A Choice Between Raisins and Pickles

By Kurt Thelen | October 14, 2010
Year-round supplies of quality cellulosic feedstocks will be required for future cellulosic ethanol refineries. With the exception of nonwoody biomass, the seasonal growth pattern of biomass makes proper storage a significant logistical component. Since biomass is a perishable commodity, harvest-storage strategies must be designed to minimize respiration loss and the formation of molds that inhibit bioconversion.

Fortunately, we have two model systems in the ruminant livestock industry from which to base bioenergy feedstock storage systems: 1) dry the feedstock down, usually in the field, to a level that will not support microbial activity thereby reducing respiratory loss and the formation of molds; and, 2) ensile the feedstock by harvesting wet biomass and packing to facilitate anaerobic lactic and acetic acid formation and preserve from further decomposition. In the farming vernacular, the two strategies are known respectively as 1) baling and 2) chopping. Both systems are readily deployable—the equipment is already out there on farms and growers know how to use it.

Baling has the advantage of relatively lower equipment costs, less water in the final stored product, and generally less respiratory storage loss. Baling requires considerably more harvest time, however, including a day or more of field drying, more trips across the field to mow, rake and bale, higher losses due to machine handling of fragile dry biomass, and greater risk from inclement weather.

Conversely, chopping systems require much less time for harvest and because of lower in-field losses, harvest more energy per acre relative to baling. Equipment costs are higher, however, and although dry matter density of tightly packed silage is equal to that of baled hay, much of the weight is from water resulting in heavier loads to transport. Early microbial biomass-to-ethanol conversions were inhibited by lactic acid but recently developed strains appear to have overcome this initial concern.

Field drying once was the most common system, but, as technological advancements paved the way for larger farms chopping-ensiling systems began to emerge as the preferred method east of the Mississippi River. Time savings from field to storage, less reliance on cooperative weather, and the ability to reduce field biomass loss were the main drivers for the transition to chopping systems in the more humid eastern U.S.

Managing moisture levels is key regardless the method. Biomass must be less than 20 percent (preferably less than 15 percent) moisture for bales and at least 50 percent moisture to ensure good packing and anaerobic fermentation conditions for chopping systems This results in a moisture level danger zone for biomass harvest of 15 to 50 percent moisture—too wet to preserve in a baled form and too dry to preserve in an ensiled form.

Switchgrass demonstrates the optimization challenge. The ideal time to harvest switchgrass and other warm season perennial grass crops from an ecological, economic and agronomic perspective is approximately two weeks after the first killing frost (28 degrees Fahrenheit)—generally late October in Michigan. This timing allows translocation of nutrients to the roots to ensure winter survival and facilitates leaching of remaining cytoplasm nutrients from the plant to the soil following frost-induced rupturing of cell wall integrity. Michigan experience indicates switchgrass is still about 30 percent moisture two weeks after the first frost and little additional field drying occurs before the first harvest-inhibiting snow accumulation. If switchgrass is left standing in the field until spring, 15 percent moisture can be obtained, but greater than 50 percent biomass loss can be expected from snow-induced lodging—an economically unsustainable loss for growers. A compromise system we are investigating involves direct chopping warm season perennial grasses several weeks prior to the first frost when whole plant moisture levels in the 50 percent range are conducive to excellent packing and ensiling conditions and much of the translocation of nutrients to the perennial root system has occurred. We are currently researching the impact on winter survivability.

Geography will determine which strategy emerges. Field drying and baling will be more common in the arid West while chopping-ensiling systems will likely prevail in the more humid East. The final conversion platform will also determine the harvest storage method used. The extra water present in ensiled systems will likely preclude use in thermal conversion platforms, particularly direct biomass-to-heat conversion systems, but the water may actually be beneficial for biological conversion to liquid transportation fuels.

Kurt Thelen is a professor in the Department of Crop and Soil Sciences at Michigan State University. Reach him at (517) 355-0271 or thelenk3@msu.edu.