Feb., 26/Making electricity from manure on a 700-cow dairy farm has been a long process in which all parties involved have learned and hope to profit from. This process started in the spring of 2000 when Ray Crammond, a digester engineer from Ankeny, Iowa wrote a grant proposal to the Iowa Department of Natural Resources (IDNR). Crammond had designed a digester in Michigan and the plan was to pattern the the proposed digester after that one - a plug flow design with a concrete lid.

The grant application was successful and awarded half the estimated project cost, $157,000, to Top Deck Holsteins, Inc. (Top Deck), owned by Roger Decker and his family in Westgate, Iowa. Decker asked me as the Regional Extension Agricultural Engineer for Iowa State University - what I thought about the project. My reply was "Any time somebody pays half the price, it makes the project a lot easier to justify the economic payback." And actually, future developments made the economics look even better.

With Crammond's design, and the participation of IDNR and other interested parties, I helped Top Deck construct, start up and monitor the farm's new anaerobic digestion system, which includes the digester, equipment for generating electricity, capturing waste heat recovery and related manure handling components. Construction started in the fall of 2000, and the system was operating by spring of 2002.

CONSTRUCTING THE DIGESTER

Before the anaerobic digester became a reality, Top Deck had been planning an expansion of the farm that would more than double the herd size from 300 to 700 cows. One of IDNR's requirements was that the farm had to complete this expansion before installing the digester. As the first item of business, Top Deck needed to modify existing earthen manure storage of 180 feet by 240 feet by ten feet deep to meet the latest requirements. It took several months to obtain the required permits, approval of blueprints and to complete various site modifications such as moving drain lines and monitoring wells.

The digester project started with construction of the 27 feet by 124 feet by 12 feet deep concrete digester tank. A four-inch drainage pipe was placed around the perimeter of the tank, which was insulated with polystyrene foam. A cold winter temporarily halted the work. Construction restarted in the summer of 2001 when the precast lid was installed. The four-inch thick lid was covered with four-inches of foam insulation and one foot of soil.

Two additional tanks were added at the west end of the digester - a preheat tank and a separator pump tank, both 13 feet by 13 feet by 12 feet deep. The precast concrete lids for the two smaller tanks were each cast in two sections, 6 1/2 by 13 feet This allows the lids to be removed more easily in order to provide access to the interior equipment, (e.g. the heating pipe grid in the preheat tank). Together the two tanks plus the digester provide about a 14 day detention time based on 29 gallons of manure per cow per day plus the milking center wastewater.

The purpose of the first tank is to preheat the 20,000 gallons of daily manure production to 98 deg F. The preheated manure is pumped from this tank to the east end of the digester through an eight- inch PVC pipe. The preheat tank was included for two reasons: it provides much easier access to the heat exchanger for maintenance or removal; and it can serve as a settling tank for solids from the dairy, such as sand and grit. It is preferable to have these solids settle in the tank than in the digester.

The other small tank was installed with a future use in mind. It is intended to house a pump to transfer the digested manure to a separator that can capture the manure solids for use as bedding for the cows' free stalls. The separator was not included initially because it was preferred to get the digester system up and running first. For now, the manure from the digester overflows the wood weir and enters the separator pit and then flows out by gravity through a 12-inch pipe to the earthen manure storage lagoon. When a mechanical separator is installed, it can be located just west of the current pit.

The IDNR wanted a surface outlet for the digester manure. This necessitated constructing the digester partially above ground, with the top six feet above the existing grade. This avoided the need for a pump to empty the effluent into the earthen storage, which is located about 30 feet to the south. It also allowed any gas leakage from the digester gas to remain above ground level. Underground leaks have the potential to migrate horizontally and cause an explosion elsewhere. For this reason, the engine/generator building was located 40 feet from the digester.

POWER SYSTEM SET-UP

In the spring of 2001, Alliant Energy Corporation entered the picture. In exchange for the electricity produced on the farm, Alliant offered to furnish the electricity generating equipment. The equipment purchased includes a Waukesha 150 hp engine (used) and 100 kW synchronous parallel generator that runs at 1200 rpm. A 30 kW Capstone microturbine generator was also included in the system. The microturbine incorporates a compressor, recuperator, combustion chamber turbine and permanent magnet generator. These rotating components are mounted together on a single shaft supported by air bearings that rotate up to 96,000 rpm. The microturbine is air-cooled and thus eliminates the need for a liquid coolant. Gas from the digester needs to be cleaned, compressed and dried before entering the microturbine combustion chamber.

The contract to supply the generating equipment and controls was awarded in March 2001 to Perennial Energy of West Plains, MO. The Waukesha engine and generator and much of the microturbine arrived in mid-December on an assembled skid packaged. The remaining piece of the microturbine, an oil bath system for drying the gas, was delivered in April 2002. The generating equipment can be purchased by Top Deck Holsteins at the end of the ten year contract for a ten percent salvage fee. The contract with Alliant also compensates Top Deck $500 per month (adjustable with kilowatt hours generated) as maintenance fee for monitoring the digester and the engine/ microturbine equipment.

