In­field Separation of Kenaf

L. H. Chen and Jonathan W. Pote

There are two disadvantages with forage harvesters currently being used for kenaf.

First, the harvester chops the kenaf into small pieces. The bast fiber must be separated from the core, and the short bast fibers may limit the usage. Second, harvesting normally begins after frost kills the vegetation. Weather often prohibits prompt harvest. This delay in harvesting may affect other field operations the following year. In­field separation is an alternative harvest method that can eliminate these two problems.

It is easy to separate the bast fiber from core by crushing the green kenaf. The separation machine described here is based on a prototype developed by Tainan Fiber Crops Experiment Station in Taiwan in 1975. The schematic diagram (Figure 1) shows the basic principle of the separation.

The whole kenaf stalk is fed into the machine. The first two crusher­rollers crush the stalk, which is then beaten by the beater­roller. This action separates the bast fiber from the core. The stalk is then fed into the third crusher­roller while the core falls downward. The crushing and beating processrepeats through the third crusher­roller and the second beater­roller; the last crusher­roller removes the remaining core attached to the bast fiber. The whole­length bast fiber is the end product.

The crusher roller consists of a 3/4­inch thick 5 1/2­inch OD steel pipe with twelve 1 1/4­inch by 1/2­inch steel bars welded to the periphery of the pipe. The beater­roller consists of four 2­inch by 1/2­inch steel bars supported>by four equally­spaced 3/8­inch thick steel plates welded to a 2­inch shaft. The outside diameter of the crusher­roller is 8 inches and that of the beater­roller is 13 3/8 inches. With the crusher­roller operating at 267 rpm, the linear speed of the roller is 9.2 feet per second. Therefore, for a 10­foot long kenaf stalk, the separation time takes less than 2 seconds.

The speed of crusher­roller affects the performance in bast and core separation. Our preliminary test indicates that the roller speed around 288 revolutions per minutes produced the best separation (Figure 2).

The crusher­roller did damage the skin of the bast fiber. An Instron test showed that the tensile strength of the machine­separated bast was about 75% of that of the hand­stripped bast. Whether or not the separation process reduced the strength of retted kenaf fiber remains to be tested.

The ease of bast and core separation depends on the moisture content of kenaf stalk. Green kenaf stalk has a moisture content around 70% wet basis. When the moisture content of kenaf is lower than 50%, the separation of bast from core becomes difficult. Whole stalk kenaf dries down very slowly after harvest. In our tests, it took at least 3 weeks for the kenaf to dry down from 70% to 50% moisture.

Some visualized advantages for in­field separation are:

(1) Because the resulting bast fibers are several feet in length, their use may be broadened.

(2) Bast fibers dry within a day, facilitating storage.

(3) Core can be more easily collected.

In order to fully utilize the machine capacity, it is necessary to have some means to harvest kenaf in large quantities and transport the stalks to a central location for separation. Another approach is to develop a machine that will harvest the kenaf stalk and separate the bast and core in the field just as the combine does for grain crops.

Our effort for the 1993 year was to build a once­over kenaf harvester. A used John Deere grain combine was used as the prime mover and power source. The separation unit was built to fit in the combine. A Kemper head and a feeder house of a forage harvester were attached to the front of the separation unit.

The self­propelled harvester worked satisfactorily most of the time when there was a continuous feeding of the kenaf. Occasionally, the intermittent feeding of kenaf from the Kemper head to the feeder house caused a large bundle of kenaf passing through the separation unit. This caused the breakage of the roller chain or the bearing housing. Further refinements are necessary to achieve a continuous field harvest with this unit.

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Lung-Hua Chen is a Professor and Agricultural Engineer, and Jonathan W. Pote is Associate Professor and Associate Agricultural Engineer, Department of Agricultural and Biological Engineering, Mississippi State University.