Federal policies require that biofuels, such as ethanol and biodiesel, be blended into petroleum-based transportation fuels. Typically, these fuels have been produced from corn grain (ethanol) and soybeans (biodiesel). Recent policies have mandated that a portion of biofuels be produced from lignocellulosic biomass, which includes agricultural residues (e.g., corn stover, wheat straw/stubble), energy crops (e.g., switchgrass, giant miscanthus) and forest biomass (e.g., forest residues thinnings, milling residues). These lignocellulosic biomass are comprised of cellulose, hemicellulose and lignin. While most of the cellulose and some of the hemicellulose are utilized during biofuels production, the lignin is usually separated out and burned to supply energy for the operation of the biofuels production plants. In the United States, corn stover, which is conventionally collected in limited quantities for feed and bedding material for livestock, is being considered to be a principal feedstock candidate for cellulosic biofuels production due to its widespread availability, especially in the Midwest. It is the primary feedstock choice of first generation cellulosic biorefineries, including POET-DSM’s Project Liberty (located at Emmetsburg, Iowa) and DuPont’s Cellulosic Ethanol plant (located at Nevada, Iowa). Although corn stover is considered to be the principal feedstock candidate for cellulosic biofuels production, sustainably supplying the enormous quantities that will be required for biofuels production is a major challenge, and will require an efficient, cost effective and environmentally balanced feedstock supply chain.
The corn stover feedstock supply chain for a first generation cellulosic biorefinery will include harvesting (windrowing and baling), collecting and stacking bales at the field edge, transporting bales from the field-edge to the central storage facilities, and transporting stored bales to the biorefinery plant (Figure 1). Transportation to both the storage facility and the biorefinery includes loaders for loading and unloading bales to and from the trucks. An understanding of the costs associated with each of these operations is critical to identifying how to reduce the cost of biofuels production.
|Figure 1. Discrete operations within a corn stover feedstock supply system.
Determining corn stover supply costs
|Figure 2. Cost of harvesting, collecting, storing and delivering corn stover biomass feedstock to the gate of a Midwest-based 30 MGPY cellulosic biorefinery.
The overall cost of harvesting, collecting, storing and delivering corn stover biomass to the gate of a 30 MGPY biorefinery plant was estimated to be $82.40 per ton. The two largest contributors to these costs are feedstock transportation (transports to storage and biorefinery sites), which comprises around 40 percent of the overall cost, and stover harvesting (windrowing and baling), which comprises more than 35 percent of the overall cost. The transportation cost is highly influenced by the bulk density of the feedstock, and the feedstock harvesting cost is influenced by the actual working window and farm machinery efficiency.
Considering that 80 gallons of ethanol are produced from each ton of corn stover biomass, the overall feedstock supply cost would be around $1 per gallon of cellulosic ethanol. Efforts to reduce this cost should focus on increasing bale density, extending the working window and enhancing farm machinery efficiency. Additional costs of biofuels production include feedstock producer payments, nutrient replenishment for crop production, conversion of biomass to biofuels, and biofuels distribution to the fuel pump.