Wood composite panels (WCP) are well known for their environmentally friendly attributes of being sustainable, renewable, biodegradable, and predominantly made from wood residues generated during lumber and plywood production. This paper focuses on the ability of WCPs to store carbon for long periods of time in nonstructural applications such as cabinets, furniture, and flooring. WCPs, include particleboard, medium density fiberboard (MDF), and hardboard–engineered wood siding and trim (EWST). These panels are anticipated to have an average service life of 25–30 years. In 2019, there was an estimated 291 million cubic meters (m3) of WCPs in use in North America that corresponds to a carbon pool of 354 million metric tons of carbon dioxide equivalents. This WCP carbon pool is enough to offset 24 years of cradle-to-gate cumulative carbon emissions (fossil and biogenic sources) emitted during production of these panels. In other words, producing and using WCPs stores carbon for long periods because the amount of carbon emitted during the production of the panels is far less than what the panels themselves are capable of storing over their lifetime of product use.
Demand for sustainable “green” products, desired for their favorable environmental performance, is increasing in the marketplace. Recent life cycle assessment (LCA) studies document the environmental performance of composite panels (Puettmann and Salazar 2018, 2019; Puettmann et al. 2016). Wood products (in use and landfills) store 9,786 million metric tons (mt) of carbon dioxide (as CO2e; Desai. and Camobreco 2020) representing two (2) times the amount of carbon stored in forests in United States (US) National Parks (Smith et al. 2019).
When round-shaped logs are processed into rectangular boards at sawmills, coproducts in the form of bark, hogged fuel, sawdust, shavings, and chips are generated. These coproducts may be used for heat energy onsite at the facilities, used in pulp and paper production, or in the manufacturing of wood composite panels (WCPs). WCPs represent 3.2 percent of the total harvested wood volume in the United States (Oswalt et al. 2019). In 2016, wood processing facilities in the United States generated 58 million mt (dry) of residues (Oswalt et al. 2019). These residues were primarily used for fuel (46%) and fiber products (38%) including WCPs.
The US softwood lumber industry produces an estimated 19 million mt per year of residue coproduct, which represents over half of the log mass entering sawmills (Milota and Puettmann 2017). Recent surveys indicate that softwood lumber producers use about 3.8 million dry mt per year of coproduct for onsite energy consumption (Milota and Puettmann 2017). This self-generated biofuel not only comes at a low environmental and economic cost to wood producers but is a direct substitution of fossil fuels with a direct reduction in carbon emissions. Increasing pressure to reduce greenhouse gas emissions, including the reduction of fossil fuel use, such as coal, have boosted interest in using wood residues from wood producing facilities to produce energy and transportation fuels (Kelley et al. 2019). The use of wood-based fuels reduces fossil-based carbon emissions, but the substitution may come with unintended consequences—such as higher carbon emission than would occur if the wood residues were used in long-term products such as wood composite panels.
The Composite Panel Association (CPA) represents North American (NA) manufacturers of composite wood and fiber panels. The NA composite panel industry stores more than 14.8 million metric tons of carbon (CO2e) through the manufacture of panels each year. This is equivalent to carbon emissions for over 3.2 million cars (US Environmental Protection Agency [EPA] 2021).
The purpose of this study, commissioned by CPA, was to determine the net carbon impact of WCPs by measuring the total carbon storage (embedded carbon) and embodied carbon for the WCP products produced over an estimated service life of 25–30 years (the anticipated service life of panel products). The results present carbon pools and flows for particleboard, medium density fiberboard (MDF), and hardboard–engineered wood siding and trim (EWST) manufacturing facilities located in North America (Mexico not included) for the production years 1996–2019.
Methods and Materials
The principal raw material used in manufacturing WCPs is residual fiber sourced from forests, sawmills, and other wood processing and agricultural operations that would otherwise be discarded or used for energy. Over 90 percent of all WCP feedstocks are sourced from sustainably managed forest where carbon removals do not exceed carbon stored in the wood. We used carbon flows that are based on WCP production shipment volumes and carbon pools based on the cumulative carbon stored during the life span of the WCP containing-product. With an estimated life span of 25–30 years, we used a conservative 24-year life span for this study.
Panel production = oven dry mass (wood component only),
mt = metric tons,
Estimated wood carbon content of panel = 50 percent,
Molecular weight of CO2 = 44/12.
X = carbon pool (CO2),
t = year,
The Underwriters Laboratories (UL) Product Category Rule (PCR) for North American Wood Products (UL 2018, 2019) specifies the Tool for Reduction and Assessment of Chemicals and Other Environmental Impacts (TRACI) as the default life cycle impact assessment (LCIA) method for quantifying the global warming potential (GWP; Bare 2012). The TRACI method does not account for removals or emissions of biogenic CO2. The reporting for GWP and biogenic carbon (CO2 BIO) are as follows:
CO2e TRACI includes greenhouse gases (GHG) emissions from the combustion of fossil resources, and GHG emissions other than CO2 from the combustion of biogenic resources.
CO2 BIO includes only carbon dioxide emissions emitted from the combustion of biomass (wood).
Biogenic carbon is the carbon derived from biomass. Trees absorb CO2 through the process of photosynthesis, incorporating it into plant tissue as carbon. This biogenic carbon is emitted as CO2 BIO and biogenic methane when trees or biomass are combusted or decay. During the production of wood products, biogenic carbon is emitted if wood biomass is combusted for energy during manufacturing or if forest residues are burned after a harvest. Biogenic carbon is stored in WCP as a negative emission when it enters the product life cycle. At the end of the wood product's life, biogenic carbon emissions can be released partially or all back into the atmosphere depending upon the end of life of treatment. Biogenic carbon emissions results were generated in SimaPro v. 9.1. (Pré Consultants 2020) and obtained from the life cycle inventory output.
