Key Feedstock and Process Choices
Both biodiesel and renewable diesel can be produced from plant or animal oils, fats, and wastes. In the United States, soy oil made up 65 percent, corn oil was 7.5 percent, and yellow grease and white grease was less than 8 percent of production of 8,478 million pounds of feedstock inputs for biodiesel in 2013.13
Biodiesel is produced from a chemical conversion called transesterification to produce fatty acids (triglycerides). Oil-rich biomass or animal fats are reacted with alcohol with a catalyst to form methyl or ethyl esters and a glycerin byproduct.14 Blends of 5 percent biodiesel are common, but not universally available. Infrastructure for distribution and refueling of biodiesel remains a barrier. Additionally, cold-flow properties of biodiesel can cause blockage of fuel line filters if not specced to the right local weather conditions.15
Renewable diesel is produced through thermochemical transformation, the most common of which is called hydrotreating.16 Hydrogen replaces atoms of the biomass or animal fats with hydrocarbons that can then be used as a fully renewable diesel or blended with petroleum diesel.
Second-generation biodiesel can be produced in similar ways as first-generation fuels through thermochemical and biochemical routes.17 The Fischer-Tropsch process, more commonly used for coal to liquids and gas to liquids, can be used to convert biomass from crops or waste products to biodiesel. BioDME (dimethyl ether) can be produced via catalytic dehydration of methanol or directly from syngas.18
Key Sustainability Opportunities and Impacts
Opportunity: Climate Change
According to the latest published life-cycle analysis for various feedstocks and the feedstock mix reported by the Energy Information Administration and the U.S. EPA for 2013, the average GHG reduction for biomass-based diesel exceeds 80 percent.19 20 21 As such, certain types of biodiesel and renewable diesel generate favorable low-carbon credits under both the federal RPS and California’s LCFS. If biofuels provided just over a quarter of transport fuel in 2050, it would avoid an estimated 2.1 gigatons of GHG emissions per year when produced with best sustainability practices.22 23
Opportunity: Renewable Fuels
Biofuels are among the few fuels for medium- and heavy-duty trucking that are renewable. Renewable fuels can be produced by the agricultural sector (planted crops and crop residues) and the non-agricultural sector (planted trees and tree residues, animal waste material and byproducts, slash and pre-commercial thinnings), according to the EPA.24
Impact: Air Pollutants
Transportation related emissions are estimated to be responsible for about half of deaths from outdoor air pollution, which is now the biggest environmental cause of premature death, resulting in an estimated 110,292 deaths in the Unites States in 2010.25 These impacts are largely the result of tailpipe emissions that include suspended particulate matter, nitrogen dioxide, benzene, and other pollutants.26 Research suggests that biodiesel could produce higher NOx, hydrocarbon, acetaldehyde, and ethanol emissions and lower carbon monoxide, and benzene emissions compared to diesel from fossil fuels.27 28
Increased biofuel production can result in large impacts on biological diversity through land conversion, introduction of invasive species, and the soil and water impacts common to agriculture. Studies have shown that substantially increased biofuel production can result in habitat loss, increased invasive species, and nutrient pollution, especially if crop production replaces native forest.29 30 Biofuels from palm oil are of particular concern by some global environmental stakeholders.31 32
Key Uncertainties and Unresolved Issues
Uncertainty: Direct and Indirect Land-Use Change (ILUC):
The science for characterizing impacts of emissions from land use is emergent, however recent advances in modeling and new standards and regulation have increased clarity on the issue. Advances in modeling have also enabled a likely range for impacts of indirect land use of 13.4-42.3 gCO2e/MJ for soy biodiesel in a recent California Air Resources Board (CARB) analysis.33 While increased use of lifecycle assessment improves our understanding of land-use change from biofuels, methodologies are not yet standardized, leaving this question still unresolved.
Uncertainty: Food and Commodity Competition
In the United States, study results of commodity competition for biofuel feedstocks vary. The question of biofuel’s impact on food illustrates the variation: Some studies claim that biofuels production increases food production from co-benefits and efficiencies, others show modest influence on food prices, still others claim that large-scale biofuel production would divert land from producing food needed to feed the world.34 35 36 Most studies show that impacts are minimal, and biofuels regulation addresses the issue, but more information is needed to resolve this entirely.
Uncertainty: Human Rights
Biofuel projects are at risk for the same human rights challenges faced by the agricultural sector broadly (e.g., treatment of labor and workers). Exploitation sometimes includes unlawful child labor and migrant workers. Additionally, land-use conflicts and tension with traditional livelihoods are other important factors that have the potential to produce human rights challenges.
Uncertainty: Water Availability
Certain biofuels consume more water than any type of fuel energy, though there are notable variations by feedstock type, fuel pathway, and irrigation patterns. The freshwater intensity of biofuels from soy and corn can be two orders of magnitude larger than average freshwater consumption for the oil-to-liquid fuels supply chain (primary recovery).37 38 Over 90 percent of biofuels’ water consumption impacts are related to farming crops. Yet the regions and crops that matter most to North American fuel users in the near term are mostly low- or no-irrigation crops.39 40 41