From Problem to Profit

By Jerry W. Kram
Making a buck in the ethanol industry has been getting tougher over the past year with high feedstock and energy costs. Some ethanol producers have sought to boost their bottom lines by increasing the value of their coproducts, reducing their energy consumption or using coproducts to offset fossil fuel usage. David Rein, a process engineer with Rein and Associates in Moorhead, Minn., has created a process that does all three.

Rein has been working with Otter Tail Ag Enterprises LLC and the Fergus Falls Wastewater Treatment Plant in Fergus Falls, Minn., to produce methane from thin stillage through anaerobic digestion in a study sponsored in part by Minnesota's Agricultural Utilization Research Institute. Thin stillage is what remains after ethanol and distillers grains have been removed after fermentation. The liquid is so rich in organic compounds that the stillage from Otter Tail Ag Enterprises, a 55 MMgy ethanol plant, can produce 3 million cubic feet of methane per day-worth nearly $9 million per year.

One problem with using stillage for anaerobic digestion is that it carries a large amount of inorganic material, such as ammonia, magnesium and phosphates. These three chemicals combine in a neutral or alkaline environment to create a mineral called struvite. The struvite builds up scale deposits on the sides of the bioreactor and in pipes. It can foul heat exchangers, choke pumps and pipes, and reduce digester volume. "It is especially a problem at (pipe) elbows, where the velocity increases or there is a lot of turbulence in the water," Rein says. "It is a historic problem with anaerobic digestion if you have all the components there."

Rein figures that the struvite problem could be avoided by removing the components of the mineral from the thin stillage before it's digested. He created a skid-mounted system that removes the ammonia, phosphates and magnesium from the stillage before it goes into the digester. The system then precipitates the struvite into small nodules or crystals. "It is a patented process that we have brought in," Rein says. "It's an upflow device. You get the right pH and you form the (struvite) crystals."

While struvite is a pain in the digestor, it's also a valuable slow-release fertilizer that is 8 percent nitrogen and 21 percent phosphorus. "What makes struvite valuable is the method you use to get it out," Rein says. "When it comes out naturally, it's in thin layers and doesn't have much more value than a normal fertilizer. If you get it out as crystals, it is a slow-release organic fertilizer."

The magnesium in struvite is another important plant nutrient as it is a central component of chlorophyll. "It's what gives plants a deep blue-green color," Rein says. "I've actually put it on my lawn to see if it works, and it's true. So it is really good stuff, and it's organic."

In 2006, struvite was selling for $1,500 per ton as a fertilizer and is primarily used in the turf industry. Rein says fertilizer prices have shot up since 2006, so it's probably more valuable now. A plant the size of Otter Tail Ag Enterprises could potentially produce 10 tons of struvite per day. The value of the struvite could equal or exceed that of gas that comes out of the anaerobic digester.

Rein says the struvite removal system is ready for commercial production. Black & Veatch, a consulting company in Kansas City, Mo., is evaluating the system for its value to ethanol producers.