Transportation - Engineering and Science of Biomass Feedstock Production and Provision - page 142

inversely proportional to leaf loss, timely harvest, baling, and preprocessing are of
the essence, especially for round bales [ 12 ]. At harvest time, green energy crops
have a high moisture content of up to 50 %, which makes preprocessing and storage
more challenging than in the case of dry biomass. For instance, the majority of the
existing milling machines, such as knife and hammer mills are incapable of fine size
reduction of wet lignocellulosic feedstock. The same problem is present in sugar
cane mills, where roller mills produce bagasse particles that are usually larger than
25.4 mm. Because of the high moisture content of green energy crops, typical road
transportation vehicles reach their weight limit before their volume limit, which is
suboptimal. Green energy crops also exhibit high fiber content, resulting in higher
preprocessing energy consumption compared to dry biomass crops. Preprocessing
of green energy crops such as short-rotation coppiced willow and poplar can utilize
chippers and shredders for size reduction. Energy cane and energy sorghum are cur-
rently using sugar cane technology to extract juice with high sugar content. Forest-
based biomass, including lumber wood logs, branches, and foliage, possess high
moisture content, high fiber content, and a high bulk density. Similar to green energy
crops, forest biomass preprocessing is challenging. Forest-based biomass often
grows on hill slopes and marginal lands with limited accessibility; therefore, harvest
and transportation of forest biomass is more difficult than that of dedicated energy
crops and agricultural residues, which grow in farm fields and plantations. River
transportation has been used to transport wood logs in some areas. Transpiration
methods have been proposed to dry forest-based biomass, but the method is depen-
dent upon many uncertain factors, such as weather and soil moisture. Since forest
biomass does not have a distinct harvest window, to circumvent storage, just-in-time
(JIT) harvest and transportation approaches are suitable.

6.3 Feedstock Preprocessing

The three main preprocessing methods consist of baling, size reduction, and pel-
letization. Common biomass forms include rectangular and round bales, pellets, and
briquettes generated by extrusion, chopped forms such as generated by a self-
propelled forage harvester (SPFH), and milled forms after size reduction by various
types of milling machines.

6.3.1 Baling and/or Bundling

Bales comprise the most common biomass feedstock form used for on-road trans-
portation. Baling is one of the elementary steps of one- or two-pass biomass harvest
and collection systems as discussed in Chap. 5 . For prairie grasses, the two-pass
harvest system includes cutting, conditioning, infield windrowing, and baling. For
agricultural residues such as corn stover, the two-pass harvest system includes one