Life-Cycle Carbon from Waste Wood Used in District Heating and Other Alternatives

Using wood wastes provides an opportunity to avoid fossil carbon emissions from the combustion of natural gas or other fossil fuels. Using a life-cycle assessment, a new biomass boiler sourced by forest residuals, sawmill residuals, and clean demolition material (CDM) was compared with an existing natural gas boiler for supplying heat to a large-scale district heating system. Potential alternative uses of these feedstocks, such as recycled or reprocessed products, and landfill alternatives were also evaluated for their relative impact on carbon emissions. We found a reduction in emissions from natural gas of 0.62 unit of carbon for every unit of carbon in the wood combusted. Temporary losses of forest carbon after initiating the collection of forest residuals were minimal over a short interval. These losses were more than offset by the joint production of wood products displacing fossil emissions. Carbon mitigation in the Pacific Northwest was increased from 5.5 metric tons/ha/y from the production of forest products with no collection of forest residuals to 6.5 metric tons/ha/y after completing the first rotation, an 18 percent reduction in fossil emissions per hectare of forest. The potential to recycle CDM into wood products may ultimately raise the efficiency in avoiding carbon emissions by 40 to 60 percent, although available wood quality and logistics currently favor use of CDM as biofuel feedstock. In the absence of any ‘‘incentives’’ or value for carbon mitigation, feedstock collection costs relative to low-cost fossil fuel will substantially limit the use of waste woods for biofuel or recycling alternatives. A broad, although not universal, agreement exists that releases of greenhouse gases from fossil fuels are warming our planet, which could potentially have serious environmental and social consequences (Intergovernmental Panel on Climate Change [IPCC] 2007a). Combustion of woody biomass has been considered to be carbon neutral, because the carbon absorbed from the atmosphere by tree growth is simply returned to the atmosphere upon combustion or forest decomposition when sustainable forest management is practiced (IPCC 2007b, Beauchemin and Tampier 2008, Fernholz et al. 2009, Malmsheimer et al. 2011). Under sustainable management, the rate of wood removed from the forest is not allowed to be greater than the rate of net forest growth. This two-way flow of carbon from the atmosphere, when using woody biomass for fuels, displaces the one-way flow of fossil carbon from ancient, deep pools to the atmosphere. Some studies have focused on biogenic carbon emissions at the time of combustion, suggesting that carbon must first be absorbed during new tree growth before the bioenergy from that tree contributes to reducing emissions (Manomet Center for Conservation Sciences [MCCS] 2010, Biomass Energy and Research Center [BERC] 2012). As the demand for biofuels increases, other concerns have been raised about reductions in forest carbon as a consequence of excessive removals (Schulze et al. 2012). On the other hand, biomass resources have been reported to be largely underutilized for the production of fuels (Gan and Smith 2006, US Department of Energy 2011). In spite of some loss of forestland to urban development, forest inventory information shows growth greater than removal in all US regions and potential growth per acre substantially greater than current growth (US Department of Agriculture [USDA] 2011). Greater rates of wood accumulation through forest growth compared with rates of wood removal from harvest The authors are, respectively, Professor Emeritus, College of Environment, School of Environmental and Forest Sci., Univ. of Washington, Seattle ([email protected] [corresponding author]); and Woodlife Environmental Consultants, LLC, Corvallis, Oregon ([email protected]). Any opinions findings, conclusions, or recommendations expressed in this article are those of the authors and do not necessarily reflect the views of contributing entities. This paper was received for publication in August 2012. Article no. 12-00093. Forest Products Society 2013. Forest Prod. J. 63(1/2):12–23. doi:10.13073/FPJ-D-12-00093 12 LIPPKE AND PUETTMANN and all other activities (USDA 2011) suggest that opportunities exist to displace fossil-intensive products and fuels by using more wood, not just in the Pacific Coast region, the focus of the present article, but in other regions as well. In the Pacific Coast region, potential growth exceeds current growth by more that 60 percent, whereas current growth exceeds removal by more than 50 percent. A biomass assessment for Washington State (Jamison et al. 2012) estimated that a volume of forest residuals equivalent to 40 percent of the volume of merchantable logs could be considered potentially marketable with favorable prices. Pre-existing woody material (dead trees and downed material) varied widely but could still provide additional biomass volume for bioenergy use. The burning of wood residue in boilers for heat generation has historically been restricted to those industries in which biomass or ‘‘hogged fuel’’ is readily available as a by-product, primarily in the forest products industry. Converting boilers from fossil fuels to renewable biomass is relatively new. The Seattle Steam Company (SSC) has recently installed a biomass boiler to substantially reduce its use of natural gas (NG) and carbon emissions. The SSC is using a mix of purchased mill residuals, forest residuals, and clean demolition material (CDM) as biofuel feedstock. Concerns have been raised about the effectiveness of its use of waste wood relative to other potential uses, providing a useful case study to evaluate the impact of alternative uses for different waste wood biofuel feedstocks.