Correspondence Comment on “PCDD/F, PCB, HxCBz, PAH, and PM Emission Factors for Fireplace and Woodstove Combustion in the San Francisco Bay Region” The characterization of air emissions from woodstoves and wood-burning fireplaces is pivotal in assessing exposure to air pollutants in residential settings and for serving as a benchmark for developing new space heating technologies and alternative fuels that reduce air emissions. The work of Gullett et al. (1) contributes to the body of woodstove and wood-burning fireplace air quality literature. The importance of the work is reflected in that fact that we estimate that 45 million residential wood-burning appliances are in U.S. homes alone. The mean wood burn rate and associated standard deviation for woodstoves determined from 770 in-home, week-long measurements on 202 woodstoves made in 15 studies conducted at various locations in the United States and Canada are 1.14 and 0.39 dry kg of fuel/h (2). The median wood burn rate for U.S. woodstoves is calculated as 1.17 dry kg of fuel/h from burn rate cumulative probabilities published in the Code of Federal Regulations (3). Gullett et al. (1) reported that the approximate burn rate for the woodstove used in their study was 2.5 kg/h. We interpret that the 2.5 kg/h value is on a wet basis and calculate a mean burn rate of 2.31 dry kg/h from their data. The 2.31 dry kg/h value is three standard deviations higher than the mean woodstove burn rate based on the data compiled from the 15 studies, and its cumulative probability is 0.946 based on the Code of Federal Regulation tabulations. A cumulative probability of 0.946 corresponds to 5.4% of woodstove burn rates higher than 2.31 dry kg/h. The San Francisco Bay Area has a mild climate with a 30-yr average of only 3015 Fahrenheit heating degree days (1675 Centigrade heating degree days) (4). Lower than average woodstove burn rates are characteristic of mild climates. The burn rates used by Gullett et al. (1) are higher than for typical woodstoves in the San Francisco Bay Area. The median and mode of wood burn rates for fireplaces based on 557 measurements reported in 21 studies are 4.20 and 3.50 dry kg of fuel/h (5). The median and mode are better indicators of the overall national central value than the mean when utilizing these data as many of the fireplace studies where either laboratory studies or research studies with an over-representation of large, uncommon fireplaces with high burn rates as compared to the fireplace size distribution in homes. It is general knowledge among members of the hearth industry that the small, standard, and inexpensive “36-in.” zero-clearance manufactured fireplace is overwhelmingly the most common fireplace type in U.S. homes. We have made numerous qualitative observations of burn rates in 36-in. fireplaces and have concluded that approximately 3.7 dry kg of fuel/h is a typical realistic burn rate with this most common appliance type. Gullett et al. used a larger fireplace (42-in.) and reported 7.0 ( 0.3 kg/h as the mean burn rate. We interpret that the 7.0 ( 0.3 kg/h burn rate value is on a wet basis and calculate, from their data, a mean burn rate of 5.82 dry kg/h for the oak fuel and 6.44 dry kg/h for the pine fuel. The burn rates used by Gullett et al. (1) are higher than for typical fireplaces in the San Francisco Bay Area. 1910
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Wood moisture measurements of home wood stockpiles made at various locations in the United States and Canada reported by 23 studies have been compiled (5). The mean and associated standard deviation of wood moisture on a dry basis determined from 820 measurements made in these studies are 24.1% and 12.9%. The wood heater testing protocol developed by the U.S. Environmental Protection Agency in support of standards of performance for new stationary sources pursuant to the federal Clean Air Act is designed to burn wood with realistic moisture content. Wood fuel with moisture contents between 19% and 25% on a dry basis is specified by that protocol (6). Gullett et al. (1) reported using two wood fuel typessoak and pine. The moisture content of the oak fuel was representative of typical cordwood. However the pine fuel moisture values, while statistically not outliers, were exceptionally low with some of the reported values lower than one standard deviation below the mean of the data compiled from the 23 studies. The moisture content of the pine fuel was given as 8.7% and 8.8%, depending on the measurement technique, in one table and ranged from 8.98% to 11.22% for three test runs reported in a second table. Gullett et al. (1) do not specify whether these values are on a wet or dry basis. In either case, the pine fuel moisture content is not representative of typical fuel. We have experimented with similarly dry pine fuel in a fireplace and found it to burn more rapidly and at higher temperature than fuel with a more representative moisture content. For the purposes of assessing the overall representativeness of the woodstove pollutant emissions provided by Gullett et al. (1), we selected the Method 5H equivalent particulate (PM) emission rate (g/h) among the various pollutant types and reporting conventions because less data are available for comparisons with the other pollutant types and reporting conventions. PM emission rates are reported in three other independent sources for the Quadra-Fire 3100 woodstove used by Gullett et al. (1). PM data can be converted to Method 5H equivalents to permit direct comparisons (7). Studies of Quadra-Fire 3100 woodstoves under actual in-home use conducted in Crested Butte, CO, and Klamath Falls, OR, reported mean particulate emission rates (converted to Method 5H equivalents) of 7.1 and 7.4 g/h (8, 9). The U.S. EPA certification Method 5H emission rate for the QuadraFire 3100 woodstove is 2.1 g/h (10). The Gullett et al. (1) laboratory study