Exergetic, exergoeconomic, and exergoenvironmental aspects of an industrial-scale molasses-based ethanol production plant

Highlights

• A molasses-based bioethanol production process was exergetically analyzed.

• The exergetic efficiency of the studied industrial process was 35.9%.

• The unit exergoeconomic cost of the product was calculated to be 20.9 USD/GJ.

• The unit exergoenvironmental impact of the product was found to be 31.5 mPts/GJ.

• More exergetically-sustainable alternatives should be considered for natural gas.

Abstract

An industrial-scale molasses-based bioethanol production system was modeled and studied by conducting exergy, exergoeconomic, and exergoenvironmental analyses. The entire process was represented by a control volume, and its exergoeconomic and exergoenvironmental parameters were determined using the specific exergy costing (SPECO) approach. These exergy-based analyses were carried out to measure the overall exergy dissipation, cost, and environmental impact of the bioethanol production process based on actual operational thermodynamic, economic, and environmental data. Natural gas showed the highest contribution to the total input exergy (61.1%) and total environmental impact rate (56.6%) of the process, while the highest contribution to its total cost rate (75.7%) was from molasses. The exergetic efficiency determined for the process was 35.9%, while the exergy dissipation accounted for 60.8% of its total input exergy. The unit exergoeconomic costs of the fuel and product were determined to be 6.2 and 20.9 USD/GJ, while the unit exergoenvironmental impacts of the fuel and product were 15.5 and 31.5 mPts/GJ, respectively. The exergoeconomic factor of the process was found to be 29.4%, while the exergoenvironmental factor was 0.74%. Overall, natural gas consumption was the most significant exergetic hotspot of the process, and hence more exergetically-sustainable alternatives should be considered to improve the process. Low-cost waste feedstocks need to be utilized to improve the economic viability of the process.

Introduction

Climate change and its adverse health impacts have further intensified the global interest in alternative renewable projects to produce fuels, chemicals, and materials [1], [2]. In line with this trend, significant efforts have been devoted worldwide to developing and commercializing liquid transportation fuels derived from renewable sources, particularly bioethanol and biodiesel [3], [4]. This could be attributed to the fact that these eco-friendly and clean-burning biofuels share highly similar physicochemical properties with their petroleum counterparts, and hence they can be used in in-use internal combustion engines without the need for significant modifications [5]. Bioethanol and biodiesel can be used in neat form or blended with petroleum-based gasoline and diesel fuels, respectively [6]. Global bioethanol production has been much higher than that of biodiesel during the past two decades, and it is expected to continue to grow in the coming years [7].

Among the various production pathways developed for bioethanol production to date, bioethanol production from readily fermentable sugar feedstocks such as molasses is still the most promising route owing to the highly competitive cost of production and the relative abundance of the inputs [8]. In addition to being commercially viable, this bioethanol production route is also technologically mature [9]. These are indeed the two significant aspects that are still lacking for the lignocellulosic bioethanol production route [10]. The synthesis of bioethanol from readily fermentable organic waste materials does not pose any threats to food security in contrast to the bioethanol fuel produced from edible crops such as corn, wheat, and barley [11]. As mentioned earlier, molasses, a by-product of sugar production, is a suitable feedstock for bioethanol production [12]. This sugar-rich feedstock contains over 50 wt% sugar, including sucrose, glucose, and fructose [13]. According to Darvishi and Abolhasan Moghaddami [14], 40% of the molasses generated worldwide is fermented to bioethanol. Despite the above-mentioned unique features of the existing molasses-based bioethanol plants, their performance could still be enhanced by applying comprehensive sustainability assessment frameworks to facilitate the support decision-making process. Such findings could also guide future research efforts in this domain.

It is well-documented that compared to energy analysis, exergy analysis can provide more meaningful results regarding the efficiency of energy conversion systems [15]. Exergy analysis considers both the quantity and quality of material and energy flows, while energy analysis considers only their quantities [16]. The thermodynamic property exergy can reasonably value all kinds of material and energy flows in unified dimensions (J) [17]. Accordingly, exergy analysis and its extensions have been extensively employed in the literature for evaluating bioethanol production processes from various feedstocks such as oil palm fronds [18], sugarcane bagasse [19], [20], [21], wheat grain and straw [22], various fermentable sugars [23], and corn cob [24]. Through quantifying exergy destruction (thermodynamic losses), exergy analysis can determine the sustainability level of biofuel production processes [25]. The exergy concept can also be further strengthened through integration with economic and environmental constraints, and new insights into feasibility, profitability, and renewability of biofuel production systems could be subsequently obtained [26]. These exergy-based cost and environmental analyses, also known as “exergoeconomic and exergoenvironmental” approaches, respectively, have become powerful tools for investigating the performance of energy conversion systems [27].

Accordingly, exergy analysis and its combination with economic and environmental considerations have been considered the most promising tools for analyzing biofuel production systems from the sustainability perspective [28]. Notably, invaluable practical information about the thermodynamic, economic, and environmental aspects of biofuel production systems can be obtained through exergoeconomic and exergoenvironmental approaches that cannot be attained using exergy analysis or economic life cycle cost/environmental life cycle assessment. This can be mainly attributed to the synergy between the exergy principles and the economic accounting/environmental assessment concepts. The developed synergistic effect could then facilitate a mechanistic and systematic understanding of the cost and environmental impacts of products and the hotspots of the cost losses and environmental impacts of biofuel systems.

Overall, it could be concluded that exergy-based methods could provide decision-supporting insights to guide the sustainable production of biofuels. In better words, exergy-based approaches can reliably locate, quantify, and elucidate thermodynamic imperfections, cost losses, and environmental impacts of biofuel systems. Even though there are several studies on the use of exergy-based methods for investigating bioethanol production systems, all these attempts were only based on simulation studies with numerous irrational assumptions, unrealistic oversimplifications, and erroneous approximations [19], [20], [21], [29]. Besides, most of the processes simulated in these works were not validated and assessed using reliable experimental data. It is believed that the exergy-based analyses carried out herein based on empirical data can potentially provide more accurate and reliable results than the studies mentioned above carried out using simulation data.

To the best of our knowledge, exergoeconomic and exergoenvironmental analyses have never been used before for scrutinizing bioethanol production systems in the literature. Furthermore, there is no report in the literature on the use of actual industrial data for determining the exergetic aspects of commercial bioethanol production systems, which was the motivation and focus of this study. Hence, the present survey, for the first time, was aimed at analyzing an industrial-scale bioethanol production plant from the exergetic, exergoeconomic, and exergoenvironmental viewpoints using real-world operational thermodynamic, economic, and environmental data. This study was carried out to illustrate how exergy, exergoeconomic, and exergoenvironmental analyses could be efficiently and reliably applied to uncover the hotspots of the thermodynamic losses, cost losses, and environmental impacts of the investigated bioethanol production system. The exergetic values, unit exergoeconomic costs, and unit exergoenvironmental impacts of input/output streams were determined to present a general picture of the sustainability status of the molasses-based bioethanol production process. Besides, exergy, exergoeconomic, and exergoenvironmental parameters of the process were determined to provide key insights to further improve the process. Typically, such studies should be of value in optimizing and enhancing the existing bioethanol production plants from thermodynamic, economic, and environmental viewpoints.