RNG – Livestock and Poultry Environmental Learning Community

Purpose

Anaerobic digesters are an established technology for reducing methane emissions from livestock manure. In recent years, the rapid expansion of renewable natural gas (RNG) projects, driven by economic incentives such as Renewable Identification Number (RIN) credits, Low Carbon Fuel Standard (LCFS) credits, and Investment Tax Credits (ITC) from the 2022 Inflation Reduction Act, has spurred significant growth in RNG production. These incentives, while promoting the adoption of anaerobic digestion, may only sometimes be the most cost-effective way to achieve meaningful carbon reductions within the livestock sector. RNG production, electricity generation via biogas, and flaring biogas all mitigate agricultural greenhouse gas (sometimes referred to as carbon dioxide equivalence or CO_2e) emissions from manure. Nonetheless, electric generators are significantly cheaper than the biogas upgrading systems necessary for RNG production, and flares are significantly cheaper than electric generators.

Our analysis compares system costs and emissions reductions, and investigates the societal benefit featured by each system. The only revenue we analyze is RNG sales and electricity sales; we do not incorporate carbon credits into the revenue stream. Flaring biogas, or the process of burning the methane within biogas to produce the lesser potent greenhouse gas, CO₂, greatly reduces agricultural CO_2e emissions, though this process does not generate usable renewable energy. Electricity generation via biogas is cheaper than RNG production via biogas, but electricity can be sustainably generated with more efficient methods, such as wind turbines and solar panels. RNG is primarily created via anerobic digestion; additionally, RNG is the leading renewable replacement for conventional natural gas, a fossil fuel with increasing use, traveling within 3,000,000 miles of pipelines in the U.S. Nonetheless, RNG remains an expensive and technically complex process, requiring high capital investment and persistent, local, and skilled labor for effective operation.

What Did We Do?

This study compares the economic and carbon reduction potential of various anaerobic digestion biogas uses, including RNG production, electricity generation, and flaring. By evaluating the carbon savings and cost-effectiveness of these options, the study provides policymakers insights on optimizing public funding and incentives for the livestock industry. Furthermore, we provide livestock farmers with a decision support tool that balances the environmental benefits of anaerobic digestion with the most efficient use of financial resources to foster clean and sustainable livestock production system.

What Have We Learned?

Table 1 summarizes dollars per megagram (MG) of CO_2emitigated via RNG production, electricity production via biogas, and flaring biogas for both covered manure storages and constructed anaerobic digesters. Five scenarios were compared for farms featuring dairy cows, swine with lagoon manure storages, and swine with deep pit manure storages to analyze the carbon credit value (units of dollars per MG of CO_2emitigated) necessary to financially break even on the project. Flaring biogas featured the lowest necessary break-even carbon credit for dairy, swine farms with lagoon manure storage, and swine farms with deep pit manure storages. If a farmer wants to generate power, then generating electricity requires a lesser carbon credit value per MG CO_2emitigated compared to RNG generation. If a farmer wants to generate power via RNG, and carbon credits exist in units of dollars per energy, then a dairy farmer would be more profitable with a digester, whereas a swine farmer would be more profitable with a covered manure storage.

If a governing body is interested in maximizing its livestock manure CO_2ereduction given a set amount of tax dollars, then the governing body may be most interested in incentivizing flaring systems. If a governing body is interested in both power generation via livestock manure and CO_2ereduction, then the governing body may be most interested in incentivizing electricity generation. Nonetheless, renewable electricity can be generated more efficiently by a variety of methods, whereas RNG is the most prominent fossil natural gas replacement and primarily created via anaerobic digestion. Furthermore, as the electric grid “greens”, or as the CO_2eemissions associated with grid electricity decrease, RNG generation will provide an overall greater percent CO_2ereduction.

Deep pit swine farms generating electricity or RNG demonstrated CO_2ereduction that was greater than 100%. Deep pit swine farms have less emissions than lagoon swine farms. By converting a deep pit swine farm to an outdoor covered manure storage or digester system, methane production increases, though that methane is now used for renewable energy generation, thereby offsetting fossil energy generation.

Table 1: Required Carbon Credit Value ($/MG CO₂e mitigated) to Break Even
		Head	Dairy: 2,000	Swine – Lagoon: 14,000	Swine – Deep Pit: 14,000
		Baseline CO₂e (MG/yr)	10,654	10,179	3,980
Covered Storage	Flaring	CO₂e Mitigated (MG/yr)	8,786	8,786	3,424
		% CO₂e Reduction	82%	86%	86%
		$/yr Profit (10-year life)	($84,137)	($108,151)	($342,975)
		Break-Even ($/MG CO₂e Mitigated) Carbon Credit	$10	$12	$100
Covered Storage	Electricity	CO₂e Mitigated (MG/yr)	9,734	9,735	4,373
		% CO₂e Reduction	91%	96%	110%
		$/yr Profit (10-year life)	($178,978)	($202,992)	($437,816)
		Break-Even ($/MG CO₂e Mitigated) Carbon Credit	$18	$21	$100
		Break-Even ($/kWh) Carbon Credit	$0.08	$0.09	$0.20
Covered Storage	RNG	CO₂e Mitigated (MG/yr)	9,913	9,914	4,552
		% CO₂e Reduction	93%	97%	114%
		$/yr Profit (10-year life)	($780,801)	($795,726)	($1,030,551)
		Break-Even ($/MG CO₂e Mitigated) Carbon Credit	$79	$80	$226
		Break-Even ($/MMBTU) Carbon Credit	$46	$47	$61
Digester	Electricity	CO₂e Mitigated (MG/yr)	10,042	9,826	4,464
		% CO₂e Reduction	94%	97%	112%
		$/yr Profit (10-year life)	($557,022)	($519,566)	($754,390)
		Break-Even ($/MG CO₂e Mitigated) Carbon Credit	$55	$53	$169
		Break-Even ($/kWh) Carbon Credit	$0.14	$0.19	$0.27
Digester	RNG	CO₂e Mitigated (MG/yr)	10,169	9,904	4,542
		% CO₂e Reduction	95%	97%	114%
		$/yr Profit (10-year life)	($1,207,287)	($1,056,809)	($1,291,633)
		Break-Even ($/MG CO₂e Mitigated) Carbon Credit	$119	$107	$284
		Break-Even ($/MMBTU) Carbon Credit	$39	$50	$62

