Selective Harvest of Higher Value Wheat Straw Components

Each year, millions of tons of agricultural residues such as wheat straw are produced worldwide. In this paper, we describe ongoing efforts to solve technological, infrastructural, and economic challenges to using this straw for bioenergy and bioproducts. Among these challenges, silica in straw forms a low-melting point eutectic with potassium, causing excessive slagging deposits in boilers. The presence of chlorine causes corrosion beneath the slag deposits on boiler tubes. Silica fouls kilns and fines slow paper machines, limiting the usefulness of straw in linerboard production. Poor resin bonding to the waxy outer cuticle of the straw, poor resin penetration, and high resin consumption due to fines limit the use of straw for straw-thermoplastic composites and for straw particleboard. Poor cellulase penetration limits the use of straw for production of fuels (ethanol) and chemicals. Straw consists principally of stems, leaves, sheaths, nodes, awns, and chaff. Not all of these parts of straw residue are equally valuable. Leaves and sheaths are higher in silica, while chaff, leaves and nodes comprise the source of most of the fines. The predominantly fibrous straw stem, high in cellulose, is of greater value for bioenergy, biofuels, and bioproducts. Our approach to reducing silica content is to selectively harvest the straw stems using an in-field physical separation, leaving the remaining components in the field to build soil organic matter and contribute soil nutrients. To address resin issues and cellulase penetration, we are developing small, distributed windrow systems employing white rot fungi to upgrade the straw.