Factor #1: How EPA-Certified Stoves Achieve a Clean Burn
All EPA-certified stoves have certain elements in common: Each incorporates a much-improved method of injecting combustion air into the firebox of the stove. The goal is to provide air in all the right places, to maximize the mixture of air and fuel, thereby maximizing combustion.
Air for combustion that’s regulated by the user (by opening or closing a valve, turning a dial, or pushing an air control lever one way or the other).
If you have used a pre-EPA stove, you may have noticed that the air enters the stove in one or two specific places, usually through a hole covered by an adjustable valve of some sort. This design provides relatively poor distribution of air in the firebox. In these older stoves, especially during low burns, the combustion process is incomplete, since oxygen doesn’t mix effectively with the fuel. Much of the combustible gasses emitted by the burning wood are lost up the flue, unburned.
In most EPA-certified stoves, primary air enters the stove through a long, thin slit across the top of the firebox, providing a widely-distributed flow of air into the stove. This creates a better mixture of oxygen with the fuel load for more efficient primary combustion.
Then, instead of allowing the combustible gasses emitted by the fuel load to escape up the flue, EPA-certified stoves utilize a new, second phase of burning: the burning of the gasses. This is accomplished either by means of a catalytic combustor or by means of a (non-catalytic) secondary combustion air system.
How EPA-stoves accomplish this second phase of combustion categorizes them as catalytic or non-catalytic stoves.
Note: some pre-EPA stoves use catalytic technology. Here we will focus on EPA- certified stoves, which incorporate both a catalytic combustor and the other features needed to achieve EPA certification (such as better primary combustion air designs).
Catalytic stoves achieve clean burn by means of a catalytic combustor. This is a bit complicated, but worth understanding.
Catalytic combustors work the same way catalytic converters in cars do: as smoke passes through the combustor, it is burned, creating heat instead of pollution.
The added advantage of the wood stove catalytic unit, moreover, is that you gain the benefit of the extra heat produced.
A catalytic combustor
is a ceramic insert, round, square, or rectangular, with numerous small channels, or tubes, running through it. (It looks like a honey comb.) Applied to the surface of the ceramic is a layer of a catalytic chemical, generally platinum or palladium. When sufficient temperatures are reached, this chemical coating reacts with smoke passing through the channels, reducing the ignition temperature of the smoke, and causing the smoke to burn. The chemical coating is not burned or consumed in the process.
Some people think the EPA requirements mandate the use of a catalytic combustor. In fact, they do not. Stoves simply have to pass the test, and it is up to the manufacturer come up with a design that will pass.
Non-catalytic stoves (called non-catstoves) maximize combustion efficiency by providing a secondary combustion air system. The idea is to cause the smoke and gasses emitted by the fuel load to burn by injecting more air (secondary air) into the stove in just the right places.
To accomplish this, non-cat stoves have a set of perforated stainless steel tubes (or a perforated baffle, or some similar device) across the top of the firebox. These tubes introduce air all across the inside of the stove at the top, in just the right place to mix with the combustible gasses rising from the fuel load; and this maximizes the combustion of the gasses.
The stove user has no control of the flow of secondary air into the stove: it is pre-set by the design of the stove, so even at a low burn rate (when the primary air control is set on low), this second phase of combustion will take place, providing a clean burn and an efficient use of fuel.
Which is better: Catalytic or Non-catalytic?
Each has its own benefits. Catalytic stoves are generally a bit more efficient, and offer a slightly longer burn time than similarly-sized non-cat stoves. They tend to function extremely well in long, low burn cycles, but also require a bit more maintenance, on average. Non-cat stoves display a more active, bright fire picture through the glass door (virtually all new stoves have glass doors), and are generally slightly easier to operate.
But before you decide which is best for you, finish this section, and read about Buying a New Stove, then talk to the experts at your local stove shop.
~ Todd Woofenden
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