Harvest and collection constitutes gathering and removing the biomass from field. The opera­tions depend upon the state of biomass on the field. This includes the type of biomass (grass, woody, or crop residue). The moisture content and the end use of biomass also affect the way biomass is collected. For crop residue, the operations need to be organized in companion with the grain harvest. In this section, we examined advanced systems that may be used to gather biomass residues.

Harvesting

For crop residues, grain harvest most probably will take the center stage. All other operations, such as residue management and collection, take place after grain is harvested. This situation may change in future but at the present time this is the case. Figures 7.2 and 7.3 show the present and future scenarios for harvesting crop residues. Future scenarios are highlighted in the larger boxes encompassing more than one unit operation In the case of corn, a combine takes a small portion of the corn stalks. The majority of the corn stalk left in the field is anchored to the ground. The stalks need to be shredded before a baler can pick them up. Figure 7.3 also shows the use of new stripper headers for harvesting grain. Stripper headers strip the grain from the stalk and leave the stalks standing in the field. The straw stalks are then cut and placed in a swath for baling.

Loafing is an attractive option, because collection, densification, and transport to the side of the farm can be done with a single equipment unit. Loafing of stover is practiced in Iowa but its performance with straw and dedicated crops is unknown. Corn stalk moisture is high especially early in the season. One option is to chop the high moisture stover and store it in a bunker silo as silage. This option is under investigation.

Cutting and Field Drying

In the case of straw the biomass is generally dry. For stover the leftover biomass after grain harvest may be dry or wet. In the case of switchgrass, depending on the time of harvest, the

biomass may be wet as much as 80% moisture content in the middle of summer to a low of 20% at the end of the growing season. A field shredder is used to cut the material to pieces and spread on the field for drying. For green switchgrass, mowing may be combined with conditioning, where the cut material passes through two or more rollers. The rollers break the green stems at several points along the stalk. The bruise and cut provide escape routes for the plant moisture to evaporate quickly. Various degrees of maceration or severe bruising and cutting (super conditioning) have been developed in recent years.

When switchgrass is dry and standing in the field, a mower would be adequate to cut the plant and place it in a swath for immediate baling. No conditioning or maceration is needed. This statement was validated by Venturi et al. (2004) who recommend mowing and condi­tioning during early season but only mowing late in the season as the moisture content of the plant decreases. But, they also found that round baling late in the season is difficult due to the toughness and lack of pliability of the straw.

For wheat, cutting is not required as the height of cut can be adjusted during combining. In stripper header combining, standing stalks are cut and windrowed for baling. For most cases, straw is of low moisture content at the time of grain harvest or immediately after grain harvest. Operations to expedite field drying of straw may not be needed.

For corn stover, grain and stalk are at different moisture content during harvest. Figure 7.4 is a plot of stover moisture content and grain moisture content after the kernel has matured to 40% moisture content (Sokhansanj et al. 2008). Stover moisture content initially at more than 75% (w. b.) drops to 10% toward the end of harvest season. Special operations are needed to deal with the variation in moisture content. Shredding the stover and spreading it with the combine accelerates field drying. The spread material then has to be raked into windrows for efficient baling. Many operations use a flail shredder to shred the broken stalks while gathering the shredded material in a windrow in a single operation.

Collection

Collection is defined as operations for collecting, packaging, and transporting biomass to a nearby site for temporary storage. The most conventional method for collecting biomass is baling. Bales are in the form of either rounds or squares. Round bales are popular on most U. S. farms (Cundiff 1995, 1996; Bransby and Downing 1996; Cundiff and Marsh 1996; Cundiff and Shapouri 1997; Cundiff et al. 2004). Limited experience with using round bales for biomass applications indicates that round bales are not suit­able for large scale biomass handling. Because of their round shape, round bales tend to deform under static loads in a stack. Bales that are not perfectly round cannot be loaded onto trucks to form a transportable load over open roads. Experience with switch — grass harvest at the Chariton Valley Co-Firing project in Iowa (CV—RC&D. Chariton Valley Biomass Project Design Package 2002; Miles 2006) showed that variations in the density of round bales were the cause of uneven cuts and erratic machine operation during the de — baling process.