Lessons Learned from this Project |
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Foam
Anaerobic digester demonstration with biogas fueled micro-turbines
Foaming can occur when changes occur such as: the diet of the cows the temperature of the digester, and additions of other organic loading. Spraying water over the foam can control foaming. A water source and spray device near the effluent tank is useful.
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Foam
Anaerobic digester demonstration with biogas fueled micro-turbines
As foaming occurs, it often flows into the pipe chase where the gas collection pipes leave the digester. They have water basins set to provide the pressure needed to keep the cover inflated. Foam flowing out creates a mess. Providing a drain for the pipe chases and a solid bottom and water supply makes clean up easier. Removing the top of the pipe chase allows easy access and good ventilation for those working in the area.
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Gas Conditioning
Anaerobic digester demonstration with biogas fueled micro-turbines
Gas conditioning for the compressors and micro-turbines is critical. Hydrogen Sulfide and water vapor in the gas present the potential for corrosion. Methods to remove a significant portion of these contaminants are crucial. The compressor may have sensitive components that will corrode. The output from the compressor must be reduced in moisture content and have no excess oil in it or the micro-turbines may fail.
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Heating Barn
Anaerobic digester demonstration with biogas fueled micro-turbines
Heat pipes in the barn floor may not keep manure from freezing during weather colder than 5 degrees F. Although heat can be supplied continuously from the heat recovered off the electrical generation system, during sharp cold snaps the in floor heat cannot prevent freezing.
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Marketing Needs
Anaerobic digester demonstration with biogas fueled micro-turbines
Solid sales marketing needs to be done to sell manure solids. The digested separated solids are homogeneous, dark in color, and have good tilt. They should be preferred over other forms of organic soil amendment. However, without a marketing effort it cannot be assumed that they can be sold at any price. Transportation costs make the market area relatively near to the farm.
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Moisture Content
Anaerobic digester demonstration with biogas fueled micro-turbines
The plug-flow digester relies on the proper moisture content of the influent. Dairy manure as produced will not separate into floatable and settable solids very easily. When extra moisture is added, the floatable and settable solids sort out inside the digester, leaving a floating crust and a deposit. As these two portions of the digester get larger they will decrease the usable volume and the hydraulic retention time. Lower retention times will decrease biogas production and fail to reduce the odors in the effluent.
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Temperature Control
Anaerobic digester demonstration with biogas fueled micro-turbines
Temperature control of the digester is critical. Air locks in the heat pipes can prevent the proper circulation of the heated water inside the digester to heat the incoming manure to 100° F and to keep it at that temperature throughout the 20 day retention time. Temperature gauges, if they work properly, show the lack of heat, however, if the temperature gauges are not calibrated, then the lack of heat in the digester may go undetected. Groundwater impingement on the bottom of the digester can take a significant amount of heat away from the digester making it hard to heat. When flows to the digester are curtailed, the amount of heat to the digester needs to be adjusted since heat will not be needed for incoming manure. Without adjustment higher temperatures than desired may result.
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Volatile Acid Levels
Anaerobic digester demonstration with biogas fueled micro-turbines
Odor control can be achieved by a digester system. The odors are negligible when the volatile acid levels leaving the digester are below 500 ppm. As the digester experiences difficulties with temperature or retention time the volatile acids leaving the digester may be higher and odors may occur.
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Weather
Anaerobic digester demonstration with biogas fueled micro-turbines
Heavy snow load can collapse the flexible cover on the digester if it falls faster than it can melt. Shoveling the snow off will allow the cover to inflate.
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