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The Digester Pilot-Plant PROCESS DESCRIPTION OF THE BIODIGESTER PILOT PLANT
BLOCK DIAGRAM. 
RECEPTION
The chicken litter that arrives to this pilot plant comes from commercial West Virginia poultry farms. In the pilot plant the chicken litter is stored in barrels, and has moisture content around 30%. The principal solids are: chicken manure, wood, feathers, and grit; this composition can change depending on litter age and number of flocks grown on litter before cleanout. Plastic barrels are now used to eliminate metal contamination of the litter. MIXTURE PREPARATION
First, the barrels are transported from the storage area to the mix tank, then, by means of a chain hoist, the barrels are raised up to mix tank entry and they are emptied into the mix tank. Water is pumped into the tank and mixed with the litter to achieve a slurry containing 6-8% total solids. The tank contents are homogenized by means of a mixer. Baffles on the side of the tank ensure a vigorous mix. In addition in the mix tank, a SONAR unit is installed to monitor feed level. A submersible chopper feed pump is installed in the tank. The pump is attached to a metal guide bar and can be lifted and lowered for servicing. GRIT SEPARATION
Primary grit separation involves opening the 4" drain valve and collecting the manure, water, sand, and wood mixture in a plastic tank with 378.5 liters (100 gallons) of capacity. In this tank the grit settles and is manually separated. The rest of the slurry is recycled to mix tank with a sump pump. FEEDING
A PC equipped with a custom C++ control program controls the feeding. First the mixture is stirred; then the feed slurry is pumped into the digester. Feed volume is controlled by the PC control program. The SONAR records feed volume before and after feeding and automatically adjusts the next feed to compensate for over or under feeding. Daily feed totals are within 5% of target. DIGESTION
The slurry is digested in an anaerobic thermophilic biodigester with the following characteristics: 4.11m (13 ' 6 ") of diameter, and 3.42 m (11 ' 3 ") of height and 36468.47 liters (9635 gallons) of capacity. Inside the digester, thermophilic bacteria degrade the organic compounds (manure) turning them into methane and carbon dioxide. The temperature inside the digester is around 134 Fahrenheit. Hydraulic retention time fluctuates between 10 and 30 days, depending on feed volume. The temperature inside the digester is maintained by an external heat exchanger. Liquid from the digester is pumped thru the tubes while a hot water boiler and circulating pump heat the shell. The PC automatically starts the heating cycle when the internal thermocouples read .2 F below target. Digester Mixing
The digester is mixed with its own biogas. The biogas is drawn from the headspace with a gas blower. The gas is sent thru a pipeline into a ring inside the bottom of the digester tank. The gas bubbles thru the liquid and mixes the digester. The PC turns the blower on and off as programmed into the PC control program. Sediment Elimination
A drain valve is manually opened and sediments are periodically eliminated from the bottom of the digester. The principal materials that are eliminated are grit and woody material. Biogas Production
The biogas produced in the digester tank goes out of the system and is burned in a flare. The methane content in the biogas is around 55-65 % and the rest is CO2 (30-40%), water vapor (5%) and hydrogen sulfide (>1000 ppm). The gas passes through a heat exchanger (40 degrees F) to condense and remove the water, and then the biogas passes thru a mass flow meter and flows to a flare. Effluent
The digester maintains approximately 8200 gallons of liquid volume at 10 inches of water pressure, with the remaining volume of 1500 gallons being biogas headspace. Effluent is discharged thru a 6-inch pipe when the combined digester liquid volume and pressure deviate from equilibrium. When the combination of the pressure and liquid volume reach a certain point, digester liquid is pushed up the pipe and is expelled as effluent. The typical digester effluent contains 2-4% total solids, with some large wood particles (>1 inch diameter), easily separated solids (wood fraction) and fine suspended solids (microbial protein). Effluent is typically discharged during feeding and mixing. |
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