Conclusions

The combustion of wood in Residential Woodfired Heaters (RWHs) involves highly complex chemical processes which are sensitive to a wide variety of influences. Key elements required for efficient combustion include high combustion zone temperatures, appropriate air-to-fuel ratios, adequate air (i.e., oxygen) and fuel mixing, and adequate residence time. The batch process of wood combustion in the naturally drafted RWH presents special problems in that the entire fuel charge is involved in various and changing states of a complete combustion processes throughout the fuel-load burning cycle. Ideal conditions vary during each stage, making complete and efficient combustion of the entire fuel charge in a single RWH configuration very difficult. At best, present designers of naturally drafted RWHs provide optimized averages for the burn cycle: complete and efficient combustion for all stages of the burn cycle have not been perfected.

A variety of RWH technologies have been examined for their effectiveness in emissions reduction and for their appropriate measurement and reporting units. Conclusions drawn include:

1. The most useful and technically sound reporting unit for all RWH appliances and masonry heaters would be grams of emissions discharged per unit of useful heat produced. These units have not been used because no verifiable measurement methods were available at the time applicable regulations were being written.
2. The g/kg and g/hr reporting units must be used with caution when applied to the performance of EPA-regulated or EPA-exempted RWH appliances:
   
   a) G/hr can be used to indicate the performance of EPA-regulated RWH appliances (EPA calls them affected facilities) only because of the limitations imposed by EPA's definition of affected facilities and the specificity of the test methodology utilized to measure their emissions performance. It is only because these limitations and specificity impose such a narrow range of sizes, design configurations, and test-condition operating protocols that the g/hr reporting units can be used for ranking one RWH against another. G/hr should not, however, be used to estimate the field performance of RWH appliances or typical real emissions.
   
   b) The g/hr reporting unit is not appropriate for masonry heaters because burn times are short resulting in high g/hr values when g/kg values are low. G/kg is the most useful reporting unit for masonry heaters because it does directly reflect the quality of the burning process taking place. In addition, with g/kg data and defined construction specifications, masonry heaters can be fairly ranked against one another.
   
   c) Since g/kg is the most useful unit for indicating the quality of the combustion process taking place, it would also be useful for comparing the currently regulated RWH appliances (i.e., woodstoves and pellet-fired stoves) with masonry heaters and any other wood burning appliances.
   
   
3. Smaller fuel charges required with smaller fireboxes reduce emission rates when compared to larger fuel loads consumed at comparable burn rates.
4. Air entering the firebox near or up through the coal bed ("underfire air") results in higher emissions.
5. Preheated secondary air, or more properly termed, additional wood-gas combustion air, introduced as combustion gases leave primary combustion zones and enter secondary combustion chambers can effectively reduce emissions.
6. Thermostatic air supply controls on non-catalytic RWHs cause air-starved conditions and high emissions when fuel load and firebox temperatures are high and the thermostat closes the damper.
7. Pellet-fueled RWHs utilizing mechanically assisted drafts have demonstrated emission rates below the most efficient cordwood burning RWHs. This is due to externally powered controls for fuel feed and air supply which maintain ideal air-to-fuel ratios and mixing conditions.

Wood combustion involves a large number of highly variable parameters which can cause a high degree of variability in pollutant emissions. Certification testing, conducted by any current or proposed method, necessarily allows a range of test conditions. For example, there are allowed ranges of wood moisture, fuel loading density, and starting and ending temperatures. Although in most cases these ranges are narrow, they do produce error ranges or "noise" in resulting data. This "built-in noise," combined with the variable conditions of wood combustion, makes statistically significant isolation and quantification of RWH design factors difficult. However, as presented here, generally consistent differences can be seen between many design variables using the currently available data base.