagem de um produto produzido em relação ao total de matéria-prima utilizada) comumente praticada nas usinas brasileiras, isto é, 30 a 70% mix açúcar. Foram identificadas as principais correntes de calor residuais no processo de transformação e calculada a exergia disponibilizada pelo processo para cada condição de mix produto simulada. As simulações foram baseadas em equações de conservação de massa e energia aplicadas a todo o sistema, sendo resolvidas utilizando-se o software Simulador Pentagro. Os resultados obtidos permitiram verificar que: (i) o mix produto altera significativamente o consumo térmico requerido pelo processo e a exergia disponibilizada; (ii) para todos os cenários simulados, as principais fontes de exergia disponibilizadas pelo processo foram o condensado vegetal, seguido pelo vapor de etanol, vinhaça, caldo enviado para fermentação e vapor gerado no flasheamento do caldo; (iii) a exergia disponível nos condensados vegetais aumenta consideravelmente com a elevação do mix açúcar; (iv) a exergia disponibilizada pelo processo tem potencial para autossupri-lo em até 43% da carga térmica necessária para a transformação da matéria-prima em seus produtos finais açúcar e álcool; (v) o consumo térmico requerido pelo processo diminui consideravelmente com o reaproveitamento total da exergia disponibilizada, sendo que, se o vapor economizado for destinado à geração de energia no estágio de condensação dos geradores, pode-se aumentar a exportação de energia em até 23% quando comparado a processos sem reaproveitamento exergético.
Brazilian power plants produce sugar, ethanol, and energy, given that the energy source used to generate these products is provided by the raw material itself, sugarcane bagasse. The cogeneration step in the plant uses bagasse as fuel to generate steam, which is used to produce electric energy and provide thermal energy for the process of sugar and ethanol production. The higher the thermal efficiency of the process, the higher the electric energy exportation potential in the power plant. In this study, an exergetic analysis of the transformation process from sugarcane to its final products, sugar and hydrated ethanol, was performed aiming to identify a methodology to calculate the process’s thermal efficiency, being able to work as a base to qualify the thermal efficiency level in the sucro-energy sector power plants. In order to do it, an integrated modeling of all production process steps was developed, from the feedstock entrance in the power plant until the sugar and ethanol production, calculating the thermal energy required for this transformation. The simulations were performed considering the range of product mix (percentage of a produced product in relation to the total raw material used) usually exercised in Brazilian power plants, that is, from 30 to 70% sugar mix. The main currents of residual heat in the transformation process were identified and the exergy made available by the process for each simulated condition of product was calculated. The simulations were based on mass and energy conservation equations applied to the whole system, solved through Simulador Pentagro software. The obtained results allowed to verify that: (i) the product mix significantly alters the thermal consumption required by the process and the available exergy; (ii) for all the simulated scenarios, the main exergy sources made available by the process were the vegetal condensate, followed by ethanol steam, vinasse, the juice sent to fermentation and the steam generated by the juice’s flash evaporation; (iii) the exergy available in vegetal condensates considerably increases with sugar mix elevation; (iv) the exergy made available by the process has potential to self-supply it up to 43% of the thermal load necessary for the feedstock transformation into the final products, sugar and alcohol; (v) the thermal consumption required by the process considerably decreases with the reuse of the total available energy, given that if the saved steam is destined to energy generation in the generators’ condensation stage, energy exportation may be increased up to 23% when compared to processes without exergetic reuse.
Brazilian power plants produce sugar, ethanol, and energy, given that the energy source used to generate these products is provided by the raw material itself, sugarcane bagasse. The cogeneration step in the plant uses bagasse as fuel to generate steam, which is used to produce electric energy and provide thermal energy for the process of sugar and ethanol production. The higher the thermal efficiency of the process, the higher the electric energy exportation potential in the power plant. In this study, an exergetic analysis of the transformation process from sugarcane to its final products, sugar and hydrated ethanol, was performed aiming to identify a methodology to calculate the process’s thermal efficiency, being able to work as a base to qualify the thermal efficiency level in the sucro-energy sector power plants. In order to do it, an integrated modeling of all production process steps was developed, from the feedstock entrance in the power plant until the sugar and ethanol production, calculating the thermal energy required for this transformation. The simulations were performed considering the range of product mix (percentage of a produced product in relation to the total raw material used) usually exercised in Brazilian power plants, that is, from 30 to 70% sugar mix. The main currents of residual heat in the transformation process were identified and the exergy made available by the process for each simulated condition of product was calculated. The simulations were based on mass and energy conservation equations applied to the whole system, solved through Simulador Pentagro software. The obtained results allowed to verify that: (i) the product mix significantly alters the thermal consumption required by the process and the available exergy; (ii) for all the simulated scenarios, the main exergy sources made available by the process were the vegetal condensate, followed by ethanol steam, vinasse, the juice sent to fermentation and the steam generated by the juice’s flash evaporation; (iii) the exergy available in vegetal condensates considerably increases with sugar mix elevation; (iv) the exergy made available by the process has potential to self-supply it up to 43% of the thermal load necessary for the feedstock transformation into the final products, sugar and alcohol; (v) the thermal consumption required by the process considerably decreases with the reuse of the total available energy, given that if the saved steam is destined to energy generation in the generators’ condensation stage, energy exportation may be increased up to 23% when compared to processes without exergetic reuse.
Descrição
Palavras-chave
Análise exergética, Exergia, Usinas sucroenergéticas, Cana-de-açúcar, açúcar e álcool, Cogeração, Bioenergia, Eficiência térmica, sucro-energy power plants, sugarcane, exergetic analysis