The following sections describe some important thermophysical properties of biomass that are relevant to gasification.

1.4.1 Physical Properties

Some of the physical properties of biomass affect its pyrolysis and gasification behavior. For example, permeability is an important factor in pyrolysis. High permeability allows pyrolysis gases to be trapped in the pores, increasing their residence time in the reaction zone. Thus, it increases the potential for second­ary cracking to produce char. The pores in a wood are generally oriented longitudinally. As a result, the thermal conductivity and diffusivity in the longitudinal direction are different from those in the lateral direction. This anisotropic behavior of wood can affect its thermochemical conversion. A densification process such as torrefaction (Chapter 3) can reduce the anisotropic behavior and therefore change the permeability of a biomass.

Densities

For a granular biomass, we can define four characteristic densities: true, appar­ent, bulk, and biomass (growth).

True Density

True density is the weight per unit volume occupied by the solid constituent of biomass. Total weight is divided by actual volume of the solid content to give its true density.

Total mass of biomass Solid volume in biomass

The cell walls constitute the major solid content of a biomass. For common wood, the density of the cell wall is typically 1530 kg/m3, and it is constant for most wood cells (Desch and Dinwoodie, 1981). The measurement of true density of a biomass is as difficult as the measurement of true solid volume. It can be measured with a pycnometer, or it may be estimated using ultimate analysis and the true density of its constituent elements. True densities of some elements are given in Table 2.4.

Apparent Density

Apparent density is based on the apparent or external volume of the biomass. This includes its pore volume (or that of its cell cavities). For a regularly shaped biomass, mechanical means such as micrometers can be used to measure dif­ferent sides of a particle to obtain its apparent volume. An alternative is the use of volume displacement in water. The apparent density considers the internal

pores of a biomass particle but not the interstitial volume between biomass particles packed together.

The pore volume of a biomass expressed as a fraction of its total volume is known as its porosity, ep. This is an important characteristic of the biomass.

Apparent density is most commonly used for design calculations because it is the easiest to measure and it gives the actual volume occupied by a particle in a system.

Bulk Density

Bulk density is based on the overall space occupied by an amount or a group of biomass particles.

Bulk volume includes interstitial volume between the particles, and as such it depends on how the biomass is packed. For example, after pouring the biomass particles into a vessel, if the vessel is tapped, the volume occupied by the particles settles to a lower value. The interstitial volume expressed as func­tion of the total packed volume is known as bulk porosity, eb.

To determine the biomass bulk density, we can use standards like the American Society for Testing of Materials (ASTM) E-873-06. This process involves pouring the biomass into a standard-size box (305 mm x 305 mm x 305 mm) from a height of 610 mm. The box is then dropped from a height of 150 mm three times for settlement and refilling. The final weight of the biomass in the box divided by the box volume gives its bulk density.

The total mass of the biomass may contain the green moisture of a living plant, external moisture collected in storage, and moisture inherent in the biomass. Once the biomass is dried in a standard oven, its mass reduces. Thus,
the density can be either green or oven-dry depending on if its weight includes surface moisture. The external moisture depends on the degree of wetness of the received biomass. To avoid this issue, we can completely saturate the biomass in deionized water, measure its maximum moisture density, and specify its bulk density accordingly.

Three of the preceding densities of biomass are related as follows:

papparent ptrue (1 -£p)

pbulk papparent(1 &b)

where Ep is the void fraction (voidage) in a biomass particle, and eb is the voidage of particle packing.

Biomass (Growth) Density

The term biomass (growth) density is used in bioresource industries to express how much biomass is available per unit area of land. It is defined as the total amount of above-ground living organic matter in trees expressed as oven-dry tons per unit area (e. g., tonnes per hectare) and includes all organic materials: leaves, twigs, branches, main bole, bark, and trees.