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New Technology Clean Burn Wood Stoves
First let me say, and I know this may be a let down to some of you, that this New Technology has been around for quite some time now. As a matter of fact manufacturers have been regulated since 1990 to certain particulate emission standards depending on burn rates, airflow, firebox volume, and stove weight. Two predominant technologies rose to the top of the class when these regulations were enforced. These were catalytic and the aforementioned New Technology. I’m not going to talk too much about catalytic technology because the title of this article is not “Catalytic Clean burn Wood Stoves” In fact, I’ve said more than I wanted to about it already!
I will explain particulate emissions however. They are just what they sound like, tiny particles that are in the exhaust gasses as a result of the combustion process in a wood stove. Wow sometimes I really sound like I know what I am talking about. To be more precise the particles are a result of incomplete combustion. In the 70’s and 80’s when residential wood burning really started coming in to vogue because of high oil prices, these little particles were blamed for causing some big problems. In certain valleys temperature inversions were trapping wood stove smoke and all of those little particles. The inhabitants were literally choking on them. So people who knew how important breathing can be to good health and wellbeing decided to create some particulate emission regulations. The regulations required a reduction of the particulate production by approximately 80%. Stated in grams per hour (GPH) stoves that used to produce as much as 50 GPH were limited to producing 7.5 GPH or less.
Almost 15 years ago when I first started selling wood stoves in retail I learned about something called residence time. This is a key ingredient in making a wood stove that burns cleaner. The trick is to get heat, fuel, and air in the proper ratio, all to hang around long enough in the firebox to achieve high rates of combustion efficiency. Lucky for the wood stove manufacturers this wasn’t too difficult to accomplish.
Before I explain how it was done let me portray the combustion process in painful detail. To sustain a fire we need three things heat, fuel, and oxygen.
1-The heat initially comes from an ignition source such as a match and continues to be supplied, hopefully by the ongoing combustion process.
2-The fuel in this case is fire starter, kindling, and wood.
3-The air supplies the oxygen. Actually an air control on the stove allows us to regulate the flow depending on which stage of the combustion process we are in.
There are also four distinct stages of combustion when burning a piece of wood. They are typically all happening at once in a wood stove however.
1-The first stage is moisture evaporation and takes place as the wood is heated. (Even “dry” wood has moisture in it.)
2-As the temperature rises the second stage occurs when the wood molecules begin to break down and form hydrocarbon vapor. This is referred to as pyrolysis.
3-In the third stage as the temperature rises further the vapor ignites and combusts.
4-The fourth stage occurs when pyrolysis is complete and only charcoal is left to burn. In this stage there is little or no visible flame.
Initially when wood stoves were built they were no more than a box with a load door, flue exit, and an air control. Some had a top baffle that would create more residence time but also had an air control that could limit the airflow so much that the fire would only smolder. (These were the so- called “air-tight” stoves that people often refer to.) They were on the right track however with the top baffle, which is a horizontal plate that protrudes from the back and top of the firebox. There is a space in front of the baffle that allows exhaust to flow through and then over the baffle and ultimately out the flue exit.
The real solution for a cleaner burn was introducing a secondary air source under the baffle. The air is introduced beneath the baffle through tubes with holes drilled in them, or by using a hollow baffle with holes drilled in the bottom. There is also a design that allows the air to flow through holes in the back wall of the firebox just under the baffle. The air that flows out of these holes is routed through a manifold or in some cases an opening in the back or side of the stove. It is important to keep in mind that draft in the chimney is pulling the air through these holes.
So here is how it works! The heat in the firebox is rising as it makes its way to the exit. Much of the flammable vapor escapes the primary combustion and also rises along with the heat…stay with me now because this is the good part. All we need now for more complete combustion is air and it is waiting under the baffle. We’ve got heat, we’ve got fuel, and we’ve got air and that is all we need so we have FIRE!
As Martha Stewart would say “That’s a good thing”. This of course is a simplified version of how it happens. I’m sure our engineers could give a long drawn out dissertation on all of the intricacies that go into arriving at this happy ending, but not on my watch. Some other great benefits from this technology are more efficient stoves and cleaner, safer chimneys. It just doesn’t get any better than that!
article author: Tim Gerencer, Jotul
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