Biogas Feedstock from Within

Energy self-sufficiency and a new coproduct opportunity are what biogas plant designer Krieg & Fischer has to offer. The German company is able to help ethanol plants utilize their thin stillage for on-site energy and lower carbon intensity scores.
By Katie Schroeder | May 22, 2022

The ethanol industry is known for coproducts like corn oil and DDGS, but a company in Germany is giving them the option to branch out into a new coproduct: biogas. German company Krieg & Fischer GmbH designs biogas plants around the world for a wide variety of industries, anything from the potato industry to oil and gas companies. Krieg & Fischer’s expertise in established biogas technology is being applied to the ethanol industry by using thin stillage as a feedstock for biomethane production. Raphael Thies, general manager with Krieg & Fischer, outlines the benefits, process and complexities of building a biogas plant.

Krieg & Fischer was started 23 years ago in 1999. In the beginning, their clients were primarily farmers, Thies says. The German renewable energy law created a lot of interest in the farming community due to incentives for energy using crops as the feedstock. Over the years, Krieg & Fischer started building biogas plants internationally for an array of industries. “We know our different technologies in the biogas field and our job [is] to adapt the technology to the feedstock,” he explains. The company will design and build a biogas plant onsite at whatever plant or farm will produce the feedstock.

Analyzing the feedstock is where the job begins. Krieg & Fischer first built a biogas plant integrated with an ethanol plant in 2017 with a facility in Argentina. “At the very beginning, the idea was to digest corn sileage, but our client caught on quite quickly that they had a lot of organic waste from their ethanol plant, and thin stillage,” Thies says. Krieg & Fischer then built the second biogas plant for the customer—this time using thin stillage—which started operations in 2018. “When you produce alcohol from the corn, you take out the carbohydrates to produce the alcohol and all the fat and protein, and also still some carbohydrates are left. And the residuals, the whole stillage … the usual treatment is solid/liquid separation. The solids go to farmers … for animal feeding and the liquid part is perfect substrate for biogas plants.”

Thin stillage as a feedstock for biogas is “quite easy to handle,” Thies explains. The advantage of thin stillage is that it is already in liquid form, which makes it easier to transport and begin the anerobic digestion process. Thin stillage does come out of the ethanol plant very hot, at 176 degrees Fahrenheit, so Krieg & Fischer addresses this by cooling the stillage down. “Then you need to calculate carefully the retention time in the digester tank and the organic load rate and main design parameters for every single biogas plant,” Thies says. The more organic matter in the stillage, the more biogas can be produced. The presence or absence of corn oil in the thin stillage impacts the overall yield of the biogas plant. “But it doesn’t really matter, with or without corn oil the difference is only the energy content,” Thies explains. “Of course, when you take out the corn oil there’s less energy inside, so you get less gas yield per cubic meter of thin stillage.” The amount of biogas produced depends greatly on the makeup of the thin stillage, however, the biogas plant will typically produce about 600 to 1,000 cubic meters of biogas per ton of volatile solids.

Another important parameter is the type of oil removal technology used by the ethanol plant. Thies explains that if the plant uses a technology which increases the dry matter in the thin stillage, it can cause problems by increasing the nitrogen concentration which inhibits the anaerobic digestion process. “When you increase dry matter by evaporation, and you do not remove the nitrogen, the concentration in a certain amount of stillage is getting higher and then we might get a problem for the AD process itself because it will be an inhibition of the methane bacteria due to the high nitrogen concentration,” he says.

Biogas Benefits   
Biogas production offers many opportunities to ethanol producers, according to Thies. Ethanol producers can use the biogas or biomethane produced to generate  renewable electricity or thermal energy to reduce their overall carbon intensity score, he explains. Utilizing thin stillage for biogas production can also bring independence for an ethanol producer. “The moment they have to find a customer for their stillage, someone who buys it, corn oil is dependent on biodiesel producers, so if there’s less demand from biodiesel the price will go down,” Thies says. “At the moment, they are dependent on other markets, [but] with the biomethane or biogas production at their own plant, their own facility, they are [becoming] more independent from other parties.”

Once the biogas plant is in place, the producer can depend on energy at a dependable price, without the need to rely on a volatile fossil fuel market. “It is also a way to use King Corn. Last time I was in the States—South Dakota and Nebraska—a plant manager told me, ‘I don’t know what is going on in five years or ten years, what I know for sure is that right now we can grow corn.’ The corn will survive.” The producer could also use other parts of the corn to increase biogas yield, such as corn sileage or corn stover. Thies suggests corn stover as an ideal feedstock for an ethanol plant to use alongside thin stillage. The corn stover would require different pretreatment technology to get it ready for anerobic digestion.

The Anatomy of a Biogas Plant
Although each biogas plant looks a little different due to different feedstocks, there are a few technologies which are consistent across the board. The most important of these, according to Thies, is the digester tank, which comes with a heating system, a mixing system and insulation. The thin stillage will be pumped into this tank from the ethanol plant. The anerobic digestion takes place in this tank, when the organic components are broken down into smaller chains, then through acidification processes, the specialized bacteria convert the hydrogen and carbon dioxide into methane. “The main point is that you have to provide all the bacteria there with the perfect conditions in terms of water content, temperature, and you have to homogenize this in a good way,” Thies explains.

After the digester tank, there is generally a secondary or “post digester” used to optimize the biogas production. Next, a gas buffering system used to store the biogas for a few hours, Thies explains that this system is usually a plastic membrane system on top of the tank. “Then of course, you need a gas utilization system, a CHP or biogas upgrading unit or biogas boiler where you can directly burn the gas so it can produce electricity—so electrical energy, thermal energy or simply the biomethane that can be injected into a grid,” Thies says. An ethanol plant could have a biogas plant up and running in a year to 18 months, he estimates. Other factors, such as the size of the plant or winter climate, could impact this timeline.

The technology of a biogas plant is by no means in the research and development phase, Thies explains. “All this biogas technology [has been known for] more than 40 years, industrial scale we are talking about now more than 20 years, so it’s not R&D or something. This is a really well-established technology.” There are a wide variety of plant sizes found across the world, from those producing 50 cubic meters per hour of biogas, to those producing over six thousand, these plants are utilizing proven technologies. Although the biogas industry started primarily in the agricultural market, with parts that were originally designed for construction or farming purposes, it has grown to an industrial size and has a lot to offer. “Now in Europe we have specialized companies providing the perfect technology for the different tasks: pumps, mixers, tanks, gas holding membranes, pretreatment systems,” Thies says. “All of that is not research and development anymore, CHP units, 20 years ago it was an old truck engine and you adjusted it. Now there are professional industrial suppliers.”

Coproducing energy alongside ethanol helps plants have more stability and the flexibility of biogas gives ethanol producers more options for the future. “You can use it for heating in your households, you can even use it for driving your car,” Thies says. “It can be used in the same way as natural gas today.”

Author: Katie Schroeder
Contact: [email protected]