This study presents a life-cycle
analysis of greenhouse gas (GHG)
emissions of biodiesel (fatty acid methyl ester) and renewable diesel
(RD, or hydroprocessed easters and fatty acids) production from oilseed
crops, distillers corn oil, used cooking oil, and tallow. Updated
data for biofuel production and waste fat rendering were collected
through industry surveys. Life-cycle GHG emissions reductions for
producing biodiesel and RD from soybean, canola, and carinata oils
range from 40% to 69% after considering land-use change estimations,
compared with petroleum diesel. Converting tallow, used cooking oil,
and distillers corn oil to biodiesel and RD could achieve higher GHG
reductions of 79% to 86% lower than petroleum diesel. The biodiesel
route has lower GHG emissions for oilseed-based pathways than the
RD route because transesterification is less energy-intensive than
hydro-processing. In contrast, processing feedstocks with high free
fatty acid such as tallow via the biodiesel route results in slightly
higher GHG emissions than the RD route, mainly due to higher energy
use for pretreatment. Besides land-use change and allocation methods,
key factors driving biodiesel and RD life-cycle GHG emissions include
fertilizer use and nitrous oxide emissions for crop farming, energy
use for grease rendering, and energy and chemicals input for biofuel
conversion.