Frequently Asked Questions

What is G2G (Grass2Gas)?

Grass2Gas, or G2G, is a subset of the C-CHANGE project, focused on assessing whether perennial and winter crops can be more widely used as feedstock for producing renewable natural gas (RNG) through anaerobic digestion. Ultimately, the project explores incentives for farmers and farmland owners to establish more perennial and winter crops on their land and ways to replace fossil natural gas with renewable natural gas developed from plant material. 

What is anaerobic digestion? How does it work?

Anaerobic digestion is the natural breakdown of organic matter by microorganisms in an environment without oxygen. These complex microbial communities break down – or digest – the organic matter, which in turn produces biogas.

What is biogas? How does it become methane gas?

Biogas is about two-thirds methane and one-third carbon dioxide, and contains some other gases in trace amounts. Biogas can be cleaned and purified to become renewable natural gas, which is 98% methane. 

What is an herbaceous feedstock?

A feedstock is the raw material used to create an energy source. In the context of this system, an herbaceous feedstock refers to the harvested biomass from perennial and winter crops. However, in the larger context of biogas production, feedstocks may also include manure, food waste, and other crop residue.

What are winter crops?

Similar to a cover crop, a winter crop is typically grown following the harvest of a commodity crop like soybeans or corn. Unlike a cover crop, the biomass from a winter crop is harvested for use as a feedstock in an AD system. There are a number of potential species that can be grown as winter crops; however, cereal rye is the most common.   

What are perennial grasses?  

Perennial grasses are resilient crops with many benefits and applications. They can be used for animal feed and bedding, and feedstock for renewable natural gas production, while also providing ecosystem benefits (e.g. reduced erosion, increased nutrient retention). They are best planted in areas on the farm that have resource concerns (e.g., areas prone to erosion, next to rivers and streams) or that yield poorly for annual crops (e.g., highly uneven terrain, frequently inundated areas, or turnrows).

While annuals, such as corn and soybeans, need to be replanted every year, perennials do not and thus provide a cost savings. Perennial grasses can be planted either in monocultures or in diverse mixtures, though diverse mixtures are preferred as they provide additional benefits as habitat for wildlife.

How many acres of herbaceous feedstock (perennials, winter annual crops, etc.) would be needed for an AD system?

Zero acres of perennials or winter annuals are required for an AD system. Systems that run solely on manure are prevalent today. The G2G project, however, is studying the benefits of adding herbaceous feedstocks to AD systems alongside manure. Currently, small livestock operations would need vast amounts of land (potentially greater than 5,000 acres in the case of prairie) to make a profit from digesting perennials whereas large livestock operations could add in relatively small amounts of herbaceous feedstock and see a benefit to biogas production. Community digester projects, government subsidies, and using digestible (e.g., winter rye) and cheaper (e.g., corn stover) feedstocks could help improve profitability and reduce the need for a single farm to manage thousands of acres of perennials. 

How much biogas is produced with herbaceous feedstock as opposed to other feedstock regimes based only on manure and/or food waste? 

Combining herbaceous feedstocks with manure will generally generate more biogas than either feedstock would on its own. The amount of additional biogas generated from adding an herbaceous feedstock will vary based on the type of herbaceous biomass (prairie, switchgrass, miscanthus, winter rye, etc.) and the proportions of herbaceous biomass and manure in the digester. Adding herbaceous feedstocks to a food waste digester may have the additional benefit of providing more stability to the digester output (and management), given that herbaceous feedstocks are a more predictable feedstock than food waste.

What is the optimal type of herbaceous feedstock, and what is the optimal mix of feedstock and manure?

The amount of crop biomass required to continuously feed an anaerobic digester system depends on the size of the system, but a consistent ratio of plant biomass to manure is key. G2G has been studying a 1:1 grass-to-manure ratio but is working toward a 4:1 grass-to-manure ratio. A higher amount of grass compared to manure may help smaller livestock operations make biogas production profitable, or provide operational flexibility with market changes. The amount and ratio of crop biomass will change from system to system with some requiring more manure to provide nitrogen as well as microbial inoculum to kickstart digestion. Manure from different livestock species, different perennial species, and different harvesting times will all affect nitrogen levels and thus affect the ratio of grass to manure.

