Woody Biomass Substitution for Thermal Energy at Softwood Lumber Mills in the US Inland Northwest*

Using life-cycle inventory production data, the net global warming potential (GWP) of a typical inland Northwest softwood lumber mill was evaluated for a variety of fuel types used as boiler inputs and for electricity generation. Results focused on reductions in carbon emissions in terms of GWP relative to natural gas as the fossil alternative. Woody feedstocks included mill residues, forest residuals, and wood pellets. In all fuel-substitution scenarios, increasing the use of biomass for heat generation decreased GWP. Using woody biofuels for electricity production is somewhat less effective in lowering carbon emissions than when used for heat energy. Heat generation at the mill under the current practice of using about half self-generated mill residues and half natural gas resulted in a 35 percent reduction in GWP over 100 percent natural gas. The greatest reduction in GWP (66%) was from increased use of forest residuals for heat energy, eliminating the use of fossil fuels as a direct heating fuel at the mill. We summarize the results by documenting that greater use of woody biomass for heat energy will reduce carbon emissions over fossil-based fuels. Recent technological advances have provided numerous options for the conversion of biomass to energy. These technologies include electricity production, pellet production for residential and industrial heating, woody and agricultural residue to liquid fuels, and steam generation for industrial heating or manufacturing operations. The scientific community has conflicting opinions on the use of biomass for energy, however, both for economic reasons and, more commonly, because of the environment impacts. The challenge is to use wood resources sustainably while improving our economy, yet without adversely affecting our environment. Increasing the use of wood waste as an energy fuel can reduce our need for imported fossil fuels, resulting in many benefits to the economy while at the same time reducing net carbon emissions. Unfortunately, the balancing acts between economics and environmental improvements have been problematic. On one hand, federal agencies are setting greenhouse gas (GHG) reduction standards for acceptable substitutes for fossil fuels. The US Environmental Protection Agency (US EPA) under the 2007 Energy Independence and Security Act (EISA) has set the threshold for cellulosic fuels at a minimum of 60 percent reduction in fossil emissions (EISA 2007, Sissine 2007). However, many critics are claiming that the use of woody biomass for energy actually releases more carbon into the atmosphere because of management, harvesting, and conversion (Schulze et al. 2012). Some argue that biofuels from woody resources are not carbon neutral because of decreases in the forest stand inventory, which reduces carbon sinks, together with the use of fossil fuels for stand management and harvesting, which emit carbon back to the atmosphere (Heiken 2007, Schulze et al. 2012). Life-cycle assessment (LCA) is a holistic approach to quantify environmental impacts for every stage of production and use of a product, i.e., from cradle to grave. Comparing the environmental benefits of biofuels requires The authors are, respectively, Consultant, WoodLife Environmental Consultants, LLC, Corvallis, Oregon ([email protected] [corresponding author]); and Professor Emeritus, College of Environment, School of Environmental and Forest Sci., Univ. of Washington, Seattle ([email protected]). This paper was received for publication in February 2012. Article no. 12-00023. * This article is part of a series of nine articles addressing many of the environmental performance and life-cycle issues related to the use of wood as a feedstock for bioenergy. The research reported in these articles was coordinated by the Consortium for Research on Renewable Industrial Materials (CORRIM; http://www.corrim.org). All nine articles are published in this issue of the Forest Products Journal (Vol. 62, No. 4). Forest Products Society 2012. Forest Prod. J. 62(4):273–279. FOREST PRODUCTS JOURNAL Vol. 62, No. 4 273 measurements across the total life cycle, from forest/ biomass growth through combustion (Lippke et al. 2011). Depending on the fuel combusted, carbon is emitted to the atmosphere as ‘‘biogenic CO2’’ or ‘‘fossil CO2.’’ Carbon dioxide is also sequestered, absorbed from the atmosphere by living trees during photosynthesis, and extended to carbon stored in biomass. When biogenic CO2 is released during combustion, it is considered equal to the amount of CO2 absorbed during tree growth. Under sustainable forest management, the harvest does not exceed the growth in the forest. Therefore, carbon emissions from the combustion of woody biomass are considered carbon neutral (US EPA 2006, Beauchemin and Tampier 2008, Fernholz et al. 2009). When biofuels replace fossil fuel, the net impact over the long term is a sustainable reduction in CO2 levels in the atmosphere, some well above the 60 percent threshold reduction in carbon emissions set by the EPA (EISA 2007, Sissine 2007). With the increasing interest in biofuels, it is important to understand the relative impacts of different biofuel uses on carbon. With softwood lumber manufacturers in the US Inland Northwest (INW) as an example (Puettmann et al. 2010b), the objective of this study was to evaluate the different impacts on net carbon emissions from the use of mill residues, wood pellets, and forest residuals as substitutes for natural gas for either heat production for drying or the generation of electricity. Cogeneration for the woody feedstocks