reports a mean Method 5H equivalent emission rate of 20.7 g/h, significantly higher than either the Quadra-Fire 3100 woodstove emissions measured from actual in-home use or as measured as part of the U.S. EPA certification program. Pollutant emissions from woodstoves are specific to model types due to differences in designs. The U.S. EPA lists 566 different certified woodstove models (10). In addition to the certified models, prior to the July 1, 1988, U.S. EPA certification requirement, we estimate that there were approximately 300 woodstove manufacturers, most offering multiple uncertified models. Forty percent of woodstoves in use in the San Francisco Bay Area are pre-certification models (11). Of the 60% that are certified models, 29% are catalytic and 71% are noncataltyic (11). Consequently, less than half of the woodstoves in the San Francisco Bay Area are certified noncatalytic models. The Quadra-Fire 3100 is a certified noncatalytic woodstove, and it is one of only several hundred certified noncatalytic woodstove models in use. Fireplaces have simple and large uncomplicated fireboxes without combustion air or emission controls. For this reason, 10.1021/es030660e CCC: $27.50
2004 American Chemical Society Published on Web 02/06/2004
unlike woodstoves, emissions for given burning conditions are similar among fireplace models. For fireplaces, the 5H equivalent particulate emission factor (mass pollutant per mass of fuel) is the most widely published emission pollutant and convention and is used here for emission comparisons. The emission factor published by the U.S. Environmental Protection Agency for fireplaces is 17.3 g/kg (12). The mean and standard deviation of 552 fireplace 5H equivalent particulate emission factor measurements made in 21 studies are 11.1 and 8.9 g/kg (5). Gullett et al. (1) report 5H equivalent mean emission factors of 5.56 and 2.80 g/kg for oak and pine fuel, respectively. These lower particulate emission factors, particularly for the pine fuel, reported by Gullett et al. (1) are consistent with the higher burn rates facilitated, in part, by the drier than average fuel. Those pollutants, such as PM, that are predominately products of incomplete combustion tend to be produced at lower levels at higher temperature combustion conditions. In contrast, those pollutants such as dioxins, furans, and some polycylic aromatic hydrocarbons that are formed by the combustion process have been shown to be produced at higher levels at higher combustion temperatures. Evaluation of particulate emissions reported by Gullett et al. (1) for woodstoves and fireplaces illustrate that they are not representative of particulate emissions from the use of woodstoves and fireplaces in the San Francisco Bay Area. Furthermore, by inference, the emission values for the other pollutants that do not have a large published database allowing for comparisons are also unlikely to be predictive of their emission values in the San Francisco Bay area. Unrepresentatively high burn rates, unusually dry fuel, and in the case of woodstoves utilizing a single model to represent a large number of types and models are primarily responsible.
Literature Cited (1) Gullett, B. K.; Touati, A.; Hays, M. D. Environ. Sci. Technol. 2003, 37, 1758-1765. (2) Houck, J. E.; Scott, A. T. Residential Wood Combustion Appliance Test Method for Representative Emission Factor Measurement; Report prepared by OMNI Environmental Services for U.S. Environmental Protection Agency, Air Pollution Control Division, Research Triangle Park, NC, November 19, 1999. (3) Protection of Environment. Code of Federal Regulations, Part 60, Title 40, Appendix A, Method 28, Section 17.0, Reference 1 and Section 18.0, Table 28-1, pp 1439-1441; http:// www.epa.gov/ttn/emc/pomgate/m-28.pdf.
(4) WorldClimate. http://www.worldclimate.com. (5) Houck, J. E.; Crouch, J. Updated Emissions Data for Revision of AP-42 stion 1.9, Residential Fireplaces; Report prepared by OMNI Consulting Services, Inc. and Hearth, Patio and Barbecue Association, December 18, 2002; http://www.omni- test.com/ publications.htm. (6) Protection of Environment. Code of Federal Regulations, Part 60, Title 40, Appendix A, Method 28, Section 7.1.2, p 1408; http://www.epa.gov//ttn/emc/pomgate/m-28.pdf. (7) Emission Factor Documentation for AP-42 Section 1.10, Residential Wood Stoves; Reported prepared by E.H. Pechan & Associates, Inc. for U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC, April 1993. (8) Barnett, S. G.; Bighouse, R. D. In-Home Demonstration of the Reduction of Woodstove Emissions form the Use of Densified Logs; Report prepared by OMNI Environmental Services for Oregon Department of Energy and U.S. Environmental Protection Agency, July 7, 1992. (9) Correll, R.; Jaasma, D.; Mukkamala, Y. Field Performance of Woodburning Stoves in Colorado during the 1995-96 Heating Season; Report prepared by Virginia Polytechnic Institute and State University for U.S. Environmental Protection Agency; EPA-600/R-97-112; October 1997. (10) U.S. Environmental Protection Agency. Certified Woodstoves; Wood Heater Program, Manufacturing and Transportation Division: Washington, DC, January 7, 2003. (11) Broderick, D.; Houck, J. E. Results of Wood Burning Surveys Sacramento, San Joaquin, and San Francisco Areas, University of California/Berkeley/California Air Resources Board-GIS Study; Report prepared by OMNI Consulting Services, Inc., January 15, 2003. (12) U.S. Environmental Protection Agency. External Combustion Sources, 1.9 Residential Fireplaces. Compilation of Air Pollutant Emission Factors, AP-42, 5th ed.; Vol. 1, Chapter 1; http:// www.epa.gov/ttn/chief/ap42/ch01/.
John Crouch Hearth, Patio & Barbecue Association Suite 185, 7840 Madison Avenue Fair Oaks, California 95628
James E. Houck* OMNI Consulting Services, Inc. 5465 SW Western Avenue Beaverton, Oregon 97005 ES030660E
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