The moisture content (MC) is the mass of moisture in biomass and can be either expressed as a percentage/fraction of the ovendry mass (mass with 0 % MC)

m — m0

MC = 0 x 100% (8.1)

m о

m = mass of wet wood; m0 = ovendry mass or as a percentage/fraction of the wet mass

m m0

MC = — x 100% (8.2)

m

m = mass of wet wood; m0 = ovendry mass

The MC on dry basis is commonly used in the wood processing industry, while the MC on a wet basis is used in the Forestry and Pulp & Paper industry. The MC on wet basis has a maximum value of 100 %, while the MC on dry basis can be larger than 100 %. The MC on a wet basis is mostly used for practical reasons, as all transport costs are based on the weight of the wet biomass. The problem with the MC on wet basis is that it is not well defined, because in contrast to the MC on dry basis the weight of wood is very variable.

Typically, considerable MC variation can be found in all biomass, depending on location, age, season etc. In wood, for example, heartwood generally has a lower MC than sapwood and in softwoods it tends to be larger close to the bark and higher up. Softwoods generally have a larger MC than hardwoods. Care has to be taken therefore to determine a statistically significant average value that describes the entire biomass. Typical values are displayed in Table 8.1.

The MC is the most important property for biomass utilisation as fuel, because it affects the entire supply chain and the related costs, i. e. transport, storage, energy content, conversion methods and end use.

Table 8.1 Typical MC values for different types of biomass