First of a two-part series on renewable natural gas in the agricultural sector.
Renewable natural gas, or methane from non-fossil sources, continues to penetrate the nation’s energy system, led by a replacement for diesel fuel in California.
RNG from dairy and swine production has become highly prized because of low measurements of field-to-wheels carbon emissions, known as carbon intensity. This gas is transported through pipeline networks and ultimately compressed for use in buses, trucks and other vehicles.
Across the country, landfills account for 63% of RNG production. Agricultural waste accounts for 24% of operational RNG projects, according to the Coalition for RNG, up 7 percentage points from 2023.
Agricultural RNG sources now total 243 nationwide. Dairy dominates that sector but data sources show many projects are designed for multiple feedstocks.
The coalition said that 89 agricultural RNG projects are under construction and 134 are planned, outpacing growth from landfills.
California leads in the number of total agricultural projects at work with more than 100. Texas, South Dakota and Idaho currently attract developer interest because their larger dairy operations and extensive natural gas infrastructure make projects more practical, said Mark Stoermann, chief operating officer of Newtrient, a technology and solutions provider for dairy RNG.
Available feedstocks and many federal and state policies make RNG attractive for developers, farmers and parties that want low-carbon fuels.
The technology is simpler than oil refining, the other main source of motor fuels, starting with manure lagoons covered by plastic tarps.
In a warm climate, said Daryl Maas, CEO of Maas Energy Works, which has 69 anaerobic dairy digesters in the country, mostly in California, it costs about $3,000 per milking cow to build a digester and related equipment. Operators in cold climates use heated tanks of concrete or steel which resemble a traditional sewage plant and raise the cost to $5,000 per cow.
Projects that maximize renewable energy production cost more than those focused on reducing greenhouse gas emissions. This is significant because the California Air Resources Board (CARB) approves every RNG project that will deliver gas to the state. Crucial to approval is the energy pathway or life cycle emissions for a fuel that produces the carbon intensity score.
“Probably the biggest difference between the two types of projects is if you heat the organic material to 105 degrees Fahrenheit or not,” Stoermann said. “With the additional heat, there are significant ongoing costs but also almost double the gas production over an ambient temperature system.”
After raw gas leaves the digester by pipeline, a separate upgrader reduces CO2 content of the biogas from 35%-50% to 1%-5% in the final product, chemically indistinguishable from hydrocarbon methane. RNG is then compressed to commercial pipeline pressure.
Multiple dairies and pig farms are commonly clustered for their gas to use a single upgrader.
“Cleaning up gas for pipeline quality meant going to larger scale dairies,” Maas said. “We have 10 of these clusters in the country with a single upgrader.”
It can be economical to build one upgrader for 1,000 cows, but the average dairy is smaller than that. A scale of 5,000 cows per upgrader is economic in places like Texas and South Dakota, Maas said, while that number is too low in California because “everything is so expensive.”
Daryl Maas“One of the big things for the industry is to push down the average digester size,” he said. “If we can aggregate them, we can push down the size and extend the technology.”
Swine operations differ
Swine manure is the second largest source of methane from livestock, with many operators in North Carolina, Virginia and the upper Midwest. About 50 swine RNG facilities exist now, but the Environmental Protection Agency suggests 5,400 more could be added.
The manure collection process for swine gas can be more costly than for dairy gas. Pig manure can be harder to handle than cattle manure, and the volume produced per animal is lower.
In its favor, swine gas has more methane content on a volume basis than dairy cattle gas.
Anaerobic digesters found in hog operations tend to be larger than those found in other agricultural operations. This is because hog manure is handled more easily by adding extra water, and that additional water results in needing a larger, less efficient digester.
The University of California, Davis, found last year that manure management systems on hog operations usually collect manure through slotted floors, gutters or runoff, making it more expensive to install the infrastructure needed to transport manure to an anaerobic digester.
Many manure systems currently used on swine farms, such as hoop structures or deep litter, are not ideally suited for anaerobic digestion.
Flush collection, in which flowing water is used to remove manure, is a common manure management practice, particularly in North Carolina.
The same economies of scale are pursued for swine RNG as in dairy projects. Patrick Serfass, executive director of the American Biogas Council, explained last month that Smithfield, the large buyer and owner of swine, will convince many hog farms that may be miles apart to work together. In these projects, manure pits get covered and farmers pay for their own anaerobic digesters, receiving a long-term offtake deal for gas. Smithfield covers the upgrader costs.
Monarch Bioenergy operates nine projects in Missouri creating gas from swine waste from Smithfield and is developing projects in North Carolina and elsewhere. President Sean Lock said anaerobic digestion is best deployed at farms with 4,000 or more hogs, using anaerobic lagoons and liquid slurry manure systems that are flushed or scraped each day.
Objections persist
Local communities often take dairy and swine operators to task with regulators over the perceived impacts of RNG clusters.
Lock said the approach of Monarch Energy does not require or lead farmers to increase their herd sizes. “We optimize existing operations,” he said.
Residents may have disliked these farm operations because of the smell from uncovered ponds, but experts point out that the digestate remaining after anaerobic digestion has less odor than manure.
North Carolina public interest groups have a variety of legacy complaints, based in part on the smell that accompanies pig manure and high levels of nitrates from pond water runoff. Producers in the state insist that operators conscientiously manage manure pits and remediate problems.
Groups concerned with animals and communities near dairies, and those opposed to internal combustion transport fuels, have asked CARB to limit dairy farm participation in its Low Carbon Fuel Standard (LCFS).
The RNG industry points to studies that contend the fuel standard has not increased dairy farm consolidation.
“Environmental groups don’t like the fact that RNG is still combusting gas,” said Irra Core, principal at environmental consultants Ashworth Leininger Group. “But emissions in the life cycle are still lower than for fossil gas.”
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