FA binding agent is necessary to prevent the compressed material from springing back and eventually returning to its original form. This agent can either be added to the process or, when compressing ligneous material, be part of the material itself in the form of lignin. Lignin is a constituent in most agricultural residues. It can be defined as the thermal plastic polymer, which begins to soften at temperatures above 100°C and is flowing at higher temperatures. The softening of lignin and its subsequent cooling while the material is still under pressure is the key factor in high pressure briquetting. It is a physic-chemical process related largely to the temperature reached in the briquetting process and the amount of lignin in the original material. The temperature in many machines is closely related to the pressure though in some, external heat is applied.
In general there are two immediate ways of classifying briquetting processes. One distinction is whether or not an external binding agent must be added to agglomerate the compressed material. The second way of classifying follows the pressure applied while briquetting: high, medium or low pressure.
For a rough distinction the following numbers might be adopted:
- Low pressure up to 5 MPa
- Medium pressure 5 - 100 MPa
- High pressure above 100 MPa
Usually high pressure processes will release sufficient lignin to agglomerate the briquette. Medium pressure machines may or may not require binders, depending upon the raw material whilst low-pressure machines invariably require binders. Such external binders might be: starch, clay, molasses or wood tar etc .. All briquettes using inherent binders (lignin) or external hydrophilic binders (starch, molasses, gum, clay) are not waterproof and will disintegrate when they come into contact with water or stored under humid conditions.