Firebox/ Fuel load size

The capacity of a woodstove to hold fuel is determined by the size, or usable volume, of the primary combustion chamber or firebox. Many RWH manufacturers report that the U.S. market requires large volume RWHs which enable consumers to load large amounts of wood fuel. The ability to maintain long burn duration without reloading or adjusting the air supply settings is commonly regarded as a major marketing feature. As a result, most RWHs sold in the U.S. market have firebox volumes of about 1 to 3.5 cubic feet (28 to 100 liters). Data from EPA certification tests and from previous studies on fuel-load/firebox volume burn rate, and PM-10 emissions relationships⁽³⁾ were used to construct Figure 4.

All the data were from non-catalytic RWHs. The 1 kg/hour-burn-rate curve shows that emission factors are at approximately 5 g/kg in one cubic-foot firebox and increase rapidly as firebox sizes (and hence fuel load size) increase over two cubic feet. The small decreases in the 1 kg/hour emission-factor curve when firebox sizes increase to greater than three cubic-foot, are probably due to deposition losses of emissions on firebox and chimney walls.

Each of the burn rate curves exhibits dramatic increases in emission factors as firebox size increases. As the burn rates increase, the slope of the emission-factor curves decrease and the larger the firebox is when the dramatic increases occur: i.e., larger fireboxes, and hence, fuel loads, require higher burn rates to maintain low emission factors.

It should be noted that since the standard test procedures call for a fuel loading density of 7 pounds of fuel per usable cubic-foot of firebox volume, each RWH is loaded according to its specific size. If several RWHs are each burned at seven pounds per hour, at the end of one hour, a 2 cubic-foot RWH burns half of its fuel load, a 3 cubic-foot RWH burns one third of its fuel load, while a 1 cubic-foot RWH consumes its entire charge.

The size of the fuel charge appears to be the primary critical factor. The batch process involved in fueling an RWH requires an entire fuel charge to be placed in the firebox at once. As the fuel load is heated, gasification of the wood occurs. The larger the fuel load, the greater the amount of wood subjected to gasification, resulting in greater quantities of wood gas being released over a given time. At a fixed heat output level, more wood gas will be released from a large fuel charge than from a small charge. Lower mixing intensities and more cool areas in larger RWHs will result in higher emissions (per mass unit of fuel burned), especially under low-fire conditions. Catalyst equipped RWHs are not as susceptible as the non-catalytic RWHs to the effects of firebox/fuel-load size.

As discussed previously, because of higher sustained temperatures, catalyst assisted secondary combustion accommodates a much greater range of air-to-fuel ratios and air/fuel mixing conditions. As long as the catalyst is sized correctly and an appropriate amount of air is provided, a catalyst equipped RWH will have lower emissions under just about any burning conditions.