Carnegie Mellon University
July 19, 2016

Researchers Study Whether Renewable Is Always Better

Corn graphic

Making plastics from plants is a growing trend. It's renewable, but is it better?

A recent study by Carnegie Mellon University researchers examines the life cycle greenhouse gas emissions of three plant-based plastics at each stage of production compared with that of their common fossil fuel-based counterparts.

The study by Daniel Posen, Paulina Jaramillo and Michael Griffin in the Department of Engineering and Public Policy (EPP), was published in the Journal of Environmental Science and Technology.

The study is novel in the way it treats uncertainty and looks at emissions over the life cycle of plastics. The researchers used a technique called life cycle assessment that analyzes emissions at each stage in the life of a product: resource extraction to manufacturing, to use of the product and end of life.

"At each stage, you look at what is going on, what the different inputs are, and what's likely to be coming out the other end," said Posen, a Ph.D. student in EPP and Civil and Environmental Engineering. "Adding all of these scenarios together can get quite complicated because you have to decide how far you want to go back."

While other research has been done on this topic, this study incorporates uncertainty and analyzes multiple scenarios that affect the overall carbon footprint.

For example, nitrous oxide (N2O) is emitted into the air from the fertilizer used to grow corn. And, if the crop of one cornfield is used as a feedstock to produce plastics, then a cornfield may need to be grown somewhere else for food. The consequences extend all the way to deforestation in Brazil, where forests are cut down to grow corn or other crops on the land.

"We've created a more complete model," explained Posen, "creating different scenarios, looking at uncertainty and any kind of parameter that goes into the analysis of emissions. We've built a big model to try and understand, 'Do bio-based plastics make sense?'"

The study stresses the importance of considering uncertainty.

Instead of working under assumptions for the amount of fertilizer that becomes N2O emissions, Posen used a Monte Carlo method, or an approach that runs an experiment many times to generate many samples, to treat that uncertainty. Posen created a more accurate picture of the real percentage of emissions by generating a range of percentages instead of one assumption. Using materials with lower uncertainty could give companies confidence in their ability to reduce emissions.

The researchers also looked at the plastics in terms of their "feedstocks," or the material from which the plastics are derived. Most bio-based plastics are made from corn, but switchgrass is a lower-emission material that may emit fewer pollutants than fossil fuel-based plastics.

Posen and Jaramillo, an associate professor of EPP, will use the results of the study to continue looking into how best to reduce emissions from plastics. In the future, they will continue to ask if there is a better way to reduce emissions through renewable energy in the production process.

"What we've published here is the key model and a big picture look at emissions," Posen said. "We will use it to look at more specific questions about reducing emissions through energy."