The building that houses the generation equipment is a 24 feet by 36 feet insulated wood-framed shed with 13 feet high walls. The interior walls are finished with drywall for sound insulation. The building includes a gas space heater plus two 36-inch diameter ventilation fans on one side wall and four 36-inch square louvers on the opposite wall. There is one water drain, although additional drains would have been preferred. The microturbine requires a drain to handle the moisture removed from the gas.

The concrete digester tank under construction showing the precast lid, wall insulation, and ten-inch PVC access pipes. The pre-heat tank heat exchanger sits on the lid waiting installation.

NEAT FOR THE DIGESTER AND THE DAIRY

Heat captured from the engine-generator and microturbine system is used to maintain the digester temperature and supply heat to the dairy center. When hot water from the generation equipment is not available or insufficient (such as during startup), boilers running on LP gas are used as heat sources.

To heat manure for digestion, hot water is circulated through separate steel pipe heat exchangers installed in the digester and the preheat tank. The digester contains 290 feet of two-inch pipe that is 18-inches above the floor and 12-inches in from the outer wall. In the preheat tank, the 1,000 feet of two-inch pipe is arranged in a grid loop that measures seven feet high, six feet wide and 11 feet long. The grid starts two feet above the tank floor. Manure constantly covers this pipe grid to prevent pipe rusting. Each heat exchanger has a single one-hp pump to circulate the water.

A third hot water loop runs 370 feet from the engine/generator building to the dairy center. This hot water loop feeds a heat exchanger inside a hot water heater in the dairy. The hot water from the engine/generator runs through a heat exchanger (rather than used directly) because it contains antifreeze (needed when the system is inactive). The energy from this hot water is used at the dairy center for several purposes, including: cleaning milk pipelines; space heating in the milking parlor via a radiator and fan; space heating via pipes embedded in a floor; and for high pressure hot water washing in the milking parlor.

Hot water is captured from the engine and microturbine as the heat becomes available. The generation system's computerized controts determine when and where to pump water based on temperature set points. A total of 460,000 Btu/hr is available from the engine water jacket and the engine exhaust pipe heat exchanger at full capacity. Another 270,000 Btu/hr can be obtained from the microturbine exhaust pipe heat exchanger when running at full rpm.

When the engine is running below full load, there may not be enough hot water to maintain the digester temperature. To sufficiently preheat manure for digestion manure requires about 343,000 Btu/hr in cold weather (based on a 40 deg F manure temperature and a 98 deg F digester temperature). When little or no heat is available from the generation equipment, this energy must be obtained from fuel. For this purpose, two 200,00\0 Btu/hr boilers were rented that together use about 70 gallons per day of LP gas. In hindsight, the boilers should not have been rented because it took an unexpectedly long time to get the digester up and running perfectly. A 400,000 Btu/hr boiler could have been purchased for the cost of renting them off and on for five months. The boiler would be useful when both the engine and/or microturbine go down for repairs. Fortunately, in normal situations only one is down for repairs at a time so the other one can heat the hot water.

MAKING ADJUSTMENTS

Several problems occurred that necessitated changes to the system, especially during initial start-up period. In addition, experience with the digestion suggests other changes that promise to improve its operation.

Among the first problems evident was a lack of pressure inside the digester due to poor sealing. Based on the experience at another anaerobic digestion project, a new polyurea spray sealant was used to seal the digester instead of the more conventional epoxy tar. The spray polyurea was used initially at a 100 ml thickness on the inside of the digester at the crack between the wall and lid and also on both sides of the "gas beam" (located in front of the overflow weir to hold the gas in the digestion tank). Because hydrogen sulfide in the biogas deteriorates concrete, the spray product was also used to coat the tank surfaces exposed to biogas (lid area and down three feet from the lid on the walls). The product, which costs about $4/ sq. feet, is supposed to resist up to 85 percent pure sulfuric acid and handle 520 percent elongation (it stretches with temperature). However, the digester failed to hold pressure. The remedy was to bring back the sealer company and seal the outside edge of the tank where the lid sits on the wall.

30 kW microturbine (foreground) with the compressor and oil bath biogas drying system (to the rear).

Another problem that developed was that foam and liquid rose up into the three-inch biogas line leading from the digester to the engine/generator system. This caused plugging problems in the three- inch biogas line and fouling of the engine. The remedy was to install a 30 gallon vessel with a drain and a eight-inch diameter window to check for liquid and foam in the biogas line. At the same time, a water spray nozzle was added to spray down the foam at the end of the biogas line just before the 30-gallon tank.

A flare was added to burn the gas produced during times when the engine and microturbine system are not keeping up with gas production. Other items added include a gas meter and a 100 percent shut-off valve outside the building.

BEDDING IMPACTS

The farm's bedding preferences have an impact on the digester operation. Currently the farm uses rice hulls for bedding. If the rice hulls do not decompose well enough in the digester, they will accumulate in the tank. Eventually, the farm may have to switch to another bedding material. However, in the meantime, it will be necessary to occasionally clean out sediment in the digester tank.

