Ashes are defined as the residual mass obtained after combustion in air, under controlled con­ditions of time and temperature; in a similar way to moisture, ash content can be calculated on wet basis, on dry basis and on dry and ash-free basis. Usually the value referred to dry basis is used. Ashes obtained from wood combustion are composed mainly of: silicon, calcium, potas­sium, phosphorus, manganese, iron, zinc, sodium, boron, in the form of oxides, silicates and nitrates. For their composition ashes are generally alkaline, with a pH around 12; the chemical composition may also vary depending on the combustion temperature.

Table 5.2 shows a typical ash composition for different biomasses together with their melting point which constitutes a main issue to tackle during combustion. Some biomasses show ashes with a particularly low melting temperature which may adhere to surfaces inside the combustion chamber and also cause heat-exchanger fouling. To avoid ash melting, a limit on the combustion temperature should be considered hence the resulting power cycle efficiency and emission control may be affected.

5.1.3.1 Volatile matter (EN 15148, 2009)

This represents the part of biomass that is released during heating (200-450°C) without oxygen (pyrolysis). Pyrolysis process also happens during biomass combustion, because oxygen does not reach the internal layers of the fuel with the same velocity with which heat does. For this reason while the external layer of biomass burns, the internal layer decomposes due to pyrolysis. During this process biomass decomposes in non-condensable gases (syngas), condensable gases (tars) and solid carbon (char): the first two products can be grouped under the category: volatile matter (VM) (EN 15148, 2009).

(Nussbaumer, 1993; Lewandowsky, 1996; Obernberger et al., 2000; Ruckenbauer, 1996; Schmidt et al., 1994; Obernberger etal, 1996; Channiwala and Parikh, 2002).

The content of volatile matter is determined by heating the fuel in absence of oxygen and in strictly controlled conditions and it is calculated using the following relation:

where:

VM = % of volatile matter in the air-dried sample [%] m1 = sample mass before heating [g] m2 = sample mass after heating [g].

Volatile matter as well can be expressed on a wet basis, dry basis or dry and ash-free basis. The volatile matter content is important because it determines the quantity of secondary combustion air to provide inside the combustion chamber; primary air being the one necessary to oxidize solid char.