Agricultural residues and energy crops are potential lignocellulosic feedstocks for use in biorefineries to produce biofuels and bioproducts. This fact sheet discusses the steps and logistics of collecting and handling these feedstocks, their limitations, and logistics costs.
Agricultural residues, energy crops and their importance as biomass feedstocks
The U.S. revised renewable fuel standard (RFS2) mandates the production of 16 billion gallons of cellulosic biofuels by 2022, which will annually require about 200 million U.S. tons of cellulosic feedstock. Currently, less than 1% of the total targeted capacity has been met. Thus, there is a huge opportunity for growth of cellulosic biorefineries.
Lignocellulosic biomass, such as agricultural residues (e.g., corn stover, wheat straw) and energy crops (e.g., miscanthus, switchgrass), contains cellulose and hemicellulose, which can be converted to sugars. These sugars can be further utilized to produce biofuels and bioproducts, making them a potential feedstock source for use in a biorefinery. Agricultural residues are readily available after crop harvest, while energy crops, which are usually perennial, are currently not as widespread but can be grown in most of the United States. According to the U.S. Department of Energy’s Billion-Ton Report, in the United States, 104 million tons of agricultural residues were available in 2017, and 201 million tons of agricultural residues and energy crops will be potentially available in 2022 at a farmgate price of less than $60/ton.
Feedstock logistics for agricultural residues and energy crops
Feedstock supply system logistics for agricultural residues and energy crops involve harvesting or collecting, handling, transporting, storing, and delivering the feedstock to the biorefinery (Figure 1). In addition, biomass modification options (e.g., densification or upgrading to form pellets, torrefied biomass or bio-oils) are also being considered for larger biorefinery systems. Currently, corn stover and switchgrass are the most common feedstocks for agricultural residues and energy crops in the United States.
Figure 1. Feedstock logistics for agricultural residues and energy crops for the cellulosic biorefinery.
Feedstock harvest and collection
Agricultural residues, such as corn stover, are usually collected just after crop harvesting, and have a relatively short harvest window. Unlike agricultural residues, switchgrass grows from early spring to late fall and can be harvested at any time during this period. However, a single harvest at the end of the season can maximize biomass yield, which may make it the most economical option for use as a biomass feedstock.
Agricultural residues and energy crops have similar harvesting and handling operations. Conventionally, corn stover is harvested in multiple passes through the field, in which grain harvest is followed by windrowing (shredding or raking), baling, and in-field bale collection and movement to the field edge. Multi-pass harvesting of corn stover especially in the U.S. Midwest has issues, such as poor drying conditions, a short operational window between grain harvest and snow cover, frequent delays due to weather, low harvest efficiency, soil contamination, and high costs. Harvesting of switchgrass involves mowing followed by conditioning and/or drying. Conditioning involves crimping mowed plant stems between two or more roller blades, which results in faster drying of the switchgrass.
For existing biorefineries, feedstock delivery in large rectangular bales, with dimensions of 4 ft. width, 3 ft. height, and 8 ft. length, is the most feasible packaging format compared to the round bale, loaf, and bulk formats, as it uses the safest and most efficient feedstock handling practices. Bales are collected from the field using multiple bale collection wagons and stacked at the field edge for further handling and transfer to the desired locations.
Biomass storage, handling, and transportation
Harvested feedstocks are usually stored to maintain a supply for year-round operation of biorefineries. The feedstock can be stored at the field edge, distributed feedstock collection facility, or biorefinery plant and protected using a tarp cover, plastic wrap, or permanent structure. Among these a tarp cover is currently preferred for industrial-scale feedstock storage due to its ease of use, lower cost, and relatively lower dry matter losses (5–7%) compared to the other storage methods.
Biomass is handled multiple times during its transportation from the field to biorefinery. The biomass bales from the field are collected and moved to the field edge, loaded to trucks for transportation, unloaded and stacked at storage sites, and then transported to the biorefinery. A squeeze loader can handle multiple bales at a time without piercing them and could improve industrial-scale handling, especially for loading/unloading of bales to/from the semi-trucks. Currently, a truck with an 8-ft. wide by 48-ft. or 53-ft. long flatbed trailer can transport 36–39 bales in a single trip and is the most feasible option for bale transportation. Depending on the availability of transportation modes, railroads, pipeline, barges, or ships could be used in the future when the demand for biomass increases substantially, and transportation distances increase. Biomass feedstocks may need to be further densified to form briquettes, cubes, or pellets if it needs to be stored for extended periods and/or transported long distances.
Logistical limitations of lignocellulosic biomass
Logistical limitations of agricultural residues and energy crops as feedstocks must be overcome for them to be viable for biorefinery use. Major logistical limitations of these feedstocks are their low initial bulk density, low energy density, high moisture content, and irregular shapes, which increases the complexity and cost of biomass supply logistics. Biomass harvesting and handling logistics often comprise a significant portion of the costs and energy inputs for the overall biofuels production. Despite these challenges, its widespread availability, low cost, and negligible effect on food production make these biomass ideal candidates for the next generation of biofuels, and thus efforts should be directed toward alleviating these limitations.
Feedstock supply and logistics costs for agricultural residues and energy crops
The costs for feedstock supply and logistics from the production source (usually field) to the biorefinery gate varies widely in the literature. Some studies also consider the grower’s payment and nutrient replacement cost of between $9 and $24/ton. The reported total delivered cost, including the grower’s payment and nutrient replacement costs, varies between $48 and $111/ton of delivered biomass for rectangular bales [1–5]. Further, study by Sokhansanj et al.  showed that the total delivered cost of corn stover in square bale format ($66/ton) was lower than corn stover delivered in chopped format ($78/ton) and pellet format ($76/ton). For switchgrass, the production costs are also included in the delivered costs as it is produced to be used solely as an energy crop. The delivered cost of switchgrass in rectangular bale format varies between $71 and $126/ton [5–7].
Biomass feedstock supply logistics involve different processes from harvest to storage and delivery to the biorefinery. Agricultural residues and energy crops are usually harvested and transported in the form of bales. Biomass feedstock source, availability, and proximity to the biorefinery play a vital role in the logistical complexity and economic feasibility of biomass for lignocellulosic biorefineries. One of the critical factors for the success of these biorefineries is the ability to handle and deliver enough feedstocks in a technically, environmentally and economically sustainable manner. Lignocellulosic biorefineries could be located anywhere across the United States with concentrated availability of biomass feedstock sources. This could reduce the distance for biomass feedstock or biofuel transportation, which could potentially reduce the overall cost of lignocellulosic biofuels and bioproducts production. Feasible and sustainable biorefinery operations in the future will depend on proper planning and will involve optimization of feedstocks and transport modes and routes, as well as the selection of processing, handling, and transportation equipment and facilities according to the feedstock type, biorefinery plant size, and conversion technology.
Authors thank Dr. Erdal Ozkan, Professor; Dr. Harold Keener, Professor Emeritus and Associate Chair; and Mary Wicks, Program Coordinator; Department of Food, Agricultural and Biological Engineering, The Ohio State University, for technical and editorial review of this fact sheet.
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