Changing solids to gas

LAKE BENTON, Minn. — Transitioning manure into methane takes several steps. The owners of Ash Grove Dairy can now accomplish this with help from Clean Energy Fuels, BP & Dynamic Renewables.

The dairy farm installed an on-site manure digester and renewable natural gas system that became operational in April 2024.

Ash Grove Dairy is owned by Michael Crinion and his wife, Patricia. The 2,100 lactating and dry cows are milked in a double-20 parallel parlor and housed in a freestall barn.

Between 2022 and 2023, Ash Grove Dairy expanded and added a digester to the farm.

Crinion worked with Dynamic Renewables, which launched the digester program.

Crinion said there were difficulties with adding the digester to the farm.

“We had a little bit of weather (challenges),” Crinion said. “In a startup, there are some problems, no different than a dairy. So, we got through all of that. It’s moving along pretty well (now).”

There were also challenges after the digester was fully implemented that the dairy and Clean Energy Fuels had to overcome.

“Manure is not water,” Crinion said. “The guys working here at the digester have been great to work with.”

The digester system has seven steps, starting with collecting manure and ending with natural gas leaving on the truck.

At the start of the digester, manure is intercepted before it heads to the lagoon. As the manure is collected from the manure pipe, it is brought to a building to be thickened. Ash Grove Dairy supplies about 50,000 gallons of manure a day, Crinion said.

In the first building, fine screens separate the manure solids from water. From there, the thickened manure will make its way to the dome, which holds about 1.3 million gallons of manure. The manure will remain in the dome for roughly 24 days at about 100 degrees.

Clean Energy Fuels personnel said they try to simulate the temperature of a cow because the bacteria in the manure like warm temperatures. Inside the dome, there are three mixers to agitate the manure, which helps the bacteria move inside.

The dome has two membrane layers as the ceiling. The outside layer is always round as it has air pumped into it. This allows the outer layer to protect the inside layer from the weather. The second layer will fluctuate depending on the amount of gas inside, which is how Clean Energy Fuels measures the gas.

The gas derived from digesting the manure is pumped into vessels for the next step: hydrogen sulfide scrubbing. Within the gas, there is methane, hydrogen sulfide and carbon dioxide. The fourth step is designed to convert the hydrogen sulfide into a benign sulfur cake. The leftover gas will go to the fifth step. Clean Energy Fuels personnel said the sulfur cakes are used differently depending on the farm. At some locations, the sulfur will be used as a soil amendment while at other locations the sulfur will be metered back into the lagoon or disposed of.

The fifth step is carbon dioxide scrubbing, designed to take the carbon dioxide out of the gas so all that is left is the methane. From there, the gas is pressurized and pumped into trucks. Every two days, the trucks are brought to Brookings, South Dakota, to be injected into the natural gas pipeline. Clean Energy Fuels personnel said that six cubic feet of manure makes one pound of volatile solids.

Implementing the digester on the farm has been a value add for Ash Grove Dairy.

“Dairy farming can’t be always running just on milking,” Crinion said. “For a lot of dairy farmers (back then), milking was making up 95% of their total income. Now, with beef calves that we’re selling, that alone was making up 15%. You can have a diversified income.”

Although the digester is on the farm, Ash Grove Dairy does not have to do any maintenance work on the manure digesting system.

“They’ve got good people,” Crinion said. “I just make sure that they have a good flow of manure to their project every day.”

Although there have been some challenges in the process, Ash Grove Dairy has completed one year of manure digestion for renewable energy.