Reporting units for RWH emissions

One significant difficulty which emerged early in the efforts to develop RWH emissions control and reduction regulations and which has also caused some controversy and confusion over the last 15 years, is the units for expressing the amount of emissions from RWH appliances. The two primary reporting-unit candidates considered since the regulation of RWH emissions began in the early 1980s, are:

1. The emission factor; i.e., how much (mass) of pollution is discharged per mass of fuel burned, expressed in grams per kilogram (g/kg), and
2. The emission rate; i.e., how much (mass) of pollution is discharged per unit of time, expressed in grams per hour (g/hr).

A third candidate, mass of pollution discharged per unit of useful room heat produced by an a RWH appliance expressed as grains per British thermal unit (gr/Btu, or in SI units; grams per Megajoule (g/MJ)) had been considered in the early days of regulation development. However, because a verified method for measuring efficiency did not exist at that time and it was thought that an acceptable and accurate measurement method would be very expensive, this unit of measure never gained favor by regulators or the RWH appliance manufacturing industry.

The basis for controversy between using either of the two primary-candidate reporting units is whether one or the other provides better information for ranking one RWH appliance against another and/or whether one or the other provides better and more useable information for modelling the relationship between the amount of RWH pollution being discharged into an airshed and how the RWHs are being operated. It would be simplified and ideal if RWH appliances discharged pollutants at constant rates or even consistent rates for any given burn rate or heat output level. The only RWH appliances which can emit pollutants at close to constant rates are the pellet stoves. Pellet stoves and other residential heating appliances, like oil or gas furnaces and most large combustion sources like electric power utility boilers, have controlled air supplies and fueling rates to optimize combustion processes at or near a "steady state" conditions. Having the ability to operate at optimized steady-state conditions can maintain nearly constant, low-pollutant emission rates over long periods of time.

If cordwood-burning RWH appliances had steady-state combustion conditions and constant and consistent emissions rates, relatively simple:

Calculations could be used for developing good estimates or models of airshed pollutant loading caused by RWH appliances. But the fact is that no RWH appliances, except pellet stoves, discharge pollution at a constant rate from the beginning to the end of a complete burn cycle or even consistent rates from one burn-rate/heat-output level to the next. As mentioned earlier, during the burn cycle of a cordwood-fuel load in an RWH appliance, its physical and chemical characteristics change dramatically as do the amount and the physical and chemical nature of the pollutants being produced and discharged. Everything is always changing in a cordwood-burning RWH appliance including combustion-influencing parameters like temperatures, fuel weight, and fuel-load geometry.