In order to produce good quality briquettes, feed preparation is very important. For densification of biomass, it is important to know the feed parameters that influence the extrusion process. For different briquetting machines, the required parameters of raw materials like their particle size, moisture content, temperature are different. These are discussed below.

2.6.2.1 Effect of Particle Size

Particle size and shape are of great importance for densification. It is generally agreed that biomass material of 6-8 mm size with 10-20% powdery component (<4 mesh) gives the best results. Although the screw extruder which employs high pressure (1,000-1,500 bar), is capable of briquetting material of oversized parti­cles, the briquetting will not be smooth and clogging might take place at the entrance of the die resulting in jamming of the machine. The larger particles which are not conveyed through the screw start accumulating at the entry point and the steam produced due to high temperature (due to rotation of screw, heat conducted from the die and also if the material is preheated) inside the barrel of the machine starts condensing afresh. Cold feed results in the formation of lumps and leads to jamming. That is why the processing conditions should be changed to suit the requirements of each particular biomass. Therefore, it is desirable to crush larger particles to get a random distribution of particle size so that an adequate amount of sufficiently small particles is present for embedding into the larger particles.

The presence of different size particles improves the packing dynamics and also contributes to high static strength. Only fine and powdered particles of size <1 mm are not suitable for a screw extruder because they are less dense, more cohesive, non-free flowing entities.