Much has been written about using crop residues as a bioenergy feedstock. In the mid- to late-1970s, energy prices soared, which led to discussion about using crop residues for energy [1, 18]. In 2003, the DOE shifted interest from dedicated energy crops to crop residues such as corn stover and wheat straw [70]. Kim and Dale [30] estimated that harvesting crop residues worldwide could replace 32 percent of worldwide gasoline consumption if E85 ethanol is used in midsize vehicles. E85 is 85 percent ethanol and 15 percent gasoline.
Corn stover gets the most attention as a potential feedstock for biofuel production in the United States. Corn stover includes the stalks, cobs and leaves left in the field after grain harvest. Some believe that corn stover is the largest untapped source of agricultural biomass in the United States [53]. About 5 percent of corn stover is currently used for animal bedding and feed, and less than 1 percent is used for industrial processing. This leaves more than 90 percent of corn stover in the field. According to Petrolia [55] the most abundant agricultural biomass source in the United States is corn stover, followed by manure.
Karlan et al. [28] examined harvest strategies for corn stover to evaluate the impact its removal has on the soil. After five years of study, they found that the phosphorus and potassium available to the next crop were low following stover removal. This reduced soybean yields the next year. Following the five-year analysis, it was concluded that “with good crop management practices, including routine soil testing, adequate fertilization, maintenance of soil organic matter, sustained soil structure, and prevention of wind, water or tillage erosion, a portion of the corn stover being produced in central Iowa USA can be harvested in a sustainable manner” [28].
In 2008, R. Lal [34] discussed the interactions between crop residue and soil. Crop residues provide food and energy for soil organisms, resulting in enhanced species diversity. Residues increase soil-nutrient levels by decreasing nutrient runoff and by returning nutrients to the soil as they decompose. Crop residues can also increase available water in the root zone, increase water infiltration rates and decrease erosion. However, the question still remains if it is wise or economically viable to harvest residues for bioenergy. Many agronomists and economists argue that only a few crop residues are practical as bioenergy feedstocks. They include corn, small grains, sorghum, rice and sugarcane. Crops such as cotton and soybeans leave too little residue behind or their residues decompose too quickly for harvesting [32].
Studies have shown that removing crop residues will result in decreased yields the following year [78]. Crop residues are directly related to soil organic carbon (SOC): The more residues, the greater the SOC [39]. In turn, greater SOC increases both soil quality and yields [33]. Lal stated that the long-term benefits of leaving crop residues in the field outweigh the financial gain from selling the residue to a biorefinery [34]. He goes on to say that residue removal is not a sustainable option for biofuel production. More research is needed to determine if some residue can be removed while leaving enough to prevent soil deterioration and decreased yields [27, 53].
Another problem with harvesting crop residues is the short harvest window: one to three months depending on the crop. Enough biomass has to be harvested and stored during the harvest window to supply the biorefinery year round. Storage can be a significant cost. More research is needed before crop residues can be considered a commercially viable feedstock for ethanol production.
Estimates of the costs to harvest, collect, store and transport corn stover to a biorefinery [8, 17, 23, 37, 40, 55] range from $29–$116 per dry ton (Table 16.1) [46, 47, 48]. It is difficult to estimate the costs since efficient residue-harvesting technology has yet to be developed. Current research focuses on developing equipment that can harvest both corn stover and corn grain at the same time.