Sugar and starch—residue and waste

Feb 8, 2013
by WLJ

Interested in feedstock crops for biofuel energy production, but don’t know where to start? There are a wide variety of biomass crops that you can choose from, including those that provide energy from sugar and starch, and residues and waste.

Biomass feedstocks for energy production can result from plants grown directly for energy or from plant parts, residues, processing wastes, and materials from animal and human activities. The U.S. Department of Energy conducted an assessment of these feedstocks in 2005 and concluded over 1.3 billion dry tons annually—the equivalent of more than one-third the current demand for transportation fuel—of agricultural and forestry-related biomass could sustainably be collected and delivered to biorefineries.

Sugar and starch

Corn grain makes a good biofuel feedstock due to its starch content and its comparatively easy conversion to ethanol. Unlike other sugar-based biofuel feedstocks, corn starch must be specially cooked to convert the starch to the simple sugars necessary for biofuel.

Using the current corn-toethanol conversion of 2.8 gallons of ethanol from a bushel of corn, total U.S. corn production could result in approximately 37 billion gallons of ethanol, which would provide approximately 26 percent of our 137 billion gallon-per-year gasoline consumption. However, using all of our corn for ethanol is neither realistic nor necessary and has not been proposed.

Creating the 15 billion gallons required under the RFS-2 would call for 5.4 billion bushels, or about 41 percent of our 2009 corn crop. Although this percentage seems rather high, onethird of the weight and 100 percent of the nutritional content of corn entering an ethanol dry mill biorefinery is returned to the feed market as distillers grains.

These distillers grains can be used to replace corn in the diets of cattle, swine and poultry. When this replacement is calculated into the overall consumption figures, it lowers the number to 27 percent of our 2009 corn crop, or only 3.6 billion bushels of corn to produce the 15 billion required gallons.

Corn production has been blessed with nearly 100 years of infrastructure build-up and research. Farmers have great knowledge and experience in growing corn. This infrastructure and grower intelligence make corn a natural crop for expanded uses such as ethanol. Yet high production costs and high inputs make corn a very intensive crop.

Another sugar/starch biofuel crop is sweet sorgum. Although sweet sorghum is primarily grown to produce sorghum syrup, it can also be used as a feedstock for biofuel. In favorable environments, sweet sorghum varieties can grow 14 feet tall and produce 20 to 50 tons of biomass (fresh weight) per acre. It is more drought tolerant than corn and requires less nitrogen fertilizer. Dr. Morris Bitzer, professor emeritus, University of Kentucky, calculated that corn has an energy efficiency of 1:1.8, while sweet sorghum has an efficiency of 1:8.

Unlike corn, sugar in the sweet sorghum stalk is not the primary focus. Sorghum biomass is burned by fast pyrolysis to produce syngas, bio-oil, and charcoal. In this system, the synthetic gas and bio-oil are used for transportation fuel, and the charcoal is applied to fields to improve soil structure.

Other bioenergy crops may be less intensive and require fewer inputs. The cost versus profit per acre needs to be compared, as economics is a major driver in deciding which crop is best. Growing another crop on an acre where corn could be grown carries risks that may include a new cropping system; no harvest, transport, or storage infrastructure; or no commodity market to fall back on if the biofuel market fails.

Many of the sugar and starch crops that are candidates to produce biofuels are those currently being used for grains for feed and food or for sugars for sweeteners. Root and tuber starches are typically used as food staples throughout the world. These crops and their specific products can easily be converted, via traditional fermentation methods, to ethanol and similar alcohols for use in transportation and other fuels.

The specific challenges with most of these crops will be the competition for food and the need for genetic, production, and processing modifications to enhance energy production in a sustainable way.

Residues and wastes

When grain, sugar, starch, or oil crops are harvested for feed and food parts, significant biomass residues remain. Corn stover, corn cobs, wheat, and small grain straw are abundant crop residues that can be converted to renewable fuels. Other waste products such as manure or residential and retail (particularly food service) waste are also readily available.

Corn stover is one of the largest potential annual crop-based biofuel feedstocks for several key reasons: the quantity of feedstock in the U.S. is large, the feedstock is relatively uniform, the cost of production is relatively low, and large quantities are concentrated in some regions.

Another corn-based potential for biofuel feedstock is the cob. Corn cobs were once viewed as an important biofuel feedstock early in U.S. history to heat houses, farm buildings, and small businesses. However, with the advent of combines, which left the cob in the field, the use of corn cobs as a biofuel declined dramatically. Now, however, corn cobs are reemerging as a potential biofuel feedstock for direct combustion, gasification, and cellulosic ethanol and appear to have numerous advantages over many competing feedstocks.

Corn cobs are dense and relatively uniform, and they have a high heat value, with low N and S contents, and can be collected during corn grain harvest. Harvesting cobs has little potential impact on soil residue, soil carbon, or the nutrient requirements of subsequent crops.

Corn cobs appear to be a relatively sustainable, but relatively low-yielding, feedstock that can be used effectively in cellulosic ethanol, gasification, or co-firing applications. Two of the limiting issues are the need for a significant local resource base and the development of harvesting equipment which is compatible with current corn-harvesting systems.

A key challenge with removing crop residues is sustainably managing the crop production system. Typically, crop residues are returned to the soil to enhance soil organic matter content and for soil and water conservation purposes. A great deal of research is being conducted to determine the impacts of stover removal on sustainability of crop production, effects on ecosystem services, and diversity of insects, vertebrates, and microbes.

Other common sources of residue- and waste-feedstocks for biofuel come in the form of animal manure and daily-life waste products that can be found in homes or businesses.

In addition to their typical land application for nutrient value, anaerobic digestion of manure has been employed for years to convert these and other organic wastes to methane and related gases. Methane can in turn be used directly for combustion heat, fueling gas turbines, or further cleaned to supplement natural gas.

Food processing and municipal wastes including restaurant grease, leaves, grass clippings, and other yard wastes are found in substantial amounts in metropolitan areas and can be collected and converted to energy by a variety of processes.

There are many other potential biofuel feedstocks which might work for your current operation, but these are some of the most accessible to most cattlemen farmers. For more information, contact your local Extension office. — WLJ