Environmental Engineering Reference
In-Depth Information
under current tillage practices the national average safe removal rate based on soil
erosion for corn stover is less than 30%. Actual rates vary widely depending on local
conditions. In other words, much of the generated crop residues may be out of reach
for biomass use if soil conservation goals are to be achieved (Graham et al., 2007).
The estimated delivery costs for agricultural residues vary widely depending on
crop type, load resource density, storage and handling requirements, and distance
and transportation costs. Moreover, existing estimates are largely derived from engi-
neering models, which may not account for economic conditions. Agricultural resi-
due feedstocks (such as corn stover) have a significant advantage in that they can
be readily integrated into the expanding corn ethanol industry; however, dedicated
energy crops (such as switchgrass) may have more benign environmental impacts.
Another significant biomass source is
forest biomass
, which can be immediately
available should the bioenergy market develop. Logging residues are associated with
timber industry activities and constitute significant biomass resources in many states,
particularly in the Northeast, North Central, Pacific Northwest, and Southeast. In the
western states, the predominance of public lands and environmental pressure reduce
the supply potential for logging residues, but there is a vast potential for biomass
from thinning undertaken to reduce the risk of forest fires. However, the few analy-
ses that have examined recoverability of logging residues cite the need to account for
factors such as the scale and location of biorefineries and biopower plants, as well as
regional resource density.
The potential for forest residues may be large but the actual quantities available
for biomass conversion may be low due to the economics of harvesting, handling,
and transporting the residues from forest areas to locations where they could be
used. It is not clear how these residues compete with fossil fuels in the biopower and
co-firing industries. In addition, there are competing uses for these products in the
pulp and paper industry, as well as different bioenergy end uses. Economic studies
of logging residues suggest a current lack of competitiveness with fossil fuels (coal,
gas), but logging residues could become more cost competitive with further improve-
ments in harvesting and transportation technologies and with policies that require
a fuller accounting of the social and environmental benefits from converting forest
residues to biopower or biofuels.
Another source of forest residues that could be recovered in significant quantities is
biomass from fuel treatments and thinnings. Fuel treatment residues are the byproduct
of efforts to reduce the risk of loss from fire, insects, and disease and therefore present
substantially different challenges than do logging residues. The overall value of forest
health benefits such as clean air and water is generally believed to exceed the cost of
treatment. However, treated forests are often distant from end-use markets, resulting in
high transportation costs to make use of the harvested material. Road or trail access,
steep terrain, and other factors commonly limit thinning operations in western forests.
Transportation costs can be a significant factor in the cost of recovering biomass. As
much as half the cost of the material delivered to a manufacturing facility may be attrib-
uted to transportation. The offset to the high-cost transportation of forest thinnings is
onsite densification of the biomass. This could entail pelletization, fast pyrolysis (to
produce bio-oil), or baling. The economics of transporting thinned woody residues vs.
onsite densification depend on the distance to end-use markets. Densification may be
