27 January, 2015

Wood stoves with secondary combustion: Less smoke and more performance

In this blog we have been analyzing the benefits of using biomass to heat our home for profitability and ecology.
In this post I will discuss something even more beneficial for our pocket and nature: The technology of double combustion wood stoves. With this system, lacking the vast majority of wood stoves, get even greater performance avoiding much maintenance to avoid the generation of smoke, which the outlet tubes accumulate less soot and tar with use, produced by imperfect combustion of the gases expelled by the timber to catch high temperatures.
Soot particles are carbon that failed to burn and did not pass gas as CO and CO2.

Right smoke emitted by a conventional stove,
to the left. a stove with secondary combustion

In a perfect combustion of wood, the fumes would be almost transparent, emitting only water vapor and CO2 (carbon when combined with wood atmospheric oxygen). We can see in the video below comparing classic example stoves stoves with high temperature and double combustion:

The system is all the same; timber is placed in a slightly thermally insulated from the outside with firebrick to reach the highest possible temperature brazier, and oxygen (air) supplied to it previously passed through the sides and tubes which are heated with the same flame, introducing high temperature air, whereby unburned gases released combust spontaneously in oxygen at high temperature:

Secondary combustion system using preheated sheet

In the following video (in English) is explained in detail; shown how tubes at the top, with openings, introducing preheated air into the gases released from the wood causing a flare immediately and producing the effect of a natural gas burner, in fact seems to be gas.
The air naturally enters, by the need of the fire, and a fan cools the stove body, eliminating the risk of being deformed due to high temperatures while the entire emits heat to the room.

In this video we have no explanation other third party; apart from the primary combustion where the wood is, become fumes burning on top of the hot air contact.

In this last video is a detail poorly explained; the air of the upper tubes surely not fall behind, but on the side, unprotected without refractory to pick temperature heat pipes bricks, and with them the air flowing through them. This has the side effect and increase durability cool them, as does the water in a boiler, to which the tubes are welded, injecting the air at the top. In that stove air (oxygen) is divided by a rear tube, the front and upper tubes.

Double preheated combustion by tubes
Therefore, if we want to get the most out of our wood, ideally making a combustion chamber as efficient as possible, then we collect the calories in the second step, either a heat exchanger or with a fan to remove it in mostly before letting out the chimney. In homemade wood boiler like Jose and mine , the gases are cooled prematurely, so the smoke level is much higher than in a double combustion. Modifying easily can achieve much higher performance. I will keep you informed. Furthermore, this system want to use it in my next home stove; I have to consider how to prevent warping due to overheating and the weight of the refractory at the top bricks. To heat the air we can make the tube through contact with the upper chimney, but not sure how to do it and not have problems with the deformation of the tube or veneers.Surely airflow refrigerate and avoid critical temperatures for the tube, but the problem is in managing the air outlet and where to inject, which should be at the bottom part behind another, and another superior, distributing of most optimal way possible and that works by natural draft fire.

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