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How Many Cows Does it Take to Make Biogas?

Posted: October 14, 2009

It doesn’t take a lot of cows to make biogas from manure. The real question is how does the capital and management cost of a digester fit into your farm business and management situation?

It doesn’t take a lot of cows to make biogas from manure. We had a 25 gallon demonstration mesophilic (maintained at 95 F) mixed digester at Ag Progress Days that produced about 7 cu ft of biogas a day from 5 quarts of manure slurry added daily. The real question is how does the capital and management cost of a digester fit into your farm business and management situation? Capital cost per cow is one item that usually decreases as digesters become larger. Other, perhaps more important, questions include, “How does a digester fit into my nutrient management plan?” Digesters do not reduce nutrients or quantity of manure that must be stored and ultimately land applied.

Also, there are significant knowledge and management requirements required to build and operate a biogas production and utilization system, “Does my farm business have the resources of time and interest to accumulate the knowledge and experience necessary to assemble a digester project, oversee construction, monitor the digester, recognize and trouble shoot operational problems and develop solutions before the digester has a major loss of biogas production?”

When everything is going well it is relatively easy to predict the day-to-day time and effort required to pump manure, check the digester and do routine maintenance to the combined heat and power unit (CHP). But what about when something is going wrong, the quicker it is recognized and corrected the better. The digester contents are a living and growing mass of microorganisms. Like the baby calf or high producing cow it may not take long to feed her but she also requires regular and careful observation and attention to head of digestive or other problems early and quickly.

Successful digester operators need to develop this same skill about their digester. You are taking on a whole new “animal production enterprise” – feeding and caring for the millions of “workers” in this mass of microorganisms. Acquiring and honing the necessary management skills takes just as long for a small digester as for a large one.

So how do I go about investigating the possibility of a digester? If you want to learn more about biogas production you may visit our Web site www.biogas.psu.edu. This site has important information and several case studies of Pennsylvania digesters and links to other sites in the United States.

If you burn biogas in an internal-combustion engine, approximately 30 percent will be converted to shaft horsepower.

Developing a complete feasibility study and business plan to go to the bank and borrow money or qualify for government loans/grants can be a formidable process and is an investment of a lot of your time to gather information. Many digester owners invested $10-20,000 in an outside professional time to develop a more detailed feasibility study. This study is not a digester design document; it is a “go to the banker or granting agency” document.

So how do you decide if you are really interested and should invest significant time and perhaps dollars in a more detailed study? Consider the following quick calculation for gross energy production potential from 1300 pound Holsteins. Manure production from 50 cows is predicted to provide an average of 4290 cu ft of biogas a day. Biogas is a mixture of about 60% Methane (the part that burns) and 40% Carbon Dioxide that doesn’t burn. There will also be some trace gases, especially Hydrogen Sulfide – the stuff that smells and is poisonous if trapped in small enclosed areas – and water vapor.

Biogas is very corrosive and has about 60% of the heating value of natural gas. It is not easy to liquefy so it is difficult to store large amounts. Assuming a heat value of 600 BTUs per cubic foot the manure from these 50 cows would produce about 2,574,000 BTUs of gross energy a day. This would be the equivalent of about 18.6 gallons of #2 fuel oil per day. (A gallon of #2 diesel fuel contains about 138,500 BTUs) or about 27.8 gallons of propane per day (A gallon of propane contains about 92,500 BTUs). If you were going to burn the biogas directly for heat, plan to use about 30% of it to maintain digester temperature of 95 F.

If you burn the biogas in an internal combustion engine approximately 30% will be converted to shaft horsepower. If the engine was powering an AC generator, it would produce 226 kWh in a 24 hour day. The rejected heat from the engine cooling water and exhaust can be recovered to heat the digester plus supply hot water for other uses.

Using these estimates, you can back calculate how much investment you might be able to pay for with the value of the energy produced. Remember in addition to the actual digester cost you also have the management and knowledge cost of an additional farm enterprise. This can help you to decide if you want to investigate this process further. You can quickly multiply these values times the appropriate conversion to estimate what your herd would produce.

Robert E. Graves and Patrick A. Topper, Department of Agricultural and Biological Engineering