A problem associated with the burning of biomass fuel in general is the production of air pollutants. For example, the burning of biomass fuel (and particularly the inefficient burning of biomass fuel) may produce volatile, toxic, or other undesirable gases. Furthermore, large amounts of smoke and particulate matter may also be released into the atmosphere.
In this regard open fireplaces are particularly inefficient. That is, open fireplaces usually produce larger amounts of air pollutants, as compared to enclosed fireplaces. Furthermore, an open fireplace generally only provides heat directly in front of the fireplace, with the vast majority of the heat being lost up through the chimney or out through the rear wall of the fireplace.
The inefficiency of open fire places has been addressed to a certain extent by the use of domestic furnaces. Examples can be found in U.S. Pat. No. 4,559,882 (Dobson) and U.S. Pat. No. 4,630,553 (Goetzman).
However, whilst the problems of the inefficient burning of biomass fuel for space heating can be addressed somewhat with a furnace, the extra capital cost is not always necessary, practical, or affordable. Furthermore, furnaces tend to be used mainly in very cold climates, but not in temperate climates, and are also usually coupled to some sort of central heating, which is not always desirable.
Moreover, a further issue with furnaces is that they are usually completely closed off from view, and do not therefore provide the psychological or aesthetic benefits that are derived from lazy flame as a light source. That is, people like to see flames.
Perhaps a result, “air tight wood burners” or simply “wood burners” have become increasingly more popular over the years, and are now in widespread use. “Wood Stove” is another common term for such appliances, particularly in North America.
Wood burners generally comprise a metal firebox, into which biomass fuel may be placed and burnt, an adjustable air control or damper, and an exhaust flue. Many, if not most, wood burners also have a glass door through which the fire and/or flames may be viewed.
The combustion of biomass fuel is a complex process, and includes a range of chemical reactions. As yet, there does not appear to be a wood burner or fireplace designed for space heating that adequately incorporates the various stages of combustion in relation to each other.
These stages are drying, pyrolysis, combustion and reduction, which if done correctly produce the combustible gases carbon monoxide and hydrogen. The carbon monoxide and hydrogen can then be combusted separately during what is known as secondary combustion to yield only water and carbon dioxide (and heat). However, most wood burners lack the ability to burn or convert such gases (and their precursors such as carbon dioxide and water) due to the wood burner not being able to produce enough heat to do so (conventional wood burners usually reach maximum temperatures of between 600° C.-800° C.).
Hence, attaining a high enough temperature during secondary combustion to consume all the volatiles distilled during pyrolysis or combustion is difficult because the necessary temperature is often higher than that which can ordinarily be generated. In this regard, the temperature required to adequately consume the vast majority (if not all) of the volatiles and/or particulate matter and smoke would be a minimum of approximately 900° C., and more preferably above 1000° C.
U.S. Pat. No. 4,672,946 (Craver) describes a wood burner which has a secondary combustion means for burning the particulate matter in the flue gases. However, the temperature reached within the firebox of the Craver device is stated as being only around 540° C. (1000° F.) and the secondary combustion region only reaches up to around 760° C. (1400° F.). Hence, a disadvantage associated with Craver is that the design of the wood burner does not attain high enough temperatures to adequately consume the vast majority of gases or particulate matter. Furthermore, the wood burner described in Craver is not able to be retro-fitted to an existing wood burner or other fireplace.
In more recent times, many countries or local bodies have introduced regulations to restrict the sale of inefficient and/or polluting wood burners.
For example, in New Zealand the generally allowable standard for wood burners is a maximum of 1.0 grams of particulate matter released per kilogram of wood burned, accompanied by a minimum efficiency of 65%. However, some regions have gone further than this. For example, the Canterbury Regional Council in New Zealand (which is in the region of a weather-inversion layer) has lowered these levels to 0.6 grams of particulate matter per kilogram of wood burned. The Regulations further restrict the use of wood with a moisture content higher than 25%.
However, these Regulations are not retrospective, and hence they only have effect in relation to wood burners manufactured and sold after the Regulations came into force. Moreover, to date there have been no innovations which have enabled people to bring their older wood burners up to modern compliance levels (voluntarily or otherwise).
Two factors which usually have the most detrimental effect regarding the efficiency of, and/or the release of air pollutants from, a wood burner are to do with refueling the wood burner and when shutting down or reducing the air supply to the wood burner.
Refueling causes quenching, a situation where the introduction of fresh fuel to the fire is not supported by the heat contained within the existing fire to adequately pyrolyse the biomass. As a result, visible smoke and particulate matter are often seen exiting the top of the flue or chimney at this time. This can take a while to subside as enough heat builds up in the fire to commence the correct chemical processes required to efficiently combust the fresh fuel.
A wood burner user may wish to reduce the air supply to keep the fire burning longer and/or while they are asleep. This is known as “banking”. In doing so, they generally place a full load of biomass fuel in the wood burner and shut down (or minimise) the air supply to prolong the burn time. However, the reduction in available oxygen and the corresponding detrimental effect on combustion results in more air pollutants being produced and released. Because this often results in the amount of air pollutants exceeding the minimum regulated amounts, many modern wood burner designs have denied the user the ability to shut down the air supply.
The air supply also affects the dynamics of wood burners because a greater draught causes more heat to be generated, but a greater portion of heat is lost up the flue. The higher velocity of gases also results in more particulate matter being exhausted to the atmosphere. Conversely, a lesser draught reduces the amount of particulate matter being drawn from the combustion chamber but also reduces the heat output. However it is possible in these conditions that although less heat is generated, less heat is also lost to the atmosphere as the heat has more time to radiate off before being exhausted.
Or to put it another way, greater air means greater heat, but lower efficiency, however the greater heat actually results in a cleaner burn which lowers the emissions. With a lesser air supply, the fires get greater efficiency but the lower heat increases emissions. As a result of these dynamics, there is a common saying amongst laboratory engineers which is: “You can build a hot and clean fire, and you can build an efficient fire, but you can't build both in the same fire”. I believe that my combustion system, as described herein, does in fact result in both a hot and clean fire and an efficient fire.
Another approach taken by wood burner manufacturers to address problems of fire inefficiency or to reduce the release of pollutants is by employing the use of catalytic converters (either by retrofitting to existing wood burners or by incorporating them into new wood burners). However, catalytic converters are generally very expensive, and may be considered complex to operate and/or understand by many people—and this may be prohibitive to both wood burner manufacturers and end consumers. Furthermore, the installation of catalytic converters requires specialist knowledge and significant alterations to be made to the wood burner, and flue, and this can be time consuming, complex and expensive.
It may therefore be advantageous if there was available a relatively simple and/or improved combustion system, which included primary and secondary combustion zones which were able to result in more efficient combustion of biomass fuels and/or result in a lesser amount of air pollutants being released, as compared to presently available or prior art combustion systems.
It may also be of advantage if there was available a combustion system which was able to be retrofitted to existing fireplaces, such as wood burners, for example to increase their efficiency and/or to bring them up to modern compliance standards.