Elsevier

Bioresource Technology

Volume 133, April 2013, Pages 285-292
Bioresource Technology

Life-cycle energy efficiency and environmental impacts of bioethanol production from sweet potato

https://doi.org/10.1016/j.biortech.2013.01.067Get rights and content

Abstract

Life-cycle assessment (LCA) was used to evaluate the energy efficiency and environmental impacts of sweet potato-based bioethanol production. The scope covered all stages in the life cycle of bioethanol production, including the cultivation and treatment, transport, as well as bioethanol conversion of sweet potato. Results show that the net energy ratio of sweet potato-based bioethanol is 1.48 and the net energy gain is 6.55 MJ/L. Eutrophication is identified as the most significant environmental impact category, followed by acidification, global warming, human toxicity, and photochemical oxidation. Sensitivity analysis reveals that steam consumption during bioethanol conversion exerts the most effect on the results, followed by sweet potato yields and fertilizers input. It is suggested that substituting coal with cleaner energy for steam generation in bioethanol conversion stage and promotion of better management practices in sweet potato cultivation stage could lead to a significant improvement of energy and environmental performance.

Highlights

► The net energy ratio and net energy gain values were 1.48 and 6.55 MJ/L, respectively. ► The most significant environmental impacts were eutrophication and acidification. ► The main sources contributing to energy consumption and environmental impact were analyzed. ► Sensitive factors were identified, and improvement measures were discussed.

Introduction

National crude oil consumption in China reached 461.8 million t, with imported crude oil constituting approximately 54.8% in 2011 (BP, 2012). The amount of imported crude oil continuously increases because of the rapidly rising demand for oil in the flourishing economy. In addition, China is currently the top CO2 producer worldwide (Gregg et al., 2008). Increasing reliance on imported oil, high crude oil prices, and heavy environmental burdens have prompted the Chinese government to conduct a serious review of the country’s energy policy.

Development of renewable energy becomes a critical strategy for China to maintain its rapid economic growth and improve its environmental sustainability. China’s first Renewable Energy Act took effect on January 1, 2006. The government attempted to increase renewable energy to 10% of the total energy consumption in 2010. Currently, China aims to increase renewable energy to 16% of the total energy consumption by 2020.

Bioethanol is an important factor in China’s Renewable Energy Development Plan. The government planned to raise bioethanol consumption as a fuel blending component from 1 million t in 2005 to 2 million t by 2010. The government remains intent on raising bioethanol consumption as a fuel blending component to 10 million t by 2020. However, the development of bioethanol fuel is constrained by the rising concern over food safety, prompting the government and the industry to identify non-grain feedstock such as sugar cane, cassava, sweet potato, and sweet sorghum for bioethanol fuel production.

Several publications are already available on LCA studies conducted to identify the energy efficiency and environmental performance of bioethanol production from different feedstocks by using first-generation technologies (produced from food and feed crops). Leng et al. (2008) found that the energy conversion efficiency of E10 bioethanol fuel production from cassava is 1.28 and that energy consumption from denatured bioethanol conversion contributes 70% of the total energy consumption. Papong and Malakul (2010) indicated that cassava-based bioethanol in Thailand has a negative net energy value with an energy ratio of less than 1, indicating a net energy loss. The conversion stage of cassava-based bioethanol also significantly contributes to environmental burdens because of the consumption of coal for power and steam production in bioethanol plants. Amigun et al. (2011) argued that the net performance of different biofuels in reducing non-renewable energy consumption and greenhouse gas emission depends on the type of feedstock, production process, and amount of non-renewable energy required. Liang et al. (2012) argued that corn- and wheat-based bioethanol in China exhibited higher negative economic, energy, and environmental impacts, whereas bioethanol production from sweet sorghum, cassava, sugar beet, and sugarcane showed better economic performance, increased negative energy efficiency, and higher environmental impacts. A strong controversy surrounds the first-generation biofuels, frequently referring to their negative impacts on food safety and the environment (Mueller et al., 2011). Second-generation bioethanol (produced from lignocellulosic biomass) are not as exposed to such disadvantages (Sánchez and Cardona, 2008, Roy et al., 2012, Wang et al., 2012); however, high costs hinder the establishment of cellulosic ethanol infrastructure (Giarola et al., 2012). Consequently, the use of first-generation technology for the commercial production of liquid biofuels continues (Gómez et al., 2011).

China, the world’s largest producer of sweet potato, supplies 80–85% of global production. The majority of China’s sweet potato crop is grown in seasonal rotations with other staple crops. Currently, sweet potato is mainly used as processed food, feed, and feedstock for alcohol production in China. Sweet potato is rich in starch, and approximately 80% of its dry matter consists of carbohydrates. Starch and carbohydrates in sweet potato are readily and anaerobically converted into hydrogen and bioethanol. Moreover, sweet potato contains indigenous bacteria, which may aid in the bioconversion of starch into hydrogen and bioethanol. With the current technology, approximately 8 t of fresh sweet potatoes can produce 1 t of bioethanol (Qiu et al., 2010).

Sweet potato is considered a potential source of bioethanol feedstock by policymakers in China. Numerous pilot production bases of sweet potato are established with high-yielding cultivars and highly intensive cultivation practices to provide feedstock for commercial bioethanol production. Similar to other biomass fuels, bioethanol fuel derived from sweet potato is also confronted with two controversial issues: whether bioethanol fuel produces positive net energy and whether it is environment-friendly. LCA has been proven to be a valuable tool for analyzing energy and environmental considerations of product and service systems. No LCA study has been conducted to assess the energy efficiency and environmental impacts of bioethanol production from sweet potato.

Thus, this study aims to (1) evaluate the energy efficiency of a commercial sweet potato-based bioethanol production plant in China, with net energy gain (NEG) and net energy ratio (NER) as indicators of energy efficiency, and (2) assess the life-cycle environmental impacts associated with bioethanol production from sweet potato. LCA was performed for all stages in the production of 1000 L of bioethanol from sweet potato.

Section snippets

Methodology

LCA in this study comprises four steps: definition of goal and scope, inventory analysis, impact assessment, and interpretation (ISO, 2006).

Energy analysis

Comparison between the total energy output and energy input showed that the NEG and the NER of sweet potato-based bioethanol were 6.55 MJ/L and 1.48, respectively. The production of 1 L of sweet potato bioethanol required a total energy input of 13.53 MJ/L, in which the energy usage during the bioethanol conversion stage was the highest (73.24% of total energy input). This result is similar to that of the cassava-based bioethanol conversion (Leng et al., 2008, Papong and Malakul, 2010) because

Conclusions

The sweet potato based-bioethanol shows positive energy efficiency and high significance of EP and AP. The bioethanol conversion stage is the main contributor to fossil energy consumption, PCOP, HTP, GWP, and AP as a result of coal combustion for steam generation. Nutrient losses from chemical nitrogen and phosphate use during sweet potato cultivation are the main source of EP. Substituting coal with cleaner energy for steam generation and promoting better management practices in sweet potato

Acknowledgements

This work was funded by the National Natural Science Foundation of China (No.70901035, 31071350) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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