High compaction technology or binderless technology consists of the piston press and the screw press. Most of the units currently installed, are the reciprocating type, where the biomass is pressed in a die by a reciprocating ram at a very high pressure. In a screw extruder press, the biomass is extruded continuously by a screw through a heated taper die. In a piston press the wear of the contact parts e. g., the ram and die is less compared to the wear of the screw and die in a screw extruder press. The power consumption in the former is less than that of the latter. But in terms of briquette quality and production procedure screw press is definitely superior to the piston press technology. The central hole incorporated into the briquettes produced by a screw extruder helps to achieve uniform and efficient combustion and, also, these briquettes can be carbonized. Table 2.2 shows a comparison between a screw extruder and a piston press.

2.6.1.1 Piston Press

The piston presses which are currently in operation, are also known as ram and die technology. In this case the biomass is punched into a die by a reciprocating ram with a very high pressure thereby compressing the mass to obtain a briquette.

The briquette produced is 60 mm in external diameter. This machine has a 700 kg/ h capacity and the power requirement is 25 kW. The ram moves approximately 270 times/min in this process.

Merits and Demerits of Piston Press Technology

1. There is less relative motion between the ram and the biomass hence, the wear of the ram is considerably reduced.

2. It is the most cost-effective technology currently offered.

3. Some operational experience has now been gained using different types of biomass.

4. The moisture content of the raw material should be less than 12% for the best results.

5. The quality of the briquettes goes down with an increase in production for the same power.

6. Carbonization of the outer layer is not possible. Briquettes are somewhat brittle.