List and values of wood fuel parameters - Part 2

Heating Value

The calorific heating value of dry matter does not vary a great deal from one tree species to another (18.7 -21.9 MJ/ kg), but it is slightly higher in coniferous species than in deciduous tree species. This is caused by the higher lignin and resin contents in coniferous species.

A summary of heating values for Nordic woody biomass is shown in Table 4.

Table 4: Effective heating values of Nordic woody biomass (MJ/kg)

d Fuel Parameters - Part 2

Heating Value

The calorific heating value of dry matter does not vary a great deal from one tree species to another (18.7 -21.9 MJ/ kg), but it is slightly higher in coniferous species than in deciduous tree species. This is caused by the higher lignin and resin contents in coniferous species.

A summary of heating values for Nordic woody biomass is shown in Table 4.

Table 4: Effective heating values of Nordic woody biomass (MJ/kg)

particle size varies from sawdust-like needle and bark material to sticks of wood and branch pieces. The size of the wood particles is influenced both by the original raw material being chipped and by the chipper types. The more stern wood the raw material contains, the more even the particle size distribution will be. The condition of chipper knives as well as the aperture size of the screen in the chipper also influence the particle size. Chips produced with crushers have typically coarser particles compared to the chips produced with chippers.

There is a large variety of different chippers available. Wood chip users always define the size and quality. Usually size is defined so that 95 % of chips must fall under diameter of 30 mm, 45 mm or 60 mm. Maximum moisture content (w-%) allowed is usually defined for chips to be under 40%, 50% 60% or 65%.

It is the fuel supplier's responsibility to deliver the size and quality of chips that he has signed a delivery contract for.

Density

The solid volume content of chips indicates the relationship between the masses of so-called bulk measure and solid measure, that is, how many solid m3 one bulk m3 will yield. The solid volume content of chips is influenced mostly by the technical specifications of the chipper, such as particle size distribution, blowing power and loading method. The drying time of chips and the compacting that occurs during long-distance transport, however, have no decisive effect on the solid volume content value. Solid volume content (the portion of solid measure) is needed for converting bulk measure into solid measure. The bulk density of the Northern woody biomasses is in the range of 200- 350 kg/loose m3.

The density and ability to control it will affect fuel handling systems (e.g. conveyor designed).

Ash Content and Properties

The fuel contains various impurities in the form of incombustible component parts ash. Ash itself is undesirable, since it requires purifying of the flue gas for particles with a subsequent ash and slag disposal as the result. The ash contained in wood comes primarily from soil and sand absorbed in the bark. A minor proportion also comes from salts absorbed during the period of growth of the tree.

The ash also contains heavy metals, causing an undesirable environmental effect, but the content of heavy metals is normally lower than in other solid fuels.

A special characteristic of ash is its heat conservation property. For wood stoves, the ash layer at the bottom of the stove forms a heating surface, transferring heat to the final burnout of the char. For heating systems using a grate, the ash content is important in order to protect the grate against heat from the flames.

Wood also contains salts that are of importance to the combustion process. It is primarily potassium (K) and partly sodium (Na), based salts resulting in sticky ash, which may cause deposits in the boiler unit. The Na and K content in wood is normally so low that it will not cause problems with traditional heating technologies.

Typical mineral fractions in wood chips expressed as percentage of the dry matter (DM) of the wood is shown in Table 5. Compared to straw, the K content in wood chips is approx. 10 times lower.