Concerns over energy security and the environment, along with the federal policy mandating the production of 36 billion gallons/year (BGPY) of biofuels to blend into the fossil transportation fuels by 2022, have led to accelerated biofuels production in the U.S. Out of the 36 BGPY, 16 BGPY have to be cellulosic biofuels, derived from the lignocellulosic feedstock, such as forest residues, agricultural residues, and energy crops. Lignocellulosic biomass is mainly composed of cellulose, hemicellulose and lignin. Current commercial U.S. based cellulosic biorefineries are primarily using corn stover as a major lignocellulosic feedstock source due to its immediate availability. Although corn stover is available in large quantities, it needs to be collected from a wide area and transported to the biorefinery. The frequent movement of a new fleet of heavy vehicles for feedstock transportation on the roads can cause surface deterioration, increasing the need for regular maintenance and best management practices.
Current commercial cellulosic biorefineries have the capacity to produce 20 to 30 million gallons per year (MGPY) of cellulosic ethanol. The uninterrupted operation of a 30 MGPY cellulosic biorefinery requires approximately 395 thousand tons (dry) of corn stover annually, considering 5% dry matter loss. Thus, approximately 1 million large square corn stover bales (3 feet high by 4 feet wide by 8 feet long) need to be delivered to the plant each year. Currently, the most feasible mode of transportation of corn stover bales to biorefineries is on trucks, typically 5- and 6- axle tractor semitrailers. The 5-axle semitrailers are of two types depending on the front tractor axle load of 12,000 or 14,000 pounds-force (kips), and the 6-axle semitrailers are of a single type (Fig. 1-a). The maximum payload capacities of these trucks, minimum number of truck trips based on the maximum payload capacity utilization, and annual truck trips required for completely transporting feedstock to a 30 MGPY cellulosic biorefinery are summarized in Fig. 1.
At present, truck utilization to the maximum payload capacity is limited by the volume of the trailers rather than the federal weight limit (80,000 pounds) due to low feedstock bulk density. Thus, with a current maximum payload of 36 bales with moisture content of 20%, about 22,807 annual truck trips are required to fully deliver feedstock required for a 30 MGPY cellulosic biorefinery (Fig. 1-b). The large number of trips and travel distances are critical factors for determining road damage costs. The corn stover can be either transported from the field to a central depot, and then to the biorefinery, or directly to the biorefinery. In either case, our previous study estimated an average truck travel distance for delivering corn stover to a biorefinery to be around 33 miles. Out of the 33 miles, around 10 miles was the average distance from the field to the central storage depot, with roughly 80% on gravel roads, and the remaining 23 miles was the average distance from the depot to the biorefinery, with 5% on gravel roads. The approximately 24 miles of non-gravel roads are county and state highways, which are composed of composite pavement (34%), asphalt pavement (64%), and rigid pavement (2%), as shown in Fig. 2-a. The road maintenance costs due to feedstock transportation for each truck and road type estimated using historical road maintenance data are summarized in Fig. 2-b.
Road maintenance costs
The estimated overall road maintenance cost due to delivering corn stover biomass feedstock to the gate of a 30 MGPY biorefinery plant was 21% more with a 5-axle tractor semitrailer when compared to a 6-axle tractor semitrailer. The major contributor to the additional road maintenance cost due to feedstock transportation was asphalt pavement followed by gravel roads, composite pavement, and rigid roads. Even with almost similar distances traveled on gravel roads and composite pavement, the composite pavement had a low contribution to this additional maintenance cost as it can resist 100 times more vehicular movement than gravel roads. Based on the total annual additional maintenance cost and truck types, the estimated maintenance cost was 5 cents to 6 cents per dry ton corn stover transported to the biorefinery gate. Although on a per ton basis, this cost seems to be low, it will increase with the increase in biorefinery capacities due to the need to collect feedstock from a wider area and, thus, larger distances.
Assuming the sizes of future commercial cellulosic biorefineries to be 100 MGPY, the total road maintenance cost associated with feedstock transportation is expected to increase by around five fold. Thus, when we think of the federal biofuels production target of 16 BGPY, the additional road maintenance cost will be pretty significant. Practices that help maintain the health and longevity of the road networks used for feedstock transportation need to be implemented. This can be accomplished through research and development efforts to improve the bale density that reduces truck trips, regular maintenance of road surfaces to improve road strength, and selection of truck types that reduce road impacts. All of these measures will be crucial for the overall success of the feedstock supply system for biorefineries in the future.
Dr. Ajay Shah is an Assistant Professor in the Department of Food, Agricultural and Biological Engineering of The Ohio State University. He can be reached at email@example.com and (330) 263-3858. This column is provided by the OSU Department of Food, Agricultural and Biological Engineering, OSU Extension, Ohio Agricultural Research & Development Center, and the College of Food, Agricultural and Environmental Sciences