Forest residues, including unmerchantable trees, small diameter trees, tops, limbs and chunks produced from mechanical thinning and conventional sawtimber harvesting operations provide an opportunity to produce bioenergy and bio-based forest products. New technologies are emerging that are capable of converting previously wasted or underutilized forest residues into high quality and sustainable bioenergy and useful bio-based products. These new products can help offset the costs of forest restoration and fire hazard treatments while facilitating follow-up forest management activities. Further, the use of these bio-based forest products can improve air quality, reduce greenhouse gas emissions, sequester carbon, amend soil, and create employment in rural forestry-dependent communities, while reducing the nation’s reliance on imported fossil fuels.
The central opportunity underlying these goals is to more fully utilize forest residues. Federal land managers have identified 28 million acres of National Forest lands in the western U.S. that are characterized as having unnatural or excessive amounts of woody vegetation, leaving these areas prone to catastrophic wildfires and susceptible to insect attack and degradation. In addition, forest residues generated during commercial logging operations are often left in the forest or are collected into piles and burned. Approximately 68 million dry tons of forest residues produced during traditional logging and landclearing operations go uncollected in the U.S. every year. To a large degree, the underutilization of forest residues can be attributed to the high cost of collecting and transporting these residues to end user markets (approximately more than $50/bone dry ton), and the low market prices paid for delivered forest residues (approximately $2540/bone dry ton). Several studies have developed innovative forest biomass operations that effectively improve access to harvesting sites and economic efficiency. However, the inherent inefficiency of transporting low density and high moisture content biomass feedstock to market still remains a fundamental economic barrier to its increased utilization. This project team proposes an innovative approach to this problem by using conversion technologies that add value to residues in the field while significantly reducing transportation costs. In addition, improvements in logistics and the development of new tools for the collection and transportation of forest residues will be examined.
Unmerchantable materials produced from fuel reduction thinning and restoration operations could be comminuted (i.e., chipped or ground up) for the production of bioenergy and biobased products. Integration of biomass conversion technologies with in-forest biomass comminution operations can provide an alternative to the expensive and inefficient long distance transport of high moisture, low energy density forest residues. While this concept has been discussed to a large degree within the literature, it has not been achieved because biomass conversion technologies have not been successfully deployed and the low cost production of high quality feedstocks from low quality forest residues remains a significant challenge. Biomass conversion technologies can effectively convert comminuted forest residues into energy fuels with desired characteristics (high energy density, low or no moisture content, and high market value) within the forest, resulting in a significant increase in transportation efficiency. For example, a mobile pyrolysis (torrefaction) technology has been developed by Norris Thermal Technologies, with research support from the Schatz Energy Research Center (SERC) at Humboldt State University, that produces wood pellets whose gravimetric and volumetric energy densities are increased by between 2.5 and 6.5 times, relative to green biomass.
In order to integrate in-woods biomass conversion technologies into practice, forest residues need to be cost-effectively collected and comminuted into high quality feedstock that meets requirements for use by the technologies. See our Production Pathways diagram for more information. The following six items have been identified by the research team as major issues that need to be addressed to successfully integrate biomass conversion technologies into forest management activities:
- Sorting and arranging forest residues
- Densification of loose forest residues
- Production of high quality feedstocks through comminution
- Controlling feedstock size with new screening technologies
- Centralized biomass feedstock operations supporting biomass conversion technologies
- Integration of biomass conversion technologies with landscape level planning and transportation logistics