Virgin Biomass

The production of virgin biomass for food and feed has progressed from very labor-intensive, low-efficiency agricultural practices over the 1800s and 1900s to what some consider to be a modern miracle. The invention of numerous agricultural machines in the late 1700s and 1800s that can seed the earth and reap the harvests with minimal labor and energy inputs made it possible to continuously produce biomass in quantity to help meet the massive demand for foodstuffs and other farm products caused by the growing population. In the United States today, only a few percent of the population living on farms is sufficient to produce enough food to meet all the nation’s demands for foodstuffs as well as supply surplus amounts for export. Farm equipment is available so that almost all row and grain crops can be continuously planted and harvested and separated into foodstuffs, feed, and residual materials. Eli Whitney’s cotton gin and Cyrus McCormick’s reaper are just two of the devices that helped mechanize agriculture and change the course of history by provid­ing non-labor-intensive methods of physically separating the desired products, cotton and grain for these particular inventions, from biomass. As candidate energy crops evolve, such as several of the thick-stemmed grasses that are difficult to harvest at high growth densities, new agricultural equipment designs and adaptations of existing machinery are expected to solve these problems also.

Simultaneously with the advancement of agriculture, although not via the same pathway, new hardware and improved methodologies were developed

for the planting, managing, and harvesting of trees that made large-scale com­mercial forestry operations more economic and less dependent on labor. Better methods of land clearing, thinning, and growth management, and improved hardware for harvesting, such as feller-bunchers, which were first used in the early 1970s, resulted in a modern forest products industry that supplies commercial and industrial needs for wood and wood products. As the use of trees for energy and feedstocks expands, it is expected that much of the existing commercial hardware and improvements will be applied to meet these needs.

Equipment and methods for the harvesting of the smaller short-rotation woody crops at low cost are also expected to be developed. Much of the ongoing work to design improved equipment for SRWC is directed to feller — bunchers that perform severing, bunching, and off-loading functions. The results from systems analysis studies indicate that prototype feller-buncher harvesters can be balanced with two or three small grapple skidders to move bunched SRWC to a landing for chipping or just loading in the case of whole trees (Perlack et al, 1996). However, because of the high skidding costs, a whole-tree, direct-load system for use with a track-type feller-buncher is pre­ferred.

A few of the nonmanual separation methods used for woody biomass pro­cessing that have use in energy applications are briefly described here. Delimb — ing and debarking of trees is an old technology. For the smaller trees where fiber in the form of white wood chips is the desired product, the trees can be debarked and delimbed by the use of chain flails, which remove the outer bark layer, leaving the white wood behind. Hammermilling then yields a homogeneous product (Hudson and Mitchell, 1992). In most thermochemical energy applications, however, separation of the bark and wood is not necessary. But where it is necessary to remove the bark, some efforts have been made to recover the residues for fuel from flail machines by using them together with tub grinders (Stokes, 1992). A tub grinder operating simultaneously with a chain flail was successfully used to comminute the residues (Baughman, Stokes, and Watson, 1990). The green weight of the fuel residues was about one — fourth to one-third of the total clean chip-plus-fuel weight.

In a few installations that bum hogged wood, disc and shaker screens have been employed to separate preselected, oversize pieces for subsequent size reduction and return to the fuel stream. Finely divided wood fuels such as sawdust and sanderdust are also sometimes screened to remove the larger pieces.

By-product hulls from the production of rice, cotton, peanut, soybean, and similar crops that have outer shells covering small seed or fruit are sometimes used directly as fuels or feedstocks. After the shells are fractured, most of the hulls can be separated with vibrating screens or rotating trommels having appropriately sized openings. The by-product hulls that have high ash contents

and bulk densities present a few difficulties on direct combustion or gasifica­tion, but specially designed systems are available to eliminate these problems (с/. King and Chastain, 1985; Bailey, 1990; Bailey and Bailey 1996).