Future Plans

The project life of biogas upgrading equipment, pipeline interconnects, electric generators, and flares are not always the same. We intend to further investigate the project lives of different equipment to calculate more accurate annualized costs and payback periods. Furthermore, we will analyze how the economies of scale compare between biogas upgrading equipment, electric generators, and flares by evaluating costs of equipment necessary for various farm sizes. Lastly, we would like to further define and quantify the overall societal impact created by RNG production, electricity production via biogas, and flaring biogas.

Authors

Presenting author

Luke Soko, Graduate Student, Iowa State University

Corresponding author

Dan Andersen, Associate Professor, Iowa State University, dsa@iastate.edu

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2025. Title of presentation. Waste to Worth. Boise, ID. April 7-11, 2025. URL of this page. Accessed on: today’s date.

Purpose

Co-digestion of organic material with dairy manure represents an opportunity to provide both a revenue stream to anaerobic digester operations, through the collection of a tipping fee and/or increased biogas/electricity production, as well as a means for waste generators to dispose of their product in a beneficial way.

However, there are many factors for an operator to consider when deciding on whether to accept organic waste. A major consideration is the volume of biogas that the material will generate when co-digested. This can be used both to assign a value to the waste through increased biogas production and/or electricity sales, as well as to size equipment for producing, treating and potentially selling/using the biogas. Estimating the biogas produced is a complicated process, encompassing many different factors of digester design, waste characteristics, and environmental factors.

To assist in this estimation, we have developed software that allows a user to predict the biogas production from mixed wastes and dairy manure based on changing herd sizes, as well as providing the ability to vary the timing and volume of addition of multiple organic wastes, throughout the course of a simulated year. With this user-friendly tool, we hope to enable producers to better explore the opportunities that co-digestion offers.

What Did We Do?

The originally developed Cornell Anaerobic Digester Simulations software allowed the user to input a herd size and to select how much (if any) of seven wastes would be co-digested with the dairy manure. This rudimentary method of simulation assumed that the same volume/mass would be applied to the digester in a steady-state constant fashion for the entire year that the simulations were run for. However, that is unlikely to be the case in a real-world production environment.

In the new version of the software, we have incorporated the characteristics of over 200 wastes into a user selectable interface. Once a waste type is selected, the user has the option to select when the waste is placed into the digester, whether that be on an everyday, weekly, monthly or custom basis with the option to select to which months of the year the additions occur. When selecting a weekly or monthly basis, the user can select which day(s) of the week or month wastes are added, and in the custom basis, the user can select which days of the year additions occur.

Once the timing of addition is completed, the user can select how much of the waste is applied during each addition. Whether that be a constant volume for each addition, or a custom volume for each addition.

The data for the specific wastes includes the dry matter and organic matter content as well as the biogas and methane yields. Based on the type of waste we have also assigned a “digestibility” curve to the particular waste which when assuming a first order kinetic model of gas production, can provide the production of gas a function of time. The production of biogas from all added wastes and the added manure is then summed for each day of the year to provide an estimate of the biogas production, on a daily basis, that can be summarized with a minimum/maximum/average on a monthly and annual basis.

What Have We Learned?

During the process of developing the software, we examined a few different techniques for estimating the timing of biogas production from co-digested wastes. There are more complicated models available such as Anaerobic Digestion Model #1 (ADM1), however many more parameters must be known/estimated for each waste type, (not to mention requiring a much more complicated user interface). We felt that using a simplified first order kinetic model provides a good way to add the necessary complexity to model biogas production over time without overly complicated calculations. The simplification allowed us to include a more complicated and yet more real world means of modeling the addition of wastes to a digester that wouldn’t be possible with more complicated digestion/biogas production models.

Future Plans

Currently, the Cornell Dairy Anaerobic Digestion Simulation Software is capable of predicting the amount of heat necessary to maintain digester temperatures, as well as the parasitic electrical load. Future additions will include modeling the energy usage (and effects on biogas) of treatment processes to produce Renewable Natural Gas (RNG) from biogas.

We would also like to include the ability to track nutrients through the process of digestion. Nutrient additions from the co-digestion of wastes also represent an important consideration for farm as they may or may not have the land base/crop requirements to use all of the imported nutrients. The cost of treatment of the effluent from the digester to remove nutrients, or the shipment of effluent off site may have to be added into the determination of how much of a “tipping fee” a farmer would need to charge for taking an organic waste for co-digestion.

We hope to make the program freely available to the public to use. Currently, the software is written in MATLAB which ordinarily requires a license to operate, however it is possible to create an executable standalone program that can be shared and run without the need to purchase MATLAB.

Authors

Timothy Shelford, Extension Associate, School of Integrated Plant Science, Cornell University

Corresponding author email address

tjs47@cornell.edu

Additional authors

Curt Gooch, Senior Extension Associate Emeritus, Department of Biological and Environmental Engineering, Cornell University

Peter Wright, Agricultural Engineer, Department of Animal Science, Cornell University

Lauren Ray, Agricultural Energy Systems Engineer, Cornell University

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2022. Title of presentation. Waste to Worth. Oregon, OH. April 18-22, 2022. URL of this page. Accessed on: today’s date.