Straw utilization for biofuel production: A consequential assessment of greenhouse gas emissions from bioethanol and biomethane provision with a focus on the time dependency of emissions

Benedikt Buchspies,Martin Junginger,Martin Kaltschmitt

doi:10.1111/gcbb.12734

Abstract

AbstractThe shift from straw incorporation to biofuel production entails emissions from production, changes in soil organic carbon (SOC) and through the provision of (co‐)products and entailed displacement effects. This paper analyses changes in greenhouse gas (GHG) emissions arising from the shift from straw incorporation to biomethane and bioethanol production. The biomethane concept comprises comminution, anaerobic digestion and amine washing. It additionally provides an organic fertilizer. Bioethanol production comprises energetic use of lignin, steam explosion, enzymatic hydrolysis and co‐fermentation. Additionally, feed is provided. A detailed consequential GHG balance with in‐depth focus on the time dependency of emissions is conducted: (a) the change in the atmospheric load of emissions arising from the change in the temporal occurrence of emissions comparing two steady states (before the shift and once a new steady state has established); and (b) the annual change in overall emissions over time starting from the shift are assessed. The shift from straw incorporation to biomethane production results in net changes in GHG emissions of (a) −979 (−436 to −1,654) and (b) −955 (−220 to −1,623) kg CO2‐eq. per tdry matter straw converted to biomethane (minimum and maximum). The shift to bioethanol production results in net changes of (a) −409 (−107 to −610) and (b) −361 (57 to −603) kg CO2‐eq. per tdry matter straw converted to bioethanol. If the atmospheric load of emissions arising from different timing of emissions is neglected in case (a), the change in GHG emissions differs by up to 54%. Case (b) reveals carbon payback times of 0 (0–49) and 19 (1–100) years in case of biomethane and bioethanol production, respectively. These results demonstrate that the detailed inclusion of temporal aspects into GHG balances is required to get a comprehensive understanding of changes in GHG emissions induced by the introduction of advanced biofuels from agricultural residues.

Highlights

At present, high efforts are undertaken to reduce the dependency on fossil fuels and thereby to lower anthropogenic greenhouse gas (GHG) emissions
Case (b) reveals carbon payback times of 0 (0–49) and 19 (1–100) years in case of biomethane and bioethanol production, respectively. These results demonstrate that the detailed inclusion of temporal aspects into GHG balances is required to get a comprehensive understanding of changes in GHG emissions induced by the introduction of advanced biofuels from agricultural residues
The change in GHG emissions entailed to the shift from present straw use to bioenergy production is assessed by means of a consequential life cycle assessment (cLCA) and the methodology defined by the RED Renewable Energy Directive II (II)

Summary

Introduction

High efforts are undertaken to reduce the dependency on fossil fuels and thereby to lower anthropogenic greenhouse gas (GHG) emissions. For these purposes, biofuels were promoted as a substitute for fossil energy carriers in the transportation sector in the past decades. The majority of biofuels are made from starch, sugar and oil crops that are used to fulfill the needs for food and feed. This has led to substantial criticism on biofuels made from these kind of feedstocks (Escobar et al, 2009; Koizumi, 2015). Cereal straw presents a mostly commercially unused resource that could be used to produce advanced biofuels such as bioethanol or biomethane (Monforti, Bódis, Scarlat, & Dallemand, 2013)

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