Secondary combustion is the technique that lets an outdoor fire burn with virtually no smoke. You’ll see it in smokeless fire bowls, outdoor pellet stoves and in what many people call a smokeless fire pit. But how exactly does it work, and why does a regular fire produce smoke? In this article we dive into the technology.

What is secondary combustion?

Secondary combustion is a second combustion phase in which the flue gases from the fire are burned again. In a normal fire, those gases escape as smoke. With secondary combustion they get an extra shot of very hot oxygen at the top, causing them to ignite after all. What you would normally see as smoke is thus converted into extra heat and bright flames. You’ll also hear it called double combustion or secondary burn.

Why does an ordinary fire smoke, anyway?

Smoke is caused by incomplete combustion. When wood burns, gases and small particles are released. If the fire doesn’t burn hot enough or gets too little oxygen, those particles escape unburned as smoke. Damp wood makes it worse: the moisture has to evaporate first, which lowers the temperature and creates even more smoke. A fire that smokes heavily is therefore only partially burning. Secondary combustion solves exactly that problem.

Primary and secondary combustion: the two phases

A low-smoke fire works in two phases that build on each other:

• Primary combustion: air flows into the fire from below and lets the wood burn. In a regular fire bowl or fire pit, this is where it stops and the rest escapes as smoke.
• Secondary combustion: a second airflow from above burns the rising flue gases. This phase is what makes the difference between a smoky and a clean fire.

How the double wall does its job

The secret lies in the double-walled construction. Here’s what happens step by step:

• Fresh air is drawn in at the bottom and feeds the fire.
• Part of that air flows upwards between the inner and outer wall and heats up significantly along the way.
• At the top, that hot air escapes through a ring of small holes, right in the middle of the rising flue gases.
• The hot oxygen causes the flue gases to ignite after all—and that’s the secondary combustion.

The space between the two walls acts as a kind of preheater. Cold air would only cool the flue gases, but this air is already red-hot by the time it flows into the top of the fire. As a result, the flue gases in the outdoor stove get exactly what they need to ignite: heat and oxygen at the same time. The result is a hot, bright flame with hardly any smoke.

Why temperature and oxygen make the difference

Flue gases only combust at a high temperature. A smouldering or just-lit fire is simply too cool for that, so the gases escape unburned. The double wall solves this by heating the secondary air strongly before it reaches the flue gases. This keeps the temperature at the top of the fire high enough to ignite the gases. Heat and oxygen together ensure a more complete combustion, and that completeness is exactly what low-smoke burning is all about.

What does secondary combustion deliver?

Secondary combustion makes a fire both cleaner and more efficient. You’ll notice this in four ways:

• Less smoke and particulate matter, so no stinging eyes and no nuisance for the neighbours.
• More heat, because the flue gases release their energy instead of escaping.
• Less ash and soot, because the wood burns more completely.
• More efficient burning, because you need less fuel for the same fire.

It’s especially the lower particulate emissions from wood smoke that make this a more sustainable way to burn outdoors. It’s more pleasant for you, less of a nuisance for your neighbours and much better for air quality than an old-fashioned, smoky fire.

Does the material of the fireplace matter?

The material partly determines how well the secondary combustion gets going. The wall must be able to withstand high temperatures and retain heat so that the air between the walls heats up properly. That’s why you mainly see low-smoke outdoor fireplaces in heat-resistant materials:

• Powder-coated steel: light, affordable and heat-resistant, with a sleek, modern look.
• Corten steel: forms a protective rust layer under the influence of the weather that stops further corrosion and gives a warm, industrial look.
• Stainless steel: rust-resistant and strong, popular for portable models.

Whichever material you choose, the technology stays the same. The difference is mainly in looks, weight and maintenance.

There is one misconception we’d like to clear up, because “smokeless” shouldn’t be taken completely literally.

Is a low-smoke fire really 100% smokeless?

No, an outdoor fire is never completely smoke-free. When lighting it, there’s always some initial smoke, because the construction first has to reach temperature. Only once the fire is burning well and the wall is hot enough does secondary combustion kick in and the smoke drops sharply. In practice, “smoke-free” therefore means low-smoke once the fire is fully burning. After a few minutes you’ll clearly see the difference: from some start-up smoke to a flame that hardly smokes at all.

Smokeless burning: how to get the most out of it

Smoke-free burning starts with the right fuel and good technique. Keep these points in mind to minimise smoke:

• Burn dry, untreated wood with a moisture content below 20%, or use dry wood pellets.
• Avoid wet wood, treated wood, cardboard and waste, as these immediately produce more smoke.
• Fill up to a few centimetres below the top ring of holes so the secondary air supply remains unobstructed.
• Light the fire from the top for a quick and clean start.

That last method, lighting from the top, works better than you might be used to. Place the larger pieces of wood at the bottom and the kindling on top. The fire then burns slowly downwards, so the rising smoke is immediately drawn through the hot flame above and largely combusted. This way you limit smoke precisely in the phase when a fire normally smokes the most. If you prefer logs instead of pellets, choose well-seasoned firewood. The drier and purer your fuel, the calmer and clearer the flame.

Where will you encounter secondary combustion?

You’ll find secondary combustion in almost every modern low-smoke outdoor fire. A smokeless fire bowl like the Embra uses exactly this air circulation system, making it easy to take along to the garden or campsite. If you prefer a fixed stove with automatic pellet feed, an outdoor pellet stove works on the same principle, without gas or electricity.

• Share on:
• WhatsApp
• Deel
• X
• Pin it
• Messenger
• Email

---