Many opportunities exist to use methane on the farm
Posted: June 8, 2012
My observation is that some dairy farmers yearn for the time when they will be able to sell more electricity into the grid than they use, thus having an additional income stream.
Of course, we are talking about a biogas digester on the farm being fed by the manure from the dairy animals and maybe some other compostable products (food processing plant wastes, food scraps from institutions, etc.) from the surrounding vicinity. The methane produced by the digester then fuels an internal combustion engine that drives a generator to produce electricity. The generated electricity may be used to supply the electricity needs of the farm and residence with the remainder sold into the grid. A simpler and more common approach is to sell all the generated electricity into the grid and then continue to purchase the electricity needed for the farm and home. Sounds perfect!
The big challenge is the quality of the methane produced by the digester. By the very nature of the digestion process, the methane will contain a considerable amount of hydrogen sulfide which is highly corrosive on the engine parts. It is essential that a scrubber be installed between the digester and the engine to remove most of the hydrogen sulfide. I have heard the scrubber described as “a black box”, “Pandora’s box”, and “a can of worms.” More seriously, the scrubber is identified as the weakest link (by far) in the system of using a bio-digester to generate electricity. It is critically important to establish and maintain a preventive maintenance program not just for the engine-generator set but for the scrubber as well.
So why do we use the methane to generate electricity? Are there other uses for methane on a dairy farm? You bet there are. The methane could be used as an energy source for producing the hot water needed for the dairy operation. It could be used to speed up the digestion process by heating the manure slurry as it enters the digester. The methane could be used for drying the manure compost before it is re-used as bedding. Methane could provide the energy for heat-treatment of the colostrum and other milk products being fed to the calves. Methane could reduce the energy requirements for crop drying. Perhaps the methane could be sold to a greenhouse operation in close proximity to the digester to provide some of the heating requirements of the greenhouse.
With a little creativity, many uses can be found for the methane. It must be emphasized that the methane should be scrubbed before being used for any of the heating applications discussed in this paragraph, but it does not need to be scrubbed as thoroughly as when it is used to fuel an internal combustion engine. It also needs to be emphasized that methane is explosive. Proper safety precautions must be followed when using the methane, especially when in a closed environment.
I just returned from a two-week study tour of the dairy industry in Costa Rica; the emphasis of the study tour was the energy usage on dairy farms. I visited about a dozen farms; half of the farms had manure digesters. The smallest farm with a digester had a herd of 25 cows. Another farm with a herd size of 40 cows was using the methane to: 1) heat the water for the dairy and his home, 2) heat the incoming manure stream, 3) dry cassava tubers (for home use and for sale), 4) dry the cassava stalks for animal feed, and 5) dry the composted manure (for sale and for use as bedding and soil amendment).
Manure digesters will become more common on our dairy farms in the future, perhaps driven by odor control more than anything else. It will be important to evaluate various creative opportunities for utilizing the methane in a safe, effective, and economic manner.
- By Dr. Dennis Buffington, professor, Penn State Department of Agricultural and Biological Engineering, firstname.lastname@example.org, 814-865-2971