Sustainable ethanol production from lignocellulosic biomass – Application of exergy analysis

Abstract

The sustainability of the second-generation biofuels requests to confirm that the energy produced from lignocellulosic biomass is significantly greater than the energy consumed in the process. As lignocellulosic biomass does not affect the food supply, sugarcane bagasse was analyzed as a raw material for second-generation biofuels production. Exergy analysis serves as a unified and effective tool to evaluate the global process efficiency. Exergy analysis evaluates the performance of sugarcane bagasse and its sustainability in the bioethanol production process. In this work, four ethanol production topologies using the typical daily amount of residual biomass produced by the sugar industry were compared. The exergy analysis concept is effective in screening design alternatives with the lowest environmental impact for second-generation bioethanol fuel production from renewable resources. This study was executed by the use of the Aspen Plus^® program and other software developed by the authors.

Introduction

The fast development of the world’s bioethanol industry has contributed to the debate on “food versus fuel” and the industry’s environmental impact. One of the largest potential feedstock for ethanol is lignocellulosic biomass which does not compete with food crops, specifically agricultural residues (e.g., sugarcane bagasse, crop straws, and corn stover), herbaceous crops (e.g., alfalfa, switchgrass), forestry wastes, wood, wastepaper, and municipal waste. Other point to be considered in biofuels production is “energy consumption vs. energy content in produced ethanol.” The processing of a renewable energy source usually involves the consumption of non-renewable resources (NRR). When the exergy content of an NRR is altered through an irreversible process, the environment is also considered altered. Hence, much research [1], [2], [3] has been undertaken on the exergy accounting of NRR consumption in order to measure the environmental impact of many manufacturing processes. According to WORC [4], the biomass energy should be grown or produced in a sustainable way that provides net environmental benefits. Biomass energy crops should be grown and harvested in a way that embodies best stewardship practices to maintain or improve air, water and soil quality. Among the criteria for judging sustainable biomass energy production, can be remarked the Net energy balance, which mentions that more energy should be released through biomass energy use than consumed in producing it (over its lifecycle). This includes energy consumed from planting, cultivating, and fertilizer or pesticide application, harvesting and transporting to market. Consequently, the second-generation biofuels represents a great alternative to reach the sustainability of this industry. In this sense, the global energy consumption in ethanol production from lignocellulosic residues, like bagasse, is lower than that of non residual energy crops because the residual biomass from existing sugar industry is used. As such the energy consumption for all farming stages of sugarcane is assumed by the traditional sugar production chain.

In accordance with Dincer and Rosen [5], sustainable development requires sustainable energy resources and the efficient use of such residues. Measuring the renewability of an energy resource using traditional energy accounting methods is also questionable since these methods are based on the first law of thermodynamics, which includes the principle of energy conservation. Moreover, a meaningful energy-related yield calculation, which would indicate if there is some type of actual net gain or loss during the utilization of an energy resource, should take into account the differences in all forms of energy (including the chemical energy of all materials) and the second law of thermodynamics, which recognizes changes in energy quality or usefulness [6]. Exergy analysis is a thermodynamic analysis technique based on the second law of thermodynamics which provides an alternative and illuminating means of assessing and comparing processes and systems rationally and meaningfully. In particular, exergy analysis yields efficiencies which provide a true measure of how nearly actual performance approaches the ideal, and identifies more clearly than energy analysis the causes and locations of thermodynamic losses and the impact on the natural environment. Consequently, exergy analysis can assist in improving and optimizing designs [7]. Exergy analysis is linked to sustainability because to increase the sustainability of energy use, we must be concerned not only with loss of energy, but also loss of energy quality (or exergy) [5]. One principal advantage of exergy analysis over energy analysis is that the exergy content of a process flows a better valuation of the flow than the energy content, since the exergy indicates the fraction of energy that is likely useful and thus utilizable. Application of exergy analysis to a component, process or sector can lead to insights into how to improve the sustainability of the activities comprising the system by reducing exergy losses. Thus, to justify the production of second-generation biofuels it is necessary to confirm that the energy produced from the lignocellulosic biomass is greater than the energy consumed in the ethanol production through exergy analysis. The main objective of this paper is to apply exergy analysis concept for the evaluation of the transformation of lignocellulosic biomass to bioethanol and to approach the sustainability of second-generation biofuels production. In the first part of the paper, the different processes involved in ethanol production from lignocellulosic biomass and the exergy analysis concept are described. In the second part, several case studies about lignocellulosic biomass transformation were selected and simulated using the typical daily amount of residual biomass produced by the sugar industry (1200 tonnes). Finally, the exergy analysis methodology was applied to evaluate of sustainability development of second-generation ethanol production. This methodology requires an analysis of each stage of the production process and the global evaluation of several scenarios of production to verify the sustainable development of the biofuels industry using lignocellulosic biomass.