Corn stover is the residual crop material left over after the harvesting of corn which consists of the other parts of the corn plant which are the stalks, leafs, cobs and husks. Some of these plant residuals are left to lie on the corn fields to help mulch the soil before the next planting period. However, corn stover has been collected after corn harvests in the past and reused to produce other chemicals and products. For example, due to the fiber content of corn stover, some of it was processed as non-wood fiber pulping which can be converted into other products. Even currently, the fiber processed from corn stover is being considered as use in particleboard manufacturing. Under current farming conditions and practices, corn stover is collected or produced at around 100 million tons per year. According to the USDA and DOE, this figure could increase dramatically within the next decade if corn yields are increased and farming technology is improved. The government estimates that at least 170 - 256 million tons of corn stover could be produced every year according to the USDA billion ton study, corn stover would definitely be the largest plant crop residual renewable resource in the United States. There are certain obstacles that make the use of large amounts of corn stover not as realizable. For example, since corn stover can only be collected once per year, it must be transported and stored properly before it can be bioprocessed. One of the challenges involved is ensuring that it is dry. The use of corn stover has now expanded into energy and fuels production as well as the continued production of chemicals. This essay attempts to demonstrate some of the practical uses of corn stover. One method that corn stover can be stored and partially processed into useful chemicals or energy is the ensilage process. Ensilage packs corn stovers into silos and fermentation of the stover then proceeds with the natural microorganisms. Ensilage is a method that is being used to produce hydrogen for a 1.5 kW fuel cell power plant in Germany. The ensilage process allows the corn stover to be converted into biogas through anaerobic fermentation, the biogas is then converted into hydrogen. Ensilage is also a method that could produce adequate amounts of a chemical called lactic acid which can then be further converted into poly lactide plastics or ethyl lactate. This is usually done with regular corn starch which is full of sugars which lactic acid bacteria need to make lactic acid.
The ensilage process could also make large amounts of lactic acid if other enzymes from fungi are seeded with the corn stover. Fungi such as Trichoderma Reesei or Aspergillus Niger could be grown and placed with the stover as it is going through the fermentation process to ensure that sugars from its cellulose and hemicellulose content are hydrolyzed [ 1. Ren et al 2007 ]. PLA based platics are one of the fastest growing biobased plastics on the market made from lactic acid. Another valuable chemical that can be extracted from corn stover are furfurals. These compounds are oftentimes extracted from wood and other materials that have hemicellulose content where the pentose sugars are extracted and converted into furfurals through processes like steam distillation. Furfurals constitute a large subset of plastics that are made partially from natural sources. Furfurals are mixed with a number of other chemicals such as acetone, other ketones, alcohol, phenol and aniline to make furfural based resins [ 2. Brady 1991 ]. Corn stover can also be densified in the future to produce fuel and further power and heat known as combined heat and power (CHP). The University of Minnesota is working with densified corn stover which are in the form of pellets, to show it is a legitimate fuel that can be gasified to produce electricity and heat. Corn stover is also being experimented by the government in large bioethanol refineries that can produce multi million gallons of ethanol per year along with other sources such as switchgrass. Corn stover can also be used to produce butanol, which has been considered as another alcohol based fuel source for vehicles. Butanol can be made through a process known as ABE (Acetone Butanol Ethanol) Fermentation. Other companies may also use corn stover in order to gasify it as a Fischer-Tropsch process to make biobased alternative fuels such as jet fuel. Overall, there are numerous alternative products that corn stover can produce that have not been mentioned, the uses outlined in this essay are not an exhaustive list. It is meant to provide information on how useful corn stover will become in future generations to help produce electricity, heat & power, plastics and vehicle fuels - all areas in which renewable resources such as crop residues can help solve our alternative product and energy needs when replacements for petroleum are needed.
REFERENCES
1. "The impact of enzyme characteristics of corn stover fiber degradation and acid production during ensiled storage", Applied Biochemistry and Biotechnology Vol 137 pgs 221-238 [2007] by H. Ren, K. Moore
2. Materials Handbook 13th Edition [1991] pg. 361 by G. Brady, H. Clauser
Photos taken from one of NREL photo archives
KEYWORDS: corn stover, poly lactic acid, furfurals, ethanol biorefineries, butanol ABE fermentation, ensilage of corn stover, ensilage fermentation of corn stover with fungi, Fischer-Tropsch, Combined Heat and Power with corn stover, fuel cell power with corn stover, USDA billion ton study, crop residues as renewable resources, corn stover use for energy and fuel
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