Anaerobic digestion turns grass into green fuel

Reviewed by Lexie CornerJuly 29, 2024

Scientists from Iowa State University analyzed the effects of using anaerobic digestion to generate renewable natural gas from grass biomass in different contexts and from different angles. The research, published in the journals Global change Biology Bioenergy AND Bioenergy research, helps explain the potential of the system.

One way to mitigate the unexpected environmental impacts of growing dominant row crops, such as soil erosion, fertilizer runoff, and greenhouse gas emissions, is to strategically seed perennial grasses in corn and soybean fields.

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

We look at existing markets where demand is already there, we use existing infrastructure to drive down the cost of the energy transition and create wins across 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 happen, but if it doesn’t benefit everyone in those supply chains, it won’t happen..

Schulte Moore, Professor and Director of the Consortium for the Advancement of Human and Regenerative Enterprises

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

To replace natural gas with a resource that will revitalize sustainable agriculture, we need to be able to estimate how much energy we can generate and demonstrate that 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 about $650,000 from the Walton Family Foundation supported the ongoing research. The primary goals of the work are to scale up 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 be easily converted into a fuel that can replace petroleum-based natural gas. It can also be used to produce fertilizers and power electric generators.

Iowa State researchers analyzed how a network of digesters in and around Ames could meet the city’s energy and heat needs. In addition to grass biomass, other feedstocks for up to ten digesters include sewage, food waste, manure and biofuel byproducts. The locations, sizes and numbers of the network facilities varied depending on whether their primary purpose was to generate power or natural gas.

The study found that renewable natural gas is the most cost-effective option, with its average cost being about twice the historical average cost of traditional natural gas.

Incentives to promote clean energy production could help drive up prices competitively. In either case, Mba-Wright said, knowing how digester supply chains can function to meet city needs helps city leaders understand the possibilities.

We wanted to take into account the seasonality of supply and demand throughout the year, for example, to give the mayor scenarios to analyze and develop strategies.

Mark Mba-Wright, study co-author and associate professor, Department of Mechanical Engineering, Iowa State University

Anaerobic digestion has been a topic of discussion between researchers and municipal wastewater officials in several Iowa cities, and officials have generally been curious, according to Schulte Moore, co-director of the Bioeconomy Institute and a 2021 MacArthur fellow.

“Their urgent need is to provide customers with 24/7 service. However, they work with a 15-30 year perspective, so they also think about the future“Moore said.

Roadmap from grass to gas

The study, published in the journal Global Change Biology Bioenergy, models the economic and environmental impacts of operating two hypothetical grassland reactors in the Grand River Basin in northwestern Missouri and southwestern Iowa.

The digesters would generate a cumulative benefit of more than $400 million over a projected 20-year life under ideal conditions, the researchers said. The roughly 12.5 billion kilowatt-hours, or 45 million gigajoules, of renewable natural gas produced over 20 years would have an 83 percent smaller carbon footprint than natural gas produced from fossil fuels.

Furthermore, emissions are projected to be lower than those from biodiesel produced from soy or ethanol produced from corn.

According to Mba-Wright, it is crucial to determine how anaerobic reactors that produce renewable natural gas will work on a grass-based diet, since most of those currently in use operate on dairy manure.

“It’s dotting the i’s and crossing the t’s to confirm that the benefits are what we expect. We’re providing a roadmap that will help us build the infrastructure, which in turn will reduce future costs.,” he said.

The cost-effective 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-yielding grass and prairie restoration on some of the least productive farmland are also necessary to achieve the most beneficial outcomes.

In all of the studies, researchers tried to be as realistic as possible, taking into account all known costs, including capital expenditures. But as techniques advance and new research becomes available, Schulte Moore says, they will become even more precise in the coming years.

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

Magazine reference:

Olafasakin, O., and others. (2024) Techno-economic and life-cycle analysis of renewable natural gas from anaerobic fermentation of grass biomass: a case study of the US Corn Belt watershed. Global change Biology Bioenergy…accessed .org/10.1111/gcbb.13164.

Martin, M., I. and others. (2024) Optimal production and dispatch of renewable natural gas, electricity, and fertilizers in city-scale anaerobic digestion supply chains. Bioenergy research. https://doi.org/10.1007/s12155-024-10767-y.