Secondary Combustion

Over the last 10 years, low-emission non-catalytic RWH appliances have been developed which exhibit weighted-average emissions well below the EPA NSPS - required 7.5 g/hour (approximate conversion to 5.4 g/kg). These appliances depend on fine-tuned firebox configurations and operating protocols which optimize the average, air-regulated batch-loaded-fuel combustion conditions over whole fuel-load burn cycles. Virtually all of the non-catalytic RWHs currently on the market have manual controls for setting air supplies and burn rates. One of the most notable non-catalytic RWH features is that none of them have thermostatic combustion controls. The lack of thermostatic control on non-catalytic RWHs is not some extraordinary coincidence of design. To date, no design mechanism or technology has been developed which can accommodate clean burning with the complex dynamics of batch-loaded-fuel combustion and air-supply-mediated burn rate control.

In addition, all EPA certified non-catalytic RWHs to date have utilized natural draft to drive the flow of combustion air through the combustion systems. None of the batch-loaded cordwood burning RWH appliances utilize externally powered fans for providing combustion air delivery to the combustion chamber. And, since combustion air control is the only method available for controlling the rate of combustion in batch-loaded-fuel systems, the only practical means available for modulating combustion is by the application of thermo-mechanical devices; i.e., devices which have a mechanical response to changes in temperature.

Batch-loaded non-catalytic RWHs cannot accommodate modulated air supply while maintaining clean burning conditions because clean burning is dependant on maintaining the active high temperature combustion of fuel gases generated by the heated fuel load. When a thermostat decreases the air supply to the combustion chamber, the balance of air-to-fuel ratios and mixing turbulence is shifted which very often leads to cessation of gaseous combustion activity (i.e., flame). Under these circumstances, the unburned or incompletely burned gases leave the combustion chamber as emissions (i.e., smoke). No design factor has yet been devised for batch-loaded maintaining clean burning, efficient combustion in non-catalytic RWHs where the air supply is reduced during the combustion of a fuel load.

The typical batch-loaded non-catalytic RWH is configured with primary and secondary combustion chambers. The primary combustion chamber is sized to accommodate a cordwood fuel load and is located directly below the secondary combustion chamber. The structure separating the primary and secondary combustion chambers is typically called a baffle. The term "secondary" refers to the area or chamber where combustion of only gaseous fuel materials takes place. Typically the secondary combustion chamber is smaller than the primary combustion chamber and is constructed of materials which can contain and hold as much heat as possible so the elevated temperatures can be maintained as long as possible. Also typical is the addition of heated air to the combustion gases as they leave the primary combustion chamber and enter the secondary combustion chamber. This heated air is usually referred to as "secondary air." The size of the secondary chamber, the amount and temperature of added secondary air, and the temperatures and turbulence within the secondary combustion chamber govern the quality of secondary combustion which can take place.

If this primary and secondary combustion system is optimized for an air supply which is governed only by natural draft, it is very difficult to maintain the appropriate air-to-fuel ratio, temperature, and mixing conditions for sustaining active and clean combustion. This is especially true when the primary air supply is mechanically altered to reduce the overall combustion rate. Gaseous combustion ceases when air-to-fuel ratios drop below approximately 15:1 and when combustion gas temperatures drop below approximately 950°F. Once gaseous combustion ceases, temperatures, of course, fall rapidly and air-to-fuel ratios decrease as does turbulence which is needed for adequate mixing of the air and fuel gases.

Most catalyst equipped batch-loaded RWH appliances currently on the market, are also equipped with devices which provide thermostatic control of their combustion air supply. Thermostats utilized on these stoves are universally powered by the thermo-mechanical response of bimetallic coils. As temperatures on the surfaces or in the spaces where the bimetallic coils are placed change, the bimetallic coil physically expands or contracts. This mechanical action is then translated into supplying more or less combustion air to the combustion chamber.

The catalyst in catalyst-equipped RWHs replaces the secondary combustion chamber of non-catalytic RWHs. Some catalyst have a metal substrate base but most are manufactured with ceramic substrates and active catalytic coatings which contain mostly precious metal oxides such as platinum and palladium. Catalysts are available in a variety of overall shapes and sizes, from 2- to 10-inch squares and rectangles to 6- to 8-inch circles. Most of them are from 2- to 3-inches in depth along the path of combustion gas flow and have a monolithic, honeycomb structure with 4 to 6 cells per inch.

Catalytic activity works to reduce the temperature at which chemical reactions such as wood-gas combustion take place. The same amount of chemical energy is released from the combustion of wood generated gases when a catalyst mediates the chemical reactions but the temperatures at which the chemical reactions start taking place are reduced. Without catalytic mediation, the lowest temperature at which wood-gas combustion appears to be initiated is approximately 950°F. With the same gas mixtures, and when the gases are passed through a catalyst, this temperature is reduced to the range of 500 to 600°F. In addition, once catalyst mediated combustion is initiated the energy released generates temperatures in excess of the 950°F level so that wood-gases passing through the honeycomb spaces are combusted without coming in contact with the actual catalytically active surface. This phenomenon adds even more heat to the catalytic structure which is then capable of sustaining clean burning conditions under a wide range of catalyst inlet gas temperatures.

Most catalyst equipped RWHs also introduce heated secondary air to the gases leaving the primary combustion chamber. As in non-catalytic RWHs, this is done to ensure that adequate air-to-fuel ratios are maintained in the catalyst mediated combustion zone even if the primary air supplies are reduced by the action of a thermostat. Like non-catalytic RWH design features, catalyst equipped RWHs typically incorporate measures like insulating ceramic materials to conserve and hold heat in the secondary (catalyst) chamber. Because very high temperatures above 1000°F are maintained in catalysts for long periods of time during full fuel-load burn cycles, a wider range of air-to-fuel ratios and mixing turbulence can produce cleaner burning results than occur in non-catalytic RWHs.