Embodied carbon is the sum of the cradle-to-gate upfront carbon emissions, which includes resource extraction, material transportation, and product manufacturing and does not include the carbon stored in WCPs or changes in soil carbon pools. Embodied carbon is reported as GWP measured in carbon dioxide equivalents (CO2e). We present carbon emissions from both biogenic sources (CO2 BIO) and fossil sources (CO2e TRACI).
Both biogenic and fossil-based carbon emissions are released as a result of combustion of biomass or fossil-based fuels used during the cradle-to-gate production of WCPs. Wood composite panels producers use wood residue or waste for heat energy to operate onsite dryers and boilers. The onsite use of biomass for energy represents 23, 29, and 56 percent of the total energy consumed for particleboard, MDF, and hardboard–EWST manufacturing, respectively (Puettmann and Salazar 2018, 2019; Puettmann et al. 2016; Table 1). Natural gas is the most commonly used fossil fuel for heat energy (drying and pressing) at WCP facilities in NA.
For this paper, previous particleboard and MDF life cycle inventory (LCI) models were updated with new consumption data of fuels, electricity, and ancillary materials. For hardboard–EWST no new data were collected, and only new electricity and fuel processes were updated. Production volumes of WCP were obtained using 24 years of shipment volumes from production years 1996–2019 (CPA 2017, 2019). All LCA modeling was constructed in SimaPro. New GWP values and biogenic carbon emissions were calculated and applied to annual WCP production flows to obtain a carbon flow profile over 24 years. Excluded from the study were the fluxes in carbon pools from removals and decay.
For particleboard, the updated NA GWP per cubic meter (m3) value is 0.351 metric ton (mt) CO2e TRACI and 0.639 mt CO2e TRACI for MDF. For hardboard–EWST the CO2e TRACI is 0.680 mt CO2e/m3. In 2019, there was an estimated cumulative 291 million m3 of WCP (particleboard, MDF, and hardboard–EWST) in use in NA that represents a cumulative carbon pool (24 yr) of 354 million mt of CO2 where the total carbon emissions from manufacturing (cradle-to-gate) is 224 million mt CO2e (Table 2). The panel cumulative carbon pool is enough to more than offset 24 years of all the CO2 emissions (CO2 BIO and CO2e TRACI) from producing particleboard, MDF, and hardboard–EWST cradle-to-gate (Table 2). More carbon is stored in WCP than is released from cradle-to-gate.
Figure 1 shows carbon emissions as CO2e TRACI (orange bars) and biogenic CO2 BIO (blue bars), embedded carbon (stored) as CO2 based on total shipments for a given year (light green bars), and net GWP (embedded carbon minus emissions [CO2e TRACI and CO2 BIO] shown in dark green bars). For particleboard and MDF, more carbon is stored in the product than is released from all cradle-to-gate emissions, while for hardboard–EWST the carbon storage is not enough to offset the carbon emissions from cradle-to-gate. As a collective, the carbon pools of all WCPs are enough to offset the carbon emissions for all three panel types. Following the reporting requirements of the PCR, all panel products would store more carbon then was released during production (Table 3). This assumes that biogenic CO2 emissions from combustion do not exceed the CO2 uptake during tree growth (assuming no land-use change), leaving the balance of the biogenic carbon as carbon stored in the wood product for its lifetime.
Production of WCPs from cradle-to-gate releases more fossil carbon (CO2e TRACI) than biogenic carbon (CO2 BIO). This is driven by regional electricity grids, resins, transportation fuels, and heat energy from natural gas. Particleboard and MDF facilities use more fossil-based fuels for heat energy generation onsite because the majority of the wood fiber at WCP facilities is incorporated into panels. WCP facilities would need to purchase additional wood fuel and transport it to completely substitute biomass fuels for fossil-based fuels. For example, using biomass instead of natural gas as the primary heat source can avoid over 211,000 metric tons of fossil-based carbon emission (CO2e TRACI) from the cradle-to-gate (based on 2019 production data only), but only if the biogenic emissions are considered carbon neutral. In actuality, more carbon emissions are released when using biogenic fuels, owing to their lower carbon density and associated heating value. In a recent white paper by Puettmann (2021), biogenic carbon emissions for particleboard increased by 154 percent and CO2e TRACI decreased by 12 percent. For MDF, biogenic carbon emissions increased by 56 percent while fossil-based carbon emissions (CO2e TRACI) decreased by 24 percent.
Wood is a biobased material and thus contains biogenic carbon. Carbon is stored in WCP as a negative emission when it enters the product life cycle referred to as CO2 BIO. During the production of WCPs, biogenic carbon is emitted if wood biomass is combusted for energy use during manufacturing or if forest residues are burned after a harvest. Carbon flows are based on shipment volumes, while carbon pools are the cumulative carbon stored during the life span of the WCP containing-product. Assuming a life span of 25–30 years, the system boundary for this study did not consider fluxes in carbon pools (removal and decay changes). By the year 2019 there was an estimated 291 m3 of WCP in use in North America that corresponds to a cumulative carbon pool of 354 million mt of CO2. This cumulative WCP carbon pool is enough to offset 24 years of cumulative cradle-to-gate greenhouse gas emission (as CO2e) emissions from both fossil and biogenic sources. In summary, North American WCP store much more carbon than they release as a consequence of their production. In summary, WCP are the key to maximizing tree use by providing society a useful long-lived product made from wood waste (i.e., wood fiber, chips, sawdust, plywood trim) from the production of primary wood products, while storing carbon for many years.
Principal, WoodLife Environ. Consultants LLC, Corvallis, Oregon (email@example.com). This paper was received for publication in January 2022. Article no. FPJ-D-22-00010.