Do yield and quality of big bluestem and switchgrass feedstock decline over winter?

Authors: Johnson, Jane; GRESHAM, GAROLD - Idaho National Laboratory

Submitted to: BioEnergy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/13/2013
Publication Date: 3/4/2014
Publication URL: http://handle.nal.usda.gov/10113/58337
Citation: Johnson, J.M., Gresham, G.L. 2014. Do yield and quality of big bluestem and switchgrass feedstock decline over winter? BioEnergy Research. 7(1):68-77.

Interpretive Summary: Domestic and renewable energy sources can provide economic and environmental benefits. Perennial grasses like switchgrass and big bluestem may be used as a natural gas substitute in appropriately designed heating and cooling power plants. However, information is needed about their heating value and mineral concentration. Plant biomass normally contains some mineral elements that are needed for plant growth, but these same elements can cause problems when they are gasified or combusted. Potassium is an essential plant nutrient but is undesirable in bioenergy feedstock. Because elements like potassium may be leached (washed out) of plant material, overwinter feedstock quality may increase if harvest is delayed until spring. A study was conducted that compared quantity and quality of big bluestem and switchgrass changes if harvested in the fall or the following spring. In general, the quantity and heating value remained the same but undesirable elements like potassium decreased. One of the springs had very muddy conditions when the plant samples were collected causing the biomass to contain more soil than usual. The soil reduced the heating value and increased unwanted elements in the feedstock. Based on these results if fall harvest is compromised waiting until early spring is an option, but late snow and wet spring conditions can interfere with early spring harvest. This information will benefit engineers and facilities managers designing flexible fuel heat and cooling systems. It also provides harvest information for producers.

Technical Abstract: Switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerdardii Vitman) are potential bioenergy feedstocks for thermochemical platforms. Feedstock storage, fall harvest constraints, and environmental benefits provided by perennials are rationales for developing localized perennial feedstock. However, little information is available on mineral and thermochemical properties of native species or on the impact of delaying harvest until spring. Objectives of this study were to compare the feedstock quantity and quality of grasses harvested in the fall (Fall Harvest) or the following spring (Spring Harvest). It was hypothesized that biomass yield may decline, but the feedstock quality would increase due to leaching of minerals. Big bluestem and switchgrass yields differed among years, but not by harvest timing between fall and the subsequent spring as both grasses produced an average 5.7 Mg ha-1 yr-1. Thermal quality properties analysis for both grasses detected significant decreases in ultimate C, volatile C, fixed C, H, and O concentration and subsequently a decline in caloric content between Fall and Spring Harvest times. However, these observations were attributed primarily to one very muddy spring harvest, as the effect was not observed between Harvest time pairs in other years. Potassium and Cl concentration of both grasses declined between Fall and Spring. Feedstock quantity and quality were maintained or improved by K reduction most years making spring harvest a viable option if fall conditions delayed harvest. However, late snows and wet spring conditions can also hamper early field access; thus, we recommend fall harvest when feasible and storage space is available.