Fortunately, there are five ten-inch diameter perimeter pipes on the north side of the digester that were placed at a 60 degree angle in case the digester needs to be pumped empty. Also, in the event that a person must enter the digester pit for clean-out, the ten-- inch PVC pipes can be used for ventilation by inserting a small fan in a pipe. There is also a 12-inch valve that can be opened to drain the digester into the 13 by 13 preheat tank where there is a 20 hp manure pump (with a propeller) to remove manure if necessary. Also, there is a 14" by 14" opening in the common wall between the preheat tank and the separator pit. If the preheat tank is ever overloaded, manure will discharge by gravity into the separator pit and then flow into the storage lagoon.

PERFORMANCE AND ECONOMICS

Manure is pumped independently from the North and the South barns, which each house about 350 cows, to the preheat pit at the end of the digester. The North barn's manure has the milking center wastewater added to it. Located in the pit, a 20 hp Houle pump with a propeller agitates and pumps the manure into the digester. The pump operates 17 cycles per day, transferring about 1,000 gallons of manure with each cycle. The propeller was added to the pump because solids were building up in the preheat tank. A sonar control on the two barn pumps and preheat tank allows flexibility in the manure volumes pumped.

The manure will continue to be analyzed monthly for the next year. The pH of the digester effluent was found to be 7.0. The biogas has tested at 65 to 72 percent methane.

The anaerobic digestion system at Top Deck has been generating about 100 kW of electricity from manure from about 700 cows. It ran initially at about 95 kW and later peaked at 125 kW. Of the two generating options, the microturbines have had fewer mechanical problems than the engine/generator combination because there are fewer mechanical parts. Down time costs money two ways. First, it costs in LP gas to keep the digester and preheat tank warm when the engine and microturbine are not providing the heat. The second cost of downtime is in electricity production. It is good to have both systems because the microturbine can almost heat the digester and preheat tanks if the engine/generator system is down. The 30 kW microturbine runs about 25.5 kW when the weather is hot and increased to about 28.5 with cooler October weather.

Approximate costs for the digester and electricity generation system: Of the total $586,500 cost, the digester partners - Alliant Energy, IDNR and Top Deck - paid for $335,000, $157,000 and $94,500 respectively.

There are several possible ways to benefit from the dairy manure digester. These benefits include: Generation of electricity; Improved odor control on the manure storage; Excess heated water from the engine/micro turbine set; and Savings from the use of the digested manure solids for free stall barn bedding. It is hard to put a cost on improved odor control because the farm is not in a high traffic area. The digested solids are not being separated and used yet. Therefore, the primary financial return is currently from electricity and heat.

Anaerobic digestion system in operation. The mound-covered digester is to the right. The engine/generator shed is on the left.

Top Deck Holstein's average electric usage varies from 28,880 kWh in winter (cost = $1,813) to its peak in the summer of 45,400 kWh ($3,197). So the average savings for a month would be $2,505 if they only generated electricity for their farm. This translates to a saving of $30,000 per year. The excess power generated would have been sold to the power company for two cents per kWh. Assuming that the electricity can be generated at 125 kW, the digester's best output so far, the amount of electricity potentially generated amounts to 3000 kWh per day or 90,000 kWh per month. If an average of 37,100 kWh per month is used on the farm, the excess available for sale would be 52,900 kWh per month. At two cents per kWh, this would amount to an income of $1,058 per month or $12,700 per year. The revenue would then be $30,000 (electricity used on farm) plus $12,700 (surplus electricity sold to power company) plus $4,000 (saved on heat in the dairy center) for a total of $46,700 per year. Based on the $586,500 this would represent a simple return of 8.0 percent.

In the future, the manure solids separator is a good possibility. This would essentially eliminate the farm's $30,000 annual bill for rice hull bedding. The separator and shed are estimated to cost $80,000. With the additional savings on bedding, the new annual return would be $76,700 on an investment of $666,500 ($586,500 + $80,000). The simple annual return would then be 11.5 percent.

Certainly, this economic exercise is an optimistic one. In reality, a portion of the revenue must go to depreciation, interest, repairs, taxes, insurance, and other cost factors. Also, the revenue and cost estimates do not recognize the effects of downtime and extraordinary repairs. Nevertheless, this simple analysis shows that anaerobic digestion has the potential to bring positive returns to dairy farms -from electricity saved, electricity sold, and recovery of heat and bedding.

Operations at an Iowa dairy farm use a microturbine and generator to produce power, provide heat and reduce manure management costs.

In exchange for the electricity produced on the farm, Alliant Energy Corporation offered to furnish the electricity generating equipment.

In hindsight, the boilers should not have been rented because it took an unexpectedly long time to get the digester up and running perfectly. A 400,000 Btu/hr boiler could have been purchased for the cost of renting them.

The anaerobic digestion system at Top Deck has been generating about 100 kW of electricity from manure from about 700 cows. It ran initially at about 95 kW.

Dan Meyer is an Iowa State agricultural engineer with the extension service based in Fayette. This report is based on his presentation at the BioCycle Conference on Renewable Energy from Organics Recycling.