1.2 Liquid state combustible biomasses

Before being introduced into the market, the lignocellulose biomasses are usually subjected to a transformation process to give them the necessary physical and energy characteristics for their use in the energy plants. Firewood (logs or stub pipes), chips, pellets and briquettes are the main commercial forms for this biomass category.

1.2.1 Firewood

It is sold in logs or stub pipes with variable coal sizes from 50 to 500 mm and humidity values of lower than 50%. This kind of fuel at the domestic level is used mainly in small hand-feed plants, and its use is less than the use of briquettes and pellets (dense forms). The wood boiler, in fact, also does not allow an automatic loading of fuel and shows a lower energy efficiency (50-60% against 75-90% for the chips boilers and wooden pellet) [1, 2].

1.2.2 Chips

To make the wooden and material composition homogeneous and appro­priate for the automatic feed in the energy plants, we can resort to the use of

chips, a mechanical operation that reduces the sortings into flakes of small dimensions that are called chips. Such an operation can be applied, with no differ­ence, to the wooden or herbaceous biomasses. The flakes can be obtained through a crushing-milling process that is essentially based on the percussion and the defibering. The chip is obtained with the cut action.

The typologies of wood that are prepared for chipping are forest, agricultural and urban pruning residuals, slashes and lops or sawmill subproducts and wood coming from the brief rotation plants (SRF) [1, 2, 20].

The expense during chipping production energy, which is variable depending on the humidity of the biomass used, varies from 2 to 5 kWh/t, which corresponds to

less than 0.5% of the energy contained in the wood. Hard and dry wood chipping requires 18% more of energy than the just demolished humid wood working.

The geometry of the chips varies with the cut techniques adopted, as a function of the dimensions required by the type of the energy plant and, especially, of its power system. The chips usually show a variable length from 15 to 50 mm, a width that is equal to half of the length and a variable thickness from 1/5 to 1/10 of the length. A typical dimension is 40 x 20 x 3 mm. The homogeneity (obtained with chip screener adjusting) is the most important parameter for the chips that are destined for combustion, given that non-homogeneous chip dimensions result in annoying blockages of the plant’s feed systems.

The standard of the desired humidity is usually obtained prior to cumulus stock­ing, for an appropriate time: the maximum humidity that is accepted from the disposal of the chips combustion technologies is equal to 50%. It is also important, to avoid chips’ working problems, that the standard of the biomass humidity is limited between 25% and 59% [2].

The usable high quality chips in automated combustion systems do not have bark (or it contains only a minimum part) ensuring optimal combustion with a minimum cinder content, which is lower than 0.5%.

Similar to the analogy of the pellet, even for chips it is important to use pure wood. Impurities such as plastic or paints result in higher polluting emissions and cinder content. For this purpose, their use is generally restricted in biomass boilers that do not have a provision for waste gas purification. The chips are good for feed­ing all the types of biomass boilers with powers that vary from a few kilowatts

to tens of megawatts [1, 2]. The market has at its disposal different kinds of chips that are able to provide different quality and dimension wood (up to 30 cm diam­eter), with a working capacity that varies from a few tons to some tens of tons in an hour, both railcars and carried from agricultural tractors. The cut system can be disks (more diffused and used in the little power chips) or in rolls (that are availa­ble in very heavy and powerful versions). For the chips of herbaceous biomasses, cut-waders are often used, machines that work by directly cutting the trunks at the base, chipping them and pushing them on towing [2].