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From Science & Research

From Science & Research Bioplastic products from citrus wastes by Mohammad Pourbafrani 1 Jon McKechnie 2 Heather L. MacLean 1,3 Bradley A. Saville 1 1 Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, ON, M5S 3E5, Canada 2 Division of Energy and Sustainability, University of Nottingham, University Park, Nottingham NG7 2RD, UK 3 Department of Civil Engineering, University of Toronto, 35 St George Street, Toronto, ON, M5S 1A4, Canada Introduction: Biomass-derived plastics have the potential to displace relatively high market value products, while also contributing to sustainability objectives. In particular, second generation feedstocks such as agricultural residues offer great potential. Employing citrus wastes (CW) as a feedstock for bioplastics production has potential as a low-cost alternative, while providing other environmental advantages. Approximately 30 million tonnes of CW is estimated to be produced annually, representing half of the citrus fruit used for juice production [1]. New strategies for processing CW are required to address disposal challenges, including high costs, a lack of disposal sites, and concerns about negative environmental impacts of current practices. Citrus waste contains simple sugars and carbohydrate polymers such as cellulose and hemicellulose. The proposed CW biorefinery discussed in this article could convert these sugars and carbohydrates into bioethanol, while recovering limonene (natural solvent), and producing biomethane and nutrientrich digestate (fertilizer) from residual materials [1]. The bioethanol could then be further processed to renewable low density polyethylene (LDPE) following ethanol dehydration to ethylene. To evaluate the CW to LDPE process, it is important to understand the associated environmental implications from a life cycle perspective (from feedstock production through to the final product) and to compare with current production technologies. Understanding the greenhouse gas (GHG) emissions of the process is important due to the GHGintensity of current LDPE production from fossil fuels. In this article, the potential to reduce life cycle GHGs when LDPE is produced from citrus wastes is evaluated. Citrus waste to bioethanol A biorefinery for production of bioethanol from CW is presented in Fig.1. The technical data related to the 28 bioplastics MAGAZINE [03/13] Vol. 8

From Science & Research Citrus Waste Acid Ethanol Hydrolysis Reactor Fermentation Distillation Flash Liquid Solid Flash Stillage Limonene Recovery Biogas Purification Anaerobic Digestion Limonene Methane Steam Boiler Steam Citrus Waste Biorefinery Methane Ethanol Power Plant Electricity and Heat Ethylene and LDPE Production Plant Excess to grid LDPE Limonene and Digestate Fig. 1. Block Flow Diagram of Ethanol Production from Citrus Wastes [1] Fig.2. Production of LDPE from Citrus Wastes biorefinery were published previously [1]. The biorefinery’s main stages include hydrolysis, fermentation, distillation and anaerobic digestion. Citrus waste carbohydrate polymers are converted into sugars during hydrolysis, and then fermented to produce bioethanol. The ethanol is purified by distillation and the non-fermentable sugars and other process residues are converted to biomethane by anaerobic digestion. Some of the biomethane is combusted to satisfy the thermal energy requirements for the biorefinery; excess biomethane is converted to electricity. In this biorefinery design, one dry tonne of CW yields 198 liters of ethanol, 45 liters of limonene, 270 m 3 of biomethane and 220 kg digestate. For a hypothetical 40,000 dry tonne per year CW biorefinery, the ethanol production cost is estimated to be 0.65 USD per litre [1]. Bioethanol to bioplastic The ethanol produced by the CW biorefinery is dehydrated to ethylene in a catalytic process at high pressure and temperature [2]. Each kg of ethanol yields 0.59 kg of ethylene. This process is energy intensive and requires 5.6 MJ of thermal energy and 1.8 MJ of electricity per kg of ethylene produced. The ethylene is polymerized to LDPE, consuming 0.3 MJ of thermal energy and 6.4 MJ of electricity per kg of LDPE. With 1 kg of ethylene yielding 1 kg of LDPE, each dry tonne of CW can produce ~92 kg of LDPE. Life Cycle Assessment of renewable LDPE from citrus wastes Although LDPE production from CW is an energy intensive process, biomethane generated in the biorefinery can provide the required energy (Fig. 1 and Fig. 2). The biomethane is utilized in a power plant that generates heat and electricity, which are consumed by the ethanol and ethylene production processes and the ethylene polymerization process; excess electricity is exported to the grid. Therefore, the production of LDPE from CW is energy self-sufficient. A life cycle assessment was performed to calculate the life cycle GHG emissions associated with LDPE production from CW. The key inputs, outputs and processes are shown in Figure 2, and include CW transportation, bioethanol production, ethylene production and LDPE polymerization. The emissions associated with LDPE production include all process steps, inputs and outputs. In addition, emissions credits resulting from the biorefinery’s co-products (limonene, digestate and biomethane) displacing chemical and fossil fuel products (acetone, biofertilizer and natural gas, respectively) are assigned to the LDPE. This method of co-product treatment, termed displacement or system expansion, is recommended under the International Organisation for Standardisation guidelines for life cycle assessment [3]. Since generation of electricity and heat from biomethane is considered to be a carbon neutral process, the life cycle GHG emissions of LDPE production are dominated by chemical inputs to the process stages, fossil fuel use in transportation of CW to the biorefinery, and biomethane emissions from the biorefinery’s anaerobic digesters [4]. The net life cycle emissions for the production of renewable LDPE are -4,100 g CO 2 eq./kg. Negative emissions are achieved because of two factors: CW LDPE sequesters biomass carbon that would otherwise be released to the atmosphere; and emissions credits for co-products more than offset the production-related emissions. By comparison, the life cycle GHG emissions values for LDPE produced from crude oil are significantly greater (2,130 g CO 2 eq./kg of LDPE) [5]. Prior work has assessed LDPE production from sugar cane [2], which found emissions to exceed those of crude oil-derived LDPE when including land use change-related emissions (e.g., land clearing directly or indirectly linked to sugar cane cultivation for ethanol production). In contrast, when using CW, no land use change related GHG emissions are incurred since CW is a byproduct of juice manufacture. bioplastics MAGAZINE [03/13] Vol. 8 29

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