was not part of this assessment because the mills in the INW region did not have these operations. We wanted to show the carbon impacts of using waste residues for energy in the mills to offset fossil fuels and to address the issue of collection options for forest residuals for direct heat energy or for electricity. The process models assessed used mill generated wood waste for either direct heat generation or electricity, not both. In the western United States, wood is an important part of the economy. Western softwood lumber production uses self-generated mill residues for about half of the energy required for drying, with the remainder from natural gas (Milota et al. 2005, Puettmann et al. 2010a). Previous LCA studies have shown that wood drying is the dominant use of energy in the production of lumber, regardless of the geographical region in which the lumber is produced (Milota et al. 2005, Bergmann and Bowe 2010, Puettmann et al. 2010b). Cradle-to-gate impact assessments for softwood lumber consistently show that manufacturing energy is the dominant energy life-cycle stage, consuming 89 to 92 percent of the total energy (Puettmann et al. 2010a). With nearly half of the fuel coming from natural gas, significant carbon emission reductions are possible by converting softwood lumber mills to all woody biomass energy, at least for drying. This article will focus on the impacts on net global warming potential (GWP) of softwood lumber production from cradle to gate, using fuel substitution as boiler inputs for steam and electricity generation alternatives. GWP is an indicator, expressed as a factor of CO2, that reflects the relative effect of a GHG in terms of climate change over a fixed time period, commonly 20, 100, or 500 years. For example, the 20-year GWP with substitution for electricity at the mill of methane is 56, which means if the same weights of methane and carbon dioxide were introduced into the atmosphere, methane will trap 56 times more heat than will the carbon dioxide over the next 20 years. Values were converted to kilograms of carbon dioxide equivalents (kg CO2 eq). GWP compares the amount of heat trapped by a certain mass of the gas in question with the amount of heat trapped by a similar mass of carbon dioxide. Questions remain on whether it might be better to raise the share of biofuel in solid wood processing facilities to displace the emissions from natural gas or possibly even lower the biomass share. For example, collecting more forest residuals as a part of the harvest for boiler inputs has the potential to provide as much as four times the energy needed for processing energy, resulting in better than selfsufficiency in solid wood production mills (Lippke et al. 2011). Although the cost of collecting forest residuals has been the primary deterrent to their use as biofuel feedstocks, this may not be an obstacle if the value of carbon is increased through incentives to reduce carbon emissions or if the cost of fossil fuels or their emissions increases. When forest residuals are densified at the landing, hauling costs are reduced, potentially making residuals more competitive in serving energy needs, especially where hauling distances have been the primary obstacle to their use (Johnson et al. 2012). Common forest practices in the West for forest residuals have been to collect the debris into ‘‘slash piles’’ and burn them on site or leave the piles to decompose. Lee et al. (2010) reported that in the Pacific Northwest, one-third of a ton of woody biomass residuals is generated for every ton of merchantable logs harvested; however, there is substantial variation in the amount, depending on the source of the biomass. Oneil and Lippke (2009) found that residuals for eastern Washington were almost equal to the volume of merchantable logs, but only about 24 percent of the total standing volume was likely to be recoverable. In either case, if made accessible and affordable, this material could provide a significant source of energy. Alternatively, it might be better, both economically and environmentally, to increase the use of natural gas in lumber mills and use the forest residuals for electricity production or to bypass the mill completely and convert the residuals to liquid fuels. The conversion of biomass to electricity could offset the wood processing use of fossil fuels by reducing the fossil emissions generated for electricity production. Because of the variety of fuel sources used for electricity production, air emissions vary substantially across the country and are heavily influenced by the availability of alternative energy sources, such as hydropower. These consequences should be noted when considering different fuels for electricity production. The development of largescale biomass-to-electricity utilities could be limited based on the cost of collection, sustainable feedstock resources, and environmental impacts. Small electric utilities or on-site electricity generation, such as cogeneration at wood product mills, could provide the production of electricity from non– fossil fuel sources, resulting in reductions in GHG emissions. Although cogeneration is not prevalent in INW softwood lumber mills, several US sawmills do generate some of their own electricity (Bergman and Bowe 2010), which does offset the use of electricity from the grid. Bergman and Bowe reported that, on average, 18 percent of the total wood waste used in the mills went to cogeneration. In the northeastern United States, where coal is the primary fuel for electricity generation (63% share), using mill residues for electricity generation could significantly reduce carbon emissions. 274 PUETTMANN AND LIPPKE