Anaerobic Digestion Turns Grass into Green Fuel

Researchers from Iowa State University analyzed the effects of employing anaerobic digestion to generate renewable natural gas from grassy biomass in various contexts and from different angles. The research, published in the journals Global Change Biology Bioenergy and BioEnergy Research, helps to clarify the system's potential.

One way to mitigate the unexpected environmental effects of growing the dominant row crops, such as soil erosion, fertilizer runoff, and greenhouse gas emissions, is to strategically sow perennial grass throughout corn and soybean fields.

However, farmers must be able to afford to convert portions of farmland back to prairie.

We are looking at existing markets where there is already a demand, use existing infrastructure to reduce costs of the energy transition, and create wins in multiple categories. We want wins for farmers, wins for businesses, wins for municipalities, and wins for society. We can have great conversations about what could be, but unless it benefits everyone along these supply chains, it would not happen.

Schulte Moore, Professor and Director, Consortium for Cultivating Human and Naturally reGenerative Enterprises

Schulte-Moore's research group analyzed the economic viability of grass-to-gas generation in two recently published peer-reviewed studies from various viewpoints and situations. This study helps clarify the system's potential for mutual benefit.

To replace natural gas with resources that revitalize sustainable agriculture, we have to be able to quantify how much energy we can produce and show it can be cost-effective and environmentally friendly.

Mark Mba-Wright, Study Co-Author and Associate Professor, Department of Mechanical Engineering, Iowa State University

City-Based Scenarios

A $10 million federal grant in 2020, an additional $10 million in federal support in 2022, and roughly $650,000 from the Walton Family Foundation all contributed to the ongoing research. The main goals of the work are to increase and optimize the use of anaerobic digesters. Anaerobic digestion, which is the natural process of biodegrading organic matter without oxygen, releases biogas.

Captured in tank-like digesters, biogas can easily be converted into a fuel that can replace petroleum-based natural gas. It can also be used to make fertilizer and run electrical generators.

The Iowa State researchers analyzed how a network of digesters in and around Ames could meet the city's needs for power and heat. In addition to grassy biomass, other feedstock sources for up to ten digesters include wastewater, food waste, livestock manure, and biofuel byproducts. The networks' locations, sizes, and quantity of facilities varied according to whether their primary purpose was to generate power or natural gas.

The study determined that renewable natural gas was the most financially viable option since its levelized cost was around double that of traditional natural gas's historical average.

Incentives to promote the production of clean energy might help raise prices competitively. In any case, Mba-Wright said, knowing how digester supply chains might function to meet municipal needs helps city leaders grasp the possibilities.

We wanted to consider the seasonality of the supply and demand over a year to give a mayor, for instance, scenarios to look at and strategize around.

Mark Mba-Wright, Study Co-Author and Associate Professor, Department of Mechanical Engineering, Iowa State University

Anaerobic digestion has been the subject of discussions between researchers and municipal wastewater officials in several Iowan cities. According to Schulte Moore, Co-Director of the Bioeconomy Institute and a 2021 MacArthur Fellow, these officials have typically shown curiosity.

“Their immediate need is to provide a service to their customers 24-7. But they work on 15- to 30-year planning horizons, so they are also thinking about the future,” Moore said.

A Grass-to-Gas Road Map

The economic and environmental effects of two hypothetical digesters digesting grassy biomass in the Grand River Basin of northwest Missouri and southwest Iowa were modeled in a study published in Global Change Biology Bioenergy.

According to the researchers' study, the digesters would yield a cumulative profit of more than $400 million during their anticipated 20-year lifespan under ideal circumstances. Approximately 12.5 billion kilowatt hours, or 45 million gigajoules, of renewable natural gas produced over 20 years would have an 83 % smaller carbon footprint than natural gas produced from fossil fuels.

Additionally, emissions are predicted to be lower than those from biodiesel made from soybeans or ethanol made from maize.

According to Mba-Wright, it is critical to determine how anaerobic digesters that generate renewable natural gas would function on a grass diet because the majority of them currently in use run on dairy dung.

“This is dotting our ‘i’s and crossing our ‘t’s to confirm the benefits are what we would expect. We are providing a road map to help build infrastructure, which will, in turn, reduce future costs,” he said.

The profitable scenarios analyzed in the study depend on current carbon credit systems, such as the federal Renewable Fuel Standard and the California Low Carbon Fuel Standard. High-yield grass and prairie restoration on some of the least productive farmland is also necessary to achieve the most beneficial results.

In all investigations, researchers tried to be as realistic as possible by taking into account all known costs, including capital expenses. However, according to Schulte Moore, as techniques advance and fresh study findings become available, they will become even more precise in the upcoming years.

“In the future, we will refine our models by plugging in data our research teams have collected right here in Iowa,” Moore said.

Journal Reference:

Olafasakin, O., et al. (2024) Techno‐economic and life cycle analysis of renewable natural gas derived from anaerobic digestion of grassy biomass: A US Corn Belt watershed case study. Global Change Biology Bioenergy. doi.org/10.1111/gcbb.13164.

Martin, M., I. et al. (2024) Optimal Production and Dispatch of Renewable Natural Gas, Electricity, and Fertilizer in Municipal-Scale Anaerobic Digestion Supply Chains. BioEnergy Research. https://doi.org/10.1007/s12155-024-10767-y.