Global warming intensity of biofuel derived from switchgrass grown on marginal land in Michigan

Abstract

Energy crops for biofuel production, especially switchgrass (Panicum virgatum), are of interest from a climate change perspective. Here, we use outputs from a crop growth model and life cycle assessment (LCA) to examine the global warming intensity (GWI; g CO2 MJ−1) and greenhouse gas (GHG) mitigation potential (Mg CO2 year−1) of biofuel systems based on a spatially explicit analysis of switchgrass grown on marginal land (abandoned former cropland) in Michigan, USA. We find that marginal lands in Michigan can annually produce over 0.57 hm3 of liquid biofuel derived from nitrogen‐fertilized switchgrass, mitigating 1.2–1.5 Tg of CO2 year−1. About 96% of these biofuels can meet the Renewable Fuel Standard (60% reduction in lifecycle GHG emissions compared with conventional gasoline; GWI ≤37.2 g CO2 MJ−1). Furthermore, 73%–75% of these biofuels are carbon‐negative (GWI less than zero) due to enhanced soil organic carbon (SOC) sequestration. However, simulations indicate that SOC levels would fail to increase and even decrease on the 11% of lands where SOC stocks >>200 Mg C ha−1, leading to carbon intensities greater than gasoline. Results highlight the strong climate mitigation potential of switchgrass grown on marginal lands as well as the needs to avoid carbon rich soils such as histosols and wetlands and to ensure that productivity will be sufficient to provide net mitigation.

Supply chain system for a centralized biorefinery system based on switchgrass grown on marginal land in Michigan

“…However, the difference in the CIB between biorefinery locations is slightly greater than the difference in biofuel prices due to spatial variations, particularly variations in the initial carbon stock (see Supporting Information, Fig. S11), which result in different SOC sequestration rates 26 …”

“…Switchgrass grown on marginal lands in Michigan was used as cellulosic feedstock for biofuel production in a centralized biorefinery. Data for marginal lands in Michigan (marginal land coordinates and land parcel size) and switchgrass production (biomass yield and soil organic carbon level) were obtained from a previous study, 26 in which biomass yield and changes in SOC for every 30 × 30 m marginal grid cell were estimated over 30 years by the System Approach to Land Use Sustainability (SALUS) model 27,28 . Marginal land was defined as land with United States Department of Agriculture Land Capability Classifications V–VII, excluding urban and carbon dense habitats (e.g., forests and wetlands, etc.)…”

“…The LCI data for static GHG emission factors are calculated using the 2016 electricity fuel mix, 37 while the LCI data for dynamic GHG emission factors are calculated using the electricity fuel mix projections by the US Energy Information Administration 38 . More detailed information on the GHG emission factors can be found elsewhere 26 …”

“…38 More detailed information on the GHG emission factors can be found elsewhere. 26 The carbon intensity of biofuel (CIB) refers to the GHG emissions associated with all processes in the biofuel production system, including switchgrass production, transport of switchgrass bales and pellets, pelletization (if applicable), storage, biorefinery operations, transportation/ distribution of biofuel, avoided grid electricity, biofuel combustion and upstream processes (e.g., materials and fuels). Greenhouse gas emissions from switchgrass production include GHG emissions from agronomic inputs (i.e., seed, fertilizers, and herbicides) and fuel for field operations, CO 2 emissions from SOC change, N 2 O emissions, and GHG emissions due to carbon stock change.…”

“…Various scenarios for the supply of cellulosic biomass to the centralized biorefinery are therefore examined here to understand better the effects of annual The reference scenario (referred to as scenario O) assumes that the biorefinery is always supplied with enough cellulosic biomass to operate at full capacity from its suppliers, and static life cycle assessment (LCA) calculations are used to estimate the global warming impact. The other scenarios account for annual variations in biomass production and use dynamic life cycle assessment (LCA) calculations (including dynamic GHG emission factors and time-dependent global warming potentials (GWPs)) 25,26 due to temporal variations.…”