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October 26, 1995
October 1998 Revision
October 1998 Revision
Several times a year Vincent receives inquiries in regards to forage grass processing. The need arises because field dry hay will have lost a lot of the nutrients, especially proteins. The best nutritional value of grasses exists if they are harvested while young and green.
Back in the 1950’s and 1960’s Vincent Corporation sold rotating drum dryers for use on forage grasses. Some of these included a screw press ahead of the dryer, and a few included pelleting equipment. If the grass was pelleted, there was generally provision for adding molasses to the dried grass (so that the pellet mill would operate properly).
Most of these facilities were shut down in the 1970’s and 1980’s following the Oil Embargo. This was related to the cost of fuel required to evaporate moisture from the grass, in relation to the cost of alternative animal feeds (especially soybean meal).
If a dryer is used to dry forage grasses, the fuel needs can be analyzed as follows: taking 2,000 pounds of grass at 80% moisture down to 15% moisture requires removing 1,530 pounds of water. The dryer output will be 470 pounds of feed material made up of 400 pounds of solids and 70 pounds of water.
We figure that it takes 1,500 BTU of fuel consumption to evaporate a pound of water in a Vincent dryer. Thus 2,300,000 BTU will be required to process the ton of grass. If this is done with natural gas costing $0.27 a therm, the fuel bill will be $6.20 per ton of grass dried, or $26.40 per ton of feed produced.
The alternative of using a screw press ahead of the dryer has the advantage of lowering the fuel cost per ton of animal feed produced. This is because the screw press is very efficient in removing moisture. This disadvantage of using the screw press, however, is that protein nutrients that come out with the press liquor are apt to be lost.
The economics using a press would look like this: The press will easily take one ton of 80% forage grass down to 70% moisture press cake. Assuming both some suspended and some dissolved solids are lost in the press liquor, we would expect the ton of grass to turn into 800 pounds of press liquor and 1,200 pounds of press cake. The energy cost to do this (i.e., run the press) can be considered as negligible.
This gives us 1,200 pounds of grass with 70% moisture, which can be dried down to 15% in a smaller dryer. It will come out as 425 pounds of material at 15% moisture. This means the dryer must evaporate only 775 pounds of water from the press cake. At $0.27 therm, the fuel cost to evaporate this water works out to $14.80 per ton of animal feed.
It is important to note that in this second example we get only 425 pounds of dry feed per ton of grass, as compared to 470 pounds in the dryer-only example. The difference arises from the solids lost in the press liquor from the screw press.
Generally the press liquor was disposed of through a spray field application, where it was used to irrigate a pasture. As an alternate, Vincent has supplied equipment for a number of Leaf Protein Concentrate facilities. In an LPC operation the press liquor is centrifuged and concentrated in an evaporator so as to produce a high protein liquid. The process, which uses alfalfa, has not proven economical in most instances.
An interesting note was found in regards to the solids in forage press liquor. It stated that about half of the press liquor solids are potash, other mineral salts, and immature ammoniates, which are more detrimental than valuable as a feedstuff. This sheet was loose in our files; it is undated, but the cost of fuel oil was $0.08 a gallon at the time.
To get the maximum capacity out of a screw press, the grass should be chopped very short, ½” to ¾” in length. We would expect a model VP-22 to handle 30 tons per hour of such forage chopped grass. (If longer pieces are pressed, the capacity must be de-rated by 50%.)
Note: March 9, 2003, the price of natural gas is now about $0.50 a therm. A therm is 100,000 BTU’s.
Issue 34