Briquetting of powdery biomass

The process of briquetting consists of applying pressure to a mass of particles, with or without a binder and converting it into a compact mass. The end product may be in a solid geometrical form or in the form of hollow cylinders which may be a solution to the large volume, low-density products having high transportation/handling cost problems associated with biomass. Cow dung cakes, fuel balls made with coal dust, half burnt cinder as well as hand compressed special fuel lumps are some of the prime examples. High-density briquettes are also made by power driven machines to make wood like solid briquettes from powders/dust of various materials.The vital requirement of briquette formation form wood biomass is the destruction of the elasticity of the wood, which could be done either by high pressure or by previous heat treatment or by a combination of both. There are two processes of briquetting, namely, direct compaction and indirect compaction after pyrolysing or carbonization of the residue.

Direct compaction

In this ease, the briquettes may be prepared with or without binding agents.

a) Without binding agents: This process involves two steps: Pretreatment of biomass through application of high pressure in the range of 120 - 200 MPa at which condition the biomass gets heated to a temperature of about 182°C and the lignin begins to flow and acts as binder. Compaction or densification of material: The powdered biomass is densified through briquetting machines available in the capacity range of 100 - 300 kg/h or more operating by electric power. The cost of such briquetting units depends upon the capacity.

b) With binding agents: In this process, the briquetting machines operate at lower pressure range of 50 - 100 MPa and are powered by electricity. This process requires additional binders like molasses, lingo-sulphonate, sodium silicate, dung slurry etc. Such machines are available in the capacity range of 100 to 400 t / h.

Indirect compaction

Pyrolysis is the process or destructive distillation of organic materials heated at slow rate to about 270°C in the absence of oxygen. During process of pyrolysis, solid char, liquid tar and combustible gases are produced besides organic liquids. The nature and quantum of these products depend on various factors such as composition of biomass, and residence time in kiln and temperature. During the pyrolysis, the fibre content of the biomass is broken which latter facilitates in briquetting of charcoal. The obtained charcoal is briquetted through extrusion process.