Wood heating, as a renewable energy source, is viewed by many as an environmentally beneficial, “green” heating option. From a pollution perspective, this emission source has an outsized contribution to fine particulate matter (PM2.5) emissions, accounting for approximately 2% of residential energy but responsible for 95–98% of PM2.5 from the residential heating sector and is a major source of air toxics (EIA, 2020; NEI, 2017). Wood heaters also have carbon monoxide emissions that are far greater than allowed from other residential heating technologies. In many states throughout the country, residential wood heating is one of the largest sources of PM. High wood combustion emissions have resulted in rural regions with elevated PM2.5 including some that are in non-attainment. Yet in areas without attainment issues, wood heating has not met with the same level of concern as other sources emitting far less criteria and air toxic pollution. As emissions decrease from industrial and electrical generation units, emissions from area sources, such as residential wood heating, represent opportunities to realize significant emission reductions and achieve public health goals in the coming decades.
Government agencies are taking steps to reduce emissions from this source. In many areas of the country, agencies trying to reduce wood smoke are attempting to reduce pollution through wood stove changeouts – removing older wood stoves and replacing them with newer, presumably cleaner models. Federal programs, such as the Biomass Thermal Utilization Act provide 26% tax credits to consumers for appliances with efficiency levels tested at greater than 75%. Unfortunately, there has been a major disconnect between appliance certification test methods and in-use performance. The programs meant to reward cleaner technology may instead be rewarding technologies that have tested well in certification testing but have actual in-use operations that are much different.
The New York State Energy Research and Development Authority, the Northeast States for Coordinated Air Use Management, Brookhaven National Laboratory, Cornell University, Clarkson University, ClearStak LLC, State University of New York at Stony Brook, New York State Department of Health, New York State Department of Environmental Conservation and other partners set about conducting ambient air quality studies of wood smoke, characterizing wood-burning appliance performance, evaluating existing appliance test methods, and developing new, more representative test protocols. This effort was undertaken with the philosophy that innovation and improved performance can only be promoted by challenging the technology to repeatably perform well under a broad range of conditions encountered when the appliance is installed in the home. Additionally, technology assessment must move beyond PM emissions. Research and inventory work suggests that residential wood heating is a large source of hazardous air pollutants but we know little about the efficacy of new technologies to reduce air toxic emissions. This special issue on residential wood combustion provides insights into the findings of some of that research.
The first article by Marin et al. presents an overview of air quality and public health impacts of residential wood combustion and wood heater regulations. The article identifies challenges with the test protocols and weaknesses in the design and implementation of the 2015 New Source Performance Standard for Residential Wood Heaters.
The following articles by Morin et al., Morin et al., and Allen et al., present the development of an Integrated-Duty-Cycle test protocol for cordwood stoves that includes for the first-time, high emitting periods of the burn cycle such as cold start-up and fuel reloading; development of a fuel loading protocol; impacts of wood fuel species and moisture content; and the application of real-time PM measurements. The achievement of real-time PM measurement instead of a multi-hour composite filter sample provides a new capability necessary for understanding when the high emitting periods occur during the burn so that combustion designs may be improved.
O’Brien et al. present foundational research for cordwood fueling protocols for hydronic heaters and their impact on emissions. Capturing performance during start-up and refueling, the periods of highest emissions, is essential to the development of a representative test protocol for these appliances. This in turn is important for evaluating and improving technology designs and reducing air pollution and public health impacts.
Rousta et al. present computational modeling that evaluates the efficacy of dilution tunnel designs in two laboratories to produce adequate turbulence and mixing of wood stove emissions to result in a uniform concentration profile at the outlet. This is a critical need for representative emissions sampling to characterize appliance performance, especially for certification testing.
Li et al. present their work on ambient sampling of residential wood combustion plumes to measure the absorption Ångström exponent that can be used to characterize wood burning conditions and identify different wood smoke sources.
Lindberg et al. evaluated two cordwood-fired boilers and a chip-fired boiler. The testing was done for start-up, low-output, high output, and burn-out conditions. Particle number concentration, particle mass concentration, lung-deposited surface area, and black carbon were all measured simultaneously. Findings suggest particles of different compositions and morphology are produced by different combustion conditions during different operating conditions.
The final three articles by Trojanowski et al., Lindberg et al., and Lindberg et al. investigate two high mass cordwood-fired hydronic heaters, often called outdoor wood boilers. These units were characterized during cold-start, at several heat loads as well as during cyclic operations and burn-out. Measurements include efficiency, PM mass, particle number and size, black carbon, brown carbon, and gaseous emissions under these realistic combustion conditions. Results indicate outdoor wood hydronic heater emissions during transient operations can be significantly greater than during steady-state test conditions. It is important that these conditions be included in certification test protocols. Additionally, there are emissions that may be considered beyond PM2.5 for more comprehensive air quality, public health, and climate considerations.
Emission reductions, regulations, and test methods go hand-in-hand. Regulatory, incentive, and change-out programs cannot achieve emission reductions and efficiency improvements without robust test methods. The Integrated-Duty-Cycle test method approach represents the next generation of test methods that can finally provide information to accurately and appropriately assess residential wood heating appliances. To date, the U.S. Environmental Protection Agency (EPA) has accepted Integrated-Duty-Cycle test methods for cordwood stoves (Alternative Test Method-140) and pellet-fired boilers, important steps for facilitating technology innovation and cleaner products. In spring 2021, seven state environmental agencies and nine state Attorneys General requested EPA revoke Alt-125 and Alt-127 for cordwood stoves because of difficulty reproducing certification test outcomes and “foundational flaws, making it unsuitable for determining compliance with the New Source Performance Standard for Residential Wood Heater requirements.” On January 24, 2022, EPA announced the withdrawal of those methods in the Federal Register (Vol. 87, No. 15) and that renewal or recertification of a wood heater model previously certified using Alt-125 or Alt-127 would not be granted a waiver and must be retested using a valid test method at the time for renewal or recertification. Additional Integrated-Duty-Cycle protocols are under development for pellet stoves, hydronic heaters, and furnaces to provide a common platform to assess a broad suite of residential wood heating appliances. The next New Source Performance Standard for Residential Wood Heaters provides an opportunity for bold improvements that can further the Integrated-Duty-Cycle approach for all wood heating technologies. This will promote the innovation in appliance design necessary for emissions reductions leading to better air quality and improved public health in communities that utilize wood heating.
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Ellen Burkhard
Dr. Ellen Burkhard is Senior Advisor for the New York State Energy Research and Development Authority's (NYSERDA’s) Environmental Research Program where Ellen manages a broad portfolio of research projects to inform energy and environmental policy decisions. These projects are focused on technology evaluations, emissions, air quality, source apportionment, health impact analysis and accountability studies. NYSERDA has made a major effort to improve how wood heating appliances are evaluated and supports the development of integrated-duty cycle test methods and multi-pollutant emissions testing for all combustion heating technologies.