Frequently Asked Questions

1. Does the analysis account for carbon sequestration from a parcel that has been replanted after harvesting timber?

Although carbon sequestration occurs after a parcel of land is replanted, this analysis was designed to account only for emissions that are a consequence of use or disposal of the woody biomass residues, not the emissions and carbon sinks that are a consequence of land use decisions. Therefore, this analysis accounts for emissions that result from gathering, processing, transporting, and end use of woody biomass residues. The reason why this scope of analysis was chosen is that, whereas carbon sequestration and other consequences resulting from land use decisions are a primary area of research, there is not a comprehensive assessment of the emissions implications of alternatives for woody biomass residues. This analysis was designed specifically to fill this information gap.

In summary, this study quantifies and compares “Post Harvest to Grave” air emissions from alternatives for woody biomass residues and is not intended to account for air emissions over the entire woody biomass life cycle nor evaluate the sustainability of wood bioenergy or the carbon sequestration implications of different forest land management practices. Emissions and/or carbon sequestration associated with forest management practices (e.g. harvesting, planting, and growth) are assumed to be identical for a given source of residues being compared and are by design not included in this analysis.

2. What is a “Post Harvest to Grave” analysis?

This means that the analysis starts after the point where timber is harvested and woody biomass residues are generated; it begins at the point where the residue is collected in the forest and ends with its ultimate use or disposal. Based on these starting and ending points, it is referred to as a “Post Harvest to Grave” analysis of fates for woody biomass residue utilization. By design, our analysis does not encompass the woody biomass life cycle “upstream” of the point where the woody biomass residues are collected, and therefore does not include the emissions and/ or carbon sequestration from forest growth, forest practices or forest re-growth.

Because it is a “Post Harvest to Grave” analysis, it is important that results from either the final report or accompanying emissions calculator tool not be used out of the context or beyond the explicit limits of the study boundaries. Results should only be used for comparing alternative fates for existing streams of woody biomass residues.  This analysis has no implication or bearing on forest management choices, which affect forest growth or forest harvest.

3. Is woody biomass a renewable fuel?

The majority of Washington’s timber lands are managed to sustain timber production and, therefore, can be considered a renewable resource. To get a more in-depth answer to this and other questions regarding woody biomass resources in Washington, please visit the Forest Biomass page of the Washington Department of Natural Resources website: http://www.dnr.wa.gov

4. What “Embedded Emissions” were considered in this analysis?

The analysis accounts for emissions associated with energy use for the gathering, processing, transport, use and disposal of the woody biomass residue. It also accounts for air emissions associated with the manufacture of equipment used to harvest, process and transport the woody biomass (e.g. loaders, grinders and transport vehicles).  Emissions and/or carbon sequestration associated with forest management practices (e.g. harvesting, planting, and growth) are assumed to be identical for a given source of residues being compared and are by design not included in this analysis. In summary, this study quantifies and compares “post-harvest to grave” air emissions from alternatives for woody biomass residues and is not intended to account for air emissions over the entire woody biomass life cycle nor evaluate the sustainability of wood bioenergy or the carbon sequestration implications of different forest land management practices.

5. What “Capital Emissions” were considered in this analysis?

The analysis accounts for air emissions associated with the manufacture of equipment used to harvest, process and transport the woody biomass (e.g. loaders, grinders and transport vehicles).

6. Can emissions results from this study be compared to emissions factors from fossil and other biomass fuels?

Results presented in this analysis are intended to focus on comparing the emissions implications of one alternative vs. another (e.g. “What is the difference in emissions if residues are burned on-site vs. being used in an industrial boiler?”). Interpretation of results should emphasize relative differences between alternatives considered. Comparison with other published emission factor estimates requires ensuring that an appropriate “Apples-to-Apples” comparison is being made. For the purposes of comparison designed in this study, emissions results focus after the point of harvest and do not distinguish between emissions from biomass vs. fossil fuel sources. In contrast, many studies providing a “cradle-to-grave” life-cycle assessment of biomass fuels or fossil fuels will include sources and sinks of emissions not included in this “post-harvest to grave” analysis. As well, many studies comparing biomass fuels to fossil fuel sources account for GHG emissions from biomass and fossil fuel sources differently.  In order to compare results from this study to others you must confirm that the same sources and sinks such as forest sequestration are being included, and emissions are being accounted for in the same way.

7. What combustion control technology was assumed for wood fired boilers?

Emissions factors for combustion of hog fuel in a boiler are based on EPA’s Compilation of Air Pollutant Emissions Factors (AP-42) and emissions reported from hog fuel boiler facilities in Washington State provided by ORCAA. The emissions factors reflect of particulate emissions using a mechanical separator such as a multicyclone followed by a fabric filter baghouse.

8. What were the assumptions for displaced electricity?

Avoided emissions attributable to displaced electricity generation are based on the marginal electric emissions for the PNW as reported by the Northwest Power and Conservation Council and a combined-cycle NG turbine.