A life cycle assessment of biopolymer production from material recovery facility residuals

Abstract

This study applies life cycle assessment to a potential production pathway for polyhydroxybutyrate (PHB), from the cellulosic fraction of organic residuals from material recovery facilities that otherwise are disposed of in a landfill.The cradle-to-gate production of PHB from this proposed pathway results in primary energy consumption and emissions of approximately 55–76MJ/kg and 3.4–5.0kg carbon dioxide-equivalent/kg, respectively. By comparison, producing PHB from a dedicated agricultural feedstock such as corn is estimated to require approximately twice the energy and greenhouse gas emissions as PHB from material recovery facility residuals.The study also considers an expanded modeling system boundary that assesses the consequences of diverting the organic fraction of material recovery facility residuals from landfills for biopolymer production. This diversion leads to reduced fugitive emissions of landfill gas but also reduced power generation from landfills that combust landfill gas. Including the changes to landfill gas generation leads to greater variability in carbon-intensity estimates for PHB: from as low as −1.7, to as high as 6.3kg carbon dioxide-equivalent/kg PHB. Key contributors to this variability include the selection of an analytical time horizon for landfill gas generation, assumptions about landfill gas collection and combustion, and disposal options for PHB material. The expanded system boundary highlights current gaps in life cycle assessment methodology; in particular, the lack of clear conventions on analytical time horizon selection for processes with long-term consequences, such as landfill gas generation.