BioCycle September 2009, Vol. 50, No. 9, p. 38
Viewing swine manure as a resource, codigestion project in Minnesota plans to supply a nearby community with 50 percent of its natural gas needs while producing fertilizer for crops grown for the digester.
J.G. Ingersoll, F.A. Hoover, C.A. Kaendler, J.C. Madole, G. Peichel and A. Barka

A BIOGAS facility to process partially dewatered swine manure from farms owned and/or controlled by Christensen Farms, is under development in Sleepy Eye, Minnesota. Corn stover and other readily available cellulosic materials will be cofermented to increase the biogas yield. This facility will utilize dry, thermophilic anaerobic digestion to process swine manure from about 10,000 animals and corn stover from about 1,500 acres of farmland. The facility will occupy almost two acres of land, although the footprints of the various on-site structures cover an area of less than one-half acre.
Biogas will be upgraded to pipeline quality natural gas and injected into the local Center Point Energy pipeline. A net amount of 80 million cf of natural gas is projected to be supplied annually comprising an estimated 50 percent of the natural gas needs of Sleepy Eye, a community of about 2,500 people with a significant agricultural processing industry. Biogas also will supply all the power and process heat needs of the facility. There is a longer-term plan to utilize carbon dioxide separated out of the biogas, along with surplus waste heat and wastewater, in on-site greenhouses. The facility also is permitted to produce biomethane – the first of its type in Minnesota. Full-scale operations are expected to begin in the fourth quarter of 2010.

PROJECT BACKGROUND
Christensen Family Farms is a major Midwestern hog breeder and marketer, with operations in Minnesota, Nebraska and Iowa. It partnered with ECOCORP, a developer, owner and operator of biogas facilities. The design is based off of the configuration of a biogas facility in Saxony, Germany, which codigests cow manure and agricultural corn and rye crop residues.
Based on the analyzed data of some three dozen swine farms owned or controlled by Christensen Farms in that general area of Minnesota, composition of the swine manure, distance between farms, availability of corn stover and other agricultural crop residues in the area, and capacity of commercial biogas facilities already in operation, it was determined that the Sleepy Eye plant should be designed to process the partially dewatered manure from a combined maximum of 12.8 million gallons of liquid manure generated at one or more hog farms annually. This represents the manure output of almost 10,000 animals.

DIGESTER DESIGN
The liquid swine manure discharged daily into the existing small holding areas in each farm will be processed via a screw press. The wet solids will be digested. The wastewater will have total solids of less than 1 percent, about one-third the nitrogen amount in the original swine manure, and will be returned to the existing large storage lagoons on the farm to be land applied.
Corn stover will be the primary source of codigestion substrate, with the possible utilization of corn silage and a few other agricultural industry wastes generated locally. The amount of corn stover or another substrate high in carbon content will be dictated by the desire to obtain a carbon to nitrogen ratio of about 25 to 1, to employ the available amount of nitrogen in the swine manure to its maximum biogas generating potential.
Total solids in the digester will range from 12 to 15 percent by weight; operational temperature will be in the thermophilic range of 136°F to 140°F. The continuous (vs. batch) process consists of two steps: aerobic hydrolysis and anaerobic digestion. The premix or conditioning/hydrolysis step has a design retention time ranging from 2 to 3 days. Design retention time in the digester is up to 21 days.
The receiving area will accept the swine manure and corn stover daily. From there, the substrates pass though a macerator-pump, are reduced to one-quarter-inch nominal size, and are then pumped into the premix vessel. The hydrolyzed substrates are pumped from the premix vessel into the digester. The digester effluent is extracted and dewatered via a high-speed centrifuge or a screw press. Wastewater is returned into the receiving pit to dilute the fresh substrates.
The proprietary loop-digester has a draft tube to supply process heat and facilitate substrate mixing for process stability. A portion of the biogas will be injected back into the digester for additional substrate mixing. Metered amounts of air will also be injected into the digester to suppress formation of sulfides in the biogas (instead converting sulfur into sulfates that will remain in solution in the digester liquid). Lastly, a recirculation of the bottom of the digester content back to the top facilitates inoculation of the fresh materials and eliminates sedimentation.

BIOGAS TO PIPELINE, CHP SYSTEM
Digested solids (over 15 percent TS) will be stored indoors for a week before being land applied to adjacent farms. Biogas will be stored temporarily in a double-wall fabric sphere at an operational gauge pressure of 10 inches water maximum. (The sphere can store up to 2.5 hours of maximum biogas production rate.) The biogas then will be compressed to 85 psig and directed through the two-stage pressure swing adsorption unit (PSA). The PSA has two exit streams: the product and the exhaust. The product stream consists of 98 percent methane, 2 percent carbon dioxide, has no sulfur or water content and is connected to the Center Point Energy (CPE) mass spectrograph, flow rate and other monitoring equipment before it is injected into the 4-inch distribution natural gas pipeline at 60 psig.
A portion of the product stream will be diverted prior to entering the CPE equipment into the combined heat power (CHP) equipment in order to supply the process heat and power needs of the biogas facility. The exhaust stream contains over 92 percent carbon dioxide (the remainder is methane) and is also directed into the CHP to supply additional heat and power (or to a greenhouse heater to supply heat, if necessary). The PSA system design throughput is 17,500 cf of biogas per hour. The CHP equipment consists of microturbines with an integrated hot water generation component.
The Sleepy Eye Biogas Facility is being permitted as a biofuels producing entity rather than as an animal feedlot operation, which has been typically the classification of the few existing anaerobic digesters in the state. As such, it will be analogous to bioethanol and biodiesel facilities. Long-term contracts are being developed for supply of the various feedstocks, as well as for sale of the renewable biomethane and the organic fertilizer.
The estimated avoided greenhouse gas emissions from this biogas facility are 5,500 tons/year of CO2 from displacement of fossil natural gas, and about 17,700 tons/year of CO2 equivalent from avoidance of methane emissions from the swine manure lagoons.
It is estimated that about 2,500 tons/year (dry basis) of organic fertilizer will be produced with an NPK content of at least 10 percent by weight. The typical application rate of nitrogen in Minnesota for corn production is 140 to 180 lbs/acre, depending on whether a rotation with soybeans takes place from year to year. Thus, the land to which the generated organic fertilizer can be applied will range between 1,500 and 2,000 acres. Incidentally, the required land to obtain corn stover for the process will range from 1,400 to 1,800 acres depending on the amount of corn stover that can be removed from each field without causing soil depletion in nutrients and/or soil erosion. Ideally the same corn growing farms that supply stover and silage will also receive the organic fertilizer.
Last year, the Minnesota Department of Commerce awarded ECOCORP a grant to help with facility development. Permits are expected later in 2009. Facility financing, including long-term agreements for securing feedstocks and selling natural gas and organic fertilizer, is expected to be in place by mid-2010. The target for full-scale operations is mid-2011.

John Ingersoll, Frederick Hoover, Christian Kaendler and John Madole are with ECOCORP. George Peichel and Adam Barka are with Christensen Farms in Sleepy Eye, Minnesota. The authors acknowledge the financial support provided by the Minnesota Department of Commerce in getting this project off the ground in 2008.