Forestry Residue, Logging and Mill Residue
Forest residues include logging residues produced from harvest operations, fuel wood extracted from forestlands, and primary and secondary wood processing mill residues. There are a number of important factors in the use of forest residues for bioethanol production, such as the costs of transportation, and limited accessibility to forest areas largely increase operation costs of logging/collection activities. Another factor is a potential reduction of recoverability in forest areas and environmental considerations. Techno-economics of ethanol production from forest residues are discussed in a number of recent review articles; according to most of the analysis the impact of using forest residues for cellulosic ethanol production on deforestation is ambiguous [112-114].
The environmental impacts of the use of forest residues for cel — lulosic ethanol and fuel productions have been discussed in some recent publications as well. In 2011, one article by McKechine et al. discussed the potential of forest-based bioenergy to reduce greenhouse gas (GHG) emissions when displacing fossil-based energy sources [115]. Application of the method to case studies of wood pellet and ethanol production from forest biomass reveals a substantial reduction in forest carbon due to bioenergy production. For all cases, harvest-related forest carbon reductions and associated GHG emissions initially exceeded fossil fuel related emissions, temporarily increasing overall emissions. According to the analysis of McKechine et al., over the long term, electricity generation from pellets reduces overall emissions relative to coal, although forest carbon losses delay net GHG mitigation by 16-38 years, depending on biomass source (harvest residues/standing trees). Furthermore, ethanol produced from standing trees may increase overall emissions throughout 100 years of continuous production; ethanol from residues achieves reductions after a 74 year delay. Analysis revealed that forest carbon more significantly affects bioenergy emissions when biomass is sourced from standing trees compared to residues, and forest carbon dynamics are significant. Although study results are not generalizable to all forests, they suggest that an integrated life-cycle analysis/forest carbon approach be undertaken for bioenergy studies [115].
In a recent energy policy analysis, Kocoloski et al. estimated the costs, benefits, and potential for cellulosic ethanol production from forest thinnings in the United States [113]. According to their analysis, since 2004, wildfires have been responsible for the destruction of 3.5 million hectares of forestland per year in the United States. Fuel reduction activities such as prescribed fires, cutting and burning in situ, and biomass removal (thinning) have been shown to reduce wildfire severity, but with mounting costs of fighting wildfires and a tightening budget, public agencies such as the USDA Forest Service may find it difficult to continue funding these preventative treatments. They argued that, by using thinned biomass as a cellulosic ethanol feedstock, these agencies may be able to generate funds for these treatments. In this study Kocoloski et al. estimated that 27 to 34 Tg of biomass could be removed from overcrowded forests per year at collection costs of $55 to $110 per dry Mg. Given a mature cellulosic ethanol industry, ethanol produced from these thinnings could generate revenue at gasoline prices of $0.5 to $0.8 per liter. Further, they proposed that by using thinned biomass as an ethanol feedstock, it may be possible to generate significant funds for socially beneficial thinning treatments [113].
A number of common pretreatment techniques have been tested on forest residues as well; however, it is difficult to compare the results of these studies as the composition of forest residues varies from one study to the other. Some of the techniques used include dilute acid [116], sulfite [117], ionic liquids [118], steam explosion [114], and ammonia fiber explosion [114].