September 21, 2008
“Ag Products to Energy
By Womac, Alvin R
Biofuels, feedstocks, and processing technology What roles do ASABE members play in bioenergy? The breadth of the answer may surprise you: sustainable production, harvest, and storage; feedstock characterization; processing; conversion; and testing and standards development. It spans the entire bioenergy production value chain.Biofuels and bioproducts begin in fields, forests, or lakes as biology that converts the sun's energy into a harvestable "feedstock." ASABE members' expertise in effective production and mechanized harvest systems remove the high labor requirements to produce and harvest feedstock - from grain and grasses to growth of many terrestrial and aquatic feedstock. Current U.S. ethanol for gasoline is mostly derived from grain (corn) that is produced, harvested, and supplied with systems perfected by years of solid engineering often attributed to members.
The same is true for biodiesel made from oilseed crops such as soybeans. Corn and soybean supplies have some degree of established feedstock specifications to benefit seller and buyer. This minimizes unpleasant surprises during transactions. Renewed emphases are on cellulose-based feedstock for energy. Challenges for sustainable production, harvest, and storage lie ahead - especially to economically supply crop residues, dedicated energy crops, and wood residues. The large number of potential sources and crops, estimated at 50 to 60 in the southeast United States alone, are often dissimilar enough to require different mechanized harvest systems. Opportunities abound.
Feedstock has various physical and chemical properties depending on the selected plant anatomy, growing conditions, and time of harvest. The quality of feedstock is important. Members have determined how various handling and storage systems affect the most efficient use of feedstock chemicals. For example, members determined that corn stover fractionation and storage method affected glucose production during enzymatic hydrolysis, which ultimately affects ethanol (EtOH) production. Corn cobs, leaves, and husks produce more than 300 percent more glucose than corn stalks. This raises engineering optimization questions. Should single-pass harvesters harvest grain and the highest EtOH yield stover components? Or is it more efficient to collect all stover residue for EtOH production, taking into account soil sustainability, EtOH production efficiency, and transport cost of stalks that yield less EtOH? Similar questions are raised with respect to other feed stock; is it more efficient to "cherry pick" the components that are most chemically-targeted for conversion?
Feedstock processing includes a wide range of "processes" that improve and prepare the feedstock for conversion to a biofuel or bioproduct. Categories of processes range from physical to chemical to biological. Processing is a value-added step that converts raw biomass to a safe highquality feedstock with tighter-toleranced specifications. In collaboration with other ASABE members, my research involves improving size reduction and separation of cellulosic biomass feed stock. Energy-instrumented knife, hammer, and refiner disk mills are used while investigating chopping and grinding of switch grass, corn stover, and wheat straw under a range of moisture contents. Particle-size distributions were determined using ASABE-specified sieve designs and corrected for actual particle dimensions using image analysis. Resulting particle spectra may impact the degree of chemical degradation needed for conversion reactions. If size reduction takes place nearer the production field, this has implications of transport savings and the handling of a bulk product with tighter specifications compared to bales.
Biomass conversion to biofuel and bioproducts can take several pathways: chemical, biological, thermochemical, or combinations of these. Members use their broad knowledge of basic biology and engineering to ensure that the most efficient pathway is used for the particular biological characteristics of the selected feedstock. Driving factors include finding higher conversion efficiencies, newer conversion pathways, greater robustness of organisms, wider range of operating conditions, and greater economic viability. Innovative minds of ASABE members scour to find the right conversion pathway to ensure the success of bioenergy.
Testing of the biofuel or bioproduct for compatibility with equipment and the environment is another member strength. Performance testing enables accurate information for adjusting any step in the biofuel development process. Problems have been headed off before and during biofuel or bioproduct production bound for end- users. Many members apply expertise to develop published, consensus- based ASABE standards for the public good.
There are behind-the-scenes efforts to build on the achievements in biofuels, feed stock, and processing technology, and ASABE can take a greater leadership role within the growing bioenergy industry. Organization and packaging of current ASABE member efforts will help raise the visibility to potential members, conferences, and the associated bioenergy industry.
ASABE member Alvin R. Womac is chair of FPE-709 (Biomass Energy and Industrial Products) and professor of biosystems engineering at the University of Tennessee, Knoxville, USA; email@example.com.
Copyright American Society of Agricultural Engineers Apr 2008
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