Energy Density

The energy density of a fuel is the heating value per unit volume. Energy density has been found to be highest in chips from high-density species such as oaks. The southern scrub oak forest type has been shown to be an important component of southern fuelwood resources for industrial heat generation in South Carolina (Harris et al. 2004).

Energy density is relatively low in biomass chips. The space required for transporting and storing chips is 11 to 15 times greater than the space needed for oil, and 3 to 4 times greater than that required for coal (Young 1980). These differences result in higher utilization costs and the biomass material being utilized closer to the source. A commonly used conversion factor shows that a solid cubic meter of wood will produce roughly 2.5 cubic meters of loose chips (Standish et al. 1985).

Factors that influence the solid content of chips include:

1. Particle shape: Reduced ratio between diagonal length and thickness increases solid content (Edberg et al. 1973).
2. Size distribution: Heterogeneous size distribution increases solid content.
3. Tree species: Brittle, low-density materials have a higher solid content.
4. Branch content: Fresh branches and twigs reduce the solid content value.
5. Loading: Compaction from blowing increases solid content versus free-fall loading systems.
6. Storage: Stored materials tend to contain more fine materials and slightly higher solid content.

Comparisons of fuel sources often examine the energy balance ratio or the output energy versus the input to create energy. The energy balance ratio of chips from logging residues and thinnings has been shown to be high compared with other energy crops such as alfalfa, rapeseed, canary reed, and potato (Richardson et al. 2002). Thus, as long as forest fuel is a by-product of other operations and transportation is kept to a minimum, forest residuals make an excellent energy source.