In the screw press technology, the biomass is extruded continuously by a screw through a taper die which is heated externally to reduce the friction.

Merits and Demerits of this Technology

1. The output is continuous and the briquette is uniform in size.

2. The outer surface of the briquette is partially carbonized facilitating easy ignition and combustion. This also protects the briquettes from ambient moisture.

3. A concentric hole in the briquette helps in combustion because of sufficient circulation of air.

4. The machine runs very smoothly without any shock load.

5. The machine is lightweight compared to the piston press because of the absence of reciprocating parts and flywheel. The machine parts and the oil used in the machine are free from dust or raw material contamination.

6. The power requirement of the machine is high compared to that of piston press.

At present, screw press and piston press technologies are becoming more important commercially. As the piston press technology is comparatively older than the screw press technology, more piston presses are operating today. How­ever, the screw press technology is also rapidly gaining in importance. The lack of basic research to improve the piston press and the manufacturers’ inability to understand the technology are the two prime reasons that these presses are not performing satisfactorily on a commercial basis [6]. Entrepreneurs face many problems due to frequent wear in the ram and the die. The life of the ram has been observed from 33 to 300 h. This is the most frequently used briquetting equipment and is manufactured throughout the world. It consists of a flywheel that operates a piston, which presses the material through a tapered die where the briquette is formed. But piston presses have not been successful due to a lack of understanding of the characteristics of raw material which in turn affects machine design parameters like flywheel size and speed, crank shaft size and piston stroke length. The feeding mechanism also needs to be perfected, in this case according to the bulk density of the raw material.