Elsevier

Bioresource Technology

Volume 163, July 2014, Pages 74-81
Bioresource Technology

Energy efficiency and environmental performance of bioethanol production from sweet sorghum stem based on life cycle analysis

https://doi.org/10.1016/j.biortech.2014.04.014Get rights and content

Highlights

  • Bioethanol based on sweet sorghum stem can produce positive net energy.

  • Most negative environmental impacts were human toxicity and eutrophication.

  • Main factors contributed to fossil energy consumption and emissions were analyzed.

  • Vinasse treatment and inventory allocation exerted significant effects on results.

  • Key points to better energy efficiency and environmental performance were discussed.

Abstract

Life cycle analysis method was used to evaluate the energy efficiency and environmental performance of bioethanol production from sweet sorghum stem in China. The scope covers three units, including plant cultivation, feedstock transport, and bioethanol conversion. Results show that the net energy ratio was 1.56 and the net energy gain was 8.37 MJ/L. Human toxicity was identified as the most significant negative environmental impact, followed by eutrophication and acidification. Steam generation in the bioethanol conversion unit contributed 82.28% and 48.26% to total human toxicity and acidification potential, respectively. Fertilizers loss from farmland represented 67.23% of total eutrophication potential. The results were significantly affected by the inventory allocation methods, vinasse reusing approaches, and feedstock yields. Reusing vinasse as fuel for steam generation and better cultivation practice to control fertilizer loss could significantly contribute to enhance the energy efficiency and environmental performance of bioethanol production from sweet sorghum stem.

Introduction

China is currently the largest fossil energy consumer and greenhouse gases (GHGs) emitter worldwide (Gregg et al., 2008, IEA, 2010). The increasing dependence on imported oil and excessive GHGs emission has resulted in the emergence of renewable energy as an important energy and environmental concern in China. Bioethanol based on energy crop has been promoted by the Chinese government in many provinces. However, the development of bioethanol fuel is constrained by the increasing concern over food safety (Qiu et al., 2010), prompting the government and enterprises to identify non-grain crops, which include cassava, sweet potato, sugar cane, and sweet sorghum, as feedstock for bioethanol fuel production.

Bioethanol produced from various feedstocks using different technologies faces several controversial issues, such as energy efficiency, environmental impact and cost-effectiveness. A highly controversy issue of first-generation biofuels (produced from food and feed crops) frequently involves their negative impacts on food safety and environmental performance (Papong and Malakul, 2010, Mueller et al., 2011, Liang et al., 2012). Although the second-generation bioethanol (produced from lignocellulosic biomass) does not have such disadvantages (Sánchez and Cardona, 2008, Roy et al., 2012), the main controversy of second-generation bioethanol is the high cost for the establishment of cellulosic ethanol infrastructure (Gómez et al., 2011, Giarola et al., 2012).

Sorghum is a fast growing C4 plant native to tropical zones, but it can adapt to different environmental conditions. This plant grows in tropical, subtropical and temperate zones (Zegada-Lizarazu and Monti, 2012). Sweet sorghum has attracted attention as one of the most promising non-food feedstock crop for bioethanol production in China because of its rich germplasm resource, high biomass yield, rapid growth, wide adaptability, and rich sugar content in stem, as well as clean and relatively low production cost. Therefore, bioethanol from sweet sorghum is regarded as a 1.5-generation biofuel (Li et al., 2013). China has launched numerous research projects and pilot production activities involving sweet sorghum, especially on alkaline and saline lands, as feedstock for bioethanol production. For instance, several International Scientific and Technological Cooperation projects and National Key Technology R&D programs have been funded by the Ministry of Science and Technology of China (Li et al., 2013).

However, bioethanol production from sweet sorghum stem is also confronted with two controversial issues: whether the bioethanol produces positive net energy, and whether it is environmental friendly. The net energy efficiency and environmental performance of different biofuels depend on the type of feedstock, production process, and amount of nonrenewable energy required (Amigun et al., 2011, Liang et al., 2012). Life cycle assessment (LCA) is a useful tool to analyze these two issues. LCA is a technique to evaluate environmental impacts associated with all the stages of a product’s life cycle from cradle to grave, so it has been extensively used to evaluate the energy efficiency and environmental performance of bioethanol and biodiesel (Jury et al., 2010, Hou et al., 2011). Several studies have assessed the productive potentials, conversion technology, energy balance, GHGs, and economic performance of bioethanol production from sweet sorghum stem (Zhang et al., 2010, Tao et al., 2011, Liang et al., 2012, Li et al., 2013, Yu et al., 2014). However, few studies have evaluated the energy efficiency and environmental performance of sweet sorghum from the life cycle perspective.

Thus, this study aims to evaluate the fossil energy consumption, energy gain, and energy efficiency associated with bioethanol production from sweet sorghum stem and to assess its environmental performances using attributional LCA method. In addition, the main sources of energy consumption and environmental impacts, as well as the uncertainty of the evaluation results were discussed. Hence, this study provides insight into the reasonable use of sweet sorghum stem as feedstock for bioethanol production.

Section snippets

System boundary and functional unit (FU)

The product system boundary used in the study is presented in Fig. 1. A product system is a collection of unit processes connected by flows of intermediate products that perform one or more defined functions. The system is subdivided into a set of units. Units are linked to one another through flows of intermediate products and waste, to other product systems through product flows, and to the environment through elementary flows. According to the above definition, the product system is

Energy efficiency

Comparison between the total energy input and output shows that the NEG and NER of sweet sorghum stem based bioethanol were 8.37 MJ/L and 1.56, respectively. The production of 1 L of bioethanol requires a total fossil energy input of 14.90 MJ/L, in which the energy consumption quantity during the bioethanol conversion unit is the highest (68.93% of total fossil energy input), and energy usage in the plant cultivation unit is second highest, constituting 29.80%. Steam generation consumes 64.50% of

Conclusions

The bioethanol production from sweet sorghum stem shows positive energy efficiency and high negative impacts of HTP, EP and AP. The bioethanol conversion unit is the main contributor to fossil energy consumption, AP, PCOP, HTP and MAETP because of large coal consumption for steam generation. Agrochemicals loss to surrounding environment in plant cultivation unit is the main source of EP, FATEP, and TETP. Reusing vinasse as energy for steam generation and promoting better management practices in

Acknowledgements

This work was funded by the National Natural Science Foundation of China (No. 70901035), the National Science and Technology Support Program (No. 2014BAL02B02), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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