What is the best model or scale for these systems (e.g., on-farm energy only, individual farms connecting to the energy grid, multiple farms connecting to a central hub, a co-op model, etc.)?

There is no one-size-fits-all approach to the G2G scenario, and what works best for any individual operation will vary. What type of model makes sense for an operation depends on, among other things, farm size, number and type of livestock, and proximity to other farms and NG pipelines. For example, a small dairy farm may choose to operate a small digester supplemented with some herbaceous feedstock and use the biogas to produce on-farm heat and electricity. However, if that dairy were located near several other dairies and were close to a natural gas pipeline, those dairies may choose to combine their feedstocks at a centralized digester, upgrade the biogas to RNG, sell that RNG, and claim LCFS and RIN credits. 

What are the end uses of biogas/RNG, and is there a climate benefit? 

Biogas can be used on-site for heat and electricity, reducing the power costs for farm operations, or it can be upgraded to renewable natural gas. RNG can be linked to existing infrastructure and used in any application that relies on traditional natural gas, such as producing heat and electricity for homes and businesses or as a fuel for vehicles. The climate benefits of producing biogas and RNG could be described in multiple ways. Including RNG in a national energy portfolio significantly shortens the timeline to a 100% renewable energy system as compared to using only solar and wind. Solar and wind storage is difficult and may have to rely on batteries which have their own negative impacts. RNG can be used in hard-to-decarbonize heavy industry and to fill in gaps from the intermittency of sun and wind. Methane capture from livestock operations mitigates global warming in the short-term by reducing potent methane emissions. Using perennials can augment climate benefits by acting as a biological form of carbon capture and storage. The G2G project, however, holistically considers potential benefits and tradeoffs. The project has multiple sustainability aims, including supporting farms and rural economies; reducing soil erosion; reducing nutrient loss through runoff, leaching, and volatilization which lead to eutrophication and greenhouse gas emissions; reducing odors, pathogens, and other forms of pollution; and improving biodiversity. 

Concern has been expressed about nitrous oxide emissions from digestate. Is this a legitimate concern, and if so, what can be done to mitigate these emissions? 

Nitrogen management of digestate is a concern, though in a properly sealed digester, the challenge comes from managing the digestate after it leaves the digester, rather than while the feedstock is digesting. An improperly sealed digester would allow oxygen into the system which could lead to the generation of nitrous oxide. In a properly functioning system, the nitrogen will leave the digester primarily in the form of ammonium (NH4+). Results suggest that sequencing injection (as opposed to direct field application) of digestate to correspond with cover/winter annual crop and primary/commodity crop schedules can reduce runoff (i.e., loss of nitrogen to flowing water), leaching (loss of nitrogen to ground water), and volatilization (loss of nitrogen to the air). Environmental conditions, including soil saturation, soil pH and oxygen levels, and post-application rainfall also have a big impact on nitrogen loss. The digestate from a digester which processes only manure will have a higher amount of nitrogen relative to carbon than digestate from a digester which processes manure and herbaceous feedstock. Thus, a “grassy” digester (i.e., one which digests herbaceous feedstock) could potentially make nitrogen management easier.

Why does California's low carbon fuel standard (LCFS) matter so much? 

The LCFS is designed to encourage the use of cleaner low-carbon transportation fuels in California, encourage the production of those fuels and, therefore, reduce GHG emissions and decrease petroleum dependence in the transportation sector. As the largest state economy in the US, California’s LCFS is an important driver of emerging renewable energy sources. 

Who is funding this project? What is the timeline?

The USDA National Institute for Food and Agriculture (NIFA) awarded a $10 million grant for a five-year period (2020-2025). 

Who is involved in the Grass2Gas project? 

Collaborating organizations include Iowa State University, Penn State University, and Roeslein Alternative Energy.