Introduction

Residential wood-fired heaters (RWH) have long been known to be significant polluters, especially in the category of PM10 -loading to localized airsheds. In many western valley areas of the United States, woodstove-generated particulate and carbon monoxide are major components of wintertime National Ambient Air Quality Standards (NAAQS) violations. Excessive woodsmoke loading to local airsheds also causes nuisance odors and degrades visibility and vista values.

The physical and chemical characteristics of RWH emissions has further accentuated problems in this source category. RWH generated particulates are virtually all submicron-sized organic condensate materials. In addition, several of the organic compounds found in woodsmoke have demonstrated carcinogenic and mutagenic properties (1). A 1986 study by OMNI also showed stove emissions to be highly acidic (pH 2.5 to 3.8), with high acidity persistence (i.e., buffering) due to organic acids (2).

Subsequent to the major increases in RWH use in the early 1980s and the evolution of widespread woodsmoke pollutant problems, the RWH industry embarked on the development of low emissions technologies. The industry's fast response to the developing problems produced RWHs that emitted 60 to 80% less emissions, under laboratory test conditions, than the original "conventional" RWHs sold and in use during the early 1980s. These were the first reduced-emission woodstoves which qualified for the first state-wide woodstove certification programs promulgated by Oregon and Colorado. In 1987, the U.S. Environmental Protection Agency (EPA) promulgated an even more stringent 2-stage, national standard for the reduction RWH emissions. This New Source Performance Standard (NSPS, 40 CFR Part 60, Subpart AAA), mandated that RWH appliances manufactured after 1990, have laboratory-tested emission reductions that represent more than an 80% reduction from the conventional woodstoves of the early 1980s.

The RWH manufacturing industry responded anew with appliances that generated even lower emissions. RWHs that were built and sold in the early 1980s generated laboratory-tested particulate emission factors ranging from 30 to 60 grams per kilogram (g/kg) of dry fuel burned. They are now being replaced by models which generate less than 7.5 g/kg (at weighted-average dry burn rates of about 1.4 kilogram per hour).

This paper reviews known operating factors which affect wood-fired heater emissions with updated test results and analyses of new technology design concepts. Batch-loaded cordwood, as well as processed wood-pellet parameters are analyzed. A review of batch-load versus air-to-fuel-mixture controlled technologies and reporting units are included in the main body of this discussion paper.

All values reported in this paper, unless otherwise noted, were obtained at the laboratories of OMNI Environmental Services, Inc., using the Oregon Department of Environmental Quality (ODEQ) "Standard Method for Measuring the Emissions and Efficiencies of Residential Woodstoves," June 8, 1984 edition, the methods stipulated by the 1987 EPA NSPS, or the OMNI Automated Woodstove Emissions Sampler procedures which have had EPA performance audits performed four times over the last seven years.

It is important to note that if non-standard methods and protocols are used to operate woodstoves or to measure and calculate performance and emissions parameters such as fuel loading densities and configurations, efficiency algorithms, particulate sampling systems, or laboratory altitude, there can be significant differences in results. Care should be taken when comparing data generated using different procedures or methods unless equivalency has been demonstrated and appropriate correction factors are applied.