Shahab Sokhansanj, Erin Webb 2016-10-25 00:28:09
In the 1970s, the U.S. and other countries began commercial production of ethanol to supplement transportation fuels, especially gasoline. The production of ethanol from corn grain was slow until the early 2000s, when petroleum prices became volatile and the U.S. relied heavily on imports. To reduce this reliance and boost domestic production, federal and state governments legislated renewable fuel standards, such as the RFS2 established by the Energy Independence and Security Act of 2007. This legislation includes a cap on ethanol production from corn grain to minimize the impact on feed and food markets. The graph below shows transportation fuel production in the U.S. based on the RFS2. The mandated 15 billion gallon plateau for corn ethanol was achieved in 2014. The remaining growth in biofuel production—more than 20 billion gallons by 2022—will be sourced from non-food crops and residues. Those 20 billion gallons of biofuel will require a minimum of 280 million dry tons of feedstock, assuming a conversion rate of 75 gallons per dry ton. This implies that an additional 40 million tons of biomass must be produced each year to reach the target of 280 million tons in 2022. In addition to biomass for liquid biofuels, there has been fast growth in wood pellet production in the U.S., increasing from 1 million tons per year a decade ago to about 8 million tons per year in 2015. This growth is due to the demand for wood pellets to reduce the use of coal for power generation in Europe. The growing demand for pellets in Europe may push U.S. production up to 15 million dry tons per year by 2022. At the same time, domestic growth in pellet demand for power production may require another 9 million tons per year of lignocellulosic biomass when the EPA’s mandated Clean Power Plan is put in place in 2022. Using a factor of 1.2 dry tons of biomass to produce one dry ton of pellets, an additional 24 million dry tons of biomass will be needed for pellet production in 2022. Harvesting, storing, and transporting these huge quantities of biomass will create opportunities for equipment manufacturers, construction contractors, the rural workforce, and a whole range of services to support this emerging biofuel economy. Our analysis estimates that the annual purchase of new and replacement equipment for harvest, handling, and transport of biomass could exceed $4 billion annually. This does not include the economic and social activities that would result from the demand for new processing and handling equipment. However, industrial-scale biomass harvest and supply systems face challenges beyond those of traditional agricultural operations and require development of new technologies to improve efficiency, reduce environmental impacts, and maintain feedstock quality. Biomass ash content The biomass ash content—whether biogenic ash that is part of the plant matter or environmental ash that is picked up from the soil during harvest—can have significant impacts on biomass conversion. Excluding bark, the biogenic ash in woody biomass is often less than 1% but may range from 2% to 10% in herbaceous species. Picking up soil particles during biomass collection adds to the overall ash content. With soil contamination comes undesirable levels of iron and manganese, which can catalyze reactions and reduce process efficiency. Some soils neutralize the acids used in pretreatment. This can reduce the efficiency of the conversion process or increase the production cost because additional acid must be added to achieve the pH necessary for optimum processing. High ash content has been shown to reduce conversion of cellulose to sugars by 6% to 10%. The components of ash also affect machinery life and maintenance costs. Early experiences with logging equipment and pellet mills showed that excessive silica abrades steel surfaces. Sensing instruments, along with design and operational strategies, could be developed to avoid soil particles and/or shake excess soil from the plant matter during harvest. Post-harvest fractionation and washing strategies to reduce ash are usually not economically feasible. Soil compaction Another challenge facing biomass harvest operations, especially in residue collection, is the potential for soil compaction. Frequent travel in the field tends to press the soil down, and dense soil is less productive. Collecting crop residue currently means bringing additional equipment into the field. For example, after grain harvest, additional vehicles are needed to bale, transport, and store the straw at the field edge. Soil compaction is exacerbated by the fact that the machines used for handling biomass are often heavier than conventional agricultural equipment. Likewise, forest equipment that gathers logging residue may lead to excessive forest floor compaction. These extra operations are often at the end of the season, when the soil may have become soft due to rain and low evaporation. Machines can be designed to spread their ground force over a larger track area. In addition to developing lighter equipment, soil compaction can be reduced by combining operations to avoid repeated travel in the field. For example, grain and stover harvesting can be performed simultaneously without using an extra tractor to pull the baler. An added benefit of single- pass harvesting is less entrainment of soil particles in the biomass because the cut stover does not touch the ground. Densification Increasing the bulk density of biomass feedstocks significantly reduces the costs of transport and storage. Denser bales lower the cost of on-farm transport and stacking. Briquetting, cubing, and pelletizing reduce the costs of longhaul transport and long-term storage. However, operations that increase density are power intensive and can be expensive. The power input required to make biomass denser increases exponentially with the increase in bulk density. Recent research shows that the density of current bales can be economically increased by 15%—beyond that, the power demand becomes excessive. The densification parameters, such as moisture content and particle size, are important factors. The lignin content in woody biomass provides natural binding to form durable pellets. In contrast, the low lignin in herbaceous biomass requires the addition of external binders to form pellets. Lignocellulose content The lignocellulosic portions of agricultural and forest materials are tough and more difficult to process than softer tissues, such as fruit pulp, green forages, and grains. Many processing machines, such as choppers, grinders, and densification equipment, exhibit early wear when grinding and pressing lignocellulosic materials. When working with dry residue that is contaminated with soil, this equipment can require more frequent service and even replacement. Future equipment should be able to handle abrasive lignocellulosic materials. Up to 80% of the dry matter in a mature plant consists of highly lignified cell wall materials. Corn stover, for example, has dry, thick, hard stalks with high resistance to shear and bending. Energy crops like switchgrass, miscanthus, and energy sorghum are also tough to cut, dry, and bale. Moisture content Moisture is one of the most critical factors in handling agricultural and forestry materials. Researchers have established that the ultimate shear strength of herbaceous crops is inversely proportional to the moisture content and directly proportional to the dry matter density. Some technologies have been developed to harvest, process, and store high-moisture materials. Natural and artificial drying methods have been developed for food grains and other perishables. Wet storage methods, such as ensilage, have been adapted for animal housing. Unfortunately, our knowledge of handling high-moisture cellulosic materials is still limited. An early analysis of weather data indicated that almost 1/3 of the stover in the U.S. after grain harvest could have a moisture content greater than 40% (wet mass basis). While machinery systems are widely used for harvesting and handling forage crops, similar systems for collecting crop residues and dedicated energy crops are yet to be commercially developed. The functionality of the equipment used for collecting, handling, and processing of biomass is affected by the variability of the biomass, including its morphology, composition, and physical properties. To overcome these challenges, agricultural and biological engineers must understand the factors that govern the interactions between biological materials and machines—as well as the environmental factors of traditional agricultural operations. ASABE Fellow Shahab Sokhansanj, P.Eng., Distinguished R&D Staff, and ASABE member Erin Webb, P.E., Senior R&D Staff, Environmental Sciences Division, Oak Ridge National Laboratory, Tenn., USA, firstname.lastname@example.org and email@example.com.
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