Preparation Protocol

Biomass fuel used at Koda Energy must be dried, sized and ground according to specifications for a proprietary fuel blend recipe.
By Anna Simet | July 22, 2013

In Shakopee, Minn., Koda Energy generates heat and power on a 76-year-old campus of Rahr Malting Co. The energy plant began operations in 2009 and is very new compared to Rahr, but the Shakopee Mdewakanton, the other project partner, have a rich history in the region and have called it home for hundreds of years.

In 2006, the Shakopee Mdewakanton Sioux Community and Rahr partnered to develop Koda Energy, and the plant was built by Norcross, Ga.-based McBurney Corp. on the southwest edge of the Minneapolis-St. Paul metropolitan area. It provides Rahr Malting with its thermal requirements, replacing natural gas, and supplies an average of 12.5 MW of electricity to the grid, though its maximum capacity is 23.4 MW.

About 20 percent of the facility’s fuel comes from neighbor Rahr, and other sources include oat hulls from General Mills, wood, grass seed and corn cobs. The fuel mix burned at the plant is a specific blend of materials designed to maintain a consistent heat output and limit emissions, and Koda Energy has deployed two satellite biomass preparation facilities that ensure it meets specifications upon delivery to the boiler.

Points A and B

The Fort Snelling wood tipping and processing facility is where whole tree material is received from the City of Minneapolis Parks and Recreation trimming crews, as well as private tree-trimming crews within Hennepin County. There, initial resizing is performed, according to Stacy Cook, Koda Energy general manager. He says resizing at this point is necessary for two reasons. “First, woody biomass  collected within Hennepin County is under quarantine for the emerald ash borer, and cannot be removed from the county prior to an accepted treatment protocol under an emerald ash borer compliance plan,” explains Cook. 

One of the accepted treatment methods is reducing the size of the wood material to the point where the insects cannot survive. Once it meets specification for size under the compliance plan, it can be removed from the county.

The second reason for initial resizing is related to logistics, says Cook. A 100-cubic-yard semitrailer can haul 10 tons of whole tree and limb material. After it is resized, however, the same size trailer can haul 24 tons in the same space. “Fewer trips equals reduced transportation expense,” says Cook.
As product is chipped, wheel loaders pick up piles of material and place them in a self-unloading semi-trailers for transport to the second facility, the 7-acre Koda Energy biomass processing and storage facility. “There, we process the material further by running it through our wood chip dryer, and we resize the chips to three-fourths of an inch,” Cook says. “Once it is dried and resized to meet our fuel specification, we keep it in a covered storage building until it is scheduled to be delivered into the Koda Energy plant.

The Ft. Snelling site is open six days per week to receive waste tree material, and during that time product may be moving between all sites. Once treated at the processing and storage facility, it’s hauled to the CHP plant. “These deliveries occur every day of the week normally, and the number of loads can vary daily, depending on the specific volume fuel requirement that is required to supplement our other fuel sources,” Cook explains. “It undergoes pretty much the same process again—the dried fuel is under covered storage and loaded into the same type of trailer with a wheel loader, and then it’s sent into the CHP plant to be unloaded and fed into the process.”

What’s the reasoning for having the facilities separate from the energy plant? “The entire footprint of our power plant is on a 2.5-acre site, so we don’t have the space available for preprocessing large quantities of material,” says Cook. “The processing and storage facility is only 7 miles from the plant, and is also valuable as a surge facility where we can store enough fuel to run our plant for an additional week beyond the storage capability at the power plant.

About 30 miles away from Koda Energy, the Fort Snelling site has been an established wood tipping site used by the City of Minneapolis for several years, and it’s close to the locations where the tree material is being removed. It’s also inside of the EAB quarantine area, Cook points out, reiterating that no whole tree material can leave the quarantine area without further processing.

The fact that the processing and storage facility, which is located at the 20-acre SMSC organics recycling facility, is only 7 miles away from the power plant keeps processed fuel transportation costs low.

Both facilities are key components in ensuring that the product delivered to the energy facility meets certain specifications for size and moisture content for the proprietary fuel blend used at the energy plant.

Meeting Specifications

For the woody biomass component of Koda’s fuel blend, chippers and grinders are used to reach the specifications of three-fourths of an inch for size, and an MEC wood dryer is used to achieve less than 14 percent moisture content by weight. “We use several other types of fuel too, hulls from various grains, aged seed corn, undersized whole grain products, sunflower seed material, corn screenings, other grain dusts, and many other dry agricultural byproducts,” Cook says. “These grain-derived fuels are typically less than 10 percent moisture and the size of a kernel of wheat, and usually arrive meeting our specifications.”

Koda also contracts with several pallet manufacturing companies to receive recycled pallet board material, most of which, Cook says, use horizontal wood grinders for size reduction and can meet the moisture specification without further processing.

When fuel arrives at the CHP plant, it’s segregated by material type and combustion characteristics, including ash content, Btu value and alkali per million Btu. “The separate fuel types are then blended in a specific ratio so that the combined fuel recipe has favorable characteristics for our combustion process,” Cook explains. Drag conveyors move fuel from individual silos to four Bliss hammermills, which are  commonly found in flour production facilities, and they process the material into a biomass flour. “We produce 20 to 25 tons per hour of biomass flour on average to feed our combustion process,” says Cook.

 The biomass flour is pneumatically conveyed into a metering bin on the boiler house, and then metered into the stream of primary air that conveys the fuel mixture to the burners and into the boiler. “In our process, we use a controlled and sustained dust explosion inside the furnace,” Cook adds. “That converts the chemical energy in the fuel into heat energy.”

The resulting heat energy is used by Rahr Malting, and the power generated  is distributed between Koda Energy and Rahr, with the balance supplied to Xcel Energy.

Though currently experiencing down-time due to a late-April explosion, Koda is currently salvaging existing equipment not damaged in the event and will reuse it to build bigger, better and safer. The company is currently working a redesign of the fuel receiving, storage, and blending system, and is in the planning and funding stage for reconstruction, Cook adds. “We have performed a thorough safety assessment on all potential aspects of the design to improve the safety factor of the fuel side of the plant.”

In the meantime, Koda Energy is processing tree storm debris for the Minneapolis Park and Recreation Board and providing the resulting wood chips to Minneapolis residents. Since a June 21 storm, the MPRB has hauled away more than 1,000 semi truckloads of boulevard and residential tree debris that has ash trees intermingled with other tree debris. Koda Energy is grinding and double chipping the material to meet the Minnesota Department of Agriculture's compliance standards for preventing the spread of emerald ash borer.

“We try to be good neighbors in our community and provide service where it is feasible to do so,” Cook says.

After all, in the Dakota language, Koda means “friend.”

Author: Anna Simet
Managing Editor, Biomass Magazine