Similar to pelletization, the briquette also represents a particularly interesting technology because by reducing the material density it allows to concentrate high energy reserves in a contained volume. The briquette, in fact, is a bio-fuel that has a parallelepiped or cylindrical shape; it is obtained by compressing some pulverized biomass with or without the help of pressing additives. Dur­ing the process of production, the wood is desiccated so that humidity is not higher than 10%. Briquettes can be stored easily and their calorific power is

18.5 MJ/kg [1, 2].

A complete line of briquetting is usually composed of subsequent steps that change the raw biomass, with variable humidity and pressing characteristics, into a standardized briquette that is ready to be sold in the market.

The line comprises steps that are very similar to those for pelletization, but with a relatively simpler technology. In principle, there is a biomass pre-treatment which is followed by compaction and by briquette changing.

Crushing, drying and biomass heating, which are necessary to reach the optimal characteristics of granulometry and water content, form the pre-treatment phase.

Through a feed system, which comprises holding under pressure, or by a con­veyor belt pipe, the pre-treated biomass is brought to the briquette phase, where it is compacted and transformed into briquettes. We can have low-, medium — and high-pressure briquetting systems depending on the pressure exerted. In the first two cases, the biomass is mixed with a binding substance, whereas the high — pressure systems work on the biomass as well as the biding effect obtained after the high pressures are exerted.

The most important high-pressure briquette technologies are screw briquetting and the piston briquetting (mechanic oleodynamic). In screw briquetting, the bio­mass is continuously extruded because of the rotation of one or more screws without the end inside a cone room (heated during the process). During mechanic piston briquetting, the compaction of the biomass is achieved through an alternative piston which is actuated by an electrical engine, whereas during the oleodynamic circuit piston process, there are two pistons which are actuated by the holding pressure of a closed-circuit oil which compress the material in orthogonal directions.

The most used system, which allows the treatment of the biomass with higher humidity and a better control of the applied pressure, is piston briquetting.

The final transformation of the product which is obtained from the briquetting phase includes the steps of the eventual briquette cut (only for screw systems where the material continuously exits), eventual cooling (for screw systems with heating), packing and final stocking of the briquettes [2].

The densification of the biomass in briquettes has the same advantages and disadvantages as the transformation into pellets: in fact, after the briquetting proc­ess we obtain an improvement in the physical biomass characteristics (density, homogeneity, etc.), a reduction in the volume, a reduction in the storage and trans­port costs, and an improvement in the behaviour during combustion. At the same time, the briquetting process, just as the pelletization process, needs, as we have seen, a preventive material conditioning, in particular the biomass drying. The briquettes can be used in the place of firewood and coal, by adjusting some opera­tive parameters such as the primary and secondary air distribution; relative to the two fuels, in fact, the briquettes require a higher quantity of secondary air and a lower quantity of primary air.

The briquettes with a higher thermal capacity (retain heat for a longer period of time and keep the temperature inside the oven high to allow easy combustion of the newly entering combustible) are a ‘better’ combustible than uncompressed wood.

This type of combustible is used most frequently in both domestic (they do not generate sparks which makes them appropriate for the chimney) and industrial applications [1, 2].