Wondering whether a biofuel is really greener? The life cycle assessment of biofuels answers that by adding up emissions from field to tailpipe, then comparing the total with the fossil fuel it replaces.
This guide sits within our wider look at what is renewable energy, and it explains how a fuel’s footprint gets measured.
| Item | Value |
|---|---|
| Scope of EPA’s lifecycle assessment | Full lifecycle, direct and indirect emissions, including land use change |
| Comparison baseline in the US RFS2 rule | 2005 average gasoline or diesel |
| Required reduction levels under RFS2 | Four, one for each renewable fuel standard |
| Greenhouse gas threshold for renewable fuel | 20 percent |
| Grandfathering cut-off for facilities | Construction begun before December 19, 2007 |
These figures come from EPA’s final rule in the Federal Register. Rules and figures can change, so check the source before you rely on them.
What a life cycle assessment actually measures
A life cycle assessment (LCA) tracks the environmental burdens of a product across every stage of its existence. For a fuel, that usually means growing or collecting the feedstock, moving it, converting it, distributing the fuel and finally burning it.
The result is normally expressed as greenhouse gas emissions per unit of energy delivered. That unit lets analysts compare corn ethanol, sugarcane ethanol, biodiesel and petroleum on the same footing.
Two labels you will meet often are “well-to-wheels” and “cradle-to-grave”. Both describe a boundary that starts at the resource and ends with use of the fuel in a vehicle.
The four stages of an LCA
When reading an LCA, use these four questions to check its scope and conclusions:
- Which fuel is being assessed?
- Does the assessment include direct and indirect emissions, including land use change?
- Which baseline fuel is used for comparison?
- What assumptions does the assessment make about co-products such as animal feed?
Direct and indirect emissions
Direct emissions come from the activities in the fuel’s own supply chain, such as tractor diesel, fertiliser production and refinery heat. Indirect emissions are knock-on effects elsewhere in the economy.
The best-known indirect effect is land use change. If farmland is diverted to fuel crops, farming may expand somewhere else and release carbon stored in soil and vegetation.
US regulators chose to count these effects. In its final rule, EPA said “our greenhouse gas emission assessments consider the full lifecycle emission impacts of fuel production from both direct and indirect emissions, including significant emissions from land use changes.”
Co-products and allocation choices
Corn ethanol production also yields distillers grains and solubles, an animal feed co-product.
EPA’s rule reported that new research showed distillers grains and solubles to be “more efficient as an animal feed (meaning less corn is needed for animal feed) than we had assumed in the proposal”.
How LCA results feed into fuel policy
Regulators use LCA to decide which fuels qualify for support. Under the US Renewable Fuel Standard, a qualifying fuel must emit less than a petroleum baseline over its lifecycle. The detail is covered in our guide to renewable fuel standard explained.
The EPA rule states that “the lifecycle GHG emissions of a qualifying renewable fuel must be less than the lifecycle GHG emissions of the 2005 baseline average gasoline or diesel fuel that it replaces; four different levels of reductions are required for the four different renewable fuel standards.”
The rule also grandfathered some plants. It adopted an “indefinite exemption from the 20 percent GHG threshold for renewable fuel facilities which have commenced construction prior to December 19, 2007.”
When a single score is needed for comparison or credits, it is usually reported as a carbon intensity figure. Our explainer on the carbon intensity of fuels shows how those numbers are read.
Why two LCAs of the same fuel can disagree
Results differ because studies make different choices, not because the arithmetic is wrong. When you compare two reports, look for these differences:
- The system boundary, especially whether land use change is included.
- The assumptions about co-products.
- Regional data, such as grid electricity mix, crop yields and fertiliser use.
- The reference fuel and the year of the baseline.
- Technology assumptions, such as plant efficiency and process energy source.
A sound study states each of these and tests how sensitive the result is to them. Treat a single headline percentage with caution unless the method is shown.
How to read an LCA claim as a non-specialist
You do not need to rebuild a model to judge a claim. A few plain questions will tell you most of what you need to know about how much weight a number deserves.
- Who ran the study? Regulators, universities and national laboratories usually publish their method and data. Be wary of a figure with no method behind it.
- What is it compared with? A reduction is always relative to a reference fuel, so a percentage means little until you know the reference.
- Which feedstock and which region? Waste oils, crop residues and dedicated crops carry very different burdens, and so do different farming systems.
- Is the date stated? Grids get cleaner, yields improve and plants become more efficient, so an older study may no longer describe current production.
Applying these checks takes a few minutes and often explains why two headlines about the same fuel point in opposite directions.
Limits of life cycle assessment
LCA is a modelling tool, so it carries uncertainty. Land use change in particular depends on assumptions about global markets that cannot be observed directly, and experts still debate how large it is for some feedstocks.
The method also focuses on what it is told to measure. Greenhouse gases are the usual headline, but water use, soil health, air quality and biodiversity can matter just as much to a community near a plant.
For that reason, analysts often pair an LCA with other sustainability checks. Used that way, it is a strong comparison tool, but it is not a verdict on whether a fuel is good or bad for every purpose.
Frequently asked questions
What is a life cycle assessment of biofuels?
Show answer
It is a structured method that totals the emissions and energy use of a biofuel from feedstock production to final use, then compares the total with a reference fuel such as petroleum.
Does an LCA include land use change?
Show answer
It can. EPA’s final rule included significant emissions from land use changes, but other studies may draw the boundary differently, so check the scope.
Why do co-products matter?
Show answer
EPA reported research indicating that distillers grains and solubles, a corn ethanol co-product, were more efficient as animal feed than it had assumed in its proposal, meaning less corn was needed for animal feed.
Is one LCA result final for a fuel?
Show answer
No. Results depend on the boundary, data and assumptions, and they can be updated as research